David W. Flaherty
Catalysis, surface science and materials synthesis
We urgently need to develop “greener” alternatives for existing chemical processes; alternatives that produce less waste, use benign solvents, and are more energy efficient. As part of this goal, our group works to discover and understand catalytic processes that convert renewable feed stocks (e.g., biomass) and recalcitrant, heavy oil, into liquid energy carriers and chemicals. We also identify opportunities to replace potentially harmful and polluting chemical oxidizers in industry with cleaner and safer alternatives. Our solutions must be simple and involve a minimum number of operations, because we lose energy and resources at every intermediate step. To achieve these goals we develop highly selective catalysts that suppress undesirable products and improve the overall energy- and atom-efficiency.
Projects in our group address targeted sets chemistry, which we are motivated to study by the need to produce fuels and chemicals for our society in the most efficient and environmentally responsible way possible. We draw chemical connections between systematic changes in the physical properties of catalysts and their chemical reactivity by synthesizing and studying highly uniform materials. Each project in our group aligns with one or several of the following research themes.
Utilizing “new” feed stocks
We are repurposing old chemical reactions (e.g., aldol addition) for new applications including catalytically coupling small oxygenates from biomass fermentation into larger species suitable for use as diesel fuel or lubricants. By doing so, we are reducing the amount of petroleum needed to form these products. Our current challenge is to reduce the size of the reactors needed for this upgrading process and to minimize the amount of carbon loss from non-productive reaction pathways (e.g., esterification and decarbonylation).
Hydrotreating is critical for removing sulfur and nitrogen contaminants from heavy, sour oils and for removing oxygen from biomass. Although these reactions are widely-used in industry, there are many questions about the mechanisms of this chemistry and how to improve the selectivity for removing the heteroatoms (S-, N-, and O-atoms) while maintaining the carbon number (Cn) of the reactants. Therefore, we are investigating fundamental aspects of the surface chemistry in hydrotreating to develop approaches to improve our ability to cleave C-S, C-N, and C-O bonds in large hydrocarbons without breaking C-C bonds.
Producing and using green oxidants
Chlorine-based oxidants form hazardous byproduct and threaten environmental health. Yet, these harmful species are still used in many industrial oxidations (e.g., pulp, paper, and textile bleaching, oxidation of alkenes to aldehydes and epoxides, alkane activation), because there are no viable alternatives. To solve this problem, we are exploring ways to produce the “green” oxidant hydrogen peroxide (H2O2), because H2O2 does not persist in the environment and only forms water as a byproduct.
We have identified new reaction pathways that allow us to form H2O2 from its elements (i.e., H2 + O2 → H2O2) in pure water on metal nanoclusters. We are using this mechanistic insight to design more selective catalysts by exploiting the fundamental differences between the elementary reaction steps that produce H2O2 and those that form H2O. Our approaches involve simple and scalable synthesis procedures by which we change the electronic properties of metal surfaces.
Design of Cooperative Catalysts
We could save incredible amounts of energy and resources by designing more effective catalysts. One method for doing this is to synthesize catalysts which cooperatively stabilize reactive intermediates and transition states through simultaneous interactions with multiple chemical functions. The ability to design such materials would allow us to develop enzyme-like reactivity and selectivity on inorganic surfaces and provide many exciting possibilities. Cooperative catalysis is an important research area, however, little is known about how these materials work and there are few appropriate characterization methods. We are creating, characterizing and testing cooperative catalysts in the form of bimetallic nanoclusters and mixed metal oxides. We control the structure of these materials to create surfaces that possess pairs of metallic, acidic, or basic chemical functions at sub-nanometer distances. We use microcalorimetry, vibrational spectroscopy, and other methods to probe how molecules interact with these cooperative sites and compare these data to reactivity tests to show how cooperativity influences catalysis.
Experimental and Conceptual Methodology
We apply a systematic approach to build our understanding of each of these problems. We use kinetic and spectroscopic methods to determine a functional model for the overall reaction. Our group has a unique set of experimental capabilities that allow us to study catalysis over a wide-range of conditions from high pressures (50-100 atmospheres) for hydrotreating, within the liquid-phase for H2O2 synthesis, and down to low pressures (10-13 atmospheres, ultra high vacuum) for investigations of atomically-designed model catalysts. When necessary we use transient kinetic, isotope exchange/labelling, and in situ spectroscopic methods to identify reactive intermediate, reaction pathways, and the relevance of specific steps in determining selectivities and rates. Methods recently developed in our group allow us to obtain incredibly high quality, time-resolved spectra of the reactive intermediates on catalyst surfaces. We use this information to reveal the chemical processes responsible that differentiate selective and useful catalysts from others. This information allows us to predict ways to change the composition, structure, or electronic state of a catalyst to increase selectivities and reaction rates.
The understanding of reaction mechanisms that we develop is critical for catalyst and process development. We cannot rationally design either without knowing why the rate of a reaction changes with temperature, species concentrations, or solvent identity. Likewise, we must understand how the elemental identity, oxidation state, and coordination of metals and metal oxides influence the chemistry they promote. If we do not understand why the state-of-the-art catalyst performs better than others we have no hope for making further improvements.