Optimizing Shape-selectivity Of HZSM-5 Zeolites For The Alkylation Of Toluene With Methanol

Para-xylene?p-xylene?is an important chemical raw material for the production of terephthalic acid.Its downstream product polyethylene terephthalate?PET?is widely used for the production of polyester products.Among the methods for synthesizing p-xylene,the alkylation of toluene with methanol on HZSM-5 zeolite is the most competitive production technology.Compared with other molecular sieve catalyst system,HZSM-5 not only has high catalytic activity,but also has good selectivity.Surface modification and functionalization of HZSM-5 zeolite can improve its catalytic activity,especially the selectivity.Recently,people continue to explore more effective post-processing methods for further improving the catalytic activity of HZSM-based catalysts.In this work,MgO/ZSM-5 catalyst and MgO/HZSM-5@SiO₂ catalyst with core-shell structure were respectively prepared by two-step impregnation method and cationic surfactant method,and using HZSM-5 molecular sieve as matrix.Then the catalytic performance of the catalysts was investigated in the alkylation of toluene with methanol.In addition,the physical and chemical properties of the modified catalyst were studied by XRD,SEM,TPD,N2 physical adsorption and TG-DTA.The main factors influencing the catalyst activity and selectivity of the catalysts were discussed.The main contents and the results of this work are as follows:MgO modified ZSM-5 zeolite catalysts were prepared by two-step method of complex impregnation?CI?and ordinary impregnation?GI?,using ethyl acetoacetate as complexing agent and magnesium nitrate as magnesium source.By adjusting the amount of MgO and complexing agent,highly selective catalyst for p-xylene could be obtained.Under the optimized reaction conditions?React.Temp.520 ?C,molar ratio of toluene to methanol is 2,the mass space velocity is 2 h-1,N2 as the carrier gas?,3.0%Mg?GI?/15%Mg?CI?/ZSM-5 exhibits a 15.5% toluene conversion and a 96.1% p-xylene selectivity,which is much selective than the MgO-supported ZSM-5 catalyst prepared by ordinary impregnation method.The characterization results show that most of MgO species are distributed on the external surface of the zeolites,which can cover the nonselective acidic sites on it,while a trace amount of Mg2+ ions may enter the channel of zeolites and regulate the pore size.These features result in the significant improvement of the selectivity to p-xylene without obvious loss of the catalytic activity of the zeolite catalysts.Mesoporous silica-coated core-shell structure ZSM-5?ZSM-5@SiO₂?zeolites were prepared by modifying the HZSM-5 zeolites with TEOS through a cationic surfactant method.A series of MgO/ZSM-5@SiO₂ catalysts were obtained by introducing suitable amount of MgO on the surface of ZSM-5@SiO₂.Under the optimized reaction conditions,the catalyst of 20%MgO/ZSM-5@0.75 SiO₂ shows a 17.7% toluene conversion and a 93.7% p-xylene selectivity.The mesoporous channels of silica distributed on the external layer of the core-shell ZSM-5@SiO₂ have good connectivity with the micropores of interior HZSM-5,resulting in the good accessibility for the reagents.The silica layer could cover the acidic sites located on the external surface of HZSM-5 zeolites,while the introduced Mg2+ ions may regulate the pore size of zeolites to a certain extent.These features bring positive effects on improving the catalytic properties of the MgO-modified core-shell structure ZSM-5 zeolite catalyst for the alkylation of toluene with methanol to p-xylene.