Thermal decomposition of furans with oxygenated substituents: A combined experimental and quantum chemical study

Furans are an important class of compounds that can be thermochemical or enzymatically produced from biomass. Despite of their importance little is known about the thermal decomposition of furans with oxygenated substituents. In this work, the influence of the -CH₃, -CH₂OH and -CHO functional groups on the molecular and radical decomposition chemistry is studied with a combined quantum chemical and experimental approach using 2-furfuryl alcohol and 5-methyl furfural as model components.The quantum chemistry calculations show that both reactants can decompose by a ring-opening isomerization reaction and through carbene intermediates. The latter are formed by the shift of a hydrogen atom or a -CHO functional group within the furan ring structure. The -CHO functional group on the furan ring structure accelerates the molecular ring-opening isomerization reaction, while it suppresses carbene formation channels compared to other functional groups.The weaker CH and CO bonds in 2-furfuryl alcohol and 5-methyl furfural compared to furan and furfural respectively result in a higher importance of radical chemistry that cannot be neglected. This is confirmed experimentally by analyzing the product spectrum with molecular beam synchrotron VUV photoionization mass spectrometry at a pressure of 0.04 bar and for temperatures between 923 K to 1223 K for 2-furfuryl alcohol and 973 K to 1273 K for 5-methyl furfural. For both reactants several radical intermediates are observed starting from 923 K for 2-furfuryl alcohol and from 973 K for 5-methyl furfural. Examples of measured radicals are those initial formed from the reactant by a CH homolytic bond scission and methyl, allyl, propargyl, 1,2-butadiene-4-yl, 2-furanyl-methyl, 2,5-dihydrofuran-2-yl and 1?hydroxyl-2-furanyl-methyl radicals.     

2-糠酸甲酯与羟基反应动力学的理论研究

糠酸甲酯是随着2,5-呋喃二甲酸二甲酯新合成方法的发展而发展起来的一种新型可再生生物燃料. 本文用CCSD(T)/CBS//M062X/cc-pVTZ方法研究了糠酸甲酯与羟基自由基之间的势能面,包括夺氢反应和加成反应. 确定了异构化和分解反应生成的初级自由基. 结果表明,支链甲基上的夺氢反应是主要的反应通道,呋喃环上的OH加成具有明显的压力依赖性. 本文提出的速率系数为糠酸甲酯燃烧机理的改进提供了重要的动力学数据,为进一步研究实际燃料的燃烧过程奠定了良好的基础.     

羟基自由基与乙烯反应的化学动力学模拟

使用电子结构理论计算和直接动力学模拟对羟基自由基与乙烯反应体系进行了理论研究．在高水平的CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ上获得了包括羟基加成和氢抽取在内的多种反应通道的准确势能面信息,并在此基础上对OH+C₂H₄发展了一套准确性较高的MSINDO半经验哈密顿参数．特定反应参数哈密顿(SRP-MSINDO)能准确的再现高水平从头算结果．在2～10 kcal/mol的碰撞能应用SRP-MSINDO对OH+C₂H₄反应进行了直接准经典轨线模拟,获得的反应截面表明羟基加成反应为主导地位．另外,激发函数的模拟结果表明羟基加成是一个无势垒的捕获过程,而氢抽取则对应活化过程,这与两条反应通道的过渡态能量紧密相关．研究发现对OH+C₂H₄发展准确的半经验哈密顿需要考虑弥散矫正,这对准确描述分子间的长程吸引尤为重要     

A high temperature and atmospheric pressure experimental and detailed chemical kinetic modelling study of 2-methyl furan oxidation

