Large PAHs Research Articles

Gas-phase hydrogenation processes of cationic carbon clusters

ABSTRACT In this work, the gas-phase ion–atom collision reaction between large cationic carbon clusters and H-atoms is investigated. The carbon cluster cations (C$_{48-2*n}$$^+$, n = [0$-$8]) are produced from the photo-fragmentation processes of large PAH (dicoronylene, DC, C$_{48}$H$_{20}$) cations. The hydrogenated carbon cluster cations are efficiently formed (e.g. C$_{44/46}$H$_{9}$$^+$), and no even–odd hydrogenated mass patterns are observed. The hydrogenation behaviour and hydrogenation rate for these carbon cluster cations are the same. With theoretical calculations, the formation and bending processes of carbon cluster cations, the structure of these newly formed hydrogenated carbon cluster cations, and the bonding energies for the hydrogenation pathways are investigated. During the formation process of carbon clusters, the zigzagged edges gradually increase, and the planar configuration tends towards a bent and folded molecular configuration, i.e. from graphene to fullerene structures. The bending process with higher exothermic energies provides a reasonable explanation for the formation of the ‘magic numbers’ (e.g. C-atoms = 44) carbon clusters and their greater stability. The exothermic energy for each hydrogenation reaction pathway is relatively high; consequently, the forms and the hydrogenated states of carbon clusters are complex. The hydrogenation ability of edge carbon sites is higher than that of internal carbon sites; after bending and folding, the hydrogenation ability of these originally internal carbon sites becomes higher due to structural caged. As a result, under the co-evolution interstellar chemistry network, the (hydrogenation) states and forms of carbon compounds are complicated and diverse in the ISM.

An updated functional-group-based approach to modeling the vacuum residue oil gasification kinetics

Gasification of heavy petroleum residues can convert low-value feedstock to hydrogen-rich syngas, which can further be used for power generation and/or chemical production. The present work proposed an updated functional-group-based approach (FGMech) to modeling the gasification of a vacuum residue oil (VRO), which is helpful for better understanding the detailed kinetics of the gasification and improving gasifier performance. Elemental, average molecular weight (AMW) and nuclear magnetic resonance (NMR) analyses were conducted experimentally to characterize VRO, including elemental composition, average molecular formula and the functional group distribution, which were further used for model construction. A lumped mechanism for VRO devolatilization was constructed based on the updated FGMech approach: stoichiometric parameters of stable gases, tars and char were obtained from experiments, while those of radicals were based on the multiple linear regression (MLR) correlations; thermodynamic and kinetic parameters were derived from Benson group additivity method and rate rules, respectively. A merged detailed model was adopted for describing the conversion of gases and tars, and a global model was used for char. To test the reliability of the present model approach, Orimulsion gasification experiments from literature were simulated using an integrated perfectly stirred reactor (PSR) and plug flow reactor (PFR) model. It shows that the present model can reasonably predict measured results under various equivalence ratios, and has better performance on the prediction of CH4 compared with most literature models. Based on model analyses, syngas comes from the conversion of C2H4, H2S, CH4 and char in different gasification stages. Benzene, toluene, naphthalene and 1-methylnaphthalene are initial tar species considered in the devolatilization of the VRO. They can undergo hydrogen abstraction acetylene addition (HACA) and C3H3/C5H5 addition reaction pathways to produce large PAHs.

Comprehensive study on particle size distribution, MAH and PAH profiles during alkylbenzene pyrolysis

