Synergistic Octahedral Iron and Brønsted Acid Sites in Fe-ZSM-5 for Selective Phenol Hydroxylation to Dihydroxybenzenes

Iron-containing zeolites have shown great potential in catalyzing phenol hydroxylation with H2O2 to produce dihydroxybenzenes (catechol and hydroquinone). However, their industrial application has been hindered by low selectivity and the complexity of active-site identification. This study addresses these challenges by synthesizing a series of Fe-ZSM-5 catalysts with iron contents up to 4 wt % via an ionic liquid-assisted interzeolite transformation route. Through systematic regulation of transformation temperature and calcination atmosphere, combined with an acid treatment step, a catalytically inert benchmark sample (T180-Air) was strategically prepared and compared with three active variants (T200-Air, T180-N2, T200-N2) to achieve the isolation and identification of active sites. Employing a “differential comparison strategy”, combined with DR UV–vis, Raman, Mössbauer, XANES/EXAFS, EPR, H2-TPR, in situ FT-IR characterizations, and DFT calculations, we systematically excluded the possibility of binuclear iron species, small FexOy clusters, and distorted tetrahedral iron as primary active sites. The evidence consistently points to extra-framework octahedrally coordinated mononuclear iron as the key active species, which effectively activates both phenol and H2O2 to generate reactive intermediates. Moreover, the high-selectivity formation of dihydroxybenzenes is driven by a synergistic interaction between these mononuclear octahedral iron sites and neighboring strong acid centers. The optimal catalyst (T200-Air) achieved a phenol conversion of 46.7% and a dihydroxybenzene selectivity of 95.6% under dark conditions, with its dihydroxybenzene selectivity surpassing that of state-of-the-art Fe-based catalysts at comparable phenol conversion levels. This work elucidates the critical importance of the synergistic design between the active iron species and the local acidic microenvironment, providing clear experimental evidence and structure-performance guidelines for the rational design of high-performance iron-zeolite catalysts for selective phenol hydroxylation reactions.