SOLVENT-ASSISTED PROTON-TRANSFER IN CATALYSIS BY ZEOLITE SOLID ACIDS

Intermolecular proton transfer in the gas phase is usually strongly di sfavoured because the charged products are highly unstable. It is prom oted in aqueous solution, however, because the high dielectric constan t (epsilon = 78.3) of water allows efficient stabilization of the corr esponding cations and anions. Zeolites-microporous catalysts used in p etroleum refining and the synthesis of chemical feedstocks-provide ano ther medium for proton-transfer reactions(1-3), because their anionic aluminosilicate frameworks are highly acidic. The low dielectric const ant of zeolites (epsilon approximate to 1.6; ref. 3) suggests, however , that such processes in the zeolite channels should involve concerted action rather than strong charge separation, and thus resemble gas-ph ase reactions. Here we demonstrate that solution-like proton-transfer behaviour can be induced in zeolites. We find that the co-injection of nitromethane into a catalytic flow reactor clearly enhances the conve rsion of methanol, isopropanol and acetone over the zeolite catalyst H ZSM-5. Conservation of nitromethane during-the course of reaction and its effects on the reactant-zeolite interaction complex seen by solid- state NMR indicate that nitromethane behaves in a manner similar to po lar solvents: it promotes proton transfer by stabilizing ion-pair stru ctures. These findings suggest that rationally selected solvents might provide a simple means to increase the efficiency of these industrial ly important catalysts.