V-containing molecular sieves

Surprisingly, V-containing molecular sieves were not active in the oxidation of aniline with hydrogen peroxide, even when the reaction was performed in various solvents. This could be easily understood over VAPO-5 because of the hydrophobic character of the framework. Nevertheless, the lack of activity over VS-1 was more unexpected. In contrast the use of TBHP converted aniline into nitrobenzene (NB) over VAPO-5 and V-HMS. Traces of nitrosobenzene (NSB) were detected at the beginning of the reaction but AZY was never observed. 

Generally, V-containing molecular sieves are thermally stable. However, V-containing MFI, MFI/MEL and ZSM-48 prepared with VCI3 are reported to collapse to cristobaHte upon calcination in air at 550 °C, but not in an atmosphere of H2 [104]. The explanation that the structural collapse is due to oxidation of the atomically dispersed to is not convincing since the presence of air during the synthesis probably induces the oxidation of even before the... 

The chemistry of V is rather complex because of the existence of three stable oxidation states, depending on the synthesis procedure adopted as well as on the post-synthesis treatments. Most of the V-containing molecular sieves described in the Hterature contain and/or species, and only few ions. Several authors pointed out that the transition, taking place upon calcination... 

As for Ti- and V-containing molecular sieves, the synthesis conditions must assure the suppression of the polymerization/precipitation reactions of iron oxide/hydroxide species. Though the aquo-chemistry of Fe + is well documented (see [133] and references therein), little information is available on the behavior of Fe sources in the presence of silica, particularly under the hydrothermal conditions necessary for the crystallization of molecular sieves. Monomeric octahedral aquo-hydroxocomplexes of iron are stable only at pFl < 2. At higher... 

Recently the large pore vanadium containing molecular sieve, V-NCL-1 with a pore size of 7 A, has been shown to be an active catalyst for the oxidation of larger molecules, such as napthalenes, 1,4-napthoquinones and phthalic anhydride (Scheme 22)[187]. The as synthesised form of V-NCL-1 contains atomically dispersed V4+ ions located in fiamework postions although not neccessarily in tetrahedral coordination. The vandium ions can be oxidised to the pentavalent state by calcination, as evidenced by ESR [157], with some... 

Generally zeolites of the MFI, MEL and BEA types are synthesized from alkaline media. Occasionally acidic media containing fluoride ions have also been used for the synthesis of zeolites [13]. We will now examine the influence of the mode of preparation (acidic or alkaline medium) on the nature of the V-ions in V-MFI molecular sieves. Two V-MFI samples, A from acidic and B from basic media were prepared. Analytical data of the precursor gels and the crystalline samples are presented in Table 1. 

Similar to Ti-containing molecular sieves, an IR band at ca. 960 cm was also reported for [Si,V]MFI structures (cf., e.g., [340] and Volume 1, Chapter 7, pages 208-209, of the present series and references therein). Thus, Hongbin Du et al. [341] observed by the FTIR/KBr technique in synthesized large pore V-contain-ing zeolite Beta a band at 960 cm" and took the appearance of this signal as an indication of vanadium incorporation into the framework to produce H-[Si,Al,V]Beta. 

As for Ti-containing molecular sieves, the incorporation of V has been mainly attempted in pentasil-type frameworks (MFI and MFI/MEL), while little information is available about the possibility to synthesize vanadosilicates with the framework topology of other zeolites. Table 6 summarizes the data reported in the literature. Hydrothermal synthesis is the route used in most cases. 

Similarly to Ti-containing molecular sieves (Sect. 3.2), incorporation of V is supported by spectroscopic and structural evidence. In calcined samples, a band located at ca. 960 cm was observed in the IR region, not present in the silica... 

Figure 2. Fluorine NMR relaxation times for a sample of Linde molecular sieve 13X containing about 6.6 molecules of SFg per cage O, spin lattice relaxation time , spin-spin relaxation time T2 characterized by exponential decay V and A, T2 characterizedby two relaxation times ticked O, decay as r2. Solid lines are theory to the left of 10Z/T = 6 based on molecular diffusion to the right of 10Z/T controlled by Tu. For dashed lines see text (20)...

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... 

Between 0.02 and 0.15 typically 0.05 equivalents3 of TPAP (MW = 351.43) are slowlyb added to a ca. 0.02 0.3 M solution of the alcohol in CH2C12,c containing ca. 0.2 0.7 g of 4 A molecular sieves 1 per mmol of alcohol and ca. 1.1 to 2.5—typically 1.5 equivalents of TV-methylmor-pholine /V-oxide (NMO, MW = 117.15). The resultant mixture is stirred at room temperature1 till most of the alcohol is consumed.8 This is followed by a work-up that can be carried out according to two alternative protocols ... 

Although a number of books now contain chapters or sections dealing with zeolites, this subject has been the primary focus in very few volumes. The most noteworthy of these is the treatise by D. W. Breck, Zeolite Molecular Sieves, published by Wiley in 1974. The earliest modem ones are the Russian texts on Molecular Sieves and Their Use, published by V. A. Sokolov, N. S. Torocheshnikov, and N. V. Kel tsev in 1964, and S. P. Zhdanov s Chemistry of Zeolites in 1968. Other significant works include the 1976 ACS Monograph on Zeolite Chemistry and Catalysis edited by J. A. Rabo, and the volume on Zeolites and Clay Minerals as Sorbents and Molecular Sieves by R. M. Barrer, published in England in 1978. 

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