Microstructure and aggregate structure of Liulin No.3 coal after pyridine extraction

Coal is a complex molecular system formed by the interaction of organic matter with different composition and structure. The chemical structure and spatial arrangement of different components control the microstructure and aggregation structure of coal,which directly affects the properties and utilization of coal. A series of unique phenomena will occur during the second coalification evolution. In order to reveal the nature of this phenomenon,it is essential to understand the microscopic chemical structure characteristics of different components in coal with this metamorphic degree,as well as the aggregation structure characteristics within and between different components controlled by intermolecular interaction and molecular recognition mechanism. The Liulin No.3 raw coal,which is in the second coalification evolution,is selected for the study,and three samples of raw coal(YM),post-extraction residual coal(CM) and extraction product asphaltene (LQZ) are obtained by soxhlet extraction using pyridine as the solvent. Combined with ¹³C Nuclear Magnetic Resonance carbon spectroscopy(¹³C NMR) to quantify the carbon skeleton information of coal structure,the laser Raman spectroscopy(Raman) is used to analyze the degree of order of aromatic lamellae structure in coal,and the High Resolution Transmission Electron Microscopy(HRTEM) is used to quantitatively characterize the fringes length,orientation and stacking distribution of aromatic lamellae in coal. The results show that ¹³C NMR,Raman spectroscopy and HRTEM have confirmed that the aromatic CM structure in the Liulin No.3 coal had a high degree of condensation and fewer defects,while the aliphatic carbon structure had a high degree of condensation,and the aliphatic cyclization and aromatization were obvious. Under the action of molecular forces such as hydroxyl π hydrogen bond,these molecules had obvious cambium lamellar orientation,prominent stacking scale and high order aggregation structure. LQZ structure was mainly composed of some aromatic structures with low condensation degree,oxygen-containing functional groups and aliphatic chain(including branched chain) carbon structures,and some of its fringes were arranged into weakly ordered aggregation structures with approximately the same orientation with adjacent CM layers through intermolecular interaction. The abundant aliphatic rings and branched structures in the CM and LQZ molecules are speculated to have been formed during the massive fracture of methyl groups directly connected with the aromatic system during the second coalification evolution,which provides the molecular basis for the rapid increase of CH₄,H₂ and other gases. These branched chain and alicyclic substances hindered the formation of a compact spatial structure of coal,causing coal at this coalification stage to have the lowest specific gravity. At the same time,it was also the evolution basis for further aromatic nucleation,polycondensation and graphitization of coal in subsequent coalification. The hydrogen bonding force formed between the LQZ molecules and between the LQZ and CM phase molecules was the molecular basis for the high cohesiveness of the coal.