吡啶抽提前后柳林3号煤的微观与聚集态结构特征分析

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

煤是由不同组成和结构的有机物质相互作用而形成的复杂分子体系,不同组分的化学结构和空间排列控制形成煤的微观结构与聚集态结构特征,直接影响着煤的性质与利用。煤在第2次煤化作用跃变时,会伴随出现一系列独特的现象。为揭示这些现象的本质,需从分子角度出发,了解该变质程度煤中不同组分的微观化学结构特征,以及由分子间相互作用、分子识别机制等控制所形成的不同组分内部、不同组分之间的聚集结构特征。研究选取处于第2次煤化作用跃变的柳林3号原煤为实验对象,利用吡啶作为溶剂,对其进行索氏抽提,获取了原煤(YM)、抽提后残煤(CM)和抽提产物沥青质(LQZ)3种样品。结合^(13)C核磁共振碳谱(^(13)C NMR)定量分析煤结构的碳骨架信息,利用激光拉曼光谱(Raman)分析了煤中芳香层片结构的有序程度,采用高分辨透射电镜(HRTEM)定量表征煤中芳香层片的条纹长度、取向和堆垛分布。结果表明:^(13)C NMR,Raman光谱与HRTEM三种测试技术一致地印证了柳林3号煤中,CM芳香结构缩合度高且形成的缺陷少,脂肪碳结构则包含缩合度较高的脂环结构,脂环化与芳香化作用明显,这些分子在羟基π氢键等分子力作用下形成层片取向明显,堆垛规模较突出、有序度较高的聚集态结构。LQZ结构主要为缩合度较低的芳香族结构、含氧官能团与脂肪链(含支链)碳结构,其部分条纹通过分子间相互作用与邻近CM层片排列为取向近似一致的弱有序聚集态结构。CM与LQZ分子中丰富的脂环与支链化结构,推测是在第2次煤化跃变过程中与芳香体系直接相连的甲基大量断裂过程中形成,为CH_(4),H_(2)等气体急剧增加提供了分子基础;这些支链、脂环类物质,阻碍了煤大分子紧密立体结构的形成,使得处于该变质程度的煤具有最低的比重,同时也是后续煤化作用中煤进一步芳核化、缩聚化、石墨化的演化基础;LQZ分子间、LQZ与CM相分子间形成的氢键作用力是该煤形成高黏结性的分子基础。

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 ^(13)C Nuclear Magnetic Resonance carbon spectroscopy(13 C 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 ^(13)C 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 CH_(4),H_(2) 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.