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
University of California
Berkeley, CA 94720
(2)
Introduction
Natural Gas:
CH
4,
C
2H
6, C
3H
8, C
4H
10Chemical Feedstocks
: CO/H
2,
CH
2=CH
2, CH
3CH=CH
2,
CH
2=CH-CH=CH
2, C
6H
6(3)
Introduction
Natural Gas:
CH
4,
C
2H
6, C
3H
8, C
4H
10Chemical Feedstocks
: CO/H
2,
CH
2=CH
2, CH
3CH=CH
2,
CH
2=CH-CH=CH
2, C
6H
6• How do heterogeneous catalysts facilitate the conversion of NG
(4)
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)
(5)
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)
(6)
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?
(7)
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
(8)
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)
(9)
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
(10)
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)
(11)
Kinetics of Steam and
Dry
Reforming of CH
4
Kinetics for the dry reforming of CH4 at 873 K on Ni/MgO
(12)
Kinetics of
Steam
and
Dry
Reforming of CH
4
R
f= k
fP
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
(13)
Kinetics of C Accumulation on Ni during
Steam and Dry Reforming of CH
4
(14)
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
(15)
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
(16)
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
(17)
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
(18)
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
(19)
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
(20)
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
(21)
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)
(22)
Methane Oxidation to Methanol
• DFT calculations support the conclusion that the active center is a
[Cu-O-Cu]2+ cation
(23)
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
(24)
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
nH
2n+2C
nH
2n+ H
2•Light alkenes can be used as monomers for oligomers or polymers
(25)
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
(26)
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
(27)
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
(28)
Effect of Space Time on Coke Accumulation
C
2H
6+ s C
2H
5s+ H
sC
2H
5s+ s
C
2H
4s+ H
sC
2H
4+ s
or
C
2H
5s+ s …
CH
3C
s+ 2H
sCH
3C
sC
s+ CH
3scoke 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
(29)
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.
(30)
Graphene Initiation at Pt Steps
Reaction Conditions: PC2H6 = 0.2 bar, PH2 = 0.25 bar, T= 873 K; 2 h
(31)
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
(32)
<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
(33)
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
(34)
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
(35)
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
(36)
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
(37)
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
(38)
Acknowledgements
Office of Basic Energy Sciences
US Department of Energy
Chevron Energy Technology Co.
(1)
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
(2)
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
(3)
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
(4)
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
(5)
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
(6)