Diffusional kinetics in the catalytic hydrodechlorination of chlorobenzene in multiphase aqueous mixtures

The catalytic hydrodechlorination of aqueous mixtures of monochlorobenzene has been investigated experimentally and theoretically. The reaction runs were conducted in a stirred autoclave at T=30 °C, in the presence of pure hydrogen at pressures (kept constant during each reaction run) in the range 2≤pH2≤60 bar, and using a Pd/C catalyst. Chlorobenzene was loaded in excess with respect to its solubility in water. Therefore, the reacting process involves four phases: (i) gaseous hydrogen; (ii) aqueous solution of monochlorobenzene and hydrogen; (iii) oily droplets of undissolved chlorobenzene; (iv) catalyst solid particles. The theoretical analysis for modelling the overall reactive process has been based on the general results and methods of transport phenomena and of kinetics of heterogeneous catalytic reactions. The experimental data, interpreted in the light of the theoretical model, permit to gain an insight into the Langmuir–Hinshelwood mechanism of the reaction: hydrogen is adsorbed according to a Langmuir-type dissociative adsorption and the rate determining step is the surface reaction. According to this mechanism the reaction rate has a fractional order with respect to hydrogen.