Vol 56 Issue 2 April 2012 - Johnson Matthey Technology Review · Published by Johnson Matthey Plc Vol 56 Issue 2 April 2012 E-ISSN 1471-0676 A quarterly journal of research on the

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E-ISSN 1471-0676 • Platinum Metals Rev., 2012, 56, (2), 61•

Editorial Team: Jonathan Butler (Publications Manager); Sara Coles (Assistant Editor); Ming Chung (Editorial Assistant);Keith White (Principal Information Scientist)

Platinum Metals Review, Johnson Matthey Plc, Orchard Road, Royston, Hertfordshire SG8 5HE, UKEmail: [email protected]

Platinum Metals ReviewA quarterly journal of research on the platinum group metals

and developments in their application in industryhttp://www.platinummetalsreview.com/

APRIL 2012 VOL. 56 NO. 2

Contents Catalysis in the Service of Green Chemistry: Nobel Prize-Winning 62 Palladium-Catalysed Cross-Couplings, Run in Water at Room Temperature By Bruce H. Lipshutz, Benjamin R. Taft, Alexander R. Abela, Subir Ghorai, Arkady Krasovskiy and Christophe Duplais

SAE 2011 World Congress 75 A conference review by Timothy V. Johnson

The Preparation of Palladium Nanoparticles 83 By James Cookson

Challenges in Catalysis for Pharmaceuticals and Fine Chemicals III 99 A conference review by John B. Brazier

“Higher Oxidation State Organopalladium and Platinum Chemistry” 104 An essay book review by Martyn V. Twigg

Commercial Development of Palladium(0) Catalysts for Highly 110 Selective Cross-Coupling Reactions By Thomas J. Colacot

PGM Highlights: Palladium-Based Membranes for Hydrogen Separation 117 By Hugh Hamilton

EuropaCat X 124

A conference review by Gordon Kelly and Steve Bailey

Publications in Brief 131

Abstracts 134

Patents 137

Final Analysis: Biological or Chemical Catalysis? 140 By David Ager

•Platinum Metals Rev., 2012, 56, (2), 62–74•


http://dx.doi.org/10.1595/147106712X629761 http://www.platinummetalsreview.com/

By Bruce H. Lipshutz*, Benjamin R. Taft and Alexander R. Abela

Department of Chemistry, University of California, Santa Barbara, CA 93106, USA

*Email: [email protected]

Subir Ghorai

New Product Research & Development, Sigma-Aldrich Chemical Company, Sheboygan Falls, WI 53085, USA

Email: [email protected]

Arkady Krasovskiy

Dow Chemical Company, 1776 Building, E-10C, Midland, MI 48674, USA


Christophe Duplais

UMR-CNRS Ecofog, Institut Pasteur de la Guyane, 23 Avenue Pasteur, 97306 Cayenne, French Guyana


Palladium-catalysed cross-couplings, in particular Heck,

Suzuki-Miyaura and Negishi reactions developed over

three decades ago, are routinely carried out in organic

solvents. However, alternative media are currently of

considerable interest given an increasing emphasis

on making organic processes ‘greener’; for example,

by minimising organic waste in the form of organic

solvents. Water is the obvious leading candidate in this

regard. Hence, this review focuses on the application

of micellar catalysis, in which a ‘designer’ surfactant

enables these award-winning coupling reactions to be

run in water at room temperature.

1. IntroductionDecades ago, before palladium-catalysed cross-

couplings arrived, copper was the transition metal of

choice for mediating carbon–carbon bond formation,

regardless of which organometallic complex was used

as the precursor to arrive at various Cu(I) reagents.

However, palladium eventually gained in popularity,

and in 2010 with the recognition of Heck, Suzuki and

Negishi as Nobel Prize recipients (1), the importance of

Pd-catalysed carbon-carbon bond-forming reactions

in organic synthesis was confi rmed.

Each modern organic synthetic reaction was

developed along traditional lines; that is, the chemistry

was matched, not surprisingly, to an organic solvent in

which the coupling best took place (Scheme I). And

while the presence of varying percentages of water

is not an issue for Heck reactions that lead (2–4) to

products such as cinnamates, and Suzuki-Miyaura

reactions that afford (5–7), for example, arylated

aromatics, the far more basic nature of organozinc

halides (RZnX; R = alkyl) (8–11) in Negishi couplings

(12, 13) that give products such as alkylated aromatics

Heck, Suzuki-Miyaura and Negishi reactions carried out in the absence of organic solvents, enabled by micellar catalysis

Catalysis in the Service of Green Chemistry: Nobel Prize-Winning Palladium-Catalysed Cross-Couplings, Run in Water at Room Temperature

http://dx.doi.org/10.1595/147106712X629761 •Platinum Metals Rev., 2012, 56, (2)•


precludes their use under aqueous conditions. None

of these would typically be thought of as amenable to

use in pure water especially at ambient temperatures,

pKa issues aside, if for no other reason than that

organic substrates are not normally soluble in water.

The world is paying increased attention to organic

waste produced by the chemical enterprise, and

organic solvents play a considerable role in this regard

(14).‘Sustainability’ is becoming a guiding principle in

many areas of science and engineering (15), and the

concept of ‘green chemistry’ is gaining in importance

(16–21). Much emphasis, therefore, is being directed

towards ‘alternative media’ (22, 23) in which synthetic

chemistry can be conducted. In this way, our

dependence on organic solvents, whether derived

from petroleum reserves or otherwise, is minimised.

It should be appreciated that green chemistry is an

all-inclusive term, and an evaluation of the full cycle

(a full life cycle assessment (LCA)) of all components

associated with a given process should be considered

in order to fully evaluate the ‘greenness’ of that

process (24–26). Clearly, reductions in major factors

such as the number of steps, the number of changes

in solvent(s), and the number of product isolations

can play a huge role in controlling the generation of

organic waste. Notwithstanding the many virtues of

water as a medium, its use could be costly in terms

of its downstream handling and processing, and if

heating is required either during a reaction or for its

eventual evaporation. But it is also acknowledged that

total LCAs can be both challenging to produce, and

expensive, while on the other hand, solvents, whether

organic or otherwise, are well known entities that can

be readily assessed, quantifi ed, and analysed in terms

of their use in manufacturing and waste disposal.

Scheme I. Traditional cross-coupling reactions going ‘green’

Replace with

water

Product

Organic solvent

Heck Suzuki Negishi

CO2R R–B(OH)2 R–ZnX

Therefore this review considers solvents alone with

regard to ‘greener’ processes.

The most likely alternative among the various

choices available (for example, ionic liquids (27–30),

supercritical carbon dioxide (31–37) etc.), in terms

of potential generality, is water (38–41). To get around

the substrate solubility issue, the leading candidate

technology appears to be micellar catalysis, in which

reactants can be ‘solubilised’ within the surrounding

aqueous phase by the addition of surfactants (42,

43). Although this approach to mixing ‘oil and water’

is decades old, the nature of the surfactants available

to the organic chemist through normal commercial

channels is actually very modest; a handful of each

type (ionic, nonionic and zwitterionic) is all that is

normally seen in the literature. It seems odd, given the

importance of solvent effects in organic chemistry

(24–26), that the choice of amphiphile supplying the

organic medium in which the chemistry is to take

place would be so limited. Moreover, most common

surfactants were created for use in the manufacture of

paint, cosmetics, oil, cleaning fl uids, leather, etc. rather

than for their use in organic synthesis.

To be able to run Heck, Suzuki-Miyaura and Negishi

cross-couplings in water at room temperature, thereby

totally bypassing organic solvents as the reaction

medium, as well as to derive energy savings by

avoiding any need for either heating or cooling reaction

mixtures, the requirements are: (a) identifi cation of an

existing, or possibly newly designed and synthesised

surfactant that leads to results that are as good as or

better than those in organic media; and (b) assurance

that any amphiphile chosen is fully compliant with

‘The 12 Principles of Green Chemistry’ (44).

2. The Amphiphile TPGS-750-MUnlike many surfactants that contain lipophilic, usually

hydrocarbon fragments that have been ‘PEGylated’

(PEG = polyethylene glycol), the newly designed

amphiphile DL--tocopherol methoxypolyethylene

glycol succinate (TPGS-750-M) (1, Figure 1) has three

components (45): non-natural -tocopherol (vitamin

E), a succinic acid linker, and methoxy polyethylene

glycol (‘MPEG-750’). This latter, hydrophilic portion

is monomethylated at one terminus and contains

on average 17 oxyethanyl units (750 divided by 44).

The ‘TPGS’ nomenclature derives originally from

Kodak’s ‘TPGS’, which by analogy is TPGS-1000, and

contains the same -tocopherol (albeit in its natural,

nonracemic form) and succinic acid linker, while the

PEG is PEG-1000 (which has ca. 23 oxyethanyl units

Catalyst LnPdR’

X



and a free hydroxyl residue at its terminus) (46).

These look very similar on paper, but their chemistry

is very different, allowing opportunities to fi ne-tune

the nanoreactors formed during micellar catalysis

that serve as an organic solvent-like medium for

homogeneous cross-coupling reactions in water.

In designing TPGS-750-M (45), it was anticipated

that the economics of its synthesis would be quite

favourable, since it is based on racemic vitamin E,

succinic anhydride, and a commercially available

MPEG. It can be prepared in >90% overall yield using

a simple two-step procedure (45). Couplings within

its larger nanomicellar interior should be as fast or

faster than those in other surfactants that form smaller

particles in water, since both size and shape appear

to be signifi cant (for example, TPGS-1000 forms ca. 13

nm spherical micelles, while TPGS-750-M forms ca. 60

nm particles). An important implication was that a Pd

catalyst would be found for each reaction type that

would function well under the high concentrations

typically found in micelles. However, it was far from

obvious whether the ‘rules’ of modern homogeneous

catalysis, in which each process and catalyst is

precisely matched with a particular organic solvent,

would apply to homogeneous catalysis taking place

at much higher concentrations within nanomicelles.

Therefore, a wide range of Pd catalysts, obtained from

Johnson Matthey, were screened for applicability

to transition metal-catalysed couplings under the

infl uence of the hydrophobic effect.

3. Heck ReactionsCinnamate-forming reactions between aryl bromides

and acrylates take place very smoothly at room

temperature within the nanomicellar environment of

TPGS-750-M (5 wt%) (45, 47), akin to those seen earlier

in the fi rst generation surfactant polyoxyethanyl

-tocopheryl sebacate PTS (PTS-600; Figure 2)

(48, 49). The keys to success in this type of coupling

are the use of 3 M sodium chloride solutions in

Fig. 1. Comparison of structures between ‘TPGS’ (TPGS-1000) and newly engineered TPGS-750-M

TPGS-750-M1(ca. 60 nm micelles)

Racemic vitamin E

MPEG-750

Succinic acid

Succinic acid

Nonracemic vitamin E

PEG-1000

TPGS-1000‘TPGS’(ca. 13 nm micelles)

H3C

CH3 CH3 CH3

CH3 CH3

CH3

CH3

O

OO

OO

H23

O

H3C

CH3 CH3 CH3

CH3 CH3

CH3

CH3

O

OO

OO

Me17

O



place of water alone, and either of the electron-rich

Johnson Matthey catalysts bis(tri-tert-butylphosphine)-

palladium(0) (Pd(tBu3P)2) or dichloro[1,1’-bis(di-tert-

butylphosphino)]ferrocene palladium(II) (PdCl2(dtbpf))

(Figure 2). Other catalysts, such as those derived

from various Pd(II) salts (for example palladium(II)

acetate (Pd(OAc)2), palladium(II) chloride (PdCl2), or

tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3))

as Pd(0) precursors in the presence of various

bidentate phosphine ligands were all less effective.

The ‘salting out’ effect (50) of the NaCl leads to a

complete alteration from spherical to worm-like

micellar arrays (as seen by cryogenic transmission

electron microscopy (cryo-TEM) analysis) (49) with

presumably greater binding constants of substrates

within the particles and hence, faster rates of reactions.

Triethylamine appears to be the common base for

these Heck couplings. Representative examples, in

general, of (E)-favoured arylated products are shown

in Scheme II.Likewise, (E)-stilbenes constructed from aryl halides

and styrene derivatives can be readily fashioned under

similar micellar conditions (Scheme III) (45, 47).

Reactions with aryl iodides and styrene derivatives

can be performed at room temperature in aqueous

micellar solutions using either deionised water or 3

M NaCl and typically are complete in less than four

hours (global concentration = 0.50 M). The product

stilbenes are obtained in excellent yields with >8:1

E:Z selectivity, and often precipitate directly from

the reaction mixture. As with the case of acrylates

(Scheme II), 3 M NaCl can be used to enhance the

rates of otherwise sluggish reactions.

4. Suzuki-Miyaura Cross-CouplingsBiaryl-forming reactions of arylboronic acids and

aryl or heteroaryl bromides can be effi ciently

catalysed by PdCl2(dtbpf) at room temperature in

aqueous solutions of either PTS or TPGS-750-M. Three

representative cases are illustrated in Scheme IV,

including the cross-coupling of a relatively hindered

2,4,6-triisopropyl-substituted bromide.

The air stable complex, PdCl2(dtbpf), has seen less

extensive use in organic solvents for Suzuki-Miyaura

chemistry than its diphenyl analogue, dichloro-

[1,1'-bis(diphenylphosphino)ferrocene]palladium(II)

(PdCl2(dppf)). Nonetheless, early reports from

Johnson Matthey suggested far greater activity

Fig. 2. Catalysts used in Heck reactions run in water at room temperature

Bis(tri-tert-butylphosphine)palladium(0)

Dichloro[1,1’-bis(di-tert-butylphosphino)]ferrocene palladium(II)

Pd(tBu3P)2

PdCl2(dtbpf)

PTS-600‘PTS’(ca. 25 nm micelles) Sebacic acid

Racemic vitamin E

PEG-600

H3C

CH3 CH3 CH3

CH3 CH3

CH3

CH3

O

OO

O

13O

H

O

P Pd PFe

P

P

PdCl2



of PdCl2(dtbpf) in promoting cross-couplings of

aryl chlorides over its more established relative

(51). However, under micellar conditions, with

challenging aryl chlorides, PdCl2(dtbpf) appears

not to be the catalyst of choice (Scheme V). The

Pd(0) catalyst Pd(tBu3P)2 showed little improvement,

despite the tBu3P ligand’s effectiveness at promoting

Suzuki-Miyaura couplings at room temperature in

tetrahydrofuran (THF) (52). And while dichlorobis(p-

methylaminophenyl-di-tert-butylphosphine)palladium(II)

(PdCl2(Amphos)2) showed a marked improvement,

the N-heterocyclic carbene (NHC) bound catalyst

phenylallylchloro[1,3-bis(diisopropylphenyl)imidazole-

2-ylidene]palladium(II) ([(IPr)Pd(cinnamyl)Cl] or

Neolyst CX31), which has been shown to be highly

effi cient in an organic solvent (53), was the most

effective for these reactions being carried out in

nanomicelles.

Interestingly, the scope of Suzuki-Miyaura couplings

in surfactant-water catalysed by this NHC-ligated

catalyst proved to be rather narrow, as PdCl2(Amphos)2

generally provided superior results with heteroaryl

chlorides (Scheme VI). More surprisingly, a large

number of aryl bromide combinations for which

PdCl2(dtbpf) had previously proven effective

(Scheme VII) failed to reach completion under

catalysis with the NHC complex, despite this catalyst’s

demonstrated competence with these substrate

types in organic solvents (53). The effectiveness of

PdCl2(Amphos)2 with heteroaryl coupling partners

was not unexpected given early reports by Amgen

researchers of its high effi ciency with educts of

this type in Suzuki-Miyaura reactions (54, 55).

trans-Dichlorobis(tricyclohexylphosphine)palladium(II)

(PdCl2(Cy3P)2) notably led to very low conversions with

heteroaromatic chlorides under PTS-water conditions,

in spite of previous reports of the Cy3P ligand’s

effectiveness in this role for reactions in dioxane/

water (albeit at elevated temperatures) (56). While an

extensive study comparing specifi c catalysts’ effi cacy

in water vs. organic solvent has not been undertaken,

and notwithstanding the corresponding change of

other parameters (most notably the choice of base),

catalysts PdCl2(dtbpf) and PdCl2(Amphos)2 clearly do

seem especially well-suited to use in water relative to

organic solvents.

5. Negishi-Like Couplings in WaterNegishi couplings today have come to imply a Group 10

metal-catalysed cross-coupling between an organozinc

reagent and an sp2-hybridised electrophilic partner

(12, 13). In fact, there were several other organometallics

Scheme II. Representative Heck couplings in water at room temperature using [Pd(tBu3P)]2

Pd(tBu3P)2 (2 mol%) Et3N (3 equiv.)

PTS or TPGS-750-M (5 wt%) 3 M NaCl, H2O, 4–14 h, RT

86%

90%

82%

84%

76%

O

O

O

OMeO

MeO

OMe

O O

OMe

O

O

O

O O

Cl

Cl

Cl NO2

CO2R’RR

Br+ CO2R’

NH



Scheme III. Representative Heck couplings to form stilbenes using PdCl2(dtbpf) in water at room temperature

PdCl2(dtbpf) (2 mol%) Et3N (3 equiv.)

PTS or TPGS-750-M (5 wt%) H2O or 3 M NaCl, 2–4 h, RT

95% 94% 94%

95% 96% 97%

(>8:1 E/Z)

Scheme IV. Representative Suzuki-Miyaura couplings in water at room temperature

PdCl2(dtbpf) (2 mol%) Surfactant-H2O (2 wt%)

Et3N (3.0 equiv.), RT

93% (2 h) (TPGS-750-M)

88% (24 h) (TPGS-750-M)

99% (6 h) (PTS)

that were utilised by the Negishi school before zinc

(for example, aluminium, zirconium and boron)

(57–61). The synthetic potential of zinc reagents, RZnX,

however, is undeniable, as they possess a suitable level

of nucleophilicity to participate in a transmetalation

step to a Pd(II) intermediate, while being especially

IR

BnOOH

OBn

NC

+ Ar RAr

NNN

octyl

NTs

Me

MeOMe

F

MeO

Cl

OMe OMe

EtO2C

ArBr + Ar’B(OH)2 Ar Ar’

OMeN

N

S

N



tolerant of most functionality (62). Unfortunately,

however, they are defi nitely not tolerant of water (12,

13). Nonetheless, Negishi couplings can be, and have

been, conducted in pure water (63–65). The secret

to success in this unlikely methodology is avoidance

of the traditional up-front use of stoichiometric Zn

reagents; this is achieved by generating a zinc reagent

over time on the surface of the metal, surrounded

by the hydrophobic pocket of a nanomicelle. The

catalyst must: (a) be readily available while tolerating

exposure to water; and (b) respond favourably to the

hydrophobic effect associated with micellar catalysis.

One species has been identifi ed to date that meets

these criteria: PdCl2(Amphos)2 (54, 55) (2, Figure 3). Other species, including the parent bis-desamino

system PdCl2(tBu2PhP)2 (Figure 3), were screened,

but the rates of conversion were too low to make the

reaction synthetically viable. Interestingly, among the

Scheme V. Comparison of catalysts for Suzuki-Miyaura couplings of aryl chlorides in water at room temperature

Scheme VI. Comparison of NHC-containing catalysts with phosphine-ligand containing catalysts

GC conversion, Catalyst %, 23 h, RT

CX31 68CX32 68PdCl2(Cy3P)2 10PdCl2(Amphos)2 100

Conditions: 2% catalyst, 1 wt% PTS in water, 3.0 equiv. Et3N [(SIPr)Pd(cinnamyl)Cl] (Umicore Neolyst CX32)

Catalyst (2 mol%) PTS-H2O (1 wt%)

Et3N (3.0 equiv.)ArCl + Ar’B(OH)2 Ar Ar’

NF3C

NN

PdCl

Catalyst Time, h Temp., ºC Yield, %

PdCl2(dtbpf) 24 50 28CX31 4 RT 98

GC conversion after 24 h at 50ºC: PdCl2(dtbpf) 38%PdCl2(Amphos)2 83%PdCl2(

tBu3P)2 47%CX31 100%

Catalyst Time, h Temp., ºC Yield, %

PdCl2(dtbpf) 24 50 82CX31 11 RT 99

[(IPr)Pd(cinnamyl)Cl] (Umicore Neolyst CX31)

OMe

NN

PdCl



catalysts shown in Figure 3 are selected cases that

are well known to mediate Negishi couplings in THF

(66, 67). Clearly, in a nanomicelle, they are not the

preferred species.

There is a third essential component: the ‘gatekeeper’,

without which there is no coupling whatsoever:

tetramethylethylenediamine (TMEDA). TMEDA likely

plays several important roles in these couplings: (i)

to clean the metal surface for subsequent electron

transfer; (ii) to chelate and thereby stabilise the newly

formed RZnX; and (iii) to enhance the transfer of

RZnX into the nanomicellar interior.

When an alkyl halide (iodide or bromide), an

aryl bromide, excess TMEDA, and PdCl2(Amphos)2

are mixed together in water containing 2 wt% TPGS-

750-M (or PTS), nanomicelles are formed containing

high concentrations of these species. Upon addition

of either Zn powder or dust, chemistry takes place in

water at room temperature. The selective insertion of

Zn into the sp3-carbon-halogen bond via successive

electron transfer steps is presumed to form RZnX

on the metal surface, protected momentarily by the

surrounding micelle. The newly formed organozinc

halide, thought to be chelated by TMEDA, then enters

the hydrophobic interior where a coupling partner

and associated reagents are located.

As illustrated in Scheme VIII, C–C bond formation

takes place smoothly with aryl bromides, including

coupling with a secondary alkylzinc reagent to give

product 3 in good yield (45, 63). It is especially

worthy of note that traditional Negishi couplings

with aryl bromides in THF do not typically occur

at room temperature (Scheme IX; C) (12, 13);

heating at reflux is common, especially for non-

activated substrates. A control experiment using

RZnX (prepared in THF) in an aqueous surfactant

environment gave the expected low level of

conversion due to quenching of the organozinc

halide (A in Scheme IX). Thus, the hydrophobic

effect adds yet another benefit to these reactions

(B in Scheme IX).

Several heteroaromatic halides have also

been studied under micellar catalysis conditions

(Scheme X) (64). Here again, PdCl2(Amphos)2, used

in catalytic amounts (2 mol%), is crucial for success.

Bromides located on each position on a substituted

pyridyl ring lead to good yields of alkylated products,

although the parent 2-, 3- or 4-bromopyridines gave

Scheme VII. Comparison of catalysts for Suzuki-Miyaura couplings of aryl bromidesArBr + Ar’B(OH)2 Ar Ar’

Catalyst (2 mol%) PTS-H2O (2 wt%)

Et3N (3.0 equiv.)

PdCl2(dtbpf): 78% yield CX31: 47% GC conversion


OMe

F OMe

MeO


F




Fig. 3. Ligands/catalysts screened for Zn-mediated, Pd-catalysed cross-couplings in water at room temperature

Pd PCy3Cy3PCl

Cl

only traces of substitution. Heteroaromatics including

thiophenes, benzothiophenes, indoles and quinolines

appear to be amenable. As expected for organozinc

reagents, excellent tolerance to functionality in either

partner is observed. Noteworthy is the case of indole

derivative 4, where a secondary centre can be directly

inserted onto the ring in high yield.

In addition to aryl bromides, cross-couplings

involving alkenyl halides are also amenable using

catalyst PdCl2(Amphos)2, although in these cases the

added feature of olefi n geometry is present (Scheme

XI) (65). As anticipated, (E)-alkenyl halides, whether

iodides or bromides, retain their original geometry in

the coupled products. Likewise, (Z)-alkenyl halides

maintain their stereochemical integrity (45) which

was unexpectedly found not to be the case for the

corresponding reactions under traditional Negishi

coupling conditions in THF (66, 67). The positive

stereochemical outcome for these reactions in water is

a fortunate occurrence, since the choice of ligands that

help mediate these couplings (as noted above), at least

to date, is not broad.

[Pd(OAc)2]3 +

PCy2iPr

iPr

iPr Pd(OAc)2 + P(tBu)2Me

PhPh

PPd(dba)2 +

Pd PP OHHOCl

Cl

N

NN

iPr iPr

iPriPrPd ClCl

Cl

FeP

PPd

tButBu

tButBu

ClCl

Pd P(tBu)3(tBu)3P

Br

BrPdPd P(tBu)3(tBu)3P

Pd PPh3Ph3PCl

Cl

Pd PPh3Ph3PPPh3

PPh3

Pd PPCl

Cl

Pd PPCl

ClMe2N NMe2

2, PdCl2(Amphos)2



Scheme VIII. Representative Negishi-like couplings between two halides, in water at room temperature

R–X +

(R = primary or secondary alkyl)

Br

R’

PdCl2(Amphos)2 (0.5 mol%) TMEDA (3–5 equiv.)

Zn dust (3 equiv.) Surfactant-H2O (2 wt%), RT

R

R’

EtO2C

C7H15-n

82% (with PTS; X = I) 93% (with PTS; X = I) 74% (with PTS; X = I)MOM = methoxymethyl

3, 75% (with TPGS-750-M; X = Br)

Coupling with a secondary alkyl halide

71% (with TPGS-750-M; X = Br)

EtO2C EtO2C

CO2Et

BocOMOM

C10H21-n

O

N

Scheme IX. Comparison reactions: traditional Negishi coupling conditions vs. micellar conditions

OMe

Br

OMe

C7H15–n

n-C7H15ZnI in THF 2 mol% catalyst 2

2% PTS-H2O, RT, 12 h

n-C7H15I, Zn, TMEDA 2 mol% catalyst 2

2% PTS-H2O, RT, 12 h

n-C7H15ZnI or n-C7H15I, Zn 2 mol% catalyst 2

THF, RT, 12 h

A

C

B

Conversion = 30%

Conversion < 20%

Isolated yield = 90%



Scheme X. Cross-couplings of heteroaromatic and alkyl halides using catalyst PdCl2(Amphos)2 in water at room temperature

+

Zn/TMEDAPdCl2(Amphos)2

PTS-H2O, RT

(FG)

X

(FG’)

X

Scheme XI. Representative cross-couplings of alkenyl and alkyl halides, in water at room temperature

6. ConclusionsThe Nobel Prize awarded for the Heck, Negishi and

Suzuki cross-coupling reactions is further recognition

of the importance of the role that catalysis, in

particular by Pd, plays in society. But catalysis is

also a key component of green chemistry, and this

requires ligands on the metal that adjust catalyst

reactivity and selectivity. Catalysts that work well in

traditional organic solvents at modest concentrations

(usually 0.1 to 1 M in substrate) may not be the

species of choice under far higher concentrations in

an alternative medium such as nanomicelles in water.

A completely new set of factors that control catalyst

motion in and out of micelles and result in greater time

spent within nanoreactors (greater binding constants)

may require alternative or even newly devised ligands

for Pd-catalysed cross-couplings. Thus, as an outgrowth

of the hydrophobic effect, some rules for catalysis

may change, giving rise to both new discoveries and

opportunities for new catalysts.

Heteroaromatic

(FG’)

Heteroaromatic-alkyl

(FG)

Alkyl+

FG = functional group

N

OTBS

TIPS86%

N

SOO

S O N

O

CO2EtPh

Cl

CO2Et

Boc

71% 74%

82% 89% 4, 97%

3

n-C7H15BnO

85% (from the iodide; PTS) 87% (from the bromide; TPGS-750-M)

CO2Et4

(Z)-Alkenyl halides give (Z)-alkenyl products

CO2Et

91% (from the bromide; PTS)



AcknowledgementsFinancial support provided by the National Institutes

of Health (GM 86485) is warmly acknowledged. We are

indebted to Johnson Matthey for generously providing

the catalysts and ligands that were successfully applied

to the cross-coupling chemistry discussed herein.

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The AuthorsBruce Lipshutz began his career at University of California (UC) Santa Barbara, USA, in 1979, where today he is Professor of Chemistry. His programme has recently shifted towards green chemistry, with the specifi c goal of getting organic solvents out of organic reactions. For this, ‘designer’ surfactants have been introduced that allow for transition metal-catalysed cross-couplings to be carried out in water at room temperature.

Ben Taft received his BS in Chemistry at California State University, Chico, in 2004. He took a PhD from UC Santa Barbara in 2008 with Bruce H. Lipshutz. He then moved to Stanford as an NIH postdoctoral fellow under Barry Trost. He began at Novartis, in Emeryville, in 2011, where he works in the oncology discovery group.

Alexander Abela carried out his graduate work in the Lipshutz group at UC Santa Barbara, focusing on transition metal-catalysed Suzuki-Miyaura cross-coupling reactions in water and C–H activation chemistry. Following completion of his PhD, he joined the Guerrero group at UC San Diego in 2012 to continue his studies as a postdoctoral scholar.

