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

Abstract

We present a mechanistic study on the formation of an active ligand layer over Pd(111), turning the catalytic surface highly active and selective in partial hydrogenation of an α,β-unsaturated aldehyde acrolein. Specifically, we investigate the chemical composition of a ligand layer consisting of allyl cyanide deposited on Pd(111) and its dynamic changes under the hydrogenation conditions. On pristine surface, allyl cyanide largely retains its chemical structure and forms a layer of molecular species with the CN bond oriented nearly parallel to the underlying metal. In the presence of hydrogen, the chemical composition of allyl cyanide strongly changes. At 100 K, allyl cyanide transforms to unsaturated imine species, containing the C=C and C=N double bonds. At increasing temperatures, these species undergo two competing reaction pathways. First, the C=C bond become hydrogenated and the stable N-butylimine species are produced. In the competing pathway, the unsaturated imine reacts with hydrogen to fully hydrogenate the imine group and produce butylamine. The latter species are unstable under the hydrogenation reaction conditions and desorb from the surface, while the N-butylimine adsorbates formed in the first reaction pathway remain adsorbed and act as an active ligand layer in selective hydrogenation of acrolein.

Mechanisms Acting in Ligand-Directed Heterogeneous Catalysis: Electronic, Geometric, and Enol-Induced Effects

“…Previously we have shown 16,17 that the main surface species formed are N-butylimine species, resulting from partial hydrogenation of AC at the CC and CN bonds, with the CC bond being hydrogenated to C−C bond and the CN bond forming imine species (CN•••Pd bond, ν(C N), appearing at 1755 cm −1 in spectrum 2b). There is also a second AC derivativethe amine species (the characteristic band δ(NH 2 ) at 1582 cm −1 ) resulting from the alternative reaction pathway of AC hydrogenation.…”

“…Spectrum 2b shows the composition of the surface, which was pretreated with AC and H 2 at the reaction temperature 250 K. It can be clearly seen that the chemical nature of the adsorbed species drastically changes under these conditions. Previously we have shown , that the main surface species formed are N -butylimine species, resulting from partial hydrogenation of AC at the CC and CN bonds, with the CC bond being hydrogenated to C–C bond and the CN bond forming imine species (CN···Pd bond, ν­(CN), appearing at 1755 cm –1 in spectrum 2b). There is also a second AC derivativethe amine species (the characteristic band δ­(NH 2 ) at 1582 cm –1 ) resulting from the alternative reaction pathway of AC hydrogenation.…”

“…In our recent studies, employing a rigorous surface science approach, we addressed the mechanistic aspects of ligand-induced heterogeneous catalysis. − Specifically, selective hydrogenation of acroleinthe simplest α,β-unsaturated aldehydeto the desired product unsaturated alcohol (propenol) over ligand-functionalized surfaces was investigated. For this system, we provided first microscopic-level insights into the mechanisms acting in ligand-directed catalysis.…”

“…It was shown that a densely packed ligand layer can be formed on Pd(111) under the operational conditions, which promotes the adsorption of acrolein in the desired adsorption configurationvia the CO double bondthus enabling hydrogenation of this bond to form propenol with close to 100% selectivity. Two different types of ligand surface species were identified in these studiesoxopropyl , and N -butylimine , surface specieswhich are formed and dynamically change under the operational condition. In contrast to the ligand-functionalized surfaces, the other entitythe CC double bondtends to be predominantly hydrogenated on the pristine Pd(111) surface. , …”

Infrared spectroscopic monitoring of solid-state processes

“…291 For sure, one of the most interesting and important applications of IR spectroscopy in heterogeneous catalysis is monitoring of the catalytic reactions, in order to detect and characterize the intermediates and their evolution across the reaction coordinate, thus enabling a reconstruction of mechanistic details at molecular level. 164,283,[292][293][294][295][296][297][298][299][300][301][302][303][304] Although the majority of work is focused on simple compounds, catalytic conversion of complex species are also traced. Examples include catalytic hydroformylation of ethylene on the Rh/Al 2 O 3 surface.…”