First synthesised in the 1990s, metal-organic frameworks (MOFs) are porous crystalline materials consisting of metal ions connected by organic linker molecules (ligands). Their repeating, cage-like structure gives them a high surface-area-to-volume ratio and makes them useful for a variety of applications, including gas storage or separation and catalysis. Over 90,000 different MOFs have been synthesised, and their properties can be tailored by modifying the metal ions and organic ligands used in their construction. However, little is known about their mechanical properties, such as how they react under high pressure. In work recently published in Angewandte Chemie, researchers from Technische Universität Dortmund, the Technical University of Munich and the University of Edinburgh investigated the high-pressure behaviour of flexible zeolitic imidazolate frameworks (ZIFs) of the ZIF-62 family. Their results show that it is possible to tune the mechanical behaviour of the MOF, without altering its crystal structure, by changing the linking molecules used. Their work opens up new potential applications for MOFs, such as pressure-switchable devices, membranes, and actuators.
Materials that dramatically change their physical properties in response to their environment offer huge potential in novel, energy-efficient applications. For example, flexible metal-organic frameworks (MOFs) can be used for sensing, gas and energy storage and molecular separation.
As porous materials, MOFs can adsorb and desorb guest molecules. Flexible MOFs typically exhibit two distinct states in terms of porosity and density, transitioning between the two in response to environmental stimuli such as gas pressure or temperature. Changing the metal ions and/or the organic linker molecules in a MOF adjusts its responsive properties, allowing fine-tuning of the material for different applications.
A deeper understanding of how flexible MOFs respond to changes in pressure would aid in shaping and pelletising MOFs, and would also open up new potential applications, such as shock absorbers or dampers.
In this work, the researchers demonstrated that it is possible to fine-tune the mechanical response of a MOF to high pressure by using mixed-linker solid solutions.
Prof Sebastian Henke from Technische Universität Dortmund is the senior author on the paper. He said,
We have a materials platform that has a certain structure type, and we can exchange some of the organic building units while maintaining the overall structure of the material. And we wanted to understand how this change in composition, while maintaining the same structure, changes the mechanical properties. And so we came to Diamond to do powder X-ray diffraction experiments (PXRD) on the I15 beamline, which has a dedicated hydraulic pressure cell that was very good for these experiments. We studied the material's structure as a function of pressure, up to 4000 bar.
The research team created eight different solid solutions in the ZIF-62 family and used I15 to investigate their structural behaviour at increasing pressure. Starting at ambient pressure, they examined each material in steps of 100 bar through to 2000 bar, and then in 250 bar steps to 4000 bar. The I15 setup allows the collection of high-quality PXRD patterns enabling atomistic structure refinement.
Dr Henke explains,
We used a solid solution, mixing different organic molecules in the same material. This was statistically mixed, and we learned how this influences the mechanical properties of these porous materials. In a porous material, there's empty space available. So we placed larger molecules in there to try to reinforce the framework. And the main question was whether that was possible and did it reinforce the framework. And the answer is yes.
The expected behaviour for these porous materials at ambient pressure is that they will rapidly compress as the pressure rises, closing the pores. The researchers showed that, by changing the composition of the material, they could precisely tune the pressure at which that happens.
However, they also made an unexpected discovery. The original, unmodified material flips between the two states. Below a critical pressure, it is in an open state, and above the critical pressure, it is closed. But as they increased the concentration of the modified linker molecules, the researchers found that this behaviour changed so that the transition happened continuously with pressure. That means it is possible to tune the pore size of the material using pressure, which opens up a lot of potential applications.
Dr Henke concludes;
We are looking at transferring this concept to other MOFs where this synthetic strategy should also apply. The materials we investigated in these experiments have a relatively low porosity, and if we can establish the concept in higher porosity MOFs, they would have more potential applications. And we're working on moving from this fundamental finding in a high-pressure cell to translate this into a membrane, where we implement the material in a polymer and see how the mechanical treatment of the polymer membrane changes the performance of the membrane, based on structural changes of the embedded MOF.
To find out more about the I15 beamline or discuss potential applications, please contact Principal Beamline Scientist Annette Kleppe: annette.kleppe@diamond.ac.uk
Song J et al. Tuning the HighâPressure Phase Behaviour of Highly Compressible Zeolitic Imidazolate Frameworks: From Discontinuous to Continuous Pore Closure by Linker Substitution. Angewandte Chemie International Edition (2022): e202117565. DOI:10.1002/anie.202117565.
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