Fluorochemical Decomposition Processes - April 4,2001 

David A. Dixon 

Theory, Modeling, and Simulation, William R. Wiley Environmental Molecular 

Sciences Laboratory, Pacific Northwest National Laboratory, Richland WA 

Overview 

This document describes results on potential degradation pathways for 

fluorochemicals in incineration processes based on modeling of the thermodynamics of 

the processes. The primary goal of this work is to evaluate if perfluoroalkyl sulfonates, or 

potential precursors of these compounds, are likely emission products of the incineration 

of perfluoroalkyl sulfonates or of monomers or polymers containing perfluoroalkyl 

sulfonamides. Ab initio electronic structure theory was used to predict the 

thermodynamics of various degradation pathways for model sulfonamides. Key 

conclusions from this work include: 

1. If the temperature in the incineration process (e.g. incinerator, cement kiln, etc.) is 

high enough, the fluorocarbon will completely decompose. Thus, the engineering 

aspects of the incineration system in terms of mixing and residence time will be the 

dominant issue in minimizing by-product formation as long as the incineration 

temperature is high enough. One must also have the proper fluorocarbodfuel ratio. 

2. If CF4, the most stable perfluoroalkyl compound, were added as a tracer to the 

incineration system, its complete destruction would show that the incineration 

conditions should fully destroy all other perfluoroalkyl compounds. 

Thermal Decomposition Processes 

In an incineration system as well as in a cement kiln, the waste material is heated 

to a high temperature (temperatures up to -3000" F - 1650' C - 1925 K based on a 

burner flame temperature of 3500 to 4000 ' F) in an air stream that provides 02 as the 

oxidizing agent together with a hydrocarbon fuel. Typical residence times are -3 sec at 

temperatures between 2000' and 3000' F and -8 sec at temperatures above 1500 ' F and 

below 2000'. Combustion can then occur leading to the destruction of the starting 

material and under the proper conditions, the formation of products with low or zero 

environmental impact. 

The material can either burn as a solid which tends to be a slower process (and 

even this is often dominated by vaporization) or it can be vaporized leading to a faster 

process. We have focused on the vapor phase reactions of the materials of interest as 

these are likely to be the dominant processes at high temperature. Once vaporized, the 

compound can start to decompose and react with oxygen (or with other radicals generated 

by the fuel that is present). The decomposition reactions usually occur by bond scission 

(2-center) or by a 3-center or 4-center elimination process and continue via a radical 

chain mechanism. If bond breaking is the initial reaction, one must determine the bond 

that breaks first. Different bonds with different strengths will have different 

dependencies on the temperature. The weakest bond will break at the lowest temperature 

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and at very high temperatures there will be less dependence on the bond strengths and a 

broader distribution of radicals species can be formed. 

The compounds of interest to the present study are predominantly sulfonamides of 

the form RfS02NRR” where Rf= n-CsF17 or other normal or branched perfluoro alkyl 

chains, R’ = CH3 or C2H5, and R” is CH2CH20H or it is often a very complex organic 

linked to the N by a (N)-CH2CH20-C(0)1 linker group. We show, based on high level ab 

initio electronic structure calculations at the density hnctional theory and molecular 

orbital theory levels, that the most likely bond to break in compounds of the form 

RfS02R’ is the C-S bond. We focus on the differences in the bond energies of the C-S 

and the C-C bonds in the Rf fragment. We used Rf = n-C3F7 and CF3 in our model 

studies and examined R’ = OH, F, NH2, and 0. In all cases where a neutral, nonradical 

compound was studied, we find that the average C-S bond energy was - 64 kcal/mol and 

the average C-C bond energy was -85 kcal/mol. This is the energy required for breaking 

the CrC(S02)R’ bond. (C-F bond energies are much higher, near 120 kcal/mol.) The 

average bond energies of C-C bonds in the perfluoroalkyl fragment further from the SO2 

group are likely to be on the order of 95 kcal/mol based on other studies of 

perfluorocarbon bond energies. Bond energies of simple unsubstituted hydrocarbon 

alkane substituents are expected to be on the order of 85 to 90 kcal/mol. Substituents that 

can stabilize radical carbon centers will clearly lead to lower C-C bond energies. 

If a C-C or a C-S bond breaks, the decomposition chemistry of the 

perfluoroalkane radical formed in the process will follow the mechanism described 

below. If 02 intercepts the radical (a likely process), then a peroxy radical ROO. is 

formed which quickly decomposes to an alkoxy radical ROO. The R@* radical will 

undergo a p-scission process as shown in Reaction (1) to form carbonyl fluoride COF2 

and a one carbon shorter perfluoroalkyl radical. The chain decomposition will then 

continue, as above, until CF3 is formed. 

Rf‘CF203Rf‘ + F2C(O) (1) 

The =CF3 can react with 02 to eventually give the alkoxide CF3O. which usually abstracts 

a hydrogen atom from the hydrocarbon fuel that is present to form the alcohol CF30H. 

The alcohol can undergo 4-center HF elimination to form carbonyl fluoride COF2 again. 

