Oxidation of dimethyl ether: Absolute rate constants for the self reaction of CH3OCH2 radicals, the reaction of CH3OCH2 radicals with O-2, and the thermal decomposition of CH3OCH2 radicals

The rate constant for the reaction of CH3OCH2 radicals with O-2 (reaction (1)) and the self reaction of CH3OCH2 radicals (reaction (5)) were measured using pulse radiolysis coupled with time resolved UV absorption spectroscopy. k(1) was studied at 296 K over the pressure range 0.025-1 bar and in the temperature range 296-473 K at 18 bar total pressure. Reaction (1) is known to proceed through the following mechanism: CH3OCH2 + O-2 <----> CH3OCH2O2# --> CH2OCH2O2H# --> 2HCHO + OH (k(prod)) CH3OCH2 + O-2 <----> CH3OCH2O2# + M --> CH3OCH2O2 + M (k(RO2)) 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 formaldehyde production. The k(1) values obtained at 296 K together with the available literature values for k(1) determined at low pressures were fitted using a modified Lindemann mechanism and the following parameters were obtained: k(RO2,0) = (9.4 +/- 4.2) X 10(-30) cm(6) molecule(-2) s(-1), k(RO2,infinity) = (1.14 +/- 0.04) x 10(-11) cm(3) molecule(-1) s(-1), and k(prod,0) = (6.0 +/- 0.5) X 10(-12) cm(3) molecule(-1) s(-1) where k(RO2,0) and k(RO2,infinity) are the overall termolecular and bimolecular rate constants for formation of CH3OCH2O2 radicals and k(prod,0) represents the bimolecular rate constant for the reaction of CH3OCH2 radicals with O-2 to yield formaldehyde in the limit of low pressure. k(RO2,infinity) = (1.07 +/- 0.08) X 10(-11) exp(-(46 +/- 27)/T) cm(3) molecule(-1) s(-1) was determined at 18 bar total pressure over the temperature range 296-473 K. At 1 bar total pressure and 296 K, k(5) = (4.1 +/- 0.5) X 10(-11) cm(3) molecule(-1) s(-1) and at 18 bar total pressure over the temperature range 296-523 K, k(5) = (4.7 +/- 0.6) X 10(-11) cm(3) molecule(-1) s(-1). As a part of this study the decay rate of CH3OCH2 radicals was used to study the thermal decomposition of CH3OCH2 radicals in the temperature range 573-666 K at 18 bar total pressure. The observed decay rates of CH3OCH2 radicals were consistent with the literature value of k(2) = 1.6 X 10(13)exp[- 12800/T) s(-1). The results are discussed in the context of dimethyl ether as an alternative diesel fuel. (C) 1997 John Wiley & Sons, Inc.