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Biobased chemicals from ligninKloekhorst, Arjan
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Publication date:2015
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Citation for published version (APA):Kloekhorst, A. (2015). Biobased chemicals from lignin. [S.l.]: [S.n.].
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9
Chapter 1
Introduction
INTRO
DU
CTION
1
10
1.1 INTRODUCTIONThe prospects of declining crude oil reserves, a steady increase in energy demand and concerns for the environment have intensified the search for renewable sources for energy generation, transportation fuels, materials and chemicals. Renewable electric energy is already commercialized in the form of wind-, solar, tidal-, and geothermal energy. However, it is expected that the transportation sector can be electrified only partly and that renewable replacements for diesel, gasoline, and kerosene will still be carbon based alternatives and thus produced from biomass. Currently, the commercial first generation bio-fuels are made from food crops, well known examples are bio- ethanol (from e.g. sugar cane, sugar beet, and starch) and bio-diesel (from vegetable oils and waste cooking oils) [1]. Ethical concerns related to the use of edible food sources for bio-fuels have led to an intensification in research and development activities on the use of more sustainable resources, like lignocellulosic (woody) biomass. The latter has high potential as a feed for bioenergy, second generation bio-fuels, and bio-based chemicals and performance products due to its high abundance at relatively low costs [2-4].
Cellulose (35-50 wt%)
Hemicellulose (20-30 wt%)
Lignin (10-25 wt%)
OOHO O
OO
HOO
OHHO
O
OH
O
O
OH
HOOCOHO
H3CO
O
OCH3OHOHH3CO
O OCH3
OHO
HO
H3CO
OH
HO
HO HO OH
lignin
lignin
O
OH
OHO OH
O O
HO OHO
OH
Figure 1.1. Pie chart of the major components of lignocellulosic biomass.
The last decade, a major proportion of the research on lignocellulosic biomass has focused on the development of second generation biofuels. Recently, focus has shifted to biobased chemicals and particularly the replacement for existing petrochemical products. The use of lignocellulosic biomass for chemicals production is not new. Currently, about 10 % of the carbon based chemicals is already produced from biomass, examples are oleochemicals and starch/cellulose derived products. However, these are mainly high molecular weight performance products and not the typical low molecular weight bulk/base chemicals. Early examples of biobased bulk chemicals from lignocellulosic biomass are methanol, acetic acid, and ethyl acetate from wood by continuous distillation [5]. Several approaches have been proposed to produce
INTRO
DU
CTION
1
11
biobased chemicals from biomass. Often used approaches involves modification of the biopolymer like starch, cellulose, or proteins while keeping the biopolymer intact. Examples of these are starch modifications by for example carboxymethylation or oxidation to obtain high added value starch products. An alternative approach involves breaking down the bio-polymers to low molecular weight building blocks, also known as platform chemicals or biomass building blocks. These are subsequently converted to secondary products, intermediates and finally to end products for introduction in the market [2, 3].
1.2 PLATFORM CHEMICALS FROM LIGNOCELLULOSIC BIOMASS
1.2.3 Valorisation of cellulose and hemi-celluloseThe chemical composition of lignocellulosic biomass depends on the source material, but woody biomass (wood, straw, grass, nuts) usually consists of cellulose (35-50 wt%), hemi-cellulose (20-30 wt%), and lignin (10-25 wt%), see Figure 1.1 for details. Cellulose and hemi-cellulose are biopolymers consisting of sugar monomers. While cellulose contains primarily glucose monomers, hemi-cellulose consits of mainly pentose sugar monomers like xylose and arabinose. A large number of valorisation technologies has been developed for cellulose and hemi-cellulose to generate a wide range of biobases platform chemicals, see Figure 1.2. Examples are fermentation, (e.g. lactic acid, succinic acid) and chemo-catalytic processes (furfural, sorbitol) [3].
HOOH
O OH
O
HOOH
O
O
OH
NH2
O
OH
OO HO
O
O
HO OHOH OH
OH
HO OHNH2
OO
HOOH
OH
OH
OH
OH
HOOH
O
OH
OH
OH
OH
NH2
HO
ONH2
HO OHOH
OHOOO
OOHO
HO OH
OOO
OH
O
OHOH
OHHO OH
HO OH
O
NH2
OH
O
HemicelulloseCellulose
L-Lactic acid
Propanicacid
Malonic acid
5-Hydroxymethylfurfural
Citric acid
Lysine
D-Gluconic acid
D-Sorbitol
Glutamic acid
Xylitol
Levulinic acidFurfural
Threonine
Succinic acidMalic acid
Acetoin
L-Serine
Glycerol
C3
C4
C6
C5
Figure 1.2. Pathways for the production of chemicals from cellulose and hemi-cellulose [3].
INTRO
DU
CTION
1
12
1.2.2 Lignin
Lignin is the third largest bio-polymer found in lignocellulosic biomass. It has a rigid structure and gives the plant tissue strength and hydrophobicity. The lignin content varies between plant species; for instance the lignin content of softwood (e.g. pine, spruce) is about 28 %, whereas it is about 20 % for hardwood (e.g. oak, birch) [6].
OH
HO
OH
HO
OCH3
OH
HO
OCH3H3CO
Coumaryl alcohol Coniferyl alcohol Syringyl alcohol
Figure 1.3. The three building blocks of lignin; coumaryl, coniferyl, and syringyl alcohol.
From a chemical perspective, lignin is an amorphous polyphenolic thermoset which is made in the plant/tree by radical coupling of three aromatic monomers, namely, coniferyl, sinapyl, and p-coumaryl alcohols (Figure 1.3) [7]. A highly complex three-dimensional polymer is formed with different types of linkages between the aromatic nuclei. Of these, the β-O-4, α-O-4, β-5, and 5-5 are the most common, see Figure 1.4 for details.
Lignin
H3COO
O
OOCH3
H3CO OOCH3
H3CO
HO
HO
O
OCH3
OCH3O
OCH3
OHOH
OCH3
O
OCH3
O
HO
OH
O
OCH3
OH3CO
O
O
H3CO
HO
OLignin
O
O
OHOH
HO
OH
OH
OH
OH
OH
OHHO
OH
OH
HO
OHO
HO
OH
H
OH
5-5 (18-10%)
β-O-4 (50-60%)
α-O-4 (8%)β-5 (11-6%)
β-β (2%)
OH
OCH3
OH
HOβ-1 (7%)
Figure 1.4. Structural model of spruce lignin (softwood) based on Adler et al. [13], with added percentage of several linkages in soft-& hardwood [14].
INTRO
DU
CTION
1
13
The main linkages (~ 70 %) in lignin are C-O-C ether bonds (β-O-4 and a smaller amount of α-O-4) with the remaining ones primarily consiting of C-C double bonds (5-5 and β-5). The high number of aromatic fragments in lignin makes it an excellent feedstock for the production of aromatic biobased chemicals, like benzene, toluene, xylene, and alkylphenolics. This is widely recognized now and the topic has attracted considerable interest by academia and industry [8-12]. The mayor producer of lignin is the paper industry with an estimated yearly production of 50 million tons world-wide, which is primarily used for steam and power production [15, 16]. Two important processes are used in the paper industry, namely Kraft and sulphite pulping [17]. During sulphite pulping, the lignin is depolymerised and sulphonate groups are introduced upon the treatment with various salts of sulphurous acid. Kraft pulping uses alkaline conditions (sodium sulphite) to solubilize the lignin.
