Key Difference – Homogeneous vs Heterogeneous Catalyst
Catalysts are of many types, but they can be mainly categorized into two groups as homogeneous catalysts and heterogeneous catalysts. The key difference between homogeneous and heterogeneous catalysts is that homogeneous catalysts can always be found in the liquid phase whereas heterogeneous catalysts can be found in all three phases of matter: solid phase, liquid phase, and gas phase.
Catalysts are compounds that are used to obtain maximum yield under mild conditions. This means, catalysts can increase the yield of a particular reaction and speed-up the reaction.
CONTENTS
1. Overview and Key Difference
2. What is a Homogeneous Catalyst
3. What is a Heterogeneous Catalyst
4. Side by Side Comparison – Homogeneous vs Heterogeneous Catalyst in Tabular Form
5. Summary
What is a Homogeneous Catalyst?
Homogeneous catalysts are catalytic compounds that are in the same phase as the substances which are going into the reaction phase. These homogeneous catalysts are often in the liquid phase. The recovery of homogeneous catalysts is comparatively difficult and expensive since the catalyst is in the same phase as the reaction mixture is. This means the catalyst separation is difficult. The thermal stability of homogeneous catalysts is also poor. The most common example for homogeneous catalysts are meta complexes.
Figure 01: Action of Catalysts
Homogeneous catalysts are efficiently active in low-temperature conditions (less than 250◦C). The catalytic activity of these catalysts is moderate when compared to heterogeneous catalysts. However, the selectivity is high. The recycling of the homogeneous catalyst is expensive since the catalytic recovery is difficult. But the modification of the catalyst is easy because it is in the liquid phase.
The diffusivity of homogeneous catalysts is high. This because all reactants and the catalyst are in the same liquid phase and proper stirring leads to proper diffusion of the catalyst throughout the reaction mixture. Homogeneous catalysts usually have well-defined active sites. This means there are many proper active sites on the surface of catalyst compound onto which the reactants are bound, and the reaction progresses in those active sites.
What is a Heterogeneous Catalyst?
Heterogeneous catalysts are catalytic compounds that are in a different phase from that of the phase of the reaction mixture. These catalysts can be found in all three phases of matter: solid phase, liquid phase or gas phase. The catalytic recovery is easy and cheap in heterogeneous catalysts since the catalyst is in a different phase from that of the phase of the reaction mixture. Common examples for heterogeneous catalysts are metals, metal oxides, etc.
Figure 2: Reaction Mixture is in Liquid Phase whereas Catalyst is a Metal in Solid Phase
The thermal stability of heterogeneous catalysts is very good compared to homogeneous catalysts. These catalysts efficiently act in high-temperature conditions, around 250-500◦C. The catalytic activity is also high when compared to homogeneous catalysts. However, the selectivity of reactants is poor than that of homogeneous catalysts. The active sites of heterogeneous catalysts are not well-defined. This reduces selectivity.
The diffusivity of heterogeneous catalyst is poor if the surface area of the catalyst is low since the catalyst and the reaction mixture are in two phases. But the separation of the catalyst is usually straightforward. Then the recycling of the catalyst is also easy.
What is the Difference Between Homogeneous and Heterogeneous Catalyst?
Homogeneous vs Heterogeneous Catalyst |
|
| Homogeneous catalysts are catalytic compounds that are in the same phase as the substances which are going into the reaction phase. | Heterogeneous catalysts are catalytic compounds that are in a different phase from that of the phase of the reaction mixture. |
| Phase | |
| Homogeneous catalysts can be found mostly in the liquid phase. | Heterogeneous catalysts can be found in all three phases; solid phase, liquid phase or gas phase. |
| Thermal Stability | |
| The thermal stability of homogeneous catalysts is poor. | The thermal stability of heterogeneous catalysts is good. |
| Catalyst Recovery | |
| The recovery of homogeneous catalysts is difficult and expensive. | The recovery of heterogeneous catalysts is easy and cheap. |
| Active Site | |
| The active site of homogeneous catalysts are well-defined and has good selectivity. | The active site of heterogeneous catalysts are not well-defined and has poor selectivity. |
| Catalyst Recycling | |
| The recycling of homogeneous catalysts is difficult. | The recycling of heterogeneous catalysts is easy. |
| Catalyst Separation | |
| The separation of homogeneous catalyst from the reaction mixture is difficult. | The separation of heterogeneous catalyst from the reaction mixture is easy. |
| Temperature Dependence | |
| Homogeneous catalysts work better in low-temperature conditions (less than 250◦C). | Heterogeneous catalysts work better in high-temperature conditions (around 250 to 500◦C). |
| Catalyst Modification | |
| The modification of homogeneous catalysts is easy. | The modification of heterogeneous catalysts is difficult. |
Summary – Homogeneous vs Heterogeneous Catalyst
Catalysts are compounds that can increase the reaction rate of a particular reaction to give an optimum yield in a short time period. There are two main types of catalysts named as homogeneous catalysts and heterogeneous catalysts. The key difference between homogeneous and heterogeneous catalysts is that homogeneous catalysts can always be found in the liquid phase whereas heterogeneous catalysts can be found in all three phases of matter; solid phase, liquid phase, and gas phase.
Reference:
1. Types of Catalysis, Chemguide, Available here.
2. “Homogeneous Catalysis.” Wikipedia, Wikimedia Foundation, 25 Feb. 2018, Available here.
3. “Heterogeneous Catalysis.” Wikipedia, Wikimedia Foundation, 16 Mar. 2018, Available here.
Image Courtesy:
1. “CatalysisScheme” By Smokefoot assumed- Own work assumed (based on copyright claims) (Public Domain) via Commons Wikimedia
2. “Heterogeneous Asymmetric Hydrogenation Pyridines” By Bmalbrecht – Own work (CC BY-SA 3.0) via Commons Wikimedia