Comparative study of the impacts of three common fractionation methods on the physicochemical features of lignin.

The intrinsic structural heterogeneity and complexity of lignin arising from diverse botanical sources and isolation processes limit its efficient utilization. Molecular weight strongly governs lignin's physicochemical properties and valorization potential, making fractionation an essential technique to reduce dispersity and tailor desired structures. Here, we systematically compared three common fractionation methods, solvent exchange (SE), acid precipitation (AP), and membrane ultrafiltration (MU), to tailor Indulin AT kraft lignin into low-, moderate-, and high-molecular-weight fractions. The physicochemical features of fractions were comprehensively characterized, including molecular weight distribution, functionality, surface charge, interunit linkages, morphology, and thermostability. The SE method demonstrated clear advantages in producing well-separated fractions with narrow molecular weight distribution and redistributed hydroxyl groups, thereby achieving superior reactivity for polymer synthesis. The AP method was conducive to large-scale lignin fractionation but exhibited poor molecular separation, as revealed by the closely overlapped molecular weights in the fractions. Moreover, acid-induced condensation likely reduced hydroxyl content yet produced more condensed lignin structures and higher thermal stability. The MU method showed large discrepancies in fractionation behavior. The low-molecular-weight fractions displayed low dispersity, with intermediate thermal properties and hydroxyl content compared with AP and SE fractions. However, its separation effectiveness among moderate- and high-molecular-weight lignin heavily relied on the type of membranes employed. This study establishes direct links between common fractionation techniques and key physicochemical properties of lignin fractions, providing important guidance for tailoring the characteristics and structures of lignin toward effective and desired valorization.