(311f) Diffusion of Hydrocarbons In SSZ42 Materials

The novel high-silica zeolite SSZ-42 was firstly synthesized in 1997 [1]. It is a representative of an undulating one-dimensional, 12-membered-ring channel system. Although SSZ-42 is one-dimensional material, it has been shown to have remarkably high adsorption capacities for n-hexane, 2,2-dimethylbutane and cyclohexane [2]. The material is also stable up to at least 800 oC under thermal and hydrothermal conditions [2]. These properties suggest that SSZ-42 could be a promising material as catalyst for hydrocarbon processing, and as adsorbent for adsorption separation.

To explore the potential application of SSZ-42 material in adsorption separation and catalysis involving large molecules, it is essential to carry out kinetic characterizations. The present study focuses on a systematic investigation of diffusion properties involving selected organic vapors in SSZ-42 materials, by employing the zero length column (ZLC) technique. Toluene, o-xylene and cumene were used as probe molecules of different (increasing) kinetic diameters, since they are important representatives of aromatics. Two samples of boron SSZ-42 materials (Na- and H-forms designated as SSZ-42-Na+ and SSZ-42-H+, respectively) have been synthesized by using N-benzyl-1,4-diazabicyclo[2.2.2]octane cation as the template molecule following hydrothermal crystallization [2]. The diffusion results reveal that transport processes of all probe molecules in SSZ-42-Na+ and toluene in SSZ-42-H+ were controlled by pore diffusion. On the other hand, transport of both o-xylene and cumene in SSZ-42-H+ was found to be controlled by surface barrier resistance at low temperatures, but pore diffusion resistance became more dominant with the increasing temperature. The transport mechanisms of probe molecules in both samples were verified by the partial saturation method proposed by Brandani and Ruthven [3]. The ion-exchange process may lead to a partial collapse of pore channels of SSZ-42-H+ leading to surface barrier effects, while at relatively higher temperatures pore openings appear to be expanding, rendering negligible barrier resistance. The diffusivities of probe molecules in SSZ-42-Na+ were systematically higher in comparison with SSZ-42-H+, while the activation energies showed the opposite trends. This confirms that the probe molecule interactions with the Bronstead acid sites cause a decrease in the diffusion rate, consequently requiring more energy for the activated transport.

References:

[1] Chen, C.Y., Finger, L.W., Medrud, R.C., Crozier, P.A., Chan, I.Y., Harris, T.V. and Zones, S.I., ?SSZ-42: the first high-silica large pore zeolite with an undulating, one-dimensional channel system,? Chem. Commun., 1775-1776 (1997).

[2] Chen, C.Y., Finger, L.W., Medrud, R.C., Kibby, C.L., Crozier, P.A., Chan, I.Y., Harris, T.V., Beck, L.W. and Zones, S.I., ?Synthesis, structure, and phsicochemical and catalytic characterization of the novel high-silica large-pore zeolite SSZ-42,? Chem. Eur. Journal, 4, 1312-1323 (1998).

[3] Brandani, S. and Ruthven, D. M., ?Analysis of ZLC desorption curves for gases systems,? Adsorption, 2, 133-143 (1996).