Synthesis of niobium silicate molecular sieves of the MFI structure: Evidence for framework incorporation of the niobium ion

A new electron spin resonance (ESR) method to identify incorporation of transition metals into the framework of zeolite molecular sieves is reported. A novel niobium silicate molecular sieve of MFI topology, designated here as NbS-1, is synthesized hydrothermally and characterized by ESR and other physical methods such as X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, electron microprobe analysis, surface area measurements, and Si-29 MAS NMR, UV-visible, FTIR, and Raman spectroscopies. Substitution of niobium into the framework has been established by these various methods, gamma-Irradiation of activated NbS-1 shows by ESR two radiation-induced hole centers (V centers) located on Si-O-Si and Nb-O-Si units of the framework. The latter can be identified through a 10-line hyperfine structure from Nb-93 (I = 9/2, 100% abundance) and provides convincing evidence for the incorporation of Nb into the silica framework. Nb(IV) ions are also formed as a consequence of radiation-induced reduction of Nb(V) associated with isolated tetrahedral NbO4 units and exhibit an axially symmetric ESR signal with a 10-line hyperfine structure due to Nb-93. Silicalite-l, on the other hand, shows radiation-induced hole centers located only on Si-O-Si units.