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argon flames
Carbon Vol.
34, No. 3, pp. 317-326,1996 Copyright 0 1996 ElsevierScienceLtd
Pergamon
Printed in Great Britain.All rights reserved 0008-6223/96$15.00+ 0.00
0008-6223(95)00175-1
EFFECT
OF HBr, HCI AND Cl2 ON FULLERENE FORMATION IN BENZENE/OXYGEN/ARGON FLAMES
H. RICHTER,~*’ A. FONSECA,~ J.-M.
GILLES,~ J. B.NAGY,’ P. A. THIRY,’
R. ROZENBERG,~ E. DE HoFFMANN,~ and H. PASCH~ ‘Laboratoire de Physico-Chimie de la Combustion, Universitt Catholique de Louvain, Place Louis Pasteur, 1 B-1348 Louvain-la-Neuve Belgium 21nstitute for Studies in Interface Sciences, Facultes Universitaires Notre-Dame de la Paix, Rue de Bruxelles, 61 B-5000 Namur Belgium 3Laboratoire de Spectromttrie de Masse, UniversitC Catholique de Louvain, Place Louis Pasteur, 1 B-1348 Louvain-la-Neuve Belgium 4Deutsches Kunststoff-Institut (DKI) SchloDgartenstraBe 6, D-64289 Darmstadt, Germany (Received 10 July 1995; accepted in revised form 29 August 1995) Abstract-Amounts between 2.5 and 5.9% of HBr, HCl and CIZ were added to fullerene forming benzene/oxygen/argon flat-flames burning at 75 mbar in an attempt to affect halogen substitutions in the carbon framework. The inhibition effect of HBr dramatically reduced the yield of fullerene, while it stayed in the same order of magnitude after addition of HCl or C12. No formation of halogenated fullerenic compounds could be detected by mass spectrometry using chemical ionization with CH,/N,O or MALDI. The measurement of bromine- and chlorine-precursor mass spectra showed the formation of a great number of bromine or chlorine containing compounds, essentially polycyclic aromatic hydrocarbons (PAH). In the case of HBr addition, only relatively small molecules could be observed because reactions with bromine atoms are thermodynamically less favourable in regions far from the burner with relatively high temperatures. Key Words-Fullerenes,
flames, combustion, bromine, chlorine. inhibition.
1. INTRODUCTION Combustion is a large research field involving many different physical and chemical methods. It has great practical interest owing to its importance for energy production, locomotion and waste treatment. There are a great number of research groups working on the increase of combustion efficiency and the reduction of pollution. An important aspect is the formation of soot. The chemical mechanism of soot formation has been investigated using mass spectrometric methods usually with aliphatic (acetylene) or aromatic (benzene) combustibles [ 1,2]. The role of polycyclic aromatic hydrocarbons (PAH) was described [l] and their formation was investigated for neutral [2-41 and ionic [1,5,6] species. After the discovery of fullerenes in 1985 [7], the presence of fullerene ions could be shown in fuel-rich acetylene and benzene flames by the Homann group [ 8,9]. The synthesis of macroscopic amounts of fullerenes by means of the electrical arc method [lo] was followed by the extraction of fullerenes from soot generated in benzene flames by the Howard group [ 1 l-131. In fact, the use of hydrocarbon flames for fullerene synthesis seems to be a potential industrial method due to its easy up-scaling [14]. Based on their long experience in soot formation, both groups suggested first mechanisms for fullerene formation in flames [15-181. The addition of halogenated compounds in flames has been relatively well-studied because of their inhibiting effect [19-211. At first, only the macroscopic
inhibiting effect on the flame propagating velocity was studied [19,21,22]. Later, detailed flame structures, i.e. the mole fraction profiles of the species involved in the combustion process as a function of the height above the burner, essentially of halogenated methane derivatives, was measured by means of molecular beam sampling coupled to mass spectrometry [23,24]. Numerical modelling has also been used [24-261. The inhibiting role of halogens has been pointed out [27]. In the present work, the influence of the addition of HBr, HCl and Cl, to fullerene-forming benzene/oxygen/argon low pressure flat-flames was studied. The generated soot was collected and analyzed. HPLC was used after Soxhlet extraction with toluene in order to determine the quantity of C6,, and C,. fullerenes formed. The other species present in the flame soot were measured by mass spectrometry. We used chemical ionization by means of a CH4/N20 mixture as the ionization technique, as well as Matrix Assisted Laser Desorption Ionization (MALDI). Measurements of precursor ion spectra allowed the identification of chlorine- and brominecontaining compounds. Some conclusions concerning the inhibition in benzene/oxygen flames were also drawn. Another objective was the search for halogenated fullerenic compounds. 2. EXPERIMENTAL The flames were stabilized vertically on a movable stainless steel burner, 100 mm in diameter, mounted 317
H. RICHTERet al.
318
Table 1. Experimental conditions of the benzene/oxygen/argon Flame
Added compound
C/O
%Ar
P [mbar]
v [cm s-‘]25”C
time [min]
HBr : 2.5% HCI : 5.9% Cl, : 4.3%
1.07 1.06 1.06 1.06
38.6 38.6 37.2 37.9
75 75 75 75
43.5 43.8 45.4 44.6
40 2 2.5 4
1 2 3 4
in a stainless steel combustion chamber of inner diameter 280mm and height 300 mm. The burner temperature was controlled by a thermostat. Low pressure conditions (75 mbar) were realized by means of a two stage rotary pump with a pumping capacity of 65 m3 h-‘. Pressure control was operated by a motor-driven throttle valve coupled to a pressure controller which is connected to a capacitance gauge. The gas flows of argon, oxygen, benzene and HBr, HCl or Cl, were controlled by means of commercial mass flow controllers. Since benzene is liquid under the experimental conditions used, it was evaporated from a heated 10 1 container, and a magnetic stirrer was used in order to avoid irregular boiling. The benzene vapour pressures were measured before the flow controller with a capacitance gauge coupled to a temperature controller, regulating the temperature in order to maintain a constant vapour pressure. Because a source of unaccuracy could stem from the gas flows, the gas flow calibrations of argon, oxygen and acetylene were checked directly, by means of a conventional gas flow meter. In the case of benzene, we calibrated by the measurement of the time necessary for the evaporation of a known volume of liquid benzene; the corresponding gas volume was calculated by means of the general gas law. The flames were ignited by an ordinary spark.
