Synthesis Of ZSM-5 And Catalytic Alkylation Of Benzene With Dilute Ethylene

Ethylbenzene is an important organic chemical raw material,which is mainly used to produce styrene intermediates,and then produce polystyrene,styrene butadiene rubber,ABS resin and other polymer materials.The gas-phase alkylation of benzene with dilute ethylene can make full and effective use of the dry gas used as fuel,which not only reduces carbon dioxide emissions,which is in line with the overall plan of"carbon peaking and carbon neutralization"proposed by China.The aim of this paper is to prepare high-performance ZSM-5 nanosheet,and to explore the effects of the diffusion properties,aluminum distribution and the number of acid sites on the outer surface of the molecular sieves on the reaction stability and product quality.The specific research contents are as follows:ZSM-5 nanosheets with b-axis thickness of 40-170 nm were synthesized hydrothermally by adjusting the amount of seed crystals by the seed-guided method to investigate the effect of diffusion properties on the alkylation of benzene with dilute ethylene.The catalysts were analyzed by characterization methods such as XRD,SEM,N₂-physical adsorption,NH₃-TPD and TG.It was found that all the samples showed sheet shape,uniform size,high crystallinity and basically similar acidity,which was attributed to their similar silicon-aluminum ratio.The reaction results showed that under the premise that the high conversion rate of benzene at 48.0%,with the decrease of the b-axis thickness,the selectivity of ethyl increased from 91.2%to 94%,the content of by-product xylene decreased by 22.4%,and the carbon deposition rate decreased by 24.1%.This is attributed to the improvement of diffusion performance that promotes the diffusion of aromatic hydrocarbon molecules,inhibits the occurrence of side reactions and the formation of carbon precursors.ZSM-5 molecular sieve with b-axis thickness of 50 nm was synthesized by hydrothermal method at different crystallization temperatures（80～170°C）.The structure,framework aluminum distribution and acid properties of the catalysts were characterized by XRD,SEM,NH₃-TPD,Py-IR,XPS,ICP-AES,²⁷Al MAS NMR and UV-vis-DRS.The effect of crystallization temperature on the crystal growth and aluminum distribution of ZSM-5 zeolite was studied,and the catalytic performance of catalysts with different crystallization temperatures in the alkylation of benzene with dilute ethylene was investigated.The results show that the crystallization temperature has little effect on the morphology and size of the molecular sieve,but obviously changes the Al distribution of the zeolite.The low-temperature crystallization makes the crystal growth rate slower,which is conducive to the uniform distribution of Al,showing less active sites on the outer surface,and its framework aluminum is mainly distributed in the straight channels and the intersections of the channels,which is conducive to the effective contact between benzene and acidic sites.The ZSM-5 zeolite crystallized at low temperature（especially at 80℃）showed the highest ethyl selectivity and the lowest xylene content of 95.4%and 873 ppm in the alkylation of benzene with dilute ethylene,respectively.The ZSM-5@S-1 core-shell catalyst was synthesized by using the synthesized ZSM-5nanosheet with b-axis thickness of 50 nm as the parent for isomorphic epitaxial growth.The effects of silicon source,coating amount and solvent amount on the physicochemical properties and catalytic performance of core-shell catalysts were investigated.By XRD,SEM,NH₃-TPD,N₂-physical adsorption and XPS and other characterization methods,it was found that the prepared ZSM-5@S-1 not only had good crystallinity and physical properties,but also was effectively passivated at the acidic sites on the outer surface.The reaction results of benzene with dilute ethylene showed that the selectivity of ethylbenzene was increased by about 3%,the content of xylene was decreased by 28.5%,and the carbon deposition rate was decreased by26.1%,but the change of solvent content had little effect on the catalytic performance.