Role of Peroxides on La2O3 Catalysts in Oxidative Coupling of

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Role of Peroxides on La2O3 Catalysts in Oxidative Coupling of Methane Changqing Chu, Yonghui Zhao, Shenggang Li,* and Yuhan Sun CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, 100 Haike Road, Shanghai 201210, China S Supporting Information *

ABSTRACT: Density functional theory and coupled cluster theory [CCSD(T)] calculations reveal an important pathway for the one-step CH3OH formation upon CH4 activation at the peroxide (O22−) site of La2O3-based catalysts for the oxidative coupling of methane (OCM) reaction. Using modest-sized La4O7 and La6O10 clusters as catalyst models, two types of structures for the O22− site were predicted, with the less stable structure (type II) more reactive with CH4 than the more stable structure (type I). CH4 activation at the O22− site can always occur via the above pathway, and for the smaller La2O4 cluster and the type I structure of La4O7, an alternative pathway leading to La−CH3 bond formation was also predicted, similar to that at the oxide (O2−) site from our previous study. For the type I structure of La4O7, the energy barrier for La−CH3 bond formation is lower than that for CH3OH formation, but both are higher than that for CH3OH formation for the type II structure of La4O7. The O22− site was predicted to be much less reactive with CH4 than the oxide (O2−) site, and can lead to CH3OH formation, which is considered as a side reaction. Thus, our calculations do not appear to support the central role previously proposed for the O22− site for La2O3-based catalysts for the OCM reaction. However, considering the catalytic and redox nature of this reaction, both the O2− and O22− sites may still play important roles in the whole catalytic cycle.

1. INTRODUCTION

The above studies seem to support the proposed central role of the O22− site on La2O3-based catalysts as the active oxygen site in the OCM reaction. However, we note that Neurock and co-workers did not show the actual structure of the transition state for CH4 activation at the O22− site on the La2O3(001) surface, neither did they further characterize this transition state regarding its reactant and product states, which can be nontrivial for surface calculations using the periodic DFT method. Thus, it is unclear whether their transition state actually leads to CH3 radical formation as they claimed. Furthermore, they only examined two elementary steps, O2 dissociation on the La2O3(001) surface and CH4 activation at the O22− site, so their catalytic mechanism is incomplete, and further reaction steps must occur for regenerating the catalyst by H2O elimination. In our previous work,12 we used DFT and coupled cluster (CCSD(T))13−15 methods and modest-sized stoichiometric La2O3 clusters as catalyst models to study CH4 activation at the O2− site on La2O3-based catalysts. Although La(OH)3 may form upon reaction of La2O3 with H2O present at typical OCM reaction conditions, La2O3 is the major species detected5 due to its higher thermal stability than La(OH)3, justifying our choice of catalyst models. In addition, despite its high basicity, the

1,2

Oxidative coupling of methane (OCM) is one of the most promising reactions for directly converting natural gas into value-added chemicals. Many catalysts have been developed for this reaction over the past three decades,3 and there remain significant interests in further improving the known catalysts and developing new catalysts.4−6 Despite the many experimental and theoretical efforts, the detailed catalytic mechanism even for the most widely studied catalysts such as the La2O3based catalysts remains unsettled, for example, the nature of the active oxygen site.1,2 This is partially due to the complex heterogeneous and homogeneous nature of the reaction, widely assumed to occur via the Lunsford mechanism.7,8 Neurock and co-workers previously studied CH4 activation at several oxygen sites such as the oxide (O2−), oxygen radical (O−), and peroxide (O22−) sites on model La2O3(001) surfaces using the periodic density functional theory (DFT) method.9 With an estimate for the energy barrier for O2 dissociation on the La2O3(001) surface leading to O22− site formation,10 they suggested the O22− site as the active site for La2O3-based catalysts in the OCM reaction. Weng and co-workers used Raman spectroscopy to detect the formation of the O22− species on La2O3 surfaces upon laser irradiation in the presence of O2, and further showed its catalytic activity in C2H6 oxidation, although a detailed product analysis was not carried out.11 © 2014 American Chemical Society

