Two-stage synthesis of Fe-UZM-35 zeolite promoted by Fenton's reagent and its application in hydroxylation of phenol

Abstract

The synthesis of UZM-35 zeolite suffers from long crystallization times (4–14 days), irregular product morphology, and low purity and crystallinity, which significantly limit the material's potential applications. In this study, Fe-UZM-35 zeolite was synthesized using a two-stage crystallization method to address these issues. Initially, Fenton's reagent was employed as both an iron source and a hydroxyl radical generator, taking advantage of the hydroxyl radical's ability to accelerate zeolite nucleation. The system was first microwave-heated at low temperatures to achieve rapid and uniform nucleation, followed by conventional heating at higher temperatures to promote uniform growth of the zeolite crystal nuclei. The effects of Fenton's reagent dosage, low-temperature pretreatment duration, and high-temperature crystallization time on the zeolite's crystallization process were thoroughly investigated. It was found that Fe-UZM-35 zeolite with high crystallinity could be obtained under the conditions of a H₂O₂/Fe^IIO ratio of 2.5, a 5-h low-temperature pretreatment, and a 3-day high-temperature crystallization period. Several Fe-UZM-35 catalysts with varying iron contents were synthesized under these optimized conditions. The catalysts crystallinity, morphology, pore structure, iron species distribution, and acid properties were characterized using XRD, SEM, N₂ physisorption, UV–Vis, H₂-TPR, and NH₃-TPD, respectively. Selected catalysts were evaluated for catalytic performance in the hydroxylation of phenol as a model reaction. Results showed that the catalytic activity of iron-based zeolites is influenced by multiple factors, with Fe-UZM-35 zeolites exhibiting higher relative crystallinity, surface area, Lewis acid density, and hydrophobicity delivering the best catalytic performance.