Chlorodifluoromethane, pyrolysis

Pyrolysis of chlorodifluoromethane is a noncatalytic gas-phase reaction carried out in a flow reactor at atmospheric or sub atmospheric pressure yields can be as high as 95% at 590—900°C. The economics of monomer production is highly dependent on the yields of this process. A significant amount of hydrogen chloride waste product is generated during the formation of the carbon—fluorine bonds. 

A large number of by-products are formed in this process, mostly in trace amounts more significant quantities are obtained of hexafluoropropylene, perfluorocyclobutane, l-chloro-l,l,2,2-tetrafluoroethane, and 2-chloro-l,l,l,2,3,3-hexafluoropropane. Small amounts of highly toxic perfluoroisobutylene, CF2=C(CF2)2, are formed by the pyrolysis of chlorodifluoromethane. 

In addition to the pyrolysis of chlorodifluoromethane [9], another commercially important synthesis of TFE is based on tnfluoromethane [/O] (equation 1). 

All fluorocarbenes are ground state singlets. For laboratory use there are some precursors which thermally generate difluorocarbene.42 Its identification is usually made by a subsequent chemical insertion reaction. A few industrially important processes proceed via difluorocarbene. The thermal pyrolysis of chlorodifluoromethane (CHF2C1) for the production of tetrafluoroethene and hexafluoropropene gives the intermediate CF2 which dimerizes to the alkene. 

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... 

The formation of dimeric fluorinated products via heat-induced bond breaking has been more or less restricted to industrial processes. Thus, the pyrolysis of trifluoromethane (1) and chloro-difluoromethane (3) are the two industrial sources for tetrafluoroethene (2), hexafluoropropenc (4). and their corresponding oligomers and polymers.1-2 Hexafluoropropene (4) is formed as a byproduct in the pyrolysis of chlorodifluoromethane (3) however, a temperature-controlled pyrolysis of 3 has been worked out as a commercial process for the production of 4.3... 

Examples exist of other processes, in which microwave heating of microwave-absorbents is used as a way to transfer energy to a microwave-transparent material in order to accomplish the pyrolysis of the latter. For example, the pyrolysis of chlorodifluoromethane has been carried out in a microwave-heated fluidized bed with a performance comparable to that of tubular reactors, the best traditional equipment for the pyrolysis of this compound [45]. 

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... 

Pyrolysis of hexafluoropropylene oxide generates difluorocarbene for cyclo-propanation reactions, as does the action of heat on chlorodifluoromethane. Halogenocarbenes formed in the thermal decomposition of polyhalogenomethanes at 500—650°C can be trapped by cyclopentene and cyclohexene. The primary adducts are unstable under these conditions, suffering dehydrohalogenation to halogeno-benzenes and cycloheptadienes, respectively. ... 

The hrst rehable report of the preparation of tetrafluoroethene (TFE) was given in 1933 by Rulf and Bretschneider [643], who decomposed tetrafluoromethane in an electric arc. Other syntheses are based on the dechlorination of yn-chlorodifluoromethane [644], the pyrolysis of chlorodifluoromethane [645], and the decarboxylation of sodium perfluoroproprionate [646]. Since then, a number of synthetic routes have been developed [644-646]. On a laboratory scale, depolymerization of PTFE by heating at about 600 °C is probably the preferred method to obtain a smah amount of pure monomer (97%) [647], along with the highly toxic perfluoroisobutene as well as octafluorocyclobutane as a side product formed by the thermal (2ji -I- 2n) cyclodimerization of TFE [648,649]. The most common commercial approach for the preparation of TFE is the pyrolysis of chlorodifluoromethane [645,650]. The noncatalytic gas-phase reaction is carried out in a flow reactor at atmospheric pressure, yields over 95% TFE at 590 to 900 °C. The synthesis of TFE involves the following steps ... 

HFP and a small amount of highly toxic perfluo-roisobutylene are among other by-products of TFE. Edwards et al. [12A] demonstrated the impact of adding steam on the conversion of cWorodifluoro-methane and the yield of TFE at different residence times. A ratio of 3 mol of steam for each 1 mol of chlorodifluoromethane was used. The mixture with steam was preheated to 400 C and then held in a tubular reactor for a brief period of time at 700°C. In comparison with controlled pyrolysis reaction, in... 

TFE and HFP can be produced by pyrolysis of one or more of the following compounds fluoroform, chlorodifluoromethane, chlorotetrafluoroethane, a mixture of chlorodifluoromethane and chlorotetrafluoroethane, or a mixture of chlorodifluoromethane and perfluorocyclobutane [22]. The products include fluoroolefins such as TFE and HFP. The reaction performed by Gelblum et al. took place in a gold-plated tubular reactor at a temperature in the range of 600°C-1000°C. 

Sherratt ] has provided a detailed description of the preparation of TFE. The overall yield of TFE production depends on the pyrolysis reaction. It proceeds to yield better than 90% TFE at short contact times, low conversions, and subatmospheric pressure in the temperature range of 590-900°C. Results similar to subatmospheric pyrolysis can be achieved if superheated steam is present during the pyrolysis. Tetrafluoroethylene yields approaching 95% can be achieved at 80% chlorodifluoromethane conversion if the molar ratio ofsteamtoCHClF2isintherangeof7 l to 10 1. 

TFE is prepared from pyrolysis of chlorodifluoromethane (CDFM). The pyrolysis products are cooled, scrubbed to remove HCl and dried. The resulting gas is compressed and distiled to recover the unreacted CDFM and highly pure TFE. The TFE polymerization requires high purity. 

In the first step, chloroform is converted into chlorodifluoromethane by catalytic vapor phase fluorination reaction with anhydrous hydrogen fluoride and in the second step, chlorodifluoromethane is subjected to a noncatalytic gas phase pyrolysis at temperature 590-900°C and at atmospheric or subat-mospheric pressures to obtain tetrafluoroethylene in about 95% yield. 

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