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

Low-temperature propane oxidative dehydrogenation over UiO-66 supported vanadia catalysts: Role of support confinement effects

Overoxidation is the principal barrier against commercializing propane oxidative dehydrogenation (PODH) catalysts for propylene production. The current approach to reducing overoxidation, i.e., coating the non-selective support surface with a monolayer of active phase, can itself increase the probability of overoxidation of the produced propylene due to polymerization of active phase species. Incorporating the "confinement agents" onto the metal oxide support might be considered as an alternative solution to prevent hydrocarbons from reaching the support and overoxidizing. Herein, the UiO-66 metal-organic framework, which contains numerous organic ligands connected to the zirconia nodes, was used as support for the vanadia active phase to highlight the role of support's confinement effects on the overall catalytic performance toward the PODH. The UiO-66 supported vanadia catalysts with various vanadium loadings were fabricated via an ultrasonic-assisted wet impregnation procedure. The catalytic function is related to the underlying chemical processes at catalyst surfaces using physicochemical characterization techniques, PODH performance measurements, and machine learning tools. The results showed that the catalyst with a relatively low vanadia density of 2.7 nm-2, equivalent to less than half of the entire support surface coverage, could achieve propylene productivity of 4.43 [Formula: see text] , propane conversion of 17.1%, and propylene selectivity of 49.7% at 350 °C.

Creating self-assembled arrays of mono-oxo (MoO3)1 species on TiO2(101) via deposition and decomposition of (MoO3)n oligomers

Hierarchically ordered oxides are of critical importance in material science and catalysis. Unfortunately, the design and synthesis of such systems remains a key challenge to realizing their potential. In this study, we demonstrate how the deposition of small oligomeric (MoO3)1-6 clusters-formed by the facile sublimation of MoO3 powders-leads to the self-assembly of locally ordered arrays of immobilized mono-oxo (MoO3)1 species on anatase TiO2(101). Using both high-resolution imaging and theoretical calculations, we reveal the dynamic behavior of the oligomers as they spontaneously decompose at room temperature, with the TiO2 surface acting as a template for the growth of this hierarchically structured oxide. Transient mobility of the oligomers on both bare and (MoO3)1-covered TiO2(101) areas is identified as key to the formation of a complete (MoO3)1 overlayer with a saturation coverage of one (MoO3)1 per two undercoordinated surface Ti sites. Simulations reveal a dynamic coupling of the reaction steps to the TiO2 lattice fluctuations, the absence of which kinetically prevents decomposition. Further experimental and theoretical characterizations demonstrate that (MoO3)1 within this material are thermally stable up to 500 K and remain chemically identical with a single empty gap state produced within the TiO2 band structure. Finally, we see that the constituent (MoO3)1 of this material show no proclivity for step and defect sites, suggesting they can reliably be grown on the (101) facet of TiO2 nanoparticles without compromising their chemistry.

Propane Oxidative Dehydrogenation on Vanadium-Based Catalysts under Oxygen-Free Atmospheres

Catalytic propane oxidative dehydrogenation (PODH) in the absence of gas phase oxygen is a promising approach for propylene manufacturing. PODH can overcome the issues of over-oxidation, which lower propylene selectivity. PODH has a reduced environmental footprint when compared with conventional oxidative dehydrogenation, which uses molecular oxygen and/or carbon dioxide. This review discusses both the stoichiometry and the thermodynamics of PODH under both oxygen-rich and oxygen-free atmospheres. This article provides a critical review of the promising PODH approach, while also considering vanadium-based catalysts, with lattice oxygen being the only oxygen source. Furthermore, this critical review focuses on the advances that were made in the 2010–2018 period, while considering vanadium-based catalysts, their reaction mechanisms and performances and their postulated kinetics. The resulting kinetic parameters at selected PODH conditions are also addressed.

