STRUCTURAL CORRELATIONS BETWEEN THE FRAMEWORK SYMMETRY OF HIGHLY SILICEOUS MFI ZEOLITIC MATERIALS (SILICALITE, ZSM-5 FOR SI/AL-GREATER-THAN-75) AND THE LOCATION OR THE GEOMETRY OF SORBED MOLECULES

Several MFI.nsorbate complexes presenting the H0.72Al0.72Si95.28O192 framework composition (Si/Al = 1 3 2) have been prepared. The observed crystal symmetries, which depend on the nature of the sorbate and on the sorbed amount, are P2(1)/n.1.1(monoclinic), Pnma or P2(1)2(1)2(1) (both orthorhombic). These three polymorphic varieties present characteristical a/b and a/c ratios. A preliminary structure investigation shows that for 0<n<4 two space groups are observed. 1 - if the symmetry is P2(1)/n.1.1 the sorbate molecules reside near the channel-intersections and are either isolated (benzene, toluene, ethylbenzene, carbon tetrachloride, bromoform) or do not present any interactions through the straight-channels (pyridine, dibromomethane, n-decane and linear alpha, omega-dibromo C2-C6 alcanes). 2 - if the symmetry is Pnma the molecules are still located near the channel-intersections, but sorbate/sorbate or sorbent/sorbate interactions exist through or in the straight-channels (p-xylene, p-chlorotoluene, p-dibromobenzene, p-dichlorobenzene, aniline, naphtalene). For n > 4 only the Pnma or P2(1)2(1)2(1) space groups are observed and the interactions between the molecules and/or the framework oxygen atoms are sorbate-dependent (dibromomethane, mono- or disubstituted benzene derivatives). The crystallographically evidenced adsorption sites correspond to at least five distinct locations in the MFI channel-system (I, II, II', III[III'] and IV).