Acid-Modulated Synthesis of Ti-MWW Zeolites with Rich Framework Ti Species for Efficient Epoxidation
Yuanling Huo, Yang Zhang, Wen Xu, Kang Tang, Xinqing Lu, Rui Ma, Yanghe Fu, Weidong Zhu
Abstract
Ti-MWW, with tetrahedral Ti isomorphously substituted into an MWW-type zeolite framework, has exhibited a superior catalytic performance in selective oxidation with hydrogen peroxide, because of its specific solvent effects and unique pore-structure system. However, in the synthesis of Ti-MWW by prevailing methods, some extra-framework octahedral Ti species are usually formed, which are detrimental to selective oxidation. In the present study, an acid-modulated synthesis method was developed to prepare Ti-MWW without the formation of extra-framework octahedral Ti species. Different techniques were then used to characterize the synthesized zeolites that were evaluated in the catalytic epoxidation of 1-hexene. The results show that the lamellar structure of Ti-MWW is more beneficial to introduce the framework Ti species in comparison to the three-dimensional structure of the zeolite. In addition, the Ti-MWW zeolite with some unusual open sites of Ti(OSi)3OH, prepared by the acid-modulated method, possesses a stronger Lewis acidity and better catalytic performance in 1-hexene epoxidation than the zeolites with the closed sites of Ti(OSi)4, synthesized by the prevailing methods. Furthermore, the developed Ti-MWW is recyclable after the regeneration of the deactivated catalyst calcined at high temperatures.