Raoul Naumann d'Alnoncourt

Synthesis of MoVTeNb Oxide Catalysts with Tunable Particle Dimensions

Reliable procedures for the controlled synthesis of phase‐pure MoVTeNb mixed oxides with M1 structure (ICSD 55097) and tunable crystal dimensions were developed to study the structure sensitivity of the selective oxidation of propane to acrylic acid. A series of powdered M1 catalysts was successfully prepared on a gram scale by using a hydrothermal‐based route, purification of biphasic M1‐M2 (M2 phase – ICSD 55098) oxide systems, and an innovative approach utilizing a superheated water vapor treatment of calcined precursors. The influence of the preparation technique on the particle morphology and the size is discussed. Detailed experimental studies highlight that the as‐derived catalytic materials were indeed phase‐pure and compositionally uniform MoVTeNb oxide M1 powders, composed of single‐crystalline and structural defect‐free crystals grown along the c axis. The morphologically different catalysts were studied in the selective oxidation of propane to acrylic acid, revealing that active sites appear on the entire M1 surface and illustrating the high sensitivity of catalyst performance on the catalyst synthesis method.

A New Way of Probing Reaction Networks: Analyzing Multidimensional Parameter Space

Technically relevant partial oxidation reactions represent complex reaction networks. Establishing a kinetic model for a system of multiple consecutive and parallel reaction steps is a challenging goal. The synthesis of acrylic acid by oxidation of propane using MoVTeNb mixed oxide as catalyst is such a reaction network. In an on-going study, a 10- fold parallel reactor set-up is used to vary systematically reaction conditions in a broad range over a single, well-defined MoVTeNb oxide. Selectivity and product yield in a multidimensional parameter space can give insight into the reaction network. Apparent activation energies and reaction orders of propane are derived for several conditions. Optimum reaction conditions within the investigated parameter space are specified. The results presented within this contribution contain about 200 data points measured in steady states each corresponding to reaction conditions that differ in temperature, contact time, and propane feed concentration. The fact that this data was collected in less than two months shows clearly the advantage of parallel screening of reaction conditions for mechanistic studies.

Unusual Phase Evolution in MoVTeNb Oxide Catalysts Prepared by a Novel Acrylamide‐Gelation Route

The phase evolution of MoVTeNb mixed oxides derived from a novel acrylamide‐gelation approach combined with subsequent washing by hydrogen peroxide and/or annealing in argon was investigated by using powder X‐ray diffraction and scanning electron microscopy in conjunction with energy‐dispersive X‐ray spectroscopy. The calcination of acrylamide‐gelation derived carbon‐based xerogels in air at 410 °C afforded at unexpected low temperature the formation of a phase mixture containing a significant amount of small MoVTeNb oxide crystallites consisting of the M1 phase (ICSD no. 55097). Hydrogen peroxide treatment drastically changed the phase evolution upon annealing in argon at 600 °C. Whereas further enrichment of the M1 phase took place for the untreated precursor, annealing of the washed material resulted in complete collapse of the M1 phase, yielding a ternary MoVNb mixed oxide with tetragonal Mo5O14‐type structure (ICSD no. 27202). The reducing effect of organic residues on phase evolution during calcination of the carbon‐inorganic xerogel in air is discussed. The presence of side phases and the deficiency in tellurium upon annealing have a critical impact on the phase composition of the final product. The catalysts exhibit significant activity in partial oxidation of propane to acrylic acid. The analysis of the product distribution suggested that the activation of both propane and propylene intermediates proceed exclusively over the M1 phase, while side phases do not play a significant role in the reaction.