An experimental ignition delay time study for the promising biofuel 2-methyl furan (2MF) was performed at equivalence ratios of 0.5, 1.0 and 2.0 for mixtures of 1% fuel in argon in the temperature range 1200–1800 K at atmospheric pressure. Laminar burning velocities were determined using the heat-flux method for mixtures of 2MF in air at equivalence ratios of 0.55–1.65, initial temperatures of 298–398 K and atmospheric pressure. A detailed chemical kinetic mechanism consisting of 2059 reactions and 391 species has been constructed to describe the oxidation of 2MF and is used to simulate experiment. Accurate reproduction of the experimental data has been obtained over all conditions with the developed mechanism. Rate of production and sensitivity analyses have been carried out to identify important consumption pathways of the fuel and key kinetic parameters under these conditions. The reactions of hydrogen atom with the fuel are highlighted as important under all experimental conditions studied, with abstraction by the hydrogen atom promoting reactivity and hydrogen atom addition to the furan ring inhibiting reactivity. This work, to the authors knowledge, is the first to combine theoretical and experimental work to describe the oxidation of any of the alkylated furans. The mechanism developed herein to describe 2MF combustion should also function as a sub-mechanism to describe the oxidation of 2,5-dimethyl furan whilst also providing key insights into the oxidation of this similar biofuel candidate.     

On the low-temperature chemistry of 1,3-butadiene

In this paper, species versus temperature profiles were measured during the oxidation of 1,3-butadiene in a jet-stirred reactor (JSR) at 1 atm, at different equivalence ratios (φ = 0.5, 1.0 and 2.0), in the temperature range 600 – 1020 K. Both synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC) methods were used to analyze the species. The experimental results show that a large proportion of the products are aldehydes (formaldehyde, acetaldehyde, acrolein, etc.) and ketenes (ketene, methyl-ketene), with acrolein being one of the major products. Moreover, furan, 1,3-cyclopentadiene and benzene are also present as intermediates in significant amounts. The reaction pathways leading to the formation of these species are discussed in detail. A new detailed mechanism, NUIGMech1.3, was developed to simulate these new data as well as other experimental data available in the literature. The validation results indicate that quantum calculations are also needed to explore the formation of some important species formed in the oxidation of 1,3-butadiene. Overall, the new 1,3-butadiene mechanism agrees well with various experimental data in the low- to high-temperature regimes and at different pressures. Flux and sensitivity analyses show that 1,3-butadiene shares some common reaction chemistry pathways with 1- and 2-butene via Ḣ atom and HȮ₂ radical addition to the C = C double bond in 1,3-butadiene, reactions which are important for both systems. The low temperature chemistry of 1,3-butadiene is mainly controlled by the reaction pathways of ȮH radical addition to the C = C double bond of the fuel molecule. The 1-buten-4-ol-3-yl radicals so formed subsequently add to O₂ and react via the Waddington mechanism, which is important in accurately simulating the oxidation and auto-ignition of 1,3-butadiene at engine relevant conditions.     

Experimental and modeling study of the high-temperature combustion chemistry of tetrahydrofurfuryl alcohol

Lignocellulosic tetrahydrofuranic (THF) biofuels have been identified as promising fuel candidates for spark-ignition (SI) engines. To support the potential use as transportation biofuels, fundamental studies of their combustion and emission behavior are highly important. In the present study, the high-temperature (HT) combustion chemistry of tetrahydrofurfuryl alcohol (THFA), a THF based biofuel, was investigated using a comprehensive experimental and numerical approach.Representative chemical species profiles in a stoichiometric premixed methane flame doped with ～20% (molar) THFA at 5.3 kPa were measured using online gas chromatography. The flame temperature was obtained by NO laser-induced fluorescence (LIF) thermometry. More than 40 chemical products were identified and quantified. Many of them such as ethylene, formaldehyde, acrolein, allyl alcohol, 2,3-dihydrofuran, 3,4-dihydropyran, 4-pentenal, and tetrahydrofuran-2-carbaldehyde are fuel-specific decomposition products formed in rather high concentrations. In the numerical part, as a complement to kinetic modeling, high-level theoretical calculations were performed to identify plausible reaction pathways that lead to the observed products. Furthermore, the rate coefficients of important reactions and the thermochemical properties of the related species were calculated. A detailed kinetic model for high-temperature combustion of THFA was developed, which reasonably predicts the experimental data. Subsequent rate analysis showed that THFA is mainly consumed by H-abstraction reactions yielding several fuel radicals that in turn undergo either β-scission reactions or intramolecular radical addition that effectively leads to ring enlargement. The importance of specific reaction channels generally correlates with bond dissociation energies. Along THFA reaction routes, the derived species with cis configuration were found to be thermodynamically more stable than their corresponding trans configuration, which differs from usual observations for hydrocarbons.     

