The Study Of 1-hexene Skeletal Isomerization On ZSM-35 Zeolites

In recent years,with the development of green chemical industry,the demand for highly processed chemical products increased dramatically.Isomeric olefins and their derivative products which have great industrial chains are widely used in chemical industry.At present,there are various ways to produce isomeric olefins,and the skeletal isomerization is the best process which takes the greatest advantages,but the development of the process is limited to the performance of catalysts.Therefore,it is of great importance to research and develop the catalysts of olefins skeletal isomerization.In this paper,ZSM-35 zeolites produced commercially are chosen as the catalyst of the reaction and the C6 linear olefin is used as a model compound to explore the preparation of highly efficient catalysts.The reaction temperature of molecular sieve catalyst was investigated based on the reaction conditions of Al2O3 catalyst in laboratory and the optimum temperature of ZSM-35 catalyst was found as 300 ?.Under certain reaction conditions,the effect of calcination on the performance of ZSM-35 was further studied.The results show that the calcination of ZSM-35 can lead to the removal of template,thus resulting in a substantial decline of the catalyst selectivity,and a increase of the yield of cracking and polymerization products.This is because the removal of the template causes the increase of the acidity on ZSM-35 and results in a large amount of L acid center which is the active center of the side reactions.Furthermore,the calcinations can also lead to the increase of a large number of mesoporous pores in small scales which have a negative effect on the selectivity of the reaction.Therefore,the modification of the catalysts should centere around the acidity and pore structure of ZSM-35.Firstly,the modification of HZSM-35 with Na2CO3 solution as well as themodification NaZSM-35 with hydrochloric acid solution is conducted.The porechannels of the ZSM-35 can be dredged by Na2CO3 solution and hydrochloric acid solution,but the silicon and aluminum elements of the framework cannot be removed.So the solution cannot damage the skeleton structure of ZSM-35,the main role it plays is changing the acidity of the catalyst.The results are as follows: if the amount of the acid is too high,the cracking and polymerization products will form,while the amount of the acid is too low,the activity of the catalyst will not be guaranteed.Therefore,the suitable amount of acid sites and a certain amount of strong acid sites is needed for the skeletal isomerization on 1-hexene.The carbon analysis on catalysts showed that the amount of carbon deposition has direct relationship with the acid site amount of the catalyst,and the carbon deposition mainly derives from the formation of cracking and polymerization product.Besides,the amount of carbon decreases with the decline of acid site amount,the low-temperature-carbon is mainly generated on catalysts with small amount of acid site,and the type of carbon gradually changed from low-temperature-carbon to high-temperature-carbon with the increase of acidity of the catalyst.Secondly,the modification of ZSM-35 with phosphoric acid is conducted.The performance of catalysts becomes poor by hydrothermal treatment while the performance can be promoted by impregnation treatment.This is because the impregnation treatment with suitable amount phosphoric acid can help to make the transition of strong B acid to weak B acid,decrease the catalyst pore surface area and pore size of mesopores,and increase the pore surface area and pore size of micropores.It is the combination of acidity modulation and pore modification that leads to the high catalytic selectivity.The carbon analysis shows that the stability of ZSM-35 modified with phosphoric acid impregnation treatment are promoted as the carbon deposited on catalysts is decreased.Finally,the impregnation modification with phosphorus,boron and aluminum elements of the same content is conducted.The performances of boron and aluminum modified catalysts are worse than the catalyst modified with phosphoric acid because the loading of boron and aluminum increases the amount of L acid site of the catalyst.The result has further proved that L acid site is the active center of the side reactions.