Preparation of strongly acidic cation-exchange resins from gymnosperm acid hydrolysis lignin

The chemical preparation of strongly acidic cation-exchange resin from sulfuric acid lignin (Klason lignin) (SAL), a typical acid hydrolysis lignin, was investigated. Sulfonation of resinified SAL itself gave a resin with an ion-exchange capacity of 2.3 mEq/g. After resinification with formaldehyde, the phenolized SAL with a reactivep-hydroxyphenyl group yielded a resin with an ion-exchange capacity of 3.2 mEq/g. The latter capacity is superior to that of the corresponding commercial phenol-type resins (2–3 mEq/g), but did not reach the level of the corresponding commercial styrene-type resins (4-5 mEq/g).This paper was presented at the 43th Lignin Symposium, Fuchu, October 1998     

Reactivity of a condensed–type lignin model compound in the Mannich reaction and preparation of cationic surfactant from sulfuric acid lignin

 The chemical conversion of phenolized sulfuric acid lignin (P-SAL), prepared from sulfuric acid lignin (SAL) by phenolation with sulfuric acid catalyst, to novel cationic surfactant was investigated. To elucidate the chemical reactivity of the P-SAL to a Mannich reaction, 1-guaiacyl-1-p-hydroxyphenylethane (I) as a simple phenolized sulfuric acid lignin model compound was reacted with dimethylamine and formaldehyde. Quantitative analysis of the products by gas-liquid chromatography suggested that the p-hydroxyphenyl nucleus was more reactive than the guaiacyl nucleus. The Mannich reaction of SAL with dimethylamine did not yield a soluble cationic surfactant, but P-SAL produced water-soluble cationic surfactant in a quantitative yield. The Mannich reaction products (MP-SAL) of P-SAL had 1,3-dimethylaminomethyl groups/C₉-C₆. The results of the surface tension measurements showed that the decrease in surface tension of MP-SAL was much larger than that of lignosulfonate as a commercial surfactant from lignin.     

Preparation of anion-exchange resins from pine sulfuric acid lignin,one of the acid hydrolysis lignins

To utilize acid hydrolysis lignin effectively, chemical conversion to anion-exchange resin was investigated by two methods. Sulfuric acid lignin (SAL) was selected as a typical acid hydrolysis lignin in this experiment. Because it is less reactive, SAL was phenolated with sulfuric acid catalyst to yield reactive phenolized SAL (P-SAL) with p-hydroxyphenyl nuclei. One method was the restricted resinification of P-SAL followed by the Mannich reaction with formaldehyde and dimethylamine to yield a weakly basic anion-exchange resin with an ion-exchange capacity of 2.4mEq/g. Another method was to react resinified P-SAL with glycidyltrimethylammonium chloride to yield a strongly basic anion-exchange resin with an ion-exchange capacity of 2.0mEq/g. The reaction of a simple P-SAL model compound with an epoxide suggested that the phenolic hydroxyl group of the p-hydroxyphenyl nucleus had slightly higher reactivity than that of the guaiacyl nucleus.Part of this report was presented at the 47th Lignin Symposium, Fukuoka, October 2002     

Phenolization of hardwood sulfuric acid lignin and comparison of the behavior of the syringyl and guaiacyl units in lignin

To study the behavior of hardwood sulfuric acid lignin (SAL) during phenolization, we compared the product yield, average molecular weight, methoxy content, and reactions of simple model compounds with those of softwood SAL, focusing on the difference between syringyl and guaiacyl units. The beech SAL reacted with phenol more readily than red pine SAL and yielded a larger soluble fraction of phenolized SAL. To investigate the difference in the phenolization activity of the syringyl and guaiacyl units in beech lignin, we prepared syringyl-nucleus-rich sulfuric acid lignin (S-rich-SAL) and guaiacyl-nucleus-rich sulfuric acid lignin (G-rich-SAL) from beech, which were subjected to phenolization. The results suggest that the syringyl unit in SAL had greater phenolization activity and its phenolized products were more soluble in acidic aqueous medium and introduced less phenol than the guaiacyl unit. Using model compounds, the study also showed that the syringyl unit had higher phenolization reactivity than the guaiacyl unit.     

