A CLASSICAL VALENCE BOND VIEW OF CYLINDER-SHAPED AND FULLY BENZENOID-LIKE POLYARENES (POLYCYCLIC AROMATIC HYDROCARBONS) OF LARGE SIZE

This note first defines a class of cylindrical polyarenes polycyclic aromatic hydrocarbons (PAHs) C_{2(m+ 1)n}H_2n, for which the periphery is acenic. From this class the leapfrog algorithm derives the second class of cylindrical PAHs C_6nmH_4n, where the periphery is phenacenic. One further application of this algorithm to the second class leads to the third class of cylindrical PAHs C_12nmH_8n, where the less regular periphery has 2n capes (i.e., hexagons exposed on four adjacent sides to the external “sea” of space). Each molecule in the second and third classes is totally covered by aromatic sextets, that is, appears related to a fully benzenoid hydrocarbon (FBH). The Pauling bond order, and several new indices derived from the Pauling bond order and Clar sextet counts are calculated for the three classes with increasing numbers of carbons. This calculation suggests that a typical cylindrical PAH becomes more graphite-like than a typical flat FBH.     

Identification of large polycyclic aromatic hydrocarbons in carbon particulates formed in a fuel-rich premixed ethylene flame

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 C₂ and/or C₂H₂ 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.     

Treatment of PAH‐contaminated soil by combination of Fenton's reaction and biodegradation

A combination of modified Fenton and biological treatment was used to remove polycyclic aromatic hydrocarbons (PAHs) from creosote oil‐contaminated soil. After modified Fenton reaction the toxicity of column leachate and soil to Vibrio fischeri increased. The number of intact bacterial cells and utilisation of PAHs in PAH utilisation microplate assay decreased after modified Fenton reaction. However, bacteria in chemically treated soil utilised PAHs without addition of other carbon sources. The activity of extracellular esterases increased during incubation of modified Fenton‐treated soil. PAH removal in combined chemical oxidation and incubation (43–59%) was higher than in incubation alone (22–30%). Residual H₂O₂ in soil allowed chemical oxidation of PAHs during incubation. Copyright © 2006 Society of Chemical Industry     

Kinetics of surface reactions in carbon deposition from light hydrocarbons

Carbon deposition from ethene, ethine and propene as a function of pressure was studied at various temperatures and two different surface area/volume ratios. Deposition rates as a function of pressure of all hydrocarbons indicate Langmuir-Hinshelwood kinetics which suggests that the deposition process is controlled by the heterogeneous surface reactions (growth mechanism). These kinetics are favored at decreasing reactivity (C₃H₆>C₂H₂>C₂H₄), decreasing temperature and residence time as well as increasing surface area/volume ratio. A linear rate increase at high pressures suggests that carbon is additionally or preferentially deposited by aromatic condensation reactions between polycyclic aromatic hydrocarbons large enough to be physisorbed or condensed on the substrate surface (nucleation mechanism). The results completely agree with earlier results obtained with methane.     

Infrared studies of so2 on carbons—I. Interaction of SO2 with carbon films

IR spectroscopic studies were carried out on the bond character of SO₂ molecules adsorbed on the surface of carbons. No spectroscopic evidence was found for the formation of any stable surface carbon-sulfur complexes after interaction of SO₂ with a carbon surface in the temperature range 25–600 Simultaneous adsorption of SO₂ and O₂ occurs with the participation of π electrons of condensed aromatic systems. It can be concluded from IR investigations that SO₂ retards the oxidation of carbon by O₂.     

New polycyclic aromatic hydrocarbon (PAH) surface processes to improve the model prediction of the composition of combustion-generated PAHs and soot

Two new polycyclic aromatic hydrocarbon (PAH) surface processes are proposed, which can cause dehydrogenation and “rounding” of PAH molecules. The reaction pathways for both the processes involve decyclisation of a 6-member ring present on the PAH surface. 1,6-Hydrogen migration in the bay region formed by nearby carbon atoms provides an alternate route for both the processes to proceed. The energetics and kinetics of the proposed processes are investigated using density functional theory and transition state theory, respectively. The B3LYP functional with the 6-311++G(d,p) basis set is employed for the geometry optimisation and vibrational frequency analysis of the chemical species and the transition states. The current PAH growth mechanism is extended by including the new processes. A detailed PAH growth model, the kinetic Monte Carlo-aromatic site (KMC-ARS) model [Raj A, Celnik M, Shirley R, Sander M, Patterson R, West R, et al. A statistical approach to develop a detailed soot growth model using PAH characteristics. Combust Flame 2009;156:896-913] is used to study PAH growth with the extended mechanism. Computed ensembles are generated for large PAHs present in a C₂H₂ flame with 70-320 carbon atoms, and are compared to the experimentally observed ensembles. The inclusion of the new PAH processes in the chemical mechanism is found to improve the predicted composition of the large PAH molecules, especially for the PAHs with 70-200 carbon atoms.     

