Study Of The Structural Factors On The Catalytic Oxidation Activity Over TS-1/H₂O₂System

Since TS-1was successfully synthesized in1983, titanosilicate zeolite has been widely studied as a heterogeneous catalyst in various hydrocarbon oxidations. TS-1/H2O2catalytic system has also been carried out in green industrial application of the bulk chemicals. The research of catalytic nature in the TS-1/H2O2catalytic reaction system has provided scientific basis to fully utilization of the catalytic properties. In this dissertation, based on the structural factors of TS-1/H2O2on the catalytic oxidation activity, we have a systematic research on the TS-1/H2O2catalytic reaction system, such as synthesis of high-perfrmance TS-1, formation of high-coordination Ti species with high catalytic activity, build of intermediates with double hydrogen bonds and the structural stability of TS-1. Through the study of the TS-1/H2O2catalytic reaction system, the further understanding was attempted to gain about the influencing factors to the catalytic activity.In the first part, starting from the exploration on the formation law of Ti active site in the systhesis of TS-1, TS-1has been successfully synthesized in ultralow molar ratio of TPAOH/SiO2(0.08and0.065) by two-step and multistep hydrolysis process through a controlled hydrolysis process. We found that the catalytic activity depends on hydrosis of the first step. Combined characterization of XRD, UV-Vis, SEM, FT-IR, BET and TG shows that currently synthesized TS-1samples have the typical "blackberry" morphology, similar to that of the conventional TS-1synthesized at a high molar ratio of TPAOH/SiO2(>0.18). The hydrolysis of the organic esters is the key step in the synthesis of TS-1. Fast hydrolysis of a small part of TEOS at a high concentration of TPA+in the first step induces more nuclei In the second step, the silicon source could be either organic or inorganic silicon, which broadens synthetic raw sources of TS-1. This strategy can greatly reduce waste emissions and the synthesis cost of TS-1, which is suitable for synthesis of TS-1on an industrial scale.In the second part, there are three coordinated state of Ti species in TS-1zeolite. Part of framework four-coordination Ti(IV) species are inverted to framework high-coordination Ti(VI) species through hydrothermal posttreatment. Combined characterization of UV-Vis, UV-Raman, FT-IR,29Si MAS NMR shows that the vibration peak at700cm-1excited at244nm laser line in UV-Raman spectra are assigned to the characteristic peak of hexacoordinated Ti species (TiO6). The absorption band at～280nm in UV-Vis spectra and the vibration peak at130,650,670cm"1excited at325nm laser line in UV-Raman spectra are related with the formation of framework high-coordination Ti. Moreover, the intensity of the absorption band at210nm in UV-Vis spectra and the vibration peak at960and1125cm-1in UV-Raman spectra are decreased. The catalytic results show that the framework high-coordination Ti species exhibite more excellent catalytic activity in H2O2activation, thereby significantly enhancing its catalytic epoxidation of alkene. The TON in epoxidation is also2-3times higher than the four-coordination Ti(IV) species.In the third part, the stability of the intermediate Ti-Oα-Oβ-Hend is related with the catalytic activity of titanosilicates. A new intermediate with double hydrogen bonds is formed in a catalytic system over titanosilicates by introducing a hydrogen bond acceptor, and the catalytic activity is enhanced significantly as a result of the new intermediate with suitable stability. Furthermore, DFT result indicates the possibility of double hydrogen-banding reactive intermediates. The result of reaction kinetics shows that the apparent activation energy of Ti-Oα-Oβ-Hend formed over TS-1was decreased when the hydrogen bond acceptor was introduced into the catalytic oxidation system. Its catalytic performance is improved and the relatively highest increase is almost50%. This new intermediate is universal in titanosilicate/H2O2catalytic system. Except that, the new intermediate with good hydrothermal stability(≤373K) could successfully be in situ built in the pores of TS-1by hydrothermal or impregnation method. In the fourth part, we examine the structural stability of TS-1using the continuous reaction of cyclohexanone ammoximation. The results shows that in the single-run period, TS-1has a high structural stability with little loss of framework Ti species, and the main reason of TS-1inactivation is due to blockage of the pore. The introduced porous silicon may adsorb part of organic compounds, further to slo w the adsorption rate of TS-1, and to improve its operating life.