A comparative study of the formation of aromatics in rich methane flames doped by unsaturated compounds

For a better modeling of the importance of the different channels leading to the first aromatic ring, we have compared the structures of laminar rich premixed methane flames doped with several unsaturated hydrocarbons: allene and propyne, because they are precursors of propargyl radicals which are well known as having an important role in forming benzene, 1,3-butadiene to put in evidence a possible production of benzene due to reactions of C₄ compounds, and, finally, cyclopentene which is a source of cyclopentadienyl methylene radicals which in turn are expected to easily isomerizes to give benzene. These flames have been stabilized on a burner at a pressure of 6.7 kPa (50 Torr) using argon as dilutant, for equivalence ratios (?) from 1.55 to 1.79. A unique mechanism, including the formation and decomposition of benzene and toluene, has been used to model the oxidation of allene, propyne, 1,3-butadiene and cyclopentene. The main reaction pathways of aromatics formation have been derived from reaction rate and sensitivity analyses and have been compared for the three types of additives. These combined analyses and comparisons can only been performed when a unique mechanism is available for all the studied additives.     

Phenomenological characteristics of cool-flame oxidation of cyclohexane

The phenomenological characteristics of cool flames of cyclohexane are studied. It is shown that cyclohexane flames arise at lower temperatures (beginning at T ≈ 230°C) and lower pressures (lower than 10 torr at T = 290°C) than alkane and alkene flames. The temperature dependence of the lower pressure limit of the cool flame is determined. It is established that, depending on the conditions (p, T, and C₆H₁₂/O₂ ratio), cool-flame ignition is characterized by a rise and drop of the temperature ΔT or by several peaks imposed on each other. The dependence of the cool-flame structure (for ΔT) on the oxygen content in the reaction mixture is studied. It is shown that the cool flame resulting from cyclohexane oxidation is accompanied by ring opening and formation of products with a smaller number of carbon atoms. It is established that cool flames of cyclohexane are characterized by a negative temperature coefficient. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 26–28, January–February, 2008.     

Method of estimating absolute concentrations of C<Subscript>2</Subscript>H<Subscript>5</Subscript> and H radicals in hydrocarbon diffusion flames

A method for estimating absolute concentrations of C₂H₅ and H radicals in hydrocarbon diffusion flames is proposed and substantiated. Concentration profiles of C₂H₅ and H on the flame axis are obtained. In the method proposed, the concentration of C₂H₅ is determined from the equality of two quantities — the rate of loss of n-butane by diffusion and the rate of its formation by recombination of two C₂H₅ radicals. The concentration of H radicals is determined from the relation between the ratio C₂H₅/H and experimental profiles of C₂H₄, C₂H₆, and O₂. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 6, pp. 13–20, November–December, 2007.     

Effect of ethanol on the chemistry of formation of precursors of polyaromatic hydrocarbons in a fuel-rich ethylene flame at atmospheric pressure

The effect of the addition of ethanol (EtOH) to the initial combustible mixture on the concentration of various compounds, in particular, those preceding the formation of polyaromatic hydrocarbons in a fuel-rich (equivalence ratio of fuel ? = 1.7) flat premixed ethylene/oxygen/argon flame at atmospheric pressure was studied experimentally and by numerical modeling using a detailed mechanism of chemical reactions. Concentrations of various stable and labile species, including reactants, major combustion products, and intermediates in C₂H₄/O₂/Ar and C₂H₄/EtOH/O₂/Ar flames were measured along the height above the burner using molecular beam mass spectrometry. Experimental mole fraction profiles were compared with those calculated using the previously proposed mechanisms of chemical reactions. This mechanism was analyzed to determine the cause of the ethanol effect on the flame concentration of propargyl, the main precursor of polyaromatic hydrocarbons.     

