Scientists have discovered a method to control the gas-phase
selective catalytic combustion of methane, so finely that if done at room
temperature the reaction produces ethylene, while at lower temperatures it
yields formaldehyde. The process involves using gold dimer cations as catalysts
— that is, positively charged diatomic gold clusters. Being able to catalyze
these reactions, at or below room temperature,
may lead to significant cost savings in the synthesis of plastics,
synthetic fuels and other materials. The
research was conducted by scientists at the Georgia Institute of Technology and
the University of Ulm. It appears in the April 14, 2011, edition of The Journal of Physical Chemistry C.
“The beauty of this process is that it allows us
to selectively control the products of this catalytic system, so that if one
wishes to create formaldehyde, and potentially methyl alcohol, one burns
methane by tuning its reaction with oxygen to run at lower temperatures, but if it’s ethylene one is after,
the reaction can be tuned to run at room temperature,” said Uzi Landman,
Regents’ and Institute Professor of Physics and director of the Center for
Computational Materials Science at Georgia Tech.
Reporting last year in the journal Angewandte Chemie International Edition, a team that included
theorists Landman and Robert Barnett from Georgia Tech and experimentalists
Thorsten Bernhardt and Sandra Lang from the University of Ulm, found that by using
gold dimer cations as catalysts, they can convert methane into ethylene at room
temperature.
This time around, the team has discovered that, by using the
same gas-phase gold dimer cation catalyst, methane partially combusts to
produce formaldehyde at temperatures below 250 Kelvin or -9 degrees Fahrenheit.
What’s more, in both the room temperature reaction-producing ethylene, and the
formaldehyde generation colder reaction, the gold dimer catalyst is freed at
the end of the reaction, thus enabling the catalytic cycle to repeat again and
again.
The temperature-tuned catalyzed methane partial combustion
process involves activating the methane carbon-to-hydrogen bond to react with
molecular oxygen. In the first step of the reaction process, methane and oxygen
molecules coadsorb on the gold dimer cation at low temperature. Subsequently, water is released and the
remaining oxygen atom binds with the methane molecule to form formaldehyde. If
done at higher temperatures, the oxygen molecule comes off the gold catalyst,
and the adsorbed methane molecules combine to form ethylene through the
elimination of hydrogen molecules.
In both the current work, as well as in the earlier one,
Bernhardt’s team at Ulm conducted experiments using a radio-frequency trap,
which allows temperature-controlled measurement of the reaction products under
conditions that simulate realistic catalytic reactor environment. Landman’s
team at Georgia Tech performed first-principles quantum mechanical simulations,
which predicted the mechanisms of the catalyzed reactions and allowed a
consistent interpretation of the experimental observations.
In future work, the two research groups plan to explore the
use of multi-functional alloy cluster catalysts in low temperature-controlled
catalytic generation of synthetic fuels and selective partial combustion
reactions.
