Aromatic structures of carbonaceous materials and soot inferred by spectroscopic analysis (original) (raw)
Carbon, 2008
A B S T R A C T Large polycyclic aromatic hydrocarbons were identified in carbon particulate sampled in a fuel-rich premixed ethylene flame. The particulate was extracted with dichloromethane (DCM) in order to separate the soluble organic species (DCM-extract) from the solid carbon (soot). After DCM extraction soot was re-extracted with N-methyl pyrrolidone (NMP) obtaining the NMP-extract. Both the DCM-extract and NMP-extract were further fractionated by size exclusion chromatography in selected molecular weight (MW) ranges. Large polycyclic aromatic hydrocarbons obtained by regular incorporation of C 2 and/or C 2 H 2 unit (24/26 rule) occurring in both odd and even series of carbon atom number of polycyclic aromatic hydrocarbons, were identified by laser desorption ionization-mass spectrometry (LDI-MS) analysis of the lighter MW fractions of both the DCM-extract and NMP-extract (100-400 u MW of the DCM-extract and 200-600 u fractions of the NMP-extract). The LDI-MS spectra of the heaviest MW fractions of DCM-extract and NMP-extract (600-2000 u and 600-5000 u fraction) showed a continuous spectrum of masses typical of polymeric structures. The UV-visible absorption and emission spectral analysis corroborated the assignment of lighter and of heavier fractions of DCM-extract and NMP-extract to PAH and to polymeric aromatic structures, respectively.
Aromatic site description of soot particles
Combustion and Flame, 2008
A new, advanced soot particle model is developed that describes soot particles by their aromatic structure, including functional site descriptions and a detailed surface chemistry mechanism. A methodology is presented for the description of polyaromatic hydrocarbon (PAH) structures by their functional sites. The model is based on statistics that describe aromatic structural information in the form of easily computed correlations, which were generated using a kinetic Monte Carlo algorithm to study the growth of single PAH molecules. A comprehensive surface reaction mechanism is presented to describe the growth and desorption of aromatic rings on PAHs. The model is capable of simulating whole particle ensembles which allows bulk properties such as soot volume fraction and number density to be found, as well as joint particle size and surface area distributions. The model is compared to the literature-standard soot model [J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame 121 (2000) 122-136] in a plug-flow reactor and is shown to predict well the experimental results of soot mass, average particle size, and particle size distributions at different flow times. Finally, the carbon/hydrogen ratio and the distribution of average PAH sizes in the ensemble, as predicted by the model, are discussed.
Fast Analysis of PAH in Complex Organic Carbon Mixtures by Reconstruction of UV-Visible Spectra
Chemical Engineering Transactions, 2017
Polycyclic aromatic hydrocarbons (PAH) are components of complex organic mixtures featuring liquid and solid fossil fuels as well as tars derived from combustion and/or pyrolysis of coal and hydrocarbon fuels. PAH can also be detected at the exhaust of combustion systems, often associated to carbon particulate matter emissions. Due to their high toxicological potential the concentration levels of PAH detection should be very low (order of ppm) to meet the limits provided by the regulation on combustion emissions and atmosphere quality control. The PAH detection is mainly achieved by conventional, mainly chromatographic, analytical techniques applied to the fuel or to the organic carbon extracted from carbon particulate matter. Preliminary extraction, purification and pre-separation methods are the time-consuming methods necessary to isolate the PAH-rich mixtures from whichever organic matrix for further analysis with liquid or gas-chromatography. In this paper an alternative method ...
Reflectance spectroscopy (350-2500 nm) of solid-state polycyclic aromatic hydrocarbons (PAHs)
Icarus, 2014
Polycyclic aromatic hydrocarbons (PAHs) are organic compounds based on fused aromatic rings, and are formed in a variety of astrophysical, solar nebula and planetary processes. Polycyclic aromatic hydrocarbons are known or suspected to occur in a wide variety of planetary settings including icy satellites, Titan's hazes, carbonaceous meteorites, comet nuclei, ring particles; and terrestrial organic-rich lithologies such as coals, asphaltites, and bituminous sands. Relatively few measurements of the visible and near-infrared spectra of PAHs exist, yet this wavelength region (350-2500 nm) is widely used for remote sensing. This study presents detailed analyses of the 350-2500 nm reflectance spectra of 47 fine-grained powders of different high-purity solid-state PAHs. Spectral properties of PAHs change with variations in the number and connectivity of linked aromatic rings and the presence and type of side-groups and heterocycles. PAH spectra are characterized by three strong features near 880 nm, 1145 nm, and 1687 nm due to overtones of mCH fundamental stretching vibrations. Some PAHs are amenable to remote detection due to the presence of diagnostic spectral features, including: NAH stretching overtones at 1490-1515 nm in NH-and NH 2 -bearing PAHs, aliphatic or saturated bond CAH overtone vibrations at 1180-1280 nm and 1700-1860 nm; a broad asymmetric feature between 1450 nm and 1900 nm due to OAH stretching overtones in aromatic alcohols, CAH and C@O combinations near 2000-2010 nm and 2060-2270 nm in acetyl and carboxyl-bearing PAHs. Other substituents such as sulphonyl, thioether ether and carboxyl heterocycles, or cyano, nitrate, and aromatic side groups, do not produce well-resolved diagnostic spectral features but do cause shifts in the positions of the aromatic CAH vibrational overtone features. Fluorescence is commonly suppressed by the presence of heterocycles, side-groups and in many non-alternant PAHs. The spectral characteristics of PAHs offer the potential, under suitable circumstances, for remote characterization of the classes of PAH present and in some cases, identification of particular heterocyclic or side-group substituents.
