Fractionation of beech wood cell walls into digestible cellulose-rich residues and photoluminescent lignin-rich precipitates via semi-flow hot-compressed water treatment with 2-naphthol.

Utilization of cell wall components of woody biomass has attracted attention as alternatives for fossil fuels towards a sustainable society. A semi-flow hydrothermal treatment was used to fractionate the beech (Fagus crenata) wood into cellulose-rich residues and lignin-rich precipitates. The enzymatic saccharification of the cellulose component in the residue was enhanced significantly because the preferential delignification from the secondary wall increased enzyme accessibility. Meanwhile, the precipitated lignin was soluble in organic solvent and exhibited clear photoluminescence (PL) according to the chromophore distances. Furthermore, the carbocation scavenger, 2-naphthol, was impregnated into the beech wood to inhibit the lignin re-condensation reaction. As a result, the digestibility of the cellulose component in the residue increased because unproductive enzymatic binding of lignin and lignin re-condensation were both suppressed. In addition, the PL intensity of the precipitates was significantly enhanced, indicating that 2-naphthol bound to the lignin molecules influenced the PL properties. Overall, fractionation using a semi-flow hydrothermal treatment efficiently uses both polysaccharides and lignin, especially the impregnation of 2-naphthol provided advantages for both saccharides and lignin. Monosaccharides can be converted into valuable products via a sugar platform, and the lignin precipitates exhibit useful PL properties that give them significant potential as a feedstock for numerous valuable materials, such as fluorescence reagents and spectral conversion agents. The results presented herein provide insights that are crucial for the comprehensive utilization of cell wall components for sustainable biorefinery systems.

Solubilization of sulfuric acid lignin by ball mill treatment with excess amounts of organic compounds.

In order to improve the solubility of sulfuric acid lignin (SL) in N,N-dimethylformamide (DMF), dry ball milling with excess amounts of additives such as l-tartaric acid was performed. Although the ball-milled SL without any additives was not soluble in DMF, when the SL was ball milled with an excessive amount of l-tartaric acid (the concentration of SL to be 0.1%), the dispersion and solubility of SL in DMF detected by the dynamic light scattering was greatly improved. Furthermore, the DMF solution showed clear photoluminescence, indicating that the distance between luminophores was modulated due to dispersion on the nanoscale. The structural analysis of the isolated lignin showed a decrease in molecular weight and the introduction of carboxylic acid groups. In other words, the introduction of hydrophilic functional groups into the lignin and simultaneously decrease in the molecular weight due to the cleavage of lignin linkages is considered to result in good dispersion in DMF on both the micro and macro scales. Similar effects were observed with the other chemicals containing several hydrophilic groups such as citric acid, d-glucose, and polyacrylic acid. Furthermore, this method is applicable to various lignins other than SL, and it is expected to utilize unused lignin resources.

Influence of Proteins on the Lignin Decomposition Behavior of Japanese Cedar (Cryptomeria japonica) Wood by Supercritical Methanol Treatment.

The effect of adding protein on the decomposition behavior of lignin in Japanese cedar under supercritical methanol conditions (270 °C/27 MPa) was studied. The Klason method was used to detect the lignin content in the insoluble residue following to a 30 min treatment. Adding either an animal (bovine serum albumin) or plant (soy) protein enhanced delignification from 50 to 65% of the lignin-based wt %. This result was attributed to enhanced lignin depolymerization owing to inhibited lignin recondensation and/or the suppressed formation of polysaccharide-derived char via reactions between the protein and polysaccharides. Although the solubilization of lignin was promoted and the yield of lignin-derived low-molecular-weight compounds increased, the selectivity of major monomers such as coniferyl alcohol (CA) and γ-methylated CA decreased. The addition of proteins has a substantial impact on the decomposition behavior of cell wall components under supercritical methanol conditions. This information provides insights into the use of protein-rich lignocelluloses.

Tunable Light Emission from Lignin: Various Photoluminescence Properties Controlled by the Lignocellulosic Species, Extraction Method, Solvent, and Polymer.

This report describes the tunable light emission from lignin, which was achieved by carefully selecting the lignocellulosic species, extraction method, solvent, and polymer. Lignins comprising various taxonomic species with distinct primary structures exhibited diverse photoluminescence (PL) intensities and spectral patterns. Investigations probing how the solvent affects the PL properties revealed that the PL quenching phenomenon originated from the decreasing distance between aromatic moieties (luminophores). Therefore, polymers can play key roles as media to modulate the distance between luminophores, and the PL intensity can be enhanced by employing a relatively stiff polymer. In terms of the emission color, the PL spectral pattern can be tuned by changing the lignin primary structures or by deprotonating the phenolic hydroxyl groups. By modulating these influencing factors, various light emissions were obtained from lignins in solutions and transparent solid materials.

Topochemistry of the Delignification of Japanese Beech (Fagus crenata) Wood by Supercritical Methanol Treatment.

The topochemistry of Japanese beech (Fagus crenata) wood delignification was evaluated in this study following a supercritical methanol treatment (270 °C, 27 MPa). Ultraviolet microscopic analysis of the insoluble residue revealed that the lignin in the secondary wall was easily decomposed and removed because of the preferential cleavage of ether-type linkages. In contrast, the middle lamella lignin was initially resistant to supercritical methanol but eventually decomposed and was removed. In addition, UV-absorbing secondary products formed selectively inside the parenchyma cells. Results from the supercritical methanol treatment of demineralized beech wood indicated that inorganic substances in the lumen of parenchyma affected the formation of these secondary products, thus leading to an overestimation of the residual lignin. Therefore, the topochemistry of delignification was more precisely evaluated when using demineralized beech wood.

