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

Tracking the reaction networks of acetaldehyde oxide and glyoxal oxide Criegee intermediates in the ozone-assisted oxidation reaction of crotonaldehyde

The reaction of unsaturated compounds with ozone (O3) is recognized to lead to the formation of Criegee intermediates (CIs), which play a key role in controlling the atmospheric budget of hydroxyl radicals and secondary organic aerosols. The reaction network of two CIs with different functionality, i.e. acetaldehyde oxide (CH3CHOO) and glyoxal oxide (CHOCHOO) formed in the ozone-assisted oxidation reaction of crotanaldehyde (CA), is investigated over a temperature range between 390 K and 840 K in an atmospheric pressure jet-stirred reactor (JSR) at a residence time of 1.3 s, stoichiometry of 0.5 with a mixture of 1% crotonaldehyde, 10% O2, at an fixed ozone concentration of 1000 ppm and 89% Ar dilution. Molecular-beam mass spectrometry in conjunction with single photon tunable synchrotron vacuum-ultraviolet (VUV) radiation is used to identify elusive intermediates by means of experimental photoionization energy scans and ab initio threshold energy calculations for isomer identification. Addition of ozone (1000 ppm) is observed to trigger the oxidation of CA already at 390 K, which is below the temperature where the oxidation reaction of CA was observed in the absence of ozone. The observed CA + O3 product, C4H6O4, is found to be linked to a ketohydroperoxide (2-hydroperoxy-3-oxobutanal) resulting from the isomerization of the primary ozonide. Products corresponding to the CIs uni- and bi-molecular reactions were observed and identified. A network of CI reactions is identified in the temperature region below 600 K, characterized by CIs bimolecular reactions with species like aldehydes, i.e., formaldehyde, acetaldehyde, and crotonaldehyde and alkenes, i.e., ethene and propene. The region below 600 K is also characterized by the formation of important amounts of typical low-temperature oxidation products, such as hydrogen peroxide (H2O2), methyl hydroperoxide (CH3OOH), and ethyl hydroperoxide (C2H5OOH). Detection of additional oxygenated species such as alcohols, ketene, and aldehydes are indicative of multiple active oxidation routes. This study provides important information about the initial step involved in the CIs assisted oligomerization reactions in complex reactive environments where CIs with different functionalities are reacting simultaneously. It provides new mechanistic insights into ozone-assisted oxidation reactions of unsaturated aldehydes, which is critical for the development of improved atmospheric and combustion kinetics models.

The photoionization of methoxymethanol: Fingerprinting a reactive C2 oxygenate in a complex reactive mixture

Methoxymethanol (CH3OCH2OH) is a reactive C2 ether-alcohol that is formed by coupling events in both heterogeneous and homogeneous systems. It is found in complex reactive environments-for example those associated with catalytic reactors, combustion systems, and liquid-phase mixtures of oxygenates. Using tunable synchrotron-generated vacuum-ultraviolet photons between 10.0 and 11.5 eV, we report on the photoionization spectroscopy of methoxymethanol. We determine that the lowest-energy photoionization process is the dissociative ionization of methoxymethanol via H-atom loss to produce [C2H5O2]+, a fragment cation with a mass-to-charge ratio (m/z) = 61.029. We measure the appearance energy of this fragment ion to be 10.24 ± 0.05 eV. The parent cation is not detected in the energy range examined. To elucidate the origin of the m/z = 61.029 (C2H5O2) fragment, we used automated electronic structure calculations to identify key stationary points on the cation potential energy surface and compute conformer-specific microcanonical rate coefficients for the important unimolecular processes. The calculated H-atom dissociation pathway results in a [C2H5O2]+ fragment appearance at 10.21 eV, in excellent agreement with experimental results.

Atmospheric Oxidation of Hydroperoxy Amides

Recently, hydroperoxy amides were identified as major products of OH-initiated autoxidation of tertiary amines in the atmosphere. The formation mechanism is analogous to that found for ethers and sulfides but substantially faster. However, the atmospheric fate of the hydroperoxy amides remains unknown. Using high-level theoretical methods, we study the most likely OH-initiated oxidation pathways of the hydroperoxy and dihydroperoxy amides derived from trimethylamine autoxidation. Overall, we find that the OH-initiated oxidation of the hydroperoxy amides predominantly leads to the formation of imides under NO-dominated conditions and more highly oxidized hydroperoxy amides under HO2-dominated conditions. Unimolecular reactions are found to be surprisingly slow, likely due to the restricting, planar structure of the amide moiety.

Combustion, Chemistry, and Carbon Neutrality

Combustion is a reactive oxidation process that releases energy bound in chemical compounds used as fuels─energy that is needed for power generation, transportation, heating, and industrial purposes. Because of greenhouse gas and local pollutant emissions associated with fossil fuels, combustion science and applications are challenged to abandon conventional pathways and to adapt toward the demand of future carbon neutrality. For the design of efficient, low-emission processes, understanding the details of the relevant chemical transformations is essential. Comprehensive knowledge gained from decades of fossil-fuel combustion research includes general principles for establishing and validating reaction mechanisms and process models, relying on both theory and experiments with a suite of analytic monitoring and sensing techniques. Such knowledge can be advantageously applied and extended to configure, analyze, and control new systems using different, nonfossil, potentially zero-carbon fuels. Understanding the impact of combustion and its links with chemistry needs some background. The introduction therefore combines information on exemplary cultural and technological achievements using combustion and on nature and effects of combustion emissions. Subsequently, the methodology of combustion chemistry research is described. A major part is devoted to fuels, followed by a discussion of selected combustion applications, illustrating the chemical information needed for the future.

