Mechanism Of Oxidative Dehydrogenation Research Articles

Oxidative Dehydrogenation of 1-Butene to 1,3-Butadiene Over Metal Ferrite Catalysts

The oxidative dehydrogenation (ODH) of 1-butene to 1,3-butadiene was studied over a series of AFe2O4 catalysts, where A = Zn, Mn, Ni, Cu, Mg and Fe. The catalysts were characterised by XPS, EPR spectroscopy, BET surface area analysis, Raman spectroscopy and XRD. All the ferrites were active for ODH and gave an order of activity after 80 h on-stream of ZnFe2O4 > NiFe2O4 > MnFe2O4 > MgFe2O4 > CuFe2O4 > FeFe2O4. All catalysts lost significant surface area (up to ~ 80%) under reaction conditions of 0.75:1:15 oxygen:1-butene:steam with an overall GHSV of 10,050 h−1 at 693 K. Fe3O4 was unstable under reaction conditions and was converted to Fe2O3, which showed very low activity. Nickel ferrite was the only material that gave carbon dioxide as a significant product, all others were selective to 1,3-butadiene. Zinc ferrite gave a steady-state yield of 1,3-butadiene of ~ 80%. Inversion parameters were determined for the ferrites from XPS and a correlation was obtained between 1,3-butadiene yield and inversion parameter, indicating that Fe3+ in an octahedral hole is a key species in the mechanism of oxidative dehydrogenation. Butene isomerisation and ODH were shown to occur on different sites.

Ammoxidation of Ethane to Acetonitrile and Ethylene: Reaction Transient Analysis for the Co/HZSM-5 Catalyst.

Ethane ammoxidation to acetonitrile and ethylene over the Co/HZSM-5 catalysts was revisited based on both transient and steady-state performance evaluation to elucidate the structure/reactivity relationships. We suggested that the exchanged Co2+ cation encapsulated in the zeolite favors the formation of acetonitrile and ethylene, whereas nanosized cobalt oxide particles without close proximity with the HZSM-5 only favor CO2 formation. Excess Brønsted acid sites of the zeolites may act as a reservoir for NH3, which inhibits the CO2 formation through the NH3-mediated oxidative dehydrogenation mechanism. According to the transient kinetic analysis, the time constants τ from the back-transient decay for NH3 and CO2 are both 7.7 min, which decreased to 2.7 min for acetonitrile and further decreased to 3–4 s for ethane, ethylene, and O2. Assuming first-order reaction kinetics, the rate constants for the formation of acetonitrile and CO2 are 0.37 and 0.13 min–1, respectively, from their corresponding reactive intermediates.

Catalytic activity of Mg–Al hydrotalcites and derived mixed oxides for imination reactions via an oxidative-dehydrogenation mechanism

A Mg–Al hydrotalcite derived mixed oxide (Mg/Al ratio = 3.0) showed excellent catalytic activity in imination and tandem reactions via an oxidative-dehydrogenation mechanism.

Numerical Modeling of Hydroperoxyl-Mediated Oxidative Dehydrogenation of Formic Acid under SCR-Relevant Conditions

Formic acid decomposition to carbon dioxide is an important step during the decomposition of alternative formate-based ammonia precursor compounds, such as ammonium formate, guanidinium formate, and methanamide, in the selective catalytic reduction (SCR) process. At oxygen and water concentrations prevalent in a typical SCR feed, an oxidative dehydrogenation (ODH) type pathway of formic acid decomposition proceeds on titania-supported gold catalysts. Using the surface perfectly stirred reactor (SPSR) model, the ODH mechanism is demonstrated to satisfactorily predict the experimentally observed conversions. The single-site mechanistic model captured the negative trend in conversion with increasing formic acid concentrations, stemming from an extensive coverage of the active sites by formates. This in turn rendered the active sites unavailable for the formation of the active hydroperoxyls required for the rate-determining step of C–H bond scission of formates. The positive order dependency on oxygen concentration and the promotional effect of water are qualitatively and semi-quantitatively described by the model. Predicted trends in the relative surface coverages of different reaction intermediates are in agreement with previously reported kinetic and spectroscopic measurements. These results are important fundamentally for the understanding of formic acid decomposition chemistry and in general catalysis by gold, as well as for the design of dedicated hydrolysis catalysts for applications in the SCR process.

