The Study On Effect Of Alkali Treatment On High-silicon ZSM-5 Catalyst As Alkylation Of Benzene With Methanol

HZSM-5 zeolite has been widely studied in alkylation of benzene with methanol because of its high catalytic activity.However,HZSM-5 has poor stability,and come easily carbon deposit,then leads to deactivation of catalyst,which is the limitation of its application.So that,in the paper,the desiliconization of HZSM-5 zeolite was treated with alkali solution.The effects of inorganic base,organic base,mixed base system and metal oxide loading on the alkylation performance and physi-chemical properties of ZSM-5 catalysts were investigated.Then,catalyst with a high alkylation activity and an excellent stability was obtained.In this paper,the effect of modification of inorganic/organic base on the alkylation of benzene and methanol over ZSM-5 catalysts was studied.The results showed that in a low treatment amount of inorganic/organic base,the acid content increases moderately and alkylation activity increases.However,in the high treatment amount,the acid content of the catalyst increases too much,which leads to the serious carbon deposition and the reduction of benzene conversion.The treatment amount of inorganic base is 2.4mmol/g_cat,the activity of the catalyst is about 16%higher than HZSM-5,while the activity increases by nearly 24%when the treatment amount of organic base is 4.0mmol/g_cat.The effects of mixed base system factors:total base amount,the ratio of inorganic base to organic base,and the amount of organic base on the performance of the catalysts were investigated.The results indicated that the alkylation activity increases first and then decreases with the increasing of total alkali content.When the total alkali content is6.4 mmol/g_cat,the activity of ZSM-5 catalyst is up to 48%,and the catalyst has better crystal structure preservation,the more mesoporous,high capacity of carbon and favourable stability.The number of weak acid sites of ZSM-5 catalyst treated with the ratio of inorganic base to organic base of 0.6 and the amount of organic base of 4.0mmol/g_cat is reasonable,and the benzene conversion is 43%.However,the exorbitant of ratio of inorganic base to organic base,and the overdose of organic base content,would lead to the excessive increase of weak acid sites of catalysts,which causes more secondary reaction,severe carbon deposit,and the decrease of benzene conversion.ZSM-5 catalyst treated with organic base content of 4.0 mmol/g_cat was modified to the metal oxide,and its performance was studied.The results demonstrated that with the increase of MgO loading,the coverage of acid centers on the external surface increases gradually,the total acid content reduces,and the benzene conversion decreases,but stability of catalysts enhance.The stability evaluation of catalysts pre-and post-modified by MgO was carried out under liquid hourly space velocity of 85 h^-1.It's showed that the pre-modified catalyst deactivated quickly,and performed a poor stability.The catalyst with MgO loading of 10%has a minimal decline in activity,the lowest carbon deposition and the optimal stability.The further stability investigation of catalyst with a MgO loading of 10%was carried out under liquid hourly space velocity of 15 h^-1,the benzene conversion of the catalyst is still 46%after stable operation of 100 h,and exhibited an excellent stability.