Understanding Hβ Zeolite in 1,4-Dioxane Efficiently Converts Hemicellulose-Related Sugars to Furfural

Furfural represents one of the most valuable biomass-derived platform molecules. Its selective production with a smart and tractable catalytic system is highly attractive for technical implementation, yet remains challenging. We herein report a promising protocol involving the mature Hβ zeolites in pure 1,4-dioxane to convert hemicellulose-related sugars into furfural with high rate and impressive selectivity under mild operating conditions. Furfural yields as high as 93.6% (from xylose) and 85.9% (from xylan) were accomplished at 140 °C in 40 min. The solvent 1,4-dioxane served as a shield to maintain the crystal structure and acidic center of Hβ as well as to inhibit the polymerization of furfural. The synergistic action of Hβ with 1,4-dioxane largely promoted the selective conversion of xylose to furfural, as the multiple immobilized acid sites of Hβ in 1,4-dioxane medium accelerated xylose-to-xylulose isomerization. The dehydration process was favorable based on its apparent activation energy of 39.1 kJ mol–1, leading to a high turnover frequency of 53.8 h–1. Density functional theory calculations revealed that the superior catalytic behavior could be ascribed to the higher adsorption of xylose than 1,4-dioxane, allowing xylose molecules to fully access the open Al sites of Hβ and the low Gibbs free energy barrier of xylose isomerization to xylulose in 1,4-dioxane over the Hβ catalyst. Moreover, the heterogeneous zeolite catalyst and the single low-boiling-point solvent 1,4-dioxane (101 °C) were readily recovered from the product with outstanding reusability. This contribution not only opens a high potential avenue for the selective production of furfural but also sheds light on the coordination mechanism of Hβ zeolite and 1,4-dioxane toward developing tailored processes.