Print version ISSN 0104-6632
Braz. J. Chem. Eng. vol. 15 no. 2 São Paulo June 1998
THE INFLUENCE OF NIOBIUM ON THE ACIDITY AND STRUCTURE OF GAMMA-ALUMINA-SUPPORTED VANADIUM OXIDES
M.N.B. Sathler and J.G. Eon
Instituto de Química, Universidade Federal do Rio de Janeiro - Cidade Universitaria
Bloco A, CT - Ilha do Fundão
21945-970 Brazil - Tel: (021) 590-3544, Ext. 246 - Fax: (021) 290-4746
(Received: November 5, 1997; Accepted: March 12, 1998)
Abstract - Gamma-alumina-supported niobium oxide was used as a support for vanadium oxides. The influence of the addition of niobium oxide was studied by looking for changes in the structure and acid-base character of superficial species. Vanadium oxide was deposited using the continuous adsorption method; niobium oxide was impregnated using the incipient wetness method. The catalysts were characterized by XPS, UV-visible and IR spectroscopy. Catalytic tests were performed using propane oxidation reaction at 400oC. For coverage below the monolayer, both vanadium and niobium oxides were observed in slightly condensed superficial species. The presence of vanadium oxide on the support was found to increase the Lewis acidity and create some Bronsted acidity. Higher catalytic activity and selectivity for propene were associated with vanadium oxides. The presence of niobium did not contribute to the modification of the chemical properties of superficial vanadium but did decrease the adsorption of vanadium on the alumina.
Keywords: Supported vanadium oxides, supported niobium oxides, oxidative dehydrogenation of propane.
Supported vanadium oxides are well-known catalysts for oxidation reactions. It is currently accepted that the catalyst obeys the Mars and Van Krevelen redox mechanism. The specificity of these solids is generally explained on the basis of the nature and distribution of various vanadate species, which are determined by vanadium coverage and the acid-base properties of the support (Blasco et al., 1995). However, the first step of the reaction which corresponds to the dissociative adsorption of the reactant involves the acid-base properties of the surface. It is thus expected that catalytic performance should correlate to both redox and acid-base properties. This paper discusses the influence of niobium oxide, a well-known acidic solid, on alumina-supported vanadium oxides, emphasizing structure, acidity and catalytic properties of the material for propane activation.
Gamma-alumina (Engelhard, 200 m2/g) was used as the support for three catalysts containing vanadium oxides, niobium oxides and both vanadium and niobium oxides, with less than an equivalent monolayer in all cases. The solids were denominated VOx, NbOx and VOx/NbOx, respectively. The NbOx catalyst was obtained by impregnation of the support with an aqueous solution of potassium hexaniobate (CBMM, Companhia Brasileira de Mineralogia e Mineração) and its concentration was calculated to be equal to one monolayer, i.e., 19 wt% niobium oxide. The solid was subsequently washed with dilute aqueous nitric acid solution to remove potassium and calcined at 500oC. Vanadium oxide was adsorbed on pure alumina and on the previous NbOx solid using continuous adsorption (Scheefer et al., 1995) of 0.5 M ammonium vanadate solution adjusted to a pH of 4.0 with nitric acid. The resultant solids were also calcined at 500oC for three hours.
Characterization of the Catalysts
Superficial compositions were obtained by X-ray photoelectron spectroscopy (XPS) using a Perkin-Elmer system 1257 instrument and MgKa excitation at 1283 eV. The carbon peak at 284.6 eV was taken as the reference for correction of binding energies due to the charging of the sample. Baseline subtraction and Gaussian profiles were used for peak deconvolution in order to carry out quantitative analysis.
UV-Visible spectra were taken using a double Varian CARY-5 apparatus. Spectra were obtained at RT in the range of 200 to 600 nm, using a Herrick diffuse reflectance accessory with Praying Mantis geometry and the alumina support as reference. Reflectance spectra were presented using the Schulz-Munk-Kubelka equation.
