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Combicat: Selective Oxidation of C3 molecules
Abstract:
The industrial and economic relevance of research in selective partial oxidation catalysts are clear as about one quarter of all organic products produced worldwide are synthesized via catalytic selective partial oxidation reactions [1]. The Mars-van Krevelen mechanism is widely accepted for selective partial oxidation reactions. The mechanism describes the role of the so-called active 'lattice' oxygen for selective partial oxidation [2]. The nature of the 'lattice' oxygen is yet not fully unravelled and it is still under debate which of the possible metal-oxygen species actually take part in the oxygen insertion reaction [3,4], but it is clear that the diffusion kinetics are strongly affected by the elemental composition and geometric structures of the catalyst phases. Several groups of binary and ternary molybdenum suboxide structures doped with elements such as Nb, W, V, and Ta have been synthesized at a broad variation of the element ratios [5,6].
Here we report on the synthesis and characterization of pure molybdenum oxide compounds as catalysts for selective oxidation of propene. The solid molybdenum oxides are obtained by precipitation. Ammonium HeptaMolybdate (AHM) dissolved in water at a concentration ranging from 0.28 mol/l to 2 mol/l calculated on Mo, is used as starting material. The precipitation agent, HNO3 (1 mol/l – 5 mol/l) is applied at temperatures between 30ºC and 70ºC. The samples are filtered and dried in an desiccator over dry gel. It is crucial not to wash the samples.
The resulting solid reveals a broad spectrum of reflections in X-ray diffraction (XRD) spectra, as shown in Figure 1. To get more insight in the local microstructure of the precipitated material, transmission electron microscopy (TEM) is used. Figure 2 shows a representative TEM image. The sample consists of large agglomerates sized in the range above 1m and smaller (<100nm) agglomerates. The large agglomerates are unsuitable for high-resolution transmission electron microscopy, as they are not transparent for the electron beam. However, lattice fringe images are obtainable at the smaller agglomerates, as shown in Figure 3. The figure reveals 3-5nm crystalline particles with a characteristic lattice distance of 0.37-0.38nm, which coincides with the distance between Mo atoms in corner sharing MoO6 octahedra. The XRD pattern in Figure 1 reflects rather a large variety of different ordered building blocks, than a single phase of the solid. These building blocks are considered to serve as seeds for the catalytic relevant phase, and furthermore prevent the structure to recrystallize to well-defined, well-ordered oxides under reaction conditions, as orthorhombic MoO3, which is well known to be an inactive catalyst for the oxidation process.
1: R. K.Grasselli, Catal. Today 49 (1999) 41.
2: P. Mars, D. W. Krevelen, Chem. Eng. Sci. 3 (1954) 41.
3: K. Brückman, R. Grabowski, J. Haber, A. Maurkiewicz, J. Slocynski, T. Wiltowski, J. Catal 104 (1987) 71.
4: J. Haber, E. Lalik, Catal. Today 33 (1997) 119.
5: T. Ekström, M. Nygren, Acta Chem. Scand. 26 (1972) 1827.
6: T. Ekström, M. Nygren, Acta Chem. Scand. 26 (1972) 1836.
