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Turning Carbon Dioxide and Methane into Liquid Fuels

Liviu M. Mirica, PhD, assistant professor of chemistry at Washington University in St. Louis may have found and is developing a novel metal catalyst that would be able to turn greenhouse gases like methane and carbon dioxide into liquid fuels without producing more carbon waste in the process.

Mirica describes a new metal complex that can combine methyl groups (CH3) in the presence of oxygen to produce ethane (CH3-CH3) in the Journal of the American Chemical Society.

So far the return to fuel from combustion products has been a losing proposition because making carbon dioxide into a fuel uses up more energy than combustion releases and produces more carbon dioxide than it reclaims.  Mirica asserts, with some evidence now, it’s not impossible.

This could put a whole new take on petroleum and carbohydrate fuels, instead of being a polluting one-way street, hydrocarbon chemistry could circle back on itself and become a clean carbon-neutral cycle, even though still consuming some energy.

The new catalyst combines methyl groups (CH3) molecules in the presence of oxygen to produce ethane, the second step in the conversion of methane (CH4), the main component of natural gas, into a longer-chain hydrocarbon, or liquid fuel.  Mirica’s team is currently tweaking the complex so that it will be perform the firs step in the methane-to-ethane conversion, too.

Hydrocarbons are so useful because they pack energy in their chemical bonds and release that energy when they are burned. Thus, they’re essentially convenient little energy packages.  Reactions that release energy, however, are reluctant to reverse themselves and the more energy they release, the more reluctant they are to return.

So far there’s no way around this problem; if a reaction released energy both going forward and going backward, it could fuel a perpetual motion machine, which, of course, is an impossibility.  But, it’s possible to make hydrocarbon combustion reactions run backward — either by brute force or by finesse.

The brute force way is to pump in energy.  Old technology such as used to convert coal to oil worked only at high temperatures and pressures and much more energy was used to drive the reactions than was ultimately stored in synthetic oil they produced.

The finesse path is to devise a chemical compound, a catalyst that takes the reactants up an alternative, lower energy pathway to the reaction-produced products. In effect, instead of going straight up the energy hill, the reaction takes a more manageable — ideally the minimal-energy– series of switchbacks up to the top.

The background:  Last year Mirica’s group was working with a palladium compound that they hoped could catalyze the splitting of water. “The catalyst we made for that reaction worked,” says Mirica, “but not as well as we hoped. But we noticed it was easily oxidized, even by the oxygen in air.  This was our first hint that this might be an interesting system. So then we asked what else we could use it for.”

“One of our ideas was to use it to turn methane into ethane.” Methane, the main component of natural gas, is released sometimes in large amounts when an oil well is first tapped.  Turning methane to ethane, says Mirica, could be the first step in a process of building longer-chain hydrocarbons such as butane and octane, which are liquid at convenient temperatures and pressures and so could easily be transported to distant users.

Big oil will notice, and national oil companies should because Mirica’s metal complex solves half the problem of methane-to-ethane conversion. It takes two methyl groups (CH3) and, in the presence of oxygen and light, binds the carbon atoms to one another to form ethane getting closer to convenient transport.

Technically speaking the complex set up by the catalyst consists of an organic molecule that binds a central palladium atom through four nitrogen atoms, holding it like a ball in a glove.  The organic molecule is key to the metal complex’s function, since it stabilizes it in the unusual +3 oxidation state (it has given up three electrons), which is responsible for its unprecedented chemical activity.  Once in the glove, the palladium atom still has two docking spots that can be occupied by chemical species whose reaction it might catalyze.

In the paper the sites are occupied by methyl groups, which the palladium atom joins to produce ethane.  However, Mirica emphasizes the sites could easily be occupied by other chemical species. What’s more the reactions could be reducing ones (where electrons are donated to reactants) rather than the oxidizing ones (where electrons are removed from reactants) like the methyl-to-ethane conversion.

Now that’s a whole new arena of palladium chemistry potential.

Mirca’s work hinges on solving a certain problem:  Carbon dioxide is a highly stable molecule, so anything you do with it is going to require an energy input.  If the energy penalty turns out to be low enough both the ethane and methanol reactions taking greenhouse gases and transforming them to liquid or easily liquefied compounds that could then be reused as fuels would be economically viable, low enough and the carbon could be recycled many times.

Mirica’s ultimate goal is a recycling carbon chemistry that requires so little energy that it can run off sunlight.

Mirica has certainly made a noteworthy name for himself.  The CO2 back to fuel is great news, but the new potential opened from palladium chemistry is new territory.

By. Brian Westenhaus

Source: CO2 Back to Fuel and More




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Leave a comment
  • Dr Robert L Jacobson on August 22 2016 said:
    Conversion of methane to ethane has been a premiere goal of oil company research
    The use of carbon dioxide to take up hydrogen in converting CH4+ CH4 to ethane is very logical

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