The silicalite/p-xylene system : Part I — Flexibility of the MFI framework and sorption mechanism observed during p-xylene pore-filling by X-ray powder diffraction at room temperature

During p-xylene(XYL) sorption in MFI type materials presenting various framework compositions at room temperature, the M (monoclinic, centric P21n · 1 · 1 group) ↔ O2 (orthorhombic, acentric P212121 group) transition is never observed : for high SiAl ratios (> ≈ 75) the M → O1 is first observed at lower sorbate loadings (0 to 4 molecules/uc), and the O1 (orthorhombic, centric Pnma group) → O2 transition is only achieved at saturation (8 molecules/uc); in the case of low SiAl ratios (< 75), which is the usual situation (genuine ZSM-5 phases), only O1 → O2 is observed. On sorbate desorption, both O2 → O1 and O1 → M reversible transitions are observed. In the present work the deformations of the tenring (10R) channel-openings in the straight and zig-zag channels are investigated versus p-xylene loadings in silicalite (Si/Al ≈ 5000). It is shown that during the O1 → O2 phase transition occuring at higher sorbate loadings the mean/maximum displacements of framework oxygen atoms are close to ≈ 0.57/1.03Å, which is significantly higher than the mean displacements calculated for all the framework atoms(Si and O) in a theoretical (molecular mechanics) study (0.36Å) during the hypothetical and unobserved M → O2 transition. It is shown that during the threestep p-xylene sorption mechanism the framework is highly flexible. The effective pore-openings in silicalite · 4XYL are 6.16 × 4.78 and 5.65 × 5.25Å for the straight and zig-zag channels respectively.