Purification of C60 by fractional crystallization

J. Phys. ChemSolids Vol 58, No. ll,pp. 1839-1843, 1997

Pergamon

PURIFICATION OF

PlI: S0022-3697(97)00094-2

C60

C 1997 Elsevier Science Lid Printed in Great Britain. All rights reserved 0022-3697/97 $17.00+ 0,00

BY FRACTIONAL CRYSTALLIZATION

R.J. DOOME, A. F O N S E C A , H. RICHTER, J.B. NAGY, P.A. THIRY and A.A. L U C A S Institute for Studies in Interface Sciences, Facult6s UniversitairesNotre-Dame de la Paix, 61 rue de Bruxelles, B-5000 Namur, Belgium Abstract--We describe a new and inexpensive method of purifying C6o by fractional crystallization in 1,3diphenylacetone. After three steps, 99.5% pure C6o was obtained with a global yield of 69%. This method is simpler and could be cheaper than any other methodpresently used. Furthermore,the purity of C 6ofullerene can be increasedup to 99.99% by adsorptionof the residualC 70fullerene on charcoal. The temperaturedependenceof the solubility of C6oand C7oin 1,3-diphenylacetonewas studied and a maximum of solubility was observed at 136°(2 for C6o and at 41°C for C70. This peculiar solubility behaviour is discussed in the light of possible aggregation of C6o and C70 molecules in the solution phase. © 1997 Elsevier Science Ltd.

Keywords:fullerenes, C6o, solubility, crystallization 1. INTRODUCTION Fullerenes are usually prepared by evaporating graphite rods in arc-discharge reactors under a helium atmosphere, producing a soot that contains C6o, C70, higher fullerenes and uncharacterized insoluble material [1-3]. The soot is then stirred with benzene or toluene to extract the soluble compounds. The most common methods for the separation of C6o and C7o are column liquid chromatography carded out on charcoal [4], polystyragel [5, 6] or alumina [7]. Among these stationary phases, activated charcoal is very efficient and inexpensive since it uses pure toluene as eluent, but the drawback of this successful method is the irreversible adsorption of some C6o and nearly all of the C70 and higher fullerenes. Neutral alumina is less suitable for the purification of fullerenes in gram quantities since it is time and solvent consuming. Ultrastyragel columns are very expensive and present the disadvantage that they are not easily scaled up. In the purification procedure of C 60by fractional crystallization in 1,3-diphenylacetone described in this paper, there are no losses because all the remaining fullerenes are recovered in the mother liquor and the solvent is salvaged. We have also investigated the temperaturedependent solubility of C6o and C7o in 1,3-diphenylacetone and compared our results with those of the literature.

2. EXPERIMENTAL As-produced fullerene soot was purchased from MER (Tucson, AZ). Soxhlet extraction using toluene as the solvent yielded a mixture of fullerenes containing 84 wt% C60, 15 wt% C70 and 1 wt% higher fullerenes. The C~o purity was determined by high performance liquid chromatography. HPLC analyses were performed

at 35°C on a Waters System 600 chromatograph equipped with a Buckyclutcher I, Trident tri-DNP column (4.6 x 250 mm; Regis). The detection was performed with U V visible light at 330 nm. Toluene/2-propanol (1:1 v/v) was used as the mobile phase and the flow was set to 1 mlmin -l. The solubility measurements were also determined by HPLC under the same conditions. The eventual co-crystallization of the solvent with solid C6o was investigated by MAS-IaC-NMR using a Bruker CXP-200 spectrometer operating at 50.3 MHz.

3. PROCEDURE FOR PURIFYING Cse Solubility measurements of a C60/C70 mixture (84/16) performed in various organic solvents revealed that 1,3diphenylacetone is a very good solvent for fullerenes. 1,3-diphenylacetone is not a usual solvent since it is a solid at room temperature (melting point: 33°C). However, this solvent contains 2% of ethanol and the presence of such impurities results in a liquid at 20°C. 1,3-diphenylacetone was tested as a solvent since it appears that fullerenes are more soluble in aromatic solvents [8, 9]. Moreover, C6o is less soluble than C7o in this solvent. The solubility of C6o and C70 in 1,3diphenylacetone was determined at room temperature. A saturated solution of C6o and C7o in 1,3-diphenylacetone was filtered on a 0.45 #m polytetrafluoroethylene (PTFE) filter (Gelman Acrodisc). The solubilities measured by HPLC are 1.40 mg ml -~ for C60 and 2.20 mg ml -l for C7o. For purifying C6o, 1 g of the crude fullerene soot extract (containing ca. 84 wt% C6o, 15 wt% C7o and 1 wt% higher fullerenes), prepared as previously described, is completely dissolved in toluene (500 ml) by sonication for 30 min at room temperature. Then 1,3diphenylacetone is added--30% in excess relative to the

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R.J. DOOME et al.

