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 initial TOF (turnover frequency) for citral hydrogenation varied by three orders of magnitude and exhibited the following trend: Pd>Pt>Ir>Os>Ru>Rh>Ni>Co⪢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 different metals, with Os, Ru, and Co exhibiting high selectivity toward the unsaturated alcohol isomers (geraniol and nerol), while Rh, Ni, and Pd were more selective for citronellal and isopulegol. The variation in product distribution 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 exception of Ni/SiO2, all catalysts exhibited substantial deactivation that was attributed to inhibition by adsorbed CO produced by the decomposition of either citral or the unsaturated alcohol. In contrast, Ni/SiO2 exhibited an initially low, stable TOF during the first 7 h that then increased fourfold and again remained constant to 84% citral conversion, at which point the run was terminated.