Lignin may also be isolated by advanced lignocellulosic biomass pre-treatment techniques like steam explosion [18, 19], and organosolv processes. In the latter, the biomass source is treated with a mixture of an organic solvent (e.g. ethanol) and water under pressure to separate the lignin from the other biopolymers [20-22].
1.2.3 Valorisation of ligninLignin is an interesting renewable source for aromatic chemicals like benzene, toluene, xylene (also known as BTX), and phenolic compounds (see Figure 1.5). These are important base chemicals which are commercially applied as intermediates for a wide range of other derivatives like styrene, benzoic acid, cyclohexane, and isophtalic acid [2].
Lignin
OH
R
OH
R
OCH3
CO2H
Benzoic acid
NO2O2NDinitrotoluene
Styrene
Cumene
Cyclohexane
OH
Cyclohexanol
HO2C CO2H
Isopthalicacid
HO OHBisphenol A
OH
NO2O2NNitrophenol
OH
NH2H2NDiaminotoluene
OHOH
Catechol
OHOCH3
Guaiacol
O
Cyclohexanone
Figure 1.5. Examples of chemicals from lignin [2].
INTRO
DU
CTION
1
14
Another interesting class of chemicals from lignin are alkyl substituted phenolics. The parent compound phenol is globally produced at a level of about 8 Mt/y (2008) and is a widely used as a base chemical for the production of bisphenol-A, an important monomer for polycarbonate synthesis (48 %), phenolic resins e.g. phenol formaldehyde resins (25 %), cyclohexanone production (the feed for caprolactam production) (11 %), and for conversion to other base chemicals [23]. The production of renewable aromatic and phenolic compounds from lignin thus would have a beneficial effect on the carbon footprint of the petrochemical industry.
1.3 LIGNIN UPGRADING TECHNOLOGY
Lignin valorisation to low molecular weight aromatics and phenolics requires
Table 1.1. Bond dissociation enthalpies (BDE) for lignin linkages.
Lignin compounds BDE (KJ/mol) Lignin compounds BDE
(KJ/mol)
O
R1R2
R4
R3
α-O-4
220 ±16b 369 ±4a
O
376 ±4a
O
OH235 ±4a
OH
398 ±4a
O
263 ±4aO
419 ±4a
OOH
HO
R1
R2
R3
R4HO
β-O-4
292 ±5b
281 ±1c
427 ±4a
O
OCH3
OH
R2
R3
R4
R1
β-5
430 ±8b
OH
R1OH
R2
R3
R4
β-1
289 ±2b
435 ±4a
O
R1 4-O-5332 ±7c
OH
463 ±4a
OH 334 ±4a R1 R2
H3COR3 R4
OCH3
5-5
482 ±12b
488 ±12c
356 ±4a
358 ±4a
a) BDE results from Benson [24], b) Calculated BDE values from Parthasarathi et al. [25], c) Calculated BDE values from Kim et al. [26].
INTRO
DU
CTION
1
15
depolymerisation of lignin. However, this is not sufficient, as the aromatic nuclei in lignin are heavily substituted, e.g. with at least a three carbon side chain with multiple substituents and additional methoxy- and hydroxyl groups. For aromatics production, full deoxygenation is required, for alkylphenolics methoxy group removal is essential. Lignin contains a large amount of ether bonds and thermodynamic calculations indicate that these bonds have the lowest bond dissociation energy. As such, they are in principle the most reactive of all bonds within the structure (Table 1.1). The main challenge is to cleave the remaining (up to 30 %) C-C linkages, which have a much higher bond dissociation energy.
Lignin conversions to low molecular weight compounds has been achieved by various (catalytic) methodologies (Figure 1.6) and can be classified according to process severity (pressure and temperature). Examples are lignin pyrolysis and catalytic versions thereof [8, 27], hydrothermal liquefaction in sub- or super critical water [28], and related versions in organic solvents in the absence or presence of catalysts (catalytic) solvolysis [29, 30]. Lignin conversion at milder conditions (< 300 °C) have also been developed, examples are enzymatic breakdown by enzymes (lignin peroxidase) [31, 32], oxidative depolymerisation [12, 33], and base catalysed depolymerisation [34, 35].
The focus of this thesis will be on lignin upgrading by catalytic hydrotreatment and catalytic solvolysis [9, 36]. Details will be discussed in the following sections.
5
20
40
60
80
100
150
200
300 400 500 600 700
(Catalytic)Solvolysis
Org. solvent
Hydrotreatment
Hydroconversion
H2, catalyst
Catalytic crackingCatalyst, no solvent
PyrolysisNo catalyst, no solvent
Hydr
othe
rmal
pro
cess
ing
wat
er
Lignin
Pressure (bar)
Temperature (°C)
Chemicals(Phenolic
compounds or BTX)
Fuels
Figure 1.6. Various thermochemical pathways and process parameters (P, T) for the depolymerisation of lignin [37]. Reproduced with permission.
INTRO
DU
CTION
1
16
1.4 CATALYTIC HYDROTREATMENT OF LIGNIN
Catalytic hydrotreatment is a process where a feed, here lignin, is reacted with hydrogen in the presence of a catalyst at elevated temperatures and pressures. Typical reaction temperatures are between 200 and 450 °C and hydrogen pressures between 50 and 250 bar. The hydrogen during the reaction can be supplied ex-situ by the addition of molecular hydrogen or formed in-situ by the use of hydrogen donating compounds (e.g. isopropanol or formic acid). Catalytic hydrotreatment of lignin aims for i) depolymerisation of the lignin to lower molecular weight fragments by cleavage of ether and C-C bonds, and ii) (partly) removal of bound oxygen for the production of phenolics and aromatics. A simplified reaction for the catalytic hydrotreatment of lignin to phenol is given in eq. 1.1.
C6H6.36O2 + 0.82 H2 C6H6O + H2O (eq. 1.1)
The elemental composition of the lignin source given in eq 1.1 (C6H6.36O2) is that of a typical technical lignin (Alcell lignin) and will be different for other types of lignin. Graphically, the catalytic hydrotreatment reaction can be represented in a van Krevelen plot [38]. Here, the molar oxygen/carbon ratio is shown on the y-axis and the hydrogen/carbon molar ratio on the x-axis. When aiming for alkylphenolics, the O/C ratio should be reduced by oxygen rejection, in the form of water or CO/CO2. For aromatics, the O/C ratio should be reduced to zero.
Figure 1.7. van Krevelen plot with relevant products and the deoxygenation route for lignin, adapted from Kersten et al. with additional data from Schorr et al. [39, 40].
Various types of reactions may take place when performing the catalytic hydrotreatment of lignin, see Figure 1.8 for details. Examples are hydrogenation, hydrodeoxygenation, cracking, hydrocracking, decarbonylation and decarboxylation.
INTRO
DU
CTION
1
17
In addition, re-polymerization reactions of highly reactive intermediates, ultimately leading to char, may also occur to a certain extent and should be taken into consideration.
R1R1O +HR2
R1 H
OR1 H + CO
R1 OH
OR1 H + CO2
R1R2 + H2 R1 CH3 + H3C R2
R1 OH + H2 R1 H + H2O
R1 H
O
R2 H
O
+ H2 R1 OH
R1O
CH3+ H2 R1 H + CH3OH
Cracking:
Decarbonylation:
Decarboxylation:
Hydrocracking:
Hydrodeoxygenation:
Hydrogenation:
Demethoxylation:
R1O
CH3+ H2 R1 OH + CH4Hydrogenolysis:
Figure 1.8. Examples of reactions occurring during the catalytic hydrotreatment of lignin.
