Occurrence Modes of AAEMs (Na⁺ and Ca²⁺) and the Effect on the Molecular Structures of Zhundong Coal via Quantum Chemistry

Zhundong coal is known for its high content of alkali and alkaline earth metals (AAEMs), which greatly influences coal processing and utilization. To reveal the occurrence modes and the effect of AAEM ions on the molecular structures of Zhundong coal, the previously constructed molecular structure models of vitrinite-rich and inertinite-rich Zhundong coal (ZD-V and ZD-I) were selected to simulate using quantum chemical methods. By focusing on Na+ and Ca2+, the adsorption capacity at different adsorption sites was investigated based on the density functional theory (DFT), and the effects of adsorption of Na+ and Ca2+ on nearby atomic charges, chemical bonds, and molecular orbitals were investigated. Results show that compared with ZD-I, ZD-V contains a more negative electrostatic potential (ESP) distribution and lower bond order, indicating that vitrinite contains more adsorption sites for AAEM ions and exhibits stronger chemical reactivity. Na+ and Ca2+ are easily adsorbed to the most negative ESP with the optimal adsorption site near the carbonyl group (C & boxH;O). Compared with adsorbed Na+, Ca2+ has a smaller adsorption distance from the molecule and a higher adsorption energy. Ca2+ can transfer more charge than Na+ and has more affinity with the coal molecule. Ca2+ at all adsorption sites is bound to organic molecules by chemisorption, which also reveals the reason for the low water-soluble Ca content in coal at the molecular level. Adsorption of AAEM ions has a more significant effect on the chemical bond of oxygen-containing functional groups near AAEM ions compared to the overall molecular fragments, which makes the nearby chemical bonds (C-O/C & boxH;O) decrease in bond order and increase in bond length. Ca2+ makes the nearby chemical bonds more prone to break than Na+. Additionally, Ca2+ has a more significant impact on the energy gaps (Delta E-gap) compared to Na+. Adsorption of Ca2+ near the carbonyl and carboxyl groups leads to a significant decrease in Delta E-gap, indicating an enhanced chemical reactivity of coal molecular fragments.