Catalytic Partial Oxidationof low molecular-mass hydrocarbons
CH4 → C2H6 → C2H4 → COx
C3H8 → C3H6 → COx
M. Baerns
Lecture Series on Catalysis at FHI 2007/2008Modern Methods in Heterogeneous Catalysis Research
Catalysis and Catalyst Properties
Reaction mechanisms and kinetic schemes
Effect of properties of solid materials on their catalytic performance
Subjects to be considered
• Interactions of gas-phase reactants with the catalystsurface, i.e., transformations of reactants on thecatalyst surface
• Solid state transformations in catalyst preparationand operation
• Solid state properties
Reaction steps and kinetic schemes
• Single and multiple surface reaction steps(with and without intermediate desorption steps)
• Parallel and consecutive reaction steps originatingfrom one molecule influencing the selectivity of thedesired product in such complex reaction networks asexperienced in almost all hydrocarbon oxidation reactions.
• Quantitative description of reaction schemes by rateequations of the various reaction steps and correlating the rate constants with catalyst properties
Temperature profile in and around a tubularcontinuous-flow catalytic reactor
Reaction Network in Selective Oxidation
HC → Intermediate → COx
Selective Oxidation Product(s)
Parallel and consecutive reactionsC = f(τ); S = f(X)
SP = nP / (nk,0 – nk); (ν !!!)
Xk = (nk,0 – nk) / nk,0
YP = SP . Xk
Parallel and consecutive reactions
C = f(τ); S, Y = f(X)
SP = nP / (nk,0 – nk); (ν !!)
Xk = (nk,0 – nk) / nk,0
YP = SP . Xk
Elucidation of reaction mechanisms
• solid state transformation in catalyst preparation and operation:e.g. amorphicity, crystallinity, phase transformations, ....
• solid state properties:e.g. bulk and surface structure & composition, basicity/acidity,el. conductivity, redox, ....
• gas-phase reactants:kinetic scheme
• interactions between gas-phase reactants and catalystsurface, i.e. transformation of reactants: e.g. ad- & desorption, bond breaking and forming, atominsertion & abstraction, ...
Selected Experimental Methodsof Solids Characterization
R. Schlögl: In-situ Characterization of Pratical Heterogeneous Catalysts, in Ref. 2
XRD LIF
XPS FT-IR
SEM/EDX FT-Laser-Raman
HREM UV-vis
S(BET) DRIFTS
(EELS) ESR
(EXAFS) DSC, DTA, DTG
(NEXAFS) TPR, TPD, TPO, TPRS
Interplay between surface and bulk properties of oxide catalysts for the selective oxidation
Relationships between
properties of solids catalytic properties
reaction conditions
AcknowledgementsAcknowledgements
Part I
OCM
Oxidative coupling of methane-Reaction scheme –
CH4 C2H6 C2H4
COx
Typical pattern of dependence of selectivity on degree of conversion in the OCM rection
CH4 + O2 C2H6, C2H4over oxide-catalysts
O2(g) COx
CH4 CH3 C2H6 C2H4
O2- O- O22- O2-
O2
O2- M(n-1)+ [ ] Mn+ O2 - Mn+ O2- M n+ O 2-
O2- M(n-1)+ O2- Mn+ [ ] Mn+ O2- M n+ O 2-
O2- O2- O2- O2-h+
e-
D. Wolf; Ruhr-Universität Bochum 1999,DEGUSSA / EVONIK
Oxidative coupling of methaneheterogeneous-homogeneous mechanism
CH4
Oads or O2-(lattice)
Activation of CH4Heterogeneous process
C2H6 formationHomogenous process
O.V. Buyevskaya et al., J.Catal. 146 (1994) 346 J. Lunsford et al., J.Catal. 147 (1994) 301
CH3
.
CH3 + CH3 C2H6
. .
