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
Introduction
Natural Gas: CH4,
C2H6, C3H8, C4H10
Chemical Feedstocks: CO/H2,
CH2=CH2, CH3CH=CH2,
CH2=CH-CH=CH2, C6H6
Introduction
Natural Gas: CH4,
C2H6, C3H8, C4H10
Chemical Feedstocks: CO/H2,
CH2=CH2, CH3CH=CH2,
CH2=CH-CH=CH2, C6H6
• How do heterogeneous catalysts facilitate the conversion of NG
to chemical feedstocks?
Catalyzed Conversion of Natural Gas to Chemicals
Conversion of Methane
Pyrolysis:
Steam Reforming:
Dry Reforming:
Oxidative Coupling:
Partial Oxidation:
CH4(g) ⇋ 1/6 C6H6(g) + 1.5 H2(g)
CH4(g) ⇋ 1/2 C2H4(g) + H2(g)
CH4(g) + H2O(g) ⇋ CO(g) + 3 H2(g)
CH4(g) + CO2(g) ⇋ 2 CO(g) + 2 H2(g)
CH4(g) + ½ O2(g) ⇋ ½ C2H4(g) + 2 H2O(g)
CH4(g) + ½ O2(g) ⇋ CH3OH(g)
Catalyzed Conversion of Natural Gas to Chemicals
Conversion of Light Alkanes
Thermal Dehydrogenation:
Oxidative dehydrogenation:
Partial Oxidation:
C2H6(g) ⇋ C2H4(g) + H2(g)
C2H6(g) + ½ O2(g) ⇋ C2H4(g) + H2O(g)
C3H8(g) + O2(g) ⇋ CH3CH=CHO(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?
• What is on the horizon and beyond?
Steam Reforming of Methane (SRM) to Syngas
Mechanism of SRM
CH4(g) + H2O(g) ⇋ CO(g) + 3 H2(g)
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
CH4(g) + CO2(g) ⇋ 2 CO(g) + 2 H2(g)
Ni(111)
Relationship of TOF (s-1) and H and CH3
binding energies for T = 500 K
E. D. German, M. Sheintuch, J. Phys. Chem. C, 107,
10229, 2013
• 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 CH4
Kinetics for the steam reforming of CH4 at 873 K on Ni/MgO
CH4(g) + H2O(g) ⇋ CO(g) + 3 H2(g)
j. Wei and E. Igelsia, J. Catal., 224, 370, 2004
Kinetics of Steam and Dry Reforming of CH4
Kinetics for the dry reforming of CH4 at 873 K on Ni/MgO
CH4(g) + CO2(g) ⇋ 2 CO(g) + 2 H2(g)
Kinetics of Steam and Dry Reforming of CH4
Kinetics for the dry reforming of CH4 at 873 K on Ni/MgO
• The kinetics for the forward reaction in steam and dry reforming are identical
Kinetics of Steam and Dry Reforming of CH4
Ni/MgO
Rf = kf PCH4
• 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 CH4
• The kinetics of carbon accumulation are the same for steam and dry reforming of CH4
Effects of Surface Structure and Surface Composition
on Coke Deposition on Ni
CH4(g) → CH3(s) + H(s)
F. Abild-Pedersen et al., Surf. Sci, 590, 127, 2005
• 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
Carbon Growth Model
Energy-driven Carbon Growth1 :
∆𝐺𝐺 = −𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 ∆𝜇𝜇𝑐𝑐 + 3�𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 𝐸𝐸𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 + 2�𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 𝐸𝐸𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
Graphene
nucleus
Bulk energy
Step edge
Surface cost
Lattice mismatch
(strain) cost
Graphene growth 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
• Graphene growth nucleates at steps
• To nucleate the step width has to be greater than a critical value
1. Nørskov and coworkers, J. Phys. Chem C, 114, 2010
Carbon Growth Model
∆𝐺𝐺 = −𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 ∆𝜇𝜇𝑐𝑐 + 3�𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 𝐸𝐸𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 + 2�𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 𝐸𝐸𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
Edge Energy
Bulk Energy
Strain Energy
Estrain (eV/atom)
Ni
Au
Ni
• Introduction of Au into Ni introduces
additional strain and raises Ntot required to
nucleate the growth of graphene
1. Nørskov and coworkers, J. Phys. Chem C, 114, 2010
NiAu
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
MoCx/ZSM-5, Fe/SiO2
Methane Pyrolysis
Fe@SiO2
X. Bao and coworkers, Science, 344, 616, 2014
• 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
Methane Pyrolysis on Fe@SiO2
• 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
• The high stability is attributed to isolated FeC2 sites
Methane Pyrolysis on Fe@SiO2
Active site for Fe@SiO2
X. Bao and coworkers, Sci., 344, 616, 2014
• 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
Methane Oxidation to Methanol
CH4(g) + ½ O2(g) ⇋ CH3OH(g)
CH4 + [Cu2(µ-O)2]2+
[Cu2(CH3O)(OH)]2+
[Cu2(CH3O)(OH)]2+ + H2O
[Cu2(µ-OH)2]2+
[Cu2(µ-OH)2]2+ + CH3OH
[Cu2(µ-O)2]2+ + H2O
M. H. Groothaert et al., J. Am. Chem. Soc. 127, 1394 2005
• 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 CH3O species that can
then be hydrolyzed to form CH3OH
• Catalyst reactivation in O2 at elevated temperature is required
Methane Oxidation to Methanol
J. Woortnik et al., PNAS 106, 18908, 2009
• 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
S. Grunder et al., Nature Comm. DOI: 10.1038/ncomms8546
Dehydrogenation of Light Alkanes
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
CnH2n+2
CnH2n + H2
• Light alkenes can be used as monomers
for oligomers or polymers
• H2 can be used for HDS, HDN, etc.
