Solvent-assisted proton transfer in catalysis by zeolite solid acids

Abstract

Intermolecular proton transfer in the gas phase is usually strongly disfavoured because the charged products are highly unstable. It is promoted in aqueous solution, however, because the high dielectric constant (e = 78.3) of water allows efficient stabilization of the corresponding cations and anions. Zeolites—microporous catalysts used in petroleum refining and the synthesis of chemical feedstocks—provide another medium for proton-transfer reactions1,2,3, because their anionic aluminosilicate frameworks are highly acidic. The low dielectric constant of zeolites (e ≈ 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-phase 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 conversion of methanol, isopropanol and acetone over the zeolite catalyst HZSM-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 polar solvents: it promotes proton transfer by stabilizing ion-pair structures. These findings suggest that rationally selected solvents might provide a simple means to increase the efficiency of these industrially important catalysts.