NICKEL-CATALYZED HYDROGENATION OF CYCLOHEXENE IN THE LIQUID PHASE

Cyclohexene hydrogenation has been studied on platinum, palladium and nickel, and the reaction has been found to be structure insensitive. Nickel has been shown to catalyze the disproportionation of cyclohexene to benzene and cyclohexane in the gas phase. The objectives of this investigation were to determine if disproportionation on nickel would also occur in the liquid phase and how this reaction and the hydrogenation of cyclohexene would be affected by the structure of nickel catalysts.;The turnover rate of cyclohexene hydrogenation, when carried out in cyclohexane at 299 K and 101 kPa dihydrogen, varied between 0.98 s('-1) for the 29% metal exposed sample and 2.15 s('-1) for the 66% metal exposed sample. Thus the hydrogenation of cyclohexene in the liquid phase on nickel should be considered structure insensitive. In the absence of dihydrogen, benzene production by disproportionation was observed by U.V. spectrometry. This reaction was inhibited by dihydrogen. Disproportionation also occurred during cyclohexene hydrogenation and was parasitic, as one of its products, benzene, made the surface unavailable to cyclohexene. The deactivation of the catalyst did not proceed to completion. The activity of the catalyst stabilized after a fraction of the sites had been poisoned. Smaller nickel particles appeared to be more resistant to poisoning. These results were explained on the basis of the difference in the size of the site required for disproportionation and hydrogenation. Disproportionation required a larger site and was therefore more easily poisoned that hydrogenation. Smaller particles of nickel, which had a lower concentration of large sites, catalyzed disproportionation to a lesser extent and deactivated less. The main conclusions from this investigation are that cyclohexene hydrogenation on nickel is structure insensitive and cyclohexene disproportionation on nickel is structure sensitive.;Silica-supported nickel catalysts were prepared by incipient wetness impregnation, controlled pH precipitation and ion-exchange. For these samples, the number of surface nickel atoms was determined by dihydrogen chemisorption and the total number of nickel zero valent atoms was measured by reaction with dioxygen. In addition, the samples were characterized by magnetic susceptibility measurements, Ferromagnetic Resonance (FMR) and Extended X-ray Absorption Fine Structure (EXAFS). The percentage metal exposed varied between 29% and 66%, with low values and high values for samples prepared by impregnation and ion-exchange respectively.