Computational studies of the reaction of methanol at aluminosilicate Brønsted acid sites

Abstract

Quantum chemical calculations of the reaction of two methanol molecules at model aluminosilicate Brønsted acid sites are reported. The reaction, involving dehydration of one methanol molecule, resulting in formation of a surface bound CH₃ group was found to have an activation barrier of 130–160 kJ mol^–1 at the MP2/6-31G**//HF/3–21G level of theory when calculated using model acid sites containing one and two aluminosilicate tetrahedral centres. The calculated activation barrier was found to be substantially lower than those reported previously for a similar reaction involving only one methanol molecule. Different ‘2T’ models are used to analyse the importance of the difference in the proton affinity of neighbouring lattice oxygen sites involved in the dehydration process. The calculated difference in the reaction energetics strongly suggests that, as reported previously by Kramer et al., (G. J. Kramer, R. A. van Santen, C. A. Emeis and A. K. Nowak, Nature (London) 1993, 363, 529) the difference in proton affinities of lattice oxygen sites involved in a concerted reaction is an important, structure-sensitive parameter in zeolite reactivity.