Catalysts For Propane Ammoxidation Research Articles

A study of M1/M2 phase synergy in the MoVTe(Nb,Ta)O catalysts for propane ammoxidation to acrylonitrile

The bulk mixed MoVTe(Nb,Ta)O catalysts containing nanoscale intergrowths of so-called M1 and M2 phases display uniquely high reactivity in propane ammoxidation to acrylonitrile (ACN) associated with the surface ab planes of the M1 phase. The current controversy surrounding this catalytic system is focused on the role of the M2 phase, which is unable to activate propane but was suggested in some studies to function in synergy with the M1 phase by efficiently converting the propylene intermediate to ACN. The present study systematically examined the catalytic behavior of pure M1 phases prepared by selective dissolution of the M2 phase in MoVTe(Nb,Ta)O catalysts in aqueous hydrogen peroxide. It confirmed that the MoVTe(Nb,Ta)O M1 phases are highly active and selective for propane and propylene ammoxidation, while the MoVTe(Nb,Ta)O M2 phases were active and selective in propylene ammoxidation only. Crucially, the present study of the pure MoVTe(Nb,Ta)O M1 and M2 phases in propylene ammoxidation revealed for the very first time that the M2 phases are significantly less active than the corresponding M1 phases in propylene ammoxidation. The findings of this study do not support the existence of the synergy effect for the MoVTe(Nb,Ta)O M1 and M2 phases. Instead, the trends previously associated with the M1 and M2 phase synergy in propane ammoxidation can be explained by two effects that were unaccounted for previously: (i) the preferential loss of surface active TeOx species from the M1 phase during the H2O2 treatment, and (ii) the generation of fresh ab planes via mechanical crushing of the H2O2-treated M1 phase. These findings confirmed that the M1 phase is the only phase required for the activity and selectivity of the MoVTe(Nb,Ta)O catalysts in propane ammoxidation to ACN.

Quantitative Analysis of HAADF–STEM Images of MoVTeTaO M1 Phase Catalyst for Propane Ammoxidation to Acrylonitrile

AbstractHigh‐angle annular dark‐field scanning transmission electron microscopy (HAADF–STEM) image simulations were performed to determine accurate metal distributions in the MoVTeTaO M1 phase catalyst for propane ammoxidation. QSTEM simulation, based on a multislice method with the frozen phonon approximation, was chosen because of the excellent agreement between experimental and simulated HAADF–STEM images. This simulation method successfully provided accurate metal distributions in the MoVTeTaO M1 phase and indicated significantly lower V occupancies of the linking sites 1, 2, 3, 4, and 7, and higher Ta occupancy in site 9 than the previously reported metal occupancies. This novel methodology for accurate HAADF–STEM image analysis is a valuable tool for accurate analysis of local chemical compositions in a wide range of ordered catalytic materials.

Better Catalysts through Microscopy: Mesoscale M1/M2 Intergrowth in Molybdenum-Vanadium Based Complex Oxide Catalysts for Propane Ammoxidation.

In recent decades, catalysis research has transformed from the predominantly empirical field to one where it is possible to control the catalytic properties via characterization and modification of the atomic-scale active centers. Many phenomena in catalysis, such as synergistic effect, however, transcend the atomic scale and also require the knowledge and control of the mesoscale structure of the specimen to harness. In this paper, we use our discovery of atomic-scale epitaxial interfaces in molybdenum-vanadium based complex oxide catalysts systems (i.e., Mo-V-M-O, M = Ta, Te, Sb, Nb, etc.) to achieve control of the mesoscale structure of this complex mixture of very different active phases. We can now achieve true epitaxial intergrowth between the catalytically critical M1 and M2 phases in the system that are hypothesized to have synergistic interactions, and demonstrate that the resulting catalyst has improved selectivity in the initial studies. Finally, we highlight the crucial role atomic scale characterization and mesoscale structure control play in uncovering the complex underpinnings of the synergistic effect in catalysis.

The structure and catalytic properties of amorphous phase in MoVTeO catalysts for propane ammoxidation

This work presents an investigation of the chemical and phase composition of Mo1.0V0.3Te0.23Ox catalyst prepared by spray drying. The changes in the catalyst after heat treatment at 220–550°C were characterized by IR and Raman spectroscopy, XRD, HRTEM and differential dissolution methods. In the range of temperature 400–420°C HRTEM revealed the presence of nanoscaled crystallites that are structurally and chemically related to M1 and M2 phases and randomly distributed in the amorphous matrix. The characteristic diffraction pattern of the amorphous product can be described by a model of nano-sized clusters built from the structure of M1 phase. Nanostructured catalysts are active and selective in propane ammoxidation. The increase of temperature of heat treatment of the catalyst above 450°C leads to decomposition of nanostructured M1 phase and crystallization of M2 and a series of vanadium–molybdenum phases. The process is accompanied by a decrease in activity.

