Red mud (RM) is an abundant alkaline industrial residue whose disposal creates persistent environmental and land-use burdens. Its Fe-rich composition offers a potential raw material to synthesize iron-based catalysts for catalytic oxidation of volatile organic compounds (VOCs). Here we report an environmentally friendly route to convert red mud into an active catalyst for toluene oxidation that combines magnetic separation and microwave irradiation. Magnetic separation can enrich the iron oxide fraction, while microwave treatment drives the reconstruction of RM’s structure, in which Ti migrates from FeTiO₃ into the Fe₂O₃ lattice. Magnetic measurements were conducted to probe the spin state of Fe species in the catalysts. Additionally, DFT calculations were performed to evaluate the oxygen vacancy formation energy, clarifying the effect of Ti induction on electronic density and the d-band center for Fe. This Ti migration and spin-state regulation improve O₂ activation by decreasing the energy required to form oxygen vacancies and promoting toluene adsorption by shifting the d-band center from -2.96 eV to -1.87 eV. The optimized microwave-activated RM (MRM) catalyst achieves 90% toluene conversion at 261 °C under a gas hourly space velocity of 30000 mL g^-1 h^-1, showing a comparable catalytic performance among RM-derived catalysts. In addition, MRM maintains approximately 95% conversion for 100 h at 350 °C under a high gas hourly space velocity of 120,000 mL g^-1 h^-1. This work demonstrates a rapid and environmentally friendly route for converting red mud into high-activity Fe-based catalysts for toluene oxidation, offering a promising strategy to combine industrial solid-waste valorization and air pollution control.
 

Red mud valorization; Microwave-assisted reconstruction; Fe-based oxide catalysts; Spin-state regulation; Oxygen vacancies;