Energy & Fuels 1998, 12, 399-408
Solid State
399
13C
and 19F NMR Characterization of Fluorinated Charcoal
Edward W. Hagaman* and David K. Murray Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6201
G. D. Del Cul Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6201 Received September 2, 1997. Revised Manuscript Received November 14, 1997
The preparation of CFx by elemental fluorination of charcoal was studied using solid state 13C and 19F NMR spectroscopy. 19F-13C CP/MAS NMR experiments determine the extent of fluorination vs reaction temperature. Three types of carbon species observed over the temperature range -80 to 350 °C were assigned to graphitic carbon (C), CF, and CF2 on the basis of chemical shift. These assignments were confirmed by measurement of cross polarization and dipolar dephasing time constants, TCF and TDD, respectively. The fluorinated carbons fully cross-polarize in tenths of milliseconds, while polarization transfer among graphitic carbon is slower and is explained by a two-component model. One component, with TCF less than 1 ms, is assigned to sp2 carbons adjacent to fluorinated carbons, viz., interfacial graphitic carbon. The other component, with TCF on the order of milliseconds, is assigned to more remote carbon species, viz., bulk graphitic carbon. The concentrations of CF and CF2 found in the 19F-13C CP/MAS NMR experiments are confirmed by direct measurement of the 19F NMR spectrum. NMR results are presented along with gravimetric and XPS results to provide new insight into fluorocharcoal structure. An average platelet size of 2-4 nm for the fully fluorinated charcoal is derived from these measurements and is proposed as representative of the graphitic carbon platelet size of the charcoal. The fluorination of charcoal is initiated by addition of fluorine to the surface of platelets. At the lowest fluorination temperature, -80 °C, a substance of formula CF0.16 is produced which is diamagnetic, as is CF1.1-1.2, carbon monofluoride, the white end-product from complete fluorination (350 °C). The low free electron density in these materials stands in stark contrast to that of the charcoal and CFx samples prepared at intermediate temperatures.
Introduction The fluorination of organic substances with dilute elemental fluorine can achieve highly selective fluorinations in which unactivated tertiary Csp3H bonds are converted into Csp3F bonds, with the remainder of the molecule unaffected.1,2 Alternatively, the fluorination may be accomplished in such a manner that the perfluorohydrocarbon is produced by exploiting both the substitution of F for H in CH bonds (CH f CF) and the addition of fluorine to unsaturated bonds.3 This conversion can be effected by conducting the fluorination at low temperature and low F2 concentration and then gradually increasing the reaction severity as the fluorination proceeds. This reaction was exploited to prepare a series of related fluorocarbons, CFx, 0.16 < x < 1.2, from charcoal. Charcoals are carbonaceous solids which serve as raw materials for the production of graphites and activated carbons. The basic structural units of these materials (1) Purrington, S. T.; Kagen, B. S.; Patrick, T. B. Chem. Rev. 1986, 86, 997-1018. (2) Wilkinson, J. A. Chem. Rev. 1992, 92, 505-519. (3) Huston, J. L.; Scott, R. G.; Studier, M. H. Fuel 1976, 55, 281.
are microcrystallites composed of stacked sp2 carbon platelets. Graphite consists of microcrystallites composed of parallel-oriented platelets of fused trigonally hybridized carbon, as characterized by X-ray diffraction.4 Microcrystallites in graphite are composed of 1-10 nm diameter platelets separated by 0.335 nm.5 Charcoals exhibit disorder in both microcrystallite orientation and platelet composition. This heterogeneity is partly responsible for the wide range in physical characteristics observed for different charcoals, including differences in density, hardness, porosity, and ease of conversion to graphite. Fluorine incorporation into highly crystalline graphites, carbons with low orders of crystallinity (petroleum coke, activated carbons, carbon blacks, and pitch), and fullerenes6 has been reviewed.7,8 Partial fluorination (4) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: New York, 1984; p 304. (5) Jankowska, H.; Swiatkowski, A.; Choma, J. Active Carbon, Series in Physical Chemistry; Ellis Horwood: New York, 1991; Chapter 3. (6) Halloway, J. H. In Fluorine-Carbon and Fluoride-Carbon Materials; Nakajima, T., Ed.; Marcel Dekker Inc.: New York, 1995; Chapter 6. (7) Watanabe, N.; Nakajima, T.; Touhara, H. Graphite Fluorides; Elsevier: Amsterdam, 1988; Chapter 2.
