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Carbon multiwall nanotubes (MWNTs) can be used for separation processes if the mechanisms for sorption and desorption are known. This study describes the sorption mechanism for butane on MWNTs at room temperature and relative pressures ranging from 0 to 0.9. Previous workers have studied the sorption of hydrogen, − neon, − helium, nitrogen, and methane8-10 on nanotubes for storage purposes. Molecular dynamic simulations have been done to show that carbon nanotubes can be used as a separation tool to selectively separate isomers of monomethylnaphthalenes. Experiments have established that refrigerant mixtures, such as CHF2CF3 and CClF2CF3, can be successfully separated by using carbon nanotubes. Previous work in this lab has shown that carbon MWNTs can separate butane from methane when both are at low levels in a gas flow. This experimental result is in agreement with recent molecular dynamic simulations made for sorption of alkane mixtures on different types of single-walled carbon nanotubes (SWNTs). Morphology characterization of the MWNTs has been used to interpret the sorption data. Most of the butane was sorbed to the external surface of the MWNTs and only a small fraction of the butane condensed in the pores. No hysteresis was observed between sorption and desorption experiments. The weight fraction of butane sorbed depended inversely on the diameter of the MWNTs and was 5.3 wt % for one of the samples studied. Adsorption isotherms were modeled using a modified BET equation with coefficients consistent with the known morphology. Fixed bed adsorption systems that could use the exterior surface of MWNTs might be attractive for separations, particularly if electrical heating could be used for rapid desorption of sorbed molecules.

We report the isosteric heats of adsorption, q st , for butane on multiwalled carbon nanotubes (MWNTs) over a range of surface loadings at temperatures below the normal boiling point. Butane is a nonspherical, nonpolar molecule that may exhibit different orientations in multilayer adsorption. The morphology of our MWNTs is such that only surface adsorption or capillary condensation can occur: no interstitial sites between nanotubes are available to butane. For these nanotubes, exterior surface adsorption is the principal mechanism measured by gravimetric analysis as the internal pores are only a small volume fraction of the total solid volume. The isosteric heat of adsorption varied with the surface coverage, θ, defined as the ratio of adsorbed butane to the MWNT butane monolayer capacity. The initial heat of adsorption was in the region ∼22-26 kJ/mol, which is approximately 1 / 3 lower than the corresponding value for various graphite-butane systems. At θ ) 1, q st displays a minima (13.5-15.9 kJ/mol). The isosteric heat of adsorption approaches the butane heat of condensation for θ > 2.5, suggesting that the surface film is similar to a bulk phase. The isosteric heat of adsorption of butane on MWNTs can be related to the morphology of the MWNTs. The higher heats of adsorption at low coverages are likely related to the presence of surface defects. The minima in q st near the monolayer coverage relates to a high self-association of butane. At high loadings, the butane surface film is similar to a bulk phase.