Activated Transport of Molecular Clusters in
Molecular Sieve Micropores
David S. Sholl
Dept. of Chemical Engineering, Carnegie Mellon University
Pittsburgh, PA 15213-3890
(Extended abstract for presentation K7.2 at 1998 MRS Fall Meeting)
Understanding the transport properties of molecules adsorbed in molecular sieves is of importance for the successful application of these materials as selective adsorbents and in microporous membranes. Considerable attention in recent years has focused on the single-file diffusion of molecules in unidimensional micropores, in which adsorbates cannot pass one another. Much of this attention has been motivated by recent experimental measurements of single-file diffusion which, in addition to providing the first direct examples of this phenomena, have yielded unusually large estimates for the diffusion rate of single molecules in unidimensional micropores .
Most theories of single-file diffusion assume that the diffusion can be described in terms of activated single-molecule processes. Detailed models of single-file diffusion in AlPO4-5 indicate that this assumption is incorrect, since molecular transport also occurs by concerted motions of weakly-bound clusters of adsorbed molecules . To develop accurate coarse-grained models of transport by single-file diffusion, accurate characterization of cluster diffusion and dissociation is necessary. Transition State Theory can be applied to these clusters at low temperatures, but this approach cannot be extended to the temperatures of experimental interest. Molecular Dynamics simulations have been used to investigate the activated diffusion and dissociation of isolated metastable clusters in the pores of AlPO4-5. The processes observed in these simulations (and their rates) provide a basis for constructing coarse-grained models that can in turn be used to examine the experimentally relevant situation of a pore containing many interacting clusters.
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