2D Surface Structures in Small Zeolite MFI Crystals

Utilization of new hierarchical zeolites comprised of small crystallites (<50 nm) provides enhanced processing rates in separation and catalytic applications. However, molecular permeation through surface pores in small zeolite crystals has been shown to control the overall rate, resulting in apparent site-to-site intracrystalline kinetics that vary by as much as three orders of magnitude. In this work, the external surface of small zeolite MFI crystals was characterized by combining experimental kinetics and molecular simulation to evaluate the hypothesis of a two-dimensional distribution of surface pores that are either open or closed (i.e., fraction, p open ). The movement of benzene through the MFI lattice and surface was evaluated with a discrete space kinetic Monte Carlo algorithm with varying particle sizes (1.0 nm ≤ R ≤ 640 nm) and fraction of surface pore openings (10 –4 < p open < 10 0 ). Introduction of randomly distributed surface pore blockages was shown to increase the molecular path length for adsorbates exiting the particle while maintaining a constant apparent activation energy, consistent with additional molecular motion near the internal surface. Comparison of the dimensionless surface barrier (λ = τ Surface /τ Bulk ) obtained by simulation to experimental measurements (zero length chromatography) for silicalite-1 particles (62 nm to 3 μm, 50–110 °C) reveals that the vast majority (>99.9%) of surface pores must be blocked ( p open < 10 –3 ) to justify the observed surface permeation rate of hydrocarbon adsorbates.