Catalytic wet air oxidation of phenol with modified activated carbons and Fe/activated carbon catalysts

A commercial activated carbon was modified by different oxidative treatments to introduce different oxygen surface groups. A gentle oxidation in gas phase with a poor oxygen stream and a more severe oxidation in liquid phase with a HNO3 solution were used. The resulting activated carbons and the corresponding Fe-impregnated (2.5% Fe) ones were tested for the wet air oxidation of phenol. A trickle-bed reactor at 400 K and 8 atm of total pressure in a wide range of space time (20–320 gCAT h/gPhenol) was used. The performance of the carbon supports and Fe catalysts was assessed by following phenol and TOC conversions, as well as the toxicity of the effluents. The fresh and used catalysts were characterized by nitrogen adsorption, TPD analyses and pHslurry. Gas-phase oxidation increased mainly the carbonyl/quinone groups and also carboxylic anhydride groups, whereas HNO3 oxidation significantly increased all the oxygen surface groups, but more significantly phenol, carbonyl/quinone and lactone groups. Regarding the preparation of the catalysts, Fe itself plays a promoting role in the formation or destruction of oxygen surface groups during calcinations, the latter being only observed when the activated carbon was oxidized with HNO3.Phenol and TOC conversions increased with the initial acidity of the catalyst. The introduction of Fe enhanced the activity and toxicity abatement at high enough space times, compared to an activated carbon with the same amounts of oxygen surface groups. This can be attributed to the increase in the acidity that the Fe itself provokes and the promoter role of Fe in the oxidation reaction of phenol. After the catalytic wet air oxidation process, surface area and micropore volume decreased whereas the amount of oxygen surface groups increased, as well as the total acidity, except for the activated carbon treated with HNO3. Also Fe dispersion significantly diminishes. However, the final surface distribution did not affect the catalyst activity. The presence of Fe reduced the detrimental effect of wet oxidation on the porous structure of the activated carbons.