Conceptual model shows how porphyrin COFs embedded in a cathode could be used to split carbon dioxide (CO2) into carbon monoxide (CO) and oxygen for making renewable fuels and other valuable chemical products. Credit: Courtesy of Omar Yaghi, Berkeley Lab/UC Berkeley
Porphyrin CO2 catalysts have been incorporated into the sponge-like crystals of covalent organic frameworks (COFs) to create a molecular system that not only absorbs carbon dioxide, but also selectively reduces it to CO, a primary building block for a wide range of chemical products including fuels, pharmaceuticals and plastics.
With the reduction of atmospheric CO2 emissions in mind, Yaghi and his MIU group designed and developed the first COFs as a means of separating CO2 from flue gases. A COF is a porous 3D crystal consisting of a tightly folded, compact framework that features an extraordinarily large internal surface area – a COF the size of a sugar cube were it to be opened and unfolded would blanket a football field, which enables the system to absorb and store enormous quantities of targeted molecules, such as carbon dioxide.
Now, through “reticular chemistry,” which enables molecular systems to be “stitched” into netlike structures that are held together by strong chemical bonds, the Berkeley Lab researchers were able to embed the molecular backbone of COFs with a porphyrin catalyst, a ring-shaped organic molecule with a cobalt atom at its core. Porphyrins are electrical conductors that are especially proficient at transporting electrons to carbon dioxide.
Structural model showing a covalent organic framework (COF) embedded with a cobalt porphyrin.
“A key feature of COFs is the ability to modify chemically active sites at will with molecular-level control by tuning the building blocks constituting a COF’s framework,” Yaghi says. “Because the porphyrin COFs are stable in water, they can operate in aqueous electrolyte with high selectivity over competing water reduction reactions, an essential requirement for working with flue gas emissions.”
COFs displayed exceptionally high catalytic activity – a turnover number up to 290,000 = 1 porphyrin COF can reduce 290,000 molecules of CO2 to CO/second, a 60X increase over the catalytic activity of molecular cobalt porphyrin catalyst and places porphyrin COFs among the fastest and most efficient catalysts of all known CO2 reduction agents. “We’re now seeking to increase the number of electroactive cobalt centers and achieve lower over-potentials while maintaining high activity and selectivity for carbon dioxide reduction over proton reduction,” Chang says. “In addition we are working towards expanding the types of value-added carbon products that can be made using COFs and related frameworks.” http://newscenter.lbl.gov/2015/08/27/soaking-up-carbon-dioxide-and-turning-it-into-valuable-products
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