Conversion of Methane and Light Alkanes to Chemicals over Heterogeneous Catalysts - Lessons Learned from Experiment and Theory

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Conversion of Methane and Light Alkanes to

Chemicals over Heterogeneous Catalysts –

Lessons Learned from Experiment and Theory

March 8, 201 6

Alexis T. Bell

Department of Chemical and Biomolecular

Engineering

University of California

Berkeley, CA 94720


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Introduction

Natural Gas:

CH

4

,

C

2

H

6

, C

3

H

8

, C

4

H

10

Chemical Feedstocks

: CO/H

2

,

CH

2

=CH

2

, CH

3

CH=CH

2

,

CH

2

=CH-CH=CH

2

, C

6

H

6


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Introduction

Natural Gas:

CH

4

,

C

2

H

6

, C

3

H

8

, C

4

H

10

Chemical Feedstocks

: CO/H

2

,

CH

2

=CH

2

, CH

3

CH=CH

2

,

CH

2

=CH-CH=CH

2

, C

6

H

6

• How do heterogeneous catalysts facilitate the conversion of NG


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Catalyzed Conversion of Natural Gas to Chemicals

Conversion of Methane

Pyrolysis: CH4(g) ⇋ 1/6 C6H6(g) + 1.5 H2(g) CH4(g) ⇋ 1/2 C2H4(g) + H2(g)

Steam Reforming: CH4(g) + H2O(g) ⇋ CO(g) + 3 H2(g) Dry Reforming: CH4(g) + CO2(g) ⇋ 2 CO(g) + 2 H2(g)

Oxidative Coupling: CH4(g) + ½ O2(g) ⇋ ½ C2H4(g) + 2 H2O(g) Partial Oxidation: CH4(g) + ½ O2(g) ⇋ CH3OH(g)


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Catalyzed Conversion of Natural Gas to Chemicals

Conversion of Light Alkanes

Thermal Dehydrogenation: C2H6(g) ⇋ C2H4(g) + H2(g)

Oxidative dehydrogenation: C2H6(g) + ½ O2(g) ⇋ C2H4(g) + H2O(g)


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Central Questions

What is the rate-limiting step in the activation of methane

and light alkanes?

What factors govern the formation of coke during the

conversion of methane and light alkanes?

Can oxygenated compounds be formed directly from

methane and light alkanes?


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Mechanism of SRM

Steam Reforming of Methane (SRM) to Syngas

TOF (s-1)

T= 773 K; P = 1 atm; CH4 conversion 10%

• Experiment show that TOF decreases in the order Ru ~ Rh > Ni ~ Ir ~ Pt ~ Pd

• Theory shows that TOF decreases in the order Ru > Rh > Ni > Ir > Pt ~ Pd

G. Jones et al., J. Catal., 259, 147, 2008


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Dry Reforming of Methane to Syngas

E. D. German, M. Sheintuch, J. Phys. Chem. C, 107, 10229, 2013

Relationship of TOF (s-1) and H and CH 3

binding energies for T = 500 K

• TOF for CH4 dissociation decrease in the order Rh > Ru ~ Ir > Ni ~ Pd ~ Pt

• For Ni(111), CO is formed from CHO

Dissociation of CH to C and H is disfavored

on Ni(111) S. G. Wang et al., Surf. Sci. 601, 1271, 2007

Ni(111) CH4(g) + CO2(g)2 CO(g) + 2 H2(g)


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Kinetics of

Steam

and Dry Reforming of CH

4

Kinetics for the steam reforming of CH4 at 873 K on Ni/MgO

j. Wei and E. Igelsia, J. Catal., 224, 370, 2004


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Kinetics of Steam and

Dry

Reforming of CH

4

Kinetics for the dry reforming of CH4 at 873 K on Ni/MgO CH4(g) + CO2(g)2 CO(g) + 2 H2(g)


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Kinetics of Steam and

Dry

Reforming of CH

4

Kinetics for the dry reforming of CH4 at 873 K on Ni/MgO


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Kinetics of

Steam

and

Dry

Reforming of CH

4

R

f

= k

f

P

CH4

The rate expression of steam and dry reforming and for CH4 decomposition on Ni are the same

