Isotopic Tracer and Kinetic Studies of Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts

Chen, Kaidong; Khodakov, Andreï Y.; Yang, Jun; Bell, Alexis T.; Iglesia, Enrique

doi:10.1006/jcat.1999.2510

Cited by 313 publications

(398 citation statements)

References 23 publications

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“…Consistent with this, the best fit of the model to the data was insensitive to the values of 1 , 2 , and 3 . The insensitivity of the model to the value of 1 suggests that isoproproxide conversion rate constants are significantly larger than those for its formation; hence, θ reaches steady-state more rapidly than n r . In agreement with this conclusion, the steady-state values of θ are small, e.g., θ < 4 × 10 -4 at 16 kPa C 3 H 8 , 4 kPa O 2 on 10 wt% V 2 O 5 /Al 2 O 3 .…”

Section: Resultsmentioning

confidence: 99%

“…[21][22][23] Also, none of the reported work has examined the reduction and oxidation cycles that occur during the oxidative dehydrogenation (ODH) of alkanes, a reaction that proceeds via a Mars-van Krevelen redox cycle using lattice oxygen atoms in metal oxides. [1][2][3][4][24][25][26][27][28][29][30] In this study, we report the use of UV-visible spectroscopy in a transient mode to study the dynamics of redox cycles for catalytically active centers involved in propane ODH on alumina-supported vanadia catalysts. The analysis of these data leads to the determination of the rate coefficients for the reduction and reoxidation of active centers.…”

Section: Introductionmentioning

confidence: 99%

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In situ UV−Visible Spectroscopic Measurements of Kinetic Parameters and Active Sites for Catalytic Oxidation of Alkanes on Vanadium Oxides

et al. 2004

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In situ diffuse reflectance UV-visible spectroscopy was used to measure the dynamics of catalyst reduction and oxidation during propane oxidative dehydrogenation (ODH) on VO x /γ-Al 2 O 3 . Transients in UV-visible intensity in the near-edge region were analyzed using a mechanistic model of ODH reactions. Rate constants per site for the kinetically relevant reduction step (C-H bond activation) measured using this analysis are slightly larger than those obtained from steady-state ODH rates normalized by surface V. The ratio of these values provides a measure of the fraction of the V surface sites that are active for ODH (0.6-0.7, for V surface densities of 2.3-34 V nm -2 ). This suggests that some of the V atoms are either inaccessible or inactive. Reoxidation rate constants, which cannot be obtained from steady-state analysis, are 10 3 -10 5 times larger than those for the C-H bond activation reduction step.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

In situ UV−Visible Spectroscopic Measurements of Kinetic Parameters and Active Sites for Catalytic Oxidation of Alkanes on Vanadium Oxides

et al. 2004

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“…C 3 H 6 combustion rates (r 3 ) were obtained from the slope of C 3 H 6 selectivity data as a function of residence time. [16][17][18][19][20][21][22][23] Figure 2 shows Raman spectra at ambient temperature for VAl samples with a range of VO x surface densities treated in flowing dry air at 673 K for 1 h, and denoted as dehydrated samples throughout. The band at 1033 cm -1 was assigned to VdO stretches in monovanadates and polyvanadates, and the broad 750-1000 cm -1 bands were assigned to V-O-V stretches in two-dimensional polyvanadates.…”

Section: Methodsmentioning

confidence: 99%

“…17,20 ODH reactions on supported VO x catalysts involve redox cycles and kinetically relevant C-H bond activation steps that require electron transfer from O to V within VO x domains. 18,19,23,35,47 Chen et al 23 showed that ODH turnover rates increase with decreasing VO x UV-visible absorption edge energies and with increasing reducibility of VO x domains in H 2 . ODH turnover rates increased as polyvanadates became the predominant VO x species, suggesting that linkages between monovanadate structures and support cations decrease the reactivity and reducibility of V-oxo species.…”

Section: Introductionmentioning

confidence: 99%

Oxidative Dehydrogenation of Propane over V₂O₅/MoO₃/Al₂O₃ and V₂O₅/Cr₂O₃/Al₂O₃: Structural Characterization and Catalytic Function

