| Fluorine, the most electronegative element, is considered to change easily the electropositivity of the elements in neighbourhood through electron-withdrawing effect. MWW type titanosilicalite originating from a lamellar precursor is readily subjected to various structural modifications. This thesis focuses on implanting fluorine effectively into the framework of Ti-MWW zeolite.Firstly, F-Ti-MWW was post-synthesized by implanting fluorine species into Ti-MWW framework through acid treatment process in the presence of ammonium fluoride. The effects of NH4F addition amount, acid treatment temperature and precursor Ti content were investigated on the incorporation ofF species, and further discussed the mechanism of F implanted into Ti-MWW zeolite.Fluorine-implanting improved the surface hydrophobicity of Ti-MWW and altered the electropositivity nearby the tetraihedral Ti sites through forming the SiO3/2F and SiO4/2F-units, meanwhile, the state of four coordination Ti had also been changed and make more Ti species exist as Ti-OH in the framework. Moreover, it had been verified that the formation of Si-F groups were realized through the interaction with Si-OH groups, the possible formation mechanism of SiO3/2F units had been discussed. The F species would be incorporated through substituting the Si-OH groups with Si-F in the neighbour of the tetrahedral Ti sites existing either in "closed"[Ti(OSi)4] units or in "open"[Ti(OSi)3OH] units.Meanwhile, we had investigated the catalytic properties of F-Ti-MWW zeolite, SiO4/2F-units showed negative effects of zeolite, whereas SiO3/2F units are beneficial to increase the electropositivity nearby the Ti sites. Presumably, the formation of SiO3/2F units is the dominant factor govening the outstanding activity of F-Ti-MWW. F-Ti-MWW containing the SiO3/2F units possessed excellent catalytic activity for the liquid-phase oxidation of a variety of organic substrates using H2O2as an oxidant, for instance, the epoxidation of functional alkenes and ammoximation of ketones. Furthermore, F-Ti-MWW containing the SiO3/2F units possessed a longer catalyst lifetime and reusability than conventional Ti-MWW. F could exist in the framework of F-Ti-MWW robustly and stablely, which have potential industrial application prospect.The negative SiO4/2F-species was formed inevitably in the treatment process with ammonium fluoride. Those negatively polarized SiO4/2F-units were claimed to lower the electrophilicity of the active sites via hydrogen bonding, and decrease the epoxidation ability. The adverse effect of F" can be eliminated by the synergistic effect between anion and cation in the inorganic salt. Inorganic salt post-treatment process basically has following two effects:anion can availably exchange part of the F" in the F-Ti-MWW zeolite, meanwhile, the metal cation itself has strong electropositive which could neutralize the electronegative of residual F-. Hence, the catalytic performance of F-Ti-MWW had been further enhanced.We had also tried two kinds of methods to synthesis F-TS-1which possess SiO3/2F unit. On the one hand, we acid treated TS-1by NH4F, on the other hand, we synthesis F-TS-lvia F-Ti-MWW transform treatment. However, both of this F-TS-1showed lower catalytic properties than conventional TS-1. This is mainly due to the fluorine in the prepared F-TS-1are mainly as the units of which provided negative effect, such as SiO4/2F-and Ti-F.Finally, the reaction mechanism for epoxidation of allyl chloride (ALC) with H2O2catalyzed by Ti-MWW was studied in detail. The first step of titanosilicate-catalyzed reactions involves the interaction of H2O2molecules with the tetrahedral Ti sites. Than the C=C double bond in alkenes would capture the oxygen directly coordinated to the Ti sites to generate corresponding epoxides. So, the critical issue is how to enhance the effective availability of H2O2. This provide certain inspiration of synthesize new type titanosilicalite zeolites and the industrialization of allyl chloride epoxidation reaction. |
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