Investigation of the stepwise mechanism and selectivity of the reduction of aceto-and propiophenone on the catalyst Rh-Al2O3

Abstract

1. In the presence of a Rh-Al2O3 catalyst, the first mole of hydrogen is added to an aliphatic-aromatic ketone, forming an alkylphenylcarbinol and an alkyl cyclohexyl ketone. The basic direction of the process, just as in the presence of Ni, Co, and Pd catalysts, is determined by the greater ease of hydrogenation of the carbonyl group. On all these catalysts the reduction proceeds chiefly through the intermediate formation of alkylphenylcarbinol. 2. At the second step of the reduction, the direction and selectivity of the process are determined by the relative ease of hydrogenolysis of the C-OH bond and the hydrogenation of the aromatic ring of the alkylphenylcarbinol, which depends on the nature of the catalyst and on the conditions of the reaction. 3. Under mild conditions, hydrogenolysis of the C-OH bond also prevails on a Rh-Al2O3 catalyst, while the aromatic ring of the alkylphenylcarbinol is weakly hydrogenated. Under rigorous conditions hydrogenation of the aromatic ring of the alkylphenylcarbinol outstrips the hydrogenolysis of the C-OH bond, and primarily the alkylcyclohexylcarbinol is formed. 4. It was found by the method of partial poisoning of the catalyst with cadmium that the aromatic ring of the alkylphenylcarbinol and the ring of the alkyl phenyl ketone are hydrogenated on different portions of the active surface and with different relative ease, each depending differently on the conditions of the process.