Studies on the catalytic hydrotreatment of lignin have been reported in the literature and will be reviewed in the following sections. A distinction is made between experimental studies in batch, mainly exploratory catalyst screening studies, and process studies in (semi)continuous set-ups. For batch studies, a distinction is made between studies using an external solvent and those in the absence of an external solvent, in the latter case, molten lignin and, in a later stage of the reaction, in combination with low molecular weight products, served as the solvent for the reaction. Model studies using molecular weight model components like anisole, phenol, or guaiacol will not be reviewed here, the reader is referred to recent reviews on this topic [9, 10].
1.4.1 Catalytic hydrotreatment of lignin in batch set-ups using an external solvent An overview of relevant studies on the catalytic hydrotreatment of various lignin sources in batch set-ups using an external solvent is given in Table 1.2 (entries 1-10). Typical reaction conditions are relatively harsh, with temperatures between 175-450 °C, pressures between 20 and 220 bar, and reaction times between 0.25 and 18 h (typically 1-4 h). Various solvents have been explored with a wide range of polarity (from dodecane to water). Both noble metal based supported catalysts (Pd, Rh, Ru, Pt) as well as non-noble metal systems based on Ni (Ni on alumina and carbon, Raney Ni, NiMo on alumina), Cu (Cu on CrO), Co (CoMo on alumina) and Fe (Fe2O3, red mud) have been explored. In most studies, though not all, the product (oil) yield is reported. Detailed characterisation in terms of molecular composition is often not provided. In the following, the individual entries in Table 1.2 will be discussed briefly.
INTRO
DU
CTION
1
18
Tabl
e 1.
2. S
tate
of t
he a
rt fo
r the
cat
alyt
ic h
ydro
trea
tmen
t of l
igni
ns in
bat
ch s
et u
ps u
sing
mol
ecul
ar h
ydro
gen.
#Fe
edSo
lven
tCa
taly
stT
(°C)
Tim
e (h
)P
(bar
)Li
quid
pha
se p
rodu
cts
Oil
(wt%
on
ligni
n in
take
)Re
f.
1Et
hano
l-be
nzen
e ex
trac
ted
Asp
en li
gnin
Dio
xane
Cu-C
rO26
018
-22
220-
400
Met
hano
l (28
% is
olat
ed y
ield
) 4-n
-pro
pycy
cloh
exan
ol (i
sola
ted
yiel
d 11
%) 4
-n-p
ropy
lcyc
lohe
xane
diol
-1,2
and
3-(4
-hyd
roxy
cy-
cloh
exyl
)-pro
pano
l (to
geth
er 2
9%)
70, s
um o
f fra
ctio
ns[4
1]
2M
aple
/Spr
uce
woo
d lig
nin
Dio
xane
/Wat
er
1:1
NaO
H (3
%)
Rane
y ni
ckel
17
35-
634
-200
4-et
hylg
uaia
col (
2.16
wt%
on
ligni
n) 4
-eth
ylsy
ringo
l (16
.4 w
t%
on li
gnin
) hom
o va
nilly
l alc
ohol
(6.2
wt%
on
ligni
n) 4
-eth
ylcy
-cl
ohex
anol
(1.8
7 w
t% o
n lig
nin)
66
[42]
3O
rgan
osol
v Li
gnin
1-M
ethy
l-na
ptha
lene
Ni/M
o, C
oMo/
γ-
Al 2O
3 (S
)40
4-42
81
70O
nly
com
bine
d ph
enol
ics
yiel
d re
port
ed, b
etw
een
0-10
wt%
on
lign
in. M
ain
prod
ucts
are
phe
nol a
nd c
reso
lsn.
d.[4
3]
4Ry
e St
raw
Lig
nin
Tetr
alin
Red
Mud
(S),
CoM
o γ-
Al 2O
340
04
150
10%
of p
heno
l on
carb
on b
asis
,Gas
olin
e (1
6.2%
) and
hea
vy o
il fr
actio
n (3
7.9)
aft
er d
istil
lativ
e w
ork-
up75
-78
(sum
of t
he li
quid
frac
tions
, C-
base
d)[4
4]
5Sp
ruce
lign
inD
ioxa
ne/ w
ater
(1
:1)
Pd, R
h, R
u/C,
Ru
,Ru/
Al 2O
319
55
35Ph
enol
ic c
ompo
unds
like
, phe
nol,
guai
acol
, 4-m
ethy
l-gui
aiac
ol,
ethy
lgui
aiac
ol, 4
-n-p
ropy
lgui
acol
, dih
ydro
coni
fery
lalc
ohol
. 16
- 25
(Chl
orof
orm
sol
uble
pro
duct
)[4
5,
46]
6W
hite
birc
h lig
nin
Wat
er, d
ioxa
ne
(1%
) H3P
O4
Ru,P
d,Rh
,Pt/
C20
04
40
Mon
omer
ic a
nd d
imer
frac
tion.
Mon
omer
ic fr
actio
n co
nsis
t of
gua
iacy
lpro
pane
(0.6
-9.9
wt%
), gu
aiac
ylpr
opan
ol (0
.5-5
.3
wt%
), sy
ringy
lpro
pane
(1.1
-34.
8 w
t%),
and
syrin
gylp
ropa
nol
(0.2
-15.
7 w
t%)
-[4
7]
7Py
roly
tic A
lcel
l lig
nin
oil
Dod
ecan
eRu
/C35
01
100
Cycl
ocal
kane
s, al
kyla
ted
cycl
ohex
anol
ics,
linea
r/br
anch
ed
alka
nes
-[2
7]
8O
rgan
osol
v Co
rnst
alk
ligni
n65
% v
olEt
hano
l/wat
erRu
/C, P
t/C,
Pd/
C22
5-30
00.
5-3
20M
ain
iden
tified
pro
duct
s: 4
-eth
ylph
enol
ics
(3.1
wt%
eth
ylph
e-no
l and
1.3
wt%
4-e
thyl
phen
ol o
n lig
nin
inta
ke)
Liqu
id y
ield
of u
p to
76%
,[4
8,
49]
9A
lkal
i lig
nin
1:1
Etha
nol/
Wat
er, a
nd
pure
eth
anol
Ru/A
l 2O3,P
t/C,
Ru/C
, Ni/A
l 2O3,
Ni/C
200-
450
0.25
-650
Prod
ucts
iden
tified
but
not
qua
ntifi
ed o
n th
e ba
sis
of li
gnin
in
take
. Maj
or c
ompo
nent
s ba
sed
on G
C ar
ea fo
r eth
anol
/w
ater
: die
thyl
-phe
nol,
2-et
hyl-5
-pro
py-p
heno
l, he
xano
ic a
cid,
3-
met
hyl p
enta
noic
aci
d
Liqu
id p
rodu
ct y
ield
s fo
r eth
anol
/w
ater
: bet
wee
n 52
and
89
[50]
10O
rgan
osol
v lig
nin
1:1
Etha
nol/
wat
er a
nd p
ure
etha
nol
Ru/A
l 2O3,
Ni/C
300-
350
250
No
deta
iled
prod
uct c
ompo
sitio
n re
port
edLi
quid
pro
duct
yie
lds
for e
than
ol/
wat
er b
etw
een
66 a
nd 9
2[5
1]
11M
ultip
le li
gnin
sou
rces
none
Fe2O
3, Ra
ney
Nic
kel,
Pd/C
,35
0-45
00-
230
-120
Prod
ucts
iden
tified
with
GC
and
mai
nly
phen
ols,
guai
acol
s, su
bstit
uted
cyc
lohe
xano
nes
Liqu
id p
rodu
ct y
ield
bet
wee
n 15
-66
[52]
12M
ultip
le li
gnin
sou
rces
none
Ni/M
o on
AlS
i, ze
olite
s, Cr
2O3 o
n A
l 2O3,
and
mix
ture
s th
ereo
f. (S
) and
no
n-(S
) for
m
395-
430
20-6
090
-100
Mon
ocyc
lic a
rom
atic
s: 6
.1-1
0.8
wt%
on
ligni
nPh
enol
s: 4
.7-9
.4 w
t% o
n lig
nin
Poly
cycl
ic a
rom
atic
s: 1
.3-2
.2 w
t% o
n lig
nin
Liqu
id p
rodu
ct y
ield
bet
wee
n 49
-63
[53]
n.d
. = n
ot d
eter
min
ed.
