Understanding adsorption of CO2, N2, CH4 and their mixtures in functionalized carbon nanopipe arrays

Abstract

The selective adsorption behaviours of carbon dioxide, methane and nitrogen on bundles of functionalized CMK-5 are investigated at 303 K using grand-canonical Monte Carlo simulations. Functional groups (–OH, –COOH) cause a significant enhancement in CO₂ uptake (up to 19.5% at a pressure of 38.13 bar for –COOH). On the other hand, the adsorption amount of methane decreases with respect to bare CMK-5 by ∼13% (at 38.13 bar) upon functionalization. Furthermore, functionalized CMK-5 with different pore sizes (4 nm, 6 nm, 8 nm) and inter-tube distances (d = 0 to 1.5 nm) are used to investigate the adsorption behaviour of flue gases. While the pore diameter is seen to reduce the isosteric heat of adsorption, the inter-tube distance of 0.25 nm shows the highest uptake of CO₂ at p ≤ 18 bar, followed by 0.5 nm for the pressure range of 18 < p ≤ 30 bar, whereas for p > 30 bar, d = 1.0 nm shows the maximum uptake. For methane and nitrogen, the maximum adsorption is obtained at d = 0.25 nm in the studied pressure range. The selective adsorption of CO₂ in binary mixtures is investigated using ideal adsorption solution theory. CO₂–N₂ selectivity is found to increase significantly by surface functionalization of CMK-5 compared to pure CMK-5. The maximum selectivity of CO₂–CH₄ using –COOH functionalized CMK-5 is found to be ∼10 for an equimolar CO₂–CH₄ mixture at a pressure of 38.13 bar.