Selective Oxidative Dehydrogenation Research Articles

Low-Temperature Selective Oxidative Dehydrogenation of Cyclohexene by Titania-Supported Nanostructured Pd, Pt, and Pt-Pd Catalytic Films.

Films of titania-supported monometallic Pd, Pt, and bimetallic Pt-Pd catalysts made of metallic nanoparticles were prepared by magnetron sputtering and studied in the oxidative dehydrogenation (ODH) of cyclohexene. Pd/TiOx and Pt-Pd/TiOx were found active at as low temperature as 150 °C and showed high catalytic activity with high conversion (up to 81%) and benzene selectivity exceeding 97% above 200 °C. In turn, the Pt/TiOx catalyst performed poorly with the onset of benzene production at 200 °C only and conversions not exceeding 5%. The activity of bimetallic Pt-Pd catalysts far exceeded all of the other investigated catalysts at temperatures below 250 °C. However, the production of benzene significantly dropped with a further temperature increase due to the enhanced combustion of CO2 at the expense of benzene formation. As in situ NAP-XPS measurement of the Pt-Pd/TiOx catalyst in the reaction conditions of the ODH of cyclohexene revealed Pd surface enrichment during the first temperature ramp, we assume that Pd surface enrichment is responsible for enhanced activity at low temperatures in the bimetallic catalyst. At the same time, the Pt constituent contributes to stronger cyclohexene adsorption and oxygen activation at elevated temperatures, leading to changes in conversion and selectivity with a drop in benzene formation and increased combustion to CO2. Both the monometallic Pd and the Pt-Pd-based catalysts produced a small amount of the second valuable product, cyclohexadiene, and below 250 °C produced only a negligible amount of CO2 (<0.2%). To summarize, Pd- and Pt-Pd-based catalysts were found to be promising candidates for highly selective low-temperature dehydrogenation of cyclic hydrocarbons that showcased reproducibility and stability after the temperature activation. Importantly, these catalysts were fabricated by utilizing proven methods suitable for large-scale production on extended surfaces.

Photocatalytic ethylene production by oxidative dehydrogenation of ethane with dioxygen on ZnO-supported PdZn intermetallic nanoparticles

The selective oxidative dehydrogenation of ethane (ODHE) is attracting increasing attention as a method for ethylene production. Typically, thermocatalysts operating at high temperatures are needed for C–H activation in ethane. In this study, we describe a low temperature ( < 140 °C) photocatalytic route for ODHE, using O2 as the oxidant. A photocatalyst containing PdZn intermetallic nanoparticles supported on ZnO is prepared, affording an ethylene production rate of 46.4 mmol g–1 h–1 with 92.6% ethylene selectivity under 365 nm irradiation. When we employ a simulated shale gas feed, the photocatalytic ODHE system achieves nearly 20% ethane conversion while maintaining an ethylene selectivity of about 87%. The robust interface between the PdZn intermetallic nanoparticles and ZnO support plays a crucial role in ethane activation through a photo-assisted Mars-van Krevelen mechanism, followed by a rapid lattice oxygen replenishment to complete the reaction cycle. Our findings demonstrate that photocatalytic ODHE is a promising method for alkane-to-alkene conversions under mild conditions.

Facilitating CO2 dissociation via Fe doping on supported vanadium oxides for intensified oxidative dehydrogenation of propane

Selective oxidative dehydrogenation of propane with CO2 (CO2-ODP) represents a promising pathway of propylene production and CO2 utilization. Presently, vanadium (V) based catalysts are commonly used due to the excellent redox ability originated from the multivalent V. However, the CO2-ODP reaction mechanism remains elusive, limiting the rational design of highly efficient catalytic systems. Herein, we introduced Fe into impregnated V-Al2O3 to build a dual-sites catalyst including Fe and V, achieving a C3H8 conversion at ∼43 % and C3H6 selectivity exceed 80 %. Via regulating the Fe/V molar ratio, we clearly describe that the formation of unique Fe-O-V facilitated the CO2 dissociation. Intrinsically, V is the main active site contributing for C3H8 dehydrogenation while Fe site is responsible for the CO2 dissociation, replenishing lattice oxygen to enhance oxidative dehydrogenation of C3H8. The design of dual-sites catalyst and extraction of molecular understanding provide guidance for the rational development of highly active CO2-ODP catalysts.

