Oxidative Dehydrogenation Of Cyclohexane Research Articles

Cyclohexane Oxidative Dehydrogenation on Graphene-Oxide-Supported Cobalt Ferrite Nanohybrids: Effect of Dynamic Nature of Active Sites on Reaction Selectivity.

In this work, we investigated cyclohexane oxidative dehydrogenation (ODH) catalyzed by cobalt ferrite nanoparticles supported on reduced graphene oxide (RGO). We aim to identify the active sites that are specifically responsible for full and partial dehydrogenation using advanced spectroscopic techniques such as X-ray photoelectron emission microscopy (XPEEM) and X-ray photoelectron spectroscopy (XPS) along with kinetic analysis. Spectroscopically, we propose that Fe3+/Td sites could exclusively produce benzene through full cyclohexane dehydrogenation, while kinetic analysis shows that oxygen-derived species (O*) are responsible for partial dehydrogenation to form cyclohexene in a single catalytic sojourn. We unravel the dynamic cooperativity between octahedral and tetrahedral sites and the unique role of the support in masking undesired active (Fe3+/Td) sites. This phenomenon was strategically used to control the abundance of these species on the catalyst surface by varying the particle size and the wt % content of the nanoparticles on the RGO support in order to control the reaction selectivity without compromising reaction rates which are otherwise extremely challenging due to the much favorable thermodynamics for complete dehydrogenation and complete combustion under oxidative conditions.

Comparative study of the oxidative dehydrogenation of cyclohexane over vanadium isomorphic-substituted hydroxyapatite and hydroxyapatite-supported vanadium oxide

Vanadium oxide-modified hydroxyapatite (HAP) with varying vanadium content was synthesized by the coprecipitation and impregnation methods. The structures of the two series of catalysts were compared, and the effects of vanadium content on catalytic properties in cyclohexane oxidative dehydrogenation (ODH) are evaluated. The XRD, UV‒Vis and XPS analyses indicate that the V species in the two series of samples exist in different chemical environments. The samples prepared by the impregnation method are mainly composed of V2O5 species, while the samples prepared by the coprecipitation method show a replacement of a part of the P in the HAP lattice by VO4. The influence of the preparation method and vanadium content on the reaction rate and reaction route for the ODH of cyclohexane is investigated. VHAP shows higher cyclohexene selectivity. The effect of VOx species on the ODH reaction of cyclohexane was investigated and the reaction mechanism was proposed.

Oxidative dehydrogenation of cyclohexene on atomically precise subnanometer Cu4-nPdn (0 ≤ n ≤ 4) tetramer clusters: the effect of cluster composition and support on performance.

The pronounced effects of the composition of four-atom monometallic Cu and Pd and bimetallic CuPd clusters and the support on the catalytic activity and selectivity in the oxidative dehydrogenation of cyclohexene are reported. The ultra-nanocrystalline diamond supported clusters are highly active and dominantly produce benzene; some of the mixed clusters also produce cyclohexadiene, which are all clusters with a much suppressed combustion channel. The also highly active TiO2-supported tetramers solely produce benzene, without any combustion to CO2. The selectivity of the zirconia-supported mixed CuPd clusters and the monometallic Cu cluster is entirely different; though they are less active in comparison to clusters with other supports, these clusters produce significant fractions of cyclohexadiene, with their selectivity towards cyclohexadiene gradually increasing with the increasing number of copper atoms in the cluster, reaching about 50% for Cu3Pd1. The zirconia-supported copper tetramer stands out from among all the other tetramers in this reaction, with a selectivity towards cyclohexadiene of 70%, which far exceeds those of all the other cluster-support combinations. The findings from this study indicate a positive effect of copper on the stability of the mixed tetramers and potential new ways of fine-tuning catalyst performance by controlling the composition of the active site and via cluster-support interactions in complex oxidative reactions under the suppression of the undesired combustion of the feed.

