Polyaromatic Species Research Articles

Thermal degradation of polyurethane and polyurethane/expandable graphite coatings

This study deals with the thermal degradation of polyurethane (PU) and polyurethane/expandable graphite (PU/EG) coatings. EG represents a new generation of intumescent additives which provides good fire retardancy to various materials and in particular to PU. The thermal analyses have shown that the thermo-oxidative degradation as well as the pyrolysis of PU are not affected by the presence of EG. On the other hand, they have enabled the determination of the temperatures which correspond to different steps of the development of the intumescence : formation, stabilisation and degradation of the intumescent shield. Spectroscopic analysis of the residues resulting from each of these steps has demonstrated that the structure of the char is similar for PU and PU/EG and consists in a carbonaceous polyaromatic species. However, the results have suggested that the char formed from PU/EG may contained trapped free radicals.

The Ideal Gas Thermodynamics of Diesel Fuel Ingredients. I. Naphthalene Derivatives and Their Radicals

The molecular fundamentals of 21 naphthalene derivatives were investigated, calculated and evaluated, and their ideal gas thermodynamic properties were calculated, for the sake of simulating the combustion properties of diesel fuel. Ten of these species are stable molecules and 11 are radicals. The molecular fundamentals are calculated using Gaussian 94 ab initio and MOPAC 6 semiempirical programs. The results can be used to estimate the MOPAC performance with polyaromatic species.

Pyrolysis and oxidation of unsaturated and species

Abstract In this work we analyse and study several experimental data on pyrolysis, partial oxidation and combusion of C 2 and C 3 acetylenics as well as allene and ketene. These data, already presented in the literature, refer to turbulent flow reactors, jet stirred reactors (JSRs) and shock tube experiments and span wide ranges of concentration, pressure and temperature. In order to stress the importance of condensation reactions of unsaturated species some data of ethylene and propylene pyrolysis are also discussed. C 2 and C 3 unsaturated species are always present as important intermediates in combustion processes and their reactions play a crucial role in explaining the formation of aromatic and polyaromatic species in sooting flames. A detailed kinetic scheme able to describe the partial oxidation and the combustion of hydrocarbon mixtures up to commercial gasolines and liquid fuels is extended and more carefully validated on these sets of experimental data. The key reactions are discussed and compared with literature kinetic data. Moreover, this kinetic analysis is very useful for the study of `mild combustion' in the intermediate temperature region where the flameless combustion seems to offer the great advantage of a reduced formation of toxic components such as nitric oxides, polyraromatics and soot.

An experimental and kinetic modeling study of propyne and allene oxidation

New experimental data for propyne and allene oxidation were obtained in a jet-stirred reactor (JSR) in the temperature range 800–1200 K at 1–10 atm for different fuel/oxygen equivalent ratios (0.2–2.0). Experimental data clearly indicate the different oxidation behavior of the two isomers and provide valuable information for extending the simulation capabilities of an already existing comprehensive kinetic scheine. Critical reactions are presented and discussed together with extended comparisons of model predictions with experimental data obtained in the JSR conditions, in the Princeton turbulen flow reactor (PTFR). and in a shock tube at higher temperatures. Finally, the effects on reaction paths of allene addition to a fuel-rich acetylene premixed flame, are also discussed. Isomerization reactions proceeding via direct and H addition routes are significant in the oxidation. As a result of H-abstraction reactions, both propyne and allen form the resonantly stabilized propargyl radical. These species are important intermediates in all the combustion processes, and their successive reactions are relevant candidates in explaining the formation of aromatic and polyaromatic species, possible presursors of particulate and soot. The analysis of the mild combustion conditions of the JSR reactor also allows the reliability of the overall kinetic scheme to be extended to the so-called flameless conditions.

