Nitrogen-containing Polycyclic Aromatic Compounds Research Articles

Production of biocrude from organic waste: Influence of feedstock composition on hydrodenitrogenation reactivity in biocrude upgrading

Hydrothermal liquefaction (HTL) process of protein-rich biomass produces biocrude with a high nitrogen content that hinders its use as a biofuel intermediate. Nitrogen compounds in the biocrudes are mainly present as fatty acid amides (FAAs) and nitrogen-containing polycyclic aromatic compounds (NAs), having different hydrodenitrogenation reactivity during the biocrude upgrading process. Therefore, it is crucial to shift reaction pathways towards the formation of less recalcitrant compounds and reduce the nitrogen content of the biocrudes. Herein, the impact of the chemical composition, in particular a lipid content, on the biocrude yield, nitrogen content and types of nitrogen species has been studied using macromolecular model compounds. By comparing the hydrodenitrogenation (HDN) reactivity of FAAs and NAs, it was found that the latter are more recalcitrant compounds compared to the FAAs and that the higher lipid content inhibits the generation of NAs. These results suggest that lipids-rich waste can be supplemented with food waste to produce the biocrudes highly suitable for conventional hydrotreatment processes.

Construction of Tricyclic Nitrogen Heterocycles by a Gold(I)-Catalyzed Cascade Cyclization of Allenynes and Application to Polycyclic π-Electron Systems.

A novel approach to the direct construction of tricyclic nitrogen heterocycles based on gold-catalyzed cascade cyclization of aminoallenynes is described. The expected biscyclization reaction of hydroxyisobutyryl-protected aminoallenynes was efficiently promoted by a catalytic amount of BrettPhosAuNTf2 in the presence of iPrOH to produce 1,2-dihydrobenzo[cd]indole derivatives in good yields. When the reaction was combined with Friedel-Crafts acylation or palladium-catalyzed N-arylation, the resulting tricyclic products were efficiently converted into nitrogen-containing polycyclic aromatic compounds (N-PACs) with highly conjugated π-electron systems. A newly obtained hexacyclic indolium salt showed characteristic concentration-dependent absorption and emission properties.

Construction of Tricyclic Nitrogen Heterocycles by a Gold(I)‐Catalyzed Cascade Cyclization of Allenynes and Application to Polycyclic π‐Electron Systems

AbstractA novel approach to the direct construction of tricyclic nitrogen heterocycles based on gold‐catalyzed cascade cyclization of aminoallenynes is described. The expected biscyclization reaction of hydroxyisobutyryl‐protected aminoallenynes was efficiently promoted by a catalytic amount of BrettPhosAuNTf2 in the presence of iPrOH to produce 1,2‐dihydrobenzo[cd]indole derivatives in good yields. When the reaction was combined with Friedel–Crafts acylation or palladium‐catalyzed N‐arylation, the resulting tricyclic products were efficiently converted into nitrogen‐containing polycyclic aromatic compounds (N‐PACs) with highly conjugated π‐electron systems. A newly obtained hexacyclic indolium salt showed characteristic concentration‐dependent absorption and emission properties.

Characterizations of Size-segregated Ultrafine Particles in Diesel Exhaust

Size-segregated ultrafine particles (UFPs) in diesel exhaust were investigated to characterize carbonaceous substances, metals, and organic compounds originating from a medium-duty diesel engine dynamometer using the 13 driving mode. Organic carbon (OC) and elemental carbon (EC) peaked at 330–550 nm, but the OC/EC ratio showed two peaks in the ultrafine and accumulation modes. The distribution trend of metal elements was opposite to that of the size-segregated OC/EC ratio. The amounts of toxic Pb, As, and Cd were less than 0.03–2.5% in diesel exhaust particles (DEPs), but their cumulative fractions in the ultrafine mode exceeded 50%. Most organic compounds (76.6%) and alkanes (67.0%) were emitted in the accumulation mode (170–1000 nm). More than 70% of the identified polycyclic aromatic hydrocarbons (PAHs) were emitted in the accumulation mode (94–1000 nm), with phenanthrene being the most abundant. Two significant size ranges of toxicity equivalent quantity peaks in the ultrafine (34–66 nm) and accumulation (170–330 nm) modes were observed for the size-segregated DEPs. Contrary to the trends for PAHs and organic compounds, the identifiable nitrogen-containing polycyclic aromatic compounds were more abundant in the ultrafine mode. Overall toxicity was high as UFPs can be deposited with high efficiency throughout the human respiratory tract.

