Isomerization of glucose into fructose in the presence of cation-exchanged zeolites and hydrotalcites

Isomerization of glucose into fructose was performed in a batch mode in the presence of a series of alkaline solid catalysts such as cation-exchanged A, X, Y zeolites and hydrotalcites. Under optimized operating conditions, water as the solvent, 95 degrees C, up to 250 g/l of initial glucose concentration, up to 20 wt.% of catalyst, 700-1200 rpm and 8 bar of nitrogen pressure to avoid oxidation reactions, the reaction is not controlled by external or internal diffusional limitations. Among the different catalysts used, Li-, Na-, K-, Cs-, Ca- and Ba-exchanged A, X and Y zeolites, and hydrotalcites, Ca- and Ba-exchanged A, X and Y zeolites were found less selective, whereas those with a moderate basicity such as NaX and KX were found to achieve isomerization of glucose into fructose with a selectivity to fructose of about 90%, but at low glucose conversion, around 10-20%. However, the high selectivity to fructose is only obtained at glucose conversions lower than 25%. Furthermore, it was found that a significant amount of the cation passed into water (around 15% in the case of monovalent cation-exchanged X zeolites). This leaching phenomenon is no longer observed after a second run. The conversion of glucose is stabilized at about 10% without loss of selectivity to fructose, so that a continuous process may be considered. (C) 2000 Elsevier Science B.V. All rights reserved.