Catalytic Performance Of Phosphate Impregnated And Silylated TS-1 In The Liquid-phase Epoxidation Of Propylene

The liquid-phase epoxidation of propylene and H₂O₂ over TS-1 zeolite is an environment benign route for propylene oxide（PO）synthesis.However,because of the existences of both non-framework Ti species and framework defects in the TS-1 zeolite,the epoxidation is always accompanied by the ring-opening reactions of PO,i.e.the solvolysis of PO with methanol and the hydrolysis of PO with water introduced by H₂O₂.These side-reactions lead to ethers and diol by-products and weaken the competitiveness of the liquid-phase epoxidation route.Therefore,this thesis focuses on the modifications of TS-1 zeolite with phosphate impregnation and hexmethyl-disilazane（HMDS）silylation,in order to improve the selectivity of the liquid-phase epoxidation of propylene and H₂O₂.Main research activities and findings are as follows.First,the impregnation modification of TS-1 was carried out with five kinds of phosphates.Results indicate that,the impregnation of TS-1 with acidic phosphates is in favor of the enhancement of catalyst selectivity.This modification can also increase the activity of catalyst.A catalyst obtained by Na H₂PO₄impregnation（TS-1-Na H₂PO₄）increased the H₂O₂conversion and PO selectivity by 7 and 5 percentages,respectively,referring to its TS-1parent.With the helps of ³¹P MAS NMR,FT-IR,UV-Raman and UV-Vis characterization tools,as well as the experiment of water rinse treatment,it is inferred that,in the phosphate modified TS-1 zeolites,there should be phosphate anion monomer and oligomers.These species might interact with both tetra-coordinated framework Ti active sites and hexa-coordinated non-framework Ti species,and thus passivate the non-framework Ti species meanwhile modify the local environment of the active sites.Second,a preliminary silylation modification of TS-1 with HMDS was made.Results show that,this method is also useful for improving the activity and selectivity of TS-1 zeolite.When the concentration of HMDS in the cyclohexane solution was smaller than 0.05 mol/L,quick increases of both H₂O₂ conversion and PO selectivity as the increase of HMDS concentration were observed with the silylated TS-1 catalysts.The catalyst obtained with0.05 mol/L HMDS solution（corresponding to a maximum 10 wt.%loading of Si（CH₃）₃ group）exhibited the highest PO selectivity,which was about 95%.The catalyst surpassed its parent by 18 percentages in terms of PO selectivity and by 7 percentages in terms of H₂O₂conversion.Based on the characterizations of hydroxyl FT-IR and pyridine adsorption FT-IR,as well as the alcoholysis（methanol）of PO and static H₂O adsorption experiments,it is found that at least part of the Si（CH₃）₃ group were grafted into the hydorxyl nests of the defective TS-1 zeolite by the silylation of HMDS,and the improvement of catalytic performance of the silylated TS-1 catalysts could be attributed to the reduction of their acidity and the increase of their hydrophobicity.