Abstract
In order to better understand the low-temperature oxidation chemistry of alkenes, 1-butene and i-butene oxidation experiments triggered by dimethyl ether (DME) were conducted in a jet-stirred reactor at 790 Torr, 500–725 K and the equivalence ratio of 0.35. Low-temperature oxidation intermediates involved in alcoholic radical chemistry and allylic radical chemistry were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). To better interpret the experimental data, a kinetic model was proposed based on our low-temperature oxidation model of DME and comprehensive oxidation models of 1-butene and i-butene in literature. Based on present experimental results and modeling analysis, alcoholic radical chemistry initiated by OH addition is mainly responsible for the low-temperature chain propagation of butenes, since the Waddington mechanism plays a dominant role compared with the chain-branching pathways through the second O2 addition. Allylic radical+HO2 reactions producing alkenyl hydroperoxides and fuel+O2 serve as the major chain-branching and chain-termination pathways, respectively, and they are competitive in the negative temperature coefficient (NTC) region. In contrast, chain-branching pathways originating from allylic radical+O2 and alkyl-like radical+O2 reactions have little contribution to the OH formation. Comparison with the simulation results of butane/DME mixtures demonstrates that butenes can largely inhibit the reactivity of DME at low temperatures due to its reduced low-temperature chain-branching process. However, in the NTC region, butenes may not be good OH absorbents since the allylic radicals can convert HO2 to OH and consequently enhance the oxidation reactivity.