Ethane oxidative dehydrogenation mechanism on MoO3(010) surface: A first-principle study using on-site Coulomb correction

We applied the PBE+U method to investigate the mechanism of ethane oxidative dehydrogenation reaction (ODH) on the MoO3(010) surface. Our systematic study shows that ethylene generation is energetically a favorable pathway from the horizontal C2H6 adsorption. The vertical adsorption of C2H6 leads to the aldehyde generation along the α-oxidative dehydrogenation pathway (α-ODH). However, along the β-ODH pathway, the ethylene oxide formation is identified as the final product but may transform to produce ethylene from the CH2CH2O intermediate. The formation energy of oxygen vacancy (ΔEfv) of all types of oxygen atoms of the MoO3(010) surface are characterized with the on-site Coulomb correction, being reasonably consistent with the HSE results. The U correction leads to the mismatching between the dz2 orbital of Mo and pz orbitals of the terminal oxygen and consequently weakens the Mo–O chemical bond. Therefore, the weakened Mo–O chemical bond can increase the possibility of forming the oxygenated species, such as aldehyde and ethylene oxide.

Insights into gold-catalyzed synthesis of azelaic acid

A novel green route for the synthesis of azelaic and pelargonic acid via aerobic gold-catalyzed cleavage of 9,10-dihydroxystearic acid (DSA) was investigated recently. In this study, the examination of the reaction mechanism is described. The results of the application of 18O-labeled molecular oxygen and sodium hydroxide as well as of diastereomeric pure erythro- and threo-DSA were discussed. Assumed reaction intermediates were synthesized and subjected to the same reaction conditions as with DSA. As a conclusion from the obtained data, an oxidative dehydrogenation mechanism was postulated. Additionally, the aging of the gold catalyst used under different storage conditions was explored.

Fundamental kinetic aspects and the mechanism of oxidative dehydrogenation of 4-vinylcyclohexene into ethylbenzene and styrene on metal-containing zeolites of the pentasil family

In promotion of a Pt-Ga-containing pentasil with a binary mixture of REE oxides, a synergism of their effect on the catalyst activity in the reaction of oxidative dehydrogenation of 4-vinylcyclohexene is observed. A kinetic model of the process of oxidative dehydrogenation of 4-vinylcyclohexene is suggested. The model includes a stage-by-stage scheme and the corresponding reaction rate equations. It was found that the accumulation rate of 4-vinylcyclohexene dehydrogenation products is correlated with the isomerization capacity of the catalytic system.

Colorimetric Sensing of Cu(II) in Aqueous Media with a Spiropyran Derivative via a Oxidative Dehydrogenation Mechanism

A spiropyran derivative containing a simple NO bidentate ligand (2) was synthesized. The ligand acts as a colorimetric chemodosimeter for selective Cu(2+) detection in aqueous media. The ligand, even when treated at elevated temperature (60 °C), exists as a spirocyclic form and shows almost no absorption in the visible region. This is because the electron donation by an amine substituent suppresses spirocycle-opening. Treatment of the solution with Cu(2+) at 60 °C, however, creates a strong absorption at 450-650 nm, whereas other metal cations do not promote spectral change. IR, NMR, and ab initio calculation revealed that Cu(2+) is coordinated with two 2 ligands and produces 1:2 Cu(2+)-amine complex intermediately. This is converted to the 1:2 Cu(+)-imine complex via oxidative dehydrogenation of amine moieties, along with the reduction of Cu(2+). The formation of the imine complex decreases the electron density of nitrogen atoms and weakens the electron donation ability. This thus promotes thermal spirocycle opening and facilitates coloration of the solution.

