Separation of CO2–CH4 mixtures on defective single walled carbon nanohorns – tip does matter

Abstract

Using realistic models of single-walled carbon nanohorns and their single-walled carbon nanotube counterparts, we study the equilibrium separation of CO₂–CH₄ mixtures near ambient operating conditions by using molecular simulations. We show that regardless of the studied operating conditions (i.e., total CO₂–CH₄ mixture pressures and mole fractions of mixture components in the bulk phase), single-walled carbon nanohorns maximize the CO₂–CH₄ equilibrium separation factor. Optimized samples of single-walled carbon nanohorns consisting of narrow tubular parts capped with horn-shaped tips show highly selective adsorption of CO₂ over the CH₄ mixture component, with the CO₂–CH₄ equilibrium separation factor of ∼8–12. A large surface-to-volume ratio (i.e., enhanced surface forces) and unique defective morphology (i.e., packing of adsorbed molecules in quasi-one/quasi-zero dimensional nanospaces) of single-walled carbon nanohorns are their key structural properties responsible for the excellent separation performance. Our theoretical simulation results are in quantitative agreement with a recent experimental/theoretical study of the CO₂–CH₄ adsorption and separation on oxidized single-walled carbon nanohorns [Ohba et al., Chem. Lett., 40, 2011, 1089]. Both experiment and theory showed that the CO₂–CH₄ equilibrium separation factor of oxidized samples of single-walled nanohorns measured near ambient operating conditions is ∼2–5. This reduction in the separation efficiency as compared to optimized samples of single-walled carbon nanohorns is theoretically justified by their lower surface-to-volume ratio (i.e., larger diameters of tubular parts and horn-shaped tips).