2018 Spring Symposium
Daniel Bregante, Alayna Johnson, Ami Patel, David Flaherty, Department of Chemical and Biomolecular Engineering, University of Illinois, Urbana-Champaign
Abstract – Early transition metal atoms (groups IV-VI) dispersed on silica and substituted into zeolites effectively catalyze the epoxidation of alkenes with hydrogen peroxide or alkyl peroxide reactants, yet the underlying properties that determine the selectivities and turnover rates of these catalysts are unclear. Here, a combination of kinetic, thermodynamic, and in situ spectroscopic measurements show that when group IV – VI transition metals are dispersed on silica or substituted into zeolite *BEA, the metals that form the most electrophilic sites give greater selectivities and rates for the desired epoxidation pathway and present smaller enthalpic barriers for both epoxidation and H2O2 decomposition reactions.
In situ UV–vis spectroscopy shows that these group IV and V materials activate H2O2 to form pools of hydroperoxide and peroxide intermediates. Time-resolved UV–vis measurements and the isomeric distributions of cis-stilbene epoxidation products suggest that the active species for epoxidations on group IV and V transition metals are only M-OOH and M-(O2)2– species, respectively. Mechanistic interpretations of turnover rates show that these group IV and V materials catalyze epoxidations (e.g., of cyclohexene, styrene, and 1-octene) and H2O2 decomposition through similar mechanisms that involve the irreversible activation of coordinated H2O2 followed by reaction with an olefin or H2O2. Epoxidation rates and selectivities vary over five- and two-orders of magnitude, respectively, among these catalysts and depend exponentially on both the energy for ligand-to-metal charge transfer (LMCT) and chemical probes of the difference in Lewis acid strength between metal centers. Together, these observations show that more electrophilic active-oxygen species (i.e., lower-energy LMCT) are more reactive and selective for epoxidations of electron-rich olefins. The micropores of zeolites about active sites can serve to preferentially stabilize reactive states that lead to epoxidations by changing the mean diameter of the pore or the density of nearby silanol groups. Consequently, these properties provide opportunities to increase rates of epoxide formation over that within mesoporous silicas. Consistently, H2O2 decomposition rates possess a weaker dependence on the electrophilicity of the active sites and the surrounding pore environment, which indicates that catalysts with both greater rates and selectivities may be designed following these structure-function relationships.