Effect Of Hydrothermal Modification On The Structure Of TS-1 Zeolites And Its Catalytic Performance For Alkene Epoxidation

TS-1 is a type of titanium-silicalite zeolite with MFI topology.It exhibits excellent catalytic performance in a variety of selective catalytic oxidation reactions using hydrogen peroxide as the green oxidant.The TS-1 catalyzed alkene epoxidation is one of the most important industrial catalytic reactions,which has been successfully applied to the industrial production of propylene oxide.However,TS-1 zeolites synthesized by traditional hydrothermal methods still have some problems,such as high synthesis cost（e.g.using expensive template of tetrapropylammonium hydroxide）and difficulty in precisely controlling the active titanium species.For these reasons,researchers have been trying to improve the synthesis processes of TS-1 zeolites,and to combine some post-processing methods to control the morphology,porosity,polarity and titanium species states of the zeolites.Studies have revealed that using the basic characteristic of tetrapropylammonium hydroxide（TPAOH）,the as-synthesized TS-1 zeolite could be hydrothermally modified,that may regulate the pore structure of the zeolite and the distribution states of titanium species.The modified TS-1 zeolites exhibit an enhanced catalytic activity for phenol hydroxylation and alkene epoxidation.However,these kinds of hydrothermal post-treatments usually cause the generation of more defect sites in the TS-1 zeolite crystals,leading to the formation of some unfavorable hydroxyl groups（e.g.Si-OH）that can decrease the epoxy selectivity of the reaction.In this thesis,we tried to use different approaches to conducting hydrothermal modification of the TS-1 zeolites synthesized with low-cost organic template（TPABr）.The effect of the post-modification methods and conditions on the catalytic performance of TS-1 zeolites was investigated by the epoxidation reactions of cyclopentene and propylene with H₂O₂ as oxidant.In addition,combined with a variety of characterization techniques,the pore structure of the post-modified TS-1 zeolites and the state of titanium species were also studied for building a relationship between the nature of the catalyst active centers and the catalytic performance of the TS-1 zeolites.The main research contents and results are as follows:1.Low-cost TS-1 zeolites were firstly hydrothermally synthesized by using ethanolamine（EA）as the alkali source,tetrapropylammonium bromide（TPABr）as the template,silica sol as the silicon source,butyl titanate（TBOT）as the titanium source in the presence of TPABr（template）and isopropanol（co-solvent with water）.The calcined TS-1 zeolites were then hydrothermally post-treated with a mixed solution containing a certain amount of ammonia,TPABr and KCl（or Na Cl）at 170°C for some while.A variety of characterization results demonstrated that the modified TS-1 zeolites possess a large number of hierarchical pores and increased specific surface area.In addition,the content of the six-coordinated framework titanium species in the modified TS-1 zeolites increased somewhat,and the number of the surface hydroxyl groups decreased considerably,implying the reduction of weak acid sites and the enhancement of the hydrophobicity on the surface of the zeolites.Compared with the parent TS-1zeolite,the modified TS-1 zeolite which was obtained by optimized post-treatment conditions exhibited much higher catalytic activity and epoxy selectivity for the epoxidation of cyclopentene.After reacting at 40℃for 2 hours,a 52.0%conversion of cyclopentene was reached with a 98.2%epoxide selectivity.In addition,the modified TS-1 zeolite could be easily recycled,and its catalytic activity remained well with increasing recycling numbers,indicating the excellent catalytic activity and stability of the catalyst.The enhancement in the surface hydrophobicity of the modified TS-1zeolite as well as the generation of more highly active hexa-coordinated titanium species should be mainly responsible for the improvement of the catalyst performance.2.Another kind of TS-1 zeolite was synthesized by the same hydrothermal synthesis method as mentioned above,except using H₂O₂ replacing isopropanol（as a complexing agent for dispersing titanium source）.The calcined TS-1 zeolite was firstly subjected to hydrothermal treatment in an aqueous ethanolamine solution at 170°C.After cooling to room temperature,TPABr and KCl were added into the mixed system,then the secondary hydrothermal modification treatment was conducted at 170°C again.The characterization results showed that the secondary modified TS-1 zeolite has relatively high mesopore volume and large specific surface area.With the increase of the concentration of KCl,the number of surface hydroxyl groups of TS-1 zeolite gradually decreased,accompanied with the decrease of the weak acid sites and the enhancement of the hydrophobicity of the zeolite.The modified TS-1 catalyst obtained by optimization of conditions exhibited exceptional catalytic activity and stability for the epoxidation of propylene,with very high utilization rate of H₂O₂（99.8%）and epoxide selectivity（98.7%）at very mild conditions（25℃and 0.4 MPa）after 1 h reaction.The catalytic performance of the modified TS-1 zeolite,especially the low-temperature activity,is not only much higher than that of the parent TS-1 zeolite,but also significantly better than other types of titanium-silica zeolite catalysts reported in literatures.In summary,by adopting suitable hydrothermal modification treatment methods and conditions,the pore structure,the state of framework titanium species,the surface acidity and the hydrophobicity of the TS-1 zeolite can be effectively controlled within a certain range,which could lead to the formation of modified TS-1 zeolites with excellent catalytic activity,selectivity and stability for the epoxidation of alkenes.The above research results could be beneficial to designing low-cost TS-1 zeolites with excellent catalytic performance for alkene epoxidation.