Density Functional Calculation of H₂O/CO₂/CH₄ for Oxygen-Containing Functional Groups in Coal Molecules

To investigate the adsorption mechanism of H₂O, CO₂, and CH₄ molecules on oxygen-containing functional groups (OFGs) in coal molecules, quantum chemical density functional theory (DFT) simulations were performed to study the partial density of states and Mulliken bond layout of H₂O molecules bonded to different OFGs. The adsorption energy and Mulliken charge distribution of the H₂O, CO₂, and CH₄ molecules for each OFG were determined. The results showed that H₂O molecules form 2, 1, 1, and 1 hydrogen bonds with -COOH, -OH, -C=O, and -O-R groups, respectively. Double hydrogen bonds connected the H₂O molecules to -COOH with the smallest adsorption distances and highest Mulliken bond layout values, resulting in the strongest bonding between the H₂O molecules and -COOH. The most stable configuration for the adsorption of these molecules by the -OH group was when the O-H bond in the OFG served as a hydrogen bond donor and the O atom in the H₂O molecule served as a hydrogen bond acceptor. The order of the bonding strength between the OFGs and H₂O molecules was Ph-COOH > Ph-OH > Ph-C=O > Ph-O-R. The adsorption energy calculation results showed that H₂O molecules have a higher adsorption stability than CO₂ and CH₄ molecules. Compared with the -OH, -C=O, and -O-R groups, the -COOH group had a higher adsorption capacity for H₂O, CO₂, and CH₄ molecules. The adsorption stability of the CO₂ molecules for each OFG was higher than that of the CH₄ molecules. From the Mulliken charge layout, it was clear that after the adsorption of the H₂O molecules onto the OFGs, the O atoms in the OFGs tend to gain electrons, while the H atoms involved in bonding with the H₂O molecules tend to lose electrons. The formation of hydrogen bonds weakens the strength of the bonds in the H₂O molecule and OFGs, and thus, the bond lengths were elongated.