Rationalizing kinetic behaviors of isolated boron sites catalyzed oxidative dehydrogenation of propane

Boron-based catalysts exhibit high alkene selectivity in oxidative dehydrogenation of propane (ODHP) but the mechanistic understanding remains incomplete. In this work, we show that the hydroxylation of framework boron species via steaming not only enhances the ODHP rate on both B-MFI and B-BEA, but also impacts the kinetics of the reaction. The altered activity, propane reaction order and the activation energy could be attributed to the hydrolysis of framework [B(OSi≡)3] unit to [B(OSi≡)3−x(OH···O(H)Si≡)x] (x = 1, 2, “···” represents hydrogen bonding). DFT calculations confirm that hydroxylated framework boron sites could stabilize radical species, e.g., hydroperoxyl radical, further facilitating the gas-phase radical mechanism. Variations in the contributions from gas-phase radical mechanisms in ODHP lead to the linear correlation between activation enthalpy and entropy on borosilicate zeolites. Insights gained in this work offer a general mechanistic framework to rationalize the kinetic behavior of the ODHP on boron-based catalysts.

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper‐Containing Mordenite: Insights into Reaction Mechanism and Product Protection

AbstractCopper(II)‐containing mordenite (CuMOR) is capable of activation of C−H bonds in C1‐C3 alkanes, albeit there are remarkable differences between the functionalization of ethane and propane compared to methane. The reaction of ethane and propane with CuMOR results in the formation of ethylene and propylene, while the reaction of methane predominantly yields methanol and dimethyl ether. By combining in situ FTIR and MAS NMR spectroscopies as well as time‐resolved Cu K‐edge X‐ray absorption spectroscopy, the reaction mechanism was derived, which differs significantly for each alkane. The formation of ethylene and propylene proceeds via oxidative dehydrogenation of the corresponding alkanes with selectivity above 95 % for ethane and above 85 % for propane. The formation of stable π‐complexes of olefins with CuI sites, formed upon reduction of CuII‐oxo species, protects olefins from further oxidation and/or oligomerization. This is different from methane, the activation of which proceeds via oxidative hydroxylation leading to the formation of surface methoxy species bonded to the zeolite framework. Our findings constitute one of the major steps in the direct conversion of alkanes to important commodities and open a novel research direction aiming at the selective synthesis of olefins.

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper-Containing Mordenite: Insights into Reaction Mechanism and Product Protection.

Copper(II)-containing mordenite (CuMOR) is capable of activation of C-H bonds in C1 -C3 alkanes, albeit there are remarkable differences between the functionalization of ethane and propane compared to methane. The reaction of ethane and propane with CuMOR results in the formation of ethylene and propylene, while the reaction of methane predominantly yields methanol and dimethyl ether. By combining in situ FTIR and MAS NMR spectroscopies as well as time-resolved Cu K-edge X-ray absorption spectroscopy, the reaction mechanism was derived, which differs significantly for each alkane. The formation of ethylene and propylene proceeds via oxidative dehydrogenation of the corresponding alkanes with selectivity above 95 % for ethane and above 85 % for propane. The formation of stable π-complexes of olefins with CuI sites, formed upon reduction of CuII -oxo species, protects olefins from further oxidation and/or oligomerization. This is different from methane, the activation of which proceeds via oxidative hydroxylation leading to the formation of surface methoxy species bonded to the zeolite framework. Our findings constitute one of the major steps in the direct conversion of alkanes to important commodities and open a novel research direction aiming at the selective synthesis of olefins.

