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

Abstract

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.