Surface-Active Hollow Titanosilicate Particles as a Pickering Interfacial Catalyst for Liquid-Phase Alkene Epoxidation Reactions

The design of catalyst particles bearing excellent catalytic activity and suitable surface wettability is the key to successful application of Pickering interfacial catalysis. In this study, the epoxidation of 1-hexene and cyclohexene with aqueous hydrogen peroxide over hollow TS-1 (HTS-1) zeolite was studied as a probe reaction to investigate the influence of catalyst surface wettability on catalytic activity. Hydrophobized HTS-1 particles were fabricated via a postsynthesis desilication treatment with tetrapropylammonium hydroxide and a postsynthesis silylation treatment with hexamethyldisiloxane (HMDSO). The successful preparation of HTS-1 particles was verified by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, and Fourier-transform infrared spectroscopy. X-ray diffraction patterns and ultraviolet-visible spectra confirmed that the hydrophobic modification had no effect on the zeolite structure of HTS-1 particles. Stable Pickering emulsions of aqueous hydrogen peroxide in either 1-hexene or cyclohexene could be prepared using HTS-1 particles as emulsifiers and confirmed by cryo-SEM images. The catalytic behavior in the obtained Pickering emulsions revealed a parabolic distribution of turnover frequency values with respect to the hydrophobization degrees with 0.2-HMDSO/HTS-1 particles possessing the maximum values of 20.6 h^-1 for 1-hexene epoxidation and 8.1 h^-1 for cyclohexene epoxidation. In addition, these 0.2-HMDSO/HTS-1 particles showed good reusability for more than five cycles.