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Hydrogen Peroxide Induced Efficient Mineralization of Poly(vinylidene fluoride) and Related Copolymers in Subcritical Water Hisao Hori,*,† Hirotaka Tanaka,† Kengo Watanabe,† Takahiro Tsuge,† Takehiko Sakamoto,† Abdellatif Manseri,‡ and Bruno Ameduri‡ Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 08/25/15. For personal use only.

†

Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan Ingenierie et Architectures Macromoléculaires, Institut Charles Gerhardt UMR 5253, École Nationale Supérieure de Chimie de Montpellier, 8 Rue École Normale, 34296 Montpellier Cedex 1, France

‡

S Supporting Information *

ABSTRACT: Decompositions of poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-co-chlorotrifluoroethylene) copolymer (poly(VDF-co-CTFE)), and poly(vinylidene fluoride-co-hexafluoropropylene) copolymer (poly(VDF-co-HFP)) in subcritical water were investigated with the aim of developing a technique to recover the fluorine component. By use of H2O2, these (co)polymers can be efficiently mineralized at a relatively low temperature (300 °C). When PVDF was heated with 3.0 M H2O2 for 6 h, which corresponds to 31 times the molar amount of fluorine and 32 times the molar amount of carbon in the polymer, both F− and CO2 yields reached 98%. Poly(VDF-co-CTFE) copolymer was also mineralized under the same reaction conditions (the yields of F−, CO2, and Cl− were 98, 95, and 97%, respectively). Poly(VDF-co-HFP) copolymer was more readily decomposed than poly(VDF-co-CTFE), leading to almost complete mineralization (F− yield, 96%; CO2 yield, 92%) with 2.0 M H2O2. Addition of stoichiometric Ca(OH)2 to the reactions formed CaF2 well-identified by X-ray diffraction spectrometry.

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INTRODUCTION Fluoropolymers, olefinic polymers in which some or all of the hydrogen atoms are replaced by fluorine atoms, are used in many industrial applications owing to their high chemical and thermal stability and other specific characteristics.1−8 Among these polymers, poly(tetrafluoroethylene) (PTFE, −(CF2CF2)n−) is most frequently used.1−6 However, PTFE cannot be processed by melt molding, a conventional technique for fabricating thermoplastic polymers, because the viscosity of the PTFE melt (109−1011 Pa s) is about 6 orders of magnitude higher than that of common thermoplastic polymers.4,6 To overcome this limitation, poly(vinylidene fluoride) (PVDF, −(CF2CH2)n−) has been developed and introduced in industry.1−7 Besides PVDF, poly(chlorotrifluoroethylene) (PCTFE, −(CF 2 CFCl) n −) is also a melt processable commercially available fluoropolymer endowed with many properties for various applications (gas barrier packaging, coatings, liners for protections) while CTFE copolymers can be involved in paints and materials for energy applications (e.g., fuel cell membranes and polymer electrolyte for lithium ion batteries).8 PVDF shows a high resistance to temperature, chemicals, ignition, mechanical stresses, and UV irradiation so that it has been used for various applications including chemical process equipment, electrical equipment, and especially energyrelated applications such as lithium ion battery electrode binders.1−7 Recently, the production of PVDF reached the largest volume of fluoropolymer after PTFE.5,7 Furthermore, several copolymers based on VDF and other monomers, which enhanced the properties such as the softness and impact resistance, have been developed. Wider use of PVDF and related copolymers will require the establishment of waste © XXXX American Chemical Society

treatment. These (co)polymers can be incinerated. However, incineration requires high temperatures to break the strong C− F bonds, and the released hydrogen fluoride gas can damage the firebrick of an incinerator. Thus, in many cases, the wastes of these (co)polymers are disposed of in landfills. If the (co)polymers could be decomposed to F− ions (i.e., undergo mineralization) by means of environmentally benign techniques, the well-established protocol for treatment of F− ions could be used, whereby 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 the recycling of fluorine, the global demand for which is increasing. Several studies reported the degradation of PVDF9−17 and related copolymers,10−12 as well as that of PCTFE.11,18 However, most previous reports, including studies related to batteries’ performance,13−16 examined the thermal stability or aging characteristics of the (co)polymers and did not focus on their decomposition to obtain F− ions for waste treatment. Only our previous study reported that PVDF can be efficiently decomposed into F− and CO2 in supercritical water at 380 °C in the presence of an ca. 5.8-fold molar excess of O2 relative to fluorine content in PVDF.17 Reactions in subcritical or supercritical water are recognized as an innovative and environmentally benign waste-treatment technique, owing to the high diffusivity and low viscosity of these media, as well as their ability to hydrolyze many organic Received: May 7, 2015 Revised: July 25, 2015 Accepted: August 18, 2015

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DOI: 10.1021/acs.iecr.5b01716 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 08/25/15. For personal use only.

