Influence of Temperature on Reaction Pathways in Fructose Conversion over Mixed Oxide Catalysts Based on Ce and Nb

The influence of temperature on the product distribution during fructose conversion was systematically investigated using Ce-(P), Nb-(P), CeNb5-(P), CeNb15-(P), and CeNb25-(P) materials prepared by the Pechini method. Reactions were performed in aqueous medium for 2 h at temperatures ranging from 130 to 160 °C. Increasing temperature enhanced fructose conversion but also modified the relative contributions of dehydration, fragmentation, and consecutive degradation pathways. Nb-(P) reached the highest fructose conversion, increasing from 17.6% at 130 °C to 67.9% at 160 °C, but also exhibited a greater contribution from unidentified and degradation products under the most severe conditions. In contrast, the Ce-Nb mixed oxides, particularly CeNb15-(P) and CeNb25-(P), more effectively directed the identified soluble products toward 5-hydroxymethylfurfural (5-HMF), while limiting some competing pathways. The formation of C3 compounds, including dihydroxyacetone and pyruvaldehyde, indicates the occurrence of C-C bond cleavage reactions, whose contribution increased with temperature and depended on material composition. The observed product distributions result from the combined influence of temperature, acid-site nature, concentration, strength, and accessibility, together with the redox properties of the Ce containing materials. These findings demonstrate that the principal contribution of the studied oxides lies in their catalyst dependent modulation of the fructose reaction network rather than in the effect of temperature alone.