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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