Catalytic Properties of Fullerene Materials
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W.L. BELL*
ABSTRACT Fullerenes were found to catalyze coupling and transalkylation reactions of mesitylene, engage in transfer hydrogenations with dihydroaromatics, and cleave strong bonds such as those in diarylmethanes. In all of these reactions, fullerenes show a marked ability to accept and to transfer hydrogen atoms. The key structural feature that endows fullerenes with many of its characteristics is the presence of a pentagon surrounded by hexagons. We suspected that fullerene soot, unlike graphitic carbon, contained pentagons in a hexagonal lattice, and that these sites imparted the soot with the desired chemical attributes of strong electrophilic nature and an ability to stabilize radicals. In subsequent studies, we have shown fullerene soot to be very effective in catalyzing various H-transfer reactions, including the conversion of methane into higher hydrocarbons. When compared with other carbons, such as activated carbons and acetylene black, the fullerene soot is much more reactive for oligomerization and hydrodealkylation of alkylbenzenes. Because this activity remains, even in chemically extracted and partially oxidized soot, the observed catalysis is not a result of residual soluble fullerenes. INTRODUCTION Fullerene Materials The discovery of fullerenes in the 1980s and the development of a method for their bulk production in 1990 provides the basis for the development of completely new carbon materials. Fullerenes are all-carbon cage molecules, the most celebrated among which is the soccer-ball shaped C 6O. While such structures were hypothesized in 1970, it was not until 1985 that the first experimental evidence of their existence was found. The structure of fullerenes can be readily understood by considering the structure of a graphene sheet, which consists of carbon atoms in a flat hexagonal lattice. In this lattice, the valencies of the atoms in the middle are fully satisfied, but those at the edges are not, giving rise to slight destabilization. If the lattice is very large, the overall destabilization on a per carbon basis is small. Moreover, many flat lattices can assemble in layers and gain stability due to interplanar van der Waals interactions. However, if the lattice consists of relatively few atoms, destabilization is significant and the system seeks to minimize its internal energy by folding onto itself. Incorporating pentagons in the lattice is one way to achieve folding without having "unsatisfied" valences. Each pentagon introduces a curvature of 600 (n/3 solid angle). It takes twelve pentagons in a hexagonal lattice for complete closure (4n solid angle). One can also arrive at the same conclusion from Euler's theorem on the relationship between the number of edges, faces and vertices in a closed polyhedron. Thus, C60 consists of 20 hexagons and 12 pentagons, C70 consists of 25 hexagons and 12 pentagons and C84 consists of 32 hexagons and 12 pentagons. The relative location of the pentagons in these strucures creates different shapes, and consequently there can be a wide range of
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