Oxidation Of Methyl Formate Research Articles

A revised reaction kinetic mechanism for the oxidation of methyl formate

The combustion kinetics of methyl formate (MeFo), a promising oxygenated, highly knock-resistant e-fuel, is investigated in this work. The potential energy surfaces (PESs) of the low-temperature oxidation reactions of MeFo radicals were determined at the CCSD(T)-F12/cc-pVTZ-F12//B2PLYP-D3/cc-pVTZ level of theory and their rate coefficients were obtained with RRKM/master equation. The thermochemical properties of relevant intermediate species were calculated by using the G4 method. A chemical mechanism for the oxidation of MeFo was developed by adding fuel-specific reactions taken from a literature model into a well-validated base mechanism, and incorporating the newly calculated rate constants of elementary reactions and thermochemical properties of species. The fuel-specific mechanism was further revised in terms of reaction channels and rate coefficients for improved prediction accuracy. In particular, the O2 addition to hydroperoxy radicals (QOOH), the isomerization of peroxy hydroperoxy radicals (OOQOOH), the subsequent ketohydroperoxide (KET) decomposition, and the formally direct pathways of MeFo radicals (R) with O2 were added for the completeness of consumption pathways, in conjunction with the reaction channels of alkoxy radicals (RO) and hydroperoxides (ROOH). For good model prediction accuracy of ignition delay times, especially in the high-temperature range, the resulting mechanism was optimized by modifying the rate constants of several important H-atom abstraction reactions of MeFo within their uncertainty limits. The performance of the proposed mechanism is demonstrated through extensive validation against literature data obtained from different experimental configurations. Finally, the underlying reaction pathways of MeFo are explored by means of reaction flux analysis with the newly developed chemical mechanism.Novelty and significance statement: This work proposes a new reaction kinetic mechanism for the oxidation of methyl formate. Quantum chemistry calculation at high level of theory was performed to explore the low-temperature chemistry of MeFo. The reaction mechanism was improved by including missing reaction channels, incorporating theoretically calculated rate and thermochemical data, and modifying rate constants of sensitive reactions. The mechanism has been validated successfully against extensive data available in the literature covering a wide range of conditions and facilities. By using this new mechanism, reaction pathway analysis is performed to provide insights into the oxidation chemistry of MeFo.

Experimental and theoretical investigations of methyl formate oxidation including hot β-scission

Abstract Recently the possibility of hot β-scission pathways gained attention. These reactions give a shortcut during the important fuel consumption phase in combustion processes leading from H-atom abstraction directly to the β-scission products without fuel radical thermalization. Methyl formate (MF) was shown to be prone to hot β-scission due to a low β-scission barrier height. Furthermore, MF as smallest methyl ester can be considered as biodiesel surrogate and it is an important intermediate product during combustion of various ethers. In this work a predominantly ab-initio derived detailed kinetic model of MF combustion is developed including hot β-scission pathways and compared to a sophisticated literature model based on classical estimation methods. For this, new stoichiometric MF in air ignition delay time measurements in a shock tube and a rapid compression machine over a wide temperature range (790 K–1250 K) and pressures of 10, 20 and 40 bar served as validation targets. The experimental ignition delay times (IDT) show Arrhenius type behavior in both facilities at all conditions. The newly developed quantum-based model catches the pressure dependency and low-temperature reactivity well although overpredicting the IDT at higher temperatures. It was found that hot β-scission is the major depletion pathway of formate group-centered MF radicals. This, however, does not change the overall reactivity of MF combustion due to the low stability of the alkyl peroxide (RO2) at the formate group. For species with competing thermal β-scission and RO2 formation, however, hot β-scission may have a significant impact.

Propagation of sub-atmospheric methyl formate flames

Laminar flame speeds of methyl formate/air mixtures were measured at sub-atmospheric pressures for which limited data exist. The experiments were carried out in the counterflow configuration at an unburned mixture temperature of 333 K. The flow velocities were measured using particle image velocimetry. Particle phase slip correction was applied to low-pressure data sets for which the density disparity between the flow tracers and the gaseous phase is notable. The data were modeled using two recently developed kinetic models of methyl formate oxidation, and significant disagreements were realized at all pressures especially under fuel-rich conditions. Additionally, the computed species profiles of CO and CO2 in the burner-stabilized flame configuration using the two models were found to differ significantly. Reaction path analysis revealed that the kinetics of CH2OCHO that is produced directly from the fuel affects the overall reactivity, and the attendant rate constants differ between the two models. The variation of laminar flame speed with pressure revealed also a different behavior between experiments and simulations. Further insight into the sources causing the observed discrepancies were investigated and it was determined that reactions involving formyl radical, methanol, and formaldehyde could also be responsible for the reduction in reactivity specifically under fuel-rich conditions.

