Collaborative Research: Understanding and controlling solvent effects in TiO2-catalyzed aldol condensation

Converting renewable or waste materials to useful products is important for energy security and U.S. manufacturing. Upgrading plant-based materials and plastic wastes often requires converting small molecules into larger molecules. The larger molecules can be used in various products, including aviation fuel. The required reactions use catalysts to speed up product formation and suppress byproduct formation. Unfortunately, many catalysts do not perform well. Their short lifespan also limits their practical use. Recent studies show that dissolving small-molecule reactants in solvents improves catalyst performance. However, the reasons for the improvements are not well understood. This project will combine laboratory experiments and computer modeling to determine how solvent properties influence key reactions. It will establish design principles for use of solvents or other chemicals to improve catalyst efficiency. The outcomes will enable more efficient production of chemical products from domestic sources. The research will be carried out by a team of researchers who will learn to communicate and work with a cross-disciplinary team. Students at graduate, undergraduate, and high school levels will be trained in research skills. Solvents are widely known to affect rates and selectivity in heterogeneous catalysis. Changes in solvation environment can increase the rate of aldol condensation from lighter carbonyl compounds. This also decreases condensation rates for heavier molecules, leading to improved resistance to deactivation from carbonaceous deposits. The project will develop a framework for understanding these changes in relative reaction rates (i.e., selectivities) for aldol condensation on TiO2 catalysts. Experimental kinetic studies will benchmark computational models, which in turn will suggest new solvent combinations to further improve selectivity. Solvation effects will be probed using vapor phase condensation within catalyst mesopores. Controlling the extent of pore condensation will enable kinetics measurements in the presence and absence of a solvating environment. This methodology facilitates comparisons between experiment and theory by providing experimental information on how the addition of a solvent environment perturbs surface chemistry at the gas-catalyst interface. In the first phase, researchers will study acetaldehyde self-aldol condensation and develop models for a relatively simple reaction that is strongly impacted by solvents. These models will be expanded to include mixed reactant systems, including both acetaldehyde/acetone and acetaldehyde/ethanol systems. Mixed systems have been shown to exhibit complex reaction kinetics due to changes in surface intermediate populations. These studies are therefore designed to determine the sensitivity of solvent effects to surface intermediate concentrations. Finally, a similar approach will be used to investigate how catalyst materials can be designed to exhibit transfer of the promotional solvent effects to the catalyst surface. The material modifications will be achieved by depositing ligands with different chemical functionalities within the TiO2 mesopores. Cumulatively, the goal of this research is to better understand the role of solvent functional groups — and the impacts of tethering those groups within catalyst pores — in directing the reactivity of oxygenates. Because solvent effects are ubiquitous in catalysis, the proposed research will help develop methods to effectively model them and, in turn, predict the effects of solvent design. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria. NSF Award ID: 2552855 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Will Medlin | Institution: University of Colorado at Boulder, Boulder, CO | Award Amount: $411,000 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2552855 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2552855.html