CH4 + [O] CH3 + [OH]
CH3 + [O] [ ] + CH3O
2[OH] H2O + [O] + [ ]
O2 + [ ] [O2]
[O2] + [ ] 2[O]
2CH3 C2H6
C2H6 + [O2] [ ] + 2CH3O
Reaction Scheme (basis of kinetic analysis)
D. Wolf; Ruhr-Universität Bochum, 1999,DEGUSSA / EVONIK
Electronic properties for O2 activation
(O2-)ads(O2
2-)ads(O-)ads(O2-)ads
eee(O2
-)ads(O22-)ads(O-)ads(O2-)ads
eeeeeeeee(O2
-)ads(O22-)ads(O-)ads(O2-)ads
eeeeeeeee(O2
-)ads(O22-)ads(O-)ads(O2-)ads
eeeeeeeeeeee
0.00 0.05 0.10 0.15 0.2020
30
40
50
60
70
80
Na0.0001CaOx Na0.012CaOx Na0.064CaOx
S(C
2H6)
/ %
ΘO/ΘO2
CHCH44 + O+ O22 CC22HH66 + O+ O22
0.0 0.1 0.2 0.3 0.4 0.550
60
70
80
Na0.0001CaOx Na0.012CaOx Na0.064CaOx
S(C
2H4)
/ %
ΘO/ΘO2ΘO/ΘO
2ΘO/ΘO
2
Effect of Composition/Anion Conductivity on Selectivityin OCM over a CeO2/CaO Catalyst
Effect of Anion Conductivity on C2 Selectivity
in OCM Reaction over CeO2/CaO Catalysts
Effect of Oxygen Anion Conductivity on C2 Selectivityfor Different Oxides
Effect of Electronegativity of different Oxides on C2 Selectivity
Selectivity-determining factors in OCM - electronic properties of catalysts -
3.5
4.0
4.5
5.0
5.5
6.0
Band
gap
/ e.
V.
LaCePrNdSmEuGdTbDyHoErTmYbLu0
20
40
60
80
100
C2-s
elec
tivity
/ %
Rare earth oxides0 20 40 60 80 100
0
20
40
60
80
Ca in CaO-CeO2 / wt.%
C2-s
elec
tivity
/ %
0.00
0.02
0.04
0.06
0.08
0.10
σ ion /
Ω-1 c
m-1
An optimized ratio of p-type to ionic-type conductivities is required for well-performing OCM catalysts
Scientific Knowledge on the OCM Catalysis
• Co-feeding of O2 and CH4
• Basic catalytic materials are considered to be more efficient than the acidic ones (activation of C-H bond)
• p-type semi-conductors are more selective than n-type ones. Band gap should be in the range of 5-6 eV.
• An optimal ratio of p-type conductivity to O2--conductivity is required (fast dissociation of adsorbed bi-atomic O species)
• Structural point defects (anion vacancies, impurity transitionalmetal ions)
• Periodic mode of operation
• High catalyst stability under reducing and oxidizing conditions• Catalyst ability for storing oxygen and offering high amounts of
lattice oxygen• Minimal time of catalyst re-oxidation as compared to period of
OCM reaction
Concluding Comments to OCM Catalysis
• Inspite of all the extensive knowledge on OCM catalysisscientific breakthroughs are still required.
• Suppression of consecutive total oxidation of ethane andethylene at high degrees of methane conversion
Yield Y of C2 (Y = S . X / 100 %)
Presently Y = 25 - 28 %
• This results in high expenditures for separation and recycle of
C2H6 / C2H4 / CH4 / (CO2, CO)
making industrial application uneconomic
AcknowledgementsAcknowledgements
Part II
ODP
Oxidative dehydrogenation of propane to propene on different metal oxides
(ODP)
Overall Scheme of Oxidative Dehydrogenation of Alkanes to Olefins
on Transition Metal Oxides Catalysts
CnH2n+2
CnH2n
COx
Selection of potential catalysts: Primary reaction steps of the oxidative dehydrogenation of alkanes on metal oxides
CnH2n+2CnH2n+1 + MeOxH
CnH2n + MeOx-1 + H2OMeOx
MeOx
CnH2n+1 + MeOx-OH
MeOx + 0.5O2
MeOx-Oad.