Exploring the Effect of Different Reactivity Promoters on the Oxidation of Ammonia in a Jet-Stirred Reactor

The low reactivity of ammonia (NH₃) is the main barrier to applying neat NH₃ as fuel in technical applications, such as internal combustion engines and gas turbines. Introducing combustion promoters as additives in NH₃-based fuel can be a feasible solution. In this work, the oxidation of ammonia by adding different reactivity promoters, i.e., hydrogen (H₂), methane (CH₄), and methanol (CH₃OH), was investigated in a jet-stirred reactor (JSR) at temperatures between 700 and 1200 K and at a pressure of 1 bar. The effect of ozone (O₃) was also studied, starting from an extremely low temperature (450 K). Species mole fraction profiles as a function of the temperature were measured by molecular-beam mass spectrometry (MBMS). With the help of the promoters, NH₃ consumption can be triggered at lower temperatures than in the neat NH₃ case. CH₃OH has the most prominent effect on enhancing the reactivity, followed by H₂ and CH₄. Furthermore, two-stage NH₃ consumption was observed in NH₃/CH₃OH blends, whereas no such phenomenon was found by adding H₂ or CH₄. The mechanism constructed in this work can reasonably reproduce the promoting effect of the additives on NH₃ oxidation. The cyanide chemistry is validated by the measurement of HCN and HNCO. The reaction CH₂O + NH₂ ⇄ HCO + NH₃ is responsible for the underestimation of CH₂O in NH₃/CH₄ fuel blends. The discrepancies observed in the modeling of NH₃ fuel blends are mainly due to the deviations in the neat NH₃ case. The total rate coefficient and the branching ratio of NH₂ + HO₂ are still controversial. The high branching fraction of the chain-propagating channel NH₂ + HO₂ ⇄ H₂NO + OH improves the model performance under low-pressure JSR conditions for neat NH₃ but overestimates the reactivity for NH₃ fuel blends. Based on this mechanism, the reaction pathway and rate of production analyses were conducted. The HONO-related reaction routine was found to be activated uniquely by adding CH₃OH, which enhances the reactivity most significantly. It was observed from the experiment that adding ozone to the oxidant can effectively initiate NH₃ consumption at temperatures below 450 K but unexpectedly inhibit the NH₃ consumption at temperatures higher than 900 K. The preliminary mechanism reveals that adding the elementary reactions between NH₃-related species and O₃ is effective for improving the model performance, but their rate coefficients have to be refined.

Plasma-Assisted Chemical-Looping Combustion: Low-Temperature Methane and Ethylene Oxidation with Nickel Oxide

The chemical reaction network of low-temperature plasma-assisted oxidation of methane (CH₄) and ethylene (C₂H₄) with nickel oxide (NiO) was investigated in a heated plasma reactor through time-dependent species measurements by electron-ionization molecular beam mass spectrometry (EI-MBMS). Methane (ethylene) oxidation by NiO was explored in temperature ranges from 300-700 °C (300-500 °C) and 300-800 °C (300-600 °C) for the plasma and nonplasma conditions. Significant enhancement of methane oxidation was observed with plasma between 400 and 500 °C, where no oxidation was observed under nonplasma conditions. For the oxidation of methane at higher temperatures, three different oxidation stages were observed: (I) a period of complete oxidation, (II) a period of incomplete CO oxidation, and (III) a period of carbon buildup. For the C₂H₄ experiments, and unlike the CH₄ experiments, the plasma resulted in a significant amount of new intermediate oxygenated species, such as CH₂O, CH₃OH, C₂H₄O, and C₂H₆O. Carbon deposits were observed under both methane and ethylene conditions and verified by X-ray photoelectron spectroscopy (XPS). ReaxFF (reactive force field) simulations were performed for the oxidation of CH₄ and C₂H₄ in a nonplasma environment. The simulated intermediates and products largely agree with the species measured in the experiments, though the predicted intermediate oxygenated species such as CH₂O and C₂H₆O were not observed in experiments under nonplasma conditions. A reaction pathway analysis for CH₄ and C₂H₄ reacting with NiO was created based on the observed species from the MBMS spectra along with ReaxFF simulations.

A Comparative Study on the Combustion Chemistry of Two Bio-hybrid Fuels: 1,3-Dioxane and 1,3-Dioxolane

Bio-hybrid fuels are a promising solution to accomplish a carbon-neutral and low-emission future for the transportation sector. Two potential candidates are the heterocyclic acetals 1,3-dioxane (C₄H₈O₂) and 1,3-dioxolane (C₃H₆O₂), which can be produced from the combination of biobased feedstocks, carbon dioxide, and renewable electricity. In this work, comprehensive experimental and numerical investigations of 1,3-dioxane and 1,3-dioxolane were performed to support their application in internal combustion engines. Ignition delay times and laminar flame speeds were measured to reveal the combustion chemistry on the macroscale, while speciation measurements in a jet-stirred reactor and ethylene-based counterflow diffusion flames provided insights into combustion chemistry and pollutant formation on the microscale. Comparing the experimental and numerical data using either available or proposed kinetic models revealed that the combustion chemistry and pollutant formation differ substantially between 1,3-dioxane and 1,3-dioxolane, although their molecular structures are similar. For example, 1,3-dioxane showed higher reactivity in the low-temperature regime (500-800 K), while 1,3-dioxolane addition to ethylene increased polycyclic aromatic hydrocarbons and soot formation in high-temperature (>800 K) counterflow diffusion flames. Reaction pathway analyses were performed to examine and explain the differences between these two bio-hybrid fuels, which originate from the chemical bond dissociation energies in their molecular structures.

