Department of Chemistry and Catalysis Research Center, Technische Universität München, Garching 85747, Germany.
School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu 225009, China.
J Am Chem Soc. 2021 Dec 8;143(48):20133-20143. doi: 10.1021/jacs.1c07203. Epub 2021 Nov 23.
Tungstate domains supported on ZrO, AlO, TiO, and activated carbon drastically influence the hydronium-ion-catalyzed aqueous-phase dehydration of alcohols. For all catalysts, the rate of cyclohexanol dehydration normalized to the concentration of Brønsted acid sites (turnover frequencies, TOFs) was lower for monotungstates than for polytungstates and larger crystallites of WO. TOFs were constant when reaching or exceeding the monolayer coverage of tungstate, irrespective of the specific nature of surface structures that continuously evolve with the surface W loading. However, the TOFs with polytungstates and large WO crystallites depend strongly on the underlying support (e.g., WO/C catalysts are 10-50-fold more active than WO/AlO catalysts). The electrical double layer (EDL) surrounding the negatively charged WO domains contains hydrated hydronium ions, whose local concentrations change with the support. This varying concentration of interfacial hydronium ions ("local ionic strength") impacts the excess chemical potential of the reacting alcohols and induces the marked differences in the TOFs. Primary H/D kinetic isotope effects (∼3), together with the substantially positive entropy of activation (111-195 J mol K), indicate that C-H(D) bond cleavage is involved in the kinetically relevant step of an E1-type mechanistic sequence, regardless of the support identity. The remarkable support dependence of the catalytic activity observed here for the aqueous-phase dehydration of cycloalkanols likely applies to a broad set of hydronium-ion-catalyzed organic reactions sensitive to ionic strength.
钨酸盐在 ZrO、AlO、TiO 和活性炭上的存在极大地影响了水合氢离子催化的醇类在水溶液中的脱水反应。对于所有催化剂，归一化到 Brønsted 酸位浓度的环己醇脱水速率（ turnover frequencies，TOFs）均低于单钨酸盐和 WO 的较大晶粒。当达到或超过钨酸盐的单层覆盖时，TOFs 保持不变，而不管表面结构的具体性质如何，这些表面结构会随着表面 W 负载的增加而不断演变。然而，多钨酸盐和大 WO 晶粒的 TOFs强烈依赖于基础支撑物（例如，WO/C 催化剂比 WO/AlO 催化剂的活性高 10-50 倍）。带负电荷的 WO 域周围的双电层（EDL）包含水合的水合氢离子，其局部浓度随支撑物而变化。界面上水合氢离子的这种变化浓度（“局部离子强度”）会影响反应醇的超额化学势，并导致 TOFs 产生明显差异。主要的 H/D 动力学同位素效应（∼3），以及显著的正活化熵（111-195 J mol K），表明 C-H(D)键的断裂涉及到 E1 型反应机制序列中动力学相关的步骤，而与支撑物的性质无关。这里观察到的环己醇水溶液脱水反应的催化活性对支撑物的显著依赖性可能适用于广泛的对离子强度敏感的水合氢离子催化的有机反应。