3. RESULTS One benzene/oxygen/argon flame without any addition was kept burning for 40 minutes and three flames under addition of HBr, HCl or Cl, were kept burning for between 2 and 4 minutes. The experimental conditions shown in Table 1. were kept similar for the four flames in order to get comparable results. After each flame, the combustion chamber was opened for collecting the as-formed soot. The soot was analyzed by means of a Waters HPLC system equipped with an UV/VIS detector operating at 350 nm and a Regis analytical Buckyclutcher I column (Trident-Tri-DNP, 250 x 4.6 mm). The samples were prepared for analysis by Soxhlet extraction with toluene for 40 hours. As chromatographic Table 2. Soot and fullerene formation Flame 1 2 3 4
flames
soot [g h-‘1
% CC,, + C,,,)
C&0 + C,, [mg h-l1
4.1 25.9 8.8 10.9
2.61 0.0014 3.15 3.42
107.3 0.36 278.4 372.0
%&3 1.42 0.9 1 1.26 1.31
eluent we used a mixture of 50% toluene and 50% hexane at a flow rate of 1 ml min-‘. The identification of C,, and C,, fullerenes was performed by using a solution of known concentration prepared with commercially available pure fullerenes, peak integration allowing quantitative analysis. The results are summarized in Table 2 and the chromatogram of Flame 4 soot is shown in Fig. 1 for reference. An increase of soot formation after the addition of halogenated compounds can be observed. The most important variation was stated for Flame 2. The soot formation became very high, the flame was visibly less stable and it burnt further from the burner surface. Due to these variations of the burning conditions, the fullerene formation nearly stopped. Such variations are directly related to flame inhibition. In fact, from a chemical point of view, a flame is maintained by means of propagation and ramification steps including reactive radicals. In the case of the simplest flame, the H,/Oz system, the most important reactions are: (1) OH + H, o H,O + H: propagation (2) H + 0, o OH + 0: ramification (3) 0 + H, o OH + H: ramification. (4) 2H A
H2,
Owing to the increase of the number of radicals due to the ramification reactions (2) and (3), this system should explode, but chain terminating reactions like for example on the metallic burner surface, lead to an equilibrium between radical formation and consumption which allows a deflagration instead of a detonation. In the case of hydrocarbon flames, the number of chemical species and reactions increases rapidly as a function of the number of carbon atoms in the combustible. Nevertheless, these reaction mechanisms will always include the reactions of the HZ/O, system, the simplest one containing no carbon atoms [28]. The flame structure measurements already mentioned allow the determination of the concentrations of reactants, products and intermediate species as a function of the height above the burner, i.e. important information for a better understanding of the combustion chemistry. Generally, these measurements are done in low pressure flames because a diminution of the pressure leads to a larger reaction zone (flame front) so that the evolution of the different species concentrations can be observed more easily. Chemical flame inhibition must be understood as a modification of the described reaction mechanism, where radical species are removed from the system
The addition of HBr, HCI and Cl, to benzene/oxygen/argon
flames used in fullerene production
319
5.750: c,
0. lo-
0.09:
0.08 -
0.07 -
.z 9
0.06 -
8.033: c,,
ia” .g e 4
0.05 -
0.04 -
I
I
10
I
1
I
20
Min
Fig. 1. HPLC
chromatogram
and are then unavailable for combustion. The most efficient method is the scavenging of hydrogen atoms because the chain branching reaction with 0, molecules, (2) H + O2 o 0 + OH, is the slowest one and so determines the velocity of the combustion process. In the case of HBr, the inhibiting effect results from the following cycle of reactions [26]: (5) H + HBr o H, + Br (6) H + Br, o HBr + Br (7)Br + Br + MoBr,+ M. The sum of these three reactions gives the recombination of two H atoms into a relatively unreactive H, molecule: (8) H + H o H,.
3.1 Precursor ion mass spectra Precursor ion spectra were measured by means of a Finnigan-Mat TSQ 70 triple-stage Quadrupole mass spectrometer. As an evaporation technique, we used Desorption Chemical Ionization (DCI). A rhenium filament on which the sample had been deposited was rapidly heated (40 to 1000 mA, at a rate of 400 mA s-r); the ionization of the desorbed species was performed chemically with a CHJN,O mixture. In the second chamber the ions formed reacted unimolecularly after collision with xenon atoms forming chlorine or bromine ions which were focused in the third stage. A scan with the first mass spectrome-
of Flame 4 soot.
ter thus detected all species containing chlorine or bromine. The negative ion mode was used. For the interpretation it must be considered that the chemical ionization in the negative mode leads to the capture of an electron or to the removal of a proton. The latter case is denoted in the tables by [M-H] after the detected masses. These ions may be identified by the odd parity of the masses, as no nitrogen is present. 3.1.1 Flame 2 The bromine precursor mass spectrum measured in the negative ion scan mode is shown in Fig. 2. Only compounds with molecular masses up to 332 Dalton could be detected. This observation shows that reactions were only occurring relatively near the burner, because heavier species are only formed in the zone beyond about 10 mm from the burner surface [S]. This phenomenon can be explained by the relative stability of bromine atoms at high temperature [23] so that further reactions are not thermodynamically favourable. The temperature in the near-sooting premixed benzene/oxygen/argon flame [ 31 - well known and relatively similar to the present flames - reached a maximum value of about 1900 K. Radical species (R*) will react with HBr in order to generate a stable compound (R-H) and a Br-radical. Table 3 indicates the masses of some typical peaks and suggests molecular formulae as well as possible structures. The composition and the structures of the compounds given in Table 3 are only suggestions con-
320
H.
RICHTER et al.
158.1
,
VI
.z 8
0
a a
80
100
120
7 L.L
.”
