STRUCTURAL CHARACTERIZATION OF PARA-XYLEN E AND META-XYLENE IN BAX ZEOLITE

The separation of para-xylene from C-8 aromatic by adsorption on a mol ecular sieve is a field of research in which much work has been done a t Institut Francais du Petrole (IFP) in recent years. With a view to o btaining a better understanding of the phenomena involved in this sepa ration, the ensuing research aims to characterize the adsorption of pa ra-xylene and meta-xylene isomers in the state of pure bodies in a BaX zeolite, which is a sieve recognized for its high-performance selecti ve properties during competitive adsorptions. The originality of our a pproach consists in characterizing, on a molecular scale, the adsorpti on of two isomers in the zeolitic network so as to work out a molecula r 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 prope rties of the adsorbate provide exact information concerning the local environment of the xylene molecule in the zeolite as well as on the ex istence of adsorbate-adsorbent and adsorbate-adsorbate interactions. I nfrared spectroscopy was used to examine both the influence of adsorpt ion on the vibrationel properties - integrated frequencies and adsorba nces of fundamental modes - of the xylene molecules and the way these same properties evolve as a function of the zeolite coverage. At the s ame time, neutron diffraction was used to determine, atome by atome, t he chrystallographic position of the xylene molecules in the zeolitic network as a function of the coverage. Two coverages were examined, co rresponding 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 in teratomic 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 s cale for low coverage and at saturation. A very good correlation was t hus obtained between infrared analysis and crystallographic analyses. The changes in the crystallographic positions of the adsorbed molecule s, 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 o f an analysis of the adsorbate-adsorbate interactions caused by the in crease in the coverage. For both isomers, it appears that the increase steric hindrance determines the relative arrangement of the molecules in the supercage. We thus confirmed that the crystallographic positio ns revealed by diffraction can be used to obtain the optimum arrangeme nt of the adsorbate molecules so as to minimize intermolecular repulsi ons. 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 ar e adsorbed in the vicinity of the Ba2+ cations of the supercages in an identical crystallographic position A. In its adsorption site, the mo lecule is stabilized by M-type interaction between the benzene ring an d 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 t he existence of specific ring-cation and methyl-oxygen interatomic dis tances. Infrared analysis confirms that the adsorption sites are ident ical 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), a n analysis of the steric stresses shows that a third para-xyelene mole cule cannot be adsorbed in the vicinity of a Ba2+ cation without causi ng strong intermolecular repulsions. A crystallographic analysis shows that this last molecule is adsorbed in any posiition, involving no pi -type interaction between the benzene ring and a cation. This result i s confirmed by a huge change in the vibration frequencies and of the i ntegrated 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 adsorp tion showed the existence of two filling ranges characterized by speci fic crystallographic positions and specific infrared vibrations. In th e domain (0-2 mol/sc), the meta-xylene molecules are adsorbed in cryst allographic position B. This position makes for an optimum interaction , not only between the benzene ring and the Ba2+ cation, but also betw een the two methyl groups of the molecule and the square windows in th e 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 reo rientation of the other two molecules in a new position B'. The molecu lar rearrangement thus observed by diffraction is confirmed by a huge change in all the infrared properties of the meta-xylene adsorbed in t he 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 minimu m methyl-methyl intermolecular repulsions.