Research in the Estes Laboratory
Heterogeneous catalysis has been around for well over a century and is used in an overwhelming majority of all processes in the chemical industry. However, despite their importance, the molecular processes and structures that take place on their surfaces are still largely unknown. This is due largely to 1) the difficulty of studying heterogeneous catalytic systems (e.g. spectroscopically or microscopically) and 2) the overwhelming complexity of their surface chemistry. It is therefore no surprise that, in spite of recent progress, heterogeneous catalysis is still largely an empirical science.
Homogeneous catalysis, on the other hand, uses well-defined structures to catalyze reactions with high selectivity. The understanding of the relationships between structure and activity in homogeneous catalysis allows researchers to taylor catalysts for specific applications, not empirically, but in a rational way. In order to design better heterogeneous catalysts, it is necessary to understand the relationship between structure and activity.
In the Estes Group at the University of Stuttgart, we bridge the gap between homogeneous and heterogeneous catalysis by synthesizing well-defined active sites on the surfaces of metal oxides in a controlled way using the techniques of Surface Organometallic Chemistry and Molecular Heterogeneous Catalysis. The species so synthesized are molecularly uniform and can be characterized using the tools of homogeneous catalysis (IR, Solid State NMR, X-ray Spectroscopy). The uniformity allows us to establish structure-activity relationships for environmentally sustainable catalytic reactions that will be important for the future of our society.
The Role of Hydrogen Spillover in Cu Catalysts for CO2 Hydrogenation to Methanol
The unmitigated carbon emissions caused by burning fossil resources has dire environmental consequences and represents a breakdown of the natural carbon cycle, in which equal amounts of carbon dioxide are emitted and taken up. An elegant solution to this problem is to make fuels (MeOH) directly from CO2 and renewable energy, as is done by nature. If the CO2 comes from renewable sources, such as biomass or directly out of the atmosphere, the fuels so derived would be overall nearly carbon-neutral.
Hydrogenation of CO2 selectively to methanol could offer a solution to this problem, provided that the H2 is obtained renewably. Common catalysts for this reaction are mixtures of Cu metal nanoparticles dispersed on reducible metal oxide supports, such as ZnO and ZrO2. The metal oxide is critical for the reactivity and is thought to interact with the Cu through a so-called Strong Metal Support Interaction (SMSI). We hope to shed light on the nature of the SMSI between Cu and reducible metal oxides using well-defined model species synthesized on the surfaces of ZnO and ZrO2 by Surface Organometallic Chemistry.
Another Strategy to understand the interaction between SMSI between metal oxides and metal hydrides is to immobilize transition metal hydrides onto active metal oxides. The Lewis acid centers have synergistic activity with the hydride complexes, allowing them to hydrogenate CO2 to methanol, which is otherwise not possible.
Metal Oxides as Catalysts for Organic Radical Reactions
Organic radical reactions, such as radical cyclizations and radical defunctionalizations, are versatile reactions that have been widely used in organic synthesis. Radical organic reactions in the laboratory are typically carried out by reacting an R-X bond (X = halide, sulfide, selenide, thionoester) with a stoichiometric amount of alkyl tin reagent, such as Bu3SnH. Due to their potent neurotoxicity (LD50 = 44 – 234 mg kg-1, TRGS-900 workplace exposure limit 0.0018 ppm), difficult separation from most organic products, and their poor atom economy, radical reactions are rarely used in industrial scale syntheses of complex molecules (with the exception of radical polymerizations, which do not rely on tin reagents).
One possible solution to this problem is to use replace alkyl tin reagents with other chain-carrying radical precursors. One way this could be done is to replace the tin reagent with a catalytic amount of metal oxide that reacts with H2. This generates reactive species on the surface of the catalyst that then can initiate and mediate radical cyclizations. This has the advantage that the only stoichiometric reagent is hydrogen gas and that the reactions can be performed in flow. Our group is currently developing catalytic systems based on many types of metal oxide catalysts in order to selectively perform both radical defunctionalizations and radical cyclizations. Such reactions could eliminate the need for tin reagents and allow the use of these reactions industrially.