The effect of Si/Al ratio of ZSM-12 zeolite on its morphology, acidity and crystal size for the catalytic performance in the HTO process

Abstract

In this research, ZSM-12 zeolite with six Si/Al ratios (20 to 320) was synthesized by a hydrothermal method and systematically investigated. The physicochemical properties of the synthesized nano zeolites were evaluated and compared by XRD, FE-SEM,ICP-AES, NH₃-TPD, BET, FT-IR, and TGA analyses. The results show that when the Si/Al ratio increases, the amount of microcrystals increases with the dominant competitive phase of cristobalite by decreasing the MTW phase. The catalytic assessment of synthesized zeolites in the (n-hexane to olefins) HTO process in a fixed bed reactor under atmospheric pressure and WHSV equal to 4 h⁻¹ at 550 °C was evaluated and various parameters such as selectivity towards light olefins, P/E ratio, production of light alkanes, and aromatic compounds (BTX) were investigated. The result of the n-hexane to olefins process indicated that the presence of cristobalite as an impurity phase strongly affects the activity of the catalysts. The Z80 zeolite, with a Si/Al ratio of 80, corresponds to the pure form of ZSM-12 and exhibits the highest light olefin yield at 52.5%. This zeolite demonstrates superior propylene selectivity (P/E = 1.75) owing to its well-suited pore structure, wide channels, and optimal acidity derived from the MTW zeolite. On the other hand, zeolite Z320 has a lower light olefin yield (19.4%) and a lower P/E (1.1) ratio. In addition, according to the results of the TGA analysis, the content of coke on the Z80 catalyst after the catalytic reaction is much less than other catalysts after the catalytic reactor test.