Synthesis, Characterization And Acid Catalytic Properties Of Titanium Silicalite-1

TS-1 has been studied for more than twenty years for low-temperature selective oxidations that use dilute hydrogen peroxide as oxidant.Although the acidic essence of TS-1 has been clearly shown by the methanol solvolysis of propylene oxide-a major side-reaction in the epoxidation of propylene,little attention has been paid to the characterization and exploitation of the acidity of TS-1.This may be owing to the fact that the acidity of TS-1 is usually too weak to be measured by those familiar methods like NH₃-TPD and pyridine-FT-IR.In this paper,the acidity characterization of TS-1 was made with the Hammett indicator titration method for the first time.In addition,two mode reactions,the cycloamination reaction of ethyldiamine to piperazine and DABCO and the etherification of propylene oxide and methanol,were also employed to study the acid catalysis performance of TS-1.It will be seen that,by using proper indicators,Hammett indicator titration was an effective method for TS-1 acidity characterization.The acid strength of TS-1 studied fell in H₀=+3.3～+4.8.The weaker acid sites in H₀=+3.85～+4.8 probably originated from framework Ti,while the stronger acid sites in H₀=+3.3～+3.85 related to amorphous octahedral coordination extraframework Ti.In the epoxidation of propylene only the stronger acid sites could trigger the side-reaction of propylene oxide with methanol.However,the weaker acid sites,although very weak,was surprisingly active and selective to convert ethyldiamine to piperazine and DABCO,and showed higher activity and selectivity than HZSM-5.It is possible to use TS-1 as acidic catalyst for reactions other than selective oxidations.XRD,DRUV-Vis,UV-Raman indicate that the forms of extraframework Ti species were closely related to the content of Na⁺ ion in the gel.Gels with low Na⁺ content inclined to have anatase as extraframework Ti species.But gels with high Na⁺ content inclined to have amorphous octahedral coordination Ti as extraframework Ti species.