While metal-organic frameworks (MOFs) are valued for their inexpensive nature and ability to pull gases out of air or other mixed gas streams, they have never been able to do so with oxygen, until now. A team of scientists from the Department of Energy's Pacific Northwest National Laboratory (PNNL) has solved this problem by creating a composite of an MOF and a helper molecule, allowing the two to work together in order to separate oxygen from other gases in a simple, cost-effective manner.
The findings, which could help in numerous applications such as oxygen for fuel cells and oxygen sensors, is being touted as an alternative to the current industry process called cryogenic distillation. Although this process can effectively separate oxygen from other gases, it is costly and requires a great deal of energy.
Prior to the development of the new method, less than a handful of MOFs could absorb molecular oxygen, and even they reacted with oxygen chemically, leading to the formation of oxides and rendering the material useless.
"When we first worked with MOFs for oxygen separation, we could only use the MOFs a few times," said Praveen Thallapally, a PNNL materials scientist who worked on the project. "We thought maybe there's a better way to do it."
The team used a second helper molecule, ferrocene, in combination with the MOF MIL-101 to mediate oxygen separation. While this molecule is attracted to the MOF, it does not react with it chemically, instead reacting with oxygen, thus stimulating its separation from the other gases.
After choosing these materials, the team created a composite of MIL-101 and ferrocene by mixing them, heating them up and sending the gases through a black composite material. The results showed that the material captured a large percentage of oxygen and none of the added nitrogen, argon or carbon dioxide. Furthermore, the team discovered that heating up ferrocene caused it to decompose into the pores of the material, allowing iron to react with oxygen.
The results show promise for using MOFs to purify oxygen, and future research should explore the other potential combinations of MOFs and helper molecules.
The findings were published in the March 8 issue of Advanced Materials.
