The ability of copper-containing enzymes to activate molecular dioxygen in energy metabolism and in the selective oxidation of biological substrates far surpasses the control and efficiency achieved by experimental chemists. The binuclear copper enzyme tyrosinase activates molecular dioxygen to a μ-η2:η2-peroxide dicopper(II) species in the catalytic, regioselective hydroxylation of tyrosine to an o-catechol - the first biosynthetic step invoked in pigment formation and oxidative fruit browning. The intimate mechanistic details of this remarkable transformation have eluded biochemical investigation, largely due to the transient nature of intermediates formed in the reaction in aqueous conditions. Another pressing academic challenge and economic opportunity is the conversion of methane to methanol. Methanotropic bacteria selectively and efficiently perform this transformation under benign conditions by action of the enzyme pMMO. Systematic investigation by several research groups has shown an asymmetric binuclear copper site, in which one copper is ligated by two histidine imidazole groups and another is ligated by an N-terminal histidine, to be the probable active site. This work has shown transient absorption features upon exposure to O2. However the active-site oxidant remains poorly understood, as XAS and rRaman studies have proven unfeasible. The use of small-molecule active-site models in the bioinorganic chemistry of copper has yielded a wealth of information, previously unobtainable through enzymatic studies, concerning structure-reactivity relationships and the potential role of different oxidation states of copper in biosynthetic pathways.
The link between carbon dioxide emissions and climate change has resulted in extensive research toward post-combustion CO2 sequestration, as this is anticipated to be a primary component of the CO2 mitigation portfolio. The current state-of-the art technology for CO2 capture at scale is solution amine scrubbing, though widespread implementation is prohibitive due to the high energy cost associated with sorbent regeneration. Alternative solid sorbent technologies confer certain advantages over solution based methods, including reduced regeneration energies, minimization of the environmental hazards associated with corrosive amine solutions, and greater structural variability of the sorbent.
In collaboration with Professor Jennifer Wilcox, we are developing strategies for the synthesis and covalent modification of mesoporous silica- and carbon-based materials to achieve highly stable, selective, and recyclable post-combustion CO2 sorbents. Careful adjustment of the electronic and geometric structure of the active sorbents provides detailed understanding of the rates and thermodynamic properties associated with CO2 adsorption/desorption cycling at a fundamental chemical level.
A comprehensive picture of homogeneous transition-metal catalyzed reactions is often difficult to achieve due to an incomplete understanding of the speciation of the active catalyst and intermediates in the reaction process. In the solution phase, unrestricted interactions of catalyst molecules can lead to complexes of unknown composition through multimerization or transformation of the ligand scaffold. These processes obscure insight into the mechanism of catalysis and may also contribute to deactivation pathways.
Covalent immobilization of discrete molecular complexes to a bulk material, such as mesoporous silica (SBA-15), enables investigation of outstanding questions in homogeneous catalysis, most notably catalyst speciation and the effects of catalyst-catalyst interactions. Tuning the distribution of tethered catalyst molecules allows exploration of molecular species under the limiting cases of "site-isolated" and "site-dense", as well as intermediate cases. In the site-isolated case, catalyst molecules are spatially far apart; catalyst-catalyst interactions are disabled, and speciation is straightforward under such conditions. Conversely, in the site-dense case, catalyst molecules are spatially close, and catalyst-catalyst interactions can occur readily. Comparisons of reactivity under these limiting heterogenized conditions and homogeneous conditions can clarify salient mechanistic details and illuminate possible strategies for improvements to catalysis.