Sequential extractions and structural characterization of lignin with ethanol and alkali from bamboo (Neosinocalamus affinis)

A sequential process with the combination of ethanol and alkali aqueous solutions was utilized to extract lignin from bamboo (Neosinocalamus affinis), a potential lignocellulosic material. In this case, the successive treatments of dewaxed bamboo with 70% ethanol at 80 °C, 0.2 and 0.5 M NaOH, 70% ethanol containing 0.6 M NaOH, and 1.0, 2.0, and 3.0 M NaOH at 50 °C, resulted in a total yield of acid-insoluble lignin fractions of 10.06%, corresponding to release of 62.25% original lignin from the cell walls. The lignin fractions obtained were then characterized by GPC, FT-IR, NMR spectroscopy, and sugar analysis. As compared to the alkali lignin fractions, the ethanol-soluble lignin fraction had a relatively higher molecular weight (2670 g/mol) and the content of carbohydrates primarily consisted of glucose 2.01% and xylose 1.90%. This suggested that the carbohydrate chains linked to lignin may increase the hydrodynamic volume of lignin and therefore increase the apparent molecular weight of the ethanol-soluble lignin. HSQC spectra analysis revealed that the alkali lignin fractions consisted mainly of β-O-4′ linkages combined with small amounts of β-β′, β-5′, β-1′ linkages, and p-hydroxycinnamyl alcohol end groups. Furthermore, minor amounts of esterified p-coumaric and ferulic acids were also detected in the lignins isolated.

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► Seven acid-insoluble lignin fractions were successively isolated with aqueous ethanol, alkali, and alkaline ethanol from the dewaxed bamboo (Neosinocalamus affinis).
► The successive treatments resulted in a total yield of acid-insoluble lignin fractions of 10.06%, corresponding to release of 62.25% original lignin from the cell walls.
► The lignin fractions extracted with 0.5 and 1.0 M NaOH had a predominance of β-O-4′ (over 80% of all linkages), followed by low amounts of β-β′, β-5′, β-1′ linkages, and p-hydroxycinnamyl alcohol end groups.
► The lignin side chains were found to be partially acylated at the γ-carbon with p-coumarate and/or acetate groups.