Comparative Study of Effective Pretreatments on the Structural Disruption and Hydrodepolymerization of Rice Straw

Rice straw (RS) is the most potentially renewable agricultural waste resource widely distributed in nature. Considering the complex recalcitrant structure and components of RS, three pretreatment methods, including high-temperature hydrothermal, medium-temperature microwave, and low-temperature cryocrushing pretreatment were performed. The components and structure of RS residues were examined and analyzed after the pretreatments. Pretreatment with hydrothermal yielded the lowest rice straw recovery (59.0%); after being pretreated at 180 °C for 10 min, the hemicellulose recovery was only 14.1%, and the removal efficiency of lignin was the largest (41.3%), which was 32.2% and 18.8% higher than that achieved from cryocrushing and microwave pretreatment, respectively. Pretreatment with cryocrushing yielded the highest recovery rates of rice straw (92.9%), hemicellulose and cellulose (88.8% and 90.4%, respectively). Results of scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and the analysis of specific surface area and apertures demonstrated that all three pretreatments could effectively disrupt the structure of RS and reduce its cellulose crystallinity. The three pretreatments were found to enhance the hydrodepolymerization of RS residues. Furthermore, cryocrushing pretreatment yielded the highest cellulose conversion rate (56.8%), and the yields of glucose, xylose, and arabinose were 29.6%, 56.2%, and 17.8%, respectively. Apart from the use of acids and enzymes, hydrodepolymerization of RS was among the few methods that can effectively degrade cellulose, presenting an ideal solution for the degradation of biomass.