Oxidation of 2,4,6-trimethylphenol using iron-based catalysts Original Research ArticleOxidation of 2,4,6-trimethylphenol using iron-based catalysts Original Research Article

The catalytic oxidation of 2,4,6-trimethylphenol to 3,5-dimethyl-4-hydroxybenzaldehyde (DMHB) was studied in alcohols under 10 kg/cm2 pure oxygen pressure. Five iron halides were used as the catalysts, it was found that the oxidation activity and the DMHB yield decreased in the order FeCl2 > FeBr3 > FeCl3 >> FeF2 > FeI2. The solvent had significant effect on the FeCl2 catalyst performances, but the solvent effect was opposite to that observed earlier for a CuCl2 catalyst. Among alcohols with 1–6 carbon numbers, methanol was the best solvent for FeCl2, but it was reported earlier to be the worst solvent for CuCl2. In the temperature range of 70–110 °C, the maximum DMHB yield obtained with FeCl2 was around 75% (this yield was higher than that obtained with CuCl2), which occurred at the 2,4,6-TMP conversion of around 93%. Further increase of 2,4,6-TMP conversion resulted in the rapid increase of the by-product (2,6-dimethyl-p-benzoquinone, DMBQ), which suggested that most of the DMBQ came from the oxidation of the desired product. An electron transfer (from 2,4,6-TMP to iron (III)) mechanism involving the intermediacy of an aromatic radical cation was proposed for the oxidation of 2,4,6-TMP to DMHB. Based on the mechanism, the effects of solvents and halide ligands were explained.