INTRO
DU
CTION
1
19
Early reports on the catalytic hydrotreatment of lignin involve the use of CuCrO as the catalyst (Table 1.2 entry 1) [41]. Experiments were performed at 260 °C with 220 bar of initial hydrogen pressure at room temperature (up to 400 bar at reaction temperature) with reaction times up to 22 h in dioxane (Scheme 1.1). After reaction, the products were distilled in fractions and analysed (melting- and boiling point and elemental analysis). The main compounds found in the lower boiling fractions were 4-propylcyclohexanol and 4-propylcyclohexandiol-1,2 (11 wt% on lignin), two type of glycols (28 wt% on lignin), and methanol (27 wt% on lignin). Higher boiling point fractions (33 wt% on lignin) were first dehydrated over alumina at 400 °C and further hydrogenated with Raney nickel (200 °C, 3 h, 300 bar hydrogen) to give a mixture primarily consisting of hydrocarbons (CnH2n-2, CnH2n-4) (6-8 wt% on lignin).
Aspen organosolvlignin 260 °C, 220 bar H2
CuCrO, 18-22 h
Dioxane
Low boiling fraction (<130°C) vacuum
67 wt%
OH
C3H6OH
OH
CH3OH
High boiling fraction (>130°C) vacuum
33 wt%Raney Ni
400 °C
CnH2n-2 CnH2n-4-H2O
Alumina400 °C
+H2
11 wt%
27 wt%Methanol
C3H7
OH
C3H7
OH
29 wt%
Scheme 1.1.
In 1948, Pepper et al. reported the use of Raney nickel as catalyst for the hydrotreatment of organosolv maple lignin in dioxane-water solutions (1:1) containing NaOH (3 %) [42], see Table 1.2 entry 2 and Scheme 1.2. The reactions conditions were relatively mild (173 °C, 206 bar of initial H2 pressure, 6 h) and led to the complete conversion of the lignin. Main identifiable compounds were 4-ethylguaiacol (2.2 wt% on lignin), 4-ethylsyringol (15.4 wt% on lignin), and homovanillyl alcohol (6.2 wt%). Compounds were identified by boiling- and melting points.
Maple organosolvlignin 173 °C, 206 bar H2
Raney Ni, 6 h
3% NaOHDioxane/water 1:1
OOH
OOH
O OOH
OH
4-Ethylguaiacol 4-Ethylsyringol Homovanillyalcohol
Scheme 1.2.
In 1988, typical hydrodesulphurisation (HDS) catalysts like sulphided NiMo and CoMo on alumina were applied for organosolv lignin hydrotreatment in 1-methylnapthalene (1:2 ratio) [43], see Table 1.2 entry 3. Best results regarding total phenolic yields were achieved with sulphided CoMo/Al2O3 catalyst at 404 °C, 69 bar of H2 at start of the experiment, and a reaction time of 1 h. The reported combined phenolic yield was
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10.0 wt% on lignin when using a continuous flow of hydrogen, the main product being phenol (30 %) and cresols (40 % on total phenolics).
Organosolvlignin CoMo/Al2O3 (S)
1-Methylnapthalene 1:2404 °C, 69 bar H2, 1 h
OH OH
Phenol Cresols
+
30% 40%
Phenolics10 wt% on lignin intake
~ ~
Scheme 1.3.
Klopries et al. (Table 1.2 entry 4) explored the conversion of rye straw lignin with sulphided red mud and sulphided CoMo with tetraline as the solvent [44], see Scheme 1.4. After reaction, the oil phase was collected, washed with water to isolate phenolic content and subsequently fractionated using distillation. The best results regarding product yields were achieved with sulphided red mud at 400 °C, with an initial pressure of 150 bar H2 for 4 h reaction time. This resulted in 19.1 % carbon conversion to permanent gases, 16.2 % to a gasoline distillate fraction, 21.3 % of phenol, 37.9 % of a heavy oil fraction, and 5.5 % of char. It is not clear whether phenol is isolated or if this fraction consist of a mixture of phenolics.
Rye strawlignin Red mud (S),
Tetraline400 °C, 150 bar H2, 4 h Phenol
21.3 C%
Gasoline
16.2 C%
Heavy oil
37.9 C%
Char
5.5 C%
Scheme 1.4.
In the late 1960’s, noble metal catalyst like palladium, platinum, rhodium, and ruthenium were introduced for the catalytic hydrotreatment of lignin. Pepper et al. reported the use of noble metal catalyst like Pd, Rh, and Ru on carbon or alumina for the hydrotreatment of extracted spruce lignin in a dioxane/water 1:1 solution [45, 46], see Table 1.2 entry 5. The most interesting catalyst was Rh/C which gave the largest amount of a chloroform soluble fraction (~23-26 wt%). The main compounds detected after an diethylether extraction were 4-n-propylguaiacol and dihydroconiferylalcohol (Scheme 1.5, GC-FID).
Extracted sprucelignin Raney Ni, Rh/C,
Rh/Al2O3, Pd/C
Dioxane/water 1:1195 °C, 35 bar H2, 5 h
OOH
OOH
OH
OOH
OOH
+ ++
4-Methyl-guaiacol 4-Ethyl-
guaiacol 4-Propyl-guaiacol Dihydroconiferyl-
alcohol
Scheme 1.5.
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The catalytic hydrotreatment of ethanol-benzene extracted birch wood lignin using Ru/C, Pd/C, Rh/C, and Pt/C was investigated by Yan et al. [47], see Table 1.2 entry 6. Reactions were performed in water, either pure or in combination with phosphoric acid (1 %) and/or dioxane (1:1 v/v), at 40 bar of initial H2, at 200 °C, for 4 h. After reaction, the suspension was filtered and the liquid phase was analysed by GC and GC-MS. The emphasis was on the quantification of the individual components in the monomer and dimer fraction. Highest amounts of the monomer fraction were found for Pt/C (46.4 wt% on lignin) using dioxane/water/phosphoric acid as the solvent. The main components were 4-propylsyringol, 4-propylguaiacol, dihydroconiferylalcohol and syringylpropanol, see Scheme 1.6 for details.
Extracted birch lignin Ru/C, Pd/C, Rh/C ,Pt/C
Water, 1% H3PO4, dioxane (1:1 v/v)
200 °C, 40 bar H2, 4 h
Max 46.4 wt% on lignin for Pt/C
OHO
OHO
OH
OHO
OH
OOH
OO
Propyl-guaiacol
Dihydro-coniferylalcohol
Propyl-syringol
Syringyl-propanol
Scheme 1.6.