Low‐Temperature Gas‐Phase Oxidation of Benzyl Alcohol on Mesoporous K‐Cu‐TiO2 through Oxidative Dehydrogenation

AbstractA highly selective, gas‐phase benzyl alcohol‐to‐benzaldehyde transformation is achieved over mesoporous multi‐component metal oxide K‐Cu‐TiO2 (prepared by AcHE process) at a surprisingly low temperature: the b.p. of benzyl alcohol, 203 °C. We propose that highly efficient selective oxidation over K‐Cu‐TiO2 occurs through an oxidative dehydrogenation mechanism based on the catalytic performance with controlled oxygen addition and the observation of surface hydride species on the catalyst surface by electron paramagnetic resonance spectroscopy and spin‐trapping techniques. It was observed that the benzyl alcohol conversion increased sharply with the increase in p and reached a maximum of 80.4 % at p/pBA=0.4–0.6. A further increase in the p slightly decreased the benzyl alcohol conversion. The fact that the reaction rate was independent of the oxygen pressure after the stoichiometric p/pBA ratio (0.5) could indicate a fast reoxidation of CuIH by molecular oxygen, which is, therefore, not the rate‐controlling step. No hydride species have been detected on the catalyst that was collected from the alcohol oxidation reaction performed with added O2, suggesting that O2 acts as efficient hydrogen acceptor and accelerate the reaction by liberating free active CuI sites. Based on understanding of the reaction mechanism, in addition to the low‐temperature reaction condition, stoichiometrical addition of O2 proves effective in the stabilisation of the CuI oxidation state for highly selective benzyl alcohol oxidative dehydrogenation over a wide reaction temperature range (up to 310 °C).

Kinetics of l-arabinoseoxidation over supported goldcatalysts with in situcatalyst electrical potential measurements

The kinetics of L-arabinose selective oxidation over Au/Al2O3 catalysts was studied with simultaneous in situ catalyst electrical potential measurements. An oxidative dehydrogenation mechanism was proposed and a kinetic model taking into account the catalyst potential changes was developed. The kinetic model was implemented into the three-phase system and validated with the experimental observations. The agreement of the kinetic model to the experimental data, in line with the suggested reaction mechanism, was satisfactory.

A BODIPY-based fluorescent chemodosimeter for Cu(ii) driven by an oxidative dehydrogenation mechanism

A boradiazaindacene (BODIPY) derivative containing a simple NO bidentate ligand shows a Cu(2+)-selective fluorescence in aqueous media. This is promoted via a coordination of Cu(2+) followed by oxidative dehydrogenation of an amine moiety, leading to a formation of a fluorescent Cu(+)-Schiff base complex.

On the Mechanism of Iron(III)-Dependent Oxidative Dehydrogenation of Amines

Kinetic and structural data are presented for the iron-promoted dehydrogenation of the amine, [Fe(III)L3]3+ (1), L3 = 1,9-bis(2'-pyridyl)-5-[(ethoxy-2''-pyridyl)methyl]-2,5,8-triazanonane. Spectroscopic and electrochemical experiments under the exclusion of dioxygen helped to identify reaction intermediates and the final product, the Fe(II)-monoimine complex [Fe(II)L4]2+ (2), L4 = 1,9-bis(2'-pyridyl)-5-[(ethoxy-2''-pyridyl)methyl]-2,5,8-triazanon-1-ene. 2 is formed by disproportionation of the starting complex 1 by a three-step reaction mechanism, most likely via ligand-centered radical intermediates. The rate law can be described by the second-order rate equation, -d[(Fe(III)L3)3+]/dt = k(EtO)- [(Fe(III)L3)3+][EtO-], with k(EtO)- = 4.92 +/- 0.01 x 104 M(-1) s(-1) (60 degrees C, mu = 0.01 M). The detection of general base catalysis and a primary kinetic isotope effect (k(EtO)-(H)/k(EtO)-(D) = 1.73) represents the first kinetic demonstration that the deprotonation becomes rate determining followed by electron transfer in the oxidative dehydrogenation mechanism. We also isolated the Fe(II)-monoimine complex 2 and determined its structure in solution (NMR) and in the solid state (X-ray).

