Insights into the role of hierarchical porosity in zeolite architectures for selective uptake of metal ions in solution

The ability to match two levels of porosity in hierarchical structures obtained <i>via</i> the top-down approach greatly enhances the adsorption capacity of zeolites. The effect of top-down desilication treatment on different topological frameworks (MFI, FAU and natural HEU) was evaluated by varying the pH (from 8 to 13.3) of alkaline solutions to minimize the loss of crystallinity and acidity. The structural, textural and acidity properties of the hierarchical zeolites were examined using a range of physicochemical characterization tools. In particular, the amount, location and accessibility of the Brønsted acid sites, the locus for cation exchange, processes were studied by FTIR spectroscopy using basic probe molecules with different kinetic diameters, and the accessibility factor (AF) was calculated. Finally, an improvement in the uptake capacity of Cu²⁺ and Co²⁺ cations from aqueous solutions, with different affinities and selectivities depending on the topological structures, was observed.