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.