Comparative study of three lignin fractions isolated from mild ball‐milled Tamarix austromogoliac and Caragana sepium

Abstract

Six lignin fractions from mild ball-milled Tamarix austromogoliac (TA) and Caragana sepium (CS) were sequentially isolated with 80% dioxane containing 0.05M HCl at 758C for 4 h, 50% aqueous ethanol containing 1M triethylamine at 708C for 4 h, and 8% aqueous NaOH at 458C for 3 h. The results showed that the successive treatments made it possible to isolate lignin from wood with a high yield and purity, in which 89.4 and 90.6% of the original lignin from TA and CS were released, respectively. The lignin fractions isolated with the three-step method were analyzed with Fourier transform infrared, 1 H-and 13 C-NMR, alkaline nitrobenzene oxidation, and gel permeation chromatography. It was found that the three lignin fractions isolated from TA were rich in syringyl units, and the molar ratio of the relatively total moles of vanillin, vanillic acid, and acetovanillin to the relatively total moles of syringaldehyde, syringic acid, and acetosyringone decreased from 1 : 2.6 to 1 : 3.2 to 1 : 3.6 in the lignin preparations, whereas this ratio in the corresponding lignin fractions isolated from CS was found to be 1.4 : 1, 1.1 : 1, and 1 : 1.4, respectively. More importantly, the results revealed that the sequential extractions of the mild ball-milled TA and CS with 80% acidic dioxane, 50% alkaline ethanol, and 8% aqueous NaOH under the conditions used did not significantly cleave the b-O-4 and a-O-4 linkages in lignin macromolecules.

Structural Changes of Lignin from Wheat Straw by Steam Explosion and Ethanol Pretreatments

“…The distinction between the absorption peak intensities among the samples indicates that the proportion of the three-unit basic structural units of wheat straw (H, G, and S) differed between the LW, LS, LE, LB, and LL fractions. This variation can be attributed to a reaction on the benzene ring, and mutual changes between the three types of aromatic structures (Xu et al 2008;Wen et al 2013), which is similar to the results of the 1 H NMR spectra of the lignin fractions. The absorption peak intensities of LB with a two-step pretreatment process were strongest and multiform, and the peaks of the three basic structural units of lignin became increasingly apparent, and were most obvious in samples subjected to the ethanol pretreatment process.…”

“…The peaks between 6.0 and 8.0 ppm observed in the spectra of LE and LB became more vague and diverse compared with those of LW, LS, and LL. This difference is attributed to a reaction on the benzene ring, and a mutual change between the three types of aromatic structures (Xu et al 2008;Wen et al 2013). The strong signal at 3.75 ppm and disappearance of the signal at 3.37 ppm in the spectra of LE and LB were derived from methoxyl protons, confirming the demethoxylation reaction during the ethanol pretreatment process, which was not observed in the LW, LS, and LL spectra.…”

“…The peak around 9 to 12 ppm was assigned to carboxylic acids and aldehydes. The signals observed between 0.8 and 1.5 ppm were caused by the protons of the aliphatic side chain in the lignin, and the signals observed at 7.03 and 6.73 ppm can be attributed to the aromatic protons in the G-and S-units, respectively (Xu et al 2008), and the signal at 7.6 ppm can be attributed to p-hydroxybenzene units. There were weak signals observed between 4.0 and 6.0 ppm, which can be assigned to protons of the aromatic ring on the side chain.…”

Structural Characterization of Lignin and Its Degradation Products with Spectroscopic Methods

“…UV Spectroscopy. The content of acid-soluble lignin, the purity, and the components of isolated lignin, can be determined by using UV spectroscopy [53]. National Renewable Energy Laboratory (NREL) proposed an accurate method for the determination of lignin, by which the absorbance of lignin was recorded at the recommended wavelength [54].…”

“…National Renewable Energy Laboratory (NREL) proposed an accurate method for the determination of lignin, by which the absorbance of lignin was recorded at the recommended wavelength [54]. According to the intrinsic structure of lignin, several absorption maxima attributed to different functional groups were observed, as shown in Table 1 [25,30,50,[53][54][55][56]. The determination of phenolic hydroxyl groups can be achieved basing on the difference in absorption at 292 and 370 nm between phenolic units in neutral and alkaline solutions [30,50,55,56].…”