2005

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The structure and catalytic properties of binary dispersed oxide structures prepared by sequential deposition of VO x and MoO x or VO x and CrO x on Al 2 O 3 were examined using Raman and UV-visible spectroscopies, the dynamics of stoichiometric reduction in H 2 , and the oxidative dehydrogenation of propane. VO x domains on Al 2 O 3 modified by an equivalent MoO x monolayer led to dispersed binary structures at all surface densities. MoO x layers led to higher reactivity for VO x domains present at low VO x surface densities by replacing V-O-Al structures with more reactive V-O-Mo species. At higher surface densities, V-O-V structures in prevalent polyvanadates were replaced with less reactive V-O-Mo, leading to lower reducibility and oxidative dehydrogenation rates. Raman, reduction, and UV-visible data indicate that polyvanadates predominant on Al 2 O 3 convert to dispersed binary oxide structures when MoO x is deposited before or after VO x deposition; these structures are less reducible and show higher UV-visible absorption energies than polyvanadate structures on Al 2 O 3 . The deposition sequence in binary Mo-V catalysts did not lead to significant differences in structure or catalytic rates, suggesting that the two active oxide components become intimately mixed. The deposition of CrO x on Al 2 O 3 led to more reactive VO x domains than those deposited on pure Al 2 O 3 at similar VO x surface densities. At all surface densities, the replacement of V-O-Al or V-O-V structures with V-O-Cr increased the reducibility and catalytic reactivity of VO x domains; it also led to higher propene selectivities via the selective inhibition of secondary C 3 H 6 combustion pathways, prevalent in VO x -Al 2 O 3 , and of C 3 H 8 combustion routes that lead to low alkene selectivities on CrO x -Al 2 O 3 . VO x and CrO x mix significantly during synthesis or thermal treatment to form CrVO 4 domains. The deposition sequence, however, influences catalytic selectivities and reduction rates, suggesting the retention of some of the component deposited last as unmixed domains exposed at catalyst surfaces. These findings suggest that the reduction and catalytic properties of active VO x domains can be modified significantly by the formation of binary dispersed structures. VO x -CrO x structures, in particular, lead to higher oxidative dehydrogenation rates and selectivities than do VO x domains present at similar surface densities on pure Al 2 O 3 supports.

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“…79 Focusing on the oxidation of a hydrocarbon, This reaction scheme occurs in many catalysed oxidations of hydrocarbons by metal oxides, such as: (i) propene oxidation to acrolein by molybdenum and bismuth oxides 81 (ii) total oxidation of volatile organic compounds like benzene and hexane by cerium oxides, 82 and (iii) oxidative dehydrogenation by vanadium oxides. 83 This mechanism is quite important both in industrial applications and for fundamental studies. In fact, due to the transfer of lattice components from the metal oxide to the products, this also leads to correlations between the catalytic activity and thermodynamic parameters characterizing the lattice, like the metal-oxygen bond strength.…”

Section: Oxygen Activation By a Metalmentioning

confidence: 99%

Catalytic oxygen activation versus autoxidation for industrial applications: a physicochemical approach

Liu

Ryabenkova

Conte

2015

Phys. Chem. Chem. Phys.

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The activation and use of oxygen for the oxidation and functionalization of organic substrates is among the most important reactions in a chemist's toolbox. Nevertheless, despite the vast literature on catalytic oxidation, the phenomenon of autoxidation, an ever-present background reaction that occurs in virtually every oxidation process, is often neglected. In contrast, autoxidation can affect the selectivity to a desired product, to those dictated by pure freeradical chain pathways, thus affecting the activity of any catalyst used to carry out a reaction.This critical review compares catalytic oxidation routes by transition metals versus autoxidation, particularly focusing on the industrial context, where highly selective and "green" processes are needed. Furthermore, the application of useful tests to discriminate between different oxygen activation routes, especially in the area of hydrocarbon oxidation, with the aim of an enhanced catalyst design, is described and discussed. In fact, one of the major targets of selective oxidation is the use of molecular oxygen as ultimate oxidant, combined with the development of catalysts capable to perform the catalytic cycle in a real energy and cost effective manner on a large scale. To achieve this goal, insights from metallo-proteins that could find application in some areas of industrial catalysis are presented, as well as considering the physicochemical principles that are at the fundament of oxidation and autoxidation processes.3

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Isotopic Tracer and Kinetic Studies of Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts

Cited by 313 publications

References 23 publications

In situ UV−Visible Spectroscopic Measurements of Kinetic Parameters and Active Sites for Catalytic Oxidation of Alkanes on Vanadium Oxides

In situ UV−Visible Spectroscopic Measurements of Kinetic Parameters and Active Sites for Catalytic Oxidation of Alkanes on Vanadium Oxides

Oxidative Dehydrogenation of Propane over V₂O₅/MoO₃/Al₂O₃ and V₂O₅/Cr₂O₃/Al₂O₃: Structural Characterization and Catalytic Function

Catalytic oxygen activation versus autoxidation for industrial applications: a physicochemical approach

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Isotopic Tracer and Kinetic Studies of Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts

Cited by 313 publications

References 23 publications

In situ UV−Visible Spectroscopic Measurements of Kinetic Parameters and Active Sites for Catalytic Oxidation of Alkanes on Vanadium Oxides

In situ UV−Visible Spectroscopic Measurements of Kinetic Parameters and Active Sites for Catalytic Oxidation of Alkanes on Vanadium Oxides

Oxidative Dehydrogenation of Propane over V2O5/MoO3/Al2O3 and V2O5/Cr2O3/Al2O3: Structural Characterization and Catalytic Function

Catalytic oxygen activation versus autoxidation for industrial applications: a physicochemical approach

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Oxidative Dehydrogenation of Propane over V₂O₅/MoO₃/Al₂O₃ and V₂O₅/Cr₂O₃/Al₂O₃: Structural Characterization and Catalytic Function