Energy & Fuels 1988,2, 709-716
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Identification of Aromatic Alkynes and Acyclic Polyunsaturated Hydrocarbons in the Output of a Jet-Stirred Combustor Arthur L. Lafleur,* John J. Gagel, John P. Longwell, and Peter A. Monchamp Center for Environmental Health Sciences, Department of Chemical Engineering, Department of Chemistry, and Energy Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Received February 29, 1988. Revised Manuscript Received June 22, 1988
As part of a study of the combustion of ethylene in a jet-stirred reactor, combustion products in the C4-CI2 range were characterized by gas chromatography coupled with mass spectrometry and Fourier transform infrared spectrometry. Some components yielded equivocal mass spectra whose simple patterns appeared to indicate the presence of aromatic rings but whose molecular mass information ruled out such structures. Several others gave mass spectra nearly identical with those of benzene and toluene while still others gave mass spectra similar to those of toluene and benzene, but with molecular ions shifted 2 and 4 mass units lower. These data strongly suggested that many components were highly unsaturated acyclic molecules; however, the lack of structural detail in their mass spectra combined with the scarcity of reference compounds restricted the usefulness of the GC/MS data. The IR data confirmed the presence of acyclic benzene and toluene isomers and demonstrated the existence of other acyclic polyunsaturated hydrocarbons. A number of aromatic acetylene derivatives were identified as well. Many of the compounds found have been postulated as intermediates in theoretical models for the formation and growth of polycyclic aromatic compounds.
The identification of polycyclic aromatic compounds (PAC) has been the subject of much inquiry because of the potential health hazard posed by a number of PAC when they are introduced to humans by the inhalation of combustion products, particularly soot.' Toxicology studies have shown that bioactivity of PAC is associated with certain structural types and that it may vary greatly among structural and positional isomers.2 An understanding of the pathways that give rise to the more active PAC can help in controlling the formation of these species in combustion systems. The classic source of environmental PAC is combustion of coal where polycyclic aromatics are a major component of the coal; however, PAC are also found in combustion of aliphatic hydrocarbons such as methane and the paraffins found in diesel fuel and domestic heating oil. It is believed that PAC increase in number of rings by reaction of acetylenic species with smaller aromatic compounds.36 The initial formation of single-ring compounds is therefore a subject of special i n t e r e ~ t . ~Formation ,~ of benzene is believed to involve reactions of acetylene with C4species such as butadienyl radical (C4H5.)or the radical formed from methylacetylene by hydrogen abstraction to form linear (26 radicals that subsequently cyclize to form benzene and a hydrogen atom or a phenyl radical. An alternative result is formation of stable compounds such as the linear C,H, species found in this study. In this case, linear C6H6is a byproduct that is part of the network of reactions involved in benzene synthesis. These linear acetylenic compounds are therefore of considerable importance in establishing an understanding of the chemistry involved in aromatic synthesis in flames. The formation of PAC more complex than benzene can also be elucidated by the study of polyunsaturated inter-
* To whom correspondence should be addressed at the Center for Environmental Health Sciences. 0887-0624/88/2502-0709$01.50/0
mediate^.^ One of the proposed mechanisms for the formation of PAC in flames involves phenyl radical addition to acetylene as one of several steps. A predicted byproduct of this mechanism is phenylacetylene, and indeed a 102 amu species was seen by sampling a flame using molecular beam mass spectrometry.' However, the identity of this species could not be determined with the mass spectrometer used. In this work, our unequivocal identification of phenylacetylene confirms and extends the previous experimental work and supports the proposed mechanism, thereby contributing significantly to the understanding of how polycyclic aromatic hydrocarbons are formed in flames. Structural elucidation of flame-generated intermediates involved in the formation of aromatics and PAC requires the identification and differentiation of many closely related aromatic and polyunsaturated hydrocarbons in complex combustion mixtures. The current method of choice is gas chromatography-mass spectrometry (GC/ MS) but it has at least two disadvantages that are difficult to overcome. First, these types of molecules produce few characteristic fragment ions and are difficult to identify by GC/MS, (1)Grimmer, G. Enuironmental Carcinogens: Polycyclic Aromatic Hydrocarbons; CRC: Boca Raton, FL, 1983. (2)Dipple, A.;Moschel, R. C.; Bigger, C. A. H. In Chemical Carcinogens, Second Edition; Searle, C. E., Ed.; ACS Monograph 182;American Chemical Society: Washington, DC, 1984;Vol. 1, pp 41-164. (3)Crittenden, B. D.; Long, R. In Carcinogenesis-A Comprehensiue Suruey; Freudenthal, R. I., Jones, P. W., Eds.; Raven: New York, 1976; Vol. 1, p 209. (4)Lee, M.L.; Novotny, M. V.; Bartle, K. D. Anal. Chem. 1976,48, 405-416. (5)Lee, M. L.;Hites, R. A. Anal. Chem. 1976,48, 1890-1893. (6)Cole, J. A.;Bittner, J. D.; Longwell, J. P.; Howard, J. B. Combust. Flame 1984,56, 51-70. (7)Bittner, J. D.;Howard, J. B. Eighteenth Symposium (International) on Combustion; The Combustion Institute Pittsburgh, PA, 1981; pp 1105-1116.