Ready chemical conversion of acid hydrolysis lignin into water-soluble lignosulfonate II: Hydroxymethylation and subsequent sulfonation of phenolized lignin model compounds

Highly condensed lignin can be transformed by three reactions — phenolation, hydroxymethylation, and neutral sulfonation — to water-soluble lignosulfonate. To elucidate reactivities and products in the latter two reactions, simple compounds were selected as lignin model compounds. With hydroxymethylation of creosol at 60°C, the yield of a condensed-type product with the diarylmethane structure was less than 10%. Hydroxymethylation of 1-guaiacyl-1-p-hydroxy-phenylethane (compound VI) as a phenolized guaiacyl lignin model compound gave four compounds. The initial reaction introduced the hydroxymethyl group mainly in the guaiacyl nucleus, and the additional reaction created two hydroxymethyl groups in the p-hydroxyphenyl nucleus. Contrary to our estimation, treatment of the models with ¹³C-labeled formaldehyde (H¹³CIIO) did not form any diarylmethane structure. Neutral sulfite treatment of hydroxymethylated products gave corresponding sulfonates in high yields. Phenolized guaiacylglycerol--aryl ether (compound XVI) showed a reactivity similar to that of compound VI.This paper was presented at the 45th and 46th annual meetings of the Japan Wood Research Society at Tokyo and Kumamoto, April 1995 and April 1996, respectively     

Application of cationic polymer prepared from sulfuric acid lignin as a retention aid for usual rosin sizes to neutral papermaking

Cationic polymers that acted as the retention aids for usual rosin sizes in neutral papermaking were prepared from sulfuric acid lignin (SAL), one type of acid lignin. To convert SAL to the cationic polymer (MP-SAL), SAL was phenolated and then treated by the Mannich reaction to introduce the amino groups. In the MP-SAL single system, MP-SAL exhibited high sizing effectiveness in neutral papermaking with the rosin emulsion size. However, MP-SAL showed no sizing effectiveness when soap rosin size was used. MP-SAL showed increased sizing effectiveness as the pK _a of the introduced amino group increased. From this and comparison of the sizing degrees of MP-SAL and polyethylenimine, which possesses a linear structure, it was suggested that the sizing effectiveness was not only affected by the charge density and molecular weight, but also by the basicity of the introduced amino groups and the molecular structure of the retention aid. In the alum–MP-SAL dual system, alum and MP-SAL synergistically enhanced the sizing effectiveness in the rosin emulsion sizing at neutral pH. In turn, this allowed a decrease in the MP-SAL dosage and resulted in a small decrease in brightness.Part of this report was presented at the 48th Lignin Symposium, October 2003, Fukui     

Formation and chemical structures of acid-soluble lignin II: reaction of aromatic nuclei model compounds with xylan in the presence of a counterpart for condensation,and behavior of lignin model compounds with guaiacyl and syringyl nuclei in 72% sulfuric acid

To elucidate the formation and chemical structures of water-soluble material in acid-soluble lignin (ASL), lignin aromatic nuclei model compounds of creosol (I) and 5-methoxycreosol (II) were reacted with xylose or xylan in the presence of apocynol as a counterpart for condensation in 72% sulfuric acid (SA). The reaction of I gave mainly condensation product. However, the condensation reaction of II with apocynol was suppressed because of steric hindrance from the methoxyl group, and II yielded a C-xyloside after refluxing in 3% SA together with condensation products. To obtain information on CHCl₃-soluble material in ASL, model compounds of arylglycerol--aryl ethers with guaiacyl (VIII) and syringyl (X) nuclei were treated by the Klason procedure. VIII gave only insoluble polymerized product, while X gave insoluble polymerized product and CHCl₃-soluble low molecular weight products, which were dissolved in 3% SA. These results prove earlier views that water-soluble material in ASL consists of condensation products formed from syringyl lignin and monosaccharide units in hemicellulose. In addition, the CHCl₃-soluble material in ASL appears to be composed of low molecular weight degradation products from SA treatment of Klason lignin with the syringyl nucleus.Part of this report was presented at the 51st Annual Meeting of the Japan Wood Research Society, Tokyo, April 2001 and at the 47th Lignin Symposium, Fukuoka, October 2002, and was reviewed in Mokuzai Gakkaishi (2002) 48:55–62     

