Metal-organic frameworks as regeneration optimized sorbents for atmospheric water harvesting

As the freshwater crisis looms, metal-organic frameworks (MOFs) with stepped isotherms lie at the forefront of desiccant development for atmospheric water harvesting (AWH). Despite numerous studies on water sorption kinetics in MOF desiccants, the kinetics of AWH sorbents are a challenge to quantify. Here, we report that the AWH kinetics of seven known MOFs and the industry-standard desiccant Syloid are limited by diffusion to the sorbent bed surface. A quantitative model that exploits isotherm shape enables simulation of sorption cycling to evaluate sorbent performance through productivity contour plots ("heatmaps"). These heatmaps reveal two key findings: steady-state oscillation around partial loading optimizes productivity, and dense ultramicroporous MOFs with a step at low relative humidity afford superior volumetric performance under practically relevant temperature swing conditions (27°C, 30% relative humidity [RH] − 60°C, 5.4% RH). Cellulose-desiccant composites of two such regeneration optimized sorbents retain the kinetics of powders, producing up to 7.3 L/kg/day of water under these conditions.