Subir Ghorai took his PhD in 2005 from the Indian Institute of Chemical Biology. After a year of research at UC Riverside, he joined the Lipshutz group at UC Santa Barbara. Currently, he is in the Catalysis and Organometallics group at Sigma-Aldrich in Wisconsin, USA. His research interests are focused on catalysis, organometallics, and green chemistry.

Arkady Krasovskiy holds a PhD in organic chemistry from M. V. Lomonosov Moscow State University, Russia. He has extensive training in synthetic/organometallic chemistry gained during his postdoctoral time in the Knochel (2003–2006), Nicolaou (2006–2008), and Lipshutz (2008–2011) laboratories. Currently he is working in Core Research & Development at the Dow Chemical Company in Midland, MI, USA.

Christophe Duplais took his PhD in 2008 from the University Cergy-Pontoise in France, under the guidance of Gerard Cahiez. He then did a postdoctoral stay in the labs of Bruce Lipshutz at UC Santa Barbara, before accepting a position in 2011 with the CNRS, located in French Guyana.




SAE 2011 World Congress

Reviewed by Timothy V. Johnson

Corning Environmental Technologies, Corning Incorporated, HP-CB-2-4, Corning, NY 14831, USA


The annual SAE Congress is the vehicle industry’s

largest conference and covers all aspects of automotive

engineering. The 2011 World Congress was held in

Detroit, USA, from 12th–14th April 2011. There were

upwards of a dozen sessions focused on vehicle

emissions technology, with most of them on diesel

emissions. About 60 papers were presented on the

topic. In addition, there was one session on gasoline

engine emissions control with about six papers

presented.

This review focuses on key developments related to

the platinum group metals (pgms) for both diesel and

gasoline engine emissions control from the conference.

Papers can be purchased and downloaded from the

SAE website (1). As in earlier years, the diesel sessions

were opened with a review paper of key developments

in diesel emissions and their control from 2010 (2).

Lean NOx Traps Joseph Theis (Ford Motor Co, USA) reported (3) on

an interesting study in which they alternated lean

NOx trap (LNT) and selective catalytic reduction

(SCR) slices in one can to check the effect of NOx,

ammonia and hydrocarbon (HC) distribution on

deNOx performance. The total volume of the LNT

and SCR was constant as was the pgm loading on

the LNT, at about 3.0 g l–1. The system performance

improved as the number of alternating slices of the

LNT and SCR increased. As shown in Figure 1,

deNOx effi ciency for the eight segment system (four

pairs of LNT and SCR catalysts) was 81% in a reference

test at 275ºC, vs. 78% for four segments and 60% for

two segments. The reference single LNT with no SCR

catalyst had only 30% deNOx effi ciency. The authors

estimate that the pgm loading on the LNT-only system

would need to double to reach the performance of

the four- or eight-segment system. The authors also

show reduced nitrous oxide (N2O), ammonia, HC

and carbon monoxide (CO) emissions with the

segmented systems. Various dynamics are operative,

but the segmented systems tend to better match the

nitric oxide (NO) and ammonia concentrations in the

Vehicular emissions control highlights of the annual Society of Automotive Engineers (SAE) international congress



SCR, and alternating SCR slices better adsorb HCs for

enhanced utility.

Ford also reported vehicle and laboratory testing

on a second generation LNT + SCR system (4). The

diesel oxidation catalyst (DOC) (2.2 l, 7 g pgm) + LNT

(3.6 l, 10.8 g pgm) + SCR (4.9 l) + diesel particulate

fi lter (DPF) (6.6 l, 1.2 g pgm) system was installed

on a prototype Ford F-150 pick-up truck (2610 kg,

4.4 litre V8, turbo-diesel). The aged system (64 h,

750ºC) reduced NOx by 96% to 13.5 mg mile–1, and

HC emissions were 14 mg mile–1 (reduced by 99%),

bringing the vehicle to within the emerging California

third generation Low Emission Vehicle (LEV III)

programme limit values (30 mg mile–1 HC + NOx) on

the standard certifi cation test cycle. The laboratory

work focused on HC reductions from the system. The

SCR component reduced HCs by about 75%, mainly

by adsorption under rich conditions and oxidation

under lean conditions.

Low load (low temperature) NOx exhaust emissions

control is diffi cult using urea-SCR because urea cannot

be properly evaporated and decomposed to form

ammonia at low temperatures. Hiroshi Hirabayashi

(Hino Motors, Ltd, Japan) reported on a new HC-SCR

approach using a platinum catalyst on a front DOC,

and palladium/platinum catalysts on the DPF and

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Length, cm

0

10

20

30

40

50

60

70

80

90

100 KeyNet NOx conv. 8 × 0.64 cm ‘sandwich’

Net NOx conv. 4 × 1.27 cm

Net NOx conv. 2 × 2.54 cm

Net NOx conv. all LNT

N2O production

Net

NO

x co

nv. a

nd N

2O y

ield

, %

0.64 cm

All LNT system

2 × 2.54 cm system

4 × 1.27 cm system

8 × 0.64 cm system

Flow

LNT SCR

Fig. 1. LNT + SCR system performance improves if the catalysts are applied in alternating segments (3). Solid lines show net NOx conversion for each catalyst system; dotted lines show N2O production



rear DOC, in addition to an undisclosed HC-adsorbant

material (5). Fuel is dosed ahead of the front DOC

to provide reductant. The combination system has

a peak deNOx effi ciency at 200ºC of about 70%,

but it rapidly decreases to 20% at 275ºC. The system

achieved 37% deNOx effi ciency on the Japanese

2005 emission standards (‘JE05’) heavy-duty transient

certifi cation test cycle. Engine methods are used at

higher temperatures to reduce NOx.

Diesel Particulate Filters Papers at the 2011 Congress were offered on DPF

regeneration, and several papers were presented on

next generation DPF substrates.

More than ten years ago, in the fi rst wide-scale

application of DPFs for particulate control on light-

duty diesels, Peugeot chose a ceria-based fuel borne

catalyst (FBC) to facilitate the regeneration of the

DPF. Researchers at Rhodia and Lubrizol described a

new generation of FBC based on iron that improves

DPF regeneration characteristics with or without

pgms on the DPF (6). Compared to the original

which contained 30 ppm cerium and 10 ppm Ce/Fe,

the new formulation uses only 5 ppm Fe and gives

similar performance, resulting in half the ash load on

the DPF. The new FBC lowered the DPF regenerating

start temperature of a stock pgm-catalysed DPF (also

known as a catalysed soot fi lter (CSF)) from 410ºC to

360ºC, and increased the total soot burn from 12% in

the baseline ramp-up test (to 500ºC) to 75% with the

FBC-CSF combination. The improved regeneration

effi ciency and decreased temperature will reduce

thermal exposure of the SCR catalyst in Euro 6 systems,

as well as reducing the DPF regeneration fuel penalty

when the SCR system is located upstream of the DPF.

Shingo Iwasaki (NGK Insulators, Ltd, Japan)

updated the fi eld on their development of inorganic

DPF membranes to enhance fi ltration and reduce

back pressure (7). The membrane is added to the

inlet cells of the DPF and keeps the soot from entering

the wall, preventing rapid back pressure build up

in the early stages of fi ltration. In vehicle testing,

pressure drop was reduced by 30%–40% depending

on speed and soot load, relative to the same fi lter

without a membrane. This membrane benefi t was

also demonstrated on SCR-coated DPFs in engine

dynamometer testing. Alternatively, in engine tests the

investigators demonstrated that the membrane can

be used to increase the soot mass limit of a cordierite

DPF by about 2 g l–1 without a back pressure penalty

by applying it to a lower porosity substrate.

Thorsten Boger (Corning Inc, USA) and his team

took a different approach to reducing back pressure

in DPFs (8). They tightened the pore size distribution

and decreased porosity in the next generation

aluminium titanate fi lter to provide either a 2–3 g l–1

increase in soot mass limit, or a 20%–25% reduction

in back pressure, depending on cell geometry.

Catalysed samples of the low-pressure-drop version

had 20%–30% lower back pressure with no soot on the

fi lter and 15%–20% lower back pressure with 6 g l–1

soot loading shown in Figure 2. The soot mass limit

–20%

–15%

–30%

–20%

Ø 5.66” × 8”4 cyl. engine

DuraTrap AT 300/13 ACTDEV AT TW 300/10 ACT

0 g l–1 Soot 6 g l–1 Soot 0 g l–1 Soot 6 g l–1 Soot

Coating A Coating B(higher washcoat coating)

0

2

4

6

8

10

12

14

16

Pres

sure

dro

p, K

Pa

–30%DPF size: 5.66” × 6”Soot load: 6 g l–1

SiC commercial partAT with similar coating

SiC 300/10 OctosquareDuraTrap® AT 300/13 ACTDEV AT TW 300/10 ACTDEV AT TW 300/10 ACT

0 100 200 300 400 500 6000

5

10

15

20

25

30

Volume fl ow, m3 h–1

Pres

sure

dro

p, k

Pa

Fig. 2. Pressure drop comparisons for the next generation aluminium titanate fi lter (DEV AT), in the low-pressure-drop thin wall version. 300/10 refers to 300 cells per square inch with 10 mm wall thickness. Two different catalyst coatings and two soot loadings are shown at the left. Both asymmetrical cell technology (ACT) and ‘Octosquare’ have the entry cell larger than the exit cell to add ash storage capacity and lower pressure drop (8)



was similar to that of a silicon carbide (SiC) fi lter with

the same cell geometry, but the SiC version had 50%

higher back pressure. As a result of lower thermal

conductivity, the regeneration effi ciency of the new

fi lter in a standard drop-to-idle test at 575ºC was 6%

higher than the earlier version and 16% higher than

the SiC comparison.

Taking advantage of improvements in SCR

technology, future heavy-duty diesel engines will

be calibrated to produce higher NOx and lower

particulate matter (PM) in order to save fuel. This will

result in favourable conditions for passive oxidation

of soot by NO2 and will dramatically decrease the

need for active regeneration of the DPF at high soot

loadings. Less thermal mass will be needed in the

DPF to provide a buffer against uncontrolled active

regenerations. Boger and his colleagues gave a paper

on their next generation thin wall cordierite fi lter to

address this trend (9). Relative to the current offering,

the pore size distribution was tightened and made

nominally smaller, and the porosity was increased to

~55%. Wall thickness was reduced by 33% in the 200

cells per square inch (cpsi) geometry. To enable this,

the inherent strength of the cordierite was increased.

As with membrane technology, this redesigned

porosity allows little, if any, soot penetration into

the wall that causes rapid build-up of back pressure.

Also, there is little difference between the back

pressures of coated and uncoated fi lters. The result

is that soot-laden fi lters have 40%–50% lower back

pressure than their 2010 predecessors under a variety

of conditions. Interestingly, because of the reduced

thermal mass, skin temperatures are higher but

centreline temperatures are the same during active

regeneration, reducing the thermal stress in the part.

Although the authors made no mention of soot mass

limit impacts, the fi lter survived worst case drop-to-

idle testing at 3.5 g l–1 soot. In the presentation, the

authors added that the lower thermal mass of the DPF

allowed faster heat-up of a downstream SCR catalyst,

resulting in 10% more time for urea injection in the US

certifi cation test cycle. This can result in 15% lower

cumulative NOx emissions in the cold-start test (10).

The impact of catalyst washcoat on DPF

performance will become more important as more

functionality is added to the DPF. Koji Tsuneyoshi

and Osamu Takagi (TYK Corp, Japan) and Kazuhiro

Yamamoto (Nagoya University, Japan) showed

how washcoat loading can affect back pressure

and fi ltration effi ciency (11). They tested -alumina

washcoats on SiC fi lters with 16.5 μm average pore

size and 47% porosity. Doubling the washcoat loading

from an undisclosed reference decreased the time to

achieve 90% particle number fi ltration effi ciency by

50%, to 20 seconds. The time to reach 99% effi ciency

dropped by 40%, to 60 seconds. Back pressure

increased by 28%.

Diesel Oxidation Catalysts DOCs play two primary roles in commercial emissions

control systems: (a) to oxidise HCs and CO, either to

reduce emissions coming from the engine, or to create

exothermic heat used to regenerate a DPF; and (b)

to oxidise NO to NO2, which is used for continuously

oxidising soot on a DPF, and/or for enhancing the SCR

deNOx reactions, particularly at low temperatures.

Cary Henry et al. (Cummins Inc, USA) and Mario

Castagnola et al. (Johnson Matthey Inc, USA) used a

series of iterative reaction decoupling experiments to

explain interactions between HC and NO oxidation

(12). They showed that inhibition of NO oxidation in

the presence of HCs on Pt/Pd is due to the reduction

of NO2 during HC oxidation. Long chain alkanes had

a more adverse effect than short chain alkenes due

to their slower oxidation rate with oxygen. Decreasing

space velocity was shown to help NO2 formation in

the presence of HCs. Pre-storing HCs on the DOC

improved NO oxidation performance up to 300ºC. At

higher temperatures, coke formation from propylene,

for example, inhibited NO oxidation, but stored non-

coking HCs, like dodecane, maintained a positive

impact at the higher temperatures. It is hypothesised

that the stored HCs prevent over-oxidation of the

Pt surface, thus enabling more NO adsorption and

oxidation.

Researchers have been substituting Pd for Pt in

DOCs for several years. However, there have been few

published systematic studies on the effects of varying

the Pt:Pd ratio on DOC HC and NO oxidation and

durability under different conditions. Chang Hwan

Kim et al. (General Motors Co, USA) and Michelle

Schmid (Optimial, Inc, USA) investigated (13) the

relationship between the Pt:Pd ratio and catalyst

activity and stability by evaluating a series of catalysts

with various Pt:Pd ratios (1:0, 7:1, 2:1, 1:2, 1:5 and 0:1).

All bimetallic Pt-Pd catalysts showed better HC light-

off activity and thermal stability than the Pt- or Pd-only

catalyst. Small amounts of Pd (Pt:Pd = 7:1) reduced the

propylene light-off temperature from 205ºC to 155ºC,

but no further improvements were observed at higher

Pd levels. However, too much Pd (more than Pt:Pd

= 1:5) caused light-off to deteriorate. Hydrothermal

stability improves with even small Pd additions,

but further additions have minimal additional

impact. NO oxidation to NO2 was found to depend

directly on Pt content, with better NO oxidation at



higher Pt loadings. Similar durability trends were

observed as with HCs. Figure 3 shows a schematic

representation of these fi ndings. High propylene

concentration (1190 ppm vs. 260 ppm) increased

light-off temperatures by 20ºC independently of the

Pd level, but higher HC concentrations had only a

small impact on the NO2 yield. The presence of CO

promoted HC oxidation, with increasing impact as

Pd levels increased. There was little or no impact of

CO on NO oxidation. At 300ºC, high concentrations

(5000 ppm) of either light or heavy HCs (propylene

or n-dodecane and m-xylene) increased HC light-off

temperature by 20ºC, although there was increasing

deterioration with increasing Pd content, especially

for the heavy HCs. HC type or levels had no impact on

NO oxidation, confi rming that Pt content is the only

driver for NO oxidation found in this study.

The use of biodiesel in place of petroleum-based

fuel is mandated in the US and Europe for the coming

years. The transesterifi cation step of the vegetable oil,

animal fat, or cooking oil feedstocks with methanol

involves the use of an alkali catalyst, which can result

in sodium and potassium ash in the fuel. Potential

adverse effects of this ash on diesel emissions control

components was described by a large collaborative

research group led by Aaron Williams (National

Renewable Energy Laboratory, USA) (14). Using an

accelerated fuel ash loading method, with alkali

components added at the maximum specifi cation,

combined with extensive dynamometer testing and

chemical and physical analyses, the group reported

that, after a simulated 150,000 miles of durability testing,

HC slip increased nominally by 20%–25% over the

range of temperatures in steady-state tests (240–390ºC)

as a result of alkali exposure from the biodiesel ash.

NO2 formation declined from 35% to 20%. In addition,

the thermal shock parameter of the DPF, as indicated

by mechanical property measurements, declined by

69% after simulated exposure of 435,000 miles, again

due to alkali attack of the cordierite substrate. NOx

emissions from the SCR increased by about 50%, but

more work was needed to determine whether this was

due to alkali attack of the zeolite catalyst. The group

concluded that operating with fuel at the maximum

alkali ash specifi cation will signifi cantly deteriorate

emissions control system performance.

Gasoline Emissions ControlIn the last year or two, the automotive industry has

become increasingly interested in gasoline emissions

control technology. This is driven by two regulatory

developments. The fi rst is the proposed tightening

of criteria pollutant tailpipe regulations in California

via the LEV III programme. Beginning in 2015 with

the aim of being fully phased-in by 2025, the formal

proposal is to tighten non-methane HCs (NMHCs) and

NOx limits by 70% (to 30 mg mile–1 NMHCs + NOx)

from the current LEV II standards, which are already

the tightest in the world. Improved three-way catalyst

(TWC) technology is the main approach to meeting

these standards. The second regulatory development

is the introduction of a particle number (PN) standard

for Euro 6 gasoline engines beginning in 2014. Auto

companies are moving towards direct injection

gasoline engines to meet tightening CO2 standards, but

these engines have higher PN emissions than multi-

port injection engines. As such, gasoline particulate

fi lters (GPFs) are emerging as a possible solution to

meet this regulation.

Yuki Aoki et al. (Toyota Motor Corp, Japan)

and Shingo Sakagami and Masaaki Kawai (Cataler

Corp, Japan) reported on complex TWC coating

architectures as a way of improving performance

and reducing pgm loadings (15). They showed

that HC light-off time is reduced by 50% if all the

Pd is concentrated in the front 20% of the catalyst

substrate. Conversely, because rhodium is poisoned

by phosphorus (from lubricant oil ash), the Rh should

be concentrated in the back 20% of the substrate.

They also show that ceria-zirconia washcoats can be

formulated for different properties and distributed

on the substrate accordingly. Zirconia-rich recipes

(0 to 0.40 ceria:zirconia mole ratio) release oxygen

fastest, and therefore should be in the front half of

the catalyst, while ceria-rich formulations (0.8 to

1.2) store more oxygen, and are best located in the

back half. To wrap up the study, they showed that an

Fig. 3. Conceptual impact of substituting platinum with palladium in DOCs, according to General Motors Co and Optimial, Inc (13). Moderate substitutions improve durability and HC oxidation, without signifi cant deterioration of NO oxidation

Stability after ageing

NO2 production

Light-off temperature

0.0 1.0Mass fraction Pd



alumina addition can prevent zirconia sintering and

allow better Rh dispersion, and niobia can prevent

grain growth of the Rh catalyst. Figure 4 shows the

improved performance of the new technologies

relative to a baseline catalyst.

Douglas Ball, Carlos Buitrago (Umicore Autocat USA

Inc) and Michael Zammit and Jeffery Wuttke (Chrylser

Group LLC, USA) (16) reported on meeting the LEV III

challenge more effi ciently by moving the under body

catalyst to a position directly behind the close-coupled

catalyst. HC and NOx emissions were cut by 25%.

They also looked at optimising pgm loadings with the

new design. Six formulations reduced pgm loading

by up to 25% from the previous partial zero emission

vehicle (PZEV) design, while meeting a 20 mg mile–1

NMHC + NOx limit on the federal test procedure (FTP)

light-duty transient cycle. This is low enough to meet

the lowest certifi cation level in the proposed LEV III

regulation.

In another contribution, Douglas Ball showed why

low-sulfur gasoline is an important enabler for modern

catalysts to meet the LEV III regulations (17). Figure 5

shows the results. If the fuel contains 33 ppm sulfur, the

poison builds up on the catalyst in back-to-back tests

(T1–T3, fi rst set of data). If a hot US06 high-load test

cycle is run, some of the sulfur is purged, dropping NOx

emissions by 30% (second set of bars). Alternatively,

and quite pertinent to urban low-load operation, if the

fuel sulfur is dropped to 3 ppm, the NOx emissions are

cut by 40% without the need for a high-load purge.

Lean burn direct injection engines are emerging

to meet the current and upcoming CO2 regulations

in the major world automotive markets. LNTs can

in some cases be used to meet the NOx regulations,

but Chang Hwan Kim et al. and Orgun et al. (General

Motors Co, USA) showed in two papers an alternative

design (18, 19). They describe a TWC + SCR

approach wherein ammonia generated from the

TWC during rich tip-ins in normal operation is stored

and utilised in a downstream SCR catalyst for use

during lean operation. The fi rst paper (18) shows

that ammonia production in the TWC is enhanced

with Pd-only formulations. Rh, although benefi cial

for CO oxidation and stoichiometric NOx reduction,

contributes little to ammonia generation. An oxygen

storage catalyst (OSC) detracts from ammonia

generation, but is benefi cial for CO and HC oxidation.

However, CO and HC oxidation can be promoted

with higher Pd loadings (200 g ft–3 vs. 60 g ft–3 in the

base formulation). The authors conclude that the

system might be suitable for European applications,

where the NOx regulations are not as tight, but more

research is needed on optimising pgm and washcoat

formulations, and improving the SCR catalyst

durability and performance. In the second paper (19),

the researchers used the TWC + SCR system in a

stoichiometric direct injection engine adapted for

lean idle and aggressive deceleration fuel cutoff

(DFCO), saving 11% on fuel consumption. DFCO

sends air through the system, cooling the catalyst and

Fig. 4. New TWC coating architecture decreases stoichiometric gasoline emissions relative to a conventional double-layer catalyst. The new designs can be used to decrease the rhodium loading by 45%, while still delivering a 20% NOx reduction (‘Developed Catalyst 2’), or can reduce emissions by 30 to 45% at the same pgm loadings (15)

NMHC CO NOx0

20

40

60

80

100

120

Rela

tive

emis

sion

s, %

Developed catalyst-1 Developed catalyst-2Conventional catalyst

Conventional

Developed

1 2

Zone-coatDouble layerCoat

Noble Pd/Rh Rh 45%Pd/Rh

Substrate volume: 0.9 l

Ageing: equivalent of 120K miles

Vehicle: ‘05MY ULEV/CAMRY

catalyst

metal reduced



saturating the OSC with oxygen, adversely impacting

stoichiometric deNOx functionality. The combination

aftertreatment system lowers NOx levels by 95% with

this engine strategy compared to a TWC only system.

ConclusionsTo meet tightening of criteria pollutant emissions

regulations and new fuel effi ciency demands, much

effort is being placed on decreasing emissions on

lean burn engines with less or better utility of pgm.

Lean NOx trap – SCR systems are being optimised

and are now moving onto much more vehicle testing.

Ammonia generation from the LNT and fi nding

optimum system confi gurations are at the forefront.

Lean burn engines have high particulate emissions,

and new fi lter designs (gasoline and diesel) are

improving performance. DOCs have complex

functionality, and even though they are decades

old, work is continuing to decrease pgm costs and

understand fundamental behaviour. Finally, studies

on meeting the next round of tailpipe emissions

tightening on stoichiometric gasoline engines show

impressive pgm utilisation gains, and the adverse

effects of fuel sulfur in advanced systems.

References 1 SAE International: http://www.sae.org/ (Accessed on 6th

February 2012)

2 T. V. Johnson, ‘Diesel Emissions in Review’, SAE Paper 2011-01-0304

3 J. R. Theis, M. Dearth and R. McCabe, ‘LNT + SCR Catalyst Systems Optimized for NOx Conversion on Diesel Applications’, SAE Paper 2011-01-0305

4 L. Xu, R. McCabe, P. Tennison and H.-W. Jen, ‘Laboratory and Vehicle Demonstration of “2nd-Generation” LNT + in-situ SCR Diesel Emission Control Systems’, SAE Paper 2011-01-0308

5 H. Hirabayashi, T. Furukawa, W. Koizumi, W. Koyanagi, Y. Saitou and S. Tanaka, ‘Development of New Diesel Particulate Active Reduction System for Both NOx and PM Reduction’, SAE Paper 2011-01-1277

6 L. Rocher, T. Seguelong, V. Harle, M. Lallemand, M. Pudlarz and M. Macduff, ‘New Generation Fuel Borne Catalyst for Reliable DPF Operation in Globally Diverse Fuels’, SAE Paper 2011-01-0297

7 S. Iwasaki, T. Mizutani, Y. Miyairi, K. Yuuki and M. Makino, ‘New Design Concept for Diesel Particulate Filter’, SAE Paper 2011-01-0603

8 T. Boger, J. Jamison, J. Warkins, N. Golomb, C. Warren and A. Heibel, ‘Next Generation Aluminum Titanate Filter for Light Duty Diesel Applications’, SAE Paper 2011-01-0816

9 T. Boger, S. He, T. Collins, A. Heibel, D. Beall and C. Remy,‘A Next Generation Cordierite Diesel Particle Filter with Signifi cantly Reduced Pressure Drop’, SAE Paper 2011-01-0813

10 A. Heibel, ‘Advancements in Substrate Technology’, SAE 2010 Heavy Duty Diesel Emissions Control Symposium, Gothenburg, Sweden, 21st–22nd September, 2010

18.8

24.9

2.9

26.4

4.3 5.6

9.3

12.0 11.6

6.65.7 9.2

Hot clean + 2 LA4s

no US06’s between

15.6 15.0

17.9

1.8

7.7

6.1

3.8

6.4

4.8

3.6

10.1

4.2

2 LA4s + US06,

US06’s between

13.9

2.9

10.3

16.0

5.5

4.8

5.6

11.5

3.6

7.3

Hot clean + 2 LA4s

no US06’s between

23.4

4.2

11.0

8.2

16.1

3.1

8.1

5.0

2.3

13.8

3.8

7.7

Averages

13 ppm 33 ppm 33 ppm 33 ppm 33 ppm 33 ppm 3 ppm 3 ppm 3 ppm 33 ppm 33 ppm 3 ppm

T1 T2 T3 US06 T1 US06 T2 US06 T3 T1 T2 T3 AVE US06AVE

AVE

0

4

8

12

16

20

24

28

32

36

40

Bag 3

Bag 2

Bag 1

NO

x, m

g m

ile–1

Summary of NOx Tailpipe Emissions by Bag

Fig. 5. Sulfur adversely impacts NOx emissions from TWC. It builds up in sequential urban testing (LA4 cycle, fi rst set of bars), but is partially purged in high-load cycles (US06, second set of bars). Purges are not needed if the fuel sulfur level is 3 ppm (17)



11 K. Tsuneyoshi, O. Takagi and K. Yamamoto, ‘Effects of Washcoat on Initial PM Filtration Effi ciency and Pressure Drop in SiC DPF’, SAE Paper 2011-01-0817

12 C. Henry, N. Currier, N. Ottinger, A. Yezerets, M. Castagnola, H.-Y. Chen and H. Hess, ‘Decoupling the Interactions of Hydrocarbons and Oxides of Nitrogen over Diesel Oxidation Catalysts’, SAE Paper 2001-01-1137

13 C. H. Kim, M. Schmid, S. J. Schmieg, J. Tan and W. Li, ‘The Effect of Pt-Pd Ratio on Oxidation Catalysts under Simulated Diesel Exhaust’, SAE Paper 2011-01-1134

14 A. Williams, R. McCormick, J. Luecke, R. Brezny, A. Geisselmann, K. Voss, K. Hallstrom, M. Leustek, J. Parsons and H. Abi-Akar, ‘Impact of Biodiesel Impurities on the Performance and Durability of DOC, DPF and SCR Technologies’, SAE Paper 2011-01-1136

15 Y. Aoki, S. Sakagami, M. Kawai, N. Takahashi, T. Tanabe and T. Sunada, ‘Development of Advanced Zone-Coated Three-Way Catalysts’, SAE Paper 2011-01-0296

16 D. Ball, M. Zammit, J. Wuttke and C. Buitrago, ‘Investigation of LEV-III Aftertreatment Designs’, SAE

Paper 2011-01-0301

17 D. Ball, D. Clark and D. Moser, ‘Effects of Fuel Sulfur on FTP NOx Emissions from a PZEV 4 Cylinder Application’, SAE Paper 2011-01-0300

18 C. H. Kim, K. Perry, M. Viola, W. Li and K. Narayanaswamy, ‘Three-Way Catalyst Design for Urealess Passive Ammonia SCR: Lean-Burn SIDI Aftertreatment System’, SAE Paper 2011-01-0306

19 O. Guralp, G. Qi, W. Li and P. Najt, ‘Experimental Study of NOx Reduction by Passive Ammonia-SCR for Stoichiometric SIDI Engines’, SAE Technical Paper 2011-01-0307

The ReviewerTimothy V. Johnson is Director –Emerging Regulations and Technologies for Corning Environmental Technologies, Corning Incorporated. Dr Johnson is responsible for tracking emerging mobile emissions regulations and technologies and helps develop strategic positioning via new products.




