Oxygenated Polycyclic Aromatic Hydrocarbons Formation Research Articles

Computational thermochemistry of oxygenated polycyclic aromatic hydrocarbons and relevant radicals

Oxygenated polycyclic aromatic hydrocarbons (OPAHs) have attracted growing attention due to their toxicological harmfulness and significant role in soot formation. This study comprehensively investigates the thermochemistry of OPAH species and relevant radicals via quantum-chemical calculations. Temperature-dependent enthalpy of formation, entropy, and heat capacity for C5C18 OPAHs (59 molecules and 33 radicals) are consistently determined at the M06–2X/6-311++G(d,p) level of theory. Considerable differences are found to be introduced by different methods when calculating electronic energies, and the G3 method outperforms the composite compound methods G3/G4/CBS-APNO. The calculated thermochemical properties from the G3 method show excellent agreement with literature data. An accurate thermochemistry database for OPAHs is thus developed. In addition, the existing group additivity (GA) method does not apply to OPAHs since the group additivity values (GAVs) derived from small hydrocarbons fail to predict large polycyclic species. Based on our dataset, GAVs are obtained from combinatorial considerations. The updated GAVs can be applied with enhanced confidence to estimate the thermochemical parameters at different temperatures for larger OPAHs where such high-accuracy quantum chemistry calculations are intractable. These thermodynamic properties and GAVs are crucial for the development of accurate kinetic models for OPAH formation chemistry and for achieving emission control.

Blending effect of methanol on the formation of polycyclic aromatic hydrocarbons in the oxidation of toluene

Methanol has been considered as a potential renewable liquid fuel and blending it with gasoline and diesel is an effective way to reduce greenhouse gas emissions from the transport sector. To understand the mixing effect of methanol on the formation of polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs), the fuel-rich oxidation of toluene with and without methanol was studied using a flow reactor at atmospheric pressure, temperatures from 1050 to 1350 K, equivalence ratio of 9.0, and residence time of 1.2 s. The blending ratio of methanol was varied as 0% and 50% on a molar basis. Gas chromatograph mass spectrometer was employed to identify and quantify PAHs and OPAHs in gaseous products. A kinetic model on PAH growth up to five ring structures was used to investigate the blending effect on PAH and OPAH formation. Both experiment and modeling showed that PAH and OPAH production at lower temperatures was unexpectedly promoted in toluene/methanol oxidation compared with toluene oxidation, while their production in toluene oxidation was identical with or larger than that in toluene/methanol oxidation at elevated temperatures. In methanol oxidation, no PAHs were produced under the current experimental conditions. Kinetic analysis indicated that high methanol reactivity produced several radicals, such as OH, H, and HO2, which promoted toluene reactivity at lower temperatures, resulting in the enlargement of PAH and OPAH formation in toluene/methanol oxidation compared to neat toluene oxidation. When the temperature was increased, the effect of methanol blending was diminished based on the kinetic analysis. These results suggest that oxygenated fuels do not necessarily reduce PAH production, but promote it under some conditions.

Formation of PAHs, phenol, benzofuran, and dibenzofuran in a flow reactor from the oxidation of ethylene, toluene, and n-decane

The necessity to reveal the formation mechanism of not only polycyclic aromatic hydrocarbons (PAHs) but also oxygenated PAHs (OPAHs) during combustion is increasing. Although many studies on PAHs have been conducted, fundamental studies investigating OPAH formation are still limited. Phenol, benzofuran, and dibenzofuran were selected as OPAHs in this study. We experimentally investigated fuel-rich oxidation in a flow reactor at atmospheric pressure, mean gas temperatures from 1050 to 1350 K, residence times from 0.2 to 1.5 s, and equivalence ratios from 3.0 to 12.0. Ethylene, toluene, and n-decane were used as fuels. Three kinds of OPAHs as well as 23 kinds of PAHs including monocyclic structures were quantitatively measured through a direct sampling using gas chromatography mass spectrometry. The results showed that concentrations of PAHs and OPAHs were strongly affected by the fuel type, with increasing concentrations in the following order: ethylene < n-decane < toluene. A chemical kinetic model for OPAH formation was developed based on a recent PAH growth model. The model showed that the predicted concentrations of OPAHs and PAHs were in reasonable agreement with the measured data in this study. Some modifications were made to the previous model based on recent literature studies and on the comparison of the simulated and measured results. The effect of the fuel type on the formation of PAHs and OPAHs was investigated through kinetic analysis using the model to discuss their reaction pathways.

The formation and evolution of parent and oxygenated polycyclic aromatic hydrocarbons during a severe winter haze-fog event over Xi'an, China.

