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BACK TO MAIN 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 0 2 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 1 BACK TO MAIN 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 R f = 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' = compound was studied, we OH, find F, NH2, and 0. that the average In all cases where a C-S bond energy was -neutral, nonradical 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 0 2 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 =CF3can react with 0 2 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 BACK TO MAIN Bond Cleavage Products C3F7* + *S02NH2 C2F5* + *CF2S02NH2 B.E.(kcal/mol) 63 85 C3F7* + SO2 7 C2F5* + CF2S02 (singlet) 68 C3F7* + SO3 (singlet) 20 C2F5* + *CF2S03*(triplet) 84 C3F7* + *S02OH 64 C2F5* + *CF2S020H 85 C3F7* + *S02F 64 C2F5. + aCF2S02F 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 BACK TO MAIN CF3S02NH2 + CF3. + *S02NH2 3 NH2. + .SO:? CF3 68.3 kcal/mol 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 3 CF3S02. + aNHCH3 94.3 kcal/mol 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 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 4 BACK TO MAIN temperature does however need to be well above -lOOOC. 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 ho 177 161 157 156 Excitation Energy (eV) 7.00 7.69 7.90 7.93 Oscillator Strength f 0.0008 0.0228 0.0066 0.005 1 Conclusions Based on the above assessment of the thermal destruction of fluorochemicals, the following conclusions may be drawn: 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 to form C02 and HF. 3. The S02R' radical will likely decompose to SO2 and R' with the R' radical following its normal decomposition path. 5 BACK TO MAIN 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). 6