Refrigerants for the 21st Century
5. Fluorocarbon Preparations

PREPARATION OF FLUOROCARBONS RESULT FROM THE CORE CONCEPTS OF CARBON-BASED MATERIALS, THE HALOGENS AND EQUILIBRIUM

The chemical compounds we use must be manufactured safely, in large volumes and at a cost that allows the manufacturer to sell at a price the customer is willing to pay.  When mechanical refrigeration was first invented, the chlorofluorocarbons (CFCs) were laboratory curiosities with no practical applications or methods for synthesizing them.

The pioneer in the synthesis of fluorinated alkanes (= fluorocarbons) was Frederic Swarts, a Belgian chemist. In the early 1890's he generated an effective fluorine (F) exchange agent, SbF3Br2, from antimony (III) fluoride (SbF3) and bromine (Br2). This reagent is capable of exchanging some of the chlorines (Cl) in carbon tetrachloride (CCl4) and chloroform (CHCl3) with F to produce CFCl3 and CHFCl2, respectively. Little, if any, exchange by Br occurs.  This is an example of a reaction Swarts carried out:

SbF3Br2 + CCl4 -->   CFCl3 + SbF2Br2 Cl

Later Swarts and then O. Ruff in Germany also described CF2Cl2 (CFC-12) prepared by an entirely different process. Thomas Midgley of General Motors working with Albert Henne of Ohio State University improved the synthesis of CFC-12 by treating CCl4 with SbF3 plus a small amount of antimony (V) chloride (SbCl5).

Henne eventually showed that SbF3Cl2, which probably forms when SbF3 and SbCl5 are combined (SbCl5 is known to be a chlorinating agent), can be used with chlorinated ethanes to generate chlorofluoroethanes. Finally, researchers at Dupont found that mixing SbCl5 with anhydrous hydrogen fluoride (AHF) gives a F-exchange system capable of the following transformations:

Chlorofluorocarbon synthesis in the presence of SbCl5 as catalyst.

CCl4 + HF --> CFCl3 + CF2Cl2 + HCl

                   (CFC-11) (CFC-12)

CHCl3 + HF --> CHFCl2 + CHF2Cl + HCl

                            (HCFC*-21) (HCFC*-22)

                            * -Hydrochlorofluorocarbon

CCl2=CCl2 + Cl2 +  HF --> CF2ClCFCl2 + CF2ClCF2Cl + HCl

                                  (CFC-113) (CFC-114)


CF2ClCF2Cl (CFC-114) was the source of tetrafluoroethylene (TFE) by dechlorination with zinc:

CF2ClCF2Cl + Zn --> CF2=CF2 + ZnCl2

Dr. Roy Plunkett discovered the polymer of TFE, polytetrafluoroethylene, when, in  1938, he filled a steel cylinder with TFE gas.   Later, Plunkett noticed the pressure gauge on the cylinder indicated the cylinder no longer contained a gaseous material - the pressure had dropped to near zero.

Dr. Plunkett's astute technician, Jack rebok, cut the cylinder in half and found a white polymer, polytetrafluoroethylene, now known as Teflon.

CF2=CF2   -->   Polytetrafluoroethylene (PTFE)

Dr. Michael Dove and his students, in 1976 at the University of Nottingham (UK), found that a series of equilibria exists between SbCl5 and HF:


SbCl5 + excess HF <--> HCl + SbFCl4 <--> HCl + SbF2Cl3 <--> HCl + SbF3Cl2 <-->

HCl + SbF4Cl <--> HCl + SbF5

In each case the SbFxCl5-x (x = 1 to 5) can be isolated as a discreet compound. However, it is quite difficult to force the equilibria all the way to antimony pentafluoride (SbF5).    SbF5 is more easily prepared from the elements (Sb + F2) or from SbF3 + F2. Each of the SbFxCl5-x's is capable of exchanging F for the Cl in chloroalkanes, with SbFCl4 being the least and SbF5 being the most effective.

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 Dupont and other manufacturers take advantage of the equilibria between SbCl5 and AHF to produce fluorocarbons from chlorocarbons, SbCl5 and HF in a continuous manner.  In these large manufacturing facitlities, they co-feed the raw materials while removing the more volatile (lower boiling point) fluorinated products along with HCl.
The SbCl5 acts as a catalyst, lowering the activation energy for the reaction between the chlorocarbons and AHF.  But SbCl5 is really a carrier for fluorine according to the stated equilibria. By controlling the temperature and pressure inside the reactor, manufacturers optimize the production of one particular fluorocarbon when a series of them is possible. Thus, using CCl4 as a starting material, efficient production of both CFC-11 and CFC-12 can be achieved.

With the expanding markets since their 1931 introduction, the worldwide production of all CFC's grew to a maximum of 1.25 million metric tons/year by 1988.

Note:  Chemists call the processes for making chemical compounds in the laboratory or in a manufacturing facility, synthesis. 

Concept Map for this ChemCase

Fluorocarbon Alternatives
Case Study: Ozone Layer Degradation

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Principal Investigator Laurence Peterson; Project Director Matthew Hermes;
Author of this module William Gumprecht.