HYDROGENATION OF CYCLOHEXENE WITH LANI5-XALXHN METAL-HYDRIDES SUSPENDED IN CYCLOHEXANE OR ETHANOL

The hydrogenation of cyclohexene on the metal hydride forming alloys L aNi4.8Al0.2, LaNi4.9Al0.1 and LaNi5, all suspended in cyclohexane and LaNi5 suspended in ethanol, has been investigated. Two sources for hyd rogen are recognized: hydrogen supplied by the gas phase and hydrogen which is available inside the metal hydride particles. For hydrogen wh ich is supplied by the gas phase, the kinetics can be described with a two-site Langmuir-Hinshelwood relation, assuming a fast dissociative adsorption of hydrogen. The values of the rate constant, k(r), and ads orption coefficient for cyclohexene, K(C6H10), are lower if the hydrog enation is carried out on the metal (alpha) phase of the metal alloys instead of on the hydride (beta phase. Also, increasing the aluminum c ontent results in a decrease of k(r) and K(C6H10). In ethanol, a highe r reaction rate constant and a lower adsorption coefficient were obser ved. The hydrogenation of cyclohexene with hydrogen provided by the me tal hydride particles has been described with a combination of the rat e equation for the hydrogenation and the relation for the hydrogen des orption from the hydride. It was found that the reaction rate decrease s during the cyclohexene conversion, because the nature of hydride par ticles changes from the beta into the a phase as the reaction proceeds . Initially, the hydrogenation is partly limited by the transport of h ydrogen from the centre of the particle to the surface.