Investigation of adsorption hysteresis in microporous materials

The adsorption and desorption isotherms of nitrogen, methane, benzene, cyclopentane, p-xylene, and p-dichlorbenzene in either silicalite-1 or AIPO(4)-5, in which hysteresis loops were found, have been investigated experimentally and theoretically. The experimental isotherms indicated that a hysteresis loop call be obtained in the adsorption/desorption isotherms of benzene in silicalite-1 at 303 K with loadings in the range of 6 to 8 molecules per unit cell (m./u.c.). The isotherms of cyclopentane, a molecule similar to benzene, however, exhibits hysteresis at loadings between ca. 9 to 12 m./u.c. and at temperatures around 273 K. For p-xylene and p-dichlorbenzene, two molecules with size and shape very similar to one another, the hysteresis loops are found in the isotherms in silicalite-1 at 323 K with loadings over the range of 4 to 8 m./u.c. In the case of the small molecules, methane shows a hysteresis loop at loadings between 4 to 6 m./u.c. in AlPO4-5 at 96.5 K and nitrogen shows a hysteresis loop with loadings in the range of 26 to 31 m./u.c. in silicalite-1 at 77 K. The fundamental nature of this adsorption hysteresis is described in this work using both experimental results and molecular simulation calculations.