(487i) Drying of Supported Catalysts – Impact of Film-Breakage and Crystallization On Metal Distribution

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.

Supported catalysts are usually prepared by impregnation, where a porous support is contacted with a liquid solution that contains the desired metal as a dissolved salt. This step is usually followed by the evaporation of the liquid solvent and that is drying. After drying reduction and calcination are carried out. 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 the impregnation, but we also need to have a fundamental understanding of the drying stage. 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 focus on the impact of film-breakage and crystallization on drying of supported catalysts, using experiments and simulations based on a Nickel/Alumina system. Film-breakage is an important phenomenon during drying. At the beginning of drying, the liquid water phase is continuous in the support. As evaporation proceeds, isolated domains are gradually formed. Finally, the liquid is only found in the isolated domains. We have known that for a regular drying process, convection dominates the early stage of drying, driving the metal to move toward the support surface, and diffusion may dominate the late stage of drying, leading the metal to move back to the support center. Since both convection and diffusion are enhanced with an increase in the liquid flux and the liquid flux greatly reduces once film-breakage occurs, we believe that film-breakage suppresses the egg-shell distribution at the early stage of drying and favors the egg-shell distribution at the late stage of drying. This has been validated from our experiments and simulations. In general our simulation results can match our experimental measurements fairly well if the effect of film-breakage is considered in the model. For high metal loading conditions, crystallization may become important during drying because the metal concentration is likely to be above its solubility. We find that crystallization can greatly enhance the egg-shell profiles by increasing the amount of the metal deposited near the surface, and decreasing the dissolved metal concentration near the surface and thus reducing the back-diffusion. The comparison of the importance of crystallization and film-breakage on the generation of egg-shell profiles is also considered in this work.