Controlled precipitation of nanostructured Molybdenum oxides for selective oxidation reactions

The selective oxidation of small organic molecules is performed over oxide catalysts comprising the elements Mo, V, Te, Nb and W as essential ingredients. It is commonly accepted that highly specific local electronic structures of the active metal sites are essential for the catalytic performance [1]. As simple Mo oxides contain only a small selection of these structures, it is easy to understand that surface defects that inevitably will alter the local connectivity are the tool by which the optimisation of the catalyst is performed. The control variable is usually the addition of hetero-cations that are believed to add specific functions to the base properties of MoO3 [2;3]. Their role as structural promoters distorting neighbouring molybdate polyhedra is rarely considered. In order to minimise chemical complexity and to optimise synthetic strategies towards catalytically relevant molybdates it is desirable to explore the potential of controlled precipitation chemistry coupled with avoiding the usual high-temperature calcination that eliminates all non-orthorhombic binary molybdates. The present study investigates the sequence of events during decreasing pH precipitation that is the normal method of preparation. It highlights the potential to synthesise complex connectivity of molybdate polyhedra without having to use thermal defect formation procedures. EXPERIMENTAL Ammonium heptamolybdate (AHM) was employed as Mo source and HNO3 was used as precipitation agent. Most experiments were carried out at ambient temperature. The concentration of the AHM solution was chosen to be 0.1 molar (equivalent to 0.7 molar in [MoO4]2-). A titration automat (Mettler Toldedo DL 77) was used with 100 ml vessels. The pH data were recorded and analysed digitally. The precipitates were characterised by XRD and Raman spectroscopy. RESULTS AND DISCUSSION Principally the herein described method allows producing four different families of materials in a controlled manner. High molybdenum concentrations and low temperature (30 °C) lead to a spontaneous precipitation of a supramolecular compound, which is very similar to the Mo36O112 reported by Krebs [4] whereas low concentration and high temperature leads to the formation of a hexagonal MoO3. Whilst the use of lithium as counterion leads to orthorhombic MoO3, the use of potassium together with high temperatures produces Trimolybdate. The Mo36O112 species is the major compound in solution at low pH and precipitates as soon as the solubility product is reached. It is remarkable that the connectivity has changed compared to the starting material. In the AHM precursor only corner sharing octahedra are observed. In the Mo36O112 edge sharing connectivity prevails and a pentagonal bipyramid is formed. This structural motif appears also in catalytic active material such the M1 phase of Mo1V0.33Te0.22Nb0.11Ox or Mo(VW)5O14, whilst the hexagonal phase observed shows a big similarity to the M2 phase of Mo1V0.33Te0.22Nb0.11Ox. REFERENCES 1. Grasselli,RK (2002) Topics in Catalysis 21: 79 2. Grasselli,RK (2001) Topics in Catalysis 15: 93 3. Mestl,G, Linsmeier,C, Gottschall,R, Dieterle,M, Find,J, Herein,D, Jager,J, Uchida,Y, and Schlogl,R (2000) Journal of Molecular Catalysis A-Chemical 162: 455 4. Paulat - Böschen, I., Buss, B., Krebs, B., Acta Cryst. 1974, 30, 48