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.

中文翻译：

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在高度分散的4和5族金属氧化物催化剂上用过氧化氢对烯烃进行环氧化的限制结果

Ti，Nb和Ta原子取代进入沸石* BEA（M-BEA）或接枝到介孔二氧化硅（M-SiO ₂）上，不可逆地激活过氧化氢（H ₂ O ₂）形成金属氢过氧化物（M -OOH）和过氧化（M-（η ² -O ₂）），用于烯烃环氧化的物种。与Z-苯乙烯反应的产物分布，以及H ₂ O ₂活化材料和环己烯之间反应的时间分辨UV-vis光谱表明，M-OOH表面中间体在Ti基催化剂上环氧化烯烃，而M-（η ² -O ₂）部分使含Nb和Ta的材料上的底物环氧化。苯乙烯的动力学测量（C ₈ H ^ ₈）环氧化揭示了这些材料的第一吸附和然后不可逆地激活ħ ₂ ö ₂至互变的形式池M-OOH和M-（η ² -O ₂）的中间体，然后与苯乙烯反应或H ₂ O ₂分别形成氧化苯乙烯或H ₂ O ₂分解产物。活化焓（Δ ħ ^⧧）对于C ₈ H ^ ₈环氧化和H ₂ ö ₂随着吡啶或氘代乙腈与路易斯酸位点的吸附热增加，分解反应呈线性下降，这表明具有较高电子亲合力的材料（即较强的路易斯酸）对C ₈ H ₈环氧化活性更高。Δ的值ħ ^⧧对于C ₈ H ^ ₈环氧化和H ₂ ö ₂分解也与配位体-金属电荷转移线性减小（LMCT）带能量为活性中间体，这是对于要求更相关的度量这些催化循环中的活性位点。与H ₂ O相比，环氧化速率对LMCT谱带能的依赖性更大。_2个分解速率，这表明更亲电子M-OOH和M-（η ² -O ₂）种类（即，那些在更强的路易斯酸位点形成的），得到两个更大的速率和的环氧化反应更大的选择性。的热化学分析Δ ħ ^⧧对于C ₈ H ^ ₈环氧化和吸附焓对C ₈ H ^ ₈内* BEA和SiO的孔₂揭示，0.7纳米孔隙内M-BEA优先稳定过渡态对C ₈ H ^ ₈环氧化用相对于M-SiO ₂的5.4 nm孔，而H ₂ O ₂由于H ₂ O _2的斯托克斯直径小，分解不受这些孔径之间的差异的影响。因此，M-BEA和M-SiO ₂材料之间的反应性和选择性的差异仅归因于过渡态的限制，而不是反应性中间体的身份，烯烃环氧化的机理或内在活化障​​碍的差异。因此，烯烃环氧化的速率和选择性至少反映了两个正交的催化剂设计标准：过渡金属原子的电负性（决定活性络合物的电子结构）和周围孔的平均直径（可以选择性地稳定特定化合物的过渡态）反应途径。