Direct Measurement of High-Temperature Rate Constants of the

Subscriber access provided by University of South Dakota

A: Kinetics, Dynamics, Photochemistry, and Excited States

Direct Measurement of High-Temperature Rate Constants of the Thermal Decomposition of Dimethoxymethane – A Shock Tube and Modeling Study Sebastian Peukert, Paul Sela, Damien Nativel, Jürgen Herzler, Mustapha Fikri, and Christof Schulz J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b06558 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Direct Measurement of High-Temperature Rate Constants of the Thermal Decomposition of Dimethoxymethane – a Shock Tube and Modeling Study

Sebastian Peukert*, Paul Sela, Damien Nativel, Jürgen Herzler, Mustapha Fikri, Christof Schulz

IVG, Institute for Combustion and Gas Dynamics – Reactive Fluids and CENIDE, Center for Nanointegration Duisburg-Essen, University of Duisburg-Essen, 47048 Duisburg, Germany

Corresponding Author: Sebastian Peukert IVG, Institute for Combustion and Gas Dynamics – Reactive Fluids University of Duisburg-Essen 47048 Duisburg, Germany Phone: +49 202 3793511 Email: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Shock-tube experiments have been performed to investigate the thermal decomposition of the oxygenated hydrocarbon dimethoxymethane (DMM; CH3OCH2OCH3). The primary initial reaction channels of DMM decomposition are considered to be the two bond fissions: CH3OCH2OCH3 → CH3O + CH2OCH3 (1) and CH3OCH2OCH3 → CH3 + OCH2OCH3 (2). In the present work, two shock-tube facilities and three different detection techniques have been combined: Behind reflected shock waves, we have carried out time-resolved measurements of (i) the formation of H atoms using the highly sensitive H-ARAS (Atomic Resonance Absorption Spectrometry) technique and (ii) the depletion of the DMM reactant by high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS). In addition, (iii) the temperature-dependent composition of stable reaction products was measured in single-pulse shock-tube experiments via gas chromatography (GC/MS). The experiments span a temperature range of 1100–1430 K, a pressure range of 1.2–2.5 bar and initial reactant mole fractions from 0.5 ppm (for H-ARAS experiments) up to 10000 ppm (for HRR-TOF-MS experiments). Experimental rate constants ktotal, ktotal = k1 + k2, obtained from these three completely different methods were in excellent agreement among each other, i.e., deviations are within ±30–40%, and they can be well represented by the Arrhenius expression ktotal(T) = 1013.28±0.27 exp(–247.90±6.36 kJ mol−1/RT) s−1 (valid over the 1100–1400 K temperature and the 1.2–2.5 bar pressure range). By replacing the respective ktotal values used in a recently published DMM chemical kinetics combustion mechanism (Vermeire et al., Combust. Flame 2018, 190, 270-283), it was also possible to successfully reproduce measured product distributions.


Page 2 of 38

Page 3 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Oxygenated hydrocarbons are becoming more important worldwide as neat alternative fuels (“biofuels”) and as blending agents in gasoline and Diesel fuels. In the group of oxygencontaining organic compounds, ethanol is currently the most widely used biofuel1. Besides alcohols, also ethers, in particular dimethyl and tert-butyl methyl ether, are used as additives and components of synthetic fuels2. Another group of ether compounds that can be used both, as additives and fuels, are oxymethylene ethers (OMEs)3-4. These compounds have the general chemical structure CH3-O-(CH2-O)n-CH3. OMEs can be produced in several steps from methanol, thus providing an interesting energy storage medium that can ultimately be made from electrolytically generated hydrogen. Therefore, they gain additional relevance as a result of electrochemically/catalytically producible fuels (“e-fuels“), which in the context of the energy transition can play an important key role in harnessing renewable electricity5. Dimethoxymethane (CH3OCH2OCH3; DMM) is the smallest member of the family of OMEs. In a series of engine tests, Härtl et al.6 performed a “fuel screening” of various oxygenated compounds. These measurements have shown that DMM had the greatest decreasing effect on both, NOx and soot particulate emissions. Since DMM represents a model compound for the group of OMEs, its combustion chemistry has been subject of previous flow reactor and premixed-flame studies7-12. Daly et al.9 were the first ones who devised a chemical kinetics model describing the combustion chemistry of DMM based on flow-reactor experiments. In their model, the Arrhenius parameters of the initial unimolecular and bimolecular fuelconsumption steps were estimated by reactivity analogies to reactions of dimethyl and diethyl ether. One of the most recent kinetics model on DMM combustion has been published by Vermeire et al.12. They measured the formation of products obtained during the pyrolysis and oxidation of DMM at temperatures between 500 and 1100 K in a jet stirred reactor setup. A substantial part of their chemical kinetics mechanism was generated by applying a code named “Genesys” for the automatic construction of reaction mechanisms13. Rate-constant expressions and 3 ACS Paragon Plus Environment


thermochemical data are based on the results of electronic structure calculations. The latest experimental and modelling study was reported by Sun et al.14, who performed new species measurements from the combustion of DMM in another jet stirred reactor in combination with photoionization molecular-beam mass spectrometry. In contrast, Kopp et al.15 investigated particular chemical reactions, especially H abstractions by H atoms and CH3 radicals as well as subsequent radical dissociation channels, by applying transition state theory and master equation analyses based on high level ab initio calculations. The only shock-tube study on the high-temperature gas-phase kinetics of DMM, has been published by Golka et al.16 so far. By performing shock tube experiments in combination with atomic resonance absorption spectrometry (ARAS) and time-of-flight mass-spectrometry (TOF-MS), Golka et al.16 applied an experimental methodology that is very similar to the one presented in this work. The high relevance of DMM as a model compound for OMEs and the fact that directly determined hightemperature rate-constant data on unimolecular reactions of DMM are scarce motivates the study of the kinetics of DMM. Based on the mechanism provided by Vermeire et al.12, DMM decomposes by two bond fissions: CH3OCH2OCH3 → CH3O + CH3OCH2

(1)

CH3OCH2OCH3 → CH3 + CH3OCH2O

(2)

In the simulations shown in the present work, the contributions of unimolecular C-H bond fissions to DMM depletion are not considered because of thermochemical considerations: By applying the G4 composite method17, which can be employed for calculating thermochemical data and which is implemented in the Gaussian09 software18, in combination with the atomization method19, we calculated standard enthalpies of formation (∆Hf0) of all chemical species involved in these bond fissions and from these ∆Hf0 values, we calculated the corresponding standard reaction enthalpies (∆Hr0). For reactions (1) and (2), calculated ∆Hr0 are 355.2 and 337.8 kJ/mol, whereas ∆Hr0 values for loosing primary and secondary H atoms are calculated to be 403.7 and 407.2 kJ/mol. As expected, C-H bond fissions have dissociation energies that are around 50 to 60 kJ/mol larger than those of C-C and C-O bond fissions. 4 ACS Paragon Plus Environment

Page 4 of 38

Page 5 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Therefore, the contribution of unimolecular C-H bond fissions can be safely neglected under the conditions of the present shock-tube experiments. Even though no H atoms are directly formed in reactions (1) and (2), both result in the formation of H atoms after subsequent dissociation of the primarily formed products. At temperatures above 1000 K, CH3O radicals formed in (1) rapidly decompose to formaldehyde (CH2O) and H atoms. The decomposition of the other radical, CH3OCH2, yields CH3 radicals and CH2O again. The intermediate radical CH3OCH2O formed in (2) can dissociate either by C-H bond fission (3a) or by β-bond-fission (3b): CH3OCH2O → H + CH3OCHO

(3a)

CH3OCH2O → CH3O + CH2O

(3b)

Again, CH3O radicals will generate CH2O and H atoms. No matter in which way DMM decomposes, reactions (1) and (2) will release H atoms due to fast secondary reactions so that each decomposed DMM molecule will effectively result in the formation of one H atom. Therefore we have first applied the highly sensitive H-ARAS technique for monitoring temporal H-atom profiles, [H](t) to determine the overall DMM decomposition rate constant ktotal, in which ktotal = k1 + k2. According to the Arrhenius expressions provided in the Vermeire et al.12 mechanism, channel (1) is the dominant decomposition step, i.e., the branching ratio BR1, in which BR1 = k1/(k1+k2), equals approximately 0.98 at temperatures around 1000 K. The H-ARAS experiments, however, do not permit to extract information about a branching ratio. Therefore, the consumption of DMM during its pyrolysis was also monitored time-resolved by highrepetition-rate time-of-flight mass-spectrometry (HRR-TOF-MS) behind reflected shock waves and by single-pulse shock-tube-experiments measuring DMM concentrations in dependence of temperature via GC/MS. The combination of these techniques has been previously demonstrated by Sela et al.20 By applying HRR-TOF-MS and GC/MS, we also intended to obtain species profiles from stable reaction products to further validate the Vermeire et al.12 mechanism and to obtain information about the branching ratio BR1. Therefore, this work reports experimental high-temperature rate constants determined from three complementary 5 ACS Paragon Plus Environment


experimental methods: H-ARAS, HRR-TOF-MS, and GC/MS. This combination is a unique feature of the present work, especially since these methods have completely different sensitivities and capabilities, i.e., DMM mole fractions in the reactant gas mixtures covered a range of ∼0.5 ppm DMM (for H-ARAS) up to 10,000 ppm DMM (for HRR-TOF-MS).

