Theoretical Study of Ethylene Hydroformylation on Atomically Dispersed Rh/Al2O3 Catalysts: Reaction Mechanism and Influence of the ReOx Promoter
Theoretical Study of Ethylene Hydroformylation on Atomically Dispersed Rh/Al2O3 Catalysts: Reaction Mechanism and Influence of the ReOx Promoter
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DOI：
10.1021/acscatal.1c00705
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发表时间：
2021-08
期刊：
影响因子：
12.9
通讯作者：
Seungyeon Lee;Abhirup Patra;P. Christopher;D. Vlachos;S. Caratzoulas
中科院分区：
文献类型：
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作者：
Seungyeon Lee;Abhirup Patra;P. Christopher;D. Vlachos;S. Caratzoulas
与金属簇相比，载体上原子分散的后过渡金属催化剂表现出出乎意料的高活性和选择性，这归因于由金属-载体界面决定的独特电子特性以及改变单原子位点局部环境的促进剂的存在。通过协同作用和协同作用，这些结构为催化剂的发现提供了肥沃的土壤。在原子尺度上了解这些材料以及如何调整它们的电子特性将是设计用于选择性化学的新型催化剂的关键。在这里，我们使用密度泛函理论计算和第一原理微动力学模型来揭示有关原子分散的 Rh-ReOxpairs 在 γ-氧化铝载体上乙烯加氢甲酰化为丙醛的协同作用的广泛机械知识。通过考虑多种可能的催化剂前配合物，我们确认最稳定的一种是 Rh 宝石二羰基物质 Rh(CO)2，与均质威尔金森配合物相反，它通过与两个氧化铝表面氧原子配位而呈现 16 电子方形平面几何形状。我们发现随着 ReOxloading 的增加，Rh-CO 配位键减弱，证实了早期的实验工作。我们开发了两种竞争反应（乙烯加氢甲酰化和氢化）的机制，并表明它们再现了实验观察结果和趋势，例如反应动力学，最关键的是，在 ReOx 存在下加氢甲酰化选择性的增加。在这些机制中，催化剂通过 Rh(CO)2 的两个 CO 配体之一的解离而被激活，以允许乙烯和 H2 依次配位，并且我们提供了证据表明 Rh 上的 H2 解离不是氧化性的。我们确定了加氢甲酰化限速步骤，并表明它取决于Rh的局部环境：在没有ReOx的情况下，加氢甲酰化由酰化步骤控制，并且需要八面体配位的Rh，即在酰化之前重新结合CO配体；在存在 ReOx 的情况下，并且由于 Rh-CO 键的减弱，插入步骤之前必要的 CO 配位变得速率控制。我们断言，ReOx 通过阻止 Rh 配体的关键重排来控制向丙醛的反应，从而有利于竞争反应，即乙烯氢化。
Atomically dispersed late transition-metal catalysts on supports have demonstrated unexpectedly high activity and selectivity compared to metal clusters, attributed to the unique electronic properties determined by the metal–support interface and the presence of promoters that modify the local environment of the single-atom site. Through cooperativity and synergism, these structures provide a fertile ground for catalyst discovery. Understanding these materials at the atomic scale and how to tune their electronic properties will be key for designing novel catalysts for selective chemistries. Here, we use density functional theory calculations and first-principles microkinetic modeling to unveil extensive mechanistic knowledge about the cooperativity of atomically dispersed Rh–ReOxpairs on the γ-alumina support for the hydroformylation of ethylene to propanal. By considering a number of possible pre-catalyst complexes, we confirm that the most stable one is a Rh gem-dicarbonyl species, Rh(CO)2, which, contrary to the homogeneous Wilkinson complex, assumes a 16-electron square-planar geometry by coordinating to two alumina surface oxygen atoms. We find the weakening of the Rh–CO coordinative bonds with increasing ReOxloading, confirming an earlier experimental work. We develop mechanisms for two competing reactions, ethylene hydroformylation and hydrogenation, and show that they reproduce experimental observations and trends such as reaction kinetics and, most critically, the increase in hydroformylation selectivity in the presence of ReOx. In these mechanisms, the catalyst is activated by the dissociation of one of the two CO ligands of Rh(CO)2to allow ethylene and H2coordination, in that order, and we provide evidence that H2dissociation on Rh is not oxidative. We determine the hydroformylation rate-limiting step and show that it depends on the local environment of Rh: in the absence of ReOx, the hydroformylation is controlled by the acylation step and requires octahedrally coordinated Rh, namely, re-binding of a CO ligand prior to the acylation; in the presence of ReOx, and owing to the weakening of the Rh–CO bonds, the requisite CO coordination prior to the insertion step becomes rate-controlling. We assert that ReOxsteers the reaction toward propanal by impeding a critical rearrangement of the Rh ligands that favors the competing reaction, ethylene hydrogenation.