Pyridine FTIR spectra were recorded in a Perkin-Elmer 2000 spectrometer. Self-supported wafers were prepared using 25 mg of the sample corresponding to an area of 2.54 10-2 cm2. The catalysts were first treated in the IR cell at 400oC under oxygen flow (50 mL.min-1) for one hour. Then pyridine (4 Torr) was introduced for one hour at 150oC for adsorption.
Oxidative dehydrogenation of propane was studied using a conventional flow system. The solid (200 mg) was deposited on a fixed bed in a Pyrex reactor operating under atmospheric pressure. The gas mixture containing 2 mol. % propane in air was fed at a flow rate of 36 mL. min-1 for six hours. Analysis of the reactants and products was done by on-line chromatography using a CG IGC gas chromatograph.
RESULTS AND DISCUSSION
UV-Visible reflectance spectra are shown in Figure 1. Only bands in the 200 to 500 nm region are observed. They are attributed to ligand to metal charge transfer (LMCT) transitions and indicate the absence of reduction in the samples. The asymmetric band at 323 nm observed in the VOx spectrum is attributed to weakly condensed vanadate species (Eon et al., 1994). The asymmetric band at 272 nm observed in the NbOx spectrum is attributed to weakly condensed niobate species, as observed in the spectrum of potassium hexaniobate (Medeiros et al., 1995). Both spectra show that no bulk compound was formed. The VOx/NbOx sample also shows only one asymmetric band at 299 nm which is mainly attributed to vanadate species because of the weak intensity of the band corresponding to the niobate band in the NbOx sample.
Table 1 shows the binding energy values for vanadium and niobium, which correspond to V5+ and Nb5+, and the results for superficial analysis by XPS. The results indicate that the vanadium and niobium contents are below the monolayer value which is M/Al=9.0 (M=V, Nb) (Turek and Wachs, 1992). Moreover, the values observed for the VOx/NbOx sample suggest that the previous adsorption of niobium on the alumina surface decreases its capacity to adsorb vanadium.
Figure 2 shows the pyridine FTIR spectra for vanadia-containing samples compared to the spectrum acquired on the alumina support. The bands at 1450, 1493 and 1620 cm-1, which characterize pyridine adsorbed on Lewis acid sites, are observed in all spectra. A band at 1545 cm-1, attributed to pyridine adsorbed on Bronsted acid sites, is observed only in the spectrum of the VOx catalyst, although it has a low intensity almost comparable to the noise signal of the spectrum. These results are in agreement with those reported by Datka et al. (1992), who found that addition of niobia to the alumina surface changes its Lewis acidity but does not create Bronsted acidity at low niobium coverage. On the other hand, Blasco et al. (1995) also found a new band at 1545 cm-1 for alumina-supported vanadia.
The catalytic results for oxidative dehydrogenation of propane are shown in Table 2. In the case of the NbOx sample, conversion is very low and analysis of the products was not possible. The only products observed for vanadia-containing samples are propene and carbon oxides. Thus the presence of vanadium in the sample is the determining factor for propane conversion. By comparison with the data for superficial composition given by the XPS analysis, it can be seen that the conversion is approximately proportional to the vanadium content of the sample. On the other hand, selectivity is barely modified by the presence of niobium on the surface.
The XPS data reported in Table 1 show that vanadium adsorption is greatly decreased when niobium oxide has been previously deposited on the alumina support. In agreement with Turek and Wachs (1992), we assume that the M/Al ratio at the monolayer is approximately equal to 9.0. Thus, total concentration for vanadium and niobium is less than the monolayer value. This suggests that during the adsorption process, vanadium polyanions in solution do not interact with supported niobium oxides and can only adsorb on the free alumina surface, which is therefore divided into patches of different compositions. These results are in agreement with those of Blasco et al. (1995), who showed that the vanadium content depends upon modification of the support.
Figure 1: UV-Visible spectra of VOx (a), NbOx (b) and VOx/NbOx (c) catalysts.
