Molecular simulation of CO2/CH4 adsorption in brown coal: Effect of oxygen-, nitrogen-, and sulfur-containing functional groups

摘要

The CO2/CH4 adsorption behaviors in brown coal at the temperatures of 298, 313, and 373 K and in the pressure range of 0.005-10 MPa were investigated by molecular dynamics (MD), density functional theory (DFT), and grand canonical Monte Carlo (GCMC) simulations. The absolute adsorption isotherms of single-component CH4 and CO2 exhibit type-I Langmuir adsorption behavior showing a negative influence of temperature. For the binary CO2/CH4 mixture, brown coal shows super high selectivity of CO2 over CH4 at pressures below 0.2 MPa, which then decreases quickly and finally tends to be constant when the pressure increases. The high competitive adsorption of CO2 originates from the effects of (i) the large electrostatic contributions, (ii) the conducive micropore environment with pore sizes below 0.56 nm, and (iii) the stronger adsorption of CO2 with respect to CH4. These effects are strengthened by the highdensity oxygen-containing, pyridine, and thiophene functional groups contained in brown coal, which provide abundant and strong adsorption sites for CO2, but show weaker affinity to CH4. Furthermore, the influence of various nitrogen-and sulfur-containing functional groups on the CO2 adsorption capacity was also investigated. The results indicate that the basicity of the oxygen-and nitrogen-containing groups has a large influence on the CO2 adsorption, while for the sulfur functional groups the determining factor is the polarity. (C) 2017 Elsevier B.V. All rights reserved.