Targeted phenolation and rapid extraction of light-colored lignin from pine using nucleophilic reagent in combination with acid hydrotrope p-TsOH

Structural change, reduced reactivity, and deepened color of lignin caused by uncontrollable reaction in the process of separation are the main problems that hinder its effective utilization. Based on lignin-first strategy, nucleophilic phenol (PH) or syringol (SL) in conjunction with hydrotrope p-toluenesulfonic acid (p-TsOH) was employed to separate phenolated and light-colored lignin from pine under mild conditions. It was found that the quick and targeted reaction of the nucleophilic PH or SL with lignin at the sidechain C_α position inhibited the cleavage of β-O-4 bonds thus reduced the intermolecular condensation of lignin and the formation of chromophoric groups. The introduction of phenolic hydroxyl groups and low condensation degree enhanced the solubility of lignin in p-TsOH solution, resulting in a high extract yield of lignin. The separated lignin had a lighter color, more phenolic hydroxyl and methoxyl groups, abundant β-O-4 bond and more uniform molecular distribution and exhibited an exceptional UV shielding and antioxidant activity. The phenolated lignins were easily self-assembled into regular spherical nanoparticles by anti-solvent method, which had good mixing effect with face cream to prepare sunscreen with adjustable sun protection factor (SPF) value.