Fractionation of industrial lignins: opportunities and challenges
Lignin is a readily available yet heavily underutilised resource due to its severe degree of variability related to the plant source and to the processes employed for its isolation. Being in most cases a by-product of technologies targeting other biopolymers, e.g. cellulose, less attention has been paid to the physicochemical features of the obtained lignin, which can therefore dramatically vary even on a batch to batch basis. To fully valorise the enormous potential of this material, the development of robust and economically sustainable strategies leading to lignin structures with well-defined and homogenous characteristics is of crucial importance. In this respect, fractionation represents a valid option, and thus is attracting increasing interest from the scientific community. Different methodologies have been established and tested on many lignins in the last few years. With the aim of extrapolating general trends in terms of the properties of the obtained lignin cuts and providing useful guidelines for researchers working in this topic, this contribution presents a critical review of the existing approaches for the fractionation of technical lignins. The applications where lignin fractions have found use are also overviewed, with a particular focus on the benefits gained as compared to pristine lignin.