Palladium Compound May Have Future in Turning CO2 into Fuel
Washington University study is tweaking the catalyst to complete the methane to ethane reaction.
Liviu M. Mirica, Ph.D., assistant professor of chemistry in Arts & Sciences at Washington University in St. Louis, is working with catalysts to more effectively turn carbon dioxide into such fuels as methanol or hydrocarbons.
Catalysts might provide alternative reaction pathways with lower energy barriers. The reactants then could be bumped over those lower barriers with carbonless energy sources such as sunlight. Instead of being a polluting one-way street, hydrocarbon chemistry could circle back on itself and become a clean carbon-neutral cycle, although one that still consumed energy.
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 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 perform the first step in the methane-to-ethane conversion as well.
Fossil fuels 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. Still, it is possible to make hydrocarbon combustion reactions run backward — either by brute force or by finesse.
The brute force way is to pump in energy. That’s how the Nazis turned coal into oil during World War II. Saddled with an abundance of coal but short on oil, Germany solved the problem by transmuting coal to oil by chemical means. But Nazi synthetic oil plants 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 option uses a chemical compound, a catalyst, that takes the reactants up an alternative, lower energy pathway to the reaction products. In effect, instead of going straight up the energy hill, the reaction takes a more manageable — ideally the minimal-energy — series of switchbacks to the top.
Mirica’s group has been working with a palladium compound that they hoped could catalyze the splitting of water. “The catalyst we made for that reaction worked,” Mirica said. “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 could we use it for? One of our ideas was to use it to turn methane into ethane,” Mirica said. 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, Mirica said, 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, holding it like a ball in a glove. The organic molecule is key to the metal complex’s function, because 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.
Mirica’s lab is currently trying to tweak the metal complex so that it can perform the entire methane-to-ethane reaction.
Ultimately, Mirica’s goal is a recycling carbon chemistry that requires so little energy that it can run off sunlight.
“If we’re going to keep using these carbon-containing fuels that make CO2, we should be trying to make combustion carbon-neutral by using catalysts and the sun’s energy to convert CO2 back into fuel,” he said.