Hierarchical Nanosized ZSM-5 for Enhanced Selectivity of Para-Xylene in the Alkylation of Toluene with Methanol: Synergy of External Acid Sites and Diffusion

Alkylation of toluene with methanol to produce para-xylene using ZSM-5 zeolite is a critical shape selectivity process for enhancing the selectivity of para-xylene, a key intermediate in poly(ethylene terephthalate) (PET) production. In this work, we synthesized ZSM-5 zeolites with varying crystal sizes and subjected them to different posttreatment methods (alkaline or steaming treatments) to optimize their catalytic performance. Our results demonstrate that modifications in the crystal size and surface treatments significantly influence the number of external acid sites and diffusion characteristics of the zeolites, thereby affecting para-xylene selectivity. The reduced number of external acid sites and enhanced diffusion properties in hierarchical nanosized ZSM-5 zeolite enhance para-xylene selectivity. Specifically, a para-xylene selectivity exceeding 98.0%, with a 5-fold increase in catalyst stability compared to conventional micron-sized zeolites and a long-term stable operation (>135 h), can be achieved by high-concentration alkaline treatment, which disrupts the intersectional channels, facilitating the timely diffusion of para-xylene generated from straight and/or sinusoidal channels. This study demonstrates the potential of hierarchical nanosized ZSM-5 zeolites in achieving high selectivity and stability in para-xylene production, offering a promising approach to meet the growing industrial demand efficiently.