Variation of the contents of biphenyl structures in lignins among wood species

Biphenyl structures (5–5 linkages) are one of the significant structural units in plant cell wall lignin. They can potentially play a role in providing branch points in lignins and influence the polymer structure of lignins. Variation of the biphenyl content was investigated for 21 wood species by alkaline nitrobenzene oxidation. The total yields of biphenyl-type products were similar among the six softwoods, and at least 6% of the phenylpropanoid units in lignin were involved in the biphenyl structures (>0.06 per C6-C3). The biphenyl product yield of the 15 hardwoods ranged widely (0.002–0.05 per C6-C3) and was lower in species having a higher syringyl/guaiacyl ratio (i.e. a species with less guaiacyl units). In addition, the proportion of biphenyl products in all guaiacyl-type products was not constant but decreased as the proportion of syringyl units increased. This indicates that the content of guaiacyl units is not likely the only factor influencing the biphenyl content.

Characterization of Lignin Precipitated From The Soda Black Liquor of Oil Palm Empty Fruit Bunch Fibers by Various Mineral Acids

Soda lignin from oil palm empty fruit bunch was directly isolated by various mineral acids i.e. sulfuric acid, hydrochloric acid, phosphoric acid and nitric acid at three levels of concentration (20% v/v, 60%v/v and concentrated). A comparison study was performed through physicochemical properties and structural features using FT-IR, UV, 13C-NMR and nitrobenzene oxidation. The FT-IR results showed that there is no significant difference between the main structures of the lignin isolated by various acids. However, low concentration of phosphoric acid is preferable because of its highest yield. The S: V: H ratio of 7-15:6-11:1 as evaluated by the nitrobenzene oxidation procedure suggests that soda lignin can be classified as belonging to either the cereal straw on grass type. The UV results indicate that phosphoric acid consistently gave the highest absorbance value among the four acids tested in this study regardless of its concentration level. The C13-FTNMR spectra, suggest that the lignin structure is independent of the type of acid used for precipitation.

Characterization of the precipitated lignin from Japanese beech as treated by semi-flow hot-compressed water

Japanese beech (Fagus crenata) was decomposed by a semi-flow process in hot-compressed water (HCW) at 150~230°C/10 MPa for 25 min. Mainly hemicelluloses and partly the lignin moiety of the wood was affected and liquefied in water. A part of the liquefied products can be precipitated after 12 h standing at ambient conditions. The precipitates are composed of around 90% of lignin, which was quantified and characterized by various chemical and spectroscopic methods in comparison of milled wood lignin (MWL), which is considered to be a representative compound for the native lignin in wood. The yield of the precipitated lignin (Lprecip) is higher than that of MWL and its syringyl content is also higher as demonstrated by alkaline nitrobenzene oxidation (NBO) giving rise to higher syringaldehyde/vanillin (SA/VA) ratios. Its OHphen group content was also high. The interpretation is that Lprecip is mainly from the secondary wall, and accordingly, the presented semi-flow HCW treatment offers the possibility for an efficient lignin isolation.

Revisiting alkaline nitrobenzene oxidation: quantitative evaluation of biphenyl structures in cedar wood lignin (Cryptomeria japonica) by a modified nitrobenzene oxidation method

A modified alkaline nitrobenzene oxidation (NBO) method was developed to enable the analysis of the biphenyl structures of cedar wood lignin. The most essential point of the process is a modified work-up process in pyridine and a prolonged gas chromatography analysis of the silylated products. By applying this mean to cedar wood meal, a novel biphenyl product, dehydrovanillin-vanillic acid (1-carboxy-1′-formyl-4,4′-dihydroxy-3,3′-dimethoxy-5,5′-biphenyl), was detected together with known products, dehydrodivanillin and dehydrodivanillic acid. The highest total yield of biphenyl products was detected when NBO was carried out for 2–4 h at 170°C. The work-up procedure was slightly modified so that the biphenyl products can be quantified. The NBO conversion rates of biphenyl linkages of lignin were also examined with a biphenyl-type lignin model compound, and this gave rise to 75% biphenyl-type NBO degradation products. Under the same condition, the total yield of vanillin and vanillic acid (VA) from a non-condensed-type β-O-4 model compound was 89%. Because the latter did not exhibit any peak of the biphenyl products, it can be concluded that all the biphenyl products obtained by NBO (0.17 mmol g-1) were from the lignin of the native wood. It was calculated that at least 6.7 of 100 phenylpropanoid units of cedar lignin were involved in biphenyl structures even if the NBO conversion rate was not taken into consideration. The synthesis and analytical data of 11 lignin model compounds is described.

Production of Monomeric Aromatic Compounds from Oil Palm Empty Fruit Bunch Fiber Lignin by Chemical and Enzymatic Methods

In this study, oil palm empty fruit bunch (OPEFBF) was pretreated with alkali, and lignin was extracted for further degradation into lower molecular weight phenolic compounds using enzymes and chemical means. Efficiency of monomeric aromatic compounds production from OPEFBF lignin via chemical (nitrobenzene versus oxygen) and enzymatic [cutinase versus manganese peroxidase (MnP)] approaches was investigated. The effects of sodium hydroxide concentration (2, 5, and 10% wt.) and reaction time (30, 90, and 180 minutes) on the yield of aromatic compounds were studied. The results obtained indicated that nitrobenzene oxidation produced the highest yield (333.17±49.44 ppm hydroxybenzoic acid,5.67±0.25 ppmp-hydroxybenzaldehyde,25.57±1.64 ppm vanillic acid,168.68±23.23 ppm vanillin,75.44±6.71 ppm syringic acid,815.26±41.77 ppm syringaldehyde,15.21±2.19 ppmp-coumaric acid, and44.75±3.40 ppm ferulic acid), among the tested methods. High sodium hydroxide concentration (10% wt.) was needed to promote efficient nitrobenzene oxidation. However, less severe oxidation condition was preferred to preserve the hydroxycinnamic acids (p-coumaric acid and ferulic acid). Cutinase-catalyzed hydrolysis was found to be more efficient than MnP-catalyzed oxidation in the production of aromatic compounds. By hydrolyzed 8% wt. of lignin with 0.625 mL cutinase g−1lignin at pH 8 and 55°C for 24 hours, about642.83±14.45 ppm hydroxybenzoic acid,70.19±3.31 ppm syringaldehyde,22.80±1.04 ppm vanillin,27.06±1.20 ppmp-coumaric acid, and50.19±2.23 ppm ferulic acid were produced.

Effect of black liquor replacement in wheat straw soda-AQ cooking and lignin structure of pulps

The effects of black liquor replacement cooking (BLRC) on wheat straw soda-anthraquinone (AQ) pulps and their lignin structures were investigated by using different black liquor replacement ratios (BLRR) and alkaline nitrobenzene oxidation and ozonation. The residual AQ, alkali, and dissolved lignin, as well as carbohydrates in the wheat straw black liquor, greatly influence wheat straw soda-AQ cooking. The influence could be controlled by different BLRR. The BLRC with around 60% BLRR resulted in superior delignification selectivity. The residual AQ and dissolved carbohydrates in the black liquor are beneficial to delignification selectivity, while the dissolved and degraded lignin can slow the delignification rate. No obvious differences were found in the lignin condensation and β-O-4 structure degradation by the BLRC at different BLRR compared to control soda-AQ cooking. The BLRC technology has potential application because it reduces chemical charge and chemical recovery load and results in higher yield and quality of pulp.