Low-temperature Oxidation Of Dimethyl Ether Research Articles

Unsaturated Penta-Coordinated Mo5c5+ Sites Enabled Low-Temperature Oxidation of C-H Bonds in Ethers.

Selective oxidation of C-H bonds under mild conditions is one of the most important and challenging issues in utilization of energy-related molecules. Molybdenum oxide nanostructures containing Mo5+ species are effective for these reactions, but the accurate identification of the structure of active Mo5+ species and the catalytic mechanism remain unclear. Herein, unsaturated penta-coordinated Mo5c5+ with a high fraction in MoOx fabricated by the hydrothermal method were identified as the active sites for low-temperature oxidation of dimethyl ether (DME) by the deep correlation of characterizations, density functional theory calculations, and activity results, giving a methyl formate selectivity of 96.3% and DME conversion of 12.5% at unreported 110 °C. Low-temperature electron spin resonance (ESR) and quasi in situ X-ray photoelectron spectra (XPS) with the designed experiments confirm that the Mo5c5+ species can be formed in situ. Molybdenum located at the pentachronic site is preferable to significantly promote the oxidation of the C-H bond in CH3O* at lower temperatures.

Quantification of Key Peroxy and Hydroperoxide Intermediates in the Low-Temperature Oxidation of Dimethyl Ether

Dimethyl ether (DME) oxidation is a model chemical system with a small number of prototypical reaction intermediates that also has practical importance for low-carbon transportation. Although it has been studied experimentally and theoretically, ambiguity remains in the relative importance of competing DME oxidation pathways in the low-temperature autoignition regime. To focus on the primary reactions in DME autoignition, we measured the time-resolved concentration of five intermediates, CH3OCH2OO (ROO), OOCH2OCH2OOH (OOQOOH), HOOCH2OCHO (hydroperoxymethyl formate, HPMF), CH2O, and CH3OCHO (methyl formate, MF), from photolytically initiated experiments. We performed these studies at P = 10 bar and T = 450-575 K, using a high-pressure photolysis reactor coupled to a time-of-flight mass spectrometer with tunable vacuum-ultraviolet synchrotron ionization at the Advanced Light Source. Our measurements reveal that the timescale of ROO decay and product formation is much shorter than predicted by current DME combustion models. The models also strongly underpredict the observed yields of CH2O and MF and do not capture the temperature dependence of OOQOOH and HPMF yields. Adding the ROO + OH → RO + HO2 reaction to the chemical mechanism (with a rate coefficient approximated from similar reactions) improves the prediction of MF. Increasing the rate coefficients of ROO ↔ QOOH and QOOH + O2 ↔ OOQOOH reactions brings the model predictions closer to experimental observations for OOQOOH and HPMF, while increasing the rate coefficient for the QOOH → 2CH2O + OH reaction is needed to improve the predictions of formaldehyde. To aid future quantification of DME oxidation intermediates by photoionization mass spectrometry, we report experimentally determined ionization cross-sections for ROO, OOQOOH, and HPMF.

Inhibiting and promoting effects of NO on dimethyl ether and dimethoxymethane oxidation in a plug-flow reactor

The effects of NO addition (1000, 2000 ppm) on the low-temperature oxidation of dimethyl ether (DME) and dimethoxymethane (DMM), as particular cases of oxymethylene ethers (OMEn) with n = 0 and 1, have been investigated in a plug-flow reactor at near-atmospheric pressure in a temperature range of 400–1000 K. An in-situ electron ionization molecular-beam mass spectrometer (EI-MBMS) was used to measure the reactants, intermediates, and products, with particular attention on nitrogenous species that were scarcely detected previously. Explorative modeling with published mechanisms was performed, indicating the necessity of further model development. Potential kinetic fuel/NO interactions are discussed based on the experimental observations. The results reveal an overall inhibiting effect of NO addition on DME reactivity in the low-temperature regime, but a pronounced promoting effect at higher temperatures. For DMM, a similar temperature-dependent effect of NO was observed, but only for high NO concentration (2000 ppm). NO addition significantly suppresses the formation of hydrocarbon intermediates for both DME and DMM, but remarkably promotes the formation of methyl formate and methanol for DME. Several nitrogenous species were detected upon NO addition. The interactions of NO + HO2 and NO + OH, together with the regeneration routes of NO, are thought to be influential for both DME and DMM oxidation, while the significance of the NO + RO2 (R, fuel radical) reaction depends on the reactivity of the respective RO2 radical of DME and DMM. These results contribute to the understanding of OMEn/NO interactions and serve as a basis for further model development by providing new and detailed speciation data for DME/NO and DMM/NO oxidation.