Chlorine-Decorated Ceria Nanocubes for Facilitating Low-Temperature Cyclohexane Oxidative Dehydrogenation: Unveiling the Decisive Role of Surface Species and Acid Properties

Chlorine-Decorated Ceria Nanocubes for Facilitating Low-Temperature Cyclohexane Oxidative Dehydrogenation: Unveiling the Decisive Role of Surface Species and Acid Properties

Photocatalytic oxidative dehydrogenation of cyclohexane to cyclohexene over oxygen-deficient tungsten trioxide

Oxidative dehydrogenation of cyclohexane (C6H12) to cyclohexene (C6H10) suffers from low selectivity due to its over-dehydrogenation and oxidation to other products like benzene or cyclohexanol. In this study, we report that oxygen-deficient tungsten trioxide (WO3-x) can realize photocatalytic oxidative dehydrogenation of C6H12 with high C6H10 selectivity (∼99 %) and stability. Experimental results and theoretical calculations revealed that the introduction of oxygen vacancy favored the adsorption of C6H12 over tungsten trioxide and the low-coordinated W atom of WO3-x would donate electrons to the C atom of C6H12, weakening its CH bond. During the photocatalysis, the photogenerated holes acted as oxidative species for the dehydrogenation of C6H12 along with the H2O formation through the combination with adjacent O atom in WO3-x. Meanwhile, the product C6H10 could easily desorb from the surface of WO3-x to avoid the over-dehydrogenation and oxidation, contributing to the high selectivity.

Unveiling the morphology dependence of ceria nanocrystals for boosting low−temperature cyclohexane oxidative dehydrogenation

Tailoring the oxygen vacancy concentration by controlling the morphology of CeO2 has been proved to be a feasible strategy to construct excellent catalysts for oxidative dehydrogenation (ODH) reactions. Here, the morphology dependence of ceria nanocrystals for low−temperature cyclohexane ODH was investigated. The results reflected the variation of oxygen vacancy concentration of ceria with morphologies, resulting in different reactivity. Temperature−programmed re-oxidation (TPRO) and temperature−programmed reduction (TPR) tests demonstrated that prepared catalysts possessed the opposite replenishment and consumption capacity of active oxygen species. In addition to superficially correlating the facet effect of ceria with the formation of oxygen vacancies, as reported in most literatures, the facet dependence on the selectivity of the target product also has come to light by density functional theory (DFT). Furthermore, the temperature programmed isotope exchange (TPIE) and kinetic experiments revealed the Langmuir−Hinshelwood (LH) mechanism rather than the recognized Mars−van Krevelen (MvK) mechanism. That is, the lower reaction temperature makes the main active agent electrophilic oxygen species, and the competitive adsorption of cyclohexane and oxygen on the intrinsic oxygen vacancies causes the dependence of reaction rate on oxygen concentration. The findings provide fresh insights into the optimization of the CeO2−based catalysts in cyclohexane oxidative dehydrogenation.

COx‐Resistant Oxidative Dehydrogenation of Cyclohexane Catalyzed by sp3@sp2Nanodiamonds towards Highly Selective Cyclohexene Production

AbstractDeep oxidation/dehydrogenation are longstanding problems for decades in catalytic oxidative dehydrogenation (ODH) of cyclohexane and other alkanes. Here we show a metal‐free catalyst of nanodiamonds (NDs) with unique sp3@sp2hybrid structure that catalyzes COx‐resistant cyclohexane ODH with remarkable reactivity towards cyclohexene production. The selectivity of cyclohexene can reach as high as 67 % with significantly suppressed COxemission (<5 %), which is on top of the highest reported values among other metal(oxide)/metal‐free catalysts. Structural evolution of sp3@sp2NDs under annealing treatments and their specific surface functional groups are systematically studied using TEM, XPS, Raman and TPD. By comparing with carbon nanotubes (CNTs), we found that the carbonyl groups stabilized on strained sp3@sp2core‐shell NDs enhanced the cyclohexene selectivity via preferential cleavage of C−H over C−C bond. Kinetic studies further revealed the underlying reaction pathways that cyclohexane is rapidly dehydrogenated to cyclohexene which subsequently transforms into benzene (fast) and COx(slow). Deep oxidation of both cyclic hydrocarbons is largely suppressed due to the low density of electrophilic functional groups on strongly curved graphitic surface of sp3@sp2NDs.