In this study, 3- or 4-h high time-resolved PM2.5 was observed during a severe winter haze-fog event (1 to 6 January 2017) to investigate the formation and evolution of parent polycyclic aromatic hydrocarbons (pPAHs) and oxygenated polycyclic aromatic hydrocarbons (OPAHs) in Xi'an, a typical city in northwestern China. Three episodes (episode I, episode II, and rainy day (EI, EII, and RD)) have been identified during this haze-fog event. Nine water-soluble ions, 8 carbonaceous fractions, 18 pPAHs, and 3 OPAHs in PM2.5 were measured. pPAHs showed two peaks at around 12:00 local standard time (LST) and 24:00 LST and two troughs at around 2:00 LST and 18:00 LST during EI. However, the OPAHs presented highest at around 18:00 LST and lowest at around 2:00 LST. During EII, pPAHs and OPAHs displayed similar diurnal variations with the highest values at noon but lowest values at around 2:00-5:00 LST.In addition, no obvious diurnal variations of pPAHs and OPAHs were observed during RDwere absent during RD. Diurnal variations of pPAH ring distributions demonstrated coal combustion, and vehicle emissions contributed to pPAHs for three episodes, which is further confirmed by diagnostic ratio results. High oxygenation (Ro) rates were found during the sampling time, which favored OPAH formation. The study herein indicates that OPAH formation through complex atmospheric reactions provides us new insights into the severe haze-fog events.

Experimental and theoretical evidence for the temperature-determined evolution of PAH functional groups

Elucidating the chemical evolution of various functional groups in polycyclic aromatic hydrocarbons (PAH) and soot aids in understanding soot formation chemistry. In this work, the chemical evolution of various functional groups, including aromatic CH, aliphatic CH, C=O, COH and COC bonds, was experimentally investigated online, rather than with offline diagnostics. Oxidation was performed in a jet-stirred reactor (JSR), fueled with benzene/C2H2/air/N2 and benzene/phenol/C2H2/N2 for a temperature range of 600-1400 K. Kinetic modelling, including ab initio quantum chemistry calculations, reaction rate coefficient calculations and JSR simulations, were conducted to interpret the experimental data and the evolutionary chemistry of the various functional groups. Results show that the formation of functional groups on PAH and oxygenated PAH (OPAH) are highly sensitive to temperature. Aliphatic CH bonds survive mainly in the form of CCH2C, CCH2CH2C or CCH functional groups above 1200 K, and exist in the CHCH2 functional group below 1000 K. For the OPAH, the COC functional group presents stronger thermal stability than COH and C=O functional groups. Simulation results indicate that HACA-like pathway (hydrogen abstraction carbon addition), in which C2H2 attacks the O atom, followed by cyclization and H-atom elimination reactions, qualitatively describe the formation of OPAH with the CO-C functional group at different temperatures. The addition reaction involving PAH radical and C2H4 / C2H3 captures the evolution of PAH with the CHCH2 functional group, but fails to explain the formation of CCH2C and CCH2CH2C functional groups.

Formation of Polycyclic Aromatic Hydrocarbons (PAHs) and Oxygenated PAHs in the Oxidation of Ethylene Using a Flow Reactor

ABSTRACT Interest in polycyclic aromatic hydrocarbons (PAHs) including a functional group in its chemical structure, particularly oxygenated PAHs (OPAHs), has been growing due to the toxicity and wide presence of the compound. Understanding the formation mechanism of OPAHs is very important for reducing their emission during combustion. However, studies on the quantitative evaluation of OPAHs have been very limited. In this study, the formation of OPAHs was quantitatively evaluated in addition to that of PAHs using a flow reactor system in which a mixture of ethylene, O2, and N2 reacted at atmospheric pressure. 5 types of OPAHs were targeted in this study: phenol, benzofuran, dibenzofuran, 9-fluorenone, and benzanthrone. 3 types of monocyclic aromatic hydrocarbons (MAHs) and 17 types of PAHs were also analyzed by gas chromatography mass spectrometry. Measurements were performed at different reaction gas temperatures (1050 to 1350 K), residence times (0.40 to 1.5 second), and equivalence ratios (3.0 to 12.0). Some measured data, mainly MAHs and PAHs, were compared with the concentrations predicted using existing kinetic models. Finally, we evaluated the reaction pathways that contributed to the formation of the OPAHs such as benzofuran, dibenzofuran, 9-fluorenone, and benzanthrone quantified in this study.

