How is freon used as a catalyst

Chlorofluorocarbons

 

Chlorofluorocarbons (CFC), chemical nomenclature according to IUPAC: Chlorofluorocarbons (CFC), are an extensive chemical group of low molecular weight organic compounds that are used as propellants, refrigerants or solvents. CFCs are hydrocarbons in which hydrogen atoms have been replaced by the halogens chlorine and fluorine; they are a subgroup of the halogenated hydrocarbons. CFCs that contain only single bonds are called saturated CFC. If the compound no longer contains hydrogen, it is called Chlorofluorocarbons. During the 1970s and 1980s it became clear that the release of CFCs into the atmosphere is to a large extent responsible for the depletion of the ozone layer in the stratosphere (ozone hole), which is why the use of CFCs is now prohibited in many areas of application.

HCFCs are partially halogenated chlorofluorocarbons whose hydrogen atoms are only partially replaced by chlorine and fluorine atoms. They have a far lower ozone depletion potential than CFCs, and their global warming potential is also far below that of CFCs. In addition, the HCFCs are already broken down in the troposphere and only partially reach the stratosphere.

properties

CFCs are very stable, non-flammable, odorless, transparent (colorless) and are often non-toxic or have only a low level of toxicity. The CFCs of the methane and ethane series have a low boiling point and can be easily liquefied by compression. Because of their inertia, CFCs remain in the atmosphere for a long time. They therefore rise up into the stratosphere, where they are broken down by the UV rays. This releases chlorine or fluorine radicals, which react with the ozone in the ozone layer and damage it. In 2007, three CFCs with atypical properties - very reactive and toxic - were detected in the atmosphere.

Manufacturing

Direct fluorination of alkanes is difficult to carry out because the highly exothermic reaction usually takes place in an explosive manner and almost always leads to a mixture of perfluorinated compounds. Technically, chlorofluoroalkanes are obtained by fluorinating the corresponding chloroalkanes with anhydrous hydrogen fluoride on fixed-bed catalysts made from aluminum or chromium fluorides. It is also possible to use an antimony (V) chloride catalyst.

  • Example 2: Reaction of chloroform with anhydrous hydrogen fluoride in chlorodifluoromethane (Freon 22). Antimony (V) chloride is used as a catalyst here.

Electrofluorination according to Simons is also possible. The anodic fluorination is carried out in anhydrous hydrogen fluoride at a voltage that is not yet sufficient to release elemental fluorine.

History and use

At the end of the 19th century, the first halogenated hydrocarbons were produced by direct fluorination (Moissan) and electrophilically catalyzed halogen exchange (Swarts). The first CFCs (CFCl3 and CF2Cl2) were synthesized in 1929 by Thomas Midgley at General Motors. From 1930, CFCs were technically produced and increasingly used as refrigerants in refrigeration machines, as propellants for spray cans, as propellants for foams, as cleaning agents and solvents. Its use as a refrigerant in refrigerators has been banned since 1995 because CFCs contribute to the destruction of the ozone layer.

CFCs containing bromine were used as fire extinguishing agents and are also known as halons.

The use of CFCs was first warned in 1974, but it was not taken seriously. The discovery of the ozone hole in 1985 caused a change of opinion. In the Montreal Protocol of September 16, 1987, many countries committed themselves to drastically reducing the production of CFCs. On June 29, 1990, the international conference on the protection of the ozone layer in London decided to ban or at least severely restrict the production and use of CFRP and CFC from the year 2000. The agreement envisaged reducing CFC use by 50% by 1995 and by 85% by 1997. The chemical stability makes these gases in the atmosphere difficult to break down (average residence time between 44 and 180 years, depending on the product).

The main CFC refrigerants:

descriptionCommon nameMolecular formulaboiling point
TrichlorofluoromethaneFriday 11CCl3F.24.9 ° C
DichlorodifluoromethaneFriday 12CCl2F.2−30 ° C
DichlorofluoromethaneFriday 21stCHCl2F.8.9 ° C
ChlorodifluoromethaneFrigen 22CHClF2−40.7 ° C
1,1,2-trichloro-1,2,2-trifluoroethaneFrigen 113CClF2–CCl2F.48 ° C
1,2-dichloro-1,1,2,2-tetrafluoroethaneFrigen 114 or CryofluoranCClF2-CClF23.5 ° C

Environmental impact

The low molecular weight, hydrogen-free CFRPs get into the stratosphere due to their chemical stability and their high volatility and react with the ozone layer. Example:

Thereby means a photon of suitable frequency, and a chlorine radical.

The chlorine radical breaks down ozone into biatomic oxygen. The chlorine bound to the oxygen is released again, creating molecular chlorine. By means of a photon of suitable energy, chlorine radicals are released again, allowing the cycle to start over:

This destroys the ozone layer. Without their protective effect, hard UV radiation can penetrate to the surface of the earth and damage plants, animals and people.

CFCs also absorb solar radiation in the infrared range (more strongly than CO2) and carry according to their respective global warming potential (in CO2-Equivalent) contributes differently to global warming. Some CFCs exceed the global warming potential of carbon dioxide by ten thousand times.

Alternatives

Alternatives to CFC-based propellants for aerosol atomization include HFA-134a, which does not affect the ozone layer but promotes the greenhouse effect. Most of the time, however, an alkane mixture of propane and butane that is easy to liquefy under pressure is used, which is why these spray cans carry the extremely flammable hazard symbol. When it comes to refrigerants, propane, butane, pentane, ammonia, 2,3,3,3-tetrafluoropropene or carbon dioxide as well as chlorine-free refrigerants such as R134a, R404a etc. are available as alternatives, although it should be noted that the first three substances are flammable, Ammonia and 2,3,3,3-tetrafluoropropene are corrosive and toxic.

Nitrogen trifluoride, which has been used since then, was recommended as an alternative to CFCs in the electronics industry in the manufacture of flat screens, solar cells and microcircuits. In 2008, new measuring methods demonstrated its concentration in the atmosphere and the significant damage to the climate.

Improper recycling of old refrigerators still releases large amounts of CFCs into the atmosphere.

Based on an article in Wikipedia.de
 
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Date of the last change: Jena, the: 28.06. 2020