2. Experimental arrangement All experiments were performed in stainless-steel diaphragm-type shock tubes. For modeling of all the results, i.e., [H](t) from ARAS and species-concentration profiles from HRR-TOF-MS measurements as well as end product compositions from single-pulse GC/MS experiments, the program Chemical Workbench21 (Version 4.1.15340) was used. 2.1 Shock-tube with H-ARAS detection Driver and driven section of the shock-tube have a length of 3.0 and 5.5 m, respectively, and an inner diameter of 80 mm. Aluminum sheets with 50 µm thickness were used as diaphragm between driver and driven sections. The driver section was evacuated down to 3×10–3 mbar. Between experiments, the driven section was routinely pumped down to pressures of 1.0×10–6 mbar. To generate a shock wave, helium (Air Liquide, 99.999%) is filled into the driver section until the diaphragm bursts. The driven gas in the low-pressure section was argon of high purity (Air Liquide, 99.9999%) with small quantities of DMM (0.5–0.6 ppm). The gas mixtures were prepared in a 50-l stainless-steel mixing vessel based on the partial pressure method. Due to the high sensitivity of the H-ARAS method, initial reactant mole fractions were below 1 ppm. In order to prepare these highly diluted gas mixtures of DMM in Ar, it was necessary to conduct four dilution steps. A more concentrated gas mixture was first prepared, and then this mixture was successively further diluted with argon with manometric control. In each dilution step, the gas mixture was allowed to homogenize for two hours. The calculations necessary for determining the post-shock conditions (T5 and p5) are based on 1-D gas-dynamic equations and require the pre-shock conditions (T1 and p1) as well as the incident 6 ACS Paragon Plus Environment

Page 6 of 38

Page 7 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shock-wave velocities as input parameters. The velocity of the incident shock wave was measured by four pressure transducers (PCB model 112A21) that were mounted at equal distances of 150 mm on the top of the driven section. One additional pressure transducer (Kistler 603A) was installed 20 mm upstream of the end wall to measure the arrival times of the reflected shock waves. Two lithium fluoride windows transparent in the vacuum ultraviolet (VUV) were installed 20 mm upstream of the end wall of the driven section at opposite sides as optical access for absorption measurements. Behind the reflected shock waves, the reaction progress was monitored by time-resolved HARAS at the Lyman-α line (121.6 nm). The Lyman-α radiation is generated in a microwavedischarge light source consisting of a quartz tube and an antenna driven by a microwave generator (Sairem, 2.45 GHz, 200 W). By using a 5 dB damping resistor, the power is reduced to 60 W. A gas mixture containing 1% H2 in He is flown through the quartz tube and adjusted to a pressure of 7 mbar. One end of the quartz tube is connected to the optical port of the shock tube, and the generated VUV radiation was transmitted across the shock tube through the VUVLiF windows and collected by a solar-blind photomultiplier (Hamamatsu/R8487). The photomultiplier is located in a metal housing that is purged by O2, at a pressure around 200 mbar, that acts as a spectral filter that isolates the Lyman-α radiation and hence achieves the required spectral resolution according to the concept introduced by Appel and Appleton22. At atmospheric pressure the O2 VUV spectrum provides a narrow region of high transmission at 121.6 nm while suppressing neighboring lines. Signals of the photomultiplier as well as the pressure transducers are recorded by two oscilloscopes (PicoScope 5442A). The experimental observation period was 1000 µs. The oscilloscopes are triggered by a pulse derived from the first pressure transducer in the driven section of the shock tube. For the purpose of chemical kinetics modeling, the measured absorption–time profiles need to be converted into absolute H-atom concentration–time profiles. Within the VUV light source, self-absorption at the resonance frequency causes partial reversal of the emission profile. Moreover, the emission line is broader then the absorption line of the detected H-atoms. Therefore, the Beer-Lambert law is not directly applicable for interpreting the measured 7 ACS Paragon Plus Environment


absorption. Therefore, calibration experiments were performed under experimental conditions that provide a wide range of well-known H-atom concentrations. The well-established reaction sequence starting with N2O → N2 + O, followed by H2 + O → OH + H and OH + H2 → H2O + H was applied for the controlled generation of H atoms, with known concentrations of N2O and H2 diluted in Ar23. This procedure is suitable because the thermal decomposition of H2 is very slow at temperatures below 2000 K. Below 2000 K, a quasi-stationary concentration profile of H atoms cannot be achieved within the observation period of 1 ms and the dynamic increase of the observed absorption is used to correlate it with the respective H-atom concentration23. The functional relationship between absorber concentration and absorption A can be fitted to a modified Beer-Lambert equation, A = 1 – exp(l σ [H]n), where A is the measured absorption, l the absorption path length in cm, [H] is the concentration of the absorber (H atoms) in cm–3, and σ and n are fit parameters providing the correlation [H] = f(A). We found σ = (3.74±0.12)×10−9 cm0.818 and n = 0.606±0.001. Within the investigated temperature range, no temperature dependence of the calibration constants was found. 2.2 Shock tube apparatus for HRR-TOF-MS and GC/MS The HRR-TOF-MS/GC/MS stainless-steel shock tube has a total length of 8.8 m and an inner diameter of 80 mm. The shock tube is divided into a driver section with a length of 2.5 m and a driven section with a length of 6.3 m, separated by an aluminum diaphragm with thicknesses of 30–70 µm. The driver and driven sections were evacuated by a four-stage rotary pump (Edwards dry star, Model QDP 80) combined with a mechanical booster pump (Edwards, EH 500A) to achieve a final pressure of ~3×10−3 mbar. The initial pressure p1 of the test gas in the driven section was measured using a capacitance manometer (Edwards, Trans 600 AB) and the diaphragm burst pressure p4 was measured with a manometer (Keller, Mano 2000). The facility can either be operated as a conventional shock tube (for HRR-TOF-MS) or as a singlepulse shock tube (for GC/MS). For the single-pulse mode, the shock tube is connected with a dump tank (0.35-m3) via a ball valve (60 mm inner diameter). The dump tank is filled with


Page 8 of 38

Page 9 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nitrogen at the same filling pressures (50–150 mbar) as the driven section and the valve is opened just before an experiment is started. The velocity of the shock wave was determined using a set of four pressure transducers (Kistler, 603B) placed at the end of the driven section in distances of 150 mm. To derive the post-shock conditions T5 and p5, the velocity was extrapolated to the end plate of the driven section with an observed attenuation of less than 1%. A fifth pressure transducer (PCB, 112A05) located 20 mm from the end plate was used to monitor the pressure history and all signals from the pressure transducers were recorded by an oscilloscope with a resolution of 500 MS/s at 14 bit (Pico Technology, PicoScope 5442A). For further details of the experimental facility see Refs.24,25. All mixtures were prepared in a 50 l stainless-steel vessel and stirred for at least one hour. A turbomolecular pump (Pfeiffer, TMH 071 P) evacuated the vessel to a final pressure of ∼7×10−7 mbar and the mixtures were composed of DMM as reactant, Kr as an internal standard and the bath gas. Depending on the detection technique, Ar or Ne was used as bath gas (Ne is selected for the HRR-TOF-MS measurements because of its low ionization cross section). Before each experiment, the driven section of the shock was flushed twice with the initial mixture from the mixing vessel and evacuated to reduce impurities. In one set of experiments, toluene (C7H8) was used as a chemical inhibitor, which suppresses secondary reactions of the highly reactive hydrogen and methyl radicals formed during DMM pyrolysis. In this case, methyl and hydrogen radicals abstract hydrogen directly from the side chain of C7H8 to form CH4 or H2 and less reactive benzyl radicals26-27. 2.2.1 GC/MS The composition of the stable products of the DMM decomposition was assigned and quantified with a highly sensitive GC/MS (Agilent 7890A and MSD 5975C). The experiments were performed in the single-pulse mode of the shock tube. The shock tube is coupled to a heated (363 K) gas sampling system, which consists of a solenoid valve (Balzers, EVI 005) and a probe chamber with a volume of 55 cm³. After each experiment, the valve opens automatically by a delayed (∼5 ms) signal of the fifth pressure transducer and stays open for about 1 s and the 9 ACS Paragon Plus Environment


probe chamber is filled with the product gas to pressures of 110–160 mbar, depending on the pressure p5. The sampling system automatically extracts product gas behind the reflected shock wave after almost all reactions had been quenched by the gas expansion and consequently a rapid decrease in temperature; only reactions with very low activation energies (e.g., radical reactions) could still continue after the gas-dynamic quenching typical for single-pulse operation of the shock tube. Because of the long sampling time and the large core-gas volume, effects of the gases, which could be trapped from the cold boundary layer, can be neglected. Sela et al.25 estimated that the uncertainty in the reactant concentration measurements caused by boundary layer effects is 0.1% of the initial concentration. To reduce the uncertainty in the GC/MS measurements, each sample was analyzed three times by GC/MS.

