Nonalkaline Fabrication of Al-Based Metal–Organic Frameworks with Tailored Water Sorption Properties via Polymeric Hydroxy-Aluminum Basicity Modulation

Metal–organic frameworks (MOFs) are porous crystalline materials composed of metallic nodes and organic ligands, demonstrating increasing potential in water harvesting in arid and semiarid regions. This study presents a nonalkaline, water-based, and scalable synthesis strategy designed to adjust the water sorption properties of aluminum-based MOFs (Al-MOFs), specifically, AlFum and MOF-303, by modifying the basicity of the metal source, polymeric hydroxy-aluminum, as an alternative. Characterizations, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analyses (TGA), confirmed the successful synthesis of Al-MOFs. The results revealed that high-basicity polymeric hydroxy-aluminum introduced additional mesoscopic intraparticle defects, interparticle voids, and hydrophilic surface sites to the primary microporous Al-MOFs. This led to an enhanced external surface area and uniformity in the particle size. Consequently, the water sorption performance of basicity-modulated Al-MOFs was significantly improved. Specifically, within the typical working humidity between 0.05 and 0.3, using polymeric hydroxy-aluminum of the highest basicity resulted in a 23% and 68% increase in water uptake for AlFum and MOF-303, respectively, achieving capacities of 0.43 and 0.37 g·g–1. Cyclic water adsorption–desorption tests further indicated the hydrolytic stability of prepared Al-MOFs. This study offers a novel approach to engineering MOF properties through metal source modulation, with important implications for applications in water harvesting and heat transfer.