New research could lead to practical uses for metal-organic frameworks

By Jared SagoffSeptember 24, 2010

ARGONNE, Ill. – Scientists at U.S. Department of Energy's Argonne National laboratory are putting the pressure on metal-organic frameworks (MOF).

In MOF materials, organic molecules can connect metal ions to form scaffolding-like structures similar to a molecular Tinker toy. The struts that make up the structure do not fill space efficiently, in the way that Lego blocks might, leaving extra spaces in the structure that can contain guest molecules.

Acting like molecular-scale sponges, these MOFs have wide ranging potential uses for filtering, capturing or detecting molecules such as carbon dioxide or hydrogen storage for fuel cells.

"By examining the framework at various pressures," scientist Karena Chapman said, "we found that the MOF compresses rapidly at high pressures."

Since the MOF frameworks do not fill space efficiently, the structures are particularly sensitive to even relatively moderate applied pressures. For any carbon dioxide or hydrogen gas storage application, the MOF materials – which generally form as fine particles or small crystals – will need to be compressed into pellets to optimize their volume capacity. This compression would subject the structure to pressures up to several gigapascals (GPa).

While a few GPa of pressure would have minimal impact on denser oxide-based materials, the MOFs' structure may show significant and possibly irreversible distortions, altering their ability to store gas selectively.

Understanding how MOF materials behave under pressure is an important step in taking MOF technology beyond the lab.

Using a diamond anvil cell at the laboratory's Advanced Photon Source, Chapman, along with Argonne scientists Gregory Halder and Peter Chupas, synthesized a copper-benzenetricarboxylate MOF and subjected its framework to various pressures with and without pressure-transmitting fluids.

X-ray diffraction from Advanced Photon Source data showed a transition from the hard regime, where pressure transmitting fluid penetrates the framework cavities, to a soft regime, where the MOF compresses concertedly.

This uncharacteristic behavior is caused by smaller molecules in the pressure-transmitting fluid that can permeate the framework's cavities. This leads to a supersaturated state that counteracts the external pressure until a threshold pressure is reached, when the MOF rapidly compresses and cannot allow any additional guest molecules into the cavities.

"MOFs have wide and varied potential applications in the real world," Chapman said. "By exploring high-pressure phenomena, we come a step closer to realizing these advanced applications."

A paper on their work can be seen in a recent edition of the Journal of the American Chemical Society. Funding for this research was provided by the Office of Basic Energy Sciences (BES) in the U.S. Department of Energy's Office of Science. BES manages a multipurpose, scientific research effort to foster and support fundamental research to expand the scientific foundations for new and improved energy technologies and for understanding and mitigating the environmental impacts of energy use.