2.2.2 HRR-TOF-MS For time-resolved, simultaneous multi-species concentration measurements, the shock tube was equipped with a high-repetition-rate time-of-flight mass spectrometer (HRR-TOF-MS, Kaesdorf). This setup was described in Refs. 24,25, therefore only a brief description is given here. The HRR-TOF-MS is connected by an ISO flange to the end plate of the driven section of the shock tube. The end plate contained a changeable nozzle (45–100 µm in diameter) protruding 1 mm into the shock tube; in this study, a nozzle diameter of 60 µm was used. The ion source of the HRR-TOF-MS is evacuated to ∼1×10−7 mbar by a turbomolecular pump (Pfeiffer, TMU 521 P) and the time-of-flight section is evacuated to ∼4×10−8 mbar by another turbomolecular pump (Pfeiffer, TMU 261 P). The sample is ionized by electron-impact ionization and the nominal ionization energy can be varied from 5–85 eV. The repetition rate of the ion source can be set up to 150 kHz, thus providing a time resolution of ∼6.7 µs and the detection of masses up to 105 u. The TOF-MS is operated in reflectron mode and the ions are detected with a two-stage micro-channel plate (MCP) detector. In this study, the TOF-MS was operated with a repetition rate of 100 kHz and an electron energy of 45 eV. These settings yield full mass spectra with masses up to 175 u every 10 µs. For data acquisition, an 8 bit A/D card (Agilent acqiris, DP214) 10 ACS Paragon Plus Environment

Page 10 of 38

Page 11 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


was used with a sampling rate of 2 GS/s. To prevent overloading of the MCP, Ne was used as bath gas in all HRR-TOF-MS measurements, because of its low electron ionization cross-section. To achieve the best signal-to-noise ratio of the reactant signal, the MCP voltage was set to its maximum and thereby the Ne peak is saturated only slightly without affecting the sensitivity of the detector at neighboring masses. A Matlab-based routine was used to convert the raw data into intensity–time profiles by summation of the peak area at a certain time step by using the equation m/z = a (t − b)2. The time of flight t of the detected ions was converted into the mass-to-charge ratio (m/z) using the constants a and b that were determined in separate calibration measurements28. Behind the reflected shock wave, the temperature T5 and pressure p5 increase and the pressure build-up causes a greater mass flow through the nozzle into the ion source and therefore, an increase in signal at t = 0 ms is observed. In this initial phase, the measured signal can depend both on the build-up of the gas expansion after the pressure rise (i.e., a temporal variation of the gas density in the ionization chamber) and the variation in concentration of the respective species as a consequence of thermal reactions initiated by the shock-wave-induced heating of the gas. To separate the kinetics from the gas-dynamic effects, the signal intensities of all species of interest were normalized to the peak area of the signal measured for Kr that was added to the initial gas mixture as chemically-inert internal standard28-29. Before this normalization, the intensity–time profile of Kr was smoothed by a moving average to minimize deterioration of the signal-to-noise ratio.

3. Results and Discussion 3.1 H-ARAS measurements The observed [H](t) profiles were simulated using a reaction mechanism that contains 16 elementary reactions. This mechanism is presented in table 1. Thermodynamic data of all chemical species were taken from the Vermeire et al.12 mechanism. 11 ACS Paragon Plus Environment


Page 12 of 38

Table 1. Reactions used to simulate [H](t) from the pyrolysis of DMM (CH3OCH2OCH3). Reaction CH3OCH2OCH3 = CH3O + CH3OCH2

Rate constant See text

Reference This work

CH3OCH2OCH3 = CH3 + CH3OCH2O

See text

This work

k∞(T) = 6.8×1013 exp(−13170 K/T) s–1 k0(T) = 1.9×1025 T–3.0 exp(−12232 K/T) cm3mol–1s–1 α = 0.9/T*** = 2500/T* = 1300/T** = 1.0×1099 k∞(T) = 1.1×1012 T0.48 exp(131 K/T) cm3mol–1s–1 k0(T) = 1.4×1024 T–2.57 exp(−717 K/T) cm3mol–1s–1 α = 0.78/T*** = 271/T* = 2755/T** = 6570

[11]

HCO + M = H + CO + M

k(T) = 5.7×1011 T0.66 exp(−7483 K/T) cm3mol–1s-1

[11]

CH3OCH2 = CH2O + CH3

k(T) = 1.4×1011 T1.01 exp(−12828 K/T) s–1

[11]

CH3OCHO+H=CH3OCH2O

k(T) = 1.0×1013 exp(−3944 K/T) cm3mol–1s–1

[11]

CH3O + CH2O = CH3OCH2O

k(T) = 1.0×1011 exp(−4006 K/T) cm3mol–1s–1

[11]

C2H6(+M) = CH3 + CH3 (+M)

k∞ = 8.03×1028 T–3.52 exp(−47983 K/T) s–1 k0 =2.80×1072 T–15.10 exp(−54222 K/T) cm3mol–1s–1 α = 0.21/T*** = 1.00×10–30/T* = 1.00×1030

[30]

CH3 + CH3 = 2H + C2H4

k(T) = 3.2×1013 exp(−7395 K/T) cm3mol–1s–1

[31]

CH3 + H (+M) = CH4 (+M)

k∞ = 7.2 × 1013 T0.19 exp(−T/25200 K) cm3mol−1s−1 k0 = [M] 2.3×1021 exp[(−T/21.22 K)0.5] cm6 mol–2s–1 Fc = 0.262 + [(T − 2950 K)/6100 K]2 F(x)=1−(1 − Fc) exp(−[log(1.5x)/N]2/N*)a)

[32]

C2H6 + CH3 = C2H4 + CH4 + H

k(T) = 34.5 T3.44 exp(−5237 K/T) cm3mol–1s–1

[33]

C2H6 + H = C2H4 + H2 + H

k(T) = 8553 T3.06 exp(−2559 K/T) cm3mol–1s–1

[34]

CH4 + H = CH3 + H2

k(T) = 1.8×1014 exp(−6945 K/T) cm3mol–1s–1

[35]

CH3 + H = CH2 + H2

k(T) = 6.0×1013 exp(−7610 K/T) cm3mol–1s–1

[36]

CH3O (+M) = CH2O + H (+M)

HCO + H (+M) = CH2O (+M)

[36] k(T) = 2.2×10 exp(−48352 K/T) cm mol s x = k0/k∞; N = 0.75−1.27 log(Fc); N* = 2 for log(1.5x) > 0; N* = 2 [1−0.15 log(1.5x)] for log(1.5x) < 0.

H2 + M = H + H + M a)

[11]

14

3

–1 –1

[H](t) profiles from two DMM pyrolysis experiments are shown in Fig. 1. Best-fit simulations were achieved by using the table 1 mechanism and adjusting the total DMM decomposition rate constant, ktotal = k1 + k2. 12 ACS Paragon Plus Environment

Page 13 of 38

4.0x1012

1.0

3.5x1012

CH3OCH2OCH3 = CH3O + CH3OCH2

3.0x1012

CH3OCH2OCH3 = CH3 + CH3OCH2O

H sensitivity

[H] / cm-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5x1012 2.0x1012 1.5x10

12

T5 = 1370 K

1.0x1012

0.5

p5 = 1.33 bar

5.0x1011

0.0

0.0 0

200

400

600

800

1000

0

t / µs

200

400

600

800

1000

t / µs

Fig. 1: Left: Measured [H](t) profile for an experiment at T5 = 1370 K and p5 = 1.33 bar; initial mole fraction of DMM: x0,DMM = 0.48 ppm. The red solid curve represents the best-fit simulation using the table 1 mechanism; green dotted curve: see text. Right: Local H-atom sensitivity-analysis using the table 1 mechanism and the modeled ktotal (ktotal = k1+k2) rate constant; the branching ratio BR1, BR1 = k1/ktotal, is adopted from the Vermeire et al. model; the H-atom sensitivity SH is defined as SH = dxH/dki.

The initial mole fractions of DMM in the two gas mixtures used for the ARAS experiments were 0.48 and 0.56 ppm. Due to the high sensitivity of the H-ARAS detection technique, even small uncertainties in the initial mole fractions of DMM (x0,DMM) within ±3–4% slightly affect simulated temporal H-atom concentration profiles at longer observation times (for example in Fig. 1: t > 400 µs). In order to derive best-fit profiles over the entire time range, also the initial mole fraction of CH3OCH2OCH3 was allowed to vary at most within ±4% along with the total rate constant for DMM decomposition. The red solid curve in Fig. 1 represents the best-fit simulation with ktotal = 7018 s−1. The green dotted curve represents a simulation using the ktotal value, which is calculated based on the Arrhenius expressions provided in the reaction model of Vermeire et al.12. In this investigation, flow reactor experiments were performed up to temperatures of 1100 K, whereas the present H-ARAS shock tube experiments were conducted between 1250 and 1420 K. Therefore, extrapolating ktotal(T) to the present temperature range is not expected to yield best fit simulations right away. According to the Vermeire et al.12 mechanism, ktotal = 20100 s−1 at T = 1370 K. In general, the present rate constant data deviate from those provided in this literature 13 ACS Paragon Plus Environment


Page 14 of 38

mechanism by a factor between two and three. Inspection of the local H-atom-sensitivity analysis in Fig. 1 shows that seemingly only one of the two DMM bond-fission channels is relevant for the measured H-atom-formation. The reason is related to the branching ratio BR1. During the simulations, only ktotal was adjusted, whereas BR1 was adopted from the Vermeire et al.12 mechanism. Even at temperatures of 1400 K, bond fission (1) seems to be the absolutely dominant reaction channel, i.e., BR1 ≈ 0.98. With a branching fraction of less than 3%, channel (2) does almost not show up in the sensitivity plot at all. However, the simulations of the HARAS experiments do not depend on BR1. Even reversing the branching ratio has no influence on the best-fit rate-constant data of ktotal. Therefore, the H-ARAS experiment only permit to extract information about the overall rate constant ktotal. Experimental conditions and simulated best-fit ktotal data are tabulated in table 2.

Table 2. Summary of experimental conditions for DMM pyrolysis experiments using H-ARAS. Quantities with the subscript 5 refer to the thermodynamic state of the gas in the reflected shock wave region.

T5 / K

p5 / bar

ktotal / s−1

Mixture: x0,DMM = 0.48±0.01 ppm in Ar 1292

1.29

1312

1308

1.33

3012

1324

1.33

2762

1355

1.35

6209

1370

1.33

7018

1412

1.31

14591

Mixture: x0,DMM = 0.56±0.02 ppm in Ar 1251

1.34

638

1267

1.32

899

1292

1.33

2612

1316

1.33

3384

1355

1.27

7647

1388

1.27

15350


Page 15 of 38

3.2 HRR-TOF-MS measurements Time-resolved concentration measurements were also carried out via HRR-TOF-MS. The HRRTOF-MS was operated with a repetition rate of 100 kHz and an electron energy of 45 eV. Two mass spectra are shown in Fig. 2.

