NMR Studies of the Thermal Degradation of a Perfluoropolyether on

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Langmuir 2007, 23, 8855-8860

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NMR Studies of the Thermal Degradation of a Perfluoropolyether on the Surfaces of γ-Alumina and Kaolinite Kerri A. Denkenberger,† Ruth A. Bowers, A. Daniel Jones,‡ and Karl T. Mueller* Department of Chemistry, PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed February 14, 2007. In Final Form: June 4, 2007 Solid-state nuclear magnetic resonance (NMR) methods are used to follow the thermal degradation of Krytox 1506, a common perfluoropolyether, following adsorption onto the surfaces of γ-Al2O3 and a model clay (kaolinite). The alumina studies are complemented with thermogravimetric analysis (TGA) to follow the degradation process macroscopically. Molecular-level details are revealed through 19F magic-angle spinning (MAS), 27Al MAS, and 19F f 27Al cross-polarization MAS (CPMAS) NMR. The CPMAS results show the time-dependent formation of probable VIAl(O6 - nFn) (n ) 1, 2, 3) species in which the fluorine atoms are selectively associated with octahedrally coordinated aluminum atoms. For the alumina system, the changes in peak shapes of the CP spectra over time suggest the early formation of catalytically active degradation products, which in turn lead to the formation of additional perfluoropolyether degradation products. Similar to the alumina system, the kaolinite system also displays new resonances in both the 27Al MAS and 19F f 27Al CPMAS spectra after thermal treatment at 300 °C for up to 20 h but reveals a more distinct species at -15.5 ppm that forms at the expense of an initial species (3 ppm), which is in greater abundance at shorter heating times.

Introduction Perfluoropolyethers (PFPEs) are commonly used as lubricants in magnetic recording material, aerospace engines, and satellite instruments.1 Similar to other perfluorinated compounds, PFPEs are resistant to degradation because of strong C-F bonds that confer high-temperature stability and low chemical reactivity under a wide range of conditions.2,3 Because of the global distribution of perfluorinated compounds in wildlife and humans,4 the degradation and environmental fate of fluorinated polymers are being investigated in detail.5,6 In addition, PFPEs have been identified as atmospheric pollutants with long lifetimes and large global warming potentials.7 Studies have shown that upon heating, the degradation of PFPEs is catalyzed by the Lewis acid Al2O3 (alumina).1,2,8-14 Alumina and other metal oxides are present in computer disks and the reading heads of magnetic hard disk drive systems, and the metal oxides come into contact with the PFPE lubricant and therefore have the potential to degrade the lubricant.8-11,15 The surface * Corresponding author. E-mail: [email protected]. Tel: 814-863-8674. Fax: 814-863-8403. † Current address: Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093. ‡ Current address: Department of Biochemistry and Molecular Biology and Department of Chemistry, Michigan State University, East Lansing, Michigan 48824. (1) Kasai, P. H. AdV. Inf. Stor. Syst. 1992, 4, 291. (2) Kasai, P. H.; Tang, W. T.; Wheeler, P. Appl. Surf. Sci. 1991, 51, 201. (3) Fuchs, B.; Scheler, U. Macromolecules 2000, 33, 120. (4) Houde, M.; Martin, J. W.; Letcher, R. J.; Solomon, K. R.; Muir, D. C. G. EnViron. Sci. Technol. 2006, 40, 3463. (5) Ellis, D. A.; Mabury, S. A.; Martin, J. W.; Muir, D. C. G. Nature 2001, 412, 321. (6) Prevedouros, K.; Cousins, I. T.; Buck, R. C.; Korzeniowski, S. H. EnViron. Sci. Technol. 2006, 40, 32. (7) Young, C. J.; Hurley, M. D.; Wallington, T. J.; Mabury, S. A. EnViron. Sci. Technol. 2006, 40, 2242. (8) Koka, R.; Armatis, F. Tribol. Trans. 1997, 40, 63. (9) Kasai, P. H.; Wheeler, P. Appl. Surf. Sci. 1991, 52, 91. (10) Zhang, J. Y.; Cheng, T.; Cheng, P.; Chao, J. Wear 2003, 254, 321. (11) Li, P.; Lyth, E.; Munro, D.; Ng, L. M. Tribol. Lett. 1998, 4, 109. (12) Kasai, P. H. J. Inf. Stor. Proc. Syst. 1999, 1, 23. (13) Kasai, P. H. Macromolecules 1992, 25, 6791. (14) Liu, J. W.; Stirniman, M. J.; Gui, J. IEEE Trans. Magn. 2003, 39, 749. (15) Kumar, S.; Srivastava, P. K. Tribol. Int. 2005, 38, 687.

