Hydrogel-based heterogeneous-acid-catalysts for converting carbohydrates into the platform chemical: 5-hydroxymethylfurfural

Background: 5-hydroxymethylfurfural (5-HMF) is a significant platform chemical with the potential to be converted into hydrocarbon biofuels. Herein, a facile and reusable catalyst was investigated to synthesize 5-HMF from monosaccharides in a green solvent medium following eight green chemistry principles. Methods: A hydrogel-based heterogeneous acid catalyst was synthesized through photoreaction to determine its ability to dehydrate fructose in the invert sugar syrup into 5-HMF. The main structure of the hydrogel catalyst was formed with polyethylene glycol diacrylate (PEGDA), and this backbone structure was modified by 3-sulfopropyl methacrylate potassium salt (3SMP) and further acidified by 3M HCl to form PEGDA-3SMP-H with 32.54 mmole/g of H⁺ amount on the catalyst. Significant findings: With the PEGDA-3SMP-H catalyst, fructose conversion of 85.80% with a 5-HMF yield of 46.99% could be attained at 120°C for a 24 h reaction time in an aqueous medium with the co-solvent gamma-butyrolactone (GBL), which is considered a green solvent medium. PEGDA-3SMP-M (M=metal) hydrogel catalyst was synthesized using 8 different metal ions fixed on the 3SMP through ion exchange. The catalytic ability sequence for synthesized hydrogel catalysts followed H⁺> Cr³⁺> Cu²⁺> Al³⁺> Ni²⁺> Co²⁺ > Cu⁺> Fe³⁺> Fe²⁺ for 5-HMF formation. The 5-HMF yield was improved with consecutive cycles of using a regenerated PEGDA-3SMP-H catalyst. In the first cycle, the 5-HMF yield was 36.86%; in the second and third cycles, it was 39.39% and 41.49%, respectively. The prepared heterogeneous catalyst is preferred in terms of industrial applications owing to its facile synthesis, recyclability, stability, simple separation method, low equipment corrosion, and minimization of final product contamination with acid catalysts.