By James Cookson

Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, UK


Palladium nanoparticles are of great importance

as catalytic materials, as well as for a number of

other applications such as hydrogen storage and

sensing. Their synthesis has been wi dely studied

and interest in their properties is growing. Here the

synthesis of palladium nanoparticles by chemical and

electrochemical methods using a variety of stabilisers

including organic ligands, salts/surfactants, polymers

and dendrimers is reviewed and their potential benefi ts

in catalytic applications are introduced.

1. IntroductionNanotechnology represents one of the major

breakthroughs of modern science, enabling materials of

distinctive size, structure and composition to be formed.

Such nanodimensional materials (in the 1–100 nm

size domain) are seen as a bridge between atomic

and bulk materials and have been shown to exhibit

a variety of unique chemical, physical and electronic

properties (1). The study of these properties has

become an increasingly important area in chemistry,

physics, biology, medicine, and material sciences.

However, reliable preparations of the nanomaterials

are required for their exploitation, and this remains an

area of active research.

Whilst much research has focussed on

nanomaterials of the coinage metals (especially

those of gold) (2), interest in the properties of other

transition metal nanomaterials is also considerable

and growing (3). The high surface-area-to-volume

ratio makes nanomaterials highly desirable for use

as potential catalysts. Given that palladium is one of

the most effi cient metals in catalysis (4, 5), the study

of palladium-based materials is hugely important

and valuable. As a consequence, nanoparticles of

palladium have been heavily studied in a wide range

of catalytic applications including hydrogenations

(6, 7), oxidations (8, 9), carbon–carbon bond

formation (10, 11), and electrochemical reactions

in fuel cells (12). However, it should be noted that

Controlled particle sizes are key to producing more effective and effi cient materials

The Preparation of Palladium Nanoparticles



the applications of palladium go beyond catalysis.

For example, the propensity of palladium to adsorb

hydrogen has also led to palladium nanoparticles

being utilised in hydrogen storage (13, 14) and

sensing applications (15, 16).

In the present article, the synthesis of palladium

nanoparticles prepared via chemical and

electrochemical routes is reviewed. The preparation of

palladium nanoparticles with well-controlled particle

sizes and shapes (17, 18) of a high monodispersity is a

key technology in producing materials that are more

effective and effi cient than the current state of the

art. For example, particle size can play a critical role

in a catalytic process and a monodispersed particle

with an optimal size enables the most effi cient use of

the valuable metal and the highest selectivity in the

subsequent reaction.

2. Types of StabilisationAs nanoparticles are essentially fi nely divided bulk

materials, they are typically thermodynamically

unstable with respect to agglomeration. Consequently,

they need to be kinetically stabilised and this is

typically done using a protective stabiliser. The

stabilisation is achieved by electrostatic or steric forces

or a combination of the two (electrosteric forces). The

stabiliser is typically introduced during the formation

of the nanoparticles, and this is achieved via the

chemical or electrochemical reduction or thermal

decomposition of metallic precursors. The subsequent

interaction between the stabiliser and the surface of the

nanoparticle is a highly dynamic one, with its strength

and nature often controlling the long-term stability of

a dispersion of the nanoparticles. This interaction can

take many forms, such as a strong covalent linkage

(as in the case of a thiol), a chemisorbed atom (for

example via a lone pair of a heteroatom in a polymer)

or an electrostatic interaction with a layer of anions

(within a double layer structure of a surfactant).

The formation of palladium nanoparticles stabilised

by the most common stabilisers (organic ligands,

surfactants, polymers and dendrimers) (Figure 1) are

discussed below.

3. LigandsOne of the most frequent methods of stabilising

palladium nanoparticles is by the addition of an

organic ligand that typically contains a heteroelement

bearing an accessible lone pair. The organic chain

of the ligand prevents agglomeration, whilst the

heteroatom binds strongly to the surface of the metal.

3.1 Sulfur-Based LigandsThe strong interaction between the platinum group

metals and soft sulfur-based donors make sulfur-

containing ligands highly effi cient stabilisers for

nanoparticles. In the 1990s, Brust demonstrated that

thiols made excellent stabilisers in the two-phase

preparation of gold nanoparticles (19). The 1–3 nm

diameter nanoparticles were stabilised by a monolayer

of thiolate ligands and were readily isolable as dry

powders and, subsequently, redispersable into non-

polar solvents. This methodology has been used to

prepare nanoparticles containing a wide range of

precious metals including palladium. In general,

it is the use of thiol and thioether ligands that

(a) (b) (c)

Fig. 1. Schematic representing the stabilisation of palladium nanoparticles using different protecting groups: (a) surfactants; (b) polymers; and (c) ligands



dominates the literature due to the high stability of the

nanostructures that they can generate.

The use of sulfur-based ligands to stabilise palladium

nanoparticles usually restricts their use as potential

catalysts due to the poisoning effect of sulfur (20).

However, this effect is not universal. For example,

dodecylthiolate-stabilised palladium nanoparticles

have been shown to be active catalysts for the

formation of carbon nanotubes (21). Furthermore,

they have been demonstrated to be a stable and

recyclable catalyst in the Suzuki-Miyaura C–C coupling

reaction of halogenoarenes and phenylboronic acid

(Figure 2) (22), while thiolated -cyclodextrin (HS-

-CD)-stabilised palladium nanoparticles have shown

good activity in the hydrogenation of allylamine

(Figure 2) (23).

3.1.1 Thiol LigandsAlkanethiol-protected palladium nanoparticles can be

directly prepared via the two-phase Brust methodology

(Figure 3) (24). Typically, a tetrachloropalladate salt

is initially phase transferred from aqueous to organic

solution (in solvents such as dichloromethane or

toluene) by the addition of a long-chain ammonium

salt (for example, tetra-n-octylammonium bromide or

Aliquat 336®). The stabilising ligand is then added to

the organic phase prior to reduction with an aqueous

solution of sodium borohydride. The ligand is usually

introduced as a long-chain thiol molecule; however,

the in situ reduction of an alkylsulfate (such as

S-dodecylthiosulfate (25)) has been demonstrated to

be an effective alternative strategy.

The size and morphology of the resulting palladium

nanoparticles are sensitive to a number of different

reaction conditions ( 26). These include the surfactant

used (27), the reducing agent employed, the reaction

time, the nature of the stabilising ligand (28), and the

ratio of the palladium precursor to the other reagents

(29). In general, to prepare small nanoparticles it is

benefi cial to use a large excess of long thiols with an

excess of reducing agent (26).

The Brust route has proved to be an excellent

preparative technique for the formation of

nanoparticles stabilised by simple lipophilic

R = H, CH3, CH3OX = Cl, Br, I

Pd(C12H25S–) NPs

THF/H2O (2:1)NaOH, rt

Pd(-CD-S–) NPs

H2 (1 atm)D2O, rt

(a)

(b)

Na2PdCl4 (aq)(i) N(C8H17)4Br (toluene or CH2Cl2)

(ii) RSH(iii) NaBH4 (aq)

Fig. 3. Scheme illustrating the synthesis of thiol-stabilised palladium nanoparticles using the modifi ed Brust method (24)

Fig. 2. Reported catalytic transformations using thiol-stabilised palladium nanoparticles (NPs): (a) Suzuki-Miyaura carbon–carbon coupling reaction of halogenoarenes and phenylboronic acid (22); and (b) hydrogenation of allylamine (23)



n-alkanethiols (24). The use of a two-phase system

also allows the organically soluble nanoparticles to

be simply separated from the aqueous byproducts.

However, when more polar ligands are used, the

purifi cation is less facile. There have been several

variations that have enabled the fabrication of

complementary nanoparticles that are soluble in

aqueous or polar solvent systems.

An alternative synthetic approach allowing a

wider variety of ligands to be used was achieved by

Ulman and coworkers, who employed Super-Hydride®

(lithium triethylborohydride, LiEt3BH) as a reductant

(30). The combination of palladium(II) acetate with

octylthiol resulted in a soluble metal-thiolate complex

in tetrahydrofuran (THF). Addition of Super-Hydride®

solution to this resulted in stable nanoparticles being

formed with an average diameter of 2.3 nm.

Whilst the Super-Hydride® route allows stabilisers

(such as hydroxyl-terminated alkylthiols) to be used that

cannot be used in the two-phase reaction, and also avoids

the use of phase transfer surfactants, there are limited

solvent systems that can be used and the reductant is

less easy to handle than sodium borohydride.

An alternative strategy for preparing palladium

nanoparticles that are soluble in aqueous systems

has been the use of -substituted thiol ligands (those

terminated with a charged functional group, such as

ammonium or carboxylate salts). Indeed, the one-

phase sodium borohydride reduction of potassium

tetrachloropalladate(II) in the presence of the chloride

salt of the N,N-trimethyl(undecylmercapto)ammonium

ligand yielded water-soluble palladium nanoparticles

with an average diameter of 2.7 nm (± 1.1 nm) (31).

Similarly, the functionalisation of water-soluble hosts

with thiol groups enables the preparation of ligands that

can stabilise the formation of palladium nanoparticles

in aqueous solvent systems. Cyclodextrins are

highly functionalised water-soluble hosts that can

easily be synthetically modifi ed (32). The sodium

borohydride reduction of a dimethyl sulfoxide-

water solution of the per-6-thio--cyclodextrin and

sodium tetrachloropalladate(II) produces a colloidal

dispersion (Figure 4) (23). The nanoparticles were

shown to be active in hydrogenation reactions and

could also be easily recovered by precipitation with

ethanol.

For many precious metal nanoparticles, a successful

way to introduce functionality to preformed

nanoparticles is via ligand exchange (33, 34). Typically,

the addition of a second ligand can statistically

displace those bound to the surface of the particle

without destroying the size of the metallic core. For

example, the addition of a thiol ligand containing

electrochemically active ferrocene derivatives could

displace hexanethiol- and dodecanethiol-stabilising

molecules (35), thus providing additional functionality

to the nanomaterial. In addition, the replacement of

one ligand by another may be desirable in order to

alter the solubility of the nanoparticles (for example,

from organic to aqueous solvent systems (36)) or

Fig. 4. Structure of thiol functionalised beta-cyclodextrin per-6-thio--cyclodextrin (HS--CD) (23)

7



to improve their solubility by introducing a better

stabilising group (26, 37).

3.1.2 Other Sulfur-Based LigandsGiven that there is much literature on the stabilisation

of palladium nanoparticles by thiol ligands and that

there is a strong interaction between palladium and

sulfur, there are surprisingly few other sulfur-based

ligands that have been reported as stabilisers. Two

such examples are the thioether (38) and thioester

(39) functional groups. The interaction between these

groups and the metal surface is weaker than for thiols

as the ligand is unable to form anionic species via

the same mechanism by which thiols readily form

thiolates. However, this can lead to advantages, with

greater surface accessibility in applications such as

catalysis and facile post-synthetic modifi cations via

ligand displacement reactions (40).

The use of thioethers has enabled stabilised

palladium nanoparticles to be prepared on a gram

scale by heating palladium(II) acetate in the presence

of an excess of the ligand (38). It was found that

thioether chains greater than six carbons in length

were required to form stable dispersions with a narrow

particle size distribution, with longer chain lengths

giving rise to smaller particle sizes. Furthermore,

n-dodecylsulfi de stabilised palladium nanoparticles

were shown to be active hydrogenation catalysts for

a range of olefi ns when used either unsupported or

following deposition onto silica. This illustrates the

accessibility of the surface when such ligands are

employed and their potential benefi ts in catalytic

applications.

3.2 Phosphorus-Based LigandsThe use of phosphorus-containing compounds in

the formation of nanosystems dates back to the 19th

century, when Faraday prepared gold nanoparticles

by reducing tetrachloroaurate salts with phosphorus

vapours (41). It is therefore not surprising that they

have subsequently been exploited in the stabilisation

of precious metal nanoparticles and subsequent

catalytic transformations (42, 43).

The use of phosphine ligands has been reported by

several authors for the preparation of monodisperse

palladium nanoparticles. Hyeon and coworkers

reported how the thermolysis of a preformed

palladium-trioctlyphosphine (TOP) complex yielded

such nanoparticles (44). Th e mean size could be

varied depending on the reaction conditions. For

example, 3.5 nm nanoparticles were formed when

Pd(acac)2 (acac = acetylacetonate) and TOP were

heated to 300ºC under an argon atmosphere. When TOP

was used in combination with oleylamine as a stabiliser

and solvent, larger monodispersed nanoparticles up to

7.5 nm could be formed.

A further benefi t of phosphine ligands is the

presence of a nuclear magnetic resonance (NMR)

active nucleus in close proximity to the metal surface.

Examination of 31P NMR spectra has given insights

into the coordination of phosphorus-based ligands to

metal surfaces (45). The coordination of the phosphine

ligand to a palladium nanoparticle typically results in

a downfi eld shift in the 31P NMR peaks compared to

that of the free ligand, such that it resembles that of a

Pd(0)-phosphine coordination complex. While this is

the case for triphenylphosphine-stabilised palladium

nanoparticles, the bond to the surface is weaker in TOP

and, consequently, the shift is not as great. It is thought

that strong van der Waals interactions between the

alkyl chains in the different TOP ligands hinder the

effective adsorption of TOP ligands onto the surface of

the palladium nanoparticles.

In addition to the direct preparation of phosphine-

stabilised nanoparticles, indirect ligand exchange

reactions can also be successfully undertaken. These

reactions are particularly effective when TOP-stabilised

palladium nanoparticles are used, as the weak binding

of the ligand on the surface allows easy displacement.

Son et al. demonstrated that a wide variety of mono-

and bi-dentate phosphines could be used to exchange

with TOP (Figure 5) (45). The methodology was also

used to introduce hydrophilic phosphines, which

were able to solubilise the resulting nanoparticles in

aqueous solvent systems. All of the exchange reactions

were achieved while maintaining the monodispersity

and particle size of the initial nanoparticles.

The preparation of phosphine-stabilised palladium

nanoparticles has also been undertaken by Fujihara

and coworkers using a two-phase process. They

adapted an existing route for preparing analogous gold

nanoparticles (46) by phase transferring potassium

tetrachloropalladate(II) into dichloromethane

using tetraoctylammonium bromide before sodium

borohydride reduction in the presence of optically

active bidentate BINAP (2,2-bis(diphenylphosphino)-

1,1-binaphthyl) ligands (Figure 6) (47).

The use of chiral (R)- and (S)-BINAP gave rise to

the corresponding chiral palladium nanoparticles, for

which circular dichroism spectra showed positive and

negative Cotton effects, respectively. The nanoparticles

were also effective as a catalyst in the hydrosilylation



of styrene with trichlorosilane, leading to the formation

of an asymmetric product. Subsequent oxidation

using hydrogen peroxide retained the chiral centre

and yielded optically active 1-phenylethanol in an

enantiomeric excess of 75%. This revealed the active

role of the chiral ligand surrounding the palladium

nanoparticles as both a promoter and an asymmetric

induction reagent.

Palladium nanoparticles stabilised by BINAP-

thioether-derivatives have also been used in the (quasi)

homogeneous C–C coupling reactions (Figure 7)

(48). The presence of the chelating diphosphine ligand

provided remarkable stability to the nanoparticles,

Pd(acac)2Excess ‘A’

300ºC

A = tri-n-octylphosphine (TOP)

B = PPh3 (TPP), PCy3 (TCP), P(2-furyl)3 (TFP),

Excess ‘B’

CH3Cl, rt

DPPE DPPP DPPBDPPF

(–)-BINAP (R,R)-NORPHOS BIPHEP (–)-DIOP

TPPDS BDSPPB

Fig. 5. Formation of phosphine-stabilised palladium nanoparticles via direct thermolysis and subsequent ligand exchange reactions (45)

Fig. 6. Structure of the optically active (R)-BINAP ligand (47)



preventing their aggregation throughout the process.

Furthermore, the nanoparticles could be subsequently

isolated and reused without any loss of catalytic

activity.

3.3 Nitrogen-Based LigandsThe use of electron-rich nitrogen-containing ligands

has been extensively used to stabilise precious metal

nanoparticles. The lone pair of the nitrogen species

(such as long-chain primary amines) is able to strongly

chemically adsorb onto the surface of the metal, with

the alkyl group preventing agglomeration via steric

stabilisation.

Mazumder and Sun prepared monodispersed

palladium nanoparticles by reducing Pd(acac)2

in oleylamine with boron tributylamine (BTB) at

90ºC (Figure 8) (49). The oleylamine served as a

solvent, stabilising ligand and reductant, with BTB as

a coreductant. When oleylamine alone was used as a

stabiliser/solvent, higher reaction temperatures were

required and, subsequently, transmission electron

microscopy (TEM) showed that larger and more

polydispersed nanoparticles were formed (44).

A wide range of alternate primary amines can also be

employed in this one-phase reaction. However, using

the saturated analogue dodecylamine did not provide

such high quality nanoparticles, suggesting that the

double bond present in oleylamine plays a crucial

role in particle stabilisation and the control of the

growth process. Furthermore, the palladium precursor

used was also observed to have a signifi cant effect on

the nanoparticles formed. The acac salt gave more

monodispersed distributions than the corresponding

nitrate or chloride salts. It is possible that the carbon

monoxide molecules generated in situ from the

decomposition of the acac ligand also play a key role

in controlling the particle growth process (44).

Primary amines have also been employed in two-

phase reactions. Choi and coworkers demonstrated

that particle size could be altered by varying the initial

ratio of palladium(II) chloride to amine and the length

of the alkyl chain (50). The particle size decreased

C8-BINAP-Pd NPs

THF, rt

Yield = 91%

C8-BINAP-Pd NPs

THF, KF, rt

Yield = 90%

C8-BINAP =

Fig. 7. (Quasi)-homogeneous carbon–carbon coupling reactions undertaken by BINAP thioether-derivatised palladium nanoparticles (48)



with increasing chain length, which corresponded to

superior catalytic performance in the electrochemical

oxidation of methane.

This demonstrates that, despite the presence of

amine groups, palladium is still able to act effectively

as a catalyst, largely due to the resulting accessibility

of the metal surface. The surface accessibility has been

examined by 1H NMR studies, which show that fast

exchange occurs between free and uncoordinated

amine ligands at the surface of the palladium

nanoparticles (51). In contrast, polyphosphine ligands

do not display fast exchange, with the ligands fi rmly

coordinated on the surface of the nanoparticles.

The weaker binding of the amino ligands makes

them more attractive for use in catalytic processes.

Furthermore, the presence of the ligand on the

surface may even provide additional protection to

the palladium nanoparticle when corrosive solvent

systems are employed.

In addition to aliphatic amines, several other

nitrogen-containing molecules have been used to

stabilise palladium nanoparticles. These include

aromatic amines (52), porphyrins (53), pyridyl groups

(54) and imidazole derivatives (55).

The readily available 4-dimethylaminopyridine

(DMAP) ligand has been widely explored as a stabiliser

for metal nanoparticles. Of these, palladium-based

systems have been used as catalytic microcapsules,

exploiting the non-bulky nature of the ligand, thereby

allowing access to the particle surface by organic

reactants (56).

Although the synthesis of DMAP-stabilised

palladium nanoparticles has been undertaken via

the reduction of sodium tetrachloropalladate(II)

using sodium borohydride in aqueous conditions

(57), its use has been better documented in a ligand

exchange reaction (54). Gittins and Caruso prepared

tetraalkylammonium bromide-stabilised palladium

nanoparticles in a two-phase (toluene/water) reaction

(Figure 9) (54). Addition of an aqueous solution of

DMAP gave rise to rapid and complete transfer of the

nanoparticles into the aqueous phase. This not only

offers a simple and effective method of transferring

nanoparticles into aqueous media, but also allows the

particle size of the initial palladium nanoparticle to be

maintained.

Recently, Serpell et al. have demonstrated that

imidazole derivatives can also be used as effective

stabilisers for palladium nanoparticles and the

subsequent deposition of these onto activated carbon

gives rise to an active catalyst for hydrogenation

reactions (55). Furthermore, the addition of hydrogen

bond donors into the aliphatic chain of the imidazole

derivative has proved effi cient in preparing core-shell

nanoparticle structures. For example, an appended

amide group is able to bind an anionic metallic salt

in close proximity to the surface of the nanoparticle

and its subsequent reduction generates the desired

core-shell structure (Figure 10). When gold is

added to a preformed palladium nanoparticle in

this way, the selectivity of the resulting catalyst in the

catalytic hydrogenation of 2-chloronitrobenzene to

2-chloroaniline is dramatically enhanced by the core-

shell system.

3.4 Other LigandsIn general, ligands containing heteroatoms stabilise

precious metal nanoparticles by forming strong

bonds with the surface. However, recently there have

been examples of nanoparticles being stabilised

with carbon-based ligands (58, 59). Palladium has

been extensively used as the contact metal of

choice in the fabrication of carbon nanotube-based

nanoelectronic devices and circuitries because of

its low contact resistance (60, 61), and so it is not

unexpected that carbon can be used to stabilise

nanoparticles. In fact, the bonding energy for a Pd–C

single bond is 436 kJ mol–1, even larger than that of

the Pd–S linkage (380 kJ mol–1) (62).

Stable palladium nanoparticles have been

prepared by passivating the metal cores with Pd–C

covalent linkages by using diazonium derivatives

as precursors (62). The addition of Super-Hydride®

to palladium(II) chloride resulted in the generation

of nanoparticles. Simultaneously, aliphatic radicals

generated by the reduction of diazonium ligands

formed the strong Pd–C linkages. A range of

50 nm 2 nm

(a) (b)

Fig. 8. (a) TEM; and (b) high resolution-TEM images of 4.5 nm Pd nanoparticles prepared by the reduction of Pd(acac)2 in oleylamine and BTB (Reprinted with permission from (49). Copyright 2009 American Chemical Society)



Toluene Water

DMAP

Fig. 9. Partitioning of palladium nanoparticles from organic to aqueous solvent systems via the addition of DMAP to tetraalkylammonium bromide-stabilised nanoparticles. Inset shows the resonance structures of DMAP illustrating the high electron density present on the donor nitrogen

(a)

(b) (c)

2 nm 1 nm

Reduce

Fig. 10. (a) Preparation of imidazole-stabilised palladium-based nanoparticles. The use of an anion binding group on the backbone of the ligand enables well-defi ned core-shell nanoparticles to be prepared. Aberration-corrected high angle annular dark fi eld STEM images clearly show the formation of the (core@shell) materials; (b) Au@Pd; and (c) Pd@Au nanoparticles (55)



spectroscopic and microscopic measurements were

consistent with the formation of carbon-stabilised

palladium nanoparticles (Figure 11).

4. SurfactantsThe use of salts/surfactants is a popular route to

stabilising metal nanoparticles. Frequently, tetra-

N-alkylammonium halide salts are chosen for

this purpose, although other analogous materials

(such as imidazolium-based ionic liquids (63–65))

can act as stabilisers using the same mechanism.

Here the stabiliser is able to prevent irreversible

agglomeration of the metal via a combination of

electrostatic and steric effects (66). It is thought that

surfactant-stabilised nanoparticles strongly adsorb a

layer of anions to the surface of the metal, which in

turn are surrounded by a layer of countercations, in

order to retain electroneutrality (Figure 12) (4, 67,

68). Both elements of the surfactant play a key role

in protecting the metal from agglomeration. Varying

the nature of the cationic component allows the

nanoparticles to be dispersed in either organic or

aqueous media. Furthermore, surfactants such as

cetyltrimethylammonium bromide (CTAB) enable

the generation of anisotropic particle shapes (69).

Bönneman demonstrated that the choice of metal

salt used was also vitally important (70). For example,

the hydrogen reduction of [(octyl)4N)]2PdCl4 only

gave a metallic precipitate, whereas the reduction of

[(octyl)4N)]2PdBr4 by hydrogen does not occur, even

at 50 bar H2. In contrast, the hydrogen reduction of

[(octyl)4N)]2PdCl2Br2 in THF yields 4 nm nanoparticles,

albeit after fourteen days.

Many routes have been employed for the synthesis

of surfactant-stabilised palladium nanoparticles.

Gittins and Caruso prepared tetra-n-octylammonium

bromide (TOAB) stabilised palladium nanoparticles

in a two-phase (toluene/water) reaction (54).

This is a modifi cation of the Brust route, which

used thiol ligands. In the modifi ed method, the

ammonium salt acts as both a quantitative phase

transfer agent and a stabiliser. Subsequent work has

illustrated that the two key parameters to obtain

small nanoparticles with a narrow size distribution

(a)

(b)

(c)

(d)

9 8 7 6 5 4 3 2 1 ppm

Fig. 11. 1H NMR spectra of Pd nanoparticles stabilised by metal-carbon bonds: (a) Pd–BP; (b) biphenyldiazonium (BP); (c) Pd–DP; and the parental diazonium ligands; and (d) decylphenyldiazonium (DP). The samples were all prepared in CDCl3 (62) (Copyright © 2008, Royal Society of Chemistry. Reproduced with permission)



were the concentration of the capping agent and

the stirring rate used upon addition of the reductant

(71). The latter can be explained by considering

that the reduction of Pd(II) to Pd(0) occurs at the

phase boundary (organic to aqueous). Therefore,

controlling the size of the water droplets formed in

the toluene/water mixture has a dramatic effect on

the subsequent particle size.

A similar approach was adopted by Bönneman

and coworkers (72), who combined the stabilising

agent (NR4+) with the reducing agent to allow a high

local concentration of the stabiliser to build up at the

reduction centre (entropic stabilisation factor), which

reduces the need to add an excess of the capping agent

or reductant. However, this has the disadvantage that a

stoichiometric ratio of the two is always maintained

and an additional cost of preparing the reducing agent

is present (Figure 13) (72).

Reetz also employed surfactants as stabilisers for

palladium nanoparticles, but using an electrochemical

synthetic procedure (Figure 14) (3). By using an

electrochemical cell with a sacrifi cial palladium

anode as the metal source, and with the supporting

electrolyte also acting as a source of stabilising

surfactant, stable nanoparticles can be formed (73).

Varying the current density infl uences the particle

size, with higher current densities giving rise to

smaller nanoparticles. The synthesised nanoparticles

precipitate out of the initial acetonitrile/THF

solvent system, but are subsequently redispersible.

Altering the surfactant can dramatically alter the

solubility of the nanoparticles – for example, using

tetraoctadecylammonium bromide gives nanoparticles that

are soluble in organic solvents; whereas using sulfobetaine

3-(N,N-dimethyldodecylammonio)propanesulfonate gives

nanoparticles that are soluble in aqueous solvent

systems.

One of the major advantages of using surfactants

as protecting agents for nanoparticles is that their

relatively weak and poorly defi ned interactions

with the metal surface give reagents a high degree

of accessibility to the surface of the nanoparticles.

This feature has been exploited in ligand exchange

reactions (where the surfactant can be displaced by

a stronger binding ligand (54)) as well as in catalysis.

For example, tetra-n-alkylammonium halide-stabilised

palladium nanoparticles have demonstrated good

catalytic activity in liquid phase hydrogenation

reactions. However, as alluded to earlier, over time

the nanoparticles precipitate out of solution under a

hydrogen atmosphere and are therefore ineffective

as semi-homogeneous catalysts. However, once

immobilised onto a solid support, the nanoparticles

remain active and the heterogeneous catalyst can

undergo numerous turnovers (74).

5. Steric Stabilisation (Polymers and Dendrimers)The stabilisation of nanosystems can also be achieved

by incorporating them within an organic matrix,

which can be either a fl exible polymer or a more

preorganised dendritic structure. The steric bulk of

this class of stabilising agents prevents agglomeration

of the nanoparticles to bulk metal (75, 76).

Cl–Cl– Cl–

Cl–

Cl–

Cl–

Cl–

Cl–Cl–Cl–

Cl–Cl–

Cl–Cl–

Fig. 12. ‘Electrosteric’ stabilisation of a nanoparticle by a surfactant. Halide ions are in close proximity to the positively charged nanoparticle surface and surrounded by the sterically bulky tetrabutylammonium countercations (Reprinted with permission from (4). Copyright 2007 American Chemical Society)

MXn + nNR4(BEt3H) MNP + nNR4X + nBEt3 + n/2H2

M = metals of Groups 6–11; X = Cl, Br; n = 1, 2, 3; R = alkyl, C6–C20

Fig. 13. Bönneman‘s method of preparing metal nanoparticles by combining the reductant and the stabilising surfactant (72)



5.1 PolymersPolymers, such as poly(N-vinyl-2-pyrrolidone) (PVP)

and poly(vinyl alcohol) (PVA), are widely used to

protect nanoparticles because of their commercial

availability at relatively low cost and their solubility in a

range of solvents, including water (Figure 15) (77, 78).

The use of polymers is often associated with the ‘polyol

method’, in which a metal precursor is dissolved and

reduced at high temperatures by an alcohol (typically

ethylene glycol) (78, 79). The standard procedure

requires several hours of refl ux to fully reduce the

precursors to their metallic state, although the

process can be hasted by irradiating the sample with

microwaves (80, 81). The use of alcohols as reducing

agents offers the advantage that any byproducts are

simple organic compounds, unlike the residues of

other reducing agents, such as borane derivatives (82).

