Studies On Catalytic Properties And Stucture Stability Of Titanium Silicalite Synthesized By Cheap Method

Titanium silicalite (TS-1) was first synthesized in 1980's. The system of TS-1 and H2O2 exhibits excellent catalytic and oxidative properties under the mild reaction conditions, which has overcome the shortcomings of serious pollution and unsafe factors of traditional oxidative technologies, so TS-1 has attracted more and more attention of researchers. However the cost of TS-1 synthesized with traditional classical method is too high to make it in commercial process. In past two decades, many studies had been carried out on decreasing the synthesis cost of TS-1. Above them, the method of replacing costly tetrapropylammonium hydroxide with tetrapropylammonium bromide as template (called cheap method in this paper) can decrease the synthesis cost to a great extent.In this paper, the structure stability and catalytic properties of TS-1 synthesized by cheap method are studied. Combined with characterizations of XRD, UV-Vis, FT-IR, SEM, EPR, BET, TPD and NMR, some important information and data are acquired. The main contents of the dissertation are as follows:1. Investigation on hydrothermal stability of TS-1 synthesized by cheap method under neutral (weak acidic) reaction circumstanceSome of catalytic systems of TS-1 with hydrogen peroxide are neutral of weak acidic, such as propylene epoxidation. In the view of Clerici et al., TS-1 experiences Ti-0 bond's rupture, then forms an active "five-member-cycle" intermediate with H2O2 and solvent. Some literatures reported whether or not reversibility of interaction between TS-1 and H2O2 is dependent on the synthesis system and conditions of TS-1. Hydrothermal stability of the TS-1 synthesized by cheap method in our lab is studied in the simulated circumstance of propylene oxidation (PH=6~7). After treatment for 1200h, the framework of TS-1 is stable, the acidity and activity is unchangeable. So it is induced that reaction of this TS-1 interacting with H2O2 is reversible.2. Investigation on hydrothermal stability of TS-1 synthesized by cheap method under ammonia alkaline reaction circumstanceCyclohexanone ammoxidation catalyzed by TS-1 is a promising method in place of traditional hydroxylamine method to produce cylcohexanone oxime. The literatures reported that TS-1 synthesized by classical method in ammoxidation system was unstable, the leaching framework Ti accumulated on the surface of TS-1 as bulk TiO2 and leaching Si dissolved inreaction solution that deactivated TS-1. In order to investigate the situation, the TS-1 synthesized by cheap method is put into the simulative circumstance of cyclohexanone ammoxidation in our experiment. The results show that structure of TS-1 is destroyed after treatment for a long time. The acidity of TS-1 increases evidently. Leaching Si is dissolved in solution, while leaching Ti accumulates on surface of TS-1 as tetra-coordinate Ti. When used in propylene epoxidation, the conversion of H2O2 doesn't change, while the selectivity of PO decreases.3. Increasing the structure stability of TS-1 under alkaline circumstance by introducing Al into frameworkCompared the TS-1 with the Al-TS-1 (which framework is introduced into Al when synthesized by cheap method), it is founded that introducing Al into TS-1 doesn't influence the crystallization, but restrains the content of framework Ti. Nevertheless the morphology of zeolite will change with increase of Al addition. When used in cyclohexanone ammoxidation, framework Al doesn't affect activity and selectivity of Al-TS-1, but increase its stability under ammonia alkaline circumstance.4. Improving activity of TS-1 by ammonia solution hydrothermal modification Compared the nano-sized TS-1 by classical method, the TS-1 synthesized by cheap methodhas large crystal and relatively low activity. In order to improve its activity, several modification systems are proposed. It is found that the effect of ammonia modification is best among H2O, citric acid, sodium hydroxide and sodium carbonate solutions. Sodium ion residues forming the un-reactive species with tetra-coordinated Ti of TS-1 deactivate TS-1. H2O has weak effect and citric acid has little effect on TS-1.Through investigation of ammonia solution modification conditions, the results show that temperature, time and concentration of ammonia solution are main parameters on effect of TS-1, while medium phase has no effect. The optimum modification is under the condition of 170°C, 24h, 14mol/L NH3-H2O corresponding temperature, time, concentration respectively. When used on batch reaction, the parent TS-1 and modified TS-1 have a 62.24% and 97.40% conversion of H2O2 respectively and a similar selectivity of PO under the given same conditions. When used on in fixed-bed reaction, the parent TS-1 and modified TS-1 have an average 76.17% and 90.12% conversion of H2O2 respectively and a similar selectivity of PO under the given same conditions. Combined the characterizations of Pyridine-IR, NMR, EPR et al. and theoretic analysis, the excellent modification effect mainly contributes to improving diffusibility of TS-1 and generating more reactive Ti sites located adjacent to Si vacancies in TS-1 than fully coordinated Ti sites.