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

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

Introduction

Natural Gas:

CH

₄

,

C

₂

H

₆

, C

₃

H

₈

, C

₄

H

₁₀

Chemical Feedstocks

: CO/H

₂

,

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

Catalyzed Conversion of Natural Gas to Chemicals

Conversion of Methane

Pyrolysis: CH_4(g) ⇋ 1/6 C₆H_6(g) + 1.5 H_2(g) CH_4(g) ⇋ 1/2 C₂H_4(g) + H_2(g)

Steam Reforming: CH_4(g) + H₂O_(g) ⇋ CO_(g) + 3 H_2(g) Dry Reforming: CH_4(g) + CO_2(g) ⇋ 2 CO_(g) + 2 H_2(g)

Catalyzed Conversion of Natural Gas to Chemicals

Conversion of Light Alkanes

Thermal Dehydrogenation: C₂H_6(g) ⇋ C₂H_4(g) + H_2(g)

Oxidative dehydrogenation: C₂H_6(g) + ½ O_2(g) ⇋ C₂H_4(g) + H₂O_(g)

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?

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

Dry Reforming of Methane to Syngas

• 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

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

Kinetics of Steam and

Dry

Reforming of CH

₄

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

Kinetics of Steam and

Dry

Reforming of CH

₄

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

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

Kinetics of C Accumulation on Ni during

Steam and Dry Reforming of CH

₄

Effects of Surface Structure and Surface Composition

on Coke Deposition on Ni

CH_4(g) → CH_3(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

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

N_tot = total # atoms in graphene island

ΔμC = carbon chemical potential

E_edge = energy/C atom on edge of island

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

• Graphene growth nucleates at steps

Ni NiAu

• Introduction of Au into Ni introduces

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

Ni

Bulk Energy Edge Energy Strain Energy

∆�

=

−�

_���

∆�

_�

+ 3

��

_���

�

_����

+ 2

��

_���

�

_{������}

Thermodynamics of Methane Pyrolysis

• Thermodynamics predicts that the preferred products should be

C(s) >> C₆H_6(g) > C₂H_4(g)

• Carbon deposition along with C₆H₆ and C₂H₄ is observed for

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

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

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

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

Methane Oxidation to Methanol

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

[Cu-O-Cu]2+ _cation

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

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

Effects of Catalyst Sn/Pt Ratio and Particle Size on

• TOF increases with Sn/Pt ratio

• TOF increases with increasing particle size

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

Effect of Space Time on Coke Accumulation

experiments, which show low coke depositions at high space velocities

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

Graphene Initiation at Pt Steps

Reaction Conditions: P_C2H6 = 0.2 bar, P_H2 = 0.25 bar, T= 873 K; 2 h

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-_/(Å2_s)

In situ

<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

Oxidative Dehydrogenation of Light Alkanes

Isolated monovanadate

2-D VO_x

Polyvanadate oligomer

2.3 V nm-2

C_nH_2n+2(g) + ½ O_2(g) ⇋ C_nH_2n(g) + H₂O_(g) n = 2-4

Al₂O₃

3 wt% V₂O₅/Al₂O₃

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

present as isolated vanadate species

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

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

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

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

Acknowledgements

Office of Basic Energy Sciences

US Department of Energy