Unveiling high-temperature oxidation kinetics of PODE2: Theoretical studies on the H-abstraction, isomerization, and β-dissociation reactions

Abstract

Polyoxymethylene dimethyl ether (PODEn) is a promising fuel additive to reduce soot emissions. In this work, a theoretical study on the H-abstraction reaction from PODE2 (CH3OCH2OCH2OCH3) by the H atom and CH3 radical, as well as the subsequent isomerization and β-dissociation reactions of PODE2 radicals were carried out. The geometry optimization and frequency analysis for all the stationary points on the potential energy surfaces (PESs) were performed at the M08-HX/6-311+G(2df,2p) level of theory. Single point energies were calculated using the CCSD(T)-F12 method together with the cc-pVTZ basis set. The high-pressure limit rate constants of the H-abstraction reactions from PODE2 were determined through the multistructural canonical variational transition state theory (MS-CVT) with small-curvature tunneling (SCT) correction. The temperature and pressure dependent rate constants for the unimolecular reactions of radicals produced by the H-abstraction reactions from PODE2 were calculated by solving the energy-dependent master equation using the Tsinghua University Minnesota Master Equation (TUMME) program. For the H-abstraction channels of PODE2, reaction pathways initiated via H atom attack are more energetically and kinetically favored over those via CH3 radical within the studied temperature range. The PODE2 radicals with H atoms abstracted from CH3 and CH2 sites are denoted as PODE2A (CH3OCH2OCH2OCH2) and PODE2B (CH3OCH2OCHOCH3), respectively. For unimolecular reaction channels of PODE2A radical, with increased temperatures, the dominant reaction shifts from isomerization to the decomposition process. In the case of the PODE2B radical, the two decomposition channels are consistently preferred across all temperature ranges, resulting in a competitive relationship between them. The thermodynamic properties of PODE2 and its associated radicals were calculated using the isodesmic reaction method in conjunction with multistructural torsional partition functions. Based upon the newly calculated rate constants and existing literature models, an updated kinetic model describing PODE2 combustion chemistry is developed in this work. This model incorporates the latest findings and contributes to a more comprehensive understanding of the oxidation and auto-ignition characteristics of PODE2.