Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts

Ti, Nb, and Ta atoms substituted into the framework of zeolite∗BEA (M-BEA) or grafted onto mesoporous silica (M-SiO₂) irreversibly activate hydrogen peroxide (H₂O₂) to form pools of metal-hydroperoxide (M-OOH) and peroxide (M-(η²-O₂)) species for alkene epoxidation. The product distributions from reactions with Z-stilbene, in combination with time-resolved UV-vis spectra of the reaction between H₂O₂-activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η²-O₂) moieties epoxidize substrates on the Nb- and Ta-containing materials. Kinetic measurements of styrene (C₈H₈) epoxidation reveal that these materials first adsorb and then irreversibly activate H₂O₂ to form pools of interconverting M-OOH and M-(η²-O₂) intermediates, which then react with styrene or H₂O₂ to form either styrene oxide or H₂O₂ decomposition products, respectively. Activation enthalpies (ΔH^‡) for C₈H₈ epoxidation and H₂O₂ decomposition decrease linearly with increasing heats of adsorption for pyridine or deuterated acetonitrile coordinated to Lewis acid sites, which suggests that materials with greater electron affinities (i.e., stronger Lewis acids) are more active for C₈H₈ epoxidation. Values of ΔH^‡ for C₈H₈ epoxidation and H₂O₂ decomposition also decrease linearly with the ligand-to-metal charge-transfer (LMCT) band energies for the reactive intermediates, which is a more relevant measure of the requirements for the active sites in these catalytic cycles. Epoxidation rates depend more strongly on the LMCT band energy than H₂O₂ decomposition rates, which shows that more electrophilic M-OOH and M-(η²-O₂) species (i.e., those formed at stronger Lewis acid sites) give both greater rates and greater selectivities for epoxidations. Thermochemical analysis of ΔH^‡ for C₈H₈ epoxidation and adsorption enthalpies for C₈H₈ within the pores of∗BEA and SiO₂ reveal that the 0.7 nm pores within M-BEA preferentially stabilize transition states for C₈H₈ epoxidation with respect to the 5.4 nm pores of M-SiO₂, while H₂O₂ decomposition is unaffected by the differences between these pore diameters due to the small Stokes diameter of H₂O₂. Thus, the differences in reactivity and selectivity between M-BEA and M-SiO₂ materials is solely attributed to confinement of the transition state and not differences in the identity of the reactive intermediates, mechanism for alkene epoxidation, or intrinsic activation barriers. Consequently, the rates and selectivities for alkene epoxidation reflect at least two orthogonal catalyst design criteria - the electronegativities of the transition metal atoms that determine the electronic structure of the active complex and the mean diameters of the surrounding pores that can selectively stabilize transition states for specific reaction pathways.