Comparison of the structures of methane-air and propane-air partially premixed flames

A comparison of flame structures between methane-air and propane-air laminar partially premixed flames has been made through the centerline concentration distributions of selected species measured using gas chromatography. The concentrations of fuel, major species like O₂, CO and CO₂ and those of the intermediate hydrocarbons like C₂H₆, C₂H₄, C₂H₂ and CH₄ (for the propane flame only) have been compared. Distinct double flame structures are observed for the experimental conditions under study. With approximately the same equivalence ratio and jet velocity for the primary mixture, the height of the inner flame for propane is less than that of methane. The peak concentration of C₂H₆ in the propane flame is found to be only a little higher than that in the methane flame, while the peak concentrations of C₂H₄ and C₂H₂ are much greater in the propane flame than in the methane flame. In a methane partially premixed flame, the hydrocarbon concentrations drop from their peak values very rapidly at the inner flame tip, but in the propane flames it is more gradual. In a methane partially premixed flame, CO is formed at the inner flame and burns at the outer flame to CO₂. Similar distributions of CO and CO₂ are found in the propane flame also. However, the peak CO concentration in the propane flame is found to be higher than in methane flame. A radial measurement of species distribution for a particular case in the propane partially premixed flame is also done to ascertain the species distributions across the flame.     

Development of processes for C<Subscript>4</Subscript> hydrocarbons separation and 1,3-butadiene purification

Physicochemical substantiation and improved more effective technologies for C₄-hydrocarbons separation and 1,3-butadiene purification with the help of polar extragents are given, such as compressorless extractive rectification with the use of an intermediate desorbent, extractive rectification of the butane-n-butene fraction with preliminary isomerization of 1-butene into 2-butenes, the combined removal of -acetylenes C₄ and methylallene by rectification with a small introduction of the extragent, and the hydration of butenyne in the C₄ fraction and distillation of propyne from it.     

Effect of the addition of triphenylphosphine oxide,hexabromocyclododecane, and ethyl bromide on a CH<Subscript>4</Subscript>/O<Subscript>2</Subscript>/N<Subscript>2</Subscript> flame at atmospheric pressure

The chemical and thermal structure of a Mache-Hebra burner stabilized premixed rich CH₄/O₂/N₂ flame with additives of vapors of triphenylphosphine oxide [(C₆H₅)₃PO], hexabromocyclododecane (C₁₂H₁₈Br₆), and ethyl bromide (C₂H₅Br) was studied experimentally using molecular beam mass spectrometry (MBMS) and a microthermocouple method. The concentration profiles of stable and active species, including atoms and free radicals, and flame temperature pro.les were determined at a pressure of 1 atm. A comparison of the experimental and modeling results on the flame structure shows that MBMS is a suitable method for studying the structure of flames stabilized on a Mache-Hebra burner under near-adiabatic conditions. The relative flame inhibition effectiveness of the added compounds is estimated from changes in the peak concentrations of H and OH radicals in the flame and from changes in the flame propagation velocity. The results of the investigation suggest that place of action of the examined flame retardants is the gas phase. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 12–20, September–October, 2007.     

Reforming of C6 Hydrocarbons Over Model Pt Nanoparticle Catalysts

Size-controlled model Pt nanoparticle catalysts, synthesized by colloidal chemistry, were used to study the hydrogenative reforming of three C₆ hydrocarbons in mixtures with 5:1 excess of H₂: methylcyclopentane, n-hexane and 2-methylpentane. We found a strong particle size dependence on the distribution of different reaction products for the hydrogenolysis of methylcyclopentane. The reactions of 50?Torr methylcyclopentane in 250?Torr H₂ at 320 °C, using 1.5 and 3.0 nm Pt nanoparticles produced predominantly C₆ isomers, especially 2-methylpentane, whereas 5.2 and 11.3 nm Pt nanoparticles were more selective for the formation of benzene. For the hydrogenolysis of n-hexane and 2-methylpentane, strong particle size effects on the turnover rates were observed. Hexane and 2-methylpentane reacted up to an order of magnitude slower over 3.0 nm Pt than over the other particle sizes. At 360 °C the isomerization reactions were more selective than the other reaction pathways over 3.0?nm Pt, which also yielded relatively less benzene.     

