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.

Automated summary

As the freshwater crisis looms, metal-organic frameworks (MOFs) with stepped isotherms are at the forefront of desiccant development for atmospheric water harvesting (AWH). Diffusion to the sorbent bed surface limits the AWH kinetics of seven known MOFs and the industry-standard desiccant Syloid. A quantitative model exploiting isotherm shape reveals that steady-state oscillation optimizes productivity and dense ultramicroporous MOFs with a step at low relative humidity offer superior volumetric performance.