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Contribution to the Isolation and Characterization of Buckminsterfullerenes Moustapha Diack,tJ R. L. Hettich) R. N. Compton,t*land Georges Guiochon'VtJ Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996-1501, Division of Analytical Chemistry, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -6120, and Chemical Physics Section, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6125
Buckmlnsterfullerenesare extractedfrom samples of the soot generated by a high-energy electric arc discharge between two graphite electrodes. A rapld and quantitative procedure basedon uitrasonk extractionwas developed. I t permitsthe evaluation of adjustments made In the different parameters involvedIn the synthesis process by relatlngthem to the ma88 of the fractions obtained. Purified sample8 of COO, CTO,and Cu are obtained by high-performancellquld chromatography, using chemkally bonded Cr dllca as the datlonary phase and Mexane as the mobile phase. The purified fullerenes were identified by laser desorption Fourier-transform ma88 spectrometry and verified by UV-vldble spectroscopy. The rewits are consistent with those prevloudy reported.
INTRODUCTION The separationand characterizationof the newly discovered third allotropic form of carbon,' known as buckminsterfullerenes or fullerenes, has become increasingly important since the report2of the contact-arc method of generatinggram quantities of these materials. This is demonstrated by the large number of publications relating their unique structure,lVz4stability,5b6and physical properties.'-15 There is also much speculation about their different potential applications.16 Some even anticipate a revolution in chemistry, harkening back to the discovery of benzene in 1825.16 The future development of fullerene chemistry and its applications depends, in part, upon two developments (1) the optimization of the different parameters involved in the synthesis process, e.g., the high current discharge2and the ~
t University of Tennessee. 8 Division of Analytical Chemistry, Oak Ridge National Laboratory. 1 Chemical Physics Section, Oak Ridge National Laboratory.
(1) Kroto, H. W.; Heath, J. R.; OBrien, S. C.; Curl, R. F.; Smalley, R.
E. Nature 1985,318,162.
(2) Kratachner, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990,347, 354. (3) Ben-Amotz,D.;Cooks,R. G.;Dejme,L.; Gunderson, J. C.;Hokell, S.H.; Kahr, B.; Payne, G. L.; Wood, J. M. Chem. Phys. Lett. 1991,183, 149. (4) Manolopoulos,D. E.J. Chem. Soc.,Faraday Trans. 1991,87,2861. ( 5 ) Newton, M. D.; Stanton, R. E. J.Am. Chem. SOC.1986,108,2469. (6) Luthi, H. P.; Almlof, J. J. Chem. Phys. Lett. 1987,135, 357. (7) Satpathy, S. Chem. Phys. Lett. 1986,130, 545. (8) Haddon, R. C.; Brus, L. E.; Raghavachari, K. Chem. Phys. Lett. 1986,125,459. (9) Larson, S.; Volosov, A.; Rosen, A. Chem. Phys. Lett. 1987,137,501. (10) Wu,Z.C.; Jelski,D.A.;George,T.F. Chem.Phys.Lett.1987,137, 291. (11)Stanton, R. E.; Newton, M. D. J. Phys. Chem. 1988'92, 2141. (12) Fowler, P. W.; Lazzereti, P.; Zanasi, R. Chem. Phys. Lett. 1990, 165,79. (13) Haddon, R. C.; Elser, V. Chem. Phys. Lett. 1990, 169, 362. (14) Saxby, J. D.; Chatfield, S. P.; Palmisano, A. J.; Vassallo, A. M.; Wilson, M. A.; Pang,L. S. K. J. Phys. Chem. 1992,96, 17. (15) Coulombeau, C.; Jobic, H.; Bemier, P.; Fabre, C.; Schutz, D.; Rassat, A. J. Phys. Chem. 1992, 96,22. (16) Edelson, E. Pop. Sci. 1991, August, 51. 0003-2700/92/0364-2 143$03.00/0
laser vaporization'J7 processes, to ensure a sufficiently largescale production and a high selectivity; (2) the development of rapid and reliable analytical methods to evaluate and control the different processes in terms of conversion yield (fraction of the graphite used converted into fullerenee) and of production yield of a given cluster (fraction of the graphite used converted into a given carbon cluster) and to separately characterize the products. For the electrical discharge process, the conversion yield and the selectivity between the different fullerenes depends on the quality and size of the graphite rod, the pressure between the rods, the intensity of the electrical current, and the nature of the buffer gas used.18Jg The selectivity of synthesis is an important parameter to study because of the large number of possible stable, closed polyhedral structures found from CZOup through CZ~O.