Bo Jonson

Activity measurements and spectroscopic studies of the catalytic oxidation of toluene over silica-supported vanadium oxides

Vanadium oxides on silica gel (Davison 952) with varying vanadium concentrations have been investigated for the activities and selectivities in the oxidation of toluene. These results were correlated with the features shown by X.r.d., ESCA, u.v.–visible and i.r. studies of adsorbed CO. The catalysts show a low activity in comparison with V/alumina. The support alone shows 60% selectivity for benzaldehyde formation. From i.r. data it is suggested that this benzaldehyde is formed on sites in association with sodium impurities. Introduction of vanadium increases the rate of reaction but the selectivity is shifted towards carbon oxides. The increased activity at low vanadium content is due to isolated four-coordinated vanadium species. U.v.–visible data show that the isolated species agglomerate at further loading to polyvanadium chains. These polymers possess an increased activity as a further route of reduction of VV to VIV is present. Five-coordinated vanadium of the same activity as the chains and producing selective oxidation products other than benzaldehyde is found with a loading of 2% V. Larger agglomerates and V2O5 crystallites are present on the 10% V catalyst. These species only give a somewhat increased activity, but with a product pattern different from crystalline V2O5 and 10% V/alumina.

Activity measurements and spectroscopic studies of the catalytic oxidation of toluene over vanadium oxides supported on titania

Catalysts containing 0.1–10 wt% V have been prepared by impregnating TiO2(Degussa P25) by aqueous NH4VO3 and calcining at 773 K. The catalytic performance in the oxidation of toluene was investigated and correlated with the features shown by X.r.d., ESCA, u.v.–visible spectroscopy and i.r. spectroscopy of adsorbed CO. Both the activity in the oxidation of toluene and the initial selectivity for benzaldehyde increase rapidly with increasing vanadium loading up to 2 wt% V. The changes between 2 and 10 wt% V are small. Surface areas, pore structure, primary particle size and anatase/rutile ratio change with the vanadium content. Up to 2 wt% V, a small increase in the anatase/rutile ratio is obtained. In these catalysts, surface-bonded vanadium species are present, and it is suggested that they induce a reconstruction in the titania surface. Larger and opposite effects are observed for 10 wt% V, where crystalline V2O5 facilitates the anatase to rutile phase transition. Sintering of the primary particles and a large reduction in the specific surface area are also obtained. U.v.–visible spectroscopy of reduced catalysts indicated the predominance of VIV species in tetrahedral coordination. VIII species could not be observed by i.r. spectroscopy of adsorbed CO. I.r. spectra indicate the presence of V—OH groups. Breakpoints in the spectral data were obtained at ca. 1.1 wt% V corresponding to the consumption of two support OH groups per vanadium species. It is suggested that the isolated vanadium species are oxohydroxyvanadium bonded to titania through two oxygen bridges. These constitute the active site at low loadings. Two adjacent vanadium species are thought to form the reduced tetrahedral structure. New active sites present at higher loadings may be formed by the reaction between V—OH groups and vanadium precursors. The new sites possibly represent a VV/VIV redox couple: It is suggested that its presence is an important factor governing the catalytic activity.

Activity measurements and spectroscopic studies of the catalytic oxidation of toluene over V2O5/Al2O3-C catalysts

Vanadium oxide catalysts based on Al2O3-C (γ-Al2O3) as support contain at least four different forms of vanadium. The distribution between these depends on the vanadium loading. The activity (per gram of vanadium) of these catalysts for toluene oxidation increases with increasing loading. The selectivity for benzene formation decreases from 2% for the support to 0% for 0.5 wt% V, while the selectivity for benzaldehyde formation first appears at this concentration and rises to 29% for 10 wt% V. It is suggested that benzene is formed at Lewis acid sites on the support, whereas benzaldehyde is formed on vanadium sites. At low loadings (0.1 and 0.2 wt% V) single vandium species with tetrahedral coordination are formed. The oxidised forms have u.v. bands at 35 500 and 42000 cm–1 and the reduced forms have i.r. bands of adsorbed CO at 2200 cm–1(room temperature) and at 2190 and 2158 cm–1(133 K). At medium loadings, vanadium surface clusters with varying degrees of agglomeration are formed in addition to the other species. These are suggested to be single chain species with vanadium in tetrahedral coordination, double chain species with vanadium in square pyramidal coordination and aggregates of octahedral vanadium formed by the coupling of double chains. The oxidised forms have u.v. bands at 34 500 and 43000 cm–1 and the reduced forms have i.r. bands of adsorbed CO at 2178 cm–1(room temperature) and at 2178 and 2158 cm–1(133 K). The agglomerates are more active than the isolated species and show some selectivity for benzaldehyde. Both the activity and the selectivity appear to increase with the degree of agglomeration. At high vanadium loadings (10 wt% V), surface crystallites of vanadium oxide are formed in addition to the other species. The oxidised forms have u.v. bands at 30 000 and 43 000 cm–1 and the reduced forms have i.r. bands of adsorbed CO at 2181 cm–1(room temperature) and at 2181 and 2145 cm–1(133 K). These crystallites are more active and selective than other species with less agglomeration. It is suggested that the increased activity for the larger species is due to the possibility of a transition from corner to edge sharing octahedra at the release of oxygen, which increases the activity of the double-bonded oxygen.

