New molecule may help convert CO2 into liquid fuel

Scientists have developed a new metal complex that could be key to converting carbon dioxide gas into liquid fuels, an advance that can help reduce the accumulation of the greenhouse gas in atmosphere.
The findings provide a foundation for the development of technologies that may one day help neutralise the negative effects of atmospheric accumulation of carbon dioxide (CO2) by converting it back into fuel.
Theo Agapie, professor at the California Institute of Technology in US and graduate student Joshua Buss have developed a model system to demonstrate what the initial steps of a process for the conversion of carbon monoxide (CO) to hydrocarbons might look like.

Although methods exist to transform CO2 into CO, a crucial next step, the deoxygenation of CO molecules and their coupling to form C-C bonds, is more difficult.
In their study, Agapie and Buss synthesised a new transition metal complex - a metal atom, in this case molybdenum, bound by one or more supporting molecules known as ligands - that can facilitate the activation and cleavage of a CO molecule.
Incremental reduction of the molecule leads to substantial weakening of the C-O bonds of CO.

Once weakened, the bond is broken entirely by introducing silyl electrophiles, a class of silicon-containing reagents that can be used as surrogates for protons.

This cleavage results in the formation of a terminal carbide - a single carbon atom bound to a metal centre - that subsequently makes a bond with the second CO molecule coordinated to the metal.
Although a carbide is commonly proposed as an intermediate in CO reductive coupling, this is the first direct demonstration of its role in this type of chemistry, the researchers said.
Upon C-C bond formation, the metal centre releases the C2 product. Overall, this process converts the two CO units to an ethynol derivative and proceeds easily even at temperatures lower than room temperature.
"To our knowledge, this is the first example of a well-defined reaction that can take two carbon monoxide molecules and convert them into a metal-free ethynol derivative, a molecule related to ethanol; the fact that we can release the C2 product from the metal is important," Agapie said.

While the generated ethynol derivative is not useful as a fuel, it represents a step toward being able to generate synthetic multicarbon fuels from carbon dioxide.
The researchers are now applying the knowledge gained in this initial study to improve the process.
"Ideally, our insight will facilitate the development of practical catalytic systems," Buss said.
The scientists are also working on a way to cleave the C-O bond using protons instead of silyl electrophiles.
"Ultimately, we'd like to use protons from water and electron equivalents derived from sunlight," Agapie said.
The study was published in the journal Nature.