Environ. Sci. Technol. 2008, 42, 1301–1307
Global Warming Potentials of Hydrofluoroethers PAUL BLOWERS,* DENA MARIE MOLINE, KYLE FRANKLIN TETRAULT, R’NLD RUTH WHEELER, AND SHANE LEE TUCHAWENA Department of Chemical and Environmental Engineering, The University of Arizona, P.O. Box 210011, Tucson, Arizona 85721-0011
Received March 13, 2007. Revised manuscript received November 20, 2007. Accepted November 26, 2007.
the time dependent concentration of a pulse of species i, while the corresponding quantities for a reference gas are in the denominator and can be represented as the absolute global warming potential (AGWP) of the reference species. HFEs contain at least one C-H bond that leads to shorter atmospheric lifetimes through hydrogen abstraction by hydroxyl radicals, which is the predominant loss mechanism for most atmospheric releases of volatile organic compounds (11–13). In general, as we will see in the summary of HFE kinetic data, shorter atmospheric lifetimes are characteristic with more hydrogen atoms on the HFE molecule. A standard transformation of eq 1 uses the atmospheric lifetime of a species, τ, instead of integrating xi(t): TH
ai
Global warming potentials are estimated for hydrofluoroethers, which are an emerging class of compounds for industrial use. Comparisons are made to the limited data previously available before observations about molecular design are discussed. We quantify how molecular structure can be manipulated to reduce environmental impacts due to global warming. We further highlight the need for additional research on this class of compounds so environmental performance can be assessed for green design.
Introduction Soon after chlorofluorocarbons (CFCs) were identified as ozone depleting substances and their phase out began, the Environmental Protection Agency conducted an assessment of replacement alternatives (1). Hydrofluoroethers (HFEs), were selected as one class of potential replacement materials because the introduction of an ether moiety reduced atmospheric lifetimes (2) over those of hydrofluorocarbons (HFCs). Table S1 in the Supporting Information highlights the different HFE species and their potential uses that have been the subject of U.S. patents over the last 1.5 years. HFEs do not cause ozone depletion due to the lack of chlorine, iodine, and bromine atoms (3). Early claims about HFEs suggested that their global warming potentials (GWPs) would be near zero as well (4). Other work identified HFEs as having lower GWPs compared to the CFCs they would replace (5–7). Recently, concerns have arisen that HFEs may contribute to global warming, and some work has been done to investigate their GWPs to identify if this will lead to their eventual phase out. Global warming potentials can be calculated from knowledge of degradation rates of species in the troposphere combined with infrared vibrational spectroscopy using the following equation (8–10): TH
GWPi )
∫
ai[xi(t)]dt
0
(1)
TH
∫a
ref[xref(t)]dt
0
where TH is the time horizon for GWP the species will be considered over, ai is the radiative forcing due to a unit increase in atmospheric concentration of species i, [xi(t)] is * Corresponding author phone: 520-626-5319; fax: 520-621-6048; e-mail:
[email protected]. 10.1021/es0706201 CCC: $40.75
Published on Web 01/17/2008
2008 American Chemical Society
GWPi )
∫e
t -( ) τ dt
0
(2)
AGWPCO2(TH)
where atmospheric lifetime is denoted as follows: τlifetime,R ) τlifetime,CH3CCl3 ×
kOH+CH3CCl3(277K)
(3)
kOH+R(277K)
Atmospheric lifetimes of species can be obtained by multiplying the global atmospheric lifetime of a well characterized reference compound like CH3CCl3 by the ratio of the OH reaction rate constant at 277 K for CH3CCl3 to the one for a new species (14, 15). In this work, the data from DeMore (16), et al., was used to get 6.7 × 10-15 cm3/molecule-sec (14) for the rate constant of CH3CCl3 at 277 K. For the atmospheric lifetime of the reference compound, we will use a value of 5.7 years (17). Many techniques for measuring OH radical concentrations have given differences of 20-30% (18), suggesting that atmospheric lifetimes may have similar differences, leading to differences in GWPs. This paper uses eq 1-3 combined with radiative forcing results from density functional theory reported in our earlier work (19) and expanded here to compute GWPs for 49 hydrofluoroethers where kinetic rate data is available at 277 K or where other proxy kinetic measures have been reported, like atmospheric lifetime. Where rate data is not available at this temperature, extrapolation is made from rate constants available at 298 K using a method similar to Andersen, et al.’s work (20). Finally, comparisons are made between previous GWP results for HFEs and the work here before new results are detailed. The end result is an expanded database of GWPs to allow for comparisons between alternatives being contemplated in industrial applications. Currently, GWPs at some time horizon have been reported for only 19 HFEs.
