(582y) Group IV and V Periodic Trends in Olefin Epoxidation: Effects of Electronic Structure and Local Environment
Turnover rates for styrene (C8H8O) oxide formation through primary reaction pathways as a function of styrene (C8H8) and H2O2 concentrations over M-BEA and M-SiO2 (M = Ti, Nb, or Ta) reveal two distinct regimes, which correspond to a change in the most abundant reactive intermediate (MARI). The observed dependencies are consistent with a mechanism that describes the irreversible activation of H2O2 to form a pool of M-(O2)* intermediates, which then react with C8H8 or H2O2 via an Eley-Rideal mechanism to form C8H8O or H2O2 decomposition products, respectively. Time-resolved in situ UV-vis measurements acquired during reaction of cyclohexene with H2O2-activated M-BEA and M-SiO2, in conjunction with probe reactions with cis-stilbene, support the trend that the reactive intermediate on Ti-based catalysts is the hydroperoxide (i.e., M-OOH), while Nb- and Ta-materials react through a superoxide (i.e., M-(O2)-) intermediate.
Understanding of the fundamental properties of the catalysts and reactive intermediates that are responsible for the differences in rates for epoxidation require comparisons of these catalysts made under similar surface coverages (i.e., MARI). Activation enthalpies for C8H8O formation (ÎHâ¡E) are measured on similar M-(O2)* saturated surfaces on all catalysts (shown by rates that are proportional to [C8H8] and independent of [H2O2]), and correlate linearly with both the measured adsorption enthalpies of Lewis-acid bound pyridine (ÎHPy, determined via vanât Hoff analysis of FTIR spectra of pyridine coordinated to the Ti, Nb, and Ta atoms within the BEA framework or grafted onto SiO2) and also to the LMCT band energy of the reactive intermediates on these materials. These comparisons reveal two critical differences among these materials. First, ÎHâ¡E and ÎHPy values for M-SiO2 materials are systematically greater by 19 Â± 2 and 11 Â± 2 kJ mol-1 than M-BEA materials, respectively, which is consistent with the difference in bulk-averaged adsorption enthalpies for C8H8 and pyridine in the Si-BEA framework and in mesoporous SiO2 (i.e., measured in the absence of metal atoms). This difference is attributed to the microporous environment of *BEA solvating the transition state (relative to the initial state) to a larger extent than SiO2, which results in the systematic decrease of apparent activation and adsorption enthalpies. Second, the linear relationship between ÎHâ¡E and the LMCT band energy of the active intermediate possesses the same systematic offset in ÎHâ¡E between *BEA and SiO2 that matches the differences in adsorption enthalpies of C8H8 (see above). Interestingly, the M-SiO2 catalysts possess systematically lower energy LMCT bands, which may be attributed to one of two phenomena, or both. First, the cleavage of a M-OSi bond in the zeolite framework (upon H2O2 activation) leaves a vicinal SiOH group in a tetrahedral position that may contribute a small amount of electron density towards the metal atom, which, in turn, decreases the Lewis acid strength of the metal atom. Second, the adjacent SiOH group in *BEA (after H2O2 activation) likely distorts the geometry of the M-(O2)* intermediates, which will lower the extent of orbital overlap, lengthening the M-O bonds, and thus increase the LMCT band energy.
The overarching trend relating ÎHâ¡E and the LMCT band energy suggests that metal centers that are highly electron withdrawing (i.e., possess more highly polarized M-(O2)* bonds) result in electrophilic intermediates. To develop an intuition for the electrophilicity of the reactive intermediates, turnover rates for the epoxidation of p-substituted styrene (X-C8H8, where X = OMe, Me, H, Br, and NO2) were measured under identical reaction conditions (3 mM X-C8H8, 0.01 M H2O2, 313 K). The corresponding Hammett plot reveals a reaction constant (Ï) of -0.9 Â± 0.05 for all M-Î² and M-SiO2, which strongly suggests that the reactive intermediates implicated in olefin epoxidation have a strong preference to react with electron-rich olefins. Collectively, these data show that the rational design of increasingly active and selective epoxidation catalysts should seek to increase the Lewis acid strength of the metal center, thus increasing the electrophilicity and reactivity of the active intermediates, and leverage the decrease in apparent activation barriers that are attributed to the stabilization of the transition states in microporous environments.
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