Acid\nCatalysis Mediated by Aqueous Hydronium Ions\nFormed by Contacting Zeolite Crystals with Liquid Water

Zeolites\nare crystalline microporous aluminosilicates widely used\nas solid acids in catalytic routes to clean and sustainable energy\ncarriers and chemicals from biogenic and fossil feedstocks. This study\naddresses how zeolites act as weak polyprotic acids and dissociate\nto form extra-crystalline hydronium (H<sub>3</sub>O<sup>+</sup>) ions\nin liquid water. The extent of their dissociation depends on the energy\nrequired to form the conjugate framework anions, which becomes unfavorable\nas the extent of dissociation increases intracrystalline charge densities\nbecause repulsive interactions ultimately preclude the detachment\nof all protons as catalytically relevant H<sub>3</sub>O<sup>+</sup>(aq) ions. The extent of dissociation is accurately described using\nelectrostatic repulsion formalisms that account for aqueous H<sub>3</sub>O<sup>+</sup> concentrations for all zeolite concentrations,\nAl densities, and frameworks. Probed by hydrolysis of cellulose, the\nmost abundant biogenic polymer, this study demonstrates that zeolites\ncatalyze this reaction exclusively through the formation of the extra-crystalline\nH<sub>3</sub>O<sup>+</sup> ions at rates strictly proportional to\ntheir concentrations in the aqueous phase, irrespective of their provenance\nfrom zeolites differing in framework structure or Al content, without\nthe purported involvement of acid sites at extracrystalline surfaces\nor intervening formation of smaller cellulose oligomers. The results\nand mechanistic interpretations seamlessly and rigorously bridge the\nchemistry of solid and liquid acids in aqueous media, while resolving\nthe enduring puzzle of solid acids that catalyze transformations of\nsubstrates that cannot enter the voids where acid sites reside.