0.5 I / arb. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ne+ Ne+ (20) (22)

C2H5O+ pre-shock (45) C3H7O+2 (75)

Kr+ (84)

0.0

-0.5

CH3OH+ (32)

-1.0 CH+4 (16) 20

C2H+2 (26) 30

CO+ (28) 40

CH3OCHO+ (60) post-shock 50

60

70

80

m/z

Fig. 2: Mass spectra measured with HRR-TOF-MS averaged for 0.5 ms before the arrival of the reflected shock wave at pre-shock conditions (T1 = 295 K, p1 = 47.5 mbar: blue top line) and for post-shock conditions (T5 = 1424 K, p5 = 1.46 bar: red lower line) averaged for 1.5 ms after arrival of the reflected shock wave. Both mass spectra belong to one experiment with an initial mixture of 1% DMM (CH3OCH2OCH3), and 1% Kr in Ne. For better illustration, the upper spectrum was inverted, and the strong Ne signals are cut off. The lower spectrum shows the effect of thermal decomposition of DMM and the formation of the products. The repetition rate of the ionization was set to 100 kHz and the energy of ionization was 45 eV.

The top spectrum represents pre-shock conditions before the arrival of the incident shock, time averaged for 0.5 ms at T1 = 295 K and p1 = 47.5 mbar, and the lower spectrum depicts post15 ACS Paragon Plus Environment


shock conditions after the reflected shock wave, time averaged for 1.5 ms at T5 = 1424 K, and p5 = 1.46 bar. The electron-ionization process leads to fragmented ion signals of DMM with the main peak at m/z = 45 (CH3OCH2+), a strong fragment peak at m/z = 75 (C3H7O2+) and many smaller peaks spread over the complete mass spectrum. The parent peak at m/z = 76 (CH3OCH2OCH3+) was too weak with an electron-ionization energy of 45 eV and was not detected. For the evaluation of the concentration–time profiles of DMM the main peak at m/z = 45 was used, which corresponds to the DMM fragment CH3OCH2+. The peak at m/z = 84 is related to the internal standard of Kr and the peaks at m/z = 20 and 22 to the bath gas Ne. Furthermore, the post-shock spectrum of Fig. 2 depicts the signals of the product species methane at m/z = 16 (CH4+), ethylene at m/z = 26 (C2H2+), carbon monoxide at m/z = 28 (CO+), and methyl formate at m/z = 60 (CH3OCHO+). The fragmentation patterns of CO, C2H4, and of DMM overlap at m/z = 28. In addition, in experiments where C2H6 was formed up to temperatures of T5 = 1350 K, the fragmentation pattern of C2H6 overlaps with those of CO, C2H4, and DMM too. For the evaluation of C2H4 the fragment signal at m/z = 26 (C2H2+) was used, but for extracting temporal CO concentration profiles, it was necessary to subtract the signal contribution at m/z = 28 that are not caused by CO. The fragmentation spectra of DMM and various products (C2H4, CH3OH, CH3OCHO) were determined in calibration experiments, the fragmentation spectrum of CH2O was taken from literature. Using the ratio of the main peak of these substances to their peak at m/z = 28 and the signal of the main peak at a certain time of the measurement, the signal of all this substances at m/z = 28 could be determined at each time of the measurement and subtracted from the total signal at m/z = 28. Therefore, only the signal of CO and fragmentation signals of some minor products, which can be neglected, remain. For most of the substances like DMM, CH3OH and CH3OCHO, the peak at m/z = 28 is less than 10% of the main peak, so that more than 50% of the measured signal at m/z = 28 is caused by CO. However, this procedure leads to a lower signal-to-noise (S/N) ratio of the measured CO concentrations compared to other measured concentrations, which could be measured. Single-pulse GC/MS experiments reveal that CH3OH and CH3OCHO are formed, but it was not possible to derive concentration–time profiles of these products because their signal intensities 16 ACS Paragon Plus Environment

Page 16 of 38

Page 17 of 38

were small and at m/z = 31 the signals of DMM, CH3OH and CH3OCHO were overlapping. The occurrence of the peak at m/z = 60 shows qualitatively the formation of CH3OCHO. The timedependent decomposition of DMM and the formation of the products CH4, CO, and C2H4 are shown in Fig. 3. It can be seen, that DMM decomposes and instantaneously CO, C2H4, and CH4 are formed.

c/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0

DMM CO CH4

1.5

C2H4

1.0 0.5 0.0 0.0

0.1

0.2

0.3

0.4

0.5

t / ms

Fig. 3: Concentration–time profiles of DMM (CH3OCH2OCH3), CH4, CO, and C2H4 measured with HRR-TOFMS for the post-shock region at T5 = 1358 K and p5 = 1.42 bar. Mixture: 1% DMM, and 1% Kr in Ne. The declaration c/% on the y-axis represents normalized mole fractions, given in mol%.

All the concentrations were measured relative to the internal standard Kr, which was already part of the initial mixtures and the calibration mixtures, therefore normalized mole fractions, xi = xi,products(xKr,educts/xKr,products) = xi,products(ntotal,educts/ntotal,products) were determined, given in mol%. The simulated data shown in the figures were normalized with this equation. The thermal decomposition of DMM consumes heat and this effect increases with larger concentrations of DMM, which results in temperature variations during the pyrolysis. In order to derive ktotal rate constants of DMM decomposition, the measured [DMM](t) profiles were 17 ACS Paragon Plus Environment


simulated using the Vermeire et al. model12. Simulations were performed with Chemical Workbench21 and are based on non-reactive pressure profiles of pure Ar and Ne. ktotal rate coefficients were simulated accounting for the reaction-induced temperature variation during the reaction. Fig. 4 gives an example for modelling a [DMM](t)-profile and a brute-forcesensitivity-analysis, in which ktotal was varied by factors of 2 and 0.5. Best-fit profiles were obtained by only adjusting ktotal. As in case of the H-ARAS-experiments, modelled rate constants do not depend on the branching ratio BR1.

1

DMM this work this work x2 this work x0.5

0,8 0,6

0

DMM sensitivity

1,0

c/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

T5 = 1358 K p5 = 1.42 bar

0,4 0,2

-1 -2 -3

DMM = CH3O + CH3OCH2 H + DMM = H2 + CH3OCH2OCH2

-4

H + DMM = H2 + CH3OCHOCH3 CH2O + CH3 = HCO + CH4

-5

CH2O + H = HCO + H2 CH3 + CH3 + M = C2H6 + M

0,0

-6 0,00

0,25

0,50

0,75

C2H6 + H = C2H5 + H2

0.00

t / ms

0.25

0.50

0.75

t / ms

Fig. 4. Left: Measured [DMM](t)-profile for an experiment at T5 = 1358 K und p5 = 1.42 bar; initial mole fraction of DMM: x0,DMM = 1%. The black solid curve represents the best fit simulation using the Vermeire et al. mechanism. Right: Local DMM-sensitivity-analysis with the simulated best fit ktotal rate constant; the local DMM-sensitivity SDMM is defined as SDMM = dxDMM/dki.

Due to much higher reactant mole fraction, the local DMM-sensitivity-analysis illustrated in Fig. 4 is more complex than those for the conditions of the H-ARAS measurements (see Fig. 1). For example, also bimolecular reactions of the type H + CH3OCH2OCH3 = products + H2 contribute to the consumption of the DMM reactant. Experimental conditions and simulated best-fit ktotal data obtained from the HRR-TOF-MS experiments are tabulated in table 3.


Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Summary of experimental conditions for DMM pyrolysis experiments with HRR-TOF-MS.

T5 / K

p5 / bar

ktotal / s–1

Mixture: x0,DMM = 1% and xKr = 1% in Ne 1195

1.83

168

1273

1.78

1095

1306

1.72

1451

1343

1.67

3363

1347

1.54

3673

1358

1.42

4665

1424

1.46

13266

3.3 Single-pulse GC/MS experiments Using GC/MS detection reveals that C2H4, C2H6, CH3OCHO, and CH3OH are the most abundant stable products formed during thermal decomposition of DMM. This is shown in Fig. 5a. It can be seen that the concentration of C2H6 reaches a maximum at around 1230 K and that the concentration of CH3OCHO reaches a maximum at 1180 K. The formation of CH4 starts around 1130 K and the concentration increases continuously up to temperatures of 1350 K. In terms of temperature, C2H4 is formed at the latest and the concentration increases continuously up to temperatures of 1350 K. Acetylene (C2H2) was found in much smaller concentrations at temperatures above 1250 K.



DMM CH4 C2H4

1,00

[X] / [DMM]0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0,75

(a)

0,50

(b)

1,00 0,75

C2H6 CH3OCHO CH3OH

Page 20 of 38

0,50

DMM CH4 CH3OH DMM (C7H8)

0,25

0,25

0,00

0,00 1050

1100

1150

1200

1250

1300

1000

CH4 (C7H8) CH3OH (C7H8)

1050

T/K

1100

1150

1200

1250

1300

T/K

Fig. 5: (a) Measured DMM, CH4, C2H4, C2H6, CH3OCHO, and CH3OH concentrations as a function of the temperature for two mixtures, 510 ppm DMM + 500 ppm Kr + Ar as well as 560 ppm DMM + 500 ppm Kr + Ar. (b) Measured DMM, CH4, and CH3OH concentrations for experiments with and without toluene (C7H8); solid symbols: experimental results from measurements without C7H8; open symbols: 200 ppm DMM + 200 ppm Kr + 1.78% C7H8 +Ar.

