Effects of Pore Connectivity on Water Adsorption in Metal-Organic Frameworks

Atmospheric water harvesting (AWH) has emerged as a promising solution to help address global freshwater scarcity. Metal-organic frameworks (MOFs) are premier adsorbent candidates for their highly tunable nature through combinations of metal nodes and organic linkers. Ideally, they should exhibit an "S-shaped" isotherm, featuring large water uptake that can be released within a narrow humidity range. While the effects of pore dimension and chemical affinity on their adsorption properties have been studied, the influence of spatial pore connectivity remains largely unexplored. To this end, this study investigates the role of topological connectivity in their water adsorption properties using flat-histogram Monte Carlo simulations. Specifically, two representative types of MOFs (i.e., one with connected cages and another with individual channels) are studied. Moreover, a strategic pore-blocking approach is employed to systematically modulate their structural connectivity without altering local host-adsorbate interactions. Simulation results show that enhanced spatial connectivity fundamentally promotes cooperative adsorption, notably shifting the step pressure toward a lower value. Grand potential free energy and configurational analyses further reveal that interconnected pores facilitate the formation of a more stable condensed phase with extended hydrogen-bonding networks. Interestingly, in MOFs with effectively isolated one-dimensional channels, minor interpore associative behaviors through long-range Coulombic interactions can still persist, exerting a non-negligible influence on their water adsorption behavior. Comparative simulations for nonpolar gases also confirm that this connectivity-driven step shift is a unique characteristic of strongly associated molecules like water. Overall, this work demonstrates that pore connectivity is a critical structural parameter dictating water phase behavior, offering key insights into the rational design of highly efficient AWH adsorbents.