Formation of lignin alkyl-O-alkyl ether structures via 1,6-addition of aliphatic alcohols to ß-O-4-aryl ether quinone methides.

Quinone methides (QMs) are formed as the intermediates during lignin biosynthesis and chemical transformation; the chemical structure of the resulting lignin can then be significantly modified via the corresponding aromatization. Herein, the structure-reactivity relationship of ß-O-4-aryl ether QMs (GS-QM, GG-QM and GH-QM, which are 3-monomethoxylated QMs carrying syringyl, guaiacyl and p-hydroxyphenyl ß-etherified aromatic rings, respectively) was investigated to clarify the formation of alkyl-O-alkyl ether structures in lignin. The structural features of these QMs were characterized by NMR spectroscopy, and their alcohol-addition experiment was well performed at 25 °C to generate alkyl-O-alkyl/ß-O-4 products. The preferential conformation of GS-QM contains a stable directional intramolecular H-bond between γ-OH hydrogen and ß-phenoxy oxygen, which makes the ß-phenoxy group locate on the same side with γ-OH. In contrast, the ß-phenoxy groups in both GG- and GH-QM conformations are distant from the γ-OH; thus, the stable intermolecular H-bond is associated with the γ-OH hydrogen. Based on UV spectroscopy, the addition of methanol and ethanol occurs in QMs with a half-life of 1.7-2.1 and 12.8-19.3 minutes, respectively. With the same nucleophile, these QMs react faster in the order GH-QM > GG-QM > GS-QM. However, the reaction rate appears to be more influenced by the type of nucleophile than by the ß-etherified aromatic ring. Furthermore, the NMR spectra of products indicate that the steric bulkiness of both the ß-etherified aromatic ring and nucleophile contributes to the erythro-preferential formation of adducts from QMs. Moreover, the effect is more pronounced for the ß-etherified aromatic ring of QMs than the nucleophiles. The structure-reactivity relationship study demonstrates that the competition effect between H-bonds and steric hindrance determines the approaching direction and the accessibility of nucleophiles to planar QMs, resulting in stereo-differentiating formation of adducts. This model experiment may provide implications for the biosynthetic route and structural information of the alkyl-O-alkyl ether structure in lignin. Its results can also be further utilized to design innovative extraction methods of organosolv lignins for subsequent selective depolymerization or material preparation.

Exploring Alkyl-O-Alkyl Ether Structures in Softwood Milled Wood Lignins.

Recent studies have suggested that there are significant amounts of various alkyl ether (Alk-O-Alk; Alk = alkyl) moieties in a spruce native lignin preparation, milled wood lignin (SMWL). However, the comprehensive NMR assignment to these moieties has not been addressed yet. This study focused on investigating different types of Alk-O-Alk structures at the α- and γ-positions of the lignin side chain in an heteronuclear single-quantum coherence (HSQC) spectrum of SMWL using experimental NMR data of lignin and synthesized model compounds. Ambiguous structural features were predicted by computer simulation of 1H and 13C NMR spectra to complement the experimental NMR data. As a result, specific regions in the HSQC spectrum were attributed to different Alk-O-Alk moieties of Alk-O-Alk/ß-O-4 and Alk-O-Alk/ß-ß' structures. However, the differences between the specific regions were rather subtle; they were not well separated from each other and some major lignin moieties. Furthermore, SMWL contained a large variety of Alk-O-Alk moieties but in minute individual amounts, resulting in rather broad, superimposing resonances. Thus, evaluation did not allow assigning individual types of Alk-O-Alk moieties from the HSQC spectra; instead, they were quantified as total (α- and γ-linked) Alk-O-Alk based on the balance of structural units in the 13C NMR spectra. At last, potential formation mechanisms of various Alk-O-Alk ether structures in lignin biosynthesis, lignin aging, and during ball milling of wood were hypothesized and discussed.

Stereo-Recognition of Hydrogen Bond and Its Implications for Lignin Biomimetic Synthesis.

The hydrogen bond (H-bond) is essential to stabilizing the three-dimensional biological structure such as protein, cellulose, and lignin, which are integral parts of animal and plant cells; thus, stereo-recognition of the H-bond is extremely attractive. Herein, a methodology combining the variable-temperature 1H NMR technique with the density functional theory was established to recognize the underlying H-bonding patterns in lignin diastereomers. This method successfully classified the intramolecular and intermolecular H-bonds with slope values varying between 50.2-201.5 and 221.9-655.4, respectively, from the natural logarithm of the hydroxyl proton chemical shift versus the inverse of the temperature plot. Moreover, this slope was found to be correlated with the interaction distance between the H-bond donor and acceptor. Finally, it was proposed that the stereo-preferential formation of the ß-O-4 structure (erythro vs threo form) during lignin biomimetic synthesis was probably influenced by their intramolecular H-bonding patterns, thus making it easier to reach thermodynamic equilibrium.

Structural Analysis of Lignin-Based Furan Resin.

