Preparation And Performance Evaluation Of Zeolite For The Alkylation Of Benzene With 1-Dodecene

A series of MCM-22 zeolite catalysts modified with phosphor were prepared by means of incipient wetness impregnation.The effects of phosphor on the physical chemistry characteristics and catalytic performance of the modified MCM-22 zeolite catalyst were investigated based on the combined characterizations of XRD,BET,NH₃-TPD,TG and FT-IR,with experimental evaluation.The results showed that the crystal structure was not destroyed after modified,but the relative crystallinity of zeolite samples decreased. Impregnating zeolite with proper amount of phosphor could enlarge the pore size of the zeolite,the phosphor played a role of expanding agent.However, the specific pore volume and average size of zeolite minished rapidly when phosphor was too much,that because polyphosphate chains were increased. But the specific surface areas of the zeolite samples changed a little.Moreover, the acidity of MCM-22 zeolite was changed when phosphor was introduced, with the increase of the phosphor content,the quantity of the weak acid sites increased gradually,the quantity of the strong acid sites decreased,the quantities of total acid sites and the Brφnsted acid sites increased firstly and then decreased,and the quantity of Lewis acid sites decreased.Proper phosphor content could improve the selectivity to 2-phenyldodecane and the catalytic stability of MCM-22 catalyst for the alkylation of benzene with 1-dodecene.The maximum yield of 2-phenyldodecane(2-LAB)was obtained in case that the phosphor content was 0.5%.Long time impregnation at lower temperature was benefic to the interactions between P species and framework atoms of zeolite,therefore,the catalytic performance of modified zeolites was improved.High temperature calcination couldn't reach the purpose of modification with P.The characterization of compounds deposited in deactive zeolites showed that the deactivation of catalysts was caused by a jam of bulkier molecules, such as linear alkylbenzenes,branch alkylbenzenes and aromatic compounds in raw material.About modifying mechanism,IR spectras of modified zeolites showed that symmetric stretch vibrations of T-O-T tended to low frequency,that probably because the strong interactions between the P species and the framework atoms of zeolite.²⁷Al MAS NMR demonstrated that partial Al-O-Si bonds were broken,and some tetrahedral aluminum was converted to octahedral aluminum.³¹p MAS NMR found the formation of aluminum phosphate species and polyphosphate chains.At low phosphor content,short chain polyphosphate species were more,as phosphor content increasing, highly condensed branching polyphosphates increased obviously. The zeolite modified with phosphorus has two possible structures,their main difference was the bridge oxygen of phosphorus coordination with aluminum or terminal oxygen coordination.The two forms of model cluster were established by atomic H saturation approach.Gaussian98 program and PM3 semi-empirical quantum chemical calculation were employed to study the stablility of two possible phosphorus modified structures,the modelling results showed that the terminal oxygen coordination was more possible according to the heat of formation.