Production, Isolation and Purification of Fullerenes

ROGER TAYLOR AND GLENN A. BURLEY

Faculty of Chemistry and Pharmacy, Ludwig-Maximilians University, Munich, Germany

1.1 Introduction

Eight fullerenes have been obtained in significant quantities. These are [60-Ih]-, [70-D5h]-, [76-D2]-, [78-D3]-, [78-C2v(I)]-, [78-C2v(II)]-, [84-D2(IV)]-, [84-D2d(II)]-fullerenes and are depicted in Figure 1. Of the fullerene family, [60]fullerene and [70]fullerene are the major isomers obtained in 75 and 24%, respectively, via the arc-discharge method of Hufmann and Krätschmer. The remaining 1% constitutes a variety of higher order fullerenes ranging from C74 to beyond C100. The colours of the fullerene family vary according to their molecular weight and symmetry. Their colours in solution are magenta (C60), port-wine red (C70), brown (C76 and C78), and yellow-green (C84). A major obstacle in higher order fullerene research is confronted when investigating these fullerenes [Figure 1(c)–(h)]. A gradual increase in the size of fullerenes is accompanied by an increase in the number of isomers of the same symmetry, therefore making definitive assignment of the fullerene structure difficult. Coupled with the difficulty of separation and decreasing solubility of higher order fullerenes with increasing size, makes further studies of these larger fullerenes unattractive.

The numbering system adopted for fullerene assignment in this review is the IUPAC system that has been in place for over a decade. The Roman numerals used for subdividing fullerenes exhibiting the same symmetry are those given by Fowler and Manolopoulos.

1.2 Production

Three methods have been used to make fullerenes. These are

(i) The Hufmann–Krätschmer procedure involving arc-discharge between graphite rods in an atmosphere of helium.

(ii) Combustion of benzene in a deficiency of oxygen.

(iii) Condensation of polycyclic aromatic hydrocarbons through pyrolytic dehydrogenation or dehydrohalogenation.

1.2.1 The Hufmann–Krätschmer Method

Hitherto this has been the most important method for fullerene production, and its introduction marked the real beginning of fullerene science. It is the preferred method because the only by-product is graphite, which if necessary, can be reformed into rods and recycled. The Hufmann–Krätschmer method involves arc-discharge between high-purity carbons rods of ca. 6 mm diameter in an atmosphere of 100–200 torr helium. Argon may also be used but is less effective. The temperature required for fullerene formation is ca. 2000°C, and obviously a small gap between the rods is necessary to prevent a fall in temperature. The need for this gap was shown at an early stage and was confirmed subsequently. The yield of fullerenes in the soot produced is approximately 5% and is higher when taken from the reactor at greater distances from the arc source, which implies that the initially formed fullerenes are subsequently degraded by UV irradiation.

Numerous ingenious variations in reactor design were introduced at an early stage, including, for example, a carousel and one with an autoloading device (Figure 2). The latter method, designed by Bezmelnitsyn and Eletskii is especially notable, using carbon strips 7x3.5x400 mm cut from a reactor moderator block. A stack of 24 of these (anode) are housed in a 450 mm long by 280 mm diameter water-cooled chamber equipped with a hinged door fitted with a O-ring seal. The cathode consists of slowly rotating 70 mm-diameter carbon wheel which passes a scraper to remove accumulated slag. The strips are gravity fed and the lowest strip is slowly wheel-driven into the cathode. When consumed it drops away exposing the next strip, and the process continues during 24 h to yield 100–200 g of fullerene-containing soot, accessed by opening the end door of the reactor.

1.2.2 The Combustion Process

Howard and co-workers discovered that combustion of benzene in a deficiency of oxygen resulted in the formation of both [60]- and [70]fullerenes. This continuous method has been developed to the extent that a purpose-built factory has been erected in Japan, capable of producing 5000 ton of fullerenes per year, but currently running at about one-tenth of that capacity. One envisages that this investment must be driven by the expectation or knowledge that large-scale applications of fullerene lie ahead.

1.2.3 Condensation of Polycyclic Aromatic Hydrocarbons through Pyrolytic Dehydrogenation or Dehydrohalogenation

These methods produce fullerenes but not in sufficient quantities for practical applications. Rather, they provide a means of deducing the mechanisms of fullerene synthesis. For example, C60 consists of six dehydronaphthalene moieties located at the octahedral sites, and pyrolysis of naphthalene does indeed produce C60, as does corannulene, (which has been detected as a precursor in the combustion process), 7,10-bis(2,20-dibromovinyl)fluoranthene, and 11,12- benzofluoranthene. The dehydrogenation involved is a high-energy process and dehydrohalogenation of precursors is more successful, a feature made use of in formation of C60 from a chloroaromatic precursor (Scheme 1). No other fullerene has yet been made in this way.

1.3 Isolation and Purification of Fullerenes

The fullerene-containing soot is extracted using a Soxhlet apparatus with either chloroform (slowest), toluene, or 1,2-dichlorobenzene (fastest, but removal of traces of solvent requires high vacuum). If carbon disulfide is used at any stage during the extraction/concentration, then it must be vigorously removed under vacuum, otherwise the fullerene will be contaminated with sulfur.

Purification of...