sites present on alumina are similar to certain species found on clay mineral surfaces; therefore, soil particles could catalyze the degradation of PFPEs through mechanisms similar to those proposed for degradation on aluminum oxides. In fact, highmolecular-weight organic compounds have been shown previously to adsorb strongly to alumina and aluminosilicates found in natural waters.16 Furthermore, the use of perfluorinated alumina has been recommended to assist in the degradation of gasoline compounds that contaminate soil and water.17 Therefore, the degradation of PFPEs, especially that induced by metal oxides, must be examined to understand the fate of PFPEs in the environment. The catalytic centers on alumina are suggested to be defect Al3+ surface sites and hydroxyls.9,11,13,18 Using Fourier transform infrared spectroscopy (FTIR), Li et al. examined the interactions of CF3CF2OCF2CF3 and CF3OCF3 with Al2O3 under vacuum and proposed that hydroxyl sites on the Al2O3 surface play an important role in the decomposition of these simpler PFPEs.11 Upon the basis of a study using an ultrahigh vacuum drag test, Zhang et al. suggested that a coordination bond forms between the Al atom of Al2O3 and the O atom of the PFPE, leading to the degradation of the PFPE.10 In other studies of interest, Kasai et al. found that the rate of catalytic degradation of PFPEs on oxide surfaces is timedependent; this is due to an induction period caused by the conversion of surface oxygen sites to fluorine sites.2 Because of slower kinetics of the conversion of surface sites, the apparent length of the induction period increases with decreasing temperature.1 It was found that the addition of fluorine to Al2O3 increases the number of strong Brønsted acid sites on the alumina by creating a through-lattice inductive effect due to the high electronegativity of fluorine; this leads to higher acid-catalytic activity than for non-fluorinated Al2O3.19 Therefore, fluorinated (16) Davis, J. A.; Gloor, R. EnViron. Sci. Technol. 1981, 15, 1223. (17) Kasprzyk-Hordern, B.; Andrzejewski, P.; Nawrocki, J. Ozone: Sci. Eng. 2005, 27, 301. (18) Chupas, P. J.; Ciraolo, M. F.; Hanson, J. C.; Grey, C. P. J. Am. Chem. Soc. 2001, 123, 1694. (19) Ghosh, A. K.; Kydd, R. A. Catal. ReV. Sci. Eng. 1985, 27, 539.