The catalytic hydrotreament of a lignin pyrolysis oil obtained by fast pyrolysis of Alcell lignin was explored with Ru/C in dodecane as the solvent at 350 °C with 100 bar H2 for 1 h [27], see Table 1.2 entry 7. The main components in the feed, syringols (3.3 wt%), methoxybenzenes (5.7 wt%), alkylated phenolics (7.0 wt%) were hydro(-deoxy)genated to alkylated cyclohexanols (8.6 wt%), and cyclic- and linear alkanes (2.7 wt%), Scheme 1.7.
Pyrolytic Alcelllignin oil Ru/C
Dodecane350°C, 100 bar H2, 1 h
OHHO
Cyclohexanol Methyl-cyclohexane
4-Methyl-cyclohexanol
Hexadecane
Scheme 1.7.
Ye et al. (Table 1.2 entry 8) studied the use of Ru/C, Pt/C, and Pd/C for the hydrotreatment of organolsolv cornstalk lignin in 65 vol% ethanol-water mixtures (250 °C, 20 bar hydrogen, 1.5 h) [48]. The liquid and solid reaction products were quantified using a distillation/extraction protocol. Typical liquid yields were between 69 and 76 %. The highest yield (76 wt%) was achieved with Ru/C at 250 °C, 90 min reaction time, and 20 bar of H2. 4-Ethylphenolics, the target products of this study, were obtained in yields of up to 3.1 wt% for 4-ethylphenol and 1.3 wt% of 4-ethylguiaiacol on lignin intake (Scheme 1.8).
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Organsolvcornstalk lignin Ru/C, Pt/C, Pd/C
65 vol% Ethanol/water250 °C, 20 bar H2, 1.5 h
HO HO
O
4-Ethylphenol 4-Ethylguaiacol
3.1 wt% on lignin 1.3 wt% on lignin
Scheme 1.8.
More recently, Cheng et al. converted alkali lignin in pure ethanol and water/ethanol mixtures with Pt/C, and Ni or Ru on alumina or carbon as the catalysts at 200-450 °C, 50 bar H2 and reaction times up to 6 h (Table 1.2 entry 9, Scheme 1.9) [50]. Best yields (14.8 wt% on lignin) for reactions in ethanol were obtained with the Pt/C catalyst. Main components were esters (~44 % peak area), phenolic compounds (~19 % peak area) and alcohols (~10 % peak area). The liquid product yield was substantially better when using ethanol/water mixtures (1 to 1) and yields up to 89 % were obtained using Ru/C. Monomeric phenolic compounds were the primary products (~65 % GC peak area) and include 2-ethyl-5-propylphenol, diethylphenol, 3-ethyl-5-methylphenol, and 2-ethyl-5-methylphenol.
Alkali ligninPt/C
Ethanol/water 1:1300 °C, 50 bar H2, 2 h
OH
2-Ethyl-5-propylphenol
OH
2,6-Diethylphenol
OH
3-Ethyl-5-methylphenol
+ +
Scheme 1.9.
In subsequent studies, Cheng et al. depolymerised organosolv lignin in water-ethanol (1:1 v/v) and pure ethanol with Ni/C, and Ru/Al2O3 [51], see Table 1.2 entry 10. The reaction temperature determined to a large extent the liquid product yield and molecular weight distribution of the depolymerised lignin. Optimum results considering both depolymerised lignin yield (81 %) and lowest molecular weight (Mn 181 g/mol) were achieved with a Ni/C catalyst at 340 °C with 50 bar of H2 and a 1:1 v/v water-ethanol mixture. The molecular composition of the depolymerised lignins was not reported.
1.4.2 Catalytic hydrotreatment of lignin in batch in the absence of a solvent using molecular hydrogenThe studies reported in the previous paragraph were all performed in the presence of a solvent and using molecular hydrogen. However, for large scale processing, the use of an external solvent is best avoided to reduce extensive solvent recycling. For catalytic hydrotreatment studies in batch at elevated temperatures, the use of a solvent is not necessary as most lignins are known to melt above 200 °C and as such may act as the solvent. In addition, the lower molecular weight products may, at a later stage of the reaction, also serve as a solvent. A potential drawback of the absence of
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the solvent is a higher rate of repolymerisation reactions ultimately leading to char. Such repolymerisation reactions are expected to occur to a larger extent in more concentrated solutions.
Meier et al. performed experiments with mainly organocell lignin using Pd/C, red mud, Raney Nickel, and NiMo on several supports as the catalyst in the absence of a solvent [52], see Table 1.2 entry 12. After reaction, the oil was isolated after extraction of the reactor content with dichloromethane. The highest oil yield (80.6 wt% on lignin intake) was obtained with Pd/C (380 °C, 100 bar initial H2, for 15 min reaction time). The total GC detectables in the oil was about 11.5 %, indicating that the majority of the products is of higher molecular weight and not sufficiently volatile for GC detection. The main compounds were methyl-, and ethylcylcohexanones together with minor quantities of phenols, guaiacols, and catechols, see Scheme 1.10 for details (total 8.8 wt% on lignin intake). The product composition of the oil is a function of the catalyst employed. For instance, with NiMo, cyclohexanones were absent and higher amounts of phenolics were detected (3.68 wt% on lignin). Process optimisation studies with the NiMo catalyst led to GC detectable fractions of up to 37.8 wt% (22 wt% on lignin), with a phenol content of 6.4 wt% (3.7 wt% on lignin).
Organosolvlignin Pd/C
380 °C, 100 bar H2, 0.25 h
OO OH OHO
OHOH
4-Methylcyclo-hexanone
4-Ethylcyclo-hexanone
4-Ethyl-phenol
4-Ethyl-guaiacol
4-Ethyl-catechol
0.31 wt% 0.76 wt% 0.82 wt% 1.17 wt% 0.61 wt%
Scheme 1.10.
Oasmaa et al. performed catalytic hydrotreatment reactions using a (non-)sulphided NiMo/Al2O3 catalyst and several lignin sources at elevated temperatures (395-400 °C) and pressures (90-101 bar H2) in the absence of an external solvent (Table 1.2 entry 11) [53]. Best results were obtained with Pine Kraft lignin hydrotreated at 430 °C for 60 min with a mixture of sulphided NiMo on alumina and Cr2O3 on alumina (1:1), see Scheme 1.11. For this catalyst composition, the oil consisted of 10.8 wt% of monocyclic aromatics and 9.4 wt% of phenols on lignin intake (GC), the main phenolic compounds being phenol, methylphenol, and ethylphenol.
Organosolvlignin (S) NiMo/Al2O3 :
Cr2O3/Al2O3 (1:1)
430 °C, 90 bar H2, 1 h
OHOH OH
3-Methyl-phenol
Phenol 4-Ethyl-phenol
1,2-Dimethyl-benzene
1,2,3-Trimethyl-benzene
+ +
9.4 wt% Phenolics 10.8 wt% Aromatics
Scheme 1.11.
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24
Tabl
e 1.
3. S
tate
of t
he a
rt fo
r cat
alyt
ic h
ydro
trea
tmen
t of l
igni
n us
ing
exte
rnal
hyd
roge
n in
con
tinuo
us s
et u
ps.
#Fe
edSo
lven
tCa
taly
stT
(°C)
WH
SVP
(bar
)M
ajor
pro
duct
sO
il (w
t%)a
Ref.
