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J. Am. Chem. Soc. 1998, 120, 564-572
Methane-Methanol Conversion by MnO+, FeO+, and CoO+: A Theoretical Study of Catalytic Selectivity Kazunari Yoshizawa,*,† Yoshihito Shiota,† and Tokio Yamabe†,‡ Contribution from the Department of Molecular Engineering, Kyoto UniVersity, Sakyo-ku, Kyoto, 606-01 Japan, and Institute for Fundamental Chemistry, 34-4 Takano-Nishihiraki-cho, Sakyo-ku, Kyoto, 606 Japan ReceiVed May 27, 1997
Abstract: The entire reaction pathway for the gas-phase methane-methanol conversion by late transitionmetal-oxide ions, MnO+, FeO+, and CoO+, is studied using an ab initio hybrid (Hartree-Fock/density-functional) method. For these oxo complexes, the methane-methanol conversion is proposed to proceed via two transition states (TSs) in such a way MO+ + CH4 f OM+(CH4) f [TS1] f HO-M+-CH3 f [TS2] f M+(CH3OH) f M+ + CH3OH, where M is Mn, Fe, and Co. A crossing between high-spin and low-spin potential energy surfaces occurs both at the entrance channel and at the exit channel for FeO+ and CoO+, but it occurs only once near TS2 for MnO+. The activation energy from OMn+(CH4) to HO-Mn+-CH3 via TS1 is calculated to be 9.4 kcal/mol, being much smaller than 22.1 and 30.9 kcal/mol for FeO+ and CoO+, respectively. This agrees with the experimentally reported efficiencies for the reactions. The excellent agreement between theory and experiment indicates that HO-M+-CH3 plays a central role as an intermediate in the reaction between MO+ and methane and that the reaction efficiency is most likely to be determined by the activation energy from OM+(CH4) to HO-M+-CH3 via TS1. We discuss in terms of qualitative orbital interactions why MnO+ (d4 oxo complex) is most effective for methane C-H bond activation. The activation energy from HOM+-CH3 to M+(CH3OH) via TS2 is computed to be 24.6, 28.6, and 35.9 kcal/mol for CoO+, FeO+, and MnO+, respectively. This result explains an experimental result that the methanol-branching ratio in the reaction between MO+ and methane is 100% in CoO+, 41% in FeO+, and < 1% in MnO+. We demonstrate that both the barrier heights of TS1 and TS2 would determine general catalytic selectivity for the methane-methanol conversion by the MO+ complexes.
Introduction The activation of methane1-9 has attracted increased attention in recent years because of its scientific and industrial importance. In particular, methanol converted from methane is likely to be of great interest as an important energy source in the 21st century. An industrial process for the production of methanol from natural gas that mainly consists of methane involves twostep reactions associated with the formation of synthesis gas, i.e., carbon monoxide.6 The so-called Fischer-Tropsch reaction converts carbon monoxide into a mixture of long-chain alkanes and alcohols. The conversion of methane to methanol is also known to occur very efficiently when catalyzed enzymatically, †
