Lignin dissolution model in formic acid-acetic acid-water systems based on lignin chemical structure

The separation of lignin from woody biomass and subsequent conversion into useful products requires a solution to the problem of its solubility. The expanded C₉ formula of lignin, along with its atomic and functional groups, was determined by elemental analysis and NMRs spectroscopy. Based on the thus-obtained expanded C₉ formula, the cohesion parameters of lignin dispersion (10.8-11.1 cal^1/2·cm^-3/2), polarity (4.15-4.31 cal^1/2·cm^-3/2), hydrogen bonding (6.30-7.38 cal^1/2·cm^-3/2), and solubility (13.2-14.0 cal^1/2·cm^-3/2) were respectively calculated using atomic and functional group contributions method. We established the relationship between lignin structure and lignin solubility parameters. The dissolution characteristics of wheat straw organic acid lignin, industrial eucalyptus kraft lignin, bamboo kraft lignin, and softwood kraft lignin in formic acid-H₂O, acetic acid-H₂O, and formic acid-acetic acid-H₂O solvent systems were analyzed. The results indicate that the dissolution behavior of lignins follows the solubility parameters theory. We have developed a lignin dissolution model according to the lignin structure. This model obeys the solubility parameter theory, overcomes the limitations of the "like dissolves like" principle in organic acid-water systems, and provides a concise method for the selection of lignin solvent systems and the quantitative determination of their solvent composition.