(492f) Mechanisms of Drying of Supported Catalysts for Low and High Metal Loadings

Supported catalysts are used in a variety of industrial processes, ranging from catalytic converters to the production of new drugs. These catalysts have many advantages, such as a high surface area, a low amount of the often expensive active component (Pd, Pt, etc.) and high mechanical and thermal stability. Clearly, the catalyst design has a pronounced effect on the performance of a catalytic process. With respect to the distribution of the active component in the support materials, four main categories of metal profiles can be distinguished, i.e., uniform, egg-yolk, egg-shell and egg-white profiles. The choice of the desired metal profile is determined by the required activity and selectivity, and tailored for specific reactions and/or processes. Although the development and preparation of supported catalysts have been investigated for many years, many aspects of the various catalyst manufacturing steps are still not fully understood, and in industry the design of catalysts is predominated by trial and error experiments, which are expensive and time-consuming, and do 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 where the metal contacts the solid support for the first time. However, experimental work has shown that drying may 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 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 developed a theoretic model for drying of supported catalysts, and compared simulations and experiments based on a Nickel/Alumina system. For low metal precursor concentrations, initially convection drives the metal to move toward the external surface, leading to a pronounced egg-shell profile. After a certain point, back-diffusion becomes more important, driving the metal to move away from the surface. Once the water content inside the support is below a certain amount, film-breakage starts to dominate the system. Most of liquid solutions are located in the isolated domains. Therefore, further drying can not change the metal distribution any more. The drying mechanism for high metal loadings is quite different. For metal precursors with a high concentration and a low melting point, such as nickel nitrate hexahydrate, most of the metal precursor is dissolved or melted in the liquid phase during drying if the drying temperature is higher than the melting point. At the beginning of drying, convection drives the liquid solution to move toward the external surface, and then water evaporation leads to an increase in the metal precursor concentration near the surface. Once the metal precursor concentration on the surface is close to the pure metal precursor concentration, the drying rate becomes very low on the surface and the drying front starts to move inside the particle. This results in a high metal concentration regime near the surface. After a certain point, capillary force starts to dominate the liquid flow, sucking the solution from the surface to the pores. This results in an egg-white profile inside the support. At the end of drying, the metal precursor flows to the center due to the capillary force and flows to the surface due to diffusion, leading to a nearly uniform distribution. For metal precursors with a high concentration and a high melting point, pore-blockage and crystallization become important. Generally, our simulations match the experiments fairly well.

The goal of this study is to better understand the fundamental mechanisms during drying, and to develop a drying strategy that can generate desired metal profiles, using theoretical simulations and experiments. The models used in the present work can capture the essential physics of drying while still maintaining a level of generality. Although the results presented are based on a particular metal/support system, they serve to provide physical insight into the fundamentals of the drying process.