Decomposition of Perfluorinated Ion-Exchange Membrane to Fluoride

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Ind. Eng. Chem. Res. 2010, 49, 464–471

KINETICS, CATALYSIS, AND REACTION ENGINEERING Decomposition of Perfluorinated Ion-Exchange Membrane to Fluoride Ions Using Zerovalent Metals in Subcritical Water Hisao Hori,* Misako Murayama, Taizo Sano, and Shuzo Kutsuna National Institute of AdVanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba 305-8569, Japan

The decomposition of Nafion NRE-212, a typical perfluorinated ion-exchange membrane used for fuel cells, in subcritical water was investigated. This is the first report of the decomposition of a perfluorinated ionexchange membrane aimed at the development of a technique to recover the fluorine component for waste treatment. Although the membrane showed little decomposition in pure subcritical water, the addition of several zerovalent metals to the reaction system accelerated the membrane decomposition to F- ions, and the acceleration increased in the order Al < no metal < Zn < Cu , Fe. When the membrane and iron powder were heated in subcritical water at 350 °C for 17 h, 73.2% of the fluorine content in the initial membrane was successfully transformed into F- ions. In addition to F- ions, trifluoroacetic acid and HCF(CF3)OC2F4SO3were also detected in the reaction solutions as intermediates, and CO2 and CF3H were detected in the gas phase. Time profiles of the products suggest that one pathway of the decomposition of the membrane proceeded by decomposition of the pendant-chain part, followed by decomposition of the polymer backbone. Introduction Fuel cells have been extensively investigated because they have a great potential to replace internal combustion engines in vehicles and to provide power in stationary and portable applications.1 Polymer electrolyte membrane (PEM) fuel cells are among the most promising candidates because they can deliver high power densities and offer the advantages of low weight and volume. Perfluorinated ion-exchange membranes, manufactured by copolymerization of tetrafluoroethylene and a functionalized perfluorinated vinyl ether (Scheme 1), that is, perfluorinated sulfonic acid membranes, are widely used for PEM fuel cells owing to their high proton conductivity and high chemical, mechanical, and thermal stabilities.2-5 For their wider use in fuel cells, waste treatment techniques for these membranes should be established. These membranes are also extensively used in the production of chlorine and sodium hydroxide from brine (chlor-alkali process), in desalination to produce potable waters, and so forth. Perfluorinated membranes can be decomposed by incineration. This, however, requires high temperatures because the membranes consist of strong C-F bonds, and hydrogen fluoride gas is formed, which can seriously damage the firebricks of incinerators. Thus, there is virtually no wastetreatment method for these membranes except for disposal in a landfill. If the membranes could be decomposed into F- ions using environmentally benign techniques, the well-established protocol for the treatment of F- ions could be used: Ca2+ is added to the system to form CaF2, which is a raw material for hydrofluoric acid. Thus, the development of such a method would allow for the recycling of a fluorine resource. Several reports have focused on the degradation of perfluorinated ion-exchange membranes.4,6-10 However, these previous studies were conducted to elucidate the mechanism for the deterioration of fuel-cell performance, by use of hydrogen peroxide or Fenton’s reagent (H2O2/Fe2+); none focused on the

decomposition of the membranes in view of waste treatment. The level of membrane degradation that reduces cell performance is substantially lower than the level required for effective waste treatment. Furthermore, little is known about the organofluorine compounds ascribed to the degradation of the membranes. One previous report10 indicated that the degradation of an earlier perfluorinated ion-exchange membrane formed analogues to perfluorocarboxylic acids (PFCAs; CnF2n+1COOH, n ) 1, 2, 3) or perfluoroalkylsulfonates (CnF2n+1SO3-), which are ubiquitous environmental contaminants,11-15 and species with longer perfluoroalkyl chains are bioaccumulative.13-15 We previously demonstrated that environmentally persistent and bioaccumulative perfluoroalkylsulfonates such as perfluorooctanesulfonate (PFOS; C8F17SO3-) are efficiently decomposed in subcritical (250-350 °C) or supercritical (∼380 °C) water with the addition of a metal such as iron.16,17 We also reported that PFCAs such as perfluorooctanoic acid (PFOA; C7F15COOH) are efficiently decomposed in hot water at quite low temperatures (∼80 °C) in the presence of persulfate.18 Reaction in subcritical or supercritical water has been recognized as an innovative and environmentally benign reaction technique,19 which has been applied to plant-scale decomposition of hazardous compounds such as trinitrotoluene20 and polychlorinated biphenyls.21 Subcritical water is defined as hot water Scheme 1. Schematic of the Synthesis of a Perfluorinated Ion-Exchange Membrane

