%D, %d %M %y
Time: %h~:~%m

Home  / GENERAL CHEMISTRY Textbook / Chapter 10.** CATALYSIS

Chapter 10.** CATALYSIS

Catalysis is the acceleration of a chemical reaction under the influence of a catalyst.

How does the theory of elementary interactions explain the phenomenon of catalysis?

The interaction of saturated molecules with each other, as mentioned above, proceeds according to a chain mechanism: association - electron isomerization - dissociation. Active particles in a chemical reaction are the radicals, the ions, the conences. The speed of chemical reaction is determined by the concentration of active particles in the system.

What is a catalyst and its role in chemical reactions ?

Catalyst is a substance that increases the number of active particles in the system.

The catalyst introduced in a chemical reaction creates active particles at lower temperatures and easier than the initial compounds. In concurrence with this the active particles supplied by the catalyst can form with the molecules of the precursor an intermediate compound with one or both of the initial components.

After a full cycle of the intermediate chemical interactions catalyst restores its chemical composition.

As examples of the acceleration of catalytic reactions under injection of active particles into the system the following reactions are most commonly cited:

  1. RCO2R + H2O - H+-→RCO2 H + ROH
  2. SO2 + O2 -NO˙-→SO3
  3. (CH2 = CH2)n - R˙-→(-CH2 - CH2-)n
  4. O3 - Cl˙-→O2 + O.
  5. RCO2 R - OH¯-→ RCO2 + ROH
  6. Cl2 + H2- Cl˙-→2H Cl

The catalytic reaction also proceeds through three stages: association - electronic isomerization - dissociation.

Let's consider the last example. The mechanism of this transformation is described in the following way:

  1. Cl˙ +H:H → Cl...H:H
  2. Cl...H:H ⇆ Cl:H...H
  3. Cl:H...H → H:Cl+H˙

At the stage of association a new weak van der Waals bond (denoted here as ...) of the initial component occurs with the active particles supplied by catalyst. The intermediate compound is formed.

At the stage of isomerization an old strong covalent bond (denoted here as:) in the intermediate compound becomes the van der Waals bond in the result of the isomerization, and a new van der Waals bond becomes a covalent one.

At the stage of dissociation "weak" bonds in the intermediate compound break, and the intermediate compound dissociates into the final product and the active particle.

The rate-limiting step in this interaction is the stage of dissociation. The speed of this stage is determined by the concentration of intermediate compound. The concentration of the intermediate compound depends on the concentration of active particles in the system and on the difference between the energies of a breakable bond and a formed bond. The lower the energy of the breakable bond, the higher the energy of the formed bond and the higher the concentration of the intermediate compound, the faster the dissociation reaction proceeds.

Mechanism of catalysis

The word "catalysis" comes from the Greek κατάλυσις which goes back to καταλύειν - "destruction".

Acceleration mechanism of hydrolysis process in esters in the presence of bases is an illustration of the above provisions (see fig. 10.1).

catalysis under water leverage
Figure 10.1. The scheme of hydrolysis process in esters
in the presence of bases

Under the injection of the base into the system, the concentration of hydroxide ions occurs which is many orders of magnitude greater than in water.

When the concentration (NaOH)- is equal to 0.01 mol/L the concentration of hydroxyl is 105 times greater than its concentration in pure water. As can be seen from the scheme, the catalyst does not change the reaction mechanism but it increases the concentration of the hydroxyl anion which determines the speed of the process.

The theory of elementary interactions does not prohibit the proceeding of molecular reactions, the speeds of such reactions are much lower than the speeds of catalytic transformations. The main reason for a low speed of molecular interaction is a low concentration of intermediate associates and significantly lower rate and extent of electron isomerization.

An illustration of the interaction of the catalyst with both initial components is the hydroformylation reaction which we will consider below. Such a mechanism of catalysis is typical for the biological systems in which the enzymes are catalysts.

Active particles in these reactions are the coordinately-unsaturated compounds called conences.

Coordinately-unsaturated compound is called a coordination compound in which the number of electrons in the outer layer in the elements of the 4th and 5th periods calculated according to the rules of valence schemes is less than 18.

