Toward liquid fuels from carbon dioxide

December 23, 2015 by Ker Than, California Institute of Technology
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

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 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 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.

In their study, Agapie and Buss synthesized 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 center—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 say. Upon C–C bond formation, the metal center 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 says.

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 says.

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 says. "But protons are very reactive, and right now we can't control that chemistry."

Explore further: Plant-inspired power plants: Developing catalysts that turn excess atmospheric CO2 into liquid fuel

More information: Joshua A. Buss et al. Four-electron deoxygenative reductive coupling of carbon monoxide at a single metal site, Nature (2015). DOI: 10.1038/nature16154

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1 / 5 (4) Dec 23, 2015
All of this is to save us from the ravages of fossil fuels. It is time to transition from filthy fuels such as coal and nukes. Yes, nukes. The pollution from them is invisible and deadly, . . and forever.
3 / 5 (2) Dec 27, 2015
Agreed fossil fuels, UNLESS the processes discussed in this article can be made efficient and high-volume. As far as nukes, I disagree. I'd far rather get my energy from a reactor than from a coal burner, and face it, REALITY TODAY IS you couldn't be doing this communication without one or the other. Which do you prefer?
1 / 5 (3) Dec 27, 2015

And we would not be here without stone tools long ago. We outgrew them, too.
5 / 5 (2) Dec 29, 2015
We outgrew them, too.

Actually, we didn't. We simply refined them.

Some surgeons for example use scalpel blades made of obsidian - a stone-age invention - to make precise cuts because they're much sharper than any metal blade . Flint-knapping also survives in the trade of microscopy because you need atomically thin blades to cut your samples.

Similiar to that, we've started to make kitchen knives out of ceramics because they don't dull so easily. A mortar and pestle is basically a stone-age tool as well. Grindstones and wheels are also stone-age inventions.

The only stone-age tool we've really "outgrown" is the stone handaxe. Every other stone tool is still in use somewhere, somehow.
1 / 5 (3) Dec 29, 2015
Thanks for the book report.
not rated yet Dec 29, 2015
Notice Gkam when the gasoline engine was invented and mechanical transportation was possible the government did not offer huge rebates to people who purchased the new technology. The government did not impose a livestock tax to support the fledgling transportation industry.
1 / 5 (5) Dec 29, 2015
The livestock was not killing us with pollution yet. The auto saved us from that eventuality, just like alternative renewables will save us from petroleum pollution.
3 / 5 (2) Dec 29, 2015
Livestock was totally killing us with pollution. The amount of horse shit alone in the cities was a health catastrophe.
not rated yet Dec 29, 2015
Although I have huge reservations about the efficiency of any process that turns CO2 into a fuel the general study of catalytic processes can only help mankind.

Most of these CO2 or H2O into fuel schemes depend on excess, almost cost free, renewable or fusion energy sources. If and when these sources become a reality it will become a golden age for society. In fact, energy has always been a substitute for and equivalent to human labor. Thus the availability of cheap energy is paramount to the existence of today's affluent society.

Oh we, the US, are not all affluent! Just compare the "Poor" of today with the poor of the 30s. In reality if you want to judge well-being of the population of a nation just look at their per capita consumption of fossil fuels. That figure pretty much defines the affluence of a nation.
5 / 5 (1) Dec 29, 2015
The livestock was not killing us with pollution yet.

Actually, it was.

Horses were a huge sanitation problem in cities because they kept shitting everywhere and dying on the streets, and causing a large number of accidents by panicking and trampling people dead.

just like alternative renewables will save us from petroleum pollution.

Unlikely. The only practical way to store large amounts of renewable energy is in the form of hydrocarbons, which implies that the internal combustion engine will be around for a long time to come. That's what this whole article is about, if you didn't happen to notice.

The pollution is not due to the fuel being petroleum per se - biodiesel makes NOx emissions just the same.
4.5 / 5 (2) Dec 29, 2015
Most of these CO2 or H2O into fuel schemes depend on excess, almost cost free, renewable or fusion energy sources.

Most of the world besides the US hasn't got an issue with that. They're already used to fuel prices at $8-10 per gallon due to taxation, which they can drop at will. For an energy source that delivers a kWh for 10 cents, the raw gasoline equivalent cost is at $3.30 per gallon, and an efficiency loss of 50% will bump that up to $6.60

Most of the EU for example would be quite happy paying only that. The difference to the US is that American cars use a lot more fuel and people drive three times as much despite living in cities/regions with population densities equivalent to central Europe because the public infrastructure is built too sparsely.

A social restructuring in the US, to cope with more expensive fuel without a loss of living standards, is clearly possible.
1 / 5 (1) Dec 29, 2015
"Most of the world besides the US hasn't got an issue with that. They're already used to fuel prices at $8-10 per gallon due to taxation, which they can drop at will."

I see Eikka, if only the US would double or triple their taxes on energy then all of these renewable energy schemes would be feasible and make economic sense.
5 / 5 (1) Dec 30, 2015
I see Eikka, if only the US would double or triple their taxes on energy then all of these renewable energy schemes would be feasible and make economic sense.

Taxation has its own problems, namely that it makes the governments dependent on the source of tax, which is why they're a bad idea for controlling the behaviour of the public. I didn't suggest that the US should raise the fuel taxes - that wouldn't really accomplish anything.

I was simply saying that the people in the US can tolerate much more expensive fuel prices without a corresponding drop in living standards by simply re-structuring the social infrastructure around the fact that fuel costs more.

That's because fuel - being as cheap as it is now - is simply being wasted on an infrastructure that was drawn up and built in the 50's and 60's when transportation cost next to nothing.

Europe was re-built after WW2 around the fact that they simply had no fuel - cars were even made to run on wood.

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