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 V<inf>3</inf>(PET), a MOF containing hexatopic peripherally extended triptycene (H<inf>6</inf>PET) ligands, which show promise for hydrogen storage. While V<inf>3</inf>(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 V<inf>3</inf>(PET) with dangling ligands (+1.96 eV) is significantly lower than that for V<inf>3</inf>(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 V<inf>3</inf>(PET). Additionally, thermally treated V<inf>3</inf>(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. © 2025 The Authors