160
140
*
180
220
200
1
100
4
L..
80
B
232.2
I?’
123.4
20
I
60 40
20
0
sidering molecules.
the
260
280
Fig. 2. Negative
thermodynamical
300
320
340
360
380
ion bromine precursor mass spectrum of Flame 2 soot.
stability
of
the
In fact, they are stable species formed by the scavenging of intermediate compounds during the combustion process. In the case of the peaks 232, 280 and 304, similar compounds - without bromine - were detected in the near-sooting premixed benzene/oxygen/argon flame by Bittner and Howard [ 31. Potentially, all polycyclic aromatic hydrocarbons (PAH) and oxygenated PAH described in the literature [ 1,3,5,6] could be scavenged by reaction with bromine-atoms, but only a limited number of compounds like C&H,Br, C16H9Br or C,,H9Br were detected in important quantities. However, no fullerenic compound resulting from an addition of HBr or Br was detected. 3.1.2 Flame 3 A large number of species covering the whole mass range investigated could be scavenged in the HCl-containing flame. Figure 3 shows the chlorine precursor mass spectrum of Flame 3 soot measured in the negative ion scan mode. Table 4 summarizes some important peaks and suggests their molecular formulae and their structures. Besides these species, a large number of heavier chlorinated species could be detected. Due to their high molecular mass, molecular structures cannot be suggested. The species reported in Table 4 shows the easy formation of species chlorinated more than once. 3.1.3 Flame 4 Figure 4 shows the chlorine precursor mass spectrum in the negative ion scan mode of Flame 4 soot. Table 5 summarizes some important peaks and suggests their molecular formulae and eventually their structures. The large number of detected chlorine-containing
species shows the scavenging of intermediate polycyclic aromatic hydrocarbon (PAH) radicals in each flame region during their growing process. The scavenging as stable chlorinated compounds allows their detection in the soot by mass spectrometry. The presence of mono-chlorinated species confirms the chlorination by radical mechanism; it is the only reasonable mechanism because it maintains the aromatic character of the species involved.
3.2 MALDI mass spectra An important question in the present work was the possible formation of halogenated fullerenes. Because of the probable instability of such compounds we used a Kratos Kompact MALD13, a Matrix Assisted Laser Desorption Ionisation (MALDI) using a N,-LASER with 337 nm, coupled to a time-of-flight (TOF) mass spectrometer - a very soft ionization method which has been developed for the analysis of large molecules in biology, avoiding fragmentations [ 291. No halogenated fullerenes could be detected. However, some interesting scavenged species could be observed. Figure 5 shows the first part of the negative ion MALDI spectrum of Flame 2 soot. The triplets indicate the presence of two bromines because of the two isotopes 79Br and “Br. The peaks at 220,222 and 224 Dalton could represent the compound C,H,Br, (4,4-dibromo-1,3-pentadiyne). In the case of the peaks at 292, 294 and 296 Dalton we suggest C,,H,,Br2, i.e. an aliphatic hydrocarbon with two bromine atoms, one double and one triple bond. The differences between the spectra measured using chemical ionization and with the MALDI
The addition of HBr, HCl and Cl, to benzene/oxygen/argon
flames used in fullerene production
321
Table 3. Some major bromine precursor peaks of Flame 2 soot Mass (dalton)
Identity
19.2 123.4 [M-H] 151.4 [M-H]
Br CzHd(OH)Br, CdHs(OH)Br,
158.1
2-bromoethanol bromobutanol
Br2 79Br-81Br
159.9
232.2
Structure
Cl2H9Br.
bromoacenaphthene
Br
or bromobiphenyl
280.4
Cl&Br,
bromofluoranthene
or bromopyrene Br
304.3
ClsHgBr, bromobenzo[ghi]fluoranthene Br
or bromocyclopenta[cd]pyrene
332.2
CldbBr2 Br
technique can be explained by the different ionization conditions: a possible thermal instability during the heating up to about 1000 K must be considered. The masses 373, 375 and 377 Dalton could be explained
BI
by dibromo-benzofluorene, dibromomethylfluoranthene or dibromomethyl-pyrene shifted by one unit. The analysis of Flames 3 and 4 soot gave very similar MALDI-spectra. Figure 6 shows the first part
H.
322 I
813.2 ’ 7.40
11
1
2
100
RICHTER
et al.
x60
80
60 117.1 I
40
142.4
20
0 100
200
300
400
600
700
800
900
501
’ x60
100
80
60
Fig. 3. Negative
I
ion chlorine
mass spectrum
of Flame
3 soot.
1 x6o
83.1
LOO
precursor
125.1
1105
137.3
1
80 60
-139.5
161.0
235.1
40 20 0
.
.I 50
I,
, 100
150
200
0
250
x60 100 , r-
105 210 I
CT
80
60 368.7 40 20 0
100
332.5 ~~Il~~~~~~~~l~~~~~~~l~~U 300 I
465.6 350
400
450
,1
0
500
’10s
x60
80 60 40 20 0 !u&d
Hti
550
illadbdtibW~l~,d 600
650
700
750 IlO5
100
x6o
1
60 80 3 40
t
20 0
800
Fig. 4. Negative
,
850
ion chlorine
0
1000
precursor mass spectrum of Flame 4 soot
of the flame 4 soot negative ion spectrum. Important peaks for phenyldichloromethane or chloronaphthalene and phenyltrichloromethane or dichloronaphthalene could be recognized.
3.3 Oxygen-containing aromatic hydrocarbons There are a great number of mass-spectrometric peaks which are attributed to oxygenated aromatic hydrocarbons. Some of these compounds have been
The addition of HBr, HCl and Cl, to benzene/oxygen/argon
flames used in fullerene production
323
294 100
95 90 85 80 75 70 65 60 r; 55 z 3 v
50 45 40 35 30 25
48 I
:;
Mass/charge Fig. 5. First part of the negative ion MALDI - TOF mass spectrum of Flame 2 soot.
