Adsorption of CO2/CH4/N2 by different functional groups in coal doped with N, S, and Na atoms: A DFT and GCMC study

•The synergistic adsorption of functional groups and doped atoms is investigated.•The Ph-OH-N system shows the highest absorption capacity of CO2.•Na doping enhances the strength of adsorption of CH4 in Ph-X system.•N-modification of highly oxygenated coal can significantly capture CO2. Low-cost and high-performance carbon-based adsorbents can be obtained by doping modification to enhance the adsorption characteristics for carbon dioxide. In this work, the microscopic mechanism of synergistic adsorption of CO2/CH4/N2 by doping atoms and functional groups in coal was studied. Through density functional theory (DFT) and grand canonical Monte Carlo (GCMC) calculation methods, the interaction between doped N, S, Na atoms and functional groups of –OH, –COOH, –OCH3, –CH3 and –CH2CH3 in coal was simulated. According to the results, the acid group –COOH was more likely to interact with the alkali metal atom Na, which made doped system more stable. The Ph-oxygen-containing functional group-N system had the largest adsorption capacity for CO2, and the adsorbate accumulated in clumps near oxygen-containing functional groups and N atoms. The interaction energy between Ph-COOH-N and CO2 (−15.65 kcal/mol) was stronger than that of other systems, that is, the adsorption state was the most stable. Compared with other systems, the Ph-CH2CH3-Na system had the highest adsorption heat of CH4 and N2, 7.298 kcal/mol and 5.746 kcal/mol, respectively, which means Na doping would enhance the adsorption strength of the Ph-CH2CH3 system on CH4 and N2. These results show that N-modification of coal samples with high oxygen content can obtain significant carbon capture and stable adsorption state, which is conducive to large-scale industrialized CO2 capture.