Effect of noncovalent bonds on the successive sequential extraction of Xianfeng lignite

In order to examine the effect of noncovalent bonds on the extraction yield of Xianfeng lignite (XL), XL is subjected to NMP extraction, acid treatment, and subsequent successive extraction with N-methyl-2-pyrrolididinone (NMP), 1-butyl-3-methyl-imidazolium tetrafluoroborate ([Bmim]BF4) and 1-butyl-3-methyl-imidazolium chloride ([Bmim]Cl) at 200 °C. The extraction yields in each solvent and step are investigated, respectively. The residues and extracts obtained in each step are characterized by Fourier transform infrared (FTIR) spectra. The results show that there are only small amount of the thermally induced relaxation of the aggregate structure of XL during NMP extraction. Furthermore acid treatment significantly increases the extraction yield of XL with NMP about 41.4%, which results from the release of ionic cross-links. [Bmim]BF4 and [Bmim]Cl can break the weak and strong hydrogen bands in lignite, respectively. The extraction yield of XL is strongly dependent on noncovalent interactions, such as ionic cross-links and hydrogen bonds.