Functional groups of sequential extracts and corresponding residues from Hefeng sub-bituminous coal based on FT-IR analysis

Petroleum ether, carbon disulfide, methanol, acetone, and acetone/carbon disulfide were selected as solvents for ultrasonic-assisted extraction of acid-washed Hefeng sub-bituminous coal. The extract and residue were identified as E_i and R_i (i = 1, 2, 3, 4, 5) for each stage. By FT-IR characterization of E_i and R_i, molecular structure of the extracted product was analyzed by means of segmented peak fitting. The results show that the hydroxy hydrogen bond in the fifth-order extract is dominated by self-associating hydroxy hydrogen bond; in aliphatic substances, only E₃ was dominated by aliphatic −CH₃ and asymmetric −CH₂ stretching vibration, while the other extracts were dominated by symmetric and asymmetric −CH₂ stretching vibration. E₁ is dominated by symmetric bending vibration of aliphatic chain terminal −CH₃ and asymmetric deformation vibration of −CH₃ and −CH₂, indicating that petroleum ether mainly break the easily dissociated chemical bonds in coal samples; CS₂ dissolve a higher proportion of aromatic structure containing aliphatic side chains. The functional groups contained in the five residues are the same, indicating that the main structure of the coal sample is not changed due to the stepwise extraction. Extraction has an obvious influence on aromatic structure and hydroxy hydrogen bond in the residue. Aromatic structures change from di-substituted benzene dominant to tetra-substituted benzene dominant. Before extraction, the hydroxyl hydrogen bond in the acid-washed coal sample was dominated by hydroxyl ether hydrogen bonds, and after extraction, it was transformed into self-associated hydroxyl ones. In addition, sequential extraction has little effect on oxygen-containing functional groups and aliphatic functional groups. By comparing structural parameters, it is found that E₁, E₃, and R₅ has a higher degree of aromatic condensation, and E₄ has a longer straight-chain and less branched chain.