Synergistic effects of Fe species distribution and acid site balance in Fe/zeolite catalysts for 5-HMF production from glucose

Abstract

The selective conversion of glucose to 5-hydroxymethylfurfural (5-HMF) remains a key challenge in biomass valorization due to the complex interplay between Brønsted and Lewis acid sites. In this work, iron-supported HMOR and HZSM-5 zeolites were synthesized and systematically investigated to elucidate the roles of acid site nature, strength, and iron speciation in glucose conversion. Comprehensive characterization using XANES, UV-vis DRS, XPS, NH₃-TPD, and pyridine-IR revealed that Fe incorporation generates bifunctional catalysts containing Fe-based Lewis acid sites (Fe₂O₃ and Fe₃O₄) together with Brønsted acid sites derived from framework Si–OH–Al, surface Fe–OH, and extra-framework Al–OH species. Py-IR analysis showed that Fe/HMOR exhibits a higher Brønsted/Lewis (B/L) acid ratio than Fe/HZSM-5 due to the formation of oligomeric and hydrated Fe species. Catalytic evaluation in a biphasic n-butanol/water system demonstrated that moderate acidity and an optimized B/L ratio are critical for enhancing 5-HMF selectivity while suppressing levulinic acid formation. The highest 5-HMF yield (∼44–45%) was achieved with 6Fe/HMOR at 190 °C, which correlated strongly with a high fraction of Fe₃O₄ species. These findings provide mechanistic insight into acid site synergy and offer design principles for improved glucose to 5-HMF catalysts.