Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
J Am Chem Soc. 2017 May 24;139(20):6888-6898. doi: 10.1021/jacs.7b01422. Epub 2017 May 9.
Group IV and V framework-substituted zeolites have been used for olefin epoxidation reactions for decades, yet the underlying properties that determine the selectivities and turnover rates of these catalysts have not yet been elucidated. Here, a combination of kinetic, thermodynamic, and in situ spectroscopic measurements show that when group IV (i.e., Ti, Zr, and Hf) or V (i.e., Nb and Ta) transition metals are substituted into zeolite *BEA, the metals that form stronger Lewis acids give greater selectivities and rates for the desired epoxidation pathway and present smaller enthalpic barriers for both epoxidation and HO decomposition reactions. In situ UV-vis spectroscopy shows that these group IV and V materials activate HO to form pools of hydroperoxide, peroxide, and superoxide intermediates. Time-resolved UV-vis measurements and the isomeric distributions of Z-stilbene epoxidation products demonstrate that the active species for epoxidations on group IV and V transition metals are only M-OOH/-(O) and M-(O) species, respectively. Mechanistic interpretations of kinetic data suggest that these group IV and V materials catalyze cyclohexene epoxidation and HO decomposition through largely identical Eley-Rideal mechanisms that involve the irreversible activation of coordinated HO followed by reaction with an olefin or HO. Epoxidation rates and selectivities vary over five- and two-orders of magnitude, respectively, among these catalysts and depend exponentially on the energy for ligand-to-metal charge transfer (LMCT) and the functional Lewis acid strength of the metal centers. Together, these observations show that more electrophilic active-oxygen species (i.e., lower-energy LMCT) are more reactive and selective for epoxidations of electron-rich olefins and explain why Ti-based catalysts have been identified as the most active among early transition metals for these reactions. Further, HO decomposition (the undesirable reaction pathway) possesses a weaker dependence on Lewis acidity than epoxidation, which suggests that the design of catalysts with increased Lewis acid strength will simultaneously increase the reactivity and selectivity of olefin epoxidation.
IV 族和 V 族骨架取代沸石已被用于烯烃环氧化反应数十年，但决定这些催化剂选择性和周转率的基本性质尚未阐明。在这里，动力学、热力学和原位光谱测量的结合表明，当 IV 族（即 Ti、Zr 和 Hf）或 V 族（即 Nb 和 Ta）过渡金属取代沸石 *BEA 时，形成更强路易斯酸的金属赋予所需环氧化途径更高的选择性和速率，并为环氧化和 HO 分解反应呈现更小的焓垒。原位紫外-可见光谱表明，这些 IV 族和 V 族材料将 HO 激活为过氧化物、过氧化物和超氧化物中间体的池。时间分辨紫外-可见测量和 Z-联苯乙烯环氧化产物的异构体分布表明，IV 族和 V 族过渡金属上的环氧化活性物质分别仅为 M-OOH/-(O)和 M-(O)物种。对动力学数据的机理解释表明，这些 IV 族和 V 族材料通过主要涉及配位 HO 的不可逆活化，随后与烯烃或 HO 反应的不可逆 Eley-Rideal 机制，催化环己烯环氧化和 HO 分解。这些催化剂的环氧化速率和选择性分别在五个和两个数量级上变化，并且分别与配体到金属电荷转移（LMCT）的能量和金属中心的功能路易斯酸度呈指数关系。总的来说，这些观察结果表明，更亲电的活性氧物种（即更低能量的 LMCT）对于富电子烯烃的环氧化更具反应性和选择性，并解释了为什么 Ti 基催化剂被确定为这些反应中最活跃的早期过渡金属催化剂。此外，HO 分解（不希望的反应途径）比环氧化对路易斯酸度的依赖性更小，这表明设计路易斯酸度增加的催化剂将同时增加烯烃环氧化的反应性和选择性。
