A review of the computational studies of proton- and metal-exchanged chabazites as media for molecular hydrogen storage performed with the CRYSTAL code

In the present paper, a review of our previously published results about the ab initio modeling on the H₂ interaction with the polarizing centers of proton- and metal-exchanged chabazites performed with the periodic ab initio CRYSTAL code is reported. The paper highlights how ab initio modeling allows to: (i) understand the H₂ interaction with solids at an atomistic level; (ii) infer the potential of zeolites as hydrogen storage materials; (iii) give insights in the design of new H₂ storage materials for applications. The calculations were performed within the periodic approach using the B3LYP functional with all-electron double zeta polarized Gaussian type basis sets. New data are reported using a richer basis set for both zeolite and H₂, which ensures a better description of the intermolecular interaction components. In all the considered systems, H₂ mainly interacts side-on with zeolites' charge balancer cations. Geometrical and energetic features were also refined at MP2 level adopting clusters cut out from the chabazite framework in order to gauge the dispersive contribution to the interaction energy, not accounted for by the B3LYP functional. Computed BSSE-free binding energies (BE^c) showed that H₂ interacts poorly with the protons (H⁺) of the Brønsted sites (BE^c ∼ 5 kJ / mol), whereas the interaction with Li⁺, Na⁺ and K⁺ in metal-exchanged chabazites was stronger, particularly for Li⁺ located in small framework rings (BE^c ∼ 12 kJ / mol). When divalent Mg^{2 +} cation is used as a charge balancer, the BE^c becomes ∼ 18 kJ / mol, showing that the interplay between cationic polarization and its spatial position in the zeolite framework is crucial in determining the sorption capacity of the material.