DIMETHYL ETHER OXIDATION - KINETICS AND MECHANISM OF THE CH3OCH2-2 REACTION AT 296 K AND 0.38-940 TORR TOTAL PRESSURE(O)

The title reaction was studied at 296 K and 0.38-940 Torr total pressu re using a FTIR smog chamber technique. The overall rate constant for reaction of CH3OCH2 radicals with O-2 may be written, k(1) = k(RO2) k(prod), where k(RO2) is the rate constant for peroxy radical producti on and k(prod) is the rate constant for the production of other specie s from reaction 1. k(1) was measured relative to the pressure independ ent reaction of CH3OCH2 radidals with Cl-2 (k(4)) There was no discern ible 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, CH3O CH2O2#, that is either collisionally stabilized to form the peroxy rad ical, CH3OCH2O2, or undergoes intramolecular H-atom abstraction follow ed by decomposition to give two formaldehyde molecules and an OH radic al: CH3OCH2 + O-2 reversible arrow CH3OCH2O2#, CH3OCH2O2# + M --> CH3O CH2O2 + M, CH3OCH2O2# --> CH2OCH2O2H# --> 2HCHO + OH. The products fro m this reaction were studied as a function of total pressure. The mola r yield of formaldehyde increased from <2% at 700 Torr total pressure to similar to 200% at 0.38 Torr total pressure, while the combined yie ld of methyl formate and methoxy methylhydroperoxide decreased from si milar to 100% to 4% over the same pressure range. Fitting the product yields and relative rate data using a modified Lindemann expression ga ve the following rate constants: k(RO2.0)/k(4) = (1.97 +/- 0.28) x 10( -19) cm(3) molecule(-1), k(RO2 infinity)/k(4) = 0.108 +/- 0.004, and k (prod,0)/k(4) = (6.3 +/- 0.5) x 10(-2) where k(RO2.0) and k(RO2,infini ty) are the overall termolecular and bimolecular rate constants for fo rmation of the CH3OCH2O2 radical and k(prod,0) represents the bimolecu lar rate constant for the reaction of CH3OCH2 radicals with O-2 to yie ld 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 CH3OCH2 radicals with Cl-2 of (1.00 +/- 0.16) x 10(-10 ) cm(3) molecule(-1) s(-1). The results are discussed in the context o f the use of dimethyl ether as an alternative diesel fuel.