Hydrogenation and Hydrogenolysis. IX. The Hydrogenation of Benzyl Alcohol with Platinum Metal Catalysts

The rate of the hydrogenation of benzyl alcohol with a rhodium catalyst decreases markedly in the course of the reaction, accompanied by the formation of an increasing amount of cyclohexanecarboxaldehyde with the conversion of the alcohol. The decrease in the rate is only slight with a platinum catalyst, however, and the formation of the aldehyde is much less than with the rhodium catalyst. It has been shown that cyclohexanecarboxaldehyde is an effective catalyst poison in the hydrogenation of benzyl alcohol, but that cyclohexylcarbinol is not. 1-Cyclohexenylcarbinol, an intermediate of the hydrogenation, isomerizes to cyclohexanecarboxaldehyde during the hydrogenation on rhodium and 7 : 3 rhodium-platinum catalysts to a greater extent than on the platinum catalyst. Cyclohexanecarboxaldehyde is hydrogenated only very slowly, but the rate with the platinum catalyst is about four times greater than that with the rhodium catalyst. These results may account for the different behaviors of the rhodium or 7 : 3 ...