Squeezing iron-containing ferrocene (not to scale) in the pores of the MOF known as MIL-101 allows the resultant composite to snag oxygen from passing air. Image: PNNL.
Squeezing iron-containing ferrocene (not to scale) in the pores of the MOF known as MIL-101 allows the resultant composite to snag oxygen from passing air. Image: PNNL.

Inexpensive materials called metal-organic frameworks (MOFs) can pull gases out of air or other mixed gas streams, but fail to do so with oxygen. Now, a team of researchers has overcome this limitation by creating a composite of a MOF and a helper molecule in which the two work together to separate oxygen from other gases simply and cheaply.

This work, reported in Advanced Materials, might prove of use in a wide variety of applications, including making pure oxygen for fuel cells and then using that oxygen in fuel cells, removing oxygen from food packaging, making oxygen sensors, and for many other industrial processes. The technique could also be used with gases other than oxygen by simply switching the helper molecule.

Currently, industry uses a common process called cryogenic distillation to separate oxygen from other gases, but it is costly and uses a lot of energy to chill the gases. Also, it can't be used for specialty applications like sensors or getting the last bit of oxygen out of food packaging.

A great oxygen separator would be easy to prepare and use, inexpensive and reusable. MOFs are materials containing lots of pores that can suck up gases like sponges suck up water. They have potential for use in nuclear fuel separation and in lightweight dehumidifiers.

But of the thousands of MOFs produced to date, less than a handful can absorb molecular oxygen. And those MOFs chemically react with the oxygen, forming oxides that render the material unusable.

"When we first worked with MOFs for oxygen separation, we could only use the MOFs a few times. We thought maybe there's a better way to do it," said materials scientist Praveen Thallapally at the US Department of Energy's Pacific Northwest National Laboratory (PNNL).

The new approach that Thallapally and his colleagues at PNNL came up with involves using a second molecule to mediate the oxygen separation. This helper molecule should be attracted to, but chemically uninterested in, the MOF. Instead, the helper should react with oxygen to separate it from the other gases.

The researchers chose a MOF called MIL-101 that is known for its high surface area – making it a powerful sponge – and its lack of reactivity: one teaspoon of MIL-101 has the same surface area as a football field. The high surface area comes from a MOF's pores, where reactive MOFs work their magic.

MOFs that react with oxygen usually need to be handled carefully in the laboratory, but MIL-101 is stable at ambient temperatures and in the open atmosphere of a lab. For their helper molecule, Thallapally and his colleagues tried ferrocene, an inexpensive iron-containing molecule.

The researchers made a composite of MIL-101 and ferrocene by simply mixing them and heating them up. Initial tests showed that MIL-101 took up more than its weight in ferrocene and at the same time lost surface area. This indicated that ferrocene was taking up space within the MOF's pores, where they need to be to snag oxygen molecules.

Then the team tried sending gases through the black composite material. They found that the composite bound up a large percentage of oxygen, but almost none of the added nitrogen, argon or carbon dioxide. The material behaved this way whether the gases went through individually or as a mix, showing that the composite could separate oxygen from the other gases.

Additional analysis showed that heating caused the ferrocene to decompose in the MOF pores to nanometer-sized clusters, making the iron available to react with oxygen. This reaction formed a stable mineral known as maghemite, all within the MOF pores. Handily, maghemite could be removed from the MOF, allowing the composite to be used again.

Together, the results on the composite showed that a MOF might be able to do unexpected things – like purify oxygen – with a little help. Future research will explore other combinations of MOF and helper molecules. In addition to PNNL, other researchers taking part in this study hailed from Argonne National Laboratory and the University of Amsterdam in the Netherlands.

This story is adapted from material from PNNL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.