Liquid-phase hydrogenation reactions over supported group VIII metals (Silicon dioxide, Aluminum oxide, Titanium dioxide)

The kinetics of liquid-phase hydrogenation of benzene and citral (3,7-dimethyl-2,6-octadienal) were studied over SiO2, Al 2O3, and TiO2-supported catalysts with particular emphasis on obtaining quantitative kinetics free of transport limitations to probe the effect of solvent, process parameters, metal-support interactions, and metal specificity. Thermodynamic arguments indicated that solvent effects can influence the surface coverage of hydrogen. The rate of citral hydrogenation exhibited an activity minimum with respect to temperature, which was attributed to the relative rates of concurrent alcohol decomposition and CO desorption, with the decomposition reaction having a lower activation energy compared to CO desorption. Metal-support interactions also had a significant influence on reaction kinetics. Pt/TiO2-HTR (high temperature reduced at 773 K) exhibited a 50-fold enhancement in the initial TOF for citral disappearance compared to Pt/TiO2-LTR (low temperature reduced at 473 K) and 100-fold enhancement in TOF compared to Pt/SiO2. Similar to the kinetics exhibited by Pt/SiO2, Pt/TiO2-LTR exhibited an activity minimum with respect to temperature and a near zero and first order dependencies on citral concentration and hydrogen pressure, respectively. In contrast, Pt/TiO2-HTR exhibited conventional Arrhenius behavior and negative first and near zero order dependencies on citral concentration and hydrogen pressure, respectively. The initial TOF for citral hydrogenation over SiO2-supported Group VIII metals at 300 K and 1 atm H 2 varied 1000-fold and correlated with % d-character. The initial TOF for citral hydrogenation exhibited the following trend: Pd > Pt > Ir > Os > Ru > Rh > Ni > Co and no activity was detected with Fe/SiO2. There were significant differences in product distribution among the different metals, which was rationalized on the basis of the width of the d-band, which affects the relative contributions of the stabilizing two-electron interactions and destabilizing four-electron interactions. DRIFTS study of vapor-phase citral, citronellal, and geraniol decomposition revealed that all three organic compounds decomposed on Pt to yield adsorbed CO, and the activation energy for citronellal decarbonylation was the largest. Furthermore, qualitative arguments indicated that the apparent rate constant for geraniol decomposition at 300 K is greater than that for citral.