Influence of ozone addition on the low-temperature oxidation of dimethyl ether in a jet-stirred reactor

The influence of ozone addition on the low-temperature oxidation of dimethyl ether (DME) was investigated experimentally in an atmospheric-pressure jet-stirred reactor, over the temperature range of 400–800 K. Detailed speciation information was obtained by employing synchrotron vacuum ultraviolet photoionization mass spectrometry. Experimental results revealed that the ozone addition had a positive influence on the production of the highly reactive intermediates. Moreover, the low-temperature reactivity of DME was significantly enhanced, which resulted in the broadening of the temperature window of fuel consumption and intermediates formation at lower temperatures. Therefore, novel experimental data of the low temperature regime (400–500 K) could be obtained. The data set of this special temperature regime yielded insights into the DME low-temperature kinetics, which were further supported with modeling analysis based on two existing DME models (Metcalfe et al., 2013; Wang et al., 2015) combined with an ozone sub-mechanism (Zhao et al., 2016). The analysis showed that temperature-sensitive reactions such as the second oxygen channel could be nearly “frozen” at this low temperature (T < 440 K). Furthermore, the production of some intermediates was found to be strongly governed by reaction pairs, such as CH3OCH2 + O2 = CH3OCH2O2 and CH3OCH2 + O2 = 2CH2O + OH for the CH2O formation. This finding could be useful for examining branching ratios in both models, and the analysis suggested the further modification of the branching ratios for the oxygen addition to CH3OCH2O2 pathways and the CH3OCH2O2 self-reactions were required. Finally, the influences of the O3 addition in the sensitive reactions of the fuel initial low-temperature oxidation were investigated in this work. It was interesting to note that O3 addition could change the dominating reactions in the initial low-temperature oxidation, by the addition of some O3-related pathways with relatively high sensitivity.

Quantification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Elusive Intermediates during Low-Temperature Oxidation of Dimethyl Ether.

This work provides new temperature-dependent mole fractions of elusive intermediates relevant to the low-temperature oxidation of dimethyl ether (DME). It extends the previous study of Moshammer et al. [ J. Phys. Chem. A 2015 , 119 , 7361 - 7374 ] in which a combination of a jet-stirred reactor and molecular beam mass spectrometry with single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation was used to identify (but not quantify) several highly oxygenated species. Here, temperature-dependent concentration profiles of 17 components were determined in the range of 450-1000 K and compared to up-to-date kinetic modeling results. Special emphasis is paid toward the validation and application of a theoretical method for predicting photoionization cross sections that are hard to obtain experimentally but essential to turn mass spectral data into mole fraction profiles. The presented approach enabled the quantification of the hydroperoxymethyl formate (HOOCH2OCH2O), which is a key intermediate in the low-temperature oxidation of DME. The quantification of this keto-hydroperoxide together with the temperature-dependent concentration profiles of other intermediates including H2O2, HCOOH, CH3OCHO, and CH3OOH reveals new opportunities for the development of a next-generation DME combustion chemistry mechanism.