Synthesis of Nitrogen-Containing Polyaromatics by Aza-Annulative π-Extension of Unfunctionalized Aromatics.

Nitrogen-containing polycyclic aromatic compounds (N-PACs) are an important class of compounds in materials science. Reported here is a new aza-annulative π-extension (aza-APEX) reaction that allows rapid access to a range of N-PACs in 11-84 % yields from readily available unfunctionalized aromatics and imidoyl chlorides. In the presence of silver hexafluorophosphate, arenes and imidoyl chlorides couple in a regioselective fashion. The follow-up oxidative treatment with p-chloranil affords structurally diverse N-PACs, which are very difficult to synthesize. DFT calculations reveal that the aza-APEX reaction proceeds through the formal [4+2] cycloaddition of an arene and an in situ generated diarylnitrilium salt, with sequential aromatizations having relatively low activation energies. Transformation of N-PACs into nitrogen-doped nanographenes and their photophysical properties are also described.

Synthesis of Nitrogen‐Containing Polyaromatics by Aza‐Annulative π‐Extension of Unfunctionalized Aromatics

AbstractNitrogen‐containing polycyclic aromatic compounds (N‐PACs) are an important class of compounds in materials science. Reported here is a new aza‐annulative π‐extension (aza‐APEX) reaction that allows rapid access to a range of N‐PACs in 11–84 % yields from readily available unfunctionalized aromatics and imidoyl chlorides. In the presence of silver hexafluorophosphate, arenes and imidoyl chlorides couple in a regioselective fashion. The follow‐up oxidative treatment with p‐chloranil affords structurally diverse N‐PACs, which are very difficult to synthesize. DFT calculations reveal that the aza‐APEX reaction proceeds through the formal [4+2] cycloaddition of an arene and an in situ generated diarylnitrilium salt, with sequential aromatizations having relatively low activation energies. Transformation of N‐PACs into nitrogen‐doped nanographenes and their photophysical properties are also described.

Construction of Nitrogen-containing Polycyclic Aromatic Compounds by Intramolecular Oxidative C-H/C-H Coupling of Bis(9H-carbazol-9-yl)benzenes and Their Properties

Treatment of 1,3- and 1,4-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)benzenes with an oxidizing system of Pd(II)/Ag(I) has been found to induce double intramolecular C-H/C-H coupling to give the corres...

Annulative π-extension of indoles and pyrroles with diiodobiaryls by Pd catalysis: rapid synthesis of nitrogen-containing polycyclic aromatic compounds.

A palladium-catalyzed one-step annulative π-extension (APEX) reaction of indoles and pyrroles that allows rapid access to nitrogen-containing polycyclic aromatic compounds is described. In the presence of palladium pivalate and silver carbonate, diverse indoles or pyrroles coupled with diiodobiaryls in a double direct C-H arylation manner to be transformed into the corresponding π-extended compounds in a single step. The newly developed catalytic system enables the use of various pyrroles and indoles as templates with a series of diiodobiaryls to provide structurally complicated and largely π-extended nitrogen-containing polycyclic aromatic compounds that are otherwise difficult to synthesize.

