Conversion of Methane and Light Alkanes to Chemicals over Heterogeneous Catalysts – Lessons Learned from Experiment and Theory
March 8, 2016
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: 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)
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)
Partial Oxidation: 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?
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
CH4(g) + H2O(g) ⇋ CO(g) + 3 H2(g)
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 CH3 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)
Kinetics of Steam and Dry Reforming of CH4
Kinetics for the steam reforming of CH4 at 873 K on Ni/MgO
j. Wei and E. Igelsia, J. Catal., 224, 370, 2004
CH4(g) + H2O(g) ⇋ CO(g) + 3 H2(g)
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
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
Ni/MgO
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)
• 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
F. Abild-Pedersen et al., Surf. Sci, 590, 127, 2005
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
Bulk energy
Step edge
Graphene nucleus
∆𝐺𝐺 = −𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 ∆𝜇𝜇𝑐𝑐 + 3�𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 𝐸𝐸𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 + 2�𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 𝐸𝐸𝑠𝑠𝑡𝑡𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
• Graphene growth nucleates at steps
• To nucleate the step width has to be greater than a critical value
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
∆𝐺𝐺 = −𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 ∆𝜇𝜇𝑐𝑐 + 3�𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 𝐸𝐸𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 + 2�𝑁𝑁𝑡𝑡𝑡𝑡𝑡𝑡 𝐸𝐸𝑠𝑠𝑡𝑡𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠
E stra
in (
eV/a
tom
) Carbon Growth Model
Au
1. Nørskov and coworkers, J. Phys. Chem C, 114, 2010
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
• 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
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
• 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
X. Bao and coworkers, Sci., 344, 616, 2014
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 CH3O species that can then be hydrolyzed to form CH3OH
• Catalyst reactivation in O2 at elevated temperature is required
CH4 + [Cu2(µ-O)2]2+ [Cu2(CH3O)(OH)]2+
[Cu2(CH3O)(OH)]2+ + H2O [Cu2(µ-OH)2]2+ + CH3OH
[Cu2(µ-OH)2]2+ [Cu2(µ-O)2]2+ + H2O
CH4(g) + ½ O2(g) ⇋ CH3OH(g)
M. H. Groothaert et al., J. Am. Chem. Soc. 127, 1394 2005
Methane Oxidation to Methanol
• DFT calculations support the conclusion that the active center is a [Cu-O-Cu]2+ cation
J. Woortnik et al., PNAS 106, 18908, 2009
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
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. 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.
CnH2n+2 CnH2n + H2
• Light alkenes can be used as monomers for oligomers or polymers • H2 can be used for HDS, HDN, etc.
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
<d> = 2.5 – 8.0 nm
Effects of Catalyst Sn/Pt Ratio and Particle Size on Catalyst Activity
0.0 0.1 0.2 0.3 0.4 0.51.2
1.3
1.4
1.5
1.6
1.7
Etha
ne T
OF
(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 8 100.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Etha
ne T
OF
(1/s
)Size (nm)
Pt/Mg(Al)O Pt3Sn/Mg(Al)O
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
Pt Loading = 0.8 wt% Pt
Effect of Space Time on Coke Accumulation
C2H6 + s C2H5s + Hs
C2H5s + s C2H4s + Hs C2H4 + s
or
C2H5s + s … CH3Cs + 2Hs
CH3Cs Cs + CH3s
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
2 Hs H2 + 2 s
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.
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 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 b
<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
J. Wu et al., J. Catal., submitted
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
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
Ea = 100 kJ/mol
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.
