Simulation study on the adsorption characteristics of CO2 and CH4 by oxygen-containing functional groups on coal surface

To investigate the adsorption mechanism of CO2 and CH4 with oxygen-containing functional groups in coal, the structural models of coal with different functional groups of carboxyl, hydroxyl, carbonyl, and ether bonds were constructed using Materials Studio software, and the adsorption isotherms, heat of adsorption, adsorption energy, energy distribution, and adsorption configurations of CO2 and CH4 adsorbed by different functional group structures were analyzed using the giant regular GCMC method. The results showed that the presence of carboxyl, hydroxyl, and carbonyl functional groups will promote the adsorption of CO2 by coal, and the intermolecular interaction energies of the model with the addition of carboxyl, hydroxyl, and carbonyl groups, and CO2 increased by 70.37%, 33.67%, and 14.26%, respectively. The order of the adsorption strength of the five structures is: AC-COOH > AC-OH > AC-O > AC > AC-OCH3, which indicates that the influence of oxygen-containing functional groups on the adsorption performance of CO2 by coal mainly depends on polarity. The presence of carboxyl, hydroxyl, and carbonyl functional groups weakens the adsorption of CH4 on coal, and the interaction energy between the model and gas molecules with the addition of carboxyl, hydroxyl, carbonyl, and ether bonds reduced by 41.83%, 36.14%, 26.22%, and 80.21%, respectively. The order of the adsorption strength of the five structures is: AC > AC-O > AC-OH > AC-COOH > AC-OCH3, which indicates that the effect of oxygen-containing functional groups on the adsorption of CH4 on coal mainly depends on hydrophobicity; the introduction of ether-bonded functional groups does not favor the adsorption of CO2 and CH4 molecules on coal. It indicates that the ether bond has a certain repulsive effect on CO2 and CH4 molecules. In different functionalized coal structures with the same carbon atom skeleton, there are different adsorption capacities for CH4 as well as CO2, but in the same functionalized structure, the adsorption magnitude relationship is CO2 > CH4, indicating that the coal body structure interacts with CO2 more than CH4.