Study On Benzene Alkylation With Methanol In Reforming Gasoline Over Silicalite-1 Catalyst

In order to make motor vehicles gasoline to be much cleaner,the refinery plant needs to develop advanced technology,and increase the content of reforming gasoline with high octane number and low sulfur content in gasoline pool.Benzene in motor vehicles gasoline is mainly from reforming gasoline,so that we need to use abundant methanol resource in China to reduce benzene in reformate;in other words,we need to develop the technology of reformate benzene alkylation with methanol,which could effectively reduce benzene in reforming gasoline,make full use of methanol resource,and increase gasoline yield.As the most widely used catalyst of benzene alkylation with methanol,ZSM-5 zeolite has strong acid property,so that it could have deeper alkylation reaction and various side reactions.Therefore,this paper used Silicalite-1zeolite which is similar to ZSM-5 zeolite as the catalyst,applied to the reaction of benzene alkylation with methanol to reduce benzene in reformate.When the Silicalite-1 zeolite is synthesized with TEOS as the silicon source and TPAOH as the template,the mole ratio of TEOS:TPAOH:H₂O is 1:0.2:50.Then Silicalite-1 zeolite and two kinds of ZSM-5 zeolites which have a different silica alumina ratio,were characterized by various techniques and applied to benzene alkylation with methanol.After that,we found ZSM-5 zeolite with the high silica alumina ratio had the best performance in alkylation reaction and it’s optimal reaction temperature is 460℃,because it had appropriate acid property,abundant mesoporous structures and moderate active sites.Because of weak acid property and lack of active sites,Silicalite-1 zeolite could not activate benzene in alkylation reaction so that methanol mainly reacted in MTO reaction.To enhance the acid amount of Silicalite-1 zeolite,Zn modified Silicalite-1 zeolites with different loading were prepared by impregnation method.After that those catalysts were characterized by various techniques and applied to benzene alkylation with methanol,we found Zn modified Silicalite-1 zeolite had the stronger acid property and better alkylation performance than unmodified one,and the best loading of Zn impregnation was 1.0%（wt）.The performance of Zn modified Silicalite-1 zeolites was related to the existent state of Zn species,not to the loading of Zn.Then we designed the experiment to explore the active sites of Zn modified Silicalite-1 zeolites,and found that ZnOH⁺,which was formed from the interaction between Zn species and the acid sites of Silicalite-1 zeolite,could activate benzene in alkylation reaction with methanol,and ZnO had no effect on benzene alkylation with methanol.To sum up,Zn modified Silicalite-1 zeolites could catalyze benzene alkylation with methanol,but the catalytic activity would become one third of the original after 32 hours because of lack of mesoporous structures.Then,this paper tried using different concentrations of NaOH solution to modify Silicalite-1 zeolites,and found that NaOH solution with low concentration could make a certain number of mesoporous structures and improve the stability of Silicalite-1 zeolite.But NaOH solution with high concentration could destroy the skeleton of Silicalite-1 zeolite,which reduced some weak acid sites and caused that Zn modified Silicalite-1 zeolites treated by high concentrations of NaOH solution lost the ability of activating benzene in alkylation reaction with methanol because of lack of ZnOH⁺.Therefore,this paper thought Zn species had difficulty in forming ZnOH⁺on Silicalite-1 zeolite treated by NaOH solution with high concentration,because NaOH modification reduced a certain number of acid sites on Silicalite-1 zeolite.