As a possible energy source for fuel cells or a substitute
for gasoline, methanol is increasingly drawing attention beyond its importance
as a feedstock for chemical industry. It can be stored much more efficiently
and cheaply than hydrogen and could be distributed by way of the existing
network of fuelling stations. The disadvantage is the truly complex synthesis
of methanol from natural gas via a detour through synthesis gas. One
interesting alternative that was pursued and then abandoned is known as the direct
low-temperature oxidation of methane to methanol.
A team led by Ferdi Schüth at the Max Plank Institute of
Coal Research in Mülheim (Germany)
and Markus Antonietti at the Max Planck Institute for Colloids and Interfaces
in Potsdam-Golm (Germany)
has now developed a novel catalyst. As the researchers report in the journal Angewandte
Chemie, this could provide a second wind, if not a major breakthrough, for
this process.
"The development of catalyst systems for the direct
low-temperature oxidation of methane to methanol has been one of the major
challenges in catalysis over the last decades," says Schüth. The problem
is that the bonds in methane are very strong and difficult to break. In
addition, under the reaction conditions required, methanol has the tendency to
react further to form carbon dioxide. The process thus requires not only highly
active but also highly selective catalysts.
One breakthrough was the development of a platinum complex
by a research group led by Roy Periana. This complex catalyzes the
low-temperature oxidation of methane in concentrated sulfuric acid at
temperatures around 200 °C to form methyl sulfate-which can be converted into
methanol-in good yield and high selectivity. Despite highly promising results,
the complex separation and difficult recycling of this dissolved catalyst,
among other things, hampered the commercial application of this process.
Development proceeded to the pilot-plant stage before being abandoned. "A
solid catalyst that can be easily separated could make such a process viable on
a small scale, making possible the efficient, decentralized consumption of
natural gas," says Schüth.
The German researchers have now been able to develop such a
solid catalyst, whose high reactivity and selectivity, and its outstanding
stability through numerous recycling steps, have raised hopes of its industrial
implementation. "Our development is based on a recently discovered class
of high-performance polymers," explains Anonietti. Polymerization of a
ring-shaped molecule, an aromatic nitrile, results in a network known to
chemists as a "covalent triazine-based framework", abbreviated as
CTF. Loading this substance with platinum results in a highly active, easily
separated, and recyclable catalyst.