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Homogeneous Catalyst

A catalyst is considered as a substance which helps in increase the rate of chemical change without itself being used up or something that could be recovered unchanged chemically at the end of chemical reaction. The presence of catalysts does not affect the position of equilibrium in any reversible change and neither does it initiate a reaction but only increase or decrease the speed. 

In homogeneous catalysis the catalysts are present in the same phase as the substances which are going into the reaction phase. The most widely used industrial unit operation are either homogeneous or heterogeneous catalysts as these are better suited for the industrial operations that are undertaken. Whether we are looking at hetero or homogeneous forms of catalysts the catalysts does not undergo chemical changes but the physical states of these substances undergo substantial changes in the size of the particles or the changes in colours. It is also noticeable that unlike other forms of catalysts, the homogenous form of catalysts supported reactions, the rate of catalytic reactions are proportional to the concentration of catalysts like what we see in cane sugar inversion process.

 

Homogeneous Catalyst Definition

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The homogeneous forms of catalysts are nothing but chemical compound which remains in same phase as the reactants and help in accelerating the process of chemical change. These chemicals helps in attaining the equilibrium more quickly by increasing the rates of both the forward and reverse reactions to an extent.

These chemicals helps in increasing the rate which is already taking place slowly and has no role in initiating the reaction. In many homogeneous catalytic processes the ligands present in the catalysts needs to be discarded or separated by some method as these are later converted into active homogeneous catalysts and finally recycled for future applications.

Homogeneous Catalysis Mechanism

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In most of the widely applied industrial catalysts, the separation of these soluble catalysts from the reacting phase or solution is carried out by distillation and it could be either by flash distillation or distillation which are external to the reacting medium.

In flash distillation, the reaction is carried out at higher temperatures to help evaporate the products continuously and final collect the catalyst which remains back. In these methods the catalysts does not need recycle process. But these also leads to decomposition of the catalysts. So the maximum temperature is always controlled in a manner that helps in containing the decomposition.

The mechanism that is involved in such activities are studied in depth under the catalysis mechanism.Homogenous catalysis in liquid phase mechanism involving the common acid catalyst in liquid phase or water is the hydronium ion and the most common base catalyst is the hydroxyl ion. Different acids have different catalytic activity as HCl has greater activity than acetic acid. And hence it’s evident that the actual catalysts are $H^{+}$ or $H_{3}O^{+}$ ions. The rates of reaction are found to be proportional to the concentration of $H_{3}O^{+}$ ions and the overall concentration of the reacting molecules or ions.

$C_{12}H_{22}O_{11} + H_{2}O$ $\xrightarrow[]{H_{3}O^{+}}$ $C_{6} H_{12} O_{6} + C_{6} H_{12}O_{6}$ 

Reactions which are catalysed by certain acids or only $H_{3}O^{+}$ are termed as specific acid catalysis and there are reactions which are catalysed by $OH^{-}$ ions only and hence are called hydroxyl ion catalysis.

$CH_{3} COCH_{3} + CH_{3} COCH_{3}$ $\xrightarrow[]{OH^{-}}$ $CH_{3} COCH_{2} C-(CH_{3}) -OH$

There are many reactions where $H_{3}O^{+}$ ions and $OH^{-}$ ions simultaneously act as catalysts along with water. Hydrolysis of ester follows the H+ ion as catalyst.

$CH_{3}-COO-R + H^{+}$ $\rightarrow$ $CH_{3} - COO - H(R)$ $\xrightarrow[]{H_{2}O}$ $CH_{3} - CO - O + (H_{2} O) (ROH)$ $\rightarrow$ $CH3-CO - OH + ROH + H^{+}$

Homogeneous Catalyst Example

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Many of the homogeneous catalysed reactions have been studied in both gas and liquid phases and some of the common examples in gas phase are as follows. 

  • In the lead chamber process during the manufacture of sulphuric acid, the presence of nitric oxide gas helps in catalysing the oxidation of sulphur dioxide. 
$2SO_{2} + O_{2}$ $\xrightarrow[]{NO}$ $2SO_{3}$

  • During the decomposition of acetaldehyde, the catalysis is carried out by iodine vapours.
$CH_{3} - CHO$ $\xrightarrow[]{I_2}$ $CH_4 + CO$

  • The presence of nitric oxide as catalyst during the combination of carbon monoxide and oxygen also clarifies the homogeneous catalysis.
$2 CO + O_{2}$ $\xrightarrow[]{NO}$ $2CO_{2}$

Examples of homogeneous catalysis in liquid phase are as follows:

The decomposition of nitroso – tri-acetone-amine by hydroxyl ion catalysis. 

Decomposition
 
The hydrolysis of nitrile is catalysed by H+ and OH- ions as well.

$RCN + 2H_{2}O \xrightarrow[OH^-]{H^+ or} RCOONH_{4}$.

Homogeneous Catalysis and Heterogeneous Catalysis

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Differences between Homogeneous Catalysis and Heterogeneous Catalysis are mentioned below:

Homogeneous catalysis  Heterogeneous catalysis 
These types of catalysts usually are common in either liquid phase or gas phase  These are found in liquid phase, gas phase and solid phase
Operative temperature for homogeneous catalysis is generally low except only when under high pressure  Operative temperature for heterogeneous catalysis is harsh as compared to homogeneous process
As the reactants and catalysts are in same phase the diffusion in homogeneous catalysis process is very high  The heterogeneous catalysis process diffusivity is low as the absorbance surface is low
The heat transfer in homogeneous catalysis is very high as all the molecules of reactants and catalysts are in same phase The heat transfer is relatively low as compared to homogeneous catalysis as the reactant molecules and catalysts are in different phase
The active site is very well defined The active site is not very well defined as different phase sites might have different catalytic properties for the same particle
Recycling methods are not very cost effective as it’s a long drawn process and as it’s a difficult treatment method for spent catalysts These catalysts although require reactivating treatment process but still quite cost effective
Modification of homogenous catalysts is very easy as it depends upon the tuning of electronic and steric properties on metal The modification of heterogeneous catalysts is relatively difficult as the controlling methods of particle site as well as the active size at molecular level  is really difficult
Reaction mechanism easier to find as varied techniques are available Reaction mechanism is difficult to fins as the techniques utilised as the product is scrutinised and not the catalysts
Selectivity of homogeneous catalysts are very high Selectivity of heterogeneous catalysts is low

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