Molecular cage nestling in the liquid-phase adsorption of n-alkanes in 5A zeolite

The liquid-phase adsorption Of C-5-C-24 linear alkanes in zeolite 5A at room temperature was studied using the configurational bias Monte Carlo molecular modeling technique and the batch experimental method. Up to C-16, a highly discontinuous variation of the saturation capacity with the carbon number occurred. Whereas the shortest chains remain as a whole within the cage, longer chains are distributed in a more bent configuration over adjacent cages or even adopt highly coiled configurations within one cage. For alkanes greater than C-16, a sharp, unexpected drop in adsorption capacity with a minimum at C-18-C-19 was observed experimentally. The almost complete exclusion of these alkanes was not observed in the molecular simulation under equilibrium conditions and is explained by an extremely slow diffusion of C-18-C-19 because of "cage nestling", i.e., an imperturbable adsorption of tightly fitting highly coiled n-alkanes in the first supercage they enter. The competitive adsorption and counterdiffusion of alkane mixtures is strongly affected by the differences in adsorption mechanism for alkanes of different chain length.