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...

Samples of condensable material from laminar low-pressure benzene/argon/oxygen diffusion flames were collected and analyzed by high-performance liquid chromatography to determine the yields of fullerenes and by high-resolution transmission electron microscopy (HRTEM) to characterize the fullerenic material (i.e., curved-layer nanostructures) on and within the soot particles. The highest concentration of fullerenes was always detected just above the visible stoichiometric surface of a flame. The percentage of fullerenes in the condensable material increases with decreasing pressure. The overall highest amount of fullerenes was found for a surprisingly high dilution of fuel with argon. A comparison of the flames with the same cold gas velocity of fuel and oxygen showed a strong dependence of fullerene content on flame length. A shorter flame, resulting from higher dilution or lower pressure, favors the formation of fullerenes rather than soot, and the amount of soot and precursors of both soot and fullerenes is less at lower pressure and higher dilution. This behavior indicates a stronger correlation of fullerene consumption to the total amount of soot than of fullerene formation to precursor concentration. The maximum flame temperature seems to be of minor importance in fullerene formation. The HRTEM analysis of the soot 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 this maximum, where soot and fullerenes are consumed by oxidation, the curvature decreases. In addition to the soot, the samples included fullerenic nanostructures such as tubes and spheroids including highly ordered multilayered or onionlike structures. The soot itself shows highly ordered regions that appear to have been cells of ongoing fullerenic nanostructure formation.

The pyrolysis of ethynylbenzene (C8H6, 1) was studied in a flow system between 700 and 1100°C (reaction time about 0.3 s) by using a mixture of 5 mol-% of 1 in nitrogen and also in hydrogen at 700°C. The products were analyzed gas chromatographically. At 700°C in nitrogen, the main products were 1-and 2-phenylnaphthalene (2, 3), l-methylene-2-phenyl-1 H-indene (4), 1 -methylene-3-phenyl-1 H-indene (5), and 5,10-dihydroindeno[2,l-a]indene (6). At higher temperatures, ethynylaromatics and more stable aromatics such as benzene, naphthalene, acenaphthylene, biphenyl, pyrene, fluoranthene, and six further C16H1O isomers where detected. With hydrogen as diluent, the dimer formation was reduced, mainly in favor of styrene. -The complex mixture of reaction products and the dependence of its composition on the pyroEthynylbenzene (CsH6, 1) combines two substructures which are important in high-temperature pyrolysis of hydrocarbons: the aromatic ring and the ethynyl group which is part of ethyne, propyne, butenyne etc. Hydrocarbons of the enyne-and oligoenyne-type (butenyne, n-C6H6s etc.) are discussed as intermediates in the formation of aromatics in fuel-rich hydrocarbon flames ['] as well as in the pyrolysis of acetylenic hydrocarbons [2]. Ethynylbenzene can be considered as an enyne system having its double bonds integrated in the phenyl group.The hydrocarbon 1 cannot cyclize in a unimolecular reaction, but its structure offers may possibilities for enlargement towards polycyclic aromatic system~ [~,~]. For example, the ethynyl group could serve as a starting group for a second ring condensed to the first. Two molecules of 1 could react at their rings to form a substituted biphenyl with the ethynyl groups in "waiting position" to form hydrocarbons with three or four condensed rings by cycloisomerization.On the other hand, decomposition of 1, finally leading to benzene and ethyne, provides further building blocks for a number of addition and cyclization reactionsr3I.The unimolecular decay of 1 has been studied behind shock waved3], but almost nothing is known about the nature of the higher products which are formed. It is therefore the purpose of this work to identify these products which are formed in the range 700°C < T < 1100°C and to deterlysis temperature cannot be explained in terms of one reaction scheme only. It is suggested that H atoms act as important chain carriers. At temperatures around 700°C they mainly add to 1 yielding the phenylvinyl radicals l a and l b . These add to 1 forming dimers C&12 via radicalic intermediates C16H13. With increasing temperature the 2-phenylvinyl radical l a not only reacts back to H + 1 but also decomposes by p(C-C) cleavage into phenyl and ethyne. The latter channel is more endothermic by 33 kJ/mol. Additionally, isomeric ethynylphenyl radicals are increasingly formed by bimolecular H abstraction. Thus, with increasing temperature product formation is controlled by reactions of phenyl-type radicals.mine their yields as a function of the temperature. To characterize the most i...

The pyrolysis of ethynylbenzene (1) in helium was studied in a tubular flow reactor at 10.7 mbar/103O0C and reaction times ranging from 5 to 30 ms. Reactive intermediates such as radicals and carbenes were scavenged with dimethyl disulfide (DMDS). Qualitative and quantitative analysis of the scavenging products and of the stable pyrolysis products were carried out by GC-MS analysis. -The radicals phenyl In the preceding paperll] we have reported on the products of the pyrolysis of ethynylbenzene (1) which is also a component in fuel-rich flames, and have discussed it as a starter in mechanisms of formation of polycyclic aromatic hydrocarbons. The formation of the complex mixture of stable products from this pyrolysis between 700 and 1000°C has been interpreted by specified radical reactions only. From the structures of the products, however, it has not always been possible to decide whether carbenes may also be involved. To obtain a better insight into the complex reaction events, reactive intermediates such as radicals and carbenes are analyzed directly in the study presented here.For this purpose, a new scavenging method using dimethyl disulfide (DMDS) has been applied for the first time to the pyrolysis of hydrocarbons. This method is superior to gas-phase scavenging since it takes place at low temperature in the condensed phase under conditions where disturbing consecutive reactions of radicals and carbenes are negligible or can easily be recognized. It is particularly well suited to the analysis of medium-sized and large hydrocarbon radicals[2a,bl and has already been successfully applied to hydrocarbon f l a m e~ [~,~] .To investigate the influence of ethyne, which isa pyrolysis product and is also a major component in fuel-rich hydrocarbon flames, mixtures of 1 with various concentrations of ethyne were pyrolyzed separately. Results RadicalsThe radicals detected in the pyrolysis of 1 are given in Scheme 1. The major radical by far is phenyl (2a) followed in concentration by the three isomeric radicals 0-, m-, and p-ethynylphenyl (lc, d, e)lsl which in total make about a fifth of the amount of 2a. For quantitative data, see the concentration-time curves in the Figures 1 and 2. For the two isomeric radicals l a and lb, which were detected in still lower concentrations than the ethynylphenyl radicals, it was confirmed by the mass spectra of their scavenging products that the radical site is at the side chain, not at the ring. It was generally found from the fragmentation pattern of methylthio compounds that the SCH3 group is more strongly bound to the aromatic ring than to an aliphatic group, resulting in a relatively weak parent peak in the latter case. From a similarly weak parent peak in the mass spectra of the scavenging products of l a and l b the structure shown in Scheme 1 was deduced.Furthermore, 1 -and 2-naphthyl radicals (3a, b) occurred in about five percent relative to phenyl radicals. Using the solvent split technique, we also detected radicals such as 4a and 5a. These radicals are probably for...

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