The oxidation characteristics of furan derivatives and binary TPGME blends under engine relevant conditions

Furan and its derivatives have been receiving attention as next generation alternative fuels, related to advanced bio-oil production. However, the ignition quality of furans allows their use only as an additive to diesel fuel in CI engines, which potentially requires the continued use of a fossil-derived base fuel. This study first adopts tri-propylene glycol mono-methyl ether (TPGME) as a substitute for diesel fuel with addition of furan and furan derivatives, including 2-methylfuran, 2,5-dimethylfuran, and furfural, thereby removing fossil-derived fuels from the mixture. With this motivation, gas-phase ignition characteristics of furans were investigated in a modified CFR motored engine, displaying an absence of low temperature heat release (LTHR), while n-heptane as a reference fuel shows a strong two-stage ignition characteristic under the same condition. The structural impact of furans is represented as global oxidation reactivities that are as follows: furan?<?2-methylfuran?<?2,5-dimethylfuran?<?furfural?<?n-heptane. The ranking of individual furans is supported by bond dissociation energies of each fuel's functional group substituent on the furan-ring. Ignition characteristics of TPGME display a strong low-temperature oxidation reactivity; however, its reactivity rapidly diminishes with increasing amounts of furan, shutting down low-temperature oxidation paths. The structural impact of furan and methyl-substituted furans on reactivity is significantly muted when blended with TPGME, as observed in a motored CFR engine and a constant volume spray combustion chamber.     

Vibrational properties of levulinic acid and furan derivatives: Raman spectroscopy and theoretical calculations

In this work, the Raman spectra of furan, furfuryl alcohol (FA), furfural, hydroxymethylfurfural (HMF), and levulinic acid were obtained in the 500 to 4000 cm⁻¹ spectral region at room temperature. Vibrational wavenumbers were calculated for these compounds with the B3LYP method using the 6‐31 + G(2df,p) basis set. The experimentally determined CC and C C wavenumbers for furan and furan derivatives were in good agreement with the calculated wavenumbers without scaling factor, while the calculated CO and C H wavenumbers at ∼1660 and 3000 cm⁻¹, respectively, showed larger deviations from the measured ones. The Raman spectra for furan and furan derivatives showed intense CC bands, whereas the levulinic acid spectrum showed intense C H vibrations with broad doublet CO bands. We also found that an empirical method based on the chemical structure similarities is able to predict the HMF Raman spectrum from the combined furfural and FA spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

Quantum chemical studies of the OH-initiated oxidation reactions of propenols in the presence of O2

ABSTRACT

The atmospheric oxidation mechanisms of 1- and 2-propenol initiated by OH radical have been theoretically investigated at the CCSD(T)//BH&;HLYP/6-311?+?+G(d,p) level of theory. Conventional transition state theory was employed to predict the rate constants for the initial reaction channels. The calculations clearly indicate that OH-addition channels contribute maximum to the total reaction, both for 1- and 2-propenol, while H-abstraction channels can be neglected at the temperature range of 220–520?K. The calculated total rate constants at 298?K are 1.66?×?10^?11 and 7.69?×?10^?12 cm³?molecule^?1?s^?1 respectively for 1- and 2-propenol, which are in reasonable agreement with the experimental values of similar systems (vinyl ethers?+?OH reactions). The deduced Arrhenius expressions are k(OH?+?1-propenol)?=?1.43?×?10^?12 exp[(743.7?K)/T] and k(OH?+?2-propenol)?=?2.86?×?10^?12 exp[(310.5?K)/T] cm³?molecule^?1?s^?1. Under atmospheric condition, the OH-addition intermediates (CH₃C?HCH(OH)₂, CH₃CH(OH)C?H(OH), CH₃CH(OH)₂?CH₂, CH₃?C(OH)CH₂(OH)) are likely to react rapidly with O₂, the theoretically identified major products for 1-propenol are HCOOH, CH₃CHO and CH₃CH(OH)CHO, and the dominant products for 2-propenol are CH₃COOH, HCHO and CH₃COCH₂OH, both companied with the regeneration of OH and HO₂ radicals (crucial reactive radicals in the atmosphere).     

