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.