Characterization of nitrogen and sulfur-containing species in Zhaotong lignite and its extracts from ultrasonic extraction

Zhaotong lignite (ZL) was sequentially extracted with cyclohexane, carbon disulfide, methanol, acetone, and isometric carbon disulfide/acetone mixed solvent to afford extracts 1-5 (E1-E5) under ultrasonic radiation at room temperature. In addition, ZL was characterized with solid-state 13C nuclear magnetic resonance (SS 13C NMR), Raman spectrometer and X-ray photoelectron spectrometer (XRPES), while the extracts were analyzed with a gas chromatograph/mass spectrometer along with a positive-ion (PI) and negative-ion (NI) electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer (ESIFTICRMS). According to the characterization with SS 13C NMR, aliphatic carbons count for 59.5% of all the organic carbons in ZL, and each aromatic cluster contains 2 or 3 aromatic rings on average. The pyrrolic nitrogen and sulfones are the most abundant nitrogen and sulfur forms, respectively, in ZL based on the analysis with XRPES. ESIFTICRMS resolved thousands of basic and non-basic species, and the molecular masses are mainly concentrated in the range of 350-600 and 300-500 u, respectively. Acidic S1O3 class species with double bond equivalent values of 3-8 and carbon numbers of 6-39 are dominant species and could be attributed to acidic group-substituted thiophenes. Main nitrogen-containing species detected in the PI mode are basic N1O0-3, N2S2, and N2O1-8S1-2. Most of the species were speculated to contain some core structures, such as pyridines, pyrimidines, or pyrazines.