Influence of pyrolysis temperature and production unit on formation of selected PAHs, oxy-PAHs, N-PACs, PCDDs, and PCDFs in biochar\u2014a screening study

The influence of reactor type and operating conditions of the pyrolysis unit on the final concentration of toxic contaminants in biochar remains unclear. Therefore, we determined the concentrations of polycyclic aromatic hydrocarbons (PAHs), oxygenated polycyclic aromatic hydrocarbons (oxy-PAHs), nitrogen-containing polycyclic aromatic compounds (N-PACs), polychlorinated dibenzo-p-dioxins (PCDDs), and dibenzofurans (PCDFs) in biochars produced from three different feedstocks (softwood, wheat straw, and anaerobic digestate). Different scaled pyrolysis units (one batch and two continuous units) at two different temperatures (550 and 700 °C) were considered. The results revealed that the type of biomass had a significant influence on the PAH, oxy-PAH, and N-PAC content of the biochars. The configuration and type of the pyrolysis unit influenced only the wheat straw pyrolyzed at 550 °C. PCDDs and PCDFs occurred at very low levels in the biochars. In terms of PAH, PCDD, and PCDF content, the biochars assessed in this study represent a low risk to the environment, regardless of the temperature and type and size of the pyrolysis unit.

Determination of nitrogen-containing polycyclic aromatic compounds in diesel and gas oil by reverse-phase high performance liquid chromatography using introduction of sample as detergentless microemulsion

Carbazoles, quinolines, indoles, pyrroles, anilines and acridines are nitrogen-containing polycyclic aromatic compounds (NPAC) that contribute to deteriorate fuels affecting storage stability, causing problems to motor engines and poisoning automotive catalysts. Carbazole, 3-ethylcarbazole, 9-ethylcarbazole, 9-methylcarbazole, quinoline, benzo[h]quinoline, indole, 3-methylindole, acridine and N,N-dimethylaniline were separated and quantified in diesel and gas oil by reverse-phase high performance liquid chromatography with fluorimetric detection and molecular absorption photometric detection using introduction of fuel samples as detergentless microemulsion (DME). The DME is composed by ethanol, propan-2-ol, water and iso-octane in specific proportions to stabilize the organic samples. The introduction of sample in the DME simplifies the procedure since it avoids extraction of the analytes and enables the determination of basic and neutral NPAC altogether. The limits of quantification for the analytes in the DME were in μgL−1 order (ng range for 10μL of injected volume). Satisfactory analyte recoveries were achieved (between 89.5% and 108.7%). Results were found to be comparable using or not using 7-methylindole and N-methylpyrrole as internal standards (since these were not found in the analyzed samples). Studies showed homogeneity of the analytes in the DME after mechanical stirring (at 7200rpm for 5min). Stability studies indicated results with coefficients of variation below 3%. The method is adequate for a rapid screening of NPACs in petroleum samples.

Characterizations of organic compounds in diesel exhaust particulates

To characterize how the speed and load of a medium-duty diesel engine affected the organic compounds in diesel particle matter (PM) below 1μm, four driving conditions were examined. At all four driving conditions, concentration of identifiable organic compounds in PM ultrafine (34–94nm) and accumulation (94–1000nm) modes ranged from 2.9 to 5.7μg/m3 and 9.5 to 16.4μg/m3, respectively. As a function of driving conditions, the non-oxygen-containing organics exhibited a reversed concentration trend to the oxygen-containing organics. The identified organic compounds were classified into eleven classes: alkanes, alkenes, alkynes, aromatic hydrocarbons, carboxylic acids, esters, ketones, alcohols, ethers, nitrogen-containing compounds, and sulfur-containing compounds. At all driving conditions, alkane class consistently showed the highest concentration (8.3 to 18.0μg/m3) followed by carboxylic acid, esters, ketones and alcohols. Twelve polycyclic aromatic hydrocarbons (PAHs) were identified with a total concentration ranging from 37.9 to 174.8ng/m3. In addition, nine nitrogen-containing polycyclic aromatic compounds (NPACs) were identified with a total concentration ranging from 7.0 to 10.3ng/m3. The most abundant PAH (phenanthrene) and NPACs (7,8-benzoquinoline and 3-nitrophenanthrene) comprise a similar molecular (3 aromatic-ring) structure under the highest engine speed and engine load.

