Successive and quantitative fractionation and extensive structural characterization of lignin from wheat straw

•In 4 fractions 82% of wheat straw lignin based on Klason lignin content was isolated.•One low molar mass cellulose–lignin deduced as free lignin with biological functions.•Two xylan lignins with differing branching degrees of the xylan part were separated.•One core cellulose–lignin showed high resistance to all applied extraction steps.

A unique process for the fractionation of lignin from wheat straw is proposed: Ball milling for 8 h, followed by a direct and an acidolysis-assisted dioxane–water extraction. Four distinctly different lignin structures were thus obtained: (1) one free non-core lignin, which is a cellulose–lignin with lowest molar mass and highest contents of p-hydroxycinnamic acids, condensed phenolic hydroxyl groups and tricin moieties, and with a detectable amount of cinnamyl alcohols; (2) two core xylan–lignins differing in their degree of branching as indicated by their xylose/arabinose ratios of >4 and ∼2, respectively; (3) one core cellulose–lignin which is the residual fraction resistant to all extractions. Based on the mass balance of Klason lignin the yields of these four fractions are 13.8%, 18.1%, 37.5% and 12.5%, respectively, thus accounting for 82% of the total KL in wheat straw. Therefore, the isolated lignin fractions could be considered as highly representative allowing a good insight into the different features of wheat straw lignin.