High temperature pyrolysis of methyl (and ethyl) nitrate

Abstract

The decomposition of methyl nitrate (0.25–1.75% in argon) has been studied behind incident shock waves by spectroscopically monitoring visible and u.v. absorptions in the reacting gas. The principal experiments covered the following temperature, density, and pressure ranges: T=880–1250 K, ρ=2.8×10−5−2.4×10−4 mol/cc, P=2.8–18.6 atm. The pyrolysis under these conditions was observed to occur in two distinct stages: the initial dissociation, and subsequent fast reactions of the methoxy radical, occurred almost instantaneously at the shock front; this was then followed by a much slower stage of reaction involving decomposition of formaldehyde in the presence of nitrogen oxides. The kinetics of this second stage reaction were determined; it is characterized by a total order slightly less than 2.0, and an apparent activation energy of about 24 kcal/mol. Second stage rates for ethyl nitrate (1.0% in argon) were virtually identical to those for methyl nitrate, even though much less NO2 is present in the case of ethyl nitrate. Second stage reactions did not occur below about 800 K. Modeling calculations show that at these temperatures, methoxy radicals will dissociate in the first reaction stage, rather than disproportionate as at lower temperatures. In the second stage of reaction, NO2 interrupts the normal chain decomposition of formaldehyde to produce OH radicals, which then propagate the chain by hydrogen abstraction from formaldehyde. It is proposed that direct attack of formyl radicals on formaldehyde, i.e., HCO+HCHO →H2+CO+HCO or HCO+HCHO→CO+CH3O may be the dominant pathway for formaldehyde decomposition (in the absence of NO2) for temperatures too low for fast unimolecular dissociation of HCO.