Polyaniline as nonmetal catalyst for styrene synthesis by oxidative dehydrogenation of ethylbenzene

Abstract Polyaniline was used as a nonmetal catalyst in the oxidative dehydrogenation of ethylbenzene and yield of 22.9% at 573 K and ∼40% at 673 K were obtained, respectively. An indirect oxidative dehydrogenation mechanism was proposed based on the results of pulse reactions.

Mechanism of n-butane oxidative dehydrogenation over tetravalent pyrophosphates catalysts

Abstract The reaction mechanism of the oxidative dehydrogenation of n-butane has been studied over titanium and zirconium pyrophosphates between 683 and 843 K, at atmospheric pressure. The catalytic results obtained in both stationary and transient conditions clearly showed that two reaction mechanisms occur on these pyrophosphates. The first mechanism is common to both ZrP2O7 and TiP2O7 at low temperature whereas the second mechanism is specific to TiP2O7 at high temperature. Catalyst characterization by techniques like ESR and electrical conductivity measurements together with TAP experiments allow proposing different elementary steps for these mechanisms. The initial step of activation of n-butane would correspond to the attack of the alkane molecules by O− species giving radicals on both catalysts. On ZrP2O7, the radical formed should rapidly undergo a second hydrogen abstraction according to a similar mechanism leading to butene. On TiP2O7, it should transfer an electron to a reducible TiIV or to an anionic vacancy, transforming to an alkoxide intermediate before a subsequent dehydrogenation. In the first case the rate-limiting step would be the attack of the alkane by the O− species whereas in the second one it would be the re-oxidation of the catalytic site by diffusion of the lattice oxygen or directly by gas phase oxygen. These hypotheses are discussed in relation with all the catalytic data obtained and the physicochemical properties of the solids.

Time-Resolved in Situ ATR Spectroscopy of 2-Propanol Oxidation over Pd/Al2O3: Evidence for 2-Propoxide Intermediate

In situ attenuated total reflection infrared spectroscopy in a flow-through cell combined with online UV−vis spectroscopy was used to investigate the oxidation of 2-propanol over Pd/Al2O3 catalyst. The state of the catalyst was driven fast between reduced and oxidized by admitting alternately dissolved hydrogen and oxygen, and the response of the catalytic solid−liquid interface was followed in time. Besides the oxidation product acetone and the water that forms, when hydrogen and oxygen are simultaneously adsorbed on the catalyst surface, an additional species was observed with a characteristic band at ∼1065 cm-1. On the basis of the transient character of the adsorbate and density functional theory calculations, we assign this species to adsorbed 2-propoxide. Its observation indicates that the second dehydrogenation step is rate limiting in an oxidative dehydrogenation mechanism. The results furthermore show that adsorbed hydrogen and oxygen limit the dissociative adsorption of 2-propanol and that 2-pro...

Selective oxidation of alcohols and aldehydes on metal catalysts

Oxidation of aldehydes, alcohols or carbohydrate derivatives can be performed with air in aqueous media, in the presence of palladium and platinum catalysts under mild conditions (293–353 K and atmospheric pressure). These reactions provide valuable products and intermediates for fine chemistry. They have been known for a long time, but much effort in the last 20 years has focused on this approach, because these catalytic reactions are environmentally friendly and could replace stoichiometric oxidations with mineral oxidizing agents. An oxidative dehydrogenation mechanism on the reduced metal surface has been generally accepted. During this process, a strong deactivation of the catalysts is often reported, which is a cause of serious concern for process development. Several causes of deactivation have been put forward: oxidation of metal, blocking of active sites by strong adsorption of side-products, metal leaching and growth of platinum crystallites. The addition of certain p-electron metal promoters (e.g. Bi, Pb) has been shown to play a useful dual role in reducing catalysts deactivation and in changing the selectivity of reactions. The performances of the catalysts can also be improved by modification of the metallic surface with strongly adsorbing nitrogen-containing bases or phosphines. A few illustrative examples will be given, which show that carbohydrates, aliphatic or aromatic alcohols, and polyols can be oxidized with high selectivities into valuable products.