Algorithmic Explorations of Unimolecular and Bimolecular Reaction Spaces

Algorithmic reaction exploration based on transition state searches has already made inroads into many niche applications, but its potential as a general-purpose tool is still largely unrealized. Computational cost and the absence of benchmark problems involving larger molecules remain obstacles to further progress. Here an ultra-low cost exploration algorithm is implemented and used to explore the reactivity of unimolecular and bimolecular reactants, comprising a total of 581 reactions involving 51 distinct reactants. The algorithm discovers all established reaction pathways, where such comparisons are possible, while also revealing a much richer reactivity landscape, including lower barrier reaction pathways and a strong dependence of reaction conformation in the apparent barriers of the reported reactions. The diversity of these benchmarks illustrate that reaction exploration algorithms are approaching general-purpose capability.

Dramatic Conformer-Dependent Reactivity of the Acetaldehyde Oxide Criegee Intermediate with Dimethylamine Via a 1,2-Insertion Mechanism

The reactivity of carbonyl oxides has previously been shown to exhibit strong conformer and substituent dependencies. Through a combination of synchrotron-multiplexed photoionization mass spectrometry experiments (298 K and 4 Torr) and high-level theory [CCSD(T)-F12/cc-pVTZ-F12//B2PLYP-D3/cc-pVTZ with an added CCSDT(Q) correction], we explore the conformer dependence of the reaction of acetaldehyde oxide (CH₃CHOO) with dimethylamine (DMA). The experimental data support the theoretically predicted 1,2-insertion mechanism and the formation of an amine-functionalized hydroperoxide reaction product. Tunable-vacuum ultraviolet photoionization probing of anti- or anti- + syn-CH₃CHOO reveals a strong conformer dependence of the title reaction. The rate coefficient of DMA with anti-CH₃CHOO is predicted to exceed that for the reaction with syn-CH₃CHOO by a factor of ∼34,000, which is attributed to submerged barrier (syn) versus barrierless (anti) mechanisms for energetically downhill reactions.

Identification of the acetaldehyde oxide Criegee intermediate reaction network in the ozone-assisted low-temperature oxidation of trans-2-butene

Uni- and bi-molecular reactions involving Criegee intermediates (CIs) have been the focus of many studies due to the role these molecules play in atmospheric chemistry. The reactivity of CIs is known to strongly depend on their structure. The reaction network of the second simplest CI, acetaldehyde oxide (CH₃CHOO), is investigated in this work in an atmospheric pressure jet-stirred reactor (JSR) during the ozonolysis of trans-2-butene to explore the kinetic pathways relevant to atmospheric chemistry and low-temperature combustion. The mole fraction profiles of reactants, intermediates, and final products are determined by means of molecular-beam mass spectrometry in conjunction with single-photon ionization employing tunable synchrotron-generated vacuum ultraviolet radiation. A network of CI reactions is identified in the temperature region below 600 K, characterized by CI addition to trans-2-butene, water, formaldehyde, formic acid, and methanol. No sequential additions of the CH₃CHOO CI are observed, in contrast with the reactivity of the simplest CI (H₂COO) and the earlier observation of an extensive reaction network with up to four H₂COO sequential additions (Phys. Chem. Chem. Phys., 2019, 21, 7341-7357). Experimental photoionization efficiency scans recorded at 300 K and 425 K and ab initio threshold energy calculations lead to the identification and quantification of previously elusive intermediates, such as ketohydroperoxide and hydroperoxide species. Specifically, the C₄H₈ + O₃ adduct is identified as a ketohydroperoxide (KHP, 3-hydroperoxybutan-2-one, CH₃C(O)CH(CH₃)OOH), while hydroxyacetaldehyde (glycolaldehyde, HCOCH₂OH) formation is attributed to unimolecular isomerization of the CIs. Other hydroperoxide species such as methyl hydroperoxide (CH₃OOH), ethyl hydroperoxide (C₂H₅OOH), butyl hydroperoxide (OOH), hydroperoxyl acetaldehyde (HOOCH₂CHO), hydroxyethyl hydroperoxide (CH₃CH(OH)OOH), but-1-enyl-3-hydroperoxide, and 4-hydroxy-3-methylpentan-2-one (HOCH(CH₃)CH(CH₃)C(O)CH₃) are also identified. Detection of additional oxygenated species such as methanol, ethanol, ketene, and aldehydes suggests multiple active oxidation routes. These results provide additional evidence that CIs are key intermediates of the ozone-unsaturated hydrocarbon reactions providing critical inputs for improved kinetics models.

Functionalized Hydroperoxide Formation from the Reaction of Methacrolein-Oxide, an Isoprene-Derived Criegee Intermediate, with Formic Acid: Experiment and Theory

Methacrolein oxide (MACR-oxide) is a four-carbon, resonance-stabilized Criegee intermediate produced from isoprene ozonolysis, yet its reactivity is not well understood. This study identifies the functionalized hydroperoxide species, 1-hydroperoxy-2-methylallyl formate (HPMAF), generated from the reaction of MACR-oxide with formic acid using multiplexed photoionization mass spectrometry (MPIMS, 298 K = 25 °C, 10 torr = 13.3 hPa). Electronic structure calculations indicate the reaction proceeds via an energetically favorable 1,4-addition mechanism. The formation of HPMAF is observed by the rapid appearance of a fragment ion at m/z 99, consistent with the proposed mechanism and characteristic loss of HO2 upon photoionization of functional hydroperoxides. The identification of HPMAF is confirmed by comparison of the appearance energy of the fragment ion with theoretical predictions of its photoionization threshold. The results are compared to analogous studies on the reaction of formic acid with methyl vinyl ketone oxide (MVK-oxide), the other four-carbon Criegee intermediate in isoprene ozonolysis.

