A Study On Titanosilicates-Catalyzed Selective Oxidation Reaction

The titanosilicate/H₂O₂ catalytic systems,which give water as the main byproduct and are featured with mild reaction conditions,high atom utilization efficiency and environmental friendliness,have been investigated and applied to the selective synthesis of furfural oxime,ethylene oxide,ethylene glycol and propylene oxide over titanosilicate in this study.In the first part,in view of the emerging problem of complicated processes,high energy consumption and serious environmental pollution in the conventional method for the production of biomass-derived furfural oxime,the clean synthesis of furfural oxime has been expected to be realized through titanosilicate-catalyzed liquid-phase ammoximation of furfural with ammonia and hydrogen peroxide.Based on the detail investigation of furfural ammoximation over three representative titanosilicates Ti-MOR,TS-1 and Ti-MWW,we found that the hydroxylamine route leads to the formation of furfural oxime and the imine route accounts for the formation of2-furylamide and 2-furoic acid.Ti-MOR showed the best catalytic performance in the furfural ammoximation among the three titanosilicates investigated,due to its high efficiency for hydroxylamine formation.Ti-MOR preferred water as a solvent and showed the highest catalytic activity and oxime selectivity among the titanosilicate catalysts when H₂O₂ was dropwise added into the reaction system.Under optimized conditions,Ti-MOR was capable of showing a furfural conversion and oxime selectivity of above 97%.In the second part,in view of the emerging problem of high energy consumption,and low atom utilization in the industrial production of ethylene oxide,the highly selective synthesis of ethylene oxide has been realized through liquid-phase epoxidation of ethylene with titanosilicate/H₂O₂ catalytic systems.By comparing four titanosilicates of different topologies and solvents,Ti-MWW/H₂O₂/MeCN was confirmed as the best catalytic system for the epoxidation of ethylene to ethylene oxide.The effects of solvent,catalyst amount,reaction pressure,reaction temperature and time on the performance of ethylene epoxidation reaction over Ti-MWW have been systematically investigated.Under the optimized reaction conditions,Ti-MWW gave a high EO selectivity of 97.9%as well as a reasonable utilization efficiency of H₂O₂ of 77.7%.The deactivation of catalysts is due to the desposition of heavy byproducts of ethylene glycol molecules with high boiling points inside micropores,and the deactivated Ti-MWW was reusable when subjected to the regeneration by high-temperature calcination.Although ethylene is a simple alkene with the smallest molecular dimension and carbon numbers,the intrinsic activity of ethylene in the epoxidation reaction is lower than other alkenes due to its electron-deficient C=C double bonds.In the third part,in view of the low intrinsic activity of ethylene in the epoxidation reaction,fluorine-implanted titanosilicate Ti-MWW has been post-synthesized by fluorinating conventional Ti-MWW with NH₄F.The effects of fluorination conditions,such as solvent,the NH₄F amount and fluorination temperature on the zeolite structure and catalytic performance of Ti-MWW have been investigated in detail.Methanol was found to be a better fluorination solvent than water in terms of preserving the zeolite structure and improving the catalytic performance.The SiO_3/2F species generated in fluorination increased the electropositively of the active sites in the neighborhood via electron-withdrawing inductive effects and a stronger hydrogen-bond was formed between H in Ti-OOH intermediates and the adjacent Si-F species,resulting in the enhanced catalytic performance of Ti-MWW in the ethylene epoxidation reaction.In the fourth part,in view of the emerging problem of complicated process and high energy consumption in the industrial production of ethylene glycol,one-pot synthesis of ethylene glycol at the mild reaction conditions has been realized by oxidative hydration of ethylene with hydrogen peroxide over titanosilicate catalysts.By comparing four titanosilicates of different topologies and solvents,deB-Ti-MWW/H₂O₂/H₂O was confirmed as the best catalytic system for the oxidative hydration of ethylene to ethylene glycol.The direct synthesis of ethylene glycol from ethylene,H₂O₂ and H₂O through oxidative hydration formally involves a combination of ethylene epoxidation and subsequent ethylene oxide hydration.Ti-OOH groups and Lewis acid sites of the framework Ti species are responsible for the hydration of ethylene oxide to ethylene glycol.The structural rearrangement proves to be an effective way to increase the catalytic activity of deB-Ti-MWW in the oxidative hydration of ethylene.In the fifth part,the liquid-phase epoxidation of propylene to propylene oxide?PO?over formed titanosilicate catalysts has been investigated in a fixed-bed reactor.The effects of reaction temperature,n?C₃⁼?/n?H₂O₂?molar ratio and weight hourly space velocity?WHSV?of H₂O₂ or solvent on the catalytic performance of the formed Ti-MWW catalyst have been extensively studied.The deactivation of catalysts is due to the deposition of heavy byproducts of propylene glycol?PG?with high boiling points inside micropores,and the deactivated Ti-MWW can be regenerated by high-temperature calcination.Adding an appropriate amount of ammonia in the reaction mixture prolonged effectively the catalyst lifetime.Fluorine-implanting enhanced remarkably the reactivity and lifetime of the catalyst in hydrogen peroxide propylene oxide?HPPO?process,exhibiting a PO selectivity of>99%.