Computational Investigation of the High-Temperature Chemistry of Small Hydroxy Ethers: Methoxymethanol,...

In the search for alternative energy carriers that can replace conventional fossil fuels, sustainably produced oxygenated hydrocarbons represent a promising class of potential candidates. An illustrative member of this class of alternative biofuels are oxymethylene ethers (OMEs). This study makes a contribution to this objective by investigating hydroxy ethers, specifically methoxymethanol, ethoxymethanol, and 2-methoxyethanol. These bifunctional oxygenated molecules are relevant intermediates formed during the combustion of OMEs or via an equilibrium reaction in methanol and formaldehyde mixtures. The high-temperature chemistry of hydroxy ethers is examined, with a particular focus on the unimolecular reactions involving H atom migration and bond fission. Quantum chemical calculations are utilized to gain insights into these processes. The results include bond dissociation energies, one-dimensional representations of the potential energy surfaces along the reaction coordinate for the simulated reactions, thermodynamic properties, and reaction rate parameters, which were derived via established ab initio methods. A detailed account of the unimolecular decomposition reactions of methoxymethanol, ethoxymethanol, and 2-methoxyethanol is provided, including the predominant reaction rates and pressure dependencies. The BDEs are benchmarked against data from different literature sources. For ME, a direct comparison of BDEs and reaction constants is performed with the results of previous studies. The derived reaction parameters for all three hydroxy ethers are compared to the results obtained via theoretical methods for dimethoxymethane and diethoxymethane, which feature similar chemical structures. The high-temperature chemistry of methoxymethanol and ethoxymethanol shares similarities and is dominated by rapid H atom migrations, forming alcohol + formaldehyde. This reaction is geometrically constrained in 2-methoxyethanol and therefore bond fission reactions dominate. A brief systematic comparison of the bifunctional oxygenated structure of hydroxy ether with other fuel species, such as n-alkanes, ethers, and alcohols, was conducted. The findings of this study are supported by the results of published chemical kinetic models of 2-methoxyethanol, formaldehyde + methanol, and OME-2. This includes simulations of chemical kinetic mechanisms, which were updated with the newly derived reaction rates, and compared with associated experimental data from literature. These experiments are concentration measurements obtained from a jet-stirred reactor and laminar burning velocities measured in an atmospheric burner. The results are in reasonable agreement with the reported experiments and thus may be considered an update to the models. The updated reaction rate constants showed no observable impact on the ignition delay times of OME-2. However, they did result in a significant increase in the concentration of methoxymethanol as an intermediate. In conclusion, this study provides crucial insights into the high-temperature combustion properties of hydroxy ethers. The data presented is intended to enhance comprehension of the decomposition behavior of these bifunctional oxygenated species and to support the development of detailed chemical kinetic models for combustion applications.