Kinetics of hydrogenation of o-nitrophenol to o-aminophenol on Pd/carbon catalysts in a stirred three-phase slurry reactor

Abstract

Kinetics of the hydrogenation of o-nitrophenol to o-aminophenol over a Pd/carbon (4.82 wt % Pd) catalyst (particle size 30 μm) in an agitated three-phase slurry reactor has been investigated in the chemical control regime at different temperatures (293-328 K), with initial concentration of o-nitrophenol (0.072-0.36 mol dm^-3 ) and H₂ pressures (442-1476 kPa), using methanol as a reaction medium. To confirm the absence of gas-liquid, liquid-solid, and intraparticle mass-transfer effects on the reaction, the effects of stirring speed (260-1290 rpm), catalyst loading (0.05-1.0 g dm^-3), and catalyst particle size (30-165 μm) on the initial reaction rate at the maximum temperature (328 K) and o-nitrophenol concentration (0.36 mol dm^-3) have been thoroughly studied. For a catalyst particle size of ≤ 45 μm and a stirring speed of ≤850 rpm, the reaction rate is not influenced by the mass-transfer processes. Effective intraparticle diffusivity of o-nitrophenol has been determined from the effectiveness factor of the catalyst for its different particle sizes. The observed large tortuosity factor (τ = 22.9 av) and activation energy (28.9 kJ mol^-1) for the diffusion indicated a strong influence of adsorption and surface diffusion of o-nitrophenol on the catalyst. From the power law analysis of the initial rate data, the reaction order is found to be 0.53 ± 0.03 for o-nitrophenol in its concentration below 0.18, 0.22, 0.23, and 0.25 mol dm^-3 at 293, 308, 318, and 328 K, respectively, and from 0.54 (at 293 K) to 1.0 (at 328 K) for hydrogen. However, the reaction is found to be zero-order for the higher o-nitrophenol concentration (>0.25 mol dm^-3). The reaction kinetic data (including the initial rate data) could be fitted well to a Hougen-Watson-type model on the basis of the mechanism involving single-site surface reaction control with all the reaction species molecularly adsorbed. The activation energy for the initial reaction obtained from the power law analysis (70.2 kJ mol^-1) is found to agree with that (68.0 kJ mol^-1) obtained from the Hougen-Watson model.