Preparation Of Supported Au Catalysts Based On Urea Deposition And Their Performance For Gas Phase Epoxidation Of Propylene

Propylene oxide(PO)is an important organic chemical raw material that is widely used in the production of various organic chemicals.At present,PO production processes that have achieved industrialization include:chlorohydrin process,co-oxidation process,and H2O2 direct oxidation process.However,these processes have the disadvantages of serious environmental pollution,large investment,multiple co-products,and difficulty in H2O2 transportation.Therefore,some researchers have devoted to a new PO production process that is greener,pollution-free,cost-effective and highly selective.In the presence of H2 and O2,the direct gas-phase epoxidation of propylene to propylene oxide is considered promising alternative to the above process,and active catalyst has become the focus and challenging direction in current research.A large number of studies have shown that the titania-silica materials supported nano-Au catalysts have a good catalytic performance for this reaction,but there has not been a catalyst yet that can simultaneously achieve industrialization goal of 10%propylene conversion,90%PO selectivity,50%H2 efficiency and high stability.Therfore,such a catalyst deserves further research and development.Based on the previous research of the research group,this study aims to further improve PO selectivity and space-time yield under the condition that the propylene conversion(>10%)meets the requirements for industrialization.The catalytic performance of propylene epoxidation was further improved through comparative selection of supports and optimization of Au catalyst preparation methods.The mesoporous titanium silicalite molecular sieve TS-1 was prepared by a modified hydrothermal method,and titanium-silicon composite oxide Ti-O-Si was prepared by sol-gel method.They were used as supports to prepare supported Au catalysts via deposition precipitation method using urea(DP urea method).The catalytic performance of Au/TS-1 and Au/Ti-O-Si catalysts were tested;then excellent TS-1 was selected as support in the following experiments.The Au/TS-1 catalysts preparation conditions,such as the loading temperature,amount of urea,Au loading and Au reduction temperature are all optimized.Their effects on Au loading,support structure and Au particle size were systematically investigated by characterization techniques such as UV-vis diffuse reflectance(DRUV-Vis),X-ray diffraction(XRD),and UV-visible absorption spectra(UV-Vis),transmission electron microscopy(TEM)and Fourier transform infrared spectroscopy(FT-IR).The results show that Au nanoparticles are easier to enter the TS-1 channel and uniform in particle size distribution when loaded on TS-1,and the Lewis acid of Au/TS-1 catalysts surface is stronger than that of Au/Ti-O-Si catalysts.That is why Au/TS-1 catalysts are more conducive to propylene epoxidation.DP urea method is a novel effective way to load Au.The optimized preparation conditions,such as the loading temperature of 80?,molar ratio of urea/Au3+ of 10,the 1.0 wt.%Au loading and Au reduction temperature of 400?,are obtained to assure the high dispersion and utilization efficiency of Au NPs(2.4±0.4 nm)on the supports.The optimized catalytic performance,propylene conversion of 13.9%,PO selectivity of 78.7%and PO space-time yield of 180.8 gpo/Kgcat/h,was obtained at the reaction temperature of 300? and the space velocity of 7000 mL-gcat-1·h-1.To further improve the PO selectivity,changing the catalytic test conditions(reaction temperature and reaction space velocity),the results showed that the optimized reaction temperature of Au/TS-1 was 275 ?,and the optimal space velocity was 9000 mL·gcat-1·h-1,At this point,the PO selectivity reached 80.3%while the propylene conversion rate reaching the industrial target(>10%).Finally,the Au/TS-1 catalyst was also modified by adding alkali and alkaline earth metal.As a result,the PO selectivity and PO space-time yield were further improved.The results show that the additive metal promoter mainly affects the catalytic performance by changing the acidity of the catalyst surface and the Au loading.Among them,the catalytic performance was promoted higher when the bivalent alkaline earth metal is added.Too much or little acid on the surface of the catalyst is detrimental to the epoxidation of propylene.When K+ and Mg2+ metal cations are added,the PO selectivity can be achieved up to 82.5%without lowering the propylene conversion.Under the optimized reaction conditions,the catalytic performance of Mg2+-Au/TS-1 catalyst can be stable within 53 h.The characterization results show that undesorbed product on the catalyst surface may be vital for the deterioration of catalyst performance,which was not resulted from the Au particle agglomeration.