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To combat water scarcity, producing water from unconventional sources is strategically important. One promising approach is atmospheric water harvesting, a technology proposed in recent years to extract water directly from the air using an emerging class of porous materials—metal–organic frameworks (MOFs).

MOFs are highly tunable, crystal-like structures with large surface areas, known for their ability to trap and hold molecules, including water vapor.

As would be anticipated, the performance of such harvesting processes depends heavily on the adsorption characteristics of water in MOFs.

To date, a variety of water adsorption behaviors in MOFs have been observed and reported in the literature. To maximize harvesting efficiency, researchers seek MOFs that exhibit S-shaped adsorption isotherms, which feature a sharp increase in water uptake with a small change in adsorption conditions.

While this principle has been known for years, a comprehensive understanding of the diverse water adsorption behaviors in MOFs has remained limited.

In a study in the Journal of the American Chemical Society, Prof. Li-Chiang Lin's group at National Taiwan University (NTU) conducted a detailed computational investigation on over 200 strategically selected MOFs to better understand how water adsorption occurs in these materials.

They employed advanced flat-histogram Monte Carlo simulations, combined with analyses of thermodynamic stability, macrostate probabilities, free energy profiles, and hydrogen bond networks.

The study revealed a surprisingly diverse range of adsorption behaviors, including both gradual and sudden uptake patterns—referred to as non-S-shaped and S-shaped isotherms, respectively.

Notably, even among MOFs with the desired S-shaped isotherms, the researchers identified distinct phase behaviors, showing that different MOFs of S-shaped behaviors can follow fundamentally different mechanisms for water adsorption. They also demonstrated a clear link between these phase behaviors and the critical temperature of confined water within the MOFs.

Furthermore, the Lin group was able to correlate these behaviors with specific chemical and structural features of the MOFs. For instance, materials with moderate heat of adsorption were more likely to exhibit the desired sharp water uptake. Additionally, pore size and adsorption site distribution were found to control both the sharpness and the position of the uptake step.

Notably, for the latter—specifically the position of the uptake step (i.e., step pressure)—the Lin group developed a novel descriptor called the connectivity index. This index incorporates not only the density but, more importantly, the connectivity and spatial homogeneity of adsorption sites, and it was shown to strongly influence the step pressure.

"This study provides a comprehensive overview of diverse adsorption behaviors in MOFs," says Prof. Li-Chiang Lin, corresponding author of the study.

"The insights obtained pave the way for the rational design of MOFs tailored for atmospheric water harvesting, potentially enabling scalable, energy-efficient solutions to help meet the world's growing water needs."