Optimizing the surface distribution of acid sites for cooperative catalysis in condensation reactions promoted by water

Abstract

Addition of small amounts of water to the organic solvent may result in different promoting or inhibiting effects in aldol condensation reactions depending on the distribution of active sites on the surface. In a combination of liquid-phase catalytic activity measurements, spectroscopies, and theoretical calculations, we have shown that water clusters around the acid site can form bridges with a ketone molecule located close to the activated carbonyl on another site. When the distance between the two acid sites is greater than a distance corresponding to about two water molecules, the efficiency of the bond polarization of the ketone by a remotely located acid site decreases significantly, and the presence of water no longer promotes the reaction. This work provides a quantitative assessment of how solvent molecules can bridge isolated active sites to enable cooperative catalysis, which offers an additional dimension for engineering catalysis at the nanoscale. • Water promotes the aldol condensation reaction by remote bond polarization • Adsorbed water on the acid site leads to H + delocalization and effective polarization • Proton delocalization is limited to about two water molecules from the original site • Water transfers the polarizing action of the acid site to the C=O bond via H bonds Nature accomplishes catalytic process with exquisite control by altering the distribution of active sites in a confining pocket and by regulating the structure of occluded solvents. In such systems, the ubiquitous water may influence reaction rates and selectivity by stabilizing reaction intermediates and transition states via a network of H bonds. Inspired by nature, here we aim at providing quantitative support and mechanistic insight into the role of water-promoting C–C bond formation on an acid catalyst by combining spectroscopic analysis, catalytic reaction measurements, and theoretical calculations. This set of experimental and computational studies reveals that the interaction between water and polar species on the surface generates diverse environments surrounding the active sites that facilitate the reaction. These findings provide an extra dimension for tailoring catalytic phenomena in industrially relevant reactions. This work emphasizes the ability of aqueous media to regulate reactivity. Upon accumulation of water around the sulfonic acid site, the proton is delocalized and transferred to two vicinal water molecules that act as a polarizing center. When the distance between two acid sites is <1 nm, water transfers the polarizing action of the acid site to the C=O bond via an H-bonded network, facilitating the C–C bond formation in condensation reactions of relevant to biomass conversion.