Natural deep eutectic solvents for efficient mild pretreatment of crop residues

Crop residues consist predominantly of lignin, cellulose, and hemicellulose, organized together as lignocellulose. With approximately 200 billion tons produced per year, lignocellulosic biomass is the most abundant source of plant-based raw materials available in nature [1]. Burning crop residues worldwide emits around 70,000 kt of CO2eq in total [2] and this can be prevented by implementing these residues in biorefinery. The prerequisite step to utilize these residues is to pretreat them and break down the complex lignocellulose structure. Natural deep eutectic solvents (NADES) are emerging as a green, biocompatible, biodegradable, and cheaper alternative to ionic liquids that are widely used as a highly efficient pretreatment. Their physicochemical properties can be fine-tuned by altering the number, type, and molar ratio of NADES constituents. Acidic NADES composed of choline chloride and carboxylic acids proved to be suitable for lignocellulosic biomass delignification. However, to increase the pretreatment’s efficiency, elevated temperatures and prolonged pretreatment duration are needed. This study aimed to test the possibility of subjecting corn stalks to NADES pretreatment at room temperature and in short time intervals to improve the sustainability of the pretreatment. Experimental: Taguchi orthogonal array design (L9) was used to select the best pretreatment conditions for achieving the highest delignification rate. Key parameters of the process, namely choline chloride to lactic acid ratio in NADES, water addition, and pretreatment duration, were optimized through nine experimental runs. Biomass was pretreated at room temperature while being stirred constantly. After pretreatment, biomass was recovered, analyzed for residual lignin content, and subsequently subjected to enzymatic hydrolysis using commercial Cellic CTec2 mixture to estimate the pretreatment’s efficiency in terms of improving both biomass delignification and digestibility. Results: The optimal conditions for achieving the highest delignification rate were defined based on the results obtained, as follows: choline chloride-tolactic acid ratio 1:5, addition of 20% water, and 10-minute-long pretreatment performed at room temperature. Analysis of variance confirmed that the composition of NADES contributed mostly to the pretreatment’s efficiency. Sugar yields obtained after 48-hour-long enzymatic hydrolysis were mostly correlated to the biomass delignification. This suggests that corn stalks could be successfully delignified and further used through various fermentation processes with reduced energy demands. However, different types of NADES could be tested to make this pretreatment energy-efficient and cost-effective on a larger scale. Moreover, the reusability of NADES and the potential valorization of extracted lignin should also be examined to shift this pretreatment toward a zero-waste approach.