Why Does the Catalyst Need to Be Wet? Active Sites, Activities, and Non-Innocent Solvents
Solvating molecules and the environment that surround catalytic sites provide numerous interactions that dramatically affect catalysis and change rates and selectivities by orders of magnitude. Although the importance of solvent effects on organic chemistry is established and provides widely used “rules of thumb”, the interactions among solvent molecules and reactive species remain challenging to quantify and describe molecularly. These challenges are compounded when solvents not only restructure about intermediates at active sites but also sense the presence and functionality of solid-liquid interfaces that extend beyond active sites. Understanding and exploiting these phenomena requires a conceptual framework, rooted in principles familiar to chemical engineers, and informed by experimental methods that probe the reactivity of intermediates, the structure of the catalyst, and changes in the solvent at the reactive interface during catalysis (i.e., in situ).
This seminar will illustrate these challenges by revealing the complexities of catalytic systems relevant for the synthesis of environmentally benign oxidants and their use in creating valuable chemical building blocks from hydrocarbons. The direct synthesis of H2O2 (H2 + O2 → H2O2) proceeds on Pd catalysts that evolve both in response to the composition of the reactants and the solvent. Here, solvent molecules can participate as co-reactants (e.g., redox mediators) but also as co-catalysts in processes that involve proton-electron transfer steps.
Once formed, H2O2 and other peroxides are co-reactants for the catalytic epoxidation of alkenes, which proceed at early transition metal active sites stabilized within the framework of zeolites such as used in the Hydrogen Peroxide-Propylene Oxide process developed by Dow-BASF. These epoxidation reactions depend on the dimensions and polarity of surrounding pores, because these surfaces change the structure of the solvation shells that form about reactive species and evolve along with the reactants. Comparisons of the results from kinetic, thermodynamic, spectroscopic, and synthetic experimentation provide insight to the coupled molecular interactions between catalyst surfaces, solvating molecules, and reactive species from which we can extract principles to design solid-liquid interfaces for catalysis. More broadly speaking, the individually weak but collectively significant interactions among critical transition states, solvent molecules, and the extended surface of solid catalysts presents new opportunities for catalyst design and reaction engineering particularly in microporous materials.