Acid-Functionalized Amorphous Silica by Chemical Grafting−Quantitative Oxidation of Thiol Groups

Abstract

This work describes the synthesis and structural features of functionalized amorphous silica with sulfonic acid groups. The approach followed involved the condensation of (3-mercaptopropyl)trimethoxysilane or (3-mercaptopropyl)methyldimethoxysilane and/or phenyltrimethoxysilane on the hydroxyl groups of the silica substrate followed by oxidation with hydrogen peroxide. This oxidation reaction was effective, although some lixiviation of sulfur occurred, and the amount of S released depended on the hydrophobicity of the surface and the temperature of the treatment. The extent of oxidation of the thiol precursor, determined by photoelectron spectroscopy (XPS), was moderate at ambient temperature and much more effective at 333 K. The XPS technique proved to be extremely useful not only to discriminate between the −SH and −SO3H, whose chemical shifts differ by 5 eV, but also to quantify the proportion of these species and hence the conditions that make the oxidation process quantitative. The solid samples prepared according to this grafting−oxidation methodology exhibited strong acid sites, as revealed by their performance in the esterification reaction of acetic acid by methanol in liquid phase. Activity data indicated that the reaction is accelerated in the presence of the sulfonic acid-functionalized silica catalyst, being higher than that of the reference Nafion silica composite. Finally, this grafting procedure and the methodologies to quantify and test these acid-functionalized silica materials can be applied to other substrates.