Figure 5b contains a comparison between GC/MS-measurements with and without the addition of toluene (C7H8) to the reactant gas mixtures. C7H8 is used as a radical chain inhibitor. Therefore, a high excess of C7H8 suppresses the influence of bimolecular reactions between radicals and DMM. It can be seen that the presence of C7H8 retards the consumption of DMM, because radical reactions involving the DMM reactant are suppressed. Molecular hydrogen (H2), the product of the H abstractions H + C7H8 = products + H2, was not observed as it is not possible to detect H2 with the GC/MS used in this study. CH3 and CH3O radicals abstract H directly from the side chain of C7H8 forming CH4 and CH3OH, respectively. Therefore, CH3 radicalrecombination reactions are inhibited and no larger hydrocarbons are formed. From the GC/MS experiments with C7H8-addition, ktotal rate constants were extracted by simulations, in which a simplified two-step reaction model was used. This two-step model considers only reactions (1) and (2) and assumes that they are not reversible. The reasons for these simplifications are that (i) due to the excess concentration of the radical inhibitor, CH3, CH3O, and other radicals like H atoms are removed from the chemical equilibrium, and (ii) the single-pulse-experiments with C7H8 addition did not exceed temperatures of 1270 K, where k[C7H8→prodcuts] ≤ 1 s−1 (see NIST 20 ACS Paragon Plus Environment

Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


data base37). From GC/MS measurements without C7H8, ktotal data were extracted from simulations again using the full Vermeire et al.12 mechanism. As before, simulations were carried out accounting for the reaction-induced temperature variation during the thermal decomposition. Table 4 summarizes experimental conditions and rate constants obtained from GC/MS-measurements with and without C7H8.

Table 4. Summary of experimental conditions of the single-pulse-GC/MS-measurements.

T5 / K

p5 / bar

ktotal / s–1

Mixture: x0,DMM = 200 ppm, xKr = 200 ppm, and xC7H8 = 1.78% in Ar 1130

2.38

65

1158

2.37

115

1185

2.34

211

1214

2.31

380

1238

2.26

630

1262

2.18

916

Mixture: x0,DMM = 510 ppm and xKr = 500 ppm in Ar 1141

2.40

69

1225

2.29

519

1227

2.24

519

1240

2.28

664

Mixture: x0,DMM = 560 ppm and xKr = 500 ppm in Ar 1114

2.30

51

1171

2.31

211

1183

2.25

259

1217

2.26

515

1141

2.30

103

1205

2.27

408

1165

2.34

201



3.4 Comparison of all experimental data and simulations of product distributions Figure 6 shows a comparison of all experimentally based ktotal data. It can be seen that the rate constants derived from the different experimental approaches, i.e., H-ARAS, HRR-TOF-MS, and GC/MS, agree very well with each other. The overall uncertainty of the experimental rate constants was estimated to be ±35%.

T/K 1500 1400 1300

1200

1100

1000

900

5

10

104 103

ktotal / s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

102 101

H-ARAS HRR-TOF-MS GC/MS with C7H8

0

10

GC/MS without C7H8

10-1 10-2 10-3 0.7

0.8

0.9

1.0

1.1

1000 K/T

Fig. 6: Comparison of present experimental high-temperature ktotal rate constants of DMM decomposition with (i) ktotal prediction based on k1 and k2 provided in the Vermeire et al. mechanism (solid and dashed grey curves; see text) and (ii) with ktotal(T) expression experimentally determined by Golka et al.16 (dotted line). Symbols represent present experimental data from our work and the red solid curve represents the 2-parameter Arrhenius fit.

The present experimental rate constants of ktotal are well represented by the following twoparameter-Arrhenius-equation:

ktotal(T) = 1013.28±0.27 exp(–247.90±6.36 kJ mol−1/RT) s−1 22 ACS Paragon Plus Environment

(E1)

Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In order to estimate overall uncertainties of the measured rate constants, following uncertainties were considered: Uncertainties in (i) reflected shock wave temperatures, (ii) initial reactant concentrations, and (iii) rate constants of secondary reactions (here: two H-abstraction channels for H + DMM). Uncertainties in measured temperatures are related to uncertainties in measured shock-wave velocities of the incident shock waves and are estimated to be ±1%. For all experimental methods (H-ARAS, GC/MS, and HRR-TOF-MS), a temperature error of ±1% results in an average uncertainty in the determined rate-constants of ±25%. In case of singlepulse GC/MS experiments, errors in initial DMM reactant concentrations of ±5% lead to uncertainties in the determined rate constants of approx. ±20%. This estimation was obtained by applying following procedure: Assuming deviations of initial DMM mole fractions of ±5%, we tested by conducting simulations using the Vermeire et al.12 mechanism, how much the best fit rate constant ktotal needed to be adjusted to obtain the measured DMM mole fraction. In case of H-ARAS, uncertainties of ±5% on [CH3OCH2OCH3]0 affect the achievable plateau in H-atom concentration at longer reactions times (see Fig. 1), but not the best fit ktotal value, since ktotal primarily determines the slope of the [H](t) profile. Also ktotal data derived from the HRR-TOFMS-experiments are not significantly influenced by uncertainties of ±5% in initial DMM mole fractions. Deviations of [CH3OCH2OCH3]0 by ±5% lead to [CH3OCH2OCH3](t) profiles that are within the noise of the measured signals. The third considered uncertainty category is caused by uncertainties in the rate constants of secondary reactions. However, this category does not apply to the H-ARAS technique because this technique has such a high sensitivity that highly diluted gas mixtures with reactant concentrations of ca. 0.5 ppm can be used where secondary reactions can be neglected. Due to larger initial reactant concentrations, secondary reactions can influence HRR-TOF-MS as well as GC/MS experiments. The GC/MS measurements with and without toluene addition have clearly shown an influence of bimolecular reactions on ktotal and the sensitivity analysis for a HRR-TOF-MS experiment presented in Fig. 4 suggests that Habstractions H + DMM are important secondary reactions. Assuming uncertainties by a factor of 2 or 0.5, leads to deviations in ktotal of ca. ±10% in both, GC/MS and HRR-TOF-MS experiments. This influence is surprisingly small because of the high rate constant values of the H + DMM H23 ACS Paragon Plus Environment


abstraction channels, i.e., these bimolecular rate constants have an order of magnitude of 1012 cm3mol−1s−1. Therefore, changing the rate constants of the H + DMM H-abstraction reactions by a factor of 2 and 0.5, respectively, does not result in a large difference of best fit ktotal data. The combined uncertainties in the determination of ktotal are: ±25% for H-ARAS, ±34% for GC/MS, and ±27% for HRR-TOF-MS. The uncertainty estimates for GC/MS and HRR-TOF-MS were derived by using the root-sum-square method: ((0.2)2+(0.25)2+(0.1)2)0.5 for ktotal from GC/MS and ((0.25)2+(0.1)2)0.5 for ktotal from HRR-TOF-MS. Figure 6 includes data from a ktotal(T) expression that was experimentally determined by Golka et al.16 These data are represented by the dotted curve. Golka et al.16 also performed H-ARAS and HRR-TOF-MS shock-tube experiments and by monitoring [H](t) and [CH3OCH2OCH3](t) they were able to derive ktotal data for DMM decomposition. For their simulations, they assumed a branching ratio of 0.5 between the two primary bond-fission channels. Due to the slightly different activation energy of overall DMM decomposition (Ea ≈ 247.9 kJ/mol in this work and Ea ≈ 264.6 kJ/mol in Ref. 16), the rate constant data differ at lower temperatures (1100–1200 K) by a factor of two. With increasing temperatures, the difference decreases and between 1250 and 1500 K, the present ktotal rate constants are 30 to 40% larger than those of Golka et al.16. They estimated the overall uncertainty of their rate constants to be ±45%. Considering the documented errors, the present results agree very well with those from Golka et al.16. Figure 6 also shows a comparison between the present experimental ktotal data and those predicted from the Arrhenius expressions given in the Vermeire et al.12 mechanism. However, one should keep in mind that the flow-reactor measurements reported by Vermeire et al.12 were conducted at atmospheric pressure and temperatures between 500 and 1100 K. The solid curve in Fig. 6, which covers the temperature range below 1100 K, refers to the ktotal prediction from Vermeire et al.12, whereas the dashed curve represents its extrapolation to the temperature range of the present shock-tube experiments. At lower temperatures, i.e., at T < 1000 K, the ktotal prediction of Vermeire et al.12 is probably close to the high-pressure limit of DMM decomposition. Present ktotal data collected below 1200 K agree well with the extrapolation of ktotal from Vermeire et al.12, which indicates that the present experiments at the 24 ACS Paragon Plus Environment

Page 24 of 38

Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lower edge of the temperature range are probably also close to the high pressure limit. At T ≈ 1200 K, falloff-behavior becomes noticeable, which results in a deviation from the extrapolation of the Vermeire et al. prediction, i.e., ktotal values obtained from H-ARAS and HRR-TOF-MSexperiments are approximately two to three times lower than the extrapolation. In an attempt to rationalize the temperature and pressure dependence of ktotal and for the purpose to get evidence for the statement that the present ktotal data are in the falloff-range, we have carried out RRKM (Rice-Ramsperger-Kassel-Marcus) calculations using the Gorin-model. The underlying concepts of the restricted-rotor Gorin model were described by Smith and Golden38 and applied by Kiefer and Tranter and their co-workers30,39. According to Vermeire et al.12, DMM almost exclusively decomposes via channel (1), i.e., BR1 ≈ 0.98. If the assumption is made that this branching ratio is correct, then one may approximately treat DMM decomposition as a single-channel process and apply a RRKM Gorin calculation. The molecular properties and parameters used in this model are summarized in table 5 and the results of the RRKM modeling are illustrated in Fig. 7.

Table 5. Summary of parameters describing the restricted rotor Gorin RRKM model for CH3OCH2OCH3 → CH3O + CH3OCH2 (channel (1)).

