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Combicat: Selective Oxidation of C3 molecules
Abstract:
There is a growing effort in Malaysia to develop a novel catalyst material that converts the vast natural resources of palm oil into higher valued oleochemicals through selective oxidation. Molybdenum oxide based catalysts are highly suitable for such processes and empirical optimization of the catalyst by doping with foreign cations has far reached a high level. The current material with outstanding activity for propane and propene selective oxidation is MoVTeNb Oxide [1]. The reason for its high activity is considered to lie in structural arrangement of the material and the huge variety of Mo-O bond lengths. Two phases are identified, the first one is pseudo hexagonal and the second one is orthorhombic with pentagonal bipyramides as general features.
Whilst the calcined catalyst has been extensively investigated, the formation mechanism of such structures is still completely unknown. The aim of this work is to prepare tailored catalyst material by using Nano Technology (Bottom up Approach) to assemble molecular units of molybdates to supramolecular structures. Understanding such a process is only successful, if the chemical complexity of the system is significantly reduced by at the same time maintaining the structural complexity.
The current preparation routine for MoVTeNb Oxides is the “slurry method”, in which a mixture of solid and liquid phases (slurry) is spray dried and calcined. Such procedure is extremely complex, as it is affected by a multitude of variables. Reproducibility and catalyst optimization can only be successfully achieved if every synthesis parameter is understood.
EXPERIMENTAL
Mo1V0.33Te0.22Nb0.11Ox was prepared according to the method mentioned in the literature. Two solutions were prepared, the first one containing appropriate amounts of ammonium heptamolybdate, ammonium metavandate and telluric acid that were heated to 80°C, and the second one niobium oxalate. The two solutions were combined at 40°C, leading to precipitation of slurry, as described in the literature. This slurry was spray dried and calcined first in air at 275°C for two hours and subsequently in helium at 600°C for another two hours. The resulting black material was analyzed by XRD and TEM. In order to study the effect of every metal compound single and binary solutions prepared and analysed by Raman spectroscopy.
RESULTS AND DISCUSSION
The Raman spectrum of the combined MoVTe solution (solution 1) showed a new terminal Mo=O band at 1006 cm-1 that is not found in the Raman spectra of the single compounds. In addition the band at 648 cm-1 that is characteristic for telluric acid was diminished. This data indicates a chemical reaction during the mixing and heating process, in which tellurium could have been incorporated into the heptamolybdate units, as observed in (NH4)6(Mo6TeO24)(TeOH)6 (H2O)7 single crystals [2]. Addition of niobium oxalate leads to precipitation. As in the synthesis process of the similar system of Mo(VW)5O14, a mechanism involving formation of oligomolybdates through connection via linker atoms (Nb in this case) can be proposed. The spray-dried solid containing Mo, V, Te and Nb shows the main Raman bands of AHM (slightly shifted), but also the additional band at 1006 cm-1, indicating that the structure developed in solution was preserved. Both the spray dried material and the precalcined material are poorly crystalline. Crystallization only occurred after calcination in inert gas at 600°C. Analysis by XRD and TEM confirms the existence of the major phases M1 and M2, described in the literature.
REFERENCES
1. P. DeSanto Jr., D. J. Buttrey, R. K. Grasselli, C. G. Lugmair, A. F. Volpe, B. H. Toby, and T. Vogt, Topics in Catalysis Vol. 23, Nos. 1–4, (2003)
2. T.Evans, Act. Cryst., B 30, 2095 (1974),
3. S. Knobl, G. A. Zenkovets, G. N. Kryukova, R. I. Maksimovskaya, T. V. Larina, N.T. Vasenin, V.F. Anufrienko, D. Niemeyer, R. Schlögl, PCCP,5,(2003) (Advance Article).
