Water Adsorption Properties and Applications of Stable Metal-organic Frameworks

Metal-organic frameworks (MOFs), featuring the ultrahigh surface area, high porosity, tunable geometrical and chemical properties, show potential applications in gas adsorption/separation, heterogenous catalysis, etc. As the ubiquity of water vapor in the ambient environment and industrial gas streams, it is necessary to study on interaction mechanism between MOFs and water molecules and develop highly water-stable MOFs with desirable water adsorption/desorption behaviors. It not only has the scientific significance, but also great importance in promoting the practical applications of MOFs. Given the tailorable abilities of pore size, pore volume, cavity hydrophilicity and water stability, MOFs provide unprecedented advantages to explore the well-defined porous sorbents in molecular level, which facilitates the realization of reversible water vapor uptake and release at expected relative pressure and temperature together with high working capacity. For now, a wide range of hydrolytically stable MOFs including high-valence metal (e.g. Cr3+, Al3+, Zr4+, Ti4+) based frameworks have emerged as the advanced and promising porous sorbents for energy efficient applications, by utilizing water as eco-friendly adsorbate media and renewable heat. This review focuses on the following aspects: (1) the degradation mechanism of MOFs in liquid phase of water and the design concepts of hydrolytically stable MOFs by modulating their coordination bond based on the Pearson' hard/soft acid/base principle; (2) the physical or chemical water ad/desorption properties of MOFs; (3) the classification of numerous MOFs sorbents and conventional desiccants based on their hydrophilicity, which is approximately reflected by the relative humidity (RH) value of the inflection points (the RH where the steep uptake starts) in isotherms; (4) a variety of water adsorption-based applications of MOFs such as industrial gas dehydration, drinking water harvesting in the desert area, adsorption-based heat pump and indoor humidity regulation. Finally, the research priorities and development outlook are summarized and the future challenge with respect to water adsorption-based applications for the next-generation MOFs are outlined.