Quantum chemical calculation of the interaction between lignite and water molecules

The high water content of lignite limits its large-scale utilization. It is of great theoretical significance to study the water-holding mechanism of lignite and reveal the nature of hydrogen bonding between oxygen-containing functional groups and water molecules for guiding the dewatering of lignite. The ordered structure of water molecules on the lignite surface and the interaction menchanism between lignite structural units and water molecules were studied by the ab initio molecular dynamics (AIMD) and density flooding theory (DFT) respectively. The results showed that the water molecules at the interface formed the first coordination layer and the second coordination layer near 0.192 nm and 0.322 nm, respectively. The extreme points of negative potential for the lignite structural unit were mainly located near the oxygen atoms in the oxygen-containing functional groups, with a maximum of -154.20 kJ/mol. Meanwhile, the extreme points of the positive potential were mainly located near the hydrogen atoms in the oxygen-containing functional groups and the benzene rings, with a maximum of 221.08 kJ/mol. The four dimer binding energies of water molecules near the carboxyl, hydroxyl, carbonyl and ether bonds were -47.76, -29.44, -40.45 and -23.36 kJ/mol, respectively. The electrostatic interactions accounted for 65.92%, 67.45%, 55.99% and 65.19% of total attraction energy, respectively. In addition, the results of atoms in molecules(AIM) topological analysis indicated that the total hydrogen bonding interaction energies were ranked as follows: carboxyl group > hydroxyl > ether bond > carbonyl group. Among these oxygen-containing functional groups, the water molecules in the vicinity of the carbonyl group were easier to interact with other parts of lignite structural unit, and thus the interaction energy between the water molecules and the carbonyl group was minimized. © 2022, Editorial Board of Journal of CUMT. All right reserved.