On the influence of the position of the double bond on the low-temperature chemistry of hexenes

Abstract

The chemistry of oxidation and autoignition of 1-, 2-, and 3-hexene has been studied after rapid compression between 630 and 850 K for stoichiometric mixtures with ''air.'' The phenomenology of autoignition has been recognized, and intermediate products formed before autoignition have been identified and analyzed. They mainly comprise of hexadienes, O-heterocycles, and aldehydes. There are many common products, because some of the intermediate alkenyl or alkenylperoxy radicals are delocalized. Saturated O-heterocycles are specific products formed by addition of HO 2 to the double bond. Unsaturated O-heterocycles are products typical of the long alkenyl chain. Saturated and unsaturated lower aldehydes are the products of OH addition to the double bond of hexenes and hexadienes. The relative abundance of the intermediates enables a better insight into the competition between the reactivity of the double bond and the reactivity of the alkenyl chain. According to the position of the double bond, the behavior of 3-hexene is dominated by the properties of the double bond whereas the behavior of 1-hexene is dominated by the properties of the alkenyl chain. The reactivity of the alkenyl chain is related to the type and number of C-H bonds, the ability of stabilized radicals to react, and the cyclic strain of the transition state of isomerization reactions. Therefore, 1-hexene reacts much more with the typical features of alkanes like a two-stage ignition with a cool flame and a negative temperature coefficient. 3-Hexene does not have typical features and 2-hexene has an intermediate behavior.

Autoignition behavior of unsaturated hydrocarbons in the low and high temperature regions

“…1) at low temperature, the reactivity of large alkenes depends on the length of the saturated portions aside the double bond: longer alkyl chains determine shorter ignition delay times, as already discussed in previous works [15][16][17];…”

“…The low temperature data presented here were collected by Vanhove et al [15] for the hexane isomers and Minetti et al [9] for 1-pentene in the Lille rapid compression machine facility. All the high temperature ignition delay times here presented were recently collected in Galway using the two shock tube described in the experimental section and have not been report previously.…”

“…The first systematic study on the effect of the position of the double bond in hexene isomers was provided by Vanhove et al [15], who determined the ignition behavior the three hexene isomers in a rapid compression machine at different pressure.…”

“…When comparing the reactivity of the different alkene isomers, the most extensive set of data is the one produced by Vanhove et al [15]. In this set of experiments is evident how the length of the saturated carbon chain determines the autoignition propensity of alkenes in the low temperature region.…”

Detailed chemical kinetic reaction mechanisms for primary reference fuels for diesel cetane number and spark-ignition octane number

“…Examples of improvements that will be included in the future include recent work of Taatjes et al [23] on alternative reaction pathways for reactions of alkyl radicals and O 2 to directly produce olefins via a concerted molecular elimination pathway will be implemented, and site-specific reactions of olefins can be improved following recent studies of Mehl et al [24] and VanHove et al [25].…”