Low‐Temperature Continuous‐Flow Dehydration of Xylose Over Water‐Tolerant Niobia–Titania Heterogeneous Catalysts

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The sustainable conversion of vegetable biomass‐derived feeds to useful chemicals requires innovative routes meeting environmental and economical criteria. The approach herein pursued is the synthesis of water‐tolerant, unconventional solid acid monolithic catalysts based on a mixed niobia–titania skeleton building up a hierarchical open‐cell network of meso‐ and macropores, and tailored for use under continuous‐flow conditions. The materials were characterized by spectroscopic, microscopy, and diffraction techniques, showing a reproducible isotropic structure and an increasing Lewis/Brønsted acid sites ratio with increasing Nb content. The catalytic dehydration reaction of xylose to furfural was investigated as a representative application. The efficiency of the catalyst was found to be dramatically affected by the niobia content in the titania lattice. The presence of as low as 2 wt % niobium resulted in the highest furfural yield at 140 °C under continuous‐flow conditions, by using H 2 O/γ‐valerolactone as a safe monophasic solvent system. The interception of a transient 2,5‐anhydroxylose species suggested the dehydration process occurs via a cyclic intermediates mechanism. The catalytic activity and the formation of the anhydro intermediate were related to the Lewis acid sites (LAS)/Brønsted acid sites (BAS) ratio and indicated a significant contribution of xylose–xylulose isomerization. No significant catalyst deactivation was observed over 4 days usage.