Structure and activity of mixed VOx-CeO2 domains supported on alumina in cyclohexane oxidative dehydrogenation

Alumina-supported VOx-CeO2 materials are synthesized by atomic layer deposition (ALD) and evaluated in cyclohexane oxidative dehydrogenation (ODH). The order of deposition of the two metal oxides by ALD influences the size, geometry and oxidation state of the resulting surface species as indicated by Raman spectroscopy coupled with density functional theory (DFT) calculations and XPS. The individual contributions of VOx and CeO2 surface sites to the cyclohexane ODH reaction mechanism are investigated through a comparison of the catalytic activity of the supported mixed metal oxide materials, VOx/Al2O3, VOx/CeO2 and a bare CeO2 support. The highest activity is observed for CeO2, and different surface active sites are distinguished by introducing cyclohexane and cyclohexene in the reactant feed mixture. The catalytic behavior is dependent on the nature of VOS (S, Support) bonds, and small clusters of CeO2 on Al2O3 favor total oxidation, similar to exposed CeO2 sites in VOx/CeO2.

Front Cover: Oxidative Dehydrogenation of Cyclohexane by Cu vs Pd Clusters: Selectivity Control by Specific Cluster Dynamics (ChemCatChem 5/2020)

Front Cover: Oxidative Dehydrogenation of Cyclohexane by Cu <i>vs</i> Pd Clusters: Selectivity Control by Specific Cluster Dynamics (ChemCatChem 5/2020)

Evidence for Copper Dimers in Low-Loaded CuOx/SiO2 Catalysts for Cyclohexane Oxidative Dehydrogenation

Copper oxide catalysts supported on KIT-6 silica were evaluated for cyclohexane oxidative dehydrogenation (ODH) to determine the effects of copper oxide domain size on ODH activity and selectivity. The catalysts were prepared by incipient wetness impregnation of KIT-6 at copper surface densities spanning 0.01–0.7 Cu/nm2 with carefully controlled drying and calcination conditions to systematically vary the average local copper oxide domain size. A distinct copper oxide active site exhibiting an order of magnitude higher activity than large copper oxide domains was identified by model cyclohexane ODH studies coupled with in situ X-ray absorption and UV–visible spectroscopies during reduction in H2. The structure of this site is experimentally identified by a combination of extended X-ray absorption fine structure analysis, resonant Raman studies, and modeling by density functional theory. All constraints imposed by these techniques indicate the active site is a mono(μ-oxo)dicopper(II) structure with copper ...

Role of Support Lewis Acid Strength in Copper-Oxide-Catalyzed Oxidative Dehydrogenation of Cyclohexane

Alkane oxidative dehydrogenation (ODH) over supported redox active metal oxides is highly sensitive to support identity, but the underlying cause of support effects has not been well-established. Here, we provide evidence that charge transfer between the support and active oxide phase impacts the rates of C–H bond abstraction and COX formation pathways in the oxidative dehydrogenation of cyclohexane over supported copper oxide catalysts. The surface Lewis acid strength of nine metal oxide supports is quantified by alizarin dye intramolecular charge transfer shifts and compared with supported copper oxide d–d transition energies to determine the relationship between support Lewis acid strength and copper oxide electronic properties. Model cyclohexane ODH reaction studies show that selectivity to C6 products increases with increasing support Lewis acid strength, with selectivities to benzene and cyclohexene over combustion products at zero conversion increasing from 20% over nucleophilic Cu/MgO to over 90% ...

Oxidative Dehydrogenation of Cyclohexane and Cyclohexene over Y-doped CeO2 Nanorods

The Y-doped and pure CeO2 nanorods were tested as dehydrogenation catalysts in the oxidative dehydrogenation of cyclohexene and cyclohexane. The dehydrogenation process involves a redox process and the increased amount of oxygen vacancies improved the catalytic activity. At a temperature as low as 150 °C, compared to the temperature of ~600 °C required for vanadium catalysts, the Y-doped CeO2 nanorods exhibits a cyclohexane conversion of ~20% with a product distribution for 38% cyclohexene, 11% benzene, 1% 1,3-cyclohexanediene and 50% combustion products.

