Abstract
The effect of ball milling expressed as the yield of milled wood lignin (MWL) on the structure and molar mass of crude milled wood lignin (MWLc) preparation is studied to better understand the process’ fundamentals and find optimal conditions for MWL isolation (i.e., to obtain the most representative sample with minimal degradation). Softwood (loblolly pine) MWLc preparations with yields of 20–75% have been isolated and characterized based on their molar mass distribution (by Size Exclusion Chromatography (SEC)), hydroxyl groups of different types (31P NMR), methoxyl groups (HS-ID GC-MS), and sugar composition (based on methanolysis). Classical MWL purification is not used to access the whole extracted lignin. The results indicate that lignin degradation during ball milling occurs predominantly in the high molar mass fraction and is less pronounced in the low molar mass fraction. This results in a significant decrease in the Mz and Mw of the extracted MWLc with an increase in the yield of MWLc, but has only a very subtle effect on the lignin structure if the yield of MWLc is kept below about 55%. Therefore, no tedious optimization of process variables is necessary to achieve the required MWLc yield in this range for structural studies of softwood MWL. The sugar composition shows higher amounts of pectin components in MWLs of low yields and higher amounts of glucan and mannan in high-yield MWLs, confirming that lignin extraction starts from the middle lamella in the earlier stages of MWL isolation, followed by lignin extraction from the secondary wall region.
Highlights
For a detailed characterization of native lignin polymers, its isolation from the accompanying matrix is still required
This, in turn, requires an isolation of soluble preparations. Such isolation protocols for lignins are usually based on ball milling and date back to the mid-1950s when Björkman [1] developed a protocol based on the extraction of extensively ball milled wood by neutral solvents at room temperature (i.e., milled wood lignin (MWL))
MWL is the most common preparation used to mimic a native lignin structure that can be used to study the reactivity of native lignin ex planta
Summary
For a detailed characterization of native lignin polymers, its isolation from the accompanying matrix is still required. This, in turn, requires an isolation of soluble (lignin) preparations. Such isolation protocols for lignins are usually based on ball milling and date back to the mid-1950s when Björkman [1] developed a protocol based on the extraction of extensively ball milled wood by neutral solvents at room temperature (i.e., milled wood lignin (MWL)). The original procedure has been widely applied throughout the wood chemistry community and further modified to increase lignin yields while minimizing structural alterations [2,3,4]. The MWL usually serves as a standard lignin considered close to the native structure of lignin in the respective raw material used. To further reduce the isolation-induced modifications, Molecules 2018, 23, 2223; doi:10.3390/molecules23092223 www.mdpi.com/journal/molecules
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