[ACS Symposium Series] Halon Replacements Volume 611 (Technology and Science) || Tropospheric...

Chapter 5

Tropospheric Degradation Products of Hydrochlorofluorocarbons

and Hydrofluorocarbons Potential Replacements for the Chlorofluorocarbons

and Halons

Ernesto C. Tuazon and Roger Atkinson

Statewide Air Pollution Research Center, University of California, Riverside, CA 92521

The products of the Cl atom-initiated photooxidations of a series of C1-C3 HCFCs and HFCs are summarized and discussed with respect to a generalized tropospheric reaction scheme. Direct observations of CFCl 2 CHO and CF 2C1CHO as products of C F C l 2 C H 3 (HCFC-141b) and C F 2 C l C H 3 (HCFC-142b), respectively, in 100% yields are reported. The measured branching ratios of the CFCl 2 ĊO and CF 2 ClĊO radicals for decomposition versus reaction with O2 are consistent and intermediate with those previously reported for CF 3 ĊO and CCl 3 ĊO.

Observations of stratospheric ozone depletion at mid-latitudes as well as in the Antarctic and Arctic stratospheres during springtime (1-3) have been linked to anthropogenic emissions of CI- and Br-containing compounds (1-3). These findings led to the 1987 Montreal protocol and its subsequent revisions at London in 1990 and Copenhagen in 1992, resulting in a phase-out of production of the Halons by January 1, 1994 and of the chlorofluorocarbons (CFCs) by January 1, 1996.

Potential hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC) replacement compounds to the Halons and CFCs contain hydrogen and are designed to react with the hydroxyl (OH) radical in the troposphere (1,3,4), minimizing or avoiding the transport of chlorine and bromine to the stratosphere (1,3). These replacement compounds are largely removed by chemical reaction in the troposphere (1,3,4), and the reaction mechanisms and products formed need to be understood prior to widespread use of these compounds (4). Over the past few years we have conducted product studies of the tropospheric degradation reactions of a series of HCFCs and HFCs (5-7), and the results of these studies are described briefly here.

0097-6156/95/0611-0041$12.00/0 © 1995 American Chemical Society

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In Halon Replacements; Miziolek, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

42 HALON REPLACEMENTS

Experimental

Studies were carried out in a 5800 liter evacuable, thermostatted, Teflon-coated chamber (8) equipped with an in situ multiple reflection optical system interfaced to a Nicolet 7199 Fourier transform infrared (FT-IR) absorption spectrometer. Radiation of wavelength >300 nm was provided by a 24 kW xenon arc, filtered through a Pyrex pane. Because of the difficulty of maintaining high OH radical concentrations and the low rate constants for the OH radical reactions with the HCFCs and HFCs (9) reactions were initiated by reaction with CI atoms

C l 2 + hv -* 2 CI

CI + R H -> HC1 + R

The initial reactant concentrations (in molecule cm"3 units) were typically: H C F C or H F C , (1.2-2.5) x 101 4; C l 2 , (1.2-16) x 1014; although a series of experiments to measure the formation yields of CF 2 C1CH0 and CFCl 2 CHO from CF 2 C1CH 3

(HCFC-142b) and CFC1 2 CH 3 (HCFC-141b) used initial HCFC and C l 2 concentrations of (4.9-25.6) x 1015 molecule cm' 3 and 1.3 x 101 4 molecule cm 3 , respectively (7). Most experiments were carried out at 298 ± 2 K and 740 Ton* total pressure of synthetic air (80% N 2 + 20% N ^ ; experiments with C H 2 F C F 3 (HFC-134a) were carried out over the temperature range 273-320 K and with the 0 2 partial pressure and total pressure varied over the ranges 150-600 Torr and 150-740 Torr, respectively (5).

The reactants and products were monitored during the irradiations by FT-IR absorption spectroscopy, using a pathlength of 57.7 m and a full-width at half-maximum (fwhm) spectral resolution of 0.7 cm 1 . The identification and analysis of products were facilitated by spectral stripping, i.e., successive subtraction of absorptions using calibrated reference spectra of the known components. Calibrations with authentic samples were made by expanding measured partial pressures of the compound in calibrated 2-L and 5-L Pyrex bulbs into the reaction chamber.

