Liquid phase hydrogenation of cinnamaldehyde on Cu-based catalysts

Abstract

The liquid phase hydrogenation of cinnamaldehyde was studied at 393 K and 10 bar on Cu-based catalysts containing about 12 wt.% of copper. Cu/SiO 2 was prepared by incipient wetness impregnation while Cu-Al, Cu-Zn-Al, and Cu-Ni(Co)-Zn-Al catalysts were obtained by coprecipitation at constant pH. Cinnamaldehyde was initially hydrogenated to cinnamyl alcohol and hydrocinnamaldehyde, and these products consecutively yielded hydrocinnamyl alcohol. Hydrogenation kinetic constants were determined by modeling catalytic data and using a pseudohomogeneous kinetic network. The reaction occurred via two different pathways depending on the composition and surface properties of the catalyst. On Cu/SiO2 and binary Cu-Al samples, cinnamaldehyde hydrogenation proceeded via a monofunctional pathway on metallic copper that produced predominantly hydrocinnamaldehyde. Ternary Cu-Zn-Al and quaternary Cu-Ni(Co)-Zn catalysts were about one order of magnitude more active than Cu/SiO2 for cinnamaldehyde conversion and produced predominantly cinnamyl alcohol. The general composition formula of reduced Cu-Zn-Al and Cu-Ni(Co)-Zn-Al catalysts was Cu0.50·[MO] 0.5·ZnAl2O4, where M is Zn, Co, or Ni. These catalysts contained the Cu0 particles highly dispersed in a super-stoichiometric zinc aluminate spinel and in close interaction with M 2+ cations. The presence of M2+ cations provided a new reaction pathway for adsorbing and hydrogenating cinnamaldehyde in addition to the metal copper active sites. Cinnamaldehyde interacts linearly via the CO group with M2+ sites and is selectively hydrogenated to unsaturated alcohol by atomic hydrogen activated in neighboring Cu0 sites. Formation of surface Cu0-M2+ sites was, therefore, crucial to efficiently catalyze the cinnamyl alcohol formation from cinnamaldehyde via a dual-site reaction pathway.