Industrial & Engineering Chemistry Research compounds.19 Subcritical water is defined as hot water with sufficient pressure to maintain the liquid state, while supercritical water is defined as water at temperatures and pressures higher than the critical point, 374 °C and 22.1 MPa. Recently, supercritical water was used for pilot and practical plant-scale decomposition of trinitrotoluene20 and polychlorinated biphenyls.21 Subcritical water was widely used for the hydrothermal biomass treatment22 and also used for decomposition of a perfluoroalkyl sulfonic acid membrane.23 Although PVDF is efficiently mineralized in supercritical water at 380 °C in the presence of an O2 excess, a technique that can work under mild conditions (lower temperature and pressure) is more preferable for real-world industrial processes. The present study reports an effective method for complete mineralization of PVDF and poly(vinylidene fluoride-cochlorotrifluoroethylene) copolymer (poly(VDF-co-CTFE), −[(CH2CF2)m(CF2CFCl)n]p−) and poly(vinylidene fluorideco-hexafluoropropylene) copolymer (poly(VDF-co-HFP), −[(CH2CF2)m(CF2CFCF3)]p−) in subcritical water at relatively low temperature (∼300 °C) by use of H 2 O 2 . Furthermore, the formation of CaF2 upon addition of a stoichiometric amount of Ca(OH)2 to the reaction system is reported.

amount of Ca(OH)2 (the molar amount was half the molar amount of fluorine atoms in the (co)polymer) were also performed. The white precipitate that formed during these reactions was collected by centrifugation, washed with pure water, dried in vacuo overnight, and then subjected to X-ray diffraction (XRD) analysis. Analysis. The fluorine content in the (co)polymers was quantified by combustion ion chromatography at Nissan Arc (Yokosuka, Japan) by means of an instrument consisting of a combustion unit (AQF-100, Mitsubishi Chemical Analytec, Chigasaki, Japan; matrix combustion temperature, 1100 °C) and an ion chromatograph unit (Dionex ICS-3000, Thermo Fisher Scientific, Waltham, MA). The F− concentrations were measured with an ionchromatography system (IC-2001, Tosoh, Tokyo, Japan) consisting of an automatic sample injector (30 μL injection volume), a degasser, a pump, a guard column (TSKguard column Super IC-A, 4.6 mm i.d., 1.0 cm length, Tosoh), a separation column (TSKgel Super IC-Anion, 4.6 mm i.d., 15 cm length, Tosoh), a column oven (40 °C), and a conductivity detector with a suppressor device. 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 ion-chromatography system (IC-2001) with a separation column (TSKgel Super IC-AP, 4.6 mm id, 7.5 cm length, Tosoh) was also used to quantify malonic acid and Cl−. The mobile phase was an aqueous solution containing NaHCO3 (1.7 mM), Na2CO3 (1.8 mM), and acetonitrile (23 vol %). A GC system (GC 323, GL Sciences) consisting of an injector (150 °C), a column oven (110 °C), and a thermal conductivity detector (130 °C) was used to quantify CO2. The column was an active carbon column (60/80 mesh, 2.17 mm i.d., 2 m length), and the carrier gas was argon. The products in the gas phase were also analyzed with a GC/MS (QP2010 SE, Shimadzu, Kyoto, Japan) system with a fused-silica capillary column (Rt-Q-BOND, Restek, Bellefonte, PA). The carrier gas was helium, and the injection temperature was held constant at 120 °C. The sample gas was transferred into the GC/MS system in split mode (ratio, 20/1), and analyses were conducted in full-scan mode (m/z 2.0−200). The oven temperature was kept at 30 °C for 5 min, raised to 200 °C at a rate of 20 °C min−1, and held at that temperature for 20 min. XRD patterns of the collected precipitates were measured with Cu Kα radiation (Multiflex, Rigaku, Tokyo, Japan).

