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STUDIES IN NUCLEAR HYDROGENATION

Our interest in nuclear hydrogenation of the benzene ring in aromatic compounds containing functional groups, as in aryl amines, prompted a study of the simpler hydrogenation of benzene and of an alkyl-substituted benzene, isobutylbenzene. The data in TABLE 1 show that the relative activities of several metals for the hydrogenations of both benzene and isobutylbenzene are in the order: rhodium > ruthenium >> platinum > palladium >> nickel > cobalt. Benzene is more reactive than isobutylbenzene with each of the metals. The relative catalyst activities and the effect of alkyl substitution are in agreement with the Rhodium and ruthenium are by far the most active of the catalysts tested. Rhodium, the more active of the two, was studied further. The results shown in TABLE 2 illustrate the effects of catalyst support, pressure, and the addition of a small quantity of acid in the rhodiumcatalyzed hydrogenation of benzene in isopropanol under very mild conditions. The reaction rates are approximately zero order with respect to the concentration of benzene and increase with increasing hydrogen pressure. The greater activity of the carbon-supported compared to the alumina-supported rhodium is probably because of the higher surface area of the carbon. The promoting effect of a small amount of acid may be due to the neutralization of a trace of a base. Basic materials are known to have an inhibitory effect on rhodium catalysts in some hydrogenations. TABLE 3 describes the hydrogenation of benzene with a rhodium on alumina catalyst in several solvents under mild conditions. Isopropanol and r-butanol are good solvents, while n-propanol, n-butanol, isobutanol, and sec-butanol inhibit hydrogenation. The addition of a small amount of acid greatly increases the rate in n-propanol. Ethyl acetate can be a fairly good solvent but seems to vary considerably from lot to lot. A decrease in rate with time suggests the gradual formation of an inhibitor from the ester. The reaction with acetic acid starts well but results in the obvious formation of an inhibitor. It is noteworthy that “acetic acid is rapidly reduced over rhodium catalysts at room temperature and pressure, but the reduction stops abruptly after a very small conversion.” The cyclic ethers, tetrahydrofuran and dioxane, give very slow reactions even after extensive purification. The results with a rhodium on carbon catalyst and several solvents are shown in TABLE 4. Approximately the same rates for hydrogenation of benzene are obtained with cyclohexane and isopropanol. Methanol and ethanol produce very slow reactions but behave similarly to isopropanol after the addition of a small amount of acid.