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 tra nsition metals into the framework of zeolite molecular sieves is reported. A novel niobium silicate molecular sieve of MFI topology, designated here a s NbS-1, is synthesized hydrothermally and characterized by ESR and other p hysical methods such as X-ray diffraction, thermogravimetric analysis, scan ning electron microscopy, electron microprobe analysis, surface area measur ements, and Si-29 MAS NMR, UV-visible, FTIR, and Raman spectroscopies. Subs titution of niobium into the framework has been established by these variou s 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 th e framework. The latter can be identified through a 10-line hyperfine struc ture 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 th e other hand, shows radiation-induced hole centers located only on Si-O-Si units.