GC-MS and IR spectroscopic analyses of the lignin-derived products from softwood and hardwood treated in supercritical water

Softwood (Cryptomeria japonica) and hardwood (Fagus crenata) were treated in supercritical water (380°C, 100 MPa) for 8 s. The treated woods were fractionated to the water-soluble portion, methanol-soluble portion, and methanol-insoluble residues. For the methanol-soluble portion, which mainly consisted of lignin-derived products, gel permeation chromatography (GPC) and gas chromatographic-mass spectrometric (GC-MS) analyses were conducted to clarify the molecular weight distribution and to identify the monomeric products, respectively. GPC analysis revealed that the methanol-soluble portion contains monomeric and some oligomeric products. GC-MS analysis identified 19 guaiacyl compounds in the methanol-soluble portion from softwood, and 15 syringyl monomeric compounds in the methanol-soluble portion from hardwood. The structures of identified products included not only phenyl propane (C₆—C₃) units but also C₆—C₂ and C₆—C₁ units. In addition, the infrared spectra suggested that the methanol-soluble portion maintains the typical structure of lignin, although it is rich in condensed-type linkages with some changes in the propyl side chain. These results indicate that the supercritical water treatment cleaves not only ether linkages but also part of the propyl chains in lignin to give various aromatic compounds.     

Formation and chemical structures of acid-soluble lignin I: sulfuric acid treatment time and acid-soluble lignin content of hardwood

To elucidate the formation mechanism of acidsoluble lignin (ASL) formed in the Klason lignin determination, beech wood meals were treated with sulfuric acid (SA) under various conditions, and the ASL solution was extracted with CHC1₃. The results indicated the following: (1) wood components yielding ASL are dissolved in 72% SA during the initial stage; (2) the quantity of ASL is highest during the initial stage, then decreases with prolonged time of 72% SA treatment and finally reaches a constant value; (3) soluble lignin prepared by 72% SA treatment and subsequent standing in 3% SA again yield insoluble Klason lignin and ASL after boiling in 3% SA; and (4) about half the amount of ASL is dissolved in CHC1₃. The foregoing suggest that wood components yielding ASL are dissolved in 72% SA at the beginning and finally change to ASL after being subjected to depolymerization, hydrolysis, and other reactions. ASL may thus be composed of low-molecular-weight degradation products and hydrophilic derivatives of lignin.     

Degradation of a nonphenolic β-O-4 lignin model dimer by horseradish peroxidase with 1-hydroxybenzotriazole

Nonphenolic β-O-4 lignin substructure model dimer, 1,3-dihydroxy-2-(2,6-dimethoxyphenoxy)-1-(4-ethoxy-3-methoxyphenyl)propane (I) was degraded by horseradish peroxidase (HRP) in the presence of hydrogen peroxide and 1-hydroxybenztriazole (HBT). 4-Ethoxy-3-methoxybenzoic acid (II), 1-(4-ethoxy-3-methoxyphenyl)-3-hydroxypropanone (III), 2,3-dihydroxy-1-(4-ethoxy-3-methoxyphenyl)-1-formyloxypropane (IV), 2,3-dihydroxy-1-(4-ethoxy-3-methoxyphenyl)propanone (V), 1-(4-ethoxy-3-methoxyphenyl)-1,2,3-trihydroxypropane (VI), 1-(4-ethoxy-3-methoxyphenyl)-1,2,3-trihydroxypropane-2,3-cyclic carbonate (VII), and 1-(4-ethoxy-3-methoxyphenyl)-1,2,3-trihydroxypropane-1,2-cyclic carbonate (VIII) were identified as degradation products by gas chromatography-mass spectrometry. These degradation products were qualitatively the same as those of substrate I in the laccase/HBT system, but the yield of the products was apparently different. The products catalyzed by the HRP/H₂O₂/HBT system contained large amounts of the aromatic ring cleavage products IV, VII, and VIII compared with those catalyzed by the laccase/HBT system, while the amount of Cα-Cβ cleavage product II is relatively low. These results suggest that the role of HBT is not in a simple one-electron transfer between the enzymes and substrates.     

