Structuration of Water in Microporous CAU-10-H under Gigapascal Pressure

Employing metal-organic frameworks (MOFs) as water adsorbents has become a flourishing field in recent years. Aluminum isophthalate [Al(OH)(bdc)]·nH₂O, referred to as CAU-10-H, is renowned for its high physical and chemical stability under external mechanical stimuli, which is crucial for use in sorption-based heat exchange. We report pressure-induced structural changes and water uptake/release of CAU-10-H in the gigapascal regime. CAU-10-H displays four different phases during pressurization in water up to 4.08(1) GPa. Upon immersion in water at ambient conditions, the unit cell volume expands by 1.3(1)%, by increasing water content from 78.7(1) to 85.6(1) H₂O per unit cell. Upon pressurization to 1.08(1) GPa, the water content increases gradually to 112.5(1) H₂O per unit cell and reduces the symmetry to P1 phase. At 1.72(1) GPa, a correlated "gate-opening/channel-closing" transition to a centrosymmetric Pnma phase occurs to increase the unit cell volume by 6.9(1)% and water content to 138.8(1) H₂O per unit cell. Further increase in pressure to 4.08(1) GPa then leads to a gradual decrease in water content to 119.2(1) H₂O per unit cell and redistribution of the inserted water molecules during a "gate-closing/channel-opening" transition to an I4₁/amd phase. After pressure removal, CAU-10-H reverts to its initial state in terms of crystallinity and water content. This pressure-induced water adsorption/desorption and concerted movements of gate-constraining ligands provides a detailed understanding of the water-MOF interplay, which can be used as a basis for designing new materials, possibly utilizing pressure as a tool for chemical modifications.