The Effect Of Acid-base Sites Distribution Of Catalysts On Side- Chain Alkylation Of Toluene

Styrene is a basic organic chemical raw material for synthetic rubber and plastics.At present,there are many ways to synthesize styrene in industry,moreover,the atalytic dehydrogenation of ethylbenzene is most wildely used.This process has many shortcomings such as long process and high energy consumption,which are not in line with the current concept of environmental protection and energy saving.The side-chain alkylation of toluene with methanol has many advantages,such as wide source of raw materials,short process flow and low energy consumption.Therefore,this reaction has become the focus of research.A large number of studies have found that side-chain alkylation of toluene with methanol is a synergistic reaction between acid and base,and the catalyst needs to have suitable weak acid sites and base sites at the same time.In our previous work,it was found that the CsX zeolite catalyst modified with K₃PO₄can reduce the acid sites amount,increase the number and strength of middle and strong base sites,moreover,the activity of side-chain alkylation and selectivity of target products increased significantly.In this paper,the relationship between the distribution of acid-base and the catalytic performance of the catalyst was analyzed from the preparation of different acid-base catalysts,and the role of the acid-base sites in the catalytic process was further refined.The catalysts were characterized by XRD,SEM,FT-IR,BET,UV-Vis,NH₃-TPD and CO₂-TPD.The main conclusions are as follows:1.AlPO₄-5 zeolite has a certain promoting effect on the formation of ethylbenzene.The proportion of strong base sites in total base sites,activity and selectivity of ethylbenzene in the catalyst with K addition increase obviously,but the number of strong acid sites increased,which leads to the high selectivity of by-product xylene.2.There are no strong acid sites in SAPO-5 zeolites modified by K₃PO₄,and the number of middle and strong base sites increased.Especially when K₃PO₄loading is 3 wt%,the relative content of middle and strong base sites in total base sites is the largest,and the selectivity of styrene and ethylbenzene is the best,reaching 62.1%and 16.0%respectively.3.The distribution of acid sites and base sites have obvious changes with the concentration of K₃PO₄ solution changed through the K₃PO₄ modified NaX zeolite catalyst,and the proportion of middle and strong base sites decreased with the reducing of K₃PO₄ concentration.When the concentration of K₃PO₄ solution was 0.1 mol/L,the selectivity of ethylbenzene was the highest?82.9%?;when the concentration of K₃PO₄ solution was 0.25 mol/L,the selectivity of styrene was the highest?18.1%?.4.Strong acid and medium acid sites in catalysts are beneficial to the formation of xylene and methane,while strong base sites in favour of the activation of methyl groups on toluene and the formation of more side-chain alkylation products ethylbenzene.When there are some strong base sites,the formaldehyde produced reacts further with activated toluene to form styrene.Therefore,it is necessary for the side chain alkylation of toluene to produce styrene that there are enough intermediate and strong base sites in the catalyst and a suitable amount of strong base sites.