10.1021/la7004453 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/18/2007

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aluminas have been examined by several researchers as catalysts of numerous reactions.18-24 Reitsma and Boelhouwer first found that catalytic sites are located between Al2O3 and AlF3 particles.23 It has also been suggested that the AlF3 species further catalyzes the degradation of PFPEs. Thus, as the oxide surface sites are fluorinated, the rate of PFPE degradation increases.2,25 NMR spectroscopy offers a sensitive, noninvasive method for studying the adsorption and thermal degradation of PFPEs on metal oxides, including clay surfaces. Solution-state NMR has been used in the past to characterize and study PFPEs and volatile PFPE degradation products.1,2,9,12,13,26,27 However, these solution-state NMR methods do not allow an examination of the catalytic surface of alumina throughout the steps of PFPE degradation. Using X-ray photoelectron spectroscopy, the surfaces of PFPE Fomblin Zdol-coated aluminas have been examined and show evidence of the formation of AlF3 species following PFPE degradation.1 Using MAS NMR and related techniques, the catalytic sites on solids and their surfaces can be examined more closely. Fluorinated aluminas and zeolites have been examined using MAS NMR,20-22,24,28-32 and VIAl(O6 - nFn) (n ) 1, 2, 3) species have been detected.20,22,29,31,32 In addition, MAS NMR methods were recently applied to the study of aluminofluoride minerals33 and perfluorobenzene in soils.34,35 To our knowledge, no studies reported to date have focused on the examination of PFPEs adsorbed onto alumina or clays using MAS NMR. In addition, no surface studies have been conducted for Krytox, a common PFPE with the general formula CF3CF2CF2O[CF(CF3)CF2O]mCF2CF3, adsorbed onto alumina or clays. In the current research, the thermal degradation of Krytox adsorbed onto alumina and a model kaolinite clay has been investigated using TGA and MAS NMR. One goal of the study was to detect possible AlxFy•nH2O species on the Krytox-coated alumina and kaolinite throughout heating and to investigate the possible catalytic role of these species in the degradation of Krytox. The strength and novelty of this study lies in the selective observation of fluorinated aluminum sites using 19F f 27Al CPMAS NMR. Although direct 19F and 27Al MAS NMR reveal information about all fluorine and aluminum species present in the samples, the heteronuclear CPMAS experiments provide direct information related to the formation of Al-F bonds during the heat treatment process. Experimental Section Krytox 1506 (Fil-Tech) was adsorbed onto powdered γ-Al2O3 (gift) and heated to catalyze degradation as described below. Krytox 1506 is characterized by an average molecular weight of 2400 amu and a density of 1.88 g/mL at 20 °C. Surface area analysis of γ-Al2O3 (20) Decanio, E.; Bruno, J. W.; Nero, V. P.; Edwards, J. C. J. Catal. 1993, 140, 84. (21) Grey, C. P.; Corbin, D. R. J. Phys. Chem. 1995, 99, 16821. (22) Zhang, W. P.; Sun, M. Y.; Prins, R. J. Phys. Chem. B 2002, 106, 11805. (23) Reitsma, H. J.; Boelhouw, C. J. Catal. 1974, 33, 39. (24) Okamoto, Y.; Imanaka, T. J. Phys. Chem. 1988, 92, 7102. (25) Stirniman, M. J.; Falcone, S. J.; Marchon, B. J. Tribol. Lett. 1999, 6, 199. (26) Kasai, P. H. J. Inf. Stor. Proc. Syst. 1999, 1, 233. (27) Karis, T. E.; Marchon, B.; Hopper, D. A.; Siemens, R. L. J. Fluorine Chem. 2002, 118, 81. (28) Kao, H. M.; Liao, Y. C. J. Phys. Chem. C 2007, 111, 4495. (29) Chupas, P. J.; Grey, C. P. J. Catal. 2004, 224, 69. (30) Kao, H. M.; Chen, Y. C. J. Phys. Chem. B 2003, 107, 3367. (31) Fischer, L.; Harle, V.; Kasztelan, S.; de la Caillerie, J. B. D. Solid State Nucl. Magn. Reson. 2000, 16, 85. (32) Chupas, P. J.; Corbin, D. R.; Rao, V. N. M.; Hanson, J. C.; Grey, C. P. J. Phys. Chem. B 2003, 107, 8327. (33) Zhou, B.; Sherriff, B. L.; Hartman, J. S.; Wu, G. Am. Mineral. 2007, 92, 34. (34) Khalaf, M.; Kohl, S. D.; Klumpp, E.; Rice, J. A.; Tombacz, E. EnViron. Sci. Technol. 2003, 37, 2855. (35) Kohl, S. D.; Toscano, P. J.; Hou, W. H.; Rice, J. A. EnViron. Sci. Technol. 2000, 34, 204.

