Promoting effect of zinc on the vapor-phase hydrogenation of crotonaldehyde over copper-based catalysts

E.L. RODRIGUES; A.J. MARCHI; C.R. APESTEGUIA; J.M.C. BUENO

APPLIED CATALYSIS A-GENERAL

Elsevier

Lugar: Amsterdam; Año: 2005 vol. 294 p. 197 - 207

0926-860X

The promoting effect of zinc for the vapor-phase hydrogenation of crotonaldehyde was studied on impregnated Cu/SiO2 and Cu-ZnO/SiO2, and on coprecipitated Cu-Al2O3 and Cux-ZnyO2y-ZnAl2O4 catalysts.  The reaction was carried out in a tubular reactor at 120°C and atmospheric pressure.  Samples were characterized by temperature-programmed reduction, X-ray diffraction, transmission electronic microscopy, diffuse reflectance FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy.  Cu/SiO2 and Cu-Al2O3 catalysts reduced in hydrogen either at 300ºC or 500ºC hydrogenated preferentially the C=C bond of crotonaldehyde and gave more than 90 % of selectivity to butyraldehyde.  In contrast, the initial butyraldehyde selectivity on Cu-ZnO/SiO2 reduced at 500ºC was only about 55 %, essentially because the selectivity to crotyl alcohol significantly increased on this zinc-containing sample as compared to Cu/SiO2.  This selectivity enhancement for hydrogenating the C=O bond on Cu-ZnO/SiO2 reduced at 500ºC was explained by considering that the high-temperature hydrogen treatment forms mobile ZnOx reduced species that strongly interact with Cu0 crystallites.  The resulting Cu0-ZnOx species preferentially catalyze the crotyl alcohol formation from crotonaldehyde via a dual-site reaction pathway.  A similar explanation was proposed to interpret the observed enhancement of the C=O hydrogenation rate on ternary Cu-Zn-Al catalysts reduced at 500ºC as compared to Cu-Al2O3.  Nevertheless, the Cu0-ZnOx species were unstable on stream and the selectivity to crotyl alcohol continuously decreased with reaction time on zinc-containing samples reduced at high temperatures.