High-Performance Vapor-Phase Selective Oxidation of Ethyl Lactate to Ethyl Pyruvate over SiO2 Supported PMoVNb Oxides

This paper describes the application of P-Mo-V-Nb/SiO2 catalysts for the selective oxidation of ethyl lactate (EL) to ethyl pyruvate (EP). The P-Mo-V-Nb/SiO2 catalysts exhibit superior performance for EP selectivity than the corresponding samples of binary V-Nb/SiO2 ternary P-Mo-V/SiO2 and P-Mo-Nb/SiO2 catalysts at same temperatures. The origin of high EP selectivity of the P-Mo-V-Nb/SiO2 catalysts is explored and attributed to the synergistic effect of P, Mo, V, and Nb mixed oxides presented on the surface of the catalyst. The highly dispersive sites separated active species under the action of phosphorus, suppressing over oxidation and improving the selectivity. The existence of MoO3 to provide higher oxidation for catalyst. The redox cycle of V and Nb oxides could be completed through electron transfer between lattice oxygen and metal cations. Moreover, the weak acidity of catalyst surface is favorable to avoid the decarboxylation reaction to target a high selectivity of EP. Therefore, the P-Mo-V-Nb/SiO2 catalyst obtained the maximum yield of 91.8% with a selectivity of 93.8% and a conversion of 99.0% simultaneously at 280 °C.

Behavior of Molybdenum⁻Vanadium Mixed Oxides in Selective Oxidation and Disproportionation of Toluene

This study deals with the behavior of molybdenum–vanadium (Mo/V) mixed oxides catalysts in both disproportionation and selective oxidation of toluene. Samples containing different Mo/V ratios were prepared by a modified method using tetradecyltrimethylammonium bromide and acetic acid. The catalysts were characterized using several techniques: nitrogen adsorption–desorption isotherms, X-Ray diffraction (XRD), ammonia temperature-programmed desorption (TPD-NH₃), temperature-programmed reduction by hydrogen (H₂-TPR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier-transform infrared-spectroscopy (FTIR) and ultraviolet-visible spectroscopies (UV–VIS). The XRD results evidenced the presence of orthorhombic α-MoO₃ and V₂O₅ phases, as well as monoclinic β-MoO₃ and V₂MoO₈ phases, their abundance depending on the Mo to V ratio, while the TPD-NH₃ emphasized that, the total amount of the acid sites diminished with the increase of the Mo loading. The TPR investigations indicated that the samples with higher Mo/V ratio possess a higher reducibility. The main findings of this study led to the conclusion that the presence of strong acid sites afforded a high conversion in toluene disproportionation (Mo/V = 1), while a higher reducibility is a prerequisite to accomplishing high conversion in toluene oxidation (Mo/V = 2). The catalyst with Mo/V = 1 acquires the best yield to xylenes from the toluene disproportionation reaction, while the catalyst with Mo/V = 0.33 presents the highest yield to benzaldehyde.

Pathways, mechanisms, and kinetics: a strategy to examine byproduct selectivity in partial oxidation catalytic transformations on reducible oxides

We review experimental practices, common reaction pathways, and kinetic modeling strategies effective in understanding partial oxidation catalysis over reducible oxides.

V-Containing Mixed Oxide Catalysts for Reduction–Oxidation-Based Reactions with Environmental Applications: A Short Review

V-containing mixed oxide catalytic materials are well known as active for partial oxidation reactions. Oxidation reactions are used in industrial chemistry and for the abatement of pollutants. An analysis of the literature in this field during the past few years shows a clear increase in the use of vanadium-based materials as catalysts for environmental applications. The present contribution makes a brief revision of the main applications of vanadium containing mixed oxides in environmental catalysis, analyzing the properties that present the catalysts with a better behavior that, in most cases, is related with the stabilization of reduced vanadium species (as V4+/V3+) during reaction.

