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Methylation of benzene is an alternative low-cost route to produce xylenes,but selectivity to xylene remains low over conventional zeolitic catalysts.In this work,a combined dry-gel-conversion and steam-assistedcrystallization method is used to synthesize hierarchically porous zeolite ZSM-5 with varied Si/AI malar ratios.X-ray diffraction(XRD),N_2 physisorption,NH_3-temperature programmed desorption(TPD),scanning electronic microscopic(SEM) measurement and Fourier transform infrared(FT-IR) are employed to characterize the structure and acidity of both hierarchically porous zeolites and their conventional counterparts.The method is found to be applicable to ZSM-5 with molar ratios of Si/AI from 20 to 180.The ZSM-5 zeolites are used as catalysts for benzene methylation at 460 °C to investigate the effect of additional porosity and Si/AI ratios.At low Si/AI ratios,the benzene conversions over conventional and hierarchical ZSM-5 are close,and selectivity to toluene is high over hierarchical ZSM-5.It is found that hierarchical porosity markedly enhances the utility of zeolite and the selectivity towards xylenes via improved mass transport at higher Si/AI ratios.Under an optimized hierarchical ZSM-5 catalyst,xylene selectivity reaches 34.9%at a Si/AI ratio of 180.
Methylation of benzene is an alternative low-cost route to produce xylenes, but selectivity to xylene remains low over conventional zeolitic catalysts. In this work, a combined dry-gel-conversion and steam-assisted crystallization process is used to synthesize hierarchically porous zeolite ZSM- 5 with varied Si / AI malar ratios. X-ray diffraction (XRD), N 2 physisorption, NH 3-temperature programmed desorption (TPD), scanning electronic microscopic (SEM) measurement and Fourier transform infrared (FT-IR) are employed to characterize the Structure and acidity of both hierarchically porous zeolites and their conventional counterparts. The method is found to be applicable to ZSM-5 with molar ratios of Si / AI from 20 to 180. The ZSM-5 zeolites are used as catalysts for benzene methylation at 460 ° C to investigate the effect of additional porosity and Si / AI ratios. At low Si / AI ratios, the benzene conversions over conventional and hierarchical ZSM-5 are close, and selectivity to toluene is high over hierarchic al ZSM-5.It is found that the hierarchical porosity markedly enhances the utility of zeolite and the selectivity towards xylenes via improved mass transport at higher Si / AI ratios. Unitized optimized ZSM-5 catalyst, xylene selectivity reaches 34.9% at a Si / AI ratio of 180.