In particular, PVP, poly(N,N-dialkylcarbodiimide)

and polyurea-stabilised palladium nanoparticles

give highly effi cient catalysts for Suzuki-Miyamura

reactions (83, 84). In addition, palladium-catalysed

Suzuki and Stille cross-couplings of aryl bromides and

chlorides were carried out under mild conditions and

with the recycling of the catalyst (84).

5.2 DendrimersDendrimers are macromolecules that, unlike polymers,

are perfectly defi ned on the molecular level (85, 86).

Their internal cavities behave as molecular boxes that

can entrap and stabilise metal nanoparticles, especially

if there are heteroatoms present in the interior of the

dendrimer. Two families of dendrimer have been

extensively studied and are commercially available:

poly(amidoamine) (PAMAM) and poly(propylene

imine) (PPI) (Figure 16). Both of these have been

employed in the synthesis of numerous metallic

nanoparticles (8 7, 88).

In order to prepare palladium-encapsulated

nanoparticles, metal ions are generally sorbed into

the interior of the dendrimer. Subsequent reduction

yields nanoparticles that remain encapsulated within

the structure of the macromolecule, which prevents

agglomeration to bulk metal (89). The size of the

nanoparticles is primarily determined by the number

of palladium(II) species preloaded into the dendritic

structure and the generation of the dendrimer

limits the particle size and maximum metal loading

that is possible. Typically, nearly monodispersed

nanoparticles in the range of 1–3 nm with a high

proportion of accessible metal sites are prepared by

this route (90). Whilst the interior of the dendrimer can

be tuned to control the metal nanoparticles, the periphery

of the dendrimer can also be modifi ed to allow solubility

in either organic (91), aqueous (92), supercritical carbon

dioxide (93), or fl uorous media (87).

There has been considerable research into using

dendrimer-stabilised nanoparticles for applications

such as catalysis (94, 95). The cavities of the dendrimer

are suffi ciently porous to allow the passage of

substrates and products to and from the reaction

media and the surface of the metal. Furthermore,

adjusting the ‘mesh’ of the dendrimer allows control

over the rate of reaction and the selectivity of the

catalytic process (Figure 17) (94, 96).

Dendrimer-encapsulated nanoparticles can also

be used in quasi-homogeneous reactions, with the

advantage that the process can be controlled due to

the well-defi ned dendritic structure. For instance, the

catalytic activity toward the hydrogenation of allyl

alcohol and N-isopropylacrylamide by hydroxyl-

terminated PAMAM dendrimer containing palladium

nanoparticles has been shown to be strongly affected

by the structure of the stabiliser. In particular, catalysts

based on high-generation dendrimers have been

found to be selective towards linear substrates,

because of their low porosity (96, 97).

Anodic dissolution

Metal ions Reductive adatom formation

Aggregation and stabilisation

Metal coreStabiliser

Precipitation

Precipitate

Ions

Adatoms

Colloid

Fig. 14. Reetz et al.’s postulated mechanism for the formation of electrochemically synthesised surfactant (R4N

+X−)-stabilised nanoclusters (3) (Copyright © 1999, Elsevier. Reproduced with permission)



5.3 Other Bulky LigandsRecently, the use of sterically bulky biomolecules

(such as proteins, polypeptides and DNA) has

attracted attention for controlling the growth of water-

soluble metal nanoparticles (98–101). Furthermore,

the functionalised nanoparticles offer a range of

bio-related applications, including the assembly of

hybrid structures, and biodetection (102). Varying the

amino acid sequence in stabilising peptide chains has

been shown to have a signifi cant effect on particle

size, as a peptide can act not only as a stabiliser but

also as a mild reducing agent. Huang and coworkers

demonstrated that increasing the proportion of

tryptophan in the peptide resulted in reduction of

sodium tetrachloropalladate(II) and tight binding to

the palladium surface (103).

The adoption of green chemistry techniques by

the use of naturally occurring products as stabilisers

for nanomaterials is also an area of huge potential.

For example, Varma has used tea and coffee extract

Mn+ +

Polymer-metal ion complex

Polymer-metal atom complex

Reduction

Mn+

Mn+

M

MPolymer-metal cluster complex (polymer protected metal cluster)

Protective polymer

Fig. 15. Schematic representation of the reduction process of metal salts in the presence of a stabilising polymer (78) (Copyright © 1998, Royal Society of Chemistry. Reproduced with permission)

Mn+

BH4–

Metal nanoparticle

Product Reactant

Fig. 17. Representation of the formation of metallic nanoparticles within a dendritic structure and their subsequent application in catalysis (Reprinted with permission from (94). Copyright 2001 American Chemical Society)

G1 PAMAM dendrimer G1 PPI dendrimer

Fig. 16. Structures of the two most commonly used dendrimer building blocks: poly(amidoamine) (PAMAM) and poly(propylene imine) (PPI)



to stabilise nanoparticles of palladium and other

precious metals (104). The polyphenols present in

the caffeine act as an effective reducing and capping

agent. The environmentally benign nature of the

caffeine (its high water solubility, low toxicity and

biodegradability) make this route highly desirable.

The use of stabilising agents with considerable

steric bulk in order to prevent agglomeration has

also been applied to resorcinarenes (105) and

non-functionalised cyclodextrins (106). Analysis

of palladium nanoparticles stabilised with

2-hydroxypropyl--cyclodextrin found that the cyclic

oligosaccharide was only physisorbed onto the surface

via hydrophobic interactions. This weak interaction

resulted in the nanoparticles showing good yields and

selectivities in Suzuki, Heck and Sonogashira reactions

in neat water under low palladium loadings.

6. ConclusionsThe utilisation of nanodimensional materials offers

signifi cant benefi ts in a range of different applications.

In order to maximise their usefulness, reliable

syntheses are required that can generate well-defi ned

nanoparticles with a high degree of monodispersity.

This aim is being achieved in the synthesis of palladium

nanoparticles by using organic ligands, surfactants and

sterically bulky molecules to control the synthesis. This

enables properties such as the size, shape, solubility

and surface functionality of the resulting nanoparticles

to be carefully tuned. Such materials are being

explored for many different applications, especially in

catalysis, where palladium can effectively catalyse a

range of different transformations.

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The AuthorJames Cookson is a Principal Scientist at Johnson Matthey Technology Centre, Sonning Common, UK. He obtained a DPhil in Inorganic Chemistry at the University of Oxford, UK, in 2004. After working at the Engineering and Physical Sciences Research Council, UK, he joined Johnson Matthey in 2005. His main interests are the synthesis of precious metal nanoparticles and their application in heterogeneous catalysis for fi ne chemical and pharmaceutical applications.




Reviewed by John B. Brazier

Department of Chemistry, Imperial College London, Exhibition Road, South Kensington, London SW7 2AZ, UK


The third meeting on “Challenges in Catalysis for

Pharmaceuticals and Fine Chemicals” took place on

2nd November 2011. Around ninety delegates, evenly

spread across academia and industry, attended the

event in London, which was organised jointly by the

Society of Chemical Industry (SCI) Fine Chemicals

Group and the Royal Society of Chemistry (RSC)

Applied Catalysis Group. Following the fi rst two

meetings of the series in 2007 (1) and 2009 (2), the

themes focused on the practical aspects of applied

catalysis including scale-up, fl ow chemistry and one-

pot multi-step procedures. Four of the seven oral

presentations and a third of the poster presentations

featured work on platinum group metals (pgms). The

following brief review brings together the pgm aspects

of the work presented.

Optimising Scalable CatalysisThe scale on which reactions are performed varies

across different areas of the chemical industry. Many

organisations employ separate groups of chemists to

work at different scales, from milligrams for screening

and assays to tonnes for the industrial production of

fi ne chemicals.

Rocco Paciello (BASF, Ludwigshafen, Germany)

leads the homogeneous catalysis group at BASF,

working across many scales from the initial chemical

idea through to the laboratory, miniplant, pilot plant

and fi nally the production plant. His group deal with

all aspects of homogeneous catalysis from computer

assisted design, synthesis and high throughput

screening to the technology necessary for the recovery

and recycling of catalysts.

L-Menthol is the world’s top selling aroma chemical

and thousands of tonnes are manufactured each

year. BASF produce around 40,000 tonnes per year of

citral at their plant in Ludwigshafen from which they

sought to develop a new enantioselective synthesis of

menthol. The most important step in this process is

the rhodium-catalysed chemo- and enantioselective

reduction of cis-citral (Scheme I). Identifying

the optimal process involved screening the Rh

source, chiral ligand, hydrogen pressure, temperature

Platinum group metals in practical homogeneous catalysis

Challenges in Catalysis for Pharmaceuticals and Fine Chemicals III



and reaction time on 1.5 ml scale. After extensive

mechanistic studies, a new catalyst system,

Rh(acac)(CO)2/Chiraphos/syngas, was developed

which gave good yields and selectivities and can be

effi ciently recycled (3). Syngas is required in order to

form the active precatalyst from the ‘resting states’ which

can then enter the catalytic cycle (Scheme II) (4).

The homogeneous catalysis group at BASF makes

considerable use of density functional theory (DFT)

calculations in their work. This has been of great

importance in aiding understanding of a process and

optimising lead structures once a ligand has been

found but has not yet been successful in predicting an

optimal catalyst system.

Practical Flow ChemistryFlow chemistry has been the focus of much research

but is still regarded by some as an interesting curiosity

rather than a useful industrial tool. Matthew Yates (Eli

Lilly, Indianapolis, USA) spoke of the advantages of

applying fl ow chemistry to catalytic processes in the

pharmaceutical industry. He highlighted three cases

in which the use of reactive gases (oxygen, hydrogen

or syngas) in conjunction with pgms is simplifi ed by

using a continuous fl ow process. Increased pressures

can be applied within the reduced volume of a fl ow

reactor without the concomitant safety problems and

plant equipment required for a batch process.

The most atom effi cient method for oxidations uses

molecular O2, however, this can present safety issues.

Use of fl ow chemistry for the oxidation of alcohols

allowed shorter reaction times, lower loading of

the homogeneous Pd(OAc)2/pyridine catalyst and

increased pressures while exposing only a small

portion of the entire reaction mixture to oxygen at any

one time. The optimised fl ow process used 8% O2 in

nitrogen, thus avoiding operating within the organic/

O2 explosion limits. Moreover the process is scalable,

providing yields up to 99% on a kilogram scale (5).

Hydroformylation chemistry is widely used in the

bulk preparation of aldehydes. A pulsed fl ow system

for hydroformylation of methyl methacrylate provided

signifi cant benefi ts over a batch process allowing

1000 psi of syngas to be used with a Rh catalyst such

as RhH(CO)(PPh3)3. 13 kg per day of purifi ed product

could be generated in this way, reducing byproduct

formation and the volume of waste.

In the arena of hydrogenations, safety and scalability

benefi ts are again evident. The use of a continuous

fl ow system allowed 1000 psi of H2 to be used, while

ensuring that the concentration of H2 present at any

given time was far below the explosive limit even if the

entire system released at once.

The presented work demonstrated that fl ow

chemistry can usefully be applied to pgm-

catalysed processes on multi-kilogram scales. These

methods have the potential to increase yields and

enantioselectivities by adjusting reactor residence

times and pressures while lowering associated risks

through reducing the volume of hazardous mixtures.

Flow chemistry is now more than a laboratory

curiosity and has a signifi cant role to play in the future

of the pharmaceutical industry.

One-Pot ReactionsAlthough processes can be optimised individually

for manufacturing, it is equally important to have

effi cient methods for the expedient synthesis of

diverse structures for discovery chemistry. This can be

achieved through multi-step, one-pot procedures.

The use of boronates as coupling partners in metal-

catalysed carbon-carbon bond forming reactions

remains a favoured choice in synthesis. Todd Marder

(Durham University, UK; now University of Würzburg,

Germany) presented his and Patrick Steel’s groups’

research into borylation reactions, including how

to combine borylations with Suzuki-Miyaura cross-

coupling reactions in a one-pot process.

Scheme I. BASF’s route to L-menthol using a rhodium-catalysed chemo- and entantioselective reduction of cis-citral

L-Menthol

OH

Asymmetric hydrogenation

Citral D-Citronellal

Cyclisation

Organoaluminium catalyst

L-Isopulegol

Hydrogenation

Nickel catalyst

O O OH?



The challenge until now has been that iridium-

catalysed borylation processes work best in non-

coordinating solvents such as hexane. Unfortunately

Suzuki-Miyaura cross-couplings, which use a Pd

catalyst, are most effective when performed in polar

solvents such as dimethylformamide (DMF), ethanol

or dioxane. Previously this meant carrying out

separate steps, hence two solvents, two work-ups and

often two purifi cations. Using an optimised protocol

with methyl tert-butyl ether (MTBE) as solvent, the

entire process could be performed sequentially in

one pot. Yields of 87%–94% of biaryls were achieved

with low catalyst loadings in a total reaction time of

9 hours under thermal conditions (6). Microwave

heating (W) signifi cantly accelerated the borylation

reactions while giving comparable yields. This allowed

the one-pot tandem C–H borylation/Suzuki-Miyaura

sequence to proceed in 95%+ yields on a range of aryl

and heteroaryl substrates with reaction times of under

an hour and in some cases as short as ten minutes (7).

The tandem one-pot principle has been extended

to a borylation/1,4-conjugate addition. A Rh catalyst

promotes conjugate addition, and microwave heating

accelerated both steps, reducing total reaction time to

minutes rather than hours. Choice of solvent proved

vital to the outcome of the reaction. After the borylation

step in MTBE, addition of the Rh catalyst and enone in

MTBE gave the 1,4-conjugate addition product in 30%–

69% yield. If iso-propanol (IPA) was used as the solvent,

reduction of the ketone via transfer hydrogenation

occurred providing the secondary alcohol product (a

three-step process) (8). The chemistry has been shown

to be suitable for array synthesis and the production of

compound libraries (Scheme III).

Borylation of C–H bonds as a means to further

functionalisation has been a major research theme

for many years (9) and John Hartwig (University

of California, Berkeley, USA) presented some of his

group’s contributions to this important area. Crude

reaction mixtures from the Ir-catalysed formation of

pinacol boronate esters can be used directly in the

synthesis of boronic acids, trifl uoroborates, halides,

nitriles, ethers and amines. A series of mechanistic

studies on the borylation reaction identifi ed that the

rate determining step in catalytic turnover is oxidative

addition of the aryl C–H to the catalyst (10). Creating

“Resting state” “Resting state”“Active catalyst”

Scheme II. Formation of the precatalyst and the catalytic cycle of the rhodium- catalysed reduction of citral (4)



a more electron-rich Ir centre by careful tuning of the

ligand gave rise to a more active catalyst. Switching

from the usual 4,4-di-tert-butylbipyridine (dtbpy)

ligand to a substituted phenanthroline resulted in

a catalyst which allowed a practical borylation of

octane catalysed by the same starting Ir complex.

As a complementary approach, intramolecular

silylation of aromatic C–H bonds can provide a useful

chemoselective handle for further functionalisation

of arenes (Scheme IV). Acetophenones can readily

be converted to the corresponding benzoxasiloles

without any inter mediate purifi cation and careful

choice of base allowed sequential Suzuki-Miyaura

and Hiyama couplings to be performed (11). This

practical approach to building molecular complexity

in two one-pot procedures not only provides rapid

accessibility to diverse structures, but also reduces

the number of manipulations required and waste

produced in the sequence.

Poster PresentationsThe award for best student poster went to Paul

Colbon (University of Liverpool, UK) who presented

a method for the direct acylation of aryl halides with

alkyl aldehydes. A palladium-amine cooperative

catalysis approach was successful in generating

alkyl aryl ketones from aryl chlorides using a

bulky, electron-rich monophosphine ligand (13).

Condensation of the amine with the aldehyde

produces an enamine which undergoes a Heck-

type arylation. Reductive elimination of Pd reforms

the enamine which hydrolyses to give the ketone

product and turns over the amine. In keeping with

a major theme of the meeting, this method can be

extended to a one-pot process where the aldehyde

is formed in situ by arylation and isomerisation of

allyl alcohol. Addition of pyrrolidine and a second

aryl bromide to the reaction mixture results in

acylation (14).

Selectivity in heterogeneous catalysis can

be achieved by modifying the formation of Pd

nanoparticles. James Cookson (Johnson Matthey,

UK) showed how nanoparticles formed in the

presence of a simple amine or amino acid could be

used in selective reductions. Alkynes were reduced

exclusively to alkenes and nitro chloro arenes could

be reduced to the corresponding amine without any

dehalogenation (15).

Probing the exact nature of pgm catalysts and the

changes they undergo during a catalytic process is

not easy. Anna Kroner (Diamond Lightsource, UK)

gave an overview of the techniques available at the

Scheme III. Scope and effect of solvent on the outcome of two-step, one-pot borylation/1,4-conjugate addition reactions (6–8)

[Ir(OMe)(cod)]2 (1.5 mol%)dtbpy (3 mol%)B2pin2 (1 equiv.)

Ar H

80 ºC, W, MTBE, 60 min

Ar BO

O

O

O

Ar

OH

Ar

aq. K3PO4

[Rh(cod)Cl]2 (1 mol%)MTBE, 100 ºC, W, 30 min

O

aq. K3PO4

[Rh(cod)Cl]2 (1 mol%)IPA, 100 ºC, W, 30 min

Ar = 3,5-substituted phenyl 2,6-substituted pyridyl 2,5-substituted furanyl 2,5-substituted thiophenyl

30–69% 30–68%

Pd(dppf)Cl2 (2 mol%)KOH (5 equiv.), H2O

80 ºC, W, MTBE

CO2Me

I

CO2Me

Ar 95–96%

Ar = 3,5-substituted phenyl 2,6-substituted pyridyl 2,5-substituted furanyl 2,5-substituted thiophenyl

[Ir(cod)OMe]2 (1.5 mol%)dtbpy (3 mol%)B2pin2 (1 eq.)80ºC, W, MTBE, 60 min.

[Rh(cod)Cl]2 (1 mol%)IPA, 100ºC, W, 30 min.

Aq. K3PO4

30–68%

Aq. K3PO4

[Rh(cod)Cl]2 (1 mol%)MTBE, 100ºC, W, 30 min.

Pd(dppf)Cl2 (2 mol%)KOH (5 eq.), H2O80ºC, W, MTBE

30–69%

95–96%pin = pinacolate

Ar

Ar

Ar–H

Ar

Ar

O

O

O

OB

O

OH

CO2Me

CO2Me

I



UK synchrotron science facility which could be useful

for the study of such processes. X-Ray scattering and

X-ray absorption spectroscopy are already available

at Diamond and a new dispersive extended X-ray

absorption fi ne structure (EXAFS) beamline should

come online in the near future to allow time resolved

studies at the microsecond level.

Concluding RemarksImproving the effi ciency and practicality of catalytic

processes can be achieved in a number of ways.

Finding optimal conditions through a directed

screening approach is one successful strategy, but

other factors play an important role. Achieving

multiple transformations in one-pot or taking

advantage of technologies such as fl ow chemistry can

have a dramatic impact on safety, waste management,

cost and hence the overall practicality of a chemical

process. In both industry and academia much

progress has been made in improving and widening

the scope of pgm methods, but signifi cant challenges

remain and many processes are still far from perfect.

What is certain is that the scientifi c community is

actively addressing these “Challenges in Catalysis for

Pharmaceuticals and Fine Chemicals”.

The next meeting in the series is expected to take

place towards the end of 2013 and will be announced

by the SCI and RSC in due course.

References 1 C. Barnard, Platinum Metals Rev., 2008, 52, (2), 110

2 F. Nerozzi, Platinum Metals Rev., 2010, 54, (3), 152

3 J. Schmidt-Leithoff, C. Jäkel and R. Paciello, BASF SE, ‘Method for Synthesizing Optically Active Carbonyl Compounds, US Patent 7,973,198; 2011

4 C. J. Scheuermann née Taylor and C. Jaekel, Adv. Synth. Catal., 2008, 350, (17), 2708

5 X. Ye, M. D. Johnson, T. Diao, M. H. Yates and S. S. Stahl, Green Chem., 2010, 12, (7), 1180

6 P. Harrisson, J. Morris, P. G. Steel and T. B. Marder, Synlett, 2009, (1), 147

7 P. Harrisson, J. Morris, T. B. Marder and P. G. Steel, Org. Lett., 2009, 11, (16), 3586

8 H. Tajuddin, L. Shukla, A. C. Maxwell, T. B. Marder and P. G. Steel, Org. Lett., 2010, 12, (24), 5700

9 I. A. I. Mkhalid, J. H. Barnard, T. B. Marder, J. M. Murphy and J. F. Hartwig, Chem. Rev., 2010, 110, (2), 890

10 T. M. Boller, J. M. Murphy, M. Hapke, T. Ishiyama, N. Miyaura and J. F. Hartwig, J. Am. Chem. Soc., 2005, 127, (41), 14263

11 E. M. Simmons and J. F. Hartwig, J. Am. Chem. Soc., 2010, 132, (48), 17092

12 C. K. Savile, J. M. Janey, E. C. Mundorff, J. C. Moore, S. Tam, W. R. Jarvis, J. C. Colbeck, A. Krebber, F. J. Fleitz, J. Brands, P. N. Devine, G. W. Huisman and G. J. Hughes, Science, 2010, 329, (5989), 305

13 P. Colbon, J. Ruan, M. Purdie and J. Xiao, Org. Lett., 2010, 12, (16), 3670

14 P. Colbon, J. Ruan, M. Purdie, K. Mulholland and J. Xiao, Org. Lett., 2011, 13, (20), 5456

15 P. T. Bishop, J. Cookson, F. E. Hancock, S. Hawker and R. J. McNair, Johnson Matthey Plc, ‘Supported Metal Catalysts’, World Appl. 2011/101,679

The ReviewerJohn Brazier is a Research Associate in the Department of Chemistry at Imperial College, London, UK. A background in mechanistic catalysis led him to his current position working on various aspects of both homogeneous and heterogeneous palladium catalysis with particular interests in the roles which solvents play in these reactions.

Scheme IV. Iridium-catalysed intramolecular silylation of aromatic C–H bonds and further functionalisation of the arene product (11)

(i) [Ir] (0.05 mol%) Et2SiH2, THF, rt

(ii) [Ir]/phen (1.0 mol%) norbornene, THF, 80ºC

[Ir] = [Ir(cod)OMe]2phen = 1,10-phenanthrolinedcpe = 1,2-bis(dicyclohexylphosphine)ethane

(i) Ar1–B(OH)2, K2CO3

Pd(PCy3)2Cl2

(ii) Ar2–I, NaOH Pd(OAc)2, dcpe

Ar1 = 3,5-dimethylphenylAr2 = 4-methoxyphenyl

75% 65%




An essay book review by Martyn V. Twigg*

Caxton, Cambridge, UK

*Correspondence may be sent via Platinum Metals Review: [email protected]

Both heterogeneous and homogeneous platinum

group metal (pgm) catalysts have remarkable

properties: invariably they have outstanding activities,

longevity, and often particularly attractive selectivities

so they are used in many demanding situations.

Therefore perhaps it is not surprising that the discovery

of heterogeneous catalysis involved platinum. In 1817

Humphry Davy discovered catalytic combustion by

placing a heated coiled platinum wire into a town

gas/air mixture that became and stayed white hot

as it sustained combustion (1, 2). This illustrated the

exceptional activity of platinum since the wire had a

very low surface area, and soon there were a number

of reports concerning catalysis by platinum, including

selective oxidations such as the oxidation of ethanol to

acetaldehyde or acetic acid (3). The interest in catalysis

continued to grow but the introduction of a successful

large scale industrial heterogeneous catalytic process

did not take place until Oswald’s work (4) culminated

in the introduction of the high-temperature selective

oxidation of ammonia to nitric oxide over platinum/

rhodium en route to nitric acid at the beginning of the

20th century (5). Now heterogeneous pgm catalysts

are at the heart of many industrial processes, and also

in environmental areas where platinum, rhodium and

palladium catalysts control exhaust emissions from

vehicles to keep urban air clean (6). In these solid

heterogeneous catalysts the reaction takes place on

surface atoms, often via mechanisms that have yet to

be completely elucidated.

In contrast, homogeneous soluble pgm catalysts

have the advantage of being discrete molecular

compounds that can be characterised in the solid

state and in solution by techniques used in organic

chemistry. The mechanisms of reactions catalysed

by them can be probed in this way, and as a result of

intense work over more than three decades much is

Edited by Allan J. Canty (School of Chemistry, University of Tasmania, Australia), Topics in Organometallic Chemistry, Vol. 35, Springer, Berlin, Heidelberg, Germany, 2011, 186 pages, ISBN: 978-3-642-17428-5, £171.00, €189.95, US$259.00

“Higher Oxidation State Organopalladium and Platinum Chemistry”



understood about the intimate mechanistic details of

homogeneously catalysed reactions. These include

hydrogenations, hydroformylations, carbonylations,

and especially carbon–carbon bond forming

processes that often provide elegant routes to desirable

organic compounds that are not easily accessed by

other means. The importance of this work has been

recognised by several Nobel Prizes, most recently

for C–C bond forming Heck-type coupling reactions

(7). Initially rhodium homogeneous catalysts were

the most signifi cant of the pgms, and now palladium

has this position, but platinum and rhodium as well

as ruthenium and iridium catalysts are important in

some areas.

During the catalytic cycle the metal centre in these

homogeneous reactions usually moves between two

oxidation states differing by two units, with the lower

oxidation state being stabilised by soft ligands like

mono- and polydentate phosphines.

These catalysts can be remarkably selective. For

example, in suitable situations when chiral ligands

are present chiral products can be obtained. The

facile oxidation state interchange is key for catalytic

activity, and for rhodium it is rhodium(III) and

rhodium(I) that are usually involved (and similarly

for iridium), while for platinum and palladium it is

the 0 and +II oxidation states. For instance, oxidative

addition of a R–X compound to a palladium(0) centre

affords a palladium(II) species, which after suitable

transformations, undergoes a reductive elimination

process to complete the catalytic cycle and reform

palladium(0) and an organic product.

Less well investigated are two oxidation state

interchanges at higher oxidation state levels in

organopalladium and organoplatinum compounds.

Almost two decades ago Allan Canty reviewed

the evidence for the possible presence of the +IV

oxidation state in homogeneous reactions catalysed

by organopa lladium species (8). A lot of work has

been done in this area since then, and the role of

palladium(IV) has been fi rmly established in some

important processes. The present monograph, edited

and partially written by Allan Canty, is concerned with

the ability of platinum and palladium species to access

these higher oxidation states, and their involvement in

stoichiometric and catalytic reactions. It is becoming

increasingly apparent that this chemistry may be

important in a variety of situations including oxidative

reactions. The book covers both basic chemistries

and applications in seven chapters written by authors

eminent in their fi elds of specialisation.

Higher Oxidation State Platinum SpeciesThe fi rst chapter, by Kyle Grice, Margaret Scheuermann

and Karen Goldberg (University of Washington,

Seattle, USA), is concerned with fi ve-coordinate

organometallic platinum(IV) complexes. Based on

kinetic studies it has long been believed that the

slow fi rst order reactions of octahedral low-spin d6

platinum(IV) ‘coordination complexes’ are dissociative

in nature and involve reactive fi ve-coordinate

intermediates (9). Five-coordinate platinum(IV)

intermediates are required as the preliminary step

in some ‘organometallic’ insertion reactions like C–C,

C–X and C–H reductive eliminations, and in -hydride

eliminations. Although isoelectronic rhodium(III) and

iridium(III) fi ve-coordinate complexes have long been

known, it is only in recent years that fi ve-coordinate

platinum(IV) complexes have been isolated and

characterised and their reactivity directly observed.

The fi rst was reported some ten years ago and is

shown in Structure 1. This is an almost perfect square

pyramidal compound, while the second one to be

isolated, Structure 2, is a distorted square pyramid

in which the metal is not in the plane of the base.

Clearly steric effects are important stabilising factors

in moderating their reactivity which enable them to be

characterised by nuclear magnetic resonance (NMR)

spectroscopy in solution and by X-ray diffraction

in the solid state. Notwithstanding this, processes

involving the addition of small molecules such as

dioxygen and carbon monoxide that can access

the metal centre have been observed, and study of

several of these compounds is shedding valuable light

on the chemistries of fi ve-coordinate platinum(IV)

complexes.

