This effect is totally unexpected and certainly earns a wow. All of
a sudden we can engineer chemical processes to sharply increase
efficiency. And once again, copper is our champion.
Read it carefully. This must apply to many reactions.
I wonder is the same trick applies to other key elements like
aluminum.
Light may recast
copper as chemical industry 'holy grail'
by Staff Writers
Ann Arbor MI (SPX) Apr 03, 2013
Wouldn't it be
convenient if you could reverse the rusting of your car by shining a
bright light on it? It turns out that this concept works for undoing
oxidation on copper nanoparticles, and it could lead to an
environmentally friendly production process for an important
industrial chemical, University of Michigan engineers have
discovered.
"We report a new
physical phenomenon that has potentially significant practical
implications," said Suljo Linic, an associate professor of
chemical engineering, who led the study, which is published in the
March 29 issue of Science.
Copper's newfound
ability to shake off oxygen attached to its surface could allow it to
act as a catalyst for a long-sought reaction, causing oxygen
molecules to bind with propylene molecules in the way that forms
propylene oxide.
Propylene oxide is a
precursor for making many plastics, toiletries and other household
products such as antifreeze, paints and insulating foams. To meet
demand for these products, the U.S. produces more than 2.4 million
metric tons of propylene oxide per year, worth about $4.9 billion.
Unfortunately,
producing propylene oxide involves a complex chain of reactions that
generate unwanted chemicals. The process that provides about half of
the propylene oxide in the U.S. also produces about twice as many
tons of salt.
A catalyst that can
coax propylene and oxygen to form propylene oxide in a direct
reaction, avoiding the waste, has been called a "holy grail"
of catalysis. Metallic copper showed promise, but it had-until
now-been written off because it tends to bind itself to oxygen,
forming copper oxide, which has poor catalytic properties.
"Copper in
metallic form has this unique electronic structure that activates the
reaction pathway for propylene oxide more than the undesired
pathways," said Marimuthu Andiappan, a graduate student in
chemical engineering and first author on the paper.
Metallic copper
prefers to bind oxygen with two of the propylene's carbon atoms,
forming propylene oxide. Copper oxide, on the other hand, tends to
break the propylene down into carbon dioxide or attach the oxygen to
only one carbon atom, resulting in the herbicide acrolein.
However, Andiappan,
Linic, and former chemical engineering graduate student Jianwen Zhang
found that if copper is cleverly structured, light can reverse its
oxidation. The team made copper nanoparticles about 40 nanometers
across, or roughly one-hundredth of the thickness of a strand of
spider silk. They peppered tiny particles of clear silica with the
nanoparticles and then floated a gas of propylene and oxygen over the
resulting dust.
In the dark, the
copper oxidized, and only 20 percent of the gas converted to
propylene oxide. But under white light, five times the sun's
intensity, the copper stayed in the metallic state and turned 50
percent of the propylene into propylene oxide.
"To our
knowledge, this is the first time anyone has shown that light can be
used to switch the oxidation state from an oxide to a metallic
state," Andiappan said.
The metallic copper
under the oxidized surface concentrated the light, freeing electrons
from copper atoms. Those electrons then broke the bonds between the
copper and oxygen.
A new kind of reactor
that can illuminate the catalyst will be needed to bring this
potentially cheap and environmentally friendly way of making
propylene oxide to industry.
"Theoretically,
it is possible to use mirrors to focus sunlight and get this much
intensity," Andiappan said.
"We are just
scratching the surface," Linic said. "I can envision many
processes that wouldn't be possible with conventional strategies,
where changing the oxidation state during the reaction or driving
reactions with light could affect the outcome dramatically."
The paper describing
this work is titled "Tuning selectivity in propylene epoxidation
by plasmon mediated photo-switching of Cu oxidation state." The
study was funded by the Department of Energy and the National Science
Foundation. The university is pursuing patent protection for the
intellectual property, and is seeking commercialization partners to
help bring the technology to market.
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