BONDING ENERGY (eV)
Figure 2: Pyridine FTIR spectra of the alumina support (a) and of vanadium oxides supported on pure alumina (b) and on niobium oxide modified alumina (c)
The UV-visible spectra show that only well-dispersed species are present on the support after calcination. The spectrum of the VOx/NbOx sample shows little difference from those of the pure materials, thereby confirming that there is no significant interaction between vanadium and niobium surface species.
Turek and Wachs (1992) studied the acidic properties of vanadium oxides and niobium oxides supported on alumina by monitoring CO2 adsorption. They found that the number of basic sites decreases after impregnation of these acid oxides on the support. The basic sites of the alumina support are thus responsible for vanadium and niobium adsorption from the solution. Our results suggest that these oxides do not interact during adsorption.
Datka et al. (1992) studied the acidic properties of alumina-supported niobium oxide with similar coverage and found that the number of Lewis acid sites decreased in relation to pure alumina. In this work, they used the value e =1.11 cm.m mol-1 for the extinction coefficient of adsorbed pyridine. Using the same value, we obtained estimates of concentrations for Lewis acid sites in our catalysts that are indicated in Table 3. The results show that adsorption of vanadium oxide increases the number of Lewis acid sites whereas adsorption of niobium oxide decreases it. Khader (1995) studied the superficial acidity of V2O5/Al2O3 catalysts and showed that catalysts with a low vanadium coverage exhibit some Bronsted acidity due to V-OH species and Lewis acidity from the bare alumina surface and also from unsaturated vanadium ions. These observations are in agreement with the results reported in this work: only the VOx sample showed Bronsted acidity and an increase in Lewis acidity.
Catalytic results for oxidative dehydrogenation of propane show that supported niobium oxides are practically inactive under these conditions. The decrease in activity observed for the VOx/NbOx catalyst compared to the VOx sample is approximately proportional to the decrease in vanadium concentration as measured by XPS. On the other hand, propene selectivity is equal for both samples. A small change is observed in the distribution of carbon oxides which suggests that the active site is not exactly identical in the two catalysts.
The results reported in this paper show that impregnation of potassium hexaniobate on alumina from aqueous solution provides a way to obtain supported niobium oxides below the monolayer content. Continuous adsorption of vanadate solutions on these solids results in vanadium and niobium coverage below the monolayer with dispersed and weakly condensed superficial species. Vanadium and niobium adsorb preferentially on the alumina surface and do not interact with one another. Niobium oxide changes the acidic character of the alumina surface but does not modify the acidic properties of the superficial vanadium oxides. The catalytic properties of supported vanadium oxides are not significantly altered by niobium oxide.
|Catalyst||VOx (9.5%)a||VOx/NbOx |
a V2O5 weight content
b Nb2O5 weight content
a according to Datka et al. (1992)
d Lewis acid sites
NOMENCLATURE AND UNITS
XPS X-ray Photoelectron Spectroscopy
FTIR Fourier Transform Infrared Spectroscopy
Vox Supported Vanadium Oxide Catalyst
NbOx Supported Niobium Oxide Catalyst
VOx/NbOx Supported Vanadium and Niobium Oxides Catalyst
e Extinction Coefficient, cm.m mol-1 (centimeter per micromole)
Wavelength, nm (nanometer)
Binding Energy, eV (electron volt, 1 eV = 1.602 10-19 J)
Wave Number, cm-1 (per centimeter)
Concentration, m mol/g (micromole per gram)
Flow rate, mL.min-1 (milliliter per minute)
Specific area, m2/g (square meter per gram)
Pressure, Torr (1 Torr = 133.3 Pa)
Mass, mg (milligram)
We would like to thank Engelhard S.A for supplying the alumina sample. We are grateful to the NUCAT/UFRJ laboratories, which carried out UV-visible, XPS and FTIR studies, and to INT, Instituto Nacional de Tecnologia, Rio de Janeiro, for the catalytic study. We also thank CNPq, Conselho Nacional de Desenvolvimento e Pesquisa of Brazil for its support.
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