Received: September 15, 2014 Revised: November 5, 2014 Published: November 10, 2014 27954

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cluster and the computing cluster at Shanghai Advanced Research Institute.

reaction of La2O3 with CH4 is not a simple acid−base reaction due to the fact that CH4 is an extremely weak acid and the CH3 radical is recognized as the key reaction intermediate, so basicity is only one of the important factors influencing the catalytic activity. The O2− site on these clusters was predicted to be much more reactive than the O22− site on the La2O3(001) surface studied by Neurock and co-workers. Following hydrogen transfer from CH4 to the O2− site, La−CH3 bond formation was predicted, which requires a substantial amount of energy to break. Some recent studies16,17 on related catalytic systems also suggest metal−CH3 bond formation and breaking as relevant steps for CH3 radical formation. Here, we report our further computational study on CH4 activation at the O22− site using similar cluster models. Compared to the O2− site, the O22− site was predicted to be much less reactive with CH4. More importantly, CH3OH formation in a single elementary step can always occur upon CH4 activation at the O22− site, which is considered as a side reaction. Our studies provide further insight into the catalytic mechanism of the La2O3catalyzed OCM reaction, where a central role for the O22− site as previously proposed is unlikely to be the case. However, considering the catalytic and redox nature of this reaction, the O22− site as well as the O2− site may still be important in the whole catalytic cycle.

3. RESULTS AND DISCUSSION 3.1. Cluster Structures and Energetics. Figure 1 presents structures and energetics of the La2nO3n and La2nO3n+1 (n = 1−

2. COMPUTATIONAL METHODS Equilibrium geometries and vibrational frequencies were calculated at the B3LYP level18 with the aug-cc-pVDZ basis set for H, C, O,19 and the Stuttgart small core relativistic effective core potential (ECP)20 basis set in the segmented contraction for La;21 this combination of basis sets is denoted as aVDZ. Single point energies were then calculated at the B3LYP and PBE22 level with the aug-cc-pVTZ basis set for H, C, O, and the above basis set for La; this combination of basis sets is denoted as aVTZ. Single point energies were also calculated at the CCSD(T) level with up to the aug-cc-pVQZ basis set for H, C, and O, and for La the above ECP basis set in the generalized contraction23 was used; this combination of basis sets is denoted as aVQZ. For open-shell systems, the R/ UCCSD(T) approach was used.24−26 In these CCSD(T) calculations, the 1s2 electrons on C and O and the 4s24p64d10 electrons on La were excluded from the correlation treatment. Unimolecular reaction rate constants at the high pressure limit at 298 and 1073 K were also calculated.36 For studying large La2O3 clusters and their reactions, the above method based on B3LYP geometry optimization and CCSD(T) energy calculations are computationally too expensive, so we benchmarked the computationally more efficient approach based on PBE geometry optimization and B3LYP energy calculations. Equilibrium geometries and vibrational frequencies were also calculated at the PBE level with the DZVP basis set for H, C, O,27,28 and the LanL2DZ basis set for La;29 this combination of basis sets is denoted as DZVP. Single point energies at the PBE/DZVP geometries were calculated at the PBE and B3LYP levels with the aVDZ and aVTZ basis sets. All DFT calculations were performed with the Gaussian 09 program package.30 The CCSD(T) calculations were performed with the MOLPRO 2012.1 program package.31,32 Molecular visualization was done using the AGUI graphics program from the AMPAC program package.33 The calculations were performed on our Xeon-based Lenovo computing

Figure 1. La2nO3n and La2nO3n+1 (n = 1−3) clusters with selected bond distances in Å calculated at the B3LYP/aVDZ (black), CCSD(T)/ aVQZ (red), and PBE/DZVP (blue) levels, and relative energies at 0 K in kcal/mol, calculated at the CCSD(T)/aVQZ (n = 1), CCSD(T)/ aVTZ (n = 2), and CCSD(T)/aVDZ (n = 3) levels.

