Thermal and structural characterization of two commercially available technical lignins for potential depolymerization via hydrothermal liquefaction

Abstract

Lignin is a complex polyaromatic macromolecule and a potential source of various sustainable materials and feedstock chemicals. To this end, researchers have made some considerable efforts in depolymerizing lignin into sustainable aromatics, even though there is limited success so far as the exact structure of native lignin is still virtually unknown due to its highly heterogeneous nature. Besides, technical lignin undergoes unintended structural modifications during the chemical pulping and extraction processes making its final structure even more complicated. Therefore, understanding the lignin structure and its macromolecular characteristics is essential for its proper utilization. In this study, two technical lignins, such as indulin AT and alkali-treated lignin, were investigated in terms of thermal and structural properties. The characterization data were linked to their potential application in hydrothermal liquefaction (HTL). Thermal behaviors were studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Indulin AT was found to be thermally more stable compared to alkali lignin. Structural characterization was performed using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and proton nuclear magnetic resonance spectroscopy (1H NMR). Cupric oxide oxidation was utilized to selectively degrade the lignin into its monomers (H/G/S-moieties), which were identified with GC-MS. The results suggested that the selected lignins are mainly composed of G-type monomers. The detailed characterization studies also revealed minor structural differences between the two lignins due to their respective delignification process. Indulin AT underwent higher structural modifications due to the harsher delignification process and hinted to show more recalcitrance toward depolymerization than alkali lignin.