Determination of Parent and Oxygenated Polycyclic Aromatic Hydrocarbons (PAHs) in Waste Cooking Oil and Oil Deodorizer Distillate by GC–QQQ–MS

Abstract Background: Polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs) are classes of contaminants that are present in the environment and food. They pose a great threat to human health because of their carcinogenicity and mutagenicity. Very few studies have focused on their concentration in waste cooking oil (WCO) and oil deodorizer distillate (ODD). Objective: This study aimed (1) to design a reliable method to determine 16 PAHs and 4 OPAHs in both WCO and ODD and (2) to determine and analyze PAH and OPAH concentrations in actual samples to provide references for further research. Method: The PAH determination approach included double liquid–liquid extraction, double solid-phase extraction, and GC–triple quadrupole tandem MS determination. Oxidation indices were determined by titrimetry. Results: The method reached good linearity (R2 &amp;gt; 0.99) and an acceptable recovery rate (55.01–126.16% for WCO and 57.48–128.97% for ODD). Ten WCO and five ODD samples were determined, and the total concentration of 16 PAHs varied from 16.34 to 239.01 and 101.08 to 198.04 μg/kg in WCO and ODD, respectively. Among the 16 PAHs, three-ring PAHs typically contributed the most. It was also found that the acid value has a strong correlation with the concentration of OPAHs, probably because of the contribution of free fatty acids to OPAH formation. Conclusions: The proposed method was effective in the analysis of PAHs and OPAHs in WCO and ODD.

Evolution of oxygenated polycyclic aromatic hydrocarbon chemistry at flame temperatures

Oxygenated polycyclic aromatic hydrocarbons (OPAH) have received increasing attention due to their toxic effect on human health. This study comprehensively investigates the evolution of OPAH chemistry at flame temperatures. Jet-stirred reactor (JSR) experiments with benzene/phenol/C2H2/N2 and benzene/C2H2/O2/N2 revealed that OPAH with oxygenated heterocycle can be formed by the addition of C2H2 at 1400 K. To further clarify the evolution of OPAH chemistry in soot systems, OPAH formation and decomposition reaction pathways and kinetic parameters have been theoretically investigated. The potential energy surfaces of 1-naphtholate and 2-naphtholate growth, and thermal decomposition reactions, were calculated by combining the density functional theory B3LYP/6–311+G(d,p) and CCSD(T)/cc-pvdz methods. The reaction rate coefficients in the temperature range of 800–2500 K and pressure range of 0.1–100 atm were calculated using RRKM theory by solving the master equations. The potential energy surface of C2H2+1-naphtholate and C2H2+2-naphtholate growth reactions showed that the O atom could be locked in a naphthofuran molecule with the formation of a COC oxygenated heterocycle; and the reaction rates were determined by adding the C2H2 elementary step with the energy barrier of 26.0 and 19.9 kcal/mol, respectively. Thermal decomposition reactions of 1-naphtholate and 2-naphtholate yielded an indenyl radical and CO. The thermal decomposition reaction rates were significantly sensitive to the zig-zag site structure next to the CO bond. The decomposition rate of 1-naphtholate at 1500 K, with a zig-zag site near the CO bond, was 14.8 times lower than that of 2-naphtholate with no zig-zag site near the CO bond. Rate comparison results indicate that the CO functional group rapidly converts to a COC functional group with the addition of C2H2. The formation, growth and thermal decomposition reactions of 1-naphtholate and 2-naphtholate were added to a detailed PAH mechanism to check the effect of OPAH reactions on PAH formation chemistry. The concentration profile of naphthalene predicted by the updated PAH mechanism was lower than current PAH mechanism predictions by 29%, indicating that the OPAH reactions had a significant effect on PAH formation chemistry, and should be included in the PAH mechanism. However, due to the relatively low concentration of OPAH compared to PAH, it is possible to ignore the correlation between OPAH and soot nucleation at flame temperatures; therefore an OPAH evolution pathway (PAH → incipient soot → OPAH formation on soot particle → selective thermal decomposition of OPAH), is proposed to explain the high content of OPAH molecules (e.g., 9,10-anthraquinone, benz(a)anthracene-7,12-dione, and benzanthrone) adsorbed on the soot particle.

Determination of Parent and Oxygenated Polycyclic Aromatic Hydrocarbons (PAHs) in Waste Cooking Oil and Oil Deodorizer Distillate by GC-QQQ-MS.

Background: Polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs) are classes of contaminants that are present in the environment and food. They pose a great threat to human health because of their carcinogenicity and mutagenicity. Very few studies have focused on their concentration in waste cooking oil (WCO) and oil deodorizer distillate (ODD). Objective: This study aimed (1) to design a reliable method to determine 16 PAHs and 4 OPAHs in both WCO and ODD and (2) to determine and analyze PAH and OPAH concentrations in actual samples to provide references for further research. Method: The PAH determination approach included double liquid-liquid extraction, double solid-phase extraction, and GC-triple quadrupole tandem MS determination. Oxidation indices were determined by titrimetry. Results: The method reached good linearity (R² > 0.99) and an acceptable recovery rate (55.01-126.16% for WCO and 57.48-128.97% for ODD). Ten WCO and five ODD samples were determined, and the total concentration of 16 PAHs varied from 16.34 to 239.01 and 101.08 to 198.04 μg/kg in WCO and ODD, respectively. Among the 16 PAHs, three-ring PAHs typically contributed the most. It was also found that the acid value has a strong correlation with the concentration of OPAHs, probably because of the contribution of free fatty acids to OPAH formation. Conclusions: The proposed method was effective in the analysis of PAHs and OPAHs in WCO and ODD.