3) clusters. The B3LYP/aVDZ geometries for La2O3 and La4O6 are taken from our previous work.12 Unless explicitly stated, equilibrium geometries were calculated at the B3LYP/aVDZ level. For La2O4, the C1 structure is the most stable. Natural charge of the O22− site was calculated to be −1.27 e, close to those for the O2− sites in La2O4 and La2O3 of −1.20 e.12 The calculated O−O bond distance in the O22− group is also very close to that of 1.49 Å known for O22−.34 These suggest the O22− site as a peroxide site. Calculated La−O bond distances in the La2O3 and La2O4 clusters at the B3LYP/aVDZ level are slightly longer than those at the CCSD(T)/aVQZ level by 2 kcal/mol at the CCSD(T)/aVDZ level than that of RC (a), showing again the preference for CH4 to weakly interact with both the La and O atoms instead of with only the O atom. Energy barriers (ΔG‡298K) from the physisorption structures were calculated to be ∼41 kcal/mol for pathway (a) and ∼49 kcal/mol for pathway (b), and chemisorption is highly endothermic for the former and highly exothermic for the latter. Thus, for the type I structure of La4O7, La−CH3 bond formation is kinetically more favorable, whereas CH3OH formation is thermodynamically more favorable. The overall reaction energy (ΔH298K) for CH4 + La4O7 (C2v) → CH3OH + La4O6 is very exothermic at −25.7 kcal/mol, whereas CH3 radical formation is very endothermic due to the breaking of the La−CH3 bond. For the less stable Cs structure of La4O7, only one pathway (c) leading to one-step CH3OH formation was located. Its physisorption energy is comparable to that of pathway (b), its energy barrier (ΔG‡298K) from the physisorption energy is the lowest at ∼37 kcal/mol, and its chemisorption energy is the most negative. Thus, the O22− site on the less stable type II

direct La−CH3 bond breaking further requires a substantial amount of energy, and the hydrogen abstraction reaction O22− + CH4 → HO2− + CH3 is predicted to be highly endothermic at ∼64 kcal/mol (ΔE0K). The endothermicity for the above reaction is much greater than that predicted for the peroxide site on the La2O3(001) surface of ∼39 kcal/mol by Neurock and co-workers.9 However, they predicted the O−O bond in the peroxide to break upon hydrogen abstraction with a bond length of ∼2.19 Å, and this differs from our predicted O−O bond distances of ∼1.48 Å upon hydrogen transfer. Our IRC calculations for pathways (a) and (b) further suggest that the O−O bond is not broken upon hydrogen transfer and even after La−CH3 bond breaking. As mentioned earlier, similar vibrational analysis or IRC calculation was not performed by Neurock and co-workers to elucidate the nature of their transition state, preventing a reliable comparison with their prediction. A distinct pathway (c) for CH4 activation at the O22− site on the C1 structure of La2O4 was also predicted, where upon hydrogen transfer the O atom bonded with only one La atom is effectively inserted into the CH3−H bond leading to CH3OH formation in a single elementary reaction step. In its physisorption structure, RC (c), CH4 weakly interacts with only that O atom on La2O4, and its physisorption energy (ΔHads,298K) is less negative by >2 kcal/mol at the CCSD(T)/ aVTZ level than those for RC (a) and RC (b). The energy barrier (ΔG‡298K) for TS (c) from RC (c) is substantially higher than those for TS (a) and TS (b) by ∼16 kcal/mol, so the onestep CH3OH formation is kinetically less favorable than La− CH3 bond formation. However, chemisorption for CH3OH formation is very exothermic, whereas La−CH3 bond formation is modestly endothermic, so the former is thermodynamically more favorable. The desorption energy (ΔH298K) of CH3OH from La2O3 is ∼22 kcal/mol, so CH3OH interacts much more strongly with La2O3 than CH4. The overall reaction energy (ΔH298K) for CH4 + La2O4 → CH3OH + La2O3 is quite exothermic at −32.2 kcal/mol, so this reaction is thermodynamically very favorable and can be considered as irreversible due to the much higher energy barrier for its reverse reaction. This reaction is also likely to be a side reaction, where CH3OH can be further transformed into carbon oxides by decomposition or oxidation. 27957

