Reoxidation Of Catalyst Research Articles

Kinetic Model for Methacrolein OxidationInfluence of Cesium and Vanadium on Heteropolyacid Catalysts CsxH3-x+y[PMo12-yVyO40

A kinetic model has been developed for the heterogeneously catalyzed methacrolein oxidation, the parameters of which reflect the redox and adsorptive properties of the heteropolyacid catalysts CsxH3-x+y[PMo12-yVyO40] (x = 0, y = 0, 1; x = 1, y = 0, 1, 2). The model based on the Mars−van Krevelen mechanism describes the reaction rates of methacrolein and oxygen consumption as well as the rate of methacrylic acid and byproduct formation as a function of the methacrolein and oxygen partial pressures. Substitution of Mo by V causes a strong decrease of the ratio of reaction rate constants of reoxidation and methacrolein oxidation. The substitution of Mo by V leads to a strong decrease of the rate of consecutive methacrylic acid oxidation. Salification of the free acid by cesium provides a decrease of byproduct formation via parallel and consecutive oxidation and also a reduction of the hindrance of catalyst reoxidation caused by strong adsorption of methacrolein.

In situ X-ray absorption spectroscopic studies at the cobalt K-edge on an Al2O3-supported rhenium-promoted cobalt Fischer-Tropsch catalyst. Comparing reductions in high and low concentration hydrogen

In situ XAFS spectroscopic studies have been carried out at 450°C on the hydrogen reduction of a rhenium-promoted Co 3 O 4 /Al 2 O 3 catalyst. Reductions carried out using 100% hydrogen and 5% hydrogen in helium gave different results. Whereas the reduction using dilute hydrogen yielded bulk-like metallic cobalt particles (hcp or fcc), the reaction with pure hydrogen led to a more dispersed system with smaller cobalt metal particles (<40 A) the crystal form of which could not be established so that the recently reported metastable nonclose-packed body-centred cubic form cannot be excluded. Reoxidation of a similar catalyst in water-containing gas mixtures has been reported in the literature; it is suggested that the different outcome in the case of the 100% hydrogen protocol may be due to a similar mechanism. This would involve the in situ water produced by the reduction with reoxidation-reduction of cobalt metal particles in the water vapour-hydrogen mixture. However, this mechanism cannot be established by the present study. Additionally, in both reduction protocols a small fraction (3-4 wt.%) of the cobalt content is randomly dispersed over the tetrahedral vacancies of the alumina support with Co-O bond lengths of 1.96±0.01 A. This dispersion occurs during reduction and not calcination.The cobalt in these sites cannot be reduced at 450°C, a temperature that is too low to permit formation of the spinel CoAl 2 O 4 .

Thermogravimetric Analysis of Vanadium Phosphorus Oxide Catalysts Doped with Cobalt and Iron

The transformation of VOHPO4·0.5H2O (VPO) precursor doped with cobalt or iron for n-butane oxidation to maleic anhydride was investigated by thermogravimetric analysis under air and nitrogen, with and without n-butane in the flow. While almost no effect was observed in nitrogen or air, a strong influence of the doping was observed when n-butane was added to the nitrogen or air. This resulted in a delay of the decomposition of the precursor and a further reoxidation of the VPO catalyst, particularly for doping with cobalt at low percentage (1%). This shows that doping can change the oxidation state of vanadium phosphorus oxide catalysts, which can explain differences in their catalytic performances and the favourable effect of doping by cobalt.

Selective synthesis of maleic anhydride by spatial separation of n-butane oxidation and catalyst reoxidation

The selective oxidation and ammoxidation of hydrocarbons in fixed-bed or fluidized-bed reactors is conventionally characterized by the simultaneous presence of oxygen gas and hydrocarbon vapor at the active sites of the catalyst. In order to increase the selectivity it has been suggested (Sze and Gelbein, 1976; Contractor and Sleight, 1987; Contractor, 1988) to separate the reaction into two parts; i.e. a first step where the catalyst chemisorbs oxygen and a second step, where the oxidized catalyst reacts with the hydrocarbons. In the present work this concept has been tested on the laboratory scale. The oxidation of n-butane to maleic anhydride (MA) has been taken as an example. The reaction step was carried out in a riser of a circulating fluidized bed, the solids return line of which was used to implement a regenerator fluidized bed for the reoxidation step. A model of this reactor has been developed which takes the oxygen loading on the catalyst particles into account. Scale-up calculations show that high solids circulation rates are necessary which, in turn, demand a high attrition-resistance of the catalyst.