High-Pressure Study of Methyl Formate Oxidation and Its Interaction with NO

An experimental and modeling study of the influence of pressure on the oxidation of methyl formate (MF) has been performed in the 1–60 bar pressure range, in an isothermal tubular quartz flow reactor in the 573–1073 K temperature range. The influence of stoichiometry, temperature, pressure, and presence of NO on the conversion of MF and the formation of the main products (CH2O, CO2, CO, CH4, and H2) has been analyzed. A detailed kinetic mechanism has been used to interpret the experimental results. The results show that the oxidation regime of MF differs significantly from atmospheric to high-pressure conditions. The impact of the NO presence has been considered, and results indicate that no net reduction of NOx is achieved, even though, at high pressure, the NO–NO2 interconversion results in a slightly increased reactivity of MF.

Oxidation of methyl formate and its interaction with nitric oxide

An experimental and kinetic modeling study of the oxidation of methyl formate (MF) has been performed. The experiments have been carried out in an isothermal tubular quartz flow reactor, at atmospheric pressure, in the temperature range 300–1100K. The influence of the temperature, oxygen concentration and the presence of nitric oxide have been analyzed on the oxidation regime of MF and on the formation of the main products (CH3OH, CO, CO2 and H2). A detailed chemical kinetic mechanism for the oxidation of MF has been used for calculations. The results show that the oxidation regime of MF for different stoichiometries is very similar both, in the absence and in the presence of nitric oxide, but some differences are found under oxidizing conditions in the presence of NO. Under these conditions, a mutually sensitized oxidation of MF and NO is seen to occur. In spite of the fact that a minimum of the concentration of NO is observed, no net reduction of NOx is found.

High-Temperature Measurements of the Reactions of OH with Small Methyl Esters: Methyl Formate, Methyl Acetate, Methyl Propanoate, and Methyl Butanoate

The overall rate constants for the reactions of hydroxyl radicals (OH) with four small methyl esters, namely methyl formate (CH(3)OCHO), methyl acetate (CH(3)OC(O)CH(3)), methyl propanoate (CH(3)OC(O)C(2)H(5)), and methyl butanoate (CH(3)OC(O)C(3)H(7)), were investigated behind reflected shock waves using UV laser absorption of OH radicals near 306.69 nm. Test gas mixtures of individual methyl esters and tert-butyl hydroperoxide (TBHP), a fast source of OH at elevated temperatures, diluted in argon were shock-heated to temperatures spanning from 876 to 1371 K at pressures near 1.5 atm. The overall rate constants were determined by matching the measured OH time-histories with the computed profiles from the comprehensive chemical kinetic mechanisms of Dooley et al. (2010) and Dooley et al. (2008), which were originally developed for the oxidation of methyl formate and methyl butanoate, respectively. These measured values can be expressed in Arrhenius form as k(CH(3)OCHO+OH) = 2.56 × 10(13) exp(-2026/T) cm(3) mol(-1) s(-1), k(CH(3)OC(O)CH(3)+OH) = 3.59 × 10(13) exp(-2438/T) cm(3) mol(-1) s(-1), k(CH(3)OC(O)C(2)H(5)+OH) = 6.65 × 10(13) exp(-2539/T) cm(3) mol(-1) s(-1), and k(CH(3)OC(O)C(3)H(7)+OH) = 1.13 × 10(14) exp(-2515/T) cm(3) mol(-1) s(-1) over the temperature ranges studied. Detailed error analyses were performed to estimate the overall uncertainties of these reactions, and the estimated (2σ) uncertainties were found to be ±29% at 913 K and ±18% at 1289 K for k(CH(3)OCHO+OH), ± 29% at 930 K and ±17% at 1299 K for k(CH(3)OC(O)CH(3)+OH), ± 25% at 909 K and ±17% at 1341 K for k(CH(3)OC(O)C2H(5)+OH), and ±24% at 925 K and ±16% at 1320 K for k(CH(3)OC(O)C(3)H(7)+OH). We believe these are the first direct high-temperature rate constant measurements for the reactions of OH with these small methyl esters. These measured rate constants were also compared with the estimated values employed in different comprehensive kinetic mechanisms. Additionally, the structure-activity relationship from Kwok and Atkinson (1995) was used to estimate these four rate constants, and the estimations from this group-additivity model are in good agreement with the measurements (within ~25%) at the present experimental conditions.

