Natural deep eutectic solvents (DES) for fractionation of waste lignocellulosic biomass and its cascade conversion to value-added bio-based chemicals

Abstract

Deep eutectic solvents (DES), prepared from choline chloride (ChCl) compound as hydrogen bond acceptor and lactic acid (LA), oxalic acid (OA), potassium hydroxide or urea (UA) as the electrostatic attracting donors in different amount ratios, were synthetized, applied and studied for lignocellulosic biomass fractionation. The mixtures of ChCl with OA or KOH were found to dissolve beech wood polymers more effectively compared to ChCl with LA or UA. In addition to DES screening test experiments, the influences of the process performance parameters, like measurement reaction time (2–24 h), temperature (60–100 °C) and the chip to solution mass relationship (1:100–1:10), on particle size distribution, solid residue's properties, functional cellulose, hemicellulose and lignin contents, the concentration of sugars, polyphenolics and volatile chemical products in raw liquid extract, as well as kinetics were experimentally determined. The further spectroscopic, microscopic and chromatographic analysis of solubilisation demonstrated that ChCl with OA selectively isolated phenols, could potentially be scalable and could be utilized in lignin-first bio-refinery plant. Purified cellulose-rich material was obtained, according to attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR), while polyphenols were above 15 g L−1 (gallic acid equivalent, examined by Folin–Ciocalteu method), revealing predominant dissolution. Conversely, for ChCl with KOH, aromatics were below detection limit value, while polysaccharides dropped for a factor of 10, paralleled to sawdust's fresh sample. The DES recovery by centrifugation, anti-solvent-assisted phase separation and vacuum distillation operation was also performed. While promising, NADES must be additionally developed, especially considering recycling, stability and economics.