The formation mechanism for OPAHs during the cellulose thermal conversion in inert atmosphere at different temperatures based on ESI(−) FT-ICR MS measurement and density functional theory (DFT)

Oxygenated polycyclic aromatic hydrocarbons (OPAHs) are an important group of components produced from the thermal conversion of cellulose. Through using gas chromatography mass spectrometry (GC–MS), Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) and ultraviolet fluorescence spectroscopy (UV-F), the information for the evolution of OPAHs is obtained in this study. The tar produced at low temperature mainly contains light components such as homologues of pyran and furan, while the components in the tars at 700 °C and 900 °C show high double bond equivalent (DBE) values, indicating the existence of large aromatic structures.High temperature promote condensation reactions during cellulose thermal conversion, thus leading to a higher percentage of OPAHs in the tar at higher temperature process. According to Density Functional Theory (DFT) calculation, the energy barrier of Diels-Alder reaction (302.65 kJ/mol) is lower than the dehydration (583.26 kJ/mol), which means that Diels-Alder reaction could be the main route for OPAH formation rather than dehydration reaction. High reaction temperature is calculated to be favorable for the formation of naphthol during the cellulose thermal conversion because of the high energy barrier of dehydration between phenol and furan. It is consistent with the ESI FT-ICR MS result that there is a higher content of naphthol in the tar at 700 °C than that at 500 °C.

Emission of Oxygenated Polycyclic Aromatic Hydrocarbons from Indoor Solid Fuel Combustion

Indoor solid fuel combustion is a dominant source of polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (OPAHs) and the latter are believed to be more toxic than the former. However, there is limited quantitative information on the emissions of OPAHs from solid fuel combustion. In this study, emission factors of OPAHs (EF(OPAH)) for nine commonly used crop residues and five coals burnt in typical residential stoves widely used in rural China were measured under simulated kitchen conditions. The total EF(OPAH) ranged from 2.8 ± 0.2 to 8.1 ± 2.2 mg/kg for tested crop residues and from 0.043 to 71 mg/kg for various coals and 9-fluorenone was the most abundant specie. The EF(OPAH) for indoor crop residue burning were 1-2 orders of magnitude higher than those from open burning, and they were affected by fuel properties and combustion conditions, like moisture and combustion efficiency. For both crop residues and coals, significantly positive correlations were found between EFs for the individual OPAHs and the parent PAHs. An oxygenation rate, R(o), was defined as the ratio of the EFs between the oxygenated and parent PAH species to describe the formation potential of OPAHs. For the studied OPAH/PAH pairs, mean R(o) values were 0.16-0.89 for crop residues and 0.03-0.25 for coals. R(o) for crop residues burned in the cooking stove were much higher than those for open burning and much lower than those in ambient air, indicating the influence of secondary formation of OPAH and loss of PAHs. In comparison with parent PAHs, OPAHs showed a higher tendency to be associated with particulate matter (PM), especially fine PM, and the dominate size ranges were 0.7-2.1 μm for crop residues and high caking coals and <0.7 μm for the tested low caking briquettes.

The formation of nitrogen species and oxygenated PAH during the combustion of coal volatiles

The combustion of coal volatiles produced by rapid pyrolysis was studied using a two-stage reactor consisting of a fluidized-bed reactor coupled to a tubular-flow reactor. Volatiles were generated in the fluidized-bed reactor under high heating rates and at 600°C such that the major volatile species produced were tars. The freshly generated tars were subsequently oxidized in the tubular-flow reactor at 900 and 1000°C. Fourier transform infrared (FTIR) analysis showed that, with an increase in oxygen concentration, the recovered tars exhibited and increased in the carbonyl, C=O, functionality. The position of the C=O peak and the presence of absorbances in the aromatic C−H out-of-plane deformation region in the FTIR spectra and GC/MS identification demonstrate that polycyclic aromatic ketones and aldehydes are significant oxygenated polycyclic aromatic hydrocarbons (OPAH) products from coal volatiles combustion. The results indicate that combustion processes are primarily responsible for OPAH formation. HNCO yield was found to increase rapidly with the addition of small amounts of oxygen. The results show that HCN oxidation is not primarily responsible for HNCO formation: reactions of other N-containing species are likely sources. The observation of HNCO suggests that previous measurements of NH 3 in coal combustion probably represent the sum of NH 3 and HNCO yields. The presence of hydrocarbon species (gases and tars) has a significant effect on fuel-N conversion. The experimental results clearly demonstrated that NO production increased significantly onee the concentration of hydrocarbons decreased.