Evaluation of Differences in Solubility in Organic Solvents of Softwood/Hardwood-Based Industrial Kraft Lignins Using Hansen Parameters and FTIR

Lignin is part of the lignocellulosic biomass found in hardwoods, softwoods, and herbaceous plants. It is isolated by fractionation and pulping processes, where it is considered a waste product and is mainly burned to generate electricity and heat. However, lignin is a biopolymer that can be a source of many chemicals of significant value after treatments of functionalisation or depolymerisation. The main processes for its valorisation require solubilisation in organic solvents, which may affect the process’s mass balance and the biopolymer’s chemical structure, as lignins are rarely completely soluble in organic solvents. In this research, two industrial Kraft lignins derived from softwood and hardwood were treated using different green organic solvents (2-methyl tetrahydrofuran, tetrahydrofuran, 1-methoxy-2-propanol, and acetone), measuring the soluble and insoluble content to determine the mass balance, and the solubility was evaluated using Hansen parameters and FTIR. The results showed that both lignins were more soluble in tetrahydrofuran and 1-methoxy-2-propanol. Also, tetrahydrofuran performed better than the 2-methyl tetrahydrofuran, which is considered the green alternative. The reason can be explained using the Hansen solubility parameters. Moreover, the solubilised fractions showed similar FTIR spectra. The same is the case for the insoluble parts. In conclusion, the two lignins studied showed different solubilities in absolute values in organic solvents, but the trends were similar.