Low-Temperature Oxidation of Dimethyl Ether to Polyoxymethylene Dimethyl Ethers over CNT-Supported Rhenium Catalyst

Due to its excellent conductivity, good thermal stability and large specific surface area, carbon nano-tubes (CNTs) were selected as support to prepare a Re-based catalyst for dimethyl ether (DME) direct oxidation to polyoxymethylene dimethyl ethers (DMMx). The catalyst performance was tested in a continuous flow type fixed-bed reactor. H3PW12O40 (PW12) was used to modify Re/CNTs to improve its activity and selectivity. The effects of PW12 content, reaction temperature, gas hourly space velocity (GHSV) and reaction time on DME oxidation to DMMx were investigated. The results showed that modification of CNT-supported Re with 30% PW12 significantly increased the selectivity of DMM and DMM2 up to 59.0% from 6.6% with a DME conversion of 8.9%; besides that, there was no COx production observed in the reaction under the optimum conditions of 513 K and 1800 h−1. The techniques of XRD, BET, NH3-TPD, H2-TPR, XPS, TEM and SEM were used to characterize the structure, surface properties and morphology of the catalysts. The optimum amount of weak acid sites and redox sites promotes the synthesis of DMM and DMM2 from DME direct oxidation.

New Insight into Competition between Decomposition Pathways of Hydroperoxymethyl Formate in Low Temperature DME Oxidation

Hydroperoxymethyl formate is a crucial intermediate formed during the low-temperature oxidation of dimethyl ether. The decomposition pathways of HOOCH2OCHO were calculated at QCISD(T)/CBS//B3LYP/6-311++G(d,p) level. The temperature- and pressure-dependent rate constants are computed using microcanonical variational transition state theory coupled with the RRKM/master equation calculations. The calculations show that a pathway leads to the formation of formic acid and a Criegee intermediate does exist, besides the direct dissociation channel to OH and OCH2OCHO radicals. However, formation of the Criegee intermediate has never been considered as an intermediate in dimethyl ether combustion before. The computed rate constants indicate that the newly confirmed pathway is competitive to the direct dissociation route and it is promising to reduce the low-temperature oxidation reactivity. Also electronic effect of groups, e.g. -CHO and O atom, is taken into account. Moreover, Hirshfeld atomic charge and natural bond order analysis are performed to explain this phenomenon from a perspective of chemical nature.

Experimental and Modeling Investigation of the Low-Temperature Oxidation of Dimethyl Ether.

The oxidation of dimethyl ether (DME) was studied using a jet-stirred reactor over a wide range of conditions: temperatures from 500 to 1100 K; equivalence ratios of 0.25, 1, and 2; residence time of 2 s; pressure of 106.7 kPa (close to the atmospheric pressure); and an inlet fuel mole fraction of 0.02 (with high dilution in helium). Reaction products were quantified using two analysis methods: gas chromatography and continuous wave cavity ring-down spectroscopy (cw-CRDS). cw-CRDS enabled the quantification of formaldehyde, which is one of the major products from DME oxidation, as well as that of hydrogen peroxide, which is an important branching agent in low-temperature oxidation chemistry. Experimental data were compared with data computed using models from the literature with important deviations being observed for the reactivity at low-temperature. A new detailed kinetic model for the oxidation of DME was developed in this study. Kinetic parameters used in this model were taken from literature or calculated in the present work using quantum calculations. This new model enables a better prediction of the reactivity in the low-temperature region. Under the present JSR conditions, error bars on predictions were given. Simulations were also successfully compared with experimental flow reactor, jet-stirred reactor, shock tube, rapid compression machine, and flame data from literature. The kinetic analysis of the model enabled the highlighting of some specificities of the oxidation chemistry of DME: (1) the early reactivity which is observed at very low-temperature (e.g., compared to propane) is explained by the absence of inhibiting reaction of the radical directly obtained from the fuel (by H atom abstraction) with oxygen yielding an olefin + HO2·; (2) the low-temperature reactivity is driven by the relative importance of the second addition to O2 (promoting the reactivity through branching chain) and the competitive decomposition reactions with an inhibiting effect.

Detection and Identification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether.