Electronic Structure of Naturally Occurring Aromatic Carbon
Energy & Fuels, 2019
Aromatic carbon in fused-ring systems can be classified into two forms of electronic structure: aromatic sextets, which have large highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gaps and resemble benzene; and isolated double bonds, which have small HOMO−LUMO gaps and resemble olefins. The relative abundance of aromatic sextets versus isolated double bonds in mixtures can be probed by carbon X-ray Raman spectroscopy. Here, we report the carbon X-ray Raman spectra of a wide variety of forms of naturally occurring aromatic carbon: kerogen (insoluble organic carbon in sedimentary rocks, which is the most abundant form of naturally occurring organic carbon in the Earth's crust) over a range of types and thermal maturities, fresh materials of a variety of forms, and coal and petroleum asphaltenes (toluene soluble and heptane insoluble materials, which represent organic carbon resulting from extensive thermal processing). It is observed that all these materials are dominated by aromatic sextets over isolated double bonds. On the basis of the diversity of materials analyzed, it is concluded that naturally occurring organic carbon is generally dominated by aromatic sextets over isolated double bonds. This conclusion is rationalized in terms of statistical and thermodynamic effects.
Energy & Fuels, 2015
The different thermal behaviors and solubilities of large and structurally different polycyclic aromatic hydrocarbon (PAH) mixtures featuring coal tar pitch (CP) and naphthalene synthetic pitch (NP) samples could be read in light of their different molecular weight (MW) distribution and spectroscopic features. The number-average MW obtained by mass spectrometry for CP (417 Da) and NP (691 Da) resulted to be lower in comparison to the values evaluated by size-exclusion chromatography (SEC) (796 and 824 Da for CP and NP, respectively) because of the different response of the detector of mass spectrometry to low-and high-MW components. Hence, SEC showed to be more suitable for the analysis of PAH mixtures overlapping and covering a higher mass range in comparison to mass spectrometry. Insights into structural PAH features were given by means of spectroscopic analysis [infrared (IR), ultraviolet−visible (UV−vis), and fluorescence], allowing for the discrimination between different families of PAHs as ortho-fused PAHs and rylenes interspersed with aliphatic (mainly naphthenic) groups, mainly featuring CP and NP, respectively. Besides showing the different aromaticity and aliphatic/aromatic hydrogen distribution, the improvement of Fourier transform infrared (FTIR) and UV−vis absorption analysis put also in evidence the contribution of carbon-rich particle impurities and PAH aggregates in CP and NP, respectively.
Probing soot structure and electronic properties by optical spectroscopy
Fuel, 2020
In this work, the physicochemical transformation of soot particles during the initial stages of formation and growth was investigated in a laminar premixed flame of ethylene and air. The selected flame condition allowed producing two classes of carbon nanoparticles, namely incipient and primary soot particles, on the basis of their size distribution. The two classes of particles have been selectively collected in well distinct flame zones for further chemico-physical characterization. The optical band gap decreases as particle size grows up. Particle medium range order and structure at molecular level have been investigated by Raman and fluorescence spectroscopy. Slight changes in particle composition were observed for incipient and primary soot. Both kinds of particles are made of a similar ensemble of fluorescence centers, which produce excitation dependent emission irrespective of particle sizes. In agreement with the Raman spectra, the molecular constituents of the particles are polyaromatic compounds with an average size of the order of ovalene molecules that only slightly increase from incipient to primary soot particles. These results suggest that the observed decrease of the optical band gap, as particle size increases, typical of a quantum-dot behavior, is not due to changes in particle composition at molecular level. Further investigations are needed to investigate possible evolution in supramolecular organization of the particles as they grow in flame and its effect on the optical band gap.
