J. Phys. Chem. 1992, 96,6657-6662
6657
Production of C60and Cl0 Fuiierenes in Benrene-Oxygen Flames Jack B. Howard,* J. Thomas McKinnon,+ M. Elaine Johnson, Yakov Makarovsky, and Arthur L. Lafleur Department of Chemical Engineering, Center for Environmental Health Sciences, and Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received: February 11, 1992; In Final Form: April 24, 1992)
Fullerenes C, and C70 were produced in varying amounts in premixed laminar benzene/oxygen/argon flames operated under ranges of conditionsincluding pressures of 12-100 Torr and C/O ratios from 0.717 to 1.072, the critical value for soot formation being 0.760. The fullerena were identifed in toluene extracts of condensed flame material including soot, using high-performance liquid chromatography (HPLC) with diode-array spectrophotometricdetection, mass spectrometry, and infrared spectroratio depend on temperature, pressure, carbon/oxygen ratio, and photometry. The yields of C, and C70 and the c7O/c@ residence time in the flame. In the sooting flames, C, and C70 first appear well after the onset of soot formation. The mass of C, + C70 produced is in the range 0.0026-9.21 of the soot, compared to 1-14% from the conventional graphite vaporization method, and the largest C, + C70yield is 0.26% of the fuel carbon, observed at a pressure of 20 Torr. A C, C70 yield of 2 X 10% of the fuel carbon was found in a nonsooting flame. The largest rate of production of C, + C70 was observed in a sooting flame at a pressure of 100 Torr. The c7O/c&, molar ratio varied over the range 0.26-5.7, compared to 0.02-0.18 for the graphite vaporization method, and can be controlled by selection of flame conditions. Depending on flame conditions, C70 fullerenes were produced with varying amounts of polycyclic aromatic hydrocarbons (PAH). The mass ratio the C, of PAH to C, + C70 varies roughly from 0.01 to 100 over the range of conditions studied. Also, the C, C70 fullerenes are accompanied by metastable C, and C70 isomers as well as CmO, C~OO, C76, (284, CW,and Cw, the identification of which by liquid chromatography/mass spectrometry is described elsewhere.
+
+
+
Iatroduction Fullerenes Ca and C70 were discovered' in carbon vapor produced by laser irradiation of graphite and later were produced in macmcopic quantities" by graphite vaporization with resistive heating. The possibility that fullerenes could also be formed in sooting flames had been suggested,@ and all-carbon ions having the same mass/charge ratios as fullerenes in carbon vapor had been detected in flames and assumed to have the closed-cage structure of fullerenes.'+I2 Recmtly, we reported our observations that both Cm and C70 fullerenes were produced in varying amounts from benzene/oxygen/argon flames operated over a range of conditi011s.l~ The proper selection of flame conditions gave fullerenes in yields up to 0.26% of the fuel carbon burned and allowed control of the C7o/Ca molar ratio over the range 0.26-5.7. The yield of c 6 0 + Goand the C7&, ratio were found to depend on temperature, pressure, carbon/oxygen ratio, and residence time in the flame. In order to characterize the flamegenerated fullerenes, samples of condensable compounds and soot were collected from flames under different conditions and analyzed using high-performance liquid chromatography with diode-array spectrophotometric detection (HPLC/DAD), mass spectrometry (MS), and infrared spectrophotometry (IR). In this paper, the combustion experiments and the chemical analysis of toluene extracts of the collected samples are provided in detail. Experimental Section Combpstioa Equipment. Premixed laminar flames of benzene and oxygen with argon diluent were stabilized on a water-cooled burner illustrated in Figure 1. The burner is in a low-pressure chamber equipped with windows and feed-throughs for visual observation, optical diagnostics, electrical ignition, and monitoring and sampling probes. The chamber is exhausted into a vacuum pump not shown in Figure 1. The burner consists of a horizontal drilled copper plate (100-mm diameter, 12 mm thick, l-mmdiameter holes centered 2.5 mm apart in a triangular array) upward through which the feed mixture is delivered. The flame is stabilized with a flat front uniformly displaced from the burner plate by a short distance which depends on the velocity of the gas leaving the burner and the flame speed of the mixture. Only the *To whom correspondence should be directed.
