日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

Titanosilicates containing tetrahedral Ti ions in zeolite framework prove to be attractive catalysts owing to their capability of oxidizing a variety of organic compounds with clean oxidant of hydrogen peroxide in liquid-phase. In the last two decades, the studies have been focused on two representative titanosilicates, MFI type TS-1 and BEA type Ti-Beta. However, these two titanosilicates have a lot of difficult problems to tackle. Medium-pore TS-1 lacks the ability in the oxidation of bulky substrates, and there is still a large space to improve its specific activity per Ti site. Ti-Beta, on the other hand, encounters serious disadvantages of easy leaching of Ti active species due to its structural stacking faults. From the viewpoint of developing new heterogeneous catalytic processes for alkene epoxidation, it is urgent to prepare more efficient titanosilicate. Very recently, we have succeeded in incorporating Ti into MWW zeolite having a very unique pore structure. Both hydrothermal synthesis using boric acid as a crystallization-supporting agent and postsynthesis through a reversible structural conversion result in Ti-MWW catalysts extremely active for alkene epoxidation. Ti-MWW is also converted to the catalysts with accessible reaction spaces suitable for bulky reactions. This paper reviews our recent studies on the preparation and catalytic properties of Ti-MWW.