Fullerene Purification Sublimation

"See Eq. (5.1) for the definition of S. Unless noted otherwise, the data refer to values at 295 K [5.30]. ''These data refer to values at 303 K [5.29], powder is heated above the sublimation temperature of C60. At T ~ 400°C, the sublimation rate for C60 in vacuum is favored by a factor of 20 over that of C70 [5.26], Therefore, since C70 is normally a factor of ~ 7 less abundant in arc soot than C60, a reasonably pure molecular beam of C60 can be obtained. For example, if a Knudsen cell [5.28] containing this C60/C70 mixture is heated to 400°C, one would expect to emit a molecular beam of C60 molecules with less than 1% C70 impurity.

5.2.3. Solubility of Fullerenes in Solvents

The solubility of C60 in organic solvents has been studied and correlated with pertinent solubility parameters [5.29,30]. In Table 5.1 we display the experimental results for the solubility of C60 near room temperature (303 K) in a variety of organic and inorganic solvents [5.30], measured by highperformance liquid chromatography, discussed below in §5.3.1. In general, the solubility of a solute in a solvent is enhanced when the polarizabil-ity, polarity, molecular size, and cohesive energy density of the solvent are equal to the corresponding parameters for the solute. In this discussion of solubility, chemists define the polarizability as (n2 — 1 )/{n2 + 2) where n is the index of refraction at a reference optical wavelength, such as 5890 A; the polarity is defined as (e - l)/(e + 2) where e is the static (dc) dielectric constant; the molecular volume parameter V is defined as the ratio of the molecular weight to the density at a reference temperature, such as 298 K; the cohesive energy is normally related to the Hildebrand solubility parameter 8 [5.31] which is defined below. From a detailed study of 47 solvents for C60 [5.30], some trends were found governing the solubility of C60 for each of these four parameters, but no single parameter could by itself be used to predict the solubility of C60 in a given solvent [5.30].

The following general trends were found to govern the solubility of C60 in various solvents. Increasing the polarizability parameter was found to yield higher solubilities, and a similar trend was found for the polarity, but with a lesser degree of correlation. Also, increasing the molecular weight of the solvent generally increases the solubility of C60. Correlations were also found with the Hildebrand solubility parameter 8, which is found from the relation

where AH is the heat of vaporization of the solvent, T is the temperature, V is the molar volume, R is the gas constant [i? = 8.31 x 107 erg/(K mol)] and AE is a measure of the cohesive energy and is defined as the energy needed to convert one mole of liquid at 298 K to one mole of noninteract-ing gas [5.32], Values of 8 for the various solvents are listed in Table 5.1

[5.30]. Whereas for conventional solutes and solvents AH in Eq. (5.1) is interpreted as the molar heat of vaporization, a better correlation for the C60 solute was obtained using the molar heat of sublimation for C60. Values of these four parameters (the polarizability, polarity, molecular volume V, and the Hildebrand solubility parameter S) for C60 are found using n = 1.96, e = 3.61, V = 4.29 cm3/mol and S = 9.8 cal1/2 cnr3/2 (where 41.4 kcal/mol is used for AH). Thus Table 5.1 shows that solvents with S values close to that for C60 (i.e., S ~ 10 cal1/2 cm~3/2) tend to have higher solubilities for C60 than solvents with other values of 8.

Since the solubility of solutes in solvents tends to be significantly temperature dependent, it is important to specify the temperature at which solubility measurements are made (see Table 5.1). This temperature dependence can also be exploited to enhance or suppress the solubility of a particular fullerene by appropriate choice of solvent and temperature in the extraction and purification process.

5.3. Fullerene Purification

In this section we describe fullerene purification using both solvent methods based on liquid chromatography (§5.3.1) and sublimation methods based on temperature gradients (§5.3.2). Gas-phase separation is discussed in §5.3.3 and vaporization studies of C60 are reviewed in §5.3.4. By purification, we mean the separation of the fullerenes in the fullerene extract into C60, C70, C84, etc. To verify the effectiveness of the purification process, the fullerenes are characterized by sensitive tools such as mass spectrometry, nuclear magnetic resonance (NMR), liquid chromatography, and infrared and optical absorption spectroscopy.

