Catalyze

History of the catalysis

The catalytic pot is undoubtedly the most known example of the catalysis, but this field of chemistry daily has repercussions for the general public. A very great number of chemical processes comprises at least a stage using the catalysis that it is in the field of synthetic fibers, the drugs, the food additives.

Discovered

The catalysis term was introduced by Berzélius in 1836 (of the Greek kata “in bottom” and luein “to dissolve”) when the beginning of XIX^ème century was favourable with studies in what will be this field. Since 1814 Kirchhoff the hydrolysis of the starch pays catalyzed by the acids, in 1817 Humphry Davy discovers that the hot platinum introduction into a mixture of air and gases resulting from coal results in heating with white metal. In 1824 Henry the poisoning of a catalyst pays: the ethylene inhibits the reaction between hydrogen and oxygen on platinum. He in addition notices selective oxidation in the reaction between oxygen and a gas mixture composed of hydrogen, carbon monoxide and methane. In 1845 Grove shows that a platinum filament is also a good catalyst for the decomposition of water out of hydrogen and oxygen. In 1871 the Deacon process, used for the oxidation of chlorine hydrochloric acid is developed, it uses a catalyst at base brick out of copper impregnated salt clay. Little time after, in 1877, Lemoine shows that the decomposition the iodic acid out of hydrogen and iodine reaches the same point of balance to 350°C that the reaction is carried out with or without catalyst (platinum). This property is confirmed two years later by Bertholet with the esterification of the organic acids and the hydrolysis of the esters whose balance of reaction remains identical with or without catalyst.

Development of chemistry

The beginning of XX^ème century marks a discovery which nowadays continues to have repercussions. Wilhelm Normann carries out the hydrogenation of the oleic acid (acid cis-9-octadecenoïque C₁₇H₃₃COOH), liquid, in stearic acid (acid octadecanoïque C₁₇H₃₅COOH), solid, on finely divided nickel. This hydrogenation is still largely used nowadays in many fields (food, pharmacy, soap factory, perfumery, painting,…) and nickel remains the catalyst headlight. The synthesis of the ammonia (NH₃) starting from nitrogen and of hydrogen is succeeded by Fritz Haber using an apparatus with high pressure and in the presence of reduced Fe₃O₄. This ammonia can be oxidized out of nitrogen oxides by oxidation on platinum to be used as a basis to manufacture nitric acid (HNO₃). In 1923 BASF orders a hydrogen and manufacturing plant of methanol starting from carbon monoxide on a catalyst containing oxide of chromium zinc and oxide. During the same period the Fischer process - Tropsch makes it possible to obtain alkanes, olefinic hydrocarbons and alcohols starting from carbon monoxide and of hydrogen using catalyst containing iron and cobalt. The catalytic oxidation of sulfur trioxyde sulfur dioxide on vanadium oxide V (V₂O₅) allows the synthesis large scales of sulphuric acid. At the end of the years 1930, catalytic cracking appears, making it possible to break connections DC. This process, the process Houdry, uses a catalyst containing clay of the montmorillonite type treated with the acid, makes it possible to crack the large molecules of oil, typically contained in the Gasoil, in smaller molecules than one finds in the gasolines. During the same decade the selective oxidation of ethylene oxide ethylene on a catalyst containing money is developed, developed and marketed by Union Carbide. All these processes make it possible to have thus opening access to an industrial scale basic commodities of chemistry to the way to the development of the basic chemicals and the specialities.

Impossible to circumvent catalysis

The glorious Thirty will benefit largely chemistry with a great development from the processes in any kind for increasingly diversified productions. The catalysis will be an important actor of this development. Polymerization develops largely while benefitting from the produced basic molecules. In the years 1950 polyethylene, polypropylene, and polybutadiene appear grace in particular to the process Ziegler - Natta using catalysts containing organometallic of titanium and aluminum. The treatment of oil continues with hydrodesulfurization on catalysts containing molybdenum and cobalt sulfide, the hydro-treating of the naphtas on catalysts cobalt-molybdenum deposited on alumina. Years 1960 mark the appearance of active and selective zeolites of synthesis for the isomerization of alkanes and catalytic cracking. Consequently these materials go is the subject of intense studies for their catalytic properties and the researchers develop many zeolites with the properties adapted according to the reactions to catalyze, but also with the shape of the molecules by the control of the size of the channels. The concerned reactions lead to increasingly various molecules: the ammoxidation of propylene on catalysts containing molybdenum and bismuth oxides led to the manufacture of acrylonitrile, the oxychloration of ethylene on catalysts containing cuprous chloride led to vinyl chloride, decade 70 sees appearing the catalytic muffler containing platinum, rhodium and palladium. It is at that time that the enzymatic catalysis with the immobilization of enzymes develops industrially, which allows the development of semi-synthetic penicillins or the isomerization of glucose in fructose. The efforts made at the time of discovered synthetic zeolites are translated industrially in the years 1980, process MTG “methanol to gasoline” methanol into gasoline makes it possible to manufacture gasoline starting from methanol thanks to a zeolite H-ZSM5, production of diesel to leave CO and H₂ thanks to catalysts containing cobalt. Fine chemistry is not remains about it with the synthesis of the K₄ vitamin using a membrane catalyst containing platinum. The list is still very long and the increasingly elaborate molecules.

