UNDERSTANDING THE ACID BEHAVIOR OF ZEOLITES FROM THEORY AND EXPERIMENT

ZEOLITES are microporous aluminosilicates which, in their protonated f orm, act as solid catalysts1, and are widely used in the oil and petro chemical industries for processes such as cracking, isomerization and alkylation if hydrocarbons2. The proposed mechanisms3-5 of these proce sses mostly involve proton transfer and formation of carbenium or carb onium ions as reactive intermediates, but the detailed function of the zeolite and in particular the relation between acidity and catalytic activity is not well understood. Here we report experimental and theor etical studies of denterium-hydrogen exchange between deuterated metha ne and protonated zeolites - a prototypical heterogeneous catalytic re action between a hydrocarbon and an acid zeolite. We monitored this sl ow exchange reaction in two different zeolites using infrared spectros copy, and used ab initio quantum chemistry calculations to determine b oth the reaction mechanism and the acidity-activity relationship. Comb ining our theoretical results with recent estimates8-11 of the acidity differences within zeolites enables us to reproduce the experimentall y observed reaction rates and thus to obtain a detailed microscopic pi cture of this heterogeneous catalytic process.