CRYSTALLOGRAPHIC MODELING OF SUPERCAGE FI LLING DURING ADSORPTION OF PARA-XYLENE AND META-XYLENE BY THE ZEOLITE BAX

Powder neutron diffraction is used in order to study the structure of BaX zeolite containing adsorbed deuterated meta- and para-xylene. The refinements allow us to localize the molecules inside zeolite supercag e and suggest a model for the filling of the pores of zeolite. At low coverage, the molecules of para- and meta-xylene are located near the site II (Ba2+) showing strong interaction between the cation and the a romatic ring. The distance between the cation and the aromatic molecul e is shorter for the meta-xylene than for para-xylene. The meta-xylene symmetry favors a good interaction between methyl groups and the zeol ite framework. When coverage increases above 2 molecules per supercage , a molecular rearrangement is observed. All three meta-xylene molecul es are still located near site II in the supercage, instead of the las t para-xylene molecule which is located in a new site (F), without spe cific interaction with Ba2+ cation. Molecular reorientation is observe d for meta-xylene in order to maximize methyl-methyl distances and min imize the sorbate-sorbant interaction. Such behavior is not observed i n the case of para-xylene adsorption.