Theoretical Study of Ethylene Hydroformylation on Atomically Dispersed Rh/Al2O3 Catalysts: Reaction Mechanism and Influence of the ReOx Promoter

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–ReOx pairs 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 ReOx loading, 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)2 to allow ethylene and H2 coordination, in that order, and we provide evidence that H2 dissociation 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 ReOx steers the reaction toward propanal by impeding a critical rearrangement of the Rh ligands that favors the competing reaction, ethylene hydrogenation.