Combustion synthesis of fullerenes and fullerenic nanostructures

Abstract

Fullerenes are molecules comprised entirely of sp 2 -bonded carbon atoms arranged in pentagonal and hexagonal rings to form a hollow, closed-cage structure. Buckyballs, a subset which contains C 60 and C 70 , are single-shell molecules while fullerenic nanostructures can contain many shells and over 300 carbon atoms. Both fullerenes and nanostructures have an array of applications in a wide variety of fields, including medical and consumer products. Fullerenes were discovered in 1985 and were first isolated from the products of a laminar low-pressure premixed benzene/oxygen/argon flame operating at fuel-rich conditions in 1991. Flame studies indicated that fullerene yields depend on operating parameters such as temperature, pressure, residence time, and equivalence ratio. High-resolution transmission electron microscopy (HRTEM) showed that the soot contains nanostructures, including onions and nanotubes.Although flame conditions for forming fullerenes have been identified, the process has not been optimized and many flame environments of potential interest are unstudied. Mechanistic characteristics of fullerene formation remain poorly understood and cost estimation of large-scale production has not been performed. Accordingly, this work focused on: 1) studying fullerene formation in diffusion and premixed flames under new conditions to identify optimal parameters; 2) investigating the reaction of fullerenes with soot; 3) positively identifying C 60 molecules in HRTEM by tethering them to carbon black; and 4) providing a cost estimation for industrial fullerenic soot production.Samples of condensable material from laminar low-pressure benzene/argon/oxygen diffusion flames were collected and analyzed by high-performance liquid chromatography (HPLC) and HRTEM. The highest concentration of fullerenes in a flame was always detected just above the height where the fuel is consumed. The percentage of fullerenes in condensable material increases with decreasing pressure and the fullerene content of flames with similar cold gas velocities shows a strong dependence on length. A shorter flame, resulting from higher dilution or lower pressure, favors the formation of fullerenes rather than soot, exhibited by the lower amount of soot and precursors in such flames. This indicates a stronger correlation of fullerene consumption to soot levels than of fullerene formation to precursor concentration. The maximum flame temperature seems to be of minor importance in formation. The overall highest amount of fullerenes was found for a surprisingly high dilution of fuel with argon. The HRTEM analysis showed an increase of the curvature of the carbon layers, and hence increased fullerenic character, with increasing distance from the burner up to the point of maximum fullerene concentration, after which it decreases, consistent with the HPLC analysis. The soot shows highly ordered regions that appear to have been cells of fullerenic nanostructure formation. The samples also included fullerenic nanostructures such as tubes and sp...

Soot Formation in Combustion Processes (Review)

“…Concentration of Propane Oxidation Products Versus the Temperature of the Second Reactor Section (p = 40 kPa, T 1 = 300 K, and t = 10 sec) T , K Partial pressure, kPa [31,90]. In [79], the maximum particle diameter measured in experiments with a low-pressure onedimensional homogeneous mixture in an acetylene flame [91,92] was correlated with the value of H/C determined under similar conditions [59].…”

Flame synthesis of single-walled carbon nanotubes

“…An extensive amount of research related to the formation of fullerenes and fullerenic nanostructures in flames has been reported in the last decade [19][20][21][22][23][24]. In particular, there have been two studies in our group at MIT where carbon nanotubes have been observed in condensed material collected from flames [19,20].…”

The HSAB principle as a means to interpret the reactivity of carbon nanotubes

“…This fullerene-like model explains many properties of so-called 'hard' synthetic activated carbons, especially their microporosity and high resistance to graphitization. A number of other scientific groups have also found evidence for fullerenerelated structures in ''conventional'' carbons [13][14][15][16][17][18]. Thus, the polycyclic curved aromatic fragments can be treated as the models representing the whole structure of both, activated carbons and/or carbon nanotubes in the quantum chemical studies.…”