Preparation of ZnFe2O4 Catalysts by a Co-precipitation Method Using Aqueous Buffer Solution and Their Catalytic Activity for Oxidative Dehydrogenation of n-Butene to 1,3-Butadiene

Zinc ferrite (ZnFe₂O₄) catalysts were prepared by a co-precipitation method using aqueous buffer solution with different pH (pH = 6−12), and applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene. Conversion of n-butene and yield for 1,3-butadiene showed volcano-shaped curves with respect to pH value employed during the co-precipitation step. NH₃-TPD experiments were conducted to correlate the acid property with the catalytic performance of zinc ferrite catalysts. It was revealed that the catalytic performance of zinc ferrite catalysts in the oxidative dehydrogenation of n-butene was closely related to the surface acidity of the catalysts. Conversion of n-butene and yield for 1,3-butadiene increased with increasing surface acidity of the catalysts. Among the catalysts tested, the zinc ferrite catalyst prepared at pH = 8 showed the best catalytic performance in the oxidative dehydrogenation of n-butene, which was attributed to its largest surface acidity.     

Selectivity controlling in the hydrogenation of 1,3-butadiene on Tl-modified Pd catalyst

The selectivity and reactivity in the hydrogenation of 1,3-butadiene catalyzed by Tl-modified 5 wt% Pd/Al₂O₃ catalysts vary with amounts of Tl loading and with the reduction temperatures, that is, the main product was 1-butene and trans-2-butene for values of Tl loading of 0.5 and 2 in Tl/Pd atomic ratio, respectively, when the catalysts were reduced at 673 K. 1,3-butadiene was hydrogenated selectively towards 1-butene and trans-2-butene when the Tl modified 5 wt% Pd/Al₂O₃ catalyst of Tl/Pd = 2 was reduced at 300 and 373 K or above, respectively. On the catalyst with Tl/Pd = 2 reduced at 373 K or above, the butenes formed are not hydrogenated to butane, even after a long reaction time. These results suggest the formation of Pd-Tl alloy or intermetallic compounds during the reduction procedure which is responsible for the selectivity controlling in the reaction. TPR and XRD results were in consistence with the reaction data.     

Experimentelle und theoretische Untersuchungen an Kohlenwasserstoffen der Formel C5H6: Pyrolyse von Cyclopenta-1,3-dien

Experimental and Theoretical Investigations on Hydrocarbons of the Formula C₅H₆: Pyrolysis of 1,3-Cyclopentadiene The details of high temperature chemistry of the conversion of 1,3-cyclopentadiene (CPD) are unknown. Therefore the pyrolysis of CPD was studied under various reaction conditions in labscale dimension. The attempt to interpret the composition of the reaction products only on the basis of convenient axiomes does not seem satisfactory. For that reason heats of formation and relative stabilities for 25 closed and open shell hydrocarbons of the formula C₅H₆ were calculated using the MINDO/3 procedure. These enthalpy values were used to estimate heats of reactions for selected start steps of the pyrolysis of CPD. The molecular-assisted hydrogen transfer under formation of cyclopentadienyl and cyclopentenyl radicals ought to be one of the most important reaction steps.     

Experimental study of the structure of laminar premixed flames of ethanol/methane/oxygen/argon

The structures of three laminar premixed stoichiometric flames at low pressure (6.7 kPa): a pure methane flame, a pure ethanol flame, and a methane flame doped by 30% of ethanol, have been investigated and compared. The results consist of mole fraction profiles of CH₄, C₂H₅OH, O₂, Ar, CO, CO₂, H₂O, H₂, C₂H₆, C₂H₄, C₂H₂, C₃H₈, C₃H₆, CH₃-C CH (propyne), CH₂ C CH₂ (allene), CH₂O, and CH₃HCO, measured as a function of the height above the burner by probe sampling followed by on-line gas chromatography analyses. Flame temperature profiles have been also obtained by using a PtRh thermocouple. The similarities and differences between the three flames have been analyzed. The results show that, in these three flames, the mole fraction of the intermediates with two carbon atoms is much larger than that of the species with three carbon atoms. In general, the mole fraction of all intermediate species in the pure ethanol flame is the largest, followed by the doped flame, and finally the pure methane flame.     