CO concentration and temperature sensor for combustion gases using quantum-cascade laser absorption near 4.7 μm

A sensor for sensitive in situ measurements of carbon monoxide and temperature in combustion gases has been developed using absorption transitions in the (v′=1←v″=0) and (v′=2←v″=1) fundamental bands of CO. Recent availability of mid-infrared quantum-cascade (QC) lasers provides convenient access to the CO fundamental band near 4.7 μm, having approximately 104 and 102 times stronger absorption line-strengths compared to the overtone bands near 1.55 μm and 2.3 μm used previously to sense CO in combustion gases. Spectroscopic parameters of the selected transitions were determined via laboratory measurements in a shock tube over the 1100–2000 K range and also at room temperature. A single-laser absorption sensor was developed for accurate CO measurements in shock-heated gases by scanning the line pair v″=0, R(12) and v″=1, R(21) at 2.5 kHz. To capture the rapidly varying CO time-histories in chemical reactions, two different QC lasers were then used to probe the line-center absorbance of transitions v″=0, P(20) and v″=1, R(21) with a bandwidth of 1 MHz using fixed-wavelength direct absorption. The sensor was applied in successful shock tube measurements of temperature and CO time-histories during the pyrolysis and oxidation of methyl formate, illustrating the capability of this sensor for chemical kinetic studies.

An experimental and kinetic modeling study of methyl formate low-pressure flames

The oxidation of methyl formate (CH 3OCHO), the simplest methyl ester, is studied in a series of burner-stabilized laminar flames at pressures of 22–30 Torr and equivalence ratios ( Φ) from 1.0 to 1.8 for flame conditions of 25–35% fuel. Flame structures are determined by quantitative measurements of species mole fractions with flame-sampling molecular-beam synchrotron photoionization mass spectrometry (PIMS). Methyl formate is observed to be converted to methanol, formaldehyde and methane as major intermediate species of mechanistic relevance. Smaller amounts of ethylene and acetylene are also formed from methyl formate oxidation. Reactant, product and major intermediate species profiles are in good agreement with the computations of a recently developed kinetic model for methyl formate oxidation [S. Dooley, M.P. Burke, M. Chaos, Y. Stein, F.L. Dryer, V.P. Zhukov, O. Finch, J.M. Simmie, H.J. Curran, Int. J. Chem. Kinet. 42 (2010) 527–529] which shows that hydrogen abstraction reactions dominate fuel consumption under the tested flame conditions. Radical–radical reactions are shown to be significant in the formation of a number of small concentration intermediates, including the production of ethyl formate (C 2H 5OCHO), the subsequent decomposition of which is the major source of observed ethylene concentrations. The good agreement of model computations with this set of experimental data provides a further test of the predictive capabilities of the proposed mechanism of methyl formate oxidation. Other salient issues in the development of this model are discussed, including recent controversy regarding the methyl formate decomposition mechanism, and uncertainties in the experimental measurement and modeling of low-pressure flame-sampling experiments. Kinetic model computations show that worst-case disturbances to the measured temperature field, which may be caused by the insertion of the sampling cone into the flame, do not alter mechanistic conclusions provided by the kinetic model. However, such perturbations are shown to be responsible for disparities in species location between measurement and computation.

Methyl formate oxidation: Speciation data, laminar burning velocities, ignition delay times, and a validated chemical kinetic model

AbstractThe oxidation of methyl formate (CH3OCHO) has been studied in three experimental environments over a range of applied combustion relevant conditions: A variable‐pressure flow reactor has been used to quantify reactant, major intermediate and product species as a function of residence time at 3 atm and 0.5% fuel concentration for oxygen/fuel stoichiometries of 0.5, 1.0, and 1.5 at 900 K, and for pyrolysis at 975 K. Shock tube ignition delays have been determined for CH3OCHO/O2/Ar mixtures at pressures of ≈ 2.7, 5.4, and 9.2 atm and temperatures of 1275–1935 K for mixture compositions of 0.5% fuel (at equivalence ratios of 1.0, 2.0, and 0.5) and 2.5% fuel (at an equivalence ratio of 1.0). Laminar burning velocities of outwardly propagating spherical CH3OCHO/air flames have been determined for stoichiometries ranging from 0.8–1.6, at atmospheric pressure using a pressure‐release‐type high‐pressure chamber. A detailed chemical kinetic model has been constructed, validated against, and used to interpret these experimental data. The kinetic model shows that methyl formate oxidation proceeds through concerted elimination reactions, principally forming methanol and carbon monoxide as well as through bimolecular hydrogen abstraction reactions. The relative importance of elimination versus abstraction was found to depend on the particular environment. In general, methyl formate is consumed exclusively through molecular decomposition in shock tube environments, while at flow reactor and freely propagating premixed flame conditions, there is significant competition between hydrogen abstraction and concerted elimination channels. It is suspected that in diffusion flame configurations the elimination channels contribute more significantly than in premixed environments. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 527–549, 2010

Enhancement of the oxidation of methyl formate at multifunctional electrocatalyst composed of Pt/Pd and Pt/Ru nanoparticles

