Molecular simulation of the adsorption and diffusion of CO2, CH4, and N2 in alkali metal-doped low/medium-rank coal

•Na doping of middle rank coal enhances its carbon capture capacity.•The binding energy required for doping is proportional to the atomic radius.•The highest adsorption for CO2 (5.934 mmol/g) was found in the Na-CYM system.•The diffusion coefficient of CO2 is minimized in the Li-coal system. Coal-based adsorbents show promise for advancing Direct Air Capture (DAC) technology as support for carbon capture grows. Doping coal with alkali metals (Li, Na, K) enhances gas molecule interactions and improves adsorption capacity. Using Density Functional Theory (DFT) and Grand Canonical Monte Carlo (GCMC), the study investigated the adsorption and diffusion performances of CO2, CH4, and N2 in modified coal. The results showed that binding energy increases with the increasing radius of the dopant atoms, with K-SM requiring the most energy (31.236 eV). The adsorption capacity followed the order: Na-coal > Li-coal > K-coal > Undoped-coal, with Na-coal achieving the highest CO2 adsorption (5.934 mmol/g). After alkali metal doping, the adsorption amounts of CO2, CH4, and N2 increased by 26.80 %, 43.29 %, and 56.60 %, respectively, with the Na-doped middle-rank coal showing an average increase of 32.33 % in CO2 adsorption. The diffusion coefficient of Na-coal was lower than that of Li-coal and K-coal, with CO2 having the smallest diffusion coefficient (1.627 × 10-8 m2/s). Overall, alkali metal doping significantly enhances the adsorption and stability for CO2, CH4, and N2, particularly with Na doping in middle-rank coal, providing insights for developing high-performance CO2 adsorbent.