Density functional theory studies of the non-catalytic and catalytic oxidative dehydrogenation reaction of n hexane to 1 and 2 hexene.

Abstract

One of the important areas in the present petrochemical industry is the catalytic production of olefins that have wide applications, including as starting materials for many valuable products, like polymers. Although the choice and advancement in the development of appropriate catalysts for such processes is a challenging effort, catalytic oxidative dehydrogenation (ODH) methods potentially provide for energy-favourable transformation of widely available hydrocarbon feedstocks into a variety of products, including olefins. The aim of this study was to use Density Functional Theory (DFT) methods to model the non-catalytic and catalytic (VMgO) ODH conversion of n-hexane hydrocarbon to 1- and 2-hexene, which are products that were characterised and obtained in low yields (< 20%) in our laboratory experiments. All the reaction pathways were modelled under experimental conditions of 573, 673 and 773 K and the relative total energies (ΔE#, ΔE, ΔG# and ΔG) were determined and in order to elucidate the non-catalytic and catalytic radical mechanisms for the reaction. In Chapter 2 of the thesis, the kinetically and thermodynamically most favourable non-catalytic reaction pathways were determined, with the rate-determining step (RDS) proposed as the interaction of the n-hexane and the O2 molecules through β-H abstraction (ΔE# = +42.4 kcal/mol) at 573 K. The most stable intermediates were found to be the alkoxy (C6H13O) and hydroxyl (OH) radicals. The propagation steps that lead to 1- and 2-hexene were proposed as likely to involve the two intermediate radicals, although the C6H13O radical may also contribute to side reactions that produce undesired oxygenates. Chapters 3 and 4 discuss the catalytic radical mechanisms of the interaction of n-hexane with H3VO4 and H4V2O7 model catalysts, respectively. Both the models comprise the vanadyl O atoms that are associated with the RDSs, through β-H abstraction at the kinetically and thermodynamically favourable temperature of 573 K. The calculated relative energies were ΔE# = +27.4 and +32.7 kcal/mol, for H3VO4 and H4V2O7, respectively. From the calculated value of ΔE# = +43.9 kcal/mol, the bridging O atom in H4V2O7 is not likely to activate n-hexane molecules. The produced C6H13 radical intermediate may either generate the desired olefin through the second H-abstraction by another vanadyl O in close proximity to it, or it may chemisorb to any of the surface O atoms, thereby enabling side-reaction channels for producing undesired products, such as the oxygenates. For both the catalyst models, the calculations show that the chemisorption pathways are kinetically and thermodynamically more favourable by ~10 kcal/mol than the H-abstraction pathways. This may be the reason for low yields (< 20%) that were obtained in our laboratory experiments for this catalytic system. The H-abstraction pathways that may lead to olefins are also likely to lead to the accumulation of OH groups on the catalyst surface. Low energy barriers for H-transfer and migration between two adjacent OH groups were calculated, with related intermediates stabilizing as a result of the formation of H2O. Barrier-less energies were also calculated for the reoxidation of the reduced V(III) by the O- species, to produce V(V) in both H3VO4 and H4V2O7. Finally, Chapter 5 discusses conclusions and the most likely mechanism for the combined non-catalytic and catalytic systems is proposed.