| 1. |
- Andersson, Arne, et al.
(författare)
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Direct Propane Ammoxidation to Acrylonitrile: Kinetics and Nature of the Active Phase
- 1993
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Ingår i: New Frontiers in Catalysis (Studies in Surface Science and Catalysis ). - 0167-2991. ; 75, s. 691-705
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Konferensbidrag (refereegranskat)abstract
- The kinetics of the direct synthesis of acrylonitrile from propane on V-Sb-Al-(W) mixed oxides indicate that acrylonitrile (ACN) forms by two parallel pathways, one directly from propane and the second, which is the prevalent path, through the intermediate formation of propylene (C3=). The limiting factor in the formation of ACN is the relative slowness of the step of allylic oxidation to ACN of the intermediate C3=, and the higher rate of C3= oxidation to carbon oxides as compared to that of ACN to COx. The step of C3= oxidation to ACN is controlled by the surface availability of NH3 which, in turn, depends considerably on the side reaction of NH3 oxidation to N2. The catalytic behavior of different modified V-Sb-(Al)-O systems and their characterization by X-ray diffraction analysis and Raman, Infrared and X-ray Photoelectron spectroscopies indicate that i) a reduction of both V and Sb occurs during the catalytic reaction, ii) the presence of vanadium not stabilized in the rutile-like phase is responsible for the side reaction of NH3 oxidation and lowering of the selectivity, iii) alumina reacts with antimony forming an AlSbO4 rutile phase which could be epitaxially intergrown or in solid solution with the VSbO4/Sb2O4 system, which, in turn, limits the presence of not stabilized (unselective) vanadium species, and iv) antimony oxide supported on alumina is also selective in propane ammoxidation, but forming acetonitrile as the main product. The doping with vanadium of this sample increases slightly the activity, but especially gives rise to the formation of acrylonitrile instead of acetonitrile.
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| 2. |
- Andersson, Arne, et al.
(författare)
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Structure-Activity Relationships in the Oxidation of Alkylaromatics over Metal Oxides
- 1991
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 67, s. 43-55
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Tidskriftsartikel (refereegranskat)abstract
- In order to accomplish oxidation of alkylaromatic compounds, both cations, which serve as adsorption centers, and oxygen species of suitable bond strength are needed. When compared to partial oxidation, relatively weakly bonded oxygen species are involved in combustion. On the basis of these criteria, using a simple model of the active ensemble, a relationship between reaction rate and bond strength is derived for partial and total oxidation. Its applicability is demonstrated using data for the oxidation of toluene over a large number of binary metal oxides. Some characteristic features following from the model are discussed, considering catalytic results on the structure sensitivity of oxidation and ammoxidation reactions over crystalline V,O and MOO,. Furthermore, kinetic results on the oxidation of toluene to benzaldehyde and carbon oxides in presence of ammonia, which serves as an electron donor to the catalyst surface, demonstrate that the oxygen species taking part in partial oxidation and combustion are nucleophilic and electrophilic in character, respectively.
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| 3. |
- Andersson, Arne, et al.
(författare)
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Surface Characterization and Reactivity in Ammoxidation Reactions of Vanadium Antimonate Catalysts
- 1994
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Ingår i: Applied Catalysis A: General. - 0926-860X. ; 113:1, s. 43-57
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Tidskriftsartikel (refereegranskat)abstract
- Unsupported vanadium antimonate catalysts with Sb/V ratios of 1 and 5 and samples with the latter ratio supported on alumina were studied in toluene and propane ammoxidation to benzonitrile and acrylonitrile, respectively, and were characterized by X-ray photoelectron spectroscopy (XPS) analysis before and after catalytic tests. Activity data for toluene ammoxidation suggest that excess antimony with respect to the stoichiometric amount required for forming the VSbO4 rutile phase affects the dispersion of the latter phase giving smaller particles. Vanadium sites are involved both in the activation of toluene and in the insertion of nitrogen in this reaction, whereas antimony does not play a specific role in the reaction mechanism. In propane ammoxidation, on the other hand, due to a higher reaction temperature with respect to toluene (500°C vs. 370°C), free vanadia on the surface of the catalyst has a negative influence on the selectivity because it promotes the conversion of ammonia to nitrogen, decreasing the surface nitrogenous species required for the selective formation of acrylonitrile. Excess antimony is thus necessary for completing the reaction between antimony and vanadium oxides, but antimony also participates in the reaction mechanism. In propane ammoxidation, in fact, XPS data show that both vanadium and antimony sites are reduced. Tentatively, vanadium sites are involved in the activation of propane, while antimony sites insert nitrogen. The differences between the toluene and propane ammoxidation mechanisms are interpreted to be primarily related to the different reaction temperatures.
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| 4. |
- Huuhtanen, Jane, et al.