In this paper we report the detection and identification of the keto-hydroperoxide (hydroperoxymethyl formate, HPMF, HOOCH2OCHO) and other partially oxidized intermediate species arising from the low-temperature (540 K) oxidation of dimethyl ether (DME). These observations were made possible by coupling a jet-stirred reactor with molecular-beam sampling capabilities, operated near atmospheric pressure, to a reflectron time-of-flight mass spectrometer that employs single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation. On the basis of experimentally observed ionization thresholds and fragmentation appearance energies, interpreted with the aid of ab initio calculations, we have identified HPMF and its conceivable decomposition products HC(O)O(O)CH (formic acid anhydride), HC(O)OOH (performic acid), and HOC(O)OH (carbonic acid). Other intermediates that were detected and identified include HC(O)OCH3 (methyl formate), cycl-CH2-O-CH2-O- (1,3-dioxetane), CH3OOH (methyl hydroperoxide), HC(O)OH (formic acid), and H2O2 (hydrogen peroxide). We show that the theoretical characterization of multiple conformeric structures of some intermediates is required when interpreting the experimentally observed ionization thresholds, and a simple method is presented for estimating the importance of multiple conformers at the estimated temperature (∼100 K) of the present molecular beam. We also discuss possible formation pathways of the detected species: for example, supported by potential energy surface calculations, we show that performic acid may be a minor channel of the O2 + ĊH2OCH2OOH reaction, resulting from the decomposition of the HOOCH2OĊHOOH intermediate, which predominantly leads to the HPMF.

Experimental and kinetic modeling study of the low- and intermediate-temperature oxidation of dimethyl ether

Recent experiments on low-temperature oxidation of dimethyl ether (DME) at atmospheric pressure reveal much lower fuel conversion than the predictions of all published models. Consistent with previous measurements, the present work on DME oxidation in a laminar flow reactor between 400 and 1160K at atmospheric pressure also confirmed this behavior. To reduce the gap between model predictions and experimental results, both regarding key oxygenated species and fuel conversion, an extended and updated kinetic model of DME oxidation was developed from the widely used model of Z. Zhao, M. Chaos, A. Kazakov, F.L. Dryer, Int. J. Chem. Kinet. 40 (2008) 1–18. The development of the model focused on the reaction sequences that affect the low-temperature oxidation reactivity. In particular, the reactivity of DME oxidation in the low-temperature regime and at atmospheric pressure can now be predicted. This is possible upon the inclusion of an additional, chemically-activated pathway of CH3OCH2 reaction with O2, as well as of the new decomposition chemistry of OCH2OCHO radical, investigated theoretically in this work. The present model was examined against species concentrations in DME oxidation from low to intermediate temperatures and shock tube ignition delay times. It shows satisfactory performance in reproducing the respective literature data.

Low-temperature deep oxidation of olefins and DME over cobalt ferrite

This work reports the facile synthesis of cobalt ferrite catalysts by pulsed spray evaporation chemical vapor deposition (PSE-CVD) for low-temperature oxidation of dimethyl ether and olefin exhaust emission. To better understand the structure–performance relationship, the prepared thin films were comprehensively characterized in terms of phases, surfaces and redox properties. A well-crystallized inverse spinel structure was revealed by both XRD and Raman analyses. Helium ion microscopy (HIM) images show that the obtained films have cauliflower structure with apparent open porosity. The catalytic performance of the as-prepared cobalt ferrite was evaluated with a fixed-bed quartz reactor–FTIR system. The results indicate that the cobalt ferrites were very active for the total oxidation of propene, n-butene and DME at low temperature and no CO was observed during the oxidation processes. The attractive performance of cobalt ferrite catalysts is attributed to a redox mechanism which involves the synergistic effect of the morphology, reducibility, and lattice oxygen mobility.

Low-temperature oxidation of dimethyl ether to methyl formate with high selectivity over MoO3-SnO2 catalysts

Low-temperature oxidation of dimethyl ether (DME) to methyl formate (MF) with high selectivity was realized in a continuous flow fixed-bed reactor over the multifunctional MoO3-SnO2 catalysts designed and prepared intentionally. The effect of the preparation methods including mechanical mixing, co-precipitation and co-precipitation-impregnation on the catalyst activity was investigated. The results showed that the selectivity to MF reaches 94.1% at 160°C over the catalyst prepared by co-precipitation-impregnation, with DME conversion of 33.9% and absence of COx in the products. The results of NH3-TPD, CO2-TPD and H2-TPR characterizations indicated that the catalysts prepared by various methods are also obviously different in the surface acidic, alkaline and redox properties. The results of Raman, XRD and TEM revealed that MoO3 structure and particle sizes have a significant influence on the catalyst activity; small particle size and oligomeric MoO3 may be responsible for the high activity of the MoO3-SnO2 catalysts from co-precipitation-impregnation in the selective oxidation of DME to MF at low temperature.