* To whom correspondence should be addressed. Tel.: (+81)-29861-8161. Fax: (+81)-298-61-8866. E-mail: [email protected]. 10.1021/ie9004699  2010 American Chemical Society Published on Web 12/03/2009

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with sufficient pressure to maintain the liquid state, and supercritical water is defined as water at temperatures and pressures higher than the critical point (374 °C, 22.1 MPa). Herein, we report the decomposition of a typical perfluorinated ion-exchange membrane, Nafion NRE-212, in subcritical water with zerovalent metals. When iron was added to the reaction system, the stable membrane was efficiently decomposed to form F- ions. This is the first report not only on the decomposition of a perfluorinated ion-exchange membrane in subcritical water, but also on a technique aimed at waste treatment that successfully achieves the efficient formation of F- ions from the membrane. Experimental Section Materials. Nafion NRE-212 was purchased from SigmaAldrich (Milwaukee, WI). The equivalent weight (EW) of the membrane, that is, the weight (g) of dry membrane (acid form) per mole of sulfonic acid groups, was reported to be 1100 by the supplier. To confirm this value, we measured the ionexchange capacity of the membrane by means of titration with aqueous NaOH, and the obtained value was 0.91 mmol g-1, which corresponds to an EW of 1100, in agreement with the value reported by the supplier. According to the EW value, the composition ratio x/y in the polymer (Scheme 1) is estimated to be 6.5. Trifluoroacetic acid (TFA; CF3COOH, >99.0%), pentafluoropropionic acid (C2F5COOH, >98%), heptafluorobutyric acid (C3F7COOH, >99%), nonafluoropentanoic acid (C4F9COOH, >98%), and undecafluorohexanoic acid (C5F11COOH, >98%) were purchased from Tokyo Kasei Kogyo Co. (Tokyo, Japan). Tridecafluoroheptanoic acid (C6F13COOH, >96%) and PFOA (>95%) were obtained from Wako Pure Chemical Industries (Osaka, Japan). The potassium salt of perfluoro(2-propoxyethane)sulfonate (C3F7OC2F4SO3-) was obtained from Research Center of Asahi Glass Co. (Yokohama, Japan). Fine metal powderss aluminum (99.9%, ∼2 µm), copper (99.9%, ∼5 µm), iron (>99.9%, 3-5 µm), and zinc (99%, ∼7 µm)swere purchased from Kojundo Chemical Laboratory (Saitama, Japan) and were used as received. (Note: The purity of metal powder described by the vendor is based on ICP atomic spectrometry. As described later, the iron powder used here contained 1.0% carbon.) H2(18O) (98 atom %) and D2O (99.8 atom %) were purchased from Taiyo Nippon Sanso Co. (Tokyo, Japan) and Acros (Geel, Belgium), respectively. Argon (99.99%) and CH4 (99.7%) gases were purchased from Tomoe Shokai Co. (Tokyo, Japan) and GL Sciences (Tokyo, Japan), respectively. The gas mixtures H2 (0.987%)/N2, CF3H (1.05%)/He, CO2 (1.005%)/N2, CO (5.12%)/ argon, and C2F4 (1000 ppmv)/N2 were purchased from Takachiho Trading (Tokyo, Japan). These gas mixtures were further diluted and then used as standards for the analyses of the products in the gas phase. Pretreatment of Membranes. Commercial perfluorinated ion-exchange membranes contain organic or metallic impurities, so all membranes used in the present study were pretreated according to the following standard method for practical fuel cell usage:6,9 Round pieces of the NRE-212 membranes (diameter, 2 cm; thickness, 51.3 µm) were boiled in 3% H2O2 solution for 1 h to remove organic impurities. The membranes were boiled in pure (Milli-Q) water for 1 h and then in 1 M H2SO4 for 1 h to remove metallic impurities. The membranes were further boiled in pure water for 1 h. After being allowed to cool, the membranes were rinsed with pure water and then stored in pure water until use. When subcritical water reactions were carried out in D2O or H2(18O), the pretreated membrane was dried under vacuum for 16 h, stored in isotopic water, and

