V-containing silicalite

V-containing silicalite, for example, has been shown to have different catalytic properties than vanadium supported on silica in the conversion of methanol to hydrocarbons, NOx reduction with ammonia and ammoxidation of substituted aromatics, butadiene oxidation to furan and propane ammoxidation to acrylonitrile (7 and references therein). However, limited information is available about the characteristics of vanadium species in V-containing silicalite samples and especially regarding correlations with the catalytic behavior (7- 6). 

V-containing silicalite (Al- and Na-free) samples were prepared hydrothermally and then treated with an ammonium acetate solution at room temperature in order to remove extralattice vanadium. Three samples with Si02A 203 ratios of 117, 237 and 545, respectively, were prepared. Hereinafter these samples will be referred to as follows V-SH117, V-SU237 and V-SH545. Details on the preparation procedure, and characterization of the samples have been reported previously (7,2). 

Characterization Data. A complete characterization of V- containing silicalite samples has been reported in a previous paper (7), but the main relevant aspects useful for a better understanding of the nature of vanadium species in V- silicalite samples and for the correlation with the catalytic behavior will be briefly summarized. 

The XRD powder patterns of V-containing silicalite samples indicate in all cases the presence of only a pentasyl-type framework structure with monoclinic lattice symmetry, characteristic of silicalite-1 no evidence was found for the presence of vanadium oxide crystallites. The analysis of cell parameters of VSU545 does not indicate significant modifications with respect to those found for pure silicalite-1. This is in agreement with that expected on the basis of the small amount of V atoms present in V-containing silicalite. 

UV-Visible diffuse reflectance spectra also show that vanadium is mainly present as V in V-containing silicalite samples. TPR and XPS results are in agreement with this conclusion. In addition, XPS data indicate that V is homogeneously dispersed in VSU545, whereas in VSill 17 part of the vanadium is segregated on the external surface of the silicalite samples. 

Propane Oxidative Dehydrogenation on V-containing Silicalite. Reported in... 

Therefore, the formation of a 7C-complex of propylene bonded to a vanadium site and to a nearlying hydroxyl group after contact of a propane and oxygen mixture with activated VSU545 indicates some interesting aspects of the surface reactivity of V-containing silicalite ... 

Nature of Vanadium Species. The data on the characterization of V- containing silicalite indicate the presence of various types of vanadium species (i) a polynuclear V-oxide containing V in various valence states (V , V and V ), (ii) octahedral sites, preferentially interacting with OH groups localized inside the pore structure of the zeolite crystals, (iii) nearly symmetrical tetrahedral species, attributed to a V species directly interacting with the zeolite framework, and (iv) after reduction, V species in a nearly tetrahedral environment. 

The good catalytic behavior of V-containing silicalite may be associated with the presence of the tetrahedral V species stabilized by the interaction with the zeolite framework as regards both redox and coordination changes. In fact, ESR and TPR data indicate the lower rate of reduction of this species as compared to that of supported vanadium-oxide, and V-NMR data indicate the stability against changes in the coordination environment. Catalytic data (Fig.s 2 and 3) indicate the better catalytic performances of this species in propane oxidative dehydrogenation as compared to supported polynuclear vanadium-oxide which can be removed by treatment with an ammonium acetate solution. 

Further studies are necessary to clarify these aspects and the details of the mechanism of propane oxidative dehydrogenation on V-containing silicalite. However, it should be noted that the amount of sites which appears to be stabilized... 

Vanadium species in V-containing silicalite in propane oxidative dehydrogenation— Continued... 

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