by Laura Buenning
US Department of Energy 2010 statistics show fossil fuel consumption accounted for 5,610 million metric tons of U.S. carbon dioxide emissions—one-sixth of the world’s total (31,780 million metric tons). That same year, the Environmental Protection Agency reported that 55 percent of all greenhouse gas emissions in the U.S. came from the electric power and other industries—much of it released into the atmosphere in flue gas.
Capturing the carbon dioxide in flue gas is particularly challenging because it is released at high temperatures (~70° Celcius = 158° Fahrenheit) and low pressure, says JDRD team leader and inorganic chemist David Jenkins. And at only 10 to 15 percent concentrations, the CO2 must be extracted from the other flue gasses, including acidic water vapor.
Jenkins, and graduate students, Brianna Hughes and Christopher Murdock, along with ORNL’s Radu Custelcean and Shun Wan want to find an alternative to the current removal technology that wet scrubs flue gas with ammonia compounds (amines). Their idea is to strip carbon dioxide from the gas by forcing it through dry, porous microcrystals designed to attract and hold CO2 until it can be released where it won’t enter the atmosphere.
“You pay a high energy penalty to regenerate and reuse amine scrubbing solutions,” Jenkins says, “not to mention their volatile, corrosive nature and long-term instability, which, when controlled by inhibitors, sap even more energy from the plant.”
The two teams’ microcrystal designs are called Covalent Organic Frameworks (COFs). While, robust bonds hold these nanostructures together under the harshest conditions, no COFs have as yet been created with a special affinity for CO2. This is the target Custelcean and Jenkins have set for themselves.
Assembled from building blocks of repeating nodes held together by linking units, carefully planned construction could make COFs capable of trapping molecules, such as carbon dioxide, in their pores. When flue gas is forced through the microcrystals under pressure, the CO2 would stick to inner-pore compounds designed to attract these molecules more strongly than other elements in the gas—which push on through. Releasing the pressure frees the CO2 from the pores.
In year one Jenkins’ team synthesized first-generation tetrazole linking units; Custelcean’s LDRD team is using these to assemble COFs designed to attract carbon dioxide based on their electronic structure. Research on porous polymer films suggests nitrogen-based tetrazoles (five-member rings with four nitrogen atoms) and triazoles (similar but with three nitrogen atoms) improve the film’s ability to capture carbon dioxide. Custelcean’s team branched out from their originally proposed three-dimensional design to two-dimensional COFs that layer like sheets of graphite. The new configuration allows for shorter linking units, which simplifies preparation.
Jenkins’ year two research continues preparation of linking units for both designs. His team will share these with Custelcean’s team and use them to synthesize 2- and 3-D COFs with nodes prepared by the LDRD team.
Triazole and tetrazole linkers for covalent organic frameworks for carbon dioxide capture
David Jenkins, UT Chemistry Department
Novel covalent organic frameworks with tailored carbon capture functionality
Radu Custelcean, ORNL Chemical Sciences Division