Solvent-directed ozonolysis dismantles lignin heterogeneity: An integrated strategy for a tunable lignin biorefinery

Impurities and the heterogeneous three-dimensional structure of alkali lignin complicate its valorization. In this study, technical alkali lignin from soda bagasse was ozonated in ethanol, tetrahydrofuran (THF), 1,4-dioxane, acetone, or methyl cellosolve to combine oxidative depolymerization with solvent fractionation. The comparison tested how solvent polarity and hydrogen-bonding behavior affect product distribution under otherwise fixed conditions. FTIR, GPC, and GC-MS showed differences among the five soluble fractions. Protic solvents retained more phenolic monomers and oligomers, whereas aprotic solvents gave more oxidized products and different H:G:S profiles. The ethanol fraction had the highest apparent thermal stability (Ea = 4.14 kJ/mol; DTG peak = 380 °C) and the lowest Mw (856 Da). Methyl cellosolve produced the highest-Mw fraction (2985 Da) and a more balanced H:G:S distribution. Guaiacol accounted for 40.6% of the ethanol fraction, coumaran for 70.4% of the THF fraction, and syringol for 17.2% of the methyl cellosolve fraction. THF-only controls did not produce a peak assigned to coumaran in the relevant retention-time window, which supports a lignin-derived origin for this signal under the tested conditions. The screening results link solvent properties to lignin fragmentation and the composition of the soluble products. Solvent recycling, residue characterization, process optimization, and atom-resolved mechanistic studies remain to be completed.