A novel titanosilicate with MWW structure: II. Catalytic properties in theselective oxidation of alkenes

The catalytic properties of Ti-MWW catalysts have been studied for the oxid ation of alkenes using hydrogen peroxide or tert-butyl hydroperoxide (TBHP) as an oxidant. Ti-MWW shows superior activity independent of the nature of oxidant when compared to the other Ti-containing catalysts both of micropo rous TS-1 and Ti-MOR and of mesoporous Ti-MCM-41 in the oxidation of cycloh exene. Compared to large-pore Ti-Beta, Ti-MWW is less active than in the ox idation of cyclohexene using H2O2, but its shows comparable activity in the case of TBHP. Ti-MWW is very stable in both the structure and the states o f Ti species in the actual cycle of reaction-regeneration. The framework B species remaining within the Ti-MWW catalysts seem to have little influence on the intrinsic activity of framework Ti species, but they act as Bronste d acid sites contributing to the hydrolysis of the oxide product to form gl ycols. The turnover number (TON) of Ti-MWW for the cyclohexene conversion d ecreases sharply with increasing Ti content, while the TON for the 1-hexene conversion shows volcanic behavior when related to the ri content. It is c larified that the bulky reaction is catalyzed mainly by the Ti species with in the supercages and the exterior pockets of the MWW structure, while in a ddition to these highly accessible Ti species, those within the channels of a 10-membered ring greatly contribute to the oxidation reactions not signi ficantly sizedemanding. (C) 2001 Academic Press.