Structural study of an MFI/naphthalene system by X-ray powder diffraction, 29Si MAS n.m.r, and energy-minimization calculations

Abstract

Combining solid-state 29Si MAS n.m.r. studies, energy-minimization calculations using the atom—atom potential method (Buckingham and Lennard-Jones models), and X-ray profile-fitting structure refinements (Rietveld method with geometric constraints) shows that it is possible to locate the naphthalene (NPH) molecules sorbed at room temperature in a saturated silicalite-4 C 10H 8 complex. Interpretation of the n.m.r. spectrum and of the diffraction profile is consistent with orthorhombic Pnma framework symmetry. The entrapped sorbate molecules, which are localized at the channel intersections of the MFI framework, have their longer symmetry axis oriented parallel to the y-axis direction [010] and form infinite and loosely associated polymeric chains in the straight channels. The NPH does not enter the zigzag channels, where the presence of an additional extraframework water molecule could be seen. NPH adsorption provokes an elliptical deformation of the straight-channel sections (6.3 × 4.8 Å). The zigzag channel sections remain almost circular. For the first time, an inversion in the a and b unit cell parameters is observed ( a b = 0.9984 ) and the NPH acronym is proposed for this deformation of the MFI framework. Time-resolved X-ray diffraction shows that at lower pore fillings (0 < n < 2 NPH molecules/u.c.), the silicalite-n NPH system is diphased (mixtures of bare silicalite and silicalite-2 C 10H 8), whereas for 2 < n < 4, the phases correspond to a solid solution. Preliminary investigations show that the NPH deformation and the inversion in the a b ratio is also observed for several other MFI/aryl systems.