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

Damoyi, Nkululeko; Friedrich, Holger B.; Kruger, Hendrik G.; Willock, David J.

doi:10.1016/j.comptc.2017.05.026

Cited by 10 publications

(7 citation statements)

References 38 publications

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“…Our modelling results agree with these experi-mental findings as demonstrated by larger negative E value (−33.5 kcal/mol) for the vanadyl O site dehydrogenation reaction re-lative to the gas phase O2 E value (−14.7 kcal/mol). In our previous results on gas-phase ODH of n-hexane [41] (N.E. Damoyi H. F.), we found that the %C6H13 radical (Fig.…”

Section: The Reaction Equation For the Pathway Ismentioning

confidence: 60%

“…If the formation of 2-hexene proceeds through gas-phase H-ab-straction by O2, then the termination step (Fig. 2) may involve dis-proportionation of the produced hydroperoxyl radical to produce H2O and O2, as recorded in Table 1 (TS12-TS14) and our previous calculations [41] (N.E. Damoyi H. F.).…”

Section: The Reaction Equation For the Pathway Ismentioning

confidence: 69%

“…Although our modelling is an oversimplification of heterogeneous surface reactions because of the methodology followed, namely, gas-phase calculations on isolated active sites, we believe that the results will be essential in acquiring a theoretical baseline for future potential periodic-DFT calculations of this system. Furthermore, comparisons of the results with those ob-tained in our recent work [41] (N.E. Damoyi H.F.) on gas-phase non-catalytic ODH of n-hexane would be useful, since under certain ex-perimental conditions, gas-phase ODH mechanisms may compete with the catalytic ODH mechanisms.…”

Section: Introductionmentioning

confidence: 92%

“…The CeH bond activation through H-abstraction by the vanadyl O (V]O) has been identified as the rate-determining step (RDS) for alkanes ODH to alkenes on V2O5 and supported vanadium oxide in several mechanistic studies [58][59][60][61]. In our previous theore-tical work on the gas-phase noncatalytic ODH of n-hexane we con-cluded that the RDS is H-abstraction by 3 O2, with a barrier height of E # = +42.4 kcal/mol [41] (N.E. Damoyi H. F.).…”

Section: Activation Of N-hexanementioning

confidence: 99%

“…Our recent publication on the gas-phase non-catalytic ODH of n-hexane show that the rate-determining step is the abstraction of -hy-drogen by molecular O2 [41] (N.E. Damoyi H. F.).…”

Section: Introductionmentioning

confidence: 99%

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

Damoyi

Friedrich

Kruger

et al. 2018

Molecular Catalysis

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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 vana-dium 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 O2 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 (E # , E, G # and 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 CeH bond activation (-hydrogen abstraction) from the C6H14 chain by the vanadyl O, with a calculated 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-hydrogen on the radical intermediate fragment (%C6H13) can take place on a diff erent active V (V) = O site. Secondly this step may involve reaction with gas-phase molecular O2. These alternatives are compared computationally and results used to discuss some existing experimental data.

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Section: The Reaction Equation For the Pathway Ismentioning

confidence: 60%

Section: The Reaction Equation For the Pathway Ismentioning

confidence: 69%

Section: Introductionmentioning

confidence: 92%

Section: Activation Of N-hexanementioning

confidence: 99%

“…Our recent publication on the gas-phase non-catalytic ODH of n-hexane show that the rate-determining step is the abstraction of -hy-drogen by molecular O2 [41] (N.E. Damoyi H. F.).…”

Section: Introductionmentioning

confidence: 99%

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