Identification of Methylene-Bridged Polycyclic Aromatic Hydrocarbon Products of Catechol Pyrolysis

As methylene-bridged polycyclic aromatic hydrocarbons (PAH) are known for their carcinogenicity and occurrence in solid-fuel byproducts and environmental samples, we have employed high-pressure liquid chromatography with diode-array ultraviolet- visible absorbance and mass spectrometric detection (HPLC/UV/MS) to search for methylene-bridged PAH among the products of the laboratory-scale pyrolysis of cate- chol, a model fuel representing aromatic moieties in coal, tobacco, and plant-based biomass fuels. Utilizing two different octadecylsilica columns and five different mobile- phase separation methods, the HPLC/UV/MS analyses have led to the unequivocal identification of six methylene-bridged PAH among the products of catechol pyrolysis at 1000°C and 0.3 s in a laminar-flow reactor: the C₁₅H₁₀ 4H-cyclopenta- [def]phenanthrene, the C₁₉H₁₂ 11H-benz[bc]aceanthrylene and 4H-cyclopenta[def]- chrysene, the C₂₁H₁₂ 11H-indeno[2,1,7-cde]pyrene and 4H-benzo[def]cyclopenta[mno]- chrysene, and the C₂₃H₁₂ 1H-benzo[ghi]cyclopenta[pqr]perylene. None of these six methylene-bridged PAH have ever before been identified as pyrolysis products of catechol or of any other pure-component fuel. The mass and UV spectra of two additional catechol pyrolysis product components have been examined in light of the finding that the UV spectra of methylene-bridged PAH generally resemble those of their respective parent benzenoid PAH (but with a 5- to 12-nm bathochromic shift in the p band)—resulting in the identification of a methylene-bridged naphtho[2,1-a]pyrene and the tentative identification of a methylene-bridged naphtho[2,3-a]pyrene, two previously unrecognized C₂₅H₁₄ PAH that would be expected to exhibit significant mutagenic and/or carcinogenic behavior.     

Precursor gas chemistry determines the crystallinity of carbon nanotubes synthesized at low temperature

Despite significant progress in carbon nanotube (CNT) synthesis by thermal chemical vapor deposition (CVD), the factors determining the structure of the resulting carbon filaments and other graphitic nanocarbons are not well understood. Here, we demonstrate that gas chemistry influences the crystal structure of carbon filaments grown at low temperatures (500 °C). Using thermal CVD, we decoupled the thermal treatment of the gaseous precursors (C₂H₄/H₂/Ar) and the substrate-supported catalyst. Varying the preheating temperature of the feedstock gas, we observed a striking transition between amorphous carbon nanofibers (CNFs) and crystalline CNTs. These results were confirmed using both a hot-wall CVD system and a cold-wall CVD reactor. Analysis of the exhaust gases (by ex situ gas chromatography) showed increasing concentrations of specific volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs) that correlated with the structural transition observed (characterized using high-resolution transmission electron microscopy). This suggests that the crystallinity of carbon filaments may be controlled by the presence of specific gas phase precursor molecules (e.g., VOCs and PAHs). Thus, direct delivery of these molecules in the CVD process may enable selective CNF or CNT formation at low substrate temperatures. The inherent scalability of this approach could impact many promising applications, especially in the electronics industry.     

Photodegradation of Selected Aromatic Constituents of Creosote in Organic Solvents

Polycyclic aromatic hydrocarbons (PAHs), naphthalene, phenanthrene, anthracene, and pyrene were irradiated in acetonitrile (CH₃CN) and in dichloromethane (CH₂Cl₂) on individual PAH compounds and in the presence of other compounds. The observed photodegradation of PAHs was dependent on the structure of the compound. Anthracene and pyrene were the most photoreactive in dichloromethane: with total degradation after 0.5 h irradiation for anthracene and 1.5 h for pyrene. The decomposition of PAHs was faster in dichloromethane than in acetonitrile.     