Unravelling the role of boron dopant in borocarbonitirde catalytic dehydrogenation reaction

Borocarbonitride (BCN) materials are newly developed metal-free catalytic materials exhibiting high selectivity in oxidative dehydrogenation (ODH) of alkanes. However, the in-depth understandings on the role of boron (B) dopants and the intrinsic activities of –C=O and –B–OH still remain unknown. Herein, we report a series of BCN materials with regulable B content and surface oxygen functional groups via self-assembly and pyrolysis of guanine and boric acid. We found that the B/C ratio is the key parameter to determine the activity of ODH and product distribution. Among them, the high ethylbenzene conversion (∼57%) and styrene selectivity (∼83%) are achieved in ODH for B1CN. The styrene selectivity can be improved by increasing of B/C ratio and this value reaches near 100% for B5CN. Structural characterizations and kinetic measurements indicate that –C=O and –B–OH dual sites on BCN are real active sites of ODH reaction. The intrinsic activity of –C=O (5.556 × 10−4 s−1) is found to be 23.7 times higher than –B–OH (0.234 × 10−4 s−1) site. More importantly, we reveal that the deep oxidation to undesirable CO2 occurs on –C=O rather than –B–OH site, and B dopant in BCN materials can reduce the nucleophilicity of –C=O site to eliminate the CO2 emission. Overall, the present work provides a new insight on the structure–function relationship of the BCN catalytic systems.

Dynamically Formed Active Sites on Liquid Boron Oxide for Selective Oxidative Dehydrogenation of Propane

Boron oxide-based catalysts have been shown to be both active and selective for driving the oxidative dehydrogenation of propane (ODHP) without the use of metal promoters. However, this reaction occurs at temperatures where boron oxide melts, challenging experimental identification of the molecular structures within the boron oxide phase under reaction conditions and thus hindering the understanding of its active sites and reaction mechanism(s). By combining density functional theory computations, ab initio molecular dynamics simulations, in situ Raman characterization, and microkinetic modeling, we propose that dimerized di-coordinated boron sites (>B–B<)─dynamically formed in liquid boron oxide─are the active species for O2 activation under reaction conditions. The resulting peroxy-like species (>B–O–O–B<) is then responsible for propane activation but is a moderate oxidant for ODHP and thus inert to propene. These peroxy-like structures rapidly activate propane, homolytically cleaving the >B–O–O–B< bond, producing a propyl radical and a >B–O• dangling bond. These > B–O• originate from the >B–O–O–B< sites as well as the liquid B2O3 structure itself and play a critical role in the abstraction of H atoms from propane and propyl. In fact, microkinetic modeling reveals that the formation of adsorbed C3H7* radicals is the main rate-controlling step due to the highly endergonic adsorption of propane into the system. Otherwise, the only activated processes were found to be the dehydration steps that lead to water formation, which exhibit an intriguing dependence on the concentration of surface hydroxyl species. These findings provide significant insights into the ODHP mechanisms on boron-based catalysts and emphasize the importance of understanding the liquid nature of the oxide to account for its catalytic activity.

A DFT study of the catalytic ODH of n-hexane over a cluster model of vanadium oxide

