Enhancing metal dispersion over an Mo/ZSM-5 catalyst for methane dehydroaromatization
Mo dispersion is the key factor promoting the performance of an Mo/ZSM-5 catalyst for the methane dehydroaromatization (MDA) reaction. Citric acid (CA) was introduced during the conventional wetness impregnation step to ameliorate the Mo dispersion over the Mo/ZSM-5 catalyst. After introducing CA, all catalysts bear high crystallinity relative to that of ZSM-5 zeolite, which proves that CA treatment does not damage the physical structure of catalysts. CA introduction also enhances the Mo dispersion significantly. The metal particle size of 3Mo/ZSM-5-CA1.5 (sample with an Mo loading of 3 wt% and nCA : nMo ratio of 1.5) is within 5–15 nm and is lower than that of pristine 3Mo/ZSM-5 (within 10–20 nm). The role of CA in improving Mo dispersion can be further certified by the higher band gap energy of molybdenum atoms, the higher proportion of surface oxygen in molybdenum oxide and the fewer Brønsted acid sites for the catalysts prepared with the aid of CA. High Mo dispersion contributes to the high activity and selectivity of the catalysts. The aromatic yield of 3Mo/ZSM-5-CA1.5 is almost twice that of pristine 3Mo/ZSM-5. The improvement of Mo dispersion is attributed to the chelation between CA and Mo species, leading to the subsequent isolation of the active components during calcination. Moreover, this effect is more distinct for the catalysts with higher Mo loading. When Mo loading increases to 9 wt%, the aromatic selectivity of 9Mo/ZSM-5-CA1.5 is more than 93% and the coke resistance is obviously elevated compared to that of a catalyst prepared without CA. Based on the above findings, it is clear that adding CA during the Mo/ZSM-5 preparation is an effective way to improve Mo dispersion.
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