Elucidating the differences in oxidation of high-performance α- and β- diisobutylene biofuels via Synchrotron photoionization mass spectrometry

Biofuels are a promising ecologically viable and renewable alternative to petroleum fuels, with the potential to reduce net greenhouse gas emissions. However, biomass sourced fuels are often produced as blends of hydrocarbons and their oxygenates. Such blending complicates the implementation of these fuels in combustion applications. Variations in a biofuel's composition will dictate combustion properties such as auto ignition temperature, reaction delay time, and reaction pathways. A handful of novel drop-in replacement biofuels for conventional transportation fuels have recently been down selected from a list of over 10,000 potential candidates as part of the U.S. Department of Energy's (DOE) Co-Optimization of Fuels and Engines (Co-Optima) initiative. Diisobutylene (DIB) is one such high-performing hydrocarbon which can readily be produced from the dehydration and dimerization of isobutanol, produced from the fermentation of biomass-derived sugars. The two most common isomers realized, from this process, are 2,4,4-trimethyl-1-pentene (α-DIB) and 2,4,4-trimethyl-2-pentene (β-DIB). Due to a difference in olefinic bond location, the α- and β- isomer exhibit dramatically different ignition temperatures at constant pressure and equivalence ratio. This may be attributed to different fragmentation pathways enabled by allylic versus vinylic carbons. For optimal implementation of these biofuel candidates, explicit identification of the intermediates formed during the combustion of each of the isomers is needed. To investigate the combustion pathways of these molecules, tunable vacuum ultraviolet (VUV) light (in the range 8.1-11.0 eV) available at the Lawrence Berkeley National Laboratory's Advanced Light Source (ALS) has been used in conjunction with a jet stirred reactor (JSR) and time-of-flight mass spectrometry to probe intermediates formed. Relative intensity curves for intermediate mass fragments produced during this process were obtained. Several important unique intermediates were identified at the lowest observable combustion temperature with static pressure of 93,325 Pa and for 1.5 s residence time. As this relatively short residence time is just after ignition, this study is targeted at the fuels' ignition events. Ignition characteristics for both isomers were found to be strongly dependent on the kinetics of C₄ and C₇ fragment production and decomposition, with the tert-butyl radical as a key intermediate species. However, the ignition of α-DIB exhibited larger concentrations of C₄ compounds over C₇, while the reverse was true for β-DIB. These identified species will allow for enhanced engineering modeling of fuel blending and engine design.

Extreme Low-Temperature Combustion Chemistry: Ozone-Initiated Oxidation of Methyl Hexanoate

The accelerating chemical effect of ozone addition on the oxidation chemistry of methyl hexanoate [CH₃(CH₂)₄C(═O)OCH₃] was investigated over a temperature range from 460 to 940 K. Using an externally heated jet-stirred reactor at p = 700 Torr (residence time τ = 1.3 s, stoichiometry φ = 0.5, 80% argon dilution), we explored the relevant chemical pathways by employing molecular-beam mass spectrometry with electron and single-photon ionization to trace the temperature dependencies of key intermediates, including many hydroperoxides. In the absence of ozone, reactivity is observed in the so-called low-temperature chemistry (LTC) regime between 550 and 700 K, which is governed by hydroperoxides formed from sequential O₂ addition and isomerization reactions. At temperatures above 700 K, we observed the negative temperature coefficient (NTC) regime, in which the reactivity decreases with increasing temperatures, until near 800 K, where the reactivity increases again. Upon addition of ozone (1000 ppm), the overall reactivity of the system is dramatically changed due to the time scale of ozone decomposition in comparison to fuel oxidation time scales of the mixtures at different temperatures. While the LTC regime seems to be only slightly affected by the addition of ozone with respect to the identity and quantity of the observed intermediates, we observed an increased reactivity in the intermediate NTC temperature range. Furthermore, we observed experimental evidence for an additional oxidation regime in the range near 500 K, herein referred to as the extreme low-temperature chemistry (ELTC) regime. Experimental evidence and theoretical rate constant calculations indicate that this ELTC regime is likely to be initiated by H abstraction from methyl hexanoate via O atoms, which originate from thermal O₃ decomposition. The theoretical calculations show that the rate constants for methyl ester initiation via abstraction by O atoms increase dramatically with the size of the methyl ester, suggesting that ELTC is likely not important for the smaller methyl esters. Experimental evidence is provided indicating that, similar to the LTC regime, the chemistry in the ELTC regime is dominated by hydroperoxide chemistry. However, mass spectra recorded at various reactor temperatures and at different photon energies provide experimental evidence of some differences in chemical species between the ELTC and the LTC temperature ranges.

Direct time-resolved detection and quantification of key reactive intermediates in diethyl ether oxidation at T = 450-600 K

High-pressure multiplexed photoionization mass spectrometry (MPIMS) with tunable vacuum ultraviolet (VUV) ionization radiation from the Lawrence Berkeley Labs Advanced Light Source is used to investigate the oxidation of diethyl ether (DEE). Kinetics and photoionization (PI) spectra are simultaneously measured for the species formed. Several stable products from DEE oxidation are identified and quantified using reference PI cross-sections. In addition, we directly detect and quantify three key chemical intermediates: peroxy (ROO˙), hydroperoxyalkyl peroxy (˙OOQOOH), and ketohydroperoxide (HOOP[double bond, length as m-dash]O, KHP). These intermediates undergo dissociative ionization (DI) into smaller fragments, making their identification by mass spectrometry challenging. With the aid of quantum chemical calculations, we identify the DI channels of these key chemical species and quantify their time-resolved concentrations from the overall carbon atom balance at T = 450 K and P = 7500 torr. This allows the determination of the absolute PI cross-sections of ROO˙, ˙OOQOOH, and KHP into each DI channel directly from experiment. The PI cross-sections in turn enable the quantification of ROO˙, ˙OOQOOH, and KHP from DEE oxidation over a range of experimental conditions that reveal the effects of pressure, O₂ concentration, and temperature on the competition among radical decomposition and second O₂ addition pathways.