AC等离子体辅助CH4低温氧化的分子激发作用

本文采用实验测量和数值模拟结合的方法,对AC放电下He/CH4/O2混合气中激发态对甲烷裂解和低温氧化的动力学贡献进行研究。基于HP-Mech,增加反应物的放电机理以及激发态参与的化学反应及其驰豫反应,建立CH4低温氧化机理。采用化学反应动力学求解器CHEMKIN中的两段式Plasma-PSR模型模拟放电过程及化学反应过程。该动力学模型能较好地预测反应物的消耗和主要产物的生成,反应路径分析表明激发态物质CH4(v),O2(v),O2(a^1△g)等通过链式反应CH4(v)+OH→CH3+H2O,O2(v)+H→OH+O,O2(a^1△g)+H→OH+O促进活性自由基和产物的生成。     

Kinetics and mechanism of uncatalysed and ruthenium(III) catalysed oxidation of allyl alcohol by diperiodatoargentate(III) in aqueous alkaline medium

The oxidation of allyl alcohol by diperiodatoargentate(III) (DPA) is carried out both in the absence and presence of ruthenium(III) catalyst in alkaline medium at 298 K and a constant ionic strength of 1.1 mol dm^?3 was studied spectrophotometrically. The oxidation products in both the cases were acrolein and Ag(I), identified by spectral studies. The stoichiometry is same in both the cases, that is, [AA]/[DPA] = 1:1. The reaction shows first order in [DPA] and has less than unit order dependence each in both [AA] and [Alkali] and retarding effect of [IO] in both the catalysed and uncatalysed cases. The order in [Ru(III)] is unity. The active species of DPA is understood to be as monoperiodatoargentate(III) (MPA) in both the cases. The uncatalysed reaction in alkaline medium has been shown to proceed via a MPA–allyl alcohol complex, which decomposes in a rate determining step to give the products. In catalysed reaction, it has been shown to proceed via a Ru(III)‐allyl alcohol complex, which further reacts with one molecule of MPA in a rate determining step to give the products. The reaction constants involved in the different steps of the mechanisms were calculated for both reactions. The catalytic constant (K_c) was also calculated for catalysed reaction at different temperatures. The activation parameters with respect to slow step of the mechanisms were computed and discussed for both the cases. The thermodynamic quantities were also determined for both reactions. Copyright © 2007 John Wiley & Sons, Ltd.     

Comparison of Rate Oscillations in Reactions of CO and Methane Oxidation on a Nickel Catalyst

Self-oscillations of the rate of oxidation reactions of CO and CH₄ on nickel foil are compared. It has been shown that, despite significant differences in the stepwise mechanism of the two reactions, the properties of oscillation regimes are very close. In both reactions, oscillation regimes result from oxidation and reduction of nickel and are accompanied by wave processes on the catalyst surface. The similarity in the properties of rate oscillations in the reactions of CO and CH₄ oxidation shows that a change in the selectivity and the formation of carbon on the nickel surface are not key factors in the mechanism of the rate oscillations in CH₄ oxidation. An observed periodic change in the concentrations of reactants and reaction products when the reaction mixture is supplied in a pulse mode proves that, in both cases, the self-oscillations of the reaction rate are due only to the reaction mechanism and are not caused by a change in the catalyst temperature.     