Laser flash photolysis study of the mechanism of photooxidation ofalkanes catalysed by decatungstate anion

Generated during light excitation, the O(2p) → W(5d) ligand-to-metal charge transfer (LMCT) excited state of decatungstate anion (W10O324-) relaxes for t < 30 ps to a transient X which persists for 5 ns. In the nanosecond time domain, it reacts with all components of the photocatalytic system (substrate, organic counter-ion and solvent) to give a one-electron reduced decatungstate species HW10O324- and an organic radical R˙. The latter is quenched by dioxygen at a near diffusion-controlled rate, forming a peroxyl radical ROO˙. In the solvent cage, the peroxyl radical ROO˙ reoxidizes HW10O324- to give the hydroperoxide ROOH and W10O324-, thus closing the catalytic cycle. The catalyst reoxidation is the rate-limiting step of the alkane oxidation.

Improvement of selectivity with a two-step process for the oxidation of isobutyric acid

The separation of catalyst reduction and reoxidation is a two step process. Its application for oxidation reactions was investigated for the oxydehydrogenation of isobutyric acid to methacrylic acid with a H 4PVMo 11O 40 heteropoly acid catalyst. Though, total oxidation is almost negligible an improvement in selectivity is obtained at certain operation conditions, especially at low temperatures. The conditions leading to selectivity improvement are described and related to catalyst properties. Deactivation experiments over many catalyst reduction/reoxidation cycles and UV-VIS-NIR and ESR spectroscopy showed reversibility of catalyst reduction if water vapor is present during reoxidation. Kinetic modeling of both steps with a Mars-van Krevelen model is presented. Calculated and experimental data show good agreement.

Steady-State and Transient Reactivity Study of TiO2-Supported V2O5−WO3 De-NOx Catalysts: Relevance of the Vanadium−Tungsten Interaction on the Catalytic Activity

The reactivity of ternary V2O5−WO3/TiO2 de-NOx catalysts (V2O5 = 0−1.47% w/w, WO3 = 0−9% w/w) in the selective catalytic reduction (SCR) reaction is investigated under steady-state and transient conditions. The results indicate that over the investigated catalysts the SCR reaction occurs via a redox mechanism and that the rate-determining step of the reaction is the catalyst reoxidation process. The reactivity of the V2O5−WO3/TiO2 catalysts increases on increasing either the V2O5 or the WO3 loading; the reactivity of V and/or W in the ternary catalysts is higher than that of the corresponding binary samples. A synergism between the TiO2-supported V and W surface oxide species in the SCR reaction is suggested, that is exploited in the enhancement of the catalyst redox properties of the samples. Accordingly, tungsta increases the rate of the SCR reaction of V2O5/TiO2 catalysts by favoring the catalyst reoxidation by gas-phase oxygen.

Controlled re-oxidation of low-pressure methanol synthesis catalyst, followed by means of DSC

The re-oxidation of low-pressure methanol synthesis catalyst was followed by means of DSC, with the aim of contributing to a better understanding of the processes taking place during catalyst blanketing, stabilization and re-oxidation. The investigations were carried out with previously hydrogen-reduced samples, and their successive re-oxidation was performed with different oxygen contents in the oxygen/nitrogen gas mixtures. A procedure for catalyst stabilization and/or catalyst discharge is proposed.

Experimental and kinetic modelling study of DeNOx on an industrial V 2O 5-WO 3/TiO 2 catalyst

The selective catalytic reduction of nitric oxide with ammonia has been studied over an industrial V 2O 5-WO 3/TiO 2 catalyst. A suitable rate equation is presented, based on mechanistic considerations, including interactions of reacting species with the active catalytic sites and rate aspects of the catalyst reoxidation. The kinetic parameters have been estimated on the basis of an extensive experimental program, whereby the process parameters, ammonia over nitric oxide ratio, oxygen level, nitric oxide level, space time and temperature, have been varied over a wide range. The proposed kinetic expression is able to predict adequately the influence of these process parameters on the DeNOx reaction.

The role of reversible changes in catalyst activity in the observed multiple steady stated during partial oxidation dynamics

Multiple steady states have been observed in fixed bed catalytic reactors for partial oxidation when operating conditions change drastically. This is thought to be caused by reversible changes in catalyst activity. Two different activity profiles apparently coexist for vanadia catalysts: one is a surface (irreversible) profile related to structural changes; the other is an oxidation-reduction profile, which is reversible and requires some time for adjustment to new conditions. A two-dimensional pseudo-heterogeneous model, incorporating both activity profiles, is used to be described the observed multiple steady states in an industrial-scale reactor for the oxidation of o-xylene. The simulation showed that the apparent activation energy for catalyst re-oxidation is a key parameter for a good qualitative description.

