Article pubs.acs.org/est
Methyl-Perfluoroheptene-Ethers (CH3OC7F13): Measured OH Radical Reaction Rate Coefficients for Several Isomers and Enantiomers and Their Atmospheric Lifetimes and Global Warming Potentials Aaron M. Jubb,†,‡ Tomasz Gierczak,†,‡,§ Munkhbayar Baasandorj,†,‡,∥ Robert L. Waterland,⊥ and James B. Burkholder†,* †
Earth System Research Laboratory, Chemical Sciences Division, National Oceanic and Atmospheric Administration, 325 Broadway, Boulder, Colorado 80305, United States ‡ Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, United States ⊥ DuPont Central R&D, Wilmington, Delaware 19880, United States S Supporting Information *
ABSTRACT: Mixtures of methyl-perfluoroheptene-ethers (CH3OC7F13, MPHEs) are currently in use as replacements for perfluorinated alkanes (PFCs) and poly-ether heat transfer fluids, which are persistent greenhouse gases with lifetimes >1000 years. At present, the atmospheric processing and environmental impact from the use of MPHEs is unknown. In this work, rate coefficients at 296 K for the gas-phase reaction of the OH radical with six key isomers (including stereoisomers and enantiomers) of MPHEs used commercially were measured using a relative rate method. Rate coefficients for the six MPHE isomers ranged from ∼0.1 to 2.9 × 10−12 cm3 molecule−1 s−1 with a strong stereoisomer and −OCH3 group position dependence; the (E)-stereoisomers with the −OCH3 group in an α- position relative to the double bond had the greatest reactivity. Rate coefficients measured for the d3-MPHE isomer analogues showed decreased reactivity consistent with a minor contribution of H atom abstraction from the −OCH3 group to the overall reactivity. Estimated atmospheric lifetimes for the MPHE isomers range from days to months. Atmospheric lifetimes, radiative efficiencies, and global warming potentials for these short-lived MPHE isomers were estimated based on the measured OH rate coefficients along with measured and theoretically calculated MPHE infrared absorption spectra. Our results highlight the importance of quantifying the atmospheric impact of individual components in an isomeric mixture.
1. INTRODUCTION Following the Montreal Protocol and its subsequent amendments, there has been societal as well as commercial interest in developing replacements for in-use halogenated compounds that are ozone depleting substances (ODSs) and greenhouse gases (GHGs).1,2 In addition, the Kyoto Protocol has targeted overall GHG emission reductions in CO2, CH4, N2O, SF6, hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs) from industrialized countries. Saturated PFCs are persistent atmospheric species with lifetimes, in many cases, greater than 1000 years. The long-lifetimes of PFCs in combination with their strong absorption in the infrared “atmospheric window” leads to them having large global warming potentials (GWPs), e.g., the 100-year time horizon GWP of CF4 and C2F6 are 7390 and 12 200, respectively.2 Recent efforts to find environmentally suitable replacements for PFCs and perfluoropolyethers have focused on compounds with the necessary physical properties for use as heat transfer fluids, fire suppressants, and lubricants,3 but with greater reactivity toward the hydroxyl radical (OH), the primary daytime oxidant in the troposphere. Unsaturated fluoro-ethers, which are commercially available as mixtures of various © 2014 American Chemical Society
isomers, are potential replacements for PFCs. Fluorocarbons that contain unsaturated carbon−carbon bonds (fluoroalkenes)4−11 or an ether group (−OR, fluoro-ethers)12−17 are known to have an increased reactivity toward the OH radical. As a result, unsaturated fluoro-ethers, which are atmospherically short-lived and expected to have low GWPs, are replacement candidates for some PFCs. Mixtures of methyl-perfluoroheptene-ethers (CH3OC7F13, MPHEs) are currently in commercial use internationally and are under consideration for use in the United States. Quantifying the atmospheric loss processes, lifetimes, and radiative properties of these mixtures is an essential element of a complete environmental risk assessment. The primary atmospheric loss process for MPHE is expected to be its reaction with the OH radical, although there are currently no OH radical rate coefficient studies available in the literature for MPHEs or for any other unsaturated fluoro-ether. Received: Revised: Accepted: Published: 4954
February 20, 2014 April 1, 2014 April 4, 2014 April 4, 2014 dx.doi.org/10.1021/es500888v | Environ. Sci. Technol. 2014, 48, 4954−4962
Environmental Science & Technology
Article
Table 1. Methyl-perfluoroheptene-ethers (MPHEs)
In this study, rate coefficients, k, for the gas-phase reaction of the OH radical with six isomers and enantiomers of MPHE
highlights the importance of quantifying reaction rate coefficients for all components of a mixture when evaluating the environmental impact of a multicomponent mixture. As part of this work, theoretical calculations were performed and used to identify the infrared spectra of the individual MPHE isomers and compared with experimentally measured spectra of a MPHE isomeric mixture. The theoretically calculated infrared spectra were used to calculate the radiative efficiencies of the individual MPHE isomers. On the basis of the OH reaction rate coefficients and infrared spectra from this work, the GWPs of the MPHE isomers were estimated. A general atmospheric degradation scheme for the MPHE isomers is also briefly discussed.