(författare)
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Catalytic and Spectroscopic Studies of Vanadium Oxide Supported on Group IVb and Vb Metal Oxides for Oxidation of Toluene
- 1993
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Ingår i: Applied Catalysis A: General. - 0926-860X. ; 97:2, s. 197-221
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Tidskriftsartikel (refereegranskat)abstract
- Vanadia was deposited on TiO2, ZrO2, HfO2, Nb2O5, and Ta2O5 supports using impregnation with either an oxalic acid solution of NH4VO3 or a solution of vanadyl acetylacetonate in ethanol. Prepared samples, with a nominal vanadia content in the range 0.5–2 monolayers, were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman and infrared spectroscopy, and were used in toluene oxidation. XRD did not reveal formation of any vanadia phase. XPS spectra showed deposited vanadium to be present mainly as V5+ on all the supports. A plot of the V:( support metal) ratio determined by XPS showed agglomeration of vanadia on TiO2, ZrO2, HfO2, and Nb2O5 to occur at loadings above 0.5 monolayer, while for Ta2O5 the dispersion seemed independent of the loading. Preparations from vanadyl acetylacetonate gave superior dispersion. Raman spectra showed bands from crystalline V2O5 on all the supports except Nb2O5. Raman and infrared bands from dispersed vanadia were present in the spectra of TiO2 (three species), ZrO2 and HfO2 (both one species) supported catalysts. No bands from vanadium-oxygen vibrational modes were seen in the spectra of Nb2O5 supported samples, but a V-OH band was observed, suggesting an amorphous structure. Some evidence was obtained for formation of amorphous VTa9O25 on Ta205. The activity for toluene oxidation increased with vanadia loading for each support, and the activity varied with respect to support at all loadings in the order TiO2 > ZrO2 > Nb2O5 > HfO2 > Ta2O5. The selectivity for formation of benzaldehyde was the highest using TiO2 and Nb2O5 supports, while for benzoic acid TiO2 was the best support.
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| 5. |
- Jansen, Susan, et al.
(författare)
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EPR Studies of the Coverage Effects in TiO2(B)-Supported Vanadia Catalysts for Toluene Ammoxidation
- 1992
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 138:1, s. 79-89
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Tidskriftsartikel (refereegranskat)abstract
- EPR spectroscopy has been employed to determine the nature of vanadium (+4) species in the TiO2(B)-supported vanadia catalysts. IR and EPR studies show that multiple vanadia species are observed as a function of loading. At coverages greater than 3.5 theoretical layers, the EPR signal is reminiscent of bulk V205. At lower coverages, a different magnetic species dominates the resonance and may be attributable to tetrahedral states of vanadium at the oxide interface. The spin concentration and (g) suggest two distinct vanadia phases as a function of coverage. In this study, we have also analyzed catalysts after use in ammoxidation of toluene. These samples show strikingly different features compared with the freshly prepared samples. Changes in both the g-anisotropy and hyperfine coupling were observed relative to the fresh catalysts. Though EPR is not surface sensitive, inference into the nature of the catalytic surface can be made by comparison of catalysts of different coverage and studies of monolayer samples. A study of the TiO2(B)-vanadia interface was made by comparing monolayer catalysts prepared from slightly different technique. Our measurements show strikingly different features in V4+ species as a function of preparation.
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| 6. |
- Sanati, Mehri, et al.
(författare)
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A Study of Spreading of Vanadia on Titania Polymorphs Using Mechanical Mixtures
- 1993
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Ingår i: New Frontiers in Catalysis (Studies in Surface Science and Catalysis ). - 0167-2991. ; 75, s. 1755-1758
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Konferensbidrag (refereegranskat)abstract
- Mechanical mixtures of V2O5 and each of the three titania polymorphs TiO2(B), anatase and rutile were heated in air at 450°C. Spreading of vanadia was found to occur on all three polymorphs. After 36 hours of heating, coverage with interacting vanadia (the part that is insoluble in aq. NH3) on TiO2(B), anatase and rutile, respectively, was 54, 48 and 38 % of a theoretical monolayer. Raman spectra confirmed spreading and interaction. HREM images revealed no formation of multilayers of vanadia on titania. Use of the monolayer catalysts for toluene ammoxidation indicated their activity for benzonitrile formation to vary with respect to support in the order TiO2(B) > anatase > rutile.
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| 7. |
- Sanati, Mehri, et al.
