Novel Metal Catalysts May Be Able to Turn Greenhouse Gases Into Liquid Fuels

Novel metal catalysts may be able to turn greenhouse gases like methane and carbon dioxide into liquid fuels without producing more carbon waste in the process. If fossil fuels burn completely, the end products are carbon dioxide and water. Today the carbon dioxide is a waste product, one that goes into the air -- adding to global warming; or the oceans -- acidifying them; or underground -- with as yet unknown consequences. Now reversing combustion has been a loser's game, because making carbon dioxide into a fuel uses up more energy than combustion releases and produces more carbon dioxide than it reclaims. But catalysts could change everything. Catalysts might provide alternative reaction pathways with lower energy barriers. The reactants could then be bumped over those lower barriers with carbonless energy sources such as sunlight. In the Journal of the American Chemical Society, Mirica describes a new metal complex that can combine methyl groups (CH₃) in the presence of oxygen to produce ethane (CH₃-CH₃). This is the second step in the conversation 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 first step in the methane-to-ethane conversion as well.

Fossil fuels are useful because they pack energy in their chemical bonds and release that energy when they are burned. Reactions that release energy, however, are reluctant to reverse themselves and the more energy they release, the more reluctant they are to back up. 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. Still, it is possible to make hydrocarbon combustion reactions run backward -- either by brute force or by finesse.

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. "One of our ideas was to use it to turn methane into ethane." Methane, the main component of natural gas, is released in large amounts when an oil well is tapped. Currently the methane from the oil fields is wasted; it is flared off on site, releasing even more carbon dioxide into the atmosphere. 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 would be liquid at normal temperatures and pressures and so could easily be transported to distant users.

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. The complex consists of an organic molecule that binds a central palladium atom through four nitrogen atoms. 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. However, these sites are occupied by methyl groups, which the palladium atom joins to produce ethane. But, 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.

The first part of that reaction is pulling methyl groups off methane molecules. "The reaction wants to run straight down the energy hill all the way to the bottom (CO₂)," Mirica says. "Our goal is to design a catalyst that stops the reaction part of the way down the hill (when only one hydrogen has been removed). His lab is also testing the metal complex's ability to perform a reduction reaction, the conversion of CO₂ into methanol (CH₃OH). "Carbon dioxide is an exceptionally stable molecule, so anything you do with it is going to require energy, " Mirica says. "We're just trying to use the metal complex to minimize the energy input. "Both the ethane and methanol reactions take greenhouse gases and transform them to liquid or easily liquefied compounds that could then be reused as fuels. If the energy penalty turns out to be low enough the carbon could be recycled in this way many times.

Ultimately Mirica's goal is a recycling carbon chemistry that requires so little energy that it can run off sunlight!

http://www.sciencedaily.com/releases/2010/11/101130103615.htm