A batch-type subcritical water system was used to perform the decomposition reaction of D-fructose to 5-hydroxymethy furfural (HMF) over a temperature range of 473-593 K. The catalytic effect of various organic and inorganic acids with pH values in the range of 1.5-5 was evaluated. It was found that not only the pH, but also the nature of the acids, had great influence on the decomposition pathway. At lower pH, a rehydration of HMF to levulinic and formic acids occurred, whereas at higher pH, polymerization reactions occurred. Phosphoric acid (pH 2) was determined to give good balance between activity, yield, and byproduct amounts. The optimum condition for the best yield of HMF (65%) was achieved at a temperature of 513 K for a residence time of 120 s. The decomposition reaction of other monosaccharides and disaccharides were also investigated, and it was determined that the ketohexoses give higher yields than aldohexoses. However, all saccharides showed the possibility of the conversion to HMF in subcritical water.

This paper deals with the dehydration of fructose (F) to 5-hydroxymethylfurfural (HMF) and the rehydration of HMF to levulinic acid (LA) along with formic acid (FA) in subcritical water (sub-CW) in the presence of HCl as catalyst. The experimental conditions were 483-543 K, 4-15 MPa, and residence times of 0.5-300 s. The pressure (in the range of 4-15 MPa) did not affect the decomposition reactions. This differed from other operating variables. The main products were found to be HMF, 2-furaldehyde (2FA), LA, FA, and soluble polymers. The HMF, LA, and FA were also individually subjected for decomposition at the same condition. We found that, during the sub-CW reaction, byproducts (soluble polymers) were produced not only from F but also from HMF. However, we distinguished between two different polymers (i.e., produced from the decomposition of F or HMF) in a proposed reaction model. The kinetics of the sub-CW reaction was developed by considering not only F, HMF, LA, and FA but also soluble polymers. The kinetic constants obtained from the proposed reaction pathway showed good agreement with experimental results.

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