Biomass valorization by integrating ultrasonication and deep eutectic solvents: Delignification, cellulose digestibility and solvent reuse

Abstract

Biomass valorization in converting biomass into value-added products and energy has gained researchers’ attention. In this study, valorization of oil palm empty fruit bunch (OPEFB) towards biofuels production was focused. Combination of deep eutectic solvent (DES) and ultrasonication was employed to pretreat the OPEFB. The pretreatment was conducted in a sonicator at 210 W, 50 °C for 30 min. Choline chloride-lactic acid (ChCl-LA) outperformed choline chloride-urea (ChCl-U) and choline chloride-glycerol (ChCl-G) because of its acidity and low viscosity properties. Besides having the highest cellulose digestibility (36.7% of reducing sugars yield), ChCl-LA pretreated biomass had the lowest lignin content of 18.8% and biomass crystallinity of 0.895. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) results further confirmed its biomass disruption ability. Ultrasonic-assisted DES pretreatment exhibited a synergetic effect, whereby the reducing sugars yield was double compared to without ultrasound. Also, its pretreatment performance was better than other pretreatment methods, such as acid, alkaline and steam explosion pretreatments. The DESs were effectively recovered with a slight decrease in delignification and cellulose digestibility efficiencies. These collectively proven the feasibility and effectiveness of ultrasound-assisted DES pretreatment in converting the biomass to sugars, which could be essential for biofuels production.