H₄SiW₁₂O₄₀-catalyzed oxidation of nitrobenzene in supercritical water: Kinetic and mechanistic aspects

The catalytic effect of a heteropolyacid, H₄SiW₁₂O₄₀, on nitrobenzene (20 and 30 μM) oxidation in supercritical water was investigated. A capillary flow-through reactor was operated at varying temperatures (T = 400-500°C; P = 30.7 MPa) and H₄SiW₁₂O₄₀ concentrations (3.5-34.8 μM) in an attempt to establish global power-law rate expressions for homogenous H₄SiW₁₂O₄₀-catalyzed and uncatalyzed supercritical water oxidation. Oxidation pathways and reaction mechanisms were further examined via primary oxidation product identification and the addition of various hydroxyl radical scavengers (2-propanol, acetone, acetone-d₆, bromide and iodide) to the reaction medium. Under our experimental conditions, nitrobenzene degradation rates were significantly enhanced in the presence of H₄SiW₁₂O₄₀. The major differences in temperature dependence observed between catalyzed and uncatalyzed nitrobenzene oxidation kinetics strongly suggest that the reaction path of H₄SiW₁₂O₄₀-catalyzed supercritical water oxidation (average activation E_a = 218 kJ/mol; k = 0.015-0.806 s^-1 energy for T = 440-500°C; E_a = 134 kJ/mol for the temperature range T = 470-490°C) apparently differs from that of uncatalyzed supercritical water oxidation (E_a = 212 kJ/mol; k = 0.37-6.6 μM s^-1). Similar primary oxidation products (i.e. phenol and 2-, 3-, and 4-nitrophenol) were identified for both treatment systems. H₄SiW₁₂O₄₀-catalyzed homogenous nitrobenzene oxidation kinetics was not sensitive to the presence of OH• scavengers.