Comparing organic solvents in a combined water precipitation and liquid–liquid extraction process to recover lignin and furanics from a lactic acid: choline chloride deep eutectic solvent used as cooking liquor for cooking of spruce

Abstract

AbstractExtracting and purifying lignin from wood without compromising cellulose quality is a challenging process. Lactic acid: choline chloride is a deep eutectic solvent (DES) that has been identified as acceptable delignification solvent, producing lignin and hemicellulose as byproducts to the cellulose. Hemicellulose is partly transformed into furanic compounds (such as furfural and 5-HMF). While the larger lignin can be obtained by water precipitation from DES, smaller lignin molecules and furanics can be recovered by liquid–liquid extraction (LLX), either directly or after precipitation of the larger lignin molecules. The presence of water in the DES after water precipitation reduced the mutual miscibility with the solvents, allowing the use of a wider range of solvents in the LLX process. In the precipitation step, all the larger molecular weight lignin (Mw > 5000 Da) can be recovered when adding at least 3.5:1 [g/g] water to DES-black liquor. For the LLX step, guaiacol was found as suitable alternative to the previously published 2-methyltetrahedrofuran (2-MTHF). In addition, here we report the use of 2,2,5,5-Tetramethyl oxolane (TMO), a recent addition to the palette of (potentially) bio-based solvents. The distribution coefficients of smaller lignin molecules and furanics in LLX with guaiacol, TMO and 2-MTHF were compared, revealing that smaller lignin molecules (500–5000 Da) can be recovered with a reasonable distribution coefficient by 2-MTHF and guaiacol. Furfural showed distribution coefficients of at least 1.27 in all three solvents. Guaiacol and TMO both showed a significantly lower lactic acid leaching than 2-MTHF. This makes them potential alternatives for 2-MTHF in this technique. Graphical abstract