Thus the main products from the decomposition of the fluorocarbon chain should be 

COF2 and HF. The COF2 will react with any water present and decompose to C02 and 

HF and, of course, as in all combustion processes with hydrocarbons present, some H20 

will be formed. As in all incineration processes, the completeness of the reaction will 

depend on the temperature and residence time. CF4 is probably the most difficult 

fluorinated compound to decompose in a normal incineration system.’ In order to detect 

whether incomplete incineration is occurring for other fluorocarbon-derived species, one 

can add a small amount of CF4 to the system. If no CF4 is present in the exit stream from 

the incineration furnace, it is likely that all of fluorinated material has been combusted. 

We have used density functional theory (DFT) at the local and nonlocal levels as 

well as molecular orbital theory at the G3 and G3-MP2 levels2 to calculate the various 

bond energies. We used correction factors from the G3 and G3-MP2 levels to correct the 

non-local DFT results. We obtain the following reaction energies. 

2 
Bond Cleavage Products 

C3F7* + *S02NH2 

C2F5* + *CF2S02NH2 

C3F7* + SO2 

C2F5* + CF2S02 (singlet) 

C3F7* + SO3 (singlet) 

C2F5* + *CF2S03* (triplet) 

C3F7* + *S02OH 

C2F5* + *CF2S020H 

C3F7* + *S02F 

C2F5. + aCF2S02F 

B.E.(kcal/mol) 

63 

85 

7 

68 

20 

84 

64 

85 

64 

84 

78 

89 

The conditions in most cement kiln incineration systems are very high 

temperatures (up to -1900 K). As the C-S bonds are of the lowest energy, they will break 

first under these high temperatures and will be the first to break at the lower temperatures 

expected in the temperature ramp region in the incineration system. This will lead to 

formation of fluorocarbon radicals that will then decompose as discussed above. If the 

S02R’ group is SO3, then the C-S bond is extremely weak, -20 kcal/mole, and the 

compound will readily decompose. The C-C bond energy is still on the order of 85 

kcal/mole. Also note that this is dissociation to the singlet ground state of SO3. The 

triplet is calculated to be 44 kcal/mole higher in energy than the singlet which places the 

S-C bond energy in the same range as those of other compounds. (Our calculated singlettriplet energy difference is likely be too low.) The C-C bond for CF$F*-CF$03 to form 

triplet CF2S03 is of a comparable energy to other C-C bonds in this series. If the S-R’ 

bond breaks first, for example in RfS02N(CH3)C2H40R (however, see below for a 

discussion of this point), then the radical RrSO2 will be formed. If an RS02* fragment is 

formed, the C-S bond is very weak, only 21 kcal/mole in CH3S02 and an even lower 7 

kcal/mol for CF3S02. Cleavage of the C-S bond in RfS02 will quickly lead to the 

fluorocarbon radical chain decomposition described above and SO2 formation. The C-C 

bond energy in RfSO2 is again much higher than the C-S bond energy although lower 

than other C-C bond energies. If the S02R’ group is SO3-, then the C-S bond is about 10 

to 15 kcal/mole stronger than in the other compounds noted above but still -10 kcal/mole 

weaker than the C-C bond. 

We also have calculated -S02-N bond anti N-C bond energies in related model 

systems. These models were chosen to get higher accuracy without having to use 

additivity corrections. The N-C bond strength in CH3NH2 is 85 kcal/mol at 298 K based 

on the latest results from experiment and theory. For the model system CF3S02NH2, we 

obtain the following bond energies at the G3-MP2 Level: 

3 
CF3S02NH2 + CF3. + *S02NH2 68.3 kcal/mol 

3 NH2. + .SO:? CF3 79.5 kcal/mol 

The C-S bond energy is consistent with the values shown above (slightly stronger as the 

CF3 radical is not as stabilizing as the C3F7 radical) and the S-N bond energy is clearly 

stronger than the C-S bond as expected. For the model system CF3S02NHCH3, we find 

the following at the G3-MP2 level: 

CF3S02NHCH3 + CF3S02NH. + .CH3 94.3 kcal/mol 

3 CF3S02. + aNHCH3 78.4 kcal/mol 

These results are consistent with those discussed above. The model systems might lead 

to changes of a few kcal/mol due to remote substituent effects but no larger changes are 

expected. 

The hydrocarbon amine decomposition chemistry and the alcohol amine 

decomposition chemistry can be worked out by similar methods as can the acrylate 

chemistry (e.g., polymers made by polymerizing RfS02N(CH3)C2H20C(O)CH=CH2 with 

methacrylates or other acrylates). As long as the fluorocarbon group degrades 

completely, it is likely that any of the other substituents bonded to the SO2 will also fully 

degrade, especially if they have a significant hydrocarbon component. 

