Reaction type

The chemical reaction carried out under the action of a catalyst is called a catalytic reaction. In a chemical reaction, some of the chemical bonds of the reaction molecule must be dissociated and form new chemical bonds, which requires a certain activation energy. In some systems where chemical reactions are difficult to occur, the addition of a third substance (catalyst) that contributes to the chemical bond rearrangement of the reactive molecules can reduce the activation energy of the reaction, thereby accelerating the chemical reaction and controlling the selectivity and stereoregularity of the product.

Catalytic principle

Figure 1. The schematic of catalytic reaction

Just as shown in Figure 1, according to the Arrhenius formula k=A*e-E_a/RT(k is the rate constant for the reaction; A referred to as the pre-exponential factor/Arrhenius constant, and its unit is the same with k; E_a is the activation energy of the reaction, and its unit is coke (J) or kilojoule (kJ), which is considered a constant when the temperature varies little; R is a gas constant; T is the temperature under the absolute temperature scale, and its unit is Kelvin (K)), the E_a value is reduced due to the catalyst participating in the reaction, so that the reaction rate is significantly increased.

There are also some reactions. After the catalyst participates in the reaction, the activation energy E_a value does not change much, but the pre-exponential factor A value increases significantly (or is explained as the activation entropy increases), which also leads to an increase in the reaction rate.

Applications

A catalyst is a chemical that changes the reaction rate of a chemical reaction without changing the thermodynamic equilibrium position of the chemical reaction and is not significantly consumed by the chemical reaction itself. It is estimated that 90% of commercially produced chemical products involve catalysts at certain stages of their manufacturing process. Adding a catalyst to the reaction can achieve the following effects:

• Accelerate chemical reaction rate and increase production capacity. Most bioactive compounds are chiral, so many drugs are produced by enantioselective catalysis (catalytic asymmetric synthesis).
• For complex reactions, the rate of the main reaction can be selectively accelerated, the side reaction can be suppressed, and the yield of the target product can be improved.
• Improve operating conditions, reduce equipment requirements, and improve production conditions.
• Develop new reaction processes, expand the use of raw materials, and simplify production processes.
• Eliminate pollution and protect the environment. A notable example is the catalysis of chlorine radicals in ozonolysis. These free radicals are formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs).

Classification

According to the reaction mechanism, the catalytic reaction can be divided into acid-base type catalytic reaction and redox type catalytic reaction.

According to reaction type classification, it can be divided into hydrogenation, dehydrogenation, oxidation, carbonylation, polymerization, halogenation, cracking, hydration, alkylation, isomerization, and the like.

According to the phase state of the reaction, it can be divided into homogeneous catalytic reaction and heterogeneous catalytic reaction. In the homogeneous catalytic reaction, organic catalysis is the main, and in the heterogeneous catalytic reaction, electrocatalysis is the main one.

References

1. Yongjie Yang,Yanfang Kang. (2008). ‘Chemical Process Safety Technology’. 32-35..
2. Laidler, K.J. and Meiser, J.H. (1982). ‘Physical Chemistry’. Benjamin/Cummings, 425-425．
3. Elschenbroich, C. (2006) ‘Organometallics’. Wiley-VCH: Weinheim.