Enhancing Enzyme-Mediated Cellulose Hydrolysis by Incorporating Acid Groups Onto the Lignin During Biomass Pretreatment.

Lignin is known to limit the enzyme-mediated hydrolysis of biomass by both restricting substrate swelling and binding to the enzymes. Pretreated mechanical pulp (MP) made from Aspen wood chips was incubated with either 16% sodium sulfite or 32% sodium percarbonate to incorporate similar amounts of sulfonic and carboxylic acid groups onto the lignin (60 mmol/kg substrate) present in the pulp without resulting in significant delignification. When Simon's stain was used to assess potential enzyme accessibility to the cellulose, it was apparent that both post-treatments enhanced accessibility and cellulose hydrolysis. To further elucidate how acid group addition might influence potential enzyme binding to lignin, Protease Treated Lignin (PTL) was isolated from the original and modified mechanical pulps and added to a cellulose rich, delignified Kraft pulp. As anticipated, the PTLs from both the oxidized and sulfonated substrates proved less inhibitory and adsorbed less enzymes than did the PTL derived from the original pulp. Subsequent analyses indicated that both the sulfonated and oxidized lignin samples contained less phenolic hydroxyl groups, resulting in enhanced hydrophilicity and a more negative charge which decreased the non-productive binding of the cellulase enzymes to the lignin.

Alkaline sulfonation and thermomechanical pulping pretreatment of softwood chips and pellets to enhance enzymatic hydrolysis.

To assess the impact of alkalinity on sulfonation and the enzyme-mediated hydrolysis of softwood cellulose, Lodgepole pine chips were impregnated with 8% sodium sulfite and increasing loadings of sodium carbonate before thermomechanical pulping. It was apparent that alkali addition enhanced lignin sulfonation with an additional 4% loading of sodium carbonate proving optimal. TEM indicated that sulfonation predominantly occurred within the secondary-cell-wall lignin, increasing cellulose accessibility to the cellulase enzymes. Although increasing alkalinity did not significantly enhance lignin sulfonation, likely due to the lower acetyl content of the softwood chips, it increases mannan solubilization. Despite their smaller particle size, softwood pellets were more poorly sulfonated, probably due to their higher lignin content and lower amount of acid groups. This more condensed lignin structure was confirmed by 2D-NMR and GPC analyses which indicated that the EMAL derived from softwood pellets contained less native ß-O-4 linkages and had a higher molecular weight.

The influence of lignin on the effectiveness of using a chemithermomechanical pulping based process to pretreat softwood chips and pellets prior to enzymatic hydrolysis.

Over the last century the pulp and paper sector has assessed various technologies to fractionate woody biomass to produce strong, bright fibers. Several of these processes have also been assessed for their potential to pretreat and fractionate biomass to enhance the subsequent enzymatic hydrolysis of the cellulosic component. Although many of these pretreatments are effective on agricultural residues, softwoods have proven more recalcitrant, primarily due to their high lignin content and structure. As delignification is too expensive to be used routinely a more economically attractive approach might be to alter the lignin. Recent work has shown that, using a modified chemithermomechanical pulping (CTMP) "front end", lignin can be modified and relocated. This significantly enhanced hemicellulose recovery and enzyme-mediated cellulose hydrolysis of woody biomass. As well as being effective on wood chips, the modified CTMP pretreatment process also enhanced the bioconversion of densified feedstocks such as pellets.

The influence of lignin migration and relocation during steam pretreatment on the enzymatic hydrolysis of softwood and corn stover biomass substrates.

To be effective, steam pretreatment is typically carried out at temperatures/pressures above the glass transition point (Tg) of biomass lignin so that it can partly fluidize and relocate. The relocation of Douglas-fir and corn stover derived lignin was compared with the expectation that, with the corn stover lignin's lower hydrophobicity and molecular weight, it would be more readily fluidized. It was apparent that the Tg of lignin decreased as the moisture increased, with the easier access of steam to the corn stover lignin promoting its plasticization. Although the softwood lignin was more recalcitrant, when it was incorporated onto filter paper, it too could be plasticized, with its relocation enhancing enzymatic hydrolysis. When lignin recondensation was minimized, the increased hydrophobicity suppressed lignin relocation. It was apparent that differences in the accessibility of the lignin present in Douglas-fir and corn stover to steam significantly impacted lignin fluidization, relocation, and subsequent cellulose hydrolysis.

Deep Eutectic Solvent Pretreatment of Transgenic Biomass With Increased C6C1 Lignin Monomers.

The complex and heterogeneous polyphenolic structure of lignin confers recalcitrance to plant cell walls and challenges biomass processing for agroindustrial applications. Recently, significant efforts have been made to alter lignin composition to overcome its inherent intractability. In this work, to overcome technical difficulties related to biomass recalcitrance, we report an integrated strategy combining biomass genetic engineering with a pretreatment using a bio-derived deep eutectic solvent (DES). In particular, we employed biomass from an Arabidopsis line that expressed a bacterial hydroxycinnamoyl-CoA hydratase-lyase (HCHL) in lignifying tissues, which results in the accumulation of unusual C6C1 lignin monomers and a slight decrease in lignin molecular weight. The transgenic biomass was pretreated with renewable DES that can be synthesized from lignin-derived phenols. Biomass from the HCHL plant line containing C6C1 monomers showed increased pretreatment efficiency and released more fermentable sugars up to 34% compared to WT biomass. The enhanced biomass saccharification of the HCHL line is likely due to a reduction of lignin recalcitrance caused by the overproduction of C6C1 aromatics that act as degree of polymerization (DP) reducers and higher chemical reactivity of lignin structures with such C6C1 aromatics. Overall, our findings demonstrate that strategic plant genetic engineering, along with renewable DES pretreatment, could enable the development of sustainable biorefinery.