Heterogeneous catalysts in industry

A catalyst is a substance added to a process to increase the reaction rate without being consumed in the reaction. Catalysts speed up the reaction process by reducing activation energy or altering the reaction mechanism. There are many types of catalysts, which makes the term “catalyst” a container term. Generally, we classify catalysts into four groups: (1) heterogeneous, (2) homogeneous, (3) heterogenized homogeneous catalysts, and (4) biocatalysts such as enzymes and acid-based catalysts.

Homogeneous and heterogeneous catalysts are often referred to as surface catalysts. We classify these catalysts based on whether they occupy the same phase as the chemical reaction mixture. If they do, they are called homogeneous; if not, they are called heterogeneous.

Typically, heterogeneous catalysts exist in a solid state. When these catalysts are submerged in a liquid or gas reaction mixture, the reactants adsorb onto binding sites on the catalyst’s surface. Heterogeneous catalysis is crucial for the industrial production of chemicals because it enables selective product creation. In fact, approximately 90% of chemicals produced by volume rely on solid catalysts.

In this article, we will specifically focus on heterogeneous catalysts and their significance in various chemical processes. We will explore their mechanisms, advantages, and applications in industrial settings.

Heterogeneous and surface catalysis play a vital role in the production processes of plastics and polymers, such as polyethylene. In this context, the catalysts used are known as Ziegler-Natta catalysts. These catalysts are essential for manufacturing a wide range of products, from plastic wrap materials to cups used in instant noodle packaging.

Another common application of heterogeneous catalysis is found in the catalytic converters of cars. These converters contain conversion metal catalysts fixed on a solid-phase support. The solid-phase catalyst reacts with the gases from the car’s exhaust, increasing the reaction rate. This process reduces the formation of less toxic byproducts from pollutants in the exhaust stream, such as carbon monoxide (CO).

The catalytic converter exemplifies a surface catalytic process. In this process, reactant molecules adsorb onto a solid surface before interacting with the catalyst to produce the desired product. The reaction rate in catalysis increases with the surface area of the catalyst that comes in contact with the reactants.

To optimize this, catalytic converters are designed to maximize surface area. They feature a honeycomb monolithic structure that provides a large surface area while minimizing pressure drop. A high surface area porous material is coupled with a low-pressure drop design to ensure efficient catalytic activity. This innovative structure guarantees the stabilization of highly dispersed catalytic species, enhancing the overall effectiveness of the catalytic converter in reducing harmful emissions.

Heterogeneous catalysts offer a distinct advantage in production processes. They can be easily separated from a reaction mixture, for example, through filtration. This ease of retrieval streamlines production processes significantly.

However, a clear disadvantage of heterogeneous catalysis is the saturation limit of the catalyst’s surface area. Once the surface becomes completely saturated with reactant molecules, the reaction process halts. The chemical reaction mixture must leave the catalyst’s porous surface, opening up space for new reactant molecules to be adsorbed. Therefore, saturation becomes a rate-limiting factor in industrial processes that utilize heterogeneous catalysts.

Despite this limitation, the benefits of heterogeneous catalysis typically outweigh the disadvantages. The catalyzed reactions are generally much more efficient in terms of reaction speed and selectivity compared to uncatalyzed reactions.

To fully understand the effectiveness of a catalyst, it is crucial to examine the active site structures. Structure-activity relationships based on the physical properties of the catalyst are vital. Aspects such as porosity, pore size, surface area, and physical transport play significant roles in determining a catalyst’s effectiveness for its intended purpose. By optimizing these properties, we can enhance the overall performance of heterogeneous catalysts in various applications.

At Delft Solids Solutions, we prioritize catalyst characterization to optimize performance. Our approach focuses on both the catalyst support and the active metal phase. We primarily use physical gas adsorption to analyze support properties, providing valuable insights into specific surface area, pore volume, and pore size distribution.

This information is crucial for understanding the transport of molecules in and out of the catalyst, impacting its effectiveness and potentially leading to issues like coke formation.

To probe the active phase, we employ chemical gas adsorption techniques using reactive gases, such as hydrogen or carbon monoxide, along with basic or acidic probe molecules. These methods yield quantitative data on metal surface area, metal dispersion, and crystallite size, allowing us to correlate these properties with catalytic performance.

For acidic or basic catalysts, counter probe molecules assess specific active sites. It’s important to consider that the size of the probe molecule can significantly affect our understanding of catalyst performance. A smaller probe molecule may overestimate the active surface area because it accesses sites more easily than the larger reactant molecule used in actual processes.

Physical transport within catalyst particles also plays a critical role and relates closely to the porous structure of both the catalyst and its support. By thoroughly characterizing your catalysts, we help you enhance their performance and improve the efficiency of your catalytic processes.

Mercury porosimetry is an effective technique for analyzing the porous characteristics of catalysts. It utilizes the non-wetting properties of mercury to gather information on porosity, pore volume, pore size distribution, and apparent density.

This method covers a wide dynamic range, allowing investigation of pore sizes from 4 nanometers to 800 micrometers. This capability makes mercury porosimetry extremely useful for studying powdered catalysts as well as shaped entities like extrudates, spheres, and trilobes.

At Delft Solids Solutions, we also offer the option to investigate the contact angle of mercury with the sample. This measurement is important because it influences pore size calculations, leading to more accurate analysis results. By using mercury porosimetry, we provide comprehensive insights into the porosity of catalysts, enabling our clients to optimize their catalytic processes effectively.

As indicated, the physical transport of molecules in and out of the catalyst structure is of great importance. We can handle both gases and vapors (water and organic vapors) under a wide variety of conditions in terms of pressure and temperature to measure real-time the interaction with the catalyst material. Transient uptake or release curves provide the desired insight into how the porosity impacts the transport characteristics and ultimately the catalyst performance.

At Delft Solids Solutions, we actively seek to interact with our peers to optimize our investigations for you. Our research is conducted in close consultation with you, ensuring that we execute our work under conditions that closely match the targeted application of the product or the specific problem at hand.

We encourage you to reach out to us. We are always eager to share our know-how and expertise. Your challenges and needs drive our research, and we look forward to collaborating to find effective solutions together.