Clean Diesel from Coal

A novel catalytic method could let you fill up your tank with coal-derived diesel, cutting U.S. dependence on foreign oil.

As the cost of oil soars and worries over the U.S. dependence on foreign petroleum escalate, coal is becoming an increasingly attractive alternative as a feedstock to make a range of fuels. Now chemists have invented a new catalytic process that could increase the yield of a clean form of diesel made from coal.

The method, described in the current issue of the journal Science, uses a pair of catalysts to improve the yield of diesel fuel from Fischer-Tropsch (F-T) synthesis, a nearly century-old chemical technique for reacting carbon monoxide and hydrogen to make hydrocarbons. The mixture of gases is produced by heating coal. Although Germany used the process during World War II to convert coal to fuel for its military vehicles, F-T synthesis has generally been too expensive to compete with oil.

Part of the problem with the F-T process is that it produces a mixture of hydrocarbons – many of which are not useful as fuel. But in the recent research, Alan Goldman, professor of chemistry and chemical biology at Rutgers University, and Maurice Brookhart, professor of chemistry at the University of North Carolina at Chapel Hill, use catalysts to convert these undesirable hydrocarbons into diesel. The catalysts work by rearranging the carbon atoms, transforming six-carbon atom hydrocarbons, for example, into two- and ten-carbon atom hydrocarbons. The ten-carbon version can power diesel engines. The first catalyst removes hydrogen atoms, which allows the second catalyst to rearrange the carbon atoms. Then the first catalyst restores the hydrogen, to form fuel.

Diesel fuel produced in this way has several potential advantages. Ordinary diesel contains molecules, called aromatics, that, when combusted, produce particulates, Goldman says. But the diesel formed by the new catalysts does not include aromatics, so it burns much cleaner, overcoming one of the major objections to diesel fuel. This could lead to more vehicles using diesel engines, which are about 30 percent more efficient than gasoline engines.

But the biggest advantage may be that the United States has huge amounts of coal: “We have as much energy in coal as the rest of the world has in oil. That’s enough to last us the next hundred years or so,” Goldman says. Thus, a more efficient, and so less expensive method of converting coal to diesel could significantly cut U.S. dependence on foreign oil, and do so for a long time.

“When I saw this I thought it was really a terrific contribution that could be very important,” says Richard Schrock, professor of chemistry at MIT, who won the Nobel Prize in Chemistry in 2005, with two other scientists, for discovering the type of catalyst used in the second step. Combining two catalysts this way “is pretty rare,” he says. “You can’t just throw any two things together and expect to get the results you anticipated.”

According to Robert Grubbs, professor of chemistry at Caltech, who shared the Nobel prize with Schrock, “The key is finding catalyst systems that are compatible, and will operate at the temperatures where you can do both processes together.”

At this time, the new catalytic method is still a proof-of-concept, and not ready for commercial use. For example, the second catalyst tends to break down. But Schrock says this problem should be solvable: “It’s theoretically possible that this could become practical. I e-mailed Alan Goldman and said, ‘Look, we’ve got a lot of catalysts, and I can think of some things that might be thermally more stable.’ So I’m going to send him some catalysts, and he’s going to try them out.”

It also might be possible to make catalysts that use products from the first reaction to regenerate themselves. “Then the catalyst wouldn’t die, and you could in fact keep the reaction going,” says Schrock.

Hope page image courtesy of Joseph Blumberg.  Caption: Postdoctoral associate Ritu Ahuja demonstrates the catalyst material to graduate student Elizabeth Pelczar and professor Alan Goldman.

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