Qualitative Structural Characterization of Two Lignin Samples and Quantitative Determination of Hydroxyl and Methoxyl Functional Groups in Kraft Lignin via Acetylation

Abstract

In this research, two types of lignin, i.e. one extracted from industrial waste water of the Iran wood and paper (choka) factory as black liquor and another provided from Aldrich company, were studied. Firstly, the lignin was precipitated from its black liquor by a dilute acid and was then purified by dissolving in tetrahydrofuran (THF). Lignin samples were characterized using Fourier transform infrared (FTIR) and proton nuclear magnetic resonance (1HNMR) spectroscopies. It was found from results that the Aldrich lignin has a structure similar with the lignosulfonate while lignin extracted from black liquor does not have a hydrophilic sulfonate group and has a Kraft lignin structure. Using acetylation of Kraft lignin and complementary characterization with 1HNMR spectroscopy, the ratio of hydroxyl functional groups to methoxy groups was calculated to be 1.11 to 1, and amount of the hydroxyl groups was calculated to be 5.91 mmol/g of acetylated lignin, where 53% of the hydroxyl groups is related to the aromatic hydroxyl groups. By having the molecular weight of the Kraft lignin which was obtained from the gel permeation chromatography (GPC) analysis of the acetylated Kraft lignin to be 1260 g/mol, average number of the hydroxyl groups per acetylated lignin chain was calculated to be 7.44