(320d) Fractionation of Lignocellulosic Components By Environmental Benign Solvents

The utilization of lignocellulosic components to materials, energy and chemicals are negative carbon processes. Fractionation of lignocellulosic components to lignin, cellulose, and semi-cellulose is a prerequisite for efficiently transformation of lignocellulosic components to valuable chemicals or energy. However, the components of lignocellulose are bonded together to form Lignin–carbohydrate complexes. It is hard to fractionation them to get pure components. We developed several environmental benign solvents to achieve the above goal.[1-5]

γ‐Valerolactone (GVL)-based solvent[1], deep eutectic solvents (DESs) with different functional groups[2] and superbase-based DESs[3] for dissolution of lignins. Several solvent systems, including biomass-derived GVL and a second component of water, ionic liquids (ILs), DMSO, or DMF; DESs employing lactic acid and N-methylthiourea as HBDs and four different quaternary ammonium salts as HBAs; 17 kinds of 1,5-diazabicyclo[5.4.0]-5-undecene-based DESs, are designed for dissolution of various industrial lignins including dealkaline lignin (DAL), enzymatic hydrolysis lignin (EHL), sodium lignosulfonate (SL), and organosolv lignin (OL). The solubility of lignin in GVL-based solvent and hydrogen bond basicity parameter β value of solvents depends on both solvent and lignin, and the dissolved lignin can be recovered by ethanol. The functional groups in DESs significantly affected their ability to dissolve lignin, and a positive correlation was found between the solubility of lignin and the subtraction value of Kamlet–Taft empirical parameters α and β for the dissolution of EHL, DAL and OL. The solubility of lignin in superbase-based DESs significantly depended on the types of the DESs and lignin. Also the relationship between lignin solubility and the solvatochromic parameters α, β, and π* varied significantly based on the DESs and the types of lignin.

Several IL-based DESs using four ILs as the HBAs and four different HBDs, including thiourea (TU), N-methylthiourea (NMTU), glycerol (Gly), and ethylene glycol (EG) were designed and found as efficient solvents for xylan.[4] The xylan solubility in DES 1-allyl-3-methylimidazolium chloride plus EG could reach 40.4 wt% at 343.15 K, which is much higher than that in the corresponding IL. The enhancement effect might be ascribed to that the decrease of viscosity and increase the hydrogen bond accepting ability. Additionally, the regeneration of the xylan from the DESs could be easily realized using ethanol as the anti-solvent, and the regenerated xylan kept a high consistency with the original xylan.

Six kinds of novel superbase-based solvents were designed by 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) with pyridine N-oxide (PyO) or 2-picoline-N-oxide (PiO) for dissolution of cellulose.[5] The solvents (denoted as DBN-PyO-x and DBN-PiO-4) could efficiently dissolve cellulose at mild temperatures (<80 â—¦C). DBN-PyO-4 has a cellulose solubility of 14.1 wt% at 70 â—¦C.

More robust solvents both environmental benign and cheap are anticipated to develop for industrial fractionation of lignocellulosic components. Moreover, the dissolution mechanism are far from disclosed. More efforts have to be devoted to this topic for the industrialization.

References:

[1] Xue, Z., Zhao, X., Sun, R. C., & Mu, T. (2016). Biomass-derived γ-valerolactone-based solvent systems for highly efficient dissolution of various lignins: Dissolution behavior and mechanism study. ACS Sustainable Chemistry & Engineering, 4(7), 3864-3870.

[2] Liu, Q., Zhao, X., Yu, D., Yu, H., Zhang, Y., Xue, Z., & Mu, T. (2019). Novel deep eutectic solvents with different functional groups towards highly efficient dissolution of lignin. Green Chemistry, 21(19), 5291-5297.

[3] Liu, Q., Mou, H., Chen, W., Zhao, X., Yu, H., Xue, Z., & Mu, T. (2019). Highly efficient dissolution of lignin by eutectic molecular liquids. Industrial & Engineering Chemistry Research, 58(51), 23438-23444.

[4] Yu, H., Xue, Z., Lan, X., Liu, Q., Shi, R., & Mu, T. (2020). Highly efficient dissolution of xylan in ionic liquid-based deep eutectic solvents. Cellulose, 27(11), 6175-6188.

[5] Liu, Q., Yu, H., Mu, T., Xue, Z., & Xu, F. (2021). Robust superbase-based emerging solvents for highly efficient dissolution of cellulose. Carbohydrate Polymers, 272, 118454.