Preparation Of Difunctional Catalysts For The Side-Chain Alkylation Of Toluene To Styrene And Their Reaction Mechanism

Styrene is an essential petrochemical byproduct and finds many applications in the synthetic polymer industry.Commercially,styrene is produced by the two steps process started from the alkylation of benzene and ethylene into ethylbenzene,and then ethylbenzene catalytic dehydrogenation.The development of a new process for styrene production has been one of the hot spots in the chemical industry.In recent years,the one-step side-chain alkylation of toluene to produce styrene is considered a new competitive process with fewer process facilities,lower cost of raw materials,and lesser energy consumption.Numerous studies revealed that the side-chain alkylation of toluene reaction requires a kind of bifunctional catalysts with adjustable acid-base sites and suitable spatial structures.Layered double oxide（LDO）is a kind of bifunctional catalyst with adjustable acid-base sites and two-dimensional layers that might be suitable for the side-chain alkylation of toluene to styrene.In this study,the thermodynamic study of the side-chain alkylation of toluene was firstly carried out.Rendering to the reaction’s characteristics,a series of bifunctional catalysts with adjustable acid-base sites and two-dimensional LDO catalytic active components were designed and synthesized.The structural characteristics of the catalysts,the relationship between the structure of catalysts and reaction performance,as well as the reaction mechanism of the side-chain alkylation of toluene to styrene were studied in detail.The obtained results are as follows:1.The thermodynamical study of the side-chain alkylation of toluene with methanol showed that the equilibrium constant of the side-chain alkylation reaction was relatively small when the temperature was?650 K and it increase apparently with the rise of temperature.The equilibrium constant of the side reaction of methanol over-decomposition increased with the increase of temperature>750 K,and the side reaction intensified when the temperature was too high.Therefore,the optimum reaction temperature of the side-chain alkylation of toluene with methanol in the gas-solid phase was650～750 K.The thermodynamic study on the side-chain alkylation of toluene with formaldehyde to styrene showed that the equilibrium constant of the main reaction side-chain alkylation was large at a lower temperature and small at a higher temperature.The equilibrium constant of excessive decomposition of formaldehyde in the side reaction increased with the increase in temperature.A decrease in temperature could inhibit the decomposition of formaldehyde.Therefore,low temperature was more favorable for the side-chain alkylation of toluene with formaldehyde in the liquid-solid phase.2.Among the Cs/Cu Al-LDO@zeolite catalysts supported by different zeolites,the Cs/Cu Al-LDO@X catalysts supported by X-zeolite,have larger specific surface area and suitable pore structure.In the prepared Cs/MAl-LDO@X catalysts with different M（Cu,Zn,Co,Mg）Al layered double oxide（MAl-LDO）as the active components,the Cs/Cu Al-LDO@X catalysts have a more suitable space structure,evenly distributed layers,and have better reaction performance of the side-chain alkylation of toluene to styrene.3.In order to increase the stability of the layers,a two-dimensional layered double hydroxide（LDH）was grown in-situ on the surface of the X-zeolites with the Al of the zeolite as the Al source and copper nitrate as the Cu source,impregnated with Cs and then calcined to prepare a series of acid-base bifunctional Cs/Cu Al（s）-LDO@X catalysts.The spatial structure of Cu Al（s）-LDO was formed on the surface of the zeolites.The LDO shell was about 230 nm,and the acid-base sites were the catalytic active sites.The Cs/Cu Al（s）-LDO@X catalyst with 25 wt%Cs and 15 wt%LDO showed good performance in the side-chain alkylation of toluene to styrene,and styrene selectivity was 54.16%when toluene conversion was 6.58%,the selectivity of the side-chain alkylation products was 91.64%.The study of the reaction mechanism on the Cs/Cu Al（s）-LDO@X catalyst showed that methanol was dehydrogenated at the Cu^δ+–O^δ–base sites in the LDO layers to form formaldehyde as a real alkylating agent,and the space limitation effect of the catalyst LDO layers promoted the stable adsorption of formaldehyde on the Cu^δ+acid sites;at this time,the toluene benzene ring was adsorbed on the Al³⁺acid sites,and the side-chain methyl group interacted with the Cu^δ+–O^δ–base sites to activate;under the continuous action of the acid-base sites,the side-chain methyl group of the activated toluene reacted with the formaldehyde transition state to form styrene.4.According to the characteristics of the side-chain alkylation of toluene with formaldehyde in the liquid-solid phase,using Al₂O₃@Fe₃O₄microspheres as the supports,a series of magnetic Cs/Cu Al-LDO@Al₂O₃@Fe₃O₄catalysts were prepared by growing Cu Al-LDH on the surface,impregnating with Cs and then calcining.The diameters of the magnetic Al₂O₃@Fe₃O₄microspheres were about 110 nm,the Cu Al-LDO layers were grown longitudinally on the surface of the supports,and the thickness of the formed LDO shell was about40 nm.To obtain a specific layer space in the catalysts,the acid-base sites were evenly distributed in the LDO layers.The magnetic Cs/Cu Al-LDO@Al₂O₃@Fe₃O₄catalyst with 20 wt%Cs and 20 wt%LDO showed good performance in the side-chain alkylation of toluene with formaldehyde.The styrene selectivity was 90.17%when toluene conversion was 7.97%,and the selectivity of the side-chain alkylation products was98.36%.After the reaction,the magnetic catalysts were separated and recovered by an external magnetic field and maintained stable performance after consecutively reusing five times.