Current address: Chemical Engineering and Petroleum Refining Department. Colorado School of Mines, Golden, CO 80404.
inner 70-mm-diameter section of the burner plate is used for the experimental flame. The 15-mm-wide outer section is used for an independently fed fuetrich but nonsooting ethylene/oxygen/argon flame. This annular flame shields the experimental flame, allowing it to approximate a onedimensional core within which temperature and species concentrations vary only with distance, or residence time, from the burner surface, thereby simplifying the mathematical analysis of data. The essential features of the burner were designed to duplicate an extensively used burner in our laboratory, so as to allow reproduction of flame conditions for which profiles of temperature and concentration of many species including radicals had already been m e a ~ u r e d . ' ~ The J ~ present burner was previously used in mechanistic studies of soot nucleation and growth,16and the flames studied are of a type for which considerable data on temperature and chemical composition are available.'"22 Operatiog Conditions. Flames were produced under different sets of conditions over the following ranges: burner chamber pressure, 12-100 Torr (1.60-13.35 H a ) ; atomic C/O ratio, 0.717-1.072; mol 5% Ar,0-39; and gas velocity at the burner (298 K),14.6-75.4 cm/s. Under these conditions, the critical C/O ratio at which soot formation is impending is 0.760. Each flame was maintained for 53-170 min, depending upon conditions, while a sample of condensable compounds and soot was withdrawn from the flame at a given distance from the burner using a quartz probe connected to a room-temperature filter, vacuum pump, and gas meter (Figure 1). The probe consists of a tube tapered to an orifce at the tip. The orifice diameter was 2 mm in all but four runs, as discussed later. The probe was held vertically with the orifice directed upstream. The mass of the sample was primarily that of soot, except at the lowest C/O ratio, where the flame was nonsooting. Soot was also collected from the inside surface of the burner chamber after each run. Most of soot deposition inside the burner chamber occurred on the top surface and hence at a large distance from the burner as compared to the probe sampling positions. Sample PrepMbiOlL The samples of condensed compounds and soot were weighed, placed in beakers and extracted with toluene using an ultrasonic bath at room temperature, and filtered. The solutions were concentrated by evaporation under a stream of nitrogen. High-Performance Liquid Chromatography. The instrument used for HPLC analysis was a Hewlett-Packard Model 1090 HPLC equipped with a ternary pumping system and diode-array
0022-3654/92/2096-6657S03.00/0 , 0 1992 American Chemical Society I
'
Howard et al.
6658 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 I
90. -$80: u
f t
~
:
Z 0 7 0'
a
: 60:
3
50
W
40. I
5
d
:
m a
!360 301
1
i
IWl I
j k
350
420
400
450
500
550
600 m /e
650
700
750
800
E50
Figure 2. Electron impact mass spectrum of a flame soot extract. i
c 70
m
Figure 1. Burner and associated equipment: a, low-pressure chamber; b, copper burner plate; c, water cooling coil; d, e, and f, windows; g, h, and i, feed-throughs;j, annular-flame feed tube, k, core-flame feed tube, 1 and m, exhaust tubes; n, sampling probe; 0, filter; p, valve; q, vacuum pump; r, gas meter.
detector and controlled with a Model 7994 analytical workstation. Injection volumes ranging from 5 to 25 pL could be selected using the data system. The HPLC column selected for analysis of fullerenes was a Nucleosil (Macherey-Nagel, Duren, FRG)octadecylsilyl-bonded (C-18) silica column. It had a 4.6-mm i.d. and was 250 mm in length. It was packed with 5-pm material having a 300-A pore size. A binary nonaqueous mobile phase of acetonitrile and dichloromethane was used in a gradient elution mode. The mobile phase program consisted of a linear increase in dichloromethane concentration from 10 to 100%in 40 min with a 10-min hold time at 100%. The flow rate was set at 1.0 mL/min. For preparative-scale separations, a semipreparativeNucleosil octadecylsilyl-bonded silica analytical column was used. This column had a diameter of 10 mm and was also 250 mm long. It was packed with 7-pm material of 60-A pore size. Both columns were obtained from American Bioanalytical, Natick, MA. UV Spectrophotometry. The UV instrument consisted of a Hewlett-Packard Model 8450A diode-array spectrophotometer with a 7225B plotter and 9121B disk drive. Ultrapure glassdistilled decahydronaphthalene (decalin) was used as the solvent in order to ensure adequate dissolution and to maximize penetration into the UV region. Pure c 6 0 and C70 fullerenes were collected by HPLC and concentrated by evaporation under a stream of nitrogen. The concentrated fullerene solutions (in HPLC solvent) were exchanged into decalin by adding a measured volume to the fullerene solution and evaporating under a stream of nitrogen until the more volatile HPLC mobile phase evaporated, leaving the higher-boiling decalin. Mass Spectrometry. The instrument used to obtain electron impact mass spectra consisted of a Varian-MAT Model 73 1 mass spectrometer interfaced to a Teknivent data system. Samples of c 6 0 and C70fullerenes were isolated by preparative HPLC and were concentrated and evaporated to dryness in a suitable probe vessel using a vacuum centrifuge. Mass spectra were acquired as the direct injection probe was heated from 100 to 400 OC. Chemicals. Methanol, acetonitrile, decalin, and dichloromethane were Caledon HPLC grade obtained from American Bioanalytical, Natick, MA. A reference mixture of 16 PAH (SRM 1647) was obtained from the U S . National Institute for Standards and Technology. Reference quantities of naphtho[8,1,2-ubc]coronene and ovalene were generously provided by John Fetzer. Results IdenMcatiOa of C, and C,Fulkreaes. A sample of flame soot suspected of containing fullerenes was extracted with toluene, using
I w
1 1
o,zojI B
0051
1
IO
AJJ1;JI;
20 30 TIME ( M I N I
40
Figure 3. HPLC chromatogram of a typical fullerene soot extract.
