BTX Production via Direct Dehydroaromatization of Methane and Propane over Gallium-based Zeolites

Abstract

Recently, due to development of shale gas, which consists of mainly methane and light paraffin, many researches about efficient conversion of methane and light paraffin have been attempted. Among them, researches on the production of BTX, which is rapidly increasing in demand, have attracted attentions. In this work, modification of pore structure of zeolites have been studied and applied for BTX (benzene, toluene, and xylene) production by co-aromatization of methane and propane. In particular, a carbon template was introduced to conventional microporous HZSM-5 and HZSM-11 (denoted as micro-HZSM-5 and micro-HZSM-11). Mesoporous HZSM-5 and HZSM-11 (denoted as meso-HZSM-5 and meso-HZSM-11, respectively) were synthesized to enhance the mass transfer and coke resistance of conventional zeolites. In order to activate the reactants, the same amount of gallium oxide (2 wt% with respect to each zeolite) was doped into prepared zeolites (denoted as GaOy/micro-HZSM-5, GaOy/micro-HZSM-11, GaOy/meso-HZSM-5, and GaOy/meso-HZSM-11) by using a wetness impregnation method. It was revealed that gallium oxide supported on mesoporous zeolites showed higher BTX selectivity and BTX yield than microporous zeolites with less coke deposition. Between GaOy/meso-HZSM-5 and GaOy/meso-HZSM-11, the effect of introduction of mesopore in meso-HZSM-11 was more significant than meso-HZSM-5. It was known that interaction between zeolites and GaO+ species played an important role on dehydrogenation of reactant, which was a rate-determining step of dehydroaromatization process. According to XPS, H2-TPR, and NH3-TPD analyses, GaOy/meso-HZSM-11 showed the stronger interaction with GaO+ species than GaOy/meso-HZSM-5. It was found that larger amount of GaO+ species in GaOy/meso-HZSM-11 was corresponding to high conversion of methane and propane.

Accordingly, a series of XGaOy/meso-HZSM-11 (X = 0, 1, 2, 4, and 8) catalysts with different amount of gallium oxide loading (X, wt%) were prepared to optimize the catalytic performance. Correlation between acid properties of XGaOy/meso-HZSM-11 and catalytic performance was investigated. Among tested catalysts, 1GaOy/meso-HZSM-11 exhibited the best catalytic performance with the largest amount of acidity. Also, 4GaOy/meso-HZSM-11 and 8GaOy/meso-HZSM-11 showed severe deactivation over times while less amount of gallium loading supported catalysts showed stable catalytic performance. Therefore, an optimum amount of gallium loading is required for improving BTX production by co-aromatization of methane and propane.