NMR study on residual lignins isolated from chemical pulps of beech wood by enzymatic hydrolysis
Introduction
All terrestrial plant cell walls are constructed with three essential organic materials: cellulose, hemicellulose and lignin. Since cellulose and hemicelluloses amount to ca. 70% of biomass weight and are the carbohydrate based macromolecules, those are the major raw materials in diverse fields including pulping and bioethanol industries. Lignin, next to cellulose, is a second most abundant (20–30% of dried biomass weight) natural organic resource in nature, and initially synthesized to phenol macromolecules via oxidative coupling of three hydrocinnamyl alcohols (p-coumaryl (H), coniferyl (G) and sinapyl alcohol (S)) in the presence of peroxidase and H2O2 (Fig. 1) [1].
In spite of its abundance the utilization potential of lignin polymer is quite underestimated due to its rigid and recalcitrant properties. So far, lignin polymers are mainly produced in the pulping processes as waste forms. In future, it will be expected that enormous amount of lignins are additionally generated when the lignocellulosic biomasses are utilized as raw materials for bioethanol production.
Recently, lignins are newly evaluated as environmental friendly and biodegradable materials due to their potentials for replacement of phenol based chemicals derived from oil refinery industries.
In the previous series of studies on residual lignins [2], [3], [4] we have showed that beech ASAM residual lignin had very distinguishable structural moieties in comparison with other residual lignins. In spite of pulping procedure, it showed close structural similarity with beech MWL. Thioacidolysis of beech ASAM lignin suggested that the lignin was still built with high contents of β-O-4 linkages and its frequency/C9-unit as even higher than MWL. The increments of phenolic hydroxyl groups, which was one of the major parameters for cleavage of β-O-4 linkages during pulping, were only few in the ASAM residual lignin. However, the ASAM residual lignin contained higher amount of methoxyl groups than the corresponding MWL, indicating different reactivity between guaiacyl and syringyl units under the delignification conditions during the ASAM pulping. Elimination of CγH2OH, demethylation and O-demethoxylation could be insignificant during the ASAM pulping of beech wood.
Nuclear magnetic resonance spectroscopic techniques (1H, 13C NMR and 2D-, 3D-NMR) were employed for structural study on the lignin macromolecules. These instrumental methods allowed us to elucidate the whole structure of lignins without complexity of sample preparation. Comparative studies on kraft lignin between modified and conventional processes were performed by NMR spectroscopic methods [5], [6]. They addressed the structural differences of lignins during both pulping procedures. Some functional groups of lignins can be evaluated by 13C NMR spectroscopy [7], [8]. A series of studies by Lundquist [9], [10], [11] and Lundquist et al. [12] showed the possibility that bonding types of lignin polymers and their frequencies could be elucidated by 1H NMR spectroscopy.
In order to verify the structural moieties in the ASAM residual lignin determined by chemical degradation methods, two fractions of ASAM residual lignin after acetylation were further characterized by 1H and 13C NMR spectroscopies. Additionally, quantitative studies of several functional groups, e.g. primary, secondary and phenolic OH as well as methoxyl groups were also evaluated.
Section snippets
Preparation of unbleached chemical pulping
Unbleached chemical pulps (kraft and ASAM pulps) were prepared with beech wood (Fagus sylvatica) at the laboratory according to standard method. Kraft pulping was run for 90 min with 22% active alkali (based on oven dried wood as NaOH) and 30% sulfidity at 165 °C. The alkaline sulfite pulping with anthraquinone and MeOH (ASAM) was carried out at 180 °C with 6.3% NaOH, 18.7% sodium sulfite, 0.1% anthraquinone (AQ) and 10% methanol (v/v) for 90 min. The liquor to wood ratios was 4:1 for the both
1H NMR spectral analysis of residual lignins
Fig. 2 illustrated 1H NMR spectra of ASAM and kraft residual lignins of beech wood. Interestingly, NMR spectrum of ASAM residual lignin was very similar with that of beech MWL. Signal for methoxyl groups was observed in the region between δH 4.0 and 3.3 ppm. The most distinct features of ASAM spectrum were signal at δH 5.9, which were assigned to Hα for β-O-4 linkage [11] and signal at δH 6.6 was corresponding to aromatic proton in syringyl protons. However, signal for Hα was hardly visible and
Conclusions
The 1H and 13C NMR spectral studies on the beech ASAM residual lignins gave agreeable results with the wet chemical degradation methods. Considering all the data obtained from NMR spectroscopic studies as well as the chemical degradation methods, the ASAM residual lignin, despite of severe chemical pulping conditions, still have high contents of aryl ether linkages. The increment of phenolic OH groups, one of the parameters for cleavage of β-O-4 linkages, was appreciably low, indicating that
Acknowledgement
The first author would like to thank Korea Research Foundation (KRF-2008-331-F00027) for providing financial support.
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