Spectroscopic properties of polycyclic aromatic compounds
Analytica Chimica Acta, 1993
BIBLIOGRAPHY iv 16. Molecular structures of PAH6 benzenoids AK-AV 17. Molecular structures of PAH6 benzenoids AW-BF 18. Molecular structures of PAH6 benzenoids derivatives A-L 19. Molecular structures of PAH6 benzenoids derivatives M-X 20. Molecular structures of PAH6 benzenoids derivatives Y-AC 21. Molecular structure of cyclopenta-PAH derivatives .... 22. Molecular structures of acenaphthylene derivatives .... 23. Molecular structures of acephenanthrylene derivatives .... 24. Molecular structures of fluoranthenoids and fluorenoids A-J. 25. Molecular structures of fluoranthenoids and fluorenoids K-Q. 26. Molecular structures of fluoranthenoid derivatives A-H 27. Molecular structures of fluoranthenoid derivatives I-P 28. Molecular structures of PAOHs and PASHs xii 65 66 76 77 78 79 pose significant health concerns due to the fact that many are known or suspected carcinogens and/or mutagens (2-10). Polycyclic aromatic compounds "are formed from the incomplete combustion of fossil fuels, tobacco products, food, and virtually any organic matter" (2). They can be found in coal, coal tars, and coal liquid factions as well as many of the refined coal products (11-13). For example, PAHs are found in automobile exhaust fumes, coal-powered factories and heating facilities' stack emissions, cigarette smoke, diesel fuels, motor oils, lubricating compounds, and even in our streams, lakes, and rivers (2, 11-13). They are priority pollutants due to the significant human exposure (2). These compounds are associated with the causes of such buzz-words as "acid-rain", "fly-ash", and tumors/cancer. For the most part, the toxicity of PACs is well characterized or easily predicted by structure activity analysis. "From the standpoint of structure activity analysis, PAHs represent an intriguing class of compounds for experimental investigation because seemingly small changes in the chemical structure can markedly affect carcinogenic activity" (2). According to Ashwood-Smith, Mitchell, and Kennedy, internal substitution on PAHs can completely alter the carcinogenic activity (4). For example, benzo[a]pyrene is one of the most studied PAH carcinogens, but by internally substituting methyl groups on the 10jb and 10c position the carcinogenic activity was completely eliminated (substitution by hydrogen atoms in this case yield an unstable compound) (4, 14). One can also add hydrogen atoms to the perimeter C=C double bonds. Such partially hydrogenated PAHs are obtained by a catalytic hydrocracking process used in the conversion of crude oil to gasoline (13). The reduction products are not very different in molecular size, although they may differ in shape, from the parent hydrocarbon and from each other. Several reduction products have been prepared in the past and have been shown either to be inactive or much less active than the parent hydrocarbons (7). For example, 7,8,9,10-tetrahydrobenzo[a]pyrene (7) is inactive; whereas, benzo[a]pyrene is a notorious carcinogen. One new technique being used to determine the reactivity of carcinogens with target DNA molecules is molecular dosimetry (15). Cancer researchers are using molecular dosimetry to measure the potency of carcinogens and determining if there are threshold levels of exposure. "This technique measures the chemical reactivity of a potential carcinogen, mutagen, or other genotoxin with a host 'target molecule' generally DNA....Molecular dosimetry may simplify animal studies-it may even shorten them by helping direct their focus on the most likely agents-but ultimately, it's not likely to replace them" (15). Large databases of known experimental mutagenic and carcinogenic properties have been developed for many PACs;
A UV spectroscopic method for monitoring aromatic hydrocarbons dissolved in water
Analytica Chimica Acta, 2000
An enhanced UV spectrometric method is applied to trace measurements of aromatic hydrocarbons dissolved in water. This approach gains selectivity and sensitivity by the use of optically generated first and second derivatives of transmission spectra. The augmented spectroscopic technique is combined with chemometric algorithms like principal component regression or partial least squares which are used for calibration of the spectrometer and quantitative evaluation of spectra. Laboratory measurements were performed on mixtures containing up to five substances, i.e. benzene, toluene, ethylbenzene, the three xylene isomers, chlorobenzene, and gasoline. Due to the difficulty of preparing precisely defined calibration and test samples of these very volatile compounds, a novel mixing device was developed and is presented. From these first investigations it can be estimated that the detection limits are down to ca. 10 g analyte per liter of water by using a 10 cm absorption pathlength and a few minutes measurement time.