## 1 Introduction  
The main fossil fuels, petroleum, natural gas, and coal are not only important energy sources but also feedstock for the modern chemical industry [1]. However, with the decline of crude oil prices and the sharp fluctuations of global oil prices, there is an urgent need to find alternative energy sources and solutions [2]. China, as the biggest developing country in the world, has significant reserves of low-rank coal deposits [3]. Through pyrolysis technology, low-rank coal is converted into semi-coke, coal tar, and carbonized gas to realize the comprehensive utilization of resources [4]. As the only liquid by-product, medium-low temperature coal tar (MLCT) can provide abundant information about the medium-low temperature pyrolysis process [5]. Through the analysis of the molecular composition and structure of coal tar, the relationship between the product characteristics and process parametersis obtained, which is helpful to reveal the mechanism of coal pyrolysis [6]. MLCT is a complex mixture of olefins, alkanes, aromatics, and variable amounts of heteroatomic organic compounds containing sulfur, nitrogen, oxygen, and metals [7]. MLCT, which has no secondary cracking and aromatization, high content of aliphatic hydrocarbon and \(<\)360 \({}^{\circ}\)C light fraction, substantial amounts of phenolic compounds, and a high H/C molar ratio, is suitable for the application in hydrogenation technology for producing clean fuels and chemicals [8]. Nonetheless, due to its high content of resin, asphaltene, and heteroatomic compounds, this will become the focal precursor, leading to the deactivation of catalysts and pipeline blockage [9]. At present, the development of the processing technology of MLCT is limited by an insufficient understanding of its chemical composition and structure [10]. Therefore, accurate qualitative and quantitative analysis of the molecular composition of MLCT will not only provide a data basis for the utilization of MLCT but also contribute to the efficient conversion of MLCT. In recent years, many researchers have used physical and chemical methods to study the chemical composition of coal tar [11; 12; 13; 14; 15]. In general, the properties of coal tar were considered comprehensively and the components of coal tar were characterized by various physicochemical analysis methods [16; 17; 18]. However, the methods of separation and characterization are quite different, according to different research purposes. The main methods for separating specific components and groups include distillation, solvent extraction, precipitation, complexing methods, liquid chromatography, liquid membrane extraction, supercritical fluid extraction, deep eutectic solvent extraction, ionic liquid extraction, column chromatography, and high-performance liquid chromatography [19; 20; 21; 22; 23; 24; 25; 26; 27]. [PERSON] et al. [19] used true boiling point distillation to separate 17 distillation fractions from initial boiling point (IBP) to 400 \({}^{\circ}\)C at an interval of 20 \({}^{\circ}\)C and distillate residues of the middle-temperature coal tar, then the narrow distillate fractions were analyzed by gas chromatography-mass spectrometer (GC-MS). High-precision real boiling distillation can effectively control the carbon number distribution of narrow fractions and make the target components more concentrated. [PERSON] et al. [20] used ethanolamine (C\({}_{2}\)H\({}_{7}\)NO) to separate phenolic compounds from coal tar and found that hydrogen-bond interactions resulted in the separation of phenolic compounds in coal tar. [PERSON] et al. [23] tested the removal efficiency of seventeen ionic liquids (ILs) and found that the nitrogen compounds present in the gas oil feed for diesel fuel production were successfully attained by means of ILs. [PERSON] et al. [25] separated the middle-temperature coal tar into acidic, basic, and neutral fractions first by acid-base extraction, and then extrography was used to fractionate the neutral fraction of into six subfractions.