Denkenberger et al. was conducted using a Micromeritics ASAP 2010 accelerated surface area and porosimetry system with nitrogen physisorption. The singlepoint surface area and multipoint BET surface area were found to be 8.7 and 8.8 m2/g, respectively; this is similar to the surface area of Al2O3 (8.4 m2/g) used by Koka and Armatis in PFPE studies.8 The micropore area and external surface area were found to be 1.8 and 6.9 m2/g, respectively. Kaolinite (KGa-1b, Washington County, Ga) was obtained from the Source Clay Repository at Purdue University. The sample was received as a crushed powder and was subsequently sieved to a particle size of less than 150 µm. No further surface pretreatment was performed. Samples were prepared by saturating 10.0 g of γ-Al2O3 in 35.04 g Krytox and 10.0 g of kaolinite in 6.01 g of Krytox. Initial mixing was performed with a Vortex Genie 2 (Scientific Industries) at speed setting 8 for 1 min, and the samples were then incubated for 5 days at room temperature. Excess Krytox was removed by vacuum filtration. Four coated samples (2 g each) for each system were removed and heated in air to 300 °C in a ThermoLyne 48000 furnace and left at 300 °C for 5, 10, 15, and 20 h. Uncoated γ-Al2O3 and kaolinite samples were also heated in the same manner as experimental controls. TGA was performed on a TA Instruments SDT 2960 to 1000 °C at 10 °C/min. All NMR experiments were performed on a Chemagnetics/Varian Infinity-500 spectrometer with a 4 mm HFXY MAS probe operating at a spinning rate of 15.5 kHz. The 19F MAS spectra were collected with a 19F resonance frequency of 470.052 MHz and were referenced relative to CFCl3 (0.0 ppm) using sodium trifluoroacetate (-79.3 ppm) (Aldrich) as a secondary reference. 19F MAS spectra were obtained using a 4.0 µs π/2 pulse, a 3.0 s pulse delay, and 128-4396 scans. The 27Al MAS spectra were obtained with an 27Al resonance frequency of 130.182 MHz using a 6.0 µs π/2 pulse, a 1.0 or 2.0 s pulse delay, and 16-2396 collected scans. All 27Al spectra were referenced to 1 M aqueous AlCl3 (0.0 ppm) using AlF3 (-16.0 ppm) as a secondary reference. The 19F f 27Al CPMAS spectra were collected with 19F and 27Al resonance frequencies of 470.027 and 130.182 MHz, respectively, using an optimized contact time of 0.400 ms, a pulse delay of 4.0 or 8.0 s, 19F π/2 pulses of 5.0 µs, and between 64 and 19 988 collected scans depending on the resulting signal strength. A relatively short contact time is required because of the reduction of the apparent spin-lattice relaxation time in the rotating frame (T1F) of the quadrupolar 27Al species under MAS conditions.36

Results TGA. TGA was performed on γ-Al2O3 (not shown), Krytox (not shown), unheated Krytox-coated γ-Al2O3 (Figure 1, top), and Krytox-coated γ-Al2O3 previously held at 300 °C for 20 h (Figure 1, bottom). For uncoated γ-Al2O3, an approximate 7.5% mass loss was observed by 300 °C. For Krytox, an 82% mass loss was observed by 300 °C. For previously unheated Krytoxcoated γ-Al2O3, an approximate 35% mass loss was observed by 300 °C. This should be compared to the much smaller percentage weight loss (∼2.5%) for a sample of Krytox-coated γ-Al2O3 that had undergone heat treatment at 300 °C for 20 h (Figure 1). 19F MAS NMR. Using the 19F NMR assignments of Karis et al.27 and Kasai et al.,1,37 resonances in the unheated Krytoxcoated γ-Al2O3 19F MAS NMR spectrum (Figure 2) are assigned as shown in Table 1. In Figure 2, the spectra for unheated, 5 h heated, 10 h heated, 15 h heated, and 20 h heated Krytox-coated γ-Al2O3 are also shown. The 19F MAS NMR spectrum of AlF3 (with a peak centered at -174 ppm) is included in Figure 3a as a comparison; the residual 19F-19F dipolar coupling contributes to the observed strong sideband pattern. For the Krytox-coated γ-Al2O3 sample examined after 10 h of heating, there is a notable (36) Vega, A. J. J. Magn. Reson. 1992, 96, 50. (37) Engelhardt, G., Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; Wiley: New York, 1987.

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Figure 1. TGA traces of samples of Krytox-coated γ-Al2O3 with no heat treatment (top) and after 20 h of heat treatment at 300 °C (bottom). Figure 3. (a) 19F MAS NMR spectrum of the AlF3 reference compound. (b) 27Al MAS NMR spectrum of AlF3. (c) 19F f 27Al MAS NMR spectrum of the AlF3 reference compound. Table 1.