One-pot synthesis of iron doped mesoporous silica catalyst for propane ammoxidation

Iron doped hexagonal mesoporous silica (Fe-HMS) was successfully prepared by one-pot direct synthesis at ambient temperature by using dodecylamine as a templating agent. The catalyst has been characterized by various techniques including XRD, DR-UV-Vis, FTIR, SEM and N2 adsorption. The obtained material exhibits very large surface area (above 1000 m(2) g(-1)), mesoporosity with narrow distribution of pores (around 4 nm) and nano-crystallites with the 20-30 nm in size. The concentration of iron in the catalyst was 0.95 wt%. The prepared material was tested in the direct ammoxidation of propane. It was shown that direct synthesis leads to better dispersion of the metal compared to the material prepared by impregnation of pure HMS silica. It is also demonstrated that this material is able to catalyze the direct ammoxidation of propane and shows quite high selectivity to acrylonitrile (52% at 11% conversion of propane).

Propane ammoxidation catalyst prepared by thermally induced spreading of Sb2O3 on V/Al/O-mixed oxide

Mechanical mixtures of Sb2O3 and V/Al/O mixed oxide have been tested as catalysts in the ammoxidation of propane. The results obtained showed that upon addition of Sb2O3 the propane conversion rate and the selectivity to acrylonitrile were decreased but to a limited extent whereas a strong decrease in selectivity to COx was observed to the advantage of selectivity to propene. Sb2O3 completely reacts with the V/Al/O mixed oxide to form a rutile-type V1−xAlxSbO4 phase that spreads over the mixed oxide. If the structure modifications have to be taken into account to explain the difference in catalytic properties, the characterization of the catalysts allowed proposing that they primarily result from the contamination of the surface of the active V/Al/O mixed oxide by Sb species. If the changes in catalytic properties evidenced in the study appear rather unproductive in terms of yield in acrylonitrile, they can be of interest if a process integrated a recycling of the reaction feeds was designed for an industrial application.

Effect of Oxalic Acid on the Structure and Catalytic Performance of Sb-V-O Catalyst for Propane Ammoxidation

Effect of Oxalic Acid on the Structure and Catalytic Performance of Sb-V-O Catalyst for Propane Ammoxidation

New rutile Ga/V/Nb/Sb mixed oxides as catalysts for propane ammoxidation to acrylonitrile

Here we report on the chemical–physical characteristics and reactivity in propane ammoxidation to acrylonitrile of a new class of rutile-type, GaSbO4-based multicomponent oxides. The GaSbO4 rutile was much less active than the conventional VSbO4-based rutile systems, but showed a good selectivity to acrylonitrile. The incorporation of V in GaSbO4 allowed for an improvement in catalytic activity, but with decreased maximum selectivity to acrylonitrile. However, the further incorporation of Nb (thereby developing a Ga/V/Nb/Sb mixed oxide) led to a mixed oxide that presented an enhanced catalytic behavior, combining both high activity and high selectivity to acrylonitrile. The Nb presence also greatly improved the stability of the catalytic system, hindering the crystallization phenomena observed with rutile Ga/V/Sb/O systems.

Alumina support-stabilized nanoscaled vanadium–phosphorous mixed oxides as new catalysts for propane ammoxidation

Much attention has recently been devoted to the surface of bulk catalysts. It has been proved that the most critical phenomena occur at the surface, which differs significantly from the bulk. To evaluate the role of the bulk in VPO catalyst, vanadium and phosphorus were supported on γ-alumina by incipient wetness impregnation at a total V + P of two monolayers so that nanoscaled VPO phases (n-VPO) form on alumina, such size maximizes the surface-to-bulk ratio. Alumina support stabilizes V 5+, which may be reduced, while V 4+ predominates in bulk vanadyl pyrophosphate phase. In addition, the support modifies acid–base and textural properties. The presence of VPO nanocrystals supported on alumina (n-VPO) facilitates investigating the interaction between alumina-dispersed vanadia and VPO-lattice vanadium ions. Both, bulk and supported phases are active for propane ammoxidation affording acrylonitrile yields of 41% and 48%, respectively, at 500 °C. The yield to acrylonitrile per mass unit of VPO increases by 150% at 500 °C in the alumina-stabilized n-VPO system when compared to the bulk VPO.

Effect of reduction-oxidation treatment on the properties of VSb3W-Al2O3 catalyst for propane ammoxidation

VSb3W-Al2O3 catalysts for propane ammoxidation were prepared by the conventional slurry method and new slurry method, namely, the reduction-oxidation slurry method. The effect of preparation methods on the physicochemical properties of VSb3W-Al2O3 catalysts were studied by N2 adsorption, XRD, XPS and Raman spectroscopy, and their catalytic performance for the ammoxidation of propane to acrylonitrile were tested. Compared with the VSb3W-Al2O3 catalyst prepared by the conventional slurry method, the VSb3W-Al2O3 catalyst prepared by the reduction-oxidation slurry method exhibited better propane conversion and acrylonitrile selectivity. This is due to that the reduction-oxidation treatment was advantageous for obtaining low valence state vanadium and high valence state antimony to form more rutile SbVO4, which is a vital active phase for propane ammoxidation.