The next chapter, by Jay Labinger and John Bercaw

(California Institute of Technology, USA), is about

higher oxidation platinum species in C–H alkane

activation. For many years low-temperature ‘methane

activation’ has been a major goal of catalysis. At high

Pt

Ar

Ar

MeMe

MeNN

1

HBN NH

N NN

N

+

Pt

SiR3

HH

2



temperatures deuterium exchange takes place over

heterogeneous nickel catalysts (and others), and

the industrially important methane steam reforming

reaction takes place also over a nickel catalyst with

a similar activation energy. Under much milder

conditions unsaturated hydrocarbons and arenes

undergo deuterium exchange in solution (10) while

remarkably methane does so in the presence of

[PtCl4]2–

in solution (11). Therefore alkane activation

per se is not necessarily the major diffi culty in

producing useful chemical products directly from

methane. The need is to intercept short lived, highly

reactive intermediate species to give desired products

more rapidly than, for instance, reaction with hydrogen

(or deuterium) which gives no overall meaningful

reaction.

The Shilov system remarkably directly produces

methanol from methane at low temperature! Originally

an aqueous mixture of platinum(II) and platinum(IV)

chloroplatinates was used, and modifi ed systems were

subsequently developed. The original reports (12,

13) of this amazing chemistry were published in the

Russian literature during the late 1960s and early 1970s,

and the overall oxidation of methane by [PtCl6]2– is

shown in Equation (i).

CH4 + [PtCl6]2– + H2O

CH3OH + [PtCl4]2– + 2HCl (i)

The key step in this process is the initial reaction

of alkane with [PtCl4]2– which is rapidly followed

by oxidation of the thus formed alkyl complex with

[PtCl6]2–, presumably via a chloride bridge, which

is a unique oxidant because it cannot destroy

platinum(II), whereas to be effective alternative

oxidants must oxidise the alkyl complex while not

oxidising the platinum(II). Copper in conjunction

with molecular oxygen appears to be the most

effective alternative oxidant system (copper alone

is ineffective so the presence of oxygen is not

simply to re-oxidise the copper), and it is pertinent

to compare this process with the Wacker reaction

where palladium is also re-oxidised by oxygen in the

presence of copper (14). The practical diffi culties

with the powerfully oxidising platinum(IV) Shilov

system are reproducibility and shortened life caused

by erratic precipitation of platinum metal. It is

mainly because of this that the highly appealing and

intriguing reaction has not been commercialised,

although the mechanistic understanding that has

been gained shows the direction in which further

development needs to go.

Reactions of Palladium(IV) Complexes and Their MechanismsThe next chapter, by Joy Racowski and Melanie

Sanford (University of Michigan, USA), deals with

the formation of C–heteroatom bonds by reductive

elimination reactions from palladium(IV) complexes.

It is a comprehensive review covering published

material from 1986 to 2010. Transient palladium(IV)

intermediates have been proposed as the product

release stage in a number of important transformations

including arene and alkane functionalisations, allylic

acetoxyations, alkene borylations etc. And indeed

palladium(IV) compounds such as Pd(bipy)(CH3)3I

(formed by oxidative addition of CH3I to Pd(bipy)(CH3)2)

undergo facile reductive elimination to give ethane.

Since this reaction was reported (15), the area has

expanded considerably with examples of C–S, C–Se,

C–O, C–I, C–Br and C–Cl bond forming reactions, and

each of these are covered in some detail, including

the considerable amount of work that is being done

to confi rm the mechanisms of these processes. Very

importantly C–F bonds can be formed using XeF2 as a

fl uorine source (16), and although this area is still in its

infancy it may be expected to be developed as a route

to important fl uoro-compounds that are otherwise

diffi cult to access.

‘Palladium(IV) Complexes as Intermediates in

Catalytic and Stoichiometric Cascade Sequences

Providing Complex Carbocycles and Heterocycles’

is the title of the next chapter, by Helena Malinakova

(University of Kansas, USA). This deals with the

capability of some palladium compounds to mediate

sequential functionalisation of one substrate to create

multiple C–C or C–heteroatom bonds in a single

operation, for example in multicomponent annulation

reactions in which palladium(IV) complexes are

implicated as intermediates. Studies on isolatable

palladium(IV) compounds have been used to obtain

evidence for the participation of such intermediates,

and stoichiometric reactions in palladium provide

routes to 1,3-dienes, norbornene derivatives,

benzoxepines, benzopyrans and benzofurans that

are discussed in detail, as is the spectroscopic and

crystallographic characterisation of the relatively

rare palladium(IV) intermediates. Further work on

moderately stable palladium(IV) compounds is likely



to clarify whether they can participate in fundamental

processes other than reductive eliminations.

Organic Reactions Mediated by Palladium and Platinum ComplexesThe fi fth chapter is by Allan Canty and Manab

Sharma (University of Tasmania, Australia) and is

concerned with higher oxidation state palladium

and platinum 1-alkynyls. That is, species in which

the metal in an oxidation state greater than +II is

bonded to alkynyl ligands, which can be prepared by

oxidation of alkynylmetal(II) complexes and reactions of

organometal(II) complexes with alkynyl(aryl) iodine(III)

reagents. The metal(IV) compounds obtained are

octahedral, and only platinum compounds have been

characterised in the solid state by X-ray crystallography.

Some of these compounds decompose via reductive

elimination, generating C–C bonds (diacetylenes)

(17). Stoichiometric reactions of the palladium(II)

and platinum(II) complexes suggest that the higher

oxidation state complexes are feasible (undetected)

intermediates in some organic synthetic procedures.

Interesting metal–metal bonded systems that can be

seen as Pt(III)–Pt(III) or Pt(II)–Pt(IV) species have

been characterised as intermediates in the oxidation

of Pt(II) to Pt(IV) compounds, and the potential

implications for mechanisms of organic reactions

mediated by higher oxidation metal centres are

discussed. Clearly this is a very fertile area for much

future research.

There then follows a chapter on palladium(III)

species in synthetic and catalytic reactions by David

Powers and Tobias Ritter (Harvard University, USA).

Unlike platinum(III) coordination complexes their

palladium(III) counterparts are very rare, and for

example, PdF3 is better described as the Pd(II) salt

of the Pd(IV) complex anion [PdF6]2–. As previously

noted, few organometallic palladium(III) compounds

have been isolated and characterised – examples of

these are given in the book. There is however growing

evidence that such species might be important in

a variety of known palladium-catalysed reactions.

For instance, silver(I) is often used as a benefi cial

additive in practical palladium-catalysed oxidative

C–H coupling reactions, and one-electron oxidation

of [(bipy)Pd(Me)2] with AgPF6 affords a moderate

yield of ethane via a suggested mechanism involving

disproportionation of the fi rst formed palladium(III)

complex [(bipy)Pd(Me)2]+ to [(bipy)Pd(Me)(solvent)]+

and [(bipy)Pd(Me)3]+, followed by elimination of

ethane from the Pd(IV) compound – this mechanism

nicely explains the 50% yield of ethane. The possible

involvement of methyl radicals was discounted

because radical traps had no effect on the reaction.

However, the addition of isopropyl iodide to Kumada

and Negishi coupling reactions results in remarkable

accelerations in reaction rates that were attributed

to a radical pathway with transient palladium(I)

intermediates and isopropyl radicals with palladium(I)

and palladium(III) chain carriers. Palladium(III) has

also been proposed in dioxygen insertion reactions

into Pd(II)–methyl bonds, consistent with a radical

process that is light sensitive, and addition of a radical

inhibitor was needed to obtain consistent reaction

rates. Photolysis of isolated palladium(III) complexes

made by electrolysis gave products that were inhibited

by radical scavengers, consistent with photoinduced

homolytic Pd–C cleavage as the pathway to the

observed organic products, although other routes are

possible. It might be thought that palladium(III) forms

dimers, and oxidation of dinuclear palladium(II)

complexes can result in diamagnetic dinuclear

palladium(III) complexes with a metal–metal -bond

that is shorter than that in a comparable palladium(II)

complex (18). There is also evidence that such

species are present as intermediates in some catalytic

reactions. Thus well defi ned palladium(III) complexes

participate in productive organometallic reactions, and

their presence in better studied processes seems likely,

processes that previously were thought to proceed

via traditional two-electron monometallic palladium

redox cycles. This area is likely to yield many intriguing

results in the near future that may substantially change

the mechanistic view of some established reactions!

The fi nal chapter is entitled ‘Organometallic

Platinum(II) and Palladium(II) Complexes as Donor

Ligands for Lewis-Acidic d10 and s2 Centers’, by Marc-

Etienne Moret (ETH Zurich, Switzerland, and California

Institute of Technology, USA). The fi lled axial orbitals

of square planar palladium and platinum complexes

enable them to be ‘metalloligands’ for Lewis-acidic

metal centres, and a wide range of such donor-acceptor

metal–metal bonded species have been reported.

This nucleophilic reactivity is key in the oxidative

addition of alkyl halides to organopalladium(II) and

organoplatinum(II) complexes via an SN2 process,

and electron-rich complexes can be protonated on

the metal to give metal(IV) hydrides, which is the fi rst

step in the protonolysis of many Pt–C bonds. The early

examples of isolated metal(II) square planar adducts

with metal–metal bonds date from the early 1980s and

have Pt–Hg or Pt–Ag bonds, and since then a wide



variety of related compounds have been prepared.

There are several bonding patterns, some involving

interactions with ligands bound to the central metal,

but all have M–M interaction. Again methyl complexes

undergo some particularly interesting reactions, as

illustrated for instance in Equation (ii), which takes

place in solution at low temperature.

[(bipy)Pt(CH3)2] + AgBF4

[(bipy)(CH3)2Pt–Ag–Pt(CH3)2(bipy)]+ + BF4– (ii)

This compound is not stable but the presence of

metal–metal bonding in solution at low temperature

was confi rmed by NMR coupling constants. The

palladium counterparts are less stable and have a

reduced tendency to form donor-acceptor metal–

metal bonds. More recently exotic complexes have

been prepared in which a central metal, for example

copper(I), is coordinated to two pyridine nitrogen

atoms and two Pt(CH3)2 moieties forming part of a

macrocycle, as shown in Structure 3.

Thallium forms stable palladium–thallium bonds,

and a remarkable complex cation has a linear chain of

four Pd(II)–Tl(I) bonds. This tendency of thallium(I) to

be involved in extended structures is expanded upon

later. The chapter then details a bewildering array of

increasingly complex structures, mostly containing

metal–metal bonds that include alkynyl complexes,

diphosphine-bridged complexes and carbene

complexes, before examining electron transfer, ligand

migration and hydrocarbyl transmetallation reactions.

The reactions of the polymetallic compounds of the

type discussed in this chapter provide further chemical

insight into why adding copper(I) and silver(I) salts

to palladium and platinum-based catalytic systems

can enhance reaction rates. This may happen via

more facile alternative mechanisms involving

donor-acceptor metal–metal bonded species and

understanding here may lead to more productive

catalytic systems.

Concluding RemarksOverall this is a nice up to date book that provides

a very readable account of highlights in a topical

and exciting area of developing chemistry. All the

contributors and the editor, as well as the publishers, are

to be congratulated for making available such a well

produced and interesting monograph. The accepted

mechanistic pathways of many established catalytic

reactions are being questioned as increasing amounts

of evidence suggest that alternative mechanisms

involving higher oxidation state species are possible

and may well take place. For example, where metal

centres are in higher oxidation states they are likely to

form reactive free radicals more readily than in lower

oxidation state processes. This monograph will be of

value to all those who are working in the area, and it

should be in libraries wishing to keep up to date in

these chemical areas that have so much potential.

Notes and References 1 H. Davy, Phil. Trans. R. Soc. Lond., 1817, 107, 77. The

term catalysis was not introduced until 1836 when J. J. Berzelius fi rst used it to describe the effect that some reactions are accelerated by the presence of a material that is unchanged after the reaction.

2 The same year, 1817, also saw the foundation of what was to become the present Johnson Matthey company. For further details see: “Percival Norton Johnson; The Biography of a Pioneer Metallurgist”, D. McDonald, Johnson Matthey, London, UK, 1951; “A History of Platinum: From the Earliest Times to the Eighteen-Eighties”, D. McDonald, Johnson Matthey, London, UK, 1960; L. B. Hunt, Platinum Metals Rev., 1979, 23, (2), 68

3 M. W. Roberts, Catal. Lett., 2000, 67, (1), 1

4 The key Ostwald patents on ammonia oxidation over platinum included: W. Ostwald, ‘Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides’, British Patent 698, 1902; W. Ostwald, ‘Improvements In and Relating to the Manufacture of Nitric Acid and Oxides of Nitrogen’, British Patent 8300, 1902; with others in France, America and Switzerland. In Germany much earlier prior art by Kuhlmann prevented granting of patents so the process was developed secretly in that country.

5 For a review of the early history of catalytic ammonia oxidation over platinum catalysts see: S. J. Green, “Industrial Catalysis”, Ernest Benn Ltd, London, UK, 1928, pp. 130–146. The fi rst plants used corrugated platinum foil, platinum-based gauzes were introduced later.

Cu PtPtMe

MeMe

Me

PPh2

PPh2Ph2P

Ph2P

+

N

N

3



6 M. V. Twigg, Appl. Catal. B: Environ., 2007, 70, (1–4), 2

7 ‘Scientifi c Background on the Nobel Prize in Chemistry 2010: Palladium-Catalyzed Cross Couplings in Organic Synthesis’, The Royal Swedish Academy of Sciences, Stockholm, Sweden, 6th October, 2010: http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2010/advanced-chemistryprize2010.pdf (Accessed on 20th February 2012)

8 A. J. Canty, Platinum Metals Rev., 1993, 37, (1), 2

9 F. Basolo and R. G. Pearson, “Mechanisms of Inorganic Reactions”, 2nd Edn., John Wiley, New York, USA, 1967

10 C. Kemball, Catal. Rev., 1971, 5, (1), 33. Methane deuterium exchange is especially discussed in pages 34–36. Interestingly this overview was presented at the Fourth International Congress on Catalysis in Moscow, Russia, 1968

11 J. L. Garnett and R. J. Hodges, J. Am. Chem. Soc., 1967, 89, (17), 4546

12 A. E. Shilov and G. B. Shul’pin, Chem. Rev. 1997, 97, (8), 2879

13 A. E. Shilov and G. B. Shul’pin, “Activation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes”, Kluwer Academic Publishers, Dordrecht, The Netherlands, 2000

14 G. W. Parshall, “Homogeneous Catalysis”, John Wiley, New York, USA, 1980

15 P. K. Byers, A. J. Canty, B. W. Skelton and A. H. White, J. Chem. Soc., Chem. Commun., 1986, (23), 1722

16 N. D. Ball and M. S. Stanford, J. Am. Chem. Soc., 2009, 131, (11), 3796

17 S. L. James, M. Younus, P. R. Raithby and J. Lewis, J. Organomet. Chem., 1997, 543, (1–2), 233

18 F. A. Cotton, J. Gu, C. A. Murillo and D. J. Timmons, J. Am. Chem. Soc., 1998, 120, (50), 13280

The ReviewerMartyn Twigg retired as the Chief Scientist of Johnson Matthey in 2010. Dr Twigg was previously European Technology Director for the Environmental Catalysts and Technologies Division of Johnson Matthey in Royston, UK. He has authored or co-authored many research papers, written numerous chapters in encyclopaedic works, and edited and contributed to several books. He edits the book series ‘Fundamental and Applied Catalysis’, and a series on the kinetics and mechanisms of inorganic and organometallic reactions. He is on the editorial board of several journals, and maintains active associations with universities in the UK and elsewhere, with honorary positions at some.




By Thomas J. Colacot

Johnson Matthey, Catalysis and Chiral Technologies,

2001 Nolte Drive, West Deptford, New Jersey 08066,

USA


Palladium-catalysed cross-coupling has huge impor-

tance for the synthesis of many organic molecules

at both laboratory and industrial scale. A range of

commercially available preformed (R3P)2Pd(0) catalysts

have now been developed and are available from

Johnson Matthey. Some unique and highly selective

applications of these palladium catalysts for various

cross-coupling reactions, including novel reactions such

as carbohalogenation reactions, Negishi coupling of

aryl chlorides and copper-free Sonogashira coupling,

are highlighted in this review.

IntroductionPalladium-catalysed cross-coupling has become one

of the most powerful synthetic methods in organic

and organometallic chemistry. It has found numerous

applications (1–6) in the laboratory, in addition to

the commercial production of pharmaceuticals,

agrochemicals and electronic materials. The

importance of the method was recognised by the

awarding of the 2010 Nobel Prize in Chemistry to

the original pioneers: Professors Heck, Negishi and

Suzuki (7). For a historical perspective of Pd catalysed

cross-coupling contextual to the Nobel Prize see the

recent review (8).

It is now commonly accepted that LnPd(0) (n =

number of ligands) is the active catalytic species to

facilitate a cross-coupling transformation. However,

until recently there were not many commercially

available examples of preformed (R3P)2Pd(0)

catalysts. Recently, we reported our development

of a novel, general and practical synthetic route

to a variety of preformed L2Pd(0) catalysts (see

Scheme I). This process produced catalysts in

excellent yield, specifi cally using the bulky, electron-

rich tertiary phosphine ligands tBu3P, Cy3P, (o-tol)3P, tBu2(Ph)P, tBu2(p-Me2N-C6H4)P, (C5H4FeC5Ph5)(

tBu)2P

(Q-Phos) and tBu2(Np)P (see Table I). These L2Pd(0)

New preformed L2Pd(0) (L = tertiary phosphine) catalysts now available

Commercial Development of Palladium(0) Catalysts for Highly Selective Cross-Coupling Reactions



catalysts are now commercially available at scales

ranging from gram to multi-kilogram quantities

(9, 10). An example of the crystal structure of one of

the complexes is given in Figure 1 (11).

Although these L2Pd(0) complexes with the

exception of Pd(tBu3P)2 have only recently been

made available commercially (Table I), several have

shown some unique and interesting applications

as summarised below. A detailed review on the

applications of these catalysts as well as the newly

developed LPd based catalysts is underway (12).

Selected Applications of Pd(tBu3P)2

Of the various catalysts listed in Table I, Pd(tBu3P)2,

the potential of which was originally identifi ed by

Fu (2, 6) in the early 2000s, is one of the best new

generation catalysts. Pd(tBu3P)2 has been shown to

be superior in many applications when compared

to the classical Pd(Ph3P)4. Selected examples of aryl

chloride couplings using Pd(tBu3P)2 are shown in

Scheme II (13–16).

Selectivities of Pd(Cy3P)2 and Pd(tBu3P)2

Laboratory studies using preformed catalysts

and literature reports of in situ systems (17) have

shown that Pd(Cy3P)2 is a unique catalyst which

Scheme I. The development of a general and practical route to various preformed (R3P)2Pd(0) catalysts

++ bbaassee ++ nnRR33PP ((RR33PP))nnPPdd ++

XX == BBrr,, CCll

RROOHHPPddXX22

ddiieennee == CCOODD,, NNBBDD

OORR

PdX2 +base + nR3P ROH

(R3P)nPd +

OR

Diene = COD, NBD X = Br, Cl

Table I

Examples of L2Pd(0) Catalysts Developed with Some of Their General Applications

Ligand (L) Product

IDa

31P-NMR

, ppmb

General applications

tBu3P Pd-116 86.5 Negishi, Suzuki, Heck, Stille – aryl chlorides and bromides

Cy3P Pd-137 39.2 Suzuki coupling – aryl trifl ate in the presence of

the halide

(o-tolyl)3P Pd-141 -7.3 Monoarylation of amines/NH3 – aryl chlorides, bromides

and tosylatestBu2(Ph)P Pd-148 67.6 Suzuki coupling – aryl bromides and chloridestBu2(p-Me2N-C6H4)P Pd-149 64.5 Suzuki, Heck, Cu-free Sonogashira – aryl chlorides and

bromides

Q-Phos Pd-150 59.0 Carbohalogenation, Suzuki – aryl chlorides, bromides and

iodides tBu2(Np)P Pd-151 45.5 -Arylation, Suzuki, Negishi – aryl chlorides and bromides

aAll Product IDs are trademarks of Johnson Matthey PlcbNMR conditions are given in Reference (9)

P1 P1Pd1

Fig. 1. X-Ray structure of Pd(tBu2(Np)P)2, showing a linear P–Pd–P orientation



Cl

NHR2

R1

R3

R4

O+

NR2

R1 R3

R4

Enamine Followed by Heck Coupling of Ar-Cl5a

Suzuki Coupling of Ar-Cl5b

PhB(OH)2 +

Cl

SO2MeN

N

Ph

SO2Me

O

F3CGlycine Transportor (GlyT-1) Inhibitor

Negishi Coupling of Ar-Cl5c

MeO

ZnCl

+

C-H Activation5d

S

Br+

SMeMe

OMe

Cl

Enamine followed by Heck coupling of Ar–Cl (13)

Suzuki coupling of Ar–Cl (14)

Glycine transporter (GlyT-1) inhibitor

Negishi coupling of Ar–Cl (15)

C–H activation (16)

R1Cl

NHR2

R4

R3OR1

R4

R3

NR2

PhB(OH)2

Cl

SO2MeSO2MeF3C

NN

O Ph

OMeMeO

ZnCl

S Me

Br

SMe

Cl

Scheme II. Selected reactions of Pd(tBu3P)2

Scheme III. Selectivity of Pd(Cy3P)2 vs. Pd(tBu3P)2 in Ar-Cl coupling (18)

Cl

OTf

1.0 equiv.

B(OH)2

TfO

(HO)2B

1% Pd(Cy3P)2

98%

23%

1% Pd(t-Bu3P)2

+

+

2% Pd(Cy3P)2

2% Pd(t-Bu3P)2

83%

76%

Cl

OTf

TfO1% Pd(Cy3P)2

(HO)2B 1% Pd(tBu3P)2

98%

23%

OTf

Cl1.0 equiv.

B(OH)22% Pd(Cy3P)2

2% Pd(tBu3P)2

83%

76%

Cl

OTf



can selectively carry out aryl trifl ate coupling

in the presence of a chloride. Interestingly,

Pd(tBu3P)2 shows the reverse selectivity

(Scheme III) (18).

Unique Selectivity of Pd(Q-Phos)2

Among the many L2Pd(0) catalysts selected from

Table I or generated in situ using commercially

available ligands, Pd(Q-Phos)2 has recently

been identifi ed as a unique catalyst for effective

carboiodination, a new type of carbon-carbon bond

forming reaction reported by Lautens (Scheme IV)

(19). In this case the Heck reaction is inhibited, and it

is thought that the reductive elimination of the halide

might be taking place after oxidative addition.

Recently the same group modifi ed the reaction

conditions to make the iodide in 95% yield

(Scheme V) from the corresponding bromide by iodide

exchange (20).

Pd(tBu2(p-Me2N-C6H4)P)2 for Sonogashira and Negishi CouplingPreliminary studies indicate that Pd(tBu2(p-Me2N-C6H4)P)2

is a very good catalyst for the Sonogashira coupling of

heteroaryl chlorides or activated aryl chlorides in the

absence of copper (Scheme VI) (9). More detailed

work will be published later (21).

Lipshutz recently identifi ed Pd(tBu2(p-Me2N-

C6H4)P)2 as a very useful catalyst for the aryl

bromide Negishi coupling of alkyl zinc iodides

very effectively with excellent regioselectivity

(Scheme VII) (22, 23).

Pd((o-tolyl)3P)2 for Monoarylation of Ammonia and Primary AminesHartwig utilised Pd((o-tolyl)3P)2 as a unique

amination catalyst, in the presence of the hindered

Josiphos ligand CyPFtBu, for the monoarylation

of primary amines using challenging aryl tosylate

substrates under very low Pd loadings (Scheme VIII) (24). Hartwig believes that the rigidity of the

chelated ring might be one of the factors favouring

this selectivity.

This chemistry has also been extended to

monoarylation of ammonia with aryl and heteroaryl

chloride substrates, with selectivities for primary

aryl amines, A, up to 98% in good yield (Scheme IX) (25).

O

IPdL2 (5 mol%)

O

I

Ligand Yield

Q-PhosP(t-Bu)3PhP(t-Bu)2PCy3P(o-tol)3

82%61%76%0%0%

Fe

Ph

P(t-Bu)2

Ph PhPhPh

Q-Phos

1a 2a

Toluene, 90 °C, 16 h

I

O

PdL2 (5 mol%)

Toluene, 90ºC, 16 h

I

O

P(tBu)2Fe

PhPh

Ph Ph

Ph

Q-Phos

Yield

Q-PhostBu3PtBu2(Ph)PCy3P(o-tol)3P

82%61%76%0%0%

1a 2a

Ligand

O

BrPd(Q-phos)2 (5 mol%)

O

I

1b 2a (95 %)

Toluene, 100 °C, 16 hKI additive (3 eq.)

Br

O

Pd(Q-Phos)2 (5 mol%)

Toluene, 100ºC, 16 h KI additive (3 equiv.)

O

I

1b2a (95%)

Scheme IV. Pd(Q-Phos)2-catalysed carbiodination (19)

Scheme V. Carbobromination followed by iodide exchange (20)



Scheme VIII. Monoarylation of primary amine with aryl tosylate

Br

Pd(t-Bu2(p-PhMe2N)P)2(5 mol%)

sec-C4H9ZnIN-Methylimidazole

THF

R= OMe (96%)R = CO2Et (98%)

R R R

Br

Pd(tBu2(p-Me2N-C6H4)P)2 (5 mol%)

sec-C4H9ZnIN-Methylimidazole

THF

R

R = OMe (96%)R = CO2Et (98%)

+

87 % 97 % 99 %

99 % 63 % 97%

XOTs RNH2

0.05 -1.0 mol % Pd((o-tolyl)3P)20.05 -1.0 mol % CyPF-t-Bu

NaO-t-Bu, toluene, 25oC XNHR

NHiBu NHoctyl NHOctyl

EtO2C

HN

N

HN

OMe N

NHiBu

OTsX

RNH2

0.05–1.0 mol% Pd((o-tolyl)3P)20.05–1.0 mol% CyPFtBu

NaOtBu, toluene, 25ºCNHR

X+

NHiBu

87% 97%

NHoctyl

EtO2C99%

NHoctyl

NH

99%

NH

N OMe63%

N

NHiBu

97%

Scheme VII. Pd(tBu2(p-Me2N-C6H4)P)2-catalysed sp2-sp3 Negishi coupling at milder temperatures

Scheme VI. Cu-free Sonogashira coupling of aryl and heteroaryl chlorides

X Cl Cs2CO3, DMF, 120°C X+ R1

0.5 mol%Pd(t-Bu2(p-PhMe2N)P)2

R1

X = N 94%

X = N 90%

X = C 85%

X = C 91%

yieldX

C6H13

C6H13

R1

X Cl

R1

X

0.5 mol%Pd(tBu2(p-Me2N-C6H4)P)2

Cs2CO3, DMF, 120ºC

R1

C6H13

C6H13

X

X = N

X = N

X = C

X = C

90%

85%

91%

R1

94%

Yield



1.4 equiv NaO-t-Bu1,4-dioxane

+NH3

5 equivR

Cl 0.1 mol % Pd((o-tolyl)3P)20.1-1.0 mol % CyPF-t -Bu

R

NH2

A

+ Ar2NH

B

R = Me, MeO, etc.

0.1 mol% Pd((o-tolyl)3P)20.1–1.0 mol% CyPFtBu

1.4 equiv. NaOtBu1,4-Dioxane

Cl

R

NH3

R

NH2

Ar2NH5 equiv.

A B

R = Me, MeO, etc.

Scheme IX. Monoarylation of ammonia

Glossary

Term Defi nitionArOTf aryl trifl ate

ArOTs aryl tosylate

COD 1,5-cyclooctadiene

CyPFtBu (2S)-1-[(1S)-1-[bis(1,1-dimethylethyl)phosphino]ethyl]-2-(dicyclohexylphosphino)ferrocene

(Josiphos SL-J009-2)

NBD norbornadiene

Np neopentyl

Q-Phos 1,2,3,4,5-pentaphenyl-1-(di-tert-butylphosphino)ferrocene

RT room temperature

ConclusionA highly effective method for the synthesis of highly

active precatalysts of general formula L2Pd(0) has

been developed, which is pivotal for the availability

of these catalysts in commercial quantities. These

tertiary phosphine (L) based L2Pd(0) complexes show

very promising applications with excellent selectivity

in cross-coupling. The complexes discussed in this

article are described in Table I, with a summary of

their specifi c applications.

The difference in selectivity for each catalyst for a

particular reaction is quite fascinating. Many more

new applications of these catalysts are yet to come.

For more applications of precatalysts developed

beyond the 2010 Nobel Prize, see the recent review

(12).

AcknowledgementWe thank Gerard Compagnoni (General Manager)

and Fred Hancock (Technical Director) of Johnson

Matthey’s Catalysis and Chiral Technologies for their

encouragement.

References 1 C. C. C. Johansson and T. J. Colacot, Angew. Chem. Int.

