H2O2 is a green oxidant, which can epoxidize olefins over site-isolated group IV and V metal oxides. Isolated metal atoms stabilized in zeolite frameworks (e.g., *BEA) possess higher reactivity and selectivity on a per-atom basis than their bulk-oxide counterparts, yet possess rates that vary by several orders of magnitude depending on metal identity. Reasons for the differences in rates and selectivities among these materials and the mechanisms for epoxidation remain unclear. Understanding the mechanism of epoxidation over substituted zeolites is needed to elucidate the reasons for the large differences in rates of selectivities. Turnover rates for cyclohexene oxide (C6H10O) formation as a function of cyclohexene (C6H10) and H2O2 concentrations on M-β (M = Nb, Ta, Ti, Zr, and Hf) contain two distinct regimes. First, rates do not depend on [C6H10], are proportional to [H2O2], and are inversely dependent on [C6H10O] at high [C6H10]/[H2O2], which suggests a C6H10O MARI. Second, rates are proportional to [C6H10] and independent of [H2O2] at low [C6H10]/[H2O2], which corresponds to an H2O2-derived MARI. UV-vis spectra obtained in situ during reaction of H2O2-activated Nb-β with ethylene reveal features assigned to Nb-(O2)- and Nb-OOH. Kinetic analysis of these features show that the rate constant for consumption of Nb-OOH is 30-times smaller than that of Nb-(O2)-, which suggests the latter is the active oxidant. These data are consistent with the irreversible activation of H2O2 over M-β to form M-(O2)-, which then reacts with C6H10 via an Eley-Rideal mechanism. Activation enthalpies for C6H10O formation (ΔH‡E) and H2O2 decomposition (ΔH‡D) are measured on similar M-(O2)- saturated surfaces of all M-β catalysts, as shown by functional dependence of epoxidation rates on [C6H10] and [H2O2]. Neither set of ΔH‡ values correlate with the metal electronegativity or ν(C≡N) of Lewis-acid bound CD3CN; however, ΔH‡E and ΔH‡D correlate linearly with the heat of adsorption of CD3CN. ΔH‡E has a stronger dependence on Lewis acidity than ΔH‡D, which suggests metals that better stabilize the electrons in the M-(O2)- give greater rates and selectivities. Moreover, this correlation shows that a direct measurement of Lewis acidity is necessary for comparisons of ΔH‡E and ΔH‡D between different M-β catalysts, and demonstrates that increasing Lewis acidity enhances both the reactivity and selectivity towards epoxidation with H2O2.

Kinetic and spectroscopic evidence for periodic trends in olefin epoxidation over group IV and V *BEA

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.