Modification Of Titanium Silicalite-1 And Its Catalytic Application In Propylene Epoxidation

Propylene oxide?PO?is an important organic chemical product and the third most valuable propylene derivative after polypropylene and acrylonitrile.At present,it is mainly produced by chlorohydrin,hydroperoxidation and hydrogen peroxide?HPPO?processes.Chlorohydrin process should suffer from serious disadvantages?high resource consumption,brine and chlorinated byproducts?,while hydroperoxidation process produces equal amounts of coproducts requiring heavy capital investment.HPPO process also meets several problems,including the short life of catalyst,low utilization of H₂O₂,high consumption of energy and solvent.In the past decade,direct gas-phase propylene epoxidation using H₂ and O₂ provided a simpler,more environmentally friendly route and has attracted considerable attentions.Therefore,it is becomimg more and more important for green PO production to develope the efficient and stable catalysts and to optimize the reaction process.In this paper,the well-crystallized and uniform titanium silicalite-1?TS-1?zeolites were synthesizd by hydrothermal method before a series of Au/TS-1 catalysts were prepared by impregnation?IMP?,deposition precipitation?DP?and sol-enhanced immobilization?SEI?methods.Compared with IMP and DP methods,SEI method showed a greater advantage in the preparation of tiny Au nanoparticles with high dispersions.The effects of preparation method,Au loading,amount of PVA and concentration of NaBH₄ have been investigated on the gas-phase propylene epoxidation with H₂/O₂.An optimal performance with C₃H₆ conversion of 5.20%and PO selectivity of 84.83%and PO formation rate of 102.94 g_PO·h^-1·kg^-1_cat was achieved over 2.0wt%Au/TS-1 catalyst via SEI method,which was prepared by controlling PVA/Au³⁺mass ratio of 1.5/1 and NaBH₄/Au³⁺mole ratio of 5/1.After 50 h test at 200 ^oC,no significant decrement of catalytic activity can be observed,indicating the excellent thermally stability of catalyst.It can be concluded that the catalytic activity of Au/TS-1 catalyst benefits from the synergetic effect of Au species and Ti species.The lager particle size and higher metal dispersion would promote PO production.In addition,a series of bimetallic catalysts were prepared and characterized by SEI method,including Au-Pd/TS-1,Au-Ag/TS-1,Au-Rh/TS-1 and Au-Ir/TS-1.The results of catalytic performance revealed that both C₃H₆ conversion and PO selectivity of Au-Ag/TS-1 catalyst were lower than those of Au/TS-1 catalyst,while the hydrogenation of propylene would be obtained on Au-Rh/TS-1 and Au-Ir/TS-1 catalysts.Despite a lower PO selectivity,the higher PO formation rate was achieved over Au-Pd/TS-1 catalyst.The 1.0wt%Au-Pd/TS-1 catalyst prepared with Au/Pd mass ratio of 0.7/0.3,exhibited the highest improvement than 1.0wt%Au/TS-1 catalyst,with PO formation rate from 49.06 g_PO·h^-1·kg^-1_cat to 62.17 g_PO·h^-1·kg^-1_cat.It was found that the Au species were the dominant active components and played a leading role in PO production.The Pd species were beneficial for the adsorption and activation of H₂,which accelerated the formation of H₂O₂from H₂ and O₂.This work focused on investigating the modification of TS-1 zeolites and its catalytic application in gas-phase propylene epoxidation.The synthesized catalyst not only showed small Au particle size and high metal dispersion,but also exhibited remarkable catalytic activity for PO production.This research has overcome the problem of low Au utilization and poor thermal stability in gold catalysts,which would promote the further development of green PO production process.