Roles of Brønsted-Base and Brønsted-Acid Catalysts for Glucose Isomerization into Fructose and Fructose Dehydration into 5-Hydroxymethylfurfural

Abstract

5-Hydroxymethylfurfural (5-HMF) holds substantial importance as a foundational chemical that can be potentially transformed into biofuels and various additional high-value products. Fructose is gaining popularity as one of the raw materials for the facile formation of 5-HMF. However, it is not as abundant in nature as glucose, the primary product derived from biomass breakdown and the most abundant monosaccharide globally. Producing fructose through glucose isomerization is an economical approach in this context. This study investigates several types of novel, facile, and reusable hydrogel catalysts for the isomerization of glucose into fructose and dehydration of fructose into 5-HMF in green solvent media. This study marks the first application of both amine and amide functional groups within a single catalyst (PEGDA-DMAPMA) for glucose isomerization. The results of glucose isomerization using Brønsted-base PEGDA-DMAPMA hydrogel catalysts reveal a 45% glucose conversion rate and a 27% fructose yield, with a 61% selectivity at 110 °C, within a 2 h reaction time. Brønsted-acid PEGDA-3SMP-H gave a fructose conversion rate of 93%, yielding 65% 5-HMF at 120 °C after 6 h. A significant observation was made for PEGDA-3SMP-H, indicating an increase in the catalytic ability with regeneration cycles. The synthesized catalysts PEGDA-DMAPMA and PEGDA-3SMP-H exhibited stability up to 120 °C despite an increase in swelling ratio at higher temperatures and times. Furthermore, PEGDA-3SMP-Cu2+ demonstrated a considerable yield of fructose compared to other metal ion-modified Brønsted-acid catalysts. Additionally, an efficient heating method for this process was identified. In conclusion, the prepared hydrogel catalysts are preferred for industrial applications in carbohydrate conversion.