PH and ionic strength effects on the binding constant between a nitrogen-containing polycyclic aromatic compound and humic acid.

Polycyclic aromatic compounds (PACs) are widespread environmental pollutants with a high potential to act as human carcinogens and mutagens. The behavior of PACs is significantly affected by their interactions with dissolved organic matter (DOM), such as their transport, solubility, bioavailability, and bioaccumulation in the aquatic environment. Being a basic PAC, benzo(h)quinoline (BQ) is the dominant species, as the solution's pH value is higher than BQ's pK a (pK a of BQ = 4.2). In contrast, benzo(h)quinolinium (BQH(+)) is the major species, as the solution's pH value is lower than its pK a. The binding constant (K DOC), measured by fluorescence quenching, between BQ/BQH(+) and Leonardite humic acid (LHA) would decrease 70 to 95% and 20 to 90% when increasing the ionic strength in acidic and neutral to basic conditions, respectively. The results can be attributed to the added cation (Na(+) and Mg(2+)), which forms a bridge with LHA and enhances the intramolecular reaction among these functional groups, therefore inducing the coiling up within the LHA molecule. In addition, the decrease of the K DOC with added MgCl2/MgSO4 (75-95%) is higher than that with added NaCl/Na2SO4 (20-75%), indicating that the K DOC was affected by the charge density of cations. The fluorescence intensity of BQH(+) in the absence of LHA (F 0) was found to decay only in the acidic solution with Cl(-), suggesting that Cl(-) might be a heavy atom serving as a quencher in an acidic solution.

Low-Temperature Mechanisms for the Formation of Substituted Azanaphthalenes through Consecutive CN and C2H Additions to Styrene and N-Methylenebenzenamine: A Theoretical Study

Ab initio G3(MP2,CC)/B3LYP/6-311G** calculations of potential energy surfaces (PESs) for the reactions of cyano and ethynyl radicals with styrene and N-methylenebenzenamine have been performed to investigate a possible formation mechanism of the prototype nitrogen-containing polycyclic aromatic compounds: (substituted) 1- and 2-azanaphthalenes. The computed PESs and molecular parameters have been used for RRKM and RRKM-Master Equation calculations of reaction rate constants and product branching ratios under single-collision conditions and at pressures from 3 to 10(-6) mbar and temperatures of 90-200 K relevant to the organic aerosol formation regions in the stratosphere of a Saturn's moon Titan. The results show that ethynyl-substituted 1- and 2-azanaphthalenes can be produced by consecutive CN and C2H additions to styrene or by two C2H additions to N-methylenebenzenamine. All CN and C2H radical addition complexes are formed in the entrance channels without barriers, and the reactions are computed to be exothermic, with all intermediates and transition states along the favorable pathways residing lower in energy than the respective initial reactants. The reactions are completed by dissociation of chemically activated radical intermediates via H losses, so that collisional stabilization of the intermediates is not required to form the final products. These features make the proposed mechanism viable even at very low temperatures and under single-collision conditions and especially significant for astrochemical environments. In Titan's stratosphere, collisional stabilization of the initial CN + styrene reaction adducts may be significant, but substantial amounts of 2-vinylbenzonitrile and 2-ethynyl-N-methylenebenzenamine can still be produced and then react with C2H to form substituted azanaphthalenes.

Quantitative Structure–Retention Relationship Study on Nitrogen-Containing Polycyclic Aromatic Compounds by Using Molecular Electronegativity Interaction Vector

The molecular structures of 117 nitrogen-containing polycyclic aromatic compounds (N-PACs) were described by a method of molecular structural characterization (MSC) called molecular electronegativity interaction vector (MEIV). The samples were divided into a training set and a test set. For the training set, a quantitative structure–retention relationship (QSRR) model was built up by multiple linear regression (MLR) and the model was evaluated by performing the cross validation with the leave-one-out (LOO) procedure. The correlation coefficient (R) and the cross-verification correlation coefficient (RCV) of the model were 0.992 and 0.991, respectively. Moreover, the model was evaluated by the test set and satisfactory results with a correlation coefficient (Rtest) of 0.993 were obtained. The results suggested good stability and predictability of the model.