INS-, SIMS- and XPS-investigation of diesel engine exhaust particles

Diesel engine exhaust particles were collected under controlled conditions with and without passing a Pt/Al2O3 exhaust after-treatment oxidation catalyst and were studied before and after extraction treatment. The surface compositions were analyzed by means of X-ray photoelectron spectrometry (XPS). The properties of the hydrogen containing carbonaceous part of the soot were selectively characterised by combining results from inelastic neutron scattering (INS) as a bulk technique and secondary ion mass spectrometry (SIMS) as a surface and selvedge related technique. The effect of the catalyst in reducing the overall hydrogen content, the relative amounts of poorly crystalline species and of polyaromatic and aliphatic contributions at the surface was observed for the original as well as for the extracted diesel soot. Complementary, INS revealed a high integral graphiticity together with the proton dynamics of the finely divided, fluffy soot as well as the changes of the amounts of polyaromatic and aliphatic species in the surface regions. The matrix dependencies of the yields of negatively charged C/H secondary ions of the diesel soot and their time profiles were compared in SIMS experiments together with related properties of selected carbon blacks of different hydrogen content and with purified activated carbon. The C2−/CH− and the C2−/C2H− fragment ion ratios were used as a sensitive measure to roughly differentiate between hydrogenous species associated with poorly crystalline contributions and with structures of enhanced graphiticity, respectively. The acetylene-like fragment ion C2H2− is helpful to indicate the progression of the erosion process from the topmost atomic layers into the near-surface regions. The combination of these surface and bulk related techniques is shown to be useful in monitoring the efficiency of the oxidizing exhaust gas purification catalyst in the abatement of polyaromatic contributions from residual particulate emissions of diesel-engine passenger cars in addition to the overall lessening of soot emission. The combination deserves more common use in characterising carbons and carbonaceous emissions from other sources as well as contributing to the evaluation of the performance of new catalysts for exhaust purification processes.

On carbonaceous deposit formation in carbon monoxide hydrogenation on a natural iron catalyst

During carbon monoxide hydrogenation on a natural iron catalyst, two different behaviors were observed when the operating pressure was changed from 3.5 to 20.4 bar at 513 K and inlet H 2/CO of 2.0. A gradual increase in the catalyst activity to a maximum followed by a gradual loss in activity was observed at lower operating pressure. The surface composition (XPS) for the used catalyst is mainly α-Fe, α-SiO 2, and a great amount of surface carbon (mainly graphitic carbon). After hydrogen reduction treatment or Ar ion etching iron carbide species appears on the surface. Aliphatic and polyaromatic species were observed by fourier transform infrared. On increasing the operating pressure up to 20.4 bar, a dramatic increase in carbon monoxide conversion (four times higher) under steady state conditions was observed. The bulk composition of the catalyst as determined by Mössbauer spectroscopy is Fe 3O 4 (50.0%), α-Fe (31.1%) and χ-Fe 5C 2 (18.9%) in correspondence to a gas phase constituted mainly by alkanes. Thus, α-Fe and Fe 3O 4 appears to be correlated with the catalytic activity whereas the deactivation process appears to be caused mainly by the formation of graphitic carbon on the catalyst surface. The formation of such deposit is inhibited at higher operating pressure.

Propene adsorption and reaction on zeolites and pillared clays

Comparative IR and UV-Vis spectroscopic studies of propene adsorption and reaction on H-mordenite, dealuminated H-mordenite, dealuminated mazzite, montmorillonite and Al13-pillared montmorillonite have been carried out. On all systems propene is first transformed into polymeric species (CnH2n+1+). On HMOR (both as such and dealuminated), allylic carbocations are successively produced by loss of H2, the monoenic species (CnH2n−1+) being formed at room temperature and the dienic (CnH2n−3+) and trienic (CnH2n−5+) species at higher temperatures. These species are not observed on the other systems, although they are presumably formed as unstable intermediates. In fact, on all the zeolites studied here two cyclic penta-atomic and hexa-atomic allylic carbocations have been observed for the first time. On all systems, the final products of reaction are polyaromatic species which, on the basis of their reaction with NH3 still exhibit unsaturated carbocation behaviour. The activity of the various samples depends on their pore dimensions and on the nature of acidic sites involved: the larger the available pore space, the more branched is the polymer and the more difficult it is to observe allylic carbocations. Evidence is provided for a Bronsted-induced mechanism.

