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-toluene-sulfonic acid (p-TsOH), 4-hydroxybenzenesulfonic acid (4-HSA), 5-sulfosalicylic acid (5-SSA), and maleic acid (MA). Under identical treatment conditions (80 degrees 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-beta-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 (similar to 60 %), the four acid hydrotropes showed variations in the beta-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 beta-O-4 bonds in the extracted lignin.

Automated summary

This study compared the dissolution of lignin in different acid hydrotropes and found that 4-HSA is the most efficient solvent. The analysis of molecular mechanism revealed that higher absorb free energy of 4-HSA and p-TsOH complex with VG contributed to a higher efficiency of lignin dissolution. The content of beta-O-4 bonds in the extracted lignin varied among the four acid hydrotropes at the same level of lignin removal and was influenced by the acidity and critical aggregation concentrations of acid hydrotropes.