Beyond synergy in MOF nanocomposites: Autocatalytic degradation, inhibition by magnetite, and reactivation within the “autocatalysis window”

The mechanisms of •OH generation in Fenton-like systems based on MIL-88B(Fe), Fe 3 O 4 nanoparticles, and Fe 3 O 4 -MIL-88B(Fe) nanocomposites were investigated using spectrophotometric assays, in situ EPR spectroscopy, time-resolved Fe-leaching analysis, and post-reaction SEM/FTIR characterization. Classical spectrophotometric tests were shown to be insufficient for describing heterogeneous self-accelerated radical generation; therefore, the kinetic analysis was based primarily on direct EPR detection of the DMPO-OH adduct. At pH 3.0, MIL-88B(Fe) exhibited an autocatalytic •OH-generation regime: acid-induced release of labile Fe initiated the Fenton cycle, while radical-assisted labilization of accessible Fe-carboxylate domains promoted further Fe release. Post-reaction SEM and FTIR data showed retention of the main bulk morphology and framework-related vibrational bands, indicating that this process corresponds to partial labilization of the chemically accessible MIL-88B(Fe) fraction rather than complete destruction of the whole solid material. Incorporation of Fe 3 O 4 suppressed the autocatalytic response; possible contributing factors include reduced effective accessibility of MIL-88B(Fe)-derived labile Fe sites and less productive interfacial H 2 O 2 conversion. Ascorbic acid partially restored the radical-generating activity by accelerating the Fe 3+ /Fe 2+ redox cycle, while residual ascorbate also acted as a temporary radical scavenger and led to an induction period before autoacceleration. A phenomenological kinetic equation describing self-accelerated •OH generation in the studied MIL-88B(Fe)-based Fenton-like systems was derived. Based on the combined EPR and Fe-leaching data, an operational “autocatalysis window” is proposed for this MIL-88B(Fe)-based catalyst series, defined by protonation-favored framework labilization, accumulation of labile Fe above a threshold level, and predominance of radical-assisted Fe release over simple acid hydrolysis or passivation. Broader application of this concept to other Fe-MOF families requires additional experimental verification.