Synthesis,Characterization And Application Of Catalysts For Selective Production Of P-xylene By Methylation Of Toluene

The process of selective production of p-xylene(PX)with toluene and methanol as the raw materials combines petrochemical industry and coal chemical industry together,which increases the utilization rate of toluene and C1 resources.The catalytic activity,selectivity and stability of catalysts are the core indexes in the process of toluene methylation reaction.The synthesis and subsequent modification of catalysts are the key to improve the catalytic performance.In this paper,IM-5 and ZSM-5 zeolites were selected as matrix and a series of modified zeolites were prepared by selective passivation of the surface acidity by meso-SiO2,phosphorus species and silicalite-1 with the assistance of cationic surfactants.The catalytic activity,selectivity and stability of modified zeolite catalysts in the methylation of toluene were investigated in a laboratory fixed bed reactor.At the same time,XRD,N2 adsorption-desorption,XRF,ICP-OES,XPS,SEM,TEM/HRTEM,STEM-EDS,solid state NMR,NH3-TPD,Py/Coll-IR,TG and Xylene-TPD were used to characterize and analyze the physical and chemical structures of the modified zeolites,and explore the function mechanism of different modification methods.Firstly,HIM-5@meso-SiO2 core-shell zeolites were prepared by PDDA pretreatment,in which IM-5 zeolite as the matrix.Among them,IM-2.4S core-shell zeolite had the best catalytic performance with toluene conversion,xylene selectivity and PX selectivity of 51.6%,74.3%and 40.3%,respectively.Then IM-2.4S was modified with phosphorus by impregnation method and IM-2.4S-P zeolite was obtained.When the phosphorus content was 2.0 wt%,the IM-2.4S-2.0P catalyst showed the best catalytic performance.The optimum conditions for the methylation of toluene were as follow:reaction temperature=440?,T/M=2.0,WHSV=2.0 h-1.Combined with the structural characteristics,acidity and catalytic performance of the modified zeolites,it can be concluded that the meso-SiO2 shell could passivate the external surface acidity of HIM-5 zeolite,which reduce the isomerization of PX and improve the selectivity of PX.The phosphorus modification method was mainly used to reduce the acid strength of HIM-5 zeolite,which could inhibit the occurrence of side reactions.In addition,the phosphorus species could enter the pores of sample and narrow the pore-opening size,which further inhibite the diffusion process of m-xylene and o-xylene and ultimately improve the selectivity of PX.Xylene-TPD results revealed that the diffusion of xylene in the pore channel of catalyst was restricted by both surface acidity and pore space.Meanwhile,the effectiveness of meso-SiO2 shell and phosphorus modification in improving the selectivity of PX was also verified from the perspective of molecular diffusion.Secondly,a series of ZSM-5 zeolites with theoretical n(SiO2)/n(Al2O3)=100?220 were synthesized in this paper,among which the HZSM-5 zeolite with actual n(SiO2)/n(Al2O3)=131 showed the best yield of PX.And then,MZS-TEOS,MZS-Coll,MZS/HDTMA-P and MZS/CPB-P zeolite catalysts were prepared by selective passivation of the external acidic sites of HZSM-5 by different methods.Among them,MZS/HDTMA-P zeolite showed the best catalytic performance.When the phosphorus loading was 1.5 wt%,the toluene conversion of MZS/HDTMA-1.5P zeolite was higher than 27.0%,and the selectivity of PX was higher than 97.6%.The optimal conditions for the methylation of toluene were as follow:reaction temperature=400?,T/M=2.0,WHSV=2.0 h-1.Under the condition of water and nitrogen,the stability of MZS/HDTMA-1.5P catalyst was improved,but the introduction of water vapor accelerated the loss of phosphorus in the catalyst,which led to a decline in selectivity of PX.The average toluene conversion and PX selectivity within 65 h reaction of MZS/HDTMA-1.5P/R catalyst were 24.9%and 93.9%,respectively.The structural and catalytic properties of the modified ZSM-5 zeolite indicated that the phosphate ions were enriched on the external surface and entrance of pores by electrostatic attraction after the pre-treatment of the external surface of ZSM-5 by HDTMA.And the phosphate ions converted into pyrophosphate or short-chain polyphosphate not connected to aluminum atoms after calcination,which ultimately passivated the external surface acidic sites selectively.Finally,ZSM-5@silicalite-1 core-shell zeolites were prepared with the coffin-shaped ZSM-5 zeolite as the matrix,which was pre-treated by NH4F,citric acid or PDDA.Among them,the PX selectivity of HZSM-5/PDDA@silicalite-1 zeolite catalyst were improved to 96.0%,and the yield of PX reached 22.9%.The optimal synthesis conditions of HZSM-5/PDDA@silicalite-1 core-shell zeolite were as follow:the composition of silicalite-1 shell precursor=1.0 SiO2:0.25 TPAOH:4.0 ethanol:60 H2O,the solid-liquid mass ratio of core to shell precursor=1:15,and the crystallization temperature=160?.Under the condition of water and hydrogen flow,the average toluene conversion and PX selectivity of 0.3%Pd/HZS-16h core-shell zeolite within 130 h were 26.4%and 96.2%,respectively.After regeneration,the average toluene conversion and PX selectivity of 0.3%Pd/HZS-16h were 25.9%and 95.1%,respectively.In the HZSM-5/PDDA@silicalite-1 core-shell zeolite,silicalite-1 crystal grew oriented in a manner that the(010)crystal plane was parallel to the external surface of ZSM-5 crystal.In the initial stage of crystallization(0?4 h),silicalite-1 crystal nucleuses were preferentially grew on the external surface EFAL of HZSM-5.In 4?8 h,silicalite-1 crystal grew rapidly along the a and c axes,and its proportion in core-shell zeolite increased rapidly.In 8?16 h,silicalite-1 shell grew slowly until it stopped.After PDDA pretreatment on the external surface of ZSM-5 crystal,a relatively continuous b-oriented monolayer silicalite-1 shell was finally generated on its external surface.The silicalite-1 shell coverage of HZS-16h core-shell zeolite was 95.7%and the thickness was about 110 nm.