HCFC-22, decomposition

Substitutes for the CFCs that cause depletion of stratospheric ozone are being developed. Those which will be used in the near future (hydrochlorofluorocarbons, HCFCs) are of concern because a decomposition product, trifluoroacetic acid, might be very persistent and, under extreme conditions, have the potential to cause an undesirable environmental impact. 

Certain undesired reactions can take place via radical intermediates. Examples are the autox-idation of ethers (see Figure 1.38) or the decomposition of ozone in the upper stratosphere. This decomposition is initiated by, among other things, the chlorofluorohydrocarbons ( HCFCs ), which form chlorine radicals under the influence of the short-wave U V light from the sun (Figure 1.12). They function as initiating radicals for the decomposition of ozone, which takes place via a radical chain. However, this does not involve a radical substitution reaction. 

The decomposition of chlorodifluoromethane (HCFC-22) using gold supported on sulfated Ti02/Zr02 proceeded as follows 117... 

The decomposition of HCFC-22 (CHCIF2) by a series of acidic single and dual component metal oxides has been studied by Li et al. [70], Initial studies over single metal oxides showed the order of reactivity ... 

Although there has been some recent interest in the catalytic fluorination of CCI4, notably over chromium(III) fluoride derived from thermal decomposition of (NH4)3CrF6 [42], most attention has been paid to dismutations within the HCFC—CHCl3 F series. Reactions on several metal oxides and halides have been studied by Kemnitz and coworkers and evidence for dismutation behaviour adduced from thermodynamic considerations and... 

Alternatives to CFCs are available. Currently, hydrochlorofluorocarbons (HCFCs), which are more benign in the stratosphere than CFCs, are being used, but because they still contain chlorine, the HCFCs are considered only a transitional solution. By 2020, it is anticipated that hydrofluorocarbons (MFCs) such as 1,1,1,2-tetrafluoroethane (CF3CH2F), which are less susceptible to photochemical decomposition in the stratosphere, will have replaced HCFCs as propellants, refrigerants, and so on. 

When a chlorine ( Cl) or fluorine ( F) in the CFC is replaced by a hydrogen, HCFCs result. Reports of widespread testing of HCFCs appear to indicate that their decomposition in the lower atmosphere will preserve the upper atmosphere ozone (O3). Of course, full-scale use of HCFCs may have unforeseen... 

HCFCs (hydrochloro/luorocarbons) and HFCs (hydro-/luorocarbons) have been developed as reasonable, but probably temporary, substitutes for CFCs. The hydrogen atoms in these substitutes make them subject to oxidation in the lower atmosphere, thus theoretically less able to carry chlorine up to stratospheric ozone. However, the identity and effects of the Cl-containing decomposition products of HCFCs are still incompletely known, and a portion of HCFC emissions and their degradation products may still reach the stratosphere, giving these compounds some potential for ozone depletion. While HCFCs are replacing CFCs in a number of commercial applications, HCFCs are also scheduled for total phase-out by 2030. 

FIGURE 11-10 Hydrochlorofl uorocarbons (HCFCs) like this one contain hydrogen, chlorine, and fluorine. Although the chlorine atoms have ozone depleting potential, the hydrogen atom makes the entire compound more reactive than CFCs and susceptible to decomposition in the troposphere. 

Fluorine Among halogens, fluorine is very different from Cl, Br and 1 in respect that it does not cause O3 dissipation. Fluorine is transported to the stratosphere in the form of anthropogenic CFCs and HCFCs as described in the following section, and also as CF4 and hydrofluorocarbons (HFCs). From the decomposition processes... 

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