CHARACTERIZATION OF GUEST MOLECULES SORBED IN ZEOLITES OF KNOWN STRUCTURE .4. LOCALIZATION OF PARA-DISUBSTITUTED BENZENE MOLECULES AT LOW SORBATE-FILLINGS IN A ZSM-5 MATERIAL (SI/AL = 159)

Para-disubstituted benzene molecules (p-xylene, p-chlorotoluene, p-dichlorobenzene and p-dibromobenzene) have been sorbed in an MFI type zeolitic material presenting the H0.60Al0.60Si95.40O192 framework composition. At room temperature, the evacuated form of this highly siliceous ZSM-5 phase presents the monoclinic symmetry. At lower sorbate-loadings (n = 2-3 molecules/unit-cell) the four MFI.nsorbate complexes crystallize in the orthorhombic Pnma space group. X-ray powder profile refinements and energy-minimization simulations show that the sorbate molecules are located at the channel-intersections (sites I). The individual molecules interact near sites III (through the straight-channels) and form dimeric molecular associations. These dimers are oriented parallel to the b-axis direction and are statistically distributed in the crystal structure. At n = 4, the associated molecules form infinite polymeric chains in the 010 direction. The atomic positions of the topology and of the guest molecules have been refined by using linear constraints.