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The diffusion of alkyl-substituted aromatic molecules in two H-ZSM5 samples consisting of very small (<100 nm) and large (∼3-4 µm) primary particles was investigated by means of the frequency response method. Analysis of the experimental data in combination with the theoretical modeling of the complete transport network shows that the details of the transport processes and the rate determining step depend on the particle size. With large particles, intracrystalline diffusion was rate determining for the overall diffusion, while for the small particles the diffusion was controlled by surface effects.

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Fast, time-resolved infrared spectroscopy was utilized to determine the transport and sorption properties of benzene, toluene, and p-xylene on a series of zeolites with increasing degree of surface modification. Postsynthetic modification of H-ZSM5 with a mesoporous silica overlayer generates novel hierarchical materials, which significantly increase the sorption rates for benzene and decrease it gradually for alkyl-substituted aromatic molecules with increasing radius of gyration of the aromatic molecule. The ratio between the sorption rates of benzene and p-xylene increased from an initial value of 4.3 to about 27 after modification. The reason for the enforced differentiation is the combination of the intrinsic size exclusion properties of the zeolite micropores and the variation of the sticking probability of the molecules on the modified surface, which enhanced the mass transfer into the porous overlayer. The size of the overlayer pores and the radius of gyration of the sorbate were identified as the parameters determining the sorption rates. Our results highlight that hierarchical composites generated by deposition of highly porous silica overlayers on microporous materials allow tailoring the separation properties of porous materials and allow introducing a new concept to kinetically separate molecules of identical minimum (kinetic) diameters.

The elementary steps of the sorption of aromatic molecules such as benzene, toluene, p-xylene, and o-xylene on nonporous amorphous SiO2 (Aerosil) and microporous silicas using HZSM-5 as an example are studied by time-resolved rapid scan IR spectroscopy. Sorption into the zeolite pores proceeds via a weakly bound physisorbed and nonlocalized state on the external surface as the dominating reaction pathway. The weak interaction leads to very low sticking probabilities on the order of 10-7 for porous and nonporous materials alike. Within the molecules investigated, the sticking coefficient increases in the series toluene, o-xylene, benzene, and p-xylene. Using a statistical thermodynamic analysis, this sequence is attributed to the symmetry of the sorbate molecule, the sorption enthalpy of the sorbate increasing with the molar mass, and the space the sorbate molecule occupies on the surface.