Characterization of lignin-derived products from Japanese beech wood as treated by two-step semi-flow hot-compressed water

Abstract

Japanese beech (Fagus crenata) wood was treated by two-step semi-flow hot-compressed water (the first stage: 230°C/10MPa/15min, the second stage: 270°C/10MPa/15min), and produced lignin-derived products in the hot-compressed water-soluble portions at the first and second stages, and the final residue of the second stage were characterized with alkaline nitrobenzene oxidation method and gel-permeation chromatographic analysis. As a result, the lignin-derived products at the first stage, where hemicellulose was also decomposed, consisted of lignin-based monomers and dimers and oligomers/polymers in the water-soluble portion. A large part of the oligomers/polymers was, however, recovered as the precipitate during 12h setting after hot-compressed water treatment. By the analysis of nitrobenzene oxidation products, there were relatively higher contents of ether-type lignin in the precipitate at the first stage than in original beech wood. Since the ether-linkages of lignin are more preferentially cleaved by this hot-compressed water, lignin-based polymeric fractions were flowed out from the porous cell walls from which hemicellulose was removed. On the other hand, at the second stage condensed-type lignin remained in the precipitate and residue. Based on these results, decomposition behavior of lignin in Japanese beech wood as treated by the two-step semi-flow hot-compressed water was discussed 3 regarding the topochemistry of lignin structure.

Characterization of lignin-derived products from Japanese cedar as treated by semi-flow hot-compressed water

“…This difference could indicate that high-molecular-weight cellulose-derived products, eluted in the 2nd stage, aggregated and precipitated with lignin-derived products under ambient condition [28]. A similar tendency was observed in the case of Japanese beech [22]. The lignin concentration of the water-insoluble residue increased as the hot-compressed water treatment was prolonged, and the final residue consisted mostly of lignin (0.875 g/g), indicating that the residual lignin has much higher resistance to hot-compressed water than other chemical components.…”

“…As a result, 12.8 % of Japanese cedar remained as water-insoluble residue after two-step treatment. This value is, in fact, much higher than that of Japanese beech only to be 4.4 % [22]. The observed difference in lignin between the two woods would be due to the fact that Japanese cedar has less ether linkages than Japanese beech.…”

“…3). These results suggest that some micropores were produced in the cell wall due to the decomposition of hemicelluloses and the cleavage of ether linkages in lignin in the 1st stage and that the micropores would have facilitated the elution of the lignin cluster, which maintains ether linkages [22]. Hydrophobic lignin could be eluted in hot-compressed water due to the low dielectric constant of hot-compressed water, whereas some large molecules cluster precipitated after hot-compressed water returned to the ordinary water under ambient condition.…”

“…They also elucidated that lignin was decomposed in both stages and that the lignin-derived monomeric products such as coniferyl alcohol and sinapyl alcohol were obtained in water-soluble portion [21]. Yamauchi et al showed that hot-compressed water can cleave only ether linkages of lignin through the structural analysis of lignin-derived products [22]. Additionally, these ligninderived products could be converted into acetic acid and phenolic compounds via microorganism, and thus expected to become valuable chemicals [23].…”

“…However, detailed analysis of the lignin-derived products obtained from Japanese cedar has not yet been conducted. Yamauchi et al [22] have studied the decomposition behavior of lignin in Japanese beech as described above, while lignin content and its structure in Japanese cedar are different from Japanese beech, and their decomposition behaviors of lignin should also differ. Thus, the aim of this study was to analyze the lignin-derived products to elucidate the decomposition behavior of lignin from Japanese cedar as treated by hot-compressed water.…”