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then subjected to the reactions. The fluorine content of the pretreated NRE-212 membrane was 65.8 wt %, quantified by combustion ion chromatography22 (details are described below in Analysis). This value was slightly lower than the ideal fluorine content in the membrane (67.7 wt %, x/y ) 6.5), calculated from the EW value. Subcritical Water Reactions. A stainless steel high-pressure reactor (35.1 mL volume) equipped with a thermocouple and a stainless steel screw cap was used. The screw cap was connected to a pressure gauge for measuring the pressure in the reactor and to a sampling port for analyzing gas products. A gold vessel (24.6 mL, 2.8-cm i.d.) was fitted into the reactor, where the inner wall of the reactor was tightly attached to the outer wall of the gold vessel to eliminate the possibility of contamination from the reactor material. In a typical run, a round piece of pretreated NRE-212 membrane (dry weight, 29.8 mg), metal powder (9.60 mmol), and argon-saturated pure water (10 mL) were introduced into the gold vessel in the reactor under an argon atmosphere. After the reactor was pressurized to 0.51 MPa with argon, the reactor was sealed. The reactor was placed in an oven, and the reactor temperature was raised to the desired temperature (250-350 °C) at a rate of about 10 °C min-1 and was held constant for a specified time (e.g., 6 h). Then, the reactor was quickly cooled to room temperature in an ice-water bath. Control reactions were also performed in the absence of the metal or the membrane. To transfer the gas, we connected the reactor to a sampling bag by means of a valve, and we analyzed the collected gas in the sampling bag by gas chromatography/mass spectrometry (GC/MS) and GC. The reaction mixture in the reactor was centrifuged to separate the reaction solution and the solid phase (metal powder), and the reaction solution was analyzed by ion chromatography, ion-exclusion chromatography, high-performance liquid chromatography (HPLC), electrospray ionization (ESI) mass spectrometry, ionchromatography/time-of-flight (IC-TOF) mass spectrometry, and size-exclusion chromatography (SEC). Analysis. For the quantification of the fluorine content in the pretreated membrane, the membrane was dried under vacuum and then subjected to combustion ion chromatography at Nissan Arc (Yokosuka, Japan). The instrument consisted of a combustion unit (AQF-100, Dia Instruments, Chigasaki, Japan; maximum combustion temperature, 1000 °C) and an ion chromatograph unit (ICS-1500, Dionex, Austin, TX); the fluorine component of the sample was converted to HF by combustion and absorbed into a solution containing Na2CO3 (2.7 mM), NaHCO3 (0.3 mM), and H2O2 (26.5 mM) and then transferred to the ion chromatograph unit. An ion-exclusion chromatograph system consisting of a guard column (TSKgel OApak-P, Tosoh Corp., Tokyo, Japan), a separation column (TSKgel OApak-A, Tosoh Corp.), a pump, a column oven, and a conductivity detector was used to determine whether short-chain PFCAs (TFA to C3F7COOH) were present in the reaction solution. The mobile phase was phthalic acid (10 mM) at a flow rate of 0.6 mL min-1. An HPLC system (IC-2001, Tosoh Corp.) with a separation column (TSKgel Super-ODS × 2) and a conductivity detector was also used to determine whether longer-chain PFCAs (C4F9COOH to PFOA) were present in the reaction solutions. The mobile phase was a mixture of methanol and aqueous NaH2PO4 (20 mM, adjusted to pH 3.0 with H3PO4) at several mixing ratios with a flow rate of 0.4 mL min-1. An ion-chromatography system (IC-2001, Tosoh Corp.) consisting of an automatic sample injector, a degasser, a pump, a guard column (TSKguard column Super IC-A, Tosoh Corp.),

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Table 1. Decomposition of NRE-212 Membrane in Subcritical Water with and without Metal Additives Induced by 6-h Reactiona polymer component remaining in the reaction solution

entry

temperature (°C)

pressure (MPa)

metal additive

Mw (×105)b

Mw/Mnc

F- (µmol) [yield (%)]d

TFA (µmol)

