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

Abstract

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.