CHARACTERIZATION OF GUEST MOLECULES SORBED IN ZEOLITES OF KNOWN STRUCTURE .5. LOCALIZATION OF PARA-DICHLOROBENZENE IN MFI MATERIALS AT MEDIUM AND HIGHER LOADINGS

Para-dichlorobenzene (PDCB) has been sorbed in MFI type zeolitic mater ials presenting the Si96O192 (silicalite) and H0.72Al0.72Si95.28O192 ( ZSM-5) framework compositions. Difference-Fourier maps clearly reveal the locations and orientations of the organic guest molecules. At room temperature and lower or medium pore-filling (two to four sorbate mol ecules/unit-cell) the sorbate is localized at the channel-intersection (site I, x/y/z almost-equal-to 0/.25/.5). At saturation (eight molecu les/unit-cell) two distinct PDCB species are detected : the first one resides at the channel-intersection (site 1), and the second one in th e zig-zag channel (site II, x/y/z almost-equal-to .2/.25/.85 or almost -equal-to .2/.25/.60, depending on the choice of the P2(1)2(1)2(1) spa ce group origin). The atomic positions of the topology and of the gues t molecules have been refined by using linear constraints. The distrib utions of the sorbate species in the investigated MFI/PDCB complexes a re comparable to those observed in the case of MFI/p-xylene systems.