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.