Abstract
The cascade dehydration of glucose to 5-hydroxymethylfurfural (HMF) was carried out in water over a series of Nb2O5 catalysts, which were derived from the thermal treatment of niobic acid at 300 and 550 °C, under air or inert atmosphere. Amorphous niobic acid showed high surface area (366 m2/g) and large acidity (2.35 mmol/g). With increasing the temperature of the thermal treatment up to 550 °C, the amorphous Nb2O5 was gradually transformed into a pseudohexagonal phase, resulting in a decrease in surface area (27–39 m2/g) and total acidity (0.05–0.19 mmol/g). The catalysts’ performance in cascade dehydration of glucose realized in pure water was strongly influenced by the total acidity of these materials. A remarkable yield of 37% HMF in one-pot reaction in water was achieved using mesoporous amorphous niobium oxide prepared by thermal treatment of niobic acid at 300 °C in air. The best-performing catalyst displayed a total acidity lower than niobic acid (1.69 mmol/g) which afforded a correct balance between a high glucose conversion and limited further conversion of the target product to numerous polymers and humins. On the other hand, the treatment of niobic acid at 550 °C, independently of the atmosphere used during the sample preparation (i.e., air or N2), resulted in Nb2O5 catalysts with a high ratio of Lewis to Brønsted acid sites and poor total acidity. These materials excelled at catalyzing the isomerization step in the tandem process.
Highlights
The efficient utilization of lignocellulosic biomass is essential for building a sustainable society
Different niobium oxide catalysts obtained by thermal treatments of niobium acid were tested in glucose dehydration to HMF, in order to find the influence of these treatments on their acidity and in turn on their catalytic performance in one-pot cascade dehydration of glucose to HMF in water
The catalytic tests revealed that the best performing catalyst was niobic acid calcined at 300 ◦C, which achieved a substantial yield of 37% of HMF with 66% conversion of glucose in 90 min of an environmentally friendly one-pot process
Summary
The efficient utilization of lignocellulosic biomass is essential for building a sustainable society. The development of an active and selective catalyst for production of HMF from hexoses is rather complex mainly because BA sites catalyze dehydration of fructose to HMF, and side reactions such as rehydration of HMF to levulinic and formic acids [7], and condensation/polymerization of HMF. Niobium pentoxide (Nb2O5) and niobic acid (hydrated form of Nb2O5) serve as highly acidic heterogeneous catalysts that display excellent activity in various reactions [31,32,34,35,36] In this regard, a variety of niobium-based catalysts has been successfully applied in dehydration of hexoses in biphasic solvent media. The relationship between the concentration of acid sites on these catalysts and their catalytic performance was established in direct dehydration of glucose to HMF carried out in neat water, without addition of organic solvents and in the absence of any additives or promoters
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