Ed., 2010, 49, (4), 676

2 G. C. Fu, Acc. Chem. Res., 2008, 41, (11), 1555

3 J. F. Hartwig, Acc. Chem. Res., 2008, 41, (11), 1534

4 R. Martin and S. L. Buchwald, Acc. Chem. Res., 2008, 41, (11), 1461

5 “Metal-Catalyzed Cross-Coupling Reactions”, 2nd Edn., eds. A. de Meijere and F. Diederich, WILEY-VCH Verlag GmbH & Co KGaA, Weinheim, Germany, 2004, Volume 1

6 A. F. Littke and G. C. Fu, Angew. Chem. Int. Ed., 2002, 41, (22), 4176

7 T. J. Colacot, Platinum Metals Rev., 2011, 55, (2), 84

8 C. C. C. Johansson Seechurn, M. O. Kitching, T. J. Colacot and V. Snieckus, Angew. Chem. Int. Ed., 2012, in press

9 H. Li, G. A. Grasa and T. J. Colacot, Org. Lett., 2010, 12, (15), 3332

10 T. J. Colacot, G. A. Grasa and H. Li, Johnson Matthey Plc, ‘Preparation of a Metal Complex’, World Appl. 2010/128,316

11 L. L. Hill, J. L. Crowell, S. L. Tutwiler, N. L. Massie, C. C. Hines, S. T. Griffi n, R. D. Rogers, K. H. Shaughnessy, G. A. Grasa, C. C. C. Johansson Seechurn, H. Li, T. J. Colacot, J. Chou and C. J. Woltermann, J. Org. Chem., 2010, 75,



(19), 6477

12 H. Li, C. C. C. Johansson Seechurn and T. Colacot, ACS Catal., 2012, in press

13 M. Nazaré, C. Schneider, A. Lindenschmidt and D. W. Will, Angew. Chem. Int. Ed., 2004, 43, (34), 4526

14 D. Alberati-Giani, S. Jolidon, R. Narquizian, M. H. Nettekoven, R. D. Norcross, E. Pinard and H. Stalder, Hoffmann-La Roche Inc, ‘Substituted Acylpiperazine Derivatives’, US Appl. 2005/0,059,668

15 C. Dai and G. C. Fu, J. Am. Chem. Soc., 2001, 123, (12), 2719

16 S. Tamba, Y. Okubo, S. Tanaka, D. Monguchi and A. Mori, J. Org. Chem., 2010, 75, (20), 6998

17 A. F. Littke, C. Dai and G. C. Fu, J. Am. Chem. Soc., 2000, 122, (17), 4020

18 H. Li and T. J. Colacot, unpublished work

19 S. G. Newman and M. Lautens, J. Am. Chem. Soc., 2011, 133, (6), 1778

20 S. G. Newman, J. K. Howell, N. Nicolaus and M. Lautens, J. Am. Chem. Soc., 2011, 133, (38), 14916

21 X. Pu, H. Li and T. J. Colacot, manuscript under preparation

22 A. Krasovskiy and B. H. Lipshutz, Org. Lett., 2011, 13,

(15), 3818

23 B. H. Lipshutz, B. R. Taft, A. R. Abela, S. Ghorai, A. Krasovskiy and C. Duplais, Platinum Metals Rev., 2012, 56, (2), 62

24 T. Ogata and J. F. Hartwig, J. Am. Chem. Soc., 2008, 130, (42), 13848

25 G. D. Vo and J. F. Hartwig, J. Am. Chem. Soc., 2009, 131, (31), 11049

The AuthorDr Thomas J. Colacot is the Research and Development Manager in Homogenous Catalysis (Global) at Johnson Matthey’s Catalysis and Chiral Technologies business unit. He is a Fellow of the Royal Society of Chemistry and holds an MBA in Strategy and New Ventures. He has developed numerous new organometallic (platinum group metal) based catalysts and processes for various organic transformations for the fi ne chemical, pharmaceutical and electronic industries. He has published numerous peer reviewed articles and holds several patents in addition to presenting at several international conferences and customer sites.


http://dx.doi.org/10.1595/147106712X632460 •Platinum Metals Rev., 2012, 56, (2), 117–123•

PGM HIGHLIGHTS

IntroductionFor some years Italian groups have been at the

forefront of research into membranes for hydrogen

separation, with thin fi lm palladium-based alloy mem-

branes being a particular area of specialisation. The

slower-than-predicted growth in the hydrogen econ-

omy has meant that the fruits of their research have

yet to make a major impact in commercial terms but

the quality of the research is well-recognised within

the global membrane fraternity. Two recent books

have been published (1, 2), which complement other

recent chapters, reviews, presentations and articles,

and highlight some of their work in the areas of mem-

brane manufacture and utilisation within catalytic

reactors, demanding applications in which marrying

the performance requirements of the membranes with

those of the catalysts has led many to suggest that nei-

ther technology is best served by the combination.

“Membrane Reactors for Hydrogen Production Processes”This book (1) provides a consolidation of the results

from a collaborative project, ‘Hydrogen Production

from Light Multi-Fuels and Storage in Porous Materials’,

started in 2006 by Italian academic and industrial

groups to translate laboratory scale membrane reac-

tors to pilot scale. Supported by the Italian Research

Ministry under the Fondo Integrativo Speciale per la

Ricerca (FISR) programme and managed by the Uni-

versity of L’Aquila, the success of the programme is

demonstrated by the construction of a 20 Nm3 h–1 pilot

plant at the Tecnimont KT SpA facility in Chieti Scalo,

about 200 km north-east of Rome, in which pre-

commercial membranes from four suppliers (ECN,

MRT, Acktar and ‘a Japanese company’) were evalu-

ated. Initially describing the general case for mem-

brane reactors, for most of the book the authors have

concentrated primarily on aspects of H2 separation

through dense Pd alloy membranes, with each chapter

providing a stand-alone account of the topic it addresses.

The fi rst chapter discusses the thermodynamic and

kinetic aspects of the integration of selective mem-

branes into chemical processes, utilising the ability

to remove one of the product gases as a means of

overcoming equilibrium constraints. The authors

clearly lay out the equations governing the perfor-

mance of membrane reactors, discussing formats in

which the membrane is either in proximity with the

catalyst or separated in order to optimise perfor-

mance. Highlighting the enormous potential of mem-

brane reactors, they acknowledge that commercial

applications have been slow to be adopted, due in

the main to a number of practical issues such as

membrane stability, mass transfer limitations and

high production costs.

These issues are ably discussed in Chapters 2 and 3,

which cover the current ‘state of the art’ and economic

requirements of small-scale Pd alloy membrane man-

ufacture. Anyone who has worked in the area of H2

separation will be aware of the rapid increase in publi-

cations associated with membrane fabrication and

use in reactor systems. The lists of references for the

two chapters, providing a good selection from groups

around the world, bear testament to the large amount

of money and time devoted to this area in the last two

decades. Primary interest has been in thin fi lm palla-

dium-silver and palladium-copper alloy membranes,

with as much time devoted to substrate preparation as

to developing methods for forming the alloy foils to be

supported. Most workers have concentrated on tubu-

lar geometries, as highlighted by three of the four sup-

pliers to the programme. Only one, Membrane

Reactor Technologies Ltd (MRT), from Canada, pro-

duces fl at foil systems for these alloys, although this

geometry has been more favoured by developers of

the base metal alloy membranes. Notable absences

among the membrane suppliers are some of the US

groups, for example, Worcester Polytechnic Institute

and Colorado School of Mines, who have long been

fabricating systems capable of extended performance.

Palladium-Based Membranes for Hydrogen Separation



Integrated Modules and Reactor-Membrane CombinationsFor the integrated membrane reactor modules, a large

amount of work must go into the module design. To

operate at maximum effi ciency, the membrane must

rapidly remove the product H2 from the reaction zone.

However, poor design can mean that, if the product H2

is removed too quickly, its partial pressure in the reac-

tion gas drops and segments of the membrane per-

form no function, adding needless cost to the module.

Conversely, if the H2 is removed too slowly, the desired

equilibrium shift is not achieved. The additional

requirements of the operational and maintenance

aspects of the fully integrated membrane reactor,

albeit having the advantage of in-reactor H2 removal,

mean that some workers have chosen to concentrate

on staged membrane reactors, in which catalyst bed

and gas separation functions are separated. Modelling

of integrated and separated reactors is a critical func-

tion, highlighted in Chapter 4. With good membrane

and catalyst performance data, coupled with algo-

rithms capable of describing the radial and axial mix-

ing, inter- and intra-particle diffusion and interfacial

gradients between solid and fl uid phases, modellers

are able to accurately predict the performance of their

membrane reactor designs.

Chapters 5–9 deal with the application of separation

membranes to example chemical processes, either as

integrated catalyst-membrane reactor units or as sepa-

rated reactor–membrane combinations, offering the

opportunity to run both at their optimum conditions.

Chapter 5 looks at natural gas steam reforming (NGSR),

modelling the performance benefi ts that could be

obtained by switching from the traditional reactors to

integrated membrane reactors. Whilst benefi ts can be

found, the limitations imposed by forcing together two

functions not running optimally lead the authors to

the conclusion that separated reactor membrane com-

binations are necessary until membranes durable at

NGSR conditions can be manufactured.

Autothermal Reforming and Water Gas ShiftAutothermal reforming (ATR) is considered in the

next chapter. In this application, not only are the tem-

peratures high but there are large thermal gradients

present within the reactor. In the preamble, the

authors point to references in which other groups

have considered use of higher operating temperature

ceramic membranes, both oxygen- and hydrogen-

transporting versions, as means of improving effi -

ciency of H2 production. Within the study reported

here, the group used a microporous alumina mem-

brane (10 cm long × 1 cm diameter) with annular

rings of foam-supported catalyst around the mem-

brane. Reforming of methane was attempted, with

some separation of the product H2 observed. How-

ever, the authors rightly point out that, whilst this

study shows encouraging results, there are many

materials problems that need to be addressed before

larger scale integrated ATR membrane reactors can

be successfully constructed.

The water gas shift reaction is much more amenable

to study within membrane reactors, due to its moderate

temperatures, moderate exothermicity and pressure

independence, meaning that operators can increase

reactor pressures to drive separation membranes with-

out adversely affecting conversion. Most investigators

have constructed reactors based around either dense

Pd or silica H2 separation membranes, employed in dif-

ferent reactor-membrane confi gurations and with base

metal and noble metal catalysts. The authors provide a

good overview of the status of work at the time of writ-

ing, with a number of groups focusing upon the appli-

cation within integrated gasifi cation combined cycle

(IGCC) power plants, where separation of H2 from car-

bon dioxide streams would provide large commercial

benefi ts. As with other candidate processes outlined in

this book, workers are considering both integrated and

staged reactor-membrane systems but the greatest

gains are to be had with the integrated systems. In addi-

tion to managing the thermal issues, however, the coal-

derived syngas will contain an array of contaminants

that will rapidly poison Pd alloy membranes, with the

result that workers must either develop effi cient gas

cleaning technologies or produce microporous mem-

branes having high selectivity on a large scale.

Hydrogen Sulfi de Cracking and Alkane DehydrogenationMembrane reactor enthusiasts are willing to trial many

reactions in attempts to commercialise their technol-

ogy, but few applications can be more demanding

than the catalytic cracking of hydrogen sulfi de. Having

very limited industrial use, H2S is generally viewed as a

pollutant. Currently separated from hydrocarbon gas

streams by amine adsorption, concentrated streams

are treated by the barely economical Claus Process.

However, the authors point out that, if H2S could be

decomposed to sulfur and hydrogen, this would yield

valuable products that could offset the high costs asso-

ciated with the decomposition process. Straightfor-

ward thermal decomposition requires temperatures



well in excess of 1000ºC to achieve any level of

commercial conversion and other potential decom-

position technologies have not progressed beyond

laboratory scale. Researchers have suggested, however,

that the combination of two reactors separated by a

microporous ceramic membrane unit could, with a

great deal of process engineering, catalyst develop-

ment and membrane scale-up, provide a commercial

alternative for H2S decomposition. To give an idea of

the engineering requirement, the authors suggest that

a 200 tonne/day plant would require a microporous

membrane area of around 1800 m2, somewhat beyond

the fabrication capabilities of current manufacturers.

Alkane dehydrogenation reactions have been

widely studied within membrane reactor modules,

since removal of the formed H2 should provide a use-

ful product gas stream as well as conversions beyond

those limited by equilibrium conditions. Chapter 9

begins with an overview of typical dehydrogenation

processes, the catalysts used, typically alumina-

supported chromium oxide and alumina-supported

platinum, both with various promoters, and, more

importantly, the drawbacks associated with the reac-

tions. Thermodynamic restrictions, endothermicity,

side reactions like cracking and isomerisation, rapid

coke formation at excessive reaction temperatures

and the need for low pressure operation are factors

that combine to complicate the incorporation of dehy-

drogenation reactions into membrane reactors. Pd

membranes have high selectivity, but their fragility

means that many researchers have turned to the more

durable microporous silica and zeolite membranes,

although these are hampered by having low H2 selec-

tivities and generally show minimal impact upon the

conversions achieved. The chapter gives an honest

assessment of the status of membrane reactors for

these processes, but points to some encouraging

recent performance data suggesting that the technol-

ogy may be developed in the future.

Pilot Plant and CommercialisationThe penultimate chapter discusses the design, success-

ful construction and current status of the 20 Nm3 h–1

pilot plant at the Tecnimont KT SpA facility in Chieti

Scalo, Italy (Figure 1), using natural gas steam reform-

ing (SR) as the studied process. Design was based on

the preliminary work described in Chapter 5. Typi-

cally, SR processes are run at 850–900ºC, too hot for

incorporation of a Pd alloy membrane. Research has

been carried out, using both base metal and platinum

Fig. 1. Membrane reforming reactor for hydrogen production (20 Nm3 h–1) in Chieti Scalo, Italy (Courtesy of Tecnimont KT SpA, Italy)



group metal (pgm) catalysts, to reduce the reaction

temperature to 550–650ºC. However, this is still above

the working temperatures for Pd alloy membranes, so

the pilot plant was constructed with two paired

reformers and membrane separation units in series.

Pre-commercial membrane modules were obtained

from the four suppliers. Three modules incorporated

tubular membranes, whilst the MRT unit was fi tted

with fl at-sheet membranes.

Feeding desulfurised natural gas into the process,

the operational parameters of the reformers were

characterised in order to control the feedgas to the

membrane units. The results presented are those from

the fi rst preliminary experiments. That the membranes

are having an effect is clear, but the conclusions sec-

tion makes the claim that ‘four types of Pd-based H2

selective membranes are tested and compared’, which

is perhaps slightly misleading. No comparative data is

shown for the membrane modules and only limited

performance data for two of them. More testing was

planned at the time of writing, so better assessment of

the modules should now be possible.

The fi nal chapter draws together the key points from

the previous chapters and examines the prospects for

membrane reactors, forecasting a very positive future

for the Pd systems. Clearly, there are several key obsta-

cles to their widespread use:

fabrication costs, although this is less the cost of

the Pd and more the labour intensive routes to

produce the membranes;

membrane stability under poisoning and process

conditions, necessitating the use of staged reac-

tors for some processes;

a current lack of industry-produced, commercial

scale units;

no acceptable accelerated ageing tests, without

which longevity can only be assessed by actually

carrying out thousands of hours of testing.

However, the authors cite the recent advances in

substrate development, Pd layer deposition, module

modelling and design and the benefi cial effi ciencies

that successful commercialisation would bring as

demonstrating the need for development to continue.

Overall, the book provides a very good summary of

both the fi eld of Pd membrane reactors and, specifi -

cally, the recent work carried out by primarily Euro-

pean groups, culminating in the successful

construction and operation of the pilot plant in Chieti

Scalo. The reviewer may not quite share the long-term

optimism of the authors but their commitment is

undeniable and progress to date has been impressive.

We await future developments with great interest.

“Preparation of Thin Film Pd Membranes for H2 Separation from Synthesis Gas and Detailed Design of a Permeability Testing Unit”The experience of the Italian research groups is fur-

ther highlighted by the second book (2), predating the

fi rst and concentrating on the manufacture and test-

ing of Pd-only membranes on porous steel supports by

the group at the University of Pisa. This study was also

fi nanced (in part) by the Italian Research Ministry

under the FISR programme.

Constructed in the format of a doctoral thesis or

extended journal publication, the 74-page book begins

with a discussion around the overall area of H2 genera-

tion and subsequent applications, before highlighting

various membrane types available for gas separation:

dense metallic and ceramic, polymeric, microporous

ceramic and composite.

The second chapter summarises the specifi c prop-

erties that give rise to the use of Pd membranes and

some of the methods used to fabricate the thin-walled,

supported fi lms desirable for large-scale, cost-effi cient

separation. As with other membrane reviews in recent

years, the authors highlight the large number of jour-

nal articles and patents associated with Pd membrane

development. The reader, and probably several fund-

ing bodies, may be left to ponder just how much more

work and how many more research dollars will be

needed before the technology is viable for long-term,

large-scale operation.

Manufacture of Thin Palladium FilmsThe manufacturing route and equipment used for the

thin Pd fi lms on discs and tubes – cleaning, surface

oxidation, preactivation and electroless plating – is

similar to that used by other groups, for example, Ma’s

group at the Worcester Polytechnic Institute, USA.

Many groups have now moved beyond the simple oxi-

dation of the steel surface to incorporate a diffusion

barrier, such as alumina or nitrides, to minimise the

risk of Pd migration into the steel substrate. Observa-

tions during the preparation are noted in Chapter 4

and here the reviewer must applaud the authors. Too

often authors gloss over the minor experimental

details, leaving out key points which may seem obvi-

ous at the time. Whilst everyone appreciates the

importance of ‘trade secrets’, the time may be

approaching when funding will begin to decline if the

promised growth in commercial applications fails to

materialise, and sharing ‘minor’ details among the

membrane community may go some way to minimise

wasted time.



Membrane thicknesses of between 10–17 μm were

produced, with higher plating effi ciencies noted for

the disc-based membranes. This was reasonably

ascribed to loss of Pd by plating out on the walls of

the recirculating equipment used for the tubular ver-

sions. Leak testing of tubular membranes at ambient

temperature, using nitrogen pressurisation testing

and immersion in water for bubble formation,

showed that, unsurprisingly, leaks decreased with Pd

thickness. The principal area for leaks to occur was

around the welded join between the porous steel

support and the dense end attached for sealing, sug-

gesting that further work is required to avoid irregu-

lar Pd deposition and delamination in that area. For

the future, the authors might consider leak testing

with helium, which will pinpoint holes more readily

than nitrogen.

The experimental setup for testing the H2 separation

capabilities of the membranes, described in Chapter 5,

is quite thorough. It includes not only the usual capa-

bility to test in pure H2 and model syngas streams, but

also can introduce poisons such as sulfur, ammonia

and hydrocarbons. Frustratingly, the book ends without

test data being presented and no indication of when

or where such data may be found, requiring the inter-

ested reader to search the usual literature sources.

Although the title specifi es the preparation and design

aspects, it is unclear why the book was published with-

out giving the reader a fl avour of the membrane per-

formance under application conditions.

A search did uncover some details of testing car-

ried out on a range of tubular membranes, having a

surface area of 19 cm2, supported on porous stainless

steel supports and prepared by varying deposition

parameters (3). Testing at elevated temperatures and

pressures was carried out in pure H2, N2 and CO2, with

the H2 selectivity being calculated and no deteriora-

tion seen over 500 hours of testing. The permeation

data was subsequently used to model the perfor-

mance of a 3 m long membrane having 50% H2:50%

CO2 feedgas. Despite having the gases available, it

appears that they did not use that mixed feedgas dur-

ing their experimental campaign, which would have

supported their calculated selectivities and model-

ling results. Perhaps that information will form the

basis of new reports.

ConclusionLooking at the details of the FISR programme, it is clear

that the Italian government is committed to investing

in a hydrogen capability and infrastructure and in

renewable energy sources. The construction of the

Chieti Scalo plant shows that the years of experience

in laboratory-scale membranes and reactors can be

translated to pilot-scale, with encouraging early results.

Membrane researchers worldwide will look forward to

future developments.

HUGH HAMILTON

Johnson Matthey Technology Centre,

Blounts Court, Sonning Common,

Reading RG4 9NH, UK


References1 “Membrane Reactors for Hydrogen Production

Processes”, eds. M. De Falco, L. Marrelli and G. Iaquaniello, Springer-Verlag, London, UK, 2011

2 M. Bientinesi and L. Petarca, “Preparation of Thin Film Pd Membranes for H2 Separation from Synthesis Gas and Detailed Design of a Permeability Testing Unit”, Nova Science Publishers, Inc, New York, USA, 2010

3 M. Bientinesi and L. Petarca, Chem. Eng. Trans., 2011, 24, 763

The ReviewerDr Hugh Hamilton has worked at the Johnson Matthey Technology Centre, Sonning Common, UK, for nearly 24 years, during which time he has researched in a range of areas including autocatalysts, palladium membranes, fuel cell membrane electrode assembly manufacture and hydrogen storage. His current role includes sorbent development for mercury from syngas, titanium powder metal injection moulding process development and modifi ed atmosphere packaging development.



About the Editors

Marcello De FalcoMarcello De Falco received a master’s degree in

Chemical Engineering from the University of

Rome “La Sapienza”, Italy, in 2004. He was

awarded a PhD in ‘Industrial Chemical Processes’

in 2008, with the thesis ‘Pd-Based Membrane

Reactor: A New Technology for the Improvement

of Methane Steam Reforming Process’. He has

been a researcher in the Faculty of Engineering at

University Campus Bio-Medico in Rome since

2010. His research activity is mainly focused on

mathematical modelling of chemical reactors,

hydrogen production processes, solar technolo-

gies, and the design of cogeneration plants. He is

the author of 34 scientifi c papers (including

20 papers in international journals and 14 in con-

ference proceedings) and 3 book chapters

about selective membrane technology and

applications.

Gaetano IaquanielloGaetano Iaquaniello is a Vice President of Technol-

ogy and Business Development at Tecnimont KT

SpA, a process and engineering company based in

Rome, Italy. He received his MSc in Chemical

Engineering from the University of Rome in 1975, a

doctorate from the University of Limoges, France,

in 1984 and an MSc degree in Management from

the London Business School, University of London,

in 1997. He has more than 35 years of experience

in designing and operating chemical plants, par-

ticularly for syngas manufacturing. After being a

technical director, he is now head of research and

development activities at Tecnimont and coordi-

nates several national and European projects. He

has authored and co-authored numerous papers

and patents on syngas production and, more

recently, on membrane reactors.

Luigi MarrelliLuigi Marrelli is full professor of ‘Chemical

Reactors’ and Dean of the Faculty of Engineering

at University Campus Bio-Medico in Rome. He was

previously professor of Physical Chemistry and of

Chemical Reactors, and President of the didactic

committee of Chemical Engineering at the Univer-

sity of Rome “La Sapienza”. He is a member of the

‘Interuniversity Research Centre on Sustainable

Development (CIRPS)’ and the ‘Research Centre

HYDRO-ECO’ at the University of Rome “La Sapi-

enza”. For many years he worked in the fi eld of

university cooperation with developing countries,

realising projects in countries including Peru,

China and Kazakhstan. His scientifi c activity, sum-

marised in over 100 papers and congress commu-

nications, is focused on the fi elds of chemical

thermodynamics, chemical and biochemical

kinetics and reactor modelling. Over the last years

he was involved in research projects on hydrogen

production by steam reforming of methane in

membrane reactors and from water by thermo-

chemical cycles.



About the Authors

Matteo BientinesiMatteo Bientinesi works at the Consorzio Polo Tecnologico Magona

(CPTM), Cecina, Italy. The CPTM deals with process studies, pilot plant

and engineering support activities at the commissioning and start-up

of plants.

Luigi PetarcaLuigi Petarca is a member of the Department of Chemical Engineering

at the University of Pisa, Italy. He has been active in research for over 30

years, and has published numerous papers in the fi elds of chemical

engineering and industrial chemistry.




Reviewed by Gordon Kelly* and Steve Bailey

Johnson Matthey PCT, PO Box 1, Belasis Avenue, Billingham TS23 1LB, UK

*Email: [email protected]

IntroductionHeld under the auspices of European Federation

of Catalysis Societies (EFCATS) the 10th EuropaCat

Congress was held in Glasgow, UK, from 28th August

to 2nd September, 2011 (1). EuropaCat is a highly

regarded biennial congress which brings together

researchers from across Europe and further afi eld for

scientifi c dissemination in catalysis. EuropaCat X was

a truly international congress with approximately 1200

delegates attending, representing some 44 countries.

The theme of the Congress was ‘catalysis across

the disciplines’ with an aim of delivering a unifi ed

conference covering all aspects of catalysis. The setting

for the conference was the Glasgow University main

building, a fi ne example of Gothic revival architecture

and a prominent local landmark.

The programme included six plenary and 21 keynote

lectures. The plenaries were presented by international

leaders from a wide range of disciplines. In total

over 210 oral presentations were given, representing

the latest in scientifi c advances and successful

commercial applications. Each day was split into fi ve

parallel sessions covering the following topics: Catalyst

Preparation, Catalyst Characterisation, Kinetics and

Mechanism, Theory and Modelling, Homogeneous

Catalysis, Industrial Application, Catalyst Deactivation

and Environmental Catalysis. In addition there were

three poster sessions with approximately 900 posters.

A parallel workshop on Selective Oxidation (ISO 2011,

X European Workshop on Selective Oxidation) was

organised independently with 17 oral presentations

and 97 posters.

There was an excellent attendance from industrial

attendees who accounted for approximately 15% of

the delegates, representing over 90 companies. The

exhibition space, with 24 companies represented, was

also well attended throughout the conference.

A full social programme was organised for

the attendees with a range of day trips available

showcasing the best of Scotland. The conference

dinner was held in the magnifi cent Kelvingrove Art

Gallery and Museum and the fi nal night’s Céilidh was

a popular and energetic event!

In terms of its original aims EuropaCat X was a huge

success in delivering a unifi ed conference covering

all aspects of catalysis and highlighting the huge

Highlights of platinum group metal catalysis from biennial congress

EuropaCat X



breadth and depth of science and technology that is

encompassed in the fi eld. In terms of the impact of

platinum group metals (pgms) in the conference, over

30% of the oral papers presented involved the use of

pgms. A more detailed analysis shows the following

split between the pgms: 32 platinum, 22 palladium,

13 ruthenium, 12 rhodium and 2 iridium papers

respectively. The industrial applications sessions

included 11 papers that incorporated pgms.

With such a range of oral presentations and parallel

sessions to choose from, this conference review can

only discuss a few highlights. Therefore, in this review

only papers which made signifi cant use of pgms have

been included.

Fischer-Tropsch SynthesisThe infl uence of pgm promoters on cobalt-supported

Fischer-Tropsch catalysts continues to be an area of

interest. Eric Marceau (Université Pierre et Marie Curie/

Centre National de la Recherche Scientifi que (CNRS),

France) presented a keynote paper characterising the

impact of sorbitol and Ru promotion in the structure

and performance of silica-supported cobalt catalysts.

Addition of sorbitol to the impregnating solution was

found to lead to a decrease in the size of the cobalt(II)(III)

oxide (Co3O4) crystallites formed from 11 nm to 7 nm.

Sorbitol was shown, by a combination of in situ quick-

scanning X-ray absorption spectroscopy (Q-XAS), in

situ UV-visible spectroscopy and thermal analysis to

retard the decomposition of the cobalt precursor into

Co3O4. In situ Q-XAS and temperature-programmed

reduction (TPR) showed that the Co3O4 particles fi rst

reduced to cobalt(II) oxide (CoO). The reducibility

of the Co-sorbitol/SiO2 catalyst was quite low due to

the formation of cobalt silicate (2). The presence of

Ru greatly enhanced the reducibility of the catalysts.

Transmission electron microscopy (TEM) images of

reduced catalysts demonstrate the homogeneous

distribution of cobalt particles in the catalyst prepared

with sorbitol (Figure 1).

Ru in its own right can, of course, be used as

a Fischer-Tropsch catalyst. Although cobalt and

iron are preferred for industrial application due

to cost considerations, Ru continues to draw

scientifi c interest due to its ability to produce high

molecular weight hydrocarbons. Xian-Yang Quek

(Schuit Institute of Catalysis, Eindhoven University,

The Netherlands) presented a very interesting

paper demonstrating unprecedented oxygenate

selectivity during Fischer-Tropsch synthesis over Ru

nanoparticles. The Ru nanoparticles, encapsulated

and stabilised in polyvinylpyrrolidone (PVP), were

prepared in a size range of between 1.3 and 3.3 nm.

The product distribution formed depended strongly

on temperature. An aldehyde selectivity as high as 70%

was obtained over 2.2 nm Ru nanoparticles at 125ºC.

