Heterogeneous zirconium and titanium catalysts for the selective synthesis of 5-hydroxymethyl-2-furaldehyde from carbohydrates

Layered zirconium- and titanium- hydrogenphosphates in the alpha and gamma structural arrangements, their corresponding alpha- and gamma-layered pyrophosphates as well as cubic pyrophosphates were used as acid catalysts in the dehydration of fructose and inulin to 5-hydroxymethyl-2-furaldehyde. The experiments were carried out either in batch or in subsequent cycles characterized by extraction of the reaction products, recovery and re-use of both the catalyst and the substrate solution. Good results, in terms of activity and selectivity, have been achieved. In particular, when cubic zirconium pyrophosphate was employed in batch aq. fructose dehydration a turnover number (TN) of about 9 mmol of HMF/g of catalyst x h was obtained with almost complete selectivity towards the target product. Under analogous conditions, gamma-titanium phosphate revealed also promising performances (TN > 6 and selectivity to HMF similar to 98%). When the reaction products were extracted and the above catalysts recycled, the cited performances were retained up to very high conversion for both fructose and inulin substrates. The catalytic performances were discussed and related to the nature of the different acid sites present on the catalyst samples. (C) 2000 Elsevier Science B.V. All rights reserved.