MARGINALIA
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.
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
Acknowledgments
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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