Catalytic oxidation of phenol over MnO₂ in supercritical water

Bulk MnO₂ was used as a catalyst for phenol oxidation in supercritical water at 380-420 °C and 219-300 atm in a flow reactor. The bulk MnO₂ catalyst enhances both the phenol disappearance and CO₂ formation rates during supercritical water oxidation (SCWO), but it does not affect the selectivity to CO₂ or to the phenol dimers at a given phenol conversion. The role of the catalyst appears to be accelerating the rate of formation of phenoxy radicals, which then react in the fluid phase by the same mechanism operative for noncatalytic SCWO of phenol. The rates of phenol disappearance and CO₂ formation are sensitive to the phenol and O₂ concentrations but independent of the water density. Both power-law and dual site Langmuir- Hinshelwood-Hougen-Watson (LHHW) rate laws were developed to correlate the catalytic kinetics. Our results show that SCWO reactor volumes can be reduced by an order of magnitude if bulk MnO₂ is used as the catalyst and by yet another order of magnitude if a supported oxidation catalyst is used.