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

The effect of cation identity on oxidative dehydrogenation (ODH) pathways was examined using two-dimensional VOx, MoOx, and WOx structures supported on ZrO2. The similar kinetic rate expressions obtained on MoOx and VOx 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 VOx/ZrO2 < MoOx/ZrO2 < WOx/ZrO2; 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 VOx/ZrO2 < MoOx/ZrO2 < WOx/ZrO2. 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 (V5+ < Mo6+ < W6+).

Selective oxidation with air on metal catalysts

Oxidation of organic molecules with air on metal catalysts has been known for a long time but there has been a renewed interest in recent years because these catalytic reactions are environmentally safe and could replace stoichiometric oxidations. This paper describes several oxidation reactions conducted either at high temperatures in the gas phase or at moderate temperatures in the liquid phase; in both cases they proceed via a mechanism of oxidative dehydrogenation on the metal surface. Ethylene glycol was converted to glyoxal at 550°C on Ag/SiC catalyst with a 70% yield provided promoters were added to the reaction feed (diethylphosphite or iodine) or deposited on the catalyst (LiPO 4 or H 3PO 4). The promoters improve the conversion and selectivity by modifying the structure and the oxygen concentration on the surface of silver. Oxidation of glyoxal to glyoxylic acid, glucose to gluconic acid and glycerol to various oxygenated derivatives were conducted in water at 60°C in the presence of carbon-supported palladium or platinum catalysts. Bismuth promoter, deposited on the platinum metals by redox reaction, improves the catalyst activity by preventing over-oxidation of the metal surface and favors the oxidation of secondary alcohol functions into keto-derivatives. At higher reaction temperatures, platinum catalysts produce C-C bond rupture with the formation of carboxylic acids with smaller chains. Thus, cyclohexanol was converted into C 6, C 5, and C 4 diacids with a 45% selectivity to adipic acid on Pt/C catalysts at 150°C.

Catalytic Oxidation of Glucose on Bismuth-Promoted Palladium Catalysts

Water solutions of glucose (1.66 mol liter −1) were oxidized with air at 313 K on palladium catalysts supported on active charcoal. High gluconate yields (99.3%) were obtained in the presence of bismuth-promoted catalysis. Bismuth was deposited via a surface redox reaction on Pd/C catalysts containing 1- to 2-nm Pd particles. A STEM-EDX study showed that bismuth atoms are selectively and homogeneously deposited on the palladium particles. The catalyst can be recycled without loss of activity and selectivity. Bismuth was not leached from the catalyst during reaction and recycling. Bismuth adatoms prevent oxygen poisoning of the palladium surface by acting as a co-catalyst in the oxidative dehydrogenation mechanism. It was determined by calorimetric measurements that oxygen should adsorb preferentially on bismuth rather than on palladium.

Oxidative dehydrogenation of ethylbenzene with carbon dioxide

An attempt to use carbon dioxide as a diluent and oxidant in the dehydrogenation of ethylbenzene to styrene was carried out over an activated carbon-supported iron catalyst ( Fe 17 wt.-% ) at 773–973 K, CO 2/ethylbenzene= 50-70 mol/mol and W/F= 30–120 g h/mol. An addition of 20–30 mol-% lithium nitrate to iron resulted in a significant increase in the catalytic activity. The highest yield of styrene (40–45%) with more than 90% selectivity was obtained at a ratio of lithium to iron of 0.1–0.2 (mol/mol). In addition to styrene, carbon monoxide and water were formed as products. This indicated that the reaction proceeds via an oxidative dehydrogenation mechanism. Added lithium nitrate was converted into lithium ferrite during the treatment of an iron-lithium co-loaded activated carbon catalyst under carbon dioxide at 973 K. Lithium ferrite thus formed would be an active center of the reaction.