Systematic evaluation of water adsorption in isoreticular UiO-type metal–organic frameworks

Abstract

Water adsorption in porous materials is important for many applications including heat allocation, dehumidification, thermal batteries, and water harvesting from air. While porous MOFs have been demonstrated as promising adsorbents for water capture, there is still a lack of a systematic study on the relationship between the MOF structure and water sorption properties. We herein comprehensively investigated how pore size, surface area and pore functionality influence water sorption properties in a family of isomorphic UiO-type MOFs. By altering the linker length or incorporating hydrophilic/hydrophobic groups, we selected and synthesized thirteen isoreticular MOFs in which the pore size, surface area and hydrophilicity are finely engineered without changing the topology. We discovered that the introduction of hydrophilic groups into UiO-66 can improve water uptake and concurrently promote water adsorption kinetics at P/P₀ = 0.2; however, the incorporation of hydrophobic groups or pore size extension exhibits negative effects. UiO-66-N functionalized with Lewis basic nitrogen sites shows the highest water uptake (0.37 g g⁻¹) and fastest adsorption rate at 298 K and P/P₀ = 0.2 among these MOFs, even comparable to the benchmark KMF-1 (0.40 g g⁻¹) and MOF-303 (0.37 g g⁻¹). UiO-66-N thus shows one of the highest coefficient of performances (0.71) at an ultra-low driving temperature of 65 °C for cooling applications. Theoretical calculations reveal that the incorporated hydrophilic nitrogen sites can serve as additional binding sites to strengthen the water binding affinity and thus enhance the low-pressure water uptake. This study reveals comprehensive insights into the structure–property relationships of water adsorption, suggesting some general perspectives to design promising MOFs for water-sorption applications.