(författare)
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Ammoxidation of Toluene over TiO2(B)-Supported Vanadium Oxide Catalysts
- 1990
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Ingår i: Journal of Molecular Catalysis. - 0304-5102. ; 59:2, s. 233-255
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Tidskriftsartikel (refereegranskat)abstract
- TiO2(B) was used as a support for vanadium oxide with loadings in the range 1/4 to 10 theoretical layers. The vanadium could be separated into two parts, which were soluble and insoluble in NH3(aq), respectively. Insoluble vanadium, in an amount corresponding to a complete monolayer at high loadings, was present as V4+ in freshly prepared catalysts. Diffuse reflectance FTIR spectroscopy was used to characterize the catalysts. The bands assigned to stretching vibrations of hydroxyl groups on TiO2(B) were found to decrease in intensity with increasing vanadium loading. Spectra in the V=O and V-O-V vibration region revealed a rather complex growth pattern. For a catalyst covered with a complete monolayer, vanadium strongly interacting with the support is present as interconnected tetrahedral V4+ units. When the loading is increased, several additional layers of V5+ in tetrahedral coordination can exist on top of the monolayer. At higher loadings, there is an abrupt change in structure, resulting in the formation of an amorphous phase built up from distorted VO6 octahedra. Also, crystalline V2O5 is formed. The catalysts were used in the ammoxidation of toluene to benzonitrile. Multilayer catalysts showed better catalytic performance in comparison with crystalline V2O5 and catalysts exposing monolayer species.
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| 8. |
- Sanati, Mehri, et al.
(författare)
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Drift Study of the Oxidation and the Ammoxidation of Toluene over a TiO2(B)-Supported Vanadia Catalyst
- 1993
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Ingår i: Journal of Molecular Catalysis. - 0304-5102. ; 81:1, s. 51-62
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Tidskriftsartikel (refereegranskat)abstract
- A DRIFT study of the adsorption and the (amm) oxidation of toluene was carried out at 150–300°C on a TiO2(B)-supported vanadia catalyst with a loading corresponding to 1.9 Mg V2O5 per m2 of the support surface area. In the absence of NH3 and gaseous oxygen, the adsorption of toluene at 150°C produces benzyl and benzoate intermediates. After one hour at the same temperature, further conversion of benzyl species, probably benzyloxy, into benzoate species is observed. Upon introduction of NH3 into the reaction cell, some benzonitrile is formed together with intermediates which possibly are (C6H5)CH(NH2)O- and (C6H5)C(NH2)(O-)2 species. Subsequent heat treatment at 300°C causes an increase in the amounts of benzonitrile and CO2 formed and the simultaneous disappearance of the precursor intermediates. On the basis of identified intermediates and observed transformations, a mechanism for the partial (amm)oxidation of toluene is proposed. The formation of CO2 is believed to pass over intermediates, which are identified as benzoquinone and butenolide species.
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| 9. |
- Sanati, Mehri, et al.
(författare)
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Kinetics and Mechanisms in the Ammoxidation of Toluene over a Titania TiO2(B)-Supported Vanadium Oxide Monolayer Catalyst. 1. Selective Reactions
- 1991
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 0888-5885 .- 1520-5045. ; 30:2, s. 312-320
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Tidskriftsartikel (refereegranskat)abstract
- A kinetic investigation of the ammoxidation of toluene was carried out over a TiO,(B)-supported vanadium oxide catalyst with a loading corresponding to a theoretical monolayer. The partial pressures of reactants toluene, oxygen, and ammonia were varied, and rates were measured for the formations of benzaldehyde and benzonitrile. By analysis of the rate dependencies on partial pressures of reactants, rate expressions completely describing the data were derived. These show that benzaldehyde is formed from two routes in which the. active ensemble accommodates one and two toluene molecules, respectively. The latter route is most facile. For the formation of benzonitrile, there are also two routes. In the route that is major at low partial pressures of ammonia, one ammonia molecule is adsorbed. With increase in the partial pressure of ammonia, a second route involving adsorption of two ammonia molecules at the active ensemble becomes increasingly important.
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| 10. |
- Sanati, Mehri, et al.
(författare)
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Kinetics and Mechanisms in the Ammoxidation of Toluene over a Titania TiO2(B)-Supported Vanadium Oxide Monolayer Catalyst. 2.Combustion Reactions
- 1991
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 0888-5885 .- 1520-5045. ; 30:2, s. 320-326
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Tidskriftsartikel (refereegranskat)abstract
- To obtain knowledge about combustion mechanisms operating in the ammoxidation of toluene, the formation of carbon oxides was studied over a Ti02(B)-supported vanadium oxide catalyst with monolayer loading. The partial pressures of reactants were varied and kinetic rate expressions, completely describing experimental data, were derived. A comparison of the expressions obtained with those for the formation of nitrile and aldehyde shows that carbon oxides are formed at sites that are not involved in the mechanism of partial oxidation. Furthermore, C02 and CO are formed in routes having a common type of active ensemble. All of the adsorption steps are fast and in equilibrium. For the formation of C02 and CO, the rate-limiting step in both cases comprises a chemical transformation of which the details are unknown. The introduction of ammonia leads to a strong decrease of the rates for formation of carbon oxides.
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