Synthesis of Niobium Silicate Molecular Sieves of the MFI Structure:  Evidence for Framework Incorporation of the Niobium Ion

Abstract

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 29Si MAS NMR, UV−visible, FTIR, and Raman spectroscopies. Substitution of niobium into the framework has been established by these various methods. γ-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 93Nb (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 93Nb. Silicalite-1, on the other hand, shows radiation-induced hole centers located only on Si−O−Si units.

Liquid-phase Dehydration of d-xylose over Microporous and Mesoporous Niobium Silicates

“…In the last few years there has been interest in the synthesis of porous niobium silicates, as the presence of niobium in molecular sieves has the potential of generating catalysts with size/shape selective properties and acidic or redox characteristics [11][12][13][14][15][16]. A novel microporous niobium silicate denoted as AM-11 was reported in 1998 and is a promising catalyst for gasphase dehydration reactions, such as the conversion of tert-butanol to isobutene [11,17,18].…”

Effect of the niobium state on the properties of NbSiBEA as bifunctional catalysts for gas- and liquid-phase tandem processes

“…The peaks with low intensity are probably due to protons of isolated and terminal silanol, respectively [30], still present in Nb0.7SiBEA and Nb2.0SiBEA after incorporation of Nb into the vacant T-atom sites of SiBEA.The DR UV-vis spectrum of Nb0.7SiBEA demonstrates the presence of two absorption bands at 220 and 234 nm(Fig. 6)corresponding to different mononuclear Nb(V) present in the framework of BEA zeolite and are assigned to oxygen-tetrahedral Nb(V) ligand-to-metal charge transfer transitions[21,22,31,32]. The absence of bands at around 330-450 nm assigned to…”

Synthesis and characterization of carbon microtube/tantalum oxide composites and their photocatalytic activity under visible irradiation

“…The peaks at 1330 cm -1 and 1573 cm -1 indicates the D-band and G-band respectively. The peaks at 740 cm -1 and 870 cm -1 are due to the octahedral TaO 33 The Raman spectral analyses clearly suggest that the formation and stability of tantalum oxide with CMT frame work. 32 The composites has the same structural integrity of CMT frame work.…”