Density Functional Calculation of H(2)O/CO(2)/CH(4) for Oxygen-Containing Functional Groups in Coal Molecules

[Image: see text] To investigate the adsorption mechanism of H(2)O, CO(2), and CH(4) 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 H(2)O molecules bonded to different OFGs. The adsorption energy and Mulliken charge distribution of the H(2)O, CO(2), and CH(4) molecules for each OFG were determined. The results showed that H(2)O molecules form 2, 1, 1, and 1 hydrogen bonds with −COOH, −OH, —C=O, and −O–R groups, respectively. Double hydrogen bonds connected the H(2)O molecules to −COOH with the smallest adsorption distances and highest Mulliken bond layout values, resulting in the strongest bonding between the H(2)O 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 H(2)O molecule served as a hydrogen bond acceptor. The order of the bonding strength between the OFGs and H(2)O molecules was Ph–COOH > Ph–OH > Ph—C=O > Ph–O–R. The adsorption energy calculation results showed that H(2)O molecules have a higher adsorption stability than CO(2) and CH(4) molecules. Compared with the −OH, —C=O, and −O–R groups, the −COOH group had a higher adsorption capacity for H(2)O, CO(2), and CH(4) molecules. The adsorption stability of the CO(2) molecules for each OFG was higher than that of the CH(4) molecules. From the Mulliken charge layout, it was clear that after the adsorption of the H(2)O molecules onto the OFGs, the O atoms in the OFGs tend to gain electrons, while the H atoms involved in bonding with the H(2)O molecules tend to lose electrons. The formation of hydrogen bonds weakens the strength of the bonds in the H(2)O molecule and OFGs, and thus, the bond lengths were elongated.