(617fg) Metal Distribution after Drying of Supported Catalysts with High Metal Loadings

Supported catalysts are essential components in a variety of industrial processes, ranging from catalytic converters to production of new drugs. They are generally required because of their high surface area and high mechanical and thermal stabilities. The performance of a catalytic process is intimately related to the catalyst design - uniform, egg-yolk, egg-shell and egg-white metal profiles. Although catalyst preparation and catalytic processing have been investigated for many years, many aspects of catalyst manufacturing are still not fully understood and in industry the design of catalysts is usually based on trial and error, which is expensive and time-consuming, and does not offer assurances on the final results.

It is generally believed that the metal profile is controlled by the conditions that are applied during impregnation, however experiments have shown that drying can also significantly impact the metal distribution within the support. Therefore, to achieve a desired metal profile we need to understand both impregnation and drying. Controlling the impregnation and drying conditions can enhance catalyst performance, and minimize the production of useless batches that have to be disposed, or recycled.

In this work we have investigated the metal distribution during the high concentration impregnation and subsequent drying of supported catalysts. Previous work found that high concentration impregnation (3-4 molar) and drying of Ni/Alumina catalysts yield a uniform metal distribution. At intermediate concentrations (~1 molar), the Ni\Alumina catalysts yielded egg-shell profiles. Theoretical models to simulate the impregnation and drying processes of Ni/Alumina had been previously developed. By not taking into account the crystallization process at higher concentrations, the drying simulation was able to obtain uniform metal distributions. Essentially, the simulation assumes that the metal remains molten throughout the drying process and that solidification during cooling does not affect the metal distribution post-drying. Good comparisons between the simulation and experimental results were obtained for Ni/Alumina. However, experimental results also yielded uniform metal distributions when drying was performed below the melting point of the metal salt when crystallization effects during drying should have been significant.

Mechanisms for the uniform metal distributions were investigated, including metal salt melting point depression due to the microporous support structure and pore blockage. However, we propose deliquescence as novel mechanism for the uniform metal distributions. We hypothesize that deliquescence, or the tendency of a crystalline solid to become a liquid, occurs at all drying temperatures due to the high relative humidity within the pore structures, allowing the metal salt to remain in a liquid state as if it was molten. Metal salts with lower solubility do not deliquesce as readily and were found to not yield uniform metal profiles at higher concentrations.

Our experiments have allowed us to better understand the fundamental mechanisms that occur during drying, and to develop a strategy that can generate desired metal profiles. Although the results presented are based on a particular metal/support system, they serve to provide physical insight into the fundamentals of the drying processes.