CnH2n + MeOx + H2O
CnH2n + MeOx + H2O
0.5O2
A) Redox-mechanism(Mars-van
Krevelen)
B) Activation byadsorbedoxygen
C) Activation bylattice oxygen(no redox-mechanism)
MeOx-1 + 0.5O2
CnH2n+1 + MeOxH
Interactions of an Alkane Molecule with a Catalytic Surface
(different oxygen species)
Propane activation by adsorbed oxygenPropane activation by adsorbed oxygenonon SmNaSmNa0.0280.028PP0.0140.014OOxx at at 723 K 723 K
Reaction of propane with adsorbed oxygen species results information of propene followed by its further oxidation to ethylene and methane (besides COx)
0.0 0.1 0.2 0.3 0.40.0
0.2
0.4
0.6
0.8
1,0
O2 (sequential pulsing of O2 and C3H8
C2H4
CH4
C3H6
O2 (single pulsing of O2)
Nor
mal
ized
inte
nsity
t / s
C3H8
Catal.Today 42 (1998) 315-323
Complex Mixtures of Metal Oxidesas Potential Catalysts for
Oxidative Dehydrogenation of Propane
Redox-metal oxides of medium metal-oxygen binding energy in the range –400 to –200 kJ/mol
Redox compounds: V2O5 Ga2O3 MoO3
Support: MgO (basic metal oxide)
Detrimental to desired catalytic performance• acidic metal oxide: B2O3
• metal oxide, on which O2 dissociates: La2O3
Propane activation by lattice oxygenPropane activation by lattice oxygen1818OO22--CC33HH88, , VV1616OOxx(9.5 %)/(9.5 %)/γγ--AlAl22OO33, T=798 K, T=798 K
0.0 0.5 1.00.0
0.5
1.0
t/ sN
orm
alis
ed in
tens
ity
C3H8
18O2 C3H6
C16O2
C16O
0.0 0.2 0.4 0.60.00
0.01
0.02 C3H8 + C3H6
18O2
C16O2
C16O+C3H8
C18O2 (=0) C18O16O (=0)
Inte
nsity
/ a.u
.
t/ s
• No labeled (18O) oxygen in COx products only lattice (16O) oxygen is involved in the reaction
• Reaction sequence: C3H8 C3H6 CO & CO2MvK mechanism
Kinetic evaluation and mechanistic assessment Kinetic evaluation and mechanistic assessment of oxygen transientsof oxygen transients
OzzO adsk −⎯⎯⎯ →⎯+ 222
1 V0.16Mg0.11Ga0.47Mo0.17Fe0.092 V0.32Mg0.18Ga0.27Mo0.04Mn0.193 V0.8Mg0.24 V0.2Mg0.85 V0.22Mg0.47Ga0.2Mo0.11
Model for O2 activation
Result: effective rate constant of O2 activation/adsorption- rate of formation of active lattice oxygen (regeneration)by gas-phase oxygen
- coverage by active lattice oxygenTopic.Catal. 15 (2001) 175-180
12
34
50.0
0.20.4
01234
5
6
7
Catalyst
Flux/ 1016 molecules/s
t/ s
Y(CY(C33HH88) versus effective rate constant of O) versus effective rate constant of O22activation determined from TAP experimentsactivation determined from TAP experiments
1−s/k effads
Y(C3H6) decreases with an increase in kadshigh concentration of near-surface lattice oxygen facilitates total oxidation
eff
V0.2Mg0.8V0.22Mg0.47Ga0.2Mo0.11V0.32Mg0.18Ga0.27Mo0.04Mn0.19V0.16Mg0.11Ga0.47Mo0.17Fe0.09V0.8Mg0.2
Steady-state experiments
Topics in Catal. 15 (2001) 175-180
0 50 100 150 2004
6
8
10
12
14
Y(C
3H6)/
%
• For the ODP reaction on vanadia-based catalysts, transient experiments proved that the dehydrogenation occurs by lattice oxygen via a Mars-van-Krevelen mechanism
• On non-transition metal oxides catalysts adsorbed oxygen plays a major role
• High concentrations of near-surface lattice oxygen as well as of adsorbed oxygen favour total oxidation
Mechanistic and Kinetic Aspects
Characterisation of catalytic Characterisation of catalytic sites of vanadiasites of vanadia--based based
catalystscatalysts
XPS & EPR: Relationship between catalytic performance and surface ratio of Mg/V
Reaction conditions: C3H8-O2-N2=40-20-40; T=773K; X(O2) ≈100 %
EPR measurements show increasing concentration of octahedral isolated VO2+ centres with increasing the Mg/V ratio (XPS). The more dispersed active vanadium species, the higher the selectivity that can be achieved.