The global "carbon emission peak" and "carbon neutrality" strategic goals promote us to replace current petroleum-based resin products with biomass-based resins. The use of technical lignins and hemicellulose-derived furfuryl alcohol in the production of biomass-based resins are among the most promising ways. Deep understanding of the resulting resin structure is a prerequisite for the optimization of biomass-based resins. Herein, a semiquantitative 2D HSQC NMR technique supplemented by the quantitative 31P NMR and methoxyl group wet chemistry analysis were employed for the structural elucidation of softwood kraft lignin-based furfuryl alcohol resin (LFA). The LFA was fractionated into water-insoluble (LFA-I) and soluble (LFA-S) parts. The analysis of methoxyl groups showed that the amount of lignin was 85 wt% and 44 wt% in LFA-I and LFA-S fractions, respectively. The HSQC spectra revealed the high diversity of linkages formed between lignin and poly FA (pFA). The HSQC and 31P results indicated the formation of new condensed structures, particularly at the 5-position of the aromatic ring. Esterification reactions between carboxyl groups of lignin and hydroxyl groups of pFA could also occur. Furthermore, it was suggested that lignin phenolic hydroxyl oxygen could attack an opened furan ring to form several aryl ethers structures. Therefore, the LFA resin was produced through crosslinking between lignin fragments and pFA chains.

Transparent, smooth, and sustainable cellulose-derived conductive film applied for the flexible electronic device.

A high-performance flexible conductive substrate is one of the key components for developing promising wearable devices. Concerning this, a sustainable, flexible, transparent, and conductive cellulose/ZnO/AZO (CZA) film was developed in this study. The cellulose was used as the transparent substrate. The added AZO was as the conductive layer and ZnO functioned as an interface buffer layer. Results showed that the interface buffer layer of ZnO effectively alleviated the intrinsic incompatibility of organic cellulose and inorganic AZO, resulting in the improvement of the performance of CZA film. In compared with the controlled cellulose/AZO (CA) film with 365 Ω/sq sheet resistance and 87% transmittance, this CZA film featured a low conductive sheet resistance of 115 Ω/sq and high transmittance of 89%, as well as low roughness of 1.85 nm Moreover, the existence of conducive ZnO buffer layer enabled the conductivity of CZA film to be stable under the bending treatment. Herein, a flexible electronic device was successfully prepared with the biomass materials, which would be available by a roll-to-roll production process.

Effect of Tungsten Species on Selective Hydrogenolysis of Glycerol to 1,3-Propanediol.

Glycerol, as the major byproduct of biodiesel industry, is a cheap and green chemical feedstock. Following the expanded production of biodiesel, the oversupply of glycerol has led to increasing research of the catalytic conversion of glycerol. The selective hydrogenolysis of glycerol is an economical and sustainable way to produce 1,3-propanediol, which experiences a global growing demand, and valorize glycerol. However, the secondary hydroxy group of glycerol is sterically hindered by two primary hydroxy groups. As a result, 1,2-propanediol is the preferential product rather than 1,3-propanediol during conventional hydrogenolysis of glycerol. Currently, tungsten-containing bifunctional catalysts with metal and Brønsted acid sites are considered as a highly effective and atom-economical catalytic system for the selective hydrogenolysis of glycerol to 1,3-propanediol. Therefore, this Minireview summarized various tungsten-containing bifunctional catalysts for the hydrogenolysis of glycerol in detail and deeply discussed the relationship between tungsten species, metal active sites, and glycerol for selectively producing 1,3-propanediol.

Stereoselective Formation of ß-O-4 Structures Mimicking Softwood Lignin Biosynthesis: Effects of Solvent and the Structures of Quinone Methide Lignin Models.

p-Quinone methide (QM) is formed as an intermediate during lignin biosynthesis. The aromatization of the QM by the attack of a nucleophile at the α-position of its side chain generates a phenolic hydroxy group in a growing polymer and creates stereoisomeric forms in the side chain. A series of ß-O-4-aryl ether QMs was reacted with water at 25 °C to replicate the formation of p-hydroxyphenyl (H) and guaiacyl (G) ß-O-4 structures in plant cell walls. Water addition occurred in 3-methoxy-substituted QMs (G-type QMs) with half-lives ( t1/2) between 13 and 15 min, at pH 7, in 50% water solution (dioxane-water, 1:1). The rate increased as the water concentration increased to 99% ( t1/2, 1.2-1.4 min). Similar solvent effects were observed for more reactive nonsubstituted QMs (H-type QMs with t1/2 of <1 min). Consequently, t1/2 of the H-type QMs was shorter than that of the G-type QMs under every solvent condition. Upon increasing the water concentration, the variation in the erythro/ threo ratios of the four dimeric ß-O-4 products increased. Interestingly, the effect of pH on the stereopreference, which was observed in 50% water solution, was small and became imperceptible as the water concentration increased to 99%, suggesting that the effect of the solvent, as well as the effect of the pH, plays an important role in understanding the reaction conditions in cell walls during lignin biosynthesis. The threo isomer was preferentially formed in the four dimeric ß-O-4 structures, which is inconsistent with the structural features of compression wood lignin rich in H-units. However, the erythro-selective formation was attained in an H-type QM at every pH studied (pH 3.5-7) by introducing a biphenyl structure into the ß-etherified ring moiety.

