A Combined Experimental and Modeling Study on Isopropyl Nitrate Pyrolysis

Alkyl nitrates thermally decompose by homolytic cleavage
of the
weak nitrate bond at very low temperatures (e.g., around 500 K at
reaction times of a few seconds). This provides the opportunity to
study the subsequent chemistry of the initially formed radical (or
its subsequent pyrolysis products, if unstable) and nitrogen dioxide
at such mild conditions. In this work this idea is applied to isopropyl
nitrate (iPN) pyrolysis, which is studied in a tubular reactor at
atmospheric pressure, temperatures ranging from 373 to 773 K, and
residence times of around 2 s. At the experimental conditions, iPN
decomposition starts at 473 K with O–N bond fission producing
isopropoxy radical (i-C3H7O) and NO2. i-C3H7O is rapidly converted to acetaldehyde
(CH3CHO), which is the most abundant product detected,
and methyl radicals. Other major products detected are formaldehyde
(CH2O), methanol (CH3OH), nitromethane (CH3NO2), NO, methane, formamide (CHONH2), and methyl nitrite (CH3ONO). Four literature nitrogen
chemistry modelsthree of those augmented with iPN specific
reactionshave been tested for their ability to predict the
iPN decomposition and product profiles. The mechanism by the Curran
group performs best, but it still underpredicts the observed high
formaldehyde and methanol yields. A rate analysis indicates that the
branching ratio of the reaction between methyl radicals and nitrogen
dioxide is of significant importance. Based on recent theoretical
and experimental data, new rate expressions for the two reactions
CH3 + NO2 → CH3O + NO and
CH3 + NO2 + He → CH3ONO2 + He are calculated and incorporated in the kinetic models.
It is shown that this change clearly improves the predictions, although
additional work is needed to achieve good agreement between calculated
and measured species profiles.