Modification Of TS-1 Supported Gold Catalyst And Its Application In Gas-phase Epoxidation Of Propylene

Direct gas-phase epoxidation of propylene is a new process,which has many advantages such as environmental protection and simple operation.However,the catalyst of Au/TS-1（TS-1 denoted as Titanium silicalite-1）for this reaction still needs to be improved in many aspects,such as low Au capture efficiency,catalytic activity and low hydrogen efficiency.In this work,Au/TS-1 catalyst was optimized by adding modifiers and applied in gas-phase epoxidation of propylene.The details are as follows:First,starting from the carrier,nickel cations as modifiers were added during the deposition process,which would change the charge state of TS-1 and result in gold complexes more easily adsorbed on TS-1.We found that with the concentration of Ni increases from 0 to 0.071 g _Ni·L^-1 in solution,the Au capture efficiency improves nearly 4 times and the Au loading increases from 0.065wt%to 0.252wt%,which leads to the increase of the number of Au active sites that results in the improvement of PO rate from 90 g _PO·h^-1·kg_cat^-1to 155 g _PO·h^-1·kg^-1_cat.However,when the concentration of Ni continues increasing to 0.355 g _Ni·L^-1 in solution,the Au loading increases to 0.396wt%,PO rate decreases to 108 g _PO·h^-1·kg^-1_cat,which is due to the formation of the larger Au particles when the Au loading is too much.Second,starting from gold complexes,ammonium chloride as a modifier was added during the deposition process.When the p H value of solution was above 9.45,gold complexs react with ammonium chloride.The new gold complexes with positive or neutral charge state can be adsorbed well with TS-1 that with negatively charge state.This situation greatly improves the Au capture efficiency.With the increase of NH₄Cl/Au molar ratio from 1 to 4 in solution,the Au capture efficiency increases from 8.32%to 97.01%.BET,quench molecular dynamics simulation and thermogravimetric analysis show that with the increase of the molar ratio of NH₄Cl/Au in solution,Au particles tend to be loaded on the surface of TS-1.With the Au loading increases from 0.080wt%to 0.931wt%,PO rate decreases from 120 g _PO·h^-1·kg^-1_catto 108 g _PO·h^-1·kg^-1_cat.Au particles were easy to agglomerate and large Au particles appeared after reaction when the Au loading was too much,which were not benefit for propylene epoxidation.Finally,the active metal Pd was introduced into the catalyst to form Au Pd alloy,and Au/TS-1,Au Pd/TS-1 and Pd/TS-1 catalysts were prepared by impregnation method.It was found that Au/TS-1 is inactive for propylene epoxidation at low temperature（50℃）.When the temperature rose to 150℃,the average of PO rate within 5 h was 29 g_PO·h^-1·kg^-1_cat,it indicates that Au/TS-1 requires a higher temperature for propylene epoxidation.However,when Pd atoms were introduced into Au/TS-1,the PO rate was up to 42 g _PO·h^-1·kg^-1_cat at low temperature.Density Functional Theory（DFT）calculation indicates that Au Pd clusters are closer to Fermi energy level compared with Au clusters,which enhance the chemical reactions on the surface.Moreover,O₂ obtains more electrons on the Au Pd clusters that is benefit for the formation of O₂^-.The transition state search shows that compared with Au slusters,O₂ adsorbed on Au Pd clusters needs to overcome a lower energy barrier to form H₂O₂,which is benefit for propylene epoxidation of Au Pd/TS-1 at low temperature.