Updated thermochemistry for renewable transportation fuels: New groups and group values for acetals and ethers, their radicals, and peroxy species

Abstract

We present new groups and group values for the gas-phase thermochemistry of ethers, polyethers, and acetals suited for combustion modeling. Our investigation comprises fuel species, their primary radicals, peroxy radicals, and hydroperoxide species. In total, 45 species are used for the parameterization of 14 groups, six of which are newly introduced here. Presently, calculated thermochemistry at the DLPNO-CCSD(T)/CBS(cc-pVTZ, cc-PVQZ) // B3LYP-D3BJ/def2-TZVP level of theory is combined with thermochemistry from the literature. Validation of the new group values against the quantum mechanical results gives deviations of 1.22 kcal/mol, 2.36 cal/(mol⋅K), and 1.20 cal/(mol⋅K) for heats of formation, standard entropies, and heat capacities, respectively. The new thermochemistry is tested with a recent OME2 and OME3 chemical kinetic model, which shows large sensitivities to the updated values in the intermediate temperature regime. This highlights the importance of using updated thermochemistry in future chemical kinetic modeling studies of oxygenated methyl ethers (OMEs) and OMErelated structures. K E Y W O R D S ab initio thermochemistry, group additivity theory, oxymethylene ethers 1 INTRODUCTION Oxygenated methyl ethers (OMEs) are interesting fuel candidates for several reasons. OMEs improve the soot-NO trade-off 1 when being used as diesel fuel additives and the smallest OME, dimethoxy methane (DMM), has already been tested as pure fuel. 2 In addition, it was found that carbon monoxide 3 and noise emissions 4 are reduced when blending diesel with DMM. Furthermore, This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

A Comprehensive Experimental and Kinetic Modeling Study of the Combustion Chemistry of Diethoxymethane

“…All calculated rate constants for the unimolecular decomposition and the β-scission reaction of Q ̇2OOHc are also visualized in the Supporting Information (section S5.1.1). 16 Furthermore, for the fuel fragments generated during the unimolecular decomposition and for the three fuel peroxyl radicals (RO ̇2) relevant to the low-temperature oxidation, the parameters were adopted from the theoretical work of Dontgen et al 52 The structure of DEM with the assignment of the corresponding radical positions and bond strength is shown in Figure 1. The naming convention in this work will follow the indicated numbering of the carbon atoms.…”

“…Thermodynamic properties including enthalpies, entropies, and heat capacities are essential for the simulation of the oxidation and pyrolysis pathways of DEM. The thermochemical parameters for DEM (RH) and its three radicals (primary C–C site: Ṙ1 = C 2 H 5 OCH 2 OCH 2 ĊH 2 ; secondary C–C site: Ṙ2 = C 2 H 5 OCH 2 OĊHCH 3 ; and central O–C–O site: Ṙ4 = C 2 H 5 OĊHOC 2 H 5 ) are taken from the theoretical investigation of Kröger et al Furthermore, for the fuel fragments generated during the unimolecular decomposition and for the three fuel peroxyl radicals (RȮ 2 ) relevant to the low-temperature oxidation, the parameters were adopted from the theoretical work of Döntgen et al The structure of DEM with the assignment of the corresponding radical positions and bond strength is shown in Figure . The naming convention in this work will follow the indicated numbering of the carbon atoms.…”

“…The naming convention in this work will follow the indicated numbering of the carbon atoms. The bond dissociation energies (BDEs) of DEM were calculated at the same high level of theory as in both previously mentioned refs and . The thermochemical data of the other relevant radicals and stable parents in the submechanism are estimated by using THERM software, which is based on the Benson group additivity theory, or are taken from the available literature. ,− An overview over all species, their naming as written in the submechanism for DEM, their molecular structure, and their source for the thermodynamic data is provided in the Supporting Information (section S5.3).…”

Higher Alcohol and Ether Biofuels for Compression-Ignition Engine Application: A Review with Emphasis on Combustion Kinetics

“…For a direct comparison between the sensitivities on rate constants of reactions and enthalpies of formation of species, the optimization potentials of kinetic and thermochemical parameters were calculated following the work of vom Lehn et al, which are defined as products of parameter sensitivities and uncertainties. An uncertainty factor of 4 was assigned for the reaction rate constants, , as they were mostly specified by using rate rules. , A 2σ uncertainty of 8.18 kcal/mol was employed for the enthalpies of formation of species according to Döntgen et al, who reported a root-mean-square deviation with 1σ confidence interval of 4.09 kcal/mol between the enthalpies of formation calculated with the group additivity approach and the quantum mechanical method for ethers, polyethers, and acetals. The results are shown in Figures and .…”

“…A comprehensive assessment of the corresponding groups specific to alcohols and ethers is expected to be of highest priority. In particular, the thermochemical property values of hydroperoxy radicals of ethers can be significantly affected by hydrogen bond interactions between the hydroperoxy and the ether moieties, which deteriorates the prediction accuracy of conventional group values for those species and points to the need of further improvement, e.g., by consideration of additional non-nearest neighbor correction groups …”