December 24, 2015

Toward Liquid Fuels from Carbon Dioxide

C1 to C2: Connecting carbons by reductive deoxygenation and coupling of CO
Credit: Kyle Horak and Joshua Buss/Caltech

(December 23, 2015)  In the quest for sustainable alternative energy and fuel sources, one viable solution may be the conversion of the greenhouse gas carbon dioxide (CO2) into liquid fuels.

Through photosynthesis, plants convert sunlight, water, and CO2 into sugars, multicarbon molecules that fuel cellular processes. CO2 is thus both the precursor to the fossil fuels that are central to modern life as well as the by-product of burning those fuels. The ability to generate synthetic liquid fuels from stable, oxygenated carbon precursors such as CO2 and carbon monoxide (CO) is reminiscent of photosynthesis in nature and is a transformation that is desirable in artificial systems. For about a century, a chemical method known as the Fischer-Tropsch process has been utilized to convert hydrogen gas (H2) and CO to liquid fuels. However, its mechanism is not well understood and, in contrast to photosynthesis, the process requires high pressures (from 1 to 100 times atmospheric pressure) and temperatures (100–300 degrees Celsius).

More recently, alternative conversion chemistries for the generation of liquid fuels from oxygenated carbon precursors have been reported. Using copper electrocatalysts, CO and CO2 can be converted to multicarbon products. The process proceeds under mild conditions, but how it takes place remains a mystery.

Now, Caltech chemistry professor Theo Agapie and his graduate student Joshua Buss have developed a model system to demonstrate what the initial steps of a process for the conversion of CO to hydrocarbons might look like.

The findings, published as an advanced online publication for the journal Nature on December 21, 2015 (and appearing in print on January 7, 2016), provide a foundation for the development of technologies that may one day help neutralize the negative effects of atmospheric accumulation of the greenhouse gas CO2 by converting it back into fuel. 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.

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