Mechanism of Phenol Alkylation in Zeolite H-BEA Using In Situ Solid-State NMR Spectroscopy,Journal of the American Chemical Society

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Mechanism of Phenol Alkylation in Zeolite H-BEA Using In Situ Solid-State NMR Spectroscopy
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2017-06-27 , DOI: 10.1021/jacs.7b02153
Zhenchao Zhao ₁ , Hui Shi ₁ , Chuan Wan ₁ , Mary Y. Hu ₁ , Yuanshuai Liu ₂ , Donghai Mei ₁ , Donald M. Camaioni ₁ , Jian Zhi Hu ₁ , Johannes A. Lercher _{1,

2}

Affiliation

The reaction mechanism of solid-acid-catalyzed phenol alkylation with cyclohexanol and cyclohexene in the apolar solvent decalin has been studied using in situ 13C MAS NMR spectroscopy. Phenol alkylation with cyclohexanol sets in only after a majority of cyclohexanol is dehydrated to cyclohexene. As phenol and cyclohexanol show similar adsorption strength, this strict reaction sequence is not caused by the limited access of phenol to cyclohexanol, but is due to the absence of a reactive electrophile as long as a significant fraction of cyclohexanol is present. 13C isotope labeling demonstrates that the reactive electrophile, the cyclohexyl carbenium ion, is directly formed in a protonation step when cyclohexene is the coreactant. In the presence of cyclohexanol, its protonated dimers at Brønsted acid sites hinder the adsorption of cyclohexene and the formation of a carbenium ion. Thus, it is demonstrated that protonated cyclohexanol dimers dehydrate without the formation of a carbenium ion, which would otherwise have contributed to the alkylation in the kinetically relevant step. Isotope scrambling shows that intramolecular rearrangement of cyclohexyl phenyl ether does not significantly contribute to alkylation at the aromatic ring.

中文翻译：

使用原位固态核磁共振光谱法研究沸石 H-BEA 中苯酚烷基化的机理

使用原位 13C MAS NMR 光谱研究了固体酸催化苯酚与环己醇和环己烯在非极性溶剂十氢化萘中的烷基化反应机理。只有在大部分环己醇脱水成环己烯后，苯酚与环己醇的烷基化才会开始。由于苯酚和环己醇显示出相似的吸附强度，因此这种严格的反应顺序并不是由于苯酚与环己醇的接触受限，而是由于存在反应性亲电试剂，只要存在大量环己醇。13C 同位素标记表明，当环己烯是共反应物时，反应性亲电子试剂环己基碳正离子在质子化步骤中直接形成。在环己醇存在下，其在 Brønsted 酸位点的质子化二聚体阻碍了环己烯的吸附和碳正离子的形成。因此，证明质子化的环己醇二聚体在不形成碳鎓离子的情况下脱水，否则碳鎓离子将有助于动力学相关步骤中的烷基化。同位素加扰表明环己基苯基醚的分子内重排对芳环的烷基化没有显着贡献。

更新日期：2017-06-27

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