Effect of Tetrahydrofuran Extraction on Surface Functional Groups of Coking Coal and Its Wettability

Abstract

Coking coal was extracted with tetrahydrofuran solvent using ultrasonic and microwave-assisted method at 50°C and atmospheric pressure. Wettability of raw coal and its residue (residual coal) was tested with capillary penetration method. The raw and residual coals were studied by Fourier transform infrared spectroscopy (FTIR) with curve-fitting analysis. The variation of main surface functional groups of coking coal before and after extraction and its effect on wettability were analyzed. The results were obtained as the following: after extraction with tetrahydrofuran, hydroxyl, ether oxygen, and carbonyl in the coal structure were dissolved, the content of hydrophilic functional groups reduced, and then the hydrophobicity of coal enhanced. At the same time, part of aliphatic hydrocarbon dissolved, the length of aliphatic chains (I2) decreased from 3.961 of raw coal to 3.636 of residual coal, the length of aliphatic chains became shorter, aliphatic CH2 side-chains decreased and aliphatic CH3 side-chains increased, and hydrophobic functional groups content increased. In the aromatic structure, four hydrogens per ring increased and two, three, and five hydrogens per ring decreased. Reduction of substitution functional groups and aliphatic hydrocarbon decreased with the side-chains breakage produce more active sites, which increases the degree of condensation of the aromatic ring (I3). The combined action of the decrease of the hydrophilic functional groups and the increase of the hydrophobic functional groups made the wettability of the coking coal become weak.

Chemical Structure Characteristics and Model Construction of Coal with Three Kinds of Coalification Degrees

“…Figure shows that the 900–700 cm –1 zone can be divided into several peaks, respectively, corresponding to the C–H out-of-plane δ mode of the aromatic structures with one H atom (1H, 870 cm –1 ), two adjacent H atoms (2H, 815 cm –1 ), three adjacent H atoms (3H, 800 cm –1 ), and four adjacent H atoms (4H, 750 cm –1 ). ,, …”

“…The 3000–2800 cm –1 region can be divided into six peaks, as shown in Figure . The peaks are assigned to the −CH 3 , −CH 2 – asymmetric υ mode (CH 3 -as, CH 2 -as) near 2950 and 2920 cm –1 , the −CH 3 , −CH 2 – symmetric υ mode (CH 3 -ss, CH 2 -ss) near 2870 and 2850 cm –1 , and the methylene C–H υ mode (Machine C–H) near 2895 cm –1 . ,, The peak near 2830 cm –1 has been reported in other studies; however, its assignment has not been determined. , …”

“…The υ mode of HBs involving −OH in coal can be divided into 5 types: free hydroxyl groups (O–H, 3750–3600 cm –1 ), HBs formed by π electrons in the aromatic structures and hydroxyl groups (OH···π, 3525 cm –1 ), self-associated HBs (OH···OH, 3420 cm –1 ), HBs formed by O atoms in ethers and hydroxyl groups (OH···O, at 3300 cm –1 ), and cyclic HBs formed by the hydrogen bonding of multiple −OH near 3200 cm –1 . , …”

“…Figure 4 shows that the 900− 700 cm −1 zone can be divided into several peaks, respectively, corresponding to the C−H out-of-plane δ mode of the aromatic structures with one H atom (1H, 870 cm −1 ), two adjacent H atoms (2H, 815 cm −1 ), three adjacent H atoms (3H, 800 cm −1 ), and four adjacent H atoms (4H, 750 cm −1 ). 35,41,42 The changes in the area of the peaks in the 900−700 cm −1 zone with the degree of coalification reflects the degree of substitution of the aromatic H atoms. 18,35,43 As shown in Figure 4d, lignite contains more 3H and 4H, while the relative content of 1H is low, indicating that the aromatic ring of lignite contains more H atoms and that the degree of condensation of the aromatic core is low.…”

“…38,39 CH 2 /CH 3 can be calculated using eq 3. 38 41,44 Figure 6d shows the distribution of HBs in lignite, bituminous coal, and anthracite. Lignite exhibited the highest relative content of OH•••OH bonds, whereas the lowest relative content was observed for free hydroxyl groups.…”