Identifying active functionalities on few-layered graphene catalysts for oxidative dehydrogenation of isobutane

The general consensus in the studies of nanostructured carbon catalysts for oxidative dehydrogenation (ODH) of alkanes to olefins is that the oxygen functionalities generated during synthesis and reaction are responsible for the catalytic activity of these nanostructured carbons. Identification of the highly active oxygen functionalities would enable engineering of nanocarbons for ODH of alkanes. Few-layered graphenes were used as model catalysts in experiments to synthesize reduced graphene oxide samples with varying oxygen concentrations, to characterize oxygen functionalities, and to measure the activation energies for ODH of isobutane. Periodic density functional theory calculations were performed on graphene nanoribbon models with a variety of oxygen functionalities at the edges to calculate their thermal stability and to model reaction mechanisms for ODH of isobutane. Comparing measured and calculated thermal stability and activation energies leads to the conclusion that dicarbonyls at the zigzag edges and quinones at armchair edges are appropriately balanced for high activity, relative to other model functionalities considered herein. In the ODH of isobutane, both dehydrogenation and regeneration of catalytic sites are relevant at the dicarbonyls, whereas regeneration is facile compared with dehydrogenation at quinones. The catalytic mechanism involves weakly adsorbed isobutane reducing functional oxygen and leaving as isobutene, and O2 in the feed, weakly adsorbed on the hydrogenated functionality, reacting with that hydrogen and regenerating the catalytic sites.

Selective catalytic oxidation of ethanol to acetic acid on dispersed Mo-V-Nb mixed oxides

The direct oxidation of ethanol to acetic acid is catalyzed by multicomponent metal oxides (Mo-V-NbO(x)) prepared by precipitation in the presence of colloidal TiO(2) (Mo(0.61)V(0.31)Nb(0.08)O(x)/TiO(2)). Acetic acid synthesis rates and selectivities (~95 % even at 100 % ethanol conversion) were much higher than in previous reports. The presence of TiO(2) during synthesis led to more highly active surface areas without detectable changes in the reactivity or selectivity of exposed active oxide surfaces. Ethanol oxidation proceeds via acetaldehyde intermediates that are converted to acetic acid. Water increases acetic acid selectivity by inhibiting acetaldehyde synthesis more strongly than its oxidation to acetic acid, thus minimizing prevalent acetaldehyde concentrations and its intervening conversion to CO(x). Kinetic and isotopic effects indicate that C-H bond activation in chemisorbed ethoxide species limits acetaldehyde synthesis rates and overall rates of ethanol conversion to acetic acid. The VO(x) component in Mo-V-Nb is responsible for the high reactivity of these materials. Mo and Nb oxide components increase the accessibility and reducibility of VO(x) domains, while concurrently decreasing the number of unselective V-O-Ti linkages in VO(x) domains dispersed on TiO(2).

Nature, Density, and Catalytic Role of Exposed Species on Dispersed VOx/CrOx/Al2O3 Catalysts

The structure and surface composition of binary oxides consisting of CrO(x) and VO(x) dispersed on alumina and their effects on the rate and selectivity of oxidative dehydrogenation (ODH) of propane were examined and compared with those for CrO(x) and VO(x) dispersed on alumina. VO(x) deposition on an equivalent CrO(x) monolayer on alumina and deposition of CrO(x) on an equivalent monolayer of VO(x) deposited on alumina led to CrVO(4) species during thermal treatment with concomitant reduction of Cr(6+) to Cr(3+). Autoreduction of Cr(6+) to Cr(3+) is also detected for CrO(x), even without the presence of VO(x). Infrared spectroscopy of NO adsorbed at 153 K probes the relative abundance of alumina and of V(5+), Cr(3+), and Cr(6+) at surfaces. This technique detects differences in the surface composition of VO(x)/CrO(x)()/Al(2)O(3) and CrO(x)/VO(x)/Al(2)O(3). The first of these samples is enriched in VO(x) relative to CrO(x) compared with the second sample. Consistent with this finding, VO(x)/CrO(x)/Al(2)O(3) and CrO(x)/VO(x)/Al(2)O(3) are distinguishable in their ODH activities and propene selectivities. The highest ODH activity and propene selectivity is observed for VO(x)/CrO(x)/Al(2)O(3), which exhibits a surface enriched in VO(x) and having a low surface concentration of Cr(6+).