Catalytic Performance of Sulfated Silica MCM-41 for the Cyclization of Pseudoionone to Ionones

A series of sulfated silica MCM-41 samples denoted as SM and ASM were prepared by calcination at different temperatures using H2SO4 and (NH4)2SO4 solution as the promoter, respectively. The mesostructure, sulfation process, and surface acidity of the two samples were investigated by N2 adsorption-desorption, temperature-programmed desorption of adsorbed NH3 (NH3-TPD), thermogravimetric analysis (TG-DTG), elemental analysis, and Fourier transform infrared (FT-IR) spectroscopy. The activity of the samples for the traditional liquid acid-catalyzed cyclization reaction of pseudoionone to ionones was investigated. For the SM and ASM samples calcined at 450°C (denoted as SM-450 and ASM-450, respectively), SO42- was successfully chelated onto the silica surface in bidentate complex forms, and no free H2SO4 molecules existed as the original promoter or intermediate of thermal decomposition inside the mesopores. The in situ pyridine adsorption FT-IR and NH3-TPD measurements revealed that ASM-450 contained both Brønsted and Lewis acid sites of weak strength. The catalytic performance of SM-450 and ASM-450 was comparable with that of the commercial acid resin Amberlyst-15, and the chelating structure of SO42-/SiO2, which was generally considered to be inactive, was responsible for the high activity. ASM-450 showed better catalytic performance compared with Amberlyst-15 at relatively low temperature, and it could be reused 5 times with good activity.