Pore with gate: Modulating hydrogen storage in metal-organic framework materials via cation exchange

A range of anionic metal-organic framework (MOF) materials has been prepared by combination of In(iii) with tetracarboxylate isophthalate-based ligands. These materials incorporate organic cations, either H ₂ppz²⁺ (ppz = piperazine) or Me₂NH ₂⁺, that are hydrogen bonded to the pore wall. These cations act as a gate controlling entry of N₂ and H₂ gas into and out of the porous host. Thus, hysteretic adsorption/desorption for N₂ and H₂ is observed in these systems, reflecting the role of the bulky hydrogen bonded organic cations in controlling the kinetic trapping of substrates. Post-synthetic cation exchange with Li⁺ leads to removal of the organic cation and the formation of the corresponding Li ⁺ salts. Replacement of the organic cation with smaller Li ⁺ leads to an increase in internal surface area and pore volume of the framework material, and in some cases to an increase in the isosteric heat of adsorption of H₂ at zero coverage, as predicted by theoretical modelling. The structures, characterisation and analysis of these charged porous materials as storage portals for H₂ are discussed. Inelastic neutron scattering experiments confirm interaction of H₂ with the carboxylate groups of the isophthalate ligands bound to In(iii) centres.