A DFT mechanistic study of the ODH of n-hexane over isolated H3VO4

作者：Danrioyi Nkululeko E^*; Friedrich Holger B; Kruger Gert H; Willock David

来源：Molecular Catalysis, 2018, 452: 83-92.

DOI：10.1016/j.mcat.2018.03.019

• DFT
• ODH
• n-hexane
• RDS
• Mechanism
• Vanadia

Catalytic (H3VO4) oxidative dehydrogenation (ODH) mechanistic studies of the activation of n-hexane have been conducted by means of Density Functional Theory (DFT).
Catalytic oxidative dehydrogenation is an important strategy for the conversion of alkanes to alkenes to provide useful chemical feedstocks from saturated hydrocarbons. Transition metal oxide catalysts based on vanadium provide an important class of catalysts for this reaction. The catalyst is usually prepared with vanadium in a high oxidation state prepared as an over layer supported on a relatively inert main group oxide (silica, alumina, etc.). Activation of the hydrocarbon is then energetically possible through the reduction of vanadium cations which can be subsequently re-oxidised using molecular O-2 to complete the catalytic cycle.
The aim of this study was to use density functional theory to explore the catalytic mechanism of this type of reaction using the conversion of n-hexane to 1- and 2-hexene as an illustrative example. Calculations are performed for the 1- and 2-hexene radical pathways and the results extrapolated to discuss the expected selectivity under laboratory experimental conditions (573, 673 and 773 K). Consideration of 3-hexene is excluded as in our earlier experimental studies and in the general literature this product is not reported. The stationary points on the potential energy surfaces were characterized and the associated geometries and relative energies (Delta E-#, Delta E, Delta G(#) and Delta G) were determined. The relative energies of all intermediates and transition states identified are used to lend insight on the mechanistic pathways for the reaction.
We have concentrated on the role of the transition metal in this chemistry and so the catalyst model chosen is an isolated, tetrahedral H3VO4 cluster containing one vanadyl bond, V(V)=O. The calculated rate-limiting step is the C-H bond activation (beta-hydrogen abstraction) from the C6H14 chain by the vanadyl O, with a calculated Delta E-# = +27.4 kcal/mol. This produces a C6H13HOH3VO3 complex as an intermediate with vanadium reduced to V(IV). There are then two possible routes for the propagation step that leads to 2-hexene. Firstly, the abstraction of the second gamma-hydrogen on the radical intermediate fragment (center dot C6H13) can take place on a different active V (V) = O site. Secondly this step may involve reaction with gas-phase molecular O-2. These alternatives are compared computationally and results used to discuss some existing experimental data.

• 出版日期2018-6