Immobilization of carbon-supported Pt/Pd nanoparticles (that have been modified with ruthenium oxo species) within the tungsten oxide film containing Pt/Ru nanoparticles results in the enhancement of the catalytic properties towards electrooxidation of methyl formate in 0.5 mol dm−3 H2SO4. The over-coating of Pt/Pd nanoparticles with cationic Ru-oxo species facilitates their attachment to tungsten oxide matrix. Because oxidation of the ester is mechanistically complex and it is expected to occur most rapidly on the formate component, the comparative measurements have been done with formic acid. In spite of modification of Pt/Pd with Ru-oxo species, the system retains high electrocatalytic activity during oxidation of formic acid. It is rationalized that not only Pt/Ru is catalytic towards oxidation of methyl formate (or products of its hydrolysis) but also oxoruthenium modified Pt/Pd is capable of promoting effectively the oxidation of formic acid or formate component of the ester. The multifunctional activity of co-deposited Pt/Ru and Pt/Pd centers is supported by good electronic and protonic conductivities of hydrated tungsten oxide matrix.

Photocatalytic oxidation of methyl formate on TiO 2: a transient DRIFTS study

Methyl formate adsorbs both molecularly and dissociatively as methoxy and formate on titanium dioxide (TiO 2). Methyl formate dissociates on TiO 2 at dual sites that adsorb methoxy and formate; formate adsorption sites are more numerous than those of methoxy and the availability of methoxy sites limits methyl formate coverage. Photocatalytic oxidation (PCO) oxidizes the α-carbon in methyl formate to CO 2 without forming any long-lived intermediates, whereas the β-carbon forms CO 2 through formaldehyde and formate. At least two types of active sites exist for PCO on TiO 2 and their activities differ by more than an order of magnitude; the more-active sites comprise approximately 30% of adsorption sites. Photocatalytic oxidation carried out at 373 K oxidizes species more quickly than at room temperature because elevated temperatures enhance inherent site activity and convert adsorbed methoxy to formate. Water does not poison methyl formate PCO and redistributes weakly bound species during PCO by displacement. Water preferentially displaces methanol and also converts some adsorbed methoxy to formate at room temperature.

Anodic oxidation of methyl formate and its relationship to the reactions of methanol and formic acid

Experiments have been performed to examine the anodic oxidation of methyl formate in sulphuric acid using a platinum–ruthenium electrocatalyst, and to relate the process to the anodic oxidation of methanol and formic acid. Formic acid is more readily anodically oxidised than methanol, with up to five times the rate being observed, dependent on potential and temperature. The rate of reaction of methyl formate is comparable with that of formic acid. It is reasoned that the oxidation of the ester occurs most rapidly on the formate component, with methanol being generated by hydrolysis. The initial activation overpotential barrier required to oxidise these materials at a significant rate is common to all three fuels.

Dye-linked dehydrogenase activities for formate and formate esters in Amycolatopsis methanolica. Characterization of a molybdoprotein enzyme active with formate esters and aldehydes.

Cell-free extracts of methanol-grown Amycolatopsis methanolica contain dye-linked dehydrogenase activities for formate and methyl formate. Fractionation of the extracts revealed that the (unstable) activity for formate resides in membrane particles, while that for methyl formate belongs to a soluble enzyme that was purified and characterized. The enzyme, indicated as formate-ester dehydrogenase, appeared to be a molybdoprotein (4 Fe, 3 or 4 S, 1 Mo and 1 FAD were found for each enzyme molecule), with a molecular mass of 186 kDa and consisting of two subunits of equal size. Product identification suggests that the formate moiety in the ester becomes hydroxylated to a carbonate group after which the unstable alkyl carbonate decomposes into CO2 and the alcohol moiety. Based on structural and catalytic characteristics, the enzyme appears to be very similar to an enzyme isolated from Comamonas testosteroni [Poels, P. A., Groen, B. W. & Duine, J. A. (1987) Eur. J. Biochem. 166, 575-579] which was at that time considered to be an aldehyde dehydrogenase. Formate-ester dehydrogenase activity appeared to be present in several other bacteria. Possible roles for the A. methanolica enzyme in C1 dissimilation (oxidation of methyl formate to methanol and CO2 or a factor-formate adduct to factor plus CO2) or in general aldehyde oxidation, are discussed.

Methyl Formate: an Experimental and Kinetic Study of its Oxidation at High-Pressure

An experimental and kinetic modeling study of the influence of pressure on the oxidation of methyl formate (MF) has been performed in the 1-60 bar pressure range. The influence of stoichiometry, temperature, pressure and presence of NO on the conversion of MF, and on the formation of the main products, has been analyzed.