Homogeneous Catalysis

Introduction

Catalysis is the process of altering the rate of a reaction. To alter the reaction rate, we add a substance called a catalyst. These catalysts are added in a small quantity and recovered after the reaction completes. Based on the catalyst phase, the type of catalysis can be classified into two broad types: Homogeneous and Heterogeneous catalysis. Homogeneous catalysis is the type of catalysis reaction in which the catalyst is of the same phase as the reactants of the reaction, whereas, heterogeneous catalysis is the type of catalysis reaction in which the catalyst is in a different phase as the reactants. The catalysts are dissolved in the same medium as the reactants in a homogeneous catalysis reaction. Examples include major organometallic reactions like Alkene Metathesis, Hydroformylation reaction, Hydrosilylation reaction, Hydrogenation of Alkene by Wilkinson catalyst, Tennessee–Eastman acetic anhydride process, etc.

Examples of Homogeneous Catalysis

Homogeneous Catalysis reactions are as follows:

Hydrolysis of Ester

The acid-catalyzed hydrolysis of ethyl acetate ester to give ethanol and ethanoic acid occurs in the presence of water and acid. Ethyl acetate is an aqueous phase, the same phase as the acid added. Hence, it comes in the category of Homogeneous catalysis.

H+ (aq) CH₃COOC₂H₅(aq) + H₂O(l)   ⇌   CH₃COOH(aq) + C₂H₅OH(aq)

The reverse of this reaction, called the Esterification reaction, is also an example of Homogeneous catalysis. This is because the reactants of the reaction become ethanoic acid and ethanol, which are in the aqueous phase, the same as acid used to increase the reaction rate (by increasing the nucleophilicity of the carboxylic carbon and also acting as dehydrating agent). So it is also an example of a homogeneous catalysis reaction.

Lead Chamber Process

The lead chamber process used in the manufacturing of sulphuric acid has a step that involves the oxidation of sulphur dioxide. This oxidation of SO₂ gas takes place along with oxygen gas, which involves using NO gas as the catalyst involved in this reaction.

Inversion of Cane Sugar

The inversion of cane sugar (sucrose) gives glucose and fructose. The rate of reaction is increased in the presence of acid and high temperature.

Carbonylation

The addition of CO into a molecule (substrate); alcohols are converted to carboxylic acids. This conversion happens through Carbonylation. For example, methanol (MeOH) reacts with carbon monoxide (CO) in the presence of a homogeneous catalyst to give acetic acid.

Advantages of Homogeneous Catalysis

1. Since the reactants and catalyst are in the same phase, the reactions see a higher rate of catalytic activity with more interaction of the catalyst by the reactants.
2. These reactions have a higher selectivity in terms of which reactions they catalyze.
3. These reactions use milder conditions of reaction as compared to usual.
4. Homogeneous catalysts are precisely and easily characterized via spectroscopic methods rather than heterogeneous catalysts in which the reaction takes place with reactants adsorbed on the catalyst surface.
5. For exothermic reactions, the homogeneous catalyst passes the heat produced in the reaction into the solvent.

Disadvantages of Homogeneous Catalysis

1. The catalyst, being in the same phase as the reactants, is difficult to isolate from the product at the end of the reaction. In some cases in which the catalyst is of high activity, removing the catalyst from products might not be possible.
2. There is a limitation associated with homogeneous catalysis in terms of thermal stability. Many homogeneous catalysts are organometallic compounds with a higher chance of getting degraded at temperatures above 100 degrees Celsius.

After recognizing the limitations of the homogeneous catalytic reaction, there has been an attempt to synthesize a heterogenized homogeneous catalyst, which combines the advantage of homogeneous catalysts of good activity and selectivity with heterogeneous catalysts’ ability to regenerate the catalyst back after the reaction.

Difference between Homogeneous and Heterogeneous Catalysis

The following table shows the difference between the two types:

Homogeneous Catalysis                                                         Heterogeneous Catalysis

High selectivity                                                                              Low selectivity

Difficult separation                                                                      Simple separation

Low reaction temperature                                                  High reaction temperature

Easy mixing and concentration control                  Complex mixing and concentration control

High adaptability                                  Low adaptability

High reproducibility                                                    Low reproducibility

Enzyme Catalysis

Enzymes also include metalloenzymes like Carbonic Anhydrase and Vitamin B12, which are those natural biological enzymes that promote the growth rate of chemical reactions in the body (all metabolic processes). These catalysis processes have properties of both homogeneous catalyst and heterogeneous catalyst. These catalysts are essential for all life processes, and they are also being harnessed industrially. Examples include:

a.) Carbonic Anhydrase catalyzes the interconversion process between carbon dioxide and water and the bicarbonate and hydrogen ions to enable the release of carbon dioxide from blood streams into the blood.

b.) Acrylamide compound is synthesized using an enzyme-catalyzed hydrolysis process from acrylonitrile. Esters and amides are rapidly hydrolyzed in neutral water with the help of metalloenzymes as catalysts present to drive the reaction forward in an otherwise slow process.

Conclusion

Catalysis is widely divided into two types – Homogeneous Catalysis and Heterogeneous Catalysis. Homogeneous Catalysis is those catalysis reactions in which the catalyst is of the same phase as the reactants of the reaction. These catalysis processes have their pros and cons associated with their utilization. For example, they are more active, selective and require milder reaction conditions and low temperature. Still, they are also more difficult and expensive to regenerate and separate once the reaction gets completed than the heterogeneous ones.