Synthesis,Modification And Catalytic Performance Of TAPO-5 Molecular Sieves

TAPO-5 is a kind of Ti-containing aluminophosphate molecular sieve with AFI configuration.It had been reported to be active in several environmentally benign chemical processes,because of variable valence state of titanium atoms.However,in comparison to titanium silicates,these TAPO-5 exhibited somewhat low catalytic performance,which can be ascribed to the following factors:the lower relative contents of tetrahedrally coordinated titanium,weak surface hydrophobicity and weak lewis acidity of the active centers.At the same time,this type of molecular sieves were mainly prepared by hydrothermal method.High autogenous pressure generated in the crystallization process,long crystallization time,and the use of toxic organic templating agents were not conducive to the promotion of this method.Therefore,the research on the modified method to improve its catalytic oxidation performance,and on the method for preparing TAPO-5 efficiently have a high scientific significance and application value.In this article,a series of fluorine modified TAPO-5 molecular sieves were synthesized by one-pot method using ammonium hexafluorotitanate as titanium and fluorine sources.The XPS and DRS UV-Vis results indicated that the existence of F could promote the formation of tetrahedrally coordinated framework Ti and Al,meanwhile,inhibit the formation of anatase TiO₂ in TAPO-5 sieves due to the unique role of F in the sol-gel process of metal ions.Furthermore,TG,contact angle test,Py-FTIR and ²⁷Al MAS NMR results revealed that fluorine modification increased the surface hydrophobicity and the Lewis acidity of Ti active sites through forming Al_xF_y units in the neighborhood.All these factors made these fluorine modified TAPO-5 sieves exhibit good catalytic performance in the ammoximation of cyclohexanone.The conversion of cyclohexanone could reach 90.4%,and the selectivity to cyclohexanone oxime was 86.7%.The recycle experiment showed that the fluorine modified TAPO-5 had good stability.In view of time-consuming and low efficiency in the synthesis of TAPO-5 by traditional method,a method for effectively preparing TAPO-5 was developed in this article.Several TAPO-5 molecular sieves were synthesized ionothermally by microwave irradiation and using eutectic mixture based on succinic acid,choline chloride and tetraethyl ammonium bromide as both solvent and template.The influences of titanium source,aluminum source,raw material ratio and crystallization conditions,etc.on the synthesis of TAPO-5zeolite were investigated.XRD,N₂ sorption,SEM and DRS UV-Vis were used to characterize the structure,texture,morphology and coordinating status of titanium of the products.The results showed that the synthesis process of TAPO-5 molecular sieve was greatly affected by aluminum source and titanium source.Titanium source and P/Al mole ratio can cause changes on the acidity of the reaction system and results in the variation of the texture and structural properties of the products.The different reactivity of aluminum sources is the main reason for the difference in product properties.The optimal titanium source is ammonium hexafluorotitanate and the optimal aluminum source is aluminum isopropoxide.The resulting TAPO-5 molecular sieves exhibited a relatively high selectivity to benzaldehyde in the reaction of styrene oxidation toward benzaldehyde.The benzaldehyde selectivity was as high as91.4%.