CHEMISTRY OF FULLERENES C-60 AND C-70 FORMATION IN FLAMES

A kinetic mechanism is constructed for the formation of fullerenes C60 and C70 in flames, based on types of reactions used in describing growth of polycyclic aromatic hydrocarbons (PAH) and including additional chemical processes needed to describe evolution of the unique structural features of fullerenes. The mechanism consists of types of reactions, each characterized by an approximate rate coefficient, including processes for ring formation (via H atom abstraction, C2H2 addition, and cyclization leading to ring closing), reactive coagulation of aromatic molecules, and cage closing via H-2 elimination and ring closing but also allowing for additional processes such as intramolecular rearrangements. Curved PAH, including benzo[ghi]fluoranthene (C18H10) and dibenzo-[ghi,mno]fluoranthene (corannulene, C20H10), are likely fullerene precursors. Corannulene is considered a key intermediate in the fullerene formation mechanism. Although alternatives to corannulene as an intermediate are mentioned, the proposed mechanism is based on corannulene and other related PAH of C5v symmetry. Trivial nomenclature for the C5v intermediates is also introduced. Preliminary kinetic testing of the mechanism, using approximate rate coefficients based on analogous flat PAH reactions, shows the mechanism to be plausible within the uncertainties of the rate coefficients and the experimental data on fullerenes formation rates. The predicted fullerene formation times extend over a range that includes the experimentally observed times, and the predicted C70/C60 ratio agrees well with flame data. Predicted fullerene formation rates are increased strongly by increased C2H2 and decreased H-2 concentrations and moderately by increased H atom concentrations. Similarities between growth species in flames and carbon vapor systems imply that much of the mechanism may be pertinent to fullerene formation in carbon vapor systems.