Effect of weak base modification on ZSM-5 catalyst for methanol to aromatics

The effect of weak base modification on the catalytic performance of ZSM-5 catalyst for conversion of methanol to aromatics was investigated. The catalysts were characterized using X-ray diffraction, X-ray fluorescence, N-2 adsorption-desorption, NH3 temperature-programmed desorption, Fourier transform infrared spectroscopy, scanning electron microscopy and thermogravimetry. The results showed that catalysts treated with weak base (NaHCO3, Na2CO3 and NH3 center dot H2O) exhibited a pore structure with interconnected micropores and mesopores. The existence of mesopores was beneficial for improving the diffusion of reactants and products, and the coke deposition resistance capacity of treated catalysts was enhanced greatly. Meanwhile, compared to traditional ZSM-5 zeolite, the ratio of Bronsted to Lewis (B/L) acid sites of ZSM-5/NH3 center dot H2O (B/L = 7.35) zeolite slightly increased but the amount of acid sites reduced, while those of ZSM-5/NaHCO3 (B/L = 0.127) and ZSM-5/Na2CO3 (B/L = 0.107) significantly reduced. Further, the catalyst treated with NH3 center dot H2O solution was evaluated in the methanol to aromatics reaction and led to an enhanced aromatization reaction rate. The liquid hydrocarbons product distribution exhibited higher aromatic hydrocarbons yield (56.12%) and selectivity (40.28%) of benzene, toluene and xylene (BTX) with isoparaffin content reducing to 26.17%, which could be explained by appropriate B/L acid sites ratio, higher pore volumes and higher surface area.