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Adsorption of Sulfur Hexafluoride on Mcm-41: Predictions from Molecular Simulations

Aurelio Olivet, Institut de Cičncia de Materials de Barcelona, Consejo Superior de Investigaciones Científicas (ICMAB-CSIC) and MatGas Research Center, Campus de la UAB, Bellaterra. Barcelona, 08193, Spain and Lourdes F. Vega, MatGas Research Center and Air Products and Chemicals, Inc, Campus de la UAB., Bellaterra. Barcelona, 08193, Spain.

Sulfur Hexafluoride (SF6) represents the best insulating gas available in the electrical industry, with excellent dielectric and arc-quenching properties. It is also used in several other applications, including thermo-acoustic insulator for windows, as a contract agent in medical applications, etc. In spite of these properties, SF6 is an efficient infrared absorbent, and due to its high chemical stability its atmospheric degradation occurs very slowly, hence, efficient methods are under development for handling and recovering SF6 after industrial uses. One of these methods is related to the optimal adsorption of SF6 in appropriate adsorbent materials. For this reason, several adsorption studies have been carried out on different micro and mesoporous materials. Since SF6 molecules are chemically inert and non-polar, no specific acid-base or pole-pole interactions come out between them and the porous solids commonly used in adsorption systems. Moreover, SF6 molecules are symmetric, large in size and highly polarizable, therefore, their interactions with adsorption sites are dispersive in nature [1]. These features simplify the development of theoretical SF6 adsorption studies, since no complex models have to be used to represent the adsorbate-adsorbate and adsorbate-solid interactions. The goal of the present work is twofold: a) to predict the adsorption capacity of SF6 in the Mobyl Catalytic Material MCM41, and b) to compare the performance of two SF6 force fields in confined media. We have developed a protocol to analyze the Pore Size Distribution (PSD) of structurally different silica-based materials, including the MCM41, by a combination of Grand Canonical Monte Carlo (GCMC) simulations and experimental nitrogen adsorption isotherms [2]. In this work, the PSD obtained for this material is combined with GCMC simulations results of SF6 in cylindrical silica pores, which were modeled by using the structure-less potential proposed by Tjatjopoulos and co-workers [3] in order to predict the overall adsorption behavior of SF6 on this material. For this purpose, SF6 was modeled using the rigid molecular force field proposed by Pawley [4], and also a flexible force field recently proposed by some of us [5]. Discussions regarding the influence of choosing different molecular models for the fluid and their impact on the adsorption behavior are presented. Since the calculated PSD used in this work is in excellent agreement with the one reported in literature for the MCM41 material, we expect a reasonable prediction for the SF6 adsorption.

This work has been partially financed by the Spanish Government (project CTQ2005-00296/PPQ) and the Catalan Government (SGR2005-00288). Additional funding from MATGAS as PhD grants are gratefully acknowledged.

[1] D. V. Cao and S. Sircar. Adsorption 2001, 7, 73-80.

[2] C. Herdes, M. A. Santos, F. Medina and L. F. Vega. Materials Science Forum 2006, 514-516, 1396-1400.; Langmuir 2005, 21, 8733-8742.

[3] G. J. Tjatjopoulos, D. L. Feke and J. A. Mann. Journal of Physical Chemistry 1988, 92, 4006-4007.

[4] G. S. Pawley. Molecular Physics 1981, 43, 1321-1330.

[5] A. Olivet and L. F. Vega. Journal of Chemical Physics 2007, 126, 144502.