The samples of HFCs and HCFCs were donated by E. I. DuPont de Nemours and Co. Inc., Allied Signal, Solvay, S. A . , and Asahi Glass Co., and were generally of stated purities >99.5%. Samples of the highest purity available for CFC1 2 CH 3 and CF 2 C1CH 3 (>99.99%, Solvay S.A.) were employed. Authentic samples of CFCl 2 CHO and CF 2 ClCHO were prepared and purified in the authors' laboratory (7).

Results and Discussion

The series of C r C 3 HFCs and HCFCs studied, the products formed and their yields (5-7) are summarized in Table I.

The haloalkyl radical R resulting from the initial H-atom abstraction by the CI atom (or OH radical) rapidly adds 0 2 , R + 0 2 ^ R 0 2 , and under the above experimental conditions where NO was absent the haloalkyl peroxy radical

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5. TUAZON & ATKINSON Tropospheric Degradation Products of HCFCs 43

Table I. Observed Products and Their Yields from the CI Atom-Initiated Photooxidation of a Series of HFCs and HCFCs

in 1 Atmosphere of Air at 298 K

Primary Products Refer-HFC or HCFC and Yields ences

CHF 2 C1 (HCFC-22) C(0)F 2 ) 100% 6 CHFC1 2 (HCFC-21) C(0)FC1, 100% 6 CH 2 FC1 (HCFC-31) HC(0)F, 100% 6 C H 3 F (HFC-41) HC(0)F, 100% 6 CH 3 CFC1 2 (HCFC-141b) CFCl 2 CHO, 100% 7 CH 3 CF 2 C1 (HCFC-142b) CF 2 ClCHO, 100% 7 CH3CHF2 (HFC-152a) C(0)F 2 , 92%; CO and C 0 2 6 CHC1 2 CF 3 (HCFC-123) CF 3C(0)C1, 98% 6 C H F C I C F 3 (HCFC-124) CF 3 C(0)F, 100% 6 C H F 2 C F 3 (HFC-125) C(0)F 2 , -100%; CF 3 OOOCF 3 6 C H 2 F C F 3 (HFC-134a) CF 3 C(0)F, 24%;

HC(0)F, 76%; C(0)F 2 , 8-14%; CF 3 OOOCF 3

and CF 3 OOC(0)F

5"

CF 3 CF 2 CHC1 2 (HCFC-225ca) CF 3 CF 2 C(0)C1, 100% 7 CF 2C1CF 2CHFC1 (HCFC-225cb) CF 2 C1CF 2 C(0)F, 99%; 7

C(0)FC1, 1.0%

aSome CF 3 C(0)F formed by the combination reaction of CF 3 CHFOO radicals; in the atmosphere the calculated yields of CF 3 C(0)F and HC(0)F at 298 K and one atmosphere of air are 18% and 82%, respectively (5).

undergoes self-reaction to produce mainly the corresponding haloalkoxy radical, 2 R 0 2 -» 2 RO + 0 2 (5-7). In the troposphere, the peroxy radical is expected to mainly react with NO to generate N 0 2 and the alkoxy radical (4). Since the irradiations of Cl 2-HFC(or HCFC)-air mixtures were conducted in the absence of added NO x , the formation of R O N 0 2 , R O O N 0 2 and RC(0)OON0 2 class product compounds was not directly investigated in these experiments.

A generalized reaction scheme which is consistent with our product data shown in Table I, and which shows the disposition of the haloalkyl peroxy and haloalkoxy radicals under tropospheric conditions, is presented in Figure 1. The haloalkoxy radical can react via three pathways: decomposition, reaction with 0 2 , or CI atom elimination. When CI atom elimination is possible from the alkoxy

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HALON REPLACEMENTS

C X 3 C Y Z + H 2 0 OH + C X 3 C H Y Z

HOp T . NOp C X 3 C Y Z O O H ^ — J C X 3 C Y Z O O C X 3 C Y Z 0 0 N 0 2

OH

C X 3 C < 0 ) Y '

Figure 1. Generalized tropospheric reaction scheme for the HFCs and HCFCs.

h v f C X 3 + H C O C X 3 C H 0 - ^ U * < J

OH

C X 3 C O

3 2

C X 3 C ( 0 ) 0 0 H + 0

C X 3 C < 0 ) O H + O 3 J

HOP I°2

. NOp

^ C H X 3 + C O

C X 3 + CO

C X 3 C < 0 ) 0 0 N 0 2 C X 3 C < 0 ) 0 0

N 0 - j ^ N 0 2

C X 3 + C 0 2

Figure 2. Generalized tropospheric reaction scheme for the haloaldehydes.

