Structural Characterization of Wheat Straw Lignin as Revealed by Analytical Pyrolysis, 2D-NMR, and Reductive Cleavage Methods

The structure of the lignin in wheat straw has been investigated by a combination of analytical pyrolysis, 2D-NMR, and derivatization followed by reductive cleavage (DFRC). It is a <i>p</i>-hydroxyphenyl-guaiacyl-syringyl lignin (with an H:G:S ratio of 6:64:30) associated with <i>p</i>-coumarates and ferulates. 2D-NMR indicated that the main substructures present are β-<i>O</i>-4′-ethers (∼75%), followed by phenylcoumarans (∼11%), with lower amounts of other typical units. A major new finding is that the flavone tricin is apparently incorporated into the lignins. NMR and DFRC indicated that the lignin is partially acylated (∼10%) at the γ-carbon, predominantly with acetates that preferentially acylate guaiacyl (12%) rather than syringyl (1%) units; in dicots, acetylation is predominantly on syringyl units. <i>p</i>-Coumarate esters were barely detectable (<1%) on monomer conjugates released by selectively cleaving β-ethers in DFRC, indicating that they might be preferentially involved in condensed or terminal structures

Variability in Lignin Composition and Structure in Cell Walls of Different Parts of Macaúba (<i>Acrocomia aculeata</i>) Palm Fruit

The lignins from different anatomical parts of macaúba (<i>Acrocomia aculeata</i>) palm fruit, namely stalks, epicarp, and endocarp, were studied. The lignin from stalks was enriched in <b>S</b>-lignin units (<b>S</b>/<b>G</b> 1.2) and β-ether linkages (84% of the total) and was partially acylated at the γ-OH of the lignin side-chains (26% lignin acylation), predominantly with <i>p</i>-hydroxybenzoates and acetates. The epicarp lignin was highly enriched in <b>G</b>-lignin units (<b>S</b>/<b>G</b> 0.2) and consequently depleted in β-ethers (65%) and enriched in condensed structures such as phenylcoumarans (24%) and dibenzodioxocins (3%). The endocarp lignin was strikingly different from the rest and presented large amounts of piceatannol units incorporated into the polymer. This resulted in a lignin polymer depleted in β-ethers but enriched in condensed structures and linked piceatannol moieties. The incorporation of piceatannol into the lignin polymer seems to have a role in seed protection

Structural Modifications of Residual Lignins from Sisal and Flax Pulps during Soda-AQ Pulping and TCF/ECF Bleaching

We have studied the structural modifications of lignins from sisal and flax during their soda-anthraquinone (AQ) pulping and subsequent totally chlorine-free (TCF) and elementary chlorine-free (ECF) bleaching. For this purpose, residual lignins were isolated from pulps, analyzed by Py-GC/MS, 2D-NMR, and GPC, and their characteristics were compared to the “milled-wood” lignin of the raw materials. Soda-AQ pulping caused a preferential removal of S-lignin and cleavage of β–<i>O</i>–4′ linkages, but the structure of the residual lignin remained relatively similar to native lignin. TCF bleaching barely affected the lignin structure, and noticeable amounts of β–<i>O</i>–4′ linkages still occur in these pulps. In contrast, ECF bleaching caused strong modifications in the lignin structure with the complete removal of lignin markers in ECF-bleached sisal pulp. However, residual lignin was still present in ECF-bleached flax pulp, with a predominance of G- and H-lignin units and the presence of β–<i>O</i>–4′ linkages

Catalytic Conversion of Organosolv Lignins to Phenolic Monomers in Different Organic Solvents and Effect of Operating Conditions on Yield with Methyl Isobutyl Ketone

Catalytic depolymerization of organosolv lignin to phenolic monomers with zeolites was investigated under various operating conditions. H-USY (Si/Al molar ratio = 5) outperformed H-USY with Si/Al ratios of 50 and 250, H-BEA, H-ZSM5, and fumed SiO₂ to produce the highest phenolic monomer yield from a commercial organosolv lignin in methanol at 300 °C for 1 h. It was then further investigated in the presence of acetone, ethyl acetate, methanol, and methyl isobutyl ketone (MIBK) on the depolymerization of organosolv bagasse lignin (BGL). The total highest phenolic monomer yield of 10.6 wt % was achieved with MIBK at 350 °C for 1 h with a catalyst loading of 10 wt %. A final total phenolic monomer yield of 19.4 wt % was obtained with an initial H₂ pressure of 2 MPa under similar processing conditions. The main phenolic monomers obtained are guaiacol (7.9 wt %), 4-ethylphenol (6.0 wt %), and phenol (3.4 wt %). The solvent properties were used to account for the differences in phenolic monomer yields obtained with different organic solvents

Modification of the Lignin Structure during Alkaline Delignification of Eucalyptus Wood by Kraft, Soda-AQ, and Soda‑O₂ Cooking

The modification of the lignin structure of an eucalyptus feedstock during alkaline delignification by kraft, soda-AQ, and soda-O₂ cooking processes has been investigated by different analytical techniques (size exclusion chromatography (SEC), pyrolysis gas chromatography–mass spectroscopy (Py-GC/MS), ¹H–¹³C two-dimensional nuclear magnetic resonance (2D-NMR), and ³¹P NMR). The characteristics of the lignins were compared at different pulp kappa levels, and with the native lignin isolated from the wood. The structural differences between the kraft, soda-AQ, and soda-O₂ residual lignins were more significant at earlier pulping stages. At the final stages, all the lignin characteristics were similar, with the exception of their phenolic content. Strong differences between lignins from pulps and cooking liquors were observed, including enrichment in guaiacyl units in pulp residual lignin and enrichment in syringyl units in black liquor lignin. A comparison of the alkaline cookings indicate that soda-O₂ process produced higher lignin degradation and provided promising results as pretreatment for the deconstruction of eucalyptus feedstocks for subsequent use in lignocellulose biorefineries