Optimizing conditions for using deep eutectic solvents to extract lignin from black liquor

Lignin extraction from black liquor (BL) was investigated employing four deep eutectic solvents (DESs) and a control solvent ([Emim][Ac]). A screening study was employed to determine the best DES, lactic acid:ChCl at a 11:1 molar ratio. La:ChCl(11:1) could extract 79.7 ± 2.1% of the lignin content of BL compared to [Emim][Ac] with 79.9 ± 2.3% at 95 °C, time of about 4 h, and a DES:BL(20:1). The DESs selected were regenerated three times, and the extraction efficiency using DES regenerated was statistically similar to the extraction values using DES before regeneration. FTIR spectra for the extracted lignin samples agreed with the peaks for a standard lignin. 1 H- 13 C-HSQC-NMR detected the side chain structure and aromatic linkage peaks in the lignin extracted. Lignin extraction was optimized for La:ChCl using RSM. A BBD was utilized to develop a model to predict the lignin extraction based on operating factors. The Anderson–Darling statistic confirmed a normal distribution of the residuals, which demonstrated a reasonable correlation between predicted and experimental data. The highest extraction of lignin predicted using D-optimality analysis was 83.8% at optimum conditions of time = 4.6 h, temperature = 99 °C, DES:BL(20:1), and La:ChCl(11:1). An experiment to test the extraction under these conditions was performed in triplicate and yielded a value of 79.2 ± 1.86%.