Molecular-Weight Growth in Ozone-Initiated Low-Temperature Oxidation of Methyl Crotonate

We report experiments of ozone-initiated low-temperature oxidation of methyl crotonate (MC, CH₃-CH═CH-C(O)OCH₃) from 420 to 660 K in a near-atmospheric-pressure jet-stirred reactor using photoionization molecular-beam mass spectrometry as a sampling technique. In this temperature regime, no typical low-temperature combustion (LTC) reactions have been observed for MC when oxygen (O₂) is used as the oxidizer. Upon ozone addition, significant oxidation of methyl crotonate is found. On the basis of experimentally observed energy-dependent mass peaks in combination with temperature-dependent mole fraction profiles and photoionization efficiency curves, we provide new insights into the methyl crotonate ozonolysis reaction network. The observed MC + O₃ products, C₅H₈O₅, are found to be related to the keto-hydroperoxides resulting from the isomerization of the primary ozonide. Evidence is also provided that molecular growth mainly results from cycloaddition reactions of the Criegee intermediate into aldehydes and alkenes as well as addition reactions of the Criegee intermediates to the double bond of methyl crotonate and sequential decomposition into ketones. Furthermore, species that contribute in large amounts to the low-temperature oxidation of methyl crotonate, like H₂O₂, CH₃OOH, CH₃OH, and HC(O)OH, are identified, and their mole fractions are reported. Additionally, preliminary modeling is performed which qualitatively captures the observed NTC behavior and reveals future research opportunities.

Combustion in the future: The importance of chemistry

Combustion involves chemical reactions that are often highly exothermic. Combustion systems utilize the energy of chemical compounds released during this reactive process for transportation, to generate electric power, or to provide heat for various applications. Chemistry and combustion are interlinked in several ways. The outcome of a combustion process in terms of its energy and material balance, regarding the delivery of useful work as well as the generation of harmful emissions, depends sensitively on the molecular nature of the respective fuel. The design of efficient, low-emission combustion processes in compliance with air quality and climate goals suggests a closer inspection of the molecular properties and reactions of conventional, bio-derived, and synthetic fuels. Information about flammability, reaction intensity, and potentially hazardous combustion by-products is important also for safety considerations. Moreover, some of the compounds that serve as fuels can assume important roles in chemical energy storage and conversion. Combustion processes can furthermore be used to synthesize materials with attractive properties. A systematic understanding of the combustion behavior thus demands chemical knowledge. Desirable information includes properties of the thermodynamic states before and after the combustion reactions and relevant details about the dynamic processes that occur during the reactive transformations from the fuel and oxidizer to the products under the given boundary conditions. Combustion systems can be described, tailored, and improved by taking chemical knowledge into account. Combining theory, experiment, model development, simulation, and a systematic analysis of uncertainties enables qualitative or even quantitative predictions for many combustion situations of practical relevance. This article can highlight only a few of the numerous investigations on chemical processes for combustion and combustion-related science and applications, with a main focus on gas-phase reaction systems. It attempts to provide a snapshot of recent progress and a guide to exciting opportunities that drive such research beyond fossil combustion.

Atmospheric Autoxidation of Amines

Autoxidation has been acknowledged as a major oxidation pathway in a broad range of atmospherically important compounds including isoprene, monoterpenes, and very recently, dimethyl sulfide. Here, we present a high-level theoretical multiconformer transition-state theory study of the atmospheric autoxidation in amines exemplified by the atmospherically important trimethylamine (TMA) and dimethylamine and generalized by the study of the larger diethylamine. Overall, we find that the initial hydrogen shift reactions have rate coefficients greater than 0.1 s^-1 and autoxidation is thus an important atmospheric pathway for amines. This autoxidation efficiently leads to the formation of hydroperoxy amides, a new type of atmospheric nitrogen-containing compounds, and for TMA, we experimentally confirm this. The conversion of amines to hydroperoxy amides may have important implications for nucleation and growth of atmospheric secondary organic aerosols and atmospheric OH recycling.

A pressurized flow reactor combustion experiment interfaced with synchrotron double imaging photoelectron photoion coincidence spectroscopy

A new pressurized low-temperature combustion experiment has been commissioned at the Swiss Light Source, Paul Scherrer Institute. The experiment uses photoionization with tunable synchrotron radiation and double imaging photoelectron photoion coincidence (i²PEPICO) detection at the vacuum ultraviolet beamline. The experimental setup is described, including the high-pressure reactor experiment, sampling interface, and reactant delivery system. The CRF-PEPICO (Combustion Reactions Followed by Photoelectron Photoion Coincidence) endstation and VUV beamline are briefly elaborated. The novel aspects of the apparatus and the new components are elucidated in detail, such as the fluid supply system to the reactor and the reactor integration into the endstation. We also present a system overview of the experimental setup. The technical details are followed by a description of the experimental procedure used to operate the pressurized flow reactor setup. Finally, first experimental results demonstrating the capability of the setup are provided and analyzed. A major advantage of this new experiment is that the excellent isomer resolution capabilities of the i²PEPICO technique can be transferred to the investigation of reactions at elevated pressures of several bars. This enables the investigation of pressure effects on the reactivity of fuel mixtures and covers more realistic conditions found in technical combustors. The capability to obtain quantitative oxidation data is confirmed, and the main and certain intermediate species are quantified for a selected condition. The results show excellent agreement with a chemical kinetics model and previously published reference measurements performed with a gas chromatography setup.