Kyoto University. Institute for Fundamental Chemistry. (1) Shilov, A. E. The ActiVation of Saturated Hydrocarbons by Transition Metal Complexes; Reidel: Dortrecht, 1984. (2) (a) Bergman, R. G. Science 1984, 223. 902. (b) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res 1995, 28, 154. (3) Hill, C. L., Ed. ActiVation and Functionalization of Alkanes; Wiley: New York, 1989. (4) Davies, J. A.; Watson, P. L.; Liebman, J. F.; Greenberg, A. SelectiVe Hydrocarbon ActiVation; VCH: New York, 1990. (5) (a) Crabtree, R. H. The Organometallic Chemistry of The Transition Metals; Wiley: New York, 1994. (b) Crabtree, R. H. Chem. ReV. (Washington, D.C.) 1985, 85, 245. (c) Crabtree, R. H. Chem. ReV. (Washington, D.C.) 1995, 95, 987. (6) Gesser, H. D.; Hunter, N. R.; Prakash, C. B. Chem. ReV. 1985, 85, 237. (7) Lunsford, J. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 970. (8) Schneider, J. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1068. (9) Hall, C.; Perutz, R. N. Chem. ReV. (Washington, D.C.) 1996, 96, 3125. ‡
e.g., by methane monooxygenase (MMO).10 We have been interested in the conversion of methane to methanol catalyzed by MMO from a theoretical viewpoint.11 The gas-phase C-H and C-C bond activations of small hydrocarbons (by various transition-metal-oxide ions and bare metal cations) have been experimentally and theoretically investigated in Schwarz’s12 and Armentrout’s13 laboratories. Detailed analyses of various gas-phase reactions of methane, higher alkanes, alkenes, benzene, and other [C, H, O] com(10) Recent reviews on methane monooxygenase and other non-heme diiron enzymes: (a) Wallar, B. J.; Lipscomb, J. D. Chem. ReV. (Washington, D.C.) 1996, 96, 2625. (b) Que, L. Jr.; Dong, Y. Acc. Chem. Res 1996, 29, 190. (c) Feig, A. L.; Lippard, S. J. Chem. ReV. (Washington, D.C.) 1994, 94, 759. (d) Stenkamp, R. E. Chem. ReV. 1994, 94, 715. (e) Lipscomb, J. D. Annu. ReV. Microbiol. 1994, 48, 371. (f) Lippard, S. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 344. (11) (a) Yoshizawa, K.; Hoffmann, R. Inorg. Chem. 1996, 35, 2409. (b) Yoshizawa, K.; Yamabe, T.; Hoffmann, R. New J. Chem. 1997, 21, 151. (c) Yoshizawa, K.; Yokomichi, Y.; Shiota, Y.; Ohta, T.; Yamabe, T. Chem. Lett. 1997, 587. (d) Yoshizawa, K.; Ohta, T.; Shiota, Y.; Yamabe, T. Chem. Lett. 1997, 1213. (e) Yoshizawa, K. J. Biol. Inorg. Chem. 1998, in press. (12) (a) Schro¨der, D.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1990, 29, 1433. (b) Schwarz, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 820. (c) Schro¨der, D.; Fiedler, A.; Hrusˇa´k, J.; Schwarz, H. J. Am. Chem. Soc. 1992, 114, 1215. (d) Fiedler, A.; Hrusˇa´k, J.; Koch, W.; Schwarz, H. Chem. Phys. Lett. 1993, 211, 242. (e) Fiedler, A.; Schro¨der, D.; Shaik, S.; Schwarz, H. J. Am. Chem. Soc. 1994, 116, 10734. (f) Schro¨der, D.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1995, 34, 1973. (g) Wesendrup, R.; Schalley, C. A.; Schro¨der, D.; Schwarz, H. Chem. Eur. J. 1995, 1, 608. (h) Shaik, S.; Danovich, D.; Fiedler, A.; Schro¨der, D.; Schwarz, H. HelV. Chim. Acta 1995, 78, 1393. (i) Ryan, M. F.; Fiedler, A.; Schro¨der, D.; Schwarz, H. J. Am. Chem. Soc. 1995, 117, 2033. (j) Holthausen, M. C.; Fiedler, A.; H. Schwarz, W. Koch, J. Phys. Chem. 1996, 100, 6236. (k) Fiedler, A.; Schro¨der, D.; Schwarz, H.; Tjelta, B. L.; Armentrout, P. B. J. Am. Chem. Soc. 1996, 118, 5047.