197 100
95 90 85 80 75 70 65 60 _: 55 5
50
E
45
160
40 35 30 25 20 15
i
10 5 0
---z?-
60
L 80
100
120
140
160
180
:oo
., 220
241 240
260
280
300
Mass/charge Fig. 6. First part of the negative ion MALDI - TOF mass spectrum of Flame 4 soot.
identified by the Homann group [ 303. They used the scavenging of radicals formed in a low pressure benzene/oxygen flame below the sooting limit by means of dimethyl disulfide (DMDS). The stable compounds formed by the scavenging reactions were collected in a cold trap and analyzed by mass spec-
trometry (MS) after separation with gas chromatography (GC). Comparing these results with the present work, the formation of bromocresol isomers (186 Dalton) must be suggested for Flame 2 (Fig. 2). In the case of Flame 3 (Fig. 3), the peak at 142.4 Dalton could
H. RICHTER et al.
324 Table 4.
Some major
chlorine
precursor
Mass (dalton)
Identity
83.2 [M-HI
CH&12
peaks of Flame
3 soot Structure
CHC13
117.1 [M-H] 142.4
C3C13H, trichloropropyne
157.2 [M-H]
C4H5C13, trichlorobutene
235.5 [M-H]
C16H&1, chlorofluoranthene
or chloropyrene
C16HsC12. dichlorofluoranthene
270.1
Cl
represent
chloro-cresol
isomers
instead
of the trichl-
3. In Flames 3 and 4 (Figs 3 and 4), the peak at 166 Dalton can be attributed to one or several chlorinated C,H,O isomers (isomers of indenol, methylbenzofuran, ethynylmethylphenol, ethenylbenzaldehyde) while the peak at 178 Dalton could be identified as a chloronaphthol. oro-propyne
suggested
4. GENERAL
in Table
DISCUSSION
AND CONCLUSIONS
It can be concluded that the addition of halogenated compounds did not lead to the formation of halogenated fullerenic species with a sufficient thermal and photochemical stability. This was confirmed by means of soft ionization techniques like Desorption Chemical Ionization (DCI) and Matrix Assisted Laser Desorption Ionization (MALDI). Nevertheless it cannot be excluded that small quantities of these compounds were destroyed during the ionization so that they could not be detected. The three added compounds HBr, HCl and Cl, intervene in the combustion processes by scavenging intermedi-
J&2 00
or dichloropyrene
Cl
ate compounds. The addition led to an important increase of soot formation in all three cases, but to a larger extent for HBr (Table 2). The comparison of the effect of HCl and Cl, addition shows a dependence of the soot quantity on the total number of added chlorine atoms (added as HCl or Cl,). However, the experimental data are not sufficient to distinguish the efficiency of HCl and Cl, for the increase of soot formation. It must be suggested that the increase of soot formation is directly related to flame inhibition. In fact, the scavenging of H radicals as shown in the reactions (5)-( 8) will also lead to a sensible reduction of the 0 and the OH formation by reaction (2) H + 0, o OH + 0. The lower concentrations of 0 and OH are responsible for the decrease of oxydation reactions so that the continuous formation of larger aromatic rings, e.g. by several subsequent acetylene additions, becomes more important. The experimental results of the present work are in agreement with the observation of a much stronger inhibition effect of bromine-containing compounds than that of chlorine-containing ones [22,27]. A related argument is
The addition
of HBr, HCI and Cl, to benzene/oxygen/argon Table 5.
Some major
chlorine
precursor
83.1 [M-H]
CH2C12
116.9 [M-H]
CHCls
137.3 [M-H]
(CsH5)CH=CHCl
139.5
(CsHs)CH2-CH2Cl
CHaCl
C6He35C137C1, phenyldichloromethane
or chloronaphthalene,
Ct&,Cl2,
195.1 [M-H]
270.1
Structure
C7H7C1, benzylchloride
125.1 [M-H]
235.1 [M-H]
325
peaks of Flame 4 soot
Identity
Mass (dalton)
161.0 [M-H]
flames used in fullerene production
CtsHsCl, Ct6HsCla,
CtcH7Cl
dichloronaphthalene
chlorofluoranthene dichlorofluoranthene
cJ@@-cl
or chloropyrene or dichloropyrene
332.5 [M-H]
C24HttC1, chlorocoronene
368.7 [M-H]
C24HtOC12, dichlorocoronene
465.6
C32HtaCl2, dichloroovalene
the fact that the C-Br bond is weaker than the C-Cl bond, so that the thermal or photochemical breaking of C-Br bonds leads simultanously to reactive intermediate species which can form larger aromatic rings and to flame-inhibiting Br radicals. The stronger C-Cl bond also explains the formation of much more chlorinated compounds after the addition of HCl or Cl, (Figs 3 and 4) than of brominated ones after the addition of HBr (Fig. 2) due to the higher stability of chlorinated species.
=Cl
Another result of this work is the discovery of a drastic decrease in fullerene formation after HBr addition. This point is directly related to the bifurcation of the soot formation mechanism and the fullerene formation mechanism as has been discussed by the Homann group [ 151. Given the complex chemistry of soot-forming flames and the consideration of homogeneous and heterogeneous reactions, actually only speculation seems to be possible. It must be stated that the decrease of fullerene formation after
H. RICHTER et al.
326
HBr addition is in agreement with the experimental observation of a sensible decrease of the fullerene yield for very high soot formation rates in low pressure benzene/oxygen/argon flames without any additive [ 13,311. One explanation could be the decrease of temperature in soot-rich flames which could favour the continuation of the formation of larger polycyclic aromatic hydrocarbons (PAH) due to relatively
higher
responsible
for
will
be necessary
chemical
activation
fullerene
mechanism
for
energies
formation.
a better of fullerene
of the reactions Further
understanding
studies of the
formation.
Acknowledgements~This work was funded by the Belgian National Programme of Interuniversity Research Projects initiated by the State Prime Minister’s Office (Services of Scientific, Technical and Cultural Affairs), by the Wallonia Region and by the Belgian National Fund for Scientific Research (FNRS, Brussels) for the mass spectrometry equipment.
