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Ziegler – Natta Catalyst

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Last updated date: 29th Mar 2024
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What is Ziegler - Natta Catalyst?

In chemistry, a catalyst is any substance that accelerates a process without being consumed. Many vital metabolic reactions are catalyzed by enzymes, which are naturally occurring catalysts.


Metals and their oxides, sulphides, and halides, as well as the semi-metallic elements boron, aluminum, and silicon, make up the majority of solid catalysts. Solid catalysts are usually disseminated in other substances known as catalyst supports; gaseous and liquid catalysts are commonly utilized in their pure form or in combination with suitable carriers or solvents.


A catalyst is a substance that either increases or decreases the rate of reaction without taking part in that reaction. Ziegler-Natta catalyst is used in the synthesis of polymers of 1-alkenes (also known as alpha-olefins). The Ziegler-Natta catalyst also increases the rate of polymerization. It is named after German Chemist Karl Ziegler and Italian Chemist Giulio Natta. 


Ziegler- Natta catalyst contains two parts – a transition metal compound and an organoaluminum compound. It contains transition metal from group IV such as Ti, Zr, Hf, etc. Organoaluminum compounds contain bonds between aluminum and carbon atoms. Examples of Ziegler-Natta catalysts include TiCl4+Et3Al and TiCl3+AlEt2Cl. So, if we want to write the chemical formula of one of the Ziegler-Natta catalysts then it can be represented as TiCl4-Al(CH3)2(CH2)2Cl.


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Ziegler Natta catalyst is used in the polymerization of -olefins. A general reaction can be represented as follows – 


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Discovery of Ziegler – Natta Catalyst 

The Ziegler Natta catalyst was discovered by Karl Ziegler. He got the Nobel Prize in chemistry in 1963 for the discovery of titanium-based catalysts. The Giulio Natta prepared stereoregular polymers from propylene. On the basis of that the catalyst is named as Ziegler-Natta Catalyst.  


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Preparation of Ziegler-Natta Catalyst 

Ziegler-Natta catalyst is formed by the reaction of transition metal compounds of group IV – VIII (derivatives of alkyl, aryl of alkoxide or halide) of the periodic table with an alkyl metal halide or alkyl halide of groups I to III. Compounds of transition metal-containing TI, V, Cr, Co, Ni, etc. are majorly used for the preparation of Ziegler- Natta catalyst. 

The reaction involved can be represented as follows -


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Classes of Ziegler Natta Catalyst 

To create or get transition metal halides belonging to groups IV-VIII in the current periodic table, they are often reacted with organometallic compounds belonging to groups I – III. Catalysts by Ziegler-Natta. A classic example is a combination of titanium tetrachloride (TiCl4) and trimethylaluminum (Al(C2H5)3). Catalysts have proven to be quite advantageous.


Ziegler – Natta catalysts can be divided into following classes – 

  • Heterogeneous catalysts 

  • Homogeneous catalysts 

Heterogeneous Catalysts - Heterogeneous catalysts are those catalysts which are based on titanium compounds and then are combined with organometallic compounds. 


In polymerization operations, heterogeneous supported catalysts based on titanium compounds are utilised in conjunction with cocatalysts, organoaluminum compounds such as triethylaluminum, Al(C2H5)3. This type of catalyst is the most common in the business.


Homogeneous Catalysts - Homogeneous catalysts are based on the compounds of Hf and Zr. They contain metallocenes (compounds that consist of two cyclopentadienyl anions) and multidentate oxygen-based ligands. These are soluble in the reaction medium. 


These are commonly composed of titanium, zirconium, or hafnium compounds. They're generally combined with methyl aluminoxane, a distinct organoaluminum cocatalyst (or methylalumoxane, MAO). Metallocenes are commonly used in these catalysts, but they also include multidentate oxygen and nitrogen-based ligands.

Mechanism of Ziegler – Natta Catalytic Polymerization 

We are explaining the mechanism with respect to TiCl4 + AlEt3. Mechanism of polymerization of Ziegler – Natta catalyst can be explained by following four steps - 

Step 1. Activation of Ziegler - Natta Catalyst 

Titanium atoms are coordinated with 6 chlorine atoms and this compound has a crystal structure. When it reacts with AlEt3, it gets one ethyl group from AlEt3. Aluminum gets attached to a chlorine atom. While one chlorine atom gets removed from titanium compounds. Thus, now the catalyst has an empty orbital on the surface. Now the catalyst is activated by the coordination of AlEt3 and titanium. 

Reaction can be written as follows


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 Step 2. Initiation 

This polymerization is initiated by formation of alkene metal complexes.


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 Step 3. Propagation 

Availability of free propylene molecules in the reaction propagates the reaction. When more propylene molecules keep coming, the process takes place again and again and gives linear polypropylene. 

 

Reaction can be written as follows :


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Step 4. Termination 

It is the final step of the chain reaction. After this step, we get the desired product. In Ziegler Natta catalytic-polymerization termination may take place by several approaches.

Application of Ziegler-Natta Catalyst

A versatile and important polymerization process is the Ziegler-Natta catalyst polymerization reaction. The following are some of the most common uses for this catalyst:

  • They're used to make high-density and low-density polyethylene.

  • Thermoplastic polyolefins, polybutylene, crystalline polypropylene, and carbon nanotube nanocomposites are all manufactured.

A Ziegler–Natta catalyst is a catalyst used in the synthesis of 1-alkene polymers. It is named after Karl Ziegler and Giulio Natta (alpha-olefins). There are two types of Ziegler–Natta catalysts used, each with its own solubility. Transition metal halides, such as titanium, vanadium, chromium, and zirconium, as well as organic non-transition metal derivatives, particularly alkyl aluminum compounds, are commonly used in today's Ziegler-Natta catalysts.


In coordination polymerizations, the Ziegler-Natta catalyst is utilized, and it involves complexes generated between a transition metal and the electrons of a monomer. The insertion of monomers at the end of the expanding chain, where the transition metal ions are attached, is typically how polymerization is performed. The entering monomers are all coordinated at the same moment at unoccupied orbital locations, resulting in lengthy polymer chains. The C=C bond is also included in the TiC bond at the active center.


Finally, the chain-growth polymerization reaches the termination stage, which produces "dead" polymers (the intended result). Anionic polymerization, which creates linear and stereo-regular polymers, is comparable to these processes.

FAQs on Ziegler – Natta Catalyst

1. What is the limitation of the Ziegler Natta catalyst?

Polymerization by Ziegler-Natta is an excellent method for producing polymers from hydrocarbon monomers such as ethylene and propylene. However, it does not function with several other monomers. We can't create poly(vinyl chloride) via Ziegler-Natta polymerization, for example. During intermediary steps of the reaction, radicals are formed when the catalyst and cocatalyst combine to form the starting complex. These can cause the vinyl chloride monomer to polymerize via free radical polymerization. Because Ziegler-Natta catalysts frequently initiate anionic vinyl polymerization in those monomers, acrylates are also out.

2. Define Heterogeneous Catalysts and Homogeneous catalysts

On the basis of solubility, they are commonly divided into two groups[1].

I: Heterogeneous Catalysts - Compounds based on titanium or vanadium that are utilized in conjunction with organoaluminum complexes. they are insoluble under the conditions of the polymerization reaction.


II: Homogeneous catalysts -  Metallocenes and/or multidentate oxygen and nitrogen-based ligands are included in complexes based on Titanium, Zirconium, or Hafnium. The complexes are activated using MAO, as is done for metallocene catalysts.

3. How has Ziegler-Natta catalysts proven advantageous in Transition metal halides?

Transition metal halides, such as titanium, vanadium, chromium, and zirconium, as well as organic non-transition metal derivatives, particularly alkyl aluminium compounds, are commonly used in today's Ziegler-Natta catalysts. To manufacture or obtain Ziegler-Natta catalysts, transition metal halides belonging to groups IV-VIII in the contemporary periodic table are generally reacted with organometallic compounds belonging to groups I – III in the modern periodic table. A classic example is a combination of titanium tetrachloride (TiCl4) and trimethylaluminum (Al(C2H5)3). Catalysts have proven to be quite advantageous.

4. What is an example of a Ziegler-Natta catalyst?

A transition metal compound and an organoaluminum compound make up the Ziegler- Natta catalyst. It includes transition metals from Group IV, such as Ti, Zr, and Hf, among others. Aluminium and carbon atoms are linked in organoaluminum compounds. TiCl4+Et3Al and TiCl3+AlEt2Cl are examples of Ziegler-Natta catalysts.

5. In the production of the Ziegler-Natta catalyst, what type of polythene is used?

The Ziegler-Natta catalyst is made up of two different compounds: triethyl aluminium and titanium tetrachloride. This catalyst is used to make polyethylene and polythenes with a high density. Each ethylene molecule is made up of two carbon atoms and four hydrogen atoms.


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