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.

Competing Reaction Pathways in Heterogeneously Catalyzed Hydrogenation of Allyl Cyanide: The Chemical Nature of Surface Species

“…Above 250 K, no IR vibrational can be detected on the surface, even though that according to the STM data, the surface is almost completely covered by AC [8] . Vanishing of all vibrational bands occurs most likely due to the nearly flat orientation of all main bands parallel to the surface plane, making them not detectable by IRAS due to MSSR [16] .…”

“…The surface ligand species were identified as N-butylimine species, which remain stable under the reaction conditions applied for acrolein hydrogenation. [8] While some aspects of the chemical transformation of AC on Pd(111) under the reaction conditions were addressed in the latter study, the full atomistic-level details of adsorption and reactivity behavior of AC under hydrogenation reaction conditions resulting in formation of an active ligand layer remain unknown. Specifically, AC is able to transform not only to Nbutylimine species but also undergo alternative reaction pathways resulting in formation of the different partly hydrogenated surface species (e. g. amines).…”

“…In that study, we found indications that the chemical nature of the originally deposited AC dynamically changes under the reactive conditions – in presence of hydrogen and at elevated temperatures – allowing formation of a specific ligand layer, which enables acrolein adsorption in the desired adsorption geometry – via the O‐end – and by this promotes its selective hydrogenation. The surface ligand species were identified as N‐butylimine species, which remain stable under the reaction conditions applied for acrolein hydrogenation [8] …”

Disordered Two-Dimensional Self-Organization of Ethyl Pyruvate Molecules on the Pt(111) Surface

“…6 However, intermolecular interactions between several surface-adsorbed molecules are also clearly important, as demonstrated by ligand-directed heterogeneous catalysis. 7,8 Hence, molecular interaction and self-assembly on (metal) surfaces 9−11 is not only a research field on its own, ranging from investigations of interaction modes 12,13 to two-dimensional crystal engineering 14 but also an important ingredient for onsurface synthesis and heterogeneous catalysis. There, it is vital to fully understand the intricate interplay between molecule− molecule and molecule−surface interactions, to be able to advance from trial-and-error to systematic, rational design.…”

“…Molecules on surfaces are key players in heterogeneous catalysis and on-surface synthesis. − One obvious reason is the influence of the surface on each single adsorbed molecule, steric or electronic or by strong local electric fields, leading to intramolecular changes, as for example, in surface-driven keto-enol tautomerization . However, intermolecular interactions between several surface-adsorbed molecules are also clearly important, as demonstrated by ligand-directed heterogeneous catalysis. , …”