0 2 4 6 8 104
6
8
10
12
14 V0.3Mg0.63Ga0.07Ox
V0.22Mg0.47Ga0.2Ox
V0.32Mg0.18Mo0.04Mn0.09Ga0.33Ox
V0.16Mg0.11Mo0.17Fe0.09Ga0.47Ox
V0.2Mg0.5Ga0.3Ox
V0.2Mg0.2Ga0.6Ox
V0.8Mg0.2Ox
V0.5Mg0.5Ox
V0.2Mg0.8Ox
V0.1Mg0.9Ox
Yiel
d/ %
Mg/V (XPS)
Not supported V-Mg-O
from theevolutionary procedure
best fromV-Mg-Ga-O/αAl2O3
Catal.Today 67 (2001) 369-378
EPR Spectra of VMgO catalysts
V0.1Mg0.9Ox
Isolated VO2+
weakly interacting VO2+
V0.8Mg0.2Ox
Strongly interacting VO2+
200 300 400 500B0/ mT
UV/VIS-DRS spectra of VMgO catalysts
200 400 600 8000.0
0.5
1.0
V(5.3)/MCM-41
Mg3V2O8 + Mg2V2O7
Mg3V2O8
λ / nm
Nor
mal
ised
Kub
elka
-Mun
k
UVUV--vvis spectrais spectra: : Interaction of Interaction of VOVOxx species species of of differentdifferent materials at 773 Kmaterials at 773 K
Weak interaction between highly dispersed V5+Ox species
No V-O-V bond !
J. Catal. 234 (2005) 131
VOx clusters
200 300 400 500 600 700 8000.0
0.1
0.2
0.3
VOx(1)/γ-Al2O3 VOx(4.6)/γ-Al2O3 VOx(5.3)/MCM-41 VOx(11.2)/MCM-41
λ / nm
F(R
)
UVUV--vvis spectrais spectra:: Interaction of Interaction of VOVOxx species species on different on different supportssupports at 773 Kat 773 K
Degree of interaction (polymerisation) of V5+Ox is a function of vanadia loading and the support material
400 800 1200
998
1026-1031
Raman shift (cm -1 )
Raman spectra: Identification of „sites“ on Raman spectra: Identification of „sites“ on VOVOxx--loaded loaded mesoporousmesoporous MCMMCM--4141
• V2O5 phase is present at high vanadia loading
• Weakly interacting VOxspecies exist under dehydrated conditions for samples with vanadia loading up to 4 to 5 wt.%
O
V
V(11.2 wt.%)
V(0.2 wt.%)
V(2.1 wt.%)
V(4.1 wt.%)
V2O5
J. Catal. 234 (2005) 131
Conclusions for ODPConclusions for ODP• Operando UV-vis as well as EPR and in-situ Raman
spectroscopy indicate the presence of highly dispersed vanadia in the form of monomeric and small 2-dimensional VOx aggregates, which might be partly considered as isolated sites.
• The highly dispersed vanadia species contribute to selective oxidation while crystalline nanoparticlesfavour formation of carbon oxides
• Catalyst preparation: For achieving high selectivity of propene supported vanadia catalysts should be designed in such a way that preferentially weakly interacting VOxspecies and preferably isolated sites are prepared.
• Reaction conditions: The concentration of near-surface lattice oxygen as well as of adsorbed oxygen should be low to avoid non-selective oxidation.
FromFrom Elucidation of catalysisElucidation of catalysisimprovement in
• understanding of interaction betweenreactant and catalyst
• catalyst development and optimization
FromFrom Kinetics of catalytic reactionKinetics of catalytic reactionimprovement in
• understanding of mechanistic aspects
• catalytic reaction-engineering procedures
What can be learnt?What can be learnt?
Ref. 1G. Centi, F. Cavani, F Trifiro
.
Selective Oxidation by Heterogeneous CatalysisFundamental and Applied Catalysis Series, Kluver Academic/Plenum Publisher, 2001 Ref. 2Basic Principles in Applied Catalysis, M. Baerns (editor) Chemical Physics Series, Springer Publisher, 2004 (a) F. Cavani, F. Trifiro: Partial Oidation of C2 to C4 Paraffins (b) R. Schlögl: In-situ Characteriszation of Practical Heterogeneous CatalystsRef. 3(a) O.Buyevskaya, M. BaernsOxidative Functionalization of Ethane and Propane(b) W. Ueda, S.W. LinMetal Halide Oxide Catalysts Active for Alkane Selective Oxidation both in : Vol. 16 of Catalysis Series, The Royal Chemical Society, 2004 Ref. 4Methane Conversion by Oxidative Processes – Fundamental and Engineering AspectsE. E. Wolf (Editor)
Van Nostrand Reinhold Catalysis Series, 1992Ref. 5B.K. HodnettHeterogeneous Catalytic OxidationWiley, 2000
References for further reading on hydrocarbon oxidation catalysis
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