Detailed kinetic mechanism of polyoxymethylene dimethyl ether 3 (PODE3). Part I: Ab initio thermochemistry and kinetic predictions for key reactions

In recent years, polyoxymethylene dimethyl ether 3 (PODE3) has emerged as a promising fuel additive to mitigate soot emissions in diesel engines, garnering significant research attention. Several detailed kinetic mechanisms of PODE3 have been developed to investigate its reaction pathways and to simulate its combustion process. Typically, these mechanisms employ analogy methods based on rate constants derived primarily from PODE1 sub-mechanisms. However, in this study, we present, for the first time, high-level ab initio quantum chemical calculations directly on PODE3 itself. (1) The bond dissociation energies (BDEs) of all C[sbnd]H and C[sbnd]O bonds in dimethyl ether (DME) and PODEn (n = 1–5) were calculated using combined compound methods (CBS-APNO/G3/G4). (2) The obtained BDE results were then utilized to investigate the unimolecular decomposition reactions of PODE3, demonstrating good agreement with previous Molecular Dynamics simulation results. (3) The rate constants of hydrogen abstraction reactions of PODE3 by H˙,C˙H3,CH3O˙,O˙H,HO˙2,O2 were calculated with all possible pre-reaction or post-reaction complexes identified. The rate constants of the H-atom abstraction reactions were found to primarily depend on the energy barriers following the order: O˙H>CH3O˙>H˙>C˙H3>HO˙2>O2. However, the abstraction by H˙ can dominate as temperature increases. (4) The rate constants of β-scission and isomerization reactions of PODE3 radicals were also computed. The calculated β-scission reaction rates supported the suitability of the rate analogy for PODEn, employing the long-chain PODE3. The energy barriers of the isomerization reactions with long chain transition states were comparable to the energy barrier of the β-scission reactions and are much lower than that in the case of PODE1 radicals. (5) The thermochemistry properties of all involved species, including PODE4–5 and their radicals, were calculated with the combined compound methods (CBS-APNO/G3/G4) for further combustion modeling. Considering PODE3′s nature as a long-chain PODEn, the rate constants computed for PODE3 can serve as a solid foundation for developing detailed mechanisms for PODE4–5.