The specific activity of technetium, rhenium, ruthenium, platinum, and palladium in catalytic reactions of benzene with hydrogen

Abstract

The specific activities of technetium, rhenium, ruthenium, platinum, and palladium catalysts supported on silica and γ-alumina at a concentration of 1% have been studied for the reactions of benzene with hydrogen in the temperature range of 100–235 ° by the microreactor chromatographic pulse method. The surface areas of the metals were determined by selective hydrogen chemisorption at 20 ° and by the X-ray line-broadening methods. The measurements on ruthenium, platinum, and rhenium powders of known surface areas were applied to determine the metal site areas effectively occupied by one hydrogen atom. Both technetium and rhenium reveal a catalytic activity in hydrogenation of benzene to cyclohexane. At sufficiently high temperatures, both metals, like ruthenium, are also catalysts in benzene hydrogenolysis, resulting in aliphatic C 1C 6 hydrocarbons. The specific activities of metals for hydrogenation and hydrogenolysis decrease in the order: Ru > Pt > Tc ≈ Pd > Re and Ru > Tc > Re, respectively. The apparent activation energies are 7–11 kcal/mole for hydrogenation and 29–32 kcal/mole for hydrogenolysis. Rhenium, ruthenium, platinum, and palladium supported on γ-alumina were found to be less active as compared with the same metals on silica carrier, but the maximum difference was only a nearly twofold change in specific activity. The activity and selectivity of technetium appear to be distinctly dependent on the nature of the support. In discussing the results the factors determining the catalytic activities of the metals are mainly considered e.g., the extent and strength of hydrogen adsorption, the mode of bonding of the benzene molecules, and the role of the carrier.