Mo-Modified ZSM-5 zeolite with intergrowth crystals for high-efficiency catalytic xylene isomerization

Abstract

Mo/ZSM-5 catalysts of xylene isomerization were prepared on the intergrowth ZSM-5 support by an impregnation–calcination–reduction procedure. The ZSM-5 zeolite with intergrowth crystals was prepared with high shape selectivity by a hydrothermal method. This ZSM-5 crystal has a higher specific surface area and more sinusoidal channels on its external surface. Further loading of Mo modified the acidity distribution of the ZSM-5 catalyst. The catalytic performance was tested by the isomerization of C8 mixed aromatic hydrocarbons (ethylbenzene, m-xylene and o-xylene) in a fixed-bed reactor. Many techniques were employed to characterize the catalysts. The results showed that this kind of Mo/ZSM-5 has a high shape selectivity in the reaction of mixed C8 aromatics. The symbiotic crystals cover most of the straight channels, which increases the diffusion resistance of MX and OX and reduces the xylene loss from cracking by xylene reaction. The interaction of metal Mo with Lewis acid and Brønsted acid sites promotes the dealkylation of EB. However, excess Mo has a negative effect on the dispersion of MoO₃, resulting in a decrease of the specific surface area. In addition, 5%Mo/ZSM-5 could achieve the excellent stability with a time-on-stream of 50 h. At 380 °C, the ethylbenzene conversion and PX yield reached 71.2% and 24.6%, with a low xylene loss of 1.65% over 5%Mo/ZSM-5, respectively.