Microwave-Assisted Efficient Fructose–HMF Conversion in Water over Sulfonated Carbon Microsphere Catalyst

A simple hydrothermal carbonization method was used to synthesize sulfonated carbon catalysts from glucose in reflux conditions. The concentration of sulfuric acid played a specific role in preparing sulfonated hydrothermal carbon microsphere (SHCS) and sulfonated hydrothermal carbon (SHC) catalysts. All of the samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) analyses, and the influence of different reaction conditions on 5-hydroxymethylfurfural (HMF) production from fructose was investigated in microwave conditions. The effect of microwave input power and energy efficiency on HMF formation was determined to understand the role of sulfonated carbon catalysts in a water medium. Results showed that the catalyst with a moderate sulfur content and spherical stacking was beneficial to enhance the heat-transfer efficiency on the catalyst surface during microwave-assisted heating, thus effectively enhancing the energy efficiency of the fructose dehydration reaction. The optimum reaction condition for HMF production was obtained at microwave input power 60 W, reaction temperature 186 °C, and reaction time 10 min. At this condition, the HMF yield and the energy efficiency were 88.3 mol % and 0.147 mmol kJ^–1, respectively. Furthermore, the prepared catalyst showed good recyclability, wherein a simple reactivation process could rapidly recover the catalyst activity with 1 M sulfuric acid solution. This work provides a way to design a potential microwave-responsive catalyst that improves the energy efficiency and HMF yield for the fructose–HMF conversion process.