OH + (E)‐4‐methoxy‐perfluorohept‐3‐ene, (E)‐4m‐3‐ene → Products
(1)
OH + (E)‐4‐methoxy‐perfluorohept‐3‐ene, (E)‐3m‐3‐ene → Products
(2)
OH + (E)‐5(RS)‐methoxy‐perfluorohept‐3‐ene, (E)‐5m‐3‐ene → Products
(3)
OH + (E)‐4(RS)‐methoxy‐perfluorohept‐2‐ene, (E)‐4m‐2‐ene → Products
2. EXPERIMENTAL DETAILS Rate coefficients for the gas-phase OH radical reaction with the MPHE isomers, reactions 1−6, and their deuterated analogues were measured at 296 K using a relative rate (RR) method. Standard samples of the individual MPHE isomers were not available for this study; all experimental work was performed using MPHE samples containing known but differing isomeric mixing ratios. The infrared absorption spectrum of each MPHE mixture was measured and, along with theoretically calculated spectra for the individual MPHE isomers, used to assess their radiative efficiencies (see Atmospheric Implications section). The RR and infrared measurements are described in this section, while the theoretical calculations are described in the next section. 2.1. Relative Rate Method and OH Rate Coefficient Determination. OH reaction rate coefficients were measured at 296 K using a relative rate method by monitoring the loss of MPHE relative to that of a reference compound with a wellknown OH rate coefficient.
(4)
OH + (Z)‐3‐methoxy‐perfluorohept‐3‐ene, (Z)‐3m‐3‐ene → Products
(5)
OH + (Z)‐4‐methoxy‐perfluorohept‐3‐ene, (Z)‐4m‐3‐ene → Products
(6)
were measured at 296 K using a relative rate method; the molecular structures of the MPHE isomers studied are given in Table 1. The (E)-5m-3-ene and (E)-4m-2-ene isomers are racemic mixtures and the R and S enantiomers, which have equivalent reactivity, are not distinguished in this work. Rate coefficients were also measured for the deuterated analogues (CD3OC7F13, d3-MPHEs) of the MPHE isomers to assess the relative importance of two competing reaction pathways, H atom abstraction from the −OCH3 group versus OH addition to the carbon−carbon double bond. The strong OH rate coefficient dependence on the MPHE isomeric and stereoisomeric geometries found in this work is discussed and
OH + Reference → Products 4955
(7)
dx.doi.org/10.1021/es500888v | Environ. Sci. Technol. 2014, 48, 4954−4962
Environmental Science & Technology
Article
The GC was equipped with a DB-5 column (length: 60 m, I.D.: 0.25 mm, film: 1 μm) operated at 296 K and an electron capture detector (ECD, 483 K). Samples were extracted from the reactor by vacuum expansion into a 0.15 cm3 sample loop maintained at 325 K. Under the flow conditions used the retention times for the MPHE isomers ranged from 500 to 1000 s; the order of elution is included in Table 1 and an example chromatogram is given in Figure S2 in the Supporting Information. The (E)-4m-3-ene and (E)-3m-3-ene isomers eluted first (peaks 1 and 2) and their GC peaks overlapped but were separable in the chromatographic and kinetic analysis (see below). The (E)-4m-2-ene and (E)-5m-3-ene enantiomers (peaks 3 and 4) eluted next; rate coefficient data for the nonseparated RS pairs were determined in the kinetic analysis. The (Z)-4m-3-ene and (Z)-3m-3-ene isomers were the last to elute and were well separated. MPHE isomer GC peak areas were determined by leastsquares fitting of the chromatograms using ten Gaussian functions. The ten Gaussian functions accounted for the six MPHE isomers included in this study and four minor sample impurities. Reaction products eluted prior to the MPHE isomers,
