Simulation of the adsorption behavior of CO2 / N2 / O2 and H2 O molecules in lignite

In order to investigate the adsorption mechanism of N2 ，O2 ，CO2 ，H2 O molecules on oxygen⁃containing func⁃ tional groups (OFGs) of lignite，the density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulation method were used to study the electrostatic potential (ESP) of OFGs，the adsorption energy of H2O，N2，O2， CO2 molecules and OFGs，and the adsorption isotherms，isosteric adsorption heat，diffusion coefficients of N2，O2，CO2 in lignite.The results show that the order of the maximum ESP of each OFG is Ph—OH>Ph—COOH>R—COOH>R— OH>Ph(R)—C =O>R2—C=O>Ph—O—R>R—O—R;the order of the minimum ESP is Ph—O—R>Ph—OH> R—O—R>Ph—COOH>R—COOH>R2—C=O>Ph(R)—C=O>R—OH.The adsorption energy calculation results show that the adsorption stability of H2 O and OFGs is greater than that of N2 ，O2 ，and CO2 .Double hydrogen bonds ex⁃ ist between H2 O and —COOH，resulting the adsorption stability of H2 O and —COOH is more stable than that of H2 O and —OH，—O—，—C=O.For—OH and —O— groups，the O atom is easy to act as the hydrogen bond receptor when it is not connected to the benzene.Compared with —OH，—O— and —C=O，—COOH has the strongest adsorption capacity for CO2 ，O2 and N2 .Except for R2 —C=O and Ph—OH，the absolute value of adsorption energy of CO2 and each OFG is greater than that of O2;except for R—O—R，the absolute value of the adsorption energy of O2 and each OFG is greater than that of N2.The relationship between the adsorption amount of gas molecules in lignite is CO2 >O2 >N2 at the same pressure and temperature;the adsorption capacity of gases decreases at 298 K to 348 K.The adsorption isotherms of the three gases in the lignite structure model are in good agreement with Langmuir adsorp⁃ tion model.At the temperature of 298 K，323 K，and 348 K，the isosteric adsorption heat of CO2 is between 31- 32 kJ / mol，and the isosteric adsorption heats of O2 and N2 are between 18-20 kJ / mol.Molecular dynamics calculation results show that the diffusion coefficient (D) of CO2，O2，N2 molecules in the lignite structure model is 1.0×10-5，8. 78×10-5 and 1.17×10-4 cm2 / s，respectively.