Low Temperature Oxidation Kinetics of Biodiesel Molecules: Rate Rules for Concerted HO₂ Elimination from Alkyl Ester Peroxy Radicals

Abstract

In an attempt to construct detailed kinetic mechanisms for biodiesel fuels on the fly, high-pressure rate rules for the concerted HO elimination reaction class were derived using a comprehensive training reaction set of more than 60 reactions involving different alkyl methyl/ethyl ester peroxy radicals (RCOOR')-OO. Using the composite electronic structure method CBS-QB3 in combination with classical statistical mechanics and the transition state theory (TST) rate model, high-pressure rate constants for the reactions in the training set as well as thermodynamic properties for the species involved were calculated. The corrections from Eckart tunneling and hindered internal rotation (HIR) treatments were also included in the calculations. The results reveal that the ester group (-COO-) selectively promotes the reaction when compared with the traditional hydrocarbon fuels; thus it is recommended that the seven derived rate rules for the title reaction class (including the thermodynamic data of the species involved in the NASA format) should be used for construction of detailed kinetic mechanisms for real biodiesel molecules.

Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene

“…The rate coefficients for O 2 elimination from NO 3 -isoprene-O 2 radicals are then obtained from the forward O 2 -addition reaction rates and theory-derived equilibrium constants, similar to the approach by Peeters et al 13 More details, the temperature-dependent rate coefficients, and the equilibrium constants are provided in the ESI. † [70][71][72][73] and can be neglected. Ring closure reactions should be divided into three categories: formation of 5-membered peroxide rings with an endo-cyclic alkyl radical site, formation of 5-membered rings with an exo-cyclic radical site, and formation of 6-membered rings (here always with an endo-cyclic radical site).…”

Investigating the kinetics of the intramolecular H-migration reaction class of methyl-ester peroxy radicals in low-temperature oxidation mechanisms of biodiesel

“…The asymmetric Eckart method is an efficient option for the unimolecular reactions of the propyl peroxy radical when compared to more complex tunneling treatments. 79 Numerous studies on the H-migration reactions of peroxy radicals 7,13,14,17,18,31,56 have effectively utilized the asymmetric Eckart method for tunneling corrections. The asymmetric Eckart tunneling correction requires an input parameter called barrier width ( L ), which is calculated using the following formula: 80 where F * is the second-order force constant corresponding to the imaginary frequency of the transition state along the reaction coordinate.…”

“…The presence of the oxygenated chemical structure (-C(QO)O-) in biodiesel molecules alters the electronic structure, leading to different chemical and physical properties of biodiesel compared to traditional hydrocarbons. 1,6,7 These properties include decreased soot formation, lower emissions of unburned hydrocarbon (HC), CO 2 , CO, sulfates, PAHs, nitrated PAHs, and particulate matter (PM), but greater NO x . [8][9][10] Therefore, understanding the influence of the ester on the combustion properties of biodiesel is significant for reliably predicting the combustion behaviour of biodiesel.…”

A computational study of the HO2+ SO3→ HOSO2+3O2reaction catalyzed by a water monomer, a water dimer and small clusters of sulfuric acid: kinetics and atmospheric implications

“…7,57,58 The Lennard-Jones (L-J) parameters ε/ k B = 71.4 K and σ = 3.798 Å were used for N 2 , 59,60 while ε/ k B = 584 K and σ = 5.4 Å were estimated for HOSO 4 7,61 and its isomers. It should be noted that the hindered internal rotation 7,57,58,62–68 (HIR) treatment has been taken into account for the HO 2 + SO 3 reaction without and with X , and the details of HIR treatment are presented in Part A of the ESI †. It should also be noted that the rate constants for the HO 2 + SO 3 reaction without and with X were computed in the temperature range of 280–320 K and the pressure range of 10–760 Torr.…”