Controlled Stability of Liquid-Phase Dispersed Catalytic Systems for Benzene Hydroxylation to Phenol: DLS, Zeta-Potential and EPR/FMR Diagnostics

Direct liquid-phase hydroxylation of benzene to phenol is considered a promising alternative to the conventional multistep cumene process, since it can shorten the reaction pathway, reduce the formation of stoichiometric by-products, and employ hydrogen peroxide as an oxidant that ideally yields water as the main reduction product under relatively mild conditions. However, practical implementation of this process is limited not only by catalytic activity and phenol selectivity, but also by the controlled stability of the dispersed catalytic medium under real reaction conditions. In this work, iron-containing catalytic systems based on boehmite, mordenite, and bentonite were investigated using dynamic light scattering, zeta-potential measurements, UV/Vis spectroscopy, and electron paramagnetic/ferromagnetic resonance. The scientific novelty of the work consists in treating these methods as an integrated diagnostic platform for describing the structural, colloidal, interfacial, and redox stability of liquid-phase catalytic dispersions. The DLS results show measurable changes in hydrodynamic particle size and polydispersity during the reaction, reflecting the balance between dispersion stabilization and aggregation of iron-containing particles. These parameters may serve as early indicators of the transition from a catalytically active dispersed state to a less stable aggregated state. ζ -Potential measurements reveal changes in the electrostatic stabilization of the catalytic medium and make it possible to evaluate the role of interparticle interactions in maintaining catalytic dispersion stability. EPR/FMR studies demonstrate the formation and transformation of paramagnetic iron centers and magnetically ordered Fe-containing domains involved in hydrogen peroxide activation and radical generation. Thus, the combined DLS, zeta-potential, and EPR/FMR data provide an experimental basis for correlating colloidal stability, interfacial charge, redox activity, and catalytic performance. Population-balance, DLVO-type, Brownian aggregation, and kinetic-stability approaches are considered as a framework for predictive control of structurally dynamic liquid-phase catalytic systems.