2461. NICOLAĻ, J., MARTIN, R., AND JUNGERS, J. C. [Kinetics of the Hydrogenation of Benzene.] Bull. soc. chim. beig., vol. 57, 1948, pp. 555-574; Chem. Abs., vol. 44, 1950, p.419.
The reaction cyclohexane+CO=benzene+CH4+H2O is thermodynamically favorable but does not go well over Ni. Since CO is readily hydrogenated, it was desired to observe the dehydrogenation of cyclohexane (I), but this is limited by equilibrium at some of the temperatures of interest, so the reverse reaction of hydrogenation of benzene (II) was studied at 60°-330°. The catalyst was prepared by reduction of NiCO3 deposited on kieselguhr. Data obtained in a static system are presented in curves. At 70°-150° the hydrogenation produces (I) and has an activation energy of (II) kcal. At 150°-250° it is complex. Above 250° the product is CH4 (III) and the activation energy is 45 kcal. Thus at low temperatures there is an additive adsorption of (II), whereas at high temperatures there is a dissociative adsorption to products that are hydrogenated to (III). The orders of reaction in H2 and (II) are fractional, about 0.5 in the low-temperature region. In the region of formation of (III) the order is first in (II) and negative in H2. When (I) is treated in H2 at 260° there is first dehydrogenation to (II) and then production of (III). The hydrogenation of (II) in the presence of various gases was observed; (I) did not inhibit at low temperatures, and (III) did not inhibit at high temperatures. The adsorption of CO was stronger than that of (II), and CO was hydrogenated first. Adsorption of CO2 was about the same as that of (II). MeOH was not strongly adsorbed and below 160° had no effect, whereas at high temperatures it was decomposed and then converted to (III). The hydrogenation of mixtures of CO and (I) at 250°-280° showed first hydrogenation of CO, then dehydrogenation of (I) to (II), then conversion of (II) to (III). Thus CO does not aid in the dehydrogenation, for it is adsorbed more strongly and is hydrogenated first. The effect of CO2 is similar though less marked. In mixtures containing CO, (I), and MeOH at 250° the first reaction was decomposition of the MeOH. Failure of the desired transfer of H2 from (I) to CO is caused by 2 effects. First at temperatures above 180° where CO can be converted to (III), (II) formed is destructively adsorbed and the catalyst activity is lowered by carbonization unless much H2 is present. Second, CO is preferentially adsorbed and keeps (I) from the catalyst.