Conversion of highly concentrated fructose into 5-hydroxymethylfurfural by acid–base bifunctional HPA nanocatalysts induced by choline chloride

Abstract

A series of acid–base bifunctional heteropolyacids (HPAs) (C₆H₁₅O₂N₂)_3−xH_xPW₁₂O₄₀ (abbreviated as Ly_3−xH_xPW) have been designed using different ratios of HPAs with amino acid lysine in order to control their acid–base properties. The amino acid group facilitated the HPAs forming micellar assemblies in choline chloride–fructose deep eutectic solvents. In the dehydration of fructose to 5-hydroxymethylfurfural (HMF), Ly_3−xH_xPW exhibited different catalytic activities because of their different acid–base properties. Among all the HPA catalysts, Ly₂HPW gave the best results with 93.3% conversion and 92.3% HMF yield within a very short time, i.e. 1 min under the conventional temperature of 110 °C using choline chloride (ChCl) as solvent, and this was almost the best result by far. The highest activity and selectivity of Ly₂HPW came from the synergistic effect between certain acidic and basic capacities, which provides ready accessibility to the nucleophilic (–NH₂) and electrophilic (H) sites. Moreover, this catalyst was tolerant to highly concentrated feedstock (∼66.7 wt%) with the additive ChCl. Ly₂HPW performed as a heterogeneous catalyst in the ChCl system and could be recycled by simple washing treatment.