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

Water sorption studies with mesoporous multivariate monoliths based on UiO-66

Hierarchical linker thermolysis has been used to enhance the porosity of monolithic UiO-66-based metal-organic frameworks (MOFs) containing 30 wt% 2-aminoterephthalic acid (BDC-NH2) linker. In this multivariate (i.e. mixed-linker) MOF, the thermolabile BDC-NH2 linker decomposed at ∼350 °C, inducing mesopore formation. The nitrogen sorption of these monolithic MOFs was probed, and an increase in gas uptake of more than 200 cm3 g−1 was observed after activation by heating, together with an increase in pore volume and mean pore width, indicating the creation of mesopores. Water sorption studies were conducted on these monoliths to explore their performance in that context. Before heating, monoUiO-66-NH2-30%-B showed maximum water vapour uptake of 61.0 wt%, which exceeded that reported for either parent monolith, while the highly mesoporous monolith (monoUiO-66-NH2-30%-A) had a lower maximum water vapour uptake of 36.2 wt%. This work extends the idea of hierarchical linker thermolysis, which has been applied to powder MOFs, to monolithic MOFs for the first time and supports the theory that it can enhance pore sizes in these materials. It also demonstrates the importance of hydrophilic functional groups (in this case, NH2) for improving water uptake in materials.</p

Rapid determination of experimental sorption isotherms from non-equilibrium sorption kinetic data

Herein, we report a new method for rapid determination of experimental sorption isotherms that exploits gravimetric non-equilibrium sorption kinetics data in a thin sorbent bed. In comparison with equilibrium-based isotherm determination methods, this sorption kinetics isotherm determination (SKID) method needs only two equilibrium points, P/P0min and P/P0max. SKID requires up to an order of magnitude less data collection time than conventional methods and is suitable for high-throughput sorbent discovery and evaluation. SKID was validated by testing a library comprising 30 sorbents, including rigid and flexible metal-organic frameworks (MOFs), inorganics (zeolites), and organics (microcrystalline cellulose). Average data collection time for water vapor was 3.1 vs. 23 h for dynamic vapor sorption experiments. SKID was also demonstrated for other vapors (C8 aromatics) and a gas (CO2), making it a promising tool for rapid screening of new or existing sorbents for applications such as water harvesting and dehumidification and carbon capture.</p