Catalytic reaction mechanism of Mn-doped nanoporous aluminophosphates for the aerobic oxidation of hydrocarbons.
Catalytic reaction mechanism of Mn-doped nanoporous aluminophosphates for the aerobic oxidation of hydrocarbons.
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DOI：
10.1002/chem.201001876
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发表时间：
2010-12
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通讯作者：
Luis Gómez-Hortigüela;F. Corà;G. Sankar;C. Zicovich-Wilson;C. Catlow
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文献类型：
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作者：
Luis Gómez-Hortigüela;F. Corà;G. Sankar;C. Zicovich-Wilson;C. Catlow
本文采用基于杂化交换密度泛函理论的电子结构计算方法，研究了掺锰纳米多孔磷酸铝(Mn-Alpos)催化碳氢化合物好氧氧化的反应机理。我们将我们的结果与现有的实验数据进行了比较。我们发现，催化氧化反应的效率与1)锰的氧化还原活性，特别是2+和3+氧化态之间的氧化还原活性有关，2)嵌入在AlPO骨架中的四面体锰的配位不饱和度，通过形成锰络合物来稳定氧型自由基，从而促进反应。我们的机理证明了Mn(III)和Mn(II)在反应机理中的重要作用：Mn(III)中心经历了一个初始反应循环，导致烷基过氧化氢中间体的产生，该中间体只能被Mn(II)转化为氧化产物(醇、醛和酸)。需要预活化步骤来产生能够分解过氧化氢中间体的还原的Mn(II)中心；这一步骤通过将碳氢化合物转化为相应的过氧衍生物而发生，通过与Mn(III)形成络合物来稳定，并同时产生还原的Mn(II)中心。这两个物种都进入了随后的繁殖周期，在这个周期中，Mn(II)催化过氧化氢的解离，直到通过两条平行的途径，通过烷氧基或羟基自由基样中间体形成氧化产物，而锰(III)-过氧基络合物使过氧化氢中间体的进一步产生成为可能。
In this work we apply state-of-the-art electronic-structure-based computational methods based on hybrid-exchange density functional theory to study the mechanism of the aerobic oxidation of hydrocarbons catalysed by Mn-doped nanoporous aluminophosphates (Mn-AlPOs). We compare our results with available experimental data. We show that the catalytic efficiency of Mn-AlPOs in oxidation reactions is intrinsically linked to 1) the Mn redox activity, in particular between 2+ and 3+ oxidation states, and 2) the coordinative insaturation of tetrahedral Mn embedded in AlPO frameworks, which facilitates the reaction by stabilising oxo-type radicals through the formation of Mn complexes. Our mechanism demonstrates the crucial role of both Mn(III) and Mn(II) in the reaction mechanism: Mn(III) sites undergo an initial reaction cycle that leads to the production of the alkyl hydroperoxide intermediate, which can only be transformed into the oxidative products (alcohol, aldehyde and acid) by Mn(II). A preactivation step is required to yield the reduced Mn(II) sites able to decompose the hydroperoxide intermediates; this step takes place through a transformation of the hydrocarbon into the corresponding peroxo-derivative, stabilised by forming a complex with Mn(III) and yielding at the same time reduced Mn(II) sites. Both species enter a subsequent propagation cycle in which Mn(II) catalyses the dissociation of the hydroperoxide that proceeds until the formation of the oxidative products by two parallel pathways, through alkoxy- or hydroxy-radical-like intermediates, whilst the Mn(III)-peroxo complex enables further production of the hydroperoxide intermediate.