Estudo cinético de reações unimoleculares nos mecanismos de pirólise e combustão do dimetóximetano

Abstract

Currently, the search for renewable and clean energy resources, important and strongly interconnected characteristics, is gaining increasing prominence in scientific research. Within this focus, great interest has been dedicated to the energy efficiency of a category of oxygenated fuels: polyoxymethylene ethers (OME). Dimethoxymethane (DMM, CH3OCH2OCH3) is found in this group, offering cleaner combustion, free from soot and polluting oxides such as SOx. This study aims at proposing a sub mechanism of initiation steps for the DMM combustion and pyrolysis kinetic models. Specifically, the kinetics and thermodynamics of DMM unimolecular reactions are discussed. Calculations were performed at the M06-2X/aug-cc-pVTZ level including geometry optimizations, vibrational frequencies and reaction paths calculations. Results suggest that the dissociation reaction pathway, forming CH3 + OCH2OCH3, is the main reaction channel, with the lowest dissociation limit, 82.40 kcal.mol-1. The reaction channels that lead to H2COCHOCH3 + H2, CH3O + CH2OCH3 and HCOCH2OCH3 + H2 are competitive, with reaction barriers of 85.90, 87.38 and 87.95 kcal.mol-1, respectively. Hydrogen atom dissociation pathways, with a dissociation limit of 94.56 and 95.86 kcal.mol-1, and the decomposition reaction that leads to H2 + CH2OCH2 + H2CO are unfavorable, both from the kinetic and thermodynamic points of view. Changing the focus of the study to the decomposition reactions of the primary radicals, the pathways that lead to HCO + CH2OCH3, H2CO + HCOCH3, H2COCHO + CH3 are highlighted because they are the most favorable, with reaction energies in the respective values 15.87, 9.97 and -1.76 kcal.mol-1, the latter an exothermic path. Results for the dissociation reactions are in agreement with the literature data. The kinetics of decomposition reactions, on the other hand, represent a new contribution to this work. Kinetic parameters, calculated with the canonical variant transition state method, for the most important reaction channels, contributed to the inclusion of these steps in the combustion model for dimethyl ether, as well as the composition of the combustion model for DMM. Finally, the thermodynamic and kinetic information on the unimolecular processes related to the combustion of DMM obtained in this work have shown to be an important contribution to the understanding of the combustion of alternative OME fuels.