Mechanisms and kinetics of the low-temperature oxidation of 2-methylfuran: insight from DFT calculations and kinetic simulations

The low-temperature oxidation (LTO) mechanisms of the 2-methylfuran (2-MF) biofuel and the corresponding thermodynamic and kinetic properties have been explored by density functional theory (DFT) and composite G4 methodologies as well as kinetic simulations. The O₂ addition to the main furylCH₂ radical from the methyl dehydrogenation in 2-MF forms three peroxide radicals PO1, PO2, and PO3 with the energy barriers of 15.1, 19.3, and 20.6 kcal mol^-1 and the reaction ΔG of -8.2, 5.7, and -0.1 kcal mol^-1 (298 K and 1 atm), respectively. Through hydrogen transfer followed by dehydroxylation, these nascent products evolve into stable aldehydes and cyclic ketones, which may further decompose into smaller species under the action of OH. Calculations and simulations show that the product P1 from the dehydroxylation of PO1 has a dominant population (higher than 96%) among the final products, although the temperature and pressure may influence the species profiles and rate constants to some extent. Based on the G4-calibrated thermodynamic parameters, the temperature and pressure dependence of the rate constants and the two- and three-parameter Arrhenius coefficients for all reactions considered here have been determined by using the transition state theory (TST) and Rice-Ramsperger-Kassel-Marcus (RRKM) methods. The present results provide a comprehensive understanding of the mechanisms and kinetics of the LTO process of the 2-MF biofuel.

Fast Peroxy Radical Isomerization and OH Recycling in the Reaction of OH Radicals with Dimethyl Sulfide

Dimethyl sulfide (DMS), produced by marine organisms, represents the most abundant, biogenic sulfur emission into the Earth's atmosphere. The gas-phase degradation of DMS is mainly initiated by the reaction with the OH radical forming first CH₃SCH₂O₂ radicals from the dominant H-abstraction channel. It is experimentally shown that these peroxy radicals undergo a two-step isomerization process finally forming a product consistent with the formula HOOCH₂SCHO. The isomerization process is accompanied by OH recycling. The rate-limiting first isomerization step, CH₃SCH₂O₂ → CH₂SCH₂OOH, followed by O₂ addition, proceeds with k = (0.23 ± 0.12) s^-1 at 295 ± 2 K. Competing bimolecular CH₃SCH₂O₂ reactions with NO, HO₂, or RO₂ radicals are less important for trace-gas conditions over the oceans. Results of atmospheric chemistry simulations demonstrate the predominance (≥95%) of CH₃SCH₂O₂ isomerization. The rapid peroxy radical isomerization, not yet considered in models, substantially changes the understanding of DMS's degradation processes in the atmosphere.

Isomer-Selective Detection of Keto-Hydroperoxides in the Low-Temperature Oxidation of Tetrahydrofuran

Keto-hydroperoxides (KHPs) are reactive, partially oxidized intermediates that play a central role in chain-branching reactions during the gas-phase low-temperature oxidation of hydrocarbons and oxygenated species. Although multiple isomeric forms of the KHP intermediate are possible in complex oxidation environments when multiple reactant radicals exist that contain nonequivalent O₂ addition sites, isomer-resolved data of KHPs have not been reported. In this work, we provide partially isomer-resolved detection and quantification of the KHPs that form during the low-temperature oxidation of tetrahydrofuran (THF, cycl.-O-CH₂CH₂CH₂CH₂-). We describe how these short-lived KHPs were detected, identified, and quantified using integrated experimental and theoretical approaches. The experimental approaches were based on direct molecular-beam sampling from a jet-stirred reactor operated at near-atmospheric pressure and at temperatures between 500 and 700 K, followed by mass spectrometry with single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation, and the identification of fragmentation patterns. The interpretation of the experiments was guided by theoretical calculations of ionization thresholds, fragment appearance energies, and photoionization cross sections. On the basis of the experimentally observed and theoretically calculated ionization and fragment appearance energies, KHP isomers could be distinguished as originating from H-abstraction reactions from either the α-C adjacent to the O atom or the β-C atoms. Temperature-dependent concentration profiles of the partially resolved isomeric KHP intermediates were determined in the range of 500-700 K, and the results indicate that the observed KHP isomers are formed overwhelmingly (∼99%) from the α-C THF radical. Comparisons of the partially isomer-resolved quantification of the KHPs to up-to-date kinetic modeling results reveal new opportunities for the development of a next-generation THF oxidation mechanism.

A new era for combustion research

Current topics in combustion chemistry include aspects of a changing fuel spectrum with a focus on reducing emissions and increasing efficiency. This article is intended to provide an overview of selected recent work in combustion chemistry, especially addressing reaction pathways from fuel decomposition to emissions. The role of the molecular fuel structure will be emphasized for the formation of certain regulated and unregulated species from individual fuels and their mixtures, exemplarily including fuel compounds such as alkanes, alkenes, ethers, alcohols, ketones, esters, and furan derivatives. Depending on the combustion conditions, different temperature regimes are important and can lead to different reaction classes. Laboratory reactors and flames are prime sources and targets from which such detailed chemical information can be obtained and verified with a number of advanced diagnostic techniques, often supported by theoretical work and simulation with combustion models developed to transfer relevant details of chemical mechanisms into practical applications. Regarding the need for cleaner combustion processes, some related background and perspectives will be provided regarding the context for future chemistry research in combustion energy science.

Identification of the Criegee intermediate reaction network in ethylene ozonolysis: impact on energy conversion strategies and atmospheric chemistry

The reaction network of the simplest Criegee intermediate (CI) CH₂OO has been studied experimentally during the ozonolysis of ethylene.

Characterization of the simplest hydroperoxide ester, hydroperoxymethyl formate, a precursor of atmospheric aerosols

Atmospheric aerosols are large clusters of molecules and particulate matter that profoundly affect the Earth's radiation budget and climate.