Figure 2. 19F MAS NMR spectra of (a) Krytox-coated γ-Al2O3 after varied heating times, with closer views of the peaks in the regions of (b) -82.6 ppm and (c) -146.1 ppm. The scaling factor is the product of the relative number of scans and the relative receiver gain.

loss in intensity of the resonances at -90.1 and -90.2 ppm, which are assigned to the -OCF2CF3 end-group. The formation of a peak at -147.2 ppm after 10 h of heating corresponds to an -OCF(CF3)2 end-group formed by the cleavage of CF2-O bonds of Krytox and requiring the displacement of an oxygencontaining group by a fluorine. Also, after 10 h of heating, a broad resonance appears in the region of the spectrum usually assigned to Al-F species (-150 to -250 ppm). Overall, the 19F MAS NMR spectra decrease in total signal with increasing heating

19

F MAS NMR Chemical Shift Assignments

assignments

chemical shift (ppm)

CF3CF2CF2OCF3CF2CF2OCF3CF2CF2O-OCF(CF3)CF2O-OCF(CF3)CF2O-OCF(CF3)CF2O-OCF2CF3 -OCF2CF3 -OCF(CF3)CF3 -OCF(CF3)2

-84.2 -132.0 -82.6 -146.1 -82.6 -82.6 -90.2 -90.1 -147.2 -82.6

time; for comparison, spectra were scaled, and magnification factors are noted in Figure 2. The relative total signal intensities were calculated assuming a linear receiver gain parameter (verified directly with standards over the gain settings used) and a direct proportionality between the number of scans and the integral of the spectrum. Relative total spectral intensities, which are directly proportional to moles of fluorine in the sample, were determined with respect to the signal from unheated Krytox-coated γ-Al2O3 (assigned an intensity of unity). 27Al MAS NMR. In Figure 4, the 27Al MAS NMR spectra for unheated, 5 h heated, 10 h heated, 15 h heated, and 20 h heated Krytox-coated γ-Al2O3 are shown. The NMR spectra contain two main resonances at 73.7 and 12.4 ppm assigned to tetrahedrally and octahedrally coordinated Al species, respectively.9 As the heating duration increases, a third peak appears as a shoulder on the lower-frequency octahedral resonance in the vicinity of -17.5 ppm. As a comparison, the 27Al MAS NMR

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Figure 6. 27Al MAS spectra of Krytox-coated kaolinite samples both before and after 20 h of heat treatment at 300 °C and 19F f 27Al MAS NMR spectrum (top) of the sample that was heat treated for 20 h.

Figure 4. 27Al MAS NMR spectra of Krytox-coated γ-Al2O3 samples after varied heating times, including the sample that did not receive any heat treatment.

Figure 7. 19F f 27Al MAS NMR spectra of Krytox-coated kaolinite samples after varied heating times.

Figure 5. 19F f 27Al MAS NMR spectra of Krytox-coated γ-Al2O3 samples after varied heating times, including the sample that did not receive any heat treatment.

spectrum of AlF3 is shown in Figure 3b and contains a single resonance at -16.0 ppm. 19F f 27Al CPMAS NMR. Application to Alumina Surfaces. The 19F f 27Al CPMAS NMR spectrum of AlF3, which serves as the setup and reference compound for this experiment, is shown in Figure 3c, again displaying a single peak at -16.0 ppm. The 19F f 27Al CPMAS NMR spectra for unheated, 5 h heated, 10 h heated, 15 h heated, and 20 h heated Krytox-coated γ-Al2O3 are shown in Figure 5. Overall, the Krytox-coated γ-Al2O3 CP spectra increase in intensity as a function of heating time when using the same pulse sequence power levels and timings in each run. Relative intensities were calculated again assuming a linear receiver gain parameter and taking into account the number of transients acquired. Relative intensities were scaled to the amount of signal in the first spectrum with detectable signal (the spectrum from the sample heated for 5 h). The signal from the unheated Krytox-coated γ-Al2O3 sample gave no signal

