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 [similar]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 [similar]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).