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34, No. 3, pp. 317-326,1996 Copyright 0 1996 ElsevierScienceLtd
Pergamon
Printed in Great Britain.All rights reserved 0008-6223/96$15.00+ 0.00
0008-6223(95)00175-1
EFFECT
OF HBr, HCI AND Cl2 ON FULLERENE FORMATION IN BENZENE/OXYGEN/ARGON FLAMES
H. RICHTER,~*’ A. FONSECA,~ J.-M.
GILLES,~ J. B.NAGY,’ P. A. THIRY,’
R. ROZENBERG,~ E. DE HoFFMANN,~ and H. PASCH~ ‘Laboratoire de Physico-Chimie de la Combustion, Universitt Catholique de Louvain, Place Louis Pasteur, 1 B-1348 Louvain-la-Neuve Belgium 21nstitute for Studies in Interface Sciences, Facultes Universitaires Notre-Dame de la Paix, Rue de Bruxelles, 61 B-5000 Namur Belgium 3Laboratoire de Spectromttrie de Masse, UniversitC Catholique de Louvain, Place Louis Pasteur, 1 B-1348 Louvain-la-Neuve Belgium 4Deutsches Kunststoff-Institut (DKI) SchloDgartenstraBe 6, D-64289 Darmstadt, Germany (Received 10 July 1995; accepted in revised form 29 August 1995) Abstract-Amounts between 2.5 and 5.9% of HBr, HCl and CIZ were added to fullerene forming benzene/oxygen/argon flat-flames burning at 75 mbar in an attempt to affect halogen substitutions in the carbon framework. The inhibition effect of HBr dramatically reduced the yield of fullerene, while it stayed in the same order of magnitude after addition of HCl or C12. No formation of halogenated fullerenic compounds could be detected by mass spectrometry using chemical ionization with CH,/N,O or MALDI. The measurement of bromine- and chlorine-precursor mass spectra showed the formation of a great number of bromine or chlorine containing compounds, essentially polycyclic aromatic hydrocarbons (PAH). In the case of HBr addition, only relatively small molecules could be observed because reactions with bromine atoms are thermodynamically less favourable in regions far from the burner with relatively high temperatures. Key Words-Fullerenes,
flames, combustion, bromine, chlorine. inhibition.
1. INTRODUCTION Combustion is a large research field involving many different physical and chemical methods. It has great practical interest owing to its importance for energy production, locomotion and waste treatment. There are a great number of research groups working on the increase of combustion efficiency and the reduction of pollution. An important aspect is the formation of soot. The chemical mechanism of soot formation has been investigated using mass spectrometric methods usually with aliphatic (acetylene) or aromatic (benzene) combustibles [ 1,2]. The role of polycyclic aromatic hydrocarbons (PAH) was described [l] and their formation was investigated for neutral [2-41 and ionic [1,5,6] species. After the discovery of fullerenes in 1985 [7], the presence of fullerene ions could be shown in fuel-rich acetylene and benzene flames by the Homann group [ 8,9]. The synthesis of macroscopic amounts of fullerenes by means of the electrical arc method [lo] was followed by the extraction of fullerenes from soot generated in benzene flames by the Howard group [ 1 l-131. In fact, the use of hydrocarbon flames for fullerene synthesis seems to be a potential industrial method due to its easy up-scaling [14]. Based on their long experience in soot formation, both groups suggested first mechanisms for fullerene formation in flames [15-181. The addition of halogenated compounds in flames has been relatively well-studied because of their inhibiting effect [19-211. At first, only the macroscopic
inhibiting effect on the flame propagating velocity was studied [19,21,22]. Later, detailed flame structures, i.e. the mole fraction profiles of the species involved in the combustion process as a function of the height above the burner, essentially of halogenated methane derivatives, was measured by means of molecular beam sampling coupled to mass spectrometry [23,24]. Numerical modelling has also been used [24-261. The inhibiting role of halogens has been pointed out [27]. In the present work, the influence of the addition of HBr, HCl and Cl, to fullerene-forming benzene/oxygen/argon low pressure flat-flames was studied. The generated soot was collected and analyzed. HPLC was used after Soxhlet extraction with toluene in order to determine the quantity of C6,, and C,. fullerenes formed. The other species present in the flame soot were measured by mass spectrometry. We used chemical ionization by means of a CH4/N20 mixture as the ionization technique, as well as Matrix Assisted Laser Desorption Ionization (MALDI). Measurements of precursor ion spectra allowed the identification of chlorine- and brominecontaining compounds. Some conclusions concerning the inhibition in benzene/oxygen flames were also drawn. Another objective was the search for halogenated fullerenic compounds. 2. EXPERIMENTAL The flames were stabilized vertically on a movable stainless steel burner, 100 mm in diameter, mounted 317
H. RICHTERet al.
318
Table 1. Experimental conditions of the benzene/oxygen/argon Flame
Added compound
C/O
%Ar
P [mbar]
v [cm s-‘]25”C
time [min]
HBr : 2.5% HCI : 5.9% Cl, : 4.3%
1.07 1.06 1.06 1.06
38.6 38.6 37.2 37.9
75 75 75 75
43.5 43.8 45.4 44.6
40 2 2.5 4
1 2 3 4
in a stainless steel combustion chamber of inner diameter 280mm and height 300 mm. The burner temperature was controlled by a thermostat. Low pressure conditions (75 mbar) were realized by means of a two stage rotary pump with a pumping capacity of 65 m3 h-‘. Pressure control was operated by a motor-driven throttle valve coupled to a pressure controller which is connected to a capacitance gauge. The gas flows of argon, oxygen, benzene and HBr, HCl or Cl, were controlled by means of commercial mass flow controllers. Since benzene is liquid under the experimental conditions used, it was evaporated from a heated 10 1 container, and a magnetic stirrer was used in order to avoid irregular boiling. The benzene vapour pressures were measured before the flow controller with a capacitance gauge coupled to a temperature controller, regulating the temperature in order to maintain a constant vapour pressure. Because a source of unaccuracy could stem from the gas flows, the gas flow calibrations of argon, oxygen and acetylene were checked directly, by means of a conventional gas flow meter. In the case of benzene, we calibrated by the measurement of the time necessary for the evaporation of a known volume of liquid benzene; the corresponding gas volume was calculated by means of the general gas law. The flames were ignited by an ordinary spark.
