Study On Influencing Factors Of Preparation Of Core-shell Zeolite Catalysts For Toluene Methanol Alkylation By Solid Phase Synthesis

Para-xylene(PX)is an important organic material,which is becoming more and more important with the prosperity of the polyester industry.Toluene alkylation with methanol,showing high PX selectivity and low cost,is considered to be one of the most attractive and promising ways in the production of PX.ZSM-5 is a commonly used catalyst in the reaction due to its unique pores and acidity.However,PX will react further on the outer surface non-selective acid sites.Therefore,it is difficult to obtain high PX selectivity on a pure HZSM-5 catalyst.Forming an inert Silicalite-1 shell on the outer surface of ZSM-5 is a strategy worth considering.Based on the preliminary research in the laboratory,the catalytic performance of the catalyst will be improved by investigating the factors of solid phase synthesis in this paper.The effect of core size on the structure and performance of the core-shell catalyst was investigated.A series of HZSM-5 zeolites with different sizes(1 μm,3μm,5 μm and 7 μm)were prepared by changing the content of TPAOH in the synthesis solution.And then,the MFI core-shell zeolite catalyst was respectively prepared by hydrothermal synthesis and solid phase synthesis using HZSM-5 zeolites with different sizes as the core crystals.In the hydrothermal synthesis,as the core size increases,the PX selectivity increased at the begining but then decreased.The dispersion of larger crystal cores in the synthesis solution will become difficult,which is difficult for shell to form.Among them,the hythermal synthesis of core-shell zeolite catalyst with 3 μm HZSM-5 crystal as the core has the best catalytic performance,with a toluene conversion of 21.6%and a PX selectivity of 89.4%.In the solid phase synthesis,as the core size increases,the PX selectivity increased.The size of core is no longer a limit,and the insert shell can be fully covered.Among them,the core-shell zeolite catalyst with 7 μm HZSM-5 crystal(Z-4)as the core has the best catalytic performance,with a toluene conversion of 18.8%and a PX selectivity of 97%.The structure of the core-shell zeolite catalyst was characterized by SEM,XRD,XPS,TEM,DTBPy-IR,EDS and other means.It is proved that a Silicalite-1 shell was successfully formed on the outer surface of large-size crystal Z-4 by solid phase synthesis.Compared with the preparation of core-shell zeolite by the liquid phase hydrothermal synthesis method,the solid phase method can ignore the influence of the size of the core crystal,breaks through the limitations caused by the mass transfer and dispersion problem,and has simple operation and less waste water,which has a good industrial application prospect.The factors affecting the stability of core-shell zeolites prepared by solid phase synthesis were investigated,such as the morphology of crystal core,the proportion of crystal core during the preparation process and the operating conditions during the toluene alkylation reaction.In the investigation of the morphology of the core crystals,it was found that the core-shell zeolite showed similar catalytic performance when crystal core had similar b-axis length.As the length of b-axis increases,the PX selectivity became higher and the stability became worse.The b-axis length of crystal core has a significant effect on the PX selectivity and stability of the core-shell zeolite catalyst;in the investigation of the crystal core addition amount,it is found that with the increase of the core ratio,PX selectivity decreases,and the stability of catalyst is obviously improved.When determining the proportion of seed,we should consider both.And as the proportion of core increases,the coating effect shows a downward trend,which is manifested as the size of the core-shell zeolite decreases after the coating,and the PX selectivity decreases.In the reaction where the catalyst loading was changed,it was found that when the catalyst loading was three times the normal loading,the catalyst deactivation was significantly improved.In the investigation of the reaction operating conditions,it was found that 400℃ was the best reaction condition from the point of view of conversion and PX selectivity.The deactivation of the catalyst could be effectively slowed down and the stability of the catalyst could be improved by reducing the space velocity and increasing the carrier gas velocity.