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Figure 4. Potential energy surface (ΔE0K, kcal/mol) for CH4 activation by La6O10, calculated at the B3LYP/aVDZ level. Relative energies for TS (c) and (d) were estimated.

the associated La atom relatively more exposed than that on its less stable type II structure. For both structures of La6O10, the relevant La atom is less exposed due to its higher coordination than La4O7, so La−CH3 bond formation does not occur even for its more stable type I structure. CH3OH formation at the O22− site on the type II structure for La4O7 and La6O10 is much faster than that on the type I structure, so the less stable type II structure are more reactive for CH3OH formation. For La4O7, CH3OH formation at the O22− site on the type II structure is even faster than La−CH3 bond formation, although at high temperature these rate constants are closer to each other. Neurock and co-workers9 previously proposed the O22− site as the active oxygen site for the La2O3-catalyzed OCM reaction. However, our studies suggest that the O22− site is much less reactive with CH4 than the O2− site.12 In addition, CH3OH can form in a single elementary reaction step at the O22− site, which is considered as a side reaction. CH3OH formation at the O22− site is not only thermodynamically more favorable than the alternative pathway, it is often the only pathway available at this site. Thus, although it is possible for the O22− site to play a positive role by first forming the La−CH3 bond and then breaking this bond leading to CH3 radical formation, the O22− site can also play a negative role by one-step CH3OH formation leading to byproducts. These are in contrast to the central role for the O22− site in the La2O3-catalyzed OCM reaction proposed by Neurock and co-workers.9 Our previous study12 predicted the O2− site to have considerable reactivity with CH4, where La−CH3 bond formation and breaking can occur. Recent works16,17 on relevant catalytic systems also suggested metal−CH3 bond formation and breaking as relevant steps for CH3 radical formation, which may be further assisted by O2,17 although this aspect has yet to be verified by theory. Considering the catalytic and redox nature of the OCM reaction, it is very unlikely that the active site is a single static oxygen species, and reactions of some active oxygen species with CH4 followed by further reactions with O2 for catalyst regeneration can be expected, where the O2− site as well as the O22− site may play a role at different stages of the catalytic cycle. Thus, further investigation on the possible catalytic cycles in the OCM reaction is required

structure of La4O7 is more reactive than that on the more stable type I structure, although the former can only lead to CH3OH formation. 3.2.3. La6O10. Figure 4 presents the PES for CH4 activation at the O22− site on the (Cs a) and (Cs b) structures of La6O10, where only transition states for CH3OH formation were located. The energy barrier (ΔG‡298K) for the more stable (Cs a) structure is ∼9 kcal/mol higher than that for the (Cs b) structure (B3LYP/aVDZ), so the less stable type II structure is more reactive, similar to the case of La4O7. Two product complexes, PC (c) and PC (d) for La−CH3 bond formation were located for the (Cs a) structure. Since the energy barrier for La−CH3 bond formation for La4O7 (C2v) is only slightly higher than its chemisorption energy, we estimate energy barriers for La−CH3 bond formation for La6O10 (Cs a) to be ∼37 kcal/mol, considerably lower than that for CH3OH formation for this structure, but comparable to that for CH3OH formation for the less stable (Cs b) structure. 3.3. Role of the Peroxide Site. As shown in Table 1, physisorption energies at 298 K are about −3 kcal/mol when CH4 interacts with both the La and O atoms and about −1 kcal/mol when CH4 interacts with only the O atom. Those at 1073 K are close to zero for the former, and ∼2 kcal/mol for the latter, so there is no physisorption at high temperature. Physisorption energies do not depend strongly on the cluster structure, due to the nonspecificity of the van der Waals interaction. On the other hand, energy barriers depend much more strongly on the temperature due to the significant entropy effect. At 298 K, CH4 activation at the O2− site on La4O6 and La6O9 occurs much faster than that at the O22− site on La4O7 and La6O10 by 6−14 orders of magnitude, but at 1073 K the former is faster by only 1−4 orders of magnitude, so the temperature effect on the relative reactivity is dramatic. For the O22− site on the more stable type I structure of La4O7, both La−CH3 bond formation and CH3OH formation can occur, and the former is much faster than the latter. On other structures of La4O7 and La6O10, only CH3OH formation can occur. This can be explained by the fact that La−CH3 formation requires the access of CH4 to both the La and O atoms in the hydrogen transfer, whereas CH3OH formation requires only the O atom. On the more stable type I structure of La4O7, the O atoms at the O22− site is less exposed making 27958

