Effect of surface polar groups in nanoporous carbon on the co-adsorption of methane and carbon dioxide

Methane (CH₄) and carbon dioxide (CO₂) are some of the most prevalent and potent greenhouse gases and are a common byproduct of the decomposition of organic waste. Adsorption in nanoporous carbon (pores in the ~ 1 nm range) is a promising method to capture, store, and separate these substances for subsequent industrial use. Here we present results of extensive Grand Canonical Monte Carlo (GCMC) simulations of the coadsorption of CH₄ and CO₂ in slit-shaped pores with separations between 8 and 20 Å both bare and functionalized with polar hydroxyl and epoxy groups on its surface for pressures between 0 and 10 MPa and gas molar compositions between 0 and 100% for each species. Due to its larger mass and interaction strength with the substrate (i.e., 60-100% larger adsorption enthalpy), even small amounts of CO₂ reduce significantly the adsorption of CH₄, whereas the reverse is only seen at high concentrations of CH₄. The presence of polar surface groups potentiates this effect, increasing substantially the adsorption enthalpy of CO₂ while that of CH₄ remains nearly unchanged, thus increasing the selectivity of the nanoporous carbon towards CO₂ adsorption. The mechanism is the strong electrostatic interaction between the surface groups and the electric quadrupole of CO₂ as observed by single-species Canonical Ensemble) Molecular Dynamics simulations of CH₄ and CO₂.