that was discernible above the noise. The 5 h treatment resulted in a spectrum assigned unit intensity. After 10 h of heating, the relative intensity was 3.0, and the signal intensity continued to increase with additional heating to totals of 15 h (10.1) and 20 h (13.8). It is important to note that different Al-F environments are expected to exhibit different cross-polarization transfer and relaxation rates and therefore even the careful reproduction of experimental parameters does not allow a fully quantitative analysis. However, the relative total amount of cross-polarization signal under these conditions does increase monotonically with heating time. Application to Clay Surfaces. 27Al MAS and 19F f 27Al CPMAS NMR experiments applied to the model clay kaolinite after saturation with Krytox display a growth of new resonances with thermal treatment as found for the alumina system but with different spectral intensities and time dependence (Figures 6 and 7). Again, the resonances are found in the region near the resonance from the octahedral aluminum species in the parent kaolinite, but the reduced width of the kaolinite resonance compared to that from the octahedrally coordinated species in the alumina allows improved resolution of a lower-frequency spectral resonance, at around -16 ppm, that arises after heating. Figure 6 displays 27Al MAS spectra from the unheated kaolinite and a Krytox-treated sample after 20 h of heat treatment at 300 °C. A new resonance appears at -15.5 ppm, after 20 h of heating, with an intensity of only 6% of the total spectral intensity. The 19F f 27Al CPMAS NMR spectrum more clearly shows the

Thermal Degradation of a Perfluoropolyether

emergence of the new resonance at -15.5 ppm, along with a less-intense resonance at 2.7 ppm. As in the alumina system, the relative and overall intensities of these resonances changes with heating duration, as displayed in Figure 7. The primary resonance near 3 ppm appears after 5 h of heat treatment, and this resonance initially grows in intensity, reaching a maximum after 15 h of thermal treatment. Ultimately, new chemical species contribute to the major aluminum resonance at -15.5 ppm, and their population increases throughout the heating process.

Discussion TGA has been shown to be an effective technique for investigating the thermal evaporation of perfluoropolyethers on catalytic surfaces.14,25,38 The approximate 7.5% mass loss observed by TGA for γ-Al2O3 corresponds to the loss of water. For Krytox, TGA showed an approximate 82% mass loss by 300 °C, which corresponds to its evaporation.39 The 35% mass loss in the previously unheated Krytox-coated γ-Al2O3 corresponds to the nearly full loss of Krytox. At first, mass loss is gradual in the 150-250 °C range, where evaporation is occurring. Near the depolymerization temperature of Krytox in the presence of metal oxides (288 °C), rapid mass loss occurs, corresponding to the loss of Krytox degradation products.38 These results are in agreement with those of Stirniman et al., who found that although thermal evaporation of PFPEs Fomblin Zdol and ZdolTX is a slow process occurring over a large temperature range, catalytic decomposition is exemplified by rapid mass loss over a much smaller temperature range.25 Furthermore, Kasai et al. observed a meager 1.4% mass loss when Krytox was heated in the presence of Al2O3 at 200 °C for 6 h.1,2,12,13 It is also important to consider the TGA results from the heated Krytox-coated γ-Al2O3 because these samples were not kept under vacuum following heating; these results show a relatively small amount of water and volatile perfluoropolyether present in the Krytoxcoated γ-Al2O3 sample heated for 20 h. The 19F MAS NMR spectral assignments of heated Krytoxcoated γ-Al2O3 (Table 1, Figure 2) agree with the volatile degradation products observed in previous studies when Krytox was heated in the presence of AlCl3 at 250 °C.1,37 As expected from the measured mass loss when heated, as shown by TGA (Figure 1), the overall 19F MAS NMR spectra decrease in intensity as a function of heating duration, showing the overall loss of 19F from the samples (Figure 2). Kasai et al. have suggested that the degradation mechanism of PFPEs in the presence of Lewis acids is dominated by an intramolecular disproportionation reaction.1,2,9,12,13,26 Krytox, a common PFPE (CF3CF2CF2O[CF(CF3)CF2O]mCF2CF3), is vulnerable to degradation at its pentafluoroethoxy end groups (CF3CF2-O-).1,9,12,26 In the 19F MAS NMR spectra (Figure 2), the loss of peaks at -90.1 and -90.2 ppm confirms the loss of this pentafluoroethoxy end group upon heating. In partially degraded Krytox, degradation occurs at the perfluoroisopropoxy end groups ((CF3)2CF-O-CF2-)).1,9,12,13,26 This is confirmed in the current study by the appearance of 19F MAS NMR peaks at -147.2 and -82.6 ppm, corresponding to the perfluoroisopropoxy end group, after 10 h of heating. In addition, a possible AlF3 species (∼ -174 ppm) is observed after 10 h of heating. However, this resonance is broad and not resolved under these experimental conditions, which is consistent with a range of inhomogeneous structures as well as strong residual dipolar coupling. Both homonuclear (19F-19F) and heteronuclear (1H-19F) dipolar (38) Hoshino, M.; Kimachi, Y.; Terada, A. J. Appl. Polym. Sci. 1996, 62, 207. (39) E. I. du Pont de Nemours & Co. Dupont Krytox Performance Lubricants, 2003.

