Vanadium silicate

The oxidation of both linear and cyclic ethers to the corresponding acids and lactones by aqueous H202 as catalyzed by TS-1 and TS-2 was reported by Sasidharan et al. (241) (Scheme 17 and Table XXXV). The titanosilicates exhibited significantly better activity (about 55% conversion) and selectivity (98%) than chromium silicates, although vanadium silicates totally failed to catalyze the reaction. Such conversions are usually accomplished using either stoichiometric amounts of chromium trioxide, lead tetraacetate, or ruthenium tetroxide as oxidants (242) or catalytic amounts of Ru04 in the presence of... 

Figure 1.17 (a) ESR spectrum from a vanadium silicate catalyst after photoreduction with H2 at 77 K [138], The ESR data obtained indicate the existence ofV4+ ions in tetrahedral coordination, (b) Addition of a small amount of water leads to a new ESR trace identified with distorted octahedral V02+ ions, indicating the easy accessibility of the vanadium surface species. (Reproduced with permission from Elsevier.)... 

In 1992, Hari Prasad Rao and Ramaswamy reported on the oxyfunctionalization of alkanes with H2O2 using a vanadium silicate molecular sieve s . With this catalyst acyclic and cyclic alkanes were oxidized to a mixture of the corresponding alcohols (primary and secondary ones), aldehydes and ketones. Unfortunately, most of the early attempts were of rather limited success due to low turnover frequencies and radical producing side reactions as observed, for example, by Mansuy and coworkers in 1988. ... 

The vanadium silicalites (with MFI and MEL stmcture) are active oxidation catalyst in gas and liquid phase reactions [180]. As for the titanium silicalites, only the ftamework associated vandium exhibits redox properties [181]. For example, in the hydroxylation of phenol, silicalite impregnated with vanadium compounds is catalytically inactive [182]. The catalytically active vanadium species is speculated to be located in non-tetrahedral positions, most probably chemically bound to the framework. Vanadium bound in that way is not extractable from the lattice [ 183]. A proposed stmcture of the vanadium site is schematically shown in Scheme 21. Note that the Si-O-V bonds are longer than the Si-O-Ti bonds and that V seems to be more exposed. The redox properties are affiliated with the changes in the oxidation state of vanadium between +IV and +V. Vanadium silicates with SiA ratios ranging from 40 to 160 have been reported and these high values suggest (in accordance with V MAS-NMR measurements) that the V sites are isolated in the lattice. 

Figure 5. Examples for the selective oxidation of naphthalene over the large pore vanadium silicate [V]-NCL-1 (after [66]).

In addition to the aluminosilicates, some obvious synthetic analogues, such as borosilicates, gallosilicates and ferrisilicates, have also been explored. The boro-silicates are more difficult to prepare, probably because the much smaller boron, with its tendency towards planar, three-fold coordination, is not an obvious substitute for the role of aluminum. Gallium and iron(III) materials, though, are well known and usually have properties similar to those of the aluminosilicates. There is evidence, however, for some unique behavior with gallosilicates, as in their ability to catalyze cyclization reactions and activate methane [30]. Other silicate-based systems include the titanosilicates, mentioned earlier, and some recently discovered vanadium silicates that show excellent thermal stability and can potentially be activated for catalysis [31]. 

O-Donor Ugands. The results of an e.p.r. study of the irradiation of vanadium silicates with adsorbed methanol have been interpreted in terms of the photoreduction sequence... 

The turnover numbers (TON) of phenol are 62.4 and 105.4 with a H2O2 efficiency of 28.9 wt.% and 48.9 wt.% for the Zr-Sil-2 samples A and B, respectively. A significant difference in the product distribution between these two runs is also observed. The catechol (CAT) to hydroquinone (HQ) ratios are 0.9 and 1.7 for Zr-Sil-2 (A) and (B) samples, respectively. A CAT/HQ ratio of 0.9 to 1.3 has been reported for titanium and vanadium silicate molecular sieves (TS-2 and VS-2) [13]. The samples synthesized using Zr(acac)4 show a nearly two fold activity in the reaction probably due to the smaller particle size. These results indicate that in the case of Zr-Sil-2 samples synthesized using ZrCU, the Zr " ions are well dispersed within the channels of the MEL structure while in the samples synthesized using Zr(acac)4, the hydroxylation occurs at the external surface as well, where a part of Zr species may be located. For small submicron crystals (<1 pm), external surface sites could be a significant fraction of the total surface area. If the external surface sites are catalytically either the same or more active than the intracrystalline active sites, then the shape selectivity of a zeolite could... 