The 5 linear alkylbenzenes, i.e., toluene, ethylbenzene, n-propylbenzene, n-butylbenzene, and n-pentylbenzene as fuels underwent pyrolysis at temperatures from 950 to 1450 °C in a flow reactor. Measurements of particle number size distribution, 11 monocyclic aromatic hydrocarbons, and 3 groups of polycyclic aromatic hydrocarbons (20 unsubstituted PAHs, 5 aliphatic-bridged PAHs, and 10 methyl PAHs) during the pyrolysis of linear alkylbenzenes were conducted. Results showed that the increase in side chain length suppressed the formation of particles, but ethylbenzene produced the highest particle mass and most extensive particle number size distribution. Total particle peak number concentrations exhibited a rise-then-fall trend (peaking at a critical temperature of 1050 °C) with increasing temperature regardless of the side chain length, indicating that high temperature promoted a transition from nucleation to agglomeration and coagulation. Total PAH mass concentrations were ranked in the same order with the variation of side-chain length as particle mass and peak number concentrations, indicating that soot nucleation was highly related to total PAH mass concentrations. Unsubstituted PAHs accounted for most PAHs while substituted·PAHs, including aliphatic-bridged PAHs and methyl PAHs presented 1–2 orders of magnitude less than the corresponding unsubstituted PAHs, probably distinct thermal stability and reactivity led to different growth routes between unsubstituted and substituted PAHs. Increasing the temperature from 950 to 1250 °C, the formation of high molecular weight PAHs was promoted. while a drop of high molecular weight fractions was observed above 1350 °C, which is attributed to the suppression of the formation, as well as their conversions of high molecular weight PAHs to heavier unidentified PAHs or soot. Among the test alkylbenzene fuels, the fuels with a long side chain benefit from the reduction of particles and PAHs. Furthermore, carbon atoms in aromatic rings are dominant sources of large PAHs and particles, while the carbon atoms in aliphatic side chains mainly form gas species.

Revisit on the assignment of electronic spectra of C11H9+ isomers

The polycyclic aromatic hydrocarbon cation C11H9+ is the reaction intermediate for the growth process of large PAHs in the interstellar space and combustion process. The assignment for the absorption and emission spectra of C11H9+ isomers has been revisited based on the vibronic structure analysis. By carefully comparing the 0–0 transition energy and their spectral profiles with the experimental spectra, we can draw the following conclusions: the previous identification of the experimental spectra of C11H9+ isomers is inadequate. We found that the absorption and emission spectra near 560 nm can be classified as isomer 1-NyMe+ but not 2-NyMe+ while the absorption and emission bands near 460 nm in the experimental spectra should be contributed to the isomer BzTr+. It is shown again that the calculated vertical excitation energy often leads to incorrect spectral identification for the isomers of PAHs cations.

Effects of diethyl ether addition on sooting transition in iso-octane counterflow diffusion flames

This study studied the effects of the addition of diethyl ether (DEE) in the fuel stream on sooting transition process in iso-octane counterflow diffusion flames with optical and chemical diagnostics. Compared with the baseline iso-octane flame, two fuel/oxygen/nitrogen flames were studied with a 20 % DEE/iso-octane and a 40 % DEE/iso-octane mixture as the fuel, while holding constant fuel mole fraction. Optical diagnosis was used for the investigation of the effect of DEE addition on macroscopic sooting transition point with a high-resolution digital camera based on image processing and transition point extraction methods. Online gas chromatograph (GC) was used for all flames to investigate the flame structure and measure quantitative mole fraction profiles of stable and reactive species formed in the sooting transition process, especially the changes caused by partial substitution of iso-octane by DEE. Meanwhile, the experimental results were compared with predictions from a model suitable for both iso-octane and DEE. Sooting transition point was delayed with the increase of DEE addition. Although the overall flame chemical structure did not change significantly with the addition of DEE, it was found that some primary decomposition products and precursors had more significant effects. In most cases, experimental trends could be predicted and explained well by the model. It was noted that the dilution of carbon flow caused by partial substitution was the main reason for the inhibition of soot formation and the delay of sooting transition point. Secondly, on the chemical effect, DEE addition could produce a large amount of C2H2 which could contribute to the production of C3H3 and the formation of large PAHs.