Laser Photolysis Kinetic Study of OH Radical Reactions with Methyl tert-Butyl Ether and Trimethyl Orthoformate under Conditions Relevant to Low Temperature Combustion: Measurements of Rate Coefficients and OH Recycling

Methyl tertiary butyl ether (MTBE) and trimethyl orthoformate (TMOF) are potential biofuel ethers and could replace conventional fossil fuels, or act as additives to aid combustion. Laser flash photolysis with laser-induced fluorescence detection of the OH radical has been used to measure the rate coefficients of the OH reaction with these ethers, from 298 K to approximately 740 K. The temperature dependence of the rate coefficients is parametrized as k_OH+MTBE(298-680 K) = 9.8 × 10^-13× ( T/298)^2.7 × exp(2500/R T) cm³ molecule^-1 s^-1 and k_OH+TMOF(298-744 K) = 8.0 × 10^-13 × [( T/298)^2.6 + ( T/298)^-8.1] × exp[2650/R T] cm³ molecule^-1 s^-1. The room temperature (298 K) bimolecular rate coefficients were measured as k_OH+MTBE = (2.81 ± 0.32) × 10^-12 cm³ molecule^-1 s^-1 and k_OH+TMOF = (4.65 ± 0.50) × 10^-12 cm³ molecule^-1 s^-1 where the errors represent statistical uncertainties at the 2σ level in combination with an estimated 10% systematic error. Regeneration of OH radicals was observed for both reactions at higher temperatures in the presence of O₂ via biexponential OH decays, which were observed above 489 K and 568 K, for TMOF and MTBE respectively. The OH yield from MTBE/O₂, between 620 and 700 K, was invariant with the concentration of oxygen (10¹⁵-10¹⁸ molecules cm^-3) at (36 ± 5)%. Mechanisms for OH regeneration from MTBE are briefly discussed and compared with those in the literature and from dimethyl and diethyl ether. The lower OH yield from MTBE, compared to these other ethers, is most likely due to competition with an HO₂ formation channel.

Low-Temperature Oxidation of Ethylene by Ozone in a Jet-Stirred Reactor

Ethylene oxidation initiated by ozone addition (ozonolysis) is carried out in a jet-stirred reactor from 300 to 1000 K to explore the kinetic pathways relevant to low-temperature oxidation. The temperature dependencies of species' mole fractions are quantified using molecular-beam mass spectrometry with electron ionization and single-photon ionization employing tunable synchrotron-generated vacuum-ultraviolet radiation. Upon ozone addition, significant ethylene oxidation is found in the low-temperature regime from 300 to 600 K. Here, we provide new insights into the ethylene ozonolysis reaction network via identification and quantification of previously elusive intermediates by combining experimental photoionization energy scans and ab initio threshold energy calculations for isomer identification. Specifically, the C₂H₄ + O₃ adduct C₂H₄O₃ is identified as a keto-hydroperoxide (hydroperoxy-acetaldehyde, HOOCH₂CHO) based on the calculated and experimentally observed ionization energy of 9.80 (±0.05) eV. Quantification using a photoionization cross-section of 5 Mb at 10.5 eV results in 5 ppm at atmospheric conditions, which decreases monotonically with temperature until 550 K. Other hydroperoxide species that contribute in larger amounts to the low-temperature oxidation of C₂H₄, like H₂O₂, CH₃OOH, and C₂H₅OOH, are identified and their temperature-dependent mole fractions are reported. The experimental evidence for additional oxygenated species such as methanol, ketene, acetaldehyde, and hydroxy-acetaldehyde suggest multiple active oxidation routes. This experimental investigation closes the gap between ozonolysis at atmospheric and elevated temperature conditions and provides a database for future modeling.

The reaction of Criegee intermediates with acids and enols

The reaction of CH₂OO, the smallest carbonyl oxide (Criegee intermediate, CI), with several acids was investigated using the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ quantum chemical method, as well as microvariational transition state theory and RRKM master equation theoretical kinetic methodologies. For oxoacids HNO₃ and HCOOH, a 1,4-insertion mechanism allows for barrierless reactions with high rate coefficients, in agreement with literature experimental data. This mechanism relies on the presence of a double bond in the α-position to the acidic OH group. We predict that reactions of CI with enols will likewise have high rate coefficients, proceeding through a similar mechanism. The hydracid HCl was found to react through a less favorable 1,2-insertion reaction, leading to lower rate coefficients, again in good agreement with the literature. We conclude that the reaction mechanism is the main indicator for the reaction rate for CH₂OO + acid reactions, with acidity only of secondary influence. At room temperature and 1 atm the main product for all reactions was found to be the thermalized hydroperoxide initial adduct, with minor yields of fragmentation products. One of the product channels characterized is a novel reaction path involving intramolecular H-abstraction after a roaming reaction in the OH + product radical complex formed by the dissociation of the hydroperoxide adduct; this channel is the lowest fragmentation route for some of the reactions studied.

Unraveling the structure and chemical mechanisms of highly oxygenated intermediates in oxidation of organic compounds

Decades of research on the autooxidation of organic compounds have provided fundamental and practical insights into these processes; however, the structure of many key autooxidation intermediates and the reactions leading to their formation still remain unclear. This work provides additional experimental evidence that highly oxygenated intermediates with one or more hydroperoxy groups are prevalent in the autooxidation of various oxygenated (e.g., alcohol, aldehyde, keto compounds, ether, and ester) and nonoxygenated (e.g., normal alkane, branched alkane, and cycloalkane) organic compounds. These findings improve our understanding of autooxidation reaction mechanisms that are routinely used to predict fuel ignition and oxidative stability of liquid hydrocarbons, while also providing insights relevant to the formation mechanisms of tropospheric aerosol building blocks. The direct observation of highly oxygenated intermediates for the autooxidation of alkanes at 500-600 K builds upon prior observations made in atmospheric conditions for the autooxidation of terpenes and other unsaturated hydrocarbons; it shows that highly oxygenated intermediates are stable at conditions above room temperature. These results further reveal that highly oxygenated intermediates are not only accessible by chemical activation but also by thermal activation. Theoretical calculations on H-atom migration reactions are presented to rationalize the relationship between the organic compound's molecular structure (n-alkane, branched alkane, and cycloalkane) and its propensity to produce highly oxygenated intermediates via extensive autooxidation of hydroperoxyalkylperoxy radicals. Finally, detailed chemical kinetic simulations demonstrate the influence of these additional reaction pathways on the ignition of practical fuels.