The rate coefficient for all three reactions is the same

The process controlling all three reactions is the dissociative adsorption of CH4


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Kinetics of C Accumulation on Ni during

Steam and Dry Reforming of CH

4


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Effects of Surface Structure and Surface Composition

on Coke Deposition on Ni

CH4(g) → CH3(s) + H(s)

CH4 dissociative adsorption occurs preferentially at Ni(211) steps

Graphene sheets nucleate at Ni(211) steps and then grow over the nanoparticle

J. Sehested, Catal. Today, 111, 103, 2006


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1. Nørskov and coworkers, J. Phys. Chem C, 114, 2010

Carbon Growth Model

Energy-driven Carbon Growth1 :

ΔG = total free energy change for a graphene island

Ntot = total # atoms in graphene island

ΔμC = carbon chemical potential

Eedge = energy/C atom on edge of island

Estretch = energy cost for stretching graphene layer to match Pt lattice

Lattice mismatch (strain) cost Surface cost Graphene growth Bulkenergy Step edge Graphene nucleus

∆� = −����∆� + 3��������� + 2�����������

• Graphene growth nucleates at steps


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Ni NiAu

• Introduction of Au into Ni introduces

additional strain and raises Ntot required to nucleate the growth of graphene

Ni

Bulk Energy Edge Energy Strain Energy

∆�

=

−�

���

∆�

+ 3

��

���

����

+ 2

��

���

������

E st ra in ( e V /ato m )

Carbon Growth Model

Au


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Thermodynamics of Methane Pyrolysis

• Thermodynamics predicts that the preferred products should be

C(s) >> C6H6(g) > C2H4(g)

• Carbon deposition along with C6H6 and C2H4 is observed for


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Methane Pyrolysis

• Only Fe@SiO2 produces ethene, benzene, and naphthalene but not

coke

• A CH4 conversion of 48% is achieved at 1363 K and a space velocity of

21,400 ml/g h

Fe@SiO2 X. Bao and coworkers, Science, 344, 616, 2014


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Methane Pyrolysis on Fe@SiO

2

• CH4 pyrolysis at 1363 K over Fe@SiO2 achieves 48% conversion and

selectivity of 48.4% to C2H4 and the rest to benzene and naphthalene

• Fe@SiO2 is stable to for 60 h


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Methane Pyrolysis on Fe@SiO

2

• Graphite is the thermodynamically preferred product of methane pyrolysis • The absence of soot or coke is attributable to the very rapid quenching of the

product gases, which inhibits the kinetics of soot formation

• Coke is not formed on Fe@SiO2 because the sites are too small to nucleate

coke

Active site for Fe@SiO2


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Methane Oxidation to Methanol

The active center is taken to be a [Cu2(µ-O)2]2+ core based on UV-Vis observations and comparison with compounds of known structure

CH4 is activated on [Cu2(µ-O)2]2+ cores to produce CH

3O species that can then be hydrolyzed to form CH3OH

Catalyst reactivation in O2 at elevated temperature is required

CH4 + [Cu2(µ-O)2]2+ [Cu

2(CH3O)(OH)]2+

[Cu2(CH3O)(OH)]2+ + H

2O [Cu2(µ-OH)2]2+ + CH3OH

[Cu2(µ-OH)2]2+ [Cu

2(µ-O)2]2+ + H2O

CH4(g) + ½ O2(g) CH3OH(g)


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Methane Oxidation to Methanol

• DFT calculations support the conclusion that the active center is a

[Cu-O-Cu]2+ cation


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Methane Oxidation to Methanol

• The activity of Cu-MOR for the

formation of CH3OH scales with Cu content

• The active center is best described

as a [Cu3O3]2+ core


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Problem

•Pt is an active catalyst for alkane

dehydrogenation but deactivates due coke accumulation

•Addition of Sn, Ga, In enhances

alkene selectivity and catalyst stability

Objective

•To identify the role of Pt particle size

and Sn addition on coke formation

• Identify the mechanism of coke

formation and the influence of coke on Pt nanoparticles

Dehydrogenation of Light Alkanes

V. Galvita et al. J. Catal. 2010, 271, 209; P. Sun et al. J. Catal. 2011, 282, 165; F. Somodi et al. J. Phys. Chem. C 2011,

115, 19084; Z. Peng et al. J. Catal., 2012, 286, 22; F. Somodi et al. Langmuir2012, 28, 3345; J. Wu et al. Appl. Catal. A, 2014 470, 208-214; J. Wu et al. J. Catal. 2014, 311, 161-168; X. Feng et al. J. Phys. Chem. C, 2015, 119,

7124-7129; J. Wu et al. Appl. Catal. A: Genl. 2015, 506, 25-32; J. Wu et al., J. Catal., 2016 in press.