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EXPERIMENTAL SECTION Materials. Powdered PVDF and 1,1,2,2-tetrafluoroethane (F2HCCHF2) were purchased from SynQuest Laboratories (Alachua, FL). The weight-average molecular weight of the PVDF was 6.47 × 105 with a polydispersity of 2.52 (relative to polystyrene as a standard). Poly(VDF-co-CTFE) copolymer with a 67.0/33.0 VDF/CTFE molar ratio and poly(VDF-coHFP) copolymer with a 95.3/4.7 VDF/HFP molar ratio were obtained from Elf Atochem (France), prepared by emulsionfree radical copolymerization of VDF and CTFE or HFP. The weight-average molecular weights of poly(VDF-co-CTFE) and poly(VDF-co-HFP) relative to poly(methyl methacrylate) in DMF were 3.5 × 105 with a polydispersity of 2.9 and 4.5 × 105 with a polydispersity of 3.2, respectively. Combustion ion chromatography revealed that the fluorine contents in PVDF, poly(VDF-co-CTFE) copolymer, and poly(VDF-co-HFP) copolymer were 60.7, 51.5, and 59.4 wt %, respectively, and the chlorine content in poly(VDF-co-CTFE) copolymer was 14.5 wt %. These analytical values were used to calculate the F− yields (and Cl− yields when poly(VDF-co-CTFE) was used) of the reactions. Argon (99.99%) and CO2 (0.995%)/N2 gases were purchased from Taiyo Nippon Sanso (Tokyo, Japan). Malonic acid (>98%), 1,3,5-trifluorobenzene, and other reagents were obtained from Wako Pure Chemical Industries (Osaka, Japan). Reaction Procedures. Reactions were carried out in a stainless steel high-pressure reactor fitted with a gold vessel to prevent contamination from the reactor material. The internal volume of the reactor was 31 mL. In a typical run, an aqueous solution of H2O2 (0.1−5.0 M, 10 mL) and the powdered (co)polymer (PVDF or copolymers, 30 mg) were introduced into the reactor, which was then pressurized to 0.60 MPa with argon, sealed, and heated to the desired temperature with a rate of ca. 10 °C min−1. After a specified time passed, the reactor was quickly cooled to room temperature, and the reaction solution was subjected to ion chromatography. The gas phase was collected with a sampling bag and subjected to gas chromatography (GC) and gas chromatography−mass spectrometry (GC/MS). Reactions involving a stoichiometric

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RESULTS AND DISCUSSION Decomposition of PVDF. H2O2 concentration dependences of the F− amount in the reaction solution and the CO2 amount in the gas phase formed after the reactions at 300 °C for 6 h are shown in Figure 1. When the reaction was carried out in the absence of H2O2, a solid residue (18 mg) formed and little CO2 formed in the gas phase. Simultaneously, 511 μmol of F− formed in the reaction solution (Table 1, entry 1), which corresponds to a yield [(moles of F− formed, i.e., 511 μmol)/ (moles of fluorine in the polymer, i.e., 959 μmol)] of 53%. We previously reported that the reaction of PVDF in pure supercritical water (without any additive, that is, in the presence of argon) at 380 °C resulted in a carbon-rich solid residue (for example, a solid after 6 h reaction consisted of 79.6 wt % carbon) and little formation of CO2; meanwhile, F− clearly formed in the reaction solution.17 The results observed here indicate that a similar phenomenon occurred, that is, the decomposition of PVDF mainly proceeded via a dehydroB


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Industrial & Engineering Chemistry Research

In the absence of H2O2, a small amount of 1,3,5trifluorobenzene (0.20 μmol) was also detected, similar to the previous result.17 The formation of 1,3,5-trifluorobenzene can be explained by chain scission during the dehydrofluorination processes (Scheme 1a). We measured the carbon ratio of the residue obtained at 300 °C. The carbon ratio was 47.1 wt %, which was higher than the carbon ratio of the initial polymer (37.5 wt %). This result supports the conclusion that the decomposition of PVDF in the absence of H2O2 proceeds via dehydrofluorination mechanism. From these data, the carbon recovery for the present reaction was calculated to be 79%, according to the equation [(moles of carbon in the products, i.e., residue + CO2 + malonic acid +1,3,5-trifluorobenzene)/(moles of carbon in the initial PVDF)]. To our surprise, when the reaction was carried out in the presence of H2O2, the F− amount showed a unique dependence on the H2O2 concentration (Figure 1a). The F− amount decreased with increasing H2O2 concentration to 0.75 M, then turned to increase around 1.0 M, and tended to saturate above 3.0 M. The solid residue observed in the absence of H2O2 disappeared when the H2O2 concentration increased above 2.0 M. When the H2O2 concentration was 3.0 M (that is, 30 mmol in the initial reaction solution), which corresponds to a 31-fold molar excess relative to the fluorine content of the polymer (959 μmol), the F− amount reached 943 μmol (98% yield, Table 1, entry 2). Meanwhile, the CO2 amount gradually increased with increasing H2O2 concentration, increased sharply above 1.0 M, and finally tended to saturate around 3.0 M. When 3.0 M of H2O2 was used, the CO2 amount reached 917 μmol, which corresponds to a CO2 yield [(moles of CO2 formed, i.e., 917 μmol)/(moles of carbon in the polymer, i.e., 937 μmol)] of 98% (Table 1, entry 2). These results indicate that virtually complete mineralization of fluorine and carbon in PVDF was achieved at a relatively low temperature (300 °C) by use of a 31-fold and a 32-fold molar excess of H2O2 with respect to the fluorine and the carbon in the polymer, respectively. Not only F− but also malonic acid was detected in the reaction solution, although the amount (0.3−1.0 μmol) was 2−3 orders of magnitude lower than the F− amount (Figure 1b). As described above, PVDF was efficiently mineralized in the presence of excess H2O2. In such conditions, we propose the reaction mechanism outlined in Scheme 1b.

