Vanadium silicalite

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. 

Sn-silicalites of MFI, MEL and MTW stmctures with Si/Sn > 30 have been synthesized hydrothemally under basic conditions. The unit cell volume eraansion in each case, though linear with respect to Sn content (upto 3 Sn per unit cell in MFI and MEL silicalites), does not correspond to theoretical T-atom substitution by Sn " ions. The well-dispersed SnOx units can be described as structural defects with octahedral coordination and are active in the oxidation of a number of organic substrates (phenol, toluene, m-cresol and m-xylene) with aqueous H2O2. These are similar to vanadium silicalites (VS-1 and VS-2), as both hydroxylation or the aromatic nucleus and the oxidation of the alkyl substituent are catalysed. Due to the presence of Sn + in large pores, Sn-ZSM-12 sample is able to oxidize bulkier naphthalene and 2-methylnaphthalene more effectively than the medium pore Sn-MFI and Sn-MEL silicalites. 

BASF has also claimed the use of metal-modified TS-1 catalysts [25]. Various catalyst compositions have been described, including (i) titanium or vanadium silicalites containing rare earth ions [25a] and (ii) titanium or vanadium silicalites containing noble metals (Ru, Rh, Pd, Os, Ir, Pt, Re, Au, Ag). In these systems the... 

Chiesa M, Meynen V, Van Doorslaer S et al (2006) Vanadium silicalite-1 nanoparticles deposition onto mesoporous walls of SBA-15. Mechanistic insights from a combined EPR and Raman study. J Am Chem Soc 128 8955-8963... 

Successful application of DRS together with complementary methods like ESR, XRD, XAS, IR, SEM, NMR etc. for characterization of the synthesized micro-porous materials with respect to incorporation into the framework has been further reported for MFI-type vanadium silicalites [74, 75], for VAPO-5, MgAPO-5 and MgVAPO-5 [76], for MFI-type ferrisilicalite [77] and for insertion of chromium in a number of molecular sieves [78]. In the case of the latter, essentially nonframework species could be identified and even in Cr-silicalite and CrAPO-5 Cr + showed no tendency to change from octahedral to tetrahedral coordination. 

---