PH dependence of binding benzo[h]quinoline and humic acid and effects on fluorescence quenching

The binding constant (K(DOC)) between humic acid and the nitrogen-containing polycyclic aromatic compound (N-PAC), benzo[h]quinoline, was measured at varying pH levels using fluorescence quenching (FQ). Because fluorescence characteristics of benzo[h]quinoline change with pH, determination required two optimum sets of excitation and emission wavelength pairs. A simple mixing model was used to eliminate the inherent fluorescence interference between benzo[h]quinoline and its protonated form, benzo[h]quinolinium, when estimating binding constants. Hydrophobic interaction is likely to control the binding between humic acid and benzo[h]quinoline and benzo[h]quinolinium, in lower and higher pH ranges (pH <3, pH >6). In contrast, cation exchange seems to control the binding affinity of benzo[h]quinolinium in the middle range of pH. The estimates of K(DOC) were up to 70% smaller after elimination of interference. This indicates that the contribution of the minor form influences estimates of the K(DOC)-pH trend for benzo[h]quinoline, and potentially explains the large discrepancy reported in the literature between results based on using FQ and those based on equilibrium dialysis methods. Previous FQ measurements overestimate K(DOC) at some pH values and lead to an underestimation of bioavailability in an aquatic environment. The application of our models appears to be necessary when using FQ for determining the K(DOC)-pH trend for organic compounds with acid-base pair analogs.

Mechanisms of formation of nitrogen-containing polycyclic aromatic compounds in low-temperature environments of planetary atmospheres: A theoretical study

Polycyclic aromatic hydrocarbons (PAHs) are believed to be responsible for the formation of organic haze layers in Titan's atmosphere, but the nature of PAHs on Titan and their formation and growth mechanisms are not well understood. Considering the high abundance of nitrogen in Titan's atmosphere, it is likely that the haze layers hold not only pure hydrocarbon PAHs but also their nitrogenated analogs, N-containing polycyclic aromatic compounds (N-PACs) with 'hetero' N atoms in aromatic rings. Laboratory studies of Titan's tholins also support the hypothesis that, together with pure PAHs and their cations, N-PACs may be the fundamental building blocks of microphysical tholin particles. In the present work, we carried out ab initio quantum chemical calculations of potential energy surfaces for various reaction mechanisms of incorporation of nitrogen atoms into aromatic rings of polycyclic aromatic compounds, which may lead to the formation of N-PACs under the low-temperature and low-pressure conditions of Titan's atmosphere. This includes mechanisms analogous to the Ethynyl Addition Mechanism (EAM) recently proposed by us for the growth of PAH by sequential C2H additions to benzene. We consider consecutive C2H and CN additions to C6H6, C6H6 + CN --> C6H5CN + H, C6HCN + C2H --> C6H4(CN)(C2H) + H, C6H5CN + CN --> C6H4(CN)2 + H, C6H4(CN)(C2H) + C2H --> 2-aza-4-ethynyl-1-naphthyl/2-aza-1-ethynyl-4-naphthyl, C6H4(CN)2 + C2H --> C6H4(CN)(NCCCH), and C6H4(CN)(NCCCH) + C2H --> 1,4-diethynylphthalazine. Although these reactions are found to be barrierless and exothermic and therefore feasible at low temperatures, the steps leading to the aza-ethynyl-naphthyl radicals, C6H4(CN)(NCCCH), and 1,4-diethynylphthalazine can give N-PACs as final products only upon their collisional or radiational stabilization. Alternatively, an N-PAC can be synthesized via the reaction of 2-methyleneaminobenzonitrile with C2H, producing 4-ethynyl-quinoline + H without an entrance barrier via a three-step sequence including C2H addition to C of CN, ring closure, and H elimination. 2-Methyleneaminobenzonitrile itself can be formed in the reaction of methyleneaminobenzene with cyano radical, C6H5(NCH2) + CN --> C6H5(NCH2)(CN) --> C6H4(NCH2)(CN) + H, which also does not have any entrance barrier. Methyleneaminobenzene can be produced through recombination of phenyl and methylene-amidogen radicals followed by collisional stabilization of the product, via the barrierless C6Hs + CH3N --> C6Hs(NCH3) --> C6H5(NCH2) + H reaction, or in the reaction of phenyl with methyleneimine, C6H5 + CH2NH --> C6Hs(NHCH3) --> C6H5(NCH2) + H. The latter would be slow at low-temperature conditions owing to the barriers of 4.5 and 2.8 kcal mol(-1) relative to the initial reactants, but feasible if the reactants possess sufficient internal energy to overcome these barriers. We anticipate that the presented mechanisms are viable to form N-PACs in hydrocarbon and nitrogen rich, low temperature atmospheres of planets and their moons such as Titan.