an ultrasonic bath at room temperature, and filtered. The solution from one of the samples was evaporated to dryness and analyzed by mass spectrometry, as described above. The electron impact mass spectrum is shown in Figure 2. Comparing these data with those reported for f ~ l l e r e n e s , ~we - ~ ,concluded ~~ that the soot sample contained a mixture of c 6 0 and C70 fullerenes showing molecular ions at m / e 720 and 840, respectively, and doubly charged molecular ions at m / e 360 and 420, respectively. This conclusion was confirmed by Fourier transform infrared spectroscopy of the soot extract pressed in a KBr pellet, which gave strong absorption peaks consistent with those reported for fullerenes Cm2-5and C70.4 Although the MS data strongly suggested that fullerenes were the major constituents of the soot, any polycyclic aromatic hydrocarbons (PAH) having molecular weights of 720 or 840 were likely to give the same mass spectra as those we observed. With improvements in chemical analysis, larger and larger PAH are being observed in combustion samples. Therefore, in order to confirm our initial findings, further analysis was performed using high-performanceliquid chromatography with spectrophotometric diode-array detection (HPLC/DAD). This technique involves the continuous acquisition of UV spectra as peaks elute from the HPLC. The UV spectra of PAH are highly characteristic and can even permit the differentiation of isomeric PAH, a task difficult to achieve by MS?4 We evaluated a number of HPLC separation schemes for PAH but focused primarily on those shown effective for PAH having upward of 10 fused rings.2s The toluene extract was analyzed by HPLC/DAD, and the chromatogram obtained is shown in Figure 3. The signal consists of the broad-band W absorption in milliabsorbance units (mAU) over the 236-500-nm wavelength interval. The broad-band UV response plotted in Figure 3 is roughly proportional to mass for PAH.26 The most striking feature of the chromatogram is the virtual absence of peaks associated with the typical PAH commonly produced in flames. The peaks labeled c 6 0 and C70 gave UV spectra closely matching those published for c 6 0 and C70 fullerenes, re~pectively.~*~ The peaks labeled A-D have been analyzed by liquid chromatography/mass spectrometry and found to include C m 0(peak A), c 6 0 isomer (peak B), C70isomer (peak
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6659
Production of Ca and C70 Fullerenes
TABLE I: Experimental C d t i o m , Extent of Soot Formtion, and Srmple Size and Compodtion soot in flame no. sample P,”Torr C/O: M U: cm/s Ar,d % dist,’ cm % of C’ vol fraction 0 3.69 0.75 3.6 X lo4 12 0.960 75.4 1A 1B 12 0.960 75.4 0 CS‘ 2A 2B 3A 3B 4A 4B 5A 5B 6A 6B 7A 8A 8B 9A 9B 1OA 10B 11A 11B 12A 12B 13A 13B
20 20 20 20 20 20 20 20 40 40 40 40 40 40 40
40 40 100 100 100 100 100 100
0.7 17 0.717 0.784 0.784 0.960 0.960 0.996 0.996 0.960 0.960 0.960 0.960 0.960 0.960 0.960 1.072 1.072 0.960 0.960 0.996 0.996 0.996 0.996
50
50 50.3 50.3 50.3 50.3 49.1 49.1 25.1 25.1 25.1 25.1 25.1 25.1 25.1 23.4 23.4 14.6 14.6 14.4 14.4 37.2 37.2
30 30 30 30 10 10 10 10 10 10
10 10 10 10 10 11 11 38 38 39 39 39 39
1.425
cs 1.68 cs 2.1 1 cs 8 cs 0.8 cs
1.175 1.5
cs 3.0 cs 2.15 cs 1.45 cs 1.21 cs 1.5 cs
0
0
0.23
1.6 X lo4
3.6
2.7 X lo-*
3.0
4.5 X IO-*
1.8
2.8 X
1.5
2.3 X
7.3
1.2 X
8.4
2.6 X
12 2.4
3.7 x 10-7 7.5 X lo-’
sample size and composition soot, g Ca, mg C70, mg 0.0736 0.855 0.0246 0.108 1.35 0.445 3.85 0.0219 1.91 0.0429 0.0132 1.75 0.0115 1.77 0.515 5.78 1.31 4.60 1.06 6.02 0.509 2.82
