Changes in neutral sugar composition, molar mass and molar mass distribution, and solid-state structures of birch and Douglas fir by repeated sodium chlorite delignification
2022
Ōno, Yūko | Takeuchi, Miyuki | Isogai, Akira
NaClO₂-treated residues or holocellulose samples were prepared from birch and Douglas fir wood powders by repeated cycles of delignification. These delignified samples were characterized from their weight recovery ratios, neutral sugar compositions, size-exclusion chromatograms using 1% (w/v) lithium chloride/N,N-dimethylacetamide as eluent, and solid-state ¹³C nuclear magnetic resonance (NMR) spectra. Neutral sugar composition analysis revealed that the weight ratios of cellulose and xylan fractions were almost constant irrespective of the number of repeated delignification cycles for up to seven and eight cycles for birch and Douglas fir, respectively. The contents of acetyl ester groups were constant between the delignified samples. The glucomannan molecules present in Douglas fir were partly removed from the delignified residues as the number of delignification cycles increased. The SEC analysis showed that the high-molar-mass (HMM) cellulose fractions of birch samples decreased slightly in molar mass with increased number of delignification cycles. The molar mass values in the HMM fractions of Douglas fir samples were much higher than those of the birch samples because of the presence of branched structures with glucomannan, and were mostly unchanged as the number of delignification cycles increased. Solid-state ¹³C NMR analysis of the NaClO₂-treated residues and their acid-insoluble fractions showed that carboxy groups were formed in the residual lignin fractions by the oxidative delignification treatment with NaClO₂. Thus, neutral sugar composition, SEC analysis, and solid-state ¹³C NMR analysis can be used for comprehensive and manifold characterization of NaClO₂-treated residues or holocellulose samples prepared from various plant samples.
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