3. RESULTS One benzene/oxygen/argon flame without any addition was kept burning for 40 minutes and three flames under addition of HBr, HCl or Cl, were kept burning for between 2 and 4 minutes. The experimental conditions shown in Table 1. were kept similar for the four flames in order to get comparable results. After each flame, the combustion chamber was opened for collecting the as-formed soot. The soot was analyzed by means of a Waters HPLC system equipped with an UV/VIS detector operating at 350 nm and a Regis analytical Buckyclutcher I column (Trident-Tri-DNP, 250 x 4.6 mm). The samples were prepared for analysis by Soxhlet extraction with toluene for 40 hours. As chromatographic Table 2. Soot and fullerene formation Flame 1 2 3 4
flames
soot [g h-‘1
% CC,, + C,,,)
C&0 + C,, [mg h-l1
4.1 25.9 8.8 10.9
2.61 0.0014 3.15 3.42
107.3 0.36 278.4 372.0
%&3 1.42 0.9 1 1.26 1.31
eluent we used a mixture of 50% toluene and 50% hexane at a flow rate of 1 ml min-‘. The identification of C,, and C,, fullerenes was performed by using a solution of known concentration prepared with commercially available pure fullerenes, peak integration allowing quantitative analysis. The results are summarized in Table 2 and the chromatogram of Flame 4 soot is shown in Fig. 1 for reference. An increase of soot formation after the addition of halogenated compounds can be observed. The most important variation was stated for Flame 2. The soot formation became very high, the flame was visibly less stable and it burnt further from the burner surface. Due to these variations of the burning conditions, the fullerene formation nearly stopped. Such variations are directly related to flame inhibition. In fact, from a chemical point of view, a flame is maintained by means of propagation and ramification steps including reactive radicals. In the case of the simplest flame, the H,/Oz system, the most important reactions are: (1) OH + H, o H,O + H: propagation (2) H + 0, o OH + 0: ramification (3) 0 + H, o OH + H: ramification. (4) 2H A
H2,
Owing to the increase of the number of radicals due to the ramification reactions (2) and (3), this system should explode, but chain terminating reactions like for example on the metallic burner surface, lead to an equilibrium between radical formation and consumption which allows a deflagration instead of a detonation. In the case of hydrocarbon flames, the number of chemical species and reactions increases rapidly as a function of the number of carbon atoms in the combustible. Nevertheless, these reaction mechanisms will always include the reactions of the HZ/O, system, the simplest one containing no carbon atoms [28]. The flame structure measurements already mentioned allow the determination of the concentrations of reactants, products and intermediate species as a function of the height above the burner, i.e. important information for a better understanding of the combustion chemistry. Generally, these measurements are done in low pressure flames because a diminution of the pressure leads to a larger reaction zone (flame front) so that the evolution of the different species concentrations can be observed more easily. Chemical flame inhibition must be understood as a modification of the described reaction mechanism, where radical species are removed from the system
The addition of HBr, HCI and Cl, to benzene/oxygen/argon
flames used in fullerene production
319
5.750: c,
0. lo-
0.09:
0.08 -
0.07 -
.z 9
0.06 -
8.033: c,,
ia” .g e 4
0.05 -
0.04 -
I
I
10
I
1
I
20
Min
Fig. 1. HPLC
chromatogram
and are then unavailable for combustion. The most efficient method is the scavenging of hydrogen atoms because the chain branching reaction with 0, molecules, (2) H + O2 o 0 + OH, is the slowest one and so determines the velocity of the combustion process. In the case of HBr, the inhibiting effect results from the following cycle of reactions [26]: (5) H + HBr o H, + Br (6) H + Br, o HBr + Br (7)Br + Br + MoBr,+ M. The sum of these three reactions gives the recombination of two H atoms into a relatively unreactive H, molecule: (8) H + H o H,.
3.1 Precursor ion mass spectra Precursor ion spectra were measured by means of a Finnigan-Mat TSQ 70 triple-stage Quadrupole mass spectrometer. As an evaporation technique, we used Desorption Chemical Ionization (DCI). A rhenium filament on which the sample had been deposited was rapidly heated (40 to 1000 mA, at a rate of 400 mA s-r); the ionization of the desorbed species was performed chemically with a CHJN,O mixture. In the second chamber the ions formed reacted unimolecularly after collision with xenon atoms forming chlorine or bromine ions which were focused in the third stage. A scan with the first mass spectrome-
of Flame 4 soot.
ter thus detected all species containing chlorine or bromine. The negative ion mode was used. For the interpretation it must be considered that the chemical ionization in the negative mode leads to the capture of an electron or to the removal of a proton. The latter case is denoted in the tables by [M-H] after the detected masses. These ions may be identified by the odd parity of the masses, as no nitrogen is present. 3.1.1 Flame 2 The bromine precursor mass spectrum measured in the negative ion scan mode is shown in Fig. 2. Only compounds with molecular masses up to 332 Dalton could be detected. This observation shows that reactions were only occurring relatively near the burner, because heavier species are only formed in the zone beyond about 10 mm from the burner surface [S]. This phenomenon can be explained by the relative stability of bromine atoms at high temperature [23] so that further reactions are not thermodynamically favourable. The temperature in the near-sooting premixed benzene/oxygen/argon flame [ 31 - well known and relatively similar to the present flames - reached a maximum value of about 1900 K. Radical species (R*) will react with HBr in order to generate a stable compound (R-H) and a Br-radical. Table 3 indicates the masses of some typical peaks and suggests molecular formulae as well as possible structures. The composition and the structures of the compounds given in Table 3 are only suggestions con-
320
H.
RICHTER et al.
158.1
,
VI
.z 8
0
a a
80
100
120
7 L.L
.”
