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.</p

Flexible coordination network exhibiting water vapor−induced reversible switching between closed and open phases

That physisorbents can reduce the energy footprint of water vapor capture and release has attracted interest because of potential applications such as moisture harvesting, dehumidification, and heat pumps. In this context, sorbents exhibiting an S-shaped single-step water sorption isotherm are desirable, most of which are structurally rigid sorbents that undergo pore-filling at low relative humidity (RH), ideally below 30% RH. Here, we report that a new flexible one-dimensional (1D) coordination network, [Cu(HQS)(TMBP)] (H2HQS = 8-hydroxyquinoline-5-sulfonic acid and TMBP = 4,4′-trimethylenedipyridine), exhibits at least five phases: two as-synthesized open phases, α ⊃ H2O and β ⊃ MeOH; an activated closed phase (γ); CO2 (δ ⊃ CO2) and C2H2 (ϵ ⊃ C2H2) loaded phases. The γ phase underwent a reversible structural transformation to α ⊃ H2O with a stepped sorption profile (Type F-IV) when exposed to water vapor at 100 cycles and only mild heating (<323 K) is required for regeneration. Unexpectedly, the kinetics of loading and unloading of [Cu(HQS)(TMBP)] compares favorably with well-studied rigid water sorbents such as Al-fumarate, MOF-303, and CAU-10-H. Furthermore, a polymer composite of [Cu(HQS)(TMBP)] was prepared and its water sorption retained its stepped profile and uptake capacity over multiple cycles.</p