Catalytic gasification of phenol in supercritical water with different metal cations

Phenol is a representative compound in coal industrial wastewater. Metal cations are the main harmful components in the wastewater of the electroplating and dyeing industries. Supercritical water gasification (SCWG) of phenol with metal cations could provide a promising way to degrade coal chemical wastewater and metal-containing wastewater simultaneously. In this study, SCWG of phenol with Mg²⁺, Mn²⁺, and Ni²⁺ was carried out to investigate the effect of metal cations on gasification results. Results showed that the gaseous product was mainly composed of H₂, CO, CH₄ and CO₂, and CO₂ was dominant. Prolonging residence time and increasing the concentration of metal cations increased the gas yield of CO₂ and carbon gasification efficiency. Higher concentrations of metal cations facilitated methane-reforming reaction and water–gas shift reaction. Metal cations could enhance the ring-opening reaction of phenolic compounds and facilitate the conversion of phenol and liquid intermediates to minor molecule gaseous products. The order of catalytic effect was ranked as Ni²⁺>Mn²⁺>Mg²⁺. Mg²⁺, Mn²⁺ and Ni²⁺ were finally transformed into the corresponding metal hydroxides and metal oxides which could be recycled for reutilization. NiO could be reduced into metallic Ni, which consumed the H₂ but effectively catalyzed SCWG of phenol.