Probing the dynamics and structure of confined benzene in MCM-41 based catalysts

Combined MD simulations and QENS experiments on benzene in MCM-41 provide insight into the dynamics and structure of benzene

Diffusion of guest molecules in MCM-41 agglomerates

The pulsed field gradient nuclear magnetic resonance method has been used to study self-diffusion of cyclohexane in a commercial MCM-41 material at different external gas pressures from zero to saturated vapor pressure. It is found that the effective diffusivities exhibit three different regions with increasing pressure: decrease at low pressures, a sudden drop at intermediate pressures, and increase at higher pressures. In addition, in the region of irreversible adsorption (hysteresis loop) the diffusivities are also found to differ on the adsorption and the desorption branches. A simple analytical model taking account of different molecular ensembles with different transport properties due to the complex architecture of the porous structure is developed which provides a quantitative prediction of the experimental data. The analysis reveals that the effective diffusivity is predominantly controlled by the adsorption properties of the individual mesoporous MCM-41 crystallites which, in combination with high transport rates, provide a simple instrument for fine tuning of the transport properties by a subtle variation of the external conditions.

Structure and diffusion characterization of SBA-15 materials

In situ formation of the micro- and mesoporous structures of SBA-15 materials was investigated. It was found that the structure is significantly different from that for cylindrical or hexagonal pores, which suggests that the SBA-15 is more complex than an array of hexagonally ordered channels. Nitrogen adsorption isotherms at 77 K provided evidence that large (primary) mesopores are accompanied by a certain amount of significantly smaller pores with a broad distribution in the micropore/small-mesopore range within the mesoporous walls of main channels. It was found that the microporosity can be controlled by the time of heating as well as the synthesis temperature. The diffusion properties of n-heptane as a probe molecule in four selected SBA-15 samples with different micropore volumes were studied by the standard zero length column technique and related to their structural characteristics. The results have shown that the diffusion process involving n-heptane at a low concentration level takes place inside the walls of main mesoporous channels and depends on the relative content of micropores. In the samples that have a relatively high content of micropores, n-heptane diffusivities are relatively low, their activation energies are high, and the process is similar to diffusion in typical microporous adsorbents, like zeolites. As the micropore content is decreased, diffusion becomes more and more controlled by secondary mesopores of the intrawall pore structure, rendering diffusion faster and activation energies lower.