The Machan Research Group (Department of Chemistry at the University of Virginia) is interested in energy-relevant catalysis, particularly at the interface of molecular electrochemistry and materials. The development of efficient and selective transformations to produce commodity chemical precursors and fuels using CO2, O2, H2, and H2O as reagents remains an ongoing challenge for the storage of electrical energy within chemical bonds. Our approach is inspired by the numerous metalloproteins capable of catalyzing kinetically challenging reactions with significant energy barriers in an efficient manner under ambient conditions. This type of reactivity is achieved through the convergent evolution of active sites with tailored coordination environments and macromolecular structures which can, among other things, transport substrates and products to and from the active site. Our research focuses on developing new inorganic complexes and materials which incorporate co-catalytic moieties, non-covalent secondary sphere interactions, and substrate relays as catalysts. These efforts represent an opportunity to address the problems posed by diminishing petrochemical reserves, increasing atmospheric carbon dioxide concentrations, and the shift to renewable energy.
In order to characterize and optimize these systems, research in the Machan group uses synthetic inorganic chemistry, electrochemistry, and advanced characterization techniques (spectroelectrochemistry, stopped-flow IR and UV-vis spectroscopies). This enables us to develop an understanding of electronic structure and mechanism in transformations of interest. A brief summary of current projects is listed below.
Dioxygen as Chemical Oxidant for C–H Bond Activation
The development of catalytic aerobic C–H activations is of general interest for petrochemical and biomass functionalization. First-row transition metals which use O2 as a substrate generally require high catalyst loadings, sacrificial reducing agents, and have limited selectivity. Transition metal oxo species, which are known to activate C–H bonds, have not been isolated to the right of Fe on the periodic table, which is a point of mechanistic divergence when comparing the reactivity of Fe and Co compounds with O2. To exploit the implications of this electronic constraint between Fe and Co, rigid and redox non-innocent ligand platforms are employed to generate tetragonal complexes with reactive metal-oxygen species, obviating the need for high oxidation states at the metal center.
Molecular Electrocatalysts for Converting Bicarbonate to Formate in Water
The efficient and cost-effective catalytic reduction of CO2 using renewable energy remains a significant challenge for molecular species. The capture and purification of gas for such transformations also requires a significant energy input. One of the simplest methods for CO2 capture is the formation of HCO3– using hydroxide, a reaction which is facile in water. The direct reduction of HCO3– would consequently circumvent the need to isolate pure CO2. In nature, small molecule transformations in aqueous systems pay lower energy penalties by using peptides to facilitate the transfer of electrons and substrate to an active site. Using biology as inspiration, we address these challenges by employing water-soluble molecular Fe compounds containing amino acids or short peptides in the secondary coordination sphere as active and selective catalysts for the reduction of HCO3– to HCO2–.
Porous Electrocatalyst Materials
Metal-organic frameworks (MOFs) and covalently linked organic frameworks (COFs) continue to attract significant interest in materials chemistry. MOFs and COFs offer many advantages in terms of porosity and stability over more amorphous materials or zeolites. Indeed, the translation of molecular properties to bulk materials in this manner has implications for the development of electrochemically responsive films and membranes. We are focused on developing new methods for synthesizing and processing conducting and semi-conducting 2D MOF and COF materials sensitive to the chemical environment. This is primarily focused on applications in molecular detection, separation, and catalysis. A fundamental understanding of how molecular properties are translated in these systems will enable future studies focusing on other applications in energy storage and optoelectronic devices.