Substituting Vanadium in the M1 Structure of (Mo,V)Ox for Oxidative Dehydrogenation of Light Alkanes
Sabrina Jung, Pierre Kube, Annette Trunschke and Robert Schlögl
Inorganic Chemistry, FHI der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195, Berlin, GER
In catalysis, mixed oxides are widely used in oxidation reactions or electrocatalysis. MoVTeNbOx and its presumed simpler version MoVOx (both possessing the complex M1 structure) are highly active catalysts in the oxidation of propane and ethane, respectively1. In the case of alkane oxidation, V5+=O surface species have been proven to not only activate the first C-H bond in the alkane, but to activate two C-H bonds simultaneously, forming the olefin in one single step2. While a certain content of vanadium is necessary to form the M1 structure during synthesis, and to perform the catalytic oxidation of small alkenes, too much vanadium has also been proven to have negative consequences for catalysis.
In the microwave-based hydrothermal synthesis of (MoV)Ox, 1 % of the original vanadium concentration was replaced by the same amount of a transition metal (Ti, Mn, Te, Zn, Co, Cr in form of sulphates). X-ray powder diffraction proves every sample to have the desired M1 structure without any additional by-phases (Fig. 1). Scanning Electron Microscopy (SEM) reveals the expected needle-like morphology of M1 crystals. According to X-ray Fluorescence (XRF) measurements, all samples possess a molybdenum content of roughly 69.4 at%, vanadium content of roughly 30.3 at%, and a substituent metal content between 0.2 and 0.3 at%. Unsubstituted MoVOx is made up of 69.2 at% and 30.8 at% of molybdenum and vanadium, respectively. Preliminary catalytic testing in propane oxidation (300 °C, 20 mL/min, propane/O2/He 10:5:85) at low propane conversion (roughly 1 %) show selectivity to propene at over 70 %, to CO2 and CO at 8 % and 15 %, respectively. Selectivity to acrylic acid and other C3 species vary depending on the applied transition metal substituent. Thorough catalytic tests in ethane and propane oxidation will give more insights into the dependence of conversion and selectivity dependency on the substituent in the M1 structure. The impact of the transition metal precursors on the synthesis of the desired M1 phase will be discussed.