The effects of oxygen on the yields of the thermal decomposition products of catechol under pyrolysis and fuel-rich oxidation conditions

In order to investigate the effects of oxygen on the distribution of thermal decomposition products from complex solid fuels, pyrolysis and fuel-rich oxidation experiments have been performed in an isothermal laminar-flow reactor, using the model fuel catechol (ortho-dihydroxybenzene), a phenol-type compound representative of structural entities in coal, wood, and biomass. The gas-phase catechol pyrolysis experiments are conducted at a residence time of 0.3 s, over a temperature range of 500-1000 °C, and at oxygen ratios ranging from 0 (pure pyrolysis) to 0.92 (near stoichiometric oxidation). The pyrolysis products are analyzed by nondispersive infrared analysis and by gas chromatography with flame-ionization and mass spectrometric detection. In addition to an abundance of polycyclic aromatic hydrocarbons, catechol pyrolysis and fuel-rich oxidation produce a range of C₁-C₅ light hydrocarbons as well as single-ring aromatics. Quantification of the products reveals that the major products are CO, acetylene, 1,3-butadiene, phenol, benzene, vinylacetylene, ethylene, methane, cyclopentadiene, styrene, and phenylacetylene; minor products are ethane, propyne, propadiene, propylene and toluene. Under oxidative conditions, CO₂ is also produced. At temperatures <850 °C, increases in oxygen concentration bring about increases in catechol conversion and yields of C₁-C₅ and single-ring aromatic products—in accordance with increased rates of pyrolytic reactions, due to the enhanced free-radical pool. At temperatures >850 °C, catechol conversion is complete, and increases in oxygen bring about drastic decreases in the yields of virtually all hydrocarbon products, as oxidative destruction reactions dominate. Reactions responsible for the formation of the C₁-C₅ and single-ring aromatic products from catechol, under pyrolytic and oxidative conditions, are discussed.     

Experimental study of the structure of a rich premixed 1,3-butadiene/CH4/O2/Ar flame

The structure of a laminar rich premixed 1,3-butadiene/CH₄/O₂/Ar flame has been investigated, 1,3-butadiene, methane, oxygen, and argon mole fractions being 0.033, 0.2073, 0.3315, and 0.4280, respectively, for an equivalence ratio of 1.80. The flame has been stabilized on a burner at a pressure of 6.7 kPa. The concentration profiles of stable species have been measured by gas chromatography after sampling with a quartz probe. The quantified species include carbon monoxide and dioxide, methane, oxygen, hydrogen, ethane, ethylene, acetylene, propyne, allene, propene, cyclopropane, 1,3-butadiene, butenes, 1-butyne, vinylacetylene, diacetylene, C₅ compounds, benzene, and toluene. The temperature distribution in the flame has also been measured. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 89–95, November–December, 2006.     

Effects of oxygenate concentration on species mole fractions in premixed n-heptane flames

Atmospheric pressure, laminar, premixed, fuel-rich flames of n-heptane/oxygen/argon and n-heptane/oxygenate/oxygen/argon were studied at an equivalence ratio of 1.97 to determine the effects of oxygenate concentration on species mole fractions. The oxygen weight percents in n-heptane/oxygenate mixtures were 2.7 and 3.4. Three different fuel oxygenates (i.e. MTBE, methanol, and ethanol) were tested. A heated quartz micro-probe coupled to an on-line gas chromatography/mass spectrometry has been used to establish the identities and absolute concentrations of stable major, minor, and trace species by the direct analysis of samples, withdrawn from the flames. The oxygenate addition has increased the maximum flame temperatures and reduced the mole fractions of CO, low-molecular-weight hydrocarbons, aromatics, and polycyclic aromatic hydrocarbons. The reduction in mole fractions of aromatic and polycyclic aromatic hydrocarbon species by an increase in oxygenate concentration was more significant.     

Kinetic study of the selective catalytic hydrogenation of 1,3-butadiene in a mixture of n-butenes

The selective hydrogenation in liquid phase of 1,3-butadiene in the presence of n-butenes, over a commercial Pd/Al₂O₃ catalyst of the egg-shell type was studied.Experiments were carried out in a batch reactor at 44 °C. Initial concentration were in the range of 1%–2% of 1,3-butadiene, up to 10% of 1-butene, trans-2-butene and cis-2-butene. Partial hydrogen pressures ($p_{{\text{H}}_2}$) were between 1 and 8 atm.Two kinetic models were proposed to fit the experimental data. Both models give an adequate representation of the experimental data.     