Exploring fuel molecular structure effects on the pyrolysis chemistry of branched hexenes

3,3-Dimethyl-1-butene (NEC₆D3) and 2,3-dimethyl-2-butene (XC₆D2) are representative branched alkene components in gasoline. This work experimentally investigated the pyrolysis of NEC₆D3 and XC₆D2 in a flow reactor (T = 950–1350 K, P = 0.04 atm) and a jet-stirred reactor (T = 730–1000 K, P = 1 atm) using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC). A pyrolysis model of branched hexenes was proposed and validated against the new experimental data. The combined experimental observations and modeling analyses provide insights into the predominant fuel decomposition pathways and specific formation pathways of products under pyrolysis conditions. NEC₆D3 exhibits a much higher reactivity than XC₆D2 due to the existence of allylic CC bonds. Unimolecular decomposition reactions play the most crucial role in NEC₆D3 decomposition, while in XC₆D2 pyrolysis, fuel consumption is dominated by H-abstraction reactions and the H-assisted isomerization reaction. Fuel-specific pathways can remarkably influence the formation of pyrolysis products, especially the key C₁C₂ products, isomer pairs and dialkenes. Furthermore, the reactions involving propargyl radical dominate the formation of fulvene and aromatic products in the pyrolysis of both fuels, leading to more abundant production of C₆ and larger cyclic products in XC₆D2 pyrolysis.     

Probing the fuel-specific intermediates in the low-temperature oxidation of 1-heptene and modeling interpretation

In this work, low temperature (low-T) oxidation of 1-heptene was investigated in a jet-stirred reactor (JSR) over the temperatures of 450–800 K, 770 Torr and equivalence ratios of 0.5–2.0. The intermediates were identified and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography combined with mass spectrometry (GC–MS). The SVUV-PIMS experiment combined with quantum chemistry calculation of ionization energy enables the identification of fuel-specific intermediates, including C₇ alkenylperoxy radical and hydroperoxides, such as diolefinic-hydroperoxide, alkenylhydroperoxide, alkenyl-ketohydroperoxide and cyclic ether hydroperoxide. Among them, alkenylperoxy radical, diolefinic-hydroperoxide and cyclic ether hydroperoxide have not been detected in alkene oxidation before. In order to accurately identify and quantify other fuel-specific intermediates such as aldehyde and cyclic ether isomers, the GC–MS experiment was conducted under the same conditions as the SVUV-PIMS experiment. On the other hand, a detailed low-T oxidation model of 1-heptene was developed, which can reasonably capture the fuel oxidation rate and negative temperature coefficient behaviors observed in this work. The present model can not only interpret the formation of different kinds of hydroperoxides and predict their temperature windows, but also capture the formation of 2-heptenal, hexanal and heptanal, and branched tetrahydrofurans, which are derived from the H-abstraction by OH, OH addition and H addition reactions of 1-heptene, respectively, revealing that the competition between these reactions can be well characterized.     

Spectroscopic Investigation on the Acrolein Hydration

The progress with time of the UV and ¹H NMR spectra of an aqueous solution of acrolein shows that, contrary to assumptions of previous authors, hydracrylaldehyde is not the only reaction product: in addition to it and its hydrated form, spontaneous oxidation products are present.     

Radiation-induced degradation of solid fluorene

Degradation of γ-irradiated solid fluorene with different γ-ray doses was investigated in the present work. Dissolution of the γ-irradiated fluorene in aqueous-methanol solution led to the formation of new products as a result of chemical interaction between the trapped electrons and fragments in the host lattice of fluorene with solvent molecules and ions. The new products were identified and separated by UV-Vis., GC/MS and NMR spectroscopy and separated by High Performance Liquid (HPL) chromatography. The new products were identified to be 1,vinyl-cyclopentene, 1,8-napthalenedicarboxylic acid and furan. For explanation of the results, probable reaction mechanisms are given.     

