Although the concept of using CO₂ as a feedstock for renewable multi-carbon compounds and synthetic fuels is attractive, practical implementation has been hampered by the difficulty of forming carbon–carbon (C–C) bonds efficiently.

CO₂ reacts readily with carbon-centered nucleophiles—a chemical intermediate species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction. However, generating these required intermediates requires high-energy reagents (such as highly reducing metals or strong organic bases), carbon–heteroatom bonds or relatively acidic carbon–hydrogen (C–H) bonds. These requirements have negated the environmental benefit of using CO₂ as a substrate and have limited the chemistry to low-volume targets, the researchers noted.

The new Stanford approach was inspired by ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO)—an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants and other photosynthetic organisms to molecules such as glucose. Rubisco effects C–C bond formation in the Calvin cycle by deprotonating a C–H bond of ribulose-1,5- bisphosphate and exposing the resulting carbon-centered nucleophile to CO₂ to form a carboxylate (C–CO₂^–).

Emulating this strategy synthetically requires de-protonating un-activated C–H bonds using a simple base that does not have a large CO₂ footprint.

The Stanford team envisioned a CO₃^2– (carbonate)-promoted C–H carboxylation reaction, wherein CO₃^2– reversibly deprotonates a C–H bond to generate HCO₃^– and a carbon-centered nucleophile that reacts with CO₂ to form C–CO₂^–. HCO₃^– decomposition results in a net consumption of one-half equivalents of CO₃^2– and CO₂ per C–CO₂^– produced.

The cycle could be closed by protonating C–CO₂^– with strong acid and using electrodialysis to regenerate the acid and base, effecting a net transformation of C–H and CO₂ into C–CO₂H without using any other stoichiometric reagents.

In the paper in Nature they showed that intermediate-temperature (200 to 350 ˚C) molten salts containing caesium or potassium cations enable carbonate ions (CO₃^2–) to deprotonate very weakly acidic C–H bonds, generating carbon-centered nucleophiles that react with CO₂ to form carboxylates.

To illustrate a potential application, we use C–H carboxylation followed by protonation to convert 2-furoic acid into furan-2,5-dicarboxylic acid (FDCA)—a highly desirable bio-based feedstock with numerous applications, including the synthesis of polyethylene furandicarboxylate (PEF), which is a potential large-scale substitute for petroleum-derived polyethylene terephthalate (PET). Since 2-furoic acid can readily be made from lignocellulose, CO₃^2–-promoted C–H carboxylation thus reveals a way to transform inedible biomass and CO₂ into a valuable feedstock chemical.

—Banerjee et al.

Despite the many desirable attributes of PEF, the plastics industry has yet to find a low-cost way to manufacture it at scale. The bottleneck has been figuring out a commercially viable way to produce FDCA sustainably.

One approach is to convert fructose from corn syrup into FDCA. The Dutch firm Avantium has been developing that technology in partnership with Coca-Cola and other companies. Instead of using sugar from corn to make FDCA, the Stanford team has been experimenting with furfural, a compound made from agricultural waste that has been widely used for decades. About 400,000 tons are produced annually for use in resins, solvents and other products.

But making FDCA from furfural and CO₂ typically requires hazardous chemicals that are expensive and energy-intensive to make. The Stanford team solved the problem using a far more benign compound: carbonate. Graduate student Aanindeeta Banerjee, lead author of the Nature study, combined carbonate with CO₂ and furoic acid, a derivative of furfural. She then heated the mixture to about 200 ˚C to form a molten salt.

After five hours, 89% of the molten-salt mixture had been converted to FDCA. The next step, transforming FDCA into PEF plastic, is a straightforward process that has been worked out by other researchers, Kanan said.

The other Stanford coauthors of the Nature study are graduate student Graham Dick and former postdoctoral scholar Tatsuhiko Yoshino, now at Hokkaido University in Japan.

Support for the research was provided by Stanford University through the Center for Molecular Analysis and Design, the Camille & Henry Dreyfus Foundation and the Japan Society for the Promotion of Science.

Resources

• Aanindeeta Banerjee, Graham R. Dick, Tatsuhiko Yoshino & Matthew W. Kanan (2016) “Carbon dioxide utilization via carbonate-promoted C–H carboxylation” Nature 531, 215–219 doi: 10.1038/nature17185