Frequencies (cm−1): CH3OCH2OCH3: 82, 133, 172, 227, 325, 384, 561, 971, 991, 1123, 1134, 1176, 1182, 1199, 1231, 1246, 1305, 1439, 1472, 1482, 1486, 1492, 1507, 1515, 1529, 2884, 2968, 2990, 3000, 3011, 3073, 3119, 3128 CH3O: 717, 965, 1120, 1365, 1368, 1521, 2896, 2962, 3001 CH3OCH2: 168, 300, 430, 643, 972, 1141, 1175, 1256, 1295, 1458, 1486, 1499, 1503, 3000, 3059, 3099, 3132, 3248 Moments of inertia (g cm2): CH3OCH2OCH3: 5.86×10−39 (a), 3.00×10−38, 3.28×10−38



CH3O: 5.32×10−40, 2.99×10−39, 3.01×10−39 CH3OCH2: 1.77×10−39, 7.85×10−39, 9.03×10−39 Critical energy E0: 352.7 kJ/mol down = 300 cm−1 Restriction parameter η: η = 1 – (0.01) Lennard-Jones parameter: Ar: ε/K = 81.1; σ/ Å = 3.41 CH3OCH2OCH3: ε/K = 327.0 ; σ/ Å = 5.9 Reaction path degeneracy: 2 (a) This is used as the molecular active moment of inertia for calculations employing the RRKMGorin model

T/K 1600 107

1400

1200

10

103

101

1000 RRKM Gorin predictions: for p = 1.3 bar for p = 10.0 bar high pressure limit

5

k1 / s-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

ktotal data: H-ARAS HRR-TOF-MS GC/MS with C7H8 GC/MS without C7H8

10-1

k1(T) from Vermeire et al.12 (at p ≈ 1 bar)

10-3 0.7

0.8

0.9

1.0

1.1

1000K/T

Fig. 7: Symbols represent the experimentally derived rate-constants, ktotal, and the RRKM model fit is shown by the dashed lines for 1.3 bar, 10.0 bar, and the high pressure limit, respectively.


Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The vibrational frequencies, moments of inertia, and the energy barrier E0 were obtained from the G4 composite method17, i.e., the molecular properties are based on the B3LYP/6-31G(2df,p) level of theory. To match the data, the restriction parameterη, which is regarded as an empirical parameter describing the “tightness“ of a transition state structure and down, the average energy transferred in collisions, were adjusted. A value of down = 300 cm-1 seems to be reasonable for a molecule like DMM. Concerning the restriction parameter η, it is more difficult to assess reasonable values. In different RRKM modeling studies employing the restricted-rotor Gorin model, Golden and co-workers used restriction parameters ranging from 0.9 to 0.99 (for bond dissociation reactions at temperatures around 1000 K)38,40. Therefore, we assume that using η = 0.99 is a reasonable choice. The RRKM calculations indicate that k1, and therefore also ktotal, are in the fall-off regime and that the lower temperature ktotal data provided in the Vermeire et al.12 mechanism are near the high pressure limit. Table 6 summarizes RRKM predictions for k1 at different pressures and each of these fits is valid over a 900 – 1500 K temperature range.

Table 6. 3-parameter fits, k(T) = A × Tn × exp(-Ea/RT), for rate constants k1, obtained from RRKM predictions based on the restricted rotor Gorin model. These modified Arrhenius fits are valid over the temperature range 900 – 1500 K. p / bar 0.001 0.01 0.1 1.0 2.0 5.0 10.0 100.0 1000.0 High pressure limit

A / s-1 1.436 × 10102 2.542 × 10103 5.864 × 10104 1.480 × 10106 3.726 × 10106 4.636 × 1098 2.925 × 1091 2.994 × 1064 1.623 × 1042 1.167 × 1026

n –26.00 –26.00 –26.00 –26.00 –26.00 –23.60 –21.45 –13.48 –6.98 –2.29

Ea/(kJ mol−1) 487.572 501.911 520.803 543.046 549.799 535.464 520.953 461.069 408.118 369.301

Despite the plausibility of these results we point out that this is only a simplified theoretical treatment assuming that DMM decomposes primarily by channel 1, which is based on the 27 ACS Paragon Plus Environment


branching ratio BR1 provided in the Vermeire et al.12 mechanism. A proper treatment would require a master equation analysis, which is beyond the scope of this paper. In the following paragraphs, we will discuss in more detail the results of our product species measurements, the possibility to extract experimentally based information on the branching ratio BR1, and the plausibility of assuming such a seemingly extended predominance of k1 over k2. Incorporating the present results of ktotal, i.e., Arrhenius equation (E1), into the Vermeire et al. mechanism, we intended to verify if we are able to reproduce the measured end-product distributions from the GC/MS measurements as well as the time-resolved species concentration profiles from the HRR-TOF-MS experiments.

1.00

DMM CO CH4

0.3

0.75

[X] / [DMM]0

C2H4

1.5 1.0

DMM CH4

0.50

0.0 0.1

0.2

0.3

0.4

C2H4

C2H6

0.2

CH3OCHO CH3OH

0.1

0.0 1000

1100

1200 T/K

1300

C2H6 CH3OCHO

0.25

0.5

0.0

DMM CH4 C2H4

[X] / [DMM]0

2.0

c/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 38

CH3OH

0.00 1000 1050 1100 1150 1200 1250 1300 1350

0.5

T/K

t / ms

Fig. 8: Comparison between measured and simulated time-resolved species profiles from HRR-TOF-MS experiments (left) and between measured and simulated temperature dependent concentration profiles from GC/MS experiments (right) applying the Vermeire et al.12 mechanism with ktotal from the present work. Symbols represent measured data points and simulations are represented by solid, dashed and dotted curves. The figure in the inset on the right depicts an expanded version of the measured product distribution.

Figure 8 shows that by using the Vermeire al.12 mechanism with the present results for ktotal, a good agreement between simulated and measured data is achieved. The second bond-fission channel, reaction (2), could contribute to the formation of methyl formate, CH3OCHO, by the subsequent reaction (3a). However, due to its low branching fraction, BR2 ≈ 0.02, reaction (2) is 28 ACS Paragon Plus Environment

Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


not the primary contributor to CH3OCHO formation. Instead, CH3OCHO is formed as a result of the H-abstraction reaction H + CH3OCH2OCH3 = H2 + CH3OCHOCH3, in which a secondary H atom is abstracted. The secondary radical CH3OCHOCH3 then dissociates to form CH3 and CH3OCHO. Once CH3OCHO is formed, it primarily decomposes to CH3OH and CO41-42, which explains the observed detection of CH3OH at higher temperatures and in similar concentrations like CH3OCHO. Applying the H-ARAS technique, Golka et al.16 also measured total rate constants for the H abstraction H + CH3OCH2OCH3 → products + H2 between temperatures of 850 and 1100 K. To provide Arrhenius expressions for abstracting H atoms from primary and secondary C-H-bonds, they adopted a theory-based branching ratio reported by Kopp et al.15. Extrapolating the kinetics data in the Vermeire et al.12 model, over the 1100–1500 K temperature range the total H-abstraction rate constants increase from 6.1×1012 cm3mol–1s–1 to 2.3×1013 cm3mol–1s–1. Extrapolating the results of Golka et al. to T = 1500 K, total H abstraction rate constants change only slightly from 5.6×1012 cm3mol–1s–1 to 7.4×1012 cm3mol–1s–1. Since H abstractions by H atoms are potentially important reactions and since there is a difference in absolute rate constant data, we have also tested the corresponding Arrhenius equations reported by Golka et al.16 in the Vermeire et al.12 mechanism. However, applying the experimentally based k(T) equations for the two H + CH3OCH2OCH3 abstractions does not substantially alter the simulated product distribution. This can be explained by the similarity of assumed branching ratios: According to Golka et al.16, the branching ratio from abstracting both secondary H atoms varies from 0.65 to 0.59 over the 1100–1500 K temperature range and according to the Vermeire et al.12 model, over the same temperature regime the branching ratio decreases from 0.58 to 0.49. Differences of 10–20% between branching ratios for H abstractions lead to only minor differences in simulated product distributions. Besides, Vermeire et al.12, Sun et al.14 as well as Marrodán et al.10 devised other DMM combustion mechanisms. In their article, Marrodán et al.10 wrote that their model includes reactions (1), (2), and two C-H bond fissions from primary and secondary C-H bonds. As 29 ACS Paragon Plus Environment


mentioned in the introduction, the contribution of unimolecular C-H bond fissions to DMM depletion is not considered in this work. Marrodán et al.10 stated that the rate constants k1 and k2 in their mechanism were adopted from Daly et al.9. Extrapolating k1 and k2 from the Marrodán et al.10 model to the 1100 – 1450 K temperature range, the branching fraction BR1 decreases from 0.53 to 0.38. One can note a substantial difference between the values of BR1 obtained from the two chemical kinetic models. Therefore, we also intended to test the influence of BR1 on the simulated product distributions. When we use the ktotal data (equation E1) determined in this work and the branching fraction BR1 based on the kinetic data provided in the Marrodán et al.10 paper, we obtain simulations which are presented as dotted line in Fig. 9.

1.00

0.75

[X] / [DMM]0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

DMM CH4

0.50

C2H4 C2H6 CH3OCHO

0.25

CH3OH

0.00 1000 1050 1100 1150 1200 1250 1300

T/K

Fig. 9: Comparison between measured and simulated temperature dependent product distributions from single-pulse GC/MS experiments. All simulations were conducted with the Vermeire et al.12 mechanism. Solid lines: Simulations with ktotal from this work and BR1 from Vermeire et al.12 (BR1 ≈ 0.98). Dotted lines: Simulations with ktotal from the present work and 0.59 ≤ BR1 ≤ 0.43 (for 1000–1300 K) from the Marrodán et al.10 model. Symbols represent measured data points.


