Phenol hydroxylation to produce dihydroxybenzenes over iron-containing UZM-35 catalysts prepared via different methods

Abstract

The chemical state of iron active centers directly influences the inherent catalytic capability of iron-based zeolites. Hence, it is very important to build highly active iron species with a single coordination state through different synthetic schemes from the perspectives of both the theoretical research and practical application. Nevertheless, the identification of dominant active sites in iron-based zeolite-catalyzed phenol hydroxylation remains ambiguous, primarily due to the heterogeneous nature of iron speciation within the zeolitic framework. In this study, iron-based UZM-35 zeolite catalysts with different coordination states were obtained by three different preparation methods using UZM-35 zeolite as the supports. A comprehensive characterization of the catalysts' structure and properties was performed using SEM, XRD, FT-IR, Raman, N₂ physisorption, H₂-TPR, UV–Vis and NH₃-TPD. The findings indicate that Fe-UZM-35 zeolite prepared via the direct synthesis method predominantly contains isolated tetrahedrally coordinated framework Fe³⁺ ion species, whereas Fe/UZM-35 zeolites produced via impregnation and solid-state ion exchange methods primarily consist of octahedrally coordinated isolated extra-framework Fe³⁺ ion species. The data from the catalytic reaction showed that Fe/UZM-35 zeolite catalysts prepared by two post-treatment methods had good activity for phenol hydroxylation, while the dihydroxybenzene selectivity of solid-state ion exchange method was slightly higher than that of impregnation method. As the iron content rose, the selectivity for dihydroxybenzene increased progressively. The sample of G-6wt%Fe/UZM-35 reached a phenol conversion of 53.65 % and a dihydroxybenzene selectivity of 89.93 % (S_CAT = 46.95 %, S_HQ = 42.99 %). Moreover, it was observed that light irradiation affected the dynamics of phenol hydroxylation reaction, resulting in diminished selectivity toward dihydroxybenzene while concomitantly enhancing tar formation. According to the findings from structural analysis and assessment of catalytic performance, we proposed that the octahedral coordinated isolated extra-framework Fe³⁺ ion species have higher hydroxylation activity and dihydroxybenzene selectivity in the phenol hydroxylation reaction catalyzed by iron-based zeolites.