Functionalization, protonation and ligand extension on MIL-53 (Al) MOFs to boost water adsorption and thermal energy storage for heat transformations

Abstract

Understanding the relationship between the geometry of metal–organic frameworks (MOFs) and the shape of H2O adsorption isotherms is of utmost importance to control the hydrophilic and hydrophobic behaviors of water in MOFs with adsorption/desorption kinetics to realize their applications in thermal energy storage (TES). The MOFs are potential candidates for TES through water adsorption process. The TES can be used as a pivot for heat transformations related to cooling, heat pump, desalination, power generation, water harvesting and dehumidification. The TES performances are mainly determined by water adsorption characteristics such as water uptake/offtake loadings, kinetics and adsorbents-water interaction energy. Therefore, this paper presents a comprehensive analysis on the restructuring of MIL-53 (Al) MOFs for heat transformation applications. In this work, the original MOF is first functionalized with amino or hydroxyl functional groups. Next, the functionalized MOF is protonated with hydrochloric acid. In addition, ligand extension is applied on the original MOF by replacing the original organic linker with the progressively longer linkers. The surface properties of these restructured MOFs are measured by XRD, FTIR, TGA and N2 adsorption analysis. The water uptakes under equilibrium and dynamic conditions are presented in the form of isotherms and kinetic plots. The structural, thermal and cyclic water transfer stabilities of these MOFs are also presented. It is found that the tailored MIL-53 (Al) MOFs modify both the hydrophilicity and hydrophobicity of the original MOFs and improve the water loadings up to 0.9 g/g. The functionalized and protonated MIL-53 (Al) MOFs show higher water transfer with faster kinetics as compared to the parent MIL-53(Al) MOFs. The ligand extended MIL-53 (Al) MOFs provide higher water transfer between humid (relative humidity RH of 80% to 90%) and regenerated (RH of 30%) conditions with promising thermal energy storage density (TESD, up to 1.54 MJ/L).