CORVALLIS, Ore. – Oregon State University scientists have found a way to more than double the uptake ability of a chemical structure that can be used for scrubbing carbon dioxide from factory flues.
The study involving metal-organic frameworks, or MOFs, is important because industrial activities, among them burning fossil fuels for energy, account for a significant percentage of the greenhouse gas in the Earth's atmosphere. In the United States, 16% of total carbon dioxide emissions are from industry, according to the Environmental Protection Agency .
OSU researchers led by Kyriakos Stylianou of the College of Science worked with a copper-based MOF and found that its effectiveness at adsorbing carbon dioxide more than doubled when first exposed to ammonia gas.
"The capture of CO2 is critical for meeting net-zero emission targets," said Stylianou, associate professor of chemistry. "MOFs have shown a lot of promise because of their porosity and their structural versatility."
MOFs are crystalline materials made up of positively charged metal ions surrounded by organic "linker" molecules known as ligands. The metal ions make nodes that bind the linkers' arms to form a repeating structure that looks something like a cage; the structure has nanosized pores that adsorb gases, similar to a sponge.
MOFs can be designed with a variety of components, which determine the MOF's properties, and there are millions of possible MOFs, Stylianou said. More than 100,000 of them have been synthesized by chemistry researchers, and the properties of hundreds of thousands of others have been predicted.
In addition to the capture of carbon dioxide and other types of gases, MOFs can be used as catalysts and for energy storage, drug delivery and water purification.
When exposed to ammonia gas, the MOF in this study, mCBMOF-1, showed a carbon uptake capacity comparable to or greater than that of the traditional amine-based sorbents that are widely used for carbon dioxide capture in industrial applications. And compared to amine-based sorbents, MOFs are more stable and can be regenerated using less energy – in this case, by immersion in water.
"The MOF is activated by removing water molecules to expose four closely positioned open copper sites," Stylianou said. "Then we introduce the ammonia gas, which causes one of the sites to be occupied by an ammonia molecule. The remaining sites attract CO2, promoting interaction with ammonia to form carbamate species."
The carbamates – compounds with a range of uses in industry, agriculture and medicine – are released during the water immersion that regenerates the MOF's pristine structure, making it reusable for ongoing carbon capture.
The findings emphasize that MOF structures can be tailored with functional groups to enhance their interactions with specific target molecules, such as carbon dioxide, Stylianou said; similar strategies could be applied to other MOFs and gases.
"Our study's use of sequential pore functionalization to enhance CO2 uptake without significantly increasing regeneration energy is a terrific development," he said. "The formation of a copper-carbamic acid complex within the pores suggests strong and selective interactions with CO2, which is crucial for ensuring that CO2 is preferentially adsorbed over other gases in flue emissions."
The study, published in JACS Au, was supported by Saudi Aramco and Baydin Inc.
Collaborators included Oregon State University graduate student Ankit Yadav and postdoctoral scholar Andrzej Gladysiak as well as scientists from the University of California, Berkeley, Nanjing Normal University and the Institute of Materials Science of Barcelona.
