Preparation Of Titanosilicate Supports And Their Catalytic Application In Propylene Epoxidation

Propylene oxide(PO)is a major organic intermediate.Its downstream products are widely used in the chemical industry.Recently,with the rapid development of economy,there has been a tremendous increase in the demand for PO.Its industrial production has attracted wide attentions.Currently the industrial manufacture for PO includes the chlorohydrin,co-oxidation and the H2O2 direct oxidation(HPPO)processes.However,there are many problems with these processes,the chlorohydrin process is associated with massive pollution,the co-oxidation process follows a complex technological process and while the HPPO process is a high cost technique.Therefore a greener and more economical production process needs to be developed.Vapor-phase epoxidation of propylene in the presence of H2 and O2 to synthesize PO has the advantages of being green and profitable.It has become one of the critical research highlights.Moreover,Titania-silica composite materials supported nano-Au catalyst for direct vapor-phase epoxidation of propylene in the presence of H2 and O2 have exhibited excellent performances,but efficiency of H2 is below 50%and selectivity of PO is below 90%.Therefore to improve the catalyst performance has become a hot spot in this research field.In this paper,titanium-containing mesoporous materials(TS-1,Ti/SiO2)supported nano-Au catalysts are applied in the vapor-phase epoxidation of propylene in the presence of H2 and O2.First,TS-1 zeolite was hydro-thermally synthesized using SiO2/TEOS as mixed silicon source and tetraptopylammonium hydroxide(TPAOH)as a template.And the SiO2/TEOS molar ratios,crystallization time,crystallization temperature were optimized.Their effects on structure of the support were systematically investigated by Characterization techniques such as X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR),UV-Vis diffuse reflectance(DRUV-Vis),and Scanning electron microscopy(SEM).The nano-Au catalyst is prepared by urea precipitation and the catalytic performance of the catalyst was evaluated by propylene epoxidation.The results showed that,the addition of mixed silicon source can prevent the formation of non-framework Ti,improve the relative crystallinity and the Ti content in the framework.The morphology of TS-1 synthesized using mixed silicon resource was six-party prismatic type.The optimal synthesis conditions were as follows:SiO2/TEOS molar ratios 8:1,crystallization time 18 h,and crystallization temperature 170 ?.Under optimal conditions,C3H6 conversion of 14.7%,H2 efficiency of 18.4%,and space-time yield of PO of 172.5 gpo/Kgcat/h were obtained.Secondly,the TS-1 was synthesized by using TEOS as silicon source and modified by NaOH aqueous solution by the microwave radiation method.And the effects of modification conditions(the concentration of NaOH,microwave radiation time)on the properties of TS-1 were investigated by Characterization techniques such as XRD,FT-IR,DRUV-Vis,and BET.A series of Au catalyst supported on NaOH-modified TS-1 were prepared and the catalytic performance of these catalysts was evaluated by propylene epoxidation.The results showed that,NaOH can selectively dissolve Si,exhibit a large BET surface area together with well-developed mesoporosity,and increase the content of framework titanium in TS-1.Furthermore,the increased catalytic active sites improve TS-1 catalytic activity in propylene expoxidation.The optimal modification conditions were as follows:modification concentration 0.015 M,microwave radiation time 9 s.Under these conditions,the NaOH-modification promoted C3H6 conversion from 11.4%to 14.7%,efficiency of H2 from 10%to 18.4%,and space-time yield of PO from 119.1 gpo/Kgcat/h to 172.5 gpo/Kgcat/h.Finally,the amorphous Ti/SiO2 support with three-dimensional wormhole-like mesoporosity was prepared by sol-gel method and the amount of triethanolamine,amount of deionized water,the amount of TPAOH were optimized.The nano-Au catalyst was prepared by urea precipitation,and added alkali metal or alkali earth metal nitrates in the preparation process.The catalytic performance of Ti/SiO2 was evaluated by propylene epoxidation.Their effects on structure of the support and Au particle size were systematically investigated by Characterization techniques such as XRD,DRUV-Vis,FT-IR,TEM and BET.The results showed that,too much or too little amount of triethanolamine or deionized water resulted in a decline in catalytic activity during the preparation of supports.The amount of TPAOH strongly affects the activity of the catalyst.The optimal synthesis conditions were as follows:Ti/triethanolamine molar ratios 1/100,Ti/deionized water molar ratios 1/550,Ti/TPAOH molar ratios 1/10.Under optimal conditions,C3H6 conversion of 12.3%,H2 efficiency of 17.4%,and space-time yield of PO of 156.1 gpo//Kgcat/h were obtained.The addition of alkali metal or alkali earth metal nitrates strongly increases the H2 efficiency from 17.4%to 29.8%,while little influence.on the C3H6 conversion and space-time yield of PO during the epoxidation of propylene is observed.