2D Surface Structures in Small Zeolite MFI Crystals

Abstract

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 < 100). 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 = τSurface/τBulk) 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.

Probing the Nature of Surface Barriers on ZSM‐5 by Surface Modification

“…A similar relation between zeolite size and apparent diffusivity was found by Dauenhauer et. al [7,8,13] as well as Gobin et al [9] The activation energies for SM-5@1200, ZSM-5@540, and ZSM-5@215 (26.7, 25.3 and 27.1 kJ mol -1 ) are very close, while the one (31.7 kJ mol -1 ) for ZSM-5@50 is 17 -25 % higher than the other ones.…”

“…Surface pore blockage exists when amorphous phase (i.e., SiO 2 ) covers the external surface of zeolites. This blockage potentially extends the diffusion path length, because additional movements are required before the probe molecule finds an open surface pore, however, the blockage should not change the activation energy [7,8,13]. In this work, HF acid was used to dissolve the amorphous phase on ZSM-5@215, and no reduction in surface barriers was found.…”

“…4c). These surface layers may be rich in structural imperfections, especially surface pore narrowing and misalignment, which can increase the activation energy [8,28].…”

“…For silicalite-1 with MFI structure, Dauenhauer et al [7,8,13] attributed its surface barriers primarily to the blockage of surface pores and estimated that over 99.9 % of its surface pores could be blocked, using ZLC and FR experiments, as well as molecular simulations. For Zn(tbip), a nanoporous metal-organic framework (MOF), Kärger et al [14,15] reached a similar conclusion that the surface barriers on this material arise from the complete blockage of surface pores, using microscopic diffusion measurements and mesoscopic modeling.…”

“…Mechanistically, the surface pore blockage could originate from the presence of an amorphous phase or uncoordinated lattice on the external surface, which has interpenetrating dangling silanol bonds [16,17]. Besides, the surface pore blockage could also arise from structural imperfections, including bridging across pores, surface pore narrowing, and pore misalignment, although existing experimental techniques are still unable to directly observe these imperfections [8,13].…”