Time-resolved measurements of product formation in the low-temperature (550-675 K) oxidation of neopentane: a probe to investigate chain-branching mechanism

Product formation, in particular ketohydroperoxide formation and decomposition, were investigated in time-resolved, Cl-atom initiated neopentane oxidation experiments in the temperature range 550-675 K using a photoionization time-of-flight mass spectrometer. Ionization light was provided either by Advanced Light Source tunable synchrotron radiation or ∼10.2 eV fixed energy radiation from a H₂-discharge lamp. Experiments were performed both at 1-2 atm pressure using a high-pressure reactor and also at ∼9 Torr pressure employing a low-pressure reactor for comparison. Because of the highly symmetric structure of neopentane, ketohydroperoxide signal can be attributed to a 3-hydroperoxy-2,2-dimethylpropanal isomer, i.e. from a γ-ketohydroperoxide (γ-KHP). The photoionization spectra of the γ-KHP measured at low- and high pressures and varying oxygen concentrations agree well with each other, further supporting they originate from the single isomer. Measurements performed in this work also suggest that the "Korcek" mechanism may play an important role in the decomposition of 3-hydroperoxy-2,2-dimethylpropanal, especially at lower temperatures. However, at higher temperatures where γ-KHP decomposition to hydroxyl radical and oxy-radical dominates, oxidation of the oxy-radical yields a new important channel leading to acetone, carbon monoxide, and OH radical. Starting from the initial neopentyl + O₂ reaction, this channel releases altogether three OH radicals. A strongly temperature-dependent reaction product is observed at m/z = 100, likely attributable to 2,2-dimethylpropanedial.

Advances in threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion coincidence (TPEPICO)

The history and evolution of molecular threshold photoelectron spectroscopy and threshold photoelectron photoion coincidence spectroscopy (TPEPICO) over the last fifty years are reviewed.

The atmospheric oxidation of dimethyl, diethyl, and diisopropyl ethers. The role of the intramolecular hydrogen shift in peroxy radicals

The atmospheric oxidation mechanisms of dimethyl ether (DME), diethyl ether (DEE) and diisopropyl ether (DiPE) are studied by using quantum chemistry and unimolecular reaction theory (RRKM-ME) calculations. For the peroxy radical CH3OCH2O2˙ from DME, a barrier height of ∼ 85 kJ mol(-1) is found for its intramolecular H-shift to ˙CH2OCH2OOH, which can recombine rapidly with the atmospheric O2. RRKM-ME calculations obtain an effective rate of ∼ 0.1 s(-1) at 298 K for the formation of ˙O2CH2OCH2OOH. For similar radicals in DEE and DiPE, effective rates are 1.6 s(-1) and 1.1 s(-1), respectively. In the atmosphere, these unimolecular reactions are fast enough to compete with the bimolecular reactions with NO and/or HO2, especially when [NO] is low. The fates of radicals after the H-shifts are also examined here. Several subsequent reactions are found to recycle OH radicals. New mechanisms are proposed on the basis of present calculations and are consistent with previous experimental results. In the atmosphere, the routes via H-shifts represent an auto-oxidation of these ethers with no involvement of NOx and therefore no O3 formation, and also a self-cleaning mechanism of organic compounds due to recycling of OH radicals. Some of the end products are highly oxidized with multifunctional groups and high O : C ratios, suggesting their low volatility and potential contribution to secondary organic aerosols.

An instrument to measure fast gas phase radical kinetics at high temperatures and pressures

Fast radical reactions are central to the chemistry of planetary atmospheres and combustion systems. Laser-induced fluorescence is a highly sensitive and selective technique that can be used to monitor a number of radical species in kinetics experiments, but is typically limited to low pressure systems owing to quenching of fluorescent states at higher pressures. The design and characterisation of an instrument are reported using laser-induced fluorescence detection to monitor fast radical kinetics (up to 25 000 s(-1)) at high temperatures and pressures by sampling from a high pressure reaction region to a low pressure detection region. Kinetics have been characterised at temperatures reaching 740 K and pressures up to 2 atm, with expected maximum operational conditions of up to ∼900 K and ∼5 atm. The distance between the point of sampling from the high pressure region and the point of probing within the low pressure region is critical to the measurement of fast kinetics. The instrumentation described in this work can be applied to the measurement of kinetics relevant to atmospheric and combustion chemistry.

Direct observation of vinyl hydroperoxide

Many alkyl-substituted Criegee intermediates are predicted to undergo an intramolecular 1,4-hydrogen transfer to form isomeric vinyl hydroperoxide species (C[double bond, length as m-dash]COOH moiety), which break apart to release OH and vinoxy radicals. We report direct detection of stabilized vinyl hydroperoxides formed via carboxylic acid-catalyzed tautomerization of Criegee intermediates. A doubly hydrogen-bonded interaction between the Criegee intermediate and carboxylic acid facilitates efficient hydrogen transfer through a double hydrogen shift. Deuteration of formic or acetic acid permits migration of a D atom to yield partially deuterated vinyl hydroperoxides, which are distinguished from the CH3CHOO, (CH3)2COO, and CH3CH2CHOO Criegee intermediates by mass. Using 10.5 eV photoionization, three prototypical vinyl hydroperoxides, CH2[double bond, length as m-dash]CHOOD, CH2[double bond, length as m-dash]C(CH3)OOD, and CH3CH[double bond, length as m-dash]CHOOD, are detected directly. Complementary electronic structure calculations reveal several reaction pathways, including the barrierless acid-catalyzed tautomerization reaction predicted previously and a barrierless addition reaction that yields hydroperoxy alkyl formate.