1M
ultip
le li
gnin
so
urce
sLi
gnin
tar,
phen
ols
FeS,
CuS
350-
400
1-2
hb15
0-30
0Pr
oduc
ts d
istil
led
in fr
actio
n, m
onop
heno
l fr
actio
n up
to 5
0 w
t%c
Up
to
95%
[54]
2Kr
aft l
igni
nPh
enol
/ wat
er/
met
hano
l
FeS
with
mod
i-fie
rs (p
repa
red
in s
itu)
300-
450
1 hb
50-1
50 P
rodu
cts
dist
illed
in fr
actio
ns. M
ain
prod
ucts
C 6-C
8 Phe
nols
, cre
sols
Up
to
98%
[55]
3Kr
aft L
igni
n-
Fe γ
-Al 2O
3 (S)
440
0.3-
0.6
g lig
-ni
n/(g
cat
.h)
40-1
35
For h
ydro
trea
tmen
t ste
p: G
as/w
ater
: 43
wt%
on
ligni
n. L
iqui
d pr
oduc
t was
dis
tille
d in
frac
tions
: Li
ght o
il: 1
4.0
wt%
on
ligni
nPh
enol
s: 3
7.5
wt%
on
ligni
nH
eavy
oil:
11.
0 w
t% o
n lig
nin
62.5
[21,
56
]
4Py
rolit
ic li
gnin
-Co
Mo/
γ-A
l 2O3
(S)
400-
415
0.51
- 1 g
lig-
nin/
(g c
at.h
)14
0Li
ght o
rgan
ic p
hase
(60-
64 w
t% o
n lig
nin)
m
ainl
y co
mpr
isin
g of
alip
hatic
and
aro
mat
ic
hydr
ocar
bons
60-6
4[5
7]
5O
rgan
ocel
l lig
nin
Lign
in s
lurr
y oi
lN
iMo
(S)
375-
450
2 hd
75-1
80O
nly
mon
omer
ic p
heno
lics
quan
tified
, up
to
12.8
wt%
on
ligni
n28
-83
[58,
59
]
6BC
D li
gnin
Alc
ohol
wat
er
mix
ture
CoM
O/A
l 2O3,
NiM
o, N
iW (S
)35
0-38
52
hb10
0-15
0A
lkyl
ated
phe
nols
, alk
oxy-
phen
ols,
alky
lben
-ze
nes,
bran
ched
par
affins
up
to 2
5 w
t%U
p to
75
[60,
61
]
a) w
t% o
n lig
nin
inta
ke, b
) for
bat
ch re
acto
r set
-up,
resi
denc
e tim
e in
the
cont
inuo
us s
et u
p no
t pro
vide
d, c
) bat
ch d
ata,
d) b
atch
tim
e fo
r the
liqu
id p
hase
.
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Despite the fact that the catalytic hydrotreatment of lignins has been studied already since the 40’s of the previous century, it appears that most studies are very exploratory in nature viz., use a large number of catalysts, a wide range of process conditions and a range of lignin feeds (variations in origin (hard and softwood), and lignin extraction methods (alkaline, organosolv)). This makes it hard to draw general conclusions regarding the best catalyst and reaction conditions. Detailed, systematic optimization studies regarding product liquid yields are lacking and feed composition-product relations and catalyst-product relations have not been determined. As such, it is very difficult to assess which catalysts are most suitable for the catalytic hydrotreatment of lignin when aiming for the highest yields of valuable bulk chemicals like aromatics and alkylphenolics.
1.4.3. Catalytic hydrotreatment of lignin in continuous set-ups. As stated earlier, most studies on lignin hydrotreatment are exploratory in nature and have been carried out in batch set ups. Investigations using dedicated continuous set-ups are scarce and an overview is provided in Table 1.3. In most cases, either cheap Fe catalysts or HDS catalysts (CoMo, NiMo) are used, whereas studies with supported noble metal catalysts have not been reported to date. Lignin feeding was performed either in the solid state or as a liquid. In the latter case, the lignin feed was dissolved in an organic solvent or a liquid reaction product. Typically harsh conditions are used, temperatures between 250 and 450 °C and pressures between 70 and 450 bar. In most cases, both the solid phase and liquid phase are fed continuously. In one example in Table 1.3 (entry 5), the hydrogen is fed continuously while the liquid phase was operated in batch.
An early patent on the conversion of several types of lignin was reported by the Japanese Noguchi institute (Table 1.3 entry 1) [54]. The reactions were carried out in a mixture of lignin and lignin oil products as the solvent (and in some cases a co-solvent) using a cheap catalyst based on FeS in combination with a co-catalyst (most examples use CuS). Typically, the lignin to solvent ratio was between 100 and 200 %, temperatures between 350 and 400 °C, hydrogen pressures between 150 and 300 bar and reaction times between 1 and 2 h. Most reactions were carried out in a batch set-up and an oil and a water phase were obtained after reaction. Lignin oil yields up to 95 wt% were reported. The lignin oil was distilled, and four fraction with a boiling point i) below 100 °C (acetone, methanol), ii) between 190 and 230 °C (monophenol fraction) iii) between 230-260 °C (catechols) and iv) a fraction above 260 °C were collected. The yield of the monophenol fraction was reported to be as high as 51 %. However, a later patent by UOP claims that these high yields of monophenols could not be reproduced [55].
The authors also describe a continuous process for lignin liquefaction. It involves mixing the lignin with the catalyst and lignin oil solvent to form a paste. This paste is preheated to 250 °C and pumped into a reactor at a typical flow rate of 3 l/h. The reactor
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26
is not mechanically stirred but hydrogen gas is introduced using a tube at the bottom of the reactor and this may result in some agitation. The liquefied reaction product is taken from the reactor by a weir system and collected. The gas phase is cooled and the liquid product is collected, whereas the gas phase is recycled. An overview of the set-up is given in Figure 1.9.
1
2
11
13 14 15
17
12
16
987
18 19
3
4
10
5
6
1. Mixing vessel 2. Paste pump 3. Reaction tower 4. Mist catcher 5. Seperator6-10. Cooler 11. Pre-heater 12. Pre-heater
13. Discharge receiver14. Product receiver15. Condensate receiver16. Recycling booster17. Compressor18. Hydrogen tube19. Overflow pipe
Figure 1.9. Schematic representation of the continuous reactor set-up provided by the Noguschi institute [54].
The residence time in the reactor is not given. The liquid product from the reactor is distilled in fractions with boiling points described above. When using a mixture of 200 parts of lignin as feed and 250 parts of lignin oil as the solvent, 6 parts of a fraction with a boiling point below 100 °C, 47 parts of a fraction boiling below 230 °C (monophenols), 4 parts of a fraction with a boiling point between 230 and 260 °C (catechols), and 17 parts with a higher boiling point were obtained. Unfortunately, a proper mass balance cannot be made with the data provided in the patent. However, the composition of the monophenol fraction is given and consists of 7 parts of o-cresol, 20.4 parts of p-cresol, 15.6 parts of 4-ethylphenol and 4 parts of 4-propylphenol (Scheme 1.12).
HydrolysisLignin FeS, CuS (10:1)
370 °C, 200 bar H2, 2 h
OH
Methyl-phenolics
OH
Ethyl-phenolics
OH
Propyl-phenolics
+ +
OH
+OH
Catechol
Scheme 1.12.
