Effect Of Surface Acidity Of HZSM-35 Zeolite On Catalytic Transformation Of Butene

With the full promotion of ethanol gasoline and increasingly stringent technical requirements,MTBE will not be added to gasoline as a blending component,which will lead to overcapacity in the production of MTBE-isobutene.The conversion of isobutene into skeletal isomerization reaction into n-butene with wide industrial application has become one of the effective methods to solve this dilemma.This is a production route with great application prospects.At present,ZSM-35 zeolite has an eight-membered ring and a ten-membered ring perpendicularly intersecting two-dimensional pore structure.Because of its suitable pore structure and acid distribution,it is a good material for catalyzing the isomerization reaction of isobuene.However,problems such as low selectivity and poor stability are still widespread.In this paper,ZSM-35 zeolite was modified by different modification reagents（dealuminizing agent and surface modifier solution）at different concentrations.Modified ZSM-35 zeolite samples with different active site compositions and distributions were obtained.A series of modified ZSM-35 zeolite samples with different acid types,density and accessibility were prepared by using different concentrations of ammonium hexafluorosilicate（AHFS）,which modify ZSM-35 zeolite by isomorphous substitution.The texture properties were characterized by XRD and N₂ adsorption isotherms.It was found that after the dealuminizing agent treatment,the samples undergo different degrees of skeletal dealuminization,but it did not cause significant damage to the zeolite structure,only the crystal fine structure has a certain impact.And,the acid properties of were characterized by NH₃-TPD and Py-FTIR.During the dealumination process,two new Lewis acid sites with different strength were produced,which due to the interaction between the dealuminizer and the extra framework aluminum hydroxyl species in the zeolite.During the reaction,the above two Lewis acid sites promoted the oligomerization-cracking reaction of isobutene,thereby,reducing the selectivity of the main reaction.The surface modification of ZSM-35 was carried out with different kinds of surface modifiers of TEOS,TETI and（NH₄）₂HPO₄ with different concentrations,and the modified zeolite samples with different acid sites were obtained.The texture properties were characterized by XRD and N₂adsorption isotherms.The acid properties of modified ZSM-35 zeolite were correlated by NH₃-TPD and Py-FTIR,and the structure-activity relationship between the acid site of modified ZSM-35 zeolite and the isomerization reaction of isobutene skeletal was explored in combination with the reaction evaluation.The results showed that the non-selective active sites on the acid sites on the outer surface of ZSM-35 to promote the side reactions such as oligomerization-cracking were covered after low concentration TEOS treatment.The selectivity of the n-butene will be slightly improved.However,after ZSM-35 is treated with high concentration TEOS,the selectivity effect will change due to the blockage of pores and the self-aggregation of Si O₂ at the pore mouth.Compared with TEOS,the effect of chemical liquid deposition on the improvement of reaction evaluation results is limited after the surface modification of ZSM-35 by high concentration TETI.However,（NH₄）₂HPO₄ used by impregnation method showed excellent surface modification performance.Under the optimal treatment concentration determined by the yield of n-butene,and the n-butene selectivity could be increased to 94%,12%higher,and the conversion rate was only 6%lower.