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

Transcription

1 Conversion of Methane and Light Alkanes to Chemicals over Heterogeneous Catalysts Lessons Learned from Experiment and Theory March 8, Alexis T. Bell Department of Chemical and Biomolecular Engineering University of California Berkeley, CA 94720

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

3 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 to chemical feedstocks?

4 Catalyzed Conversion of Natural Gas to Chemicals Conversion of Methane Pyrolysis: CH 4(g) 1/6 C 6 H 6(g) H 2(g) CH 4(g) 1/2 C 2 H 4(g) + H 2(g) Steam Reforming: Dry Reforming: Oxidative Coupling: Partial Oxidation: CH 4(g) + H 2 O (g) CO (g) + 3 H 2(g) CH 4(g) + CO 2(g) 2 CO (g) + 2 H 2(g) CH 4(g) + ½ O 2(g) ½ C 2 H 4(g) + 2 H 2 O (g) CH 4(g) + ½ O 2(g) CH 3 OH (g)

5 Catalyzed Conversion of Natural Gas to Chemicals Conversion of Light Alkanes Thermal Dehydrogenation: Oxidative dehydrogenation: Partial Oxidation: C 2 H 6(g) C 2 H 4(g) + H 2(g) C 2 H 6(g) + ½ O 2(g) C 2 H 4(g) + H 2 O (g) C 3 H 8(g) + O 2(g) CH 3 CH=CHO (g) + H 2 O (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? What is on the horizon and beyond?

7 Steam Reforming of Methane (SRM) to Syngas Mechanism of SRM CH 4(g) + H 2 O (g) CO (g) + 3 H 2(g) TOF (s -1 ) T= 773 K; P = 1 atm; CH 4 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 CH 4(g) + CO 2(g) 2 CO (g) + 2 H 2(g) Ni(111) Relationship of TOF (s -1 ) and H and CH 3 binding energies for T = 500 K E. D. German, M. Sheintuch, J. Phys. Chem. C, 107, 10229, 2013 TOF for CH 4 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

9 Kinetics of Steam and Dry Reforming of CH 4 Kinetics for the steam reforming of CH 4 at 873 K on Ni/MgO CH 4(g) + H 2 O (g) CO (g) + 3 H 2(g) 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 CH 4 at 873 K on Ni/MgO CH 4(g) + CO 2(g) 2 CO (g) + 2 H 2(g)

11 Kinetics of Steam and Dry Reforming of CH 4 Kinetics for the dry reforming of CH 4 at 873 K on Ni/MgO The kinetics for the forward reaction in steam and dry reforming are identical

12 Kinetics of Steam and Dry Reforming of CH 4 Ni/MgO R f = k f P CH4 The rate expression of steam and dry reforming and for CH 4 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 CH 4

13 Kinetics of C Accumulation on Ni during Steam and Dry Reforming of CH 4 The kinetics of carbon accumulation are the same for steam and dry reforming of CH 4

14 Effects of Surface Structure and Surface Composition on Coke Deposition on Ni CH 4(g) CH 3(s) + H (s) F. Abild-Pedersen et al., Surf. Sci, 590, 127, 2005 CH 4 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 Carbon Growth Model Energy-driven Carbon Growth 1 : GG = NN tttttt μμ cc + 3 NN tttttt EE eeeeeeee + 2 NN tttttt EE ssssssssssss Graphene nucleus Bulk energy Surface cost Lattice mismatch (strain) cost Step edge Graphene growth ΔG = total free energy change for a graphene island N tot = total # atoms in graphene island Δμ C = 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 To nucleate the step width has to be greater than a critical value 1. Nørskov and coworkers, J. Phys. Chem C, 114, 2010

16 Carbon Growth Model GG = NN tttttt μμ cc + 3 NN tttttt EE eeeeeeee + 2 NN tttttt EE ssssssssssss Bulk Energy Edge Energy Strain Energy Ni NiAu E strain (ev/atom) Ni Au Introduction of Au into Ni introduces additional strain and raises N tot required to nucleate the growth of graphene 1. Nørskov and coworkers, J. Phys. Chem C, 114, 2010

17 Thermodynamics of Methane Pyrolysis Thermodynamics predicts that the preferred products should be C(s) >> C 6 H 6(g) > C 2 H 4(g) Carbon deposition along with C 6 H 6 and C 2 H 4 is observed for MoC x /ZSM-5, Fe/SiO 2

18 Methane Pyrolysis 2 X. Bao and coworkers, Science, 344, 616, 2014 Only 2 produces ethene, benzene, and naphthalene but not coke A CH 4 conversion of 48% is achieved at 1363 K and a space velocity of 21,400 ml/g h

19 Methane Pyrolysis on 2 CH 4 pyrolysis at 1363 K over Fe@SiO 2 achieves 48% conversion and selectivity of 48.4% to C 2 H 4 and the rest to benzene and naphthalene Fe@SiO 2 is stable to for 60 h The high stability is attributed to isolated FeC 2 sites

20 Methane Pyrolysis on 2 Active site for 2 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 2 because the sites are too small to nucleate coke

