Group IV and V framework-substituted zeolites have been used for olefin epoxidation reactions for decades, yet the underlying properties that determine the selectivities and turnover rates of these catalysts have not yet been elucidated. Here, a combination of kinetic, thermodynamic, and in situ spectroscopic measurements show that when group IV (i.e., Ti, Zr, and Hf) or V (i.e., Nb and Ta) transition metals are substituted into zeolite∗BEA, the metals that form stronger Lewis acids 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, peroxide, and superoxide intermediates. Time-resolved UV-vis measurements and the isomeric distributions of Z-stilbene epoxidation products demonstrate that the active species for epoxidations on group IV and V transition metals are only M-OOH/-(O2)2- and M-(O2)- species, respectively. Mechanistic interpretations of kinetic data suggest that these group IV and V materials catalyze cyclohexene epoxidation and H2O2 decomposition through largely identical Eley-Rideal 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 the energy for ligand-to-metal charge transfer (LMCT) and the functional Lewis acid strength of the 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 and explain why Ti-based catalysts have been identified as the most active among early transition metals for these reactions. Further, H2O2 decomposition (the undesirable reaction pathway) possesses a weaker dependence on Lewis acidity than epoxidation, which suggests that the design of catalysts with increased Lewis acid strength will simultaneously increase the reactivity and selectivity of olefin epoxidation.
ASJC Scopus subject areas
- Colloid and Surface Chemistry