Study On Preparation Of MFI Type Zeolite Core-shell Catalysts For Toluene Alkylation Reaction

Para-Xylene(PX)is a basic organic raw material of the petrochemical industry and currently the most demanding C-8 aromatic hydrocarbon.Toluene alkylation reaction to produce PX may be one of the most economical and feasible technology routes in the future.Study on the catalyst is one of the core issues for this technology;and if the catalytic performance of the catalyst can achieve the requirement in industries,this technology would make a great economic benefit.If a silicalite-1 layer(shell)could be formed on the surface of the ZSM-5 zeolite(core)and the ZSM-5 zeolite pores did not blocked by the shell layer,the PX selectivity of the catalyst would be improved.In this paper,the commercial H-ZSM-5 zeolite is selected as a raw powder.Hydrothermal synthesis(liquid phase method)and solvent-free method(solid phase method)are used for preparation of the ZSM-5@Silicalite-1 zeolite core-shell catalyst,resulting in improvement of the catalytic performance for the toluene alkylation reaction,and the catalyst has a good potential for industrial application.The formation of the core-shell structures and their catalytic performance are systematically discussed and characterized by scanning electron microscopy(SEM),transmission electron microscopy(TEM),energy dispersive X-ray spectroscopy(EDS),selected area electron diffraction(SAED),N2 adsorption,X-ray powder diffraction(XRD),TiPB degradation,NH3 temperature-programmed desorption(NH3-TPD)and thermogravimetric(TG)analysis test.This thesis is mainly focused on the following two aspects:(1)Preparation of ZSM-5@Silicalite-1 zeolite core-shell catalyst by hydrothermal synthesis.The effects of the Si/Al ratio of core ZSM-5 zeolite,the water and ZSM-5 adding amounts in the synthesis solution and the hydrothermal synthesis time on the catalytic performance of ZSM-5@Silicalite-1 zeolite core-shell catalysts are systematically discussed.The commercial ZSM-5 products(such as Zeolyst CBV series)are used as raw core zeolites.The highest PX selectivity of 95.5%can be obtained for the ZSM-5(Si/Al=140)@Silicalite-1 catalyst prepared in the synthesis solution with a molar ratio of 1TEOS:0.2TPAOH:250H2O at 175 0C and 10 rpm for only 2 h(Sample ZS-W250-2h)and the corresponding toluene conversion is as high as 22.8%for the alkylation of toluene with methanol.The performance of the catalyst could be significantly improved and the sample is almost crystallized completely after synthesizing for only 2 h(Sample ZS-W250-2h).When the amount of water in the synthesis solution is more than 250,the growth of silicalite-1 shell on the surface of the ZSM-5 zeolite is difficult and the PX selectivity of the catalyst would be obviously decreased.With increasing the Si/Al ratio of the core ZSM-5 zeolite,crystallization of silicalite-1 on the surface of the ZSM-5 zeolite crystals increases,and the PX selectivity of the catalysts increases.The improvement of the PX selectivity is mainly due to the epitaxially grown of silicalite-1 shell(without strong acid sites)on the surface of the core ZSM-5 zeolite.(2)Preparation of ZSM-5@Silicalite-1 zeolite core-shell catalyst by solvent-free synthesis.The solid synthesis by mixing core ZSM-5 zeolite with the synthesis raw powder and heating can be carried out in static condition,and the synthesis condition is mild.The effects of the synthesis time,the W/V value(weight of powder/volume of the vessel),the adding of urea,NH4F and TPABr in the solvent-free condition on the catalytic performance of ZSM-5 @Silicalite-1 zeolite core-shell catalysts are systematically discussed.ZSM-5 zeolite(mass ratio to SiO2 is 0.10)and solid raw materials with a molar ratio of 1SiO2:0.25NH4F:0.035TPABr are briefly mixed and grounded by a mortar for a certain time.The highest PX selectivity of approximately 89%can be obtained for the ZSM-5 @Silicalite-1 catalyst prepared in the solvent-free condition:1.13 g of the powder(with 0.80 g SiO2)in a 50 mL autoclave and synthesis at 175 ? for 24 h(Sample S50-1-24)and the corresponding toluene conversion is as high as 25%for the alkylation of toluene with methanol.The sample S50-1-24 has ZSM-5@Silicalite-1 core-shell structure with a 50 nm pure silica(silicalite-1)shell and the core-shell zeolite is a single crystal by this solvent-free method.Moreover,the thickness of the silicalite-1 shell also can be adjusted from 20 um to 50 nm when the W/V values change from 0.013 to 0.025 g/mL.The silicalite-1 shell only passivates the surface acid sites of the core ZSM-5 zeolite.Amorphous materials with macropore/mesopore also increase the resistance ability to coking of the catalyst and can be used as binders of the catalyst.Both NH4F and TPABr are important to the crystallization.And with increasing the amount of NH4F,the relative crystallinity of the sample increases,the catalytic performance increases at the begining but then decreases.The adding of urea in the solvent-free condition not only improve the catalytic performance of the catalyst(PX selectivity=93%),but also can decrease the synthesis time to only 6 h.Compared to hydrothermal synthesis method,the solvent-free synthesis method exhibits the following benefits:high yield product,low requirement of equipment,high utilization of vessel,simple synthesis steps and low pollution.So the potential for industrialization of this method is large.At the same time,the solvent-free synthesis can also be regarded as a new strategy for preparing other core-shell structure composites.