Accelerated 5-hydroxymethylfurfural synthesis: Microwave-driven carbohydrate dehydration with mixed metal spinel catalysts

In this study, a series of spinel mixed metal oxide catalysts, namely ZnAl2O4, 1M-SO4/ZnAl2O4-TiO2, 1.5M-SO4/ZnAl2O4-TiO2, and 2M-SO4/ZnAl2O4-TiO2 were synthesized using the sol-gel method followed by sulfate impregnation. The characterization results confirmed the spinel structure of ZnAl2O4 and anatase phase of TiO2. Ternary metal oxides Ti-Zn-Al exhibit enhanced Lewis (L) acidity, while Brӧnsted (B) acidic sites were incorporated due to ammonium sulfate treatment and calcination. Total acidity and B/L ratio increased after the impregnation of sulfate groups; however, a higher sulfate loading (>1M) did not enhance acidity but instead caused agglomeration on the catalyst surface. The selective glucose dehydration to 5-hydroxymethylfurfural was performed in a microwave reactor, and the reaction parameters were optimized for a higher 5-HMF yield. Experimental results demonstrated very high glucose conversion (∼90 %) with ∼58 % of 5-HMF at 150 °C in 25 min in the presence of 1M-SO4/ZnAl2O4-TiO2 catalyst in a biphasic NaCl-H2O/THF medium. The B/L ratio and the solvent used played an important role in the yield of 5-HMF. After several cycles, the regenerated 1M-SO4/ZnAl2O4-TiO2 catalyst demonstrated its consistent activity and 5-HMF yield. This study provides detailed insights into mixed metal spinel catalysts for biomass valorization and reveals a fast, efficient microwave-assisted route for sustainable 5-HMF production.