Analysis of the microscopic mechanism of coal wettability evolution in different metamorphic states based on NMR and XPS experiments

Abstract

To investigate the evolution of carbon-and oxygen-containing groups during metamorphism and the influence of the structural variation of coal on its wettability, NMR and XPS were employed to study the two main elements, carbon and oxygen, in coal dust. Our results show that with an increase of the metamorphic degree, the number of highly reactive hydroxyl and carboxyl groups show a descending trend whereas the number of low reactivity ether and carbonyl groups increases. The number of carboncontaining groups gradually increases when the metamorphic degree increases, due to a clustering change process which includes an increase in the number of aromatic carbons (Ar-C, H) and a simultaneous decrease in the number of carboxyl carbons (-COOH), methylene carbons (CH 2 ) and ether based carbons (Ar-O, R-O). The reduction rate of carboxyl carbon (-COOH) is closest to the growth rate of aromatic carbon (Ar-C, H). The methyl (CH 3 ) and carboxyl (C]O) carbon evolve discontinuously with the metamorphic degree, which is different from the other carbon groups. Linearregression analysis suggests that the correlation coefficients between aromatic (Ar-C, H), ether (R-O), methylene (CH 2 ) and carboxyl (-COOH) carbons and wettability are 0.98, 0.97, 0.96 and 0.92, respectively, indicating that the main contribution to the wettability from carbon-containing groups comes from these groups. In contrast, hydroxyl oxygen (-OH) has a correlation coefficient as high as 0.98, and is a major contributor from the oxygen-containing groups.

An ultrasound-assisted approach to bio-derived nanoporous carbons: disclosing a linear relationship between effective micropores and capacitance

“…More hydroxyl groups (related to O3) could produce more hydrogen bonds in the electrolyte-electrode interface, possibly resulting in better wettability. 39 Reasonably, MSNPC-950 exhibits an obvious loss of C-OH at the higher activation temperature, which is consistent with the C1s spectra (Fig. 2B).…”

“…2D, lower) presents two nitrogen species: pyrrolic nitrogen (N5, 399.7 eV) and quaternary nitrogen (NQ, 400.9 eV). 40 The abundant O and N species provide great advantages for MSNPC-850: (1) NQ atoms in the carbon matrix can enhance the electronic conductivity of the material; 41 (2) hydrophilic oxygen-and nitrogen-containing functional groups can reduce the contact angle of the electrolyte-electrode interface, further capable of improving the surface wettability of the material; 39,42 (3) oxygen-and nitrogencontaining species can also provide the faradaic pseudocapacitance for the material. 22,43 XRD patterns of MSNPCs, as shown in Fig.…”

One- and Two-Dimensional NMR in Studying Wood–Water Interaction at Moisturizing Spruce. Anisotropy of Water Self-Diffusion

“…Different methods including those based on the phenomenon of nuclear magnetic resonance (NMR spectroscopy and NMR Imaging) have been applied and developed to monitor wettability and saturation of different porous materials with water [1,4,[11][12][13][14]. For instance, different NMR techniques (spin-spin relaxation time (T 2 ) as a function of saturation, spin-lattice relaxation time (T 1 ) NMR dispersion, T 2 shift, NMR diffusion and T 1 /T 2 ratio) have been probed for 2 of 14 wettability measurements on chalk, rock, sandstone, coal [14][15][16][17]. Because NMR signal is sensitive to the interaction of liquid with solid surface, it is considered to be a suitable candidate for determination of wettability [11,12,[14][15][16].…”

Experimental investigation on the effect of ethanol on micropore structure and fluid distribution of coalbed methane reservoir

“…The T 2 relaxation peak of coal samples saturated with ethanol is generally on the right side of T 2 relaxation for samples saturated with mineralized water. As the T 2 relaxation time decreases, the pore size decreases (i.e., the curve is closer to the left and the pore size is smaller), indicating that the water primarily stays in the micropores for the mineralized water samples, and the water in the immersion samples primarily stays in the micropores (Xu et al., 2017).…”