### Fluorochemical DecompositionProcesses 

## -April 

### 4,2001
David A. Dixon Theory, Modeling, andSimulation,William 

### R. Wiley Environmental Molecular Sciences Laboratory, Pacific NorthwestNational 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. Theprimary 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. Onemust alsohave 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 DecompositionProcesses 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 02as 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 compoundcan startto decomposeand react with oxygen (orwith other radicals generated by the fuel that is present). The decompositionreactions 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 isthe 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



### 1and at very high temperatures there will be less dependence on the bond strengths and a broader distribution of radicals species canbe formed. The compoundsofinterest 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 onhigh 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

# -64kcal/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 inthe 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 expectedtobe onthe order of 

### 85to 

### 90kcal/mol. Substituents that can stabilize radical carbon centers will clearlylead to lowerC-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 02intercepts 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, asabove, until CF3is formed.
Rf‘CF203Rf‘ 

## + F2C(O) (1) The =CF3canreact 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 decomposein a normal incineration system.’ In order to detect whether incomplete incineration is occurring for other fluorocarbon-derived species, one can add a small amount ofCF4to the system. Ifno CF4ispresent inthe exit stream from the incineration furnace, it is likely that all offluorinated 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 factorsfromthe G3 and G3-MP2 levels

### to correct the non-local DFTresults. 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-Sbonds are of the lowest energy, they will break first under these high temperatures and will be the firstto 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 asthose 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 ofthis point), then the radical RrSO2 will be formed. Ifan RS02*fragment is formed, the C-S bond is very weak, only 21 kcal/mole inCH3S02 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. Ifthe S02R’ groupis SO3-,then the C-S bond is about 10 to 15 kcal/mole stronger than in the other compounds noted abovebut 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 atthe 

### G3-MP2 Level:



### 3CF3S02NH2 

# + 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=CH2with methacrylates or other acrylates). As long as the fluorocarbon group degrades completely, it is likely that any of the other substituents bonded to the SO2will also fully degrade, especially ifthey have a significant hydrocarbon component. One must also consider the possibility that reactions with H or 

### 0atoms orthe 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-Fbonds 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 conditionsinthe incinerator. Flow patterns that allow the fluorocarbons to bypass the 

### flame zone and to have a short (4s) 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 inthe formation of any byproducts but rather the amount of mixing and the residence time in the high temperature region. The

4temperature 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. Ifthe amount 

### offluorinatedmaterial gets above a fewpercent, 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 decompositioncan takeplace. 

### 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 havepreviously shownthat this a good level at which to calculate such values for a broad range of inorganic and organic compounds. The predicted values showthat these types of compoundswill 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

### ofCFsS02NH2

Excitation Excitation Oscillator Energy (eV) Strength f 

## ho 177 7.00 

### 0.0008 161 7.69 

### 0.0228 

### 157 7.90 0.0066 156 7.93 0.0051


### Conclusions
following conclusions maybe 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’orthe RfSO2radicals andthe RfandR’ radicals respectively. 2. TheRf radical will then follownormal fluorocarbon combustion pathways via C(O)F2



### 3. The S02R’radical will likely decomposeto SO2 and R’ with the R’ radical following itsnormal decomposition path. to form C02and HF.

54. 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 

### ofthe 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 fluorocarbodfuelratio. In addition, it will be best ifthe cement kiln is run near the upper end ofits operating temperature range.

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### W.S.; Burgess, Jr., D.R.; Babushok, 17. 

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### Chem. Phys. 1998,lU9,7764. 3. Matsuzawa, 

### N. N.; Mori, 

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### Proc. SPIE 2000, 

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