0.54 0.053 0.0075 0.014 0.060 0.13 2.4 2.0 15 100 0 14 0 0.22 18 0.43 31 0.12 0.13 0.12 0.079 0.041 0.095 6.0 99
0.77 0.055 0.0065 0.014 0.11 0.21 3.5 5.3 15 180
0 31 0 0.40 32 0.62 52 0.037 0.40 0.32 0.29 0.27 0.062 8.5 120
“Pressure in combustion chamber. bCarbon-to-oxygenratio in flame. CGasvelocity at burner surface, at ressure P and 25 O C . dMolar percentage of argon in feed gas. ‘Perpendicular distance from burner surface to orifice at tip of sampling probe. Total carbon fed. ‘Denotes samples collected from combustion chamber surface.
P
C), and Cgq(peak D).27 Isomers of c60 and C70 fullerenes previously had not been observed experimentally. Other identifications include C700 (small unlabeled peak preceding peak C), two c 7 6 fullerenes (two unlabeled peaks preceding peak D),and (2% and Cg4(minor peaks after peak D,not easily discernible in Figure 3).27 Whether flame-generated fullerenes extend to or exceed the highest masses observed in graphite vaporizati~n~*-~~ remains to be determined. Uv-vis Spectm of C, a d C70 Fullerem. In order to obtain full-range ultraviolet-visible (UV-vis) spectra suitable for comparison with recently published data from other laboratories, the preparativescale HPLC fractionation of the flame sample extract was undertaken. After collection, fractions were concentrated by evaporation under nitrogen and the HPLC mobile phase was replaced with spectro-grade decalin. The UV-vis spectra of the C, and the C70 peaks were acquired using a spectrophotometer. WAVELENGTH ( n m l The results, illustrated in Figures 4 and 5 , are virtually identical Figure 4. UV-vis spectrum of HPLC C60fullerene peak. to those reported by Ajie et al.4 for fullerenes obtained from graphite vaporization. 0.50 Mass spectrometry of C, dCmHPLC Perks. Mass spectra of the fullerene HPLC fractions were acquired as before. The Ca peak gave a mass spectrum with the reported features of CW fullerene having a molecular ion base peak at m / e 720, showing no loss of hydrogen and having a doubly-charged molecular ion at m / e 360.2-4,23 Similarly, the C70 peak gave a mass spectrum with features closely matching those of published spectra for C70 fullerene showing a molecular ion base peak at m / e 840 and a doubly-charged molecular ion at m / e 420.24923Therefore, the identities of the HPLC peaks suggested by UV-vis spectra were confmed by mass spectra. The HPLC method, including gravimetric calibration of the c 6 0 and C70 peaks, was then used to analyze toluene extracts of all the samples. Yields of C, and C70 Fullerenes. The amounts of fullerenes 0 C , and C70 found under the different flame conditions are shown 200 300 400 500 600 in Table I along with the amount of soot in each sample. Samples WAVELENGTH (nm) 2A and 2B are from a nonsooting flame. Soot values listed in Figure 5. UV-vis spectrum of HPLC C70 fullerene peak. the table refer to the whole sample, consisting of the fraction soluble in toluene, which was largely fullerenes and polycyclic is expressed as a fraction of the total volume of flame gases at the temperature and pressure of the flame. These calculations aromatic hydrocarbons, and the toluene-insoluble material. The mass of soot collected with the probe is expressed as a fraction are based on the metered volume of flame gas withdrawn with of the carbon in the fuel fed to the flame, and the soot volume the condensed sample, the known feed rates and burner chamber
Howard et al.
