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Schlagwörter:
preparation, chemistry in solution, MoVW suboxides
8. Mixed V/W/Mo oxides for selective oxidation
Zusammenfassung:
Motivation
Mixed oxide catalysts containing molybdenum, vanadium and tungsten are widely used in industry for partial oxidation reactions [1, 2, 3]. Previous work revealed (MoVW)5O14 to be the active phase of the catalyst. The partial oxidation from acrolein to acrylic acid is performed on such a system [4, 5]. This catalytic system is characterised by high long term stability, excellent turn-over rates and high selectivity. Different preparation steps are necessary to form a single phase, crystalline, ternary oxide (MoVW)5O14 as a model catalyst. In a previous paper [6] it was suggested that a precursor of this oxide is already formed in solution. Therefore this poster is dedicated to propose a structure of the dissolved species and a reaction mechanism leading to the formation of this structure in solution.
Experimental
For the synthesis of this oxide, solutions of ammonium heptamolybdate, ammonium metatungstate, and vanadyl oxalate were spray-dried followed by different thermal treatments. The structures of the materials formed in solution were studied using UV/Vis Raman and ESR spectroscopy.
Results
It is suggested from this data that a molecular structure is already formed in solution which seems to be closely related to that of the final crystalline Mo5O14-type oxide. Raman spectroscopy shows bands at 964, 943, 912, 821, 792, 709 and 682 cm-1. The bands at 943 and 792 cm-1 could be assigned to AHM. Bands at 964, 879, 821, 709, und 682 cm-1 do not belong to AHM and point to a polymeric species This result could be corroborated by UV/Vis and ESR spectroscopy. Moreover ESR shows that the state of oxidation of molybdenum and tungsten is +6. The state of oxidation of vanadium is +4. Vanadium exists as vanadyl type.
The spray-dried sample shows bands at 943, 872, and 818 cm-1. A higher degree of polymerisation in the dried sample could be responsible for the shift of the band at 872 cm-1 compared to the spectrum in solution 879 cm-1.
Literature
[1] Hibst, H.,Unverricht, S. (BASF), DE 19815281 A 1.
[2] Tanimoto, M., Himeji-shi, H., Mihara, I., Aboshi-ku, H., H., Kawajiri, T., Himeji-shi, H., (Nippon Shokubai), EP 0 711 745 B1.
[3] Tenten, A., Hibst, H., Martin, F-G., Marosi, L., Kohl, V., (BASF), DE 4405514 A1.
[4] Mestl, G., Linsmeier, C., Gottschall, R., Dieterle, M., Find, J., Herein, D., Jäger, J., Uchida, Y., Schlögl, R., J. Mol. Catal. A 162 (2000) 455-484.
[5] Dieterle, M., Mestl, G., Jäger, J., Hibst, H., Schlögl, R., J. Mol. Catal. A 174 (2001) 169-185.
[6] Knobl, S., Zenkovets, G. A., Kryukova, G. N., Ovsitser, O., Dieterle, M., Mestl, G. , Schlögl, R., J. Catal, submitted.