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5. TUAZON & ATKINSON Tropospheric Degradation Products ofHCFCs 45

carbon, it is the dominant if not the exclusive pathway. Thus, the C r C 3 HCFCs listed in Table I which can form such haloalkoxy radicals lead to the formation of acid halide products in -100% yield.

The CX 3 CF 2 6 radical tends to undergo C-C bond fission as demonstrated by the case of C H C F 2 C F 3 (HFC-125), which formed C(0)F 2 in -100% yield along with further products of the C F 3 radical, notably the unusual trioxide CF 3 OOOCF 3 under our experimental conditions. The C X 3 C H Y O radical (Y=C1 or F) undergoes both C-C bond breakage and reaction with 0 2 , the relative importance of which is dependent on temperature, 0 2 concentration, and total pressure (and hence on altitude). This is true for the alkoxy radical CF 3 CHFO, formed from C H 2 F C F 3 (HFC-134a), with the mix of products being comprised of one- and two-carbon acyl halides and further products of the C F 3 radical. Our data (5) and those of Wallington et al (10) and Rattigan et al. (11) show the CF 3 C(0)F yields from HFC-134a increase with increasing altitude in the troposphere, with mainly formation of HC(0)F and products of the C F 3 radical at the Earth's surface and mainly formation of CF 3 C(0)F at the tropopause. Further reactions of the C F 3

radicals in laboratory systems and in the atmosphere have been investigated (3,12,13).

Earlier experiments on the reactions of CX 3 CH 2 6 radicals, such as those derived from HCFC-141b and HCFC-142b (6,14,15), could not measure the relative importance of the decomposition pathway and the reaction with 0 2 which produces the haloaldehyde C X 3 C H O (Figure 1). This is due to the very rapid reaction of the aldehydes with CI atoms or OH radicals, which lead to the same carbonyl halide products as produced by the decomposition step. A determination that the haloaldehydes are formed is important with respect to the formation of peroxy acyl nitrates, which may be sufficiently stable that they could contribute to CI transport to the stratosphere, and to the production of the C 2 acids which may be biological hazards (16). The tropospheric reaction schemes for these haloaldehydes is given in Figure 2.

Thus, photolysis experiments utilizing elevated concentrations of CFC1 2 CH 3

(HCFC-141b) and CF 2 C1CH 3 (HCFC-142b) in mixtures with C l 2 in air were carried out. Product spectra from an experiment with initial concentrations of 4.9 x 1015

molecule cm"3 of CFC1 2 CH 3 and 1.3 x 101 4 molecule cm"3 of C l 2 are shown in Figure 3. Intermittent irradiation was employed to provide for longer averaging times to obtain the spectra during the intervening dark periods. CFCl 2 CHO was positively identified by the exact position and contour of its C = 0 stretch band (1782.7 cm 1) as compared to the spectrum of the synthesized sample (7). The total amount of CFC1 2 CH 3 reacted, which amounted to 0.8% during the total of 16-min irradiation, was determined by the sum of the observed C(0)FC1 and CFCl 2 CHO. With the time-concentration data obtained and the rate constant ratio k(Cl + CFCl 2 CHO)/k(Cl + CFC1 2CH 3) = 2600 ± 700, derived from published rate constants (17-19), corrections for the secondary reaction of CFCl 2 CHO with the the CI atom were calculated. The result was a 100% yield of CFCl 2 CHO from CFC1 2 CH 3 (7), indicating that at 298 K and atmospheric pressure of air the haloalkoxy radical CFC1 2CH 26 does not decompose but solely reacts with 0 2 .

A similar experiment was carried out for CF 2 C1CH 3 (HCFC-142b). Figure 4 shows the spectra recorded from the photolysis of an air mixture with 2.6 x 1016

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0.95

0.5 min

(-1 Too iTso i6oo T5io T§oo Thso l / X (cm- 1)

Figure 3. Product spectra from the CI atom-initiated photooxidation of CFC1 2 CH 3 . Numbers shown are concentrations in units of IO13 molecule cm 3 .

