Effect of ultrasound on ionic liquid-hydrochloric acid pretreatment with rice straw

Lignocellulosic biomasses are considered the most suitable feedstocks for sustainable bioenergy generation in the future owing to their versatility, availability, and low cost. Pretreatment is considered an essential step for removing lignin and solubilizing hemicellulose to increase the sugar yield of cellulase hydrolysis of pretreated biomass because pretreatment may disrupt the crystalline structure of cellulose. To investigate the effect of ultrasound on ionic liquid-hydrochloric acid pretreatment with rice straw, this study adopted an hydrochloric acid–catalyzed process in 1-butyl-3-methylimidazolium chloride and 1-allyl-3-methylimidazolium chloride aqueous solutions under ultrasound irradiation to treat the rice straw at a medium-low temperature. The effects of temperature, acid concentration, and time on the production of reducing sugar were studied; moreover, the benefits of ultrasound during pretreatment were also explored according to the changes of enzymatic saccharification, components, morphology, crystallinity, and chemical structure. The results showed that temperature, acid concentration, and time have significant effects on enzymatic hydrolysis, and the production of reducing sugar, cellulose conversion, and delignification was increased by 20.13–28.96%, 31.69–35.23%, and 18.06–19.33%, respectively. On the other hand, the results of Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR) analysis also demonstrate that ultrasound can promote the destruction of morphology, chemical structure, and crystallinity of rice straw in the hydrochloric acid–catalyzed process in ionic liquid aqueous solutions. (1) Temperature, acidity, and processing time had significant effects on the sugar yield of rice straw pretreated with US-IL-HCl. (2) Compared with treatment without ultrasound, the sugar yield, cellulose conversion rate, and delignification increased in ultrasonic-assisted pretreatment samples. (3) Ultrasound-assisted pretreatment can further improve the damage of the surface morphology and chemical structure of rice straw, reduce the crystallinity of cellulose, and improve the enzymatic hydrolysis effect.