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.