Study of metal-organic structures reveals mechanism for capturing water from air

Researchers from Helmholtz-Zentrum Dresden-Rossendorf and Dresden University of Technology have discovered the mechanism of water adsorption in certain microporous materials – called hierarchical metal-organic frameworks (MOFs) – while probing them on the atomic scale .

Discovered only about 25 years ago, their special properties quickly earned them a reputation as “miracle materials” that, it turned out, can even harvest water from the air. Researchers describe how the material achieves this ACS Applied Materials and Interfaces.

“These very special materials are very porous solids made of metals or metal-oxygen clusters that are linked in a modular way by pillars of organic chemicals. This 3D arrangement leads to networks of cavities reminiscent of the pores of a sponge “It is precisely these cavities that interest us,” says Dr. Ahmed Attallah from the HZDR Institute of Radiophysics.

These nanoscale pores provide the basis for a multitude of potential applications, from gas storage to separation technology, catalysis and new sensors, and water harvesting is the one of the most promising.

Probing the void

The team synthesized two MOFs based on the metals zirconium and hafnium, held in place by the same organic structure. Then, the scientists delved deeper into the characteristics of the materials obtained by applying various complementary techniques.

On the one hand, they determined the amount of nitrogen or water vapor that could be trapped in the pores of the material. On the other hand, they took a closer look at the exact mechanism of water adsorption in MOFs, which until now was not well understood.

“To shed light on the process, we used a non-destructive technique known as positron annihilation lifetime spectroscopy, or PALS for short, in which a positron will interact with electrons – its antiparticles – annihilating thus then releasing gamma rays that can be detected,” said Dr. Andreas Wagner, director of the ELBE Center for High Power Radiation Sources at the HZDR.

“The time between the emission of positrons from a radioactive source and the subsequent detection of gamma rays corresponds to the lifetime of the positrons. This in turn depends on how quickly they encounter electrons.”

If voids are present in the material, such as nanopores, the positrons and electrons tend to form so-called positronium atoms, with one electron and one positron each, orbiting their common center of mass, heading straight l towards each other until the particle pair is either dispersed or annihilated, whichever comes first.

Since these exotic atoms live longer in larger voids, they reveal information about the size and distribution of the void. The researchers found that water adsorption in MOFs was mainly governed by a step-filling mechanism, including the formation of liquid bridges in the pores. Water adsorption was influenced by the formation of water clusters on the pore surface, which created small air spaces in the pores.

Expel the desert air

“Due to the close chemical resemblance of the metals zirconium and hafnium, the resulting metal-organic structures have exactly the same pore sizes and high chemical stability, which allows us to evaluate the validity of our method at the same time,” Professor Stefan Kaskel, Chair of Inorganic Chemistry I at the Technical University of Dresden, explains. His group’s research focuses on the development of novel functional materials for various applications, such as energy storage and conversion, environmental catalysis, and water adsorption.

Based on the results, the researchers conclude that their study provides new insights into the water adsorption mechanism in hierarchical MOFs, which could help design better materials for water harvesting from air, which is particularly important in arid regions. By exposing MOFs to air, they can capture water molecules from the atmosphere. Then, by applying heat or reducing pressure, the water can be released and used.

Scientists are already thinking further: is the technology suitable for commercial solutions? As another field group reported, 1.3 liters of water per kilogram of MOF per day from desert air gives an idea of ​​the magnitude of yield currently achievable.

However, to achieve a comprehensive and sustainable solution, other factors must be considered beyond yield. “To increase water harvesting with MOFs, they should be accessible in large quantities at low cost. Additionally, traditional synthesis routes require large quantities of organic solvents or the acquisition of expensive basic components,” underline Kaskel and Attallah, highlighting the possible pitfalls of this enterprise.

To avoid them, recently developed so-called “green” synthesis procedures will gain momentum in the future, ensuring environmentally friendly production of MOFs.

The Dresden team is already adhering to this idea by following the principles of green chemistry, such as using water as a solvent, performing reactions at low temperatures to save energy and operating waste as sources of metals and organic linkers.