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400 Energy & Fuels, Vol. 12, No. 2, 1998
products display useful electrochemical properties, which have fueled research into the nature of C-F bonding.9-11 XPS results indicate that the C-F bond changes from ionic to “semi-ionic” to covalent with increasing fluorination.12,13 “Semi-ionic” is a functional definition that corresponds to XPS resonances with ionization energies between those characteristic of covalent (Teflon) and ionic bonds (LiF). Carbon monofluoride (CFx, where 1 < x < 1.25) is produced if controlled fluorination of these carbonaceous materials is carried to completion. CFx is a solid fluorocarbon reported as a lubricant and an excellent cathode material for Li batteries.14 It has been produced by F2 reaction with graphite, a process which yields various nonstoichiometric products (CF0.6-1.25).15 Carbon monofluoride is composed of platelets much like graphite, but the platelets are more widely spaced (0.57 nm) to accommodate fluorine substituents and the carbon hybridization change. A second compound (C2F) with unique structural features as determined by X-ray diffractometry and XPS is formed from highly crystalline graphites under relatively mild fluorination conditions.16 Variations in CFx stoichiometry, structure, and physical properties are possible through selection of charcoal or graphite precursors or preparation conditions. In this contribution, we describe the progressive change in the statistical structure of a charcoal as it is converted to CFx by reaction with elemental fluorine. Gravimetric and XPS analyses are used with 19F-13C cross polarization (CP)/MAS 13C NMR17 and 19F MAS NMR spectroscopies18,19 to investigate fluorocharcoal structure and the fluorination process. Experimental Section Preparation of CFx. CFx was prepared by fluorination of coconut-based activated charcoal (6 × 16 mesh size) provided by Calgon Carbon Corp. According to the manufacturer’s data, the apparent density, particle density (Hg displacement), and real density (He displacement) of this charcoal are 0.44, 0.85, and 2.2 g/cm3, respectively. This compares to a real density of 2.26 g/cm3 for graphite.4 A large portion of the micropore volume in the charcoal consists of pores in the range 1.5-2.0 nm and a system of macropores larger than 100 nm. Total surface area (N2, BET method) is 1200 ( 50 m2/g. Ash content is 2.62%, most of which is iron (2.1%). Fluorine gas having a purity of 97% was obtained from Air Products. The concentration of hydrogen fluoride (HF) was determined to be about 30 ppm using an IR gas cell fitted with zinc selenide (ZnSe) windows. No attempt was made to further purify the fluorine (8) Watanabe, N.; Nakajima, T. Graphite Fluorides and CarbonFluorine Compounds; CRC Press: Boca Raton, FL, 1990; Chapters 1-2. (9) Mallouk, T.; Bartlett, N. J. Chem. Soc., Chem. Commun. 1983, 103-105. (10) Mallouk, T.; Hawkins, B. L.; Conrad, M. P.; Zilm, K.; Maciel, G. E.; Bartlett, N. Philos. Trans. R. Soc. London A 1985, 314, 179187. (11) Nakajima, T.; Touma, M. J. Fluorine Chem. 1992, 57, 83-91. (12) Matsuo, Y.; Nakajima, T. Z. Anorg. Allg. Chem. 1995, 621, 1943-1950. (13) Palchan, I.; Crespin, M.; Estrade-Szwarckopf, H.; Rousseau, B. Chem. Phys. Lett. 1989, 157(4), 321-327. (14) Nakajima, T.; Kawaguchi, M.; Watanabe, N. Electrochim. Acta 1982, 17(11), 1535-1538. (15) Kamarchik, P.; Margrave, J. L. Acc. Chem. Res 1978, 11, 296. (16) Kita, Y.; Watanabe, N.; Fujii, Y. J. Am. Chem. Soc. 1979, 101, 3832-3841. (17) Hagaman, E. W.; Burns, J. H Fuel 1993, 72, 1239. (18) Harris, R. K.; Jackson, P. Chem. Rev. 1991, 91, 1427-1440. (19) Hagaman, E. W.; Lee, S. K. Energy Fuels 1995, 9(5), 727-734.
Hagaman et al. Table 1. Concentrations (%) and Ratios of Carbon Components in Fluorinated Charcoal As Determined by 19F-13C CP/MAS 13C NMRa carbon component b
Cb Cic C ) Ci + Cb CF CF2d Ci/CF CF2/CF F/C by NMR F/C by weight
fluorination temperature (°C) -80 57 27 84 16
0 24 39 64 36
23 26 34 60 40
65
19 33 52 42 6 1.63 1.08 0.84 0.79 0.15 0.16 0.36 0.4 0.54 0.27 0.32 0.38 0.45
120
180
250
20 33 53 42 5 0.8 0.11 0.51 0.5
0 18 18 73 9 0.25 0.13 0.91 0.77
0 6 6 77 16 0.08 0.21 1.1 0.91
350 0 0 0 80 20 0.25 1.2
a % composition figures are normalized I data from curve fits 0 of variable contact time data to eq 1. b Cb designates bulk graphitic carbon two or more bonds from CF sites. c Ci designates graphitic carbon at the interface of CF regions. d not resolved from graphitic carbon at low preparation temperatures.
