Suppressing Metal Leaching and Sintering in Hydroformylation Reaction by Modulating the Coordination of Rh Single Atoms with Reactants

Abstract

Hydroformylation reaction is one of the largest homogeneously catalyzed industrial processes yet suffers from difficulty and high cost in catalyst separation and recovery. Heterogeneous single-atom catalysts (SACs), on the other hand, have emerged as a promising alternative due to their high initial activity and reasonable regioselectivity. Nevertheless, the stability of SACs against metal aggregation and leaching during the reaction has rarely been addressed. Herein, we elucidate the mechanism of Rh aggregation and leaching by investigating the structural evolution of Rh 1 @silicalite-1 SAC in response to different adsorbates (CO, H 2 , alkene, and aldehydes) by using diffuse reflectance infrared Fourier transform spectroscopy, X-ray adsorption fine structure, and scanning transmission electron microscopy techniques and kinetic studies. It is discovered that the aggregation and leaching of Rh are induced by the strong adsorption of CO and aldehydes on Rh, as well as the reduction of Rh 3+ by CO/ H 2 which weakens the binding of Rh with support. In contrast, alkene effectively counteracts this effect by the competitive adsorption on Rh atoms with CO/aldehyde, and the disintegration of Rh clusters. Based on these results, we propose a strategy to conduct the reaction under conditions of high alkene concentration, which proves to be able to stabilize Rh single atom against aggregation and/or leaching for more than 100 h time-on-stream.

Electrifying Hydroformylation Catalysts Exposes Voltage-Driven C–C Bond Formation

“…Thus, the observation that Rh sites are electrochemically accessible under any condition suggests that even during operando conditions where no voltage-dependent changes in Rh valency are observed (Figure b, black stars), the Rh sites are also electrochemically accessible. We hypothesize that the presence of CO and styrene might attenuate the response of Rh valency to applied potential by making the Rh center itself harder to reduce, preventing reductive sintering, or acting as acceptors for the electrons that would otherwise go toward reducing Rh , (Supporting Information Section 2.11.2).…”