C

n

H

2n+2

C

n

H

2n

+ H

2

Light alkenes can be used as monomers for oligomers or polymers


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Synthesis of Pt Model Catalysts

Colloidal Method

Pt(acac)2 Sn(acac)2 Pt-Sn alloy (color representing level of alloying)

Reduction

Mixing

623K, O2

Support - Mg(Al)O

Support - Mg(Al)O

Pt(acac)2, Sn(acac)2

oleylamine, oleic acid 1,2-hexadecanediol

<d> = 2.5 nm 563K

J. Wu et al., J. Catal. 311 (2014) 161


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Effects of Catalyst Sn/Pt Ratio and Particle Size on

Catalyst Activity

0.0 0.1 0.2 0.3 0.4 0.5

1.2 1.3 1.4 1.5 1.6 1.7 E thane TO F ( 1/ s) Sn/Pt

Reaction conditions: W/F = 3.75x10-3 g s-1 cm-3, T = 873 K, C2H6: 20%, H2: C2H6:1.25

• TOF increases with Sn/Pt ratio

• TOF increases with increasing particle size

<dPt> = 2.5 nm

0 2 4 6 8 10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 E thane T O F ( 1/ s) Size (nm) Pt/Mg(Al)O Pt3Sn/Mg(Al)O


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Effect of Pt Particle Size and Sn/Pt Ratio on

Carbon Accumulation

Reaction conditions: T = 873 K, C2H6: 20%, H2: C2H6:1.25

• Carbon accumulation:

- Increases with Pt particles size - Decreases with the addition of Sn

Pt particle size, nm

TOS = 2 h

τ = 3.8x10-3 g s cm-3


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Effect of Space Time on Coke Accumulation

C

2

H

6

+ s C

2

H

5s

+ H

s

C

2

H

5s

+ s

C

2

H

4s

+ H

s

C

2

H

4

+ s

or

C

2

H

5s

+ s …

CH

3

C

s

+ 2H

s

CH

3

C

s

C

s

+ CH

3s

coke methane

Desired

Undesired

• Experiments with 13C-labeled C2H4 show that coke and methane are

formed by readsorption of C2H4

• C2H4 as the source of coke is confirmed by high space velocity

experiments, which show low coke depositions at high space velocities


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Effect of Pt Particle Size on C Accumulation

Pt particle size, nm

2.0 nm

6.0 nm 3.8 nm

1 min 2 min

Pt

• Amount and morphology of

carbon change with Pt particle size.

TEM images acquired on TEAM 0.5

aberration-corrected microscope at NCEM/LBNL

Z. Peng, F. Somodi, S. Helveg, C. Kisielowski, P. Specht, A. T. Bell, J. Catal. 286, (2012) 22.


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Graphene Initiation at Pt Steps

Reaction Conditions: PC2H6 = 0.2 bar, PH2 = 0.25 bar, T= 873 K; 2 h


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Carbon Growth

Remaining questions…

Where does carbon nucleate? How do multiple layers grow? Does Pt restructure during coking?