Tetrahydrofolate-dependent vanillate and syringateO-demethylation links tightly to one-carbon metabolic pathway associated with amino acid synthesis and DNA methylation in the lignin metabolism ofSphingomonas paucimobilis SYK-6

Sphingomonas paucimobilis SYK-6 strain can degrade various lignin-related compounds. In the lignin metabolic pathway of this bacterium, vanillate and syringate are demethylated by the tetrahydrofolate (THF)-dependentO-demethylation system, which requires the enzymatic function of LigH. Upstream of theligH gene is the 5,10-methylene-THF reductase gene. Its gene product was essential for one-carbon metabolism involved in the amino acid synthesis and DNA methylation in all organisms. When themetF gene was inactivated in the genome of SYK-6, the resultant mutant, DLmetF, could not grow on vanillate and syringate as a sole carbon source. Furthermore, DLmetF showed significant accumulation of methyl-THF as a result of vanillate and syringateO-demethylation. We report here that THF-dependent vanillate and syringateO-demethylation links tightly to the one-carbon metabolic pathway that is associated with amino acid synthesis and DNA methylation, and the methyl group is the sole one-carbon source inS. paucimobilis SYK-6.     

Look back over the studies of lignin biochemistry

The role of the cinnamate pathway in monolignol biosynthesis based on feeding experiments with lignifying plant stems and characterization of the enzymes in the pathway, O-methyltransferase (OMT), cinnamyl alcohol dehydrogenase (CAD), etc. is discussed. Monolignol biosynthesis via metabolic grids according to newly characterized enzymes in the pathway is also reviewed and discussed. The cleavage mechanisms of side chains and aromatic rings by lignin peroxidase and laccase elucidated by using ¹⁸O, ²H, and ¹³C labeled lignin substructure dimers and DHP with ¹⁸O₂ and H₂ ¹⁸O are reviewed. Finally, the prospects of lignin biochemistry in the wood and paper industries are discussed according to the recent progress on gene technology on wood formation and microbial degradation of lignin.     

Development of acid soil conditioning agent from lignin by ozone treatment I

A purified softwood kraft lignin was modified by ozone treatment and its activity as an acid soil conditioning agent, mainly focusing on elimination of aluminum toxicity, was assayed by planting experiments. The growth of radish root was examined in nutrient solution containing CaCl₂ and AlCl₃ at pH 4.8 with and without modified kraft lignins. The modified kraft lignins that absorbed 1.8 and 3.9 moles of ozone per C6-C3 unit (M _w 180) showed two effects: the elimination of aluminum toxicity and the acceleration of root growth. The effect on the elimination of aluminum toxicity was observed even with modified kraft lignin that absorbed 1.0 mole of ozone per C6-C3 unit. The high molecular weight part of the modified kraft lignin that absorbed 3.9 moles of ozone per C6-C3 unit also proved to be effective not only in elimination of aluminum toxicity but also in acceleration of root growth. The acceleration effect of ozone-treated lignins on root growth was also observed under the absence of aluminum in planting experiments. This report was presented in part at the 56th Annual Meeting of the Japan Wood Research Society, Akita, Japan, August 2006     

Structural conversion of the lignin subunit at the cinnamyl alcohol stage in Eucalyptus camaldulensis

