Liquid-phase citral hydrogenation over SiO2-supported group VIII metals

Liquid-phase citral hydrogenation over SiO2-supported Group VIII metals at 300 K and 1 atm was studied in the absence of all transport limitations as verified by the Madon-Boudart test and the Weisz-Prater criterion. The init ial TOF (turnover frequency) for citral hydrogenation varied by three order s of magnitude and exhibited the following trend: Pd > Pt > Ir > Os > Ru > Ph > Ni > Co much greater than Fe (no activity was detected over Fe/SiO2). When TOF is correlated with percentage d-character, a volcano plot results. There are significant differences in product distribution among the differ ent metals, with Os, Ru, and Co exhibiting high selectivity toward the unsa turated alcohol isomers (geraniol and nerol), while Ph, Ni, and Pd were mor e selective for citronellal and isopulegol. The variation in product distri bution among the various metals is rationalized on the basis of the d-band widths that affect the relative importance of the stabilizing two-electron interactions and destabilizing four-electron interactions. With the excepti on of Ni/SiO2, all catalysts exhibited substantial deactivation that was at tributed to inhibition by adsorbed CO produced by the decomposition of eith er citral or the unsaturated alcohol. In contrast, Ni/SiO2 exhibited an ini tially low, stable TOF during the first 7 h that then increased fourfold an d again remained constant to 84% citral conversion, at which point the run was terminated, (C) 2001 Academic Press.