Carbon dioxide (CO 2 ) is the most common greenhouse gas produced by the US. It enters the atmosphere through the burning of fossil fuels, solid wastes, wood, and certain manufacturing processes. The growing abundance of CO 2 in the atmosphere makes it an attractive feedstock for commodity synthesis, which could reduce the greenhouse gases in the atmosphere.

Though scientists have explored this concept before, few have had success due to the difficulty of breaking the carbon-oxygen bond and forming carbon-carbon bonds efficiently. The current approaches require high-energy reagents, which limit the reactions to low volumes, ultimately negating any environmental benefit of drawing down CO 2 .

A team of scientists at Stanford recently published a successful route to using CO 2 to produce a highly desirable bio-based feedstock. This particular feedstock can be used to synthesize a renewable polymer that has the potential to replace a pervasive, fossil-fuel derived polymer, polyethylene terephthalate.

50 megatons of polyethylene terephthalate is produced a year. It's an industrial raw material that can be found in everything from food packaging to solar cells to clothing. The team focused on producing a similar polymer, polyethylene furandicarboxylate (PEF), which has been reported to have superior physical properties to polyethylene terephthalate.

Polyethylene furandicarboxylate production

In recent studies, scientists estimated that PEF production using the naturally occurring sugar fructose would emit half the CO 2 of the processes used to make polyethylene terephthalate. However, in order to develop a process that is efficient enough to produce the PEF feedstock on a scale that can compete with the current offerings, we need an abundant, cheap starting material. Plant-based biomass, lignocellulose, is an optimal choice for this starting material.

PEF is synthesized through polymerization of a chemical called FDCA (furan-2,5-dicarboxylic acid). There are two main pathways that are used to produce this raw material, but both have limitations. Conversion of plant material to something called furfural is a decades-old industrial process, but the current conversion routes are inefficient, unselective, and energy intensive.

This team thinks that a new route could be used to produce a modified form of FDCA as a feedstock for PEF using a lignocellulose-derived chemical (2-furoic acid, which is already on the market).

Molten salts as catalysts

This chemical conversion requires the use of a carbonate ion (CO­ 3 2-) to promote a carbon-hydrogen (C-H) carboxylation reaction. These reactions are typically very difficult to achieve when using a weak acid. In this new strategy, the scientists demonstrate that the carbonate ion could perform this reaction in the presence of a molten salt with a high concentration of alkali cations (things like potassium and cesium). These alkali cations are used to stabilize the product through ion-pairing.

The team tested their route by running a series of reactions that revealed that the use of cesium ions or potassium ions at temperatures between 200 to 350 degrees Celsius resulted in an efficient process when CO 2 was present.

For example, when cesium furan 2-carboxylate and Cs­ 2 Co 3 were heated at 260 degrees Celsius under CO 2 in a tube furnace, the desired product was formed with a 76 percent yield after 12 hours. When the reaction was then run in a high-pressure reactor (Parr reactor), scientists observed improved yields and less decomposition.

The team determined whether this could be scaled up by performing a series of reactions under CO 2 with increasing volumes. All of these reactions had yields of over 70 percent, though the largest one tested was only 100 milliMoles. As the scale is increased, the reaction slows and the yield decreases. The scientists think this is because the reaction takes place at the molten salt-CO 2 interface, and the surface area-to-volume ratio decreases with increasing scale. They expect that improved yields could be achieved at larger scales with reactors that are able to disperse the salt through reaction media more effectively.

The team also demonstrated that the carboxylation reaction works in the presence of CO 2 and potassium ions as well, although the yield was only 62 percent.

For a scalable reaction to be feasible, we need both an easy product isolation and an efficient process to recover the salts. The team developed two routes that were able to fulfill these criteria.

This investigation has established a simple strategy to use CO 2 in C-C bond formation without the need of a biological catalyst. This new development opens up the possibility that we can develop some high-volume reactions that make removing CO 2 from the air commercially viable.

Nature, 2016. DOI:10.1038/nature17185 (About DOIs).