Observe growth of carbon in situ (Haldor Topsøe)

J. Wu et al., J. Catal., submitted

Ex situ

(a) Pt/MgO carburized in 0.2 bar ethane at 873 K for 1 h. (b) Pt/MgO carburized in situ under 1 mbar C2H4 at 773 K for 20 min, taken at 500 e-/(Å2s)

In situ


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<0 min 3 min

12 min 20 min

a b

c d

Effects of Coke formation on Surface Topology of Pt

Nanoparticles

• Carbon deposition induces

step formation

• Steps serve as nucleation

points for carbon formation


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Oxidative Dehydrogenation of Light Alkanes

0-D VOx Al2O3

V O

O O O

Isolated monovanadate

2-D VOx

V O V

O O

O O

O O

Polyvanadate oligomer

2.3 V nm-2

CnH2n+2(g) + ½ O2(g)CnH2n(g) + H2O(g) n = 2-4

Al2O3

3 wt% V2O5/Al2O3

• Raman and UV-Vis spectroscopy indicate that VOx is principally

present as isolated vanadate species


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Oxidative Dehydrogenation of Light Alkanes

The overall rate of reaction depends on the strength of the weakest C-H bond

The ratio of k2/k1 is 0.1-0.4 and not very temperature sensitive


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Oxidative Dehydrogenation of Light Alkanes

• k3 depends more strongly on the

heat of alkene adsorption than on the strength of the weakest C-H bond in the alkene

• k3 is 1-5 fold higher than k1

• Alkene selectivity is limited by deep

oxidation of both the alkane and the alkene


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Concluding Remarks

• The activation of methane and light alkanes is rate limited by the

cleavage of C-H bonds

• Steam and dry reforming of methane follow identical kinetics, as do the

thermal dehydrogenation of light alkanes and the dehydroaromatization of methane

• Graphene formation is nucleated at steps on the surface of metal

particles and graphene growth can cause step formation

• Graphene formation is reduced by reducing metal particle size and

increasing the lattice mismatch between the graphene and the metal

• Soot formation is limited by very rapid thermal quenching

• The oxidation of methane to methanol is limited by catalyst reactivation • Oxidative dehydrogenation of light alkanes is limited by both primary


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Looking Over the Horizon

Identify catalysts that operate at high temperature

and are resistant to coke formation

Identify single-site catalysts that enable the

continuous conversion of methane to methanol

Identify catalysts than can promote the oxidative

dehydrogenation of alkanes to alkenes selectively

Understand the nature of oxygen species and what


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Acknowledgements

Office of Basic Energy Sciences

US Department of Energy

Chevron Energy Technology Co.


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Oxidative Dehydrogenation of Light Alkanes

0-D VOx Al2O3

V O

O O O

Isolated monovanadate

2-D VOx

V O V

O O

O O

O O

Polyvanadate oligomer

2.3 V nm-2

CnH2n+2(g) + ½ O2(g)CnH2n(g) + H2O(g) n = 2-4

Al2O3

3 wt% V2O5/Al2O3

• Raman and UV-Vis spectroscopy indicate that VOx is principally

present as isolated vanadate species M. Zboray et al., J. Phys. Chem. C, 113, 12980, 2009


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Oxidative Dehydrogenation of Light Alkanes

The overall rate of reaction depends on the strength of the weakest C-H bond The ratio of k2/k1 is 0.1-0.4 and not very temperature sensitive


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Oxidative Dehydrogenation of Light Alkanes

• k3 depends more strongly on the

heat of alkene adsorption than on the strength of the weakest C-H bond in the alkene

• k3 is 1-5 fold higher than k1

• Alkene selectivity is limited by deep

oxidation of both the alkane and the alkene


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Concluding Remarks

• The activation of methane and light alkanes is rate limited by the

cleavage of C-H bonds

• Steam and dry reforming of methane follow identical kinetics, as do the

thermal dehydrogenation of light alkanes and the dehydroaromatization of methane

• Graphene formation is nucleated at steps on the surface of metal

particles and graphene growth can cause step formation

• Graphene formation is reduced by reducing metal particle size and

increasing the lattice mismatch between the graphene and the metal

• Soot formation is limited by very rapid thermal quenching

• The oxidation of methane to methanol is limited by catalyst reactivation

• Oxidative dehydrogenation of light alkanes is limited by both primary


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Looking Over the Horizon

Identify catalysts that operate at high temperature

and are resistant to coke formation

Identify single-site catalysts that enable the

continuous conversion of methane to methanol

Identify catalysts than can promote the oxidative

dehydrogenation of alkanes to alkenes selectively

Understand the nature of oxygen species and what


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Acknowledgements

Office of Basic Energy Sciences

US Department of Energy

Chevron Energy Technology Co.