Synthesis, Characterization And Catalytic Performance Of Hierarchical TS-1 Zeolites

Titaniumsilicalite TS-1 has attracted much attention for its excellent catalytic performances in selective oxidation and epoxidation of various organic compounds with dilute H2O2 as oxidant under mild conditions. However, TS-1 suffers from obvious diffusion limitation arising from its small micropores, and this limitation is very pronounced for the reactions with bulky reactants and products. During the last decade, the synthesis of hierarchical zeolites combining the advantages of microporous zeolites and mesoporous molecular sieves has been the research focus in the field of porous materials. Hierarchical TS-1 zeolites reported in the literatures have the complicated synthesis steps and costly mesoporous templates, which greatly restricts their industrial applications. This paper aims to synthesize hierarchical TS-1 zeolites by using low-cost mesoporous templates and simple synthesis steps. Also, the obtained materials were characterized and evaluated by various technologies and catalytic reactions. The main contents are as follows:Hierarchical TS-1 zeolite was successfully synthesized by the two steps of high temperature carbonization and hydrothermal crystallization with mixing sucrose, silicon and titanium sources. The carbon template from sucrose carbonization combined with silica and titanium particles and was enwrapped in crystals easily in the crystallization process. The mesopore or macropore channels were formed after calcination. Hierarchical TS-1 not only possesses the similar coordination of titanium species to TS-1, but also has interconnected 5-100 nm mesopores or macropores. Moreover, these pores correspond to the framework structure of the carbon template trapped in TS-1 crystals. The mesopore and macropore volume of hierarchical TS-1 can be adjusted by changing the synthesis conditions. Hierarchical TS-1 shows nearly 100% removal rate of bulky benzothiophene, while TS-1 has no oxidative activity in this reaction due to the effect of diffusion limitation. Hierarchical TS-1 also exhibits higher activity than TS-1 in Th oxidation.Hierarchical TS-1 was synthesized by the one step hydrothermal crystallization with adding sucrose to the gel of TS-1 directly. The crystallization process of zeolite and the carbonization step of sucrose are simultaneous, which simplify the synthesis process effectively. The adding of sucrose changes the orientation of TS-1 crystals, but has no effect on the amount and coordination of titanium species. Compared with the carbonization of sucrose at high temparture, the carbonization of sucrose in crystallization temperature is incomplete, so that the mesopore volume of hierarchical TS-1 is lower under the same amount of sucrose. The mesoporosity of hierarchical TS-1 can be adjusted by adding different amounts of sucrose. The obtained hierarchical TS-1 shows higher removal rates of benzothiophene and thiophene than TS-1.Hierarchical TS-1 zeolites were synthesized by cationic polyquaternium-7 or polyquaternium-6 as mesoporous template and tetrapropylammonium hydroxide as microporous template. The polymers interact with negatively-charged aggregate silicate species in alkaline solution due to the presence of abundant cationic charges. With the crystallization of zeolite, the polymer can be entirely wrapped in zeolite crystals. Polyquaternium-6 shows much higher cationic density than polyquaternium-7, so that it interacts with aggregate silicate species more easily and the synthesis teps are simpler. Hierarchical TS-1 was also synthesized by using polyvinyl butyral as mesoporous template due to its abundant hydroxyl groups. Polyvinyl butyral has good compatibility with inorganic species so that it can be highly dispersed in silica and titanium composite, and the additional 5-30 nm mesopores are formed by hydrothermal synthesis and calcination steps. The crystal morphology of TS-1 changes from agglomerates of much smaller crystals to cuboidlike single crystals due to the adding of polyvinyl butyral template. Hierarchical TS-1 zeolites synthesized from the three kinds of polymers give higher catalytic activities in the oxidation of benzothiophene and show the removal rates of 100% in thiophene oxidation for 2 h.