Study on the Isomorphous Substitution of Silicon by Tetravalent Elements (Zr, Ge, Ti) in the Framework of MFI Type Zeolites

Abstract Silicalite samples containing tetravalent metals (Ge, Ti, Zr) have been hydrothermally synthesized and characterized by various spectroscopic and thermoanalytic methods. Zr-Sil. shows nearly no increase of the unit cell volume. Strong indications for an incorporation of Zr in the framework arise from a Raman band at 685 cm- 1 and a DTA peak that is about 30 K higher than for silicalite. In contrast to silicalite a symmetry change from orthorhombic to monoclinic is not observed. In Ge-Sil. a 29 Si NMR signal at -110 ppm of Si-O-Ge groups can be resolved under optimal conditions only. Combined Raman (band at 960 cm −1 ), XPS and ESR measurements of Ti-Sil. allow to distinguish between Ti isomorphously substituting Si in the framework and between extra-framework Ti