Page 30 of 38

Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Predictions of small hydrocarbons, i.e., CH4, C2H4, and C2H6, were almost not affected by the changed branching ratio. By contrast, and as expected, using BR1 values derived from Arrhenius equations provided in the Marrodán et al.10 paper, the simulation results in higher concentrations of CH3OCHO and CH3OH because BR1 is lower and, correspondingly, BR2 is larger. By increasing BR2, a larger amount of CH3OCHO is formed through the subsequent reaction (3a), which then primarily decomposes to CH3OH, thereby increasing the amount of formed CH3OH. With modified branching ratios, the new simulations clearly over predict CH3OCHO mole fractions by a factor of two. One might argue that for CH3OH, the impact of changed branching fractions is somehow ambiguous: At temperatures from 1100 to 1200 K, the new simulations are in better agreement with the measurements. However, the simulated temperaturedependent build-up of CH3OH strongly depends on the high-temperature kinetics of CH3OCHO decomposition and it can be observed that at T > 1220 K, the new simulations also tend to overpredict CH3OH formation. In the Sun et al.14 mechanism, rate constants k1 and k2 have been adopted from Glaude et al.43, who performed a modelling study on the combustion of dimethyl carbonate in an opposed-flow diffusion flame. The dimethyl carbonate mechanism from Glaude et al.43 contains a submechanism describing the thermal decomposition of DMM. According to this model, BR1 varies from 0.15 to 0.25 over the 1100–1500 K temperature range. Therefore, it is obvious that employing this branching ratio in simulations results in larger deviations to measured endproduct compositions. We noted that Sun et al.14 included an additional unimolecular DMM decomposition reaction: CH3OCH2OCH3 → CH3OH + CH3 + HCO. Following Arrhenius parameters were provided for this process: A = 5.0×1012 s−1 and Ea = 230.12 kJ/mol. Sun et al.14 adopted these Arrhenius parameters from the Glaude et al.43 mechanism. However, no information or discussion is provided about the reaction, which is supposed to yield CH3OH, CH3 and HCO. Sun et al.14 do not discuss unimolecular pathways for DMM consumption, since these reactions were not important under the oxidative conditions of the jet stirred reactor experiments. Also Glaude et al.43 did not mention and explain this particular reaction in their article. In the paper from Daly et al.9, this process is also not mentioned. Therefore, it is not clear how this reaction takes 31 ACS Paragon Plus Environment


place or if it may be regarded as a surface reaction at the reactor wall. In the following paragraph, we restrict our discussion to simulations based on the Vermeire et al.12 mechanism. Altogether, the results and comparison shown in Figs. 9 and 10 indicate that with BR1 ≥ 0.9, the Vermeire et al. mechanism is able to reproduce measured product distributions very well. However, it is not clear why the first bond fission channel, reaction (1), seems to be so dominant in comparison to reaction (2). According to the thermochemical data from Vermeire et al., reaction (1) has a standard reaction enthalpy (∆Hr0) of 370.0 kJ/mol, whereas for the second bond fission, ∆Hr0 = 348.4 kJ/mol. From the point of bond dissociation energies only, the second bond fission should be even the preferred unimolecular reaction. Even though the absolute values differ, the trends are similar: Based on the thermochemical data from Vermeire et al.12, ∆Hr0(reaction 2) is energetically approximately 21.6 kJ/mol lower than ∆Hr°(reaction 1); based on our calculated standard enthalpies of formation using the G4 method, ∆Hr0(reaction 2) is around 17.4 kJ/mol energetically lower than ∆Hr0(reaction 1). When we use the thermochemical data given in the Vermeire et al.12 model to calculate reaction Gibbs free enthalpies (∆Gr) at 1000–1500 K, ∆Gr of reaction (1) ranges from 194 to 110 kJ/mol, whereas ∆Gr of reaction 2 ranges from 199 to 128 kJ/mol. Over the 1000 – 1500 K temperature range, ∆Gr(reaction 1) < ∆Gr(reaction 2) by around 5 to 18 kJ/mol. These findings suggest that the preference of the bond fission (1) over (2) seems to be driven by the entropies. However, these differences amount only few kJ/mol and therefore it remains an open question why bond fission (1) is preferred over bond fission (2).

4. Conclusions Experiments on two shock-tube apparatuses coupled with three different detection methods were carried out to determine high-temperature rate constants for the overall thermal decomposition of DMM (CH3OCH2OCH3). The thermal decomposition of DMM primarily takes place by two bond-fission-channels: CH3OCH2OCH3 → CH3O + CH3OCH2 (reaction 1) and 32 ACS Paragon Plus Environment

Page 32 of 38

Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CH3OCH2OCH3 → CH3 + CH3OCH2O (reaction 2). Since both channels yield H-atoms due to rapid dissociation of secondary radicals, the highly sensitive ARAS (Atomic Resonance Absorption Spectrometry) technique was applied to monitor temporal H-atom-concentration profiles to derive ktotal rate constants, in which ktotal = k1 + k2. The ARAS experiments covered a temperature range of 1251–1412 K at pressures around 1.3 bar. To enhance the confidence in the measured rate-constant data, also the depletion of DMM was monitored by two sets of experiments: (i) Time-resolved by applying high-repetition time-of-flight mass-spectrometry (HRR-TOF-MS at 1195–1424 K and 1.42–1.83 bar) and (ii) in dependence on temperature by single-pulse shocktube experiments with GC/MS (1114–1262 K and 2.18–2.40 bar). In all sets of experiments, ktotal data were obtained from simulations based on the mechanism published by Vermeire et al.12 Simulated ktotal data do not depend on the branching ratio BR1, BR1 = k1/ktotal. Therefore, it was not possible to directly extract information about BR1. The present determinations for ktotal are in very good agreement with most recent experimental results from Golka et al.16, who also derived overall DMM decomposition rate-constants from shock-tube experiments in combination with H-ARAS and HRR-TOF-MS. It is interesting to note that at T > 1000 K, according to the Vermeire et al. mechanism, DMM almost completely dissociates via channel (1), i.e., BR1 ≈ 0.98. Based on the assumption of DMM decomposition to be a single-channel process, i.e., k1(p,T) ≈ ktotal(p,T), we have developed a restricted-rotor Gorin RRKM model to account for k1(T,p). Even though this is a strongly oversimplified treatment, it is nevertheless able to successfully match measured ktotal data based on reasonable fitting parameters. The HRR-TOF-MS and GC/MS measurements also enabled the observation of the formation of stable reaction products like CH4, C2H6, C2H4, CO, methyl formate (CH3OCHO) and CH3OH. By using the Vermeire et al.12 mechanism, we attempted to simulate measured species profiles. Using the present Arrhenius expression of ktotal and BR1 from the Vermeire et al. model, measured temporal species concentration profiles and product compositions can be reproduced very well. According to this mechanism, CH3OCHO is primarily formed from the bimolecular reaction H + CH3OCH2OCH3 = H2 + CH3OCHOCH3. The secondary radical CH3OCHOCH3 then dissociates to CH3 and CH3OCHO, which then primarily decomposes to CH3OH and CO. 33 ACS Paragon Plus Environment


Therefore, the formation of CH3OH is the result of CH3OCHO decomposition. Decreasing the branching ratio BR1 and hence, increasing BR2, the mechanism predicts higher CH3OCHO and hence, also higher CH3OH concentrations. The reason is that CH3OCH2O radicals produced in reaction (2) can decompose by C-H bond fission to yield CH3OCHO. The present results and simulations indicate that among the two initial bond fissions of DMM decomposition, reaction (1) is to be the dominant reaction channel, although this is somehow counterintuitive, since bond-dissociation energies suggest that the second bond-fission channel should be preferred. The measured product compositions and temporal species profiles may also be used for validating DMM chemical kinetics models in the context of pyrolysis and since the present ktotal data are the result of an experimental study combining three different methods, the present ktotal data are recommended for use in combustion modelling, although uncertainties about the branching ratio remain.

Acknowledgement Financial support of this work by the German Research Foundation within the framework of the DFG research unit FOR 1993 ‘Multi-functional conversion of chemical species and energy’ (SCHU 1369/19) and the DFG research project FI1712/1-1 is also gratefully acknowledged.

References 1. 2. 3.

4.

Sarathy, S. M.; Oßwald, P.; Hansen, N.; Kohse-Höinghaus, K., Alcohol Combustion Chemistry. Progr. Energy Combust. Sci. 2014, 44, 40-102. Boehman, A. L., Developments in Production and Utilization of Dimethyl Ether for Fuel Applications. Fuel Process. Technol. 2008, 89 (12), 1243. Burger, J.; Siegert, M.; Ströfer, E.; Hasse, H., Poly(oxymethylene) Dimethyl Ethers as Components of Tailored Diesel Fuel: Properties, Synthesis and Purification Concepts. Fuel 2010, 89 (11), 33153319. Thenert, K.; Beydoun, K.; Wiesenthal, J.; Leitner, W.; Klankermayer, J., Ruthenium-Catalyzed Synthesis of Dialkoxymethane Ethers Utilizing Carbon Dioxide and Molecular Hydrogen. Angew. Chem., Int. Ed. 2016, 55 (40), 12266-12269.


Page 34 of 38

Page 35 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5.

6.

7. 8.

9. 10. 11. 12.

13. 14.

15.

16.

17. 18.

20.

21.

22. 23.

