Selective liquid-phase hydrogenation of citral over supported bimetallic Pt-Co catalysts

The liquid-phase hydrogenation of citral was studied at 393 K and 10 bar on Pt-Co/C catalysts, having different Pt/(Pt+Co) ratios and containing a total metal load of about 2%. The monometallic and bimetallic Pt-Co/C catalysts were prepared by impregnation and co-impregnation, respectively, with cobalt tris(acetylacetonate) and platinum bis(acetylacetonate). Monometallic Pt/C and Co/C catalysts showed very low activity and selectivity to the desired products. Undesirable reactions, such as citral decarbonylation and hydrogenolysis, were observed with these monometallic catalysts. Instead, bimetallic Pt-Co/C proved very active and selective to geraniol/nerol and the main products detected were geraniol/nerol, citronellal and citronellol. Hydrogenation kinetic constants were determined by modeling catalytic data and using a pseudo-homogeneous kinetics. From the analysis of the kinetic parameters, an optimum Pt/(Pt + Co) ratio was found for both the catalytic activity and selectivity towards geraniol/nerol. Furthermore, it was determined that this optimum ratio depends on the activation conditions. Temperature-programmed reduction (TPR) experiments and X-ray absorption spectroscopy (XAS) demonstrated the existence of Pt-Co bimetallic compounds on the carbon support. On the basis of these results, it was proposed that cobalt improves the catalytic performance of platinum by electron transfer. This electron transfer is favored by the high interaction of both metals existing in these types of bimetallic compounds. (C) 2009 Elsevier B.V. All rights reserved.