Abstract
The ability of supercritical/subcritical carbon dioxide (SC/Sub-CO2) to catalyze the production of 5-hydroxymethylfurfural (5-HMF) from fructose was studied in this paper. The optimal conditions for the conversion in SC/Sub-CO2 systems were determined, and sugarcane bagasse (SCB) was pretreated with SC-CO2 using 1-butyl-3-methylimidazolium acetate ([Bmim]OAc). Structural analysis of SCB samples was done by Fourier transform infrared (FT-IR), X-ray Diffractometer (XRD), Thermal gravimetric (TG), Solid state nuclear magnetic resonance (13C NMR) and scanning electron microscope (SEM) analyses. The contents of sugars in the acid and enzymatic hydrolysates were measured. The results showed that the optimum fructose conversion conditions of the SC-CO2/[Bmim]Cl system were 9 MPa, 120 °C and 0.5 h and the Sub-CO2/H2O system were 6 MPa, 180 °C and 2 h. Besides, SC-CO2/[Bmim]OAc was the most effective pretreatment process, with the lignin and hemicellulose removal rates in SCB being 8.78 and 4.83% respectively. FT-IR, SEM, XRD and TG analysis indicated that the SC-CO2/[Bmim]OAc pretreatment changed surface morphology, increased crystallinity and decreased thermal stability of SCB. Moreover, the yield of reducing sugars obtained by enzymatic and acid hydrolysis increased by 12.57 and 22.04%, respectively. The energy balance showed that SC-CO2/[Bmim]OAc pretreatment was efficient to increase the energy output of SCB.
•Supercritical/subcritical carbon dioxide enhanced conversion of fructose to 5-HMF.•The yield of reducing sugar increased after SC-CO2 pretreatment.•The optimum condition of the SC-CO2/[Bmim]Cl pretreatment were 9 MPa, 120 °C, 0.5 h.•Microscopic give evidence of better hydrolysis of SC-CO2/[Bmim]OAc pretreated bagasse.•SC-CO2/[Bmim]OAc pretreatment improved the yield of 5-HMF.