Fundamental Study of Reversible Hydrogen Storage in Titanium- and Lithium-Functionalized Calix[4]arene

Hydrogen is the most promising candidate for a sustainable energy source in the transport sector. However, the storage of hydrogen is a major problem. Calix[4]­arene (CX) is functionalized with Ti and Li metals on the delocalized π electrons of benzene rings, and the metal-functionalized system is studied for hydrogen storage efficiency by applying density functional theory using the M06 hybrid functional and 6-311G­(d,p) basis set. The calculated binding energy indicates Ti coordinates with CX strongly while Li coordinates weakly and the binding of CX and metal is through Dewar mechanism. On saturation with hydrogen, each Ti atom traps four H₂ molecules while each Li atom traps three H₂ molecules on CX. Hydrogen molecules are adsorbed on the metal atoms by Kubas–Niu–Rao–Jena interaction. The global reactivity index obtained for the system obeys the maximum hardness and minimum electrophilicity principle. Molecular dynamics simulations are performed using spin-polarized generalized gradient approximation with the Perdew–Burke–Ernzerhof functional including Grimme diffusion parameter on H₂ saturated systems. The dissociation of H₂ molecules in the Ti-functionalized CX system begins from 273 K, while all the H₂ molecules are desorbed by 473 K. The storage capacity is found to be 8.7 wt % for Ti and 10.1 wt % for Li-functionalized CX. When the Ti atom is intercalated between the two CX moieties, the storage capacity does not reduce significantly. This study reveals that the Ti-functionalized CX is a potential reversible hydrogen storage material.