(451b) Microwave Heated Pretreatment of Agricultural Residues with Deep Eutectic Solvents to Isolate Lignin-Carbohydrate Complex for Renewable Fuel and Material Applications

Agricultural residues can be used as renewable and sustainable materials in downstream production of biochemicals and biofuels. Generally, agricultural residues contain lignin-carbohydrate complex, making conversion of agricultural residues to biochemicals and/or biofuels challenging. Fractionation of lignocellulosic biomass offers a possible solution for specific applications. Deep eutectic solvents (DESs), a combination of two or more chemical components through hydrogen bonding, can be promisingly applied to dissolve and isolate lignocellulose compositions. Microwave heating technology has been intensely developed to provide rapid, low energy, and high efficiency. It may be applied to biomass pretreatment. So far, there have only been few studies on microwave heated DES pretreatment of agricultural residues. In this work, pretreatment of corn stover with DESs was investigated in a microwave heating reactor with a fixed biomass loading of 10 wt.% at varying reaction temperature (60-150 °C) and residence time (2-10 min). The DESs were prepared form choline derivative and glycerol with different ratios of 1:3-12. The resulting solid product is considered as cellulose-rich materials (CRMs). The CRM was analyzed by thermogravimetry (TG), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). Compared to conventional heating, the microwave-assisted heating provided a significantly better efficiency for eliminating non-cellulosic compounds from the raw feedstock. SEM images showed that CRM fiber bundles were destroyed into small fragments with micro-sizes. FTIR results indicated a reduced OH peak intensity of lignin and disappearing ester linkage between lignin and hemicellulose. XRD illustrated that cellulose crystallinity was increased by 30 to over 70%. Using nuclear magnetic resonance spectroscopy, the microwave heated pretreatment was shown to affect more interactions between the DES and lignin structure, improving efficiency of breaking down lignocellulosic biomass structures. Our findings are useful for biomass utilization and applications in generating bioenergy, fuels, and chemicals.