Numerical Characteristics of the Low-Temperature Oxidation of Dimethyl Ether with Air

In the oxidation of dimethyl ether at the first stage of autoignition at low temperatures, the branching process involves not only hydroxyl radicals but also H atoms.

Measurements of H2O2 in low temperature dimethyl ether oxidation

H2O2 is one of the most important species in dimethyl ether (DME) oxidation, acting not only as a marker for low temperature kinetic activity but also responsible for the “hot ignition” transition. This study reports, for the first time, direct measurements of H2O2 and CH3OCHO, among other intermediate species concentrations in helium-diluted DME oxidation in an atmospheric pressure flow reactor from 490 to 750K, using molecular beam electron-ionization mass spectrometry (MBMS). H2O2 measurements were directly calibrated, while a number of other species were quantified by both MBMS and micro gas chromatography to achieve cross-validation of the measurements. Experimental results were compared to two different DME kinetic models with an updated rate coefficient for the H+DME reaction, under both zero-dimensional and two-dimensional physical model assumptions. The results confirm that low and intermediate temperature DME oxidation produces significant amounts of H2O2. Peroxide, as well as O2, DME, CO, and CH3OCHO profiles are reasonably well predicted, though profile predictions for H2/CO2 and CH2O are poor above and below ∼625K, respectively. The effect of the collisional efficiencies for the H+O2+M=HO2+M reaction on DME oxidation was investigated by replacing 20% He with 20% CO2. Observed changes in measured H2O2 concentrations agree well with model predictions. The new experimental characterizations of important intermediate species including H2O2, CH2O and CH3OCHO, and a path flux analysis of the oxidation pathways of DME support that kinetic parameters for decomposition reactions of HOCH2OCO and HCOOH directly to CO2 may be responsible for model under-prediction of CO2. The H abstraction reactions for DME and/or CH2O and the unimolecular decomposition of HOCH2O merit further scrutiny towards improving the prediction of H2 formation.

Mass spectrometric investigation of the low-temperature dimethyl ether oxidation in an atmospheric pressure laminar flow reactor

Low-temperature combustion is a major strategy today to reduce both soot and NOx emission. Kinetic reaction models for low-temperature combustion which are validated against a wide range of experimental data are necessary e.g. for control purposes or as a basis for subsequent model reduction.In this study, the low-temperature oxidation of dimethyl ether in a highly diluted gas mixture was investigated experimentally in an atmospheric laminar flow reactor. The respective gas composition was analyzed by a time-of-flight mass spectrometer. This technique allows detection of all species simultaneously within the investigated temperature regime. Stoichiometries of ϕ=0.8, 1.0, and 1.2 were studied with high temperature resolution in the range of 400–1200K, and quantitative species mole fraction profiles have been determined.This wide temperature range comprises the different kinetic regimes occurring during the DME oxidation, which have been clearly resolved. The distinct negative temperature coefficient (NTC) region of the system was observed and extensive speciation is available. Special attention is given to species only occurring in the low-temperature region including formic acid and methyl formate.

First-principles-derived kinetics of the reactions involved in low-temperature dimethyl ether oxidation