Supported vanadium oxides are one of the most widely reported catalysts for the selective oxidative dehydrogenation (ODH) of alkanes to alkenes. These vanadium oxide catalysts contain at least three distinct O species classified by their coordination to the vanadium cations. The involvement in ODH of the different oxygen species is thought to follow the coordination environment with singly coordinated vanadyl V(V)=O being the most reactive. A quantitative understanding of the relative activity of the different oxygen species in vanadium oxides could inform catalyst design to further enhance productivity and selectivity in the ODH reaction. Here we use computational models based on hybrid density functional theory to study the mechanistic pathway for n-hexane to 1- and 2-hexene. We compare the potential energy surfaces calculated for these reactions initiated by β-H abstraction with H transfer to the vanadyl oxygen, V(V)=O, with that involving H transfer to a two-coordinate bridging O atom. An isolated H4V2O7 cluster is used to represent the supported vanadium oxide catalyst so that the reactivity of oxygen species can be understood in the absence of support effects. Gibbs energy calculations were carried out at temperatures of 573, 673 and 773 K to mirror laboratory experimental conditions. We find that the rate-determining step (RDS) in the conversion of n-hexane to either alkene is associated with n-hexane interaction with H4V2O7 through a secondary CH bond. This leads to β-H abstraction with the calculated reaction barrier for abstraction by a V(V)=O (ΔE‡ = +32.7 kcal mol−1) significantly lower than that for the two-coordinate bridging O (ΔE‡ = +43.9 kcal mol−1). H-abstraction leads to reduction of one of the vanadium cations. Removal of the second H atom to form an alkene can follow several pathways. We have considered the second step leading to 2-hexene (γ-H abstraction) using either an adjacent V(V)=O on the reduced V(IV)-O-V(V) unit, a different active V(V)=O site on a fully oxidised cluster, or gas-phase molecular O2. Our results show that the ODH process is likely to proceed via a Mars-van Krevelen redox mechanism. In a practical catalyst the results imply that catalyst activity will depend on the surface coverage of V(V)=O active sites and the n-hexane to gas-phase molecular oxygen ratio. In addition to the pathways leading to alkene products we have noted the formation of CO bonds by the radical intermediate formed from the initial β-H abstraction. From this observation, we suggest that the low yields of 1- and 2-hexene (< 20%) obtained in our laboratory experiments with V2O5/MgO catalysts may be a result of the chemisorption properties of the radical intermediate (∙C6H13) on bridging or terminal O sites in the V(V)-O-V(V) units, leading to undesired products including oxygenates.

Coal as an Effective Catalyst for Selective Oxidative Dehydrogenation of Propane to Propene

Coal is a readily available and inexpensive material. However, its direct use as a catalyst is still rare, but attractive for practical application. In this paper, coal was directly used as a catalyst for the selective oxidative dehydrogenation of propane to propene. It exhibited a high selectivity over 90% with a yield of 8.4% at a high space velocity (12,000 mL·(h·g-cat)−1). The productivity up to 2.84 gC3H6 gcat−1 h−1 was obtained with propene selectivity above 80% (20,000 mL·(h·g-cat)−1). The kinetic showed first-order dependence with respect to propane or oxygen partial pressures. Meanwhile, electron paramagnetic resonance (EPR) and X-ray photoelectron spectrum (XPS) demonstrated that the carbonyl groups act as active sites for oxidative dehydrogenation of propane to propene. This work expands the use of earth-abundant and low-price coal in catalysis with expectable scale application.

Monolithic foam of oriented boron nitride bowl for selective oxidative dehydrogenation of propane and pollutant removal

Bowl-like structures have great potential in the field of catalysis, environmental remediation, energy storage, etc., because of the large surface area, porous structure, and buffer hollow space. Herein, we developed a unique approach to control the opening direction of bowl-like structures via self-assembly strategy at the gas/liquid interface of liquid B2O3 environment foam. BN@BPO4 self-supporting monolithic foam with oriented bowl-like structure was obtained, which is built by numerous oriented bowl-like blocks and shows a double layer structure with face-to-face opening. The opening size can be adjusted by changing the amount of Cu(NO3)2 additives that is beneficial for the production of hexagonal boron nitride (h-BN). We believe the as-generated h-BN can enhance the gas trapping capacity of liquid B2O3 environment foam to precisely control pressure differential between large and small bubbles and thus favors the oriented opening of bowl-like structure. Further, pure h-BN monolithic foam with oriented bowl-like structure prepared at higher temperature possessed multimodal hierarchically porous structure and excellent performance in the catalytic oxidative dehydrogenation of propane into propene and the removal of organic pollutants. This work provides an extended assembly interface strategy of oriented bowl-like structure materials for different applications.

Gas-phase oxidative dehydrogenation of long chain alkenols for the production of key fragrance ingredients: from Rosalva isomers to Costenal analogues

Continuous-flow, gas-phase selective oxidative dehydrogenation (ODH) of unsaturated long chain alcohols (i.e. decen-1-ol isomers) to the corresponding aldehydes over an innovative, highly dispersed Cu-based catalyst supported over ferrite.

