Aqueous Phase Oxidative Degradation of 4–Hydroxy Nitrobenzene over a CuO‐TiO₂ Catalyst

The aqueous phase oxidation of 4‐hydroxy nitrobenzene (or p‐nitrophenol) has been carried out in a semibatch slurry reactor operating at the relatively mild conditions of 373‐393 K and 80‐150 kPa oxygen pressure. The catalyst employed has a composition 5CuO:5TiO 2 :100SiO 2 and was maintained at 11.4 g/L in the reactor. Kinetic data were collected under conditions of negligible internal transport resistance. The degradation rate has an order of about 1.43 with respect to the 4‐hydroxy nitrobenzene but weakly dependent on the oxygen partial pressure (approx. 0.12). Analysis of the concentration‐time data revealed that the autocatalytic effect frequently seen during phenol oxidation was non‐existent in the reaction pathway. A Langmuir‐Hinshelwood single‐site mechanism involving the oxygen‐assisted cleavage of the benzene ring as the rate‐controlling step offered a kinetic model that accurately described the rate data. Estimates of the adsorption equilibrium constants for both p‐nitrophenol and oxygen were also thermodynamically consistent and activation energy was valued at 42 kJ/mol. Excellent activity of the catalyst was attributed to chemical synergy arising from the interaction between surface CuO and TiO2 crystallites to give a new sphene‐type phase containing the highly intrinsically active centres for the catalytic process.