Theoretical study of ignition reactions of linear symmetrical monoethers as potential diesel fuel additives: DFT calculations

Abstract

This work investigates the chemical reactivity of four linear symmetrical monoethers with molecular oxygen. Such oxygenated compounds may be considered as potential diesel fuel additives in order to reduce the ignition delay in diesel fuel engines. For this purpose, a kinetic study is proposed to clarify the relation between the molecular structure of the fuel molecule and its ignition properties. To this end, DFT calculations were performed for these reactions using B3LYP/6-311G(d,p) and BH&HLYP/6-311G(d,p) to determine structures, energies, and vibrational frequencies of stationary points as well as activated complexes involved in each gas-phase combustion initiation reaction of the monoethers CH 3 OCH 3 , C 2 H 5 OC 2 H 5 , C 3 H 7 OC 3 H 7 , or C 4 H 9 OC 4 H 9 with molecular oxygen. This theoretical kinetic study was carried out using electronic structure results and the transition state theory, to assess the rate constants for all studied combustion reactions. As it has been shown in our previous work [AbouRachid et al., J Mol Struct (Theochem) 2003, 621, 293], the cetane number (CN) of a pure organic molecule depends on the initiation rate of its homogeneous gas-phase reaction with molecular oxygen. Indeed, the calculated initiation rate constants of the Habstraction process of linear monoethers with O 2 show a very good correlation with experimental CN data of these pure compounds at T ϭ 1,000 K. This temperature is representative of the operating conditions of a diesel fuel engine.

Variational Effect and Anharmonic Torsion on Kinetic Modeling for Initiation Reaction of Dimethyl Ether Combustion

“…Francisco performed theoretical calculations on reaction and confirmed that the bimolecular reaction had a lower barrier height (217 and 195 kJ mol –1 at the PMP4/6-311G­(2df,2p)//UMP2/6-311G­(2df,2p) and CCSD­(T)/6-311G­(2df,2p)//UMP2/6-311G­(2df,2p), respectively) compared with the C–O bond fission (339–345 kJ mol –1 ), , and could be the primary initiation reaction during the oxidation of DME. Marrouni and co-workers in a theoretical study also verified that introducing an ether bridge in a hydrocarbon molecule increased the reactivity of the hydrogen atoms toward O 2 to shorten the autoignition delay time and reported barriers for reactions to be 154.64 and 178.28 kJ mol –1 , respectively, at the B3LYP/6-311G­(d,p) and BH&HLYP/6-311G­(d,p) levels.…”

New Applications for Soybean Biodiesel Glycerol

“…). The capacity of some ether oxygenated additives to improve cetane number can be explained by the presence of an oxygen bridge that increase the reactivity of the hydrogen atoms in the hydrocarbon chain of the neighboring carbons contributing to the onset of combustion [Abu-Rachid et al, 2003;Marrouni et al, 2008]. For example, the comparison between CN values of dimethyl ether (DME) (55-60) and propane (−20) suggests that the 6 hydrogen atoms of the methyl groups of CH 3 -O-CH 3 are much more reactive toward oxygen than the ones in CH 3 -CH 2 -CH 3 .…”