We depend on petroleum for so much: not just gasoline and heating oil, but the majority of our industrial chemicals, the raw materials for everything from paint to plastic to fertilizer. Natural gas, many chemists believe, could take up some of that burden. "The world reserves of natural gas are about the same as those of petroleum," says Ayusman Sen. The problem is figuring out how to make efficient use of them.
"In fact," says Sen, Penn State professor of chemistry, "it's a two-fold problem, because the price of methane, which makes up 80-90 percent of all natural gas, is actually negative. "Generally speaking," he explains, "whenever you get petroleum out of the ground, you get some methane too." While petroleum can easily be loaded onto ships and moved to where it is needed, methane's low boiling point and flammability make transportation too dangerous. "Until recently, your choices were either to release it into the atmosphere or to burn it, which produces carbon dioxide." Either way you're contributing to the build-up of greenhouse gases—and wasting a valuable resource.
These days, Sen notes, most oil-selling nations insist on another option: Pump the stuff back into the ground. Or else take it with you. "I recently talked with a fellow from Chevron, which has wells in Nigeria," he says. "The government there says if you want our oil, you have to take our natural gas too. So they have to find a way to convert it to liquid."
Currently, methane conversion requires at least a two-step process. "You allow it to react under high temperature and pressure, with steam, and you get what is called a synthesis gas—a combination of carbon monoxide and hydrogen," Sen says. From the synthesis gas, another high-temperature reaction is needed to produce methanol, which is used to make formaldehyde, as well as various plastics and solvents. A third step yields acetic acid, among other things the source of household vinegar.
Huge quantities of both chemicals are manufactured every year by these indirect processes: over 11 billion pounds of methanol and 4.7 billion pounds of acetic acid in the U.S. alone. The search for a cheaper, more direct route, Sen says, "has been something of a Holy Grail in the chemical industry." In 1994, in fact, the trade journal Chemical Week reported that some large companies, disappointed with the lack of progress, had scrapped R&D programs in methane conversion.
That same year, Sen and postdoctoral associate Minren Lin announced a breakthrough. By dissolving a powder of rhodium chloride in water, along with carbon monoxide and oxygen, they had produced acetic acid from methane directly. The reaction took place at a relatively low temperature (100 degrees centigrade), required little energy, and left no environmentally harmful solvents to throw away.
It was an important first step toward a practical process, Sen recalls, but the conversion rate was too slow for commercial applications. And the rhodium reaction produced a lot of methanol as a side product — okay if you wanted methanol, but not if you wanted only acetic acid. Sen and his group went back to the drawing board.
Looking for a new catalyst, he says, "is an enlightened random search. It's like a jig-saw puzzle, putting together known chemical principles in different ways." Most catalytic processes involve the so-called transition metals, whose electronic properties allow them to interact unusually well with organic molecules. "We have been using copper, rhodium, platinum, and palladium," Sen says. "The potential combinations are endless. We're trying to understand the fundamentals of these reactions—to see what would be optimal."
Last year, Sen and his group came up with a new combination, this time using a combination of metallic palladium and copper chloride as a catalyst to produce methanol. "It's faster and more selective than the rhodium process," Sen says, "with fewer side products."
Still not quite good enough, however, for industrial-scale production. "There's an economic factor I didn't realize when we started," Sen admits. "All those traditional chemical plants, doing indirect conversion, are already in place. In order to get somebody to build a new plant, a direct process has to be more than merely competitive. It has to be far superior."
So Sen and his team are back at work, searching for a still-faster conversion. "We continue to work on the palladium/copper system," he says, "and we're also trying other possibilities. We want a process that's economic—but we also want to understand how it works."
In the next five to 10 years, Sen predicts, a chemist somewhere will unlock the secret of direct methane conversion. "It may not be our process, but somebody will get it. There's a tremendous push now to do so."
Ayusman Sen, Ph.D., is professor of chemistry in the Eberly College of Science, 152 Davey Laboratory, University Park, PA 16802; 814-863-2460; axs20@psu.edu. The methane-conversion work reported above is funded by an international consortium and the National Science Foundation.