CF 2 CIC(0)0H

u.i«

C F 2 C I C H 0 C(0)F 2

• i Toio 1^50 i6oo i 6 5 D T5oo iUio iioo l / X (cm - 1 )

Figure 4. Product spectra from the CI atom-initiated photooxidation of CF 2 C1CH 3 . Numbers shown are concentrations in units of 1013 molecule cm 3 .

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5. TUAZON & ATKINSON Tropospheric Degradation Products ofHCFCs 47

molecule cm"3 of CF 2 C1CH 3 and 1.3 x 101 4 molecule c m 3 of C l 2 . The aldehyde product, CF 2 C1CH0, was positively identified by the exact correspondence of its C = 0 stretch band (1778.6 cm'1) with that of the prepared sample (7). In addition to the expected formation of C(0)F 2 , CF 2 C1C(0)0H was also observed as a product. A very small fraction of the starting compound was consumed ( -0 .1% with 11-min total irradiation time) such that the amount of CF 2 C1CH 3 reacted was calculated by the sum of the observed CF 2 C1CH0, CF 2 C1C(0)0H, and C(0)F 2

concentrations. The corrections for the secondary reactions of CF 2 C1CH0 with CI which were calculated with the rate constant ratio k(Cl + CF 2 ClCHO)/k(Cl + CF 2C1CH 3) = 11800 ± 2000, derived from literature rate constants (77-79), indicated a CF 2 ClCHO yield of 167%. However, a rate constant ratio of 7400, - 4 0 % lower than the above value of 11800 but within the range of errors that could occur in the measurement of rate constants for C X 3 C H O (see, for example, the case of CF 3 CHO; refs. 77, 20), predicts a unit yield of CF 2 ClCHO from CF 2 C1CH 3 . The significant amounts of CF 2 ClC(0)OH observed could also indicate that appreciable regeneration of the aldehyde possibly occurred via the reaction

CF 2 C1C(0)00 + CF 2 C1CH 2 00 -» CF 2 ClC(0)OH + CF 2 ClCHO + 0 2

which would then contribute to an overestimate of the correction for loss of C F 2 C l C H O by reaction with the CI atom. Overall, our experimental data strongly point to a 100% yield of CF 2 ClCHO from CF 2 C1CH 3 (7) and support the reaction with 0 2 as the exclusive reaction channel for the CF 2C1CH 26 radical under lower troposphere conditions.

Important details of the atmospheric transformations of HCFC-14lb and HCFC-142b required a study of the reactions of their respective aldehyde products C F C l 2 C H O and CF 2 ClCHO, specifically the disposition of the C X 3 C O radicals formed from their reactions with OH radicals or CI atoms (Figure 2). Photooxidation experiments were conducted in the 5800-L chamber at 298 K with 1 atm of air containing (1.1-1.2) x 1014 molecule cm' 3 of the aldehyde and (6.3-7.3) x 1013

molecule cm"3 of C l 2 , with spectral data being obtained after each period of short, intermittent irradiation.

As noted in Figure 2, the branching ratio of C X 3 C O with respect to decomposition and reaction with 0 2 can be determined by measuring the amounts of CO, C 0 2 , and the C(0)FC1 and C(0)F 2 end products of the C X 3 radicals. In the absence of NO, the following specific reactions, illustrated for the case of C F C l 2 C H O , occur:

2 CFC1 2 C(0)00 2 CFC12C(0)6 + 0 2

CFC12C(0)6 -> CFC1 2 + C 0 2

0 2

CFC1 2 -*^CFC1 2 6

CFC126 -* C(0)FC1 + CI

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With respect to the decomposition reaction (a) and 0 2 reaction channel (b) (Figure 2), the CFCl 2 CO branching ratio was determined as 79 ± 7% (a) and 21 ± 5 % (b), while the values for CF 2 C1C0 were measured as 39 ± 3% (a) and 61 ± 5% (b) at 298 K and 740 Torr of air. These results are intermediate and consistent with those reported in the literature for CCl 3 CO [92% (a)] and C F 3 C O [1.3% (a)] at 298 K and 750 Torr total pressure of air (27).

Acknowledgment

The authors gratefully thank the SPA-AFEAS for financial support through Contracts CTR90-3/P90-001 and CTR91-28/P91-082 (Dr. Igor Sobolev, Project Monitor).

