One-step highly selective oxidation of p-xylene to 4-hydroxymethylbenzoic acid over Cu-MOF catalysts under mild conditions

Abstract p-Xylene, a cheap industrial raw material, is mainly used to synthesize terephthalic acid. However, p-xylene was oxidized rarely to other oxygen-containing derivatives. On the other hand, current methods for synthesis of 4-hydroxymethylenzoic acid suffer from multistep reactions, long reaction time, high temperature, high pressure, and lower selectivity. In this work, p-xylene was directly oxidized into 4-hydroxymethylbenzoic acid using Cu-MOF as the catalyst, acetonitrile as the solvent, and 30% H2O2 as the oxidant under mild conditions. Very high selectivity (99.2%) of 4-hydroxymethylbenzoic acid and good substrate conversion (85.5%) of p-xylene were achieved at 30 °C for 5 h. Fast hot catalyst filtration experiments proved that Cu-MOF acted as a heterogeneous catalyst. It can be reused three times without losing its activity. Under optimized conditions, the selectivity of 4-hydroxymethyl benzoic acid and the conversion of p-xylene were 97.6% and 84.8% even though the experiment was amplified 15-folds. The efficiency of H2O2 was also up to 92.0%. The reaction mechanism was proposed combined with the analysis of XRD, FT-IR and XPS. All data displayed that Cu-MOF shows great potential as a catalyst for highly selective oxidation of stable C–H to high value-added compounds containing two diverse C–O bands under mild conditions. It also opens a new way to use of p-xylene.