Nanoporous ZSM-5 Crystals Coated with Silicalite-1 for Enhanced p-Xylene Separation

The growth of a silicalite-1 coating on ZSM-5 zeolite crystals is one of the surface modification methods allowing to improve its catalytic p-xylene selectivity. Aiming to understand this increase in p-xylene selectivity, the effect of silicalite-1 coating on the adsorptive properties of ZSM-5 was investigated. Pulse chromatographic experiments revealed the silicalite-1 coating improved the separation factors of p-xylene over o-xylene and m-xylene, resulting in a better chromatographic separation of p-xylene over m-xylene on silicalite/ZSM-5 compared to that on parent ZSM-5. Additionally, slower adsorption kinetics of m-xylene on silicalite/ZSM-5 compared to ZSM-5 was also confirmed by gravimetric adsorption measurements. It is concluded that the enhanced p-xylene selectivity is achieved by steric effects leading to a more pronounced exclusion of m-xylene in the outer silicalite-1 layer and a reduction in number of surface adsorption sites.