The Synthesis of Different Series of Cobalt BEA Zeolite Catalysts by Post-Synthesis Methods and Their Characterization

Three series of zeolite catalysts Co all-silica and Co Al-containing zeolites beta were prepared for use in the selective oxidative dehydrogenation of propane to propylene. Two series of zeolite catalysts Co all-silica were prepared by a two-step postsynthesis method at pH = 2.5 and pH = 3.0–9.0, respectively, which allows the incorporation of cobalt into SiBEA zeolite in the form of isolated framework pseudo-tetrahedral Co(II) species. The incorporation of Co ions into vacant T-atom sites and their reaction with silanol groups were demonstrated by NMR and FTIR methods. The generation of Lewis acid sites without the formation of Brønsted sites was proved by FTIR using pyridine and CO as probe molecules. The state of cobalt in three series of prepared and calcined zeolite catalysts was characterized by DR UV-vis. This technique allowed to show that for low Co content (&lt;2 wt.%) cobalt is present in the form of framework pseudo-tetrahedral Co(II) species. For higher Co content (&gt;2 wt.%), both framework pseudo-tetrahedral and extra-framework octahedral Co(II) species are present. The Co Al-containing zeolite beta series prepared on non-dealuminated support shows the presence of extra-framework octahedral Co(II) only.

The role of modified manganese perovskite oxide for selective oxidative dehydrogenation of ethane: Not only selective H2 combustion but also ethane activation

To break through the thermodynamic reaction limitation of ethane dehydrogenation to ethylene, the oxidative dehydrogenation in the presence of O2 and chemical looping oxidative dehydrogenation has been investigated with the oxygen storage oxide compound. LaMnO3 perovskite catalysts with large specific surface areas were prepared using citric acid-nitrate combustion. The oxygen vacancies and M4+ contents in the catalysts were improved by doping LaMnO3 as the active sites. The experimental results show that the catalyst not only promotes ethane dehydrogenation through selective hydrogen combustion but also promotes ethane conversion through activation of CH. The catalyst exhibits good stability in oxidative dehydrogenation reaction.

The Role of Vacancies in a Ti2CTx MXene‐Derived Catalyst for Butane Oxidative Dehydrogenation

AbstractMXenes are a new family of 2D carbides or nitrides that have attracted attention due to their layered structure, tunable surface groups and high electrical conductivity. Here, we report for the first time that the Ti2CTx MXene catalyses the selective oxidative dehydrogenation of n‐butane to butenes and 1,3‐butadiene. This catalyst showed higher intrinsic activity compared to a commercial TiC and TiO2 samples in terms of C4 olefin formation rate. We propose that the stabilisation of structural vacancies and the change in composition (from a carbide to a mixed phase oxide) in the MXene causes its higher catalytic activity. These vacancies can lead to a higher concentration of unpaired electrons in the MXene‐derived material, enhancing its nucleophilic properties and favouring the production of olefins.

Hexagonal boron nitride for selective oxidative dehydrogenation of n-hexane to olefins

Oxidative dehydrogenation (ODH) of n-hexane to olefins provides an alternative strategy for efficient utilization of n-hexane. Here, we report that hexagonal boron nitride (h-BN) is selective in ODH of n-hexane, achieving 89.6 % selectivity to olefins at n-hexane conversion of 16.3 % and 550 °C. We found that low temperatures and oxygen rich conditions favor the selective hydrogen abstraction from C-H bond over the cleavage of C-C bond, and the presence of gas phase free radical greatly improves the reaction rate of ODH of n-hexane over h-BN. In addition, an anomalous seagull type of relationship exists between the conversion of n-hexane and reaction temperature, which may be related to the re-distribution of the active sites.

Nitrogen-Doped Carbon as a Highly Active Metal-Free Catalyst for the Selective Oxidative Dehydrogenation of N-Heterocycles.

