Study On Alkylation Of Syngas And Benzene Over Zn-Based Metal Oxide/Beta Bifunctional Catalyst

Alkylbenzenes are important chemicals and had wide uses in the chemical engineering industry.The technologies of producing alkylbenzenes through the petroleum route had matured,but the domestic situation of resources is that there is more coal and less oil,and the production of alkylbenzenes by coal instead of petroleum is more in line with the current situation.Among them,the production of alkylbenzene from benzene and syngas is one of the most likely methods to achieve industrialization.This process avoids the steps of syngas to methanol and the decomposition reaction of methanol.At the same time,the cascade of the two reactions,namely synthesis of syngas to methanol and benzene with methanol to alkylbenzene,can greatly promote the reactions towards the alkylbenzene direction.Therefore,the production of alkylbenzene from benzene and syngas has received extensive attention in recent years.The tandem reaction of alkylation of benzene and syngas is usually carried out over a bifunctional catalyst.First,the syngas is converted into an intermediate product such as methanol and olefins on metal oxide,and the intermediate product is then alkylated to alkylbenzene on the acid site of the zeolite with benzene.We prepared the bifunctional catalysts by mechanical mixing of Zn-based oxide and Beta zeolite.The properties of the catalyst were characterized by XRD,SEM,NMR,NH₃-TPD,PY-IR,BET,and ICP.The synergistic relationship of Zn based oxides and Beta zeolite was deduced,and the effect of the two components’mass ratio,temperature,zeolite Si/Al,feed ratio,etc.on the reaction were studied.The main conclusions are as follows:1,Using Zn Cr Ox/Beta zeolite catalysts,optimized reaction conditions are,that the reaction temperature is 450℃,the reaction pressure is 4.0 MPa,the benzene WHSV=2.6g/h·g_cat,syngas GHSV=4800 m L/h·g_cat,the best mass ratio of Zn Cr Ox and Beta zeolite is 1:3,and the Beta zeolite with Si/Al=11 has the highest activity.Under the optimized condition,the benzene and CO conversion were 40.72%and 62.40%,respectively.The selectivity of toluene and xylene was higher than 90%,and the alkylation selectivity of CO was 47.8%.2,NMR,NH₃-TPD,and PY-IR characterizations showed that the acidity of Beta zeolite is closely correlated with Si/Al,the lower the Si/Al is,the stronger the acid amount and strength of the Br?nsted acid is.It was found that the tandem reaction needs enough Br?nsted acid of zeolite to consume the methanol in time,which is formed from syngas on the metal oxide,and the alkylation promote the methanol formation.Based on SEM,it was found that there is a certain distance between the two components,the key intermediate product should be methanol molecules rather than methoxy or methyl groups.According to XRD,the mass ratio of the two components can affect the balance of supply and consumption of the intermediate.Apart from methanol olefins are also reaction intermediates that form long-chain alkylbenzenes,such as ethylbenzene,cumene,and butylbenzene etc.The reaction temperature needs high enough to meet the requirement of benzene alkylation.3,Adding CO₂ to the syngas can effectively inhibit the water-gas shift reaction,but it will reduce the reaction activity and stability.The increase in H₂/CO is beneficial to the conversion of benzene,but it is easy to cause excessive hydrogenation and generate more alkane by-products.The experiments regulating the distance between two components evidenced that the closer between them,the higher the reaction efficiency of alkylation.It proved that the zeolites have decisive effects on methanol generating reactions.Our study found that the large 3D pore structure of Beta zeolite had a large carbon-accommodating capacity,and its relatively weak acidity can reduce the formation of carbon deposits.The benzene conversion of the composite catalyst was still higher than 20%after 100 hours on time of stream.TG characterization found that the carbon deposition amount reached 13%,indicating that the special pore structure of Beta zeolite can facilitate the diffusion of carbon deposition precursors and reduce deactivation.4,The layered structure of ZnAl-Layered Double Hydroxide（LDH）can expose the more active site of Zn²⁺to improve hydrogenation capacity.The characterization of H₂-TPD and CO-TPD showed that after hydrogen pretreatment and activation,the adsorption capacity of the ZnAl-LDH for CO and H₂ was greater than layered double oxides（LDO）and co-precipitated oxides,indicating more active sites.The LDH structures is more beneficial for the reaction of syngas to methanol.The matching of the between ZnAl-LDH and Beta zeolite has decisive effect on reaction.The results show that under the reaction conditions of 450℃,4.0 MPa,syngas GHSV=4800 m L/h·g_cat,benzene WHSV=2.6 g/h·g_cat,the catalyst of ZnAl-LDH/Beta-11=1:4 has the best performance.The conversion of benzene and CO ware 45.1%and 55.3%,respectively,and the alkylation selectivity reached 41.6%.