Water Adsorption in Metal-Organic Frameworks: Characteristics, Mechanisms, and Structure-Property Relationships

To address the increasingly limited water availability, using metal-organic frameworks (MOFs) to capture atmospheric water vapor as usable resources has emerged as a promising strategy. The adsorption characteristics of MOFs as well as their step pressure (i.e., the pressure when the steep rise in water uptake, if any, occurs) directly influence the amount of collectable water and thus their harvesting effectiveness. To date, a comprehensive understanding of water adsorption behaviors and how they are influenced by MOF properties remains lacking. This study addresses this gap by systematically investigating water adsorption in >200 strategically selected MOFs with diverse features. The results demonstrate that water adsorption in MOFs is highly complex, revealing various subtypes of both non-S-shaped and S-shaped isotherms. Moreover, among S-shaped isotherms, distinct phase behaviors that, in turn, govern different adsorption mechanisms and hydrogen-bonding environments are identified, as evidenced by analyses of their macrostate probability distributions (MPDs), thermodynamic stability limits, and grand potential free energy landscapes. Further structure-property analyses also uncover key design principles: A moderate heat of adsorption is critical for enabling S-shaped isotherms, while the size of pores critically modulates the steepness of the adsorption step by controlling the phase transition nature of confined water. The density and, more importantly, the uniformity of adsorption sites are found as well to dictate the step pressure. Overall, this study provides a comprehensive overview of diverse adsorption behaviors in MOFs as well as informs general design principles for the development of advanced adsorbents for atmospheric water harvesting.