Keggin Heteropolyacid Immobilized on Nanosilica as a Heterogeneous Catalyst for Sugar Dehydration in an Aqueous Medium

The dehydration of fructose and glucose to 5-hydroxymethylfurfural (HMF) in water solution was carried out in the presence of functionalized heteropolyanion-based heterogeneous catalysts. Two catalysts were prepared by immobilizing the Keggin polyoxometalate H₃PW₁₂O₄₀ (PW₁₂) onto nanoSiO₂ by the use of imidazoline and -SO₃^- surface species through acid-base reactions. The catalysts were characterized by N₂ adsorption-desorption isotherms, XRD, TGA, FTIR, solid-state NMR, SEM, and acidity-basicity measurements. Catalytic reactions in batch conditions were performed at 165 °C in the presence of suspended catalysts, and the yield of furfural and 5-hydroxymethylfurfural (5-HMF) was determined. The catalytic activity of the materials was tested for sugars at 1M concentration in a water solution. The valorization of sugars (fructose and glucose) was found to be more effective in the case of fructose. Furthermore, the two catalysts in which the heteropolyacid was immobilized showed activity similar to that observed for naked PW₁₂ (reaction in homogeneous phase), despite the heterogeneous nature of the process, but with the advantage of easier separation at the end of the reaction by simple filtration. The material's substantial stability was demonstrated through three consecutive catalytic test cycles, in which the same catalyst was recovered after each experiment and washed several times with hot water. Finally, tests devoted to the valorization of sugars contained in wastewater from the brewing industry provided a poor yield in 5-HMF, indicating that the catalysts prepared here were, unfortunately, not suitable for this transformation under the conditions tested. This was because the catalysts prepared in this work showed a low capacity to transform glucose (the most present sugar in the carbohydrate fraction of this biomass) into furans.