Mechanism of E-bridge formation by various PAH molecules: A theoretical study

Potential energy surfaces (PES) for dimerization of acepyrene-pyrene, aceanthracene-anthracene, and acenaphthalene-naphthalene were computed by the G3(MP2,CC) and ONIOM2(G3(MP2,CC):B3LYP/6-31G(d)) methods. Close agreement of relative energies of corresponding species along reaction paths for different reactions was found suggesting that the PES for E-bridge formation depends weakly on the number of rings in the interacting monomers. The results indicate that the reaction rate is controlled by the dynamics of the initial collision, which may depend on the monomer size. The ONIOM approach is proposed as an accurate tool for calculations of PES in large PAH systems being more reliable than various DFT methods.

Syntheses of Anthracene-Centered Large PAH Diimides and Conjugated Polymers.

Polycyclic aromatic hydrocarbon (PAH) structures with suitable electron-withdrawing groups are useful building blocks for developing optical and electron-transporting materials. Here, we report the application of a double benzannulation process to the syntheses of PAH diimides with enlarged π-frameworks featuring a central anthracene moiety. The preparations are realized by copper-catalyzed [4+2] cycloaddition of ethynyl-substituted aromatic dicarboximide to 2,5-bis(phenylethynyl)terephthalaldehyde, followed by intramolecular photocyclization or direct arylation via Heck cross coupling. A central symmetric benzo[1,2-k:4,5-k']-bis(fluoranthene)-3,4,12,13-tetracarboxyl diimide (BFDI) is acquired, with the single crystal structure revealing its completely planar polycyclic skeleton. Such a shape-persistent PAH expectedly exhibits a tendency to stack face-to-face and forms J-aggregates. Moreover, BFDI can be difunctionalized site-selectively at the reactive 9 and 10 positions of the anthracene unit and then applied to prepare conjugated polymers. When coupled with 1,4-diketopyrrolo[3,4-c]-pyrrole (DPP) via thiophene and dithiophene linkers, two polymers with significantly broadened absorption bands extended to the near-infrared regime are obtained, evidencing the effective π-conjugative extension ability of BFDI unit.

Catalytic mechanism of Na on coal pyrolysis-derived carbon black formation: Experiment and DFT simulation

Coal-derived carbon black is the main composition of PM2.5, used in the rubber industry and energy storage fields. The formation and structure control of carbon black have attracted significant attention. Alkali metals have influence on the dynamic formation of coal-derived carbon black. To analyze the mechanism of Na in the formation of carbon black, a high-temperature drop tube furnace was used to analysis of carbon black from the pyrolysis of acid-washed coal and Na-containing coal at 1250 °C. DFT was applied to simulate the stepwise inhibitory effect of Na on the polymerization of PAHs into carbon black. The results show that Na promotes the oxidative cracking of large PAHs into small PAHs during the formation of coal-derived carbon black (reaction energy barrier is reduced by 17.7%), and inhibits the condensation of small PAHs molecules to form carbon black molecules (reaction energy barrier is increased by 74.7%), which causes the length of carbon black is shortened by 14.3%, and its curvature is increased by 1.7%. Na could change the spatial structure of formed carbon black-graphite crystallites, which increases the spacing of carbon black by approximately 6.2% to 16.3%. Na enhances the electrochemical performance of carbon black by 2 to 3 times.

A hexaazatriphenylene fused large discotic polycyclic aromatic hydrocarbon with selective and sensitive metal-ion sensing properties

A HAT based large PAH discotic molecule PN8 is developed. The enlarged chromophoric core and doping heteroatoms enable colorimetric and fluorometric sensing of Cu2+ and Zn2+ with highly appreciable optical changes, good selectivity and low detection limit. Moreover, PN8 was demonstrated as an excellent adsorbent to remove Cu2+ and Zn2+ from wastewater.

