Mathematical modeling and Magnetic Resonance Imaging experimental study of the impregnation step: A new tool to optimize the preparation of heterogeneous catalysts

Optimization of supported heterogeneous catalysts requires a careful control of their synthesis conditions and in particular of the metal impregnation step. This paper presents a theoretical and experimental study of both dry and diffusional impregnation of Ni/γ-Al2O3 catalysts. The advanced characterization technique Magnetic Resonance Imaging was used to monitor in-situ the impregnation step, which provided the necessary information to develop the model. The model accurately describes the active phase distribution during impregnation by taking into account capillarity (in the case of dry impregnation), diffusion in the fluid phase and adsorption/desorption phenomena. It was demonstrated that the adsorption of nickel ions on the alumina surface is extremely fast, favoring the removal of metal ions from the fluid phase. As a consequence, the limiting step of impregnation is the diffusion of nickel ions in the fluid phase. A good agreement between experimental and simulated results was achieved by adjusting only two parameters, namely total concentration of the active sites and adsorption equilibrium constant. By neglecting capillary action and using the same optimized parameters, the model also allowed describing diffusional impregnation, which illustrates its robustness. This model can predict the distribution of the active phase in the support as a function of the impregnation conditions and can therefore be applied as a new tool to optimize the impregnation step of heterogeneous catalysts.