Supercritical water oxidation of 2-chlorophenol catalyzed by Cu²⁺ cations and copper oxide clusters

Experimentally, the selectivity to decomposition (S/D) (S/D = (amount of 2-chlorophenol (2CP) oxidized to CO₂ and H₂O)/(disappearance of 2CP)) ratio for oxidation of 2CP with Cu²⁺ cations in supercritical water was greater than that without any catalysts by 60%. The enhancement was due to a fast precipitation of copper chloride in the supercritical water oxidation (SCWO) process. Formation of toxic byproducts was significantly reduced. Because of a confined environment in the channels of ZSM-5, formation of undesired heavy chlorinated phenols and PAH (polycyclic aromatic hydrocarbon) byproducts in the SCWO of 2CP (catalyzed by CuO/ZSM-5) was also highly suppressed. Only trace light-PAHs with a molecule size less than 6 Å were found in the SCWO of 2CP. Carcinogenic PAHs (heavy-PAHs) were not observed. The extended X-ray absorption fine structural (EXAFS) spectra of the catalyst showed that the copper oxides in the channels of ZSM-5 may form Cu₃O₂ clusters with Cu-Cu and Cu-O bond distances of 2.79 and 1.91 Å, respectively. The Cu₃O₂ clusters were oxidized to Cu₃O₄ by H₂O₂ in the supercritical water. Since the diffusion coefficients of copper oxide clusters (Cu₃O₄) in the channels of ZSM-5 was greater than that of 2CP by at least 3 orders, it is possible that these clusters are relatively mobile in ZSM-5 in the SCWO process.