1 2 3 4 5 6 7 8

250 250 300 300 300 300 300 350

3.5 4.0 8.3 9.3 8.6 8.4 9.4 17.6

none Fe none Al Zn Cu Fe Fe

2.89 2.35 1.49 1.90 1.70f 1.41 1.20 -g

2.51 1.39 1.27 1.27 1.16f 1.33 1.32 -g

2.57 [0.25] 61.3 [5.95] 6.85 [0.67] 0.78 [0.08] 22.6 [2.19] 35.7 [3.47] 351 [34.1] 658 [63.9]

n.d.e 0.15 0.19 n.d. n.d. 4.00 0.66 0.48

a A pretreated NRE-212 membrane (dry weight, 29.8 mg), metal powder (9.60 mmol), and pure water (10 mL) were introduced into the reactor under an argon atmosphere, and the reactor was heated at the desired temperature for 6 h. b Mw, weight-average molecular weight of the polymer soluble in the reaction solution . c Mn, number-average molecular weight of the polymer soluble in the reaction solution . d F- yield (%) ) [(moles of F- formed)/ (moles of fluorine content in the membrane before reaction)] × 100, where the initial fluorine content in the membrane was 1.03 mmol. e n.d. ) not detected. f Insoluble polymer part was also detected. g Polymer component was not detected.

a separation column (TSKgel Super IC-Anion, Tosoh Corp.), a column oven, and a conductivity detector with a suppressor device was used to measure the F- concentrations in the reaction solutions. The mobile phase was an aqueous solution containing Na2B4O7 (6 mM), H3BO3 (15 mM), and NaHCO3 (0.2 mM), and the flow rate was 0.8 mL min-1. An ESI mass spectrometry system (LCMS-2010 EV, Shimadzu, Kyoto, Japan) was used to identify the intermediates in the reaction solutions. Analyses were carried out in negative-ion mode, and the electrospray probe voltage was 4.50 kV. Reaction samples were delivered to the electrospray probe using acetonitrile as the mobile phase at a flow rate of 0.2 mL min-1. IC-TOF mass spectrometric measurements were also performed at Sumika Chemical Analysis Service (Sodegaura, Japan) to identify reaction products. The apparatus consisted of an ion chromatograph (IP-25, Dionex) and a TOF mass spectrometer (MSD TOF system, Agilent Technologies, Palo Alto, CA). Electrospray ionization in the negative-ion mode was employed. In the IC part, the separation column was DIONEX Ion Pac AG12 + AS12, the mobile phase was an aqueous solution containing Na2CO3 (2.7 mM) and NaHCO3 (0.3 mM), and the flow rate was 1.5 mL min-1. SEC using a refractive index detector and an analytical column (PLgel Mixed-B × 2, Polymer Laboratories, Amherst, MA) was carried out at Nissan Arc to observe the changes of the molecular weights of the polymer components. The column temperature was 40 °C. The mobile phase was dimethylformamide containing LiBr (10 mM); the flow rate was 1.0 mL min-1. The reaction solutions were injected into the system after filtration through 0.45-µm filters. Molecular weights and molecular weight distributions are reported relative to polystyrene as a standard. 19 F NMR (283 MHz) spectra of some typical reaction solutions were measured at Research Center of Asahi Glass Co. (Yokohama, Japan). The reaction solutions were concentrated to approximately one-fifth of their original volumes by use of a rotary evaporator. D2O was added, and then the concentrated solutions were subjected to measurement. The chemical shifts are relative to an external standard of CFCl3. The products in the gas phase were analyzed by GC/MS. The system consisted of a gas chromatograph (HP5890, Hewlett-Packard, Wilmington, DE) with a fused silica capillary column (Poraplot Q, Chrompack, Bergen op Zoom, The Netherlands) and a mass spectrometer (HP 5972A). The carrier gas was helium, and the electron impact source was operated at 70 eV. The oven temperature was held constant at 30 °C. A GC system (GC 323, GL Sciences, Tokyo, Japan)