5.3.1. Solvent Methods

Liquid chromatography (LC) is the main technique used for fullerene purification. Briefly, LC is a wet chemistry technique in which a solution (termed the "mobile phase") containing a molecular mixture is forced to pass through a column packed with a high surface area solid (termed the "stationary phase") [5.33]. The identity of the separated fractions from the LC column is verified qualitatively by color (magenta or purple for C60 in toluene and reddish-orange for C70 in toluene) and more quantitatively, by the comparison of the observed infrared vibrational spectra (see §11.5.2), optical spectra (see §13.2 and §13.6.1), and NMR data (see §16.1) with published results [5.34]. Liquid chromatography generally allows separation of the fullerenes according to their molecular weights, but this method can also be used to isolate a single distinct chiral allotrope such as C76, or to separate isomers with the same molecular weight but having different molecular shapes, e.g., separating C78 with C2u symmetry from C7g with D^ symmetry [5.10],

The principle of the liquid chromatography process is as follows. By any one, or several, physical or chemical mechanisms, a particular molecule in the mobile phase is differentiated by being forced to experience an interaction with the stationary phase. This interaction increases (or decreases) the retention time for that molecule in the column or, equivalently, decreases (or increases) the rate of migration for that molecular species through the column. Thus, as a function of time, separated molecular components from the mixture emerge (or "elute") in the order of decreasing interaction with the stationary phase. The least retarded molecular species elutes first, and so on. Separation of molecular species is obtained when a sufficient difference in the retention time in the column can be achieved. For example, impurity PAHs might be eluted in the mobile phase first, C60 might be eluted second, followed next by C70, and then followed by the highermass fullerenes. Significant physical or chemical differences of the molecular species (e.g., mass, shape, surface adsorption) are required to achieve a clear chromatographic separation.

Historically, the earliest purification of C60, C70, and the higher fullerenes involved "flash" (or rapid) liquid chromatography of the crude fullerene extract in a column packed with neutral alumina (stationary phase) and using hexane/toluene (95/5 volume %) as the mobile phase [5.20,21,35]. This procedure was found to be reasonably effective but very labor intensive and consumed large quantities of solvent which was difficult to recycle. One of the first significant improvements to this approach was the development of an automated high-performance liquid chromatography (HPLC) separation process involving pure toluene as the mobile phase and styragel as the stationary phase [5.36-38]. This approach, which employs sophisticated LC equipment, successfully separates gram quantities of fullerenes automatically. Several other HPLC protocols using various stationary and mobile phases have also been reported [5.24,39],

Several groups have published reports of fullerene purification procedures which combine the extraction and purification steps into one apparatus using pure hexane. An example of such a "modified Soxhiet chromatography" apparatus is shown in Fig. 5.4, which is reported to be capable of extracting a gram of C60 and ~0.1 g of C70 in a day [5.40]. As can be seen, the apparatus in the figure is similar to a Soxhiet apparatus discussed above for the extraction of fullerenes from soot (Fig. 5.3), except that the Soxhiet thimble has been replaced with a liquid chromatography column. To carry out the purification step, fullerenes are first loaded by adsorption onto the top of the column, which has been packed with neutral alumina.

Working Soxhlet Apparatus Animation
Fig. 5.4. Schematic representation of the modified "Soxhlet chromatography" apparatus used to combine the extraction and purification steps and used to separate fullerenes according to mass and molecular shape [5.40],

In operation, the distilled solvent (pure hexane, 1.5 L) is then condensed at the top of the column and passes through the column, returning to the distillation flask. In this way, the hexane is recirculated continuously as the C60 molecules migrate in a purple or magenta band down the column. This process proceeds for 20-30 h until all the C60 has passed slowly through the column to the distillation flask. The valve is then closed and the first flask containing C60 in hexane is removed. A second flask of hexane is then attached to the apparatus and the C70 remaining in the column is collected in the same way.

Recently, considerable improvements in the facile extraction of pure C60 from soot have been achieved. A new inexpensive and relatively simple method has been developed, which involves a simple filtration of toluene-extracted fullerenes through a short plug of charcoal/silica gel using toluene as the elutant [5.39], The method is inexpensive and quite easy to carry out. Unfortunately, however, the extracted C70 and higher fullerenes are trapped in the charcoal/silica plug and are difficult to recover at a later time. This filtration separation of C60 [5.39] represents an improvement on the flash chromatography method reported previously [5.41], which yielded excellent separation of C60 from higher fullerenes and impurities using a solid phase consisting of silica gel/carbon Norit-A (2:1) and pure toluene as the mobile phase [5.41] and achieved a separation of ~2 g of fullerene extract in 1 h with a yield of 63% out of a possible 75%. The two improvements to the previous method were the replacement of the Norit-A carbon by activated, acid-washed Darco G60 charcoal (Fluka) and the elimination of the flash chromatography step, thus replacing the column by a simple plug-filtration flask. A concentrated solution of extracted fullerenes in toluene was loaded in a fritted funnel and eluted by the application of a slight vacuum at the side of the filter flask. Only 15 min was needed to elute 1.5 g of C60 starting from 2.5 g of fullerene extract! Furthermore, HPLC analysis of the material found a contamination of less than 0.05% C70 or other fullerenes [5.39].

For C60 or C70 powders obtained by liquid chromatography, heat treatment in a dynamic vacuum of flowing inert gas (300°C for \ to 1 day) is used to reduce the solvent impurity. Furthermore, solid C60 has been shown to intercalate or accept oxygen quite easily at 1 atm, particularly in the presence of visible or UV light [5.42], Thus, degassed powders should be stored in vacuum or in an inert gas (preferably in the dark).

