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 develop-ment 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 simula-tion of sorption cycling to evaluate sorbent performance through productivity contour plots ("heatmaps"). These heatmaps reveal two key findings: steady-state oscillation around partial loading op-timizes productivity, and dense ultramicroporous MOFs with a step at low relative humidity afford superior volumetric performance un-der practically relevant temperature swing conditions (27 degrees C, 30% relative humidity [RH] -60 degrees C, 5.4% RH). Cellulose-desiccant com-posites of two such regeneration optimized sorbents retain the ki-netics of powders, producing up to 7.3 L/kg/day of water under these conditions.