Conversion of rice husk into reducing sugars: influence of pretreatment with water and [C₁₆MIM][Br⁻] ionic liquid

Rice husk is an agricultural lignocellulosic-rich residue that can be used in the production of second-generation ethanol. A biomass pretreatment is necessary to release structural sugars because of the recalcitrance-related properties of lignin and silicon. The use of a long-chain imidazolium ionic liquid (IL), 1-hexadecyl-3-methylimidazolium bromide ([C₁₆MIM][Br⁻]), for rice husk pretreatment is reported for the first time. The influence of [C₁₆MIM][Br⁻]-to-water ratio and temperature in a hydrothermal pretreatment on reducing sugars yield obtained by enzymatic hydrolysis was investigated using an experimental design with a temperature range of 80–160 °C, varying the IL-to-water ratio between 1:5 and 1:30, with 10% (w/v) of biomass in 12 h. Structural, chemical and morphological changes were observed by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). Sugar yield was enhanced when combining the highest temperature (160 ºC) and lowest ionic liquid ratio (1:5, IL:water). The validation assay (180 °C, 1:20 IL-to-water ratio, 10% (w/v) of biomass, 12 h) resulted in 38% yield of reducing sugars and 60% dissolution of biomass, whereas untreated biomass showed only 0.18% yield of reducing sugars. The IL pretreatment affected the rice husk structure and morphology. Rice husk pretreatment with [C₁₆MIM][Br⁻] resulted in a higher concentration of reducing sugars compared to untreated biomass. Therefore, solid ionic liquids combined with water could be used as an alternative solvent for lignocellulosic biomass pretreatment.