Fractional and structural characterization of organosolv and alkaline lignins from Tamarix austromogoliac

Abstract

A comparative study was conducted on the seven lignin fractions, isolated from dewaxed Tamarix austromogoliac (TA) with DMSO, 70% ethanol containing 5% triethylamine, 70% ethanol containing 0.5% NaOH at 70°C for 5 h, and 1 M KOH, 1 M NaOH, 1 M LiOH and 1 M NH3·H2O at 50°C for 5 h, respectively. The comprehensive structural characterization of the three organic solvent-soluble and four alkali-soluble lignin isolations were examined by UV, FT-IR, TGA, 1H, 13C and 2D 13C-1H correlation (HSQC) NMR spectroscopy techniques. The results showed that the DMSO-soluble lignin fraction, which was rich in hemicelluloses, had a relatively higher molecular weight (4730 g/mol) while the other six lignin isolations had lower molecular weights ranging from 1240 - 2160 g/mol. The thermogravimetric analysis (TGA) showed a decrease of thermal stability with a decrease in the molecular weight. In addition, the HSQC spectrum of the fraction isolated with 70% ethanol containing 0.5% NaOH from TA indicated a predominance of β-O-4′ aryl ether linkages (73% of total side chains), followed by lower amounts of β-β′ resinol-type linkages (19% of total side chains), trace amounts of β-5′ phenylcoumaran (3%), β-1′ spirodienone-type (3%) substructures and cinnamyl end groups (2%). Key words: Organosolv lignins, Tamarix austromogoliac, TGA, 13C NMR, HSQC.