Post Treatment Of Titanium Silicalite-1 And Its Catalytic Properties For Propylene Epoxidation

Propylene oxide is one of the key propylene-based chemical intermediates.Conventionally,PO is commercially manufactured by two methods,the chlorohydrin method and the hydroperoxide oxidation method,which can not meet the requirements of environmental protection.The process of propylene epoxidation catalyzed by Titanium Silicalite?TS-1??HPPO process?has been studied and regarded as a process with high selectivity,mild reaction conditions,simple operation mode and environment-friendliness,which attracts much attention in recent years.The process is the development direction of industrial production of propylene oxide.Although the HPPO process has been used in industry,the catalyst of the reaction still needs to be researched because it lacks good stability and deactivates fast.The purpose of this work is to enhance the catalytic performance of TS-1 by post treatment in HPPO.Firstly,TS-1/SiO₂ extrudates were hydrothermally treated with mixtures of TPAOH and NH₃?H₂O.The influences of mixed alkaline?TPAOH/NH₃?H₂O?treatment on the physicochemical and catalytic properties were totally studied.The results demonstrated that addition of NH₃?H₂O in the post treatment could enhance the catalyst stability.With increasing NH₃?H₂O concentrations,the mesopore volumes,specific surface area and external surface area improved appreciably.However,when the concentration of NH₃?H₂O was further increased to 0.09 mol?L^-1,many visible etching marks appeared and the framework Ti amounts decreased,which led to the inferior catalyst stability.The optimized technological conditions of treated TS-1 are as followings:TPAOH=0.06 mol?L^-1 and NH₃?H₂O=0.03 mol?L^-1.Secondly,the effects of post treatment of TS-1/SiO₂ extrudates with TPAOH(0.06 mol?L^-1)and various ammonium salts??NH₄?₃PO₄,?NH₄?₂SO₄,NH₄CH₃CH₂,NH₄Cl,NH₄F,?NH₄?₂CO₃,NH₄NO₃ and?NH₄?₂TiF₆?on the physicochemical and catalytic performances were studied.The results showed that addition of various types of ammonium salts could decrease the amounts of extra-framework Ti and thus increase the catalytic performances of TS-1.Among them,TS-1/SiO₂ extrudates treated with TPAOH and?NH₄?₃PO₄ could decrease the amounts of extra-framework Ti and slightly increase the framework Ti amounts,showing the highest catalyst stability?30 h?.As a possible explanation,the introduction of PO₄^3-in the dissolution and recrystallization process could appropriately slow down the recrystallization rate and further achieve a better match between dissolution and recrystallization rates.Hence,the introduction of recrystallization-mediating agent in the post treatment is of ultra-significance to the preparation of high-performance TS-1 zeolites.