One-pot synthesis of composite metal-organic framework for enhanced water adsorption: Feasibility and mechanism exploration

Metal-organic framework (MOF) offers a promising solution to the global water crisis with atmospheric water harvesting (AWH). However, its practical application is limited by its water adsorption performance in various humidity conditions. In this work, composited MOFs (CMOFs) with hygroscopic salt were synthesized by a one-pot hydrothermal method to enhance the water uptake, and the effect of aluminum sources and dispersants was investigated. AlCl₃ and Al(NO₃)₃ were efficient aluminum sources that showed stable water adsorption performance, which exhibited excellent water adsorption performance with a water adsorption capacity of 0.4 g·g⁻¹ at 20 % RH for MOF-303. The crystal structure of MOF-303 will change and show a significantly low specific surface area as Al₂(SO₄)₃ is used as the aluminum source, and the water adsorption capacity decreases to 0.2 g·g⁻¹ at 20 % RH because of the strong binding energy (−7.196 eV) between SO₄²⁻ and MOF-303. Hygroscopic salt was incorporated in MOF successfully via the one-pot hydrothermal synthesis method, the composite ratio of salt was 0.1442, 0.1732, and 0.1607 g·g⁻¹ in MOF-303-LiOH, MOF-303-NaOH, and MOF-303-Ca(OH)₂. The new chemical state of chlorine and sodium elements demonstrate that the hygroscopic salt was adsorbed/trapped by the MOF structure. MOF-303-NaOH showed a stable water adsorption capacity of 0.22 g·g⁻¹ at 30 % RH and significantly enhanced water adsorption capacity of up to 2.05 g·g⁻¹ at 95 % RH. Meanwhile, the ratio of salts in the CMOF can be adjusted with the proportion of dispersants, the cyclic adsorption test indicated that AlFu-NaOH-2 maintained the water adsorption capacity of 1.62 g·g⁻¹ without deliquesce. This work provides a new strategy for synthesizing CMOFs with excellent water adsorption performance, which can potentially promote the application of MOF in AWH.