6660 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 TABLE II: Fullerene Yields and Coacentratiorro h F"ea fullerene mass, % of carbon fed
fullerene mass. % of scot 1A 1B 2A 2B 3A 3B 4A 4B 5A 5B 6A 6B 7A 8A 8B 9A 9B 1 OA 1 OB 11A 11B 12A 12B 13A 138
0.73 0.0063
1.1 0.0065
1.8 0.013
0.24
0.44
0.68
2.2 0.15 3.4 2.6 0 0.76 0 1.6 1.0 3.8 1.8 0.023 0.0022 0.0089 0.0017 0.0038 0.0016 1.2 3.5
3.2 0.39 3.5 4.6 0 1.6 0 3.1 1.8 5.4 2.9 0.0072 0.0070 0.024 0.0062 0.025 0.0010 1.7 4.3
5.4 0.54 6.9 7.3 0 2.4 0 4.7 2.8 9.2 4.7 0.030 0.0092 0.033 0.0079 0.029 0.0026 2.9 7.8
fullerene conc in flame. n/cm3
1.2 0.83 0.74 0.86 1.6 1.4 1.3 2.2 0.88 1.5
5.5 x 10-3
7.9 X lW3
1.3
1.1 x lo-'
9.4 X
2.0 X lo4
5.6 X lo4
1.0 X lW3
1.2 x 10-1
1.3
o/o
0
0
0 3.0 X
1.8 010 1.6 1.5 1.2 1.7 0.26 2.7 2.4 3.1 5.7 0.56 1.2 1.1
6.7 X lo-"
1.1 X
1.4 X
1.2 X
2.6 X
1.6 X lo-)
6.8 X 10-l2
1.2 X lo-"
1.9 X lo-"
2.5
1.6 X lo4
1.7 X lo+
3.3 X lo4
0
0
0
0
0 5.6 X
0 8.6 X
0 8.2 X lo-''
0 1.5
X
0 2.3
X
lo+
5.7 x 10-2
8.2 X
1.4 X lo-'
1.6 X
2.2 X
3.8
X
lo+
1.7 x 10-3
5.3
2.2 X
4.8
1.5 X lo-"
6.3 X lo-"
7.5 x 10-4
2.1 x 10-3
2.8 x 10-3
4.2 x 10-11 1.2 x 10-10 1.6 x 10-10
4.6 X lo4
3.0 X lo-)
3.5 X
2.5 X lo-''
1.7
X
2.8
3.9 X
6.7 X
1.6 X
2.3
X
X
pressure, and a nominal flame temperature of 1800 K which is representative of previous measurements under similar conditions.I5J6 The increase in number of moles accompanying combustion under the different conditions was approximated by equilibrium calculations using S T A N J A N . ~ ~The density of soot particles with associated condensables was taken as 1.8 g/cm3, based on the C/H ratio of such soots33and the observed codation of density with C/H ratio for a range of pertinent carbon materials and polycyclic aromatic hydrocarbon^.^^ These calculations are possible only for the samples withdrawn with the probe, because the volume of flame gas associated with the collected material is not known for samples removed from the chamber surface. The effective flame position represented by a sample withdrawn with the probe is a few probe orifice diameters upstream of the physical position of the probe tip.35 The orifice diameter was 2 mm except in the cases of samples 6A, 8A, and 9A, where it was 0.7 mm, and sample 7A, where it was 1.O mm. The smaller d i c e diameters were used to achieve a finer resolution of distance from the burner. If the upstream displacement is assumed to be two to three orifice diameters, sample 7A would be equivalent to a sample taken with a 0.7-mm-diameter probe orifice located approximately 1.1 cm from the burner surface. Accordingly, data from sample 7A may be combined with those from samples 6A, 8A, and 9A in the study of concentrationprofiles. With similar adjustments for the upstream displacement, samples 2A and 3A are the equivalent of samples taken with a 0.7-mmdiameter probe orifice at distances from the burner surface of 1.10 and 1.36 cm, respectively. Based on previous measurements in these two flames,14J5the last two distances mark the positions of the peak concentration of the sum of all species of molecular weight 700 g/mol or larger. The yields of Ca and C70 expressed as percentages of the soot mass, as percentages of carbon in the fuel burned, and as the molar ratio of C7,, to Ca are given in Table 11. The Ca and C70 concentrations in the flames, computed only for the probe samples as discussed above, are also included.
