Effect of silylation temperature on the physicochemical properties and catalytic performance of Ti-containing hexagonal mesoporous silica

Abstract

Ti-HMS (hexagonal mesoporous silica, HMS) was vapor silylated at 150–450°C. The samples were characterized by Si²⁹ CP/MAS NMR, FTIR, TG, XRD, and N₂-adsorption, respectively, as well as evaluated by the continuous epoxidation of propylene with cumene hydroperoxide (CHP) as oxidant. There showed only a peak at 14 ppm assigned to (CH₃)₃Si*OSi(OSi)₃ species in NMR spectra when silylated at low temperatures. The samples at higher temperatures (>250°C) gave an additional peak at 20 ppm, which was assigned to novel titanium species of (CH₃)₃Si*OTi(OSi)₃. The intensity of peaks relevant to trimethyl groups increased with the silylation temperature at 150–350°C and decreased at 350–450°C due to decomposing of organic groups. The intensity reached the highest point at 350°C. The catalytic results showed that the propylene oxide yield also changed as the same trend and reached the longest life of 100% PO yield at 350°C silylation. In conclusion, 350°C is an optimal silylation temperature.