Establishing the role of graphite as a shaping agent of vanadium - aluminum mixed (hydr)oxides and their physicochemical properties and catalytic functionalities

This work contributes to the establishment of a scientific basis for the understanding of the effect of shaping on the physicochemical properties and functionalities of catalytic powders. In particular, the influence of graphite (G) as a shaping agent for vanadium–aluminum mixed (hydr)oxides, VAlO, is presented herein. Graphite was added in different loadings (x = 1, 3, and 7 wt %). VAlO-xG hydroxides were tableted and then calcined as to obtain VAlO-xG mixed oxides. Both series of materials were characterized and tested in propane oxidative dehydrogenation (ODH). Correlation of X-ray photoelectron spectrometry (XPS) and Raman data permitted establishing that VAlO-xG hydroxides are constituted by graphite particles physically attached to the amorphous VAlO structure. The VAlO hydroxide structure was found to be composed of Al(OH)3 and AlOOH linked to decavanadates-, metavanadates, and isolated or cluster vanadyl V═O species. Calcination of VAlO-xG caused further polymerization of vanadium species into decavanate chains and V2O5-like structures connected to a γ-Al2O3-like matrix. Though graphite was not incorporated into the structure of VAlO, XPS analysis showed that it actively interacts with the VAlO surface. Such interaction favored propylene selectivity in ODH. Particularly, graphite containing VAlO-xG displayed higher production of propene over COx at high propane conversion. On the other hand, though graphite boosts the mechanical resistance of the VAlO-xG tablets, the surface area of the materials, as determined by the application of χ-theory, was found to significantly decrease after shaping and further calcination. This trend was explained considering pore widening during tableting. Finally, characterization performed on particles recovered from crushed VAlO-xG tablets showed that graphite particles were randomly distributed within the shaped catalysts.