Hydrogen Bond Promoted Lignin Solubilization and Electrospinning in Low Cost Protic Ionic Liquids

As a renewable, abundant, but underutilized aromatic component of lignocellulose, the relatively poor solubility of lignin in most pretreatment solvents including traditional ionic liquids (ILs) remains one of the biggest challenges in economic lignin processing. Herein, a series of low cost protic ILs (PILs) functionalized with various hydroxyl/carboxyl substituted groups (ethanolamine (EOA), diethanolamine (DEOA), triethanolamine (TEOA); acetic acid (HOAc), lactic acid (HLA), and oxalic acid (HOA)) were investigated to deeply understand hydrogen bond promoted lignin solubilization in the PIL. Among the nine PILs, a 35 wt % enzymatic lignin solubility was achieved at 90 °C in ethanolamine acetate ([EOA][OAc]). Deep analysis revealed that the effect of the cation on the solubility of lignin varied with the order of [EOA]⁺ > [DEOA]⁺ > [TEOA]⁺ and that of the anion was in the order of [OAc]^- > [LA]^- > [OA]^-. The possible mechanism responsible for the promoted solubility of lignin in the PILs was studied using the lignin dimer model compound, 2-phenoxy-1-phenylethanol, indicating that the high dissolution of lignin in PIL was possibly due to the synergistic effect of hydrogen bond and electrostatic interactions between the PIL and the lignin. Hydrogen bond promoted electrospinning of as-prepared concentrated lignin solution was investigated for potential generation of lignin based materials. The present work provided a new protocol of lignin valorization using low cost ILs.