Structure-Reactivity Relationships in Fuel Stability: Experimental and Kinetic Modeling Study of Isoparaffin Autoxidation

Liquid phase stability is a major concern in the transportation and the energy field where fuels, lubricants, and additives have to be stable from their production site to their application (engine, combustors). Although alkanes are major constituents of commercial fuels and well-documented solvents, their respective reactivities and selectivities in autoxidation are poorly understood. This experimental and modeling study aims at (i) enhancing the current knowledge on alkane autoxidation and (ii) reviewing and correcting the previously established structure reactivity relationships in alkane autoxidation. Experimentally, this study investigates the influence of branching [0-3] and temperature [373-433 K] on the autoxidation of alkanes using four octane isomers: n-octane (C8), 2-methylheptane (MH), 2,5-dimethylhexane (DMH), and the 2,2,4-trimethylpentane (TMP). Induction Period (IP) and qualitative species identification are used to characterize the autoxidation processes of alkanes. The present study also presents new detailed liquid-phase chemical mechanisms obtained with an automated reaction mechanism generator. Experimental results highlight a nonlinear effect of the paraffins branching on IP according to compound structure and similar oxidation products for both normal and branched paraffins. The four iso-octanes mechanisms reproduce fairly well the temperature and the branching effects on IP within a factor of 4 for high temperature range (T > 403 K). From rate-of-reaction and sensibility analyses, similarities in alkane autoxidation have been evidenced with notably the key role of peroxy radicals in both normal and branched alkane autoxidation. The origin of the structure-reactivity relations was confirmed from a kinetic point of view with the main role of the hydrogen type on the molecule. Finally, based on experimental results available in literature, an empirical relation involving simple descriptors (number of carbons, type of carbons, temperature) is proposed to estimate alkane stability.