~O A wide variety of values of the global yield of fullerenes has been reported. This variation might be related primarily to the diversity of the techniques used by the different groups working in this field to evaluate the conversion yield. Soxhlet extraction with a selective solventz1@is by far the most frequently used method for the extraction of the fullerenes from the sootlike product generated by either the discharge or the laser vaporization process. However, the results of a soxhlet extraction depend on the experimental conditions, and the extraction is time consuming. Indeed, to achieve a quantitative extraction with this method, it is necessary to carry out the procedure during several hours (5-24 h). Total elimination of the solvent is generally a difficult task, especially when toluene or chloroform is used. Thus, the determination of an accurate extraction yield with the Soxhlet method is difficult. Other methods, using direct dispersion of the soot in benzene2 or in boiling toluene19 or sublimation at 400 "C under vacuum or in an inert atmosphere,z were proposed in the literature but neither the quantitative aspect of the proposed methods nor their convenience or speed was discussed. The complete separation of the fullerenes seems to be another challenging task, as previously noted.l9 A number of attempts have been made to purify useful quantities of the different clusters by liquid chromatography, which is the obvious choice for the separation of these complex mixtures. (17) Rohlfing, E. A.; Cox, D. M.; Kaldor, A. J. Chem. Phys. 1984,81, 3322. (18) Cox, D. M.; Behal, S.; Disko, M.; Gorun, S. M.; Greaney, M., Hsu, C. S.;Kollin, E. B.; Millar, J.; Robbins, J.; Bobbins, W.; Shemood, S.D.; Tindall, P. J. Am. Chem. SOC.1991,113,2940. (19) Ajie, H.; Mvarez, M. M.; A m , 5. J.; Beck, R. D.; Diederich, F.;
Fostiropoulos, K.; Huffman, D. R.; Kratachner, W.; Rubin, Y.;Schriver, K. E.; Sensharma, D.; Whetten, R. L. J. Phys. Chem. 1990,94,8630. (20) Klein, D. J.; Seitz, W. A.; Schmalz, T. G. Nature 1986,323,703. (21) Hare, J. P.; Kroto, H. W.; Taylor, R. Chem. Phys. Lett. 1991,177, 394. (22) Haufler, R. E.; Conceicao, 3.; Chibante, L. P. F.; Chai, Y.;Byme,
N. E.; Flanagan, S.; Haley, M. M.; OBrien, S. C.; Pan, C.; Xiao, Z.;Billups, W. E.; Ciufolini, M. A,; Hauge, R. H.; Margrave, J. L.; Wilson, L. J.; Curl,R. F.; Smalley, R. E. J . Phys. Chem. 1990,94, 8634. Q 1992 American Chemical Soclety
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Charge-transfer complex chromatography using N,N-dinitrobenzoylphenylglycine (DNBPG)23or dinitroanilinopropyle (DNAP)l9 as complexing reagents are so far the most interesting approaches. Even though these phasesgive a high selectivity (a= 2.25 between Cm and C7oZ3),the analysis time is long (25min for the elution Of C70) and the chromatographic phase system is costly. The separation of Cm and CTO can be achieved by normalphase chromatography, in either the gravity19~23-25 or the HPLC mode.19~21 In this latter case, some authors2J* have suggested that, prior to the chromatographic separation, the polycyclic aromatic hydrocarbons (PAH) present in the soot should be eliminated by a diethyl ether washing. These techniques are poorly reproducible, and generally require multistep separations to achieve a reasonable degree of purification.% Also C70 seemsto be inherently unstable to prolonged exposure to alumina.21 However, chromatographic purification of C60, C70, and C76 on alumina was accomplished, or a multistepz6approach. using either a Finally, a differential sublimation technique was also employed2 as an alternative to the chromatographic purification of Cm and C~O. The authors made no mention, however, of the quantitative aspects of the recovery nor of the purity of the separated fractions. This paper represents an alternative approach to the separation and purification of the buckminsterfullerenes. We describe a rapid and quantitative extraction technique based on the use of an ultrasonic bath at room temperature, without any further treatment of the sootlike material, and a onestep purification of the fullerenes CWand c70 by nonaqueous reversed-phase chromatography, using octadecyl silica as the stationary phase and n-hexane as the mobile phase. Purified fractions were examined by laser-desorption Fourier-transform mass spectrometry and UV-visible spectroscopy for fullerene characterization.
