Understanding the molecular basics behind catalyst shaping: Preparation of suspensions of vanadium–aluminum mixed (hydr)oxides

• Shaping of vanadium–aluminum mixed (hydr)oxide (VAlO) catalysts was investigated.
• A method to obtain chemically and physically stable VAlO suspensions was developed.
• Characterization during preparation provided molecular insight on the process.
• Correlation between hydroxylation–dehydroxylation and chemical stability was found.
• Rheologically homogeneous VAlO suspensions were slip casted to shape catalysts.

Shaping is often disregarded in catalysis science. Aiming to fill the gap between catalysis in academy and industry, the objective of this work was to systematically study the shaping of vanadium aluminum (hydr)oxides (VAlO) via wet methods. These materials are formulations active in propane ODH and ammoxidation. The physical and chemical stability of suspended VAlO powders was evaluated along with the development of a method to prepare rheologically homogeneous concentrated suspensions of VAlO. Their physical stability was assessed by following changes in relevant properties; namely, their particle size distribution, density, surface area and porosity, and isoelectric point (IEP), by considering the sedimentation of VAlO in aqueous medium, and by evaluating preparation parameters such as: effect and concentration of dispersant, powder content, and sonication time on the rheology of the suspensions. Results showed that due to their physical properties, the preparation of concentrated VAlO suspensions is impracticable without the use of a dispersant, herein poly(acrylic acid) (PAA), that counteracts particle settling. The chemical stability of suspended VAlO powders was determined by preparing diluted suspensions at different pHs and then analyzing the phenomena behind the dissolution of vanadium and aluminum and its correlation with the surface chemistry of the recovered powders. On the basis of the above analysis, appropriate conditions for having physically and chemically stable concentrated VAlO suspensions were established: particularly, pH near the IEP for minimum metal dissolution, VAlO concentration ≤50 wt.%, PAA concentration 0.2–0.5 wt.%, and sonication time of 1 min. Slip casted cylinders of VAlO were obtained using these conditions.

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