A Study On The Catalytic Active Sites Of Titanosilicates And Their Migration Rules Directed By Selective Synthesis Of Oxime

Process of selective synthesis of oxime based on the titanosilicates/H2O2 catalytic oxidation systems was proved to be the development trend of liquid phase ammoximation technology. Thus, this thesis focuses on the basic catalysis scientific problems of selectivity, activity and stability in reactions on the basis of liquid-phase ammoximation process over titanosilicates. The structural features of catalytic active sites located in titanosilciates were studied in detail among the reactions of selective synthesis of oxime, deactivation progress of operational catalyst as well as preparation of monolithic titanosilicate catalyst with highly catalytic activity. We try to understand the core problems of ammoximation and provide valuable and scientific suggestions for constructing a highly selective, reactive and stable catalytic system for clear synthesis of oximes via ammoximation over titanosilicates. The main results were concluded as follows:(1) The reasons for selective synthesis of oxime (cyclohexanone oxime, methyl ethyl ketone oxime, acetone oxime and acetaldehyde oxime) via ammoximation catalyzed by various titanosilicates (Ti-MWW, F-Ti-MWW, TS-1 and TS-2) were carefully revealed through concentrating on the factors on the reaction paths of H2O2 including the main pathway of generating hydroxylamine and the typical side pathway of deep oxidation of oxime. It was found that the high concentration of NH3 merely promoted the main pathway and the diffusional restriction of catalyst as well as the highly active of oxime just intensified the side pathway, while the proceeding extent of deep oxidation of oxime was decided by the amount of free H2O2. Moreover, the reaction paths of H2O2 were controlled by Lewis acidity of titanosilicates. Specifically, strong Lewis acidic sites were able to fundamentally strengthen the path of forming hydroxylamine via lowering the activation barrier of H2O2 and thus synthesized oxime in superior selectivity.(2) The mechanisms of deactivation for titanosilicates (TS-1 and Ti-MWW) during the industrial ammoximation process of cyclohexanone and methyl ethyl ketone were carefully studied, respectively.Firstly, the deactivated microsphere TS-1 zeolite (MTS-1) aged over 2160 h from industrial cyclohexanone ammoximation (0.1 Mt/a) was chosen as research object. In respect of the deactivated MTS-1, the differences in the structural feature, acidity and catalytic performances of various Ti species in the fresh, deactivated and acid treated samples were carefully analyzed and compared. It was confirmed that a new non-framework Ti species with oligomeric structure that was rich in silica or titanium was accumulated on the surface of the deactivated MTS-1 in the form of amorphous SiO2-TiO2 binary oxides, which noticeably improved Bronsted acidity and was able to intensify the ineffective decomposition of H2O2. Simultaneously, the coordination environments of remaining framework Ti species located in deactivated sample were also changed from rigid to flexible. Hence, besides the decrease of absolute amounts and the loss of accessibility of framework Ti active sites in titanosilicates, both the change of micro coordination environment of framework Ti active sites and the non-framework Ti species induced by migration of framework Ti active sites were able to intensify the deactivation behaviors of catalyst together.Secondly, the variations in structures and properties of catalyst during the industrial one-trip ammoximation of methyl ethyl ketone (0.015 Mt/a) catalyzed by Ti-MWW were tracked analyzed. It was verified that both chemical desilication of zeolite framework and behaviors of physical coke deposition occurred to Ti-MWW during operation. It was showed that the framework Ti actives sites in Ti-MWW migrated to non-framework gradually and finally formed the isolated and dispersed hexahedral non-framework Ti species due to the chemical desilication of framework silica. Combined with the changing behaviors of framework silica, we proposed a possible migration mechanism of Ti active sites that directed by the defects of framework.Finally, the differences and similarities in deactivation mechanism showed between the TS-1 and Ti-MWW were further studied. The results showed that the non-framework Ti species which formed following the migration of framework Ti species induced by desilication significantly promoted inefficient decomposition of H2O2 and then intensified the deactivation behaviors for TS-1 and Ti-MWW. However, their acidities differed from each other due to the different structures of non-framework Ti species. On the other hand, the differences caused by physical coke deposition were compared. The coke located in the deactivated TS-1 noticeably blocked its channels and then influence the accessibility of Ti active sites, while it did not have any negative effect on the catalytic performance of Ti active sites located in Ti-MWW. As for the basic reasons for these differences between TS-1 and Ti-MWW could be summarized into two sides. Namely, the rigid structure (TS-1) and flexible structure (Ti-MWW) caused the different dissolved rates of framework silica as well as the different locations of Ti active sites in these two types of titanosilicates.(3) Following by the understanding of selectivity and stability in ammoximation, we performed to synthesize nano-TS-1 particles aggregated monolithic catalyst (microsphere TS-1) from the perspectives of industrial application. The unbroken microsphere TS-1 catalysts (MTS-1) with high active component through choosing TS-1 crystallized filter cake as active component and oligomeric SiO2 that hydrolyzed from TEOS catalyzed by TPAOH as binder were prepared successfully via spray. After a further in-situ crystallization treatment and post-treatment by TPAOH on the microsphere TS-1, a highly catalytic active MTS-1 catalyst with no binder (crystalline binder) featured with less defective as well as hollow mesopores was obtained. The MTS-1 catalyst was proved to be served as a robust catalyst for continuous ammoximation of cyclohexanone.(4) A study of liquid-phase ammoximation of cyclohexanone over titanosilicates on fixed-bed reactor was also investigated. The results showed that, under the optimum reaction conditions, the fixed-bed process not only could efficiently perform the liquid-phase ammoximation but also could remarkably improved the efficiency of H2O2 as. high as 98.7% which was higher than that obtained form a slurry reactor (91.5%).