The Role of Metal–Organic Frameworks in Adsorption-Based Atmospheric Water Harvesting

Original Authors: Ahmed A. Hassan, Mohammed Ezzeddine, Mohamed G.M.Kordy, Mohamed M.Awad

Original paper is accessible at: https://doi.org/10.1016/B978-0-323-95486-0.00030-2

The scarcity of freshwater has become a critical global challenge, intensified by climate change and rapid industrialization. Traditional solutions such as seawater desalination remain energy-intensive and environmentally costly due to their dependence on fossil fuels. Consequently, Atmospheric Water Harvesting (AWH) has emerged as a promising and sustainable method for producing potable water directly from ambient air. Among the various AWH technologies—fog collection, dew condensation, and adsorption-based harvesting—the latter stands out for its adaptability to arid climates and potential for solar-driven regeneration.

Metal–Organic Frameworks (MOFs) as Advanced Adsorbents

Metal–Organic Frameworks (MOFs) represent a breakthrough in materials science due to their exceptional porosity, tunable chemistry, and large surface areas. These hybrid crystalline materials consist of metal ions coordinated with organic linkers, creating an ordered network of pores that can selectively adsorb water molecules even at low humidity. Compared to conventional desiccants like silica gel or zeolites, MOFs exhibit faster adsorption–desorption kinetics, lower regeneration temperatures, and high structural tunability.

Mechanisms of Water Adsorption in MOFs

Water adsorption in MOFs occurs through three fundamental mechanisms:

1. Chemisorption on Open Metal Sites – Unsaturated metal centers act as strong adsorption sites where water molecules form coordination bonds. For instance, Mg-MOF-74 demonstrates high initial uptake due to strong binding at these sites, though regeneration requires significant thermal energy.

2. Micropore Filling – Polar centers within the pore framework attract water molecules, leading to hydrogen-bonded clusters and stepwise pore filling. MOF-303 and MOF-801 are prime examples exhibiting controlled sorption in this mechanism.

3. Capillary Condensation – In mesoporous MOFs with larger channels, water condenses within the pores at higher humidity levels, as demonstrated by Cr-soc-MOF-1, which achieves a record adsorption capacity of 1.95 g/g at 70% RH.

Properties and Performance in AWH

The unique attributes of MOFs—large pore volume, high thermal stability, and adjustable hydrophilicity—make them ideal candidates for atmospheric water capture, especially in desert climates. Certain MOFs can effectively adsorb water at relative humidity levels as low as 20–30%, while desorbing under sunlight, thus enabling passive solar-driven AWH. MOF-801-based systems, for example, have achieved water yields of 2.8 L/kg/day under natural sunlight without external power input.

Challenges and Strategies for Commercialization

Despite their promise, several challenges hinder large-scale adoption of MOFs in AWH:

• High production costs due to expensive ligands and solvents.

• Hydrothermal instability, leading to degradation under prolonged exposure to moisture.

• Form-factor limitations, since most MOFs are synthesized as fine powders, complicating system integration.

To address these, researchers are developing green synthesis methods using water as a solvent, mechanochemical routes to minimize energy input, and granulation and monolithic fabrication techniques to transform MOFs into stable, reusable adsorbent structures. Hybrid systems combining MOFs with polymers or photothermal layers have also demonstrated enhanced performance and scalability.

Outlook and Environmental Impact

MOF-based AWH technologies represent a paradigm shift toward decentralized, renewable water production. Integrating these systems with solar energy can significantly reduce operational costs and carbon footprint, making them ideal for off-grid communities and arid regions. Continued progress in MOF design, low-cost synthesis, and adsorption modeling will accelerate their transition from laboratory research to real-world applications.

Conclusion

Metal–Organic Frameworks stand at the frontier of atmospheric water harvesting innovation. By bridging materials chemistry, thermodynamics, and renewable energy engineering, MOFs enable efficient capture and release of water vapor from the atmosphere. Overcoming current scalability and cost barriers will pave the way for sustainable, solar-driven water generation systems—critical to addressing the intertwined challenges of water scarcity and climate resilience.