Characterization of the Molecular Structural Changes Following Ion-Exchange Treatment of Zhundong Coal

Low-rank coal usually contains the highly exchangeable metallic species content. Due to this feature, ion-exchange treatment, which can load extra exchangeable metallic species into coal matrix, is widely used to investigate the behaviors of these species during thermal conversion. However, whether any structural changes of coals are caused during ion-exchange treatment process remains unknown. In this work, to examine structural changes during ion-exchange treatment and provide evidence on reliability of this treatment, multiple techniques were used to characterize the structural features of a Zhundong coal during ion-exchange treatment. The results show that the maximum weight loss of coal becomes smaller after ion-exchange treatment, which indicates that introduction of exchangeable sodium species makes coal matrix density more compact and enhances cross-linking structures. Meanwhile, compared with raw coal, the variation of oxygen forms in the demineralized and ion-exchanged coals is negligible because the maximum differences in content for C-O, C-O, and COO are 5.3, 6.9, and 5.9%, respectively. According to Fourier transform infrared spectra, the skeletal structures of Zhundong coal are also not influenced. Raman spectra show that during ion-exchange treatment, the relative proportions of amorphous carbon structures (particularly small aromatic ring systems with 3-5 fused benzene rings) and large aromatic ring systems (more than six rings) stay almost unchanged. In light of these results, ion-exchange treatment is suggested to result in slight structural changes and is reliable in coal research.