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(5) Huang, P.; Zhao, Y.; Zhang, J.; Zhao, T.; Zhu, Y.; Sun, Y. Exploiting Shape Effects of La2O3 Nanocatalysts for Oxidative Coupling of Methane Reaction. Nanoscale 2013, 5, 10844−10848. (6) Noon, D.; Seubsai, A.; Senkan, S. Oxidative Coupling of Methane by Nanofiber Catalysts. ChemCatChem 2013, 5, 146−149. (7) Lunsford, J. H. Catalytic Conversion of Methane to More Useful Chemicals and Fuels: A Challenge for the 21st Century. Catal. Today 2000, 63, 165−174. (8) Schwarz, H. Chemistry with Methane: Concepts Rather than Recipes. Angew. Chem., Int. Ed. 2011, 50, 10096−10115. (9) Palmer, M. S.; Neurock, M.; Olken, M. M. Periodic Density Functional Theory Study of Methane Activation over La2O3: Activity of O2−, O−, O22−, Oxygen Point Defect, and Sr2+−Doped Surface Sites. J. Am. Chem. Soc. 2002, 124, 8452−8461. (10) Palmer, M. S.; Neurock, M.; Olken, M. M. Periodic Density Functional Theory Study of the Dissociative Adsorption of Molecular Oxygen over La2O3. J. Phys. Chem. B 2002, 106, 6543−6547. (11) Weng, W. Z.; Wan, H. L.; Li, J. M.; Cao, Z. X. Laser-Induced Formation of Metal-Peroxide Linkages on the Surface of Lanthanum Sesquioxide under Oxygen. Augew. Chem., Int. Ed. 2004, 43, 975−977. (12) Lei, Y.; Chu, C.; Li, S.; Sun, Y. Methane Activations by Lanthanum Oxide Clusters. J. Phys. Chem. C 2014, 118, 7932−7945. (13) Purvis, G. D., III; Bartlett, R. J. A Full Coupled−Cluster Singles and Doubles Model: The Inclusion of Disconnected Triples. J. Chem. Phys. 1982, 76, 1910−1918. (14) Watts, J. D.; Gauss, J.; Bartlett, R. J. Coupled−Cluster Methods with Noniterative Triple Excitations for Restricted Open-Shell Hartree-Fock and Other General Single Determinant Reference Functions. Energies and Analytical Gradients. J. Chem. Phys. 1993, 98, 8718−8733. (15) Bartlett, R. J.; Musial, M. Coupled-Cluster Theory in Quantum Chemistry. Rev. Mod. Phys. 2007, 79, 291−352. (16) Guo, X.; Fang, G.; Li, G.; Ma, H.; Fan, H.; Yu, L.; Ma, C.; Wu, X.; Deng, D.; Wei, M.; et al. Direct, Nonoxidative Conversion of Methane to Ethylene, Aromatics, and Hydrogen. Science 2014, 344, 616−619. (17) Kwapien, K.; Paier, J.; Sauer, J.; Geske, M.; Zavyalova, U.; Horn, R.; Schwach, P.; Trunschke, A.; Schlögl, R. Sites for Methane Activation on Lithium-Doped Magnesium Oxide Surfaces. Angew. Chem., Int. Ed. 2014, 53, 1−6. (18) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (19) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. Electron Affinities of the First-Row Atoms Revisited. Systematic Basis Sets and Wave Functions. J. Chem. Phys. 1992, 96, 6796−6806. (20) Dolg, M.; Stoll, H.; Preuss, H. Energy-Adjusted Ab Initio Pseudopotentials for the Rare Earth Elements. J. Chem. Phys. 1989, 90, 1730−1734. (21) Cao, X.; Dolg, M. Segmented Contraction Scheme for SmallCore Lanthanide Pseudopotential Basis Sets. J. Mol. Struct. (THEOCHEM) 2002, 581, 139−147. (22) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868 (Errata: idib. 1997, 78, 1396).. (23) Cao, X.; Dolg, M. Valence Basis Sets for Relativistic EnergyConsistent Small-Core Lanthanide Pseudopotentials. J. Chem. Phys. 2001, 115, 7348−7355. (24) Deegan, M. J. O.; Knowles, P. J. Perturbative Corrections to Account for Triplet Excitations in Closed and Open Shell Coupled Cluster Theories. Chem. Phys. Lett. 1994, 227, 321−326. (25) Knowles, P. J.; Hampel, C.; Werner, H.-J. Coupled Cluster Theory for High Spin, Open Shell Reference Wave Functions. J. Chem. Phys. 1993, 99, 5219−5227; Erratum. J. Chem. Phys. 2000, 112, 3106− 3107. (26) Rittby, M.; Bartlett, R. J. An Open-Shell Spin-Restricted Coupled Cluster Method: Application to Ionization Potentials in Nitrogen. J. Phys. Chem. 1988, 92, 3033−3036. (27) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Optimization of Gaussian-Type Basis Sets for Local Spin Density