With increasing temperature the total oxygenate

selectivity decreased and the amount of hydrocarbon

produced was increased. Mechanistic consideration

of the product chain lengths suggested that the

hydrocarbons and the oxygenates were formed on

different sites.

Michael Claeys (University of Cape Town, South

Africa) presented an in situ magnetometer study on

Plenary LecturesProfessor Manfred Reetz (Max-Planck-Institut für Kohlenforschung, Mülheim, Germany), ‘Tuning

Monooxygenases by Genetic and Chemical Means’

Professor Istvan Horvath (City University of Hong Kong), ‘Heterogenization of Homogeneous Catalytic

System’

Professor Charles T. Campbell (University of Washington, USA),‘Thermodynamics and Kinetics of

Elementary Reaction Steps on Late Transition Metal Catalysis, and in Their Sintering’

Professor Rutger van Santen (University of Eindhoven, The Netherlands), ‘Structure Sensitivity and

Unsensitivity in Heterogeneous Catalysis’

Professor Matthias Beller (Leibniz Institute for Catalysis, Rostock, Germany),‘Development of Practical

Molecular-Defi ned Catalysts for Industrial Applications and Hydrogen Technology’

The 2011 Michel Boudart Award for Advances in CatalysisThis was awarded to Professor James Dumesic (University of Wisconsin-Madison, USA), who presented

a plenary lecture on ‘Routes for Production of Liquid Transport Fuels by Liquid-Phase Catalytic Processing’.



the formation and stability of cobalt carbide during

Fischer-Tropsch synthesis. Novel methods that yield

different attributes of a catalytic system within the

same measurement are both information rich and

valuable from a research and development effi ciency

perspective. The application of magnetometry can

reveal structural features of Co crystallites under

authentic gas-to-liquid (GTL) process conditions. This

study concentrated upon the formation and role of

cobalt carbides during time on line of a simple Co-Pt/

alumina catalyst operated under varying conditions

typical of Fischer-Tropsch processes. The experimental

approach made use of a fi xed-bed reactor placed

within the fi eld supplied by a magnetometer, from

which it was possible to infer the state of magnetisation

of the catalyst at different times and subject to the

infl uence of reaction condition changes. Changes in

magnetisation could be correlated with the extent

of carbide formation, as confi rmed by ex situ X-ray

diffraction (XRD) measurements, and with the degree

of reduction when the catalyst was treated under

TPR conditions. Detailed analysis of magnetisation

hysteresis allows modelling of Co crystallite size and

some insight into active site mobility under reaction

conditions.

When tested under Fischer-Tropsch conditions

(230ºC, 14 bar, H2:CO = 2:1) a pre-carbided catalyst

showed an increase of magnetisation during time on

line, which is indicative of carbide decomposition

to form the metallic state. However, this carbide

decomposition took place slowly and remained

incomplete even after an extended period of testing,

suggesting that once carbides are formed, they will not

decompose completely under typical Fischer-Tropsch

conditions. The pre-carbided catalyst also displayed

inferior Fischer-Tropsch performance (activity and

methane selectivity) compared to a non-carbided

catalyst. This performance was regained after

hydrogen treatment, which also lead to completely

restored magnetisation (Figure 2).

(a)

100 nm 100 nm

100 nm

(b)

100 nm

Fig. 1. Bright-fi eld and dark-fi eld high angle annular dark fi eld (HAADF)-TEM images of: (a) CoRu/SiO2; and (b) of CoRu-sorbitol/SiO2 catalysts after reduction and passivation in He (Courtesy of Eric Marceau and Andrei Khodakov, Université Pierre et Marie Curie/Centre National de la Recherche Scientifi que (CNRS), France)



Biomass ConversionOne of the Industrial Application sessions was devoted

to the conversion of biomass into liquid fuels. In this

session it was clear that the use of pgms was key in

many of the processes described.

Two papers were presented on the use of supported

Ru catalysts to convert cellulose to sugar alcohols.

The fi rst of these from Kameh Tajvidi (Max-Planck-

Institut für Kohlenforschung, Germany) investigated

alternatives to sulfuric acid in conjunction with a 5 wt%

Ru/C catalyst for the hydrolytic hydrogenation of

cellulose. Reactions in the presence of silicotungstic

acid achieved almost full conversion of cellulose

and a sugar alcohol yield of up to 80%. The concept

was successfully transferred to the direct conversion

of spruce wood fi bres as a real feedstock again

with an almost full conversion of the cellulose and

hemicellulose content to sugar alcohols. An alternative

approach, presented by Weiping Deng (Xiamen

University, China), was to adsorb Ru nanoparticles

onto caesium salts of tungstophosphate. This

bifunctional catalyst contained both the acidity and

hydrogenation activity required for the conversion

of cellulose. A Ru/Cs3PW12O40 material was stable

in repeated use and sorbitol yields of ~75% could be

sustained after fi ve recycling uses. The sorbitol yield

increased with decreasing mean size of Ru particles

from 10.8 to 1.6 nm.

Alexey Kirilin (Åbo Akademi University, Finland)

investigated the use of a 5 wt% Pt/Al2O3 catalyst for the

aqueous phase reforming (APR) of erythritol, xylitol

and sorbitol as a means of producing hydrogen and

components of liquid fuels. Sorbitol and xylitol were

viewed as the most promising raw materials for the APR

process due to the possibility of obtaining transportation

fuel components (hexane etc.). No catalyst deactivation

was observed after more than 140 h on line. Hydrogen

yields for erythritol, xylitol and sorbitol under the

same operating conditions were 48%, 28% and 22%

respectively (Figure 3).

A series of supported 2 wt% Ru catalysts (Ru,

ruthenium(IV) oxide, ruthenium(IV) sulfi de) were

investigated by Marcelo Kaufman-Rechulski (Paul

Scherrer Institut, Villigen, Switzerland) as an alternative

to the cold scrubbers that are currently used to remove

tars and organic sulfur compounds during biomass

gasifi cation. The Ru catalysts were tested against

commercially available NiMoS and CoMoS materials.

The RuS2 catalyst was active for all three of the reactions

under investigation (sour water gas shift, sulfur

0 100 200 300 400 500 600 700

Time on line, h

Non pre-carbided catalystPre-carbided catalyst after H2 treatment

H2 at 230ºC, 1 bar

Pre-carbided catalyst

Satu

ratio

n m

agne

tisat

ion,

arb

itrar

y un

its

Fig. 2. Magnetisation of pre-carbided and non-carbided catalyst monitored during Fischer-Tropsch synthesis (Courtesy of Michael Claeys, University of Cape Town, South Africa)



resistant hydrogenation and hydrodesulfurisation of

thiophene) and appeared to be more stable under

real process conditions.

Chuan Wang (Institute of Materials Science and

Engineering, Singapore) presented an interesting

paper on low-temperature hydrogenation of furfural on

Pt catalysts supported on a multiwall carbon nanotube

(MWCNT) support. A 5 wt% Pt/MWCNT catalyst at

150ºC and 20 bar H2 achieved a furfural conversion

of >95%. Both of the products formed, furfuryl alcohol

(~90% selectivity) and 2-methylfuran, are more

chemically stable than furfural, thereby facilitating

further bio-oil upgrading at high temperature.

Aldehydes such as furfural are present in signifi cant

amounts in bio-oils obtained from the fast pyrolysis

of biomass. Under typical conditions for bio-oil

upgrading, these reactive aldehyde compounds can

form tar-like black solids. This work demonstrated

that a mild hydrogenation treatment can be used to

remove reactive compounds in bio-oils, facilitating

their storage and further high-temperature upgrading.

Methanol ReformingKarin Föttinger (Vienna University of Technology,

Austria) presented a paper investigating the use of

in situ X-ray and vibrational spectroscopic studies on

palladium/zinc(II) oxide and palladium/gallium(III)

oxide as potential methanol steam reforming (MSR)

catalysts. The localised generation of hydrogen as a fuel

for proton exchange membrane (PEM) assemblies, is

key to the successful design and deployment of fuel

cells intended to support initiatives for sustainable

means of transportation. The paper sought to address,

with some success, the issue of current steam reforming

catalysts in which the conversion of methanol generally

results in the co-production of carbon monoxide –

a potent catalyst poison. Pd supported on ZnO and

Ga2O3 had shown excellent activity and selectivity to

the steam reforming reaction, yielding only H2 and

CO2 (3). Experiments undertaken at the Swiss Light

Source (superXAS beamline), made use of a catalytic

reaction cell from which it was possible to study the

system by extended X-ray absorption fi ne structure

(EXAFS) and infrared (IR) techniques, along with gas

chromatography-mass spectrometry (GC-MS) analysis

of catalyst performance. Using time-resolved EXAFS, it

was possible to follow, in situ, the formation of the active

Pd-Zn alloy phase of a Pd/ZnO MSR catalyst, via spillover

and reduction of the ZnO by H2 generated in the

reaction. With time-on-stream, the degree of alloying

was observed to steadily increase and, in parallel, the

reactivity changed from methanol decomposition

(CO/H2) to MSR (CO2/H2), confi rming that the Pd-Zn

alloy is the selective phase for MSR. Pd-Zn alloying was

found to be reversible on contact with O2, producing

Pd metal and ZnO with a corresponding reversion of

selectivity to CO/H2. The corresponding surface state

and available adsorption sites were studied by FTIR

spectroscopy of CO adsorption. Pd-Zn alloy formation

led to the disappearance of bridge-bonded CO and a

shift of the on-top CO band from 2090 cm–1 to 2070 cm–1.

Upon O2 treatment the bands assigned to unalloyed

10 2 3WHSV, h–1

0

20

40

60

Yiel

d, %

CO2

H2

Key

0.8 1.2 1.6 2.0 2.4 2.8WHSV, h–1

0

10

20

30

40

Sele

ctiv

ity, %

C6

C3C4C2C5

Fig. 3 (a) Yields of H2 and CO2 in APR as a function of WHSV; and (b) selectivity towards compounds presented in the liquid phase during APR of sorbitol as a function of space velocity (Courtesy of Alexey V. Kirilin, Åbo Akademi University, Finland)



metallic Pd reappeared but at a lower intensity, which

may be explained by a partial decoration of the Pd by

patches of ZnO.

The theme of this paper was continued in a

subsequent presentation from the same group, given

by Christian Weilach, who extended the range of

spectroscopic methods to expose more detail of

the surface transformations associated with alloy

formation and the change in the selectivity pattern

observed during MSR (Figure 4).

CharacterisationGary Attard (Cardiff Catalysis Institute, UK) gave

a thought-provoking lecture on electrochemical

perspectives on catalysis. This addressed the

challenges of specifi cally identifying those surface

topographies and two-dimensional morphologies

that are associated with selective catalytic trans-

formations. The presentation concentrated upon the

application of cyclic voltammetry (CV) as a means of

characterising surface states – including the existence

of defects. Typical CV data collected from a series of

palladium-gold alloys supported on graphite (0.4%–3%

total metal loading) spanning the whole range of

alloy composition were presented. Referring to one

set of voltammograms, it was shown that the number

of Pd surface sites is signifi ed by the area of the CV

peaks between 0 and 0.3 V associated with hydrogen

electrosorption. The oxide stripping peaks also

provide a measure of the available Pd and Au surface

area (for example, the Au oxide stripping peak area

at 1.1 V). More interestingly, the extent to which the

surface is alloyed is given by a systematic shift in

the potential of the Pd oxide peak to more positive

potentials as surface Au composition increases.

Similar studies using single crystal electrodes

allow for direct comparison between supported

metal catalysts and shape-controlled nanoparticle

behaviour in catalytic and electrocatalytic reactions,

since it can be demonstrated that the electrosorption

charge between 0 and 0.3 V acts as a ‘fi nger-print’

analytical technique for the presence of particular

step, terrace and kink sites (Figure 5).

Concluding RemarksThe study and use of pgms continues to be an

essential component of catalyst research, both to

extend fundamental understanding and to allow

industrial exploitation. The three application areas

reviewed in this article: Fischer-Tropsch, biomass

conversion and methanol reforming, are linked by

the challenge of meeting the world’s future energy

demands and requirements. The pgms fi nd use in a

wide range of chemical reactions as demonstrated

by this conference, and their use is also essential i n

several exciting new areas of research not reviewed

here (for example, NOx storage and reduction for

lean-burn gasoline and diesel vehicles and the direct

synthesis of hydrogen peroxide from H2 and O2). The

versatility of the pgms and the reaction selectivity they

impart will make them an integral part of many future

catalytic processes.

ZnO

ZnO

ZnO

ZnO

ZnO

ZnO

Pd

Pd Pd Pd

Pd Pd

Pd-Zn Pd-Zn Pd-Zn

Pd-Zn Pd-ZnZnO ZnO

Pd Pd

(a) Room temperature reduction

(b) During MSR or upon reduction at 623 K with time

(c) In O2 at 573 K after MSR

Fig. 4. Illustration of the suggested structural changes of Pd/ZnO in various environments (Reprinted with permission from (4) © 2011 American Chemical Society)



References1 EuropaCat X: 10th European Congress on Catalysis:

http://www.europacat.co.uk/ (Accessed on 28th February 2012)

2 J. Hong, E. Marceau, A. Y. Khodakov, A. Griboval-Constant, C. La Fontaine, F. Villain, V. Briois and P. A. Chernavskii, Catal. Today, 2011, 175, (1), 528

3 N. Iwasa and N. Takezawa, Top. Catal., 2003, 22, (3–4), 215

4 K. Föttinger, J. A. van Bokhoven, M. Nachtegaal and G. Rupprechter, J. Phys. Chem. Lett., 2011, 2, (5), 428

The ReviewersGordon Kelly joined Johnson Matthey in 1994 following a Chemistry degree and PhD in Catalysis at Glasgow University, UK. He has worked in various research roles in areas such as solid base catalysis, hydrogenation, methanol synthesis, low-temperature shift and Fischer-Tropsch catalysis. He has over 40 catalysis publications including eight patents. Currently he is the Johnson Matthey Fischer-Tropsch Development Manager.

Steve Bailey has a PhD and over 20 years’ experience working in the fi elds of catalyst development and characterisation. He began his catalysis career at ICI where his principal roles were to apply surface characterisation methods to assist the understanding of catalyst performance across a broad range of industrial processes. He then moved into the roles of catalyst development and project management, focusing upon selective hydrogenation and gas purifi cation, and more recently, has led the growth of catalyst characterisation capabilities within Johnson Matthey’s Process Technologies R&D centre.

–9

–7

–5

–3

–1

1

3

5

Curr

ent,

A (×

10–4

)

Potential, V1% Pd/Gr

1% Pd, 1.8% Au/Gr

0.4% Au/Gr

1% Pd, 2.6% Au/Gr

1% Pd, 0.4% Au/Gr

0.2% Pd, 1.8% Au/Gr

1.65% Pd, 0.65% Au/Gr

0.5% Pd, 1.8% Au/Gr

1% Pd, 1% Au/Gr

AuO

PdO from PdAu alloyH on Pd

PdO

Gr = graphite

Fig. 5. Systematic changes in the CV response for a series of PdAu catalysts supported on graphite. Sweep rate = 50 mV s–1 in 0.5 M aqueous sulfuric acid. The reference electrode was a saturated Pd/H electrode in contact with the electrolyte (Courtesy of Gary Attard, Cardiff Catalysis Institute, UK)



BOOKS“Asymmetric Catalysis from a Chinese Perspective”

Edited by S. Ma (Shanghai Institute of Organic Chemistry, State Key Laboratory of Organometallic Chemistry and Department of Chemistry, East China Normal University, China), Topics in Organometallic Chemistry, Vol. 36, Springer-Verlag, Berlin, Heidelberg, Germany, 2011, 359 pages, ISBN: 978-3-642-19471-9, £206.50, €245.03, US$309.00

In 1995, a review in Chinese was published to introduce

the importance and the current state of the art of chiral

technology. The National Natural Science Foundation

of China started to actively support research in this area.

From 2000, the Ministry of Science and Technology also

started to support such research. Chinese researchers

have been making notable achievements in this area

by developing alternative effective new chiral ligands

for established enantioselective transformations, as

well as new catalytic enantioselective reactions with

known or new ligands. Chinese chemists were invited

to write an account on their own contribution in this

area by briefl y touching on the background for the

contribution from chemists outside China.

“Dendrimers: Towards Catalytic, Material and Biomedical Uses”

Edited by A.-M. Caminade, C.-O. Turrin, R. Laurent, A. Ouali and B. Delavaux-Nicot (Laboratoire de Chimie de Coordination du CNRS, Toulouse, France), John Wiley & Sons, Chichester, UK, 2011, 566 pages, ISBN: 978-0-470-74881-7, £120.00, €144.00, US$190.00

This book covers the properties and

uses of dendrimers and dendrons.

The aim of this book is to be the reference book about

dendrimers applications. It is divided in four main

parts: Part 1: Generalities, Syntheses, Characterizations

and Properties; Part 2: Applications in Catalysis; Part

3: Applications for the Elaboration or Modifi cation of

Materials; and Part 4: Applications in Biology/Medicine.

Platinum group metal complexes are involved in

catalytic applications, biological sensors, chemical

sensors and luminescent dendrimers.

“Electrochemical Science and Technology: Fundamentals and Applications”

By K. B. Oldham and J. C. Myland (Trent University, Canada) and A. M. Bond (Monash University, Australia), John Wiley & Sons, Chichester, UK, 2012, 424 pages, ISBN: 978-0-470-71084-5 (paperback), £44.95, €54.00, US$65.00

This book addresses the

scientific principles underlying

electrochemistry. It could serve as a text for a course

in electrochemistry at a university or college. To

keep the size and cost of the book reasonable,

much of the more tangential material is available

as ‘Webs’: documents devoted to a single topic that

are freely accessible from the publisher’s Student

Companion Site at: http://www.wiley.com/go/EST.

Topics include fuel cells, electrodes, corrosion

protection and pollution control, among many

others.

“Interfacial Phenomena in Electrocatalysis”Edited by C. G. Vayenas (University of Patras, Greece), Modern Aspects of Electrochemistry, Vol. 51, Springer Science+Business Media, LLC, New York, USA, 2011, 373 pages, ISBN: 978-1-4419-5579-1, £126.00, €149.75, US$189.00

This is claimed to be the first and

only volume to cover interfacial

electrochemistry, as opposed to other facets of

interfacial science. Contributors are international

leaders in their area of expertise.

Topics include:

(a) Temperature Effects on Platinum Single-Crystal/

Aqueous Solution Interphases. Combining Gibbs

Thermodynamics with Laser-Pulsed Experiments;

(b) Surface Thermodynamics of Metal/Solution

Interface: the Untapped Resources;

(c) XAS Investigations of PEM Fuel Cells;

(d) Palladium-Based Electrocatalysts for Alcohol

Oxidation in Direct Alcohol Fuel Cells;

(e) Structure and Reactivity of Transition Metal

Chalcogenides Toward the Molecular Oxygen

Reduction Reaction;

(f) Materials, Proton Conductivity and Electrocatalysis

in High-Temperature PEM Fuel Cells.

Publications in Brief



“Supported Metals in Catalysis”, 2nd EditionEdited by J. A. Anderson (University of Aberdeen, UK) and M. Fernández García (Instituto de Catálisis y Petroleoquimica, (CSIC), Spain), Catalytic Science Series, Vol. 11, Imperial College Press, London, UK, 2011, 580 pages, ISBN: 978-1-84816-677-6, £111.00, US$168.00

The second edition of “Supported

Metals in Catalysis” has new and

updated chapters containing

summaries of research in this rapidly evolving fi eld.

Chapters of interest include: ‘Supported Metal Catalysts

for Fine Chemicals Synthesis’, ‘Supported Metals in

the Production of Hydrogen’, ‘Supported Metals for

Application in Fuel Cells’ and ‘Supported Metals in

Vehicle Emission Control’.

JOURNALSChemistryOpen

Editors: K. Hindson and H. Ross; Deputy Editor: N. Ortúzar; Wiley-VCH and ChemPubSoc Europe; E-ISSN: 2191-1363

Wiley-VCH and ChemPubSoc

Europe, an association of sixteen

chemical societies, have launched

ChemistryOpen, the fi rst open

access chemical society journal.

ChemistryOpen will publish peer-reviewed primary

research in all areas of chemistry, and will thus

satisfy funding organisations and institutes which

require that the research funded by them should

be accessible to all. As an additional feature, short

summaries of PhD theses with a link to the full version

will be published. This Thesis Treasury will make PhD

theses in chemistry readily accessible while linking

them through CrossRef to all cited journal articles in

the programme.

ChemPlusChemEditor: N. A. Compton; Deputy Editor: M. Spiniello; Wiley-VCH and ChemPubSoc Europe; E-ISSN: 2192-6506

Wiley-VCH and ChemPubSoc

Europe have launched

ChemPlusChem. Original papers

published will cover at least two

different aspects (subfi elds) of

chemistry or one of chemistry and one of another

scientifi c discipline. ChemPlusChem succeeded

the Collection of Czechoslovak Chemical

Communications which ceased publication at

the end of 2011. Editorial Board Co-Chairman

Michal Hocek explained: “Research in chemical

and molecular sciences is becoming more and

more complex and inter- and multidisciplinary

and also its impact in physical, biological and

material sciences is rapidly growing. Typically

nowadays projects involve synthesis of compounds,

catalysis, spectroscopy, crystallography, biological

evaluations, physical measurements, and/or

material studies”.

Integrating Materials and Manufacturing Innovation

Editor: C. Ward; The Minerals, Metals & Materials Society (TMS) and Springer; ISSN: 2193-9764; e-ISSN: 2193-9772

Integrating Materials and Manufacturing

Innovation (IMMI) is a new open

access journal supporting the

discovery, development, and

application of materials, materials

systems, and materials processes

for practical use in manufacturing. IMMI covers

innovations from the discovery of materials to their

deployment.

Special Issue: Celebrating the 150th Anniversary of the Department of Chemistry, The University of Tokyo

Chem. Asian J., 2011, 6, (7), 1629–1895

The Department of Chemistry of

The University of Tokyo, Japan,

celebrated its 150th anniversary

in 2011. To commemorate

this occasion and to honour

the department’s tradition of

excellence, this special issue

featuring contributions from international scientists

closely linked with the department was dedicated to

this anniversary. The cover picture, provided by the

issue co-organisers, Professors Shu Kobayashi and

Eiichi Nakamura, captures the history and the many

faces of the department. Items of interest include:

‘New Design Tactics in OLEDs Using Functionalized

2-Phenylpyridine-Type Cyclometalates of Iridium(III)

and Platinum(II)’, ‘One-Pot Nitrile Aldolization/

Hydration Operation Giving -Hydroxy Carboxamides’

and ‘Platinum--Acetylide Polymers with Higher

Dimensionality for Organic Solar Cells’.



Themed Issue: Heterogeneous Catalysis for Fine Chemicals

Catal. Sci. Technol., 2011, 1, (9), 1533–1696

Mario Pagliaro (Istituto per lo

studio dei materiali nanostrutturati,

CNR, Palermo, Italy) and Graham

Hutchings (Cardiff University, UK)

introduce this themed issue. Today,

in addition to the original rhodium

homogeneous catalysts, there are

a number of commercial fi ne chemicals production

processes using highly effi cient and selective solid

catalysts. To celebrate the contribution of Professor

Michele Rossi to this fi eld on the occasion of his 70th

birthday, this themed issue aims to “grasp the current

momentum in heterogeneous catalytic chemistry

for fi ne chemicals”. Palladium with interstitial

carbon atoms is shown to be an ultraselective

hydrogenation catalyst. Other contributions

demonstrate how to discriminate between

homogeneous or heterogeneous nano-catalysis in

the conjugate addition of arylboronic acids mediated

by palladium(II) complexes; how the selective

hydrogenation of functionalised nitroarenes can be

carried out under ultra-mild conditions; and how

ruthenium-modifi cation of gold catalysts enhances

the selective oxidation of aliphatic alcohols.

ON THE WEBCatAppCatApp is a web application, from SUNCAT Center

for Interface Science and Catalysis, SLAC National

Accelerator Laboratory, Menlo Park and Department

of Chemical Engineering, Stanford University,

California, USA, for looking up calculated reaction

and activation energies for elementary reactions

occurring on metal surfaces. The database includes

reaction energies for all surface reactions that

involve C–C, C–H, C–O, O–O, O–H, N–N, C–N, O–N,

N–H splitting for molecules with

up to three C, N, or O atoms on

close-packed fcc(111), hcp(0001),

and bcc(110) surfaces, as well as

stepped fcc and hcp surfaces. The

metals included in the database are Ag, Au, Co, Cu, Fe,

Ir, Mo, Ni, Pd, Pt, Re, Rh, Ru, Sc and V.

Find this at: http://suncat.stanford.edu/catapp/

Materials Project

Researchers from the Department of Energy’s Lawrence

Berkeley National Laboratory and the Massachusetts

Institute of Technology, USA, jointly launched the

Materials Project, which is a search engine for material

properties. With the Materials Project, researchers can

use supercomputers to characterise properties of

inorganic compounds, including their stability, voltage,

capacity and oxidation state, which had previously not

been possible. The results are then organised into a

database.

Find this at: http://www.materialsproject.org/

Details of catalytic reactions, such as methanol decomposition on copper (shown in this smart phone display), can be accessed via CatApp from mobile devices. Copyright: Jens S. Hummelshøj/Stanford University



CATALYSIS – INDUSTRIAL PROCESSKilogram-Scale Asymmetric Ruthenium-Catalyzed Hydrogenation of a Tetrasubstituted FluoroenamideA. Stumpf, M. Reynolds, D. Sutherlin, S. Babu, E. Bappert, F. Spindler, M. Welch and J. Gaudino, Adv. Synth. Catal., 2011, 353, (18), 3367–3372

Ru-catalysed asymmetric homogeneous hydro-

genation (AHH) was used as the key step of a multi-

kg scale synthesis of an enantiomeric fl uoropiperidine.

The AHH of a tetrasubstituted -fl uoroenamide was

carried out under mild conditions using a Ru/Josiphos

catalyst with an ee of 98%.

CATALYSIS – REACTIONSHydrodeoxygenation of Waste Fat for Diesel Production: Study on Model Feed with Pt/Alumina CatalystA. T. Madsen, E. H. Ahmed, C. H. Christensen, R. Fehrmann and A. Riisager, Fuel, 2011, 90, (11), 3433–3438

Hydrodeoxygenation of a model feed mixture has

been investigated. Oleic acid and tripalmitin in

molar ratio 1:3 were hydrotreated at 325ºC with 20

bars H2 in a stirred batch autoclave with 5 wt% Pt/-Al2O3. Hydrogenation of both reactants was limited

and decarboxylation or decarbonylation of the ester

and carboxylic acid functionalities were highly

favoured, giving C chain lengths of odd numbers.

Pd/-Al2O3 was found to be slightly more active than

Pt/-Al2O3 and had a higher ratio of decarboxylation

and decarbonylation to hydrogenation, while Ni/-Al2O3 was substantially less active. The conversion

of oleic acid increased from 6% to 100% when the

temperature was increased from 250ºC to 325ºC.

Selective Hydrogenation of Sunfl ower Oil over Noble Metal CatalystsS. McArdle, S. Girish, J. J. Leahy and T. Curtin, J. Mol. Catal. A: Chem., 2011, 351, 179–187

Pd and Pt supported on Al2O3, ZrO2 and TiO2 were

screened as heterogeneous catalysts for the selective

hydrogenation of sunfl ower oil in a batch reactor

under different operating conditions. Metal dispersions

between 6–69% and loadings between 0.3–3.9% were

studied for promotion of selectivity in cis C18:1. Pd

catalysts were found to be much more active than a

conventional Ni catalyst. The Pt catalyst was not as

active as the Pd catalysts but produced lower trans

fatty acids. The level of trans could be further reduced

by increasing the operating pressure to 10 bar and

reducing the reaction temperature to 100ºC especially

with the Pt catalyst.

The Conversion of 1,8-Cineole Sourced from Renewable Eucalyptus Oil to p-Cymene over a Palladium Doped -Al2O3 CatalystB. A. Leita, P. Gray, M. O’Shea, N. Burke, K. Chiang and D. Trimm, Catal. Today, 2011, 178, (1), 98–102

The conversion of the title 1,8-cineole over a Pd-

doped -Al2O3 catalyst was studied. Both Pd and

-Al2O3 were found to be bifunctional catalysts but

the Pd-doped system showed very high activity and

selectivity, yielding >99% p-cymene, while producing

large amounts of H2 at a bed temperature of 250ºC.

The reaction mechanism is suggested to involve C–O

bond fi ssion in cineole, followed by dehydrogenation/

isomerisation to give p-cymene.

