A critical assessment of the roles of water molecules and solvated ions in acid-base-catalyzed reactions at solid-water interfaces

Abstract

Solid-aqueous interfaces and phenomena occurring at those interfaces are ubiquitously found in a plethora of chemical systems. When it comes to heterogeneous catalysis, however, our understanding of chemical transformations at solid-aqueous interfaces is relatively limited and primitive. This review phenomenologically describes a selection of water-engendered effects on the catalytic behavior for several prototypical acid-base-catalyzed reactions over solid catalysts, and critically assesses the general and special roles of water molecules, structural moieties derived from water, and ionic species that are dissolved in it, with an aim to extract novel concepts and principles that underpin heterogeneous acid-base catalysis in the aqueous phase. For alcohol dehydration catalyzed by solid Brønsted acids, rate inhibition by water is most typically related to the decrease in the acid strength and/or the preferential solvation of adsorbed species over the transition state as water molecules progressively solvate the acid site and form extended networks wherein protons are mobilized. Water also inhibits dehydration kinetics over most Lewis acid-base catalysts by competitive adsorption, but a few scattered reports reveal substantial rate enhancements due to the conversion of Lewis acid sites to Brønsted acid sites with higher catalytic activities upon the introduction of water. For aldol condensation on catalysts exposing Lewis acid-base pairs, the addition of water is generally observed to enhance the rate when C–C coupling is rate-limiting, but may result in rate inhibition by site-blocking when the initial unimolecular deprotonation is rate-limiting. Water can also promote aldol condensation on Brønsted acidic catalysts by facilitating inter-site communication between acid sites through hydrogen-bonding interactions. For metallozeolite-catalyzed sugar isomerization in aqueous media, the nucleation and networking of intrapore waters regulated by hydrophilic entities causes characteristic enthalpy-entropy tradeoffs as these water moieties interact with kinetically relevant hydride transfer transition states. The discussed examples collectively highlight the utmost importance of hydrogen-bonding interactions and ionization of covalently bonded surface moieties as the main factors underlying the uniqueness of water-mediated interfacial acid-base chemistries and the associated solvation effects in the aqueous phase or in the presence of water. A perspective is also provided for future research in this vibrant field. Through illustrative examples, this review highlights the utmost importance of hydrogen-bonding interactions and interfacial ionic species as the main factors underlying acid-base catalysis and the associated solvation effects at the solid-aqueous interfaces.