Solvent-Free Aerobic Oxidation of Toluene on High-Entropy (MnFeCuCoNi)₃O₄ Oxide Supported on Reduced Graphene Oxide

The growing demand for noble metal-free heterogeneous catalysts in selective, solvent-free oxidation reactions faces a persistent challenge: balancing high activity with practical scalability. Addressing this gap, we present a scalable route for the synthesis of the (MnFeCuCoNi)3O4 high-entropy oxide (HEO) catalyst and its composite with reduced graphene oxide (HEO-rGO). Comprehensive characterizations, including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM, thermogravimetry analysis (TGA), confirmed the successful formation of a single-phase spinel HEO structure and their relatively uniform dispersion on the rGO matrix, minimizing aggregation and maximizing accessible active sites. The HEO–rGO composite demonstrates exceptional performance in the solvent-free aerobic oxidation of toluene, achieving conversions of 13.6% (1 atm) and 18.5% (10 atm) within 5 h, with remarkable selectivity for benzaldehyde (>91 and 87%, respectively). These metrics rank among the highest reported values for noble metal-free systems. Mechanistic studies reveal that this activity stems from synergistic effects: (i) the multicationic HEO structure enhances lattice oxygen mobility, (ii) abundant oxygen vacancies facilitate reactant activation, and (iii) rGO promotes electron transfer and product desorption. Density functional theory (DFT) calculations further corroborate the critical role of oxygen vacancies in lowering the energy barrier for toluene oxidation. This work not only advances the design of high-entropy catalysts for selective oxidations but also provides a scalable, cost-effective strategy for converting toluene─an abundant petrochemical feedstock─into high-value benzaldehyde, with promising industrial applicability.