Softwood kraft lignin SKL was fractionated using aprotic
acetone and protic methanol to yield both previously reported “traditional”
fractions and novel refined fractions thereof. Based on differences
in mutual H-bonding affinities exerted by the solvents on one side
and the OH group characteristics
of the lignin oligo- and polymers on the other side, the 85% acetone-insoluble
kraft lignin fraction AIKL, for example, could be further
fractionated in 16% methanol-soluble and 67% methanol-insoluble parts.
Sequential use of the two solvents practically resembled a refinement
protocol that shed light on eventual “structural impurities”
contained in the traditional fractions, which is not delineable easily
in front of the background of a dominating spectroscopic image. Exploiting
H-bonding characteristics offers a valid option for the facile generation
of truly “structurally purified” lignin fractions. Acetone-insoluble,
methanol-soluble MSAIKL, for example, exhibits 25% less
aliphatic interunit bonding while being enriched in phenolic groups
up to 25%, thus representing condensed, lower-molecular-weight structures
with overall H-bond accepting character that was still contained in
the overall larger-molecular-weight acetone-insoluble AIKL. More “polyphenylpropanoidic” parts practically free
of condensed units, determining the overall structural picture of
the unrefined lignins due to their overall abundance, are represented
by the globally insoluble fractions like acetone-insoluble, methanol-insoluble MIAIKL. The original fractions and refined fractions generated
based on targeting specific H-bonding affinities give a more homogeneous
picture with respect to trends in glass transition temperatures, clearly
indicating that T
g is dominated by actually
both MW and nature of OH groups.
A new fractionation protocol for wheat straw organosolv lignin was developed on the basis of the dominating H-bonding orientations of its components. Acetone as H-bond accepting aprotic polar solvent and methanol as H-bond donating and accepting protic polar solvent were used in sequence. Obtained fractions were structurally and thermally analysed. The protocol allowed for the generation of purified lignin fractions and the isolation of a novel, yet unobserved lignin carbohydrate complex (LCC) fraction. This LCC fraction was found to contain exclusively phenyl glycosides and γ-esters as LCC motifs.
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