Influence of channel geometry on sorption in zeolites

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Abstract
In this paper we study the sorption of guest molecules in porous media of well-defined structure, specifically in zeolite A and faujasite. We introduce the concept of a site-specific maximum sorption time τi, defined as the average time required for an internalized diffusing species, starting from any cavity site i, to reach the cavity farthest removed from the surface of the zeolite, viz. the one at the geometric centre of the assembly. When averaged over all cavity sites accessible initially to the guest molecule, the consequent mean sorption time τ reflects the role of different spatial distributions of cavities and attendant channel patterns in influencing the efficiency of sorption in these systems. We study how this characteristic time τ changes as a function of the number of available cavities and the nature of the ambient conditions to which the guest/host system is exposed (two ‘boundary conditions’ are examined). We show that the enhancement in the efficiency of the sorption process when faujasite is replaced by zeolite A is bracketed between the values ∼ 110 per cent (for the smallest crystallites) and 14.2 per cent (for the very largest ones). Further studies document how this picture changes when sorption in any polyhedral cavity can take place. Let s i be the probability of sorption of a diffusing species in cavity i; for the case, all s i ≡ s > 0, we show quantitatively that the characteristic, maximal sorption time τ behaves like τ ⋍ 1/s for both zeolite structures, independent of the boundary (ambient) conditions. We explore quantitatively how the presorption of polar molecules (such as H2O and NH3) can influence the diffusivity of nonpolar molecules (such as Ar, H2, O2) by obstruction of blockage of the polyhedral cavities. If f is the fraction of cavities that remain unblocked when presorption of polar molecules occurs, we illustrate the extent to which differences between zeolite structures are suppressed with increase in the fraction f. Finally, we comment on the implications of these results for the study of diffusion-controlled reactive processes in zeolites.