Unraveling the low-temperature chemistry of nitrogenous compounds by informatively kinetic modeling of N,N-dimethylformamide

Abstract

N,N-Dimethylformamide (DMF) is a volatile organic pollutant prevalent in many modern industrial processes and an intermediate formed in nitrogenous compound low-temperature oxidation. The low-temperature oxidation of DMF was performed between 450 and 900 K under fuel-lean conditions (Φ = 0.5) in a jet-stirred reactor coupled with synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry. Weak negative temperature coefficient (NTC) behavior was observed within 520 - 650 K. A comprehensive kinetic model was developed based on the previous pyrolysis model. Key reaction parameters, including hydrogen atom abstraction, first O₂ addition to fuel radicals, and intermediate radical decomposition pathways, were determined through ab initio - transition state theory - RRKM/ME calculation. The model successfully predicts DMF consumption and major product formation, though discrepancies persist for certain intermediates due to remaining uncertainties in nitrogen chemistry. Kinetic analysis reveals that at NTC temperatures, DMF oxidation is predominantly controlled by carbonyl-site oxygen addition followed by rapid QOOH radical decomposition, generating CH₃NCH₂, CO₂, and OH through Waddington-type reactions and inhibiting second O₂ addition. In contrast, methyl-site O₂ addition exhibits higher reaction barriers for QOOH decomposition, enabling second oxygen addition and subsequent low-temperature chain-branching reaction pathways critical for NTC behavior. Based on recent advances in nitrogen compound oxidation kinetics, the generalized behavior of different types of nitrogen-containing compounds was further discussed. Compounds with primary and secondary nitrogen atoms rarely exhibit NTC behavior due to preferential HO₂ elimination from α-site ROO radicals via adjacent NH sites, effectively suppressing low-temperature reactivity. Conversely, tertiary nitrogen compounds lacking NH bonds can undergo efficient auto-oxidation through rapid intramolecular hydrogen migration. It generates highly oxygenated intermediates along with OH radicals and, therefore, is very likely to exhibit NTC behavior, though its magnitude depends on the competition between fuel-specific rates of oxygen addition and QOOH decomposition.

Novelty and significance statement

The negative temperature coefficient (NTC) behavior of nitrogenous compounds was first experimentally reported during the low-temperature oxidation of N,N-dimethylformamide. Leveraging advanced diagnostic techniques and comprehensive high-level ab initio calculations, we developed an informative kinetic model that successfully reproduces the observed oxidation behavior. Through detailed model analysis and integration with recent advances in nitrogenous compound low-temperature chemistry, we have formulated general principles governing NTC behavior in nitrogen-containing species. These findings significantly enhance our fundamental understanding of nitrogen chemical kinetics relevant to both low-temperature auto-ignition processes and pollutant abatement technologies.