21 Methane Oxidation to Methanol CH 4(g) + ½ O 2(g) CH 3 OH (g) CH 4 + [Cu 2 (µ-o) 2 ] 2+ [Cu 2 (CH 3 O)(OH)] 2+ [Cu 2 (CH 3 O)(OH)] 2+ + H 2 O [Cu 2 (µ-oh) 2 ] 2+ [Cu 2 (µ-oh) 2 ] 2+ + CH 3 OH [Cu 2 (µ-o) 2 ] 2+ + H 2 O M. H. Groothaert et al., J. Am. Chem. Soc. 127, The active center is taken to be a [Cu 2 (µ-o) 2 ] 2+ core based on UV-Vis observations and comparison with compounds of known structure CH 4 is activated on [Cu 2 (µ-o) 2 ] 2+ cores to produce CH 3 O species that can then be hydrolyzed to form CH 3 OH Catalyst reactivation in O 2 at elevated temperature is required

22 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

23 Methane Oxidation to Methanol The activity of Cu-MOR for the formation of CH 3 OH scales with Cu content The active center is best described as a [Cu 3 O 3 ] 2+ core S. Grunder et al., Nature Comm. DOI: /ncomms8546

24 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 C n H 2n+2 C n H 2n + H 2 Light alkenes can be used as monomers for oligomers or polymers H 2 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, , ; J. Wu et al. J. Catal. 2014, 311, ; X. Feng et al. J. Phys. Chem. C, 2015, 119, ; J. Wu et al. Appl. Catal. A: Genl. 2015, 506, 25-32; J. Wu et al., J. Catal., 2016 in press.

25 Synthesis of Pt Model Catalysts Colloidal Method Reduction 563K Pt(acac) 2, Sn(acac) 2 oleylamine, oleic acid 1,2-hexadecanediol Mixing <d> = 2.5 nm Support - Mg(Al)O 623K, O 2 <d> = 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

26 Effects of Catalyst Sn/Pt Ratio and Particle Size on Catalyst Activity Pt/Mg(Al)O Pt 3 Sn/Mg(Al)O Ethane TOF (1/s) <d Pt > = 2.5 nm Sn/Pt Ethane TOF (1/s) Size (nm) Reaction conditions: W/F = 3.75x10-3 g s -1 cm -3, T = 873 K, C 2 H 6 : 20%, H 2 : C 2 H 6 :1.25 TOF increases with Sn/Pt ratio TOF increases with increasing particle size

27 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, C 2 H 6 : 20%, H 2 : C 2 H 6 :1.25 Carbon accumulation: - Increases with Pt particles size - Decreases with the addition of Sn

28 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 2 H s H s or C 2 H 4 + s Desired C 2 H 5s + s CH 3 C s + 2H s CH 3 C s C s + CH 3s Undesired coke methane Experiments with 13 C-labeled C 2 H 4 show that coke and methane are formed by readsorption of C 2 H 4 C 2 H 4 as the source of coke is confirmed by high space velocity experiments, which show low coke depositions at high space velocities

29 Pt Effect of Pt Particle Size on C Accumulation 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.

30 Graphene Initiation at Pt Steps Reaction Conditions: P C2H6 = 0.2 bar, P H2 = 0.25 bar, T= 873 K; 2 h Graphene sheets form at steps on the surface of large Pt particles

31 Carbon Growth Remaining questions Where does carbon nucleate? How do multiple layers grow? Does Pt restructure during coking? Ex situ 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 C 2 H 4 at 773 K for 20 min, taken at 500 e - /(Å 2 s) J. Wu et al., J. Catal., submitted

32 Effects of Coke formation on Surface Topology of Pt Nanoparticles a b <0 min 3 min Carbon deposition induces step formation Steps serve as nucleation points for carbon formation c d 12 min 20 min J. Wu et al., J. Catal., submitted

33 Oxidative Dehydrogenation of Light Alkanes C n H 2n+2(g) + ½ O 2(g) C n H 2n(g) + H 2 O (g) n = 2-4 Isolated monovanadate O V O O O Polyvanadate oligomer O O V O V O O O O 2.3 V nm -2 Al 2 O 3 Al 2 O 3 0-D VO x 2-D VO x 3 wt% V 2 O 5 /Al 2 O 3 Raman and UV-Vis spectroscopy indicate that VO x is principally present as isolated vanadate species M. Zboray et al., J. Phys. Chem. C, 113, 12980, 2009

34 Oxidative Dehydrogenation of Light Alkanes E a = 100 kj/mol The overall rate of reaction depends on the strength of the weakest C-H bond The ratio of k 2 /k 1 is and not very temperature sensitive

35 Oxidative Dehydrogenation of Light Alkanes k 3 depends more strongly on the heat of alkene adsorption than on the strength of the weakest C-H bond in the alkene k 3 is 1-5 fold higher than k 1 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 deep oxidation of the alkane and secondary oxidation of the alkene

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 controls their activity

38 Acknowledgements Office of Basic Energy Sciences US Department of Energy Chevron Energy Technology Co.