COMPUTATIONAL STUDIES OF THE REACTION OF METHANOL AT ALUMINOSILICATE BRONSTED ACID SITES

Quantum chemical calculations of the reaction of two methanol molecule s at model aluminosilicate Bronsted acid sites are reported. The react ion, involving dehydration of one methanol molecule, resulting in form ation of a surface bound CH3 group was found to have an activation bar rier of 130-160 kJ mol(-1) at the MP2/6-31G*//HF/3-21G level of theor y when calculated using model acid sites containing one and two alumin osilicate tetrahedral centres. The calculated activation barrier was f ound to be substantially lower than those reported previously for a si milar reaction involving only one methanol molecule. Different '2T' mo dels are used to analyse the importance of the difference in the proto n affinity of neighbouring lattice oxygen sites involved in the dehydr ation process. The calculated difference in the reaction energetics st rongly 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-sen sitive parameter in zeolite reactivity.