Adsorption of alkanes, alkenes and their mixtures in single-walled carbon nanotubes and bundles

Abstract

Monte Carlo simulations are employed to calculate pure component adsorption isotherms of linear alkanes (C2–C12), alkenes (C2–C4) and some of their binary mixtures (ethane–ethene, propane–propene, cis-2-butene–trans-2-butene, propene-1-butene) in single-walled carbon nanotubes. The zigzag structures of carbon nanotubes (CNTs) of various diameters [(10,0), (20,0), (30,0) and (40,0)] are used. Furthermore, Henry coefficients and isosteric heats of adsorption are calculated. The dependence of these properties as a function of chain length (carbon number) is presented. The relation of the critical parameters and the isosteric heats of adsorption, observed earlier for zeolites, could be confirmed for CNTs. The adsorption behaviour of 1-butene, cis-2-butene and trans-2-butene are compared in detail. Radial density profiles of 1-butene in a (40,0) nanotube for various pressures reveal a build-up of three layers inside the pores with increasing pressure. For all investigated binary mixtures, one of the component isotherms shows a distinct maximum owing to an entropic effect and non-idealities of the bulk gas phase behaviour. Additionally, adsorption in CNT bundles in hexagonal arrangement is studied. Depending on the pore arrangements, pore diameters and pressures, a fraction of the adsorbed gases is located in the interstitial space.

Keywords:

• carbon nanotube
• adsorption
• alkanes
• alkenes
• molecular simulations

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (DFG) in priority program SPP 1155. We thank Niels Hansen for help with IAST calculations.