(395d) Understanding Unique Diffusion Behavior in Hierarchical Zeolites

Hierarchical zeolites are materials that contain both micro (<2 nm) and meso (2—50 nm) porosity and are proposed as potential solutions to alleviate transport limitations of conventional microporous zeolites. The introduction of mesopores adds considerable complexity to the adsorption and transport behavior exhibited by these hierarchical materials.  Both experimental [1] and computational [2] studies demonstrate a crossover phenomenon where the sorbate loading in the conventional zeolite exceeds that in the hierarchical zeolite at low pressure, but the ordering is reversed at high pressure.  Similarly, experimental measurements [3] point to unexpected transport behavior. In this talk, we present a molecular simulation study on the diffusion of linear and branched alkanes in MFI-type zeolites. Diffusion coefficients are computed at multiple temperatures from 363 – 543 K and as a function of loading. The comparison between the diffusional behaviors in conventional and hierarchical materials reveals surprising features in the latter systems, and we offer explanations in terms of free energy profiles in the two types of materials.

References:

[1] D. Xu, G. R. Swindlehurst, H. Wu, D. H. Olson, X. Zhang, M. Tsapatsis, Adv. Func. Mater. 2014, 24, 201 –208.

[2] P. Bai, D. H. Olson, M. Tsapatsis, J. I. Siepmann, ChemPhysChem 2014, 15, 2225–2229.

[3] C. C. Chang, A. R. Teixeira, C. Li, P. J. Dauenhauer, W. Fan, Langmuir 2013, 29, 13943−13950.