Infrared appearance of wind-blown bubbles around young massive stars

AbstractThousands of ring-like bubbles appear on infrared images of the Galaxy plane. Most of these infrared bubbles form during expansion of Hii regions around massive stars. However, the physical effects that determine their morphology are still under debate. Namely, the absence of the infrared emission toward the centres of the bubbles can be explained by pushing the dust grains by stellar radiation pressure. At the same time, small graphite grains and PAHs are not strongly affected by the radiation pressure and must be removed by another process. Stellar ultraviolet emission can destroy the smallest PAHs but the photodestruction is ineffective for the large PAHs. Meanwhile, the stellar wind can evacuate all types of grains from Hii regions. In the frame of our chemo-dynamical model we vary parameters of the stellar wind and illustrate their influence on the morphology and synthetic infrared images of the bubbles.

Doubly Charged Molecular Ions in GC-MS with Cold EI.

We report the finding of doubly charged molecular ions in a range of relatively large molecules including hydrocarbons upon their electron ionization as vibrationally cold molecules in supersonic molecular beams (SMB) (also named as Cold EI). Furthermore, we also report the detection by mass spectrometry of triply charged molecular ions in large PAHs such as decacyclene and ovalene upon their cooling in SMB. We found that the relative abundance of doubly charged molecular ions strongly depends on the internal vibrational cooling. While after some vibrational cooling the fragmentation pattern became cooling independent, the relative abundance of the doubly charged molecular ions was noticeably increased upon further cooling via increasing of the cooling make-up gas flow rate. In addition, the relative abundance of the doubly charged molecular ions was strongly increased with the compounds' size, and its electron energy threshold was lower than expected. These observations indicate a new mechanism that involves two separate electron ionization processes in the same compound, most likely with the same electron but at two separate atoms (places) in large molecules, to reduce Coulombic repulsion energy that can lead to fragmentation into two singly charged ions. These findings are shedding new light on electron ionization mass spectra. Accordingly, electron ionization mass spectra are the result of three separate mechanisms with relative magnitudes that depend on the compound size: (a) single electron ionization; (b) double electron ionization; and (c) single electron ionization with subsequent internal excitation by the same ionizing electron in another place.

Insights on the effect of ethanol on the formation of aromatics

For decades, ethanol has played an important role as a biofuel, as its addition to hydrocarbon fuels has been associated with a reduction in soot formation. The chemical mechanisms behind this phenomenon, however, are still not clear. In this paper, we contribute to this research area by elucidating the mechanisms that participate in the formation of polycyclic aromatic compounds (PACs) in an ethanol-doped ethylene flame, using a combination of deterministic and stochastic computational techniques. We specifically focus on the formation of oxygenated PACs and the chemical interactions of pure hydrocarbons and oxygen in six ethylene/air premixed flames with similar temperature profiles but different equivalence ratios and ethanol doping percentages. Our simulations confirm that an increase in the ethanol content results in a reduction of the formation of acetylene, small aromatics, and large PAHs. At the same time, the number of oxygenated PACs reach a maximum at a height above burner around 2–3 mm where they constitute 45% of all PACs: most of the oxygenated structures are phenols, mixed with approximately 15% of furans, and a small amount of ethers. Overall, the results indicate that the formation pathways of oxygenated PACs can compete with pure hydrocarbon growth mechanisms in the rapid growth region up to a height above burner of 2–3 mm. The rate of PACs’ growth then gradually slows down, with pure hydrocarbon growth mechanisms being the main contributors in this region of the flame.

Thermophoretic sampling of large PAH (C ≥ 22–24) formed in flames

The study of the transition from molecular to particle-like species involved in soot inception needs experimental strategies aiming to measure high molecular mass structures that could be important also for their effect on environment and human health. In this work, we have investigated the carbon particulate selectivity involved in the thermophoretic deposition used for the first time as a direct and fast sampling of high molecular weight aromatic species formed at the inception stage in an ethylene heavily sooting flame. Gravimetric analysis along with mass spectrometry, UV–Visible and fluorescence spectroscopy showed that the dichloromethane-extract of carbon particulate matter thermophoretically deposited on the substrate was much less than that sampled by a conventional water-cooled probe as it is composed only of high molecular weight condensed phases (C ≥ 22–24). Thermophoretic deposition is demonstrated to pre-separate, already at the sampling step, soot precursors of high molecular weight allowing their further analysis without the interference of the more abundant light polycyclic aromatic compounds (C < 22–24). These high molecular weight aromatic species presented interesting optical properties in terms of size and fluorescence emission peaked in the blue-region that could be of interest for carbon dots application. Size exclusion chromatography of particulate coupled with UV–Visible spectroscopy showed the presence of some aggregates already at the beginning of soot formation to be taken into account as ruling the optical properties of the whole carbon particulate matter.

