Molecular structure characterization of lignite treated with ionic liquid via FTIR and XRD spectroscopy

In order to characterize the effect of ionic liquid concentration on the molecular structure of lignite, the X-ray diffraction (XRD) and Fourier transform infrared (FTIR) experiments were performed on lignite coal samples treated with different concentrations of ionic liquids. The results show that with the increase of IL concentrations, the apparent aromaticity f(a) increases gradually. The chemical bonds on both sides of the alkyl groups are destroyed, the alkyl chains become shorter, and the molecular structure become more compact. The C-O groups such as alcohol, phenol and ether groups are converted to C=O groups such as carbonyl, aldehyde and carboxy groups. The inter-layer spacing (d(002)) ranges from 0.374 to 0.379, which is slightly higher than that in pure graphite. It indicates that the crystalline order in coal samples is in a lower degree. The L-a ranges from 3.05 to 4.05 nm and L-c ranges from 0.98 to 1.11 nm, which indicates the crystallite size increases further and the order degree of coal samples gradually approaches graphitization. The results of this paper can provide some reference for studying the characteristics of molecular structure change of coal.