UNDERSTANDING THE ACID BEHAVIOR OF ZEOLITES FROM THEORY AND EXPERIMENT

ZEOLITES are microporous aluminosilicates which, in their protonated form, act as solid catalysts1, and are widely used in the oil and petrochemical industries for processes such as cracking, isomerization and alkylation if hydrocarbons2. The proposed mechanisms3-5 of these processes mostly involve proton transfer and formation of carbenium or carbonium 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 theoretical studies of denterium-hydrogen exchange between deuterated methane and protonated zeolites - a prototypical heterogeneous catalytic reaction between a hydrocarbon and an acid zeolite. We monitored this slow exchange reaction in two different zeolites using infrared spectroscopy, and used ab initio quantum chemistry calculations to determine both the reaction mechanism and the acidity-activity relationship. Combining our theoretical results with recent estimates8-11 of the acidity differences within zeolites enables us to reproduce the experimentally observed reaction rates and thus to obtain a detailed microscopic picture of this heterogeneous catalytic process.