Towards a deep desilication/dealumination of NU-10 zeolite: Shape-selectivity regulation

The present work was aimed at performing alkaline desilication of different severities, in order to generate a broad range of mesopore volumes within the framework of the intermediate pore size NU-10 zeolite and to evaluate both physicochemical and catalytic consequences. The shape-selective sensitive o-xylene isomerisation reaction was used as a characterisation method of zeolite pore modification. Thus, several NU-10 samples, with preserved crystallinity, were obtained by treatment of a microporous parent NU-10 sample with sodium hydroxide solutions of different concentrations (0.4-0.6 M), at several contact times (15, 30, 45, 60 and 120 min) and at 353 K. The characterisation by nitrogen adsorption allowed the evaluation of the micropore volumes of the samples and the mesopore volumes generated by desilication, as well as the external surface area. Toluene adsorption experiments were also performed, an important increase in the toluene adsorption capacity for increasing desilication levels being observed, except for the most severely modified samples. The sample treated at 0.6 M for 60 min revealed lower toluene retention, probably due to a higher level of extra-framework aluminium and silicon species remaining in the internal porosity. In the case of aluminium species, they were detected by ²⁷Al NMR spectroscopy, which is also in agreement with the pronounced increase in the number of Lewis acid sites detected for the most severely treated samples. A slight decrease in the initial o-xylene isomerisation activity, at 673 K, was generally noticed as a consequence of desilication, and this can be related to a slight decrease in the concentration of protonated sites. Nevertheless, for intermediate-severity- treated samples, the activity per acid site increased significantly when compared with the parent zeolite, suggesting easier reactant accessibility. Furthermore, the para/meta-xylene product molar ratio decreased and, therefore, the zeolite shape selectivity, thus reinforcing the hypothesis of an extra space generation consequence on the catalytic properties.