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 (react ion (1)) and the self reaction of CH3OCH2 radicals (reaction (5)) were measured using pulse radiolysis coupled with time resolved UV absorpt ion 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 press ure. Reaction (1) is known to proceed through the following mechanism: CH3OCH2 + O-2 <----> CH3OCH2O2# --> CH2OCH2O2H# --> 2HCHO + OH (k(pro d)) CH3OCH2 + O-2 <----> CH3OCH2O2# + M --> CH3OCH2O2 + M (k(RO2)) k(1 ) = k(RO2) + k(prod), where k(RO2) is the rate constant for peroxy rad ical production and k(prod) is the rate constant for formaldehyde prod uction. The k(1) values obtained at 296 K together with the available literature values for k(1) determined at low pressures were fitted usi ng a modified Lindemann mechanism and the following parameters were ob tained: 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), a nd 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 bimolecu lar rate constants for formation of CH3OCH2O2 radicals and k(prod,0) r epresents the bimolecular rate constant for the reaction of CH3OCH2 ra dicals 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) mo lecule(-1) s(-1) was determined at 18 bar total pressure over the temp erature 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 pres sure 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 o f CH3OCH2 radicals was used to study the thermal decomposition of CH3O CH2 radicals in the temperature range 573-666 K at 18 bar total pressu re. 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 r esults are discussed in the context of dimethyl ether as an alternativ e diesel fuel. (C) 1997 John Wiley & Sons, Inc.