Ball Milling’s Effect on Pine Milled Wood Lignin’s Structure and Molar Mass

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.

Spruce milled wood lignin: linear, branched or cross-linked?

“…However, the structure of MWL (i.e., the extracted and purified fraction of the total wood lignin) is rather independent of the milling intensity and quite constant in a wide range of MWL yield. 26,44,46 The difference in the amount of β-O-4 structures has not been well defined yet. Hu et al did not find any difference between MWL and lignin in situ by ozonolysis, 44 while Gellerstedt et al 47 reported a 40% decrease in β-O-4 in MWL vs. biomass lignin according to TA analysis.…”

“…The main LCC linkages are of benzyl ether, phenyl glycoside and γ-ester types; their total amounts is about 0.07/C9-unit. 39,52 Obviously, the molecular weight is significantly decreasing during ball milling, 46,49 but so far it is not possible to determine the decrease quantitatively due to challenges in detection of the starting value, the molecular weight of lignin in situ.…”

Influence of Base-Catalyzed Organosolv Fractionation of Larch Wood Sawdust on Fraction Yields and Lignin Properties

“…Firstly, 20 g of European larch sawdust was milled to a fraction <60 mesh and extracted with acetone in a Soxhlet apparatus for 16h at ≥6 cycles per hour with 300 mL of solvent prior to MWL preparation. The crude milled wood lignin was prepared based on a method described by Zinovyev et al [53]. In detail, 4 g of extracted sawdust was milled in a 250-mL ZrO 2 bowl, with 34 ZrO 2 balls (Ø 5 mm) in a planetary ball mill (Pulverisette 6, Fritsch, Idar-Oberstein, Germany).…”

Rice Husk Hydrolytic Lignin Transformation in Carbonization Process

“…The application sphere of these products is very diverse [2,3,6]. It is well-known [4,7,8,9,10] that properties of lignin-based products depend largely on lignin quality. Lignin quality in turn depends on the plant feedstock and lignin isolation process used.…”

“…These methods can be divided into two groups: (1) methods based on the translation of carbohydrates in solution and obtaining lignin in the form of insoluble residue; (2) methods based on the dissolution of lignin followed by its precipitation from the resulting solutions. Classic and modified methods of wood/biomass treatment using ball milling, organic solvents, acids, and others have been presented widely in different years [9,11]. Each of them has advantages and disadvantages.…”