Fructose Dehydration to 5-Hydroxymethylfurfural over Solid Acid Catalysts in a Biphasic System

Different acidic heterogeneous catalysts like alumina, aluminosilicate, zirconium phosphate, niobic acid, ion‐exchange resin Amberlyst‐15, and zeolite MOR have been studied in fructose dehydration to 5‐hydroxymethylfurfural (HMF). The acidity of these materials was characterized using temperature‐programmed desorption of NH3 and IR spectroscopy of adsorbed pyridine. The nature and strength of acid sites was shown to play a crucial role in the selectivity towards HMF. Brønsted acid sites in the case of zeolites and ion‐exchange resin led to high selectivities in the dehydration of fructose with an increase in selectivity with the addition of an organic phase. Lewis acidity in the case of phosphate and oxides resulted in the intensive production of humins from fructose at the initial stages of the process, whereas organic phase addition did not affect selectivity. Fructose dehydration over heterogeneous acidic catalysts is studied. Lewis acidity leads to direct fructose condensation and humins are produced. Brønsted acidity results in selective 5‐hydroxymethylfurfural (HMF) formation with subsequent rehydration into acids. Organic solvent addition increases selectivity exclusively over Brønsted acid catalysts due to suppression of the rehydration reaction of HMF and primary fructose condensation with HMF (see figure).