Coal-CH4/CO2 high-low orbit adsorption characteristics based on molecular simulation

To study the competitive adsorption mechanism of CH4 and CO2, the structural parameters of Tianchi coal were obtained through proximate and ultimate, X-ray photoelectron spectroscopy (XPS), and 13C nuclear magnetic resonance (13C NMR). Then the Tianchi coal molecular structure model was established. Finally, the adsorption of CH4 and CO2 was simulated using Materials Studio (MS). The results show that: (1) The lone electron pairs on the O atoms in CO2 molecules lead to the adsorption to be dominated by electrostatic force. In addition, hydrogen bonds that can increase adsorption are formed between coal and CO2 molecules but not between coal and CH4 molecules. (2) Due to the existence of the intermolecular interaction force, the low (L) orbit is greatly affected by the repulsive force, and the high (H) orbit is less affected by the gravitational force. Therefore, the adsorption mainly occurs in the middle (M) orbit, and the adsorption sites of CO2 on the L and M orbit are more than CH4. (3) The CO2 molecule has a smaller diameter and can enter the ultrafine pores. Hence the accessible solvent pore volume and surface area obtained using CO2 as a molecular probe are greater than those obtained using CH4 molecular.