QSPR prediction of GC retention indices for nitrogen-containing polycyclic aromatic compounds from heuristically computed molecular descriptors

Gas chromatographic retention indices of nitrogen-containing polycyclic aromatic compounds (N-PACs) have been predicted by quantitative structure–property relationship (QSPR) analysis based on heuristic method (HM) implemented in CODESSA. In order to indicate the influence of different molecular descriptors on retention indices and well understand the important structural factors affecting the experimental values, three multivariable linear models derived from three groups of different molecular descriptors were built. Moreover, each molecular descriptor in these models was discussed to well understand the relationship between molecular structures and their retention indices. The proposed models gave the following results: the square of correlation coefficient, R 2, for the models with one, two and three molecular descriptors was 0.9571, 0.9776 and 0.9846, respectively.

Does glucose enhance the formation of nitrogen containing polycyclic aromatic compounds and polycyclic aromatic hydrocarbons in the pyrolysis of proline?

The gas-phase pyrolysis of proline, glucose, 1-[(2′-carboxy)pyrrolidinyl]-1-deoxy- d-fructose (the proline Amadori compound), and a 1:1 mixture by weight of proline and glucose was investigated at high temperatures (600–840 °C) and short residence time (i.e. 1.0 s) in an inert atmosphere to determine if glucose or Maillard reaction products enhance the formation of nitrogen containing polycyclic aromatic compounds (N-PACs) and polycyclic aromatic hydrocarbons (PAHs) in the pyrolysis of proline. To study the gas-phase formation of N-PACs and PAHs, the substrates were sublimed into the pyrolysis furnace at 460 °C. Thermogravimetric analysis showed that glucose, the proline/glucose mixture, and the proline Amadori compound undergo solid-state decomposition reactions before subliming. Thus, the substrates were pyrolyzed in two stages: at 460 °C during the sublimation and at 600–840 °C. At 800 °C with a residence time of 1.0 s, proline produced low yields of N-PACs, such as quinoline, isoquinoline, indole, acridine, and carbazole, and PAHs, such as phenanthrene, pyrene, benz[ a]anthracene, benzofluoranthene isomers, and benzo[ a]pyrene. Increasing the temperature and residence time increased the yield of these products. Under similar pyrolysis conditions, the proline Amadori compound produced 2–8 fold more N-PACs and PAHs than proline. A 1:1 mixture of proline and glucose produced a similar slate of pyrolysis products as the proline Amadori compound, but it is unclear whether the proline Amadori compound was an intermediate in the reaction. In general, the proline Amadori compound produced a higher yield of N-PACs and PAHs than the proline/glucose mixture, but glucose clearly enhances the low temperature gas-phase formation of N-PACs and PAHs from the pyrolysis of proline. For example, a 1:1 mixture of proline and glucose was found to produce low yields of quinoline, isoquinoline, and indole at 600 °C while proline alone does not produce these compounds until 800 °C. Thus, glucose provides a low temperature pathway for the decomposition of proline, which leads to formation of N-PACs and PAHs.