Materials and Methods Rate Constants and Kinetic Information. In the Supporting Information, we discuss in detail the sources of data used for the kinetic measurements or estimates used in this work because these choices have the largest impact on our results. We begin with a discussion of the JPL data set, followed by a discussion of data at lower temperatures, either as a function of temperature or at a single point. Then we move on to discussing how we extrapolated data from 298 K down to 277 K when only room temperature kinetics were available. Then we discuss the different computational approaches for estimating rate constants and why one of them was chosen as the source of additional lifetimes when no measurements were available. VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Radiative Forcing. Our previous paper (21) describes the methodology to calculate radiative forcing for species using quantum chemical methods, and only a brief review will highlight the methodology. The Gaussian98 software package (22) was used here to perform the quantum mechanical calculations. Due to the lesser expense and a variety of technical issues involving improved accuracy discussed in our previous paper, we used the density functional theory (DFT) B3LYP method combined with the 6–31 g* basis set for geometry optimizations and frequency calculations. All stable structures were verified to be the correct structures through frequency calculations where minima had no negative eigenvalues. Frequencies were scaled by a factor of 0.9613 (23) to reduce the rms error of frequency prediction calculations compared to experimental results to 34 cm-1. We used the simplified model for radiative forcing of Pinnock, et al., (24) where one can directly translate absorption cross sections to radiative forcing without the complex atmospheric modeling previously needed. This approach was combined with an extension first described by Papasavva, et al. (25) that uses ab initio data instead of experimental spectroscopy. In this work, we expand the work of Elrod (26) and Papasavva (25) to encompass the lower frequency contributions down to 0 cm-1. Our work found that our inexpensive method of estimating radiative forcing values compared very well to the expected 25% errors from modeled values using experimental data (27). Data for 25 of the 27 HFEs where radiative forcing values have been reported were within these error bounds. With substantial agreement between modeled results and our theoretically based ones, we can use predicted radiative forcing values with confidence in predicting GWPs.
Results and Discussion Radiative Forcing Values. With our comparison of computational radiative forcings to experimental ones demonstrating we are able to accurately estimate radiative forcing in our earlier work (21), we report radiative forcings for all species where kinetic data exists and where no cloudy sky radiative transfer values have been published. It should be noted here that the 1,1- and 1,2-HFE radiative forcings for different rotamers were used to identify trends in how rotameric structures impacted the predicted radiative transfer values. Ideally, one would use the computed energies and thermodynamic information to compute Gibbs free energies for each rotamer set to determine the fraction of time each rotamer would be expected to exist and then weight the predicted radiative forcings correspondingly. These types of analyses are outside the scope of this work and are the subject of a series of other papers currently in preparation. In our previous work, we compared the rotamers with the highest radiative transfer values among the 1,2-HFEs to find that a linear backbone with the highest molecular symmetry led to the highest radiative forcing values. It is reasonable to identify the highest radiative forcing values for each rotamer as this will give a higher estimate of GWP. The values for the planar backbone conformation