Improved water stability by thermal treatment of hexatopic ligand-based metal-organic frameworks for hydrogen storage

Abstract

One of the essential properties required for the practical application of metal–organic frameworks (MOFs) as gas storage materials is high water stability. In this study, we investigate the origin of improved water stability through thermal treatment in V₃(PET), a MOF containing hexatopic peripherally extended triptycene (H₆PET) ligands, which show promise for hydrogen storage. While V₃(PET) should be water-stable due to strong metal (hard acid, V³⁺)-ligand (hard base, carboxylate group) bonds, our experimental and theoretical findings reveal that the presence of dangling ligand—defects caused by metal-modulator (acetate) bonds— reduces its water stability. Our first-principles density functional theory (DFT) calculations show that the defect formation energy for V₃(PET) with dangling ligands (+1.96 eV) is significantly lower than that for V₃(PET) without them (+6.34 eV), making it more vulnerable to humidity. By removing acetate and restoring the original metal–ligand bonds, we significantly enhance the water stability of V₃(PET). Additionally, thermally treated V₃(PET) retains about 95 % of its hydrogen storage performance even after 7 days in 60 % relative humidity and maintains high mechanical stability over 200 hydrogen storage cycles.