Preparation, Characterization and Catalytic Test of MoVSb and MoVNb Mixed Oxide Catalysts for Propane Ammoxidation to Acrylonitrile

Preparation, Characterization and Catalytic Test of MoVSb and MoVNb Mixed Oxide Catalysts for Propane Ammoxidation to Acrylonitrile

Ruthenium and gold-doped M1 phase MoVNbTeO catalysts for propane ammoxidation to acrylonitrile

Abstract Ammoxidation of propane to acrylonitrile was investigated over the M1 phase MoVNbTeO catalysts doped with 5 wt.% Ru and Au, respectively. The MoVNbTeO mixed metal oxide catalyst was synthesized by a hydrothermal method, whereas ruthenium and gold were introduced by incipient wetness impregnation. The structures of the MoVNbTeO mixed metal oxide and ruthenium- and gold-doped catalysts were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD patterns of the original MoVNbTeO catalyst confirmed the formation of the orthorhombic M1 phase, while the XRD patterns of ruthenium- and gold-doped catalysts demonstrated that the presence of ruthenium and gold significantly affected the crystal structure of the MoVNbTeO M1 phase. The catalytic activity was studied in a tubular fixed-bed micro-reactor at atmospheric pressure and 360–445 °C. Ru- and Au-doped MoVNbTeO M1 catalysts showed lower ammoxidation activity than the original MoVNbTeO M1 catalyst. However, both doped catalysts were much more active in the oxidative dehydrogenation (ODH) of propane at relatively low temperature (

Local environment of vanadium in V/Al/O-mixed oxide catalyst for propane ammoxidation: Characterization by in situ valence-to-core X-ray emission spectroscopy and X-ray absorption spectroscopy

The local environment of vanadium in V/Al/O amorphous oxide catalyst has been studied under propane ammoxidation reaction conditions. In situ valence-to-core X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) at the V K-edge showed that acrylonitrile production starts when vanadium in the bulk structure of the oxide material changes its formal oxidation state from +4.8 to +3.8 ± 0.1. Exposure of the catalyst to pure NH 3 at 500 °C leads to further reduction of vanadium. Valence-to-core XES has also proved that the level of bulk nitridation of vanadium in the active catalyst was rather small. XPS analysis performed ex situ after the catalytic tests confirmed the presence of significant amounts of N 3−, NH x , and –NN– species on the catalyst surface. It can be concluded that activation of V/Al/O catalyst on-stream is mainly associated with reduction of vanadium in the bulk structure of the material and nitridation of vanadium atoms on the surface.

The role of Sb and Nb in rutile-type Sn/V/Nb/Sb mixed oxides, catalysts for propane ammoxidation to acrylonitrile

This paper describes the role of Sb and Nb, components of Sn/V/Nb/Sb mixed oxides catalysts for the gas-phase ammoxidation of propane to acrylonitrile. In samples without Nb and with atomic ratios Sn/V/Sb 1/0.2/ x ( x = 0 to 3), Sb in the form of amorphous oxide is necessary in order to obtain an active and selective catalyst. However, during reaction the dispersed Sb oxide segregates to α-Sb 2O 4, and the yield to acrylonitrile decreases considerably. The addition of Nb gives rise to the formation of Nb-containing SbO x and non-stoichiometric rutile-type V/Nb/Sb mixed oxides. The presence of these compounds enhances the catalytic activity and the selectivity to acrylonitrile. Moreover, the catalyst shows a stable catalytic performance, with no segregation of α-Sb 2O 4.

The role of V in rutile-type Sn/V/Nb/Sb mixed oxides, catalysts for propane ammoxidation to acrylonitrile

Rutile-type Sn/V/Nb/Sb mixed oxides of composition Sn/V/Nb/Sb 1/ x/1/3 (atomic ratios) were prepared by co-precipitation from an alcoholic medium, characterized and tested as catalysts for the ammoxidation of propane to acrylonitrile. Vanadium had a relevant effect on chemical–physical and reactivity properties of catalysts. The latter consisted of Sn oxide incorporating Sb and Nb cations, of defective rutile-type V/Nb/Sb mixed oxide and of Sb oxide. Increasing amounts of V in samples caused an increase of the crystallinity and a corresponding decrease of the specific surface area. However, a relevant enhancement of the catalyst activity (rate of propane conversion per unit surface area) was observed. This was attributed to the generation of cationic vacancies, formed in the rutile-type V/Nb/Sb mixed oxide, that enhanced the intrinsic activity of V ions in the activation of the alkane. On the other hand, the selectivity to acrylonitrile declined considerably when the content of V in samples was increased, