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

• Published in: Chemistry : a European journal Vol. 27; no. 68; pp. 17240 - 17254
• Main Authors: Schröder, Carsten, Haugg, Philipp A., Baumann, Ann‐Katrin, Schmidt, Marvin C., Smyczek, Jan, Schauermann, Swetlana
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 06.12.2021; John Wiley and Sons Inc
• Subjects: Acrolein; Adsorbates; Aldehydes; allyl cyanide; Chemical composition; Chemistry; Cyanides; Hydrogenation; infrared spectroscopy; ligand-directed catalysis; Ligands; Molecular structure; Palladium; reaction mechanisms; reactive intermediates; Species; allyl cyanide; reaction mechanisms; ligand-directed catalysis; reactive intermediates; infrared spectroscopy
• ISSN: 0947-6539; 1521-3765; 1521-3765
• DOI: 10.1002/chem.202103238

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. A mechanistic study on partial hydrogenation of allyl cyanide over Pd(111) model catalyst addressed by a combination of molecular beam techniques and infrared reflection absorption spectroscopy is presented. Two competing reaction pathways were detected spectroscopically, resulting in formation of stable N‐butylimine and unstable C‐butylamine surface species. The successive evolution of the reaction intermediates for both pathways was monitored as a function of reaction temperature.