Theoretical Investigation of Noncovalent Interactions between Low-Rank Coal and Water

Abstract

The high moisture content of low-rank coals (LRCs) is associated with the noncovalent interactions between coal and water, such as hydrogen bonding and van der Waals attraction. In this study, the molecular model of lignite was constructed and its electrostatic potential (ESP) on a van der Waals surface was analyzed. The mechanism of water absorption on the hydrophilic or hydrophobic sites of the lignite surface was investigated based on the atoms in molecules analysis and reduced density gradient analysis of seven typical lignite···water complexes. The regions with the most negative and positive ESP values were associated with oxygen in oxygen functional groups, and hydrogen was associated with smaller electronegativity. The typical hydrogen bonds O–H···O were formed between water and oxygen functional groups, whereas the weaker hydrogen bonds C–H···O were formed between water and the skeleton components of lignite, such as the benzene ring and methyl and methylene groups. Oxygen functional groups were revealed to exhibit more hydrophilic than skeleton structures of lignite, including benzene rings and aliphatic chains. The hydrophilicity sequences for the different oxygen functional groups in the lignite model were determined as carboxyl > phenolic hydroxyl > carbonyl > alcoholic hydroxyl > ether. Furthermore, the carbon bond C···O was also observed in the lignite···water complex via van der Waals interactions, exhibiting a hydrophobic effect. The color-mapped reduced density gradient isosurface accurately demonstrated the noncovalent interactions of lignite···water complexes.