The effect of silver addition on catalytic properties of vanadia catalyst for oxidation of cyclopentadiene

The catalyst systems of silver-added vanadia for selective oxidation of cyclopentadiene were studied. As silver of the amount below Ag/V atomic ratio of 0.1 being added into vanadia, the catalytic selectivity for the formation of maleic anhydride was improved. This modifying effect was seemed to be related to the weakening of oxide property of vanadia with the formation of phase AgV6O{on15} in vanadium pentoxide. However, the large-amount addition of silver caused the catalyst to lose oxide property, thereby, made the catalytic selectivity poor. Mars and van Krevelen model was applied for kinetics study. The results indicate that the reoxidation of catalyst was the rate determining step and that the addition of silver lowers the activation energy in cyclopentadiene oxidation.

Selective oxidation of isobutene over multicomponent molybdate catalyst

The kinetics of the selective oxidation of isobutene to methacrolein over multicomponent molybdate catalyst were investigated in a differential tubular flow reactor at low conversion levels. The study was carried out over the temperature range 609 to 694 K. The data were fitted to a number of standard kinetic models based on Mars-van Krevelen and Langmuir-Hinshelwood mechanisms, and also to the empirical power rate law model. The kinetics of the formation of methacrolein were best represented by the redox model in which non-dissociated oxygen is involved in the reoxidation step. The rate determining step was found to be the catalyst reduction with an energy of activation of 177 kJ/mol. The activation energy for catalyst reoxidation is found to be 71 kJ/mol.

Oxidative conversion of methane to C 2 hydrocarbons over Li, Mn, Cd and Zn promoted MgO catalysts : II: Performance of fresh and reoxidised catalysts in the absence of free oxygen

Involvement of lattice oxygen in the oxidative coupling of methane over MgO, Li-MgO, Mn-MgO, Li-Mn-MgO, Cd-MgO, Li-Cd-MgO, Zn-MgO and Li-Zn-MgO catalysts at 1073 K has been investigated by carrying out the reaction in the absence of free oxygen as a function of methane pulse number, using a pulse microreactor connected to a gas chromatograph. The influence was also studied of catalyst reoxidation after the pulse experiments on the conversion of methane and C 2-selectivity in the pulse reaction. A high C 2 selectivity in the methane pulse reaction is observed for the lithium-promoted catalysts. The order of the C 2 selectivity observed on the catalysts with and without the lithium promoter was found to be as follows: Li-Zn-MgO > Li-Cd-MgO > Li-Mn-MgO ≃Li-MgO⩾ Cd-MgO≫ Zn-MgO > Mn-MgO > MgO. Upon reoxidation of the reduced catalyst in pulse experiments, the selectivities in the formation of carbon monoxide, carbon dioxide and C 2 hydrocarbons are strongly influenced, due to the formation of active chemisorbed oxygen species and/or restructuring of the catalyst surface.

Selective oxidation of isobutene over bismuth molybdate catalyst

Initial rate data were used to study the kinetic behaviour of the selective oxidation of isobutene to methacrolein over γ-bismuth molybdate at low conversion levels. The investigation was carried out in a differential flow reactor over the temperature range 623 to 729 K. The data were fitted to a number of models based on Mars-van Krevelen and Langmuir-Hinshelwood mechanisms, and also to the empirical model known as the power rate law model. The preferred model for the formation of methacrolein is found to be the redox model with non-dissociated oxygen involved in the reoxidation step. The activation energy for catalyst reduction was found to be 52 kJ/mol while the value for catalyst reoxidation was 142 kJ/mol.

Oxidative coupling of methane with participation of oxide catalyst lattice oxygen

Redox properties of the supported Li2O/MgO, K2O/Al2O3 and PbO/Al2O3 catalysts are studied. New mechanism of the catalyst re-oxidation is suggested. Re-oxidation of the catalyst in the course of steady-state reaction can proceed as an oxidative dehydrogenation of surface OH groups.

Catalytic effects of iron compounds and the role of sulfur in coal liquefaction and hydrogenolysis of SRC

Effective iron catalysts in coal liquefaction have been explored. Reduced Fe, Fe(CO) 5, and ferrocene have shown a considerably high catalytic activity in the liquefaction of Yallourn coal at 400°C. Degree of reduction has been shown to be one of the most important factors determining the catalytic activity of the iron catalyst. In the hydrogenolysis of SRC derived from Wandoan coal, water formed in situ has been shown to exhibit a negative effect due to reoxidation of the iron catalyst, but addition of elemental sulfur is greatly effective in preventing such deleterious influence of water. This can be interpreted on the basis of the sulfidation of reduced Fe by elemental sulfur, competing with reoxidation of the iron catalyst by water. In the systems with elemental sulfur, active and oxidation-proof pyrrhotite is formed, and the degree of reduction of the remaining iron oxide is maintained higher than that in the absence of elemental sulfur.

