Effect of Catalyst Structure on Oxidative Dehydrogenation of Ethane and Propane on Alumina-Supported Vanadia

Abstract

The catalytic properties of Al 2 O 3 -supported vanadia with a wide range of VO x surface density (1.4-34.2 V/nm 2 ) and structure were examined for the oxidative dehydrogenation of ethane and propane. UV-visible and Raman spectra showed that vanadia is dispersed predominately as isolated monovanadate species below ~2.3 V/nm 2 . As surface densities increase, two-dimensional polyvanadates appear (2.3-7.0 V/nm 2 ) along with increasing amounts of V 2 O 5 crystallites at surface densities above 7.0 V/nm 2 . The rate constant for oxidative dehydrogenation (k 1 ) and its ratio with alkane and alkene combustion (k 2 /k 1 and k 3 /k 1 , respectively) were compared for both alkane reactants as a function of vanadia surface density. Propene formation rates (per V-atom) are ~8 times higher than ethene formation rates at a given reaction temperature, but the apparent ODH activation energies (E 1 ) are similar for the two reactants and relatively insensitive to vanadia surface density. Ethene and propene formation rates (per V-atom) are strongly influenced by vanadia surface density and reach a maximum value at intermediate surface densities (~8 V/nm 2 ). The ratio of k 2 /k 1 depends weakly on reaction temperature, indicating that activation energies for alkane combustion and ODH reactions are similar. The ratio of k 2 /k 1 is independent of surface density for ethane, but increase slightly with vanadia surface density for propane, suggesting that isolated structures prevalent at low surface densities are slightly more selective for alkane dehydrogenation reactions. The ratio of k 3 /k 1 decreases markedly with increasing reaction temperature for both ethane and propane ODH. Thus, the apparent activation energy for alkene combustion (E 3 ) is much lower than that for alkane dehydrogenation (E 1 ) and the difference between these two activation energies decreases with increasing surface density. The lower alkene selectivities observed at high vanadia surface densities are attributed to an increase in alkene adsorption enthalpies with increasing vanadia surface density. The highest yield of alkene is obtained for catalysts containing predominantly isolated monovanadate species and operated at high temperatures that avoid homogeneous reactions (<~800 K).2

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

“…As reported previously, 22 ODH rates on xVAl increase with increasing VO x surface density and reach constant values at ∼7 V/nm 2 . These trends reflect the higher VO x content and specific reactivity with increasing surface density, as more reactive polyvanadates replace less active and reducible monovanadate structures.…”

“…22 This trend reflects the higher reactivity of oligomeric VO x species relative to VO x monomers as the VO x coverage increases and is consistent with a concurrent decrease in absorption-edge energies and an increase in the rate of stoichiometric VO x reduction with H 2 . Figure 13d and e shows that k 2 /k 1 ratios increase and k 3 /k 1 ratios decrease with increasing VO x surface density on xVAl.…”

“…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.…”

Synthesis, Structure, and Catalytic Reactivity of Isolated V5+-Oxo Species Prepared by Sublimation of VOCl3 onto H-ZSM5

“…Larger V 5+ domains, such as in polyvanadates on Al 2 O 3 or V 2 O 4 2+ dimers on H-ZSM5, start to form as VO x surface density increases; these oligomers contain both terminal and bridging O-atoms, whereas monovandates only contain terminal oxygen. 43,70 The V-O-Al bond, which is expected to be more strained in dimers than in a monomers, a result of vicinal Al atom rearrangements when anchoring V 2 O 4 2+ , may also contribute to their enhanced activity. 69 The observed catalytic activity of V-ZSM5 for C 2 H 6 ODH, which is higher than in V-ZSM5 prepared by impregnation methods, is a result of highly disperse VO 2 + and V 2 O 4 2+ species deduced by complementary characterization techniques and studies of catalytic trends with V/Al f,f ratios.…”

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

“…[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.…”