Monday, March 27, 2006

Methanol: The New Hydrogen

Advances in methanol synthesis, coupled with improved fuel cell technology, could make it a viable alternative to gasoline.

By Chandra Shekhar

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Hydrogen has been getting plenty of hype as a potential replacement transportation fuel, for cutting carbon dioxide emissions and reducing dependence on fossil fuels. But methanol would be far better than the more reactive and volatile hydrogen, argues George Olah, a chemist and Nobel laureate, in a new book, Beyond Oil and Gas: The Methanol Economy.

Olah notes that methanol, a clean-burning liquid, would require only minor modifications to existing engines and fuel-delivery infrastructure (see "The Methanol Economy"). And manufacturing it could even make use of carbon dioxide, a source of global warming. Methanol's benefits have long been understood -- now recent advances in methanol synthesis and methanol fuel cells could make this fuel even more attractive.

Currently, about 90 percent of the worldwide production of methanol (CH₃OH) is derived from methane (CH₄), the main component of natural gas. Today's methods of making methanol have two stages: converting methane into syngas, a mixture of primarily carbon monoxide and hydrogen, and then into methanol. Although these steps have become more efficient over time, the elimination of the syngas step could save money, since it currently accounts for up to 70 percent of the cost of making methanol.

In an effort to eliminate this cost, Olah and his colleagues have explored ways of converting methane directly into methanol. "You take methane and stick in just one oxygen atom," says Olah, director of the Loker Hydrocarbon Research Institute at the University of Southern California (USC). "Easily said, but not so easily done." The problem is that methane is chemically inert, and combines readily with oxygen only at high temperatures. A catalyst helps, but commonly used catalysts themselves work only at 300 degrees Celsius or higher. At these temperatures, most of the methanol produced is oxidized to carbon dioxide and water. Indeed, methanol yields from such reactions can be as low as 2 percent.

Recently discovered lower-temperature catalysts offer better yields, says Roy Periana, associate professor of chemistry at USC. Using a platinum-based catalyst dissolved in concentrated sulfuric acid at 200 degrees Celsius, Periana has achieved a methanol yield of more than 70 percent. He's now looking for less expensive catalysts, and has found some promising ones.

Olah and his colleague Surya Prakash, professor of chemistry at the university, have developed an alternative method for converting methane to methanol, using a halogen such as bromine. In the presence of special catalysts and at less than 250 degrees Celsius, methane reacts with bromine to form methyl bromide (CH₃Br) and hydrogen bromide (HBr). Methyl bromide then reacts with water to form methanol. The bromine from the hydrogen bromide can be recovered by reaction with air, and reused.

Making methanol from natural gas -- which still involves fossil fuels and increases carbon dioxide in the atmosphere -- is just the first step, says Olah. Chemists have long known that methanol can be made by combining carbon dioxide and hydrogen. Such a process requires considerable energy, for example, to harvest the hydrogen from water, but this energy could come from carbon-free sources such as nuclear or wind power. The carbon dioxide could be captured from flue gases, and eventually directly from the atmosphere, he says.

Methanol: The New Hydrogen

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