0 1988 American Chemical Society
710 Energy & Fuels, Vol. 2, No. 5, 1988
Lafleur et al.
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Figure 1. Comparison of chromatograms for a jet-stirred combustor sample: (top) GC/MS total ion plot; (middle) GC-FTIR Gram-Schmidt reconstruction; (bottom)GC-FTIR wavelength chromatogram at 3330 cm-l corresponding to the acetylenic C-H stretching band. although abundant molecular ions can provide valuable molecular mass information.8 Second, the lack of suitable reference materials hampers structure elucidation by GC/MS. Many polyunsaturated hydrocarbons are unstable in the pure state, polyacetylenic species in particular can detonate under mild conditions, and laboratory preparation can be unsafe. Therefore these compounds are generally not available. Fortunately, chemical analysis with gas chromatography and Fourier transform infrared detection (GC-FTIR) can often provide complementary information for a more complete characterization.*12 For example, IR group (8) Lee, M. L.; Novotny, M. V.; Bartle, K. D. Analytical Chemistry Polycyclic Aromatic Compounds; Academic: New York, 1981; pp 123-125. (9) Shafer, K. H.; Hayes, T. C.; Brasch, J. W.; Jakobsen, R. J. Anal. Chem. 1984,56, 237-240. (10) Gurka, B. F.;Hiatt, M. Anal. Chem. 1984,56, 1102-1110. (11) Chiu, K. S.;Biemann, K.; Krishnana, K.; Hill, S. C. Anal. Chem. 1984,56, 1610-1615. (12) Crawford, R. W.; Hirschfeld, T.; Sanborn, R. H.; Wong, C. M. Anal. Chem. 1982,54, 817-820. of
frequencies are highly characteristic for different types of alkynes and alkenes. The strong absorbances near 3330 cm-' (C-H stretching), and near 1220 cm-' (first overtone of C-H bending) are a good indication of the presence of a terminal alkyne. The stretching band is quite unambiguous in the vapor phase and is independent of the rest of the molecular s t r u ~ t u r e . ' ~ Acetylenic C=C stretch occurs between 2220 and 2160 cm-l and is generally a rather weak band. Differentiation of mono- and disubstituted alkynes may be possible by the position of this band,14 but in the case of polyacetylenes, several bands may occur in this region due to mechanical coupling effects.15 The presence of alkenes gives rise to bands near 1400 cm-l resulting from -C=C-H in-plane bending while the (13) Nyquist, R. A. The Interpretation of Vapor Phase Infrared Spectra; Sadtler Research Laboratories: Philadelphia, PA, 1984; Vol. 1. (14) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman and Hill: New York, 1980. (15) Allan, L. J. H.; Meakins, G. D.; Whiting, M. C. J. Chem. SOC. 1955, 1878.
Energy & Fuels, Vol. 2, No. 5, 1988 711
Identification of Combustion Products peak no.' 1 2
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Table 1. GC/FTIR, GC/MS, and GC/FID Chemical Analysis Data for a Jet-Stirred want, MS." m / z 90 1R.b cm-' , (43),60 (14) 1.5 3340/3325,1222/1207(s); 2943,2927,2866,2229 64 (loo), 63 (761,62 (4361 (w); 2063 (vw) 78 (loo), 77 (37),76 (28),52 (37), 0.5 3332/3321 (9); 1242,767 (m) 51 (36),50 (28) 29.6 78 (loo),77 (52),76 (43),52 (40), 3082,3059 ( 8 ) ; 1481 (m); 1951,1801 (w); 3329, 51 (36).50 (41) 2214,1246 (vw) 0.6 78 (loo),77 (60),76 (15),52 (46), 3336/3321,1257,871,844 (s); 1963 (m) 51 (58),50 (40) d 82 (58),67 (loo),54 (79) 2935 (s); 3032,2893,2866 (m); 1450,717 (w) 76 (98),75 (42),74 (61),73 (29),50 (100) 1.6 3336/3325,1226/1215( 8 ) ; 956,933(m); 3116, 3208,2059,1843 (w) 1.4 74 (loo), 73 (60) 3332/3317,1238/1226(a) 90 (25),89 (loo),81 (52),74 (791, 0.4 3329 ( 8 ) ; 1265 (m) 73 (34).54 (71) 0.6 78 (loo), 77 (34),74 (26),52 (60), 3309 ( 8 ) ; 2989,2947,1246 (w) 51 (37),50 (31) 5.3 92 (61),91 (loo),65 (46) 3070,3039,725 (s); 2935,1496 (m); 2889,2877, 1600,1465,1422 (w) 90 (loo),89 (95),64 (23), 63 (52),62 (24)
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Figure 4. Spectral data for peaks 2 (A), 3 (B), and 4 (C). The mass spectra obtained for these three components show molecular ions at m / z 78, and only minor differencesin fragmentationcan be observed. The IR data allow benzene (B) to be differentiated from its acyclic isomers.
