Ed. Snijder et al., HYDROGENATION OF CYCLOHEXENE WITH LANI5-XALXHN METAL-HYDRIDES SUSPENDED IN CYCLOHEXANE OR ETHANOL, Chemical Engineering Science, 48(13), 1993, pp. 2429-2441
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.