Construction of Submicron Spherical ZSM-48 Zeolite: Crystallization Mechanism and Catalytic Application

Morphology control of zeolite with a one-dimensional channel, especially for ZSM-48 zeolite, is a great challenge due to its inherent characteristics of growth along the main channels. How to shorten the diffusion path of molecules is crucial to alleviating the diffusion limitation. Here, a direct synthesis route for submicron (500 nm) spherical ZSM-48 zeolite was developed with the assistance of F- ions at low crystallization temperature. Through detailed investigation of the role of F- ions and the crystallization mechanism using various characterization techniques, a reliable formation mechanism of spherical ZSM-48 zeolite was proposed. First, F- ions addition shortens the induction period by promoting the dissociation rearrangement of aluminosilicate precursors, which promotes the preferential formation of secondary structural units. Moreover, F- ions induce the formation of ZSM-48 spheres by promoting surface silanol group condensation. ZSM-48 spheres with sizes of 500-2500 nm can be successfully obtained by regulating the crystallization temperature. In m-xylene isomerization, submicron spherical ZSM-48 zeolite exhibits significantly improved stability compared with the conventional one. Moreover, the selectivity for p-xylene is much higher than that of a typical ZSM-5 zeolite (68% vs 48%). The excellent catalytic performance of submicron spherical ZSM-48 highlights not only the positive impact of shortened channels for mass transfer but also its potential as a new catalyst for m-xylene isomerization.

Automated summary

This study developed a direct synthesis route for submicron (500 nm) spherical ZSM-48 zeolite with the assistance of F- ions at low crystallization temperature. Through detailed investigation, a reliable formation mechanism of spherical ZSM-48 zeolite was proposed. The submicron spherical ZSM-48 zeolite showed significantly improved stability and higher selectivity for p-xylene in m-xylene isomerization compared to traditional zeolite.