Many organic molecules have been found to have a structural, mirror-image twin or enantiomer, which is derived from the Greek word enan-tios, meaning "against." These two structural types are also referred to as "left-handed" and "right-handed," and a common example is left- and right-handed glucose. When polarized light is made to pass through solutions containing molecules of one structural type (i.e., the left-handed form), the plane of polarization is observed to be rotated, while the other structural type also causes optical rotation, but in the opposite sense. If the polarization rotation is significant, these molecules can be used in optical applications. Because of its D2 symmetry, the C76 molecule has right- and left-handedness (see §3.2). Thus C76 is found in two mirror-image, chiral forms (see Fig. 5.5) that can be extracted in equal concentrations as a minority constituent from carbon arc soot [5.44]. One solution to the puzzle of how to effectively separate these two C76 enantiomers, with potential for more general application to the separation of other chiral fullerenes [5.43],

Fig. 5.5. Schematic view of the two C76 enantiomers. A computer graphics rendition of these enantiomers is shown in [5.43]. The two molecules are mirror images of one another.

isolates the C76 molecules using liquid chromatography, first involving neutral alumina columns and then using a 50% toluene in hexane solution as the mobile phase in a "Bucky Clutcher I" column (Regis Chemical, Inc.). In the important next step, a combination of a plant alkaloid and an osmium-containing chemical is used to promote a rapid osmylation of one enan-tiomer over the other [5.43,45-48], Using liquid chromatography involving a silica gel column, the osmylated C76 enantiomer is then separated from the unreacted twin. In a final step, the osmium adduct is removed to obtain the other C76 enantiomer as well, via a reaction with SnCl2 in pyridine. The two C76 enantiomers recovered by this process were found to exhibit a maximum specific optical rotation of 4000±400 "/decimeter g/100 ml at the sodium D line. (The definition of the specific optical rotation is in terms of a/ic where a is the angle of rotation in degrees, i is the optical path length in 10"' m, and c is the concentration of the species in g/100 ml.) The osmylation of C60 is discussed in §10.6.

5.3.2. Sublimation in a Temperature Gradient

Separation and purification of fullerenes by sublimation in a temperature gradient (STG) [5.49] does not involve solvents and therefore avoids any possible contamination from the solvent used in common solvent-based extraction and purification procedures. Furthermore, since higher fullerenes and endohedral fullerenes are, as a rule, much less soluble in organic solvents than C60 and C70, it is a natural concern that a significant quantity of these less abundant fullerenes might remain behind in the soot when solvent extraction methods are used.

In the STG process, raw soot (containing fullerenes) is obtained directly from the arc apparatus and placed in one end of an evacuated quartz tube (diameter ~15 mm, length ~30 cm). The tube is evacuated, sealed off with a torch, and placed in a resistively heated oven maintained at a temperature in the range 900-1000°C at the center of the oven. The end of the tube containing the soot is placed at the center of the oven (hottest point) and the other end of the tube protrudes out of the oven and into the ambient environment. The natural temperature gradient of this arrangement allows fullerenes to sublime from the soot at the hot end of the tube and diffuse down toward the colder end. A specific fullerene molecule eventually sticks to the wall of the tube at a location dependent on its particular sublimation temperature. Thus, fullerenes with the higher sublimation temperatures (high-masses) are located on the walls in the hotter regions, and vice versa. A low pressure of inert buffer gas (e.g., argon) as well as internal baffles can be introduced into the system to ensure a slower diffusion of molecules down the tube, thereby increasing the chance for local thermal equilibrium with the tube wall [5.50].

A modification of the STG apparatus, to allow easy use of laser desorption mass spectrographic (LDMS) methods to study the spatial variation of the fullerene deposits along the rod, is the use of a ~3-mm-diameter quartz rod inside the sealed quartz tube as shown in Fig. 5.6(a) [5.51], In the sublimation setup, 400 mg of fullerenes or metallofullerenes are placed at the closed end of the quartz tube and the fullerenes deposit on the quartz rod held at the center of the tube by a Teflon ring. The tube is in a resistively heated and temperature-controlled furnace held at 1050°C with the soot at the hot end and the Teflon ring kept 4 inches outside. In the LDMS experiments the quartz rod is placed directly in a TOF mass spectrometer for analysis, Fig. 5.6(b). Fullerenes are desorbed by a short laser

Fig. 5.6. Schematic diagrams of the sublimation and laser desorption mass spectrometer setups for separation (a) and purification (b) of fullerenes. The position-sensitive LDMS setup (b) shows the quartz rod of (a), coated with the sublimed fullerenes, hanging freely within the first extraction field of a two-stage Wiley-MacLaren type time-of-flight mass spectrometer for analysis of the mass distribution of the sample [5.51].

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