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Impact of the Local Concentration of Hydronium Ions at Tungstate Surfaces for Acid-Catalyzed Alcohol Dehydration
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2021-11-23 , DOI: 10.1021/jacs.1c07203 Niklas Pfriem 1 , Yue Liu 1 , Florian Zahn 1 , Hui Shi 2 , Gary L Haller 1, 3 , Johannes A Lercher 1, 4
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2021-11-23 , DOI: 10.1021/jacs.1c07203 Niklas Pfriem 1 , Yue Liu 1 , Florian Zahn 1 , Hui Shi 2 , Gary L Haller 1, 3 , Johannes A Lercher 1, 4
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Tungstate domains supported on ZrO2, Al2O3, TiO2, 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 WO3. 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 WO3 crystallites depend strongly on the underlying support (e.g., WOx/C catalysts are 10–50-fold more active than WOx/Al2O3 catalysts). The electrical double layer (EDL) surrounding the negatively charged WOx 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–1 K–1), 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.
中文翻译：
钨酸盐表面水合氢离子的局部浓度对酸催化酒精脱水的影响
负载在 ZrO 2、Al 2 O 3、TiO 2和活性炭上的钨酸盐畴极大地影响了水合氢离子催化的醇水相脱水。对于所有催化剂，单钨酸盐的环己醇脱水速率归一化为布朗斯台德酸位点的浓度（周转频率，TOF）低于多钨酸盐和较大的 WO 3微晶。当达到或超过钨酸盐的单层覆盖率时，TOF 是恒定的，与随着表面 W 负载不断演变的表面结构的特定性质无关。然而，具有多钨酸盐和大 WO 3的 TOF微晶强烈依赖于底层载体（例如，WO x /C 催化剂的活性是 WO x /Al 2 O 3催化剂的 10-50 倍）。围绕带负电的 WO x域的双电层 (EDL) 包含水合水合氢离子，其局部浓度随载体而变化。这种不同浓度的界面水合氢离子（“局部离子强度”）会影响反应醇的过量化学势，并导致 TOF 的显着差异。初级 H/D 动力学同位素效应 (∼3)，连同活化的基本正熵 (111–195 J mol –1 K –1），表明C-H（D）键断裂涉及E1型机械序列的动力学相关步骤，无论支持身份如何。此处观察到的催化活性对环烷醇水相脱水的显着支持依赖性可能适用于对离子强度敏感的广泛的水合氢离子催化有机反应。
更新日期：2021-12-08
中文翻译：
钨酸盐表面水合氢离子的局部浓度对酸催化酒精脱水的影响
负载在 ZrO 2、Al 2 O 3、TiO 2和活性炭上的钨酸盐畴极大地影响了水合氢离子催化的醇水相脱水。对于所有催化剂，单钨酸盐的环己醇脱水速率归一化为布朗斯台德酸位点的浓度（周转频率，TOF）低于多钨酸盐和较大的 WO 3微晶。当达到或超过钨酸盐的单层覆盖率时，TOF 是恒定的，与随着表面 W 负载不断演变的表面结构的特定性质无关。然而，具有多钨酸盐和大 WO 3的 TOF微晶强烈依赖于底层载体（例如，WO x /C 催化剂的活性是 WO x /Al 2 O 3催化剂的 10-50 倍）。围绕带负电的 WO x域的双电层 (EDL) 包含水合水合氢离子，其局部浓度随载体而变化。这种不同浓度的界面水合氢离子（“局部离子强度”）会影响反应醇的过量化学势，并导致 TOF 的显着差异。初级 H/D 动力学同位素效应 (∼3)，连同活化的基本正熵 (111–195 J mol –1 K –1），表明C-H（D）键断裂涉及E1型机械序列的动力学相关步骤，无论支持身份如何。此处观察到的催化活性对环烷醇水相脱水的显着支持依赖性可能适用于对离子强度敏感的广泛的水合氢离子催化有机反应。
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