The effect of ceria modified cryptomelane on the oxidative dehydrogenation of cyclohexane

Nanostructure cryptomelane has been synthesized in neutral medium using CTAB as a structure directing agent and was modified by impregnation with ceria (2.5, 5 and 7.5 wt%). XRD, FT-IR, TEM and N2 adsorption techniques were employed to study the characterization of the prepared samples. The catalytic activity of the prepared samples towards the oxidative dehydrogenation of cyclohexane in air was evaluated. The catalytic activity results revealed that the ceria/OMS-2 samples is the most selective catalyst towards benzene formation at all reaction temperature although the selectivity to benzene formation decreases from 99 at 300 °C to 56% with the increase of reaction temperature to 400 °C. The parent OMS-2N catalyst is an active and selective one towards the formation of cyclohexanol (oxidation reaction) which reaches maximum 47% at 375 °C.

Cyclohexane oxidative dehydrogenation over copper oxide catalysts

Here, we report on structure-reactivity trends for cyclohexane oxidative dehydrogenation (ODH) with silica supported copper oxide catalysts as a function of surface structure. Copper oxide was supported on mesostructured KIT-6 silica at low surface densities <0.2Cu/nm2 using copper (II) nitrate, ammonium and sodium copper (II) ethylenediaminetetraacetate, and a hexanuclear copper (I) siloxide complex. Copper oxide surface structures were characterized by X-ray absorption spectroscopy as well as ambient and in situ diffuse reflectance UV–visible (DRUV–vis) spectroscopy to determine trends in copper oxide nuclearity. DRUV–vis spectroscopy identifies three copper species based on Cu2+ ligand to metal transfer (LMCT) bands at 238, 266, and >300nm as well as Cu+ LMCT bands at 235, 296, and 312nm. Counterintuitively, EXAFS analysis shows that the multinuclear precursor leads to fewer average Cu–Cu interactions than syntheses with mononuclear copper salt precursors. Turnover frequency and selectivity to benzene increase with decreasing copper oxide nuclearity, and thus the multinuclear precursor leads to the highest turnover frequency and benzene production. This work shows the variety of surface species that exist even at extremely low copper surface densities, control of which can improve reactivity of an atypical ODH catalyst up to rates comparable to benchmark vanadia catalysts.

Oxidative dehydrogenation of cyclohexane over modified zeolite catalysts

The catalytic activity of modified zeolites in selective oxidative dehydrogenation of cyclohexane has been studied. It has been established that polymetallic zeolite catalyst, namely, CuZnCoCr-clinoptilolite (0.5% Cu2+, 0.2% Zn2+; 0.1% Co2+; 0.1% Cr3+) manifests the highest activity in the reaction under consideration. The kinetic scheme of the reaction mechanism has been proposed, and a kinetic model of the process has been developed on the basis of stepwise mechanism schemes. The numerical values of the kinetic parameters are calculated.

Preparation of nano-structured cryptomelane materials for catalytic oxidation reactions

Manganese-based octahedral molecular sieves of the type K-OMS-2 (cryptomelane structure) were prepared via reflux method by synproportionation of KMnO4 and Mn2+ in acidic aqueous suspension. For this method, different manganese anions (sulphate, chloride and acetate) were used, which exert a strong influence on the prepared materials. Several techniques such as X-ray diffraction, Fourier transformer infrared, Raman spectroscopy, transmission electron microscopy, differential and gravimetric thermal analysis and H2-temperature programmed reduction were used to characterize the prepared samples. The results revealed that the prepared samples were mainly pure mono-phase cryptomelane materials. The obtained K-OMS-2 material on using the sulphate anion has more available lattice oxygen as compared to that prepared by the other anions. The catalytic activity of the prepared samples was tested towards the oxidative dehydrogenation of cyclohexane. Over the K-OMS-2RS sample, cyclohexane conversion was significantly higher than the other prepared samples.

