Hydrogenation of Organic Compounds over LaNi₅H_6.0

Abstract

To elucidate the chemical reactivity of the hydrogen atom absorbed in LaNi₅, the hydrogenation of unsaturated compound such as acrylonitrile, 1-hexene, or 2-methyl-1, 3-butadiene in liquid phase was carried out over LaNi₅H_6.0. The hydrogenated alloy LaNi₅H_6.0 was pre- pared from LaNi₅ according to an ordinary procedure by repeated heating and cooling in hydrogen atmosphere. The reactions were carried out approximately at a room temperature in the absence (A) or the presence (B) of gaseous hydrogen, and in some instances in the solvent such as cyclopentane, oxolane, or etc. The rate of hydrogenation was followed by gas chromatography.
The hydrogenation of substrates except benzene and methyl acetate markedly proceeded, and the reaction rate in the presence of additional molecular hydrogen was almost the same as that in the absence of molecular hydrogen. When molecular hydrogen was added to LaNi₅, no change in hydrogenation reaction occurred. In the case of (A), a certain amount of hydrogen in the alloy was desorbed in gas phase during the reaction, and it remarkably influenced the substrate. In this case, the total decreasing rate of hydrogen in the alloy, whose hydrogen is the sum of that consumed by the hydrogenation and that desorbed as a molecular hydrogen, was relatively lower than the desorption rate in the absence of the substrate under vacuum.
The desorption rate of hydrogen from LaNi₅H_6.0 in hexane, cyclopentane, or oxolane, which was used as a solvent in the hydrogenation, was remarkably lower than that under vacuum. Considering the above results, it was concluded that the rate-determining step of the hydrogenation is the migration process of the absorbed hydrogen atoms from the bulk of the alloy to its surface which was almost completely covered with the strongly adsorbed substrate.