Oxidation of small unsaturated methyl and ethyl esters

Multistructural Variational Reaction Kinetics of the Simplest Unsaturated Methyl Ester: H-Abstraction from Methyl Acrylate by H, OH, CH3, and HO2 Radicals

The H-abstraction reaction kinetics of methyl acrylate (MA) + H/OH/CH₃/HO₂ radicals have been investigated theoretically in the present work. For these reactions, the reaction energies and barrier heights are first computed using several density functionals and compared to the coupled cluster CCSD(T)-F12/jun-cc-pVTZ benchmark calculations. The M062X/maug-cc-pVTZ method shows the best performance with the smallest mean unsigned deviation (MUD) of 0.42 kcal mol^-1. Combined with the electronic structure calculations using the M062X/maug-cc-pVTZ method, the multistructural canonical variational transition-state theory (MS-CVT) with small-curvature tunneling (SCT) is employed to calculate the reaction rate constants at 500-2000 K. The variational effect is between 0.56 and 1.0, the multistructural torsional anharmonicity factor ranges from 0.004 to 4.57, and the tunneling coefficient is in the range of 0.5-4.70. Notably, given the existence of reactant complexes (RCs) between reactants and transition states for the reaction systems MA + OH/HO₂, we further compare the rate constants under the low-pressure limit (LPL) kinetic model, which treats the reaction as a single-step process and neglects RCs, and the pre-equilibrium model, which takes RCs into account in the reaction and treats the reaction as a two-step process. The rate constants calculated by these two models are similar within the combustion temperature range, and apparent differences occur at lower temperatures. In addition, we determine the branching ratios as a function of temperature and find that the methyl site (S3) abstractions by OH and H radicals are dominant in the low- and high-temperature ranges, respectively. Moreover, we update the kinetic model with the calculated H-abstraction rate constants to simulate the ignition delay times of MA. The simulations of the updated model are in good agreement with experimental results. The accurate reaction kinetics determined in this work are useful for the understanding and prediction of consumption branching fractions and ignition properties of the unsaturated methyl esters.

Molecular-Weight Growth in Ozone-Initiated Low-Temperature Oxidation of Methyl Crotonate

We report experiments of ozone-initiated low-temperature oxidation of methyl crotonate (MC, CH₃-CH═CH-C(O)OCH₃) from 420 to 660 K in a near-atmospheric-pressure jet-stirred reactor using photoionization molecular-beam mass spectrometry as a sampling technique. In this temperature regime, no typical low-temperature combustion (LTC) reactions have been observed for MC when oxygen (O₂) is used as the oxidizer. Upon ozone addition, significant oxidation of methyl crotonate is found. On the basis of experimentally observed energy-dependent mass peaks in combination with temperature-dependent mole fraction profiles and photoionization efficiency curves, we provide new insights into the methyl crotonate ozonolysis reaction network. The observed MC + O₃ products, C₅H₈O₅, are found to be related to the keto-hydroperoxides resulting from the isomerization of the primary ozonide. Evidence is also provided that molecular growth mainly results from cycloaddition reactions of the Criegee intermediate into aldehydes and alkenes as well as addition reactions of the Criegee intermediates to the double bond of methyl crotonate and sequential decomposition into ketones. Furthermore, species that contribute in large amounts to the low-temperature oxidation of methyl crotonate, like H₂O₂, CH₃OOH, CH₃OH, and HC(O)OH, are identified, and their mole fractions are reported. Additionally, preliminary modeling is performed which qualitatively captures the observed NTC behavior and reveals future research opportunities.

Direct Kinetic Measurements and Master Equation Modelling of the Unimolecular Decomposition of Resonantly-Stabilized CH2CHCHC(O)OCH3 Radical and an Upper Limit Determination for CH2CHCHC(O)OCH3 + O2 Reaction

Methyl-Crotonate (MC, (E)-methylbut-2-enoate, CH3CHCHC(O)OCH3) is a potential component of surrogate fuels that aim to emulate the combustion of fatty acid methyl ester (FAME) biodiesels with significant unsaturated FAME content. MC has three allylic hydrogens that can be readily abstracted under autoignition and combustion conditions to form a resonantly-stabilized CH2CHCHC(O)OCH3 radical. In this study we have utilized photoionization mass spectrometry to investigate the O2 addition kinetics and thermal unimolecular decomposition of CH2CHCHC(O)OCH3 radical. First we determined an upper limit for the bimolecular rate coefficient of CH2CHCHC(O)OCH3 + O2 reaction at 600 K (k ≤ 7.5 × 10−17 cm3 molecule−1 s−1). Such a small rate coefficient suggest this reaction is unlikely to be important under combustion conditions and subsequent efforts were directed towards measuring thermal unimolecular decomposition kinetics of CH2CHCHC(O)OCH3 radical. These measurements were performed between 750 and 869 K temperatures at low pressures (<9 Torr) using both helium and nitrogen bath gases. The potential energy surface of the unimolecular decomposition reaction was probed at density functional (MN15/cc-pVTZ) level of theory and the electronic energies of the stationary points obtained were then refined using the DLPNO-CCSD(T) method with the cc-pVTZ and cc-pVQZ basis sets. Master equation simulations were subsequently carried out using MESMER code along the kinetically important reaction pathway. The master equation model was first optimized by fitting the zero-point energy corrected reaction barriers and the collisional energy transfer parameters Δ E down , ref $\Delta{E_{{\text{down}},\;{\text{ref}}}}$ and n to the measured rate coefficients data and then utilize the constrained model to extrapolate the decomposition kinetics to higher pressures and temperatures. Both the experimental results and the MESMER simulations show that the current experiments for the thermal unimolecular decomposition of CH2CHCHC(O)OCH3 radical are in the fall-off region. The experiments did not provide definite evidence about the primary decomposition products.

Selectivity and kinetics of methyl crotonate hydrogenation over Pt/Al2O3

The hydrogenation of gas-phase methyl crotonate (MC) over Pt/Al₂O₃ was investigated with the aim to understand CC hydrogenation in unsaturated methyl esters.