(661e) Functional Descriptors, Active Intermediates, and the Influence of the Porous Environment for Epoxidations at Lewis Acidic Metal Atoms in Zeolite BEA

Authors: 
Flaherty, D., University of Illinois, Urbana-Champaign
Bregante, D. T., University of Illinois, Urbana-Champaign
The epoxidation of olefins is a critical reaction to produce building blocks for the production of polyurethanes and pharmaceuticals. Group IV and V metal containing zeolites have been used for olefin epoxidation reactions for decades, yet the underlying properties of these catalysts that determine the turnover rates and the selectivities with which they utilize the H2O2 as a terminal oxidants have not yet been shown. Isolated early transition metal atoms stabilized in zeolite frameworks (e.g., BEA) exhibit large difference in epoxidation that depend on metal identity and which have been postulated to depend on the energy of the lowest unoccupied molecular orbital of the metal center among based on computational comparisons of Ti-BEA and Zr-BEA [1]. Here we show that cyclohexene epoxidation rates and selectivities vary by a factor of 105 and 102, respectively, across a series of group IV and V substituted into BEA [2,3].

Turnover rates for cyclohexene oxide (C6H10O) production exhibit two distinct sets of dependencies on the concentrations of cyclohexene (C6H10), hydrogen peroxide (H2O2), and C6H10O on all M-BEA (M = Nb, Ta, Ti, Zr, and Hf) materials. When operating at high [C6H10] to [H2O2] ratios, epoxidation rates do not depend on [C6H10] but increase in proportion to [H2O2] and depend inversely on [C6H10O]. At low [C6H10] to [H2O2] ratios, epoxidation rates are proportional to [C6H10] but do not depend on [H2O2]. These observations reflect a reaction mechanism that proceeds via irreversible activation of H2O2 over M-β to form active H2O2-derived intermediates (M-(O2)), which then reacts with weakly coordinated C6H10 on sites that are either predominantly covered by C6H10O or M-O2 at high and low ratios of [C6H10] to [H2O2], respectively. In situ UV-vis spectroscopy, attenuated total reflectance infrared spectroscopy, and X-ray photoelectron spectroscopy show that these group IV and V materials activate H2O2 to form pools of hydroperoxide, peroxide, and superoxide intermediates. Time-resolved UV-vis spectra and the isomeric distributions of stilbene epoxidation products demonstrate, however, that the active species for epoxidations on group IV and V transition metals are only M-OOH/-(O2)2- and M-(O2)- species, respectively.

Epoxidation rates and selectivities vary by a factor of 105 and 102, respectively, at a given set of conditions and on similarly M-(O2) saturated surfaces of M-BEA catalysts. These differences reflect a linear correlation between the activation enthalpies (ΔH‡)for each of the reaction pathways (i.e., epoxidation and H2O2 decomposition) and both the energy for ligand-to-metal charge transfer (LMCT; i.e., from -O2- or –OOH to the transition metal center) and also the functional Lewis acid strength of the metal centers (quantified by the heat of adsorption of base titrants on these sites). These differences in electronic properties lead to differences in ΔH‡ for epoxidation of 50 kJ mol-1 and for H2O2 decomposition of 30 kJ mol-1 across the series of M-BEA materials with largely hydrophilic pores (produced by substituting M-atoms into 20% of the framework defects created by dealumination of BEA with Si/Al of 12). H2O2 decomposition (the undesirable reaction pathways) possesses a weaker dependence on Lewis acidity than epoxidation, which suggests that the design of catalysts with increased Lewis acidity will simultaneously increase the reactivity and selectivity of olefin. These results will be compared to similar trends obtained on an analogous series of M-BEA materials containing predominantly hydrophobic pores.

References:

1) Boronat, M.; Corma, A.; Renz, M.; Viruela, P. M.; Chem. Eur. J., 2006, 12, 7067-7077.

2) Bregante, D.T.; Priyadarshini, P.; Flaherty, D.W.; J. Catal., 2017, 348, 75-89.

3) Bregante, D.T.; Flaherty, D.W.; 2017, submitted.

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