Efficient separation of 1,3-butadiene from C4 hydrocarbons by flexible metal–organic framework with gate-opening effect

Efficient and economical separation of 1,3-butadiene (C₄H₆) from C₄ hydrocarbons is imperative yet challenging in industrial separation processes. Herein, a guest-induced flexible Mn-bpdc metal–organic framework (MOF) has been employed to separate C₄H₆ from C₄ hydrocarbons, including n-butene (n-C₄H₈)_, iso-butene (iso-C₄H₈), n-butane (n-C₄H₁₀), and iso-butane (iso-C₄H₁₀). Significantly, C₄H₆ can instantaneously induce gate-opening of Mn-bpdc MOF at 0.13 bar and 298 K, thus significant amounts of C₄H₆ can be adsorbed, while other C₄ hydrocarbons cannot induce the gate-opening even at 1 bar. The uptake selectivities of Mn-bpdc MOF for C₄H₆/n-C₄H₈ and C₄H₆/iso-C₄H₈ are up to 40.0 and 45.0 at 298 K and 1 bar, respectively, both surpassing all the reported adsorbents. In addition, breakthrough experiments verify that C₄H₆/n-C₄H₈, C₄H₆/iso-C₄H₈, C₄H₆/n-C₄H₁₀, C₄H₆/iso-C₄H₁₀, C₄H₆/n-C₄H₈/n-C₄H₁₀, and C₄H₆/iso-C₄H₈/iso-C₄H₁₀ mixtures can be efficiently separated. More importantly, Mn-bpdc possesses excellent water stability and outstanding regeneration ability for C₄H₆ separation, making it a new benchmark for C₄H₆ purification.     

Analysis of flame structure by molecular-beam mass spectrometry using electron-impact and synchrotron-photon ionization

Molecular-beam mass spectrometry (MBMS) has proven to be a powerful tool for the general analysis of flame structure, providing concentrations of radical and stable species for low-pressure flat flames since the work of Homann and Wagner in the 1960’s. In this paper, we will describe complementary measurements using electron-impact ionization with a high-mass-resolution quadrupole mass spectrometer and vacuum-ultraviolet photoionization in a time-of-flight mass spectrometer. Isomers are resolved that have not been separately detectable before in MBMS studies of flames, including C₃H₂, C₃H₄, C₄H₃, C₄H₄, C₄H₅, C₆H₆, and C₂H₄O. The qualitative and quantitative results of MBMS have led to advances in modeling and applying flame chemistry. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 58–63, November–December, 2006.     

Waste water combustion in a confined swirl flame

Water with dissolved N-compounds ammonia, aminoethanol and ε-caprolactam (NH₃, C₂H₇NO and C₆H₁₁NO) was sprayed into a confined swirl flame operating under various conditions. The model waste water and the atomizing fluid influence the visible flame structure. Droplet evaporatio takes place in the recirculation zone of the flow. Measurements of the NO concentrations admit an evaluation of the conversion efficiency of the water dissolved N compounds under the present conditions. It can be shown that Fenimore's universal curve of fuel-N conversion in premixed ideal flames is applicable to describe and predict the NO formation in this very complex technical combustion systems.     

Photooxidation Products of Ethanol During Photoelectrochemical Operation Using a Nanocrystalline Titania Anode and a Two Compartment Chemically Biased Cell

ZnMe^IIIFeO₄ catalysts with different trivalent metal (Me^III = Fe, Al, Cr, Mn, and Co) were prepared by a co-precipitation method, and were applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene. Successful formation of ZnMe^IIIFeO₄ catalysts was confirmed by XRD and ICP-AES analyses. Catalytic performance of ZnMe^IIIFeO₄ catalysts in the oxidative dehydrogenation of n-butene strongly depended on the identity of trivalent metal (Me^III). Acid properties of ZnMe^IIIFeO₄ catalysts were measured by NH₃-TPD experiments, with an aim of correlating the catalytic performance with the surface acid property of the catalysts. It was revealed that yield for 1,3-butadiene increased with increasing surface weak-acid density of ZnMe^IIIFeO₄ catalyst. Among the catalysts tested, ZnFeFeO₄ catalyst with the largest surface weak-acid density showed the best catalytic performance in the oxidative dehydrogenation of n-butene.