X
X
10-'
lo4
4.7
X
X
10-1
X
X
lo-"
lo-"
1.9 X lo+
3.9 X lo4
10% Ar,and a gas velocity at the burner of 49.1 cm/s. The latter yield is estimated for the chamber surface sample SB, using probe sample SA, based on the assumption that the soot concentration in the flame gases varies little between these two samples since both represent large distances from the burner. Accordingly, the Cso C70 yield for sample 5B, expressedas a fraction of the carbon fed, is approximately the value measured for sample 5A multiplid by the ratio of the Ca C70 yield, expressed as a fraction of the soot, for sample 5B to that for sample SA. The observed yields reveal that combustion of, for example, kilograms per minute of benzene can produce grams per minute of Ca and C70 fullerenes. Given these results and the ability to scale up combustion pnxmpsar in flow reactors, flame synthesis is an alternative method for large-scale production of fullerenes, as mentioned before."." Although the largest yield of Ca C7&expressed as a fraction of the carbon fed, under the present conditions is observed at a pressure of 20 Torr, the largest rate of production of c 6 0 + C70, in units of mass/time, occurs at a pressure higher than 20 Torr. As pressure is increased within the range of conditions studied here, the decrease in the fractional yield of fullerenes is to some extent offset by the increased density and hence increased rate of mass flow through the combustion chamber for a given gas velocity through the burner. Thus, the ratio of the c 6 0 C70 production rate for the 100-Torr case with the latgest Ca +C70 yield (sample 13A) to that for the 20-Torr case with the highest Ca C70 yield (sample SA) is 0.69. However, the 100-Torr sample, taken closer to the burner (1.5 cm) t h n the 20-Torr sample (8 cm),represents an earlier stage of fullerenes production, as can be seen by comparing the fullerenes content of the soot from the probe with that from the chamber surface. The latter sample represents a large distance from the burner, as mentioned earlier, and hence a later stage of fullerenes production. From Table 11, the Ca + C70 fullerene content of the soot from the chamber surface exceeds that from the probe sample by only 1.06 for the 20-Torr case, but by 2.69 for the 100-Torr case. Recomputing the C60 C70 production rates with consideration of the larger distance from the burner as represented by the surface samples, the rate is 2.0 times larger for the 100-Torr case than for the 20-Torr case. The individual C a and C70 production rates for the 100-Torr case are 4.6 mg/min of Ca and 5.6 mg/min of
+
+
+
+
+
+
Discussion The mass of c 6 0 + C70 produced under the different sooting flame conditions is in the range 0.0026-9.2% of the soot mass (Table 11), compared to 1-14% from graphite ~aporization,3-~--'~ c70. The conditions identified above for the largest Ca + C70 yield using similar solvents and extraction procedures in both cases. (flame 5) and largest c 6 0 + C70 production rate (flame 13) obThe c 6 0 + C70 yield, expressed as a percentage of fuel carbon, served in the present study can be employed to achieve substantial varies from 2 X lo4% for the nonsooting flame (sample 2A) to fullerene production. However, the conditions for the largest 0.26%,obtained at a pressure of 20 Torr, a C/O ratio of 0.996,
Production of
c60
and C70 Fullerenes
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6661
possible yield or production rate from flames of the type studied and H2elimination. One of the most important hydrogen-ab here are not identified by the present work. The limited number stracting radicals in these flames is the H atom, whose mole fraction at the flame position discussed above is 0.008.’5 Calof sets of conditions studied does not provide sufficient resolution culations of the balance between hydrogen removal and addition of the parameter space to reveal the optimum conditions. A more indicate that a large fraction, e.g., up to 0.3 or more, of the carbon detailed systematic exploration of the parameter space is now atoms in the edge of PAH species may be without hydrogen atoms underway and will soon be reported. in important regions of flames such as these.48 The C70/CW molar ratio for the different flame conditions is Thus, the rate of growth of fullerene precursors at a given in the range 0.26-5.7 (Table 11), compared to 0.02-0.18 for position in the flame is the net effect of competing growth and graphite vaporization.’” The ratio is 0.88 for the above conditions destruction reactions, and the amount of fullerenes observed at of maximum CW+ C70 yield. The much larger yields of C70 and a given sampling position is the integral net growth occurring the ability to control the c70/c60 ratio by setting the flame through the region of the flame preceding, or upstream of, the conditions are significant differences from the graphite vaporiposition sampled. The same statement can be made with respect zation technique. The C70/c60 ratio tends to increase with into soot precursors, but there are important differences between creasing pressure and decreasing distance from the burner (or soot formation and fullerenes formation. As can be seen from decreasing residence time in the flame), but other simple trends samples 6A, 7A, 8A, and 9A in Tables I and 11, fullerenes Cm in this ratio are difficult to discern from these data alone. Over and C70 first appear after soot formation has already progressed the ranges of conditions of this study, neglecting the cases of zero Cm and C70 yields (samples 6A and 7A), the overall change in to a substantial concentration in the flame. Conversely, from sample 2A, Cm and C70 are seen to be formed in a flame producing the C70/CWratio is a factor of about 20 as compared to the overall no soot. change in the c 6 0 and C70 yields, which is a factor of order lo3 in both cases. These observations are qualitatively consistent with the known behavior of the competing growth and destruction reactions in The production of metastable fullerene isomers in flames, the case of soot precursors and the expected difference between mentioned above, is another major contrast with the well-known the reaction rates of these species and those of fullerene precursors graphite vaporization technique. The observation of isomers of when both are in the presence of the same growth and destruction c 6 0 fullereneZ7is apparently the first production and collection reactants. Given the curved and hence strained configuration of of fullerenes having adjacent five-membered rings in their fullerene intermediates, these species are expected to grow less structure. The separation of pentagons by at least one six-memrapidly than the large planar PAH envisioned to be soot prebered ring so as to avoid the strain energy added by adjacent c u r s o r ~ . ~For ~ ,the ~ ~ same reason, fullerene intermediates are pentagons has been a widely accepted constraint in the study of expected to be more reactive than the soot precursors in the fullerene structure^.