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

Abstract

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 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.

Aqueous-phase tandem catalytic conversion of xylose to furfuryl alcohol over [Al]-SBA-15 molecular sieves

“…45 It reveals that both Lewis and Brønsted acid sites on [Al]-SBA-15 catalysts identified by pyridine-adsorbed infrared spectra are active and playing a role in the pentose tandem transformation. The production of furfuryl alcohol may, therefore, be explained by the tandem reaction of xylose/xylulose dehydration to furfural, which depends predominantly on Brønsted acidity, 12,60,61 while the next cascade step (furfural transfer hydrogenation) occurs on the Lewis acid sites. 44,45,62,63 This is consistent with the formation of propanone which was observed throughout the experiments using all modified catalysts, as depicted in Fig.…”

“…In the case of xylose dehydration, significant efforts have been made to increase the sustainable aspect of this reaction, particularly by replacing the conventionally used homogeneous stoichiometric mineral acids with heterogeneous catalysts. [4][5][6][7][8][9][10][11][12] Moreover the hydrogenation of furfural to furfuryl alcohol in a second independent process is industrially carried out over CuCr-based catalyst which is unsustainable. 13,14 To date, this catalytic system remains despite extensive studies aimed at replacing it for base or noble metals supported catalytic processes.…”

Sustainable Catalytic Synthesis for a Bio‐Based Alternative to the Reach‐Restricted N‐Methyl‐2‐Pyrrolidone

“…Prompted by our previously reported engineering of bifunctional catalysts for the selective conversion of biomass‐derivatives,35 the perfluorosulfonic acid polymers Aquivion PW65S (linear) and PW79S‐02%X‐link (0.2 wt% crosslinked), featuring high Hammet acidity (≈−12), chemical and thermal stability,36 were evaluated. The hierarchically porous inorganic monoliths based on a TiO 2 (TiMNL) or mixed TiO 2 —2mol% Nb 2 O 5 (2%Nb@TiMNL) skeleton were also examined, due to their enhanced mass‐transfer and acidic properties 37,38. Lewis (LAS) and Brønsted (BAS) acid sites densities of the solid acids screened are summarized in Table 1 .…”

Catalytic Processes from Biomass-Derived Hexoses and Pentoses: A Recent Literature Overview

“…Moreno-Marrodan et al created several macroporous-mesoporous catalysts based on titania and niobia-titania, again with acid sites of the Brønsted and Lewis kind. These monolithic catalysts were prepared for and tested in continuous flow-through systems (fixed bed reactors), showing high stability on-stream for up to 30 h. However, selectivity to furfural was low (25%), while xylose conversion was stable and high (95-100%) when working at 130 • C in a liquid-liquid-solid fixed bed reactor with γ-valerolactone and water mixed in a ratio 9/1 v/v [77]. To facilitate mass transfer within the porous structure and flow-through processing, another strategy followed was to create packed structures by liquid phase synthesis and evaporation-induced-self-assembly (EISA) methods.…”