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Mesoporous materials
can be synthesized with uniformly sized "nanopores" of different
dimensions and geometries. The pore diameters (typically on the order
of a few billionths of a meter) are chosen to control the access of
molecules to the catalytic reaction sites located inside the porous
cavities. Only the molecules of certain sizes and chemical properties
are selected and guided to the reaction centers where they are
efficiently transformed to the desired products.
Scientists from Ames
Laboratory's Chemical and Biological Sciences Program are contributing
their individual expertise to prepare and study a new generation of
efficient, selective and structurally stable mesoporous catalysts. The
research team includes Marek Pruski, Victor Lin, Robert Angelici,
Andreja Bakac, James Espenson, James Evans, Mark Gordon and Edward
Yeung.
"This grant will fund a long-term, collaborative
research effort geared toward uniting the best features of homogeneous
and heterogeneous catalysis," said Pruski, a physicist and the
principal investigator who led the team of Ames Laboratory researchers.
"Ames Laboratory is in a unique position to succeed in this endeavor
because of its traditional strength in catalysis, as well as superb
analytical, theoretical and computational capabilities that already
exist here. Also, we were fortunate to have been joined by Victor Lin,
whose ideas and expertise in nanoscience and surface functionalization
of mesoporous materials are at the heart of this effort."
Lin explained that the possibility of incorporating
various catalytic functional groups into the characteristically large
and uniform channels of mesoporous materials allows control on a
molecular scale, making the materials ideal candidates for use as
catalysts that can easily be separated from the products and recycled.
But, there's a catch, warned Lin, who is also an Iowa State University
assistant professor of chemistry. Although current mesoporous
materials have the ability to enhance catalytic activity, they do not
provide a high degree of selectivity.
Providing an example, Lin said, "Suppose you have
two reactants, A and B, in your starting material and both will react
with your catalyst. But you only want the product from reactant A, so
you have to find a way to get rid of reactant B." According to Lin, it
often requires costly and time-consuming separation techniques to
produce a pure starting material that contains only reactant A.
Bypassing that problem, Lin incorporated the
separation work into the catalyst through his novel "gatekeeping"
strategy that involves what he calls "decorating the walls," in which
he fine tunes the degree of functionalization of the mesoporous
catalysts. Making use of the porous channels in a mesoporous silica,
Lin created a honeycomb-type structure and attached catalytic
functional groups to single sites on the pore walls and at the
entrances to the individual channels.
" These gatekeepers are synthetic compounds or
natural products that are able to keep unwanted reactants in the
starting material from reaching the catalyst," said Lin. Carrying out
their duties, the gatekeepers allow only the reactant of choice to
enter the cavity of the catalyst and react there, producing the
desired product. "If only one reactant is allowed to react with the
catalyst, you can actually accomplish stereochemical control without
employing sophisticated and often expensive techniques," said Lin,
"and you can play a lot of games with this kind of strategy." These "games"
resulted in several unique schemes for anchoring the groups of
catalysts and gatekeepers within the honeycomb-type structure. All of
the schemes for designing and tuning the highly selective mesoporous
catalysts will undergo extensive scrutiny.
 Several
key preliminary results obtained by Lin and Pruski have already
demonstrated the feasibility of these new catalytic principles.
Detailed characterization of these mesoporous catalysts has been
provided by solid state nuclear magnetic resonance (NMR), a
spectroscopic technique that Pruski's group uses and develops. NMR
offers unique information about the structure, location and dynamic
behavior of molecules on mesoporous surfaces, which helps to
understand how the catalysts work and drives further catalyst design.
Very importantly, theoretical/computational
modeling will provide state-of-the-art characterization capabilities
for analyzing and predicting structures, molecular dynamics and
reactivity in these systems. Other catalytic studies, including
kinetic, surface tethering, atom transfer, electron transfer and
photochemistry, will be made on the reactions that are catalyzed by
the new nanocatalysts.
"This interdisciplinary research will involve
nanoscience at its core and lay the foundations for the future
development of a wide range of novel catalytic systems," said Pruski.
Ames Laboratory is operated for the DOE by ISU. The
Lab conducts research into various areas of national concern,
including energy resources, high-speed computer design, environmental
cleanup and restoration, and the synthesis and study of new materials. |
