Effect of Sequential Solvent Treatment on the Macromolecular Structure of Organic Matter in Zichang Coal

Understanding the depolymerization and dissociation of the macromolecular structure of low-rank coal organic matter at the molecular level, is a key issue that needs to be solved urgently to efficiently use low-rank coal and obtain high value-added chemicals from it. Based on this, the Zichang low-rank coal was treated by sequential solvent treatment such as ultrasonic extraction, alcoholysis, and catalytic alcoholysis. Carbon nuclear magnetic resonance spectroscopy (C-13 NMR), X-Ray photoelectron spectroscopy (XPS) measurements, thermogravimetric (TG) and differential thermal analysis (DTG) were used to characterize the samples. The results show that the ratio of aliphatic carbon (f(al)) in coal macromolecular structure is decreased while the aromaticity (f(a)) increased during solvent treatments. The distribution of C and O elements are influenced greatly. The macromolecular structure of coal is depolymerized and dissociated to some extent, and its thermal stability is increased. After disulfide/acetone (CS2/AC) ultrasonic extraction at room temperature, the structures of oxy-aliphatic carbon (f(al)(O1),f(al)(O2),f(al)(O3)) and oxy-aromatic carbon (f(a)(O1),f(a)(O2)) in coal organic matter increase, while the proportion of carbonyl carbon (f(a)(C)), the average methylene chain length (C-n) and the ratio of aromatic bridge carbon (chi(b)) decrease, indicating that the network system mainly composed of polyhydroxyl in the macromolecular structure of coal is weakened and depolymerized during CS2/AC extraction. The structures of C-alk - O, C-ar - O are increased. Some carbonyl carbon structures are dissociated. Some aromatic and aliphatic hydrocarbon fragments bound in coal by pi-pi interaction and other non-covalent bond interactions are dissociated by CS2/AC extraction. After alcoholysis treatment, the f d in coal organic matter decreases to 24. 89% , and the methyl carbon (f(al)) increases, indicating that a nucleophilic substitution reaction occurs during alcoholysis. Some weaker bonds such as C-alk-O, C-alk -C-alk are broken. After continued catalytic alcoholysis, the content of oxygen-bound aliphatic carbon (f(a)(O3)) in coal organic matter is significantly reduced, and C-n is greatly reduced, indicating that the organic matter in the coal is further dissociated and some of the C-alk - O, C-ar -O and C-ar -C-alk bonds are broken.