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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

Recently introduced porous materials often contain interconnected networks of pores with different sizes, i.e. networks of micropores with the pore sizes approaching the sizes of gas molecules and networks of much larger meso- and/or macropores. In many cases the presence of large pores is beneficial because these pores serve as molecular “highways”, allowing fast access and removal of guest molecules to and from functional micropores where individual molecules remain in close contact with the pore walls. The knowledge of fundamentals of transport of gas and vapor molecules in such systems with a hierarchy of pore sizes is essential for their applications in catalysis and separations.  

 The research during the last year of the project was focused on studies of diffusion of small sorbate molecules (n-butane) in the pore systems of mesoporous silicas (SBA-15) containing micropores. In the studied samples the micropores serve as bridges connecting parallel cylindrical mesopores. This structure was confirmed by the measurements of nitrogen adsorption isotherms in the laboratory of Dr. Matthias Thommes, Quantachrome, FL, who has been a collaborator in this project. Diffusion studies were performed by the PI and his students using a high field (17.6 T) and high gradient (up to 36 T/m) PFG NMR spectrometer located at the he National High Magnetic Field Laboratory (NHMFL), University of Florida. The PI’s group has recently demonstrated that high field and high gradient PFG NMR at the NHMFL can be reliably used for diffusion studies of complex systems.^1,2  The PFG NMR measurements of porous samples were carried out using the 13-interval PFG NMR sequence.³ This sequence allowed eliminating disturbing influence of the magnetic susceptibility effects.  

Measurements of n-butane diffusion in SBA-15 samples have been performed in a broad range of diffusion times between around 4 and 100 ms. For small diffusion times (4 – 9 ms) the measured displacements of n-butane were smaller than or  comparable with the size of the individual SBA-15 particles. PFG NMR data obtained under these conditions allowed determining diffusivities along the direction of the mesoporous channels and the diffusivities perpendicular to this direction, i.e. along the microporous channels connecting the mesopores.⁴ The diffusion process for larger diffusion times (> 9 ms) is characterized by the presence of an additional type of diffusion, viz. the long-range diffusion.  The latter type of diffusion corresponds to sorbate transport for displacements much larger than the size of individual SBA-15 particles.⁴ For comparison, the PFG NMR diffusion studies of n-butane have also been performed in a purely microporous sample consisting of loosely packed crystals of NaY zeolite. The diffusion studies outlined above provide new insights about the interplay of diffusion in different types of pores in the porous solids with a hierarchy of pore sizes.

 References
1.         Menjoge, A. R.; Kayitmazer, A. B.; Dubin, P. L.; Jaeger, W.; Vasenkov, S., Heterogeneity of Polyelectrolyte Diffusion in Polyelectrolyte Protein Coacervates: A 1H Pulsed Field Gradient NMR Study. J. Phys. Chem. B 2008, 112, 4961-4966.
2.         Ulrich, K.; Sanders, M.; Grinberg, F.; Galvosas, P.; Vasenkov, S., Application of PFG NMR with High Gradient Strength for Studies of Self-Diffusion in Lipid Membranes on the Nanoscale. Langmuir 2008, 24(14), 7365-7370.
3.         Cotts, R. M.; Hoch, M. J. R.; Sun, T.; Markert, J. T., Pulsed field gradient stimulated echo methods for improved NMR diffusion measurements in heterogeneous systems. J. Magn. Reson. 1989, 83, 252-266.
4.         Menjoge, A. R.; Sanders, M.; Thommes, M. Diffusion anisotropy in micropore-containing SBA-15. Microporous and Mesoporous Materials, in preparation, 2008.

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