Synthesis of Fullerenes (C 60 AND C 70 ) by Combustion of Hydrocarbons in a Flat Flame Burner
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SYNTHESIS OF FULLERENES (C60 AND C70 ) BY COMBUSTION OF HYDROCARBONS IN A FLAT FLAME BURNER
S. Mitra, C. J. Pope, K. K. Gleason, Y. Makarovsky, A. L. Lafleur, and J. B. Howard,
Department of Chemical Engineering, Center for Environmental Health Sciences and Energy Laboratory, M.I.T, Cambridge, MA 02139.
ABSTRACT Fullerene (C6 o and C70) synthesis by combustion of ethylene and benzene in a flat flame burner was investigated. This method of fullerene synthesis is particularly attractive because of its potential of scale up. Also the ability to change the flame conditions and control the yield of C 6o and C70 makes this method versatile. No fullerenes were found in soot samples collected from the ethylene flame. However, fullerenes were formed in a benzene flame with C/O = 0.88 and operated at 40 torr, with cold gas velocity of 25.3 cm/s (273 K) and containing 10% argon. The concentration of fullerenes in this flame was found to depend strongly on the height above the burner surface. It exhibited a strong maxima at about 1.0 cm above the burner height suggesting the presence of both growth and destruction mechanisms. INTRODUCTION When carbon vapor is formed in an inert gas atmosphere (argon or helium) either by laser irradiation [1] or by resistive heating of graphite [2-4], fullerenes (C 60 and C 70) are found to be formed. In addition, macroscopic quantities of both C 60 and C 70 have now also been synthesized in flames operating over a wide range of conditions [5]. The possibility that fullerenes can be generated in flames was first suggested by Smalley and his coworkers [6,7] and by Kroto and McKay [8]. Indeed, all-carbon ions having the same m/e ratio as fullerenes were subsequently detected by mass spectroscopy in sooting acetylene and benzene flames by Homann and coworkers [9,10]. An advantage of synthesizing fullerenes in flames lies in the fact that by tuning the flame conditions, the fullerene yield can be varied. Also, since design of industrial scale burners is an established technology, fullerene synthesis by combustion offers the possibility of scale up. Hence, an understanding of the processes leading to fullerene synthesis in flames is particularly important. The yield of both C 60 and C70 depends on the chamber pressure, equivalence ratio (normalized C/O ratio), temperature and the residence time in flames [5,11]. In this work we investigated the formation of fullerenes as a function of residence time in the flame. Soot samples were collected with a quartz probe at different heights above burner (h) from a moderately sooting benzene/oxygen/argon and ethylene/oxygen/argon flame. The extracted soot was weighed and sonicated in toluene. Both C 6o and C70 are easily soluble in toluene and their amount was estimated from high performance liquid chromatography (HPLC) measurements. The experimental details are described below. EXPERIMENTAL DETAILS Premixed laminar benzene/oxygen/argon and ethylene/oxygen/argon flames were stabilized on a water cooled burner. The burner, which was designed to produce a one
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