Spin-polarised density functional theory (DFT-B3LYP) energies, harmonic vibrational frequencies, and moments of inertia are used to construct modified Arrhenius rate expressions for elementary steps in chain-propagation and chain-branching pathways for dimethyl ether combustion. Barrierless reactions were treated with variational RRKM theory, and global kinetics were modeled using master equation and perfectly stirred reactor simulations. Our kinetics analysis suggests that the bottleneck along the chain propagation path is the isomerisation of CH3OCH2OO, contrary to earlier interpretations. Comparing the rate constants for competing decomposition pathways of the key chain-branching intermediate hydroperoxymethyl formate (HPMF), we find that formation of formic acid and the atmospherically relevant Criegee intermediate (CH2OO) via a H-bonded adduct may be more favourable than O–O bond scission. Since the latter forms a source of a second OH radical beyond that supplied in chain propagation, which is necessary for explosive combustion, the more favourable pathway to formic acid may inhibit autoignition of the fuel. We predict that the HPMF O–O scission product, OCH2OC(=O)H, most likely directly dissociates to HCO + HC(=O)OH. This implies an overabundance of CO at 550–700 K, since HC(=O)OH is a fairly stable product in this temperature range and facile H abstraction from HCO leads to CO. We find that CO2 product yields are sensitive to the creation of CH2OO and that creation of CH2OO is competitive with the O–O scission reaction.

Hydrogen as an ignition-controlling agent for HCCI combustion engine by suppressing the low-temperature oxidation

Homogeneous charge compression ignition (HCCI) combustion enables internal combustion engines to achieve higher thermal efficiency and lower NO x emission than with conventional combustion systems. Controlling the ignition timing in accordance with the operating conditions is crucial for utilizing HCCI combustion engines. Adding hydrogen-containing gas is known to retard the autoignition of dimethyl ether (DME) considerably. The effective ignition control by hydrogen can expand the operation range of equivalence ratios and engine loads in HCCI combustion. This research investigated the mechanisms in the ignition control by the chemical kinetics analysis. The results show that the retarded ignition can be attributed to a consumption of OH by hydrogen during low-temperature oxidation of DME. The decreased OH concentration leads to retarded heat release and delays the onset of the high-temperature oxidation.

Performance of NOX reduction catalysts with simulated dimethyl ether diesel engine exhaust gas

Characteristics of dimethyl ether oxidation and formaldehyde formation with NOX reduction catalysts were investigated in comparison with noble metal catalysts. The catalysts used in this research were Co and Sn as NOX catalysts and Pt, Pd, Rh as noble metal catalysts. Moreover, all the catalysts were loaded with gamma alumina (γ-Al2O3). In addition, a pure γ-Al2O3 catalyst was also used. Laboratory experiments were conducted by using the above catalysts in the presence of oxygen-rich simulated dimethyl ether engine exhaust gas. The catalyst temperature and the concentration of NO in the simulated exhaust gas were selected as experimental parameters. The experimental results showed that low-temperature dimethyl ether oxidation was possible with Pt, Pd and Rh catalysts, whereas higher-temperature dimethyl ether oxidation was observed with Co, Sn and pure γ-Al2O3 catalysts. A large amount of formaldehyde was formed during the use of NOX reduction catalysts over a certain temperature range. However, the noble metal catalysts did not show any formation of formaldehyde. The effect of NO on dimethyl ether oxidation and formaldehyde formation with NOX reduction catalysts was significant. The dimethyl ether oxidation ratio became lower and the amount of formaldehyde formation increased with increase in NO in the simulated exhaust gas.

Formate species in the low-temperature oxidation of dimethyl ether

The oxidation of dimethyl ether (DME, 340 ppm in 10% O 2) has been studied experimentally in an atmospheric pressure laminar flow reactor in the temperature range from 240°C to 700°C for residence times in the range 2–4 s. The influence of nitric oxide additions up to 620 ppm to the feed gases has also been investigated. Products of reaction were determined by FTIR. In the absence of NO, reaction is first detected at about 260°C. The products in the low-temperature region include formaldehyde (HCHO), and formic acid (HCOOH). The addition of NO leads to the appearance of methyl formate (CH 3OCHO). While the overall behaviour of the system can be explained qualitatively in terms of typical low-temperature hydrocarbon ignition, recently published chemical kinetic models for DME ignition do not allow for the formation of these formate species. We find no experimental evidence for the formation of hydroperoxymethyl formate (HPMF, HOOCH 2OCHO) which is predicted by the models to be a significant stable intermediate at temperatures below 350°C. Since both formic acid and methyl formate have potentially harmful health effects, these observations may have significant implications for use of DME as a diesel fuel.