Literature Cited

1. Scientific Assessment of Ozone Depletion: 1991, World Meteorological Organization Global Ozone Research and Monitoring Project - Report No. 25, Geneva, Switzerland, 1992.

2. Stratospheric Ozone 1993, United Kingdom Stratospheric Ozone Review group, Department of the Environment, London, UK, November 1993.

3. Scientific Assessment of Ozone Depletion: 1994, World Meteorological Organization Global Ozone Research and Monitoring Project, Geneva, Switzerland, in press 1994.

4. Scientific Assessment of Stratospheric Ozone: 1989, World Meteorological Organization Global Ozone Research and Monitoring Project - Report No. 20, Vol. II, Appendix: AFEAS Report, Geneva, Switzerland, 1990.

5. Tuazon, E . C.; Atkinson, R. J. Atmos. Chem. 1992, 16, pp. 301-312. 6. Tuazon, E . C.; Atkinson, R. J. Atmos. Chem. 1993, 17, pp. 1799-199. 7. Tuazon, E . C.; Atkinson, R. Environ. Sci. Technol. 1994, in press. 8. Winer, A . M.; Graham, R. A . ; Doyle, G. J.; Bekowies, P. J . ; McAfee, J.

M.; Pitts, J. N . , Jr., In An Evacuable Thermostatted Environmental Chamber and Solar Simulator Facility for the Study of Atmospheric Chemistry, Pitts, J. N . , Jr. and Metcalf, R. L., Eds.; Advances in Environmental Science and Technology; John Wiley and Sons: New York, N . Y . , 1980, Vol. 10; pp. 461-511.

9. Atkinson, R.; Baulch, D. L.; Cox, R. A . ; Hampson, R. F., Jr.; Kerr, J. A . ; Troe, J . ; J. Phys. Chem. Ref. Data 1992, 27, pp. 1125-1568.

10. Wallington, T. J . ; Hurley, M. D.; Ball, J. C.; Kaiser, E. W. Environ. Sci. Technol. 1992, 26, pp. 1318-1324.

11. Rattigan, O. V . ; Rowley, D. M.; Wild, O.; Jones, R. L.; Cox, R. A . J. Chem. Soc. Faraday Trans. 1994, 90, pp. 1819-1829.

12. Sehested, J . ; Wallington, T. J. Environ. Sci. Technol. 1993, 27, pp. 146-152.

13. STEP-HALOCSIDE/AFEAS WORKSHOP, "Kinetics and Mechanisms for the Reactions of Halogenated Organic Compounds in the Troposphere," Dublin, March 23-25, 1993, University College, Dublin, Ireland.

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14. Edney, E . O.; Gay, B. W. , Jr.; Driscoll, D. J. J. Atmos. Chem. 1991, 12, pp. 105-120.

15. Edney, E . O.; Driscoll, D. J. Int. J. Chem. Kinet. 1992, 24, pp. 1067-1081.

16. Ball, J. C.; Wallington, T. J. J. Air Waste Manage. Assoc. 1993, 43, pp. 1260-1262.

17. Scollard, D. J . ; Treacy, J. J . ; Sidebottom, H . W.; Balestra-Garcia, C.; Laverdet, G . ; LeBras, G . ; MacLeod, H.; Teton, S. J. Phys. Chem. 1993, 97, pp. 4683-4688.

18. Tuazon, E . C.; Atkinson, R.; Corchnoy, S. B. Int. J. Chem. Kinet. 1992, 24, pp. 639-648.

19. Wallington, T. J . ; Hurley, M . D. Chem. Phys. Lett. 1992, 189, pp. 437-442.

20. Wallington, T. J.; Hurley, M . D. Int. J. Chem. Kinet. 1993, 25, pp. 819-824.

21. Barnes, I.; Becker, K. H.; Kirchner, F . ; Zabel, F . ; Richer, H.; Sodeau, J. In Kinetics and Mechanisms for the Reactions of Halogenated Organic Compounds in the Troposphere, STEP-HALOCSIDE/AFEAS Workshop, Dublin, March 23-25, 1993, pp. 52-58.

RECEIVED April 13,1995

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[ACS Symposium Series] Halon Replacements Volume 611 (Technology and Science) || Tropospheric...

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