Fullerenes Synthesis Extraction and Purification

There are many methods to synthesis C60 and C70 in gram quantities in the laboratory. In addition, higher mass fullerenes (larger fullerenes molecules) can be produced and isolated , albeit in very small amounts [1].

Most methods for generation of large quantities of fullerenes produce a mixture of impurity molecules and stable fullerenes. For this reason, fullerene synthesis must be followed by processes of extraction and purification of fullerenes from impurities according to mass [1].

Synthesis of Fullerenes:

Fullerene molecules can be synthesized in the laboratory in a wide variety of methods, all involving the formation of a carbon- rich vapour [1].

Early methods used laser evaporation technique which produces very small quantities of fullerenes [1]. The later approaches involve an electric discharge between graphite electrodes in He gas [10].

Laser Evaporation Technique:

This method was used in 1984 for the first time by Rohlfing and others [8]. They noticed that carbon cluster Cn with a huge number of carbon atoms (more than 190) could be produced [9]. In 1985 Kroto, Smalley and co- workers used this technique to generate and detect the most stable carbon clusters [7].

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This method involves vaporization of carbon species from the rotating graphite disk into a high density helium, using a Nd:YAG laser operation at 532nm, (fig2). The resulting carbon clusters were analysised by time -of- flight mass spectrometry. The first observation of the mass of C60 was a 720 amu peak. Although this approach produces minute quantities of fullerenes, it is still essential if when we use later modification. This modification will help to heat the dusk of graphite. Therefore, it gives remarkable control of fullerene distribution and the generation of specific fullerenes [2].

Arc Evaporation:

There is no doubt that this technique is an efficient way to produce gram quantities of fullerenes in the laboratory [1]. For the formation of fullerenes by this technique, an arc is struck between two graphite electrodes in atmosphere of 100~200 torr of He. The contact between the electrodes is maintained by the influence of gravity. The apparatus is surrounded by water to cool the soot to achieve the resulting soot which may contain approximately 10-15% of soluble fullerenes [2].

The first design by Wudl and co- workers used a pyrex cylinder for the vacuum shroud. Although this gives a suitable method for visual inspection of the graphite electrodes through the well, the glass cylinder is easily damaged. For this reason, it is appropriate to change it with a stainsteel cylinder with a window [1].

Fullerene Extraction:

In this process of fullerenes production, soluble impurity molecules and insoluble nanoscale carbon soot are generated with soluble fullerenes. Two effective methods are used to extract the fullerenes from the soot [1].

Solvent Methods:

Solvent method is the most common method is used to dissolve the fullerenes in benzene, toluene (preferred over benzene due to its toxicity is lower) or other suitable solvent. However, the solvent also contains other soluble hydrocarbon impurities [2]. It can be separated soot and other insoluble molecules from the solution by filtration. The early method used Soxhelt extraction in a hot solvent to remove fullerenes from the soot. This technique is used where the molecules to be extracted from the solid state are soluble in organic solvent, such as polyaromatic hydrocarbons (PAHs) from coal. This apparatus consists of double thimble containing soot, fullerenes and other materials and at the bottom the solvent is boiled in the flask. The solvent vapors and rises to condense in the condenser unit, the solvent distills then the solution passes through the thimble wall. The solution which contains the extracted molecules returns to the flask. The molecules that are not soluble in the solvent remain in the thimble. Another alternative method, the soot is separated in tetrahydrofuran (THF) at room temperature before sonicating the soot in an ultrasonic bath for 20 minute. Removing insoluble molecules by filtration and a rotary evaporator at 50°C are used to remove THF from the fullerenes. It can be noticed that the higher boiling point solvent and more polar isolate the higher mass fullerenes [1].

Sublimation Methods:

It can be sublimated microcrystalline C60 and C70 powder at low temperature Ts~350°C (C60) and Ts~460°C (C70). For this reason, C60 and C70 can be separated directly from the soot without introducing solvents, such as benzene, toluene, carbon disulfide or hexane. This method provides a beneficial alternative to solvent extraction for some cases which are sensitive to contamination of solvent in the sample. In this approach, the raw soot is placed in a quartz tube and the whole apparatus is heated in a furnace. Dynamic pumping is preferred because it is likely the soot may contain polyaromatic hydrocarbons impurities. The raw arc soot in the end of tube is kept at the highest temperature T~600-700° C. The higher mass fullerenes sublimate from the soot which then condenses in the colder section of the tube. Since the sublimation temperature of C70 and higher fullerenes are higher than that of C60, they will condense closer to the soot. The production of a C60 molecular beam from a microcrystalline mixture of C60 and C70 depends on the difference in sublimation temperature between C60 and C70. This microcrystalline mixture is placed in a dynamic vacuum and is heated above the sublimation temperature of C60. The sublimation rate for C60 in vacuum at T~400 °C is favored by a factor of 20 over that C70. A pure molecular beam of C60 can be obtained, because C70 is a factor of ~ 7 less abundant in arc soot than C60 [1].

Kratschmer et al [11] used the method of directly subliming fullerenes from the solid material. However, this does not provide pure fullerenes.

Fullerene Purification:

The previous methods of extraction may bring impurity molecules with the most stable fullerenes. The step of chemical purification must be carried out, if a pure fullerene microcrystalline powder or solution is desired. The step involves sublimation methods based on temperature gradients and solvent methods based on liquid chromatography. Fullerene purification means the separation of the different fullerenes in the fullerene extract into C60, C70, C76, C84 etc. Sensitive tools, such as liquid chromatography, mass spectrometry, nuclear magnetic resonance (NMR), optical absorption spectroscopy and infrared [1].

Solvent Methods:

The main technique for fullerene purification is liquid chromatography (LC). LC is a wet chemistry method which includes a solution ( called the mobile phase ) of a molecular mixture. This solution is forced to pass through a column filled with a high surface area solid (called the stationary phase ). The separation of fractions is verified qualitatively by the comparison of the observed optical spectra, vibration spectra and NMR data or by color ( magenta or purple for C60 in toluene and reddish- orange for C70 in toluene). Liquid chromatography separates molecules according to their weights. Moreover, this technique can be utilized to separate a single allotrope, such as C76, or to isolate isomers with different molecular shapes but having the same molecular weight, such as separating C78 with C2υ symmetry from C78 with D3 symmetry [1].

The liquid chromatography process involves chemical or physical interactions between a particular molecule and the stationary phase. This interaction reduces (or raises) the rate of migration for that molecule through the column or raises (or reduces) the retention time for that molecule.

Remarkable chemical or physical differences for the molecular species, such as surface absorption, shape and mass are important to provide a clear chromatographic separation. Early approaches to C60, C70, and higher fullerenes purification included flash column chromatography of the raw fullerene in a column packed with neutral alumina as the stationary phase and hexane/toluene ( 95/5 volume % ) as the mobile phase. Although this process was found useful, it used abundant quantities of solvent that was difficult to recycle [1].

One of the first important development to this method was high performance liquid chromatography (HPLC).