A Study On The Construction Of Ti Active Sites And Corresponding Reaction Behaviours Directed By Atom Economic Synthesis Of PO

Titanosilicate(TS)/H2O2 catalytic system have obtained widely attention owing to its excellent catalytic performance.How to realize the high-efficiency synthesis of propylene oxide(PO)based on TS/H2O2 catalytic system is proved to be the development trend of the hydrogen peroxide-propylene oxide(HPPO)process.Thus,this thesis focuses on the investigation of the construction of Ti active sites and corresponding reaction behaviours on the basis of improvement of the C atom economy and O atom economy in the synthesis of PO catalyzed by TS/H2O2 catalytic system,directed by the core idea of green chemistry-"atom economy".We try to understand the nature of acid sites in TS/H2O2 catalytic system,the construction of Ti active sites as well as the key factors on influencing the transfer of O atom,and expect the obtained scientific knowledge and technical methods will provide the strong support for developing a new generation of HPPO process for high-efficiency synthesis of PO.(1)Firstly,according to a typical solvolysis reaction,the ring opening of PO with solvent CH3OH and H2O as well as the characterization of the IR spectrum,based on the reaction network in the HPPO process.A comprehensive investigation was carried out to understand the acid sites type,acid strength and corresponding influence factors in TS/H2O2 catalytic system,and corresponding strategies were adopted to suppress specific acid sites.It was found that there existed four types of acid sites resulting in the ring opening of PO,and corresponding alcoholysis activity decreased in the order of Ti-OOH>Si-OH(Ti)>>Ti4+ ? Ti-OH.The Br(?)nsted acid sites of Si-OH(Ti)groups over titanosilicate and Ti-OOH species over TS/H2O2 catalytic system were the dominated acid sites to catalyze the ring opening of PO.Furthermore,the alcoholysis products ratio of 1-methoxy-2-propanol(PPM)to 2-methoxy-1-propanol(SPM)was demonstrated to be correlated with the specific acid sites for the first time:Br(?)nsted acid sites tend to generate SPM,while Lewis acid sites of tetrahedral Ti4+ centers tend to form PPM.The confirmation of PPM and SPM proportion would be beneficial to guide the suppression of acid sites in titanosilicate and TS/H2O2 catalytic system.Fine ion exchange and proper additives were adopted to suppress corresponding specific acid sites in titanosilicate and TS/H2O2 catalytic system to finally achieve high selective synthesis of PO,promoting the C atom economy.(2)Secondly,according to another perspective of the C atom economy,the improvement of reaction activity.We successfully synthesized TS-1+ catalyst containing "TiO6" octahedron species and TS-1s catalyst containing open Ti(OSi)3OH active sites based on the construction of high-performance Ti active sites,and corresponding formation mechanism of "TiO6" and Ti(OSi)3OH species and catalytic performance were investigated.TS-1+ containing "TiO6" species was synthesized via hydrothermal treatmeLt of conventional TS-1 with ethamine(EA)and TPABr agents,respectively.The characterization results of UV-Vis,UV-Raman and IR spectra demonstrated that "TiO6"species was orientated formed from the transfer of some "TiO4" species.According to the track of microstructure change over "TiO6" pecies formation process,it was demonstrated that "TiO6" species was formed based on that the fractional Si species adjacent to framework Ti sites were removed and were migrated to the surface for secondary crystallization,without leaching any Si and Ti species.The investigation into the synthesis conditions indicated that the key to form "TiO6" species was controlling the balance between dissolution and simultaneous crystallization of Si species.TS-1+showed excellent catalytic performance in alkene epoxidation and cyclohexanone ammoximation reaction,and the TON of "TiO6s" species in 1-hexene epoxidation was 2-3 times that of conventional "TiO4" species,indicating TS-1+ was a promising industrial catalyst.TS-1s with open Ti(OSi)3OH species was synthesized through hydrothermal treatment of conventional TS-1 with EA.According to the characterization of TS-1s and the track of microstructure change in the hydrothermal treatment process,it was found that Ti(OSi)3OH species was generated via the selective dissolution of Si species around closed Ti(OSi)4 sites in the surface,and no mesopores was created.TS-1s showed superior catalytic oxidation activity in alkene epoxidation and cyclohexanone ammoximation reaction.The further quantitative analysis demonstrated that open Ti(OSi)3 OH species had the highest catalytic performance,and the catalytic performance among different Ti species decreased in the order of Ti(OSi)3OH>"TiO6">Ti(OSi)4.(3)Finally,since the intrinsic nature of TS/H2O2 catalytic system was the activation of H2O2 and transmission of active "O".Thus,a thorough investigation was carried out to understand the influence on the formation and stabilization of Ti-OOH reactive intermediates with Ti active sites,solvent,oxides,additive as well as reaction substrate on the basis of H2O2 reaction pathways,and the H2O2 utilization was regarded as the evaluation standard of the transmission of active "O".The results demonstrated that with regard to alkene epoxidation reaction,the key to improve the O atom economy is to promote the activation of H2O2 by Ti active sites to form Ti-OOH species,and stabilize the Ti-OOH reactive intermediates via hydrogen-bonding with solvent or additive,facilitating the "O" heterolytic cleavage of Ti-OOH species,suppressing the competing reaction of noneffective decomposition of H2O2 or Ti-OOH,to finally realize the high-efficiency transmission of active "O".