S0002-7863(97)01723-X CCC: $15.00 © 1998 American Chemical Society Published on Web 01/06/1998
A Theoretical Study of Catalytic SelectiVity
J. Am. Chem. Soc., Vol. 120, No. 3, 1998 565
Scheme 1
pounds with transition-metal oxides have been carried out. Among these important gas-phase reactions, the methanemethanol conversion by metal-oxide ions is of particular interest since this process may be viewed as the simplest model for alkane hydroxylation. On the basis of experimentally determined isotope effects, Schro¨der and Schwarz12a,b have suggested that a reaction species, HO-Fe+-CH3, is a central intermediate in the reaction between FeO+ and CH4. Armentrout et al.13j have investigated the mechanism and energetics involved in the gas-phase methane-methanol conversion by CoO+ and its reverse reaction. An insertion intermediate, HO-Co+-CH3, has been also suggested to play an important role in this gasphase reaction. However, the efficiency of the reaction of CoO+ with methane is low as compared with that of FeO+. Recently related ion-molecular complexes of M+(CH4) and C-H bond activation of methane have been reported by many authors.14 On methane and C-H bond activation, there has been a large amount of theoretical work by Saillard and Hoffmann,15 Goddard et al.,16 Siegbahn and Blomberg and their collaborators,17 Ziegler et al.,18 Morokuma et al.,19 Sakaki and Ieki,20 and Cundari.21 In a recent paper,22 we have proposed two kinds of entire reaction pathways for the gas-phase methane-methanol conversion by FeO+ on the basis of detailed IRC (intrinsic reaction coordinate)23 analyses. The reaction pathways proposed (13) (a) Aristov, N.; Armentrout, P. B. J. Phys. Chem. 1987, 91, 6178. (b) Shultz, R. H.; Elkind, J. L.; Armentrout, P. B. J. Am. Chem. Soc. 1988, 110, 411. (c) Sunderlin, L. S.; Armentrout, P. B. J. Phys. Chem. 1988, 92, 1209. (d) Geoorgiadis, R.; Armentrout, P. B. J. Phys. Chem. 1988, 92, 7060. (e) Armentrout, P. B.; Beauchamp, J. L. Acc. Chem. Res 1989, 22, 315. (f) van Koppen, P. A. M.; Brodbelt-Lustig, J.; Bowers, M. T.; Dearden, D. V.; Beauchamp, J. L.; Fisher, E. R.; Armentrout, P. B. J. Am. Chem. Soc. 1990, 112, 5663. (g) Armentrout, P. B. Science 1991, 251, 175. (h) van Koppen, P. A. M.; Brodbelt-Lustig, J.; Bowers, M. T.; Dearden, D. V.; Beauchamp, J. L.; Fisher, E. R.; Armentrout, P. B. J. Am. Chem. Soc. 1991, 113, 2359. (i) Clemmer, D. E.; Aristov, N.; Armentrout, P. B. J. Phys. Chem. 1993, 97, 544. (j) Chen, Y.-M.; Clemmer, D. E.; Armentrout, P. B. J. Am. Chem. Soc. 1994, 116, 7815. (k) Clemmer, D. E.; Chen, Y.-M.; Kahn, F. A.; Armentrout, P. B. J. Phys. Chem. 1994, 98, 6522. (14) (a) Trevor, D. J.; Cox, D. M.; Kaldor, A. J. Am. Chem. Soc. 1990, 112, 3742. (b) Irikura, K. K.; Beauchamp, J. L. J. Phys. Chem. 1991, 95, 8344. (c) Ranasinghe, Y. A.; MacMahon, T. J.; Freiser, B. S. J. Phys. Chem. 1991, 95, 7721. (d) Van Zee, R. J.; Li, S.; Weltner, W., Jr. J. Am. Chem. Soc. 1993, 115, 2976. (e) Carrol, J. J.; Weisshaar, J. C. J. Am. Chem. Soc. 1993, 115, 800. (f) Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 10462. (g) Schaller, C. P.; Bonanno, J. B.; Wolczanski, P. T. J. Am. Chem. Soc. 1994, 116, 4133. (h) Zhang, X.-X.; Wayland, B. B. J. Am. Chem. Soc. 1994, 116, 7897. (i) Billups, W. E.; Chang, S.-C.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1995, 117, 1387. (j) van Koppen, P. A. M.; Kemper, P. R.; Bushnell, J. E.; Bowers, M. T. J. Am. Chem. Soc. 1995, 117, 2098. (k) Campbell, M. L. J. Am. Chem. Soc. 1997, 119, 5984.
