Fullerenes purification/separation

Abstract—Carbon nanotubules were produced in a large amount by catalytic decomposition of acetylene in the presence of various supported transition metal catalysts. The influence of different parameters such as the nature of the support, the size of active metal particles and the reaction conditions on the formation of nanotubules was studied. The process was optimized towards the production of nanotubules having the same diameters as the fullerene tubules obtained from the arc-discharge method. The separation of tubules from the substrate, their purification and opening were also investigated. 

Arc discharge [25] is initially used for producing C60 fullerenes. Nanotubes are produced by arc vaporization of two carbon rods placed in a chamber that is filled with low pressure inert gas (helium, argon). The composition of the graphite anode determines the type of CNTs produced. A pure graphite anode produce preferably MWNT while catalyst (Fe, Co, Ni, Y or Mo) doped graphite anode produces mainly SWNT. This technique normally produces a complex mixture of components, and requires further purification to separate the CNTs from the soot and the residual catalytic metals present in the crude product. 

Cjg, and Cg4 exist as 2, 5, and 24 possible constitutional isomers, respectively a fact that makes separation and purification procedures more difficult. As a result, only a couple of studies have been pubKshed on the electrochemistry of derivatives of the higher fullerenes [44, 54]. 

AS A SYNTHETIC TOOL IN THE SEPARATION, PURIFICATION AND ISOLATION OF HIGHER FULLERENE ISOMERS... 

As in the case of hollow fullerenes (Ajie et al., 1990 Scrivens et al., 1992 Taylor et al., 1990), liquid chromatography (LC) is the main purification technique for metallofullerenes. LC has been frequently and traditionally used in separation chemistry. One of the most powerful LC techniques is HPLC which allows separation of fullerenes according to their molecular weight, size, shape or other parameters 0inno and Saito, 1996 Kikuchi et al., 1991, 1992 Klute et al., 1992 Meier and Selegue, 1992). The HPLC technique can even allow us to separate structural isomers of various matallofullerenes (Shinohara, 2000). 

Scandium mefallofullerenes are, in particular, interesting in terms of separation and purification because, as described in Section 2.1, scandium fullerenes appear as mono-, di-, fri-and even fetra-scandium fullerenes wifh several sfrucfural isomers which can be separated completely by HPLC. As an example, the HPLC separation of scandium fullerenes is briefly described in fhe following. 

The new molecular forms of carbon, C > and C70, were prepared following the method of KrStschmer et al. in a consistently high yield (14%). The benzene-soluble material extracted from the graphite evaporation product is predominantly constituted of C(o and C70. Three independent methods, namely HPLC, mass spectroscopy, and C NMR, demonstrate that these two compounds are present in a ratio near 85 15. The two compounds can be separated by column chromatography on alumina, allowing as for now the purification of minute quantities of pure and C70. Support for the proposed symmetrical cage structures (fullerenes) of C o and C70 is inferred from the simplicity of the C NMR spectra and the strong presence of the and ions. Attempts at the X-ray determination of the Cjo molecular structure are now actively pursued. ... 

The solids produced by the arc method are often complex mixtures of fullerenes and metallofullerenes and an important part of this chemistry has involved the purification of the individual metallofullerenes and even isomers of a particular species. In general a two-stage HPLC method has been used to separate the metallofullerenes from the empty fullerenes and purify individual metallofullerenes.105,106... 

For the purification of fullerenes from the fullerene oxides the activated alumina and silica can be used. Fulllerene oxides are adsorbed strongly on such adsorbents from solution and the oxides are removed from fullerene samples. For the preparative separation of fullerenes at present activated carbons and graphite are used [11-14], For this purpose silica with the deposited carbon layer [16] can be used also. In this case it is very easily to regulate the pore diameter and specific surface area of adsorbents as well as particle diameter. Such adsorbents is very important for the decreasing of fullerenes loss. On preparative separation of fullerenes on LiChrosorb SI 60 with deposited carbon layer by modified method [15] on glass column first fractions contained quite pure Csq. 

Relatively few molecular separations have been studied from the utilitarian standpoint. One of these, the purification of fullerenes via 8 , is discussed on p. 170. In a reciprocal experiment the separation of 4 " , 6 , and 8 with a column using a chemically-bonded C o silica stationary phase has been re-ported. Chromatographic selectivity has been achieved for amino acid esters and alkali metal cations on silica-bonded calix[4]arene tetraesters, the structure of which has been explored by and Si-CP-MAS NMR. Silica-bonded calixarenes have also been used as packing materials for HPLC columns that are capable of separating disubstituted aromatics, peptides, and nucleosides. The HPLC separation of phenols using 6 ° as a constituent of the eluent has been described. ... 

---