N-methyl-2-pyrrolidone/CS2 extraction induced changes in surface and bulk structures of a lignite

Abstract

Investigation on the changes in detailed surface and bulk structures during N-methyl-2-pyrrolidone/CS2 mixed solvent extraction is crucial to improve and develop the technology of clean and value-added utilization of coals. The surface and bulk structures of Longkou lignite (LL) were investigated before and after ultrasonic-assisted extraction with mixed solvent by multiple direct tools, including, Fourier transform infrared (FTIR) spectrometer, X-ray photoelectron spectrometer (XPS), scanning electron microscope (SEM), and solid-state 13C nuclear magnetic resonance (NMR). The results show that phenols, alcohols, and ethers are the main C–O bond containing species in LL, which is further confirmed by 13C NMR and XPS analyses. The average ring number in each aromatic cluster and substituents in each aromatic ring are 4 and 2–3, respectively. XPS analysis shows that pyrrolic nitrogen is the main nitrogen occurrence and sulfur is mainly present in sulfoxide and sulfone in LL. 13C NMR and FTIR analyses all reveal that aromaticity and aromatic cluster size for LL decreased, while the length and the number of substituents on aromatics increased after extraction. C–O and multi-substituted aliphatic carbons are not easy to dissolve and concentrate in extract residue. It is the basic pyridine-N rather than pyrrolic-N species that are easy to extract with mixed solvent. The content of thiophene-S increased significantly, while other organic sulfur species decreased after extraction, especially for thiophenol and sulfone. Furthermore, solvent extraction leads to the reduction of particle size and increase of surface roughness of LL.