Sorption of butane on carbon multiwall nanotubes at room temperature

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,(1-3) neon,(4-6) helium,(6) nitrogen,(7) and methane(8-10) 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.(11) Experiments have established that refrigerant mixtures, such as CHF2CF3 and CClF2CF3, can be successfully separated by using carbon nanotubes.(12) 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).(13) 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.