N-heteroarenes represents one of the most important chemicals in pharmaceuticals and other bio-active molecules, which can be easily accessed from the oxidation of N-heterocycles over metal catalysts. Herein, the metal-free oxidative dehydrogenation of N-heterocycles into N-heteroarenes was developed using molecular oxygen as the terminal oxidant. The nitrogen-doped carbon materials were facilely prepared via the simple pyrolysis process using biomass (carboxymethyl cellulose sodium) and dicyandiamide as the carbon and nitrogen source, respectively, and they were discovered to be robust for the oxidative dehydrogenation of N-heterocycles into N-heteroarenes under mild conditions (80 °C under 1 bar O2 ) with water as the green solvent. Diverse N-heterocycles including 1,2,3,4-tetrahydroisoquinolines, indolines and 1,2,3,4-tetrahydroquinoxalines were smoothly converted into N-heteroarenes with high to excellent yields (76->99 %). Superoxide radical (⋅O2 - ) and hydroxyl radical (⋅OH) were probed as the reactive oxygen species for the oxidation of N-heterocycles into N-heteroarenes. More importantly, the nitrogen-doped carbon catalyst can be reused with a high stability. The method provides an environmentally friendly and economical route to access important N-hetero-aromatic commodities.

Highly Selective Oxidative Dehydrogenation of Ethane to Ethylene via Chemical Looping with Oxygen Uncoupling through Structural Engineering of the Oxygen Carrier

AbstractThe oxidative dehydrogenation of ethane (ODH) to produce ethylene offers advantages compared to the industry standard steam cracking, but its industrial application is hindered by costly air separation units needed to supply oxygen. A chemical‐looping‐based oxidative dehydrogenation (CL‐ODH) scheme is presented, in which oxygen carriers supply gaseous oxygen in situ, which then reacts with ethane in the presence of a catalyst at a comparatively low temperature (500 °C). A common challenge of chemical looping processes beyond combustion is to suppress the overoxidation of hydrocarbons to COx to enable high product yields. It is demonstrated that the overoxidation of ethane can be eliminated completely through structural engineering of the perovskite oxygen carrier involving alkali‐metal‐based carbonate coatings, while maintaining the materials’ ability to generate oxygen. Through CL‐ODH, higher ethylene selectivity (≈91%) and yields (≈39%) are achieved compared to the conventional ODH scheme without oxygen carrier and cofeeding air/ethane. 18O‐labeling experiments demonstrate that the carbonate layer functions like a diffusion barrier for ethane while being permeable for oxygen. Both the CL‐ODH scheme and the material design strategy can be extended to other catalytic oxidation or dehydrogenation reactions requiring oxygen at different temperatures, offering enormous potential to intensify such processes.

Atom by atom built subnanometer copper cluster catalyst for the highly selective oxidative dehydrogenation of cyclohexene.

The effect of particle size and support on the catalytic performance of supported subnanometer copper clusters was investigated in the oxidative dehydrogenation of cyclohexene. From among the investigated seven size-selected subnanometer copper particles between a single atom and clusters containing 2-7 atoms, the highest activity was observed for the titania-supported copper tetramer with 100% selectivity toward benzene production and being about an order of magnitude more active than not only all the other investigated cluster sizes on the same support but also the same tetramer on the other supports, Al2O3, SiO2, and SnO2. In addition to the profound effect of cluster size on activity and with Cu4 outstanding from the studied series, Cu4 clusters supported on SiO2 provide an example of tuning selectivity through support effects when this particular catalyst also produces cyclohexadiene with about 30% selectivity. Titania-supported Cu5 and Cu7 clusters supported on TiO2 produce a high fraction of cyclohexadiene in contrast to their neighbors, while Cu4 and Cu6 solely produce benzene without any combustion, thus representing odd-even oscillation of selectivity with the number of atoms in the cluster.

Hexagonal Boron Nitride for Selective Oxidative Dehydrogenation of N-Hexane to Olefins

Hexagonal Boron Nitride for Selective Oxidative Dehydrogenation of N-Hexane to Olefins