Spectroscopic Differences in Different Solvent Pretreated Coals in the Presence of ScCO2 and Their Mechanisms
The purpose of this study is to study the effect of supercritical carbon dioxide(ScCO2)on the chemical structure of coal after solvent pretreatment.The lean coal samples collected from Changzhi Huoerxinhe were adopted.Coal samples were pretreated with tetrahydrofuran(THF),hydrochloric acid(HCI),and hydrofluoric acid(HF),respectively.The Fourier transform infrared spectroscopy(FTIR),solid-state nuclear magnetic resonance(13C-NMR),and X-ray diffraction(XRD)were used to study the chemical composition and structure of coal samples.Results show that:①The FITR peak-fitting spectra were basically consistent with the experimental curves,but the positions and intensities of the absorption peaks of each functional group still showed some deviations,and the aliphatic peaks disappeared in part of the bands after the pretreatment with acids(HCI and HF).The two types of acid pretreatment order are different,and the effect on the oxygenated functional group,fat structure,and aromatic structure of coal is not the same.The intensity of aromatic C＝C structure and oxygenated functional group peaks were enhanced after HF-HCI treatment.In contrast,the intensity of the aromatic C＝C structure was weakened after HCI-HF treatment,and the intensity of the structure of the oxygenated functional group was insignificant.The intensity of the aromatic C＝C structure of the coal samples was enhanced,and the intensity of the structure of the C＝C oxygenated functional group was reduced after THF pretreatment.After THF pretreatment,the intensity of the aromatic C＝C structure was enhanced,and the intensity of the C＝C oxygenated functional group was reduced.Overall,the intensity of aromatic C＝C structure is much larger than that of aliphatic structure and oxygenated functional group structure.②In the 13C-NMR spectrum,the chemical shift of the main functional group peaks exhibits a certain degree of deviation.Aromatic carbon faB chemical shifts are overall shifted in the direction of increasing.The proportion of aromatic carbon is much greater than that of aliphatic carbon,indicating that aromatic carbon forms the bulk of the coal macromolecular structure.③In the XRD pattern,the diffraction intensity of the 002 and 101 peaksk increases significantly,and the microcrystal structure parameterd002shows an increasing trend.It can be seen that THF,acidic treatment,and ScCO2 all have a certain impact on the macromolecular structure of coal,causing the coal macromolecular network structure to become looser,which in turn increases the overall distance d002 of the microcrystalline layer network.Research shows that changes in the spectral characteristics of the coal after solvent action were not only related to the nature of the solven,but also to the order of inorganic acid treatment,and consequently,the functional group composition and macromolecular structure were altered to varying degrees,which in turn affected the extraction effect of ScCO2 on the pretreated coal.
ScCO2Solvent pretreatmentThe Fourier transform infrared spectroscopy(FTIR)Nuclear magnetic resonance(13C-NMR)Chemical structure
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