Aqueous-sensitive reaction sites in sulfonic acid-functionalized mesoporous silicas

Abstract

Local differences in surface hydrophilicities/hydrophobicities of propyl- and arene-sulfonic-acid modified mesoporous silica and organosilica catalysts have been compared and correlated with their bulk catalytic properties for aqueous-sensitive organic reactions. Syntheses of propyl- and arene-SO3H-modified mesoporous silicas and organosilicas yield materials with different hydrophilicities, especially when ethylsiloxane moieties are incorporated into the silica frameworks. Solid-state two-dimensional (2D) 13C{1H} and 29Si{1H} heteronuclear correlation (HETCOR) NMR spectra prove that the incorporation of hydrophobic ethylsiloxane groups into functionalized mesoporous silica frameworks result in reduced interactions of adsorbed water with the silica framework in general and, importantly, in the immediate vicinities of the SO3H active sites. The hydrophilic/hydrophobic character of the surface, as well as the active site properties depend on the functional species attached. Propyl-sulfonic acid moieties are less acidic but more hydrophobic than arene-SO3H species, leading to superior overall activities for water-mediated acid-catalyzed organic reactions. The etherification of vanillyl alcohol (4-hydroxy-3-methoxybenzylalcohol) with 1-hexanol to yield 4-hydroxy-3-methoxybenzyl-1-hexyl ether is shown to proceed significantly more effectively on SO3H-modified mesoporous organosilicas, compared to wholly siliceous mesoporous supports. The correlation of macroscopic adsorption and reaction results with 2D NMR measurements allows the hydrophilic/hydrophobic surface properties of the mesoporous support to be optimized with respect to water-retention capacities and activities for water-sensitive organic reactions.