Preparation Of Titanosilicalite-1 Zeolite Supported Nano Gold Catalysts And Study Of Catalytic Performance For Gas-phase Epoxidation Of Propylene

Propylene oxide（PO）is an important chemical intermediate and manufactured industrially by either chlorohydrin or co-production routes.The former process consumes a large amount of chlorine and causes environmental pollution.The latter process is capital intensive and produces equimolar amounts of co-products.Direct gas-phase epoxidation of propylene using hydrogen（H₂）and oxygen（O₂）exhibits an environmentally friendly process with the only by-product of water（H₂O）.In this dissertation,a research has been made on the preparation of titanosilicalite-1 zeolite supported nano gold catalysts（denoted as Au/TS-1）and the study of catalytic performance for gas-phase epoxidation of propylene.First,Au/TS-1 Cs was prepared by the deposition precipitation method with cesium carbonate（Cs₂CO₃）as the precipitation agent and tested for gas-phase epoxidation of propylene.Interestingly,the catalytic performance at lower temperature（164 ℃）was better than those at higher temperature（200 ℃）,including PO rate～300 gPO·h^-1·kgcat-1,PO selectivity～98%and H₂ efficiency～26%.Results show that the change of Au particle size plays a major role,which changed from 3.6 nm（at 164 ℃）to 5.1 nm（at 200 ℃）.The carbon deposits on catalyst surface and the surface acid amount of catalyst have little effect.Therefore,164 ℃ may be a promising reaction temperature for gas-phase epoxidation of propylene over Au/TS-1 Cs.Second,three types of catalysts,titanium silicate-1（TS-1）,uncalcined TS-1（un TS-1）and TS-1 coated with silicalite-1（S-1/TS-1）Ssupported nano-gold catalysts（denoted as Au/TS-1,Au/un TS-1 and Au/S-1/TS-1）with sodium carbonate（Na2CO3）or Cs₂CO₃ as the precipitation agent,were prepared,analyzed and compared to confirm the gold active sites for gas-phase epoxidation of propylene.The result of Au/TS-1 Cs and Au/TS-1 Na shows the importance of Au nanoclusters（<1.0 nm）inside TS-1 nanoporous channels.PO rate of Au/un TS-1 was detected at the early stage,suggesting Au nanoclusters（<1.0 nm）on the external surface also play an important role.Through designing Au/S-1/TS-1 with different thickness of S-1 shells,Au nanoclusters（<1.0 nm）on the external surface of TS-1 and inside the TS-1 nanopores are proved to be the dominant active sites for PO reaction.Furthermore,PO rate of Au nanoclusters（<1.0 nm）on the external surface was calculated to～30 gPO·h^-1 kgTS-1-1.Finally,Au/TS-1 Cs prepared by the deposition precipitation method was applied in gas-phase epoxidation of propylene with O₂ and H₂O.The average PO rate of Au/TS-1 Cs was～1.5 gPO·h^-1·kgcat-1,which was lower than that of～2.3 gPO·h^-1·kgcat-1 prepared by the solid grinding method.Up to now,the Au-Ti catalysts have poor performance in O₂-H₂O system for propylene epoxidation.