Regeneration-Optimized Chemisorbents: Evaluating Hybrid Coordination Networks for Atmospheric Water Harvesting

Atmospheric water harvesting (AWH) can be enabled by solid desiccants that exhibit S-shaped adsorption isotherms with one or more steps at low relative humidity (RH, <30%) and mild regeneration temperatures (<60°C). There remains a dearth of such regeneration-optimized sorbents (ROSs), which thus far are limited exclusively to physisorbents mainly because chemisorbents typically require elevated temperatures for water recovery. Herein, the first comprehensive water sorption study of the SIFSIX-3-Ni family of hybrid coordination networks (HCNs) and a new, related family of HCNs, reveals a shared cooperative chemisorption mechanism with markedly different RH thresholds and regeneration requirements. Whereas the SIFSIX-3-Ni family operates outside the RH and regeneration limits of ROSs, four M'FSIX-25-M analogues (M = Co²⁺, Ni²⁺; M' = Si⁴⁺, Ti⁴⁺, 25 = 1,4-bis(imidazol-1-yl)naphthalene) exhibit both low-RH chemisorption and regeneration from 34-44°C. The lead regeneration optimized chemisorbent, ROC-1 (TIFSIX-25-Ni), has a projected water-harvesting productivity of ca. 3.5  ${\mathrm{kg}}_{H_{2}O}$ L_sorbent ^-1 d^-1. Density functional theory calculations provide insight into the energetics and mechanism of hydration/dehydration in these first-generation ROCs. This work establishes ROCs as a new class of desiccants that offers reversible chemisorption within the low-RH and mild-regeneration conditions relevant to AWH in arid environments.