Optimizing Properties of n -Pentane Aromatization Catalysts via the c -Axis Length of ZSM-5 Molecular Sieves

The diffusion resistance significantly affects the dispersion of active components and coke deposition over microporous Zn/ZSM-5 catalysts in the n-pentane aromatization process. It leads to difficulties in optimizing the catalyst performance. In this study, a series of plate-like Zn/ZSM-5 catalysts with various c-axis lengths were obtained to investigate the effect of diffusion resistance on the structure and performance of the catalyst. Catalysts with different c-axis lengths feature comparable Si/Al ratios and framework structures. As the c-axis increases from ∼1.0 to 2.5 μm, an increased exposure of the [010] crystal plane reveals a greater number of straight pores. Accessibility of acid sites increases and the dispersion of Zn species improves. Meanwhile, catalysts with a longer c-axis have an appropriate Brønsted/Lewis (B/L) ratio and a higher ZnOH+/ZnO ratio, which improves the consumption of olefin intermediates and exhibits greater benzene, toluene, and xylene (BTX) selectivity. Extending the c-axis of catalysts results in more amorphous coke deposition due to enhanced hydrogen-transfer activity, yet the amorphous coke is easily removed and has little effect on catalyst activity. By increasing the c-axis length, the Zn/ZSM-5-L catalyst achieves the highest n-pentane conversion (93.8%) and BTX selectivity (39.0%). The findings are crucial for proposing an innovative approach to regulate the properties and catalytic performances of Zn/ZSM-5 catalysts through altering the c-axis length of ZSM-5 molecular sieves.