160
140
*
180
220
200
1
100
4
L..
80
B
232.2
I?’
123.4
20
I
60 40
20
0
sidering molecules.
the
260
280
Fig. 2. Negative
thermodynamical
300
320
340
360
380
ion bromine precursor mass spectrum of Flame 2 soot.
stability
of
the
In fact, they are stable species formed by the scavenging of intermediate compounds during the combustion process. In the case of the peaks 232, 280 and 304, similar compounds - without bromine - were detected in the near-sooting premixed benzene/oxygen/argon flame by Bittner and Howard [ 31. Potentially, all polycyclic aromatic hydrocarbons (PAH) and oxygenated PAH described in the literature [ 1,3,5,6] could be scavenged by reaction with bromine-atoms, but only a limited number of compounds like C&H,Br, C16H9Br or C,,H9Br were detected in important quantities. However, no fullerenic compound resulting from an addition of HBr or Br was detected. 3.1.2 Flame 3 A large number of species covering the whole mass range investigated could be scavenged in the HCl-containing flame. Figure 3 shows the chlorine precursor mass spectrum of Flame 3 soot measured in the negative ion scan mode. Table 4 summarizes some important peaks and suggests their molecular formulae and their structures. Besides these species, a large number of heavier chlorinated species could be detected. Due to their high molecular mass, molecular structures cannot be suggested. The species reported in Table 4 shows the easy formation of species chlorinated more than once. 3.1.3 Flame 4 Figure 4 shows the chlorine precursor mass spectrum in the negative ion scan mode of Flame 4 soot. Table 5 summarizes some important peaks and suggests their molecular formulae and eventually their structures. The large number of detected chlorine-containing
species shows the scavenging of intermediate polycyclic aromatic hydrocarbon (PAH) radicals in each flame region during their growing process. The scavenging as stable chlorinated compounds allows their detection in the soot by mass spectrometry. The presence of mono-chlorinated species confirms the chlorination by radical mechanism; it is the only reasonable mechanism because it maintains the aromatic character of the species involved.
3.2 MALDI mass spectra An important question in the present work was the possible formation of halogenated fullerenes. Because of the probable instability of such compounds we used a Kratos Kompact MALD13, a Matrix Assisted Laser Desorption Ionisation (MALDI) using a N,-LASER with 337 nm, coupled to a time-of-flight (TOF) mass spectrometer - a very soft ionization method which has been developed for the analysis of large molecules in biology, avoiding fragmentations [ 291. No halogenated fullerenes could be detected. However, some interesting scavenged species could be observed. Figure 5 shows the first part of the negative ion MALDI spectrum of Flame 2 soot. The triplets indicate the presence of two bromines because of the two isotopes 79Br and “Br. The peaks at 220,222 and 224 Dalton could represent the compound C,H,Br, (4,4-dibromo-1,3-pentadiyne). In the case of the peaks at 292, 294 and 296 Dalton we suggest C,,H,,Br2, i.e. an aliphatic hydrocarbon with two bromine atoms, one double and one triple bond. The differences between the spectra measured using chemical ionization and with the MALDI
The addition of HBr, HCl and Cl, to benzene/oxygen/argon
flames used in fullerene production
321
Table 3. Some major bromine precursor peaks of Flame 2 soot Mass (dalton)
Identity
19.2 123.4 [M-H] 151.4 [M-H]
Br CzHd(OH)Br, CdHs(OH)Br,
158.1
2-bromoethanol bromobutanol
Br2 79Br-81Br
159.9
232.2
Structure
Cl2H9Br.
bromoacenaphthene
Br
or bromobiphenyl
280.4
Cl&Br,
bromofluoranthene
or bromopyrene Br
304.3
ClsHgBr, bromobenzo[ghi]fluoranthene Br
or bromocyclopenta[cd]pyrene
332.2
CldbBr2 Br
technique can be explained by the different ionization conditions: a possible thermal instability during the heating up to about 1000 K must be considered. The masses 373, 375 and 377 Dalton could be explained
BI
by dibromo-benzofluorene, dibromomethylfluoranthene or dibromomethyl-pyrene shifted by one unit. The analysis of Flames 3 and 4 soot gave very similar MALDI-spectra. Figure 6 shows the first part
H.
322 I
813.2 ’ 7.40
11
1
2
100
RICHTER
et al.
x60
80
60 117.1 I
40
142.4
20
0 100
200
300
400
600
700
800
900
501
’ x60
100
80
60
Fig. 3. Negative
I
ion chlorine
mass spectrum
of Flame
3 soot.
1 x6o
83.1
LOO
precursor
125.1
1105
137.3
1
80 60
-139.5
161.0
235.1
40 20 0
.
.I 50
I,
, 100
150
200
0
250
x60 100 , r-
105 210 I
CT
80
60 368.7 40 20 0
100
332.5 ~~Il~~~~~~~~l~~~~~~~l~~U 300 I
465.6 350
400
450
,1
0
500
’10s
x60
80 60 40 20 0 !u&d
Hti
550
illadbdtibW~l~,d 600
650
700
750 IlO5
100
x6o
1
60 80 3 40
t
20 0
800
Fig. 4. Negative
,
850
ion chlorine
0
1000
precursor mass spectrum of Flame 4 soot
of the flame 4 soot negative ion spectrum. Important peaks for phenyldichloromethane or chloronaphthalene and phenyltrichloromethane or dichloronaphthalene could be recognized.
3.3 Oxygen-containing aromatic hydrocarbons There are a great number of mass-spectrometric peaks which are attributed to oxygenated aromatic hydrocarbons. Some of these compounds have been
The addition of HBr, HCl and Cl, to benzene/oxygen/argon
flames used in fullerene production
323
294 100
95 90 85 80 75 70 65 60 r; 55 z 3 v
50 45 40 35 30 25
48 I
:;
Mass/charge Fig. 5. First part of the negative ion MALDI - TOF mass spectrum of Flame 2 soot.
