Encapsulation of He and Ne in Carbon Molecular Sieves

The adsorption−desorption pattern of He and Ne from amorphous carbon molecular sieve fibers (CMSF) was found to be governed by an encapsulation mechanism. He and Ne undergo reversible and efficient entrapment by the micropores of CMSF. Selective adjustment of pore openings to meet the critical dimensions of He and Ne which allow their admission is achieved via two principal steps: (i) the well-known pore widening by means of an irreversible removal of surface oxide groups upon evacuation at elevated temperatures and (ii) regulation of the effective pore opening via reversible thermal dilation/contraction. The occurrence of a markedly high activation energy, which previously could not be solely attributed to pure adsorption, is now understood as that needed for overcoming the energetic barrier imposed by the thermally constricted pores. The present results imply that the encapsulation phenomenon can considerably affect dead volume measurements in porous materials.