Acid Catalysis Mediated by Aqueous Hydronium Ions Formed by Contacting Zeolite Crystals with Liquid Water

Abstract

Zeolites are crystalline microporous aluminosilicates widely used as solid acids in catalytic routes to clean and sustainable energy carriers and chemicals from biogenic and fossil feedstocks. This study addresses how zeolites act as weak polyprotic acids and dissociate to form extra-crystalline hydronium (H₃O⁺) ions in liquid water. The extent of their dissociation depends on the energy required to form the conjugate framework anions, which becomes unfavorable as the extent of dissociation increases intracrystalline charge densities because repulsive interactions ultimately preclude the detachment of all protons as catalytically relevant H₃O⁺(aq) ions. The extent of dissociation is accurately described using electrostatic repulsion formalisms that account for aqueous H₃O⁺ concentrations for all zeolite concentrations, Al densities, and frameworks. Probed by hydrolysis of cellulose, the most abundant biogenic polymer, this study demonstrates that zeolites catalyze this reaction exclusively through the formation of the extra-crystalline H₃O⁺ ions at rates strictly proportional to their concentrations in the aqueous phase, irrespective of their provenance from zeolites differing in framework structure or Al content, without the purported involvement of acid sites at extracrystalline surfaces or intervening formation of smaller cellulose oligomers. The results and mechanistic interpretations seamlessly and rigorously bridge the chemistry of solid and liquid acids in aqueous media, while resolving the enduring puzzle of solid acids that catalyze transformations of substrates that cannot enter the voids where acid sites reside.