Figure 1. SEC chromatograms of the aqueous polymers remaining after the reactions in pure subcritical water. A mixture of pretreated NRE-212 membrane (dry weight, 29.8 mg) and pure water (10 mL) was heated in a sealed reactor under argon atmosphere for 6 h at 250 or 300 °C, and the reaction solution was injected into the SEC system.

with a thermal conductivity detector was also used. The column was a packed column (active carbon, 60/80 mesh), and the carrier gas was argon. The oven and detector temperatures were 110 and 130 °C, respectively. Changes in the metal due to reaction were determined by X-ray diffraction (XRD) with Cu KR radiation (RU-300, Rigaku, Tokyo, Japan). Results and Discussion Decomposition of Nafion Membrane in Subcritical Water without Metal. The results of NRE-212 membrane decomposition in subcritical water, with and without metal additives, are summarized in Table 1 for reactions carried out at 250-350 °C for 6 h. In all runs, except those that used zinc, the solid membrane could not be found in the reactor following the reaction, even if the reaction had been carried out without metal. This is not surprising, because Nafion membranes become soluble in water upon heating above 230 °C in a pressurized vessel (strictly speaking, the condition is recognized as being an aqueous dispersion of the polymer).23,24 When the reaction was carried out in the absence of metal at 250 °C, the SEC chromatogram of the aqueous polymer showed two peaks at around 12.6 and 14.5 min (Figure 1, solid line). This feature is ascribed to molecular aggregation of the polymer.24 The weight-

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average molecular weight (Mw) of the aqueous polymer was 2.89 × 105, with a wide polydispersity (Mw/Mn, where Mn is the number-average molecular weight) of 2.51 (Table 1, entry 1), reflecting the coexistence of aggregated and nonaggregated polymers. Raising the reaction temperature to 300 °C caused dramatic changes in the SEC chromatogram. Only one intense peak appeared at around 15.2 min (Figure 1, dotted line), and the overall distribution shifted toward longer retention times (i.e., lower molecular masses), resulting in smaller Mw (1.49 × 105) and smaller Mw/Mn (1.27) (Table 1, entry 3). This phenomenon reflects the irreversible breakdown of the molecular aggregation of the polymer.24 Although the aggregation of the polymer dissociated after the reaction at 300 °C without metal, the reaction caused very little decomposition of the polymer. The F- yields [(moles of F- formed)/(moles of initial fluorine content in the membrane, i.e., 1.03 mmol)] were quite low: 0.25% after the reaction at 250 °C for 6 h (Table 1, entry 1) and 0.67% at 300 °C (Table 1, entry 3). Although the F- yield at 300 °C was somewhat higher than that at 250 °C, the polymer did not meaningfully decompose to F- ions without metal additive, reflecting the high thermal stability of the perfluorinated membrane. Decomposition of Nafion Membrane in Subcritical Water Induced by Metals. To accelerate the decomposition of the NRE212 membrane to F- ions, we examined the effect of the addition of zerovalent metals on the reaction system. When the reactions were carried out at 300 °C for 6 h, the addition of aluminum did not enhance the decomposition of the membrane at all; on the contrary, it inhibited F- formation in the reaction solution, as the F- yield was one-tenth of that without metal (Table 1, compare entries 3 and 4). The addition of other metals (zinc, copper, and iron) clearly enhanced the decomposition of the membrane, and the F- yield increased in the order Zn < Cu , Fe (Table 1, entries 5-7). When iron was added, the F- yield was maximized (34.1%, entry 7) at 50.9 times the yield without metal (entry 3). In addition to F- ions, small amounts of TFA were also detected in some cases. TFA has been observed in fuel cell degradation experiments10 and also in the thermal degradation of poly(tetrafluoroethylene) (PTFE).25,26 When zinc was used, the behavior of the membrane during the reaction was markedly different from reactions with other metals or without metal: The residual polymers consisted of not only a soluble part but also an insoluble part whose shape was similar to that of the original membrane, and the surface of the insoluble part was covered with metal powder. This fact indicates that zinc inhibited the dispersion of the polymer in subcritical water, although decomposition proceeded to some extent. The SEC chromatograms of the aqueous polymers after reactions at 300 °C for 6 h with copper, with iron, and without metal are shown in Figure 2. In each case, the chromatographic pattern was similar, producing similar Mw and Mw/Mn values (Table 1, entries 3, 6, and 7), whereas the peak intensity decreased in the order no metal (large) > copper . iron (small). This result indicates that the amount of polymer remaining soluble in the reaction solution decreased in the same order, which is consistent with the tendency of F- formation. We previously observed that PFOS, which has a structure similar to that of the pendant chain of the membrane polymer (although PFOS has no ether linkage in the perfluoroalkyl chain), decomposes in subcritical water at 250-350 °C with the addition of metals.16 Several phenomena, namely, aluminum not enhancing the decomposition at all, iron leading to the most effective decomposition, and copper leading to moderate decomposition, were similar in PFOS decomposi-