Pronounced Size Dependence in Structure and Morphology of Gas-Phase Produced, Partially Oxidized Cobalt Nanoparticles under Catalytic Reaction Conditions

It is generally accepted that optimal particle sizes are key for efficient nanocatalysis. Much less attention is paid to the role of morphology and atomic arrangement during catalytic reactions. Here, we unravel the structural, stoichiometric, and morphological evolution of gas-phase produced and partially oxidized cobalt nanoparticles in a broad size range. Particles with diameters between 1.4 and 22 nm generated in cluster sources are size selected and deposited on amorphous alumina (Al2O3) and ultrananocrystalline diamond (UNCD) films. A combination of different techniques is employed to monitor particle properties at the stages of production, exposure to ambient conditions, and catalytic reaction, in this case, the oxidative dehydrogenation of cyclohexane at elevated temperatures. A pronounced size dependence is found, naturally classifying the particles into three size regimes. While small and intermediate clusters essentially retain their compact morphology, large particles transform into hollow spheres due to the nanoscale Kirkendall effect. Depending on the substrate, an isotropic (Al2O3) or anisotropic (UNCD) Kirkendall effect is observed. The latter results in dramatic lateral size changes. Our results shed light on the interplay between chemical reactions and the catalyst's structure and provide an approach to tailor the cobalt oxide phase composition required for specific catalytic schemes.

Selection of Active Modified Zeolite Catalyst and Kinetics of the Reaction of Selective Oxidative Dehydrogenation of Cyclohexane to Cyclohexadiene 1,3

The catalytic activity of the modified zeolites in the reaction of selective oxidative dehydrogenation of cyclohexane to cyclohexadiene-1,3 has been investigated. It has been found that, the catalyst, Cu Zn Co Cr-clinoptilolite (Cu2+—0.5%; Zn2+—0.2%; Co2+—0.1%; Cr3+—0.1%) shows the most activity in this reaction. A kinetic scheme of the mechanism of the reaction has been suggested and on the basis of this scheme, kinetic model of the process has been developed. Numerical values of the kinetic parameters were calculated.

Effect of Steam on the Reaction Performance of Oxidative Dehydrogenation of Cyclohexane

The effect of steam, fed to the fixed-bed microreactor reactor, on the oxidative dehydrogenation of cyclohexane over Mg3(VO4)2 catalyst was reported. The catalyst characterization results showed that the crystal structure, the electron cloud distributions of the active species and the oxygen species, and the reducibility of the active species are not influence by the introduction of steam. It was observed that steam added to the oxidative dehydrogenation of cyclohexane competes for sites required for gas-phase oxygen activation, which catalyst lattice oxygen utilization is not affected. The shorter residence time and the competition for sites with oxygen causes a decrease in cyclohexane conversion and a corresponding increase in selectivity to cyclohexene.

Effect of additives doping on catalytic properties of Mg3(VO4)2 catalysts in oxidative dehydrogenation of cyclohexane

Effects of additives comprising alkali and alkaline earth metals (Li, Na, K and Ca) introduced to Mg3(VO4)2 on their structures, physicochemical properties and the catalytic behaviors in the oxidative dehydrogenation of cyclohexane were investigated. The characterization and the experimental results showed that the additive does not affect markedly the structure of the catalyst, but blocks the active sites and hinders the reducibility of the active species thus decreasing the catalytic activity. Moreover, the additive could enhance the selectivity to cyclohexene by enhancing the nucleophilicity, the redox property as well as the type and number of the oxygen species on the catalyst surface. The doped catalysts could give the catalytic activity and selectivity to cyclohexene in the orders of K-<Na-≈Ca-<Li-<non-doped and non-doped<Li-<Na-<K-<Ca-, respectively. Among the doped catalysts, Ca–Mg3(VO4)2 catalyst demonstrated a yield of cyclohexene of 8.2%, which was a promotion compared with 6.4% of Mg3(VO4)2.