spectra whose simple patterns appeared to indicate the presence of an aromatic ring but whose molecular mass information ruled against such structures. These components were thought to be acyclic polyunsaturated hydrocarbons incorporating one or more triple bonds. Four examples are compounds b, c, e, and f i n Figure 2. Although much indirect evidence pointed to the presence of important acyclic polyunsaturated compounds in the combustion sample, the limited structural information obtained from the mass spectra combined with the scarcity of reference compounds limited our confidence in these structural assignments.' Furthermore, the mass spectra obtainable from magnetic-sector instruments coupled to capillary GC's, such as the one used in this work, often show a number of anomalies. Although the scan repetition rate (one every 1.8 s) is quite fast for a magnetically scanned instrument, it is not fast enough to capture the steady-state cracking pattern from components whose concentrations are rapidly changing as they emerge from a high-efficiency capillary
column. This effect is especially apparent with early eluting components such as the ones studied here. Although wider peaks can be obtained by increasing the amount of material injected, this can cause the detector and data acquisition systems to overload. This in turn leads to the truncation of intense ions and errors in cracking pattern ratios. Although the anomalous mass spectra do not seriously detract from the value of the data, they can reduce confidence in structural assignment. Fortunately, the GC/FTIR instrume'nt is not plagued with these anomalies, and as we stated earlier, IR spectral data is highly suited for the identification of aromatics and unsaturated hydrocarbons and the application of GC/ FTIR data permitted the identification of many components whose mass spectra had proven too ambiguous for confident structural assignment. The structural elucidation of typical components from Table I is detailed below. Spectral data for peak 7 are found in Figure 3. Only the terminal acetylenic C-H stretching (3330 cm-l) and bending (1226 cm-l) bands are observed in the IR spectrum
Lafleur et al.
714 Energy & Fuels, Vol. 2, No. 5, 1988
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Figure 5. Spectral data for peak 6. The charactersistic C-H stretching and bending in the IR spectrum of this component indicates the presence of a terminal acetylene group, and the bands at 956-908 and 1843 cm-' correspond to the vinyl wagging of a terminal=CHz. A molecular ion at m / z 76 gives an empirical formula of C6H4. Combining this information gives l-hexen3,5-diyne as the most plausible assignment. of peak 7, strongly suggesting high molecular symmetry. This information combined with the mass spectral data are sufficient for the identification of the compound as 1,3,5-hexatriyne (triacetylene), a highly reactive compound difficult to obtain in pure form. The IR spectrum of peak 1,illustrated in Figure 4, also shows the terminal acetylene C-H bands, along with an aliphatic C-H band (2950-2800 cm-') and a C=C stretching band at 2229 cm-'. The molecular ion at mlz 64 in the mass spectrum of this component provides an empirical formula of C6Hb These results suggest that the component is either 1,3- or 1,4-pentadiyne. Completely unambiguous differentiation of the isomers would require reference samples. Spectral data for peak 6 are shown in Figure 5. A randomly occurring bad fit between the narrow peak elution interval and the less than optimum scan repetition rate contributed to greater than usual distortion of the mass spectra. The one shown in Figure 5 was taken on the rising edge of the peak, and intensities are skewed towards high mass. The arrow atop the base peak at mlz 76 signifies a clipped peak. The tabulated mass spectrum in Table I, obtained by averaging individual scans, indicates that the peaks at 50 and 76 m u have nearly the same intensity. This component produced a mass spectrum similar to that of benzene; however, its molecular ion at mlz 76 was 2 mass units lower than benzene's. Therefore, the MS data was highly suggestive of a polyunsaturated hydrocarbon with an empirical formula of C6Hb The IR spectrum showed the characteristic bands indicative of a terminal
