Impact of surface properties of activated carbons on oxidative coupling of phenolic compounds

Previous studies showed that activated carbons exhibit significantly different behavior for the removal of phenolic compounds in the presence and absence of molecular oxygen. Increase in capacity under oxic conditions (presence of oxygen) as high as 2.5 fold was observed for 2-methylphenol adsorption on a bituminous coal-based activated carbon. The present study was initiated to evaluate some of the possible mechanisms of oxidative coupling of phenols that are promoted by the activated carbon surface and result in such a significant increase in capacity. In particular, acidic and basic surface functional groups and metals and metal oxide complexes that are commonly present on activated carbon surface were investigated for their role in catalyzing oxidative coupling of phenolic compounds under oxic conditions. Bituminous coal-based carbon and a carbonaceous resin were used as model adsorbents, while 2-methylphenol and 2-chlorophenol were used as model adsorbates in this study. Metal content was altered either by prolonged acid washing of activated carbon or by addition of metal oxides to the carbonaceous resin using the incipient wetness method. Oxygen containing acidic and basic surface functional groups were modified by outgassing at different temperatures or by exposure of outgassed carbons to oxygenated water. Freundlich isotherm parameters and solvent extraction efficiencies for virgin and modified varieties of these adsorbents have shown that none of the parameters investigated in this study have a significant impact on the exhibited adsorptive and catalytic properties of activated carbon under oxic and anoxic conditions. It appears that oxygen-containing basic surface functional groups are primarily responsible for the catalytic properties of activated carbon towards oxidative coupling of phenolic compounds. (C) 1997 Elsevier Science Ltd.