over the bent conformation were typically 15-20% larger. For all HFEs larger than the 1,2-HFEs, we used a linear backbone and then only computed rotamers when there was the possiblity of having different end group rotamers in the cases where one or both ends of the molecule were terminated with a -CHF2 or CH2F group. It should be noted that all GWPs have been scaled by an explicit comparison to the GWP of CO2. While the AGWP of CO2 may vary by up to 20% depending on the model used, we follow the work of Wuebbles and use GWPs of CO2 equal to 0.235, 0.768, and 2.459 W yr/m2 ppmv at TH’s of 20, 100, and 500 years, respectively (28). This will allow us to scale 1302
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our GWP results for comparison to other GWPs estimated from experimental results. GWP Results. We begin the discussion of GWPs with a short description of why two sets of data were not included in this work for comparison. While Imasu, et al., performed estimates of some HFEs, their methodology is not directly comparable to the results in this work due to their lack of time horizon as a parameter in their model (29). Additionally, they used the clear sky radiative forcing value instead of the cloudy sky radiative forcing that has become the standard. Differences in their results from current work due to this could be as large as 45% as seen in Table 1. Because of this, we do not compare to their work here. Similar to the work of Imasu, Brown, et al., used a simplified first approximate method to obtain global warming potentials for HFE’s relative to CFC11 (30). This approximation skips obtaining radiative forcing results and uses integrated absorption cross sections instead, thus ignoring the effect of different band strengths across the atmospheric window region of each species. This also means that it is not possible to back-calculate GWPs relative to CO2 from their work as they did not provide full spectra information for the species. Due to this, their results are also not scalable for comparison to the standard GWP results. Table 1 shows all of the results from this work compared to the previously available data reported in the literature. We will begin the dicussion by noting there are GWPs for many of the 1,1- and 1,2-HFEs where results are given at the 20, 100, and 500 year THs. However, in many of the prior results, intermediate data like atmospheric lifetimes or kinetic data and radiative forcing values were often not reported so it may be difficult to understand why differences exist between research groups. For larger HFEs, very limited or no GWP data is available in the literature even for cases where kinetic rates have been measured. For the 1,1 HFE, CF3OCH3, we see that Orkin (31) and Good (32) have a factor of 2 difference in their GWP predications with Good, et al.’s results being higher. We see that our predictions using computed radiative forcing values with an averaged rate constant from several kinetic data sources lies in between their two values, indicating we are able to reproduce modeled results. There is good agreement between Good (32) and Myhre (33) for CHF2OCHF2, with a range of 20 year TH GWPs from 9760 to 10700. For all time horizons, our results are slightly lower even though our atmospheric lifetime is longer. This is because our estimates for radiative forcing for this species are lower than the ones reported in their work, even though, in general, our radiative forcing values are generally overpredicted (19). Good (34) and Christidis (35) both report GWPs for CF3OCHF2 where the values are within 16% of each other at the shortest TH and within 20% of each other at the longest. Their values of 11 800 at 20 years are lower than our value of 63 124 even though we have a shorter atmospheric lifetime with a similar radiative forcing value, which should lead to lower GWP values. Plugging Good’s (34) atmospheric lifetime and radiative forcing used in the