Study Of The Preparation Of TS-1 Zeolite Catalysts With High Performance For Propylene Epoxidation

Propylene oxide (PO) is one of key propylene based chemical intermediates for producing fine chemicals, such as polyether polyols (polyglycol ethers), propene glycols, and propene glycol ethers. The HPPO process, which produces PO under mild conditions based on Titanium silicalite-1 (TS-1) as catalyst, and dilute H2O2 solution as oxidant, has attracted much attention during recent years. It is an economically and ecologically state-of-the-art technology being coproduct-free and with water as the only waste product. Several HPPO process units were set up in the world. However, it is also vital necessity to develop high performance TS-1 zeolite catalysts for improving economic returns and promoting industrial application of HPPO process. In this thesis, the main purpose is to develop high activity, selectivity and stability TS-1 zeolite catalysts by simplifying the synthesis process of TS-1 zeolite, controlling the extra-framework Ti content and treating TS-1/SiO2 extrudates with different bases. The main research contents are shown as follows:1. Effects of different crystallization-mediating agents, containing urea, ammonium carbonate, ammonium acetate and nitrilotriacetic acid, on catalytic performances of TS-1 zeolite were investigated. The results indicated that the crystallization-mediating agents not only eliminated the negative influence of alcohol in the TS-1 synthesis process, but also promoted framework Ti species formation. And urea as a crystallization-mediating agent is best. The synthesis conditions were systematically studied. At the conditions of the molar ratio of Urea/SiO2 was 0.15,170 ? for 24 h, the highest catalytic properties of TS-1 (the molar ratio of Si/Ti was 40) was obtained by free alcohol removal route. The X(H2O2)?S(PO) and U(H2O2) were 92.6%?95.3% and 96.4%, respectively. The high framework Ti contents, crystallinity and uniform particle size(about 250 nm) were responsible for the high catalytic properties of the synthesized TS-1.2. The influence of the different base treatments on the catalytic properties of TS-1/SiO2 extrudates in propylene epoxidation was investigated. It was found that the introduced SiO2 and silicon species of TS-1 zeolite were dissolved and then recrystallized during the treatments with TEAOH and TPAOH, owing to the strong template ability of TEAOH and TPAOH to form MFI topology. The TEA+ and TPA+ ions played a key role in the treatment process. The diffusion property was improved and the distributions of Ti and Si ions in the catalysts were adjusted. Thus, the catalytic activity and stability of the modified catalysts were substantially enhanced.3. The methods and conditions of the TPAOH treatments were systematically studied. It was found that the catalytic activity and stability of the catalyst were increased when TS-1 zeolite was formed before the TPAOH treatment. The main reason is that the SiO2 was dissolved and then recrystallized during the TPAOH treatment, which increased the hydrophobicity and the ratio of Si/Ti on the external surface of TS-1. Moreover, the existence of SiO2 inhibited the formation of anatase TiO2 during the TPAOH treatment.4. The addition of a small amount of additives (NH3·H2O and (NH4)2CO3) into the reaction mixture can obviously enhance the catalytic activity and stability of TS-1/SiO2 catalyst for propylene epoxidation. Because the additives not only avoid the side reactions through neutralizing the acide site in the TS-1/SiO2 catalysts and reaction mixture, but also promot the adsorption of reactant on the active center, and accelerate the formation of the reactive intermediates Ti-OOH(?2).5. The TS-1/SiO2 catalyst showed high catalytic properties and selectivity to propylene epoxidation was prepared, and named as EPO-10. Under the optimized reaction conditions (the temperature was 40 ??pressure was 3.0 MPa?the molar ratio of n(C3H6)/n(H2O2) was 3:1?the concentration of H2O2 was 3.0 mol/L, the mass velocity of H2O2 was 0.4 h-1), the X(H2O2) and S(PO) still remained above 97%. At the same time high U(H2O2) was kept after a long-term run with time on stream (TOS) of 2000 h in fixed-bed reactor, indicating EPO-10 catalysts has a bright prospect for application.