Kinetic Studies of Liquid-Phase Hydrogenation of Phenol over Palladium/Carbon Catalyst

Abstract

Hydrogenation of phenol catalyzed by Pd/C suspended in cyclohexane was studied kine-tically and a most adequate rate equation was presented by assuming Langmuir-Hinshelwood model. The product distribution and the effects of hydrogen partial pressure on initial rate were measured as shown in Figs.5, 6 and 7 under the conditions that mass transfer effects were negligible. As seen from Fig.5, cyclohexanone formed selectively at first and the formation of cyclo-hexanol was negligible until the starting phenol was nearly completely consumed, so that the reaction was analyzed as two independent reactions (phenol → cyclohexanone and cyclohexanone → cyclohexanol). Assuming Langmuir-Hinshelwood or Rideal-Eley mechanism and taking into account not only the initial rate but also the change of concentration of phenol with time, the most adequate rate equations for reaction (I) and (III) were determined as Eq. (16) and Eq. (17), respectively, by means of linear or nonlinear least squares methods. Validity of these equations are demonstrated by the agreement between the experimental and calculated values as shown in Figs.5, 6 and 7. The apparent activation energy of the reaction and the heats of adsorption of hydrogen and phenol were obtained from the temperature dependency of the constants in the rate equations. The reaction mechanism based on the kinetic studies was proposed.