Xylene Selectivity at the External Surface of Hierarchical Zeolites: Experiment and Molecular Modeling

Hierarchical faujasite zeolites made up of nanoparticle aggregates (NAs) and layer-like structures (LL) are evaluated for xylene adsorption in both gas and liquid phases. For NA zeolites, several samples with varying external surfaces and meso/macropore volumes are considered. Compared to conventional faujasite, xylene adsorption isotherms as measured by means of thermogravimetric experiments display higher adsorption capacities for the NA faujasites. This adsorption capacity increase is found to be linked to the zeolite external surface area. However, in the case of the LL zeolite, despite its large external surface, the xylene adsorption capacity is found to be comparable to that of the conventional zeolite. For all hierarchical samples, the paraxylene selectivities assessed through batch adsorption experiments in the liquid phase are smaller than that for the conventional zeolite. The observed selectivity decrease seems to be associated with the decrease in functional window adsorptive sites when enhancing the external surface. To shed light on the observed phenomenon, Monte Carlo simulations in the Grand Canonical ensemble are performed to investigate xylene adsorption for a bulk infinite zeolite and a zeolite sample exhibiting an external surface. For the latter sample, the external surface chosen is made up of incomplete cavities and, therefore, defective window sites. In agreement with our experiments, the results from molecular simulation indicate a selectivity loss associated with the sample large external surface.