Study On Catalytic Synthesis Of P-Xylene By Toluene Methylation With Methyl Bromide

The catalytic process of p-xylene production is a quite important technology inpetrochemical industry and has been paid much attention. This work is for the purposeof comprehensive utilization of the natural gas resource and to synthesis p-xyleneusing methyl bromide as methylating agent. High-efficiency shape-selectivitycatalysts were obtained. The relationship between the performance andphysicochemical properties was investigated systematically.It was observed that the catalyst co-modified with Si and P compounds performedbetter than that which was modified only with Si or P under equal reaction conditions.The selectivity of p-xylene was more than93%, and the yield was over21%under theoptimal reactions (T=400oC, WHSV_toluene=2.0h^-1, n（CH3Br）:n（toluene）=2:1) onHZSM-5loaded with8wt%methyl silicon oil and3wt%P₂O₅. By combining thecharacterization results （e.g., XRD, SEM, NH3-TPD, BET） with the catalyticperformance of the catalyst, it was deduced that toluene conversion depended on theconcentration of total acid sites, while the strong acid sites showed effect on thepara-selectivity, decline in strong acid sites would result higher para-selectivity.Furthermore, the isomerization of p-xylene to other xylene isomers didn’t depend onwhether the strong acid sites are inside or on the external surface of the zeolite, andelimination of strong acid sites in either location increased para-selectivity. Thedeactivation of Si-P/HZSM-5in toluene methylation with CH3Br was mainly due toCH3Br oligomerization and coke formation.Optimizations of SiO₂/Al₂O₃and particle size were carried out for more efficientcatalyst. Silicalite-1/HZSM-5core-shell zeolites were synthesized by in-situ growthmethod, and the effects of core pretreatments and the amount of template in thecoating solution on the synthesis of Silicalite-1/HZSM-5core-shell zeolites wereinvestigated. The results showed that appropriate SiO₂/Al₂O₃and particle size werevery important to improve toluene conversion and para-selectivity. Pretreatment tothe core and the amount of TPAOH in coating solution affect the morphology andcatalytic performance of the core-shell zeolites. The conversion of toluene was morethan48%, the selectivity and yield of p-xylene were more than56%and21%,respectively when the Silicalite-1/HZSM-5core-shell zeolites were synthesized withHZSM-5（SiO₂/Al₂O₃=150, particle size about12μm, without pretreatment） as coreand the molar ratio of TPAOH to SiO₂in coating solution was0.12.