SELECTIVE OXIDATION REACTIONS OVER VANADIUM SILICATE MOLECULAR SIEVES... 

The oxidation of various hydrocarbons such as n-octane, cyclohexane, toluene, xylenes and trimethyl benzenes over two vanadium silicate molecular sieves, one a medium pore VS-2 and the other, a novei, iarge pore V-NCL-1, in presence of aqueous HjOj has been studied. These reactions were carried out in batch reactors at 358-373 K using acetonitrile as the solvent. The activation of the primary carbon atoms in addition to the preferred secondary ones in n-octane oxidation and oxidation of the methyl substituents in addition to aromatic hydroxyiation of alkyl aromatics distinguish vanadium silicates from titanium silicates. The vanadium silicates are also very active in the secondary oxidation of alcohols to the respective carbonyl compounds. V-NCL-1 is active in the oxidation of bulkier hydrocarbons wherein the medium pore VS-2 shows negligible activity. Thus, vanadium silicate molecular sieves offer the advantage of catalysing selective oxidation reactions in a shape selective manner. 

The hydrothermal crystallization of vanadium silicates was carried out using the foi-lowing molar compositions SiOj 0.2 R4-OH or (R4N + NaOH) x VOj 30 HjO. Detaiis of the synthesis and characterization of VS-2 and V-NCL-1 are given in our earlier reports [5,7]. The samples were characterized by XRD, IR/FTIR, ESR, SEM, NMR and adsorption techniques. 

All the vanadium silicate samples have been characterized to investigate their thermal stability, the location, co-ordination number and oxidation state of vanadium in different atmospheres by a variety of techniques. Some of the physical properties of representative vanadium silicates are given in Table 1. The oxidation reactions reported here are confined to calcined fin air) samples of VS-2 and V-NCL-1 with SiA = 79 and 250, respectively. Absence of ESR signals due to paramagnetic vanadium species indicated that all vanadium is in pentavalent oxidation state in these samples. For comparison, data on their respective silica polymorphs viz., silicalite-2 (S-2) and Si-NCL-1 are also included in Table 1. These samples as well as those with different Si/V ratios, (Figs. 1 and 2) probably contain vanadium in the silicalite framework as may be inferred from the following observations - 1) The unit cell parameters of V-samples increase linearly with vanadium content, as shown in Rg. 1... 

Oxidation of alkanes over vanadium silicate molecular sieves ... 

Fig. 7 The observation of strong [C(2)] and weak [C(l)] hyperfine couplings in the HYSCORE spectra after adsorption of C02 to a PHTS formed by deposition of vanadium silicate-1 nanoparticles in SB A-15 reveals equatorial and axial ligation of CO2 to the vanadyl-sites. Adapted and reprinted with permission from [164], Copyright 2011 American Chemical Society...

Zamani S, Chiesa M, Meynen V et al (2010) Accessibility and dispersion of vanadyl sites of vanadium silicate-1. J Phys Chem C 114 12966-12975... 

Reddy, K. M., Moudrakovski, I., and Sayari, A. 1994. Synthesis of mesoporous vanadium silicate molecular sieves. 7. Ghent. Soc. Ghent. Gontmun. (9) 1059-1060. 

Vanadium silicate molecular sieves are capable of selectively oxidising 4-chlorotoluene to 4-chlorobenzaldehyde using hydrogen peroxide as the source of oxygen in acetonitrile solvent (Scheme 10). ... 

The oxidation selectivity strongly depends on the specific polyaromatic substrate. Oxidation of naphthalene with H Oj in the liquid phase over a large pore zeolite, V-NCL-1 [86a], and a mesoporous vanadium-silicate, V-MCM-41 [86b], afforded 1,4-naphthoquinone (NQ) as the main product and minor amounts of 1- and 2-naphthols at the initial stage of reaction but, at conversions greater than 5%, phthalic anhydride predominated because of susceptibility of NQ to overoxidation. In contrast to NQ, AQ is rather stable to overoxidation and high selectivities can be attained at high substrate conversions. 

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