When someone mentions "clean burning fuel," methane often comes to mind. As the primary constituent of natural gas, methane's four carbon-hydrogen bonds make it a very energy-potent molecule, burning much more cleanly than other carbon-based fuels.
Methane produces carbon dioxide and water when combusted like other carbon-based fuels, but unlike larger fuels, such as octane, methane combusts almost perfectly and produces very few polluting byproducts.
In addition, methane is a plentiful resource. The U.S. is one of the largest producers of natural gas, which could provide one pathway towards decreasing the reliance on imported and dirtier fuels in gasoline and diesel-burning vehicles.
However, one major issue restricts our ability to fully utilize methane's potential as an energy resource: methane is a gas at pressures and temperatures common at the earth's surface, which makes portability an issue.
If left in the gaseous state, natural gas takes up a lot of space, but when compressed to smaller volumes, one runs the risk of an explosion. Liquefying methane can overcome these restrictions, but it must be kept at -260 degrees Fahrenheit in heavily insulted tanks to be maintained in that state, and is thus limited to large trucks and buses.
Methane can also be liquefied by reacting it to form other chemicals that are liquids at room temperature. Researchers at the University of North Carolina at Chapel Hill and the University of Washington have come one step closer to that goal, which can dramatically ease the transportation of methane gas from natural gas fields.
In their article, recently published in the journal Science, the researchers observed a metal complex that can bind to methane in solution. This complex, which utilizes the rare metal rhodium, keeps the high-energy carbon-hydrogen bonds intact.
"The idea is to turn methane into a liquid in which you preserve most of the carbon-hydrogen bonds so that you can still have all that energy," said Karen Goldberg, a University of Washington chemistry professor and co-author of the study, as told the University of North Carolina at Chapel Hill News Services.
Complexing methane to rhodium starts the process of breaking a single carbon-hydrogen bond, known as a sigma bond, which can then be more easily reacted to form chemicals like methanol. In this case, methane keeps three of its four C-H bonds, but sacrifices one bond to a hydroxyl group so that it can liquefy at room temperature.
The study demonstrated that methane is able to quickly complex with rhodium, even at temperatures as balmy as -166 degrees Fahrenheit, which makes the metal a promising candidate for the conversion of methane into a more manageable form.
Current methods for converting methane into useful chemicals require a lot of energy and high temperatures, while catalysts that have been discovered to perform at lower temperatures are too slow, inefficient or expensive for industrial applications.
According to Goldberg, although these developments are unlikely to occur in the near future, their work should spur further advances in developing catalysts to transform methane into methanol or other substances that are liquids at reasonable temperatures.
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