Elastic behavior of MFI-type zeolites: 3 – Compressibility of silicalite and mutinaite

We report the results of an in-situ synchrotron X-ray powder diffraction study – performed using silicone oil as “non-penetrating” pressure transmitting medium – of the elastic behavior of three zeolites with MFI-type framework: the natural zeolite mutinaite and two silicalites (labeled A and B) synthesized under different conditions. While in mutinaite no symmetry change is observed as a function of pressure, a phase transition from monoclinic (P21/n) to orthorhombic (Pnma) symmetry occurs at about 1.0 GPa in the silicalite samples. This phase transition is irreversible upon decompression. The second order bulk moduli of silicalite A and silicalite B, calculated after the fulfillment of the phase transition, are: K0=18.2(2) and K0=14.3 (2) GPa, respectively. These values makes silicalite the most compressible zeolite among those up to now studied in silicone oil. The structural deformations induced by HP in silicalite A were investigated by means of complete Rietveld structural refinements, before and after the phase transition, at Pamb and 0.9 GPa, respectively. The elastic behaviors of the three MFI-type zeolites here investigated were compared with those of Na-ZSM-5 and H-ZSM-5, studied in similar experimental conditions: the two silicalites – which are the phases with the highest Si/Al ratios and hence the lowest extraframework contents – show the highest compressibility. On the contrary, the most rigid material is mutinaite, which has a very complex extraframework composition characterized by a high number of cations and water molecules.