One must also consider the possibility that reactions with H or 0 atoms or the OH 

radical are occurring in the incineration regime. 'The likely radical chain carriers are H 

atoms and OH radicals. The H atom will usually abstract an F or an H depending on the 

position and the OH radical can abstract any hydrogens added to the fluorocarbon chains 

during the combustion process. One can also abstract other atoms depending on bond 

strengths. The other possibility that also must be considered is 3- center elimination of 

HF if H is added to a perfluorocarbon chain. This can be a lower energy process than 

bond breaking. However, one does not have to wisrry about 0-F bonds being formed as 

these are very weak. 

Based on the bond energies, the temperatures in incinerators and in cement kilns 

are high enough (-peak 1900 K for cement kilns) that most decomposition processes 

should readily take place. The concern about byproduct formation except for CF4 is not 

so much the temperature but the mixing conditions in the incinerator. Flow patterns that 

allow the fluorocarbons to bypass the flame zone and to have a short (4 s) contact time 

even at high temperatures can lead to the formation of unwanted byproducts if only 

partial combustion occurs. This is essentially an engineering problem, not a chemistry 

problem. In addition, we note that Tsang et.al.' provide estimated temperatures to 

achieve destruction of 99.99% of a fluorocarbon compound in 1 sec based on 

unimolecular decomposition reactions. The highest temperature needed for a 

fluorocarbon system was -1715 K for CF4. For ii fluorocarbon radical such as C2F5, a 

temperature of -1200 K was needed (based on C-C bond scission) and for C2F6, a 

temperature of -1200 K was needed. These results are consistent with our conclusions 

that the temperature is not the likely culprit in the formation of any byproducts but rather 

the amount of mixing and the residence time in the high temperature region. The 

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temperature does however need to be well above -lOOO°C. Lower temperatures can and 

will lead to the formation of undesired by-products. Thus, cement kilns will need to be 

operated at as high a temperature as possible in order to guarantee complete 

decomposition of the fluorocarbons except for CF4 and at the highest operating 

temperatures even CF4 should decompose. An additional important concern is the 

fluorocarbodfiel ratio. If the amount of fluorinated material gets above a few percent, it 

is likely to also lead to more by-product formation as there will not be as good mixing 

and less of the fluorinated material may pass through the high temperature combustion 

zone where efficient decomposition can take place. 

Photolytic Processes 

For the model compound CF3S02NH2, we also calculated the ultraviolet-visible 

(UV-Vis) absorption spectra by using time-dependent density functional theory (TDDFT). TD-DFT is an approach with reasonable computational cost and reasonable 

accuracy for the calculation of the energies of excited states of molecules that we have 

been testing for use in the development of fluorinated resist materials for 157 nm 

photo resist^.^ We have previously shown that this a good level at which to calculate such 

values for a broad range of inorganic and organic compounds. The predicted values 

show that these types of compounds will not absorb visible light and decompose. If the 

primary perfluoroalkylsulfonamides get into the atmosphere, they would only decompose 

photolytically in the upper stratosphere. In other words, they are extremely stable in 

terms of photolysis. 

TD-DFT Calculated Excitation Energies of CFsS02NH2 

Excitation Excitation Oscillator 

ho Energy (eV) Strength f 

177 7.00 0.0008 

161 7.69 0.0228 

157 7.90 0.0066 

156 7.93 0.005 1 

Conclusions 

following conclusions may be drawn: 

Based on the above assessment of the thermal destruction of fluorochemicals, the 

1. The C-S bond should break first under high temperature conditions leaving the 

substituted S02R’ or the Rf SO2 radicals and the Rfand R’ radicals respectively. 

2. The Rf radical will then follow normal fluorocarbon combustion pathways via C(O)F2 

3. The S02R’ radical will likely decompose to SO2 and R’ with the R’ radical following 

its normal decomposition path. 

to form C02 and HF. 

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4. If RfSO3 is formed, it will quickly decompose to Rf and SO3 at very low 

temperatures. 

5. At the upper operating temperatures of cement kilns (-1900 K), CF4 should be 

efficiently decomposed so that it can serve as a marker for complete decomposition. 

If CF4 was added a tracer gas and no CF4 is present at the outlet, it is unlikely that 

any significant fluorinated by-products are generated. 

6. The engineering aspects of the incineration system in terms of mixing and residence 

time will be the dominant issue in minimizing by-product formation as long as the 

incineration temperature is high enough. One wants to have the longest possible 

contact time to ensure complete degradation. One must also have the proper 

fluorocarbodfuel ratio. In addition, it will be best if the cement kiln is run near the 

upper end of its operating temperature range. 

References 

1. Tsang, W.S.; Burgess, Jr., D.R.; Babushok, 17. Combust Sci and Tech. 1998, 139, 

385. 

2. Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Pople, J. A. J. Chem. Phys. 1997, 

103, 1063; Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Pople, J. A. J. Chem. 

Phys. 1998,lU9,7764. 

3. Matsuzawa, N. N.; Mori, S.; Yano, E.; Okazaki, S.; Ishitani, A.; Dixon, D. A. Proc. 

SPIE 2000, 3999, 375; Matsuzawa, N. N.; Ishitani, A.; Dixon, D. A.; Uda, T. J. Phys. 

Chem. A, in press (2001). 

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