Influence of impregnation parameters on the axial Mo/γ-alumina profiles studied using a novel simple technique

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The development of a novel, simple technique allowed us to investigate the influence of various impregnation parameters on the axial Mo profiles achieved on γ-alumina extrudates. The effects of pH, temperature, volume, and concentration of the molybdate solutions, as well as the impregnation time, the rate of drying, the modification of γ-alumina with F − ions, and the nature and concentration of various competitors, have been systematically studied. It was found that decreases in pH as well as increases in the concentration of the molybdate solutions, of the impregnation temperature, and of the rate of drying cause a progressive transformation of the Mo profile from uniform to eggshell type. Doping of γ-alumina with F − ions and the use of NH 4F, H 3PO 4, and citric acid as competitors transformed the Mo profiles from eggshell to uniform. The change in volume of the impregnating solution had no effect on the Mo profiles. The effect of the impregnation time was found to be complicated. Increases in the impregnation time, until a critical value sufficient for the complete imbibition of the extrudates, allow for the transfer of great amounts of molybdate to the interior of the extrudates, leading to more uniform profiles. Further increases in the impregnation time caused a considerable increase in the sharpness of the Mo profile. Most of the above observations were explained on the basis of derived equations, adopting a very simple macrodistribution model. Finally, it was demonstrated that small chromatographic columns filled with powder supports may be used to study active ion profiles on catalytic supports.