Singlet-diradical character in large PAHs triggers spontaneous-ignition of coal

Chemical phenomena dictating the spontaneous-ignition of coal remain highly speculative with some of the most intriguing questions appearing unaddressed. In this communication, we deploy a synergistic experimental-theoretical approach to elucidate the definite role of singlet-biradical PAHs in driving the low-temperature oxidation of coal. We establish that, five linearly-fused benzene rings (i.e., pentacene, as a model compound for the PAH content in coal, and an example of short-chain graphene nanoribbons) ignites 200 °C lower than the corresponding temperature for naphthalene.

Effects of ionic liquid pretreatment on pyrolysis characteristics of a high-sulfur bituminous coal

Four different ionic liquids (ILs) were used to identify their extraction properties on high-sulfur bituminous coal. And the effect of ILs pretreatment on pyrolysis characteristic was investigated in a fixed-bed reactor. The imidazolium-based ILs used namely: 1-butyl-3-methyl-imidazolium chloride ([Bmim]Cl), 1-butyl-3-methyl-imidazolium bromide ([Bmim]Br), 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim]HSO4), 1-butyl-3-methyl-imidazolium tetrafluoroborate ([Bmim]BF4). All the selected ILs exhibited extraordinary extraction ability for oxygen-containing compounds, mainly furans and ethers. Among them, [Bmim]Cl got the highest extraction yields of 20.82% and performed selective extraction ability for sulfur-containing compounds with a relative content of 5.10% in light oils. Compared with pyridine pretreated coal (R-PY), the yields of CO and CO2 in pyrolysis gas showed increase trend when coal pretreated by ILs (R-ILs). The relative contents of tricyclic and larger PAHs, sulfur-containing compounds, and oxygen-containing compounds in tar from pyrolysis of [Bmim]Cl and [Bmim] Br treated coal increased significantly than R-PY tar, while phenols got decreased. High carbon number components from R-ILs tar reduced dramatically compared with R-PY tar, which was conducive to the upgrading of tar, after pretreated with ILs, the sulfur content was reduced. The crosslinks between coal macromolecules structure were destroyed after pretreated by ILs, and the pyrolysis products distribution had evident change. The oil phase yield got a substantial increase while [Bmim]Cl obtained highest oil yield of 24.89%.

Hydrogen chloride adsorption on large defective PAHs modeling soot surfaces and influence on water trapping: A DFT and AIMD study

DFT calculations both at 0 K in energy optimization procedure and at finite temperature in ab initio molecular dynamics (AIMD) calculations are used to characterize the adsorption of the hydrogen chloride molecule on a carbonaceous cluster in which a carbon vacancy has been created by removing one carbon atom. This aims at modeling the defective surface of the small graphitic basic structural units that constitute soot nanoparticles, as experimentally evidenced. The results show that HCl can be easily dissociated at the vacancy site of the surface, resulting in the formation of a chlorinated site at the carbonaceous surface, which could have different structures depending on the thermodynamics of the reactions. Then, the resulting chlorinated carbonaceous surfaces are used as model substrates to characterize the first stages of the adsorption of water molecules on chlorinated soot. It is thus shown that adsorbed chlorine atoms may act as an efficient nucleation center for water trapping at the surface of soot. These results represent a new step towards a better understanding of the soot behavior in industrial or domestic fire situations, as well as in marine atmospheric environment where oxidant and chlorinated species are ubiquitous.