EXPERIMENTAL SECTION Origin of the Samples. The first sample of homemade soot (sample 1)was prepared in our laboratoryn using the contactarc technique described earlier.2 It is obtained by evaporating graphite rods (Poco graphite) using a contact-arc discharge (ca. 80 A at 40 V), in a helium atmosphere (200Torr). Samples 2 and 3 were provided respectively by TFC (Texas Fullerene Corp., Houston, TX) and by MER (Tucson, AZ) in early 1992. Sample 4 was homemade, using the same equipment but pumping out all gases from the reactor for several hours before switching on the current. In the spring of 1992 a modified equipment was built, using essentially the same l/,-in. POCO graphite rod assembly with a 3-mm arc gap, but placed in a much larger vacuum chamber and operatedwith an ac current of 80A, under a static helium pressure of 100 Torr. Two samples of homemade soot generated in this new instrument were extracted, one of them using two different techniques (samples 1-3). A more recent sample was received from TFC (sample 4). Finally, a soot sample (sample 5) was kindlysupplied by Nori Shinohara (Mi6University, Tsu, Japan). The experimental conditions were '/Z-in. graphite (99.997% C) rods with an arc gap of 3 mm, operated with a dc current of 200 A, under a static helium pressure of 50 Torr. Extraction Procedure. The first series of soot samples (10 g) were dispersed in tetrahydrofuran (THF) (10 mL/g) at room (23) Hawkins, J. M.; Lewis, T. A.; Loren, S. D.; Meyer, A.; Health, J. R.; Shibato, Y.; Saykalli, R. J. J. Org. Chem. 1990,55, 6250. (24) Diederich, F.; Ettl, R.; Rubin, Y.; Whetten, R. L.; Beck, R.; Alvarez, M.; A m ,S.; Sensharma, D.; Wudl, F.; Khemani, K. C.; Koch, A. Science 1991, 252, 548. (25) Taylor, R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H. W. J. Chem. SOC.,Chem. Commun. 1990,1423. (26) Etti, R.; Chao, I.; Diederich, F.; Whetten, R. L. Nature 1991,353, 149. (27) Hettich, R. L.; Compton, R. N.; Ritchie, R. H. Phys. Rev. Lett. 1991,67, 1242.
temperature and submitted to sonication in an ultrasonic bath (Branson No. 52000,Danbury, CT) for 20 min. THF was selected as the extraction solvent because it is used successfullyto extract quantitatively polycyclic aromatic hydrocarbons from various coal samp1es.a After Titration of the crude extract, the undissolved material on the filter was washed repeatedly with THF until the fitrate became clear. The resulting solution together with the solvent used for the washing were collectedand treated with a rotary evaporator at 50 'C to eliminate the solvent. Benzenewas then added to remove as an azeotrope the remaining moisture resulting from the use of THF. The total extract is recovered in methylene chloride, as a brown-red solution and is dried under vacuum until a constant weight is obtained. The second series of samples was extracted by following the same procedure (sonication,filtration,washing of the precipitate, and concentration of the filtrate by evaporation), but replacing THF by boiliig toluene. The higher fullerenes are thought to be more soluble in toluene. As shown in Figure 2b, the recovery yield with toluene is slightly higher than with THF. Equipment. The chromatographic analyses were carried out with an HP1090 liquid chromatograph (Hewlett-Packard, Palto Alto, CA), equipped with a multisolvent delivery system, an automatic sample injector, a UV photodiode array detector, and a computer data acquisition station. The separated fractions were recovered using a Gilson 203 fraction collector (Middleton, WI). The mass spectra of the different HPLC fractions were obtained with an Extrel FTMS-2000 Fourier transform mass spectrometer (FTMS),equiped with a NdYAG laser. The fourth harmonic of the laser (266 nm) was used at a power density of approximately 10s W/cm2to desorb and simultaneously ionize the fullerenes. Negative ions were monitored in the FTMS. Once the ions were formed, they were trapped, manipulated, and ultimately accurately measured in the ion cell of the mass spectrometer. Details for mass measurements, resolution, and other experimental parameters have been reported.n Columns and Solvents. A 25-cm-long,4.6-mm4.d. column was used for the preparative separations. It was packed in our laboratory,with8.9-pm IMPAQ RG 2010C&lica (ThePQCorp., Conshohoken,PA), using the slurry technique (methanolat 6OOO psi). The column void volume (2.8 mL) was measured using