FLEXIBILITY OF THE FRAMEWORK STRUCTURE IN HIGHLY CRYSTALLINE SILICALITE DURING THE REVERSIBLE MONOCLINIC ORTHORHOMBIC SOLID-STATE POLYMORPHIC PHASE-TRANSITION

Highly crystalline silicalite (Si/Al>5000) obtained by hydrothermal dealumination of a ZSM-5 material (Si/Al=132) has been investigated at several temperatures by X-ray powder diffraction. Interpretation of the step-scanned XRD profiles by the Rietveld structure refinement code (DBW) shows that the displacive P2(1)/n.1.1(M) <-> Pnma(O1) polymorphic phase transition occurs between 77 and 90-degrees-C (almost-equal-to 83-degrees-C). On heating (20-127-degrees-C temperature range) the minor and major axis-directions of the elliptical D10R (double ten-ring) pore-openings corresponding to the straight-channels (8.50x7.95 and 8.55x7.98 angstrom at 20-degrees-C) progressively change and are finally inverted (7.99x8.52 angstrom at 90-degrees-C). At the same time, the elliptical D10R sections of the zig-zag channels (8.68x8.06 and 8.51x7.69 angstrom at 20-degrees-C) become more circular (8.35x8.13 and 8.43x7.96 angstrom at 90-degrees-C).