Recertification of Standard Reference Material (SRM) 1649, Urban Dust,for the Determination of Polycyclic Aromatic Hydrocarbons (PAHs)

The National Institute of Standards and Technology (NIST) recently issued SRM 1649a, Urban Dust, with certified and reference values for 44 polycyclic aromatic hydrocarbons (PAHs). This material is a recertification of SRM 1649 which was issued in 1982 with certified values for only five PAHs. The PAHs were determined using the following analytical techniques: (1) reversed-phase liquid chromatography with fluorescence detection (LC-FL) for analysis of the total PAH fraction, (2) reversed-phase LC-FL for analysis of isomeric PAH fractions isolated by normal-phase LC (i.e., multidimensional LC), and (3) gas chromatography/mass spectrometry (GC/MS) for analysis of the PAH fraction using three different stationary phases, each with different selectivity for PAH separations. The results from the different techniques are compared and discussed. SRM 1649a is currently the most extensively characterized environmental matrix SRM with respect to PAH constituents.     

Noncarotenoid hydrocarbons in palm oil and palm fatty acid distillate

Column chromatographic and gas chromatographic-mass spectroscopic (GCMS) analyses for minor and trace noncarotenoid hydrocarbons of crude palm oil and palm fatty acid distillate revealed the presence of a wide range of n-alkanes (C₁₂H₂₆ to C₃₆H₇₄) and n-alkenes in addition to the major component, squalene. Hydrocarbon components concentrated in palm fatty acid distillate where squalene was dominant, but degradation products such as alkenes (from fatty acids or glycerides), aromatic hydrocarbons (from carotenes) and diterpene hydrocarbons (from tocotrienols) were detected in significant quantities, superseding the naturally occurring n-alkanes. Mechanisms proposed suggest that degradation of the valuable vitamin E or tocotrienols needs to be minimized in physical refining.     

Evidence of PAH production under lean combustion conditions

While it is common knowledge that PAH formation can be expected at rich combustion conditions, this paper presents experimental and computational evidence that PAH formation can also be expected under lean, or cool, combustion conditions. Experimental evidence for diesel engine operation at lean conditions, as well as the literature references related to waste tire and coal combustion, are cited as supportive evidence of conclusions drawn here. A computational model based on reaction mechanisms for three surrogate fuels: C₂H₂, C₇H₁₆ and C₁₄H₂₈, was validated against the available diesel engine data, and projected to both lean and rich combustion conditions. The finding was that for the adiabatic case, minimum production of PAHs occurs at equivalence of unity and increases as equivalence becomes either lean or rich; the latter observation being consistent with the common knowledge.     

Exploring formation pathways of aromatic compounds in laboratory-based model flames of aliphatic fuels

This presentation summarizes our recent experimental and flame modeling studies focusing on understanding of the formation of small aromatic species, which potentially grow to polycyclic aromatic hydrocarbons (PAHs) and soot. In particular, we study premixed flames, which are stabilized on a flat-flame burner under a reduced pressure of ≈15–30 torr, to unravel the important chemical pathways to aromatics formation in flames fueled by small C₃–C₆ hydrocarbons. Flames of allene, propyne, 1,3-butadiene, cyclopentene, and C₆H₁₂ isomers 1-hexene, cyclohexane, 3,3-dimethyl-1-butene, and methylcyclopentane are analyzed by flame-sampling molecular-beam time-of-flight mass spectrometry. Isomer-specific experimental data and detailed modeling results reveal the dominant fuel-destruction pathways and the influence of different fuel structures on the formation of aromatic compounds and their commonly considered precursors. As a specific aspect, the role of resonance-stabilized free radical reactions is addressed for this large number of similar flames of structurally different fuels. While propargyl and allyl radicals dominate aromatics formation in most flames, contributions from reactions involving other resonance-stabilized radicals like i-C₄H₅ and C₅H₅ are revealed in flames of 1,3-butadiene, 3,3-dimethyl-1-butene, and methylcyclopentane. Dehydrogenation processes of the fuel are found to be important benzene formation steps in the cyclohexane flame and are likely to also contribute in methylcyclopentane flames.     

Quantification of polycyclic aromatic hydrocarbons (PAHs) in the smoke from six woods and comparative study of their distribution

Polycyclic aromatic hydrocarbons (PAHs) are considered as undesirable molecules in smoke. It is important to know the smoke composition applied to meat-based products. PAH extracts of the smoke from five tropical woods were fractionated, analyzed then compared to that of beechwood, a European wood commonly used in the smoking industry of meat-based products. The identification and quantification of the aromatic molecules was carried out with gas chromatography coupled to mass spectrometry (GC/MS).     

New oxygen-containing compound isolated from coal

A new type of oxygen-containing compound was isolated from benzene extract of Yubari coal. This compound has an aromatic condensed ring structure with the molecular formula C₃₀H₂₀O, and contains oxygen in heterocyclic-ring form as in diphenylene oxide. It may contain three methyl groups; possible formulae are proposed.     

