STRUCTURAL STUDY OF AN MFI NAPHTHALENE SYSTEM BY X-RAY-POWDER DIFFRACTION, SI-29 MAS NMR, AND ENERGY-MINIMIZATION CALCULATIONS

Combining solid-state Si-29 MAS n.m.r. studies, energy-minimization ca lculations using the atom-atom potential method (Buckingham and Lennar d-Jones models), and X-ray profile-fitting structure refinements (Riet veld method with geometric constraints) shows that it is possible to l ocate the naphthalene (NPH) molecules sorbed at room temperature in a saturated silicalite.4 C10H8 complex. Interpretation of the n.m.r. spe ctrum and of the diffraction profile is consistent with orthorhombic P nma framework symmetry. The entrapped sorbate molecules, which are loc alized at the channel intersections of the MFI framework, have their l onger symmetry axis oriented parallel to the y-axis direction [010] an d form infinite and loosely associated polymeric chains in the straigh t channels. The NPH does not enter the zigzag channels, where the pres ence of an additional extraframework water molecule could be seen. NPH adsorption provokes an elliptical deformation of the straight-channel sections (6.3 x 4.8 angstrom). The zigzag channel sections remain alm ost circular. For the first time, an inversion in the a and b unit cel l parameters is observed (a/b = 0.9984) and the NPH acronym is propose d for this deformation of the MFI framework. Time-resolved X-ray diffr action shows that at lower pore fillings (0 < n < 2 NPH molecules/u.c. ), the silicalite.n NPH system is diphased (mixtures of bare silicalit e and silicalite.2 C10H8), whereas for 2 < n < 4, the phases correspon d to a solid solution. Preliminary investigations show that the NPH de formation and the inversion in the a/b ratio is also observed for seve ral other MFI/aryl systems.