Carbon Structure of Coal from the CP/MAS ¹³C NMR Spectra: Effect of Contact Time and Potential Quantitative Modification

Carbon structures are important for converting coal into chemical products and carbon materials. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy is a powerful approach for determining the structural characteristics of carbon in coal. However, there is much controversy regarding the quantitative reliability of typical cross-polarization magic angle spinning (CP/MAS) C-13 NMR experiments of coal. This study systematically investigated the effect of one of the important factors, CP contact time, using six different coal types with V- daf values ranging from 6% to 44%. A series of experimental results at variable contact times from 0.01 to 10 ms indicated that aromatic and aliphatic carbons undergo polarization transfer at different rates, and the nonprotonated carbon of aromatic groups polarized 2-3 times more slowly than that of aliphatic groups. It is challenging for a single CP/MAS C-13 NMR experiment with coal to ensure that each type of carbon is sufficiently polarized before the signal decay of the proton spin-lattice relaxation. As a potential quantitative indicator, the initial magnetization intensity independent of contact time was obtained using a classical five-parameter model for CP dynamics, and the initial aromaticity was further calculated. A linear correlation between the apparent and initial aromaticity was introduced into a calibration method of model compounds to refine the effect of contact time. The modified carbon structural parameters of the coal samples showed better agreement in terms of the H/C atomic ratio, Fourier transform infrared spectroscopy, and empirical aromaticity formulas. These results will help to provide a convenient reference for the quantitative analysis of the carbon structure in coal.