Preparation Of Titanium-based Semiconductor Materials And Its Photocatalytic1 Vapor-phase Epoxidation Of Propylene

Propylene oxide（PO）is an important organic chemical product,which mainly produces polyether polyols,propylene glycol,and propylene glycol ether.Besides,it is also widely used in polyurethane foam,polyester and solvent industries.At present,industrial productions are mainly chlorine alcohol method,Hakang method and hydrogen peroxide oxidation method.Chlorine alcohol method produces waste water and waste residue.Hakang method has the disadvantages of large investment and co-products.Hydrogen peroxide oxidation method needs large solvent for circulation and high energy consumption.The search for new propylene oxide production process has been the goal of research workers.In this paper,we will study the preparation of propylene oxide by photocatalytic direct oxidation with molecular oxygen as oxidant to meet the needs of economic benefit and environmental protection.TiO₂ was prepared by hydrothermal method and Au/TiO₂ photocatalyst with different Au loading was prepared by photodeposition.The supported metal is uniformly dispersed on the TiO₂ support with a narrow particle size distribution（2-5 nm）.The effects of Au loading and reaction time on propylene oxide production rate had been discussed.The results show that the propylene oxide production rate increased with the increase of Au loading.When the loading of Au was 1wt%,the PO formation rate increased from 38.15μmol·g^-1h^-1 to 40.70μmol·g^-1h^-1,and the selectivity decreased from 27.08%to 20.60%.The improvement of photocatalytic activity is attributed to the following reasons:First,Au produces electrons which could react with O₂ to form free radicals under the conditions of plasma resonance.Next,Au acts to transfer photoelectrons to suppress electron recombination Effect,and thus improving the electronic utilization.With the increase of Au loading,the selectivity of PO is reduced,possibly because the metal load contributes to the occurrence of side effects.At T=323 K,1wt%Au/TiO₂ catalyst had the highest PO formation rate(40.70 μmol·g^-1h^-1).The highest selectivity of propylene oxide（27.08%）was obtained for TiO₂ catalyst without metal loading.For Au/TiO₂,Au plays a role in transferring electrons to suppress electron-hole recombination,thus improving the electron utilization efficiency.In addition,the microsphere-like TS-1（titanium-silicon molecular sieve）carrier was prepared with tetrapropylammonium hydroxide as template,and the Au/Ag bimetallic loading was carried out on the surface by impregnation reduction method.The effects of Au/Ag mass ratio,reaction temperature and other factors on the epoxidation rate of propylene are invesgated.The results show that the highest PO formation rate is 68.3μmol·g^-1h^-1 when T = 323K and Au/Ag mass ratio is 4/1,which is higher than TS-1(52.5μmol·g^-1h^-1)and Au/TS-1(64.9 μmol·g^-1h^-1).With the increase of Ag loading,the bimetallic particle size increases,which is not conducive to the adsorption of oxygen and electron transport,so the catalytic activity is reduced.Bimetallic loading is favorable for epoxidation of propylene,and the mechanism are followings.The introduction of bimetallic reduces the band gap of TS-1,enhances the utilization of UV light,and facilitates the adsorption and activation of O₂.The introduction of Ag promoted the transmission of photo-generated electrons,thus suppressing the recombination of electrons and holes,improving the utilization of photoelectrons and promoting the catalytic activity.