Ultraviolet Raman spectroscopy characterization of coke formation in zeolites

Ultraviolet (UV) Raman spectroscopy has been used to characterize coke formation in ZSM-5 and USY zeolites under propene at temperatures from 300K to 773K. The strong fluorescence background always present with normal Raman spectra is completely avoided in UV Raman spectra. Three groups of UV Raman bands near ≈ 1390, ≈ 1600 and ≈ 3000 cm −1 regions were detected for the two zeolites, and these bands varied significantly at different stages of coke formation. At room temperature, adsorbed propene was formed in the two zeolites and showed similar spectra. At elevated temperatures, the coke formation behavior in the two zeolites is quite different. For example, at 773K the coke species in ZSM-5 are mainly polyolefinic and aromatic species, but polyaromatic and pregraphite species are predominant in USY. The major portion of coke species formed in ZSM-5 can be removed even by He purging at 773K while the coke species in USY are very stable and can only be removed in O 2 flow at temperatures above 773K. The difference in coke formation in ZSM-5 and USY is likely due to the different pore structure and acidity of the two zeolites.

New Intumescent Formulations of Fire-retardant Polypropylene—Discussion of the Free Radical Mechanism of the Formation of Carbonaceous Protective Material During the Thermo-oxidative Treatment of the Additives

The study compares five new intumescent additive mixtures and a carbonizing additive system with the ammonium polyphosphate–pentaerythritol system and additive formulations previously developed in laboratory in terms of fire retardancy of polypropylene-based formulations. The mixture of diammonium pyrophosphate and polyols produced by agrochemical industry xylitol and d-sorbitol (carbonization agent) are FR additive mixtures of interest for polyolefins. Moreover, the FR performance of the mixture of ammonium polyphosphate and polyamide-6 is reported. It is proposed that boric acid salts have to be developed as precursors for carbonization catalytic species. A thermal analysis study shows that FR performances and amounts of carbonaceous materials resulting from the thermal degradation of the additive mixtures are not related. An additional compilation of previous spectroscopic studies by the laboratory confirms that the intumescent process results from the formation of polyaromatic species and that FR systems maintain acidic species in a relatively high temperature range. An ESR study discusses the presence of π radicals in the protective coating formed using the additive systems. It provides information on the size of the carbonaceous structures in the materials and the presence of crystalline phases in the coating. Finally, the participation of free radicals in the formation of chemical bonds between the materials produced from the additives and the products of the degradation of the polymer is discussed.

Mesophase and pyrolytic carbon formation in aircraft fuel lines

Exposure of jet fuel to high temperatures in aircraft fuel lines triggers pyrolysis reactions which eventually lead to deposition of carbonaceous solids on metal surfaces. This is a particularly important problem for advanced future aircraft which may expose fuel to very high temperatures. Different optical textures were observed in samples of deposits formed in different sections of aircraft fuel systems. Deposits from a burner fuel line consist only of pyrolytic carbon, indicating that gas phase reactions were responsible for solid formation. Afterburner line deposits, on the other hand, contain both pyrolytic carbon and carbonaceous mesophase structures, implicating also liquid phase carbonization reactions. The FTIR data shows that solids containing mesophase consist of polynuclear aromatic hydrocarbons with a low degree of condensation and alkyl substitution. In contrast, solids with pyrolytic carbon structure are composed of highly condensed, large polyaromatic species. It is clear that jet fuels go through extensive cracking, aromatization, and aromatic polymerization reactions before solid deposition takes place in the fuel lines.

Hydrodesulfurization Reactivities of Narrow-Cut Fractions in a Gas Oil

A gas oil (bp 227--377 C) was fractionated by the distillation at intervals of 20 C into five fractions. Each fraction was desulfurized over the NiMo and CoMo catalyst, respectively, in an autoclave reactor at 360 C under a total pressure of 2.9 MPa to examine its hydrodesulfurization (HDS) reactivities. The fractions exhibited the different reactivities which were expressed by the pseudo-second-order kinetics, giving the decreasing rate constants of 0.636, 0.270, 0.118, 0.039, and 0.023 [min{times}wt %]{sup {minus}1} with an increase of their boiling point ranges. Composition analyses of the fractions indicated that alkyldibenzothiophenes, polyaromatic hydrocarbons, and nitrogen compounds in the heavier fractions restricted their reactivities. Steric hindrance of methyl groups as well as the competitive retardation of latter compounds are suggested. The NiMo catalyst showed higher activity for HDS of the heavier fractions than the CoMo through its higher hydrogenation activity, while the latter was slightly superior to the former in HDS of the lighter fractions. The polyaromatic species existing dominantly in the heaviest fraction (> 340 C) are responsible for the fluorescent color in the oils. On the basis of such understanding, a deep HDS process which combined a single-stage HDS for lighter fractions and a multistage HDSmore » for the heavier fractions was proposed and examined. Such a process achieved the sulfur level below 0.05 wt % with the color removal and its superiority over the conventional HDS process was discussed.« less

