Caractérisation structurale de l'adsorption des isomères para- et meta- du xylène dans la zéolithe de type faujasite BaX Structural Characterization of Para- and Meta- Xylene in Bax Zeolite

Abstract

La séparation du para-xylène des isomères aromatiques en C8 est réalisée industriellement grâce à l'adsorption sur tamis moléculaire zéolithique. Une amélioration des propriétés de séparation des tamis, et en particulier de leur sélectivité, nécessite, entre autres, une bonne connaissance des interactions entre les molécules d'adsorbat et la structure zéolithique. Pour ce faire, nous avons fait appel à deux techniques de caractérisation physico-chimique : la diffraction des neutrons et la spectroscopie infrarouge. Nous avons étudié une zéolithe BaX de type faujasite sur laquelle nous avons adsorbé, à l'état de corps purs, les isomères para- et méta- du xylène. Cette zéolithe est connue industriellement pour ses propriétés sélectives performantes pour le paraxylène. Les analyses ont été réalisées en considérant tout d'abord un faible taux de remplissage, voisin d'une molécule par supercage de zéolithe, et ensuite à saturation où la zéolithe contient sensiblement trois molécules par supercage. La diffraction des neutrons permet, à basse température, de localiser les molécules dans la zéolithe et de préciser leur interaction avec le cation Ba²+. La spectroscopie infrarouge permet une étude des caractéristiques vibrationnelles de l'adsorbat en fonction du taux de recouvrement. Une synthèse des résultats obtenus à l'aide de ces deux méthodes d'investigation nous a permis de dégager un modèle de remplissage des supercages pour les deux isomères considérés. Ainsi, des différences significatives sont mises en évidence. En ce qui concerne le para-xylène, pour un taux de remplissage inférieur à deux molécules par supercage, les molécules de para-xylène viennent se positionner au voisinage des cations en site SII de la supercage. La troisième molécule de para-xylène introduite dans la zéolithe n'est pas en interaction avec un cation Ba²+ et sera localisée dans un nouveau site F. Pour les molécules de méta-xylène, quel que soit le taux de remplissage, les sites occupés sont à proximité des cations (sites SII); lorsque la troisième molécule est introduite, un réarrangement moléculaire est observé. The separation of para-xylene from C8 aromatic by adsorption on a molecular sieve is a field of research in which much work has been done at Institut Français du Pétrole (IFP) in recent years. With a view to obtaining a better understanding of the phenomena involved in this separation, the ensuing research aims to characterize the adsorption of para-xylene and meta-xylene isomers in the state of pure bodies in a BaX zeolite, which is a sieve recognized for its high-performance selective properties during competitive adsorptions. The originality of our approach consists in characterizing, on a molecular scale, the adsorption of two isomers in the zeolitic network so as to work out a molecular filling model of the BaX zeolite. Two principal techniques, infrared spectroscopy and neutron diffraction, were chosen for analyzing each of the two isomers, the adsorbate-adsorbent system. The infrared properties of the adsorbate provide exact information concerning the local environment of the xylene molecule in the zeolite as well as on the existence of adsorbate-adsorbent and adsorbate-adsorbate interactions. Infrared spectroscopy was used to examine both the influence of adsorption on the vibrationel properties - integrated frequencies and adsorbances of fundamental modes - of the xylene molecules and the way these same properties evolve as a function of the zeolite coverage. At the same time, neutron diffraction was used to determine, atome by atome, the chrystallographic position of the xylene molecules in the zeolitic network as a function of the coverage. Two coverages were examined, corresponding to 1 mol/sc (molecule per supercage) and 3 mol/sc. One of the major consequences was the gaining of an exact knowledge of the interatomic distances and hence of the interactions involved between the adsorbed molecule and the zeolite. The different adsorption sites of para-xylene and meta-xylene were thus characterized on the molecular scale for low coverage and at saturation. A very good correlation was thus obtained between infrared analysis and crystallographic analyses. The changes in the crystallographic positions of the adsorbed molecules, during the filling of the zeolite, could effectively be associated with concomitant changes in the infrared properties of the adsorbate. A filling model of the BaX zeolite was then worked out with the help of an analysis of the adsorbate-adsorbate interactions caused by the increase in the coverage. For both isomers, it appears that the increasing steric hindrance determines the relative arrangement of the molecules in the supercage. We thus confirmed that the crystallographic positions revealed by diffraction can be used to obtain the optimum arrangement of the adsorbate molecules so as to minimize intermolecular repulsions. For para-xylene, there are two distinct filling stages of the BaX zeolite. - In the mol/sc domain (0-2), all the para-xylene molecules are adsorbed in the vicinity of the Ba²+ cations of the supercages in an identical crystallographic position A. In its adsorption site, the molecule is stabilized by M-type interaction between the benzene ring and the Ba²+ cation as well as by van der Waals interactions between one of its methyl groups and the oxygen atoms of the zeolitic framework. These interactions were revealed by crystallographic analysis due to the existence of specific ring-cation and methyl-oxygen interatomic distances. Infrared analysis confirms that the adsorption sites are identical in all the domain of coverage (0-2 mol/sc), with the vibrational properties of para-xylene being constant. - In the mol/s domain (2-3), an analysis of the steric stresses shows that a third para-xyelene molecule cannot be adsorbed in the vicinity of a Ba²+ cation without causing strong intermolecular repulsions. A crystallographic analysis shows that this last molecule is adsorbed in any position, involving no Pi-type interaction between the benzene ring and a cation. This result is confirmed by a huge change in the vibration frequencies and of the integrated absorbances associated with the benzene ring and the methyl groups. For meta-xylene, the molecules are adsorbed in the vicinity of the Ba²+ cations of the zeolitic supercages in all the coverage range. However, a study similar to the one performed for para-xylene adsorption showed the existence of two filling ranges characterized by specific crystallographic positions and specific infrared vibrations. In the domain (0-2 mol/sc), the meta-xylene molecules are adsorbed in crystallographic position B. This position makes for an optimum interaction, not only between the benzene ring and the Ba²+ cation, but also between the two methyl groups of the molecule and the square windows in the zeolitic framework. In the mol/sc domain (2-3), the adsorption of a third molecule in the vicinity of a cation is made possible by the reorientation of the other two molecules in a new position B'. The molecular rearrangement thus observed by diffraction is confirmed by a huge change in all the infrared properties of the meta-xylene adsorbed in the mol/sc domain (2-3). Likewise, this molecular rearrangement causes an appreciable decrease in the Pi-type interactions involved among all the meta-xylene molecules and the Ba²+ cations, while ensuring minimum methyl-methyl intermolecular repulsions.