GWP equations in this work leads to GWP values of 152 952, 65 759, 33 774 at the three THs, which suggests that the currently accepted model followed in this work yields results that are quantitatively different from larger atmospheric models that were previously used. This is further supported by a comparison of Oyaro’s (36) results with Christidis’ (35) for CHF2OCH2CF3. Regardless, Good, et al.’s (34) results for CF3OCHF2 reported at the 100 year TH must have a typographical error and we suspect their value of 1400 should have been 14 000 to be selfconsistent with their other values. For the 1,2 HFEs, Christidis (35) and Oyaro (36) report results that are within a factor of 2 of each other for CHF2OCH2CF3, while our results of 4254, 1304, and 407 are
TABLE 1. Atmospheric Lifetime Estimates, Instantaneous Radiative Forcing, and Global Warming Potential Results for HFEs at Time Horizons of 20, 100, and 500 Years Compared to Previously Published Results τ (years)
radiative forcing (W m-2 ppbv-1)
20 year TH
0.2c 1.1 3.3a
0.154d 0.210 0.243
94 517 1456
29 158 446
9 49 139
CHF2OCHF2
15.5a
0.484
8645
3648
1141
CF3OCHF2
77.9
0.470
63124
31739
13053
0.240 0.259 0.345 0.389
70 284 842 4254
21 87 257 1304
7 27 80 407
0.448 0.554 0.455 0.529
1844 7076 3766 9091
566 3042 1224 2875
177 952 382 897
species 1,1 ethers CH3OCH2F CH3OCHF2 CF3OCH3
1,2 ethers CH3OCH2CF3 CF3OCH2CH3 CH3OCF2CHF2 CHF2OCH2CF3
CH3OCF2CF3 CHF2OCF2CHF2 CHF2OCF2CH2F CHF2OCHFCF3
0.1 0.3 0.3a 3.2
3.3 16.2 7.4 5.9
GWPs 100 year TH
GWPs 500 year TH
τ (years)
11.3 165.0 0.44
20 year TH
100 year TH
500 year TH
ref
1300 2200 9760 10700 11800 10137
360 656 5720 6300 1400 12015
130 202 1830 2000 9120 7596
31 32 32 33 32 35
36 151
9 47 47 570 1064 570 712 522
0.49 1871 3319 1870 4.9 6.5 14.4c 6.7c 4544
1,3 ethers CH3OCH2CF2CF3 CH3OCH2CF2CHF2 CHF2OCH2CF2CHF2 CH3OCF2CF2CHF2 CH3OCF2CF2CF3 CHF2OCF2CHFCF3 CF3OCH2CF2CF3 CF3OCF2CHFCF3 CHF2OCH2CF2CF3 CH3OCH(CF3)2 CH3OCF(CF3)2 CH2FOCH(CF3)2 CF3OCH(CF3)2 2,2 ethers CH3CH2OCF2CF2H CH3CH2OCF2CF3 CF3CH2OCH2CF3 CHF2CF2OCH2CF3 CF3CH2OCF2CF3 1,4 ethers CH3OCF2CF2CF2CF3 CHF2OCH2CF2CHFCF3 CH3OCF2CH(CF3)2 2,3 ethers CHF2CF2OCH2CH)CH2 CHF2CF2OCH2CF2CHF2 CHF2CF2OCH2CF2CF3 CH3CH2OCF2CHFCF3 CF3CH2OCF2CHFCF3 CF3CHFOCF2CF2CF3 3,3 ethers CF3CHFCF2OCH2CF2CHF2 CF3CHFCF2OCH2CF2CF3 1,5 ethers CH3OCH2CF2CF2CF2CHF2
5.6 16.9 1.3c 2.9c 33.3c
0.479 0.568 0.366e 0.439e 0.528e
7909 19305 592 1417 7992
2849 7996 181 434 5147
777 2504 57 136 1692
0.06 0.04 2.4a 2.8c 3.3a 4.1
0.320 0.221 0.447 0.401e 0.434 0.633
60 34 1080 1154 1316 5997
18 10 330 353 404 1842
6 3 103 110 126 576
1.8c 26.7 3.7 0.22 2.6 0.51 117.0
0.525f 0.641 0.479 0.360 0.439 0.424 0.563
812 22977 1641 81 1046 202 61134
248 11012 504 25 320 62 28403
78 3521 158 8 100 19 13383
0.12 0.31b 0.24a 4.0a 4.2c
0.370 0.488 0.398 0.516 0.531g
150 470 99 1935 1899
46 143 30 596 586
14 45 9 186 183
4.0 0.6c 2.8c
0.552 0.486h 0.489
4434 235 1103
1366 72 338
426 22 105
0.002 2.0c 2.3c 0.26 4.3 38.0
0.431 0.529h 0.582h 0.431 0.593 0.636
1 854 1001 290 1885 23871
0 261 306 89 582 12276
0 82 96 28 182 4122
3.0 8.8
0.621 0.670
0.1c
0.406h
1232 3291
377 1121
118 350
31
9
3
171
3801
1591 617 1311
36 37 36
>7235 6275 3400
>6516 4606 1200
36 36 38
1828 7180
522 551 4340
37 36 39
6600
4530
39
4.9
380
37
0.31
47
37
1388
432
42
127
38
36
>6589
>5936
36
4.2 CF3OCHFCHF2 CF3OCF2CF2H CF3OCH2CHF2 CF3OCH2CF3 CF3OCHFCF3
36 36 37 35 36 36 37 37
18.8c
370
c
0.1 0.1c 6.4
2.8c >67 c
1.8
a
Averaged rate constant value from numerous measurements. b Used given atmospheric lifetime from reference due to lack of kinetic data reported. c From ref 43. d Averaged value for different rotamers not reported in previous work. e Averaged value for different rotatmers reported in our previous work (21). f Only the linear rotameric form calculated in this work for the first time. g Only one rotamer found in this work (see Supporting Information). h Average of more than one shown in Supporting Information in this work.
