Dimethyl Ether Oxidation:  Kinetics and Mechanism of the CH₃OCH₂ + O₂ Reaction at 296 K and 0.38−940 Torr Total Pressure

Abstract

The title reaction was studied at 296 K and 0.38-940 Torr total pressure using a FTIR smog chamber technique. The overall rate constant for reaction of CH 3 OCH 2 radicals with O 2 may be written, k 1 ) k RO2 + k prod , where k RO2 is the rate constant for peroxy radical production and k prod is the rate constant for the production of other species from reaction 1. k 1 was measured relative to the pressure independent reaction of CH 3 OCH 2 radicals with Cl 2 (k 4 ). There was no discernible effect of pressure on k 1 in the range 200-940 Torr. Between 200 and 2 Torr total pressure k 1 decreased by approximately a factor of 2. For pressures below 2 Torr k 1 was again independent of pressure. The reaction proceeds via the formation of an activated complex, CH 3 OCH 2 O 2 # , that is either collisionally stabilized to form the peroxy radical, CH 3 OCH 2 O 2 , or undergoes intramolecular H-atom abstraction followed by decomposition to give two formaldehyde molecules and an OH radical: H # f 2HCHO + OH. The products from this reaction were studied as a function of total pressure. The molar yield of formaldehyde increased from <2% at 700 Torr total pressure to ∼200% at 0.38 Torr total pressure, while the combined yield of methyl formate and methoxy methylhydroperoxide decreased from ∼100% to 4% over the same pressure range. Fitting the product yields and relative rate data using a modified Lindemann expression gave the following rate constants: k RO2,0 /k 4 ) (1.97 ( 0.28) × 10 -19 cm 3 molecule -1 , k RO2,∞ /k 4 ) 0.108 ( 0.004, and k prod,0 /k 4 ) (6.3 ( 0.5) × 10 -2 where k RO2,0 and k RO2,∞ are the overall termolecular and bimolecular rate constants for formation of the CH 3 OCH 2 O 2 radical and k prod,0 represents the bimolecular rate constant for the reaction of CH 3 OCH 2 radicals with O 2 to yield formaldehyde in the limit of low pressure. These data and absolute rate data from the literature were used to derive a rate constant for the reaction of CH 3 OCH 2 radicals with Cl 2 of (1.00 ( 0.16) × 10 -10 cm 3 molecule -1 s -1 . The results are discussed in the context of the use of dimethyl ether as an alternative diesel fuel.Dimethyl Ether Oxidation

Pressure dependence of the rate coefficients and product yields for the reaction of CH3CO radicals with O2

“…This mechanism is analogous to that proposed to explain the production of OH in the reaction of O 3 with alkenes [27] and in the reaction of CH 3 OCH 2 with O 2 [28,29]. The ultimate fate of the biradical, CH 2 C(O)O, is unknown.…”

Comparisons of CBS-q and G2 calculations on thermodynamic properties, transition states, and kinetics of dimethyl-ether + O2 reaction system

“…The thin broken line shows CH 2 O yield based on G2-determined properties. Both calculation methods underestimate the rate constant compared to data of Sehested [2]. A QRRK fall-off plot matching the data of Sehested is obtained by lowering the E a for TS2 by 3.3 kcal/mol with CBS-q-determined thermodynamic properties.…”

“…The comparison of formaldehyde yields with CBS-q, G2, and experimental data are shown in Figure 6. The asterisk symbol represents experimental data of Sehested et al at 296 K and 0.38 to 940 Torr total pressure [2]. The thin continuous line shows CH 2 O yield based on CBS-q-determined thermodynamic properties.…”

“…The basic requirement of the isodesmic reaction is that the number of bonds of each formal chemical bond type is conserved in the reaction. In reaction (2) (2) is taken into account.…”

Rate constants for the gas-phase reactions of Cl atoms with a series of ethers

“…Then this latter concentration was considered to be suitable to prevent the RЉ ϩ Cl 2 reaction without decreasing the detection sensitivity of Cl atoms too much. In the case of DME, Sehested et al [15] reported a ratio of k(CH 3 OCH 2 ϩ O 2 )/k(CH 3 OCH 2 ϩ Cl 2 ) ϭ (0.06 -0.07) in the pressure range 15 -60 Torr. By using this ratio for our experimental conditions, we obtain (k(CH 3 …”