A Study On Synthesis,Chemical Modification And Catalysis Of Titanosilicates

The titanosilicate/H2O2 catalytic systems,appealing for environmental friendliness,mild reaction conditions and high atom utilization efficiency,are of great significance for the establishment of green chemical process.In addition,the weak acidity of titanosilicates provides possibility for application in acid catalysis.Based on the practical application of titanosilicates and the hot issues in the field of catalytic applications,the contents in this thesis mainly involve the optimization of synthesis strategy of titanosilicate,controllable construction of highly active Ti catalytic sites,the improvement of catalytic stability of titanosilicates in specific reaction system and so on.It is expected to lay the theoretical foundation for the design and synthesis of highperformance titanosilicates and their practical application.The mainly contents are as follows:(1)MSE-type titanosilicate(Ti-MSE)showed great application potential in the liquid-phase selective oxidation system,especially in the hydroxylation of phenol.In view of the problems of the usage of complicated template with complex synthesis steps,the extremely long crystallization period and the low preparation efficiency in the traditional synthesis process of Ti-MSE,a novel strategy is developed to postsynthesize Ti-MSE efficiently by the combination of interzeolite transformation and isomorphous substitution of Ti.Relied on the entirely common compositing building units of *BEA and MSE zeolite,as well as the assistance of MSE seed and tetraethylammonium hydroxide(TEAOH)as organic structure-directing agent,MSE aluminosilicate was rapidly crystallized in only 1 day with a high solid yield of 88.3%via the interzeolite conversion of siliceous Beta zeolite.After dealumination and solidgas treatment with Ti Cl4 vapor,Ti-MSE catalyst was readily obtained,which exhibited much higher catalytic activity in anisole hydroxylation than titanosilicates in other topologies,with a methoxyphenol yield of 54.9% and particularly high para-selectivity of 96.2%.Electrostatic potentials and density functional theory(DFT)calculations revealed that the abundant framework hydroxyls in Ti-MSE strongly interact with the oxygen-containing substituent groups in aromatic substrates,leading to the orientated adsorption of substrates and thus the occurrence of para-substitution.(2)Based on the great significance of regulating the microenvironment of Ti active centers in the theoretical research and practical application,the organic amines were introduced into Ti coordination environment by the hydrothermal treatment with tetrapropylammonium hydroxide and ammonium chloride.A novel six-coordination Ti speices containing organic amine ligand was constructed in the TS-1 zeolite.The presence of Ti(OSi)2(OH)2(H2O)TPA sites could reduce the apparent activation energy of H2O2 activation reaction significantly,responsible for the greatly increased catalytic activity in 1-hexene epoxidation.After removal of organics by calcination,Ti(OSi)2(OH)2(H2O)TPA was transformed into Ti(OSi)2(OH)2(H2O)2 sites,which also exhibited higher H2O2 activation capacity and catalytic activity compared to the original framework Ti(OSi)4 sites.In addition,the acid sites of Si-OH in the TS-1 zeolite were quenched by the basic amine molecules,which effectively inhibited the occurrence of side reactions such as epoxide ring-opening and thus exhibited certain application potential.(3)The green manufacture of caprolactam combined with cyclohexanone ammoximation and the rearrangement of cyclohexanone oxime is a typical case from basic research to practical application of titanosilicates.The skeleton desilication accompanied by Ti active site leaching in the alkaline media of cyclohexanone ammoximation accounts for the irreversible chemical deactivation of the titanosilicates.In view of this puzzle,aluminum species were reintroduced into Ti-MOR by hydrothermal post-treatment with aluminum salts to enhance its anticorrosion performance against alkaline medium and catalytic stability in cyclohexanone ammoximation reaction.The undesired strengthening in hydroxylamine decomposition and oxime hydrolysis derived from the implantation of aluminum into the framework could be eliminated via further Na+-exchange to quench bridging Si(OH)Al-related acidic sites selectively.With aluminum species enriched on the crystal shell,Ti-MOR after aluminated and Na+-exchanged showed a more robust performance in the simulated desilication process and a tremendously prolonged ammoximation lifetime of 460 h,which was 2.75 times that of pristine Ti-MOR.The unprecedented duration of Al-Ti-M-Na was mainly ascribed to the protective effect of the Al-rich shell as it served as the guardian crust that defended the Ti active sites against leaching caused by desilication and skeleton destruction.(4)In addition to liquid-phase selective oxidation reactions,titanosilicates also show application potential in acid catalytic systems represented by Beckmann rearrangement reactions.The selective quenching of terminal silanols in catalysts is an effective strategy to improve the caprolactam selectivity in the Beckmann rearrangement of cyclohexanone oxime.Based on the harsh conditions(over 573 K)in the gas-phase Beckmann rearrangement reaction,the highly stable ≡Si-CH3 species was constructed in the TS-1 zeolite by alcohol-refluxing and further treatment with Grignard reagent to quench the terminal silanols steadily without affecting the rearrangement active sites.In addition,the constructed ≡Si-CH3 species was extremely stable under the reaction condition,which could delay the deactivation of catalyst by inhibiting the formation of by-products steadily.Therefore,compared to the pristine and silylated TS-1 samples,the treated TS-1 sample with ≡Si-CH3 species constructed exhibited improved caprolactam selectivity and catalytic stability in the gas-phase Beckmann rearrangement of cyclohexanone oxime.