A Route without Acetic Acid/Bromide for the Aerobic Oxidation of Methyl Aromatics to Carboxylic Acids Catalyzed by Anderson-Type Polyoxometalate

The development of highly efficient heterogeneous catalytic systems for the methyl aromatics oxidation to their corresponding carboxylic acids using O2 as the oxidant is highly desirable due to the high energy barrier associated with molecular oxygen activation and the inherent stability of benzylic C(sp3)–H bonds. In this study, we present a novel method for the aerobic oxidation of methyl aromatics using NHPI in combination with Anderson-type catalysts, (NH4)3[X(III)(OH)6Mo6O18] (denoted as XMo6, where X = Co, Cr, Fe). The XMo6 catalysts were synthesized via recrystallization and systematically characterized by XRD, FT-IR, ICP-MS, NH3-TPD, and TGA. The catalysts were found to be suitable for gram-scale oxidation of methyl aromatics under environmentally benign and mild conditions, with a maximum yield of 94% in acetonitrile solvent containing 5% water. XMo6 exhibited a synergistic effect between the X-metal hybrid atom and the MoO6 inorganic ligand, as well as effective coordination with NHPI, enabling selective oxidation. Consequently, the catalytic system demonstrated excellent tolerance and selectivity for a range of methyl aromatics with various substituents. Mechanistically, the oxidation of the benzylic C(sp3)–H bond is initiated by the PINO radical generated through the interaction between X(III)Mo6 and NHPI. Subsequent activation by O2 and X(II)Mo6 leads to aldehyde formation. The aldehyde is then oxidized to the final carboxylic acid via Mo–O reactive species. The straightforward synthesis and continuous reusability of the catalysts, along with the simplified process and high product yields, render this approach both environmentally benign and cost-effective.