Density functional theory studies of the uncatalysed gas-phase oxidative dehydrogenation conversion of n -hexane to hexenes

Density Functional Theory (DFT) modelling studies were conducted for the activation of n-hexane in the gas-phase under experimental conditions of 573, 673 and 773K. The aim of the study was to establish the most favourable radical mechanism for the oxidative dehydrogenation (ODH) of n-hexane to 1- and 2-hexene. Modelling of the 3-hexene pathway was omitted due to absence of this product in laboratory experiments. Computations were performed using GAUSSIAN 09W and molecular structures were drawn using the GaussView 5.0 graphics interface. The B3LYP hybrid functional and the 6-311+g(d,p) basis set were utilized for all the atoms. The most kinetically and thermodynamically favourable pathways are proposed based on the determination of the relative total energies (ΔE#, ΔE, ΔG# and ΔG) for the different reaction pathways. The initial C-H activation step is β-H abstraction from n-hexane (C6H14) by molecular oxygen (O2) to form the alkoxy (C6H13O•) and hydroxyl (•OH) radicals. This is proposed as the rate-determining step (RDS) with the calculated ΔE# = +42.4 kcal/mol. Two propagation pathways that involve, separately, the C6H13O• and •OH radicals may lead to the formation of 2-hexene. In both the propagation pathways, the C6H13O• and •OH radicals activate further C6H14 molecules to produce C6H13OH and H2O, respectively, and the alkyl radicals (•C6H13). Thereafter, one pathway involves the interaction of the •C6H13 radical with further molecular O2, and leads to a second C-H activation step that yields 2-hexene and the peroxy radical (•OOH). The other pathway is associated with hydrogen transfer from the •OOH radical to C6H13OH that is produced earlier, leading to water and the alkyl peroxy radical (C6H13OO•). The C6H13OO• radical undergoes intramolecular H-abstraction to yield 2-hexene and the •OOH radical, and the latter disproportionate through intermediate •OH radicals to produce O2 and H2O in the termination step