471516 Understanding of Diffusion Pathway of Cyclohexane through Nanoscale MFI Zeolite
Department of Chemical Engineering, University of Massachusetts Amherst, 686 N Pleasant Street, Amherst, 01002
Zeolites have been widely used in the fields of shape selective catalysis and separation due to their unique molecular sieving property. During catalytic reactions, reactants diffuse into pore network, react at active site, and diffuse out of pore network. To minimize the effects of internal mass transfer limitation on the overall reaction rate, nanoscale zeolite crystals have been designed and synthesized.1 However, within small zeolite crystals, diffusion study has shown that the apparent diffusivity appears to be orders of magnitude smaller than that for large zeolite crystals2. This discrepancy of apparent diffusivity has led to the phenomena called “Surface Barrier”, which is a general term to describe a mass transfer resistance near the surface of zeolites. For large zeolite particles, since the internal microspore diffusion is dominant, such a surface resistance is often negligible. While for small zeolite particles, the surface resistance can contribute significantly to the overall mass transfer rate.
To explain the origin of the surface barrier, pore blockage and pore narrowing mechanisms have been proposed2. However, with such hypotheses, Kinetic Monte Carlo simulation suggested 99.9% of the micropores on external surface have to be blocked3. In this presentation, we propose a new “Pore Re-entering” mechanism. According to this mechanism, molecules undergo a surface diffusion on the external surface during the desorption step. The surface diffusion is much slower than the Knudsen diffusion. Diffusion on the external surface might lead to re-entering of micropore openings, which significantly increase apparent diffusion length for the molecules. In order to demonstrate the existence of such effects, silcalite-1/silica nanoparticle mixtures were developed where silicalite-1 particles were isolated from each other by 35 nm solid silica nanoparticles. Mass transport study using cyclohexane suggested that the diffusion of cyclohexane is still controlled by the micropore diffusion with the same activation energy, but much faster in the mixtures, providing a strong evidence for the existence of the Pore Re-entering effect. The observation provides critical information for the rational synthesis of hierarchical porous materials for the applications in separation and catalysis.  Chang, C.-C.; Teixeira, A. R.; Li, C.; Dauenhauer, P. J.; Fan, W., Enhanced Molecular Transport in Hierarchical Silicalite-1. Langmuir 2013, 29, (45), 13943-13950.
 Teixeira, A. R.; Chang, C.-C.; Coogan, T.; Kendall, R.; Fan, W.; Dauenhauer, P. J., Dominance of Surface Barriers in Molecular Transport through Silicalite-1. The Journal of Physical Chemistry C 2013, 117, (48), 25545-25555.
 Teixeira, A. R.; Qi, X. D.; Conner, C. W.; Mountziaris, T. J.; Fan, W.; Dauenhauer, P. J., 2D Surface Structures in Small Zeolite MFI Crystals. Chemistry of Materials 2015 27 (13), 4650-4660