in the paper proceed via two kinds of important reaction intermediates, HO-Fe+-CH3 and H-Fe+-OCH3, as shown in Scheme 1. Our ab initio (Hartree-Fock/density-functional) calculations showed that the reaction pathway via the hydroxy intermediate (HO-Fe+-CH3) is energetically more favorable than the other one via the methoxy intermediate (H-Fe+OCH3). This result is consistent with the prediction by Schwarz and collaborators that HO-Fe+-CH3 plays a central role in the gas-phase reaction between FeO+ and methane. The Berlin group12f has systematically examined the efficiency and product branching ratio for the gas-phase reactions of various transition-metal-oxide ions with methane. Although early transition-metal-oxide ions, ScO+, TiO+, VO+, and CrO+, do not react with methane, it is well established that oxide ions of late transition metals, MnO+, FeO+, CoO+, and NiO+, react with methane. Table 1 lists only a part of experimental results reported in ref 12f. As seen in the table, MnO+ has a high reaction efficiency, but unfortunately the methanol-branching (15) (a) Saillard, J.-Y.; Hoffmann, R. J. Am. Chem. Soc. 1984, 106, 2006. (b) Hoffmann, R. ReV. Mod. Phys. 1988, 60, 601. (16) (a) Low, J. J.; Goddard, W. A. III J. Am. Chem. Soc. 1984, 106, 8321. (b) Low, J. J.; Goddard, W. A. III J. Am. Chem. Soc. 1986, 108, 6115. (c) Perry, J. K.; Ohanessian, G.; W. A. Goddard, W. A. III J. Phys. Chem. 1993, 97, 5238. (17) (a) Blomberg, M. R. A.; Brandemark, U.; Siegbahn, P. E. M. J. Am. Chem. Soc. 1983, 105, 5557. (b) Blomberg, M. R. A.; Siegbahn, P. E. M.; Nagashima, U.; Wennerberg, J. J. Am. Chem. Soc. 1991, 113, 424. (c) Blomberg, M. R. A.; Siegbahn, P. E. M.; Svensson, M. J. Am. Chem. Soc. 1992, 114, 6095. (d) Siegbahn, P. E. M.; Blomberg, M. R. A. Organometallics 1994, 13, 354. (e) Siegbahn, P. E. M. Organometallics 1994, 13, 2833. (f) Siegbahn, P. E. M. J. Am. Chem. Soc. 1996, 118, 1487. (g) Siegbahn, P. E. M.; Crabtree, R. H. J. Am. Chem. Soc. 1996, 118, 4442. (18) (a) Ziegler, T.; Tschinke, V.; Becke, A. D. J. Am. Chem. Soc. 1987, 109, 1351. (b) Ziegler, T.; Tschinke, V.; Fan, L.; Becke, A. D. J. Am. Chem. Soc. 1989, 111, 9177. (c) Ziegler, T.; Folga, E.; Berces, A. J. Am. Chem. Soc. 1993, 115, 636. (19) (a) Koga, N.; Morokuma, K. J. Phys. Chem. 1990, 94, 5454. (b) Koga, N.; Morokuma, K. J. Am. Chem. Soc. 1993, 115, 6883. (c) Musaev, D. G.; Koga, N.; Morokuma, K. J. Phys. Chem. 1993, 97, 4064. (d) Musaev, D. G.; Morokuma, K.; Koga, N.; Ngyen, K.; Gordon, M. S.; Cundari, T. R. J. Phys. Chem. 1993, 97, 11435. (e) Musaev, D. G.; Morokuma, K. J. Chem. Phys. 1994, 101, 10697. (f) Musaev, D. G.; Morokuma, K. J. Phys. Chem. 1996, 100, 11600. (20) (a) Sakaki, S.; Ieki, M. J. Am. Chem. Soc. 1991, 113, 5063. (b) Sakaki, S.; Ieki, M. J. Am. Chem. Soc. 1993, 115, 2373. (21) (a) Cundari, T. R. J. Am. Chem. Soc. 1992, 114, 10557. (b) Cundari, T. R.; Gordon, M. S. J. Am. Chem. Soc. 1993, 115, 4210. (c) Cundari, T. R. J. Am. Chem. Soc. 1994, 116, 340. (22) Yoshizawa, K.; Shiota, Y.; Yamabe. T. Chem. Eur. J. 1997, 3, 1160. (23) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161. (b) Fukui, K. Acc. Chem. Res 1981, 14, 363.
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Table 1. Reaction Efficiencies φ (%) and Product Branching Ratios (%) for the Reactions of Methane with Late Transition-Metal-Oxide Ionsa MO+
φ
MOH+ + CH3
MCH2+ + H2O
M+ + CH3OH
MnO+ FeO+ CoO+
40 20 0.5
100 57
2