The name "coordinately-unsaturated compounds" has long been used in the literature to designate coordination compounds with the number of ligands which is smaller than the well-known coordinately-saturated compound. The introduction of short term "conence" did not introduce new concepts in science, it was dictated by the convenience of a combination "chain conence reactions". The term "conence" also has an additional meaning since it can be used to identify type of reaction differing from ion and radical reactions in the nature of intermediate particles.

The advisability of erection of conences in a separate class with a special name is determined not only by specificity of range of reactions which involve conences but also the by breadth of this range (reactions of coordination compounds, catalysis with complex catalysts).

Atoms of conences catalyze molecular interaction in a number of industrial processes, such as the synthesis of ammonia from nitrogen and hydrogen, aldehydes preparation from hydrogen (H2), carbon oxide (CO) and olefins in process of hydroformylation, etc. Catalysts for biochemical transformations are also coordinately-unsaturated molecules in the enzymes.

The interaction of conences with precursors in the reaction of hydroformylation proceeds in the following way:

HCo(CO)4+ CH3CH2Coψ(CO)3-→HCoψ(CO)3+ CH3CH2Co(CO)4.

where ψis a coordinately-unsaturated compound (conence).

This scheme explains the well-known feature of catalysis - a fast passage through the whole catalytic cycle in comparison with its modeled elementary steps.

Conences being able to active associations are considered to be statically activated, while the intermediate components receiving energy of exothermic reactions are dynamically activated. Static and dynamic activations can be schematically illustrated by the example of the formation of dynamic bonds of olefin and cobalt hydrocarbonyl and transformation of the complex of dynamic bonds into alkyl-carbonyl:

10_1


* hereinafter dynamically activated particles are denoted.

In the schematic reactions with the possibility of formation of the chain of statically activated compounds one can not see the presence of dynamically activated particles in catalytic reactions and the possibility of formation of statically activated particles according to a chain mechanism.

These examples demonstrate the mechanism of catalysts action: catalytic mechanism proceeds through statically or dynamically activated products determining a high speed of catalytic reactions.

In order to illustrate the mechanism of catalysis we can examine the scheme of hydroformylation reaction. It is represented on fig.10.2

Cobalt hydrocarbonyl dissociates according to the scheme:

hydroformylation reaction

Fig.10.2 The scheme of hydroformylation reaction

Such detalization helps to clarify the role of conences in catalysis. The presence of vacant positions in conences (with similar energy) makes the process of electron isomerization possible thereby causing the appearance of ions and radicals. The formation of associates of the ions and the radicals with saturated molecules (phase II) followed by electron isomerization leads to the formation of weak bonds that can be broken by the energy released upon binding of ligand with conense (stage IV).

There are two kinds of catalysis - homogeneous and heterogeneous (contact catalys). In case of homogeneous catalysis the catalyst is in the same phase as the reaction reagents while heterogeneous catalysts differ in phase.

In contrast to heterogeneous conence catalyses, homogeneous catalyses have maximally available surface, and thus their higher activity is not something unusual. An important role of conences in biological catalysis is explained by the possibility of conjoining of their chemical and dynamic activity.

Examples of mechanisms of reaction acceleration through the first and the second variants are well described in the book "How chemical bonds form and chemical reactions proceed" (p. 167 and 272-280).

Catalysis theories. Three groups of theories were proposed to explain the mechanism of catalytic reactions: geometric, electronic and chemical ones. Geometric theories give main attention to the correspondence between the geometric configuration of the atoms of the active centers of catalyst and the atoms of the part of the reacting molecules which is responsible for bonding with the catalyst. Electronic theories are based on the idea that chemisorption is determined by electron interaction related to charge transfer, i.e. these theories link the catalytic activity with the electronic properties of the catalyst. Chemical theory considers the catalyst as a chemical compound with the characteristic properties which forms chemical bonds with the reagents resulting in the formation of an unstable transition complex. After disintegration of the complex with the release of products the catalyst is returned to its original state. The last theory now is considered to be the most appropriate (on-line encyclopedia "Krugosvet").

The works proving the existence of the conence chain reactions and the conence chain catalysis were published by us in a number of reports. See reports on catalysis at www. itchem.ru

Chapter 6. Molecule structure >>
Chapter 7.** Chemical Bonds in Solid Bodies >>
Chapter 8. Three-dimensional structures of chemical compounds >>
Chapter 9.** Chemical Reactions >>
Chapter 10.** Catalysis
Chapter 11. Physical and chemical properties of substances >>