Hydrogenation of cyclohexene with LaNi@#5@#-@#x@#Al@#x@#Hn metal hydrides, suspended in cyclohexane or ethanol

The hydrogenation of cyclohexene on the metal hydride forming alloys LaNi4.8Al0.2, LaNi4.9Al0.1 and LaNi5, all suspended in cyclohexane and LaNi5 suspended in ethanol, has been investigated. Two sources for hydrogen are recognized: hydrogen supplied by the gas phase and hydrogen which is available inside the metal hydride particles. For hydrogen which 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, kr, and adsorption coefficient for cyclohexene, KC6H10, are lower if the hydrogenation is carried out on the metal (alpha) phase of the metal alloys instead of on the hydride (ß) phase. Also, increasing the aluminum content results in a decrease of kr and KC6H10. In ethanol, a higher reaction rate constant and a lower adsorption coefficient were observed. The hydrogenation of cyclohexene with hydrogen provided by the metal hydride particles has been described with a combination of the rate equation for the hydrogenation and the relation for the hydrogen desorption from the hydride. It was found that the reaction rate decreases during the cyclohexene conversion, because the nature of hydride particles changes from the ß into the ¿ phase as the reaction proceeds. Initially, the hydrogenation is partly limited by the transport of hydrogen from the centre of the particle to the surface.