The demand for efficient utilization of biomass induces a detailed analysis of the fundamental chemical structures of biomass, especially the complex structures of lignin polymers, which have long been recognized for their negative impact on biorefinery. Traditionally, it has been attempted to reveal the complicated and heterogeneous structure of lignin by a series of chemical analyses, such as thioacidolysis (TA), nitrobenzene oxidation (NBO), and derivatization followed by reductive cleavage (DFRC). Recent advances in nuclear magnetic resonance (NMR) technology undoubtedly have made solution-state NMR become the most widely used technique in structural characterization of lignin due to its versatility in illustrating structural features and structural transformations of lignin polymers. As one of the most promising diagnostic tools, NMR provides unambiguous evidence for specific structures as well as quantitative structural information. The recent advances in two-dimensional solution-state NMR techniques for structural analysis of lignin in isolated and whole cell wall states (in situ), as well as their applications are reviewed.

No abstract

The use of ionic liquid (IL) in biomass pretreatment has received considerable attention recently because of its effectiveness in decreasing biomass recalcitrance to subsequent enzymatic hydrolysis. To understand the structural changes of lignin after pretreatment and enzymatic hydrolysis process, ionic liquid lignin (ILL) and subsequent residual lignin (RL) were sequentially isolated from ball-milled birch wood. The quantitative structural features of ILL and RL were compared with the corresponding cellulolytic enzyme lignin (CEL) by nondestructive techniques (e.g., FTIR, GPC, quantitative (13)C, 2D and (31)P NMR). The IL pretreatment caused structural modifications of lignin (cleavage of β-O-4 ether linkages and formation of condensed structures). In addition, lignin fragments with lower S/G ratios were initially extracted, whereas the subsequently extracted lignin is rich in syringyl unit. Moreover, the maximum decomposition temperature (T(M)) was increased in the order ILL < RL < CEL, which was related to the corresponding β-O-4 ether linkage content and molecular weight (M(w)). On the basis of the results observed, a possible separation mechanism of IL lignin was proposed.

Petroleum-based polyol was replaced with different amounts of lignin (8.33−37.19% w/w) to prepare lignin-based rigid polyurethane foam (LRPF). The LRPF containing 37.19% lignin was further reinforced with different weight ratios (1, 2, and 5 wt %) of pulp fiber. The resulting foams were evaluated by their chemical structure, cellular structure, density, compressive strength, and thermal property. Fourier transform infrared (FT-IR) and 13 C CP/MAS NMR spectra indicated that typical urethane linkages in LRPF were formed. Scanning electron microscope (SEM) results showed that the cell shape is significantly affected by the lignin and pulp fiber contents, which resulted in inhomogeneous, irregular, and large cell shapes and further decreased the densities of the LRPF. Mechanical results suggested that the compressive strength of the LRPF decreased with the increase in lignin content, but the additional pulp fiber had no significant effect on the compressive strength. Thermogravimetric analysis results demonstrated that the introduction of lignin led to high "carbon residue", but the introduction of pulp fiber would slightly improve the thermal stability of the LRPF.

Milled wood lignins (MWL) were isolated from the stem (MWLS) and pith (MWLP) of bamboo (Phyllostachys pubescens). The nonacetylated and acetylated bamboo MWLs were investigated by Fourier transform infrared, quantitative 13C-nuclear magnetic resonance (NMR), 2D heteronuclear single quantum coherence (HSQC) NMR, and 31P-NMR spectroscopy. The MWL consists of p-hydroxyphenyl (1–2%), guaiacyl (21–31%), and syringyl (67–78%) units associated with p-coumarates and ferulates. A modified quantitative 13C-NMR and 2D-HSQC analysis has demonstrated that the predominant intermonomeric linkages are of the type β-O-4 (45–49 per 100 C9 units, i.e., per C900) along with small amounts of other structural units such as resinols (3.6–7.4 per C900), tetrahydrofuran (2.0–2.3 per C900), phenylcoumaran (2.8–4.5 per C900), spirodienones (1.3–2.3 per C900), and α,β-diaryl ethers (2.8–2.9 per C900). MWLP contained more p-coumarates than MWLS. The various degrees of γ-acylation (17–27%) were positively associated with S/G ratios in the lignins; however, γ-acylation was inversely correlated to the ratio between β-β and β-O-4 side chains in these lignin fractions. Moreover, a flavonoid compound (tricin) was also detected in the MWLS but not in MWLP. The two MWLs are very similar in terms of molecular weights and the contents of OHphen and OHaliph.

BACKGROUND: To achieve the goals of economically feasible auto-catalyzed organosolv pretreatments in bioethanol production, chemical conversion of the isolated lignin is needed. However, the structures and properties of lignin molecules produced after pretreatment have not been thoroughly investigated before its effective utilization. RESULTS:The study focused on the auto-catalyzed ethanol-water pretreatment of southwest birch, with the aim to clarify the structural transformations of birch lignin after pretreatment. Chemical structural elucidation of the isolated lignins was performed using multiple NMR methodologies ( 31 P-, 13 C-and 2D-HSQC NMR techniques). Results showed that the amount of β-O-4 linkages decreased in the order of AEOL (auto-catalyzed ethanol organosolv lignin) < EHL P (enzymatic hydrolysis lignin, pretreated) < EHL U (unpretreated). The homolytic cleavage of β-O-4 linkages resulted in an increase of free phenolic hydroxyl groups and carboxylic acids in AEOL and EHL P compared with that of EHL U . In addition, α-ethoxylation was the only modification in the auto-catalyzed ethanol organosolv pretreatment (AEOP). Moreover, the thermal stability of the lignin samples is related to its inherent and condensed structures. CONCLUSIONS: These findings would facilitate the further utilization of lignin as starting material for developing value-added products in chemical and catalytic process.

No abstract

A new kind of biobased material named lignin-containing polyhydroxyurethane (LPHU) is prepared from bis­(6-membered cyclic carbonate) (BCC), dimer fatty diamine, and lignin for the first time. The preparation strategy is isocyanate-free, solvent-free, and catalyst-free, representing a green and environmentally friendly method to access polyurethane (PU)/lignin composites. The resultant LPHUs possess dual networks: a dynamic covalent network and a hydrogen bonding network, exhibiting superior mechanical strength, high thermal stability, excellent reprocessability/recyclability, and smart properties such as shape memory and self-healing. Potential application investigations indicate that the resultant LPHUs can be not only used for smart packaging label fabrication for heat-sensitive commodities but also further combined with natural cellulose paper to prepare paper-based electromagnetic shielding materials with high mechanical performance.