| The titanosilicate/H2O2catalytic system has attracted much attention because of its outstanding catalytic performance in alkene epoxidation,which conforms to the concepts of"green chemistry"and"atomic economy".In view of the issues existing in the synthesis and production of high value-added fine or bulk chemicals,it is significant to develop the environmentally friendly production process based on titanosilicate/H2O2system for the realization of the goal of"green chemistry".In this thesis,from the view points of pore structure and diffusion,Ti active sites and inhibition of side reactions,the structure dimension and Ti active sites in the titanosilicates were precisely chemically modified and their effects on the catalytic ability were well investigated.It is expected to provide some theoretical basis and technical methods for the design and synthesis of high-performance catalysts and the practical process technology based on the titanosilicate/H2O2system.The main contents are as follows:(1)In view of the problems of low reactivity and PO selectivity as well poor stability in the hydrogen peroxide propylene oxide(HPPO)process over shaped Ti-MWW catalyst on a fixed-bed reactor,the structurally rearranged two-dimensional shaped Ti-MWW catalyst(RS-Ti-MWW)with good mechanical strength was successfully prepared by hydrothermal post-treatment,during which the basic piperidine(PI)molecules were embedded into the frameworks of Ti-MWW zeolites.After modification,the Ti active sites in the extrudates were changed from the original framework Ti(OSi)3OH to framework Ti(OSi)4and Ti(OSi)2(OH)2(H2O)PI.Although the introduction of PI molecules increased the diffusion limitation,the presence of Ti(OSi)2(OH)2(H2O)PI could decrease the activation enthalpy of H2O2activation process,which improved the reaction activity of propylene epoxidation significantly.The epoxidation activity of RS-Ti-MWW in protic solvent Me OH was higher than that in aprotic solvent Me CN,and the improvement of activity in solvent Me OH after modification was significantly higher than that in solvent Me CN,exhibiting a unique solvent effect.The Bronsted acid sites of Si-OH/Ti-OH defects in the Ti-MWW zeolites were quenched by the basic PI molecules inserted into the frameworks,which effectively inhibited the side reactions of PO ring-opening so as to improve the PO selectivity and stability of the catalyst in the HPPO process.In addition,the N atom in PI molecule could act as a H-bond acceptor to form the H-bond with the terminal H in Ti-OOH,which is assumed to stabilize the Ti-OOH species and to promote the effective transfer of active O atom,thus improving the H2O2utilization.(2)In view of the negative effects of inert binders in the shaped Ti-MWW catalyst on the accessibility of Ti active sites and reaction performance in HPPO process,the binder-free Ti-MWW monolithic catalyst with a Si-rich shell was synthesized by recrystallizing the binder(silica)/Ti-MWW extrudates with the assistance of dual-organic structure?directing agents of TMAda OH and HMI,which not only enhanced the mechanic strength,but also improved the porosity to relieve the internal diffusion limitation.During the recrystallization process,a partial of Si ions in the Ti-MWW matrix were dissolved,increasing the internal defect sites of Si vacancy,meanwhile a part of framework Ti O4species were transferred into the open Ti O6species with stronger Lewis acidity.These promoted the enrichment and activation of H2O2in the catalyst,thus improving the epoxidation activity.The microenvironments of the Ti active sites of the monolithic Ti-MWW catalyst were further modified by PI and NH4F treatment successively,which not only improved the epoxidation activity but inhibited the PO hydrolysis effectively.At a lower solvent space velocity,co-feeding a trace amount of(NH4)2CO3into the reactant mixture could achieve a higher reaction performance and an ultra-long lifetime of 2400 h in a single cycle.(3)The HPPO process based on the Ti-MWW/H2O2/Me CN catalytic system has been investigated in a slurry bed reactor.The Ti-MWW microspheres with a Si-rich exterior(Ti-MWW-DT)were successfully obtained by hydrothermal treatment of the spray-dried Ti-MWW microspheres with TMAda OH and HMI,during which a partial Si dissolution and recrystallization process occurred.In comparison to the original Ti-MWW,the specific surface area and mesopore volume of Ti-MWW-DT were enhanced so as to improve the diffusion efficiency.The internal hydrophilicity was enhanced,which promoted the enrichment of H2O2in the micropores of Ti-MWW zeolites.Meanwhile,more open framework Ti(OSi)3OH and a small amount of Ti(OH)2(OSi)2(H2O)2were constructed,which enhanced Lewis acidity of catalyst.And the apparent activation energy of H2O2activation was significantly decreased so as to improve the reactivity of propylene epoxidation.The effects of C3H6/H2O2molar ratio,feed H2O2concentration,Ti-MWW-DT concentration,reaction temperature and retention time on the reaction performance of HPPO were systematically investigated,and the stability and regeneration performance of Ti-MWW-DT were also investigated under the optimal operating conditions.(4)In view of the problems of serious pollution and corrosion of equipment as well poor safety in the traditional process of producing the high value-added1,2-epoxypentane(EP),a green synthesis of EP has been realized via titanosilicate/H2O2-catalyzed 1-pentene epoxidation process.By investigating the influence of topologies of titanosilicates on the catalytic performance and the solvent effects,Ti-MWW/H2O2/Me CN system was determined as the most suitable catalytic system.The effects of reaction conditions on the 1-pentene epoxidation performance over Ti-MWW were investigated.The open framework Ti(OSi)2(OH)2(H2O)PI species were constructed by introducing PI molecule as ligand,which increased the epoxidation activity of 1-pentene as well inhibited the side reaction of EP hydrolysis effectively,thus improving the reactivity and stability of Ti-MWW. |
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