Direct analysis of polycyclic aromatic hydrocarbons in cloud-water aerosol filtrates using laser desorption mass spectrometry

A novel technique for the rapid screening of polyaromatic hydrocarbons (PAHs) in atmospheric particulates is presented. Two-step laser desorption laser photoionisation time-of-flight mass spectrometry was used to assay for PAHs in cloud-water particulates collected near Peebles in southern Scotland. The particulates were examined in situ on their host filters removing the requirement for time-consuming sample extraction and separation. The mass spectra obtained from a single filter are shown. The principal components of these are assigned to PAH contaminants and phthalate contaminants. A distribution of PAHs was observed using 193 nm laser photoionisation. The mass spectrum obtained on using 248 nm laser photoionisation exhibits a different intensity distribution demonstrating an ionisation wavelength dependence. The utility of the technique for rapid screening of PAHs and other polyaromatic species is discussed.

Deactivation of toluene alkylation with methanol over magnesium-modified ZSM-5 Shape selectivity changes induced by coke formation

Abstract Coke deposition during toluene alkylation with methanol over a Mg-modified ZSM-5 catalyst leads to drastic changes in the zeolite shape selectivity with the time-on-stream. Besides a decrease in toluene and methanol conversions, a maximum in para selectivity is observed due to coke deposition. Nevertheless, the deactivated catalysts are less para-selective than the fresh ZSM-5 when they are compared at the same toluene conversion. Para selectivity of the primary product decreases due to coking below the value of 100% corresponding to the uncoked catalyst. At low times-on-stream, coke deposition takes place mainly by pore filling. At higher times-on-stream, the proportion of polyaromatic species in the coke deposits is increased, which leads to a partial blockage of the zeolite pore structure, lowering the length of the diffusional pathway. Therefore, the intracrystalline diffusional control on the internal xylene isomerization is less pronounced in the coked catalysts, which causes the lower para selectivity of the primary product observed in these samples.

Carbonization mechanisms resulting from intumescence association with the ammonium polyphosphate-pentaerythritol fire retardant system

In this work we study the thermal behavior of an ammonium polyphosphate-pentaerythritol mixture, fire-retardant additive for polyolefins and most particularly the carbonization process resulting from an intumescent phenomenon. The study has been carried out using Micro-Raman and 13C, 1H, 31P NMR of the solid state spectroscopies. It is shown that the structure consists in phoscarbonaceous and polyaromatic species. These latter form an anisotropic structure above 280°C. This structure grows when the temperature increases. Finally, a reactional scheme of the carbonization of the intumescent system is proposed.

Substrate-free gas-phase synthesis of diamond powder by CO 2 laser pyrolysis of C 2H 4

Diamond powders with grain diameters up to 0.3 μm were obtained by CO 2-laser-induced decomposition of C 2 H 4 and mixtures of C 2 H 4- H 2- SiH 4 at low pressures and temperatures in a gas flow reactor. Solid products were produced in a yellow- to orange-coloured flame (500– 550 °C) and collected in filters. The product contained polyaromatic species, high molecular polymers, graphite, amorphous carbon and spherical diamond particles. Several diamond particle populations, with mean diameters of 6.2–122 nm were observed by transmission electron microscopy. Both diamond modifications, cubic and hexagonal, were identified by electron diffraction. Hydrogen-free pyrolysis of pure ethylene produced diamond of high purity (no diffraction rings of graphite detectable).