C6o

1.20

(a) 1.00

0.80 0.60 0.40 020

0.00

C60

1.60 1.40 1.20

1.00 0.80 0.60 0.40 0.20 0.00 C 1.20

(c)

1.00 0.80 0.60 0.40 0.20 0.00 I

I

I

0.00

I

I

I

I

I

I

20.00

10.00 Elution time (rain)

Fig. 1. HPLC chromatogramsof (a) crude fullerenes extract, (b) solid obtained after the first crystallization,(c) solid recovered after the third crystallization.Column: BuckyclutcherTrident tri-DNP (250 × 4.5 ram; Regis); mobile phase: toluene/2-propanol= 1/1 v/ v; X = 330 nm. C7o solubility (73 ml)--and the solution is stirred for I h. Toluene is slowly evaporated in a rotavapor and the remaining solution is heated at 110°C for 1 h. The solid deposit is allowed to equilibrate with the solvent for about 24 h at room temperature, and this solution is filtered through a sintered-glass frit (4/zm) and washed with n-hexane. Both the deposit and the mother liquor are

analysed by HPLC. In one recrystallization, the C6o content increased from 84% to 95.3% while the C7o content in the mother liquor increased from 16% to 51%. To obtain 99.5% pure C~o, two further recrystallization steps are needed. The global yield was 69%. The progress of the purification in the three steps is shown in Table 1 and illustrated in Fig. 1. The 13C-NMR

Purification of C6o by fractional crystallization

filtered on a PTFE membrane, and the solutions were analysed by HPLC. Fig. 2 illustrates the relation between the purity level (expressed in wt%) and the yield of C60 recovered (expressed in %) as a function of the amount of Norit-A per mg of initial C6o. It must be pointed out that all of the fullerenes caught inside the pores of the Norit-A are strongly adsorbed. It is quite impossible to recover the imprisoned fullerenes, even by Soxhlet extraction using toluene as solvent.

Table 1. The deposit fraction (%) and its C 6ocontent (%) at each step of crystallization Recrystallization starting material step 1 step 2 step 3

Deposit per step(%)

Purity of Ct0(wt %)

86.1 89.2 89.7

84.3 95.3 97.7 99.5

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measurements confirm the high purity of the C6o recovered after three steps and the absence of co-crystallized 1,3-diphenylacetone. Further crystallization steps did not increase the C6o purity. The remaining impurities were essentially C70 and some oxides such as C6oO or C700, but there were no higher fullerenes. The mother liquors are conserved and concentrated for further studies on the isolation of higher fullerenes. After concentrating and recrystallizing these mother liquors three more times, we obtained a solution containing a large amount of higher fullerenes such as C76, C84, C92 . . . . . detected by HPLC and mass spectrometry. In order to attain the 99.99 + % purity level for the C6o, we also investigated an additional method of purification. It is well known that activated charcoal with specific porosity (Norit-A) can adsorb the C7o irreversibly while the C6o fullerene is partially retained in the pores of the Charcoal [4, 10]. Thus, different amounts of Norit-A were added to a solution of 97% pure C6o in toluene. The samples were stirred at room temperature for 15 min,

4. TEMPERATURE-DEPENDENT SOLUBILITY OF C~o AND C7o

This part of the work is related to the temperaturedependent solubility of C6o and C70 in 1,3-diphenylacetone. Our aim was to determine the optimal conditions for purification by fractional crystallization. For our experiments, excess C6o (C7o) and solvent are placed in a double-neck balloon equipped with a magnetic stirrer and a reflux condenser. The solution is kept under nitrogen atmosphere and protected from light in order to avoid oxidation reactions, and the balloon is heated by an oil bath. The solution is stirred for 24 h to ensure equilibration and then the stirring is interrupted for 24 h. A sample from supernatant solution is taken, diluted and analysed by HPLC. Independently, the room temperature solubility was determined by HPLC after filtration through a PTFE filter. We observed similar results by both methods within the range of experimental error. 100

100

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13

13

C60 purity (%)

A

C60 recovered (%)

8ol

$

6o

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L~ 98 40

97 20

96

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0

,

50

I

100

0

,

150

Weight of norit-A (mg) per mg of C60

200 ,.

Fig. 2. Evolutionof the purity of C 60and the amount of Cto recovered vs the amount of Norit-A (expressedin mg per mg of initial C 60).

