5-Hydroxy Methyfurfural (5-HMF) Production from Biomass Through Heterogeneous Catalysis Based on Heteropolyacids

Heteropolyacids, which are inexpensive and non-toxic strong acids, have been studied as homogeneous catalysts. However, the possibility of supporting them for use as heterogeneous catalysts has been rarely explored. In this work, a series of materials based on 12-tungstophosphoric heteropolyacid heterogenized on zirconia and niobia were prepared and applied as heterogeneous acid catalysts in the valorization of glucose, fructose and real wastewater obtained from beer production. The composites, synthesized under mild conditions, contained homogeneously well-dispersed Keggin clusters for both niobium-containing and zirconium-containing samples when prepared hydrothermally. The catalysts gave good conversions of glucose and fructose, reaching 51 and 68 %, respectively. A conversion of almost 100 % was obtained for the brewery wastewater. However, the selectivity to 5-HMF was very low in all cases. This result has been attributed to the high acidity of the solids which caused overreaction to humins. Using a Design of Experiment (DoE) optimization with a catalyst retaining the Keggin HPA structure, the best conditions were 180 °C for 5 h with 25 mg catalyst and 6 g/L glucose solution in a 25 mL reactor.