Study On Low Cost Synthesis And Catalytic Performance Of MFI Heterogeneous Molecular Sieve

MFI type zeolites modified with heteroatoms are widely applied in the field of fine chemical industry due to its unique cross channel structure and catalytic performance. However, compared with the aluminosilicate zeolite, the preparation process of the heteroatoms modified MFI type zeolites was complex and this type zeolite was expensive. In order to enlarger the application of this zeolite, these zeolites cost must be reduced. In this paper, the exploration to reduce these zeolites cost has been developed by using solid transformation method with zeolite seed gel as zeolite director form inorganic silica source.The first part of this paper described the preparation of MFI-type titanium silicon zeolites(TS-1) by using titanium(IV) sulfate and silica gel as raw materials at low usage of tetrapropylamonium hydroxide(TPAOH). In this method, the zeolite seed gel was used to direct the preparation of the TS-1 zeolite and the TPAOH was entirely provided from the zeolite seed gel. The total TPAOH/SiO2 molar ratio decreased to 0.07, which was 4 times smaller than that employed in the traditional synthesis. Especially, there were no other organic templates added in this method, which cut down the cost of the TS-1 dramatically. The results showed that the citrate added promoted the incorporation of Ti into the framework of TS-1 zeolite, despite the growth of the zeolite was delayed with the citrate addition. The crystallization processes of zeolite were characterized by X-ray diffraction(XRD), thermo gravimetric(TG) analysis, ultraviolet–visible spectroscopy(UV-vis) and Fourier transform infrared(FTIR) spectroscopy. The characterization of ultraviolet–visible spectroscopy(UV-vis) certified that only tetrahedral Ti coordination species entered the obtained TS-1 zeolite in the whole crystallization process. The morphology and textural properties were characterized by scanning electron microscopy(SEM) and nitrogen sorption isotherms. The obtained TS-1 possessed flaky crystals with a size of 0.1×1μm, well crystallinity and high surface areas(384 m2 g-1). The cyclohexanone ammoximation reaction was used as probe reaction to survey the catalytic capacity of the resulted zeolites and this zeolite showed well catalytic capacities. However, the crystal size of the obtained TS-1 zeolite is about 1μm which was larger than that of the zeolites prepared by traditional hydrothermal synthesis method(0.1 to 0.3μm), which affected the catalytic performance.In the second part of this paper, a solid-phase transformation method was used to instead hydrothermal synthesis method to prepare TS-1 zeolite. Nanosized TS-1 zeolites have been successfully synthesized in ultra low molar ratio of TPAOH/SiO2 by solid transformation process, where the molar ratio of TPAOH/SiO2 could be reduced to 0.05. The resulting samples were characterized by X-ray diffraction, IR spectroscopy, SEM and N2 adsorption to evaluate the textural properties. The obtained TS-1 was about 100-400 nm particles composed of 50 nm primary crystallites, which shown high surface areas(426 m2 g-1) and exhibited excellent catalytic oxidation performance in epoxidation of 1-hexene by H2O2. This novel strategy avoided the usage of solvent in the crystallization process of TS-1 zeolites. Especially, the usage of silica gel as raw materials and lowering the amounts of TPAOH to theoretical value reduced the costs of TS-1 zeolites dramatically.In the third part of this paper, the Mn-ZSM-5 was prepared by using hydrothermal synthesis method with addition of citrate. The obtained Mn-ZSM-5 zeolites were characterized through XRD, FT- IR, UV–vis. The results show that specific surface area of this sample is larger with high skeleton Mn content. This sample exhibits well catalytic capability in the epoxidation of styrene.