PAH clusters as interstellar very small grains

PAH clusters are one candidate species for the interstellar "very small grains" or "VSGs", i.e., dust grains small enough to be stochastically heated and contribute to the aromatic infrared emission bands (AIBs). This possibility motivated laboratory experiments on the infrared spectroscopy of PAH clusters using matrix isolation spectroscopy. The spectral shifts due to PAH clustering in argon matrices provide clues for the AIB contribution from PAH clusters in the interstellar medium. Here we review results from a number of small PAH species, extrapolation to the much larger PAHs believed to be present in the interstellar medium, and the implications for a PAH cluster contribution to the VSG population.

Laboratory formation of large molecules in the gas phase

We report the experimental study on the formation process of large molecules (e.g. a family group of molecular clusters and graphene) in the gas phase. The experiment was carried out using a quadrupole ion trap in combination with time-of-flight mass spectrometry. As the initial molecular precursor, dicoronylene (DC, C48H20)/anthracene (C14H10) cluster cations, the results show that large PAH cluster cations (e.g., (C14H10)C48Hn+, n = [1–19] and (C14H10)C62Hm+, m = [1–25]) and PAH-graphene cluster cations (e.g., (C14H10)nC48+, n = 0, 1, 2, 3 and (C14H10)mC62+, m = 0, 1, 2) are formed by gas-phase condensation under laser irradiation conditions. We infer that these results present in here provide a formation route for interstellar large molecules under the influence of a strong radiation field in the ISM. The relevance of newly formed species to the nanometer-sized dust grain in space is briefly discussed.

Temperature effects on anharmonic infrared spectra of large compact polycyclic aromatic hydrocarbons

Aims. Large compact polycyclic aromatic hydrocarbon molecules (PAHs) present special interest in the astrochemical community. A key issue in analyses of large PAHs is understanding the effect that temperature and anharmonicity have on different vibrational bands, and thus interpreting the infrared (IR) spectra for molecules under various conditions. Methods. Because of the huge amount of interactions/resonances in large PAHs, no anharmonic IR spectrum can be produced with static/time-independent ab initio method, especially for the molecules with D6h symmetry, e.g., coronene and circumcoronene. In this work, we performed molecular dynamics simulations to generate anharmonic IR spectra of coronene and circumcoronene. Results. The method is validated for small PAHs, i.e., naphthalene and pyrene. We find that the semiempirical method PM3 produces accurate band positions with an error &lt;5 cm−1. Furthermore, we calculate the spectra at multiple temperatures and find a clear trend toward band shifting and broadening.

To freeze, or not to freeze: the impact of subzero temperature on quantifying organic contaminants in ocean sediments.

Assessing the extent of ocean bed contamination by anthropogenic organic chemicals requires collecting and preserving the native state of sediments. The latter is particularly important since most sediment analyses are performed away from the sites of samples collection. Preservation, however, is presumptuous since commonly used sediment handling practices such as freezing are structurally disruptive, the impacts of which are not well understood. In this study, the impact of freezing on quantifying the total organic carbon (TOC) contents and the extent of PAH, DDT and PCB contamination in 17 split-paired sediment samples was investigated. The samples were collected from the Santa Monica Bay, California (USA). One-half of each split-pair was frozen at -20 °C and the other half was refrigerated at 4 °C for up to 11 days. The results suggest that no significant differences exist between the frozen and the refrigerated datasets for PCBs (F1,28 = 4.01, p > 0.05), DDTs (n = 16, t-Stat < t-Critical, p > 0.05) or TOC (n = 16, t-Stat < t-Critical, p > 0.05). The results however show that less PAHs were detected in the frozen sediments (F1,24 = 8.18, p < 0.05) than in the refrigerated sediments; the larger PAHs were affected the most. Interestingly, while benzo[a]pyrene, a large PAH molecule, was affected by this apparent temperature-induced difference, its structural isomer, benzo[e]pyrene, was not. Even more interesting was the finding that while non-coplanar PCBs were affected similarly, the coplanar PCBs were not. Overall, sediment freezing within this study's timeframe appears to offer little contextual advantage over sediment refrigeration.