FTIR study on structural changes of different-rank coals caused by single/multiple extraction with cyclohexanone and NMP/CS2 mixed solvent

A novel multiple extraction (ME) process integrated heat reflux extraction (HRE) with cyclohexanone (CYC) followed by ultrasonic assisted extraction (UAE) with N-methyl-2-pyrrolidone/CS2 (NMP/CS2) mixed solvent was developed for extraction of four different rank demineralized coals. Raw coals and their demineralized coals were evaluated by X-ray diffraction, X-ray fluorescence, and Fourier transform infrared (FTIR) spectroscopy analytical techniques to determine HCl/HF leaching on the properties and FTIR spectra of coals first. And then the demineralized coals as well as their extraction residues were studied using FTIR spectrometry and curve-fitting technique to clarify the performance of ME process and investigate the structural changes in coals during extraction. The results showed that acids treatment could remove most of the mineral matters in coals and did not seem to produce significant changes in IR spectra of the coals except for disappearance of those peaks caused by mineral matters. The removal of solvents soluble portions led to decrease in the amount of organic species containing CC vibration mode and carbon content in all coal samples. The alkyl side chains on aromatics of all coal samples except for NM lignite became longer and less branched after extraction according to the curve-fitting study in the region 3000-2700 cm− 1. In addition, it was found that the contents of carbon and organic portions containing CC vibration mode, CO bonds, and aliphatic hydrogen for SD, HLJ, and NX coals all decreased following the order of raw coal > extraction residue with CYC > extraction residue with NMP/CS2 > extraction residue from ME process during extraction processes. This result inferred that the NMP/CS2 mixed solvent could only show good solubility to coals after they were initially extracted using CYC. Furthermore, the detectable concentration of hydroxy groups in coals increased after extraction, especially for the HRE process of NM coal with CYC.