74 The Action of Rhodium and Ruthenium as Catalysts for Liquid-Phase Hydrogenation

Abstract

Carrier-based rhodium catalysts are specifically effective for the hydrogenation of the ring of cyclic compounds at room temperature and atmospheric pressure. The rate of hydrogenation of benzene and the influence of substitution of alkyl-, hydroxyl-, and carboxyl groups on the benzene ring has been studied. All substitutions decreased the hydrogenation rates. Upon introducing methyl groups directly in the ring, the hydrogenation rate decreases exponentially with increasing number of methyl groups. With other alkyl groups and hydroxyl and acid groups, no simple correlation to the hydrogenation rate has been found. The hydrogenation is stoichiometric, and no indication for cleavage of the groups substituted from the ring has been found. Rhodium carrier catalysts are very effective in the hydrogenation of heterocyclic compounds like pyridine, pyrrole, dimethyl furane, and furoic acid. Ruthenium catalysts on carriers are specific for the hydrogenation of the carbonyl group in aliphatic aldehydes and ketones at atmospheric conditions. They reduce preferentially the carbonyl group first in the presence of an olefinic linkage in the compound so that in certain instances the olefinic bond can be preserved. Ruthenium is specifically active for the reduction of sugars to polyhydroxy alcohols.