Aerosol Synthesis Of Mesoporous And Hierarchical Titanium-silicon Molecular Sieves And Aerosol-assisted Synthesis Of TS-1 Zeolite

Titanium-silicon molecular sieves showed excellent catalytic properties in a series of selective oxidation reactions with hydrogen peroxide.Titanium-silicon molecular sieves are green catalysts since the only by-product is water.Hydrothermal crystallization method is still the most well-developed method for the synthesis of TS-1.However,there are still some problems in hydrothermal synthesis,such as the complicatation of the synthesis process,the distribution of titanium atoms,the discharge of wastewater and so on.To solve the above problems,in this thesis,a simple and environmental benign aerosol-assisted method was used to synthesize microporous,mesoporous and hierarchical titanium-silicon molecular sieves.The influences of synthesis conditions on the structure and catalytic properties were systematically studied.The crystallization mechanism was also investigated.The main results are shown as follows:1.Titanium-silicon mesoporous molecular sieves were synthesized by an aerosol method and the influences of different synthesis conditions have been studied.The results show that titanium-silicon mesoporous molecular sieve with CTAB as template agent exhibits the uniform distribution of titanium species.Only tetrahedrally coordinated titanium species are present when Si/Ti ratio is higher than 30.The optimized synthesis conditions are as follows:CTAB/Si = 0.2,H+/Si ? 0.075,T= 200?(drying temperature).The mesoporous molecular sieves show good catalytic properties in cyclohexene epoxidation with H2O2.2.By using the aerosol technique,hierarchical titanosilicate composites(HTS)have been synthesized by solid-liquid dispersion and liquid-liquid dispersion.In this way TS-1 nanoparticles were wrapped in the spherical mesoporous molecular sieve.Compared with solid-liquid dispersion,liquid-liquid dispersion method is simpler,and provides a higher utilization efficiency of the titanium atoms.HTS prepared by liquid-liquid dispersion method shows high activity and selectivity to epoxide in cyclohexene epoxidation,and the higher yield of cyclohexene oxide than those of TS-1 and Ti-KIT-1.The conversion and products distribution can be adjusted by changing the titanium content in HTS mesoporous structure.3.TS-1 zeolite was synthesized by aerosol-assisted hydrothermal method.The optimized synthesis conditions are as follows:TPAOH/Si ? 0.05,H+/Si ? 0.067,T = 130?(crystallization temperature).TS-1 with Si/Ti= 60 shows the highest activity and yield of propylene epoxide.Compared with conventional hydrothermal synthesis,the new method shows the following advantages;(1)The synthetic process is simple and green.(2)The template amount is ultralow,which can effectively reduce the cost.(3)Volume utilization of the autoclave can be greatly improved.The samples crystallized after different crystallization times have been studied.The results show that TS-1 grew via a liquid-phase crystallization process.TS-1 crystals with low titanium contentwas firstly formed in the liquid phase.As the particles grew,the rest of the titanium atoms were gradually inserted into the MFI framework.Solid phase only provides the titanium and silicon sources for the growth of TS-1 crystals in the liquid phase.4.TS-1 zeolite was synthesized by solid-phase crystallization of SiO2-TiO2 powder obtained from aerosol method.The influences of different synthesis conditions have been studied.The results show that TS-1 can be synthesized when TPAOH/Si ratio is higher than 0.1.Lower drying temperature is advantageous to the retention of TPAOH and water for obtaining high crystalline TS-1.When crystallization temperature is higher than 150 °C,TS-1 with high crystallinity can be obtained after crystallization for 48 h.Among TS-1 with different Ti contents,TS-1 with Si/Ti = 60 shows the highest catalytic performance due to its high crystallinity.Compared with conventional hydrothermal synthesis,the solid-phase crystallization has the following obvious features:(1)The time-consuming separating and washing processes can be avoided.(2)The yield of the zeolites can be effectively increased.(3)The autoclave volume for crystallization can be effectively reduced.