Asymmetric Synthesis of 2-Alkyl-3-phosphonopropanoic Acid Derivatives via Rh-Catalyzed Asymmetric HydrogenationL.-B. Luo, D.-Y. Wang, X.-M. Zhou, Z. Zheng and X.-P. Hu, Tetrahedron: Asymmetry, 2011, 22, (24), 2117–2123

The ferrocene-based diphosphine ligand (Sc,SFc)-

TaniaPhos was shown to be highly effi cient in

the Rh-catalysed asymmetric hydrogenation of

3-aryl-2-(phosphonomethyl)propenates. Excellent

enantioselectivity (90–98% ee) and high catalytic

activity (S/C up to 1000) were exhibited.

Abstracts

Ph2P

Ph2P FeN

L.-B. Luo et al., Tetrahedron: Asymmetry, 2011, 22, (24), 2117–2123

(Sc,SFc)-TaniaPhos



,-Unsaturated Imines via Ru-Catalyzed Coupling of Allylic Alcohols and AminesJ. W. Rigoli, S. A. Moyer, S. D. Pearce and J. M. Schomaker, Org. Biomol. Chem., 2012, 10, (9), 1746–1749

,-Unsaturated imines were synthesised using only

an allylic alcohol, an amine and a Ru catalyst. The use

of large excesses of oxidants or dehydrating reagents

and the purifi cation of sensitive intermediates were

not requried. The reaction was most effective with

trisubstituted allylic alcohols. The reaction conditions

are mild enough to preserve other unsaturated

functional groups from reduction. The ability of

Milstein’s PNN Ru catalyst to catalyse the effi cient

redox isomerisation of secondary allylic alcohols to

the corresponding ketones was also demonstrated for

the fi rst time.

FUEL CELLSProducts of SO2 Adsorption on Fuel Cell Electrocatalysts by Combination of Sulfur K-Edge XANES and ElectrochemistryO. A. Baturina, B. D. Gould, A. Korovina, Y. Garsany, R. Stroman and P. A. Northrup, Langmuir, 2011, 27, (24), 14930–14939

SO2 adsorption products on Pt/Vulcan C catalyst coated

membranes (CCMs) were investigated at different

electrode potentials using a combination of in situ

S K-edge XANES spectroscopy and electrochemical

techniques. SO2 was adsorbed from a SO2/N2 gas

mixture with the Pt/Vulcan C electrode potential

held at 0.1, 0.5, 0.7 or 0.9 V vs RHE. S0 adatoms were

identified as the SO2 adsorption products at 0.1 V;

mixtures of S0, SO2 and sulfate/bisulfate ((bi)sulfate)

ions at 0.5 and 0.7 V; and (bi)sulfate ions at 0.9 V.

Effect of Deactivation and Reactivation of Palladium Anode Catalyst on Performance of Direct Formic Acid Fuel Cell (DFAFC)S. M. Baik, J. Han, J. Kim and Y. Kwon, Int. J. Hydrogen Energy, 2011, 36, (22), 14719–14724

Pd and Pt were used as anode and cathode DFAFC

catalysts, respectively, and were applied to a Nafi on

membrane by CCM spraying. As multiple repeated

DFAFC operations were performed, the cell

performance of DFAFC steadily degraded. This is

attributed to the electrooxidation of Pd into Pd–OH,

which occurs between 0.1 and 0.55 V. In CV experiments

where the voltages applied to the DFAFC single cell

are lower than 0.7 V vs DHE, the cell performance was

further deactivated due to continuous production

of Pd–OH. Conversely, in CV experiments where the

voltage was higher than 0.9 V vs DHE, cell performance

was reactivated due to redox reactions of Pd–OH into

Pd–O and Pd–O into Pd.

Development of a Catalytic Hollow Fibre Membrane Microreactor as a Microreformer Unit for Automotive ApplicationM. A. Rahman, F. R. García-García and K. Li, J. Membrane Sci., 2012, 390–391, 68–75

A catalytic hollow fi bre membrane microreactor

(CHFMMR) was prepared by electroless plating of a

uniform, defect-free Pd/Ag membrane with a total

thickness of 5.0 μm onto the outer layer of a YSZ hollow

fi bre substrate. A thin layer of a 10 wt% NiO/MgO–CeO2

ethanol steam reforming (ESR) catalyst (max. 1.3 wt%)

with a high surface area was impregnated into the

YSZ hollow fi bres using the sol–gel Pechini method.

ESR was carried out and high purity H2 was obtained

outside the shell with a yield of <53% of the total H2

produced in the ESR. This CHFMMR can be used in the

development of on-board H2 generation using EtOH as

a fuel for PEMFCs in vehicular applications.

APPARATUS AND TECHNIQUE

Brittle Failure Analysis of PtRh10 – Pt Thermocouple WireY. Wang, X. Chen, G. Li , Y. Pan, X. Chen, B. Wu and J. Wang, Precious Met. (Chin.), 2011, 32, (3), 82–84

The brittle failure of PtRh10-Pt thermocouples in

reducing atmospheres was investigated. Electron

probe microanalysis indicated that the formation of

the low melting point eutectic, Pt5Si2, is the main cause

of brittle failure.

Electrospun Carbon Nano-Felt Surface-Attached with Pd Nanoparticles for Hydrogen Sensing ApplicationL. Zhang, X. Wang, Y. Zhao, Z. Zhu and H. Fong, Mater. Lett., 2012, 68, 133–136

A C nano-felt with Pd NPs attached at the surface was

prepared from electrospun polyacrylonitrile nano-felt

surface-functionalised with amidoxime groups. The

material consisted of relatively uniform and randomly

overlaid C nanofi bres with diameters of ~300 nm. The

electrospun C nano-felt was mechanically fl exible and

resilient, and its resistance varied on exposure to H2

at room temperature. This material could therefore be

used for the fabrication of gas- or bio-sensors.



ELECTRICALS AND ELECTRONICSInjection of Synthesized FePt Nanoparticles in Hole-Patterns for Bit Patterned MediaT. Hachisu, W. Sato, S. Ishizuka, A. Sugiyama, J. Mizuno and T. Osaka, J. Magn. Magn. Mater., 2012, 324, (3), 303–308

Previously, the immobilisation of FePt NPs on a

thermal oxide Si substrate was carried out by

utilising the Pt–S bonding between the -SH in

(3-mercaptopropyl)trimethoxysilane (MPTMS) and

Pt in FePt NPs. In this study, attempts were made to

control the distortion of the arrangement of FePt NPs

using an MPTMS layer modifi ed with a silane coupling

reaction and a geometrical structure prepared by UV

nanoimprint lithography. The hole-patterns used for

the geometrical structure on Si(1 0 0) were 200 nm

wide, 40 nm deep, and had a 500 nm pitch. The 5.6 nm

FePt NPs were used to coat the hole-patterns by using

a picolitre pipette.

NANOTECHNOLOGYAtomic Force Microscopy of the Dissolution of Cubic Pt Nanoparticle on a Carbon SubstrateN. Hoshi, M. Nakamura, C. Goto and H. Kikuchi, J. Electroanal. Chem., 2012, 667, 7–10

The dissolution mechanism of cubic Pt NPs on a

C substrate in 0.1 M NaClO4 was studied using AFM.

The shape of the NPs did not change in the double

layer region. The height of the NPs increased slightly

at the onset potential of the oxide fi lm formation. The

NPs were dissolved from the sides in the oxide fi lm

formation region; the change of height was negligible.

These results differed from those of the cubic Pt NPs

on a Pt substrate, for which dissolution proceeds at the

upper terrace.

PHOTOCONVERSIONPhotodegradation of 2,4-D over PdO/Al2O3–Nd2O3 Photocatalysts Prepared by the Sol–Gel MethodA. Barrera, F. Tzompantzi, V. Lara and R. Gómez, J. Photochem. Photobiol. A: Chem., 2012, 227, (1), 45–50

The photocatalytic activity of 1.0 wt% PdO/Al2O3–

Nd2O3 was studied in the photodegradation of

2,4-dichlorophenoxyacetic acid (2,4-D). PdO/-Al2O3

photodegraded 2,4-D; however the addition of Nd2O3

to -Al2O3 improves the photocatalytic activity. As

the concentration of Nd2O3 was increased from 2 to

10 wt%, the photodegradation of 2,4-D was enhanced.

PdO/Nd2O3 showed scarce photocatalytic activity.

2,4-D was completely destroyed on the PdO/Al2O3–

Nd2O3 photocatalysts after 6 h irradiation.

REFINING AND RECOVERYRecovery of Platinum from Spent Catalysts by Sintering-LeachingM. Wang, X. Dai, J. Wu, B. Zhang, Y. Wu and T. Chen, Precious Met. (Chin.), 2011, 32, (4), 6–10

The sintering-leaching method was used to treat the

Al2O3 in spent catalysts. Pt was then recovered from

the residues after the enriching process. The residual

C was 0.54% with a high decoking rate of 92.66%, after

roasting the spent catalysts at 600ºC for 1 h. The Pt was

enriched 17.87 times by the sintering-leaching process.

The ‘cinder’ was mixed with NaOH and then sintered

at 800ºC for 2 h. The ‘clinker’ was leached at 95ºC for

10 min.

SURFACE COATINGSRoom Temperature Formation, Electro-Reduction and Dissolution of Surface Oxide Layers on Sputtered Palladium FilmsG. Macfi e, A. Cooper and M. F. Cardosi, Electrochim. Acta,

2011, 56, (24), 8394–8402

After storage under ambient conditions, peaks

corresponding to sub-monolayer coverage of surface

oxide and to chemisorbed O2 were observed on linear

sweep voltammograms measured on sputtered Pd

fi lms. The surface oxide fi lms formed on the Pd were

found to be relatively insoluble in neutral conditions

and in the absence of Cl–, where dissolution was limited

by the solubility of Pd2+. In HCl the greater solubility of

PdCl42– allowed the oxide fi lm to dissolve completely.



CATALYSIS – APPLIED AND PHYSICAL ASPECTSCore-Shell Catalyst ParticleToyota Jidosha Kabushiki Kaisha, US Appl. 2012/0,010,069

A core-shell catalyst particle is produced by

preparing a core particle containing a fi rst core

metal (selected from Pd, Ag, Rh, Os or Ir) which

has a standard electrode potential of at least 0.6 V

and a second core metal (selected from Co, Cu, Fe

or Ni) with a lower electrode potential. The second

core metal is eluted by adjusting the pH (2 to 4) and

the potential (–2 V to 1 V) of the core particle and

the shell portion is coated on the core particle by

displacement plating to replace a Cu monoatomic

layer deposited by underpotential deposition. The

shell portion includes a metal selected from Pt, Ir

and Au. The average diameter of the core particle

is 4–40 nm.

Mixed Bed Polymeric CatalystDow Global Technologies LLC, Chinese Appl. 102,309,989; 2012

A mixed bed polymeric catalyst consists of 10–90 wt%

of a fi rst catalyst having ion exchange resin loaded

with zerovalent metal selected from Pt, Pd, Rh, Ir, Ru,

Cu, Au or Ag and 10–90 wt% of a second catalyst

which has a strong acidic ion exchange resin without

metal, this comprises of a styrenic strong acid cationic

resin with a moisture hold capacity from 10–90%

and a volume capacity from 0.5–7 meq l–1. The fi rst

and second catalysts both have a particle size of

100 μm to 2 mm. The metal is uniformly distributed

throughout the mixed bed. This catalyst is used for the

hydrogenation of alkynes, alkenes, aldehydes, ketones,

alcohols, nitriles, amines and nitro groups.

CATALYSIS – REACTIONSProducing Olefi n OxideSumitomo Chem. Co, Ltd, World Appl. 2012/005,824

A method for producing an olefi n oxide e.g. propylene

oxide involves reacting an olefi n (propylene) with O2

at 100–350ºC in the presence of a catalyst consisting

of CuO, OsO2, an alkali metal or alkaline earth metal

and a halogen component. This catalyst is obtained by

impregnating a porous support (selected from Al2O3,

SiO2, TiO2 or ZrO2) with a solution containing Cu2+ ions,

Os3+ ions, alkali metal or alkaline earth metal ions and

halogen ions, followed by calcining the composition.

The molar ratio of Cu:Os in the catalyst is 1:99 to 99:1.

EMISSIONS CONTROLTwo Layers in an Exhaust Gas Purifying CatalystJohnson Matthey Plc, US Appl. 2012/0,031,085

An exhaust gas purifying catalyst comprises

two catalyst layers, each containing different

compositions of fi re resistant inorganic compound

(this is a powder selected from Al2O3, SiO2-Al2O3,

zeolite, TiO, SiO2, CeO2 and ZrO2 or a mixture and has

an average particle diameter of 2–10 μm). The fi rst

catalyst layer is selected from Pt, Pd, Rh or Pt/Pd and

the second catalyst layer is selected from Pt, Pd, Rh,

Rh/Pt and Rh/Pd, with both layers extending a length

of ≥40% to <100% of the total passage length. The fi rst

layer is extended from the exhaust gas introduction

port side to the discharge port side and the second

layer is extended from the exhaust gas discharge port

side to the introduction port side so that both layers

are partly overlapped.

Treatment of Nitrogen Oxides and/or Particulate Matter in Lean GasJohnson Matthey Plc, British Appl. 2,481,057; 2011

A method for treating NOx and PM or both comprises

of: (a) a NO conversion catalyst (for converting NO to

NO2) supported on a substrate monolith, the catalyst

contains a Mn oxide and one or more of Pt, Pd and

Rh in a loading of 1–240 g ft–3; (b) a SCR catalyst

(for selectively catalysing the reduction of NOx to

N2) supported on a fl ow-through substrate monolith

and located downstream from the NO conversion

catalyst; and optionally (c) a catalysed fi lter substrate

for removing PM to combust it in NO2.

FUEL CELLSPlatinum Alloy CatalystJohnson Matthey Plc, World Appl. 2012/017,226

A Pt alloy catalyst, PtXY, where X = Ni, Co, Cr, Cu, Ti or

Mn and Y = Ta or Nb (which is less leachable than

X in an acidic environment) with (in at%): 20.5–40

Pt; 40.5–78.5 X; and 1–19.5 Y is utilised for PAFC and

PEMFC. This catalyst is supported on a conductive

support material and is used at the fuel cell cathode.

Patents



METALLURGY AND MATERIALSPalladium-Containing Alloy for Jewellery Ware Pasquale Bruni Spa, World Appl. 2012/017,299

An alloy consists of (in wt%): 4–4.5 Au; 35–68 Ag; 8–30

Pd; and one or more elements selected from Ir, Cu, Zn,

Ni and Si in a total amount from 18–32 wt% or In in a

total amount from 25–32 wt%. This alloy can be used for

goldware or jewellery.

Platinum Jewellery AlloyKrastsvetmet, Russian Patent 2,439,180; 2012

An alloy comprising of (in wt%): 99–99.5 Pt; 0.1–0.7 Ir;

0.1–0.5 Co; and the balance Ga. This alloy is suitable

for manufacturing jewellery, especially chains, using

investment microcasting or plastic deformation.

APPARATUS AND TECHNIQUEPalladium in Heating CircuitDelphi Technol., Inc, World Appl. 2011/153,517

A planar device consists of a heater circuit which is

formed by fi rst depositing a heater precursor material

consisting of Pd, optionally alloyed with Rh to reduce

its tendency to oxidise, in a predetermined pattern on

the surface of a fi rst unfi red ceramic substrate. This is

then laminated to a second unfi red ceramic substrate

where Pd is in contact with the second substrate to

form a laminated combination, this is then fi red in an

oxidising atmosphere at 400–850ºC suffi ciently slowly

to prevent the formation of voids in the fi nal structure.

The line widths of the heater circuit are 0.1–0.25 mm

in the area which reaches the highest temperature.

MEDICAL AND DENTALStent Delivery CatheterKaneka Corp, Japanese Appl. 2012-000,328

A stent delivery catheter has a multilayer shaft tube

comprising of an outer layer, a reinforcement layer and

an inner layer. A tubular member (e.g. Pt-Ir) is formed

between the outer layer and reinforcement layer on the

shaft tube end.

PHOTOCONVERSIONRefl ective Electrode for Light-Emitting DiodeLG Innotek Co, Ltd, European Appl. 2,410,583; 2012

The structure of a LED comprises of two conductive

semiconductor layers and an active layer. An electrode

is disposed on the fi rst semiconductor layer and a

refl ective electrode (surrounded by a channel layer

selected from Pt, Pd, Rh, Ir, Ti, Ni and W, and connected to a

support substrate through an adhesive layer) is disposed

on the second semiconductor layer which also has an

ohmic layer disposed on the current blocking layer.

Solar Cells with Multiple DyesBangor Univ., British Appl. 2,481,035; 2011

A method for the preparation of dye-sensitised solar

cells with multiple dyes consists of: (a) preparing the fi rst

electrode from an electroconducting substrate, which

is a glass or polymer plate coated with a transparent

conducting oxide, preferably SnO2 which has been

preferably doped with F; (b) applying ≥1 layers of a paste

of TiO2 NPs on the conducting side of the substrate; (c)

subjecting the coated substrate to a thermal treatment

from 300–600ºC for 1 h; (d) preparing a second

electrode in the same way as the fi rst electrode and

additionally coating it with Pt; (e) optionally pre-dyeing

the fi rst coated electrode with a solution containing ≥1

dyes; (f) piercing two perforations in the fi rst and/or

second electrode and sealing these together with glue

or a thermoplastic polymer; (g) injecting or pumping

≥2 solutions containing ≥1 different dyes under vacuum

to covalently bind the dye(s) to the surface of the

metal oxide; (h) injecting or pumping the electrolyte

(selected from a liquid nitrile solvent containing a

redox couple selected from Ru bipyridyl complexes,

Ru terpyridyl complexes, coumarins, phthalocyamines,

squaraines, indolines or triarylamine dyes) through

the holes in the electrodes; (i) sealing the holes in the

electrodes with glue or a thermoplastic polymer; and

(j) providing an external connection between the two

electrodes for electron transport. The dyeing between

the sealed electrodes takes place from 10–70ºC and the

electrolyte is added <10 min after the dye, therefore, the

dyeing is completed in 10 min.

Dye solutionor electrolyte ingress

Outfl ow for excess dyesolution or electrolyte

SealHole throughelectrode

F-dopedtin oxide

Electrolyte Sealant

Dyed metal oxidephotoelectrode

Electroconductingsubstrate

Metal contact

Electroconductingsubstrate

A “cocktail” dyed solar cell using 2 different dyes within the same photoelectrode

British Appl. 2,481,035; 2011



Platinum in Dye-Sensitised Solar CellToyo Aluminium Ltd, Japanese Appl. 2011-171,133

A cathode for a dye-sensitised solar cell contains an

Al substrate. The surface of the substrate is laminated

by sputtering with at least one metal selected from Pt,

Au or Ag. The Al substrate is 7–200 μm thick and the Pt,

Au or Ag layer is 1–50 nm thick. The cathode is suitable

for a dye-sensitised solar cell using an ionic liquid free

from iodine or iodide.

REFINING AND RECOVERYRecovery of Platinum Group MetalsWintermute Metals Pty Ltd, World Appl. 2011/140,593

A method for extracting pgms from a pgm-containing

material, especially spent automotive catalysts, is

claimed. This involves mixing the pgm-containing

material with a chloride leaching solution which

consists of 10–20 wt% HCl and ≥1 trivalent metal

chloride salt selected from AlCl3, FeCl3, CrCl3 or

lanthanide chloride salts. This process occurs in an

overpressure of Cl2 gas of ~1–2 atm at ~55–90ºC for

~120 min. The leachate solution may be recycled for

use as the chloride leaching solution. The recovery

of pgms in the leachate solution involves subjecting

this solution to electrowinning, cementation, solvent

extraction, gas-reduction or adsorption.

SURFACE COATINGSCoating for Gas Turbine Engine ComponentGeneral Electric Co, US Patent, 8,084,094; 2011

A coating system is applied to the surface of a substrate

(formed from a Ni-base alloy e.g. Ni3Al) to prevent the

formation of a secondary reaction zone. This system

involves plating a stabilising layer (selected from the

pgms: Pt, Pd, Rh or Ir) on the surface of the substrate

then heat treating at ~900–1120ºC for ~1–8 h prior to

depositing an Al-containing overlay coating which has

also been coated with a ceramic coating. The stabilising

layer consists of ~75 at% pgm and optionally Ni, Co, Cr,

Al and/or Ru in a combined amount of up to 25 at%.

This layer has a thickness of ~3–12 μm.

0

1020

3040

50

60

70

8090

100

Test 111 Test 114 Test 115 Test 116

Extr

actio

n, %

Pt %

Pd %

Rh %

Leaching with 15% HCl and Cl2 gas at 90ºC for 5 min

World Appl. 2011/140,593



A common question asked by process chemists is

“Do I use a biocatalyst or one based on a transition

metal?”. The choice can be an important part of

route scouting (1). Unlike many articles that leave

the reader waiting to the end, here is the answer: “It

depends”.

The advantages of using a catalyst rather than a

stoichiometric reagent can be summed up by the

concepts of ‘green’ chemistry (2), but the simple

way to look at the catalytic approach is that less of a

catalyst is used compared to a stoichiometric reagent,

so the cost of the material and its removal is reduced.

In the past, processes were designed by chemists

who had little familiarity with biotransformations and

enzymes. Thus, the use of an enzyme was a means of

last resort (3). Now, many companies have process

groups which include members who are familiar with

enzymes and biotransformations. Indeed, to those who

are familiar with biological systems, they seem to be

the solution to most problems!

Background KnowledgeOne of the criteria for the choice between bio- and

chemical catalysis is background knowledge. Our

understanding of chemistry is far from complete.

However, there is a wealth of information available and

the key parameters to control the reaction outcome

are known for a wide range of transformations. This

knowledge has often been gained by trial and error.

As our knowledge has increased, better and more

selective catalysts have been developed. Even if we

still cannot design the ultimate catalyst from fi rst

principles, we can use prior knowledge to obtain a

workable catalyst.

For biological systems, trial and error has again been

used, but often there are subtle differences between

very similar enzymes so that interpretation of the

results can be diffi cult. Examples of this are provided

by baker’s yeast reductions and pig liver esterase. More

recently, our understanding of active sites and how to

manipulate them either in a controlled or random way

has allowed for the expansion of biomethods to make

useful compounds.

Modern screening methods make it almost as easy

to determine whether a chemical or a biocatalyst

is capable of the desired transformation. However,

chemical approaches will often require a more

involved analysis, such as high-performance liquid

chromatography (HPLC). Although biocatalysts may

also need this type of analytical method, much faster

methods have been developed and even the survival

of the strain can be used.

An Exclusive ApproachClosely related to this consideration is the ability of

one system to perform a transformation that is very

diffi cult for the other to accomplish. Examples are

the hydrolysis of a meso-diester and the coupling of

aryl groups. In the fi rst case, chemical catalysts have

been developed to hydrolyse just one ester group of a

diester to provide a chiral monoester. In contrast, there

is a wealth of information on enzymatic examples and

for some, as with pig liver esterase, a mnemonic has

been developed to predict whether the reaction can

be performed with ‘wild-type’ enzyme and what the

stereochemical outcome will be (4).

In the example of aryl couplings, there are many

examples and variations of palladium-catalysed Heck

and Suzuki reactions (5, 6), to name but two of these

types of reaction; these approaches are the methods

of choice in most cases. There are very few examples

of enzymatic aryl couplings and, thus, this will only

be used when alternative chemical methods are not

applicable.

Solvent ChoiceAnother criterion used to decide what type of catalyst

is used is that the process must include isolation of

the product in acceptable yield and purity. Although

signifi cant advances have been made for the use

of enzymes in the presence of organic solvents and

with substrates that are poorly soluble in aqueous

media, problems can still exist. Conversely, if the

product is very water soluble, isolation of the desired

material can present a signifi cant challenge. Chemical

transformations usually have the advantage of a

FINAL ANALYSIS

Biological or Chemical Catalysis?



number of solvent systems being available to perform

the transformation and subsequent isolation.

Scaling UpThe scale-up of chemical catalytic reactions is usually

an exercise in normal process chemistry. Synthesis of

the ligands and transition metal catalyst usually present

few problems, as the amounts are much lower than

for the reaction itself. However, to improve specifi city

and effi ciency new catalysts have to be prepared

and tried. This can be a time-consuming exercise. On

the biocatalytic side, modifi cations to the original

enzyme can be made by genetic modifi cations and

rapid screening. This allows selectivity, stability and

other issues to be addressed. On the other hand, these

studies can be time-consuming and may require more

resources than just making a new ligand.

Cost and SpeedThe criteria of cost and speed also contribute to

the choice of catalyst system. In the early stages of

development of a drug candidate, speed is of the

essence but cost is still important. Precedence and

the availability of the catalyst will be key factors.

For larger scale work, the pendulum may swing

from a chemical method to a biocatalytic one. For

commercial production, biological routes are often

cost effective.

However, it must be remembered that the catalytic

step is usually part of a synthetic sequence. The

substrate for the catalytic step has to be prepared and

the product converted to the fi nal target molecule.

As an example, consider the synthesis of (S)-2,3-

dihydro-1H-indole-2-carboxylic acid. It was necessary

to replace an ineffi cient route that used a late stage

resolution. Many alternatives were considered but

the most effi cient used a copper-catalysed cyclisation

to form the product. To prepare the substrate for

this reaction, three enzymatic approaches were

identifi ed (Figure 1). When the alternative methods

were compared in terms of costs and effi ciencies

(including the number of steps), the ammonia lyase

approach was the route of choice as the starting

material is readily available (7, 8). Although there are

a number of approaches to ortho-halocinnamic acid

(8), the Heck reaction provides an effi cient synthesis.

Without the use of this palladium-catalysed reaction,

the remainder of the sequence would not be effi cient.

Thus, the overall sequence shows that biocatalysts and

chemical catalysts are required to effect an effi cient,

sustainable process.

Conclusion In summary the choice between a chemical and a

biological catalyst depends on many factors. The

background literature plays an important role, as

i) Pd,

ii) Hydrolysis

Amino acid

dehydrogenase

Ammonia lyase

Hydantoinase

Cu catalysis

Fig. 1. Some routes to (S)-2,3-dihydro-1H-indole-2-carboxylic acid



does cost and speed to implement the process.

There are some transformations where both types of

catalysts excel. Chemical catalysts can work well with

a specifi c and well documented range of substrates,

and in particular palladium fi nds wide use in

essential reactions such as Heck coupling. Enzymatic

approaches can have a broader application but may

be less well understood. In the end, both transition

metal catalysts and biocatalysts may be required to

produce an effi cient process.

DAVID AGER

DSM Innovative Synthesis BV, PMB 150,

9650 Strickland Road, Suite 103,

Raleigh, NC 27615, USA


References1 D. J. Ager, ‘Route and Process Selection’, in “Process

Understanding: For Scale-Up and Manufacturing of Active Ingredients”, ed. I. Houson, Wiley-VCH Verlag GmbH & Co KGaA, Weinheim, Germany, 2011, pp. 17–58

2 P. T. Anastas and J. C. Warner, “Green Chemistry: Theory and Practice”, Oxford University Press, New York,

USA, 1998

3 H. E. Schoemaker, D. Mink and M. G. Wubbolts, Science, 2003, 299, (5613), 1694

4 D. J. Ager and M. B. East, “Asymmetric Synthetic Methodology”, CRC Press, Boca Raton, USA, 1996, Chapter 14

5 X.-F. Wu, P. Anbarasan, H. Neumann and M. Beller, Angew. Chem. Int. Ed., 2010, 49, (48), 9047

6 Angew. Chem. Int. Ed., 2010, 49, (45), 8300

7 P. Pöchlauer, S. Braune, A. de Vries and O. May, Chim. Oggi, 2010, 25, (4), 14

8 D. Ager and A. de Vries, Spec. Chem., 2011, 31, (09), 10

The AuthorDavid Ager has a PhD (University of Cambridge, UK), and was a post-doctoral worker at the University of Southampton. He worked at Liverpool and Toledo (USA) universities; NutraSweet Company’s research and development group (as a Monsanto Fellow), NSC Technologies, and Great Lakes Fine Chemicals (as a Fellow) responsible for developing new synthetic methodology. David was then a consultant on chiral and process chemistry. In 2002 he joined DSM as the Competence Manager for homogeneous catalysis. In January 2006 he became a Principal Scientist.

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Sara ColesAssistant Editor

Ming ChungEditorial Assistant

Keith WhitePrincipal Information Scientist


Platinum Metals Review is Johnson Matthey’s quarterly journal of research on the science and technologyof the platinum group metals and developments in their application in industry

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Editorial Team

Jonathan Butler Publications Manager

Sara Coles Assistant Editor

Ming Chung Editorial Assistant

Keith White Principal Information Scientist

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Vol 56 Issue 2 April 2012 - Johnson Matthey Technology Review · Published by Johnson Matthey Plc Vol 56 Issue 2 April 2012 E-ISSN 1471-0676 A quarterly journal of research on the