^^ This constraint must be violated by the destruction reactions. However, once the fullerene molecules have observed c 6 0 isomers since the separation of pentagons can be formed, they are less reactive than soot, owing to the absence of satisfied by only one c 6 0 fullerene, namely, the icosahedral edge atoms in the fullerenes. Therefore, in a given flame, the net buckminsterfullerene,’ which is the identity of the stable flamegrowth rate of fullerene precursors would be expected to be less derived Cm f~1lerene.l~ The constraint must also be violated by than that of soot precursors, consistent with fullerenes appearing the apparently C50 fullerene, seen as a prominent peak in mass well after the onset of soot formation in sooting flames. Also, spectra from molecular beam sampling of carbon vapor'^^*^^ and the lower reactivity of completed fullerene structures as compared flame’*12 systems but not yet reported from analyses of macroto the reactivity of soot would indicate that fullerenes, once formed, scopic samples. A number of different c60 isomers have been considered theoretically in structural or stability a n a l y ~ e s . ~ * ~could ~ ~ remain present even under conditions where soot would be destroyed. Such a situation seems to exist in the nomooting flame Experimental study of the structure of the flamesynthesized c 6 0 represented by sample 2A. As mentioned above, this sample was isomers is now underway. taken at the position of the maximum concentration of species Another contrast between fullerene synthesis in flames and in larger than 700 g/mol, which is the position at which the onset graphite vaporization systems, which contain only carbon, is the of soot formation would occur if the C/O ratio were increased presence of oxygen and hydrogen as well as carbon in the flames. by only 5%.15 Thus, soot formation is impending at the position For example, sample 2A is from a flame in which temperature of sample 2A but does not occur because, as the distance from and concentrations of over 50 species were measured previously the burner increases beyond this position, the rate of destruction along the centerline of the flame at different distances from the of the soot precursors exceeds the growth rate. The observation burner using molecular beam sampling with on-line mass specof fullerenes c 6 0 and C70 in this flame shows that these species trometry.lS At the flame position from which sample 2A was can indeed form and attain recoverable quantities under conditions withdrawn, the mole fraction of O2is 0.022 and the O2flux (in where the accompanying high molecular weight soot precursors, mol/(cm2 s)), computed from the concentration profile by taking although exhibiting larger peak concentrations2’than the observed diffusion and convection into account, corresponds to 26% of the fullerene’s concentration, are unable to generate soot. O2fed. Thus, O2 is still abundant at this position of fullerene formation, as is also the OH radical at a measured mole fraction The largest yields of fullerenes do not occur in the most heavily of 0.002. These species, which are known to react with PAH and sooting flames. Also,the fullerene yield increases with increasing temperature or decreasing pressure under conditions where the soot particles,@are expected to attack fullerene precursors, leading to carbon removal from the precursors and CO formation. same changes result in lower soot yields. These trends in the data reflect substantial differences between the formation mechanisms Concurrently, again drawing upon known reactions of flame of fullerenes as compared to those of soot. As discussed above, species with PAH and soot material,48 carbon addition to the both formation processes involve many of the same types of reprecursors is expected to occur through reactions with C2H2,PAH, actions, e.g., growth in small steps by addition of CzH2and other and other species including radicals. At the flame position consmall species and in large steps by reactions between polycyclic sidered above, the mole fractions of benzene and C2H2are 3.3 X and 3.1 X corresponding, from flux calculations, to compounds, competition between growth and destruction reactions, and extensive involvement of radical sites at edge carbons in the 0.4% of the carbon fed still being in the form of C6H6 while 67% growing polycyclic structure^.^^ The different trends exhibited is present as C 2 ~ 2 . * 4Also J 5 present at this position are substantial quantities of PAH, from naphthalene to species of molecular by the two processes may reflect different reactivities to, and hence weight 700 g/mol and larger, having been formed earlier in the roles of, the different reactions as required for the very different flame during the rapid destruction of benzene. Dimerization of structures being formed. Soot formation does not involve the development of curved or bowl-shaped structures as required for fullerene precursors through radical addition or recombination reactions may also be important. Both the growth and destruction fullerenes, nor would these curved structures permit the rapid reactions are expected to involve hydrogen removal from edge stacking required in the reactive coagulation of the relatively flat atoms of planar and curved PAH structures, through H abstraction sheetlike soot precursor^.^^,^^ That the very special fullerene
6662 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
structures are formed in flames and that the formation occurs in the presence or absence of soot formation provide new insight into the reactions important to both processes.
