Toward a Better Understanding of the Lignin Isolation Process from Wood

Abstract

The recently developed protocol for isolating enzymatic mild acidolysis lignins (EMAL) coupled with the novel combination of derivatization followed by reductive cleavage (DFRC) and quantitative (31)P NMR spectroscopy were used to better understand the lignin isolation process from wood. The EMAL protocol is shown to offer access at lignin samples that are more representative of the overall lignin present in milled wood. The combination of DFRC/(31)P NMR provided a detailed picture on the effects of the isolation conditions on the lignin structure. More specifically, we have used vibratory and ball milling as the two methods of wood pulverization and have compared their effects on the lignin structures and molecular weights. Vibratory-milling conditions cause substantial lignin depolymerization. Lignin depolymerization occurs via the cleavage of uncondensed beta-aryl ether linkages, while condensed beta-aryl ethers and dibenzodioxocins were found to be resistant to such mechanical action. Condensation and side chain oxidations were induced mechanochemically under vibratory-milling conditions as evidenced by the increased amounts of condensed phenolic hydroxyl and carboxylic acid groups. Alternatively, the mild mechanical treatment offered by ball milling was found not to affect the isolated lignin macromolecular structure. However, the overall lignin yields were found to be compromised when the mechanical action was less intense, necessitating longer milling times under ball-milling conditions. As compared to other lignin preparations isolated from the same batch of milled wood, the yield of EMAL was about four times greater than the corresponding milled wood lignin (MWL) and about two times greater as compared to cellulolytic enzyme lignin (CEL). Molecular weight distribution analyses also pointed out that the EMAL protocol allows the isolation of lignin fractions that are not accessed by any other lignin isolation procedures.

Propensity of Lignin to Associate: Light Scattering Photometry Study with Native Lignins

“…Lignin samples from E. globulus and Norway spruce were isolated using the enzymatic mild acidolysis isolation protocol (EMAL). 29 Then, part of the samples was acetobrominated, 17 and the resulting lignin was incubated in THF. The other part (without acetobromination) was incubated in 0.1 N NaOH.…”

“…The details of the lignin isolation and solution preparation have been described in previous reports. 17,29 Chemicals and Solvents. THF was HPLC grade and was previously equilibrated in air by standing under mild agitation in a loosely covered container overnight.…”

Determination of Arylglycerol−β-Aryl Ether Linkages in Enzymatic Mild Acidolysis Lignins (EMAL): Comparison of DFRC/31P NMR with Thioacidolysis

“…Enzymatic mild acidolysis lignins (EMALs) were isolated from Douglas fir (Pseudotsuga menziessi), white fir (Abies concolor), redwood (Sequoia semperVirens), eucalyptus (Eucalyptus globulus), and normal and compression wood from Southern pine (Pinus palustris) according to the previously described procedures. 5,9,15 Thioacidolysis. Thioacidolysis was performed on 5 mg of isolated lignins or 20 mg of Wiley-or ball-milled wood in 10 mL of reagent according to a published method.…”

“…These data are consistent with those reported in Figure 2 and support the contention that the integrity of the lignin's polymeric chains is not seriously affected by low-intensity ball-milling devices. 9,16 On the other hand, intensive milling protocols, such as vibratory-and orbital-milling devices, have been shown to seriously affect the lignin structure through oxidative reactions. 8,9 The data in Figure 3 also show significant differences among lignins from different wood species.…”

“…9,15,16 Comparison of the chemical structure of EMAL, MWL, and CEL has revealed only subtle differences, suggesting that EMAL is released by cleaving lignin-carbohydrate bonds rather than other linkages within lignin macromolecule. 9,12,15 Consequently, the aforementioned protocol presents a real opportunity to improve lignin yield and purity from different softwood and hardwood species.Another important issue in elucidating lignin structure has been the development and implementation of lignin analysis methods. Most of the conclusions regarding native lignin bonding patterns have been derived from degradative methods, such as thioacidolysis, 17 hydrogenolysis, 18 derivatization followed by reductive cleavage (DFRC), 19 acidolysis, 10,20 oxidation, 2 and ozonation.…”