Liquid-phase hydrogenation of cinnamaldehyde over Cu-Au/SiO₂ catalysts

The synthesis, characterization, and application of silica-supported Cu-Au bimetallic catalysts in selective hydrogenation of cinnamaldehyde are described. The results showed that Cu-Au/SiO₂ bimetallic catalysts were superior to monometallic Cu/SiO₂ and Au/SiO₂ catalysts under identical conditions. Adding a small amount of gold (6Cu-1.4Au/SiO₂ catalyst) afforded eightfold higher catalytic reaction rate compared to Cu/SiO₂ along with the high selectivity (53%, at 55% of conversion) toward cinnamyl alcohol. Characterization techniques such as x-ray diffraction, H₂ temperature-programmed reduction, ultraviolet-visible spectroscopy, transmission electron microscopy, Fourier-transform infrared spectra of chemisorbed CO, and x-ray photoelectron spectroscopy were employed to understand the origin of the catalytic activity. A key genesis of the high activity of the Cu-Au/SiO₂ catalyst was ascribed to the synergistic effect of Cu and Au species: the Au sites were responsible for the dissociative activation of H₂ molecules, and Cu⁰ and Cu⁺ sites contributed to the adsorption-activation of C=C and C=O bond, respectively. A combined tuning of particle dispersion and its surface electronic structure was shown as a consequence of the formation of Au-Cu alloy nanoparticles, which led to the significantly enhanced synergy. A plausible reaction pathway was proposed based on our results and the literature.