Silica functionalised sulphonic acid groups: synthesis, characterization and catalytic activity in acetalization and acetylation reactions

Abstract

Abstract A simple synthesis procedure for the immobilization of propyl thiol groups on silica is investigated using various concentrations of 3-mercapto propyl trimethoxy silane (3-MPTS) in the range of 5–40%. The thiol group functionalised silicas (SiO2–SH) were then oxidized to Bronsted sulphonic acid silica materials (SiO2–SO3H) using aqueous H2O2 as oxidizing agent. The surface structures of the functionalized catalysts were analyzed by a series of characterization techniques like elemental analysis, FTIR, TG-DTA, Surface area measurements, XPS, 13 C CP MAS NMR and 29 Si MAS NMR. The 13 C CP MAS NMR analyses confirm that disulphide species are not formed under the present preparation condition of catalysts. The acidity of the synthesized catalysts were further confirmed by the temperature programmed desorption of ammonia. The catalytic activity of the sulphonic acid functionalized silicas was evaluated in the liquid phase acetalization of ethyl acetoacetate with ethylene glycol and in the acetylation of anisole with acetic anhydride. The catalysts were found to be active in the acetalization reaction, which needs mild acidic sites, while the acetylation reactions gave lower activity, probably it needs stronger acid sites. A 30 wt.% SO3H loaded silica (SiO2–SO3H30) was recycled two times in the acetalization of ethyl acetoacetate and no major change in the conversion of EAA and selectivity to fructone is seen, which further argue against the possibility of leaching of the anchored sulphur containing species during reactions.