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27
Further improvements on the Noguchi process and particularly on the monophenolic yield were reported by IOP in 1986 using Kraft lignin and a promoted ironsulphide as the catalyst (Table 1.3 entry 2). These improvements were mainly due to the use of a proper solvent (water, phenol, and a low aliphatic alcohol). Total amounts of monophenols were reported as high as 65 % and a cresol amount of about 45 %, though it is not clear what is the basis for these yields (on lignin feed or amount in one of the distillate fractions). The process consist of two separate stages. In the first step, the Kraft lignin is suspended in water/phenol and methanol and the in-situ prepared solid catalyst. The reaction is performed at temperatures between 300 and 450 °C and a hydrogen pressure between 50 and 150 atmosphere. The product is distilled and the fraction with a boiling point below 235 °C is collected. The remaining residue is the fed to a second reactor and used as a solvent for a second reaction with fresh Kraft lignin and catalyst. The product of the second reactor is distilled and the fraction with a boiling point below 235 °C is used as the substitute for phenol in the first reaction. The concept is not demonstrated on continuous scale and only examples in a batch set-up are provided in the patent. Lignin liquefaction yields were as high as 93 %, with cresols and substituted C6-C8 phenols as the main products. Unfortunately, full mass balances are not given, making it very cumbersome to quantify the exact yields of these products (see Scheme 1.13) [55].
AlkaliLignin FeS, CuS, SnS
Lignin tars, water, methanol400 °C, 100 bar H2, 1 h
OH
Cresols
OH
Phenol
OH OH
2,3-Dimethyl-phenol
3-Ethyl-phenol
+
C6-C8 phenols
Scheme 1.13.
Another landmark in the development of efficient catalytic hydrotreatment processes for Kraft lignin is the Lignol process (1983) from the Hydrocarbon Research Institute (HRI) [21, 56], see Figure 1.10 and Table 1.3 entry 3. The process aims to co-produce phenol and benzene. It involves the reaction of a lignin feed mixed with a process oil in an ebullated hydrocracking reactor followed by a thermal hydro-dealkylation step. The final product is fractionated to produce phenols and aromatics, along with a heavy alkylated material which is recycled to the hydrodealkylation reactor to increase the product yield.
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Figure 1.10. Schematic representation of the Lignol process [56].
The first reactor contains a sulfided Fe/γ-Al2O3 catalyst and is operated at 440 °C and 70 bar of H2 and a WHSV of about 0.3-0.6 g lignin/(g cat.h). It is claimed to produce about 37.5 wt% alkylated phenolics on lignin intake, the main phenolic compounds being phenol (6.6 wt%), m-cresol (11.9 wt%), p-cresol (9.7 wt%), 2,4-xylenol (7.0 wt%), p-ethylphenol (33.2 wt%), and p-propylphenol (20.0 wt%), see Scheme 1.14. Detailed information on the second stage reactor for hydrodealkylation of the alkylated phenolic mixture is not given, though it is said that about 60 mol% of the cresols can be dealkylated to phenol.
Lignin(S) Fe/Al2O3
440 °C, 70 bar H2,WHSV 0.3-0.6 g/(g cat.h)
OH OH OH
Phenol
OH OH OH
m-Cresol p-Cresol 2,4-Xylenol p-Ethyl-phenol
p-Propyl-phenol
+ + + + +
37.5 wt% on lignin intake
Scheme 1.14.
The advantages of the Lignol process compared to the Noguschi process are a lower amount of gas and liquid produced (42 wt% on lignin versus 45 wt%), a comparable amount of light distillate (14 wt% on lignin) a higher amount of phenol fraction (37.5 versus 21 wt% on lignin) and a lower amount of heavy oils.
Piskorz et al. (Table 1.3 entry 4) studied the catalytic hydrotreatment of pyrolytic lignin (a lignin isolated from fast pyrolysis oil from hog fuel) in a continuous packed bed reactor operated co-current upflow with the objective to obtain high yields of hydrocarbons for the transportation blending pool. The catalytic hydrotreatment was performed with sulphided CoMo/Al2O3 at 400-415 °C for a run time of 94-182 min at a
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29
hydrogen pressure of 140 bar [57]. Typical WHSV’s were between 0.51 and 1 g lignin/(g cat.h). The reactor effluent was cooled in two stages and the liquid products were collected. The total liquid product yield was about 81-85 %, consisting of light organics (60-64 wt%) and water (about 20 %). The light oil contained only 0.5 wt% of oxygen and consisted of about 62 % aliphatic, 38 % aromatics, see Scheme 1.15 for details. Main compounds identified by GC-MS were cyclohexane, methylcyclohexane, toluene, ethylbenzene, propylbenzene, and dodecane.
Pyrolyticlignin (S) CoMo/Al2O3
400 °C, 140 bar H2,WHSV 0.51-1 g/(g cat.h)
Cyclo-hexane
Methyl-cyclohexane
Ethyl-benzene
Propy-benzene
Dodecane
Aliphatic 62% Aromatics 38%
Scheme 1.15.
Meier et al. depolymerised organocell lignin in a semi-continuous setup with a continuous flow of hydrogen and batch wise operation of the liquid phase with the objective to maximise phenolics yields [58, 59], see Figure 1.11 and Table 1.3 entry 5. A range of reaction temperatures (375-450 °C), hydrogen pressures (75-180 bar), catalysts (sulphided NiMo, zeolite A, and none), and the use of several different slurry oils as solvent were tested. Lowest char formation (0.3 wt%) and highest oil yield (about 82 %) were achieved with sulphided NiMo at 375 °C, 180 bar, and 120 min batch time with organocell lignin mixed with a lignin derived slurry oil. Phenolics yields were up to 12.8 wt% on lignin intake (NiMo, 450 °C, 180 bar hydrogen) with the main phenolic compounds being cresols, xylenols, phenol, and guaiacol.
Organocelllignin (S) NiMo/Al2O3
Lignin derived slurry oil450 °C, 180 bar H2,2 h
OH
Cresols
OH
Phenol
OH
Xylenols
OH
Guaiacol
+ + +
12.8 wt% on lignin intake
Scheme 1.16.
Figure 1.11. Schematic overview of the semi-continuous process set up of Meier et al. [58, 59].
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Tabl
e 1.
4 St
ate
of th
e ar
t for
(cat
alyti
c) so
lvol
ysis
of v
ario
us li
gnin
s.
#Fe
edSo
lven
tCa
taly
stT
(°C)
Tim
e (h
)O
il (w
t%)a
Solid
s (w
t%)a
Maj
or p
rodu
cts
Ref.
1En
zym
atic
/ hyd
roly
sis
ligni
nFA
, IPA
, EtO
H
optio
nal w
ater
-37
0-39
02-
16N
ot
prov
ided
16-3
0Ph
enol
ics
[62]
2M
ultip
le s
ourc
esFA
, IPA
, EtO
H,
MeO
H-
380
8-54
82-1
30n.
d.Ph
enol
ics
(25-
35 w
t%)a
H/C
1.3
-1.7
O/C
0.0
5-0.
17[2
9,
30]
3En
zym
atic
/wea
k ac
id
hydr
olys
is li
gnin
FA, I
PA/E
tOH
-38
0O
ver
nigh
t run
sN
ot
prov
ided
2-15
Alip
hatic
hyd
roca
rbon
s/ P
heno
lics
H/C
1.2
-1.8
O/C
0.1
6-0.
5[6
3]
4Va
rious
lign
ins
FA, w
ater
Pd/C
, Nafi
on
SAC-
1330
02
-10
-20
Mix
ture
of p
heno
ls, m
ax 1
0.5
wt%
Mai
nly
guai
acol
and
cat
echo
ls (p
yroc
ate-
chol
and
reso
rcin
ol)
[64]
5Sw
itchg
rass
lign
inFA
, EtO
HPt
/C13
5-80
01-
20
Max
. 21
wt%
on
lign
in o
f G
C id
entifi
ed
prod
ucts
.
-
Mai
n G
C id
entifi
ed p
rodu
cts:
4-pr
opyl
guia
col (
8 w
t% o
n lig
nin)
4-m
ethy
lgui
acol
(5 w
t% o
n lig
nin)
Hom
ovan
illyl
alco
hol (
3 w
t% o
n lig
nin)
Phen
olic
s, gu
aiac
ols
H/C
1.3
-1.2
O/C
0.3
3-0.
