Insight into the Effects of Sodium Species with Different Occurrence Modes on the Structural Features of Residues Derived from Direct Liquefaction of Zhundong Coal by Multiple Techniques

In this work, to better understand the characteristics of the direct coal liquefaction residue (DCLR) derived from the sodium-rich Zhundong coal, effects of sodium species with different occurrence modes on the structural features of the DCLR were investigated. Multiple techniques, including the element determination, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and solid-state 13C nuclear magnetic resonance (SS-NMR), were jointly used to characterize the structural features of the coal samples and DCLR. The results show that the coalescence of the high-molecular-weight substances in coals could be obviously intensified in liquefaction, and high temperatures are found to significantly enhance the cross-link structure of the DCLR. In the meantime, the DCLR derived from samples with different sodium species were observed to show different weight losses in pyrolysis. According to the amount of total sulfur content in residues, the sulfur retention ability of ammonium-acetate-soluble sodium species (AS-Na) is verified to be stronger than that of water-soluble sodium species (WS-Na). Moreover, AS-Na could also facilitate the cross-link reactions more easily than WS-Na, and the DCLR derived from Na-9.0, which was prepared by ion exchanging the demineralized sample with sodium acetate solution, contains ca. six more aromatic carbons than that from Im-Na obtained by immersing the demineralized sample with sodium chloride solution.