Quantification of the Microwave Effect in the Synthesis of 5-Hydroxymethylfurfural over Sulfonated MIL-101(Cr)

Abstract

The potential benefits of microwave irradiation for fructose dehydration into 5 hydroxymethylfurfural (5-HMF) have been quantified over a sulfonated metal–organic framework (MOF), MIL 101(Cr)-SO3H. The effects of temperature (140–170 °C), batch time (5–300 min), and catalyst-to-substrate ratio (0.1–0.01 g/g) were systematically mapped. After 10 min of microwave (MW) irradiation at 140 °C in a DMSO–acetone reaction medium, practically complete fructose conversion was obtained with a 70% yield of 5-HMF. Without MW, i.e., using conventional heating (CH) at the same conditions, the fructose conversion was limited to 13% without any 5-HMF yield. Rather, 90 min of CH was required to reach a similarly high conversion and yield. The profound impact of moving from CH towards MW conditions on the reaction kinetics, also denoted as the microwave effect, has been quantified through kinetic modeling via a change in the Gibbs free energy of the transition state. The modeling results revealed an eight-fold rate coefficient enhancement for fructose dehydration owing to MW irradiation, while the temperature dependence of the various reaction steps almost completely disappeared in the investigated range of operating conditions.