Figure 1. H2O2 concentration dependence of PVDF decomposition in subcritical water at 300 °C: detected amounts of (a) F− and CO2 and (b) malonic acid. PVDF (30 mg; fluorine content, 959 μmol; carbon content, 937 μmol) and an aqueous H2O2 solution (10 mL) were introduced into the reactor, which was pressurized with argon (0.60 MPa) and heated at 300 °C for 6 h.

fluorination mechanism (Scheme 1a), as has been reported for the thermolysis of PVDF.9−11 First, C−H scission occurs in a −CH2− group: −CF2−CH 2CF2−CH 2CF2−CH 2CF2− → −CF2−CH 2CF2−CH·CF2−CH 2CF2−

(1)

The scission leads to the formation of a CC bond in the polymer chain, eq 2, and HF formation. −CF2−CH 2CF2−CH·CF2−CH 2CF2− → −CF2−CH 2CF2−CH=CF−CH 2CF2−

(2)

Further loss of HF along the polymer chain results in the formation of carbon-rich residue.

Table 1. Decomposition of PVDF and Related Copolymers in Subcritical Watera entry 1 2 3 4 5 6

(co)polymer PVDF PVDF poly(VDF-coCTFE) poly(VDF-coCTFE) poly(VDF-coHFP) poly(VDF-coHFP)

malonic acid (μmol)

Cl−(μmol) [yield (%)]d

F2HCCHF2 (μmol)

511 [53] 27 [3] 943 ± 9e [98 ± 1] 917 ± 21e [98 ± 2] 9 [1] 2 [0]

0.4 0.8 ± 0.1e 0.2

− − 15 [12]

n.d. n.d. n.d.

11.4

796 [98]

700 [95]

n.d.

119 [97]

0.5

8.8

91 [10]

8 [1]

0.4

−

n.d.

10.0

897 [96]

832 [92]

5.7

−

n.d.

initial H2O2 conc. (M)

reaction press. (MPa)

none 3.0 none

10.1 11.1 9.8

3.0 none 2.0

F− (μmol) [yield (%)]b

CO2 (μmol) [yield (%)]c

a

The (co)polymer (30 mg) and an aqueous solution of H2O2 (10 mL) were introduced into the reactor, which was pressurized with argon and then heated at 300 °C for 6 h. For the reactions in the absence of H2O2, pure water was used instead of an aqueous solution of H2O2. bF− yield (%) = [(moles of F− formed)/(moles of fluorine in the (co)polymer)] × 100. cCO2 yield (%) = [(moles of CO2 formed)/(moles of carbon in the (co)polymer)] × 100. dCl− yield (%) = [(moles of Cl− formed)/(moles of chlorine in the copolymer)] × 100. eThese values were obtained from two reactions under the same reaction conditions. C


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Scheme 1. Proposed Mechanism for the Decomposition of PVDF (a) in the Absence of H2O2 and (b) in the Presence of a Large Excess of H2O2 (≥3.0 M)

The first step is an abstraction of a hydrogen atom from a −CH2− group, eq 1. The resulting radical reacts with H2O2 in the presence of water to produce a hydroperoxide:

Temperature dependence of PVDF decomposition in the presence of 2.0 M of H2O2 at constant reaction time of 6 h is shown in Figure 2a. At 200 °C, almost no decomposition of the polymer occurred. Both F− and CO2 clearly formed during the reaction at 250 °C, at which conditions the water is in a subcritical state, and the F− and CO2 amounts increased further with increasing reaction temperature. At 300 °C, the F− amount reached 809 μmol (yield, 84%) while that of CO2 was 732 μmol (yield, 78%). We extended the reaction time in the presence of 2.0 M H2O2 at 300 °C (Figure 2b). The prolonged reaction time increased the mineralization further. After 12 h, the F− and CO2 amounts reached 914 μmol (yield, 95%) and 846 μmol (yield, 90%), respectively. This means that most of the fluorine and carbon in the polymer also mineralized by use of 2.0 M of H2O2, a 21-fold molar excess relative to fluorine or carbon in the initial polymer, by extending the reaction time to 12 h. Decomposition of Poly(VDF-co-CTFE) Copolymer. H2O2 concentration dependences of the amounts of major products and minor products after the reactions at 300 °C for 6 h are shown in panels a and b of Figure 3, respectively. Not only F− but also Cl− formed in the reaction solution, and CO2 formed in the gas phase as major products (Figure 3a). As minor products, F2HCCHF2 in the gas phase and malonic acid in the reaction solution, the amounts of which were 3 orders of magnitude lower than those of F− and CO2, were detected (Figure 3b). The reactivity of poly(VDF-co-CTFE) copolymer was markedly different from that of PVDF. PVDF clearly formed F− in the absence of H2O2 (the F− yield was 53%; Table 1, entry 1), whereas poly(VDF-co-CTFE) copolymer released a few F− ions (9 μmol, which corresponds to a yield of 1%; Table 1, entry 3). This result suggests that poly(VDF-co-CTFE)

−CF2−CH 2CF2−CH·CF2−CH 2CF2− → −CF2−CH 2CF2−CH(OOH)CF2−CH 2CF2−

(3)

This product is unstable and can cause scission of the main chain, which results in a −CF2• terminal radical and an aldehyde: −CF2−CH 2CF2−CH(OOH)CF2−CH 2CF2− → −CF2−CH 2CF2 · + HC(O)CF2−CH 2CF2−

(4)

•

The −CF2 radical can be transformed into an acid fluoride end-group (−COF) in the presence of H2O2, and the aldehyde can either undergo a direct cleavage of the C−C bond to form a terminal −CF2• radical or can be oxidized into a carboxylic acid, which is subsequently cleaved as shown in Scheme 1b. The acid fluoride is hydrolyzed to the corresponding carboxylic acid. This sequence of steps leads to mineralization of PVDF. The formation of a carboxylic acid end-group and subsequent C−C bond cleavage can shorten the polymer chain stepwise, resulting in the formation of malonic acid as a final product. As described above, when a small amount of H2O2 was introduced into the reaction system (≤0.75 M), the F− amount decreased. This observation suggests that the decomposition of PVDF via dehydrofluorination mechanism was suppressed in the presence of a small H2O2 amount: the presence of H2O2 around the −CH2− group may inhibit the CC bond formation in the polymer chain (eq 2), at which conditions the H2O2 amount is not enough to generate a hydroperoxide group in the polymer chain. D


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Figure 3. H2O2 concentration dependence of decomposition of poly(VDF-co-CTFE) copolymer in subcritical water at 300 °C: detected amounts of (a) F−, Cl−, and CO2 and (b) malonic acid and F2HCCHF2. The copolymer (30 mg; fluorine content, 813 μmol; carbon content, 737 μmol; chlorine content, 123 μmol) and an aqueous H2O2 solution (10 mL) were introduced into the reactor, which was pressurized with argon (0.60 MPa) and heated at 300 °C for 6 h.

Figure 2. (a) Temperature dependence of PVDF decomposition in the presence of H2O2 and (b) time dependence of PVDF decomposition in subcritical water in the presence of H2O2. For the measurements shown in panel a, PVDF (30 mg) and an aqueous H2O2 solution (2.0 M, 10 mL) were introduced into the reactor, which was pressurized with argon (0.60 MPa) and heated at 200−300 °C for 6 h. For the measurements in panel b, PVDF (30 mg) and an aqueous H2O2 solution (2.0 M, 10 mL) were introduced into the reactor, which was pressurized with argon (0.60 MPa) and heated at 300 °C for 2−12 h. Besides F−, a trace of malonic acid was detected in the solutions after the reactions described in panel a (0.1−0.6 μmol) and panel b (0.3−0.5 μmol).

copolymer cannot decompose via dehydrofluorination mechanism, as proposed in the decomposition of PVDF in the absence of H2O2 (Scheme 1a). Whereas the amount of F− formed from PVDF showed a “Vshape” dependence on the H2O2 concentration (Figure 1a), the amounts of F− and CO2 from poly(VDF-co-CTFE) copolymer were monotonically increased with increasing H2O2 concentration (Figure 3a). Furthermore, a small amount of F2HCCHF2 was detected during the decomposition of poly(VDF-co-CTFE) copolymer with higher H2O2 concentrations (≥2.0 M) (Figure 3b). When the reaction was carried out in the presence of 3.0 M H2O2, the amounts of F−, CO2, and Cl− reached 796, 700, and 119 μmol, which correspond to yields of 98, 95, and 97%, respectively (Table 1, entry 4), where the Cl− yield (%) was calculated by the equation (moles of Cl− formed, i.e., 119 μmol)/(moles of chlorine in the copolymer, i.e., 123 μmol) × 100. That is, virtually complete mineralization was achieved with 30 mmol of H2O2, which is 37 times the molar amount of fluorine and 41 times the molar amount of carbon in the copolymer. Temperature dependence of the decomposition of poly(VDF-co-CTFE) copolymer in the presence of 2.0 M H2O2 at constant reaction time of 6 h is displayed in Figure 4. The formation of F− and Cl− was prominent when the reaction was carried out at 300 °C. However, while F− and Cl− amounts increased monotonically with increasing temperature, that of CO2 showed only a slight increase above 300 °C (Figure 4a),

Figure 4. Temperature dependence of decomposition of poly(VDFco-CTFE) copolymer in subcritical water in the presence of 2.0 M of H2O2: detected amounts of (a) F−, Cl−, and CO2 and (b) malonic acid and F2HCCHF2. The copolymer (30 mg) and an aqueous H2O2 solution (10 mL) were introduced into the reactor, which was pressurized with argon (0.60 MPa) and heated at 250−350 °C for 6 h.

although the amounts of the minor products, malonic acid and F2HCCHF2, increased at these temperatures (Figure 4b). When the reaction was carried out at 350 °C, while the fluorine E


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content was completely converted to F− (yield, 100%), the amounts of Cl− (88.4 μmol) and CO2 (345 μmol) resulted in lower yields, 72 and 47%, respectively, and the color of the reaction solution was slightly black. These results indicate that the H2O2 concentration of 2.0 M (that is, 20 mmol in the 10 mL reaction solution) was insufficient to induce the complete mineralization at 350 °C, suggesting that some decomposition pathway(s) which does not lead to mineralization of the carbon and chlorine of the copolymer to CO2 and Cl−, for example, the dehydrofluorination, which forms CC bonds in the polymer chain, allowing incorporation of the chlorine atoms, may participate at 350 °C. Figure 5 exhibits the time dependence of the decomposition of poly(VDF-co-CTFE) copolymer in the presence of 2.0 M

Decomposition of Poly(VDF-co-HFP) Copolymer. Compared to poly(VDF-co-CTFE), poly(VDF-co-HFP) copolymer was more readily decomposed by H2O2. Figure 6a

Figure 6. H2O2 concentration dependence of decomposition of poly(VDF-co-HFP) copolymer in subcritical water at 300 °C: detected amounts of (a) F− and CO2 and (b) malonic acid and CF3H. The copolymer (30 mg; fluorine content, 938 μmol; carbon content, 902 μmol) and an aqueous H2O2 solution (10 mL) were introduced into the reactor, which was pressurized with argon (0.60 MPa) and heated at 300 °C for 6 h.

shows H2O2 concentration dependences of the amounts of F− and CO2 after the reactions at 300 °C for 6 h. When the reaction was carried out in the absence of H2O2, the amounts of F− and CO2 were 91 μmol (yield, 10%) and 8 μmol (yield, 1%), respectively (Table 1, entry 5). Although F− formed clearly in the absence of H2O2, the yield was considerably lower than that of PVDF (53%, Table 1, entry 1). This result suggests that the decomposition of the copolymer via dehydrofluorination mechanism was limited, although 95% of the polymer chain consisted of −CH2CF2− unit. When 0.5 M H2O2 was added, the F− amount decreased to 46 μmol (yield, 5%), whereas that of CO2 increased up to 54 μmol (yield, 6%). This tendency is similar to the decomposition of PVDF (Figure 1a). Further increase in the H2O2 concentration enhanced the formation of F− and CO2. When the reaction was carried out in the presence of 2.0 M H2O2 (20 mmol), the F− and CO2 contents reached 897 μmol (yield, 96%) and 832 μmol (yield, 92%), respectively (Table 1, entry 6). Therefore, almost complete mineralization of the copolymer was achieved with 20 mmol H2O2, the molar amount of which was 21 times that of fluorine and 22 times that of carbon in the copolymer. When the reaction was carried out in the presence of medium H2O2 concentration (0.5 or 1.0 M), a small amount of CF3H was detected in the gas phase (Figure 6b). Simultaneously, a small amount of malonic acid was detected in the reaction solution. We previously reported that CF3H was detected during the decomposition of perfluoalkyl sulfonic acid membrane polymer bearing CF3 group in the pendant chain of the polymer in

Figure 5. Time dependence of decomposition of poly(VDF-co-CTFE) copolymer in subcritical water in the presence of H2O2 at 300 °C: detected amounts of (a) F−, Cl−, and CO2 and (b) malonic acid and F2HCCHF2. Poly(VDF-co-CTFE) (30 mg) copolymer and an aqueous H2O2 solution (2.0 M, 10 mL) were introduced into the reactor, which was pressurized with argon (0.60 MPa) and heated for 2−18 h.

H2O2 at 300 °C. The F−, CO2, and Cl− amounts increased monotonically with increasing reaction time (Figure 5a). After 18 h, the F−, CO2, and Cl− contents reached 732, 651, and 104 μmol, which correspond to yields of 90, 88, and 85%, respectively, indicating that most of the fluorine, carbon, and chlorine in the initial copolymer mineralized. When the F− yield is compared with the Cl− yield, both values were similar at each reaction time. This result indicates that the abstraction of chlorine atoms from the polymer chain did not proceed prior to the abstraction of the fluorine atoms and the −CH2CF2− unit in the polymer chain may decompose preferentially. As for the minor products, the amount of malonic acid decreased with increasing reaction time (Figure 5b), which indicates that malonic acid was an intermediate product in the decomposition of the copolymer. In contrast, the amount of F2HCCHF2 increased with increasing reaction time, indicating that F2HCCHF2 is a final product under these reaction conditions. F


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subcritical water and demonstrated that water was the source of hydrogen in the CF3H by means of a reaction using D2O.23 Similarly, the CF3H in the present study may form by a reaction of the CF3 radical derived from the copolymer and water. However, CF3H disappeared with increasing H2O2 concentration to 2.0 M. In the gas phase, F2HCCHF2, which appeared during the reaction of poly(VDF-co-CTFE) copolymer, was quasi not detected or only trace amounts (10−8 mol) were noted in the presence of H2O2. Temperature dependence of the decomposition of poly(VDF-co-HFP) copolymer in the presence of 2.0 M H2O2 at constant reaction time of 6 h is displayed in Figure 7. The main

Figure 8. Time dependence of decomposition of poly(VDF-co-HFP) copolymer in subcritical water in the presence of H2O2 at 300 °C: detected amounts of (a) F− and CO2 and (b) malonic acid and CF3H. The copolymer (30 mg) and an aqueous H2O2 solution (2.0 M, 10 mL) were introduced into the reactor, which was pressurized with argon (0.60 MPa) and heated at 300 °C for 1−6 h.

mL), a white precipitate was obtained. After purification by simple washing with pure water, the XRD pattern of the precipitate showed only peaks assigned to CaF2 (Figure S-1 in Supporting Information). The molar amount of the collected CaF2 was 370 μmol, which corresponds to 740 μmol of fluorine atoms. That is, 77% of the fluorine atoms in PVDF were recovered in the collected CaF2. When a reaction in the absence of H2O2 was carried out under the same reaction conditions as mentioned above, a black precipitate (51 mg), which seems to be a mixture containing carbon, was obtained instead of the white precipitate. This result indicates that H2O2 is required for the efficient CaF2 formation. In a similar manner, when poly(VDF-co-CTFE) copolymer (30 mg) was reacted in the presence of a stoichiometric amount of Ca(OH)2 (409 μmol) and H2O2 (3.0 M, 30 mmol) at 300 °C for 6 h, CaF2 (237 μmol, 58% yield) was obtained (Figure S-2 in Supporting Information). Finally, the reaction of poly(VDF-co-HFP) copolymer (30 mg) in the presence of Ca(OH)2 (476 μmol) and 2.0 M H2O2 (20 mmol, 10 mL) was carried out at 300 °C for 6 h, and CaF2 was obtained (375 μmol, 80% yield; the XRD pattern is shown in Figure S-3 in Supporting Information). The lower CaF2 yield (58%) observed for poly(VDF-co-CTFE) copolymer compared to that of other (co)polymers may be ascribed to Ca2+ reacting with not only F− but also with Cl−, although CaCl2 was removed from the reaction mixture by washing with pure water.

Figure 7. Temperature dependence of decomposition of poly(VDFco-HFP) copolymer in subcritical water in the presence of 2.0 M of H2O2: detected amounts of (a) F− and CO2 and (b) malonic acid and CF3H. The copolymer (30 mg) and an aqueous H2O2 solution (2.0 M, 10 mL) were introduced into the reactor, which was pressurized with argon (0.60 MPa) and heated at 250−300 °C for 6 h.

products, F− and CO2, formed efficiently above 270 °C (Figure 7a), and CF3H and malonic acid were also detected at 270 and 280 °C as minor products. When the reaction temperature was increased to 300 °C, CF3H disappeared and almost complete mineralization (F− yield, 96%; CO2 yield, 92%) was achieved. The time dependence of the decomposition of poly(VDF-coHFP) copolymer in the presence of 2.0 M H2O2 at 300 °C is shown in Figure 8. The amounts of F− and CO2 reached 670 μmol (yield, 71%) and 598 μmol (66%) even after a short reaction time of 1 h, and the amounts almost saturated after 4 h (Figure 8a). After 4 h, the F− and CO2 amounts reached 906 μmol (yield, 97%) and 825 μmol (yield, 91%), respectively, reflecting that the fluorine and carbon in the copolymer were efficiently mineralized. In addition, CF3H disappeared after 6 h (Figure 8b). CaF2 Formation. To determine whether CaF2 formed in such a reaction system, the (co)polymer decomposition reactions were performed in the presence of Ca(OH)2. When PVDF (30 mg, fluorine content, 959 μmol) and a stoichiometric amount of Ca(OH)2 (481 μmol) were heated at 300 °C for 12 h in the presence of 2.0 M H2O2 (20 mmol, 10

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CONCLUSION Decompositions of PVDF, poly(VDF-co-CTFE), and poly(VDF-co-HFP) copolymers in subcritical water were investigated. Addition of H2O2 into the reaction system led to an efficient mineralization of these (co)polymers that released to F− and CO2 (and also Cl− if poly(VDF-co-CTFE) copolymer) at a relatively low temperature (300 °C). When PVDF was G


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Industrial & Engineering Chemistry Research heated in subcritical water at 300 °C for 6 h in the presence of 3.0 M H2O2, which corresponds to 31 times the fluorine molar amount and 32 times the carbon molar amount in this polymer, both the F− and CO2 yields reached 98%. That result indicates a complete mineralization of this polymer. Poly(VDF-coCTFE) copolymer was also efficiently mineralized at 300 °C for 6 h by use of 3.0 M H2O2, which corresponds to 37 times the fluorine molar amount and 41 times the carbon molar amount in the copolymer, leading to F−, CO2, and Cl− yields of 98, 95, and 97%, respectively. Poly(VDF-co-HFP) copolymer was readily decomposed faster in the presence of H2O2 than poly(VDF-co-CTFE) copolymer: almost complete mineralization of the copolymer (F− and CO2 yields were 96 and 92%, respectively) was achieved after a reaction at 300 °C in the presence of 2.0 M H2O2 for 6 h, which is 21 times the fluorine molar amount and 22 times the carbon molar amount in the copolymer. The H2O2 concentration dependence of the formation of F− was different in these (co)polymers: the F− amount from PVDF for 6 h showed a “V-shape” dependence on the H2O2 concentration, whereas the F− amount from poly(VDF-co-CTFE) copolymer was monotonically increased with increasing H2O2 concentration. This difference suggests that PVDF can decompose via dehydrofluorination mechanism in the absence of H2O2, whereas poly(VDF-co-CTFE) copolymer cannot. The behavior of poly(VDF-co-HFP) copolymer was similar to that of PVDF, although the amount of F− formed in the absence of H2O2 was considerably lower than that of PVDF. Addition of stoichiometric Ca(OH)2 into the reaction system resulted in the formation of CaF2 with yields of 77, 58, and 80% for PVDF and poly(VDF-co-CTFE) and poly(VDF-co-HFP) copolymers, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01716. XRD patterns of the precipitates obtained from the reactions of PVDF, poly(VDF-co-CTFE), and poly(VDF-co-HFP) copolymers in the presence of H2O2 and Ca(OH)2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: (+81)-463-59-4111. Fax: (+81)-463-58-9688. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Japan Society for the Promotion of Science (15H02841). The authors also thank the Elf Atochem company (Pierre Benite, France) for free gifts of poly(VDF-co-CTFE) and poly(VDF-co-HFP) copolymers.

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REFERENCES

(1) Seiler, D. A. PVDF in the Chemical Process Industry. In Modern Fluoropolymers: High Performance Polymers for Diverse Applications; Scheirs, J., Ed.; John Wiley & Sons: New York, 1997; pp 487−506. (2) Fluoropolymers: Synthesis and Applications; Hougham, G., Cassidy, P. E., Johns, K., Davidson, T., Eds.; Plenum: New York, 1999; Vol. 1 and 2. H


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zerovalent metals in subcritical water. Ind. Eng. Chem. Res. 2010, 49, 464.

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