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Figure 2. Changes in the SEC chromatograms of the aqueous polymers remaining after the reactions in subcritical water containing metal additives. A mixture of pretreated NRE-212 membrane (dry weight, 29.8 mg), pure water (10 mL), and metal powder (Cu or Fe, 9.60 mmol) was heated at 300 °C in a sealed reactor under argon atmosphere for 6 h. Data obtained without metal additive are also shown.

tion and the present membrane decomposition. The enhancement of the membrane decomposition did not reflect the reducing power of the metals: The order of F- formation was different from the order of the redox potentials (E0, V) of the metals, which was in the more negative direction of Cu/Cu2+ (0.337) < Fe/Fe2+ (-0.440) < Zn/Zn2+ (-0.763) < Al/Al3+ (-1.662). These findings suggest that a particular interaction between the pendant chain of the membrane polymer and metals (such as adsorption) played an important role. Effect of Iron on Membrane Decomposition. Because the addition of iron powder led to the most efficient decomposition of the NRE-212 membrane among the metals tested at 300 °C, we investigated the membrane decomposition with iron in detail by changing the reaction conditions. Enhancement of the decomposition of the membrane was also observed at different reaction temperatures. When the reaction was carried out at 250 °C for 6 h, the F- yield in the absence of iron was 0.25% (Table 1, entry 1), whereas in the presence of iron, the yield was 5.95% (Table 1, entry 2), which is 23.8 times higher. When the reaction with iron was carried out at 350 °C for 6 h, the F- yield further increased to 63.9% (Table 1, entry 8). The SEC chromatograms of the reaction solutions after treatment with iron at several temperatures are shown in Figure 3. Raising the reaction temperature from 250 to 300 °C dramatically decreased the peak intensity of the remaining aqueous polymers. Furthermore, when the reaction was carried out at 250 °C for 6 h, the Mw value of the polymer remaining in the reaction solution was 2.35 × 105 (Table 1, entry 2), whereas at 300 °C, the value was 1.20 × 105 (Table 1, entry 7). A decrease in the peak intensity accompanied by a decrease in Mw with increasing temperature suggests that some part of the polymer, for example, the pendantchain part, was preferentially decomposed during the reaction. When the reaction was carried out at 350 °C, the polymer component was no longer seen in the chromatogram (Figure 3). This observation and the high F- yield (63.9%) after the reaction at 350 °C for 6 h indicate that the NRE-212 membrane effectively decomposed not only in the pendant-chain part, but also in the perfluorinated polymer backbone. As described in Materials, the x/y ratio in the polymer (Scheme 1) was estimated to be 6.5. This leads to a value for the ratio of the fluorine content in the functionalized perfluorinated vinyl ether unit to the fluorine content in the total polymer, that is, {moles of

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Figure 3. SEC chromatograms of the aqueous polymers remaining after reactions in subcritical water with iron at several reaction temperatures. A mixture of pretreated NRE-212 membrane (dry weight, 29.8 mg), pure water (10 mL), and iron powder (9.60 mmol) was heated at 250-350 °C in a sealed reactor under argon atmosphere for 6 h.

