On Asymmetric Surface Barriers in MFI Zeolites Revealed by Frequency Response

Diffusional mass transport in porous materials is important for shape-selective catalysis and separation technologies. To maximize turnover and catalytic site accessibility, hierarchical materials are synthesized with length scales as small as single crystal lattices (∼2 nm, MFI). While these materials are potentially efficient catalysts, they have been shown to exhibit apparent diffusivities that are orders of magnitude slower than those in bulk crystals. To evaluate the dependence of apparent diffusivity with particle size, the kinetics and mechanism have been characterized by frequency response methods for cyclohexane mass transfer into and out of silicalite-1 particles varying in size over two orders of magnitude. Development of a new mass transport model utilizes data obtained by frequency response to characterize two sequential rate limitations: intracrystalline diffusion and asymmetric surface barriers. Activation energy associated with transport into the surface (Ea,s = 20.8 kJ/mol) was observed to be significantly less than that of intracrystalline diffusion and release (Ea = 53.9 kJ/mol ≈ 54.1 kJ/ mol = Ea,−s). Surface pore blockages are proposed to dominate mass transport in small zeolite particles.