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Figure 6. Mass and IR spectra of peak 7. The high symmetry of 1,3,5-hexatriyne (triacetylene) allows only the acetylenic stretching and bending to be observed in the infrared region. acetylene as well as some near 950 cm-' due to the presence of a terminal vinyl (C=CH2) moiety. Combining this information leads to a structural assignment of l-hexen3,5-diyne although its isomer, 3-hexen-1,5-diyne,cannot be completely ruled out. Isomeric species give nearly identical mass spectra, but differentiation of isomers is possible with IR spectroscopy. In this study, IR spectral data were used to identify three peaks whose mass spectra proved to be identical with that of benzene. Spectral data for these compounds are shown in Figure 6. The vapor-phase IR spectrum of benzene is easily distinguished from those of the acyclic isomers, whose IR spectra indicate the presence of a terminal alkyne. For the identification of aromatic substitution.alisomers, the band patterns of the out-of-plane bending of ring hydrogens are often used. In this sample, two components, shown to be isomers of diethynylbenzene, were differentiated in this manner. The spectra of these compounds are shown in Figure 7. The mass spectra are nearly identical but indicate a molecular ion at 126 amu consistent with a diethynylbenzene structure. The IR spectra confirm the presence of the triple bond, and the bands from 1600-1500 cm-' can be correlated with a conjugated C=C stretch, thus indicating the presence of an aromatic ring. The region below lo00 cm-' in these spectra shows different band patterns. An important feature of the out-of-plane bending bands is that they are relatively insensitive to the nature of the substituents. Therefore, band patterns in an unknown spectrum can be compared to those of standards even if the substituents are different. The IR absorbance bands at 898 and 790 cm-' in peak 21 correspond to a meta-substituted isomer
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while the single band at 840 cm-' is characteristic of para (1,4) s~bstituti0n.l~ One important finding resulting from this experiment was the identification of the component in peak 28 that gives a mass spectrum with a molecular ion at m/z 152 indicative of a PAC isomeric with acenaphthylene. Data for this peak are shown in Figure 8. Previously, this compound had been identified as biphenylene. The IR spectrum shows ethynyl stretching and bending bands, aromatic C=C stretching at 1597 and 1504 cm-l, and out-of-plane bending below 1000 cm-'. Comparing the IR data with an authentic vapor-phase spectrum of biphenylene18clearly showed that the compound in peak 28 could not be biphenylene. The vaporphase IR spectrum of biphenylene includes two strong bands at 1150 and 1427 cm-' that are absent in the spectrum of peak 28. Also biphenylene does not have any bands above 3080 cm-' that could be mistaken for the ethynyl stretch at 3330 cm-'. A comparison of the pattern below loo0 cm-' with reference data,l'Jg allows assignment of the structure as 2-ethynylnaphthalene. The infrared data confirmed the identity of benzaldehyde and phenol, two polar species corresponding to peaks 17 and 18, respectively. Also found were a monocyclic isomer of naphthalene (C1&) having a terminal triple bond (peak 24) and a version of this compound having two additional hydrogen atoms at peak 25. Conclusion The complementary combination of GC/FTIR and GC/MS proved to be a powerful tool for the characteri-
wav e n u m b e r (cm-')
Figure 8. Spectral data for compound previously thought to be biphenylene.
(18) Pecile, C.; Lunelli, B. J. Chem. Phys. 1968, 48,1336-1350. (19) Colthrup, N. B.;Daly, L. H.; Wiebolt, S. E. Introduction t o Infrared and Raman Spectroscopy; Wiley: New York, 1975.
Energy & Fuels 1988,2, 716-719
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zation of a combustion-derived complex mixture. The exclusive use of GC/MS was inadequate for the identification of many of the ethylene combustion products because they yielded indeterminate mass spectra. Unequivocal structural assignment was found to require GC/FTIR data. On the other hand, the GC/FTIR data lacks molecular mass information and its sole use is also insufficient for the conclusive identification of many components. Many of the compounds found in this study, including some highly reactive species, have been postulated as intermediates in theoretical models for the formation and growth of PAC and soot. Our results show that many of these intermediates are stable enough to be sampled and identified by current state of the art techniques.
Acknowledgment. We thank Fred Lam, Craig Vaughn, and Edward Kruzel for their assistance in obtaining the combustion sample and for fruitful discussions. This investigation was supported by the National Institute of Environmental Health Sciences Center Grant NIH5P30-ES02109-08 and the National Institute of Environmental Health Sciences Program Grant NIH-5PO1ES01640-09.