From Overcrowded Polycyclic Aromatic Enes to Semifullerenes

Abstract

Overcrowded polycyclic aromatic enes (1), e.g., bi-9H-fluoren-9-ylidene (2) and bi-4H-cyclopenta[def]-phenanthren-4-ylidene (3) are potential starting materials for the preparation of bowl-shaped fragments of fullerenes. Semiempirical MNDO-PM3 calculations of C₂₆ H_n and C₃₀ H_n (n = 12,14,16) species 2–14 are used to analyze energetic and steric effects on the dehydrocyclization and isomerization reactions of these molecules. the out-of-plane bending and pyramidalization in these species are ascribed to intramolecular overcrowding in the fjord and cove regions and to strain introduced by C5 rings in the PAH skeleton. Oxidative photocyclization reactions on Z-2,2′-bridged derivatives of 2 and 3 are briefly outlined.     

Size-dependent melting of polycyclic aromatic hydrocarbon nano-clusters: A molecular dynamics study

The size-dependent melting behaviour of clusters of polycyclic aromatic hydrocarbon (PAH) molecules is studied computationally using the isotropic PAHAP potential (Totton et al., 2012, Phys Chem Chem Phys, 14, 4081–4094). The investigation aims to shed light on the understanding of the liquid-like behaviour of PAH clusters. Detailed molecular dynamic (MD) simulations are performed to investigate the size-dependent melting of two representative homogeneous PAH clusters composed of either pyrene (C₁₆H₁₀) or coronene (C₂₄H₁₂) molecules. The evolution of the intermolecular energy and the Lindemann index are used to estimate the melting points of individual nano-clusters. The results from the MD simulations show that individual PAH molecules within nano-clusters are highly mobile below typical flame temperatures. A detailed morphological investigation of coronene₅₀₀ further reveals that the coronene clusters evolve from a columnar particle in the solid phase to an irregular spherical particle in the liquid phase. In contrast, no such evolution is observed for pyrene₃₀₀ which remains in a spherical configuration. The nano-cluster reduced melting temperature decreases with decreasing particle size following a linear relation with reciprocal size. The melting process of these clusters starts from the surface and the liquid layer grows inwards with increasing temperature.     

The composition of the bases in a supercritical gas extract

Average structure data for the basic fraction from a supercritical toluene extract of Liddell coal are reported based on ¹³C- and ¹H-n.m.r. spectroscopy combined with elemental, functional group and molecular weight analyses. This data indicated an average aromatic ring size of two condensed rings with about a third of the possible aromatic sites substituted. The ¹³C-n.m.r. spectrum showed the presence of unsubstituted alkyl chains >C₈. A number of nitrogen heterocycles, ranging from one to five condensed rings, were identified by g.c.-m.s. and fluorescence and spectroscopy.     

The Supercritical Fluid Extraction of Anthracene and Pyrene from a Model Soil: An Equilibrium Study

Abstract

Supercritical carbon dioxide (SC-CO₂) is used to extract two polycyclic aromatic hydrocarbons (PAHs), anthracene and pyrene, from a model soil at conditions ranging from 35 to 55 [ddot]C and 7.79 to 24.13 MPa. Equilibrium partition coefficients and Freundlich isotherm constants are determined for the two PAHs on white quartz sand and the model soil. The effect on equilibrium of additional water in the soil phase is also examined.     

Quantitative appraisal of the interfacial anchoring state of polyaromatic hydrocarbons during the formation of C/C composites

The preferred anchoring state for polycyclic aromatic hydrocarbons (PAHs) pyrolyzed onto carbon substrates has been studied by semi-quantitative methods, examining their interfacial lattice arrangements. The samples were prepared by decomposing petroleum pitch inside carbon nanotubes resulting in a variety of crystallinities forming one-dimensional C/C composites. Studies indicate that the preferred anchoring state of PAH molecules depends on the nature of the substrate. Accordingly the PAHs in mesophase pitch should exhibit a face-on orientation on the carbonaceous substrates, including a graphite sheet, glassy carbon, and pyrolytic carbon. However, it showed that the anchoring state (face-on or edge-on) of PAH units can be altered even on carbonaceous substrates. The results demonstrated that in C/C composites the anchoring state is predominantly determined by the relative degree of crystallinity of the pitch/carbon substrate, and can be semi-quantitatively estimated using the I_D/I_G ratio from Raman spectra. Face-on anchoring is preferred when the I_D/I_G ratio of substrate is smaller (higher crystallinity) than that of the pyrolyzed precursor, whereas edge-on anchoring is favored when it is larger. Such an estimation method is useful in tailoring microstructures in the fabrication of C/C composites using PAH precursors.