パラジウムを用いる有機合成 アセチレンへのＯＨ，ＮＨ，ＣＯＯＨの分子内付加反応

Various heterocycles are prepared from alkynylamines, alcohols, and carboxylic acids under catalytic action of Pd (II) species. The key reactions are intramolecular addition of NH, OH or COOH to acetylene moiety. Intermediary alkenylpalladium species are protonolyzed or coupled with allyl halides affording organic compounds under the recovery of pd (II) species. During the transformation, Pd (II) species are not reduced, therfore the catalyst can recycle without reoxidation of palladium catalyst which is often observed by the use of olefinic starting materials. Prepared heterocycles are : pyrroles, furans, 1-pyrolines, tetrahydropyridines, dihydrofurans, dihydropyrans, and intramolecular acetals. Syntheses of some insect pheromones bearing intramolecular acetal linkage are also described.

Oxidative dehydrodimerization of propylene over a Bi 2O 3La 2O 2 oxide ion-conductive catalyst

The oxidative dehydrodimerization of propylene to C 3-dimers (1,5-hexadiene and benzene) has been examined at 600 °C and atmospheric pressure using a (Bi 2O 3) 0.85(La 2O 3) 0.15 oxide ion-conducting catalyst in a reactor where a catalyst disk separates a feed of propylene in helium from air. The surface of the disk exposed to propylene was reoxidized not by gaseous O 2, but by the dissociative adsorption and reduction of dioxygen at the oxidant side of the disk, followed by oxide ion conduction to replace spent lattice oxygen. Selectivity to C 3-dimers when using lattice oxide migration to reoxidize the catalyst was considerably greater than when O 2 was added to the propylene feed under the same reaction conditions. This result supports the proposal that lattice oxygen is predominantly involved in the selective oxidation of propylene to C 3-dimers, and demonstrates that Bi 2O 3-based oxide ion conductors can be used as catalysts in reactions where catalyst reoxidation can occur via oxide ion conduction.

ChemInform Abstract: Identification of Active Oxide Ions in a Bismuth Molybdate Selective Oxidation Catalyst.

Direct structural identification of catalytically active oxide ions for selective olefin oxidation has been achieved using in situ Raman spectroscopy. Spectroscopic examination of Bi2MoO6 reduced with select probe molecules such as but-1-ene, propene, methanol and ammonia, when reoxidized with oxygen-18, establishes the existence of the multifunctional nature of catalyst active site for propene oxidation and ammoxidation, wherein α-H abstraction occurs by oxide ions bridging bismuth and molybdenum atoms (Bi—O—Mo), and oxygen or NH insertion into the π-allylic intermediate occurs at centres associated with molybdenum. Sites for O2 chemisorption, reduction and dissociation are likely to be associated with the directed lone pairs of electrons of the Bi—O—Bi species in the structure. Pulse kinetic experiments reveal that diffusion of the oxide ions through the bulk of the catalyst is the rate-limiting step in catalyst reoxidation. This process is rapid when an oxide ion of only one functional type, i.e.α-H abstraction or oxygen insertion, is removed. The process becomes less facile when oxide ions associated with both α-H abstraction and oxygen insertion are removed from the catalyst. The relative rates reflect the extent of active-site reconstruction achieved by diffusion of oxide ions from the bulk to the surface of the catalyst.

Identification of active oxide ions in a bismuth molybdate selective oxidation catalyst

Direct structural identification of catalytically active oxide ions for selective olefin oxidation has been achieved using in situ Raman spectroscopy. Spectroscopic examination of Bi2MoO6 reduced with select probe molecules such as but-1-ene, propene, methanol and ammonia, when reoxidized with oxygen-18, establishes the existence of the multifunctional nature of catalyst active site for propene oxidation and ammoxidation, wherein α-H abstraction occurs by oxide ions bridging bismuth and molybdenum atoms (Bi—O—Mo), and oxygen or NH insertion into the π-allylic intermediate occurs at centres associated with molybdenum. Sites for O2 chemisorption, reduction and dissociation are likely to be associated with the directed lone pairs of electrons of the Bi—O—Bi species in the structure. Pulse kinetic experiments reveal that diffusion of the oxide ions through the bulk of the catalyst is the rate-limiting step in catalyst reoxidation. This process is rapid when an oxide ion of only one functional type, i.e.α-H abstraction or oxygen insertion, is removed. The process becomes less facile when oxide ions associated with both α-H abstraction and oxygen insertion are removed from the catalyst. The relative rates reflect the extent of active-site reconstruction achieved by diffusion of oxide ions from the bulk to the surface of the catalyst.