Determining the Solubility of Lignin in Deep Eutectic Solvents As a Function of Mono, Di, and Tricarboxylic Acids

Deep eutectic solvents (DES), capable and environmentally friendly solvents, were investigated for their ability to dissolve pure lignin and to extract lignin from wooden furniture residuals. The main challenge of this research was developing a technique that allowed for the quantification of lignin dissolution. Our solution was to develop calibration curves for lignin in a variety of deep eutectic solvents using UV/VIS. The absorption curves were measured over a range of 200 to 800 nm with special attention to wavelengths of 280 and 440 nm per the study by Skulcova et al. (2017).

In selecting our deep eutectic solvents, we concentrated on mono, di, and tricarboxylic acids combined with choline chloride. Initially, six different carboxylic acids (malonic, lactic, pyruvic, aconitic, malic, and citric) were individually combined via mortar and pestle with choline chloride. A range of molar ratios (0.1 acid:0.9 ChCl to 0.9 acid:0.1 ChCl) were examined for each acid, and the samples remaining liquid at room temperature were selected to perform lignin dissolution and wood leaching studies. For each solvent, nine samples ranging from 0.2 mg lignin/g DES to 4.0 mg lignin/g DES were created. The samples were placed into culture tubes and digested for two hours in a sample warmer at 70°C. Following the digestion, each sample was analyzed using UV/VIS over the desired range. From the absorption data collected, calibration curves of the absorption at 280 nm versus the concentration of lignin were created. Using Beer’s Law, the data was used to determine extinction coefficients for lignin in each DES. The plots demonstrate the efficacy of lignin dissolution in each type of solvent. All the mixtures exhibited some ability to dissolve pure lignin, however, malonic, lactic, malic, and citric acid were the strongest candidates. The previous procedures were then repeated for these four carboxylic acids, substituting wood furniture residuals for lignin to determine if lignin could be extracted. Due to the heterogeneity of the samples, the data is difficult to quantitatively interpret, but based on the range of absorption values, each of the DES may be ranked for their ability to remove lignin with malic and malonic acid showing the best potential. This study indicates that UV/VIS can be an effective analytical method to study a range of deep eutectic solvents for processing lignin in biomass.