Flexibility of the framework structure in highly crystalline silicalite during the reversible monoclinic/orthorhombic solid state polymorphic phase transition

• Published in: Materials research bulletin Vol. 28; no. 10; pp. 1017 - 1024
• Main Authors: Mentzen, B.F., Sacerdote-Peronnet, M.
• Format: Journal Article
• Language: English
• Published: London Elsevier Ltd 01.10.1993; Amsterdam Elsevier Science
• Subjects: 360202 - Ceramics, Cermets, & Refractories- Structure & Phase Studies; CHALCOGENIDES; COHERENT SCATTERING; Condensed matter: structure, mechanical and thermal properties; Crystalline state (including molecular motions in solids); Crystallographic aspects of phase transformations; pressure effects; DIFFRACTION; Exact sciences and technology; Inorganic compounds; INORGANIC ION EXCHANGERS; ION EXCHANGE MATERIALS; MATERIALS; MATERIALS SCIENCE; MICROSTRUCTURE; MINERALS; OXIDES; OXYGEN COMPOUNDS; PHASE TRANSFORMATIONS; Physics; SCATTERING; SILICATE MINERALS; SILICON COMPOUNDS; SILICON OXIDES; Single-crystal and powder diffraction; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; Water, oxides, hydroxides, peroxides; X-RAY DIFFRACTION; X-ray diffraction and scattering; ZEOLITES; MFI; silicalite; zeolites; Temperature dependence; Phase transformations; Pore structure; XRD; Experimental study; Temperature range 273-400 K; Zeolites; Polymorphism; Crystal structure
• ISSN: 0025-5408; 1873-4227
• DOI: 10.1016/0025-5408(93)90139-5

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°C (≈83°C). On heating (20–127°C temperature range) the minor and major axis-directions of the elliptical D10R (double ten-ring) pore-openings corresponding to the straight-channels ( 8.50×7.95 and 8.55×7.98 A ̊ at 20°C) progressively change and are finally inverted ( 7.99× 8.52 A ̊ at 90°C). At the same time, the elliptical D10R sections of the zig-zag channels ( 8.68×8.06 and 8.51×7.69 A ̊ at 20°C) become more circular ( 8.35×8.13 and 8.43×7.96 A ̊ at 90°C).