197 100
95 90 85 80 75 70 65 60 _: 55 5
50
E
45
160
40 35 30 25 20 15
i
10 5 0
---z?-
60
L 80
100
120
140
160
180
:oo
., 220
241 240
260
280
300
Mass/charge Fig. 6. First part of the negative ion MALDI - TOF mass spectrum of Flame 4 soot.
identified by the Homann group [ 303. They used the scavenging of radicals formed in a low pressure benzene/oxygen flame below the sooting limit by means of dimethyl disulfide (DMDS). The stable compounds formed by the scavenging reactions were collected in a cold trap and analyzed by mass spec-
trometry (MS) after separation with gas chromatography (GC). Comparing these results with the present work, the formation of bromocresol isomers (186 Dalton) must be suggested for Flame 2 (Fig. 2). In the case of Flame 3 (Fig. 3), the peak at 142.4 Dalton could
H. RICHTER et al.
324 Table 4.
Some major
chlorine
precursor
Mass (dalton)
Identity
83.2 [M-HI
CH&12
peaks of Flame
3 soot Structure
CHC13
117.1 [M-H] 142.4
C3C13H, trichloropropyne
157.2 [M-H]
C4H5C13, trichlorobutene
235.5 [M-H]
C16H&1, chlorofluoranthene
or chloropyrene
C16HsC12. dichlorofluoranthene
270.1
Cl
represent
chloro-cresol
isomers
instead
of the trichl-
3. In Flames 3 and 4 (Figs 3 and 4), the peak at 166 Dalton can be attributed to one or several chlorinated C,H,O isomers (isomers of indenol, methylbenzofuran, ethynylmethylphenol, ethenylbenzaldehyde) while the peak at 178 Dalton could be identified as a chloronaphthol. oro-propyne
suggested
4. GENERAL
in Table
DISCUSSION
AND CONCLUSIONS
It can be concluded that the addition of halogenated compounds did not lead to the formation of halogenated fullerenic species with a sufficient thermal and photochemical stability. This was confirmed by means of soft ionization techniques like Desorption Chemical Ionization (DCI) and Matrix Assisted Laser Desorption Ionization (MALDI). Nevertheless it cannot be excluded that small quantities of these compounds were destroyed during the ionization so that they could not be detected. The three added compounds HBr, HCl and Cl, intervene in the combustion processes by scavenging intermedi-
J&2 00
or dichloropyrene
Cl
ate compounds. The addition led to an important increase of soot formation in all three cases, but to a larger extent for HBr (Table 2). The comparison of the effect of HCl and Cl, addition shows a dependence of the soot quantity on the total number of added chlorine atoms (added as HCl or Cl,). However, the experimental data are not sufficient to distinguish the efficiency of HCl and Cl, for the increase of soot formation. It must be suggested that the increase of soot formation is directly related to flame inhibition. In fact, the scavenging of H radicals as shown in the reactions (5)-( 8) will also lead to a sensible reduction of the 0 and the OH formation by reaction (2) H + 0, o OH + 0. The lower concentrations of 0 and OH are responsible for the decrease of oxydation reactions so that the continuous formation of larger aromatic rings, e.g. by several subsequent acetylene additions, becomes more important. The experimental results of the present work are in agreement with the observation of a much stronger inhibition effect of bromine-containing compounds than that of chlorine-containing ones [22,27]. A related argument is
The addition
of HBr, HCI and Cl, to benzene/oxygen/argon Table 5.
Some major
chlorine
precursor
83.1 [M-H]
CH2C12
116.9 [M-H]
CHCls
137.3 [M-H]
(CsH5)CH=CHCl
139.5
(CsHs)CH2-CH2Cl
CHaCl
C6He35C137C1, phenyldichloromethane
or chloronaphthalene,
Ct&,Cl2,
195.1 [M-H]
270.1
Structure
C7H7C1, benzylchloride
125.1 [M-H]
235.1 [M-H]
325
peaks of Flame 4 soot
Identity
Mass (dalton)
161.0 [M-H]
flames used in fullerene production
CtsHsCl, Ct6HsCla,
CtcH7Cl
dichloronaphthalene
chlorofluoranthene dichlorofluoranthene
cJ@@-cl
or chloropyrene or dichloropyrene
332.5 [M-H]
C24HttC1, chlorocoronene
368.7 [M-H]
C24HtOC12, dichlorocoronene
465.6
C32HtaCl2, dichloroovalene
the fact that the C-Br bond is weaker than the C-Cl bond, so that the thermal or photochemical breaking of C-Br bonds leads simultanously to reactive intermediate species which can form larger aromatic rings and to flame-inhibiting Br radicals. The stronger C-Cl bond also explains the formation of much more chlorinated compounds after the addition of HCl or Cl, (Figs 3 and 4) than of brominated ones after the addition of HBr (Fig. 2) due to the higher stability of chlorinated species.
=Cl
Another result of this work is the discovery of a drastic decrease in fullerene formation after HBr addition. This point is directly related to the bifurcation of the soot formation mechanism and the fullerene formation mechanism as has been discussed by the Homann group [ 151. Given the complex chemistry of soot-forming flames and the consideration of homogeneous and heterogeneous reactions, actually only speculation seems to be possible. It must be stated that the decrease of fullerene formation after
H. RICHTER et al.
326
HBr addition is in agreement with the experimental observation of a sensible decrease of the fullerene yield for very high soot formation rates in low pressure benzene/oxygen/argon flames without any additive [ 13,311. One explanation could be the decrease of temperature in soot-rich flames which could favour the continuation of the formation of larger polycyclic aromatic hydrocarbons (PAH) due to relatively
higher
responsible
for
will
be necessary
chemical
activation
fullerene
mechanism
for
energies
formation.
a better of fullerene
of the reactions Further
understanding
studies of the
formation.
Acknowledgements~This work was funded by the Belgian National Programme of Interuniversity Research Projects initiated by the State Prime Minister’s Office (Services of Scientific, Technical and Cultural Affairs), by the Wallonia Region and by the Belgian National Fund for Scientific Research (FNRS, Brussels) for the mass spectrometry equipment.
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