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 sorptio n 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 do ne to show that carbon nanotubes can be used as a separation tool to select ively separate isomers of monomethylnaphthalenes.(11) Experiments have esta blished that refrigerant mixtures, such as CHF2CF3 and CClF2CF3, can be suc cessfully separated by using carbon nanotubes.(12) Previous work in this la b has shown that carbon MWNTs can separate butane from methane when both ar e at low levels in a gas flow. This experimental result is in agreement wit h recent molecular dynamic simulations made for sorption of alkane mixtures on different types of single-walled carbon nanotubes (SWNTs).(13) Morpholo gy characterization of the MWNTs has been used to interpret the sorption da ta. 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 w as observed between sorption and desorption experiments. The weight fractio n 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 m orphology. Fixed bed adsorption systems that could use the exterior surface of MWNTs might be attractive for separations, particularly if electrical h eating could be used for rapid desorption of sorbed molecules.