Role of the Zeolitic Environment in Catalytic Activation of Methanol I. S ˇ tich,* ,²,‡ J. D. Gale, § K. Terakura, ¶,⊥ and M. C. Payne | Contribution from JRCAT, Angstrom Technology Partnership (ATP), 1-1-4 Higashi, Tsukuba, Ibaraki 305-0046, Japan, Department of Physics, SloVak Technical UniVersity (STU), IlkoVic ˇ oVa 3, SK-812 19, BratislaVa, SloVakia, Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, U.K., JRCAT, National Institute for AdVanced Interdisciplinary Research (NAIR), 1-1-4 Higashi, Ibaraki 305-8562, Japan, CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332, Japan, and CaVendish Laboratory, Madingley Road, Cambridge CB3 0HE, U.K. ReceiVed September 29, 1998. ReVised Manuscript ReceiVed December 28, 1998 Abstract: We present an extensive study of the initial stages of the methanol to gasoline conversion in the framework of the ab initio molecular dynamics approach. We investigate the effect of different zeolite environments, methanol loading, and temperature and show that, for understanding the initial adsorption and activation of the adsorbed species, all three factors need to be considered simultaneously. The results allow us to develop a simple model for the activation of the methanol molecule, which elucidates the role of both the zeolite framework and the methanol solvent. The zeolite framework plays an active role in methanol protonation. The solvent significantly softens the C-O bond of the methoxonium, rendering it very anharmonic. High mobility of the methoxonium cation, promoted by some zeolite frameworks, prevents it from forming hydrogen bonds with the active sites and the solvent leading to the activation of the methoxonium species. This picture is shown to be consistent with the experimental infrared spectra. 1. Introduction One of the most significant industrial applications of zeolites exploits the ability of the microporous aluminosilicate environ- ment to catalyze the methanol to gasoline (MTG) process. 1 The industrial process proceeds at elevated temperatures (∼700 K) and methanol pressures which correspond to a loading of ∼5-6 methanol molecules per acidic hydroxyl group, 2 which is believed to be the active site. The catalyst used for this process is typically ZSM-5. There is a large volume of experimental evidence that the adsorbed methanol is first dehydrated to dimethyl ether (DME) that subsequently reacts with methanol to form hydrocarbons, either directly or via reconversion to methanol. Furthermore, the experiments suggest a dramatic increase by almost 2 orders of magnitude in DME formation in a narrow temperature range around 500 K. 3 However, the nature of methanol adsorption in the zeolite, the mechanism of dehydration, and formation of the C-C bonds in the hydrocar- bons are still far from understood. Methanol is known from infrared spectroscopy (IR) 2 to be initially adsorbed at acid sites in the zeolite framework. Unfortunately, the IR data cannot be unambiguously interpreted as resulting either from a physisorbed methanol complex 4,5 (Figure 1a) or from a chemisorbed complex 2,6 (Figure 1b), in which case a methoxonium cation (CH 3 - OH 2 + ) is formed. For the dehydration process two different mechanisms have been proposed. In the indirect pathway 7,8 the methyl group is first adsorbed on the acid site which subsequently reacts with the other methanol molecule Here Z stands for the zeolite framework. We assume here that the methanol is chemisorbed at an acid site as the protonated complex is expected to be more susceptible to nucleophilic attack. Alternatively, in the direct pathway 9 ² ATP. ‡ STU. § Imperial College. ¶ NAIR. ⊥ CREST. | Cavendish Laboratory. (1) Meisel, S. L. et al. Chem. Technol. 1976, 6, 86. (2) Mirth, G. et al. J. Chem. Soc., Faraday Trans. 1990, 86, 3039. (3) Wakabayashi, F. et al. Stud. Surf. Sci. Catal. 1996, 105, 1739. (4) Anderson, M. W.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1990, 112, 10. (5) Murray, D. K.; Chang, J.-W.; Haw, J. F. J. Am. Chem. Soc. 1993, 118, 4732. (6) Pope, C. G. J. Chem. Soc., Faraday Trans. 1993, 89, 1139. (7) Ono, Y.; Mori, T. J. Chem. Soc., Faraday Trans. 1981, 77, 2209. (8) Forester, T. R.; Howe, R. F. J. Am. Chem. Soc. 1987, 109, 5076. (9) Bandiera, J.; Nacchache, C. Appl. Catal. 1991, 69, 139. Figure 1. Schematic illustration of the two possible adsorption complexes of methanol at a Brønsted acid site: (a) physisorbed species with weak bonds to the zeolite framework and (b) chemisorbed methoxonium cation (CH3-OH2 + ) involving the transfer of a zeolitic proton. CH 3 -OH 2 + + ZO - f ZO-CH 3 + H 2 O (1) CH 3 -OH + ZO-CH 3 f CH 3 -O-CH 3 + ZO-H (2) 3292 J. Am. Chem. Soc. 1999, 121, 3292-3302 10.1021/ja983470q CCC: $18.00 © 1999 American Chemical Society Published on Web 03/27/1999