Composition and structural characteristics of nitrogen-containing species in the soluble organic species of Xianfeng lignite

Xianfeng lignite (XL) was firstly thermal dissolved in the isometric toluene/methanol mixed solvent at 300℃ in a stainless-steel autoclave to afford a soluble portion (SP_XL) and a residue (R_XL), then the nitrogen forms in XL and R_XL were characterized with X-ray photoelectron spectroscopy (XPS) analysis, and the nitrogen-containing species (NCSs) in SP_XL were identified using gas chromatography/mass spectrometry (GC/MS) and electrospray ionization Fourier transform ion cyclotron mass spectrometry (ESI FT-ICR MS) analyses. The results show that the amount of nitrogen forms in XL is in the order of quaternary-N>pyridinic-N>pyrrolic-N, while quaternary-N in XL is easily dissolved out during thermal dissolution. In total 20 NCSs were detected in SP_XL by GC/MS, and most of them are amines. Over three hundreds of NCSs were identified in SP_XL by ESI FT-ICR MS, and most of them are the NCSs containing one or three nitrogen atoms. The NCSs containing one nitrogen atom are mainly dominated by N₁O₁, N₁O₂ and N₁O_xS_1-2 class species, while most of NCSs containing three nitrogen atoms are N₃O_xS_1-2 class species (x=1-12). The double bond equivalent (DBE) values and carbon number of the NCSs containing one nitrogen atom increase with increasing number of oxygen atoms.