by Laura Buenning
Carbon dioxide ranks high on the list of problematic ingredients in flue gas emissions. Wet scrubbers that combine water and organic compounds derived from ammonia (amines) deliver the most effective method available to date.
But, you pay a high energy penalty to regenerate the amine solution for reuse, says JDRD team leader David Jenkins, not to mention amines’ volatile, corrosive nature and long-term instability, which, when controlled by inhibitors, sap even more energy from the plant.
Jenkins, graduate students Brianna Hughes and Christopher Murdock, and LDRD team leader Radu Custelcean and Shun Wan of ORNL’s Chemical Sciences Division are pursuing a promising alternative approach using scrubbers filled with porous, crystalline powders to attract and hold the CO2 until it’s released by applying a reverse pressure swing process. The materials involved are called Covalent Organic Frameworks, or COFs for short.
Akin to Metal Organic Frameworks (MOFs) discovered twenty years ago, COFs hit the new materials scene just ten years ago. Both materials have three-dimensional, porous nanostructures built from repeating nodes held together by linking units. With carefully planned construction, each has the potential to trap molecules, such as carbon dioxide or hydrogen, and hold them in the pores for later release. MOFs have metal-ion nodes and organic linkers; COFs are assembled solely from organic building blocks and address a problem MOFs encounter in CO2 capture from flue gas.
“Most MOFs react with water, particularly the acidic water found in flue gas,” Jenkins says, “The metal-ligand bonds are weaker, making it harder for them to hold up in high heat [and] humid and acidic conditions.”
COFs have robust covalent bonds held together by shared electrons, so they are less likely to fall apart under similar conditions. On the down side, Jenkins says, COFs with a special selective affinity for CO2 have as yet to be created.
This is the task Jenkins and Custelcean have laid out for their teams. Custelcean will synthesize frameworks and linkers designed to attract carbon dioxide based on their electronic structure. Jenkins’ linkers feature nitrogen-based compounds called tetrazoles (five-member rings with four nitrogen atoms) and triazoles (similar but with three nitrogen atoms). Recent research with porous polymer films suggests tetrazole and triazole compounds improve CO2 selectivity and uptake.
But, synthesizing COFs is not for the faint of heart. “We do combinatorial chemistry,” Jenkins says.
“It’s very much a black-box approach, in that you set up hundreds of reactions on a very small scale, making tiny changes to each, and then observe what you get. If something looks promising, we do it again on a larger scale, testing and scaling-up until we have enough material to do a full gamut of tests.
“If we can functionalize these really small pores that have no metal in them—that are much more stable once they are synthesized—and actually show they interact and bind the CO2 more strongly than with other gases, I would consider that a huge success.”
Triazole and Tetrazole Linkers for Covalent Organic Frameworks for Carbon Dioxide Capture
David Jenkins, UT Department of Chemistry
Novel Covalent Organic Frameworks with Tailored Carbon Capture Functionality
Radu Custelcean, ORNL Chemical Sciences Division