Multi-Stage Condensation Pathway Minimizes Hysteresis in Water Harvesting with Large-Pore Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have emerged as promising materials for atmospheric water harvesting (AWH). Large-pore MOFs provide high water capacity, but their significant hysteresis between sorption and desorption makes them unsuitable for AWH. Co2Cl2(BTDD) is a noteworthy exception. This MOF has large, 2.2 nm diameter one-dimensional pores, and combines both record-high water capacity and minimal hysteresis, making it an excellent material for water capture in arid areas. Sorption reversibility in Co2Cl2(BTDD) has been attributed to continuous water uptake. However, the sharp adsorption/desorption in the isotherms supports a discontinuous first-order transition. Here we use molecular simulations to compute the water adsorption and desorption pathways and isotherms in Co2Cl2(BTDD), to elucidate how does this MOF achieve reversibility despite its large pore size. The simulations reveal a multi-stage mecha-nism of discontinuous water uptake facilitated by spatial segregation of rows of hydrophilic metal sites bridged by ~1 nm hydrophobic ligands. The multi-stage mechanism breaks the barrier of capillary condensation into smaller, easier to surmount ones, resulting in a facile process despite the sharp density discontinuity between confined liquid and vapor. We find that water-metal attraction in Co2Cl2(BTDD) is optimal for AWH: stronger ones reduce the water capacity, and weaker attraction results in sharp increase of the hysteresis. A putative chemically homogeneous porous material with same structure and equilibrium pressure of Co2Cl2(BTDD) condenses water through a single-step path-way with extreme hysteresis, demonstrating that thermodynamics and kinetics of water uptake are decoupled. These findings can guide the design of water harvesting materials with maximal capacity and reversibility.