Gaining insights into water-harvesting metal-organic frameworks

The 2025 Nobel Prize for Chemistry went to the fathers of metal–organic frameworks, or MOFs, structures that contain large cavities in which molecules can flow in and out.

These are a new class of crystalline materials that could help tackle global water scarcity by pulling moisture directly from the air. Their secret lies in their extraordinary porosity and tunable chemistry, which allow scientists to design frameworks that selectively attract and trap water molecules. But to make these structures more efficient, researchers need to understand exactly how water molecules cluster and fill the pores inside the material—a process that remains surprisingly difficult to capture and observe at the molecular level.

The MIL-100 (Fe), a prototypical MOF with giant pores, has demonstrated strong potential for atmospheric water harvesting, showing promising adsorption capacity and cycling stability in several studies. Due to its large pore size and hydrophilic surface, it has been suggested as a cost-effective alternative to zirconium-based MOFs in water harvesting.

Now, a team led by the Sapienza University, in collaboration with the Swiss-Norwegian beamline at the ESRF, has analysed the framework at the structural level. Paola D’Angelo, researcher from Sapienza University and corresponding author of the publication, explains the relevance of the experiments: “This represents one of the first operando experiment conducted at a synchrotron facility on water harvesting by MOFs. The aim was to achieve a molecular-level understanding of the water adsorption mechanism, with the ultimate goal of guiding the design of more efficient MOFs for atmospheric water harvesting.”

In order to gain insights into MIL-100 (Fe), the team used a new approach to get a comprehensive view of the structure and dynamics of the water adsorbed in the framework. “It is one of the first times that we’ve combined three techniques: X-ray absorption spectroscopy (XAS), X-ray pair distribution function and powder X-ray diffraction at the beamline. The aim was to collect and extract information on all relevant length and time scales, no single technique can do that”, explains Wouter van Beek, scientist in charge of BM31.

The SNBL team has been fine tuning and developing the station for years such that these extremely complex experiments have become routine and can be performed in a fully automated fashion. “It is particularly satisfying for us to see such nice results being produced”, adds van Beek.

The results were compared with theoretical analysis, from molecular dynamics (MD) simulations to in-depth theoretical XAS calculations.

Water molecules as anchors

The complementary synchrotron X-ray techniques shed light on the behaviour of water confined in MIL-100(Fe) with unprecedented structural sensitivity at the short-, intermediate- and long-range length scales, while the MD and theoretical XAS simulations revealed the order according to which water molecules populate the MOF mesopores and tracked the evolution of the hydrogen-bond network topology as a function of water content.

The results showed that water molecules that are bound to iron atoms act as anchors, attracting more water from the air. Small pores fill first, followed by larger ones, as the molecules cluster together. With increasing water content, a dense hydrogen-bond network forms, making the trapped water less mobile and more structured inside the pores.

“This work shows an innovative approach to study porous materials for water harvesting, which can provide information that can lead to improvement in engineering of MOFs, leading to potential applications in the future”, says D’Angelo.

In the future, the researchers aim to develop and characterise photoresponsive MOFs capable of capturing water from the air and releasing it upon solar light irradiation, without the need for external heating. This approach would overcome the main limitations of conventional materials, such as the high extraction and regeneration temperatures that currently restrict their practical applicability.

Reference:

Francesco Tavani, et al., Journal of the American Chemical Society Article ASAP, DOI: 10.1021/jacs.5c12269

Text by Montserrat Capellas Espuny

Top image: Snapshots of MIL-100(Fe) loaded with N = 2, 20 and 40 water molecules per trimeric unit. The iron, carbon, oxygen and hydrogen atoms of the MOF are show in orange, gray, red, and white, respectively. The oxygen and hydrogen atoms of the water molecules are displayed in blue and white, respectively.