The lignin biosynthetic pathway in Eucalyptus camaldulensis was investigated by feeding stems with deuterium-labeled precursor. Pentadeutero[,-D₂ OCD₃] coniferyl alcohol was synthesized and supplied to shoots of E. camaldulensis, and incorporation of the labeled precursor into lignin was traced by gas chromatography-mass spectrometry. In addition to the direct incorporation of labeled precursor into the guaiacyl unit, a pentadeuterium-labeled syringyl unit was detected. This finding indicates that the -deuterium atoms in the hydroxymethyl group of labeled coniferyl alcohol remain intact during modification of the aromatic ring. The relative level of trideuterium-labeled syringyl monomer (the result of conversion via the cinnamic acid pathway) was negligible, suggesting that the pathway at the monolignol stage is used for conversion of exogenously supplied precursor. Our results provide conclusive evidence of a novel alternative pathway for generation of lignin subunits at the monolignol stage even in plants that do not accumulate coniferin in lignifying tissues.     

Pyrolysis reactions of various lignin model dimers

Primary pyrolysis reactions and relative reactivities for depolymerization and condensation/carbonization were evaluated for various lignin model dimers with α-O-4, β-O-4, β-1, and biphenyl substructures by characterizing the tetrahydrofuran (THF)-soluble and THF-insoluble fractions obtained after pyrolysis at 400°C. Reactivity was quite different depending on the model structure: depolymerization: α-O-4 [phenolic (ph), nonphenolic (nonph)], β-O-4 (ph) > β-O-4 (nonph), β-1 (ph, nonph) > biphenyl (ph, nonph); condensation/carbonization: β-1 (ph) > β-O-4 (ph) > α-O-4 (ph) > β-O-4 (nonph), biphenyl (ph, nonph), α-O-4 (nonph), β-1 (nonph). Major degradation pathways were also identified for β-O-4 and β-1 model dimers: β-O-4 types: C_β-O cleavage to form cinnamyl alcohols and phenols and C_γ-elimination yielding vinyl ethers; β-1 types: C_α-C_β cleavage yielding benzaldehydes and styrenes and C_γ-elimination yielding stilbenes. Relative reactivities of these pathways were also quite different between phenolic and nonphenolic forms even in the same types; C_β-O cleavage (β-O-4) and C_γ-elimination (β-1) were substantially enhanced in phenolic forms.     

Reactivity of lignin in supercritical methanol studied with various lignin model compounds

Behavior of lignin in supercritical methanol (250–270°C, 24–27 MPa) was studied by using lignin model compounds at the tin bath temperature of 270°C with a batch-type reaction vessel. Guaiacol and veratrole were selected as a guaiacyl type of aromatic ring in lignin, while 2,6-dimethoxyphenol and 1,2,3-trimethoxybenzene as a syringyl one. In addition, biphenyl and β-O-4 types of dimeric lignin model compounds were, respectively, studied as condensed and ether linkages between C₆-C₃ phenyl propane units. As a result, both guaiacyl and syringyl types of aromatic rings were very stable, and the biphenyl type was comparatively stable under supercritical conditions of methanol. However, β-ether linkage in the phenolic β-O-4 model compound was cleaved rapidly into guaiacol and coniferyl alcohol, which was further converted to its γ-methyl ether. Non-phenolic β-O-4 model compound was, on the other hand, converted initially into its α-methyl ether and degraded further to produce guaiacol. These lines of evidence imply that in lignin macromolecules, the new phenolic residues are continuously formed and depolymerized repeatedly in supercritical methanol into the lower molecular products, mainly by the cleavage of the dominant β-ether structure in lignin.     

Conversion of sulfuric acid lignin generated during bioethanol production from lignocellulosic materials into polyesters with <Emphasis Type="Italic">ɛ</Emphasis>-caprolactone

In this study, polyesters were prepared from sulfuric acid lignin (SAL) that was generated as a by-product during the production of bioethanol from lignocellulosic materials using ɛ-caprolactone (CL). Carboxylic groups were introduced into SAL by a hydrothermal reaction under alkali conditions. The acid form (-COOH) of the obtained polymer (HSAL) was dissolved into ɛ-caprolactone and the mixture was heated at 150°C for 12 h with ZnCl₂ as a catalyst. The structure of the obtained polymer (PCL-HSAL) was proposed based on results of differential scanning calorimetric analysis, swelling tests, and dynamic mechanical thermal analysis. The hydroxyl group in HSAL participated in the ring-opening polymerization of CL. PCL-HSAL with a high HSAL content formed a three-dimensional network that did not dissolve in organic solvents and did not melt at high temperature. PCL-HSAL showed a high swelling property for a suitable ratio of CL to HSAL.     

