Jia-Long Wen

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.

Understanding the chemical transformations of lignin during ionic liquid pretreatment

Unveiling the fundamental chemistry of lignin under ionic liquid (IL) pretreatment will facilitate the understanding of biomass recalcitrance involved in pretreatment processes. To examine in greater detail the chemical transformations of lignin under different IL pretreatment conditions without competing reactions from plant polysaccharides, the IL pretreatment of the isolated poplar alkaline lignin (hardwood lignin) under varying IL pretreatment conditions (i.e., 110–170 °C, 1–16 hours) was performed in an appropriate manner. The structural transformations of the lignin have been investigated by elemental analysis, 2D-HSQC spectra, quantitative 13C-NMR spectra, 31P NMR, and GPC analysis. Results revealed that a decrease of aliphatic OH and an increase in phenolic hydroxyl groups occurred in lignin as the pretreatment proceeded. The increased phenolic OH was mainly as a result of cleavage of β-O-4′ linkages, while the reduced aliphatic OH is probably attributed to the dehydration reaction. The cleavage of β-O-4′ linkages, degradation of β–β′ and β-5′ linkages obviously happened at high temperatures and resulted in the decrease of molecular weights. In addition, IL pretreatment selectively degraded the G-type lignin fractions and the condensation reaction took place more easily at S units than G units. Moreover, the demethoxylation preferentially occurred in G units, especially at higher temperatures. It is believed that investigating the fundamental chemistry of lignin during IL pretreatments would be beneficial to optimize and control the pretreatment process.

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.

Structural elucidation of lignin polymers of Eucalyptus chips during organosolv pretreatment and extended delignification.

Effective delignification of lignocelluloses is a very important to guarantee the economic feasibility of organosolv-based biorefinery. Eucalyptus chips were successively subjected to organosolv pretreatment (AEOP) and extended delignification (ED) process in the present study. The effects of delignification processes were scientifically evaluated by component analysis, SEM, and CP-MAS NMR techniques. It was found that the integrated process of organosolv pretreatment and subsequent delignification resulted in an effective delignification. The fundamental chemistry of the lignin obtained after these processes was thoroughly investigated by FT-IR, multidimensional NMR ((31)P-, (13)C-, and 2D-HSQC NMR), and GPC techniques. It was observed that an extensive cleavage of aryl ether linkages, ethoxylation, and some condensation reactions occurred in AEOP process, while α-oxidation mainly took place in alkaline hydrogen peroxide (AHP) process. It is believed that better understanding the fundamental chemistry of lignin facilitates the optimization of the delignification process. More importantly, well-defined of lignin polymers will facilitate their value-added applications in current and future biorefineries.

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.

Successive alkali extraction and structural characterization of hemicelluloses from sweet sorghum stem

Sweet sorghum stem was successively extracted with water at 90 °C, 0.3, 0.6, 1.0, 1.5, and 2.0% KOH aqueous solution, and 60% ethanol containing 2.5% KOH at 75 °C for 3 h, yielding 76.3% of the original hemicelluloses. Chemical composition and structural characterization of the seven hemicellulosic fractions obtained were comparatively investigated by a combination of HPAEC, GPC, FT-IR, (1)H-, (13)C-, HSQC NMR and TGA techniques. According to the spectral analysis, hemicelluloses from sweet sorghum stem are assumed to L-arabino-4-O-methyl-D-glucurono-D-xylan. In addition, the higher molecular weights of hemicelluloses resulted in a higher thermal stability of the samples. The present study suggests that successive alkali extraction is a promising approach for fractionation of hemicelluloses from sweet sorghum stem and to prepare hemicellulosic polymers with different branching and molecular weights.

Fractionation of bamboo culms by autohydrolysis, organosolv delignification and extended delignification: Understanding the fundamental chemistry of the lignin during the integrated process

Bamboo (Phyllostachys pubescens) was successfully fractionated using a three-step integrated process: (1) autohydrolysis pretreatment facilitating xylooligosaccharide (XOS) production (2) organosolv delignification with organic acids to obtain high-purity lignin, and (3) extended delignification with alkaline hydrogen peroxide (AHP) to produce purified pulp. The integrated process was comprehensively evaluated by component analysis, SEM, XRD, and CP-MAS NMR techniques. Emphatically, the fundamental chemistry of the lignin fragments obtained from the integrated process was thoroughly investigated by gel permeation chromatography and solution-state NMR techniques (quantitative (13)C, 2D-HSQC, and (31)P-NMR spectroscopies). It is believed that the integrated process facilitate the production of XOS, high-purity lignin, and purified pulp. Moreover, the enhanced understanding of structural features and chemical reactivity of lignin polymers will maximize their utilizations in a future biorefinery industry.

Structural elucidation of whole lignin from Eucalyptus based on preswelling and enzymatic hydrolysis

The structural elucidation of whole lignin in the plant cell wall is extremely important for providing a representative lignin to understand the molecular characteristics of lignin in plants, and develop lignin-based polymers and green chemicals under the current biorefinery scenario. However, research in this area still lack methodologies for effectively isolating whole lignin from the plant cell wall. In this study, an effective method based on mild alkaline preswollen (4% NaOH, 25 °C, 24 h) and enzymatic hydrolysis for the isolation of “swollen residual enzyme lignin, SREL” from Eucalyptus wood was proposed. SREL was investigated as compared to the corresponding cellulolytic enzyme lignin (CEL) and alkali lignin (AL). Observably, the yield of SREL (95%) was significantly higher than that of the corresponding CEL (20%) and AL (12%). The isolated lignin has been comparatively investigated by a combination of elemental analysis, 2D HSQC NMR, 31P-NMR, analytical pyrolysis, and GPC techniques. The major lignin linkages (β-O-4′, β–β′, β-5′, etc.) were thoroughly assigned and the frequencies of the major lignin linkages were quantitatively compared. Further experiments demonstrated that a transformation from cellulose I to cellulose II occurred during alkaline preswelling of the ball-milled Eucalyptus wood, which resulted in the efficient enzymatic hydrolysis of the substrates, thus yielding a representative lignin sample (SREL). However, the alkaline preswelling treatment has little effect on the lignin structures (typical substructures); it only tends to yield syringyl-rich lignin macromolecules as compared to CEL. Furthermore, the effective method gives us a panoramic image to understand the intrinsic structural features of whole lignin from other lignocellulosic biomasses and helps to develop more effective plant deconstruction or depolymerization strategies in the current biorefinery and catalytic conversion process.

Characterization and phenolation of biorefinery technical lignins for lignin–phenol–formaldehyde resin adhesive synthesis

Technical lignins are cheap, abundant and renewable phenolic substances that have been attracting increasing attention. In this study, the structural features and active sites of four technical lignins obtained from different biorefinery processes were thoroughly characterized. Their suitability for partial incorporation into a phenol–formaldehyde (PF) resin adhesive was also evaluated. Phenolation treatment under alkaline conditions was conducted to enhance the reactivity of the technical lignins. Composition analysis indicated that all four technical lignins had a high purity (>88%). 13C nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) analyses revealed that the technical lignins from different original feedstocks and biorefinery processes had different structural features, but all of these technical lignins could be used in the synthesis of a lignin–phenol–formaldehyde (LPF) resin adhesive. The structural features and active sites of the different technical lignins before and after phenolation treatment were determined using quantitative two-dimensional heteronuclear single-quantum correlation (2D HSQC) and 31P NMR spectroscopies. The results confirmed that the phenolation treatment under alkaline conditions could effectively increase the number of active sites on the technical lignins and could be easily included in the synthesis process of LPF resin adhesives.