Kinetics and Mechanism of Oxidative Dehydrogenation of Propane on Vanadium, Molybdenum, and Tungsten Oxides

Abstract

The effect of cation identity on oxidative dehydrogenation (ODH) pathways was examined using two-dimensional VO x , MoO x , and WO x structures supported on ZrO 2 . The similar kinetic rate expressions obtained on MoO x and VO x catalysts confirmed that oxidative dehydrogenation of propane occurs via similar pathways, which involve rate-determining C-H bond activation steps using lattice oxygen atoms. The activation energies for propane dehydrogenation and for propene combustion increase in the sequence VO x /ZrO 2 < MoO x /ZrO 2 < WO x /ZrO 2 ; the corresponding reaction rates decrease in this sequence, suggesting that turnover rates reflect C-H bond cleavage activation energies, which are in turn influenced by the reducibility of these metal oxides. Propane ODH activation energies are higher than for propene combustion. This leads to an increase in maximum alkene yields and in the ratio of rate constants for propane ODH and propene combustion as temperature increases. This difference in activation energy (48-61 kJ/mol) between propane ODH and propene combustion is larger than between bond dissociation enthalpies for the weakest C-H bond in propane and propene (40 kJ/mol) and it increases in the sequence VO x /ZrO 2 < MoO x /ZrO 2 < WO x /ZrO 2 . These results suggest that relative propane ODH and propene combustion rates depend not only on C-H bond energy differences but also on the adsorption enthalpies for propene and propane, which reflect the Lewis acidity of cations involved in π bonding of alkenes on oxide surfaces. The observed difference in activation energies between propane ODH and propene combustion increases as the Lewis acidity of the cations increases (V 5+ < Mo 6+ < W 6+ ).

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

“…Step 2 is the kinetically relevant step, [29][30] and rate constants of subsequent steps are, therefore, inaccessible from steady-state rate data.…”

“…[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.…”

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

“…In contrast, k 3 /k 1 decreases markedly with increasing temperature, as expected from the lower activation energies involved in cleaving allylic C-H groups in propene as compared to those required for activation of stronger methylene C-H bonds in propane. 19 Temperature effects on k 2 /k 1 are stronger on 10V12MoAl than on 10VAl or 10V12CrAl samples, but temperature effects on k 3 /k 1 are more marked on 10V12CrAl than on 10VAl or 10V12MoAl.…”

“…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.…”

“…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.…”