Effect of hydrothermal carbonization and eutectic salt mixture (KCl/LiCl) on the pyrolysis of Kraft lignin as revealed by thermal analysis coupled to advanced high-resolution mass spectrometry

The production of graphite requires high temperatures, and fossil petroleum, coal, or nutshells are frequently used as a carbon source. As a replacement, Kraft lignin, a by-product of the pulp and paper industry, is a promising carbon source with a prefigured aromatic network. Biomass-based feedstocks with improved characteristics can be obtained by hydrothermal carbonization, but the chemical nature of this process is not fully understood yet. Moreover, adding a eutectic salt mixture (LiCl/KCl) to the pyrolysis of Kraft lignin and HTC lignin can improve the graphitization at lower temperatures. In this study, thermal analysis with online mass spectrometric detection of the evolved gas mixture was applied to explore the influence of the eutectic salt mixture on the char conversion process. Aside from classical pyrolysis gas chromatography mass spectrometry, thermogravimetry coupled with soft photoionization mass spectrometry allowed to identify phenol, hydrogen sulfide, dimethyl sulfide, and various larger lignin fragments. These larger dimeric/trimeric methoxyphenol derivatives were successfully validated by means of high-resolution mass spectrometry equipped with soft atmospheric pressure chemical ionization. The investigations indicated the catalytic influence of the salt mixture on the production process of the biochars, achieving partially graphitization already at relatively low temperatures (700 °C). On the morphology, Raman spectroscopy and electron microscopy revealed the evolution of the carbon structures and revealed that the materials have typical features for amorphous carbon.