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.