fluorine in [CF(pendant chain)-CF2]y}/[moles of fluorine in the total polymer () membrane)], of 33.3%. The greater F- yield (63.9%) observed here clearly indicates that the decomposition efficiently proceeded not only in the functionalized perfluorinated vinyl ether unit, but also in the tetrafluoroethylene unit (CF2-CF2)x of the polymer. The reaction-time dependence of the formation of F- and the minor product TFA from the reaction of the membrane with iron at a constant temperature of 350 °C is shown in Figure 4. The amount of F- rapidly increased during the initial period (∼2 h) and then gradually increased as the reaction time increased (Figure 4a). After 17 h, the longest reaction time tested, the amount of F- reached 754 µmol, corresponding to a yield of 73.2%. In contrast, the amount of TFA increased during the first period of the reaction (∼2 h) and then decreased (Figure 4b). This result suggests that the TFA that formed further decomposed under the reaction conditions. We checked whether PFCAs other than TFA formed and confirmed that other PFCAs (C2F5COOH to C7F15COOH) were not present in the reaction solutions. Alternatively, we observed a strong peak in the HPLC chromatogram for the reaction solutions. In the absence of iron, no such peak was observed. To identify this unknown species, we subjected the reaction solutions to ESI mass spectrometry. The species showed a strong peak at m/z 297, so it was identified as HCF(CF3)OC2F4SO3-. This fact indicates that the C-C bond between the first and the second carbon atoms from the oxygen atom connected to the polymer backbone (see Scheme 1) was cleaved by the reaction with iron, and then the addition of hydrogen formed HCF(CF3)OC2F4SO3-. When the reaction was carried out in D2O, the peak at m/z 297 that appeared in the reaction in normal water was shifted to m/z 298. This result clearly indicates that the hydrogen in HCF(CF3)OC2F4SO3came from water. The reaction-time dependence of the HPLC peak intensity of HCF(CF3)OC2F4SO3- is shown in Figure 4c. The peak intensity increased during the initial period of the reaction (4-6 h). During the long reaction time, small amounts of C2F4 began to be detected at 8 h (at which time the amount was below the limit of quantification, 8.05 × 10-8 mol, determined from the signal-to-noise ratio of 10), and the amount reached 1.59 × 10-7 mol at 17 h. This observation suggests that the formation of C2F4 reflects decomposition of the polymer backbone. The decomposition of the polymer backbone might involve formation of C2F4, in a mechanism similar to that proposed for the thermal degradation of PTFE,25,26 although water might play some role in the present reaction. Conclusions In the present studies, we investigated the decomposition of a typical perfluorinated ion-exchange membrane, Nafion NRE212, in subcritical water with zerovalent metals. Although the membrane showed little decomposition in pure subcritical water, the addition of several zerovalent metals to the reaction system accelerated the membrane decomposition to F- ions. The addition of iron powder led to the most efficient decomposition of the membrane: When the membrane and iron powder were heated in subcritical water at 350 °C for 17 h, 73.2% of the fluorine content in the initial membrane was successfully transformed to F- ions. This result clearly indicates that the decomposition effectively proceeded not only in the pendantchain part, but also in the polymer backbone. Time profiles of the products suggest that one pathway for the decomposition of the membrane proceeds through decomposition of the pendant-chain part, followed by decomposition of the polymer backbone. For this work, we used a small-size reactor (35.1 mL volume). Scaleup of this reaction system and application to other perfluorinated membranes are being investigated in our laboratory. Acknowledgment The authors express their grateful acknowledgment to Dr. Y. Morizawa and Mr. T. Fujita (Asahi Glass Co.) for helpful discussion and NMR measurements. This work was supported in part by the Japan Society for the Promotion of Science (JSPS) and the Ministry of Environment. Supporting Information Available: XRD patterns of the iron powder before and after reaction. This information is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) U.S. Department of Energy Hydrogen Program, Fuel Cells. U.S. Department of Energy: Washington, DC. http://www.hydrogen.energy.gov/ fuel_cells.html (Accessed October 3, 2009). (2) Sanchez, J. Y.; Alloin, F.; Iojoiu, C. Fluorinated organic chemicals: Prospects in new electrochemical energy technologies. J. Fluorine Chem. 2006, 127, 1471. (3) Souzy, R.; Ameduri, B. Functional fluoropolymers for fuel cell membranes. Prog. Polym. Sci. 2005, 30, 644. (4) Hamrock, S. J.; Yandrasits, M. A. Proton exchange membranes for fuel cell applications. Polym. ReV. 2006, 46, 219.

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ReceiVed for reView March 23, 2009 ReVised manuscript receiVed October 4, 2009 Accepted November 23, 2009 IE9004699