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44939-G5
Molecular Transport on Various Length Scales in Novel Nanostructured Porous Materials with a Hierarchy of Pore Sizes

Sergey Vasenkov, University of Florida

Recent development of porous materials containing interconnected networks of meso- and/or macropopres in addition to microporous nanodomains or nanocrystals has the potential to drastically improve industrial applications relying on the use of micropores. Examples of such applications include catalysis, mass separations and molecular storage. Diffusion of guest molecules in pore networks with a hierarchy of pore sizes is essential for most of these applications. Large pores in hierarchical porous materials can serve as molecular “highways”, allowing fast access and removal of sorbate molecules to and from functional micropores. Due to a large ratio of the external surface of microporous nanodomains or nanocrystals to their volume the role of transport of sorbate molecules through the external surface in the overall, i.e. long-range transport in hierarchical materials becomes especially important. In our research we have developed a new approach to measure permeability of the external surface of microporous crystals. This approach takes advantage from a combined application of dynamic Monte Carlo (MC) simulations and pulsed field gradient (PFG) NMR measurements. It is based on correlating the following two quantities directly measurable by PFG NMR: (i) effective diffusivity of guest molecules, which at the beginning of a PFG NMR diffusion measurement started their trajectories inside a particular zeolite crystal (i.e. parent crystal) and remain in the same crystal by the end of the measurement, and (ii) fraction of guest molecules that remain in the same (parent) crystal over the time of the diffusion measurement. Comparison of the correlation curves for these two quantities, which are measured by PFG NMR, with the corresponding results of MC simulations allows obtaining permeability of the external surface of microporous crystals. This new method was applied to measure surface permeability of A-type zeolite crystals with different crystal sizes. For the studied samples of A-type zeolites the measured permeability of the crystal surface was found to be essentially independent of the size of the crystals. The measurements were performed in collaboration with the group of Prof. Joerg Karger, University of Leipzig, Germany. Studies of the role of the surface permeability in the overall transport in zeolite beds are currently in progress.

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