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Old Gas, New Gas

Methane—made and taken apart by microbes, in the Earth, by people

Roald Hoffmann

Activating Methane

The C-H bond in methane is very strong. But there are bacteria, methanotrophs, that have evolved to use methane as their carbon and energy source. To do so, they must break the C-H bond of CH4. This they accomplish with enzymatic finesse, using methane monooxygenases, which contain a core of one or more copper atoms.

We now use petroleum as a carbon source for fuel, plastics, fibers and pharmaceuticals. That resource will soon be exhausted. Methane will be around longer. One idea is that we might farm those bacteria to give us carbon feedstocks from methane. But could we invent an efficient industrial process that breaks the strong C-H bond?

Perhaps the nitrogen fixation story provides a lesson here. While our biochemistry craves nitrogen atoms, we cannot fix abundant  atmospheric nitrogen. Some bacteria can; biologically assimilable nitrogen also comes from minerals in the soil and naturally acid rain.

But the natural sources do not suffice. Fritz Haber and Carl Bosch devised (over ninety years ago) an industrial process to make ammonia from N2 and H2. The chemistry is so successful, so economical,  that today over half the nitrogen atoms in our bodies have seen the inside of a Haber-Bosch factory.  If all those factories disappeared, there would be enough fixed N for only half the people on earth.

To put it another way:  In providing us with a key element, N, cultural evolution (science and technology) competes effectively with nature.

Returning to methane: Of course, there is one reaction we all know—burning—which certainly activates methane. The problem is that it does it too well, taking CH4 all the way to inert CO2. Along the way, inside hydrocarbon flames, are the partially oxidized "intermediates" that industry wants. But the process of burning is nonselective; it does not stop, for example, at methanol, CH3OH, a molecule we could use.

In principle, several desirable reactions are feasible, such as the ones that lead methane to methanol or acetic acid, or produce, using methane, ammonia and hydrogen peroxide. Indeed, several commercial processes currently begin with methane and convert it to such molecules. The problem is that they require really high temperatures. For selective, low-temperature chemistry, a catalyst is needed.

This catalyst...Click to Enlarge Image

Alexander E. Shilov of the Russian Academy of Sciences came up with the first candidate in 1969. Using platinum salts, he saw hydrogen/deuterium exchange in alkanes (compounds of just C and H, such as methane) that had been mixed with deuterated water, D2O. Therefore, some of the C-H bonds must have been broken and reforged—the first evidence that the C-H bond was not untouchable.

Passing over, unwillingly, much beautiful chemistry, we come to the exciting, recent work by Roy Periana and his coworkers at the University of Southern California, who have improved on Shilov-like chemistry to the stage that they can convert methane to methanol with a yield of better than 70 percent at 220 degrees. They can also convert methane directly to acetic acid at 180 degrees—much cooler than the >800-degree conditions of other processes.

Periana thinks that within 10 years (I guess 50) we will have all the carbon-containing molecules we need—and many we haven't thought of—made from coal and methane sources. Goodbye petroleum.

© Roald Hoffmann

Thanks to Martin Fisher, John Hayes, Martin Hovland, Jay Labinger, Lynn Margulis, Roy Periana and Norman R. Scott for their enlightening comments.

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