Kinetics and mechanism of the oxidation of the products of decene-1 oligomerization

The kinetic characteristics of the autooxidation and initiated oxidation of decene oligomers PAO-2, PAO-4, PAO-6, and PAO-10⁺ at 120°C were studied. The mechanism of the oxidation of PAO-2 fraction is examined in detail using a mathematical model. The key reactions in the mechanism of PAO-2 oxidation at 120, 130, and 140°C are identified and the kinetic parameters determined. It is shown that the high oxidizability of an oligomer is determined not only by the presence of double bonds and hydroperoxides in samples, but also by high rates of chain initiation and decomposition of hydroperoxides with formation of free radicals. A comparative study of the mechanism of oxidation of a PAO-2 sample prepared using an alternative technology was carried out. It is shown that both the qualitative and quantitative parameters of the oxidation stability of the produced oligomer depend on the production technology. With the aim of enhancing oxidation stability of the initial nonhydrogenated products of oligomerization, the process of oxidation of PAO-2 sample was studied in the presence of antioxidants based on sterically hindered phenols and aromatic amines. It is demonstrated that such inhibitors can be used for anti-oxidation protection of the primary oligomerization products during their storage.     

Acrolein coupling on reduced TiO2(1 1 0): The effect of surface oxidation and the role of subsurface defects

Reactions of acrolein, water, and oxygen with the vacuum-reduced surface of TiO₂(1 1 0) are reported in a temperature programmed reaction study of the interaction of an aldehydic pollutant with a reducible metal oxide. A total of 25% of the acrolein that binds to the surface is converted to products. Notably, carbon-carbon coupling occurs with 86% selectivity for formation of C₆ products: C₆H₈, identified as 1,3-cyclohexadiene, in a peak at 500 K and benzene immediately thereafter at 530 K. Acrolein is evolved from the surface in three peaks: a peak independent of coverage at 495 K, attributed to decomposition of an intermediate that is partly converted to C₆H₈; a coverage-dependent peak that shifts from 370 K (low coverage) to 260 K (high coverage), which is attributed to adsorption at 5-fold coordinated Ti sites; and a multilayer state at 160 K. Water and acrolein compete for 5-fold coordinated titanium sites when dosed sequentially. The addition of water also opens a new reaction pathway, leading to the hydrogenation of acrolein to form propanal. Water has no effect on the yield of 1,3-cyclohexadiene. Exposure of the surface to oxygen prior to acrolein dosing quenches the evolution of acrolein at 495 K and concurrently eliminates the coupling. From these results, we propose that reduced subsurface defects such as titanium ion interstitials play a role in the reactions observed here. The notion that subsurface defects may contribute to the reactivity of organic molecules over reducible oxide substrates may prove to be general.     

Exploration on laminar flame propagation of 3,3-dimethyl-1-butene, 2,3-dimethyl-1-butene and 2,3-dimethyl-2-butene

Laminar flame propagation of branched hexene isomers/air mixtures including 3,3-dimethyl-1-butene (NEC₆D3), 2,3-dimethyl-1-butene (XC₆D1) and 2,3-dimethyl-2-butene (XC₆D2) was investigated using a high-pressure constant-volume cylindrical combustion vessel at 1–10 atm, 373 K and equivalence ratios of 0.7–1.5. The measured laminar burning velocity (LBV) decreases in the order of NEC₆D3, XC₆D1 and XC₆D2, which indicates distinct fuel molecular structure effects. A kinetic model was constructed and examined using the new experimental data. Modeling analyses were performed to reveal fuel-specific flame chemistry of branched hexene isomers. In the NEC₆D3 and XC₆D1 flames, the allylic CC bond dissociation reaction plays the most crucial role in fuel decomposition under rich conditions, while its dominance is replaced by H-abstraction reactions under lean conditions. The H-abstraction and H-assisted isomerization reactions are concluded to govern fuel consumption in the XC₆D2 flame under all investigated conditions. Both C₀C₃ reactions and fuel-specific reactions are found to be influential to the laminar flame propagation of the three branched hexene isomers. Fuel molecular structure effects were analyzed with special attentions on key intermediates distributions and fuel-specific reactions in all flames. Due to the formation selectivity of key intermediates such as 2-methyl-1,3-butadiene and 2,3-dimethyl-1,3-butadiene, the production of reactive radicals especially H follows the order of NEC₆D3 > XC₆D1 > XC₆D2, which results in the same order of fuel reactivities and LBVs.