Infrared Emission Spectroscopy Studies of Metal Oxide Catalysts

The spectral changes of a series of metal oxides, resulting from their oxidation of toluene, were studied by infrared emission spectroscopy. Comparative investigations were carried out with a grating spectrophotometer and with an interferometric Fourier transform spectrophotometer. The latter was found to be superior in the present kind of experiments.

Toluene oxidation over metal oxides in relation to oxide vibrations

Abstract The rate of the selective oxidation of toluene to benzaldehyde over metal oxides has been related to the frequencies of the metal-oxide ion vibrations. Literature data as well as new experimental data are used. It appears that a twin-peaked curve describes the relation. This result is interpreted in terms of a hypothesis on specific energy transfer by vibrational resonance in catalytic reactions.

An ESCA investigation of some dithiooxalato complexes

Abstract A series of dithiooxalato complex ions have been investigated as their tetraphenyl-phosphonium and potassium salts. Core-electron binding energies (Eb) of S, C, and O were determined, as well as those of the metals. From the first-mentioned data, effective charge values (q) were estimated for the atoms of the ligands. For this purpose linear relations (Eb = k · q + EbO) were used that had been previously derived within a scheme using C ls (phenyl) as the internal standard. From the data thus obtained the effective charge on the metal atoms could be estimated. The data is used to test if analogous linear relations also hold for the heavy elements; for example, we have found Eb(Pt) = 3.17 · qpt + 71.1 eV.

Activity measurements and spectroscopic studies on the catalytic oxidation of toluene over vanadium oxides supported on silica–alumina COK84

We describe a study in which a mixture of silica and alumina (Degussa Aerosil COK84) was impregnated with varying amounts of vanadium. These catalysts were investigated for their activities and selectivities in the oxidation of toluene. The catalytic properties were correlated with the features shown by X.r.d., ESCA and u.v.–visible and i.r. spectroscopies of adsorbed CO. The pure support produces benzene in initial selectivity exceeding 50% one acidic Al—O—Si sites formed during the preparation procedure. The benzene selectivity decreases with time owing to coking effects. Introduction of vanadium decreases the selectivity for benzene and simultaneously selectivity for benzaldehyde appears. The surface area decreases considerably with a loading of 10% V, which is due to a decrease in volume of pores <80 A. I.r. spectra revealed that the initially introduced vanadium adsorbs on alumina sites. U.v.–visible data, especially at low vanadium loadings, were similar to those for V/alumina. Comparison between experimental and calculated activities indicated that the first 0.2% V is adsorbed exclusively on alumina. At 0.5% V loading, a vanadium species on silica is also detected by i.r. spectroscopy. This loading possesses the most active vanadium, i.e. large agglomerates on alumina. The activity per g V at higher loadings levels off owing to the greater amount of low-activity vanadium on silica. An increased benzaldehyde selectivity was found for the 2 and 10% V catalysts. On these catalysts V2O5 crystallites are present. This is evident from the X.r.d. results and the product pattern of the 10% V sample.

CO Forming Carbonate Species on V2O5

Abstract Infrared Emission Spectroscopy was used to detect carbonate species on V2O5. These species are formed by reactive adsorption of CO. The type of compound on the metal oxide is identified by the degree of splitting of ν3 of the free carbonate ion.

Diffuse Reflectance Spectra of V(CP)2 + on Silica

Abstract Diffuse reflectance spectra of vanadocene adsorbed on silica were recorded in the spectral region 4000-30 000 cm−1. A d-d band around 13 000 cm−1 indicated, that vanadocene formed V(cp)2 + on the support when the number of silanol groups was decreased. This cation is suggested to be stabilized by the negative charge of deprotonated silanol groups.