n-heptane. The column efficiency (2000 plates) was measured using a standard injection of dilute Cm. The 15-cm-long,4.6-mm4.d. analytical column (Econosphere CISsilica, 3-pm particles, ALLTECH, Deerfierld, IL) has a void volume of 1.3 mL and an efficiency of 8300 plates. The solvents used were THF and methylene chloride, purchased from J. T. Baker (Phillipsburg, NJ), and n-hexane, purchased from American Burdick and Jackson (Muskegon, MI). Procedure. The fractionation experiments were carried out with the solution obtained by dissolving the sonication extract of 150 mg of the original sample in 2 mL of methylene chloride. Fractions (100pL) of this solutionwere injected in the preparative column. With such a sample amount,the column is slightlymore overloaded than would correspond to touching-band conditions,B and the resolution between the bands of the mixture components is incomplete (Figure 1). During the elution of the partially separated mixture, 20 fractions were collected on a time basis, one every 188. We show in Figure 1the chromatogram obtained on the preparative column, with the cutting points of the different fractions collected. After reinjection in the analytical column to ascertain their exact composition, the similar fractions were pooled together into six main fractions of different compositions, as indicated in the figure. All of the experiments were conducted at ambient temperature. Pure n-hexane was used as the mobile phase, at flow rates of 1mL/min with the preparative column and 1.25 mL/min with the analytical column. Detection was performed by monitoring light absorption at 260 nm. ~~
(28) Besson, M.; Bacaud, R.; Charcosset, H.; Cebolla, V. L.; Oberson, M. Fuel Process Technol. 1986, 12 91. (29) Golshan-Shirazi, S.; Guiochon, G. J. Chromatogr. 1990,517,229.
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time (min) Figure 1. Reparative chromatogram of the whole soot extract (sample 1). CondMns: injection volume, 100 p4 of the soiutlon obtained by dissolving 150 mg of soot extract in 2 mL of methylene chloride. mobile phase flow rate, 1 mL/min (+hexane); UV detection at 260 nm.
RESULTS AND DISCUSSION We compare in Figure 2a the total concentration of the soluble fullerenes extracted from four samples of graphitic soot, as measured with the first extraction scheme described above. These samples are representative of the production achieved with the mid-1991 manufacturing technology of fullerenes. The residues of the THF extraction of these samples were submitted to the classical extraction using boiling to1~ene.l~ No measurable amount of fullerene was further extracted. We note the similarity between the yields in samples 1 and 3 (conversion = 1.4% soot) compared to sample 2 (1.1%).This difference is likely related to the different parameters of the process of soot generation and shows a clear advantage of the former two samples in terms of conversion yield. Prolonged ultrasonic irradiation (several hours) of a Cm solution resulted in no losses and seems to be harmless to this component. The markedly better yield obtained with sample 4 (1.9%) was attributed to the elimination from the reaction area of a large fraction of the hydrogen and water vapor known to exist in graphite samples left in air. These results are in good agreement with the observation previously reported by Kratschner et al.,30 Le., 1.1% More recent studies claim a much higher conversion rate (8-34 % ) after process optimization.1g121-24 The data summarized in Figure 2b illustrate the dramatic improvement in production yield achieved at the beginning of 1992, as a result of the number of changes made in the manufacturing technology. Different from previous reports, all the samples have been extracted with a conventional and reliable method, solving the known difficulties encountered in the complete elimination of the extraction solvents, which can be a source of overestimation of the apparent yield.= Also, the recovery yield is based on the weight of the material actually extracted, not on a concentrationdeterminedby optical absorptionwhich is unreliable in the case of complex mixtures. It should also be noted that all of the material created in the contact-arc
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(30)Kratschner,W.;Fostiropoulos,K.; Huffman,D.Chem.Phys.Lett. 1990,170, 167.
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