The objective of designing catalyst sites to achieve high selectivity to desired products is one of the central goals of current research in heterogeneous catalysis. This objective can be particularly challenging in reactions of molecules which possess multiple functional groups, and where each functional group can interact with the catalytic surface and undergo reaction. A good example is the conversion of biomass to fuels and chemicals. Biomass-derived carbohydrates and lipids are highly oxygenated compounds that generally contain multiple functional groups (e.g., alcohols, carboxylic acids, esters, olefins, ethers, aldehydes, and ketones) on each molecule. The ability to selectively drive a specific reaction of a single functional group in such molecules is important, particularly for generating high value coproducts to fuels. Unfortunately, achieving this high selectivity on conventional supported metal catalysts is greatly complicated, as all of the key functional groups can adsorb and react on the Pt-group metals in many catalysts.
Professor J. William Medlin at the University of Colorado at Boulder proposes a different approach. He seeks to design novel catalysts for high selectivity with biological catalysts (enzymes) in mind. Enzyme catalysts exploit non-covalent interactions within binding pockets to control chemical transformations, resulting in unsurpassed selectivity, even with multifunctional reactant molecules. One approach therefore for improving the selectivity of metal surfaces is to create a near-surface environment (NSE) that fosters selective interactions of reagents with surfaces, mimicing an enzyme pocket. These NSEs can be created through attachment of organic ligands to the surface. Variations on this concept have been used in generating chiral binding pockets on Pt-group surfaces in other research. These efforts to modify catalytic surfaces sought to exploit interaction between reactants and isolated covalently-attached surface ligands. The PI proposes to use well-organized molecular surface layers to create uniform and/or stratified NSEs, much like in biomembranes. Medlin has proven the concept with highly selective Pd catalysts involving the deposition of n-alkanethiol self-assembled monolayer (SAM) coatings with dramatic improvement of the selectivity (11 to 94%) of 1-epoxybutane formation from hydrogenation of 1-epoxy-3-butene.
Medlin's proposal extends the investigation of how interactions between adsorbates and other molecules in the NSE alter the reactivity of metal surfaces, with the hypothesis being that by controlling interactions between adsorbates and the NSE, it is possible to control the adsorption geometry and subsequent reactivity of important reagents. To control the NSE, deposition on metal surfaces of organic thiol SAMs selected from a rich catalog of molecules and chemistries will be employed. Many possible demonstration reactions will be considered to evaluate the effects.
The PhD student conducting the research will benefit from a proposed international research component including beam time at the Swiss Federal Institute of Technology in Zurich. Undergraduate students will conduct independent research on small-scale projects that build on the research of PhD student. Outreach activities will be organized through existing programs in which Medlin is active, including those promoted by the Renewable Energy Materials Research Science and Engineering Center, the Colorado Center for Biorefining and Biofuels, and the Renewable and Sustainable Energy Institute.