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FIGURE 2. Plot of radiative forcing in units of W m-2 ppbv-1 versus the number of C-F bonds in the HFE. FIGURE 1. Plot of log of the atmospheric lifetime versus the number of C-H bonds in the HFE. in better agreement with Oyaro’s more recent results. For CHF2OCHFCF3, Oyaro (36) and Sekiya (37) each report GWPs at 100 years and they are within a factor of 2–3 of each other. Our results are closer to Oyaro’s (36) at 20 years with our value nearly double thiers. The only other HFE with multiple GWPs reported are from Oyaro (36) and Jain (38) for CF3OCHFCF3. Oyaro (36) reported in one work that the GWPs would be larger than 7235 at 20 years and larger than 6516 at 100 years, but then reported values of 6275 and 4606 in another publication. Jain, et al.’s (38), results are lower with values of 3400, 1200, and 370 and the three THs. Our values follow Oyaro’s original suggestion of larger GWPs, and we report values of 7992, 5147, and 1692, mostly due to a larger radiative forcing value which was discussed in our previous work (21). On the other hand, our atmospheric lifetime using updated kinetic data is about half-that used by Oyaro, et al. (36). There is even more limited GWP data for the larger HFEs with only CHF2OCF2CHFCF3 having GWPs reported by more than one group. For that species Oyaro (36) reported values that were about 6–8 times smaller than Wallington’s (39) values of 7180 and 4340 at the 20 and 100 year THs. Our results of 5977 and 1842 at the same THs are closer to Wallington’s (39) results of 1828 and 551. Our atmospheric lifetime was nearly identical to Oyaro’s (36) but our radiative forcing value was about 30% higher than theirs, leading to the discrepancy with their results. However, Wallington’s (39) much larger GWPs are due to the order of magnitude difference with their kinetic data, even with a lower radiative forcing value. In general, where data is available from more than one laboratory, our GWPs based on newer kinetic data while using radiative forcing values obtained from computational chemistry fall within the modeled results using only experimental data. A survey of Table 1 shows that when data is only available from only one research group, our results are generally within a factor of 1-2 of the reported results, with agreement most often being within about 30%. This agreement suggests that our results reported in this work for the 26 species where no GWP data were previously available is of the same quality as the currently available results. This set of data for HFEs is the largest publically available aggregation of atmospheric lifetime, radiative forcing, and GWP data assembled in one place for this class of compounds. With the extended GWP data set reported here for the first time, some trends can be analyzed in an effort to begin thinking about molecular design of this class of compounds to reduce environmental impacts. Figure 1 shows a plot of the log of atmospheric lifetime of all HFEs where kinetic data is available versus the number of C-H bonds in the HFE, regardless of the length of the 1304
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HFE. While there was some early comment about a reduction in atmospheric lifetime as the number of hydrogen atoms was increased, this was not quantified in the past (40). We see that the atmospheric lifetime reduces by almost a halforder of magnitude for each additional hydrogen atom available on the HFE for attack by hydroxyl radicals to begin the degradation mechanism of this class of compounds. In fact a linear regression of the log τ data versus number of C-H bonds leads to slope of -0.5296. A statistical analyses shows that about 83% of the atmospheric lifetime is correlated with this simple counting of C-H bonds. If one wishes to reduce the GWP of HFEs by manipulating the molecular structure, then one needs to create HFEs that have more C-H bonds as long as other physical properties are not compromised by this change. Figure 2 shows how radiative forcing values change as the number of C-F bonds in an HFE are increased. We found a linear fit where a ) 0.0486 (number of C-F bonds) + 0.1491. This linear regression had an R-squared value of 0.79, which reveals a fairly strong correlation because the average expected error in radiative forcing from the best models is about 25% (27). Again, there is a strong connection between the molecular structure and one of the two primary drivers behind GWP estimates. Here, one can see that adding more C-F bonds leads to increases in radiative forcing because new vibrational peaks are in the atmospheric region between 500 and 1500 cm-1. Figure 3 shows a suface plot of the log of the 100 year TH GWP for all of the hydrofluoroethers where kinetic data were available versus the number of C-H bonds and the number of C-F bonds. We see some interesting trends for the HFE class of compounds. We see that changes in the GWP due to the number of C-H bonds is stronger than the changes in GWP due to the number of C-F bonds affecting the radiative forcing. Second, it appears that middle-sized HFEs with 4–6 C-F bonds have lower GWPs compared to those that are smaller or those that are larger, except when the number of C-H bonds is at its largest value. This information allows one to further the discussion about how to molecularly design species that have low GWPs but may still have the physical properties one needs for different applications. Some early discussions of environmental impacts of HFEs suggested that hydrofluoroethers that are segregated, meaning that one carbon chain has only hydrogen atoms, whereas the other one has only fluorine atoms, would lead to shorter atmospheric lifetimes and thus lower GWPs (40, 41). For CH3OCF2CF3, however, improved kinetic data suggests the IPCC report by Owens has a lower atmospheric lifetime than that reported by Sekiya and that found in our work through several kinetic sources. This led to a much shorter atmospheric lifetime about half of ours and one-fourth that of Sekiya, leading to a lower GWP prediction. On the other hand, their CH3OC4F9 results had a longer atmospheric lifetime by about a year compared to more recent results, yet their GWP