15
[65]
a) w
t% o
n lig
nin
inta
ke.
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A continuous process for the conversion of lignin to hydrocarbons and aromatics by catalytic hydrotreatment was patented by Shabtai et al. [60, 61], see Table 1.3 entry 6. A two stage approach is followed. The first stage involves a base catalysed (NaOH, KOH, etc) depolymerisation (BCD) of lignin in supercritical (260-310 °C) methanol or ethanol to yield a liquefied lignin (10 minutes). In this stage, high yields of mono-, di-, and tri-oxygenated aromatics are produced. The second step involves a catalytic hydrotreatment of the liquid depolymerised lignin mixture at severe reaction conditions (350-385 °C, 97-152 bar H2) with the use of a sulphided CoMo/Al2O3 catalyst without the addition of a solvent. The main components found after 2 hours of reaction time were mono-, di-, and tri-alkylbenzenes (about 25 wt%). The process is demonstrated in a batch mode, continuous operation is not reported.
(S) CoMo/Al2O3
350-385 °C, 97-152 bar H2, 2 h
Toluene m-Xylene p-Xylene 1,2,3-Trimethyl-benzene
+ + +BCDdepolymerised
lignin
mono-, di-, tri-alkylbenzenes
~25 wt% on lignin intake
Scheme 1.17.
1.5 (CATALYTIC) SOLVOLYSIS OF LIGNIN
Historically, the catalytic hydrotreatment of lignin uses ex-situ molecular hydrogen as hydrogen source for the reaction (vide supra). A relatively new and upcoming lignin conversion methodology uses an in-situ hydrogen donor. A well-known hydrogen donor is formic acid (FA), which is, either thermally or catalytically, converted to hydrogen and CO/CO2. Commonly used solvents are alcohols, like ethanol, methanol, and isopropanol (IPA), which may also generate hydrogen upon heating in the presence of a catalyst. Kleinert and co-workers, pioneers in this research area, were able to convert lignin to a liquid product at 370-390 °C with reaction times of 2-16 h with only formic acid and an alcohol as the solvent in the absence of a catalyst (Table 1.4 entries 1-3) [29, 30, 62]. Lignin oil yields were between 60-90 %, with claimed phenolic yields on lignin intake between 25-35 wt%. Main phenolic compounds were primarily alkylated phenolics like 2-ethylphenol, thymol, and 2-ethyl-4,5-dimethylphenol (Scheme 1.18).
Formic acid,methanol/ethanol
380°C, 6-12 h
OH
Multiple sources
lignin
OH OH
2-Ethyl-4,5-dimethylphenolThymol2-Ethylphenol
+ +
25-35 wt% on intake
Scheme 1.18.
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Further work by Liguori et al. (Table 1.4 entry 4) showed that lignin solvolysis could be improved by the use of a supported noble metal catalyst (Pd/C) in combination with a strong solid acid (Nafion SAC-13) [64]. The reactions were carried out with various lignins and using formic acid and water as the solvent. The use of catalysts compared to the non-catalytic version allows reduction of the reaction temperature to 300 °C and a shorter reaction time (2 h). Typical products were monomeric phenolics like methoxyphenolics and (substituted) catechols (Scheme 1.19).
Formic acid,water, Pd/C
300°C, 2 h
OH
Multiple sources
lignin
O
+ +
OHO
OHOH
+
OHOH
Guaiacol Methyl-guaiacol
Catechol Methyl-catechol
Scheme 1.19.
Recently, Xu et al. reported the use of Pt/C as the catalyst in combination with ethanol and formic acid for the depolymerisation of switchgrass lignin at 350 °C (Table 1.4 entry 5) [65]. The reaction products were analysed with GC and a maximum of 21 wt% on lignin of GC identified product were detected after 4 h reaction time. The main GC detectable compounds were 4-propylguiacol (8 wt% on lignin), 4-methylguiacol (5 wt% on lignin) and homovanillyl alcohol (3 wt% on lignin), see Scheme 1.20 for details.
Formic acid,ethanol, Pt/C
350°C, 20 h
OH
Switchgrasslignin
O
+ +
OHO
OHO
+
Guaiacol
OHO
OHO
OH
+
p-Methyl-guaiacol p-Ethyl-
guaiacol p-Propyl-guaiacol
Homovanillylalcohol
Scheme 1.20.
1.6 THESIS OUTLINE
This thesis describes an experimental study on the valorisation of Alcell lignin to biobased products by catalytic hydrotreatment. The first part of the thesis focuses on the catalytic hydrotreatment of Alcell lignin, fractionated Alcell lignin, pyrolytic lignin, and relevant dimeric model components using mainly Ru/C as the catalyst. The main objective of this part is to gain insights in the relation between process conditions and starting lignin feed on molecular composition and the product properties of the resulting lignin oils. The last chapter of the thesis is focused on the use of catalytic solvolysis to valorise Alcell lignin with Ru/C as catalyst. Also here, relations between process conditions and molecular composition of the resulting lignin oils were determined. The study was performed in the framework of the Dutch national project
INTRO
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“LIGNOVALUE”, contract no. EOSLT-05011.
In Chapter 2, a catalyst screening study on the catalytic hydrotreatment of Alcell lignin using different noble metal catalysts (Pd, Ru) and a non-noble metal catalyst (Cu) on different supports is reported. The catalysts were tested at similar reaction conditions (400 °C, 4 h reaction time, and with an initial hydrogen pressure of 100 bar) and the level of depolymerisation and selectivity towards desired monomeric compounds (aromatics, alkylphenolics) was determined. Subsequent detailed experimental studies were performed with Ru/C to determine the effect of process conditions on the final product composition.
An exploratory study on the catalytic hydrotreatment of two pyrolytic lignins (pine and forestry residue) using Ru/C as the catalyst is described in Chapter 3. The effect of lignin feed properties on the resulting product composition was established and compared with benchmark experiments using Alcell lignin. The reactions were performed in a batch set-up (400 °C, 4 h, 100 bar initial H2 pressure). Based on the product composition, a global reaction network for the catalytic hydrotreatment of pyrolytic lignin with Ru/C is proposed.
Experimental research on the catalytic hydrotreatment of fractions of Alcell lignin, obtained by solvent extraction, using Ru/C as the catalyst (400 °C, 4 h reaction time, and with an initial hydrogen pressure of 100 bar) is provided in Chapter 4. Relevant product properties and composition of the lignin fractions and corresponding product oils were determined and rationalised.
A model compound study on the catalytic hydrotreatment of various dimeric lignin model compounds, (guaiacylglycerol-β-guaiacyl ether, 4-benzyloxy-3-methoxy-benzaldehyde, and 2,2’-biphenol) using Ru/C is reported in Chapter 5. The reactions were performed at 250 °C with 100 bar H2 using n-dodecane as the solvent. A reaction network is proposed for each model compound and a reactivity order for the various lignin linkages is proposed.
In Chapter 6, a comparative study on the differences between catalytic solvolysis (in-situ hydrogen production) and catalytic hydrotreatment (ex-situ hydrogen addition) of Alcell lignin using Ru/C is reported. Distinct differences in product yields and compositions were observed and the extent of depolymerisation and selectivity towards the desired monomeric compounds were evaluated. Furthermore, the effect of process conditions like reaction time, and type of alcohol solvent on the catalytic solvolysis approach were explored and the results were rationalised by a reaction network.
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