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

The paper reports on 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 mesoand 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 1408C under continuous-flow conditions, by using H2O/g-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.