Lignin fractionation: Effective strategy to reduce molecule weight dependent heterogeneity for upgraded lignin valorization

Conventional pulping processes and emerging lignocellulosic biorefineries provide a large quantity of lignin with heterogeneous characters, including complex chemical structure and polydisperse molecular weight distribution. The heterogeneity causes undesirable properties for down-stream applications, which limit lignin valorization seriously. Fractionation of heterogeneous lignin into several homogeneous fractions is considered as an effective approach to overcome this limitation and realize upgraded lignin valorization. To date, lignin fractionation approaches have been extensively studied and they can be broadly classified into three categories: gradient acid precipitation, step-wise solvent fractionation, and membrane separation with different cut-offs. After fractionation, the obtained lignin fractions with decreased polydispersity exhibit improved molecular weight dependent properties, and thus using them as raw materials for subsequent applications results in the upgradation of lignin valorization. In this work, the formation of lignin heterogeneity based on molecular weight is firstly elucidated, and lignin fractionation approaches are elaborated and their potentials for industrialization are discussed. Finally, the upgraded valorization using lignin fractions with high and low molecular weight is also reviewed comprehensively in this work.