Concerted Diffusion of Molecular Clusters in a Molecular Sieve

We show that attractive interactions between molecules adsorbed in molecular sieves play a vital role in the mechanisms and rates of intracrystalline diffusion. Over a wide temperature range, the single-file diffusion of CF₄, SF₆, and CCl₄ in AlPO₄- $5$ is dominated by the diffusion of molecular clusters. SF₆ and CCl₄ clusters diffuse by concerted mechanisms involving all of the cluster's molecules simultaneously. The activation energies, Eₐ, for these mechanisms are strongly size dependent and are lower than Eₐ for monomer diffusion.