Ausfelder, F.; Beilmann, C.; Bertau, M.; Bräuninger, S.; Heinzel, A.; Hoer, R.; Koch, W.; Mahlendorf, F.; Metzelthin, A.; Peuckert, M.; et al. Energy Storage as Part of a Secure Energy Supply. ChemBioEng. Rev. 2017, 4 (3), 144-210. Härtl, M.; Seidenspinner, P.; Jacob, E.; Wachtmeister, G., Oxygenate Screening on a Heavy-Duty Diesel Engine and Emission Characteristics of Highly Oxygenated Oxymethylene Ether Fuel OME1. Fuel 2015, 153 (Supplement C), 328-335. Dias, V.; Lories, X.; Vandooren, J., Lean and Rich Premixed Dimethoxymethane/Oxygen/Argon Flames: Experimental and Modeling. Combust. Sci. Technol. 2010, 182 (4-6), 350-364. Dias, V.; Vandooren, J., Experimental and Modeling Studies of C2H4/O2/Ar, C2H4/methylal/O2/Ar and C2H4/ethylal/O2/Ar Rich Flames and the Effect of Oxygenated Additives. Combust. Flame 2011, 158 (5), 848-859. Daly, C. A.; Simmie, J. M.; Dagaut, P.; Cathonnet, M., Oxidation of Dimethoxymethane in a JetStirred Reactor. Combust. Flame 2001, 125 (3), 1106-1117. Marrodán, L.; Royo, E.; Millera, Á.; Bilbao, R.; Alzueta, M. U., High Pressure Oxidation of Dimethoxymethane. Energy Fuels 2015, 29 (5), 3507-3517. Marrodán, L.; Monge, F.; Millera, Á.; Bilbao, R.; Alzueta, M. U., Dimethoxymethane Oxidation in a Flow Reactor. Combust. Sci. Technol. 2016, 188 (4-5), 719-729. Vermeire, F. H.; Carstensen, H.-H.; Herbinet, O.; Battin-Leclerc, F.; Marin, G. B.; Van Geem, K. M., Experimental and Modeling Study of the Pyrolysis and Combustion of Dimethoxymethane. Combust. Flame 2018, 190, 270-283. Vandewiele, N. M.; Van Geem, K. M.; Reyniers, M.-F.; Marin, G. B., Genesys: Kinetic Model Construction Using Chemo-Informatics. Chem. Eng. J. 2012, 207-208, 526-538. Sun, W.; Tao, T.; Lailliau, M.; Hansen, N.; Yang, B.; Dagaut, P., Exploration of the Oxidation Chemistry of Dimethoxymethane: Jet-Stirred Reactor Experiments and Kinetic Modeling. Combust. Flame 2018, 193, 491-501. Kopp, W. A.; Kröger, L. C.; Döntgen, M.; Jacobs, S.; Burke, U.; Curran, H. J.; Alexander Heufer, K.; Leonhard, K., Detailed Kinetic Modeling of Dimethoxymethane. Part I: Ab Initio Thermochemistry and Kinetics Predictions for Key Reactions. Combust. Flame 2018, 189, 433-442. Golka, L.; Weber, I.; Olzmann, M., Pyrolysis of Dimethoxymethane and the Reaction of Dimethoxymethane with H Atoms: A Shock-Tube/ARAS/TOF-MS and Modelling Study. Proc. Combust. Inst. 2019; doi.org/10.1016/j.proci.2018.05.036. Curtiss, L. A.; Redfern, P. C.; Raghavachari, K., Gaussian-4 Theory. J. Chem. Phys. 2007, 126 (8), 084108. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Gaussian. Inc.: Wallingford, CT, 2009. Sela, P.; Peukert, S.; Herzler, J.; Fikri, M.; Schulz, C., Shock-Tube Study of the Decomposition of Tetramethylsilane Using Gas Chromatography and High-Repetition-Rate Time-of-Flight Mass Spectrometry. Phys. Chem. Chem. Phys. 2018, 20 (16), 10686-10696. Deminsky, M.; Chorkov, V.; Belov, G.; Cheshigin, I.; Knizhnik, A.; Shulakova, E.; Shulakov, M.; Iskandarova, I.; Alexandrov, V.; Petrusev, A.; et al. Chemical Workbench–Integrated Environment for Materials Science. Comput. Mater. Sci. 2003, 28 (2), 169-178. Appel, D.; Appleton, J. P., Shock Tube Studies of Deuterium Dissociation and Oxidation by Atomic Resonance Absorption Spectrophotometry. Proc. Combust. Inst. 1975, 15 (1), 701-715. Frank, P.; Just, T., High Temperature Reaction Rate for H+O2=OH+O and OH+H2=H2O+H. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 181-187.



24.

25.

26. 27.

28. 29. 30.

31. 32. 33.

34. 35. 36.

37.

38.

39.

40. 41.

Dürrstein, S. H.; Aghsaee, M.; Jerig, L.; Fikri, M.; Schulz, C., A Shock Tube with a High-RepetitionRate Time-of-Flight Mass Spectrometer for Investigations of Complex Reaction Systems. Rev. Sci. Instrum. 2011, 82 (8), 084103. Sela, P.; Shu, B.; Aghsaee, M.; Herzler, J.; Welz, O.; Fikri, M.; Schulz, C., A Single-Pulse Shock Tube Coupled with High-Repetition-Rate Time-of-Flight Mass Spectrometry and Gas Chromatography for High-Temperature Gas-Phase Kinetics Studies. Rev. Sci. Instrum. 2016, 87 (10), 105103. Cher, M.; Hollingsworth, C. S.; Sicilio, F., The Vapor Phase Reaction of Methyl Radicals with Toluene at 100-300°. J. Phys. Chem. 1966, 70 (3), 877-883. Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Frank, P.; Hayman, G.; Just, T.; Kerr, J. A.; Murrells, T.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J., Evaluated Kinetic Data for Combustion Modeling. Supplement I. J. Phys. Chem. Ref. Data 1994, 23 (6), 847-848. Tranter, R. S.; Giri, B. R.; Kiefer, J. H., Shock Tube/Time-of-Flight Mass Spectrometer for High Temperature Kinetic Studies. Rev. Sci. Instrum. 2007, 78 (3), 034101. Krizancic, I.; Haluk, M.; Cho, S. H.; Trass, O., Shock Tube Coupled to the Time-of-Flight Mass Spectrometer via a Molecular Beam Sampling System. Rev. Sci. Instrum. 1979, 50 (7), 909-915. Kiefer, J. H.; Santhanam, S.; Srinivasan, N. K.; Tranter, R. S.; Klippenstein, S. J.; Oehlschlaeger, M. A., Dissociation, Relaxation, and Incubation in the High-Temperature Pyrolysis of Ethane, and a Successful RRKM Modeling. Proc. Combust. Inst. 2005, 30 (1), 1129-1135. Lim, K. P.; Michael, J. V., The Thermal Reactions of CH3. Proc. Combust. Inst. 1994, 25, 713 – 719. Troe, J.; Ushakov, V. G., The Dissociation/Recombination Reaction CH4 (+M) ⇔ CH3 + H (+M): A Case Study for Unimolecular Rate Theory. J. Chem. Phys. 2012, 136 (21), 214309. Peukert, S. L.; Labbe, N. J.; Sivaramakrishnan, R.; Michael, J. V., Direct Measurements of Rate Constants for the Reactions of CH3 Radicals with C2H6, C2H4, and C2H2 at High Temperatures. J. Phys. Chem. A 2013, 117 (40), 10228-10238. Sivaramakrishnan, R.; Michael, J. V.; Ruscic, B., High-Temperature Rate Constants for H/D + C2H6 and C3H8. Int. J. Chem. Kinet. 2012, 44 (3), 194-205. Sutherland, J. W.; Su, M. C.; Michael, J. V., Rate Constants for H + CH4, CH3 + H2, and CH4 Dissociation at High Temperature. Int. J. Chem. Kinet. 2001, 33 (11), 669-684. Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.; Frank, P.; Just, T.; Kerr, J. A.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J., Evaluated Kinetic Data for Combustion Modelling. J. Phys. Chem. Ref. Data 1992, 21, 411 – 429. Manion, J. A.; Huie, R. E.; Levin, R. D.; al., e. NIST Chemical Kinetics Database, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899-8320. available at: https://kinetics.nist.gov/kinetics/index.jsp. Smith, G. P.; Golden, D. M., Application of RRKM Theory to the Reactions OH + NO2 + N2 → HONO2 + N2 (1) and ClO + NO2 + N2 → ClONO2 + N2 (2); a Modified Gorin Model Transition State. Int. J. Chem. Kinet. 1978, 10 (5), 489-501. Srinivasan, N. K.; Kiefer, J. H.; Tranter, R. S., Dissociation, Relaxation, and Incubation in the Pyrolysis of Neopentane: Heat of Formation for Tert-Butyl Radical. J. Phys. Chem. A 2003, 107 (10), 1532-1539. Baldwin, A. C.; Lewis, K. E.; Golden, D. M., Very-Low-Pressure Pyrolysis (VLPP) of Group IV (A) Tetramethyls: Neopentane and Tetramethyltin. Int. J. Chem. Kinet. 1979, 11 (5), 529-542. Peukert, S. L.; Sivaramakrishnan, R.; Su, M.-C.; Michael, J. V., Experiment and Theory on Methylformate and Methylacetate Kinetics at High Temperatures: Rate Constants for H-Atom Abstraction and Thermal Decomposition. Combust. Flame 2012, 159 (7), 2312-2323.


Page 36 of 38

Page 37 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


42.

43.

Ren, W.; Dames, E.; Hyland, D.; Davidson, D. F.; Hanson, R. K., Shock Tube Study of Methanol, Methyl Formate Pyrolysis: CH3OH and CO Time-History Measurements. Combust. Flame 2013, 160 (12), 2669-2679. Glaude, P. A.; Pitz, W. J.; Thomson, M. J., Chemical Kinetic Modeling of Dimethyl Carbonate in an Opposed-Flow Diffusion Flame. Proc. Combust. Inst. 2005, 30 (1), 1111-1118.



TOC Graphic


Page 38 of 38

Direct Measurement of High-Temperature Rate Constants of the

Recommend Documents