A Novel Titanosilicate with MWW Structure: II. Catalytic Properties in the Selective Oxidation of Alkenes

Abstract

The catalytic properties of Ti–MWW catalysts have been studied for the oxidation 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 microporous TS-1 and Ti-MOR and of mesoporous Ti–MCM-41 in the oxidation of cyclohexene. Compared to large-pore Ti–Beta, Ti–MWW is less active than in the oxidation 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 of 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 Brønsted acid sites contributing to the hydrolysis of the oxide product to form glycols. The turnover number (TON) of Ti–MWW for the cyclohexene conversion decreases sharply with increasing Ti content, while the TON for the 1-hexene conversion shows volcanic behavior when related to the Ti content. It is clarified that the bulky reaction is catalyzed mainly by the Ti species within the supercages and the exterior pockets of the MWW structure, while in addition to these highly accessible Ti species, those within the channels of a 10-membered ring greatly contribute to the oxidation reactions not significantly sizedemanding.