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

Abstract

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 non-linear 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.

Review on the Relationship Between Liquid Aerospace Fuel Composition and Their Physicochemical Properties

“…Chatelain et al also evaluated the oxidative stability of C 8 isomers (Fig. 15b) [119]; except that of TMP (2,2,4-trimethylpentane), the IP was inversely related to the increased branching level, which can be explained by the dissociation energy of carbon-hydrogen bond, where the carbon-hydrogen bond dissociation energy follows the order primary carbon > secondary carbon > tertiary carbon. For TMP, a quaternary carbon on which no hydrogen atoms can be abstracted exists.…”

The Autoxidation of Alkenyl Succinimides—Mimics for Polyisobutenyl Succinimide Dispersants

“…The products observed indicate ASI autoxidation mechanisms are comparable with those observed in hydrocarbon autoxidation − which is initiated by the abstraction of labile hydrogen atoms by molecular oxygen (reaction ). The resultant carbon-centered radicals (R · ) can react with oxygen forming alkyl peroxyl radicals (reaction ).…”

“…No other alcohol isomers were observed, showing autoxidation was initiated by highly specific abstraction of allylic hydrogens. This can be explained by the C–H bond dissociation enthalpy (BDE), whereby a lower BDE and higher stability of the formed radical give a higher rate of abstraction. ,, BDE and carbon-centered radical stability increase in the order primary < secondary < tertiary < allylic, and so allylic alcohols were the major products observed. Allylic hydrogens at C 1 and C 4 (Figure ) were the dominant site of hydrogen abstraction.…”

“…A fully formulated engine oil lubricant comprises an additive package and a viscosity index improver dissolved in a base oil; liquid hydrocarbons either produced synthetically or refined from crude oil. Constituting around 90% (w/w) of the lubricant oil, the mechanisms of base oil autoxidation have been investigated in detail through the use of model hydrocarbons. − Smaller hydrocarbons − and fatty acid methyl esters , have been investigated to determine fuel degradation mechanisms and higher mass ester base oils , used for lubricant degradation studies. More recently, focus has shifted to the impact of lubricant degradation on friction and wear performance, such as the degradation of molybdenum , or zinc based friction modifiers. , However, studies on lubricant additive autoxidation mechanisms, are limited.…”

Kinetic Modeling of API Oxidation: (1) The AIBN/H2O/CH3OH Radical “Soup”

“…The software uses experimental and calculated thermodynamic and rate coefficient parameters from its database, and where data is missing, it is estimated . It is extensively utilized for modeling various gas-phase reactive systems − and has liquid-phase capabilities, , including an estimation scheme for temperature-dependent solvation effects on thermochemistry and a diffusion-limited kinetics correction. , While RMG applies solvent corrections to equilibrium constants and to reverse rates, it does not apply any solvent corrections to forward rate coefficients except for enforcing diffusion limits. Therefore, the solvent corrections for important reactions in the present work were separately calculated using COSMO-RS (TZVPD-Fine) and individually fed into the RMG database.…”