Niobium containing molecular sieves

Recently, there has been a growing interest into niobium- and tantalum-containing molecular sieves. The introduction of niobium into mesoporous molecular sieves has been studied by Ziolek et al [3,4], while Antonelli and Ying reported the synthesis of mesoporous niobium oxide [5], The synthesis and characterization of niobium- and tantalum-containing silicalite-1 (NbS-1 and TaS-1) was published recently [6,7,8] and some evidence has been presented for isomorphous substitution [6,8] of Nb and Ta into the silicalite-1 framework. The synthesis of NbS-2 (MEL) [9] and a new molecular sieve named NbAM-11 have been reported as well [10],... 

Figure 4. ESR spectra at 77 K of niobium- (a) and tantalum- (b) containing molecular sieves.

Characterization of mesoporous and microporous molecular sieves containing niobium and tantalum... 

Niobium- and tantalum-containing mesoporous molecular sieves MCM-41 have been studied by X-ray powder diffraction, 29Si MAS NMR, electron spin resonance, nitrogen adsorption and UV-Vis spectroscopy and compared with niobium- and tantalum-containing silicalite-1. The results of the physical characterization indicate that it is possible to prepare niobium- and tantalum-containing MCM-41 and silicalite-1, where isolated Nb(V) or Ta(V) species are connected to framework defect sites via formation of Nb-O-Si and Ta-O-Si bonds. The results of this study allow the preparation of microporous and mesoporous molecular sieves with remarkable redox properties (as revealed by ESR), making them potential catalysts for oxidation reactions. 

MCM-41 and silicalite-1 can be synthesized in the presence of niobium- and tantalum-containing compounds. The results indicated that Nb(V) and Ta(V) are well dispersed in the framework of silicalite-1 and in the amorphous walls of MCM-41 y-irradiation of activated niobium and tantalum molecular sieves show two radiation induced hole centers (V centers) located on Si-O-Si and M-O-Si (M = Nb, Ta) units. True isomorphous substitution as suggested in the literature for Ti(IV), however, is unlikely to be present Nevertheless, interesting chemical and catalytic properties can be expected from these systems and are subject to further studies... 

In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties. 

The presence of framework niobium showed an increase in activity and H202 efficiency compared to framework titanium. Over niobium containing mesoporous molecular sieves 1,2 pinane diol was also observed as the result of epoxy ring cleavage. 

Acidity of niobium containing MCM-41 mesoporous materials, as reported by Ziolek et. al [15], are sufficient to provide the relatively mild acidic sites necessary to cleave the epoxy ring leading to diols. The a-pinene conversions and H,02 selectivities were always higher with MCM-48 when compared with MCM-41 molecular sieves. This is probably due to the... 

Since the first synthesis of siliceous mesoporous molecular sieves described in the literature in 1992 [1], several mesoporous materials possessing various T atoms together with Si in the lattice have been prepared. The synthesis and properties of niobium- and siliceous-containing mesoporous sieves of MCM-41 type were first time described by our group [2,3] and almost parallely, Nb-doped mesoporous sieves were synthesized by Zhang and Ying [4]. 

Nb-containing MCM-41 sieves represent Lewis acidity proven by FTIR study conducted after pyridine adsorption [3,4], Hydrogen forms of niobium-containing MCM-41 materials exhibit lower Bransted acidity than that in hydrogen aluminosilicate mesoporous molecular sieves (see the band at 1549 cm 1 in Figure 6 [3]). The dehydroxylation of H-NbMCM-41 samples causes the formation of the following lattice species ... 

A comparative study of Cu interaction with niobium- and aluminium-containing MCM-41 molecular sieves... 

Micro- and mesoporous materials containing niobium in the framework or extra framework positions were studied in the oxidation of dibutyl sulphide with H2O2. Leaching of Nb from the solid to the liquid phase was considered. Some of the catalysts prepared via the impregnation with Nb-salts show some leaching of Nb to the liquid phase and the oxidation partially occurs homogeneously in the liquid phase. The reaction proceeds mainly on the catalyst surface when the mesoporous molecular sieves containing Nb in the framework are used. 

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