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
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. Langmuir 2012, 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.
Synthesis of Pt Model Catalysts
 Colloidal Method
Reduction
563K
Mixing
Pt(acac)2, Sn(acac)2
oleylamine, oleic acid
1,2-hexadecanediol
Support - Mg(Al)O
623K, O2
<d> = 2.5 – 8.0 nm
Support - Mg(Al)O
Pt(acac)2
Sn(acac)2
Pt-Sn alloy (color representing level of alloying)
J. Wu et al., J. Catal. 311 (2014) 161
<d> = 2.5 nm
Effects of Catalyst Sn/Pt Ratio and Particle Size on
Catalyst Activity
4.0
1.7
3.0
Ethane TOF (1/s)
1.6
Ethane TOF (1/s)
Pt/Mg(Al)O
Pt3Sn/Mg(Al)O
3.5
1.5
<dPt> = 2.5 nm
1.4
2.5
2.0
1.5
1.0
1.3
0.5
1.2
0.0
0.1
0.2
0.3
Sn/Pt
0.4
0.5
0.0
0
2
4
6
8
Size (nm)
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
10
Effect of Pt Particle Size and Sn/Pt Ratio on
Carbon Accumulation
τ = 3.8x10-3 g s cm-3
TOS = 2 h
Pt particle size, nm
Pt Loading = 0.8 wt% Pt
Reaction conditions: T = 873 K, C2H6: 20%, H2: C2H6:1.25
• Carbon accumulation:
- Increases with Pt particles size
- Decreases with the addition of Sn
Effect of Space Time on Coke Accumulation
C2H6 + s
C2H5s + s
2 Hs
C2H5s + Hs
C2H4s + Hs
H2 + 2 s
or
C2H5s + s …
CH3Cs
C2H4 + s
CH3Cs + 2Hs
Cs + CH3s
coke
Desired
Undesired
methane
• 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
Effect of Pt Particle Size on C Accumulation
Pt
1 min
2 min
2.0 nm
3.8 nm
Pt particle size, nm
• Amount and morphology of
carbon change with Pt particle size.
6.0 nm
TEM images acquired on TEAM 0.5 aberrationcorrected microscope at NCEM/LBNL
Z. Peng, F. Somodi, S. Helveg, C. Kisielowski, P. Specht,
A. T. Bell, J. Catal. 286, (2012) 22.
Graphene Initiation at Pt Steps
Reaction Conditions: PC2H6 = 0.2 bar, PH2 = 0.25 bar, T= 873 K; 2 h
• Graphene sheets form at steps on the surface of large Pt particles
Carbon Growth
Ex situ
Remaining questions…
Where does carbon nucleate?
How do multiple layers grow?
Does Pt restructure during coking?
Observe growth of carbon in situ (Haldor Topsøe)
b
In 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)
J. Wu et al., J. Catal., submitted
Effects of Coke formation on Surface Topology of Pt
Nanoparticles
a
<0 min
b
3 min
• Carbon deposition induces
step formation
• Steps serve as nucleation
points for carbon formation
c
12 min
d
20 min
J. Wu et al., J. Catal., submitted
Oxidative Dehydrogenation of Light Alkanes
CnH2n+2(g) + ½ O2(g) ⇋ CnH2n(g) + H2O(g) n = 2-4
Polyvanadate oligomer
Isolated monovanadate
O
V
O O
O
O
O
V
O
O
O O
V
O
2.3 V nm-2
Al2O3
Al2O3
0-D VOx
2-D VOx
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
Oxidative Dehydrogenation of Light Alkanes
Ea = 100 kJ/mol
• 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
oxidation of both the alkane and
the alkene
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
deep oxidation of the alkane and secondary oxidation of the alkene
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
controls their activity
Acknowledgements
Office of Basic Energy Sciences
US Department of Energy
Chevron Energy Technology Co.