Laboratory experiments of Titan tholin formed in cold plasma at various pressures: implications for nitrogen-containing polycyclic aromatic compounds in Titan haze

Titan, the largest satellite of Saturn, has a thick nitrogen/methane atmosphere with a thick global organic haze. A laboratory analogue of Titan's haze, called tholin, was formed in an inductively coupled plasma from nitrogen/methane=90/10 gas mixture at various pressures ranging from 13 to 2300 Pa. Chemical and optical properties of the resulting tholin depend on the deposition pressure in cold plasma. Structural analyses by IR and UV/VIS spectroscopy, microprobe laser desorption/ionization mass spectrometry, and Raman spectroscopy suggest that larger amounts of aromatic ring structures with larger cluster size are formed at lower pressures (13 and 26 Pa) than at higher pressures (160 and 2300 Pa). Nitrogen is more likely to incorporate into carbon networks in tholins formed at lower pressures, while nitrogen is bonded as terminal groups at higher pressures. Elemental analysis reveals that the carbon/nitrogen ratio in tholins increases from 1.5–2 at lower pressures to 3 at 2300 Pa. The increase in the aromatic compounds and the decrease in C/N ratio in tholin formed at low pressures indicate the presence of the nitrogen-containing polycyclic aromatic compounds in tholin formed at low pressures. Tholin formed at high pressure (2300 Pa) consists of a polymer-like branched chain structure terminated with CH 3, NH 2, and CN with few aromatic compounds. Reddish-brown tholin films formed at low pressures (13–26 Pa) shows stronger absorptions (almost 10 times larger k-value) in the UV/VIS range than the yellowish tholin films formed at high pressures (160 and 2300 Pa). The tholins formed at low pressures may be better representations of Titan's haze than those formed at high pressures, because the optical properties of tholin formed at low pressures agree well with that of Khare et al. (1984a, Icarus 60, 127–137), which have been shown to account for Titan's observed geometric albedo. Thus, the nitrogen-containing polycyclic aromatic compounds we find in tholin formed at low pressure may be present in Titan's haze. These aromatic compounds may have a significant influence on the thermal structure and complex organic chemistry in Titan's atmosphere, because they are efficient absorbers of UV radiation and efficient charge exchange intermediaries. Our results also indicate that the haze layers at various altitudes might have different chemical and optical properties.

Formation of low molecular weight heterocycles and polycyclic aromatic compounds (PACs) in the pyrolysis of α-amino acids

The formation of pyrolysis products, including polycyclic aromatic compounds (PACs), of three α-amino acids acids (asparagine, proline, and tryptophan) was studied in inert atmosphere. The PACs included polycyclic aromatic hydrocarbons (PAHs) and nitrogen-containing polycyclic aromatic compounds (N-PACs). A number of low temperature pyrolysis products, primarily single ring heterocyclic compounds, were also identified. The reactor consisted of a quartz tube with two-zones, each heated and controlled separately. The amino acids were pyrolyzed at 300 °C to obtain a low temperature tar (LTT) and a low temperature char (LTC). The LTC was then pyrolyzed at 625 °C to obtain a high temperature tar (HTT) and a high temperature char (HTC). In some cases, the LTT and HTT were subjected to higher temperatures in the range 700–920 °C to force secondary reactions. The various tars were analyzed by gas chromatography/mass spectrometry (GC/MS). In the absence of secondary reactions, asparagine and proline did not form any PACs. Tryptophan formed harman and norharman. After the secondary pyrolysis, all three amino acids formed three- and four-ring N-PACs and the yields generally increased with temperature. Asparagine and proline also formed PAHs in low yields, especially above 820 °C. The results suggest different decomposition pathways such as decarboxylation and deammoniation for the three amino acids leading to the formation of various PACs under different pyrolysis conditions.