Lignin-Biosynthetic Study: Reactivity of Quinone Methides in the Diastereopreferential Formation of p-Hydroxyphenyl- and Guaiacyl-Type ß- O-4 Structures.

p-Quinone methides are involved in lignin biosynthesis as transient intermediates, and the aromatization step has a great impact on the chemical structure of the resulting lignin. A series of quinone methides (QMs) were synthesized and allowed to react with water in pH 3-7 buffers at 25 °C to mimic the formation of p-hydroxyphenyl- and guaiacyl-type (H- and G-type, respectively) ß- O-4 structures in gymnosperm-plant cell walls. Water addition occurred in 3-methoxy-substituted QMs (G-type QMs) with half-lives of 1.4-15 min. In contrast, nonsubstituted QMs (H-type QMs) were very labile; they were aromatized to ß- O-4 products with half-lives of only 10-40 s. The rapid aromatization in H-type QMs may provide an advantage over G-type species for efficiently driving the lignin-polymerization cycle, which possibly contributes to the development of highly lignified compression wood. In the water-addition reaction, the threo isomers of the ß- O-4 products were stereopreferentially formed more than the erythro isomers from both G- and H-type QMs ( erythro/ threo ratios of 24:76 and 50:50, respectively). The proportion of erythro isomers was higher at lower-pH conditions. This pH-dependent trend agrees with findings from a previous study on 3,5-dimethoxy-substituted (syringyl-type, S-type) QMs; thus, this pH-dependent trend is common in H-, G-, and S-type lignin-related QMs. Higher threo-selectivity was obtained by changing the ß-etherified aromatic rings from G- to H-type. A similar but weaker effect was also observed by changing the QM moiety from G- to H-type.

TEMPO-oxidized cellulose nanofibers (TOCNs) as a green reinforcement for waterborne polyurethane coating (WPU) on wood.

In this work, TEMPO-oxidized cellulose nanofibers (TOCNs) were investigated as a green additive to the waterborne polyurethane (WPU) based coating, for improving its mechanical properties. The structure, morphology, mechanical properties and performances of the WPU/TOCNs coating were determined. Results showed that TOCNs had good compatibility to the WPU coating, and significantly enhanced the mechanical properties of the coating. The Halpin-Tsai and Ouali models were used to fit for the Young's modulus of the resulting coating, and good agreements were found between the Ouali model and experimental results when the TOCNs content exceeded the critical percolation threshold (0.7vol% or 1.0wt%). It was also found that the pencil hardness of the coating was improved with the addition of TOCNs. However, AFM and pull-off test revealed the negative effects of the TOCNs addition on the surface roughness and adhesion strength of the coating to the wood surface.

Cationic amphiphilic microfibrillated cellulose (MFC) for potential use for bile acid sorption.

In this work, Micro-fibrillated Cellulose (MFC) was cationically modified by quaternary ammonium groups with different chemical structures aiming to improve the sorption capacity to bile acid. The in-vitro bile acid sorption was performed by investigating various factors, such as quaternary ammonium group content and length of its alkyl substituent of the modified cationic MFC (CMFC), ionic strength, initial concentration and hydrophobicity of bile acid. The results showed that the sorption behavior of the modified CMFC was strongly influenced by the quaternary ammonium group content and the lengths of its alkyl substituent, the sorption capacity for the modified CMFC with a C18 alkyl substituent, was approximately 50% of that of Cholestyramine. The experimental isotherm results were well fitted into the Temkin model. The effect of salts in the solution was smaller for the bile acid sorption onto the hydrophobic CMFC than the CMFC. It was also found that the binding capacity of CMFC was higher for more hydrophobic deoxycholate in comparison with cholate.

Adsorption of polyethylene glycol (PEG) onto cellulose nano-crystals to improve its dispersity.

In this work, the adsorption of polyethylene glycol (PEG) onto cellulose nano-crystals (CNC) was investigated for preparing re-dispersible dried CNC. Results showed that the re-dispersity of CNC in water can be significantly enhanced using a PEG1000 dosage of 5wt% (based on the dry weight of CNC). The elemental analysis confirmed the adsorption of PEG onto the CNC surface. Transmission electron microscopy (TEM) was used to characterize the dry powder and indicated that the irreversible agglomeration of CNC after drying was essentially eliminated based on the PEG adsorption concept. Thermo-gravimetric analysis (TGA) and X-ray diffraction (XRD) suggested that CNC crystallinity and thermal stability were not affected by the adsorption of PEG. Thus, the adsorption of PEG has great potential for producing re-dispersible powder CNC.