Bai-Liang Xue

Recent Advances in Characterization of Lignin Polymer by Solution-State Nuclear Magnetic Resonance (NMR) Methodology

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.

Quantitative Structures and Thermal Properties of Birch Lignins after Ionic Liquid Pretreatment

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.

Quantitative structural characterization of the lignins from the stem and pith of bamboo ( Phyllostachys pubescens )

Abstract 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.

Unveiling the Structural Heterogeneity of Bamboo Lignin by In Situ HSQC NMR Technique

One of the primary challenges for efficient utilization of lignocellulosic biomass is to clarify the complicated structure of lignin. In this study, in situ heteronuclear single quantum coherence nuclear magnetic resonance (NMR) characterization of the structural heterogeneity of lignin polymers during successively treated bamboo was emphatically performed without componential separation. Specially, the NMR spectra were successfully obtained by dissolving the acetylated and non-acetylated bamboo samples in appropriate deuterated solvent (CDCl3 and DMSO-d6). The heterogeneous lignin polymers in bamboo samples were demonstrated to be HGS-type and partially acylated at the γ-carbon of the side chain by p-coumarate and acetate groups. The major lignin linkages (β–O–4, β–β, and β–5, etc.) and various lignin–carbohydrate complex linkages (benzyl ether and phenyl glycoside linkages) can be assigned, and the frequencies of the major lignin linkages were quantitatively obtained. In particular, the residual enzyme lignin (REL) contained a higher amount of syringyl units and less condensed units as compared to other samples. Inspiringly, the method gives us a vision to track the structural changes of plant cell wall (e.g., lignin polymers) during the different pretreatments.

Structural characterization of hemicelluloses fractionated by graded ethanol precipitation from Pinus yunnanensis.

Fractionation of hemicelluloses from delignified Pinus yunnanensis was carried out with KOH/H(3)BO(3) solution followed by graded precipitation in 15%, 60%, and 90% (v/v) ethanol solutions, respectively. Chemical compositions, physicochemical properties, and structures of the precipitated hemicellulosic fractions were elucidated by a combination of sugar analysis, GPC, FT-IR, and (1)H, (13)C and 2D HSQC NMR spectroscopy. Sugar analysis showed that the hemicellulosic fraction precipitated by 15% ethanol solution (H(1)) had a predominance of xylose (58.52%), while mannose was the major sugar component in the hemicellulosic fractions precipitated by 60% (H(2)) and 90% (H(3)) ethanol solutions. GPC results revealed that the hemicelluloses precipitated by low concentration of ethanol solutions had higher weight-average molecular mass (50,090-79,840 g/mol) than those obtained in the high concentration of ethanol solutions (16,500 g/mol). The fraction precipitated by 60% ethanol solution (H(2)) was composed of D-galactose, D-glucose and D-mannose in a ratio of approximately 1:1:3.5. Structural determination indicated that the hemicellulosic fraction (H(2)) had a main structure of (1→4)-linked β-glucomannans backbone with (1→6)-linked α-D-galactose as a side chain attached to C-6 of mannose units. In addition, this fraction also contained minor amounts of xylans.

Producing Lignin-Based Polyols through Microwave-Assisted Liquefaction for Rigid Polyurethane Foam Production

Lignin-based polyols were synthesized through microwave-assisted liquefaction under different microwave heating times (5–30 min). The liquefaction reactions were carried out using polyethylene glycol (PEG-400)/glycerol as liquefying solvents and 97 wt% sulfur acid as a catalyst at 140 °C. The polyols obtained were analyzed for their yield, composition and structural characteristics using gel permeation chromatography (GPC), Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectra. FT-IR and NMR spectra showed that the liquefying solvents reacted with the phenol hydroxyl groups of the lignin in the liquefied product. With increasing microwave heating time, the viscosity of polyols was slightly increased and their corresponding molecular weight (MW) was gradually reduced. The optimal condition at the microwave heating time (5 min) ensured a high liquefaction yield (97.47%) and polyol with a suitable hydroxyl number (8.628 mmol/g). Polyurethane (PU) foams were prepared by polyols and methylene diphenylene diisocyanate (MDI) using the one-shot method. With the isocyanate/hydroxyl group ([NCO]/[OH]) ratio increasing from 0.6 to 1.0, their mechanical properties were gradually increased. This study provided some insight into the microwave-assisted liquefied lignin polyols for the production of rigid PU foam.

Lignin-based polyurethane film reinforced with cellulose nanocrystals

Novel nanocomposite films were synthesized using cellulose nanocrystals (CNC) as a reactive reinforcing filler and lignin-based polyurethane (L-PU) as the matrix through a casting and evaporating method. A series of L-PU films were prepared by replacing polyol with lignin from 10 to 50% (molar percentage). The L-PU film with maximum addition of lignin content was used as the matrix, which were reinforced with 0.5, 1 and 5 wt% dosage of CNC. The structural, mechanical and thermal properties of the resulting films were evaluated by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), tensile test and thermal gravimetric analysis. The mechanical measurement showed that the tensile strength of the L-PU films was significantly improved by the addition of CNC as reinforcement. Thermal gravimetric analysis results demonstrated that the thermal stability of the L-PU film after the addition of CNC was slightly increased.