Anatase type extra-framework titanium in TS-1: A vital factor influencing the catalytic activity toward styrene epoxidation

Abstract

Anatase type extra-framework titanium in titanium silicate-1 (TS-1) molecular sieves has been observed to cause adverse catalytic impediments such as carbon–carbon bond cleavage and low H2O2 efficiency during the epoxidation of terminal alkenes. In this study, a series of TS-1 sieves were synthesized, and used as catalysts for styrene epoxidation. Among the catalysts, the TS-1 with a Si/Ti molar ratio of 35 and 0.21mol% of anatase type extra-framework titanium exhibited the best styrene conversion and styrene oxide (SO) selectivity. The results suggests that, framework Ti species have no effect on the stabilization of SO selectivity, whereas the synergetic interaction between the framework Ti species and the anatase type extra-framework titanium is essential for the outstanding catalytic performance of TS-1 in styrene epoxidation. In addition, our results demonstrate that the usage of tetraethyl orthosilicate (TEOS), tetrabutyl titanate (TBOT), and isopropyl alcohol in the synthesis of TS-1 does not guarantee the absence of extra-framework Ti species. Instead, the asynchronous degrees of hydrolysis between TEOS and TBOT might account for the formation of the anatase type extra-framework titanium. The influence of H2O2 amount, reaction temperature and time, as well as the types of solvent, oxidant, and catalyst amount, on the catalytic performance of TS-1 with anatase type extra-framework titanium was likewise investigated.