ADSORPTION OF GASEOUS P- OR M-XYLENE IN BAX ZEOLITE - CORRELATION BETWEEN THERMODYNAMIC AND CRYSTALLOGRAPHIC STUDIES

This paper presents a synthesis of macroscopic and microscopic measure ments about adsorption of xylenes in faujasite type zeolite. Adsorptio n of pure gaseous p-xylene or m-xylene in BaX zeolite was investigated by thermogravimetry, isothermal differential calorimetry, and neutron diffraction. A very good correlation was obtained between adsorption heats of both isomers in BaX zeolite and the crystallographic position s of the molecules in the zeolitic structure as a function of filling. This correlation between phenomenologic and microscopic aspects has s hown important differences in the adsorption process between both xyle nes when high fillings are considered. As for m-xylene, all molecules are adsorbed near Ba2+ ions in the alpha-cage in the whole adsorption range, i.e. up to the total filling, which is about 3 molecules per al pha-cage. Consequently, the pi-type interaction between each aromatic ring and a Ba2+ cation gives rise to rather high adsorption heats whic h decrease slightly as filling increases. As for p-xylene, the first t wo molecules adsorbed in each alpha-cage are located near Ba2+ ions, g iving rise to similar adsorption heats as for m-xylene. Strong steric hindrance prevents the third molecule of p-xylene to adsorb near a Ba2 + cation. The molecule is constrained to adopt a less energetic positi on in the alpha-cage. A strong drop in the adsorption heats is therefo re observed when the filling with p-xylene is increased above 2 molecu les per alpha-cage.