Heterogeneous catalysis

Principles

The heterogeneous catalysis is called thus when the catalyst and the reagents are not under the same phase. The vast majority of the heterogeneous case of catalysis utilizes a catalyst in solid form, the reagents being then gas and/or liquid. For example, the oxidation of the carbon monoxide (gas) by oxygen in air (gas) out of CO2 on a catalyst platinizes (solid), or the hydrogenation of the benzene (liquid) by hydrogen (gas) out of cyclohexane (liquid) on nickel (solid). The advantage of this type of catalysis which relates to 80% of the catalyzed industrial processes is the great facility of separation of catalyst of the products and reagents, since even in the second case a simple filtration is enough.

Reactional mechanisms

Let us consider a reaction in gas phase catalyzed by a solid. The first stage of any reactional mechanism in heterogeneous catalysis is the stage of adsorption.

Physical adsorption or physical absorption

A reagent of the gas phase can “settle” on the surface of catalyst. The concerned forces are weak, about 10 to 40 kJ.mol^-1, and they are molecular forces of interaction gathered under name forces van der Waals. This mode of connection is not sufficient for the catalysis, the reagent must be chemically absorbed.

Chemical adsorption or chemisorption

Chemisorption consists of the creation of a true chemical bond between the surface of catalyst and the reagent. The chemically absorbed molecule becomes more reactive then and can then undergo the chemical conversion.

Mechanism of the type Langmuir - Hinshelwood

In this type of mechanism, the reaction is done between species adsorbed on the surface of catalyst. That thus means that the species necessary to a reaction are present at the surface of catalyst. That does not want to say that all the species concerned in the reaction must be present. In the case of reactions bringing into play several species and several stages, the substrate can move on the surface of catalyst and the reagents too.

Mechanism of the type Eley - Rideal

Contrary to the preceding mechanism, here the reaction is done between a species adsorbed on the surface of catalyst and a not adsorbed species.

Desorption

Following the example first stage which consisted of an adsorption, the last stage is a stage of desorption, namely the produced molecule leaves the surface of catalyst to find itself in the ambient conditions. The product can then be pulled by flow, gas for example, to undergo other operations (typically separation, purification) it is the case in the continuous processes. It can also remain in the reactional mixture the remainder of time while waiting for the stop of handling, it is the case in the processes of the batch type (discontinuous).

Some examples of reactions and processes

Homogeneous catalysis

Principles

In homogeneous catalysis the reagents and the catalyst are presented under the same phase. One finds much this type of catalysis in organic chemistry where many reactions proceed with reagents put in solution in solvents catalyzed by such soluble complexes them.

If, contrary to the heterogeneous catalysis which makes it possible to separate catalyst easily, the homogeneous catalysis does not make it possible to separate catalyst such as it is from the reaction medium it presents other assets. A great reproducibility from one synthesis to another, a great specificity, an activity at lower temperature and from a scientific point of view a better knowledge of the reactional mechanisms.

Reactional mechanisms

It is impossible to summarize the reactional mechanisms simply. However there exist ten of elementary stages for the reactions implying of the organometallic ones, which are the catalysts in the world of the heterogeneous catalysis. These ten elementary stages form in fact only 5 reactions since one can have a reaction and its reaction reverses what thus defines two elementary stages. All these elementary stages do not appear all during a reactional mechanism, simply some of them take place in what is a catalytic cycle. The catalytic cycle is the usual manner to present the reactional mechanism. Certain mechanisms present only 3 elementary stages when others present 8 of them.

The elementary stages are characterized by three variables:

ΔVE: change of the number of electron of valence of the central atom (generally a metal atom)
ΔOS: change of the state of oxidation of the central atom
ΔCN: change of the number of coordination

Illustrations

- Video on the catalysis of the decomposition of hydrogen peroxide

Random links: