离子液体中果糖转化制备高浓度5-羟甲基糠醛

The platform chemical 5-hydroxymethylfurfural (HMF), a hexose dehydration product, is an important bio-based chemical intermediate[1]. It can be further transformed into other chemicals such as 2,5-disubstituted furan derivatives and levulinic acid, which are considered as potential sustainable substitutes for building blocks derived from petrochemicals in the production of plastics and fine chemicals[2]. However, challenges remain to attain HMF from hexoses in a selective and cost-effective way[3]. In this study, we would like to report our results on HMF production using ionic liquids as solvents. Dialkylimidazolium halide ionic liquids were used as reaction media for dehydration of fructose into 5-hydroxymethylfurfural (HMF) under conventional oil-bath heating and microwave-assisted heating, respectively. It was found that the catalyst-free reaction in ionic liquids under oil-bath heating proceeded slowly. In the presence of various strong protonic acids and acidic ionic liquids, the reaction rate was substantially enhanced and the yields of HMF reached above 90%. In this case, however, side reactions were also induced by excessive catalyst. Meanwhile, water amount affect both dehydration rate and HMF yield significantly. By N2 blowing, water formed in the dehydration process could be removed instantaneously, thus higher HMF yield could be achieved. On the other hand, weak protonic acids and some organic acids, namely phosphoric acid, acetic acid and citric acid exhibited low catalytic activity. In the second part of this paper, microwave-assisted dehydration of fructose in ionic liquid was investigated. Under microwave irradiation, outstanding dehydration activity was obtained, fructose could be converted to HMF within 5 min in high selectivity with no other additive. More significantly, HMF yield of ca. 57%-97% could be obtained from high concentration fructose (50-1200 wt%) in ionic liquid under catalyst-free condition. For example, even a syrup made with 12.0 g of fructose in 1.0 g of C4mimCl, HMF yield up to 57% could be obtained within 4.8 min at 400 W irradiation. In-situ 13C NMR and 1H NMR spectra indicated that the transformation of fructose in ionic liquid was a high selective reaction proceeded predominantly in cyclic fructofuransyl intermediate pathway. Our effort on HMF production in ionic liquids might pave a new path from carbohydrates to non-petrochemicals and biofuels