Homogeneous nucleation of diamond powder by CO2 -laser-driven gas-phase reactions

Diamond powders with grain diameters up to 0.3 μm were obtained by CO2 -laser-induced decomposition of C2H4 at low pressures and temperatures. C2H4 or mixtures of C2H4, H2, and SiH4 were irradiated in a gas-flow reactor with the 10P14 line (10.532 μm) of a grating-tunable CO2 laser with 50-W cw output power. Solid products were produced in a yellow-to-orange colored flame (500–550 °C) and collected in filters. The product contained polyaromatic species, high molecular polymers, graphite, amorphous carbon, and spherical diamond particles. Several diamond particle populations, with mean diameters of 6–120 nm, were observed by transmission electron microscopy. Both diamond modifications, cubic and hexagonal, were identified by electron diffraction. Diamond formation is dependent on the residence time in the reaction flame, but relatively independent of the reactant gas compositions. Hydrogen-free pyrolysis of pure ethylene produced diamond of high purity (no diffraction rings of graphite detectable).

Synthesis of Submicron Diamond Powders by CO₂-Laser Pyrolysis of C₂H₄

Diamond powders with grain diameters up to 0.3 μm were obtained by CO₂-laser induced decomposition of C₂H₄ and mixtures of C₂H₄/H₂/SiH₄ at low pressures and temperatures in a gas-flow reactor. Solid products were produced in a yellow-to-orange coloured flame (500–550°) and collected in filters. The product contained polyaromatic species, high molecular polymers, graphite, amorphous carbon, and spherical diamond particles. Several diamond particle populations, with mean diameters of 6.2–122 nm, were observed by transmission electron microscopy. Both diamond modifications, cubic and hexagonal, were identified by electron diffraction.

Thermal treatment of Lloydminster heavy oil: Yield and product separation by adsorption chromatography

The effect of thermal treatment on Lloydminster heavy oil has been studied under pyrolytic, catalytic and non-catalytic hydropyrolysis and hydrogen donor solvent (tetralin) regimes using a batch autoclave. Product yields are reported for each type of reaction regime in terms of gases, maltenes, asphaltenes and toluene insoluble organic matter (coke). The maltene fraction was further separated by column chromatography into four chemical compound types namely: saturates, mono- and di-aromatics, polyaromatics and polars. The asphaltene fraction was also separated in four fractions by column chromatography by elution with solvents of increasing polarity. These fractions were characterized by FTIR and 13C NMR. The evolution of these fractions was studied as a function of the treatment. Tetralin was found to be effective in eliminating coke and increasing the maltene content relative to other treatments. It has been observed that the decrease of polyaromatic and polar species in the maltenes results in a corresponding increase of saturates. The asphaltene yield also decreases with thermal treatment. In all treatments the asphaltene fractions rich in polar and highly functional groups decrease drastically.

The Suppression of Soot Particle Formation in Laminar and Turbulent Diffusion Flames

Abstract A range of aliphatic hydrocarbon fuels have been burned on a porous cylindrical burner which was housed in a duct. The approach flow velocity was adjusted so that the formation of particles was suppressed and the flame turned blue. This defined a critical velocity gradient for the stagnation point flame at which the soot inception chemistry of the flame could not cope with the rate of mixing of reactants imposed by the strain rate of the flow. Measurements of the fluorescence of polyaromatic species with an excimer pumped dye laser indicated that their formation rate in flames of the lightly sooting fuels such as ethane was very sensitive to the flow field. Fluorescence measurements in a heavily sooting fuel such as propene were not as sensitive to the velocity gradient of the flow. Measurements were made of the height to the onset of soot formation in vertical, turbulent diffusion flames of the same fuels. It was found that numerical calculations of the mean scalar dissipation rates at this location in a jet were in reasonably good agreement with the values which were derived from the laminar, counterflow flame experiments.

The lubricity of diesel fuels

The main chemical factors influencing the lubricity of diesel fuels have been investigated by carrying out wear tests on diesel fuels, diesel fuel fractions and model fuels. It has been found that, under the test conditions used, sulphur-containing components were pro-wear. Nitrogen-containing components tended to reduce wear but only at concentrations in excess of those normally found in diesel fuels. Polyaromatic species and, in particular, oxygen-containing components, were found to reduce wear significantly at concentrations normally found in fuels.