Understanding Ligand‐Directed Heterogeneous Catalysis: When the Dynamically Changing Nature of the Ligand Layer Controls the Hydrogenation Selectivity

Abstract

We present a mechanistic study on the formation and dynamic changes of a ligand‐based heterogeneous Pd catalyst for chemoselective hydrogenation of α,β‐unsaturated aldehyde acrolein. Deposition of allyl cyanide as a precursor of a ligand layer renders Pd highly active and close to 100 % selective toward propenol formation by promoting acrolein adsorption in a desired configuration via the C=O end. Employing a combination of real‐space microscopic and in‐operando spectroscopic surface‐sensitive techniques, we show that an ordered active ligand layer is formed under operational conditions, consisting of stable N‐butylimine species. In a competing process, unstable amine species evolve on the surface, which desorb in the course of the reaction. Obtained atomistic‐level insights into the formation and dynamic evolution of the active ligand layer under operational conditions provide important input required for controlling chemoselectivity by purposeful surface functionalization.

Formation of 2D Self-Assembled Layers of Allyl Cyanide on Pd(111)

“…Note that the specific chemical structure of the adsorbate deviates from the original AC. 39 Particularly, the original olefin double bond is replaced by a dotted bond. This proposed chemical structure is constructed based on the IRAS results, which will be discussed later in this manuscript.…”

“…They comprise the geometric and electronic effects altering the formation of the desired reaction intermediate, 39 the dynamic changes in the chemical structure of the ligands under the reaction conditions 40 as well as the details of mutual intermolecular interactions between ligand and reactant species. 39 The decisive effects of intermolecular interactions between different types of surface species on their reactivity were shown also for other reactive systems including hydrogenation of carbonyl and ester compounds. 12,14,42−46 These studies provided important starting ideas about the mechanistic details governing selectivity in this type of processes.…”

“…Thus, the formation of the organic ligand layers and their dynamic transformations under the reaction conditions need to be explored at the atomistic level to enable the development of the ligand-functionalized catalytic materials based on rational considerations. In our recent studies on mechanisms of chemoselective acrolein hydrogenation over surfaces functionalized with 2D layers of organic ligands, − some important aspects of mutual reactant–ligand interactions were elucidated. They comprise the geometric and electronic effects altering the formation of the desired reaction intermediate, the dynamic changes in the chemical structure of the ligands under the reaction conditions as well as the details of mutual intermolecular interactions between ligand and reactant species .…”

“…In our recent studies on mechanisms of chemoselective acrolein hydrogenation over surfaces functionalized with 2D layers of organic ligands, − some important aspects of mutual reactant–ligand interactions were elucidated. They comprise the geometric and electronic effects altering the formation of the desired reaction intermediate, the dynamic changes in the chemical structure of the ligands under the reaction conditions as well as the details of mutual intermolecular interactions between ligand and reactant species . The decisive effects of intermolecular interactions between different types of surface species on their reactivity were shown also for other reactive systems including hydrogenation of carbonyl and ester compounds. ,,− …”

“…These studies provided important starting ideas about the mechanistic details governing selectivity in this type of processes. As one of the most successful reactive systems, a Pd catalyst functionalized with allyl cyanide (AC) was recently identified. − Deposition of AC was shown to result in exceptionally high selectivity toward the desired product propenol (close to 100%) and high overall acrolein conversion rates. An especially interesting aspect of this study was the fact that deposited AC leads to the formation of a highly dynamic self-organized 2D ligand layer, whose chemical structure dramatically changes under the reaction conditions andif adjusted correctlyresults in highly efficient and selective catalysts.…”