FLEXIBILITY OF THE MFI SILICALITE FRAMEWORK UPON BENZENE ADSORPTION AT HIGHER PORE-FILLINGS: A STUDY BY X-RAY POWDER DIFFRACTION, NMR AND MOLECULAR MECHANICS

Abstract

At higher pore fillings ( n ≥ 4) the silicalite(SIL)/ nbenzene(BENZ) solid state phase diagram is very complex. The crystal structures of the 4, 6, and 8 molecules/uc loaded stoichiometric benzenates are investigated by X-ray powder diffraction profile refinements (Rietveld method) and energy minimization calculations (Buckingham model). In SIL · 4BENZ (monoclinic P2 1/ n · 1 · 1, M symmetry), the benzene molecule is disordered and presents at least two distinct orientations centred near the channel intersections (sites I). In SIL · 6BENZ (orthorhombic P2 12 12 1, O2 symmetry), the two independent benzene molecules are located at the channel intersections (sites I) and in the zigzag channels (sites II); the benzene at site I might be disordered (several equi-probable orientations as in SIL · 4BENZ). In the sorbate saturated SIL · 8BENZ complex (orthorhombic Pnma, O1 symmetry) the two independent sorbate species are ordered and located at sites I and in the straight channels (sites III). In SIL · 6BENZ and SIL · 8BENZ, the sorbate/sorbate interactions are comparable to those observed in crystalline benzene (T-association). Sorption of benzene in the MFI channels provokes a strong elliptical deformation of the pore openings. At benzene saturation, the MFI unit-cell parameters correspond to a pseudo-tetragonal deformation, but the framework symmetry remains orthorhombic O1. © 1997 Elsevier Science Ltd