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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 t...

Development of microporous materials with hierarchical structures of both micro/mesopores leads to molecular transport at nanometer length scales. For novel microporous materials including three-dimensionally ordered mesoporous imprinted (3DOm-i) zeolites and zeolite nanosheets, particle dimensions are below 35 nm resulting in surface-dominated structures. At the same time, the existence of surface-controlled mass transport including undefined “surface barriers” has been observed to reduce apparent diffusivity of hydrocarbons by orders of magnitude. This paper systematically characterizes cyclohexane transport in silicalite-1 by zero length chromatography (ZLC) to determine apparent diffusivity varying over 3 orders of magnitude in particles ranging from 35 nm to 3 μm. Three proposed mechanisms for surface barriers including surface pore narrowing, surface pore blockage, or surface desorption are evaluated by comparison with particle-size/diffusivity data. It is concluded that transport control in small p...