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couplings will contribute to the width of the resonance because neither interaction was decoupled during the acquisition period. As shown by 27Al MAS NMR (Figure 4), the relative intensities of tetrahedral and octahedral Al in the γ-alumina agree with the study of Lee et al., which found that 70 ( 2% of Al ions occupy octahedral interstitial sites in the face-centered-cubic oxygen structure, with the remaining Al ions in tetrahedral sites.40 Upon heating the Krytox-coated γ-Al2O3, a shoulder peak at -17.5 ppm appears after 10 h of heating and increases in intensity in the next 10 h of heating. By comparison to AlF3 spectra and previous literature assignments, this peak can be identified as an octahedrally coordinated Al(O6 - nFn) (n ) 1, 2, 3) species.31 This coordination is understandable because six-coordinated aluminum fluoride species are more stable than four-coordinate aluminum fluoride species.21 This shoulder peak increases in intensity with increased heating duration, strongly suggesting increased fluorination of the alumina.22 Unfortunately, further resolution of the shoulder peak is not possible at this magnetic field strength; therefore, this new resonance (or resonances) cannot be more carefully studied with one-pulse MAS techniques at this field strength. Unlike the 19F and 27Al MAS NMR methods, which provide direct detection of each of these species, 19F f 27Al CPMAS NMR takes advantage of the fluorine atoms near aluminum atoms for selective acquisition of spectra from transformed aluminum sites. The selectivity arises from the distance-dependent dipolar coupling between nuclei that allows a transfer of polarization between spins in close spatial proximity. The unheated Krytoxcoated γ-Al2O3 does not show any 19F f 27Al CPMAS NMR signal (Figure 5), which indicates that measurable transfer of fluorine species, bonded to surface aluminum sites, does not occur when the Krytox is first adsorbed onto the surface of γ-Al2O3. The accompanying increase in the 19F f 27Al CPMAS NMR signal upon heating Krytox-coated γ-Al2O3 demonstrates the increased formation of Al-F environments in which the fluorine species are selectively associated with the octahedrally coordinated aluminum atoms. At 5 h, a peak appears at 11.8 ppm in the CP spectrum corresponding to VIAl(O6 - nFn) (n ) 1, 2, 3).18,21,31 As the heating duration increases, the 27Al resonances shift to lower frequencies, indicating increased levels of fluorination with heating time.18,31 Overall, these CP spectra suggest the formation of catalytically active Al-F degradation products, which in turn lead to the formation of additional Krytox degradation products, as shown by 19F MAS NMR. The behavior of kaolinite as an acidic medium depends explicitly on the preparation conditions of the kaolinite, the degree of hydration, and whether reactions are considered in aqueous or nonaqueous media. For the experiments described here, no explicit dehydration or acidic pretreatment steps were applied. Krytox was applied directly to the kaolinite particles and then heated to 300 °C. Upon heating, the degradation of PFPE progressed significantly faster than in the alumina system as evidenced by substantial changes (on the order of 6% of the 27Al signal intensity) after 20 h of heating (Figure 7). At shorter reaction times, significant amounts of products were detected by 19F f 27Al CPMAS NMR, and these products increased in overall abundance as the reaction time progressed. Compared to the alumina system, where a group of resonances with a spread of over 20 ppm appeared as the reaction progressed, more distinct resonances, which are most probably associated with two major types of species, are apparent in the spectra from the Krytox-coated kaolinite. The major species, which grows in (40) Lee, M. H.; Cheng, C. F.; Heine, V.; Klinowski, J. Chem. Phys. Lett. 1997, 265, 673.