stream. The preparative manifold has a titanium getter heated at 450 °C which is routinely used to remove water, oxygen, and nitrogen impurities from the helium stream used to dilute the fluorine gas. A suite of eight CFx samples was prepared over the temperature range -80 to 350 °C. In this paper these materials are designated CFx(T °C) where T represents the fluorination temperature. The fluorine to carbon ratio (gravimetric and NMR) for each material is listed in Table 1. The materials were prepared using the following method. Ten to fifteen gram batches of charcoal were loaded into a passivated-nickel U-tube reactor (1/2" o.d.) having Monel bellows vacuum valves at each end. The charcoal was preconditioned by heating to 200-250 °C under flowing helium (99.99% from Air Products). The reactor was then fully immersed in a bath to maintain the desired fluorination temperature. Reactions below ambient temperatures were carried out in a methanol/dry ice bath (-78 °C) or in a water/ice bath (0 °C). A thermostatic bath was used to heat the reactor to elevated temperatures. At the highest temperatures, a hollow tube furnace was used. To minimize formation of gaseous products and to better control the highly exothermic reaction, dilute fluorine gas was used. A 5 vol % F2/He gas mixture was prepared in 3 L batches in a passivated-nickel tank. Dilute fluorine was introduced into the reactor at a very low flow rate. A temperature difference of 0.4 nm from the nearest fluorine are likely, and these will not cross polarize fully before the 19F source magnetization decays. The loss of 19F magnetization from the spin lock field, i.e., 19F spin lattice relaxation in the rotating frame, is characterized by the exponential time constant 19F T1F. This decay process is monitored indirectly by the 13C signal intensity decay observed as a function of cross polarization contact time, τ, in 19F-13C CP/MAS experiments. In materials with a wide dispersion in TCF, with long TCF comparable to T1F (which itself may be multivalued), relative signal intensities in a single spectrum will not reflect atomic ratios. The case applies for all fluorocarbon materials in this study that contain a significant graphitic carbon phase. The long TCF associated with this phase virtually guarantees that the condition for correct intensity response in a single experiment, 19F T1F . τ . TCF, is not met. Relative intensities that reflect atomic ratios, I0, can be obtained by fitting signal intensity, I, recorded as a function of τ to eq 1, yielding I0, TCF, and T1F as adjustable parameters.25
I ) I0
(
)
T1F (e-t/T1F - e-t/TCF) T1F - TCF
(1)
Variable contact time (VCT) experiments were required for all samples except those prepared at T g 250 °C. Fitting is (20) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1972, 56, 1776. (21) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569. (22) Alla, M.; Lippmaa, E. Chem. Phys. Lett. 1976, 37, 260. (23) Opella, S. J.; Frey, M. H. J. Am. Chem. Soc. 1979, 101, 5854. (24) Alemany, L. B.; Grant, D. M.; Alger, T. D.; Pugmire, R. J. J. Am. Chem. Soc. 1984, 105, 6697. (25) Mehring, M. High-Resolution NMR Spectroscopy in Solids; Springer-Verlag: Berlin, 1976; Chapter 4.
Figure 1. 19F-13C CP/MAS NMR spectra of charcoal fluorinated at various temperatures. Typically 20 000 scans were signal averaged using a 2.5 ms contact time and a 2 s recycle delay. See text for assignments. accomplished by least-squares minimization. Typically, 2540 contact times were used for each fluorinated charcoal sample, varying contact times between 40 µs and 60 ms. The Hartmann-Hahn CP match condition was optimized at an intermediate contact time, usually 10 ms. Tuning was checked before and after a VCT series to ensure that spectra throughout the series were obtained under match conditions. The I0 data from the fit, summarized in Table 1, is reported for each carbon type as a percentage of total carbon. The precision of the composition reported is on the order of (10%. XPS Analysis. XPS data were obtained using a PHI (Perkin-Elmer) 5000 series XPS spectrometer equipped with a dual anode (Al KR: hν ) 1486.6 eV and Mg KR: hν ) 1253 eV). The Al anode was utilized at a power of 400 W (15 kV). The instrument was operated in the fixed analyzer transmission (FAT) mode with a pass energy of 17.9 eV for highresolution scans. Fluorinated charcoal samples were ground to a fine powder having uniform consistency. Double-sided tape was mounted onto a sample holder. The powder was then applied so that it uniformly and completely covered the tape. The holder was then mounted for study. Pressure was
