FULLERENES SYNTHESIS IN COMBUSTION

The early suggestion in fullerenes research that fullerenes might be produced in flames was soon supported by the observation of polyhedral carbon ions in flames, and in 1991 was confirmed by the recovery and identification of fullerenes C60 and C70 from benzene/oxygen flames. Recent research has determined the effects of pressure, carbon/oxygen ratio, temperature, and the type and concentration of diluent gas, on the yields of C60 and C70 in subatmospheric pressure premixed laminar flames of benzene and oxygen. Similar flames but with acetylene as fuel have also been found to produce fullerenes, but in smaller yields than with benzene fuel. The largest observed yields of C60 + C70 from benzene/oxygen flames are substantial, being 20% of the soot produced and 0.5% of the carbon fed. The largest rate of production of C60 + C70 was observed at a pressure of 69 Torr, a C/O ratio of 0.989 and a dilution of 25% helium. Several striking differences between fullerenes formation in flames as compared to the widely used graphite vaporization method include, in the case of flames, an ability to vary the C70/C60 ratio from 0.26 to 8.8 (cf., 0.02 to 0.18 for graphite vaporization) by adjustment of flame conditions, and production of several isomers each of fullerenes C60, C70, C60O, and C70O. Many of the apparent isomers are thermally metastable, one C60 converting to the most stable form with a half-life of 1 h at 111-degrees-C. The structures of the apparent C60 and C70 isomers necessarily must include abutting five-membered rings, previously assumed to be disallowed because of their high strain energy. The chemistry of fullerenes formation in flames is in some ways similar to that of soot formation, but important differences are seen and assumed to reflect the effects of the curved, strained structures of fullerenes and their precursors.