Acknowledgment. We thank Chenhui Zeng and Catherine Costello of the MIT Mass Spectrometry Facility (NIH Grant RR00317 to K. Biemann) for obtaining the mass spectra. The combustion research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S.Department of Energy, under Grant DE-FGO284ER13282. The analytical investigation was supported by National Institute of Environmental Health Sciences Center Grant EHS-5P30-ES02109-10 and Program Grant EHS-SPOlES01640-11. References and Notes (1) Kroto, H. W.; Heath, J. R.; OBrien, S.C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Kritschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (3) Taylor, R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H. W. J . Chem. SOC.,Chem. Commun. 1990, 1423. (4) Ajie, H.; Alvarez, M. M.; Anz, S. J.; Beck, R. D.; Diederich, F.; Fostiropoulos, K.; Huffman, D. R.; Kritschmer, W.; Rubin, Y.; Schriver, K. E.; Sensharma, D.; Whetten, R. L. J . Phys. Chem. 1990, 94, 8360. ( 5 ) Haufler, R. E.; Conceicao, J.; Chibante, L. P. F.; Chai, Y.; Byme, N. E.; Flanagan, S.;Haley, M. M.; OBrien, S.C.; Pan, C.; Xiao, Z.; Billups, W. E.; Ciufolini, M. A.; Hauge, R. H.; Margrave, J. L.; Wilson, L. J.; Curl, R. F.; Smalley, R. E. J. Phys. Chem. 1990, 94, 8634. (6) Zhang, Q. L.; OBrien, S. C.; Heath, J. R.; Liu, Y.; Curl, R. F.; Kroto, H. W.; Smalley, R. E. J. Phys. Chem. 1986, 90, 525. (7) Kroto, H. W.; McKay, K. Nature 1988, 331, 328. (8) Curl, R. F.; Smalley, R. E. Science 1988, 242, 1017. (9) Kroto, H. Science 1988, 242, 1139. (10) Gerhardt, Ph.; Ldffler, S.; Homann, K. H. Chem. Phys. Lett. 1987, 137, 306. (1 1) Gerhardt, Ph.; Ldffler, S.; Homann, K. H. Proceedings of the Twenty-Second Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1989; pp 395-401. (12) Ldffler, S.; Homann, K. H. Proceedings of the Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1991; pp 355-362. (13) Howard, J. B.; McKinnon, J. T.; Makarovsky, Y.; Lafleur, A. L.; Johnson, M. E. Nature 1991, 352, 139. (14) Bittner, J. D. Sc.D. Thesis, MIT, Cambridge, MA, 1981. (1 5 ) Bittner, J. D.; Howard, J. B. Proceedings of the Eighteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1981; pp 1105-1116. (16) McKinnon, J. T. Ph.D. Thesis, MIT, Cambridge, MA, 1989. (17) Homann, K. H.; Mochizuki, M.; Wagner, H. Gg. 2.Phys. Chem. (Munich) 1963, 37, 299. (18) Homann, K. H.; Morgeneyer, W.; Wagner, H. Gg. In Proceedings of the Combustion Insiiture European Symposium; Weinberg, F. J., Ed.; Academic Press: London, 1973; pp 394-399.
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