Reaction behavior of lignin in supercritical methanol as studied with lignin model compounds

 The reaction behavior and kinetics of lignin model compounds were studied in supercritical methanol with a batch-type supercritical biomass conversion system. Guaiacol, veratrole, 2,6-dimethoxyphenol, and 1,2,3-trimethoxybenzene were used as model compounds for aromatic rings in lignin. In addition, 5-5, β-1, β-O-4, and α-O-4 types of dimeric lignin model compounds were used as representatives of linkages in lignin. As a result, aromatic rings and 5-5 (biphenyl)-type structures were stable in supercritical methanol, and the β-1 linkage was not cleaved in the β-1-type structure but converted rapidly to stilbene. On the other hand, β-ether and α-ether linkages of β-O-4 and α-O-4 lignin model compounds were cleaved rapidly, and these compounds decomposed to some monomeric compounds. Phenolic compounds were found to be more reactive than nonphenolic compounds. These results indicate that cleavages of ether linkages mainly contribute to the depolymerization of lignin, whereas condensed linkages such as the 5-5 and β-1 types are not cleaved in supercritical methanol. Therefore, it is suggested that the supercritical methanol treatment effectively depolymerizes lignin into the lower-molecular-weight products as a methanol-soluble portion mainly by cleavage of the β-ether structure, which is the dominant linkage in lignin. Received: December 19, 2001 / Accepted: April 30, 2002 Acknowledgments This research has been done under the research program for the development of technologies for establishing an ecosystem based on recycling in rural villages for the twenty-first century from the Ministry of Agriculture, Forestry and Fisheries, Japan; by a Grant-in-Aid for Scientific Research (B)(2) (no.12460144, 2001.4–2003.3) from the Ministry of Education, Culture, Sports, Science and Technology, Japan; and under the research program from Kansai Research Foundation for Technology Promotion, Japan. The authors thank them for their financial support. This study was presented in part at the 45th Lignin Symposium, Ehime, Japan, October 2000 and the 52nd Annual Meeting of the Japan Wood Research Society, Gifu, Japan, April 2002 Correspondence to:S. Saka     

Oxidative cleavage of lignin aromatics during chlorine bleaching of kraft pulp

Methanol liberation and methoxyl loss during chlorine bleaching of softwood kraft pulp were quantitatively investigated to estimate the degree of structural modification of lignin aromatics. An increase in the chlorine multiple led to enhanced methoxyl loss from lignin. Our result, using pH-adjusted chlorine water (pH 5.7), by which chlorination under oxidation-favorable conditions was achieved, strongly supported the importance of the oxidation reaction by chlorine during delignification and lignin degradation. It was also suggested that methanol can be produced not only via catalytic hydrolysis by chlorine but via oxidative cleavage of the ether bond as well. The infrared spectrum of chlorolignins suggested that chlorine oxidation can open aromatic rings to muconic acid derivatives without cleaving ether bonding of the methoxyl group. No straight relation between the methoxyl content and the kappa number of chlorinated pulps was shown. The methoxyl content of bleached kraft pulps subjected to successive chlorination and alkali extraction showed a good relation with the kappa number. This means that almost all the portions of the oxidatively modified lignin structure were successfully removed during these treatments, and the aromatic structures of residual lignin in chlorinated and alkali-extracted pulps were thought to remain intact.Part of this paper was presented at the 46th Annual Meeting of the Japan Wood Research Society, Kumamoto, April 1996; the 10th International Symposium on Wood and Pulping Chemistry, Yokohama, June 1999; and the 67th Pulp and Paper Research Conference, Tokyo, June 2000