Tetrafluoroethylene pyrolysis

Pyrolysis studies have often been applied to chlorotnfluoroethylene and tetrafluoroethylene Pyrolysis of chlorotnfluoroethylene at 560-590 °C yields 70-83% of a mixture containing both linear and cyclic dimers of chlorotri... 

Although the CFCs and HCFCs are not as stable as the PFCs, they still can be rather stable compounds (3,11)- Dichlorodifluoromethane, CCI2F2, is stable at 500°C in quartz CCl F and CHCIF2 begin to decompose at 450 and 290°C, respectively (7). The pyrolysis of CHCIF2 at 650—700°C in metal tubes is the basis of a commercial synthesis of tetrafluoroethylene ... 

Preparation. The manufacture of tetrafluoroethylene [116-14-3] (TEE) involves the following steps (2—9). The pyrolysis is often conducted at a PTFE manufacturing site because of the difficulty of handling TFE. 

The monochlorodifluoromethane may be converted to tetrafluoroethylene by pyrolysis, for example by passing through a platinum tube at 700°C. 

The decomposition products, up to a temperature of 500°C, are principally the monomer, tetrafluoroethylene, but also include perfluoropropene, other perfluro compounds containing four or five carbon atoms, and an unidentified particulate waxy fume. From 500°C to 800°C, the pyrolysis product is carbonyl fluoride, which can hydrolyze to form HE and CO2. 

Thietanes are formed by intramolecular cyclization of thiocarbenes (110) (75BCJ1490), on pyrolysis of the copolymer of sulfur, tetrafluoroethylene and thiocarbonyl fluoride (111) (69CC1274), and upon electrolysis or base treatment of dithione (112) (67JOC1562,78JOC1980, 79JCR(S)320>. 

Tetrafluoroethylene is a colorless, odorless gas which boils at —76.3° C at atmospheric pressure and freezes at —142.5° C (Renfrew and Lewis). Its critical temperature and pressure are 33.3° C and 572 psi. The heat of formation is —151.9 kcal/mole, (Duus Bryant Neugebauer and Margrave), and the heat of polymerization is —41.12 kcal/mole (Bryant). The first reliable report of the preparation of this compound was in 1933 by Ruff and Bretschneider, who decomposed tetrafluoromethane in an electric arc. Other syntheses are based on the dechlorination of sym-dichlorotetrafluoroethane (Locke, Brode, and Henne), the pyrolysis of difluorochloromethane (Downing, Benning, and McHarness Tor-kington and Thompson) or the decarboxylation of sodium perfluoro-proprionate (Hals, Reid, and Smith). 

The first step is set up to produce hydrogen fluoride and the second yields trichlo-romethane (chloroform). Chloroform is then partially fluorinated with hydrogen fluoride to chlorodifluoromethane using antimony fluoride as catalyst in the third step. Finally, in the fourth step, chlorodifluoromethane is subjected to pyrolysis in which it is converted to tetrafluoroethylene. The pyrolysis is a noncatalytic gas-phase process carried out in a flow reactor at atmospheric or subatmospheric pressure and at temperatures 590 to 900°C (1094 to 1652°F) with yields as high as 95%. This last step is often conducted at the manufacturing site for PTFE because of the difficulty of handling the monomer.9... 

There are several methods to produce HEP Eor example, thermal cracking of TFE at reduced pressure and temperatures 700°C to 800°C (1292°F to 1472°F) produces HFP in high yield [38,39]. Another process is pyrolysis of polytetrafluoroethylene under vacuum at 860°C (1580°F) with a 58% yield [40]. More recently, a technique involves the pyrolysis of a mixture of tetrafluoroethylene and carbon dioxide at atmospheric pressure and temperatures 700°C to 900°C (1292°F to 1652°F) [41]. Additional routes to HFP are described in [42,43]. 

C.M. Simon and W. Kaminsky, Chemical recycling of poly(tetrafluoroethylene) by pyrolysis, Polym. Degrad. Stab., 62, 1-7 (1998). 

One of the best-known thermal reactions of fluorine compounds is the pyrolysis of chlorodifluoromethane to tetrafluoroethylene as used in the production of Teflon polymer. This reaction was described by Park et in 1947, and Nor-ton" in 1957 reported an activation energy of 49 kcal.mole for the decomposition over silica at 425-525 °C. More recently, Gozzo and Patrick have made a kinetic study of the process using a helium flow system at 670-750 °C with a surface conditioned platinum tubular reactor. HCl is found to retard the raction and the following mechanism has been proposed... 

Starting materials for the synthesis of all perfluoroalkyl iodonium reagents are the perfluoroalkyl iodides, which themselves play a central role as building blocks in fluoroorganic chemistry. The iodides are available either by pyrolysis of the silver salts of perfluoroalkyl carboxylic acids in the presence on iodine [14] or - on a more technical scale - by iodofluorination of tetrafluoroethylene [15] with the iodine-IFp system [16] and subsequent radical telomerization of tetrafluoroethylene with the resulting intermediate perfluoroalkyl iodides [17] (Scheme 2.143). 

These compounds are synthesized by the copolymerization of tetrafluoroethylene and alkyl vinyl ether with sulfonyl acid fluoride. Preparation of sulfonyl acid fluoride takes place by pyrolysis of the respective oxide to give the olenfinated structure [7], The thermoplastic produced is extruded into a film. The sulfonyl fluoride (-S02F) group present in the thermoplastic is converted to sulfonate (-S()3 Na+) with NaOH. This is called the neutral form of Nafion, and it is converted to the acid for and cast into a thin film by heating in alcohol at 250°C. At around 650/m2, the Nafion membranes are rather expensive when compared to hydrocarbon membranes, which are also used. 

Thiocarbonyl fluoride, 9, is prepared in good yield by pyrolysis of 6 at ca. 600°C in the vapor phase or by carrying out the reaction of sulfur and tetrafluoroethylene at higher temperatures. A cleaner synthesis involves fluorination of thiophosgene dimer to tetrafluoro-l,3-dithietane, 10, followed by cracking of 10 to two molecules of 9 (7). 

The hrst report on the synthesis of fluorinated oxazetidine was published in 1955 by Barr and Haszeldine. They isolated perfluoro-2-methyloxazetidine (79) from reaction of tetrafluoroethylene with trifluoronitrosomethane, along with its polymer. This reaction proceeds even at room temperature, but higher temperature (about 100°C) favors formation of 79. The compound is inert to aqueous base or acid and UV light, but its pyrolysis at 550°C yields equimolar amounts of carbonyl fluoride and perfluoro-2-azapropene (Scheme 2.37). 

---