Postsynthesis, Characterizations And Catalytic Properties Of Ti-MWW

The dissertation focuses on improving the catalytic activity of Ti-MWW by different postsynthesis methods and investigating its possible applications to the liquid-phase epoxidation of various alkenes,particularly the clean production of epichlorohydrin(ECH).In the first part,I have developed a novel methodology for synthesizing a new titanosilicate composed of MWW sheets but with expanded interlayer pore windows. The method was based on a strategy of inserting monomeric Si source into the interlayer spaces through alkoxysilylation of interlayer-silanols followed by removing organic moieties.The effects of postsynthesis conditions,Si/Ti molar ratios of precursors,nitric acid concentration,amount of Me₂Si(OEt)₂,types of silylation agent on the structure change and properties were investigated in details.The large pore titanosilicate catalysts showed high activities in the oxidation of both linear and cyclic alkenes.Especially,in comparison to 3D Ti-MWW,they showed much higher TON in cyclohexene oxidation due to possessing much open reaction space,which were accessible to the substrates with large molecular dimensions.In the second part,the post-synthesis treatment of Ti-MWW having the three-dimensional(3D)MWW structure with aqueous amine solutions has been carried out with the purpose to improve its hydrophobicity and catalytic activity in the liquid-phase oxidations.The treatment with piperidine(PI)or hexamethyleneimine (HMI)converted the 3D MWW structure into corresponding two-dimensional(2D) lamellar precursor which returned reversibly to the 3D MWW structure by further calcination.The treatments with other amines,however,caused a structural collapse or crystalline transfer to other phases.In the case of PI treatment,the structural conversion from 3D MWW to MWW lamellar precursor occurred readily at 443 K at a PI/SiO₂ molar ratio＞0.1 within one day for the Ti-MWW samples with various Si/Ti ratios.The structural interchange did not alter the amount as well as the coordination states of the Ti active sites,but removed the internal silanols by ca.40%, leading to a defect-less Ti-MWW catalyst with a more rigid and hydrophobic framework.This kind of structural rearrangement improved the catalytic activity by up to 20%in the ammoximation of ketones and also in the epoxidation of a wide range of alkenes with various molecular dimensions.In the third part,a postsynthesis method has been developed to prepare various metallosilicates structurally analogous to MCM-56 from corresponding lamellar precursors made up of MWW sheets.The precursors were converted into high crystalline MCM-56 analogues readily by a controlled acid treatment.Acid-treating the MWW precursors at temperatures lower than 353 K led to MCM-56 analogues even after further calcination,whereas the acid treatment at higher temperatures such as refluxing only resulted in conventional 3D MWW structure.In addition,the construction of MCM-56 structure depended greatly on the crystal size of the precursors,that is,the structural transformation of MCM-56 was achieved only for the precursors with a relatively smaller crystal size.The formation of MCM-56 analogues was however neither dependent of the types of metallosilicates(containing B,Al,Ti, Ga or Fe ions)nor the amount of framework metal ions.Compared with 3D Ti-MWW, the Ti-MCM-56 analogue had a larger external surface,which mitigated effectively the steric restrictions to bulky molecules imposed by the intracrystal micropores.The Ti-MCM-56 analogue was then superior to 3D Ti-MWW in the epoxidation of a wide range of bulky alkenes with tert-butyl hydroperoxide as well as in the oxidative desulfurization of dibenzothiophene with hydrogen peroxide.Finally,the catalytic properties of Ti-MWW in the epoxidation of allyl chloride (ALC)with hydrogen peroxide to ECH were studied by comparing with those of TS-1, Ti-MOR and Ti-Beta,and the issues concerning the stability and reuse of Ti-MWW have been considered.The investigation on various reaction parameters showed that Ti-MWW was an active and selective catalyst for the ALC epoxidation.Ti-MWW preferred aprotic solvents like acetonitrile and acetone to protic alcohols,which was favorable for suppressing the formation of solvolysis by-products of ECH.The ALC conversion and ECH selectivity were achieved both as high as 99%on Ti-MWW. 3-Chloro-1,2-propanediol and other heavy by-products with high boiling points had negative effect on the ALC conversion for both TS-1 and Ti-MWW.A novel secondary synthesis caused a structural rearrangement of Ti-MWW framework and then improved its stability effectively.