FIGURE 3. Affects of molecular structure on GWP estimates on a log suface plot of GWP at the 100 year TH versus the number of C-H and C-F bonds. predictions were lower than Wallington’s (42) and our own. Due to the limited data in that report, it is unclear why this is so. In their discussion of segregated ethers, they indicated that HFEs with two more more H’s on a carbon can lead to lower atmospheric lifetimes, which is in contrast to our analysis in this work where we found it was the number of C-H bonds that mattered and not their location. The more recent patent of Milbrath (41) compared poly HFEs where more than one ether linkage was present and found that two poly HFEs with only two hydrogen atoms had longer lifetimes of about 7 years in the atmosphere compared to two HFEs with more hydrogen atoms that had τ’s of 0.8-5.0 years. They made the claim that it was the segregation of the hydrogen atoms on one carbon was important, much like Owens did. The data in Table 1 shows a different trend from the claims of Owens (40). For the 1,1 HFEs, CF3OCH3 has the middlemost atmospheric lifetime compared to those that have either fewer hydrogen atoms or more hydrogen atoms. Unfortunately, this comparison does not offer insight into the segregated ether claims due to the lack of molecular structure possibilities. However, as one moves to the 1,2 HFEs, more possibilities become available for comparison. For these ethers, one can see that CH3OCH2CF3 has an atmospheric lifetime of 0.07 years, while its segregated analog CF3OCH2CH3 has a longer atmospheric lifetime of 0.25 years. This difference leads to GWP differences that are about four times larger at short time horizons but equivalent on longer time horizons. There are four HFEs with three C-H bonds and five C-F bonds. Among these, the segregated HFE has the third longest atmospheric lifetime at 3.31 years, thus leading to relatively high GWP values compared to other molecules with the same number of bonds for each atom. As the ethers become larger, the comparisons become a little more difficult. One can compare the atmospheric lifetime of CH3OCF2CF2CF3 to that of CHF2OCH2CF2CF3 to see that the unsegregated form has an atmospheric lifetime of 3.7 years compared to 3.3 years for the segregated form. This leads to higher GWP predictions for the nonsegregated form. On the other hand, CH2FOCH(CF3)2 has an atmospheric lifetime of 0.5 years compared to 2.6 for its atomic analog CH3OCF(CF3)2. It is fairly clear that the claim that segregated HFEs have shorter lifetimes and GWPs is dubious when compared to the larger data set of GWPs now available.
Molecular design of the HFEs should not necessarily be focused on segregating the hydrogen atoms from the fluorine ones but on other characteristics applicable to the desired end use. Figure 3 showed that adding more C-F bonds does not necessarily increase GWP significantly on a log scale where orders of magnitude differences become apparent. On the other hand, adding or removing one C-H bond somewhere in the molecule more strongly affected the GWP by about a half-order of magnitude per C-H bond. This suggests that one can add more C-C linkages where the number of C-F bonds is increased by fluorinating those new carbon atoms. As long as one has a high number of C-H bonds included somewhere in the molecule, one could manipulate many of the physical properties by extending the carbon chains. Adding more fluorine atoms will decrease the temperature at which evaporation occurs by increasing the volatility of the HFE, which is a desirable trait for some applications. On the other hand, adding more hydrogen atoms onto a molecule will lead to shorter atmospheric lifetimes, but may lead to flammability concerns (40). There is also a danger in increasing molecular size to achieve lower GWPs, which is that one may create a larger species that then degrades to species which have significant atmospheric lifetimes and their own GWPs. Other environmental and safety impacts may be important (40) and one should consider water solubility and toxicity concerns as well. Highly fluorinated species like some of these HFEs tend to be insoluable in water (7), which is desirable because then the potential for aquatic pollution is reduced. This would limit exposure to airborne routes and not lead to water issues like the introduction of MTBE did. Little is known about the water solubilities of this class of compounds. There is also very little toxicity data available as well. To rationally approach design of an HFE for an application that does not have GWP or other environmental issues, one would need more physical information like the water solubility of the HFEs and the toxicity potential for humans, other mammals, amphibians, avian species, and other organisms. This information is currently unavailable in the peer reviewed literature. Clearly, more work is needed to ensure the lowest environmental impacts are achieved as this class of compounds continues to emerge as the primary VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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replacements of CFCs and HFCs. This work highlighted how changes in molecular structure can lead to reduced GWPs for this class of compounds and similar work for the other impact categories would aid in designing improved HFC replacement materials.
Supporting Information Available Additional details and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.
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