Fractionation of poplar wood with different acid hydrotropes: Lignin dissolution behavior and mechanism evaluation

Acid hydrotropes was considered a green medium for efficient wood fractionation at mild conditions. This study reported a comparative study on the dissolution of lignin in different acid hydrotropes, including p-toluenesulfonic acid (p-TsOH), 4-hydroxybenzenesulfonic acid (4-HSA), 5-sulfosalicylic acid (5-SSA), and maleic acid (MA). Under identical treatment conditions (80 °C, 60 min, and 70 % acid concentration), the removal of wood lignin varied significantly among four acid hydrotropes, 4-HSA exhibited the highest removal rate at 88.0 %, followed by p-TsOH at 81.2 %, 5-SSA at 51.1 %, and MA at 26.2 %. The molecular mechanism of the lignin dissolution was analyzed by quantum chemistry (QC) calculation and molecular dynamics (MD) simulation. The higher absorb free energy (E(absorb)) of the 4-HSA and veratrylglycerol-β-guaiacyl ether (VG) complex (E(absorb) = 17.97 kcal/mol), and the p-TsOH and VG complex (E(absorb) = 17.16 kcal/mol) contributed to a higher efficiency of lignin dissolution. Under the same level of lignin removal (~ 60 %), the four acid hydrotropes showed variations in the β-O-4 content of the extracted lignin: 4-HSA (3.1 %) < 5-SSA (10.4 %) < p-TsOH (15.9 %) < MA (63.7 %). The acidity and critical aggregation concentrations of acid hydrotropes were found to influence the content of β-O-4 bonds in the extracted lignin.