to reveal the detailed catalytic mechanisms and to provide further insight into this complex reaction.

4. CONCLUSIONS Quantum chemical calculations were carried out to study CH4 activation at the O22− site on La2O3-based catalysts using La2O3 clusters as catalyst models. Our calculations show that the O22− site is much less reactive with CH4 than the O2− site from our previous study. More importantly, we reveal two distinct CH4 activation pathways at the O22− site, one leading to La−CH3 bond formation similar to the case of the O2− site, and the other leading to one-step CH3OH formation unique for the O22− site. For many of the O22− sites studied, only the CH3OH formation pathway may occur. Our findings suggest that the O22− site on the La2O3-based catalysts, previously suggested as the active site for the oxidative coupling of methane (OCM) reaction, is not likely to play such a central role. However, considering the catalytic and redox nature of the OCM reaction, the O2− site as well as the O22− site may play a role at some stages in the catalytic cycle.

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ASSOCIATED CONTENT

S Supporting Information *

Complete lists for refs16 and 30. Figures: Potential energy surface for CH4 activation on La6O9; Detailed structures of the stationary states on the potential energy surfaces; and Potential energy surfaces for CH4 activation on La2O4 from intrinsic reaction coordinate calculations. Tables: Relative energies at 298 and 1073 K; O−O stretching frequencies; Deviations of DFT energies from CCSD(T) results; and Cartesian coordinates for the stationary states. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-21-20350994. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Hundred Talents” program of Chinese Academy of Sciences (Contract Number Y224591401), the National Science Foundation of China (Contract Number 21473233), and the “Frontier Science” program of Shell Global Solutions International B.V. (Contract Number PT32281). Part of this work was performed on the supercomputer at Shanghai Advanced Research Institute.

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REFERENCES

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