Mesoporous materials can be synthesized with uniformly sized "nanopores" of different dimensions and geometries. The pore diameters (typically on the order of 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.
Note to editors: Images of mesoporous silica nanopheres with porous holes can be viewed by visiting the following Web site and clicking on the appropriate link.
http://www.external.ameslab.gov/news/release/2003rel/catalystfunding.htm
Contacts:
Marek Pruski, Chemical and Biological Sciences, 515-294-2017
Victor Lin, Chemical and Biological Sciences, 515-294-3135
Saren Johnston, Public Affairs, 515- 294-3474