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R.J. DOOME et aL

I ;=~

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Q SolubilityC70 A ~lu~flityC60

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Temperature (*C) ) Fig. 3. Temperature dependence of the solubility of C6o and C7o fullerenes in 1,3-diphenylacetone. Fig. 3 shows the variation of C6o and C7o solubility in 1,3-diphenylacetone as a function of temperature. In both cases, we observed a maximum of solubility and the shapes are quite similar for Ceo and C7o. The solubility maxima are at 136°C for C6o and at 41°C for C70. At low temperatures, the dissolution of both C6o and C7o is endothermic while at high temperatures the dissolution is exothermic. The thermodynamic parameters are as follows: for C6o at low temperatures, AH° = 9.1 --. 0.4 kJ mol -~ and AS° = -35.0 --- 1.1 J (mol K)-I; at high temperatures, AH° = - 6 . 2 - 1.3 kJmol -l and AS°= -72.7 --- 2.9 J (mol K)-t; for C7o at low temperatures, AH° = 16.8 --. 2.1 kJ tool -~ and AS° = - 7 . 9 --2.9 J (mol K)-I; at high temperatures, AH° = -10.8 "43.3 kJ mol -~ and AS° = -96.6 - 10.0J (mol K) -l. Our curves look like those published earlier in various solvents such as toluene, hexane, naphthalene or carbon disulphide for C~0 [11, 12]. Ruoff et al. [11] have proposed the influence of the phase transition eventually modulated by the solvent as an explanation of this peculiar behaviour. It is interesting to remember that the maximum of the solubility in their experiments lies around 70C. Since a phase transition occurs in solid C~o at -13"C [13, 14], this hypothesis is quite plausible. By contrast, since our C~o curve shows the highest solubility at 1360C, it is quite difficult to imagine a solvent effect strong enough to displace the temperature of the phase transition to such an extent. Moreover, C70 shows two phase transitions, at 11"(2 and 88*C [15], and the

influence of these transitions did not appear in our experiments for C70. Hence, the influence of phase transition as an explanation of the anomalous solubility behaviour seems to be insufficient. To explain the peculiar behaviour of these globular fullerene molecules, a change in the structure of the solid phase as well as an aggregation of both C 60and C 70molecules in the solution is proposed. In order to confirm this hypothesis, experiments are proposed to determine the size of the aggregates and the nature of the solid phase at different temperatures.

5. CONCLUSIONS A new purification method for C~o in gram quantities is reported. The purity of C~o reaches 99.5% with a global yield of 69%, and an additional method is proposed to increase the purity level up to 99.99%. The main advantages of our process are its easy scaling up, the total recovery of the solvents and fullerenes and the fact that it does not require any specific apparatus. These advantages could contribute to a significant decrease of the purification cost. The unusual solubility behaviour of C60 and C70 in 1,3-diphenylacetone is explained using the hypothesis of the aggregation of C~o and C70 molecules. New experiments using colligative properties could shed some light on the peculiar temperature-dependent solubility behaviour of globular fullerenes.

Purification of C60 by fractional crystallization

Acknowledgements--This work was in part funded by the National Programme of Inter-University Research Projects initiated by the State Prime Minister's Office (Science Policy Programming), by the Regional Government of Wallonia and by the National Fund for Scientific Research. REFERENCES 1. Kraetschmer, W., Lamb, L.D., Fostiropoulos, K. and Huffman, D.R., Nature, 1990, 347, 354. 2. Kroto,H.W., Allaf, A.W. and Balm, S.P., Chem. Rev., 1991, 91, 1213. 3. Kroto, H.W., Carbon, 1992, 30 (g (entire issue)). 4. Scrivens, W.A., Bedworth, P.V. and Tour, J.M., J. Am. Chem. Soc., 1992, 114, 57. 5. Meier, M.S. and Selegue, J.P., J. Org. Chem., 1992, 57, 1924. 6. Giigel, A. and Miillen, K., J. Chromatogr., 1993, 628, 23. 7. Taylor, R., Hare, J.P., Abdul-Sada, A.K. and Kroto, H.W., J. Chem. Soc., Chem. Commun., 1992, 1423.

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8. Ruoff, R.S., Tse, D.S., Malhotra, R. and Lorents, D.C., J. Phys. Chem., 1993, 97, 3379. 9. Sivaraman, N., Dhamodaran, R., Kaliappan, I., Srinivasan, T.G., Vasudera Rao, P.R. and Mathews, C.K., J. Org. Chem., 1992, 57, 6077. 10. Manolora, N., Rashkov, I., Legras, D., Delpeux, S. and Beguin, F., Carbon, 1995, 33, 209. 11. Ruoff, R.S., Malhotra, R., Huestis, D.L., Tse, D.S. and Lorents, D.C., Nature, 1993, 362, 140. 12. Hidalgo-Quenada, R., Zhang, X.-Y. and Giessen, B.C., Mat. Res. Soc. Symp. Proc., t995, 359, 561. 13. Atake, T., Tanaka, T., Kawaji, H., Kikuchi, K., Saito, K., Suzuki, S., Achiba, Y. and Ikemoto, I., Chem. Phys. Lett., 1992, 196, 321. 14. Van Tendeloo, G., Amelinckx, S., Verheijen, M.A., Van Loosdrecht, P.H.M. and Meijer, G., Phys. Rev. Lett., 1992, 69, 1065. 15. Van Tendeloo, G., Amelinckx, S., De Boer, J.L., van Smallen, S., Verheijen, M.A., Meekes, H. and Meijer, G., Europhys. Lett., 1993, 21, 329.