Registry No. Ethylene, 74-85-1; benzene, 71-43-2; cyclohexene, 110-83-8;toluene, 108-88-3;phenylacetylene,536-74-3;styrene, 100-42-5;benzaldehyde, 100-52-7;phenol, 108-95-2;l-phenyl-lpropyne, 673-32-5;indene, 95-13-6;diethynylbenzene,30700-96-0; naphthalene, 91-20-3; acenaphthylene, 208-96-8.
Decomposition of NH3 over Quartz Sand at 840-960 "C D. A. Cooper* and E. B. Ljungstrom Department of Inorganic Chemistry, Chalmers University of Technology and University of Goteborg, S-412 96 Goteborg, Sweden Received January 5, 1988. Revised Manuscript Received M a y 31, 1988
As part of a study aimed at investigating the kinetics of possible heterogeneous gas-solid reactions pertaining to fluidized-bed combustion, the solid-catalyzed decomposition reaction of NH3 over quartz sand at 840-960 "C has been examined and shown to obey first-order kinetics: k' = 7.6 X lo4 s-l cm-2 at 900 "C and E, = 147 kJ mol-l. This reaction had been previously overlooked in earlier fixed-bed reactor studies under similar conditions using quartz materials and NH3.
Introduction Fluidized-bed technology provides an economic and efficient method of burning a variety of fuels with relatively low SO2 and NO, emissions. In view of the stricter environmental emission standards in sight, however, further steps in pollution control are recommended. A clear understanding of the chemical reactions leading to pollutant formation is thus necessary in order to effect improved control of these species. Limestone addition into the bed material rapidly yields CaO, which has been proven to be an acceptable S02-removingagent. The chemistry and control of NO, however, has provided a more challenging problem. A common strategy employed for an approximate and efficient optimization of large-scale boilers is by the use of theoretical models. The basic element of such models lies in an accurate representation of the chemical reactions occurring in fluidized-bed combustion. To this end the acquisition of kinetic data for both the homogeneous and heterogeneous reactions present is vital. Investigations aimed at uncovering and characterizing the heterogeneous reactions involved in fluidized-bed combustion often make use of a more simplified and controllable fixed-bed reactor apparatus. Previous studies using this technique have revealed several paths whereby NO formation or destruction may occur. Char particles have been shown to remove NO both by catalytic and noncatalytic rea~tions.l-~ (1)Furusawa, T.; Kunii, D.; Oguma, A.; Yamada, N. Int. Chem. Eng. 1980, 20, 239-244.
0887-0624/88/2502-0716$01.50/0
Table I. Scope of Experiment flow rate (at 25 " C ) 170-760 mL/min bed temp 840-960 O C reacn time 21-210 ms NH3 concn 4730-4930 ppm wt of sand used 8.00 g (4.00 g) sand particle size 0.20-0.80 mm specific surface area of sand 0.0179 f 0.0002 m2/g bed vol 5.6 cm3 (2.8 cm3) bed voidage fraction 0.4
In addition calcined limestone particles can allow a variety of catalytic reductions of NO to proceed in the presence of CO,4 H2,5and NH3H Kinetic data have been obtained for most of the above reactions although such information is lacking in the case for NH3. In this system the elementary decomposition reaction of NH, must also be considered. (2) Chan, L. K.; Sarofim, A. F.; Beer, J. M. Combust. Flame 1983,52,
37-45.
(3) Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Fuel 1985,
64, 1306-1309.
(4) Tsujimura, M.; Furusawa, T.; Kunii, D. J. Chem. Eng. Jpn. 1983, 16, 132-136. ( 5 ) Tsujimura, M.; Furusawa, T.; Kunii, D. J. Chem. Eng. Jpn. 1983, 16. ._ , 524-526 - - - - --. (6) Lee, Y. Y.; Sekthira, A.; Wong, C. M. Proc. Int. Conf. Fluid. Bed Combust. 1985, Bth, 1208-1218.
(7) Furusawa, T.; Tsujimura, M.; Yasunga, K.; Kojima, T. Proc. I n t . Conf. Fluid. Bed Combust. 1985, Bth, 1095-1104. (8) Lee, Y. Y.; Soares, S. M. S.; Sekthira, A. Proc. Int. Conf. Fluid. Bed Combust. 1987, 1184-1187. (9) Hirama, T.; Kochiyama, Y.; Chiba, T.; Kobayashi, H. J. Fuel SOC. Jpn. 1982, 61, 268-275.
0 1988 American Chemical Society