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at longer times and at the expense of the initially dominant species at shorter times, occurs at a resonance frequency of -15.5 ppm, which matches the resonance frequency of a pure AlF3 phase. We tentatively assign this resonance to an AlF3-like phase that forms as an ultimate product of the reaction of Krytox with the aluminum species from the kaolinite. Complete removal of aluminum from the kaolinite structure and formation of an AlF3 phase would provide a further set of surface sites for the degradation of PFPE. It is also likely that the resonance (at approximately 3 ppm) that dominates at earlier reaction times originates from fluorinated sites on the surface of the kaolinite, similar to those observed in the alumina system but with a shift of resonance frequency based on the presence of silicon species in the next-nearest-neighbor coordination sphere of the octahedrally coordinated aluminum atoms. These aluminum species are still part of the kaolinite surface, whereas the ultimate AlF3 products are not necessarily contained as an integral part of the kaolinite structure. Previous research by means of 19F f 27Al CPMAS NMR has concentrated on systems in which γ-Al2O3 is fluorinated in an effort to increase catalytic activity. In these cases, cross polarization allows for the detection of small quantities of Al-F species in the presence of high fluorine loading or alumina content.18,31 The similarity of the systems allows for the logical extension of this experiment to Krytox-coated γ-Al2O3 and kaolinite, especially when the limitations of one-pulse 19F and 27Al experiments are realized. In the Krytox-coated γ-Al2O3 system, single-pulse 19F spectra provide evidence of the degradation of Krytox through the loss of the pentafluoroethoxy end group. However, there is no distinct proof of the formation of Al-F species because of the high fluorine loading. Single-pulse 27Al experiments suffer from similar problems, as shown by potential Al-F species showing up as merely shoulders. A cross-polarization scheme is a straightforward way to combat this insensitivity to Al-F species. The selective observation of the remaining Al-F species eliminates the uncertainties introduced by high fluorine loading or aluminum peak overlap. In addition, performing the experiment in the solid state provides details about the catalytic sites themselves rather than relying on the degradation products of the PFPE alone as evidence of the catalytic process.

Conclusions Through the use of TGA and MAS NMR, the degradation of Krytox 1506, a common perfluoropolyether, has been detected

Denkenberger et al.

and investigated while adsorbed on γ-Al2O3 and kaolinite surfaces. TGA confirmed an approximate 35% weight loss when Krytox-coated γ-Al2O3 was heated to 300 °C at 10 °C/min. This weight loss can be explained by the evaporation of water and Krytox degradation products. As expected from the measured mass loss of the samples when heated, the overall 19F MAS NMR spectral intensities decrease as a function of heating time. The accompanying increase in the 19F f 27Al CPMAS NMR signal with heating demonstrates the increased formation of Al-F bonding environments on the surface of the alumina. When analyzed in concert, 19F MAS, 27Al MAS, and 19F f 27Al CPMAS NMR show the time-dependent formation of probable VIAl(O 6 - nFn) (n ) 1, 2, 3) species, in which the fluorine atoms are selectively associated with the octahedrally coordinated aluminum atoms. The changes in the peak shapes of the Krytoxcoated γ-Al2O3 CP spectra over time suggest the early formation of potential catalytically active degradation products, such as AlF3, which in turn lead to the formation of additional PFPE degradation products, as shown by 19F MAS NMR. Similar to the alumina system, the kaolinite system also displays new resonances in both 27Al MAS and 19F f 27Al CPMAS spectra after thermal treatment at 300 °C for up to 20 h but reveal a more distinct species, similar to AlF3, at -15.5 ppm that forms at the expense of an initial species (∼3 ppm), which was present in greater abundance at shorter heating times. In this study, 19F f 27Al CPMAS NMR has proven to be an effective tool in investigating the thermal degradation of Krytox on both γ-Al2O3 and the model clay kaolinite through the selective observation of fluorinated aluminum sites. Acknowledgment. We acknowledge Roderick Fry, Alan Benesi, and James Weiss for experimental assistance and both the Pennsylvania State University Department of Chemistry NMR Facility and the Pennsylvania State University Materials Characterization Laboratory for the use of instrumentation. We also acknowledge financial support from the Alfred P. Sloan Foundation and the National Science Foundation through the Pennsylvania State University Materials Research Science and Engineering Center (DMR-008019 and DMR0213623). Kerri Denkenberger was supported by a Pennsylvania State University Teas Summer Research Scholarship and a Pennsylvania State University Center for Environmental Chemistry & Geochemistry Summer Research Fellowship. LA7004453