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Selective hydroxylation of benzene to phenol remains a challenging reaction in green chemistry. In this work, liquid-phase hydroxylation of benzene with H2O2 over iron-containing mordenite was studied using dynamic light scattering (DLS) and ferromagnetic resonance (FMR) techniques to assess catalyst stability and structural changes under reaction conditions. Iron-modified mordenite samples were prepared by impregnation of H-mordenite with Fe(III) and Fe(II) salts, followed by washing, drying, and milling to ~1 μm. DLS measurements revealed a significant increase in particle size during reaction (from ~1.8 to 2.6 μm at 333 K in 6 h), indicating aggregation of iron-containing mordenite particles. FMR spectra showed only minor shifts in resonance parameters, suggesting that the main part of iron oxide species remained stabilized within the mordenite framework. The catalytic activity decreased over time due to surface blocking by reaction products but was almost fully restored after ultrasonic treatment (26 kHz, 10 min). Thermodynamic calculations (ΔG° ≈ −293 to −289 kJ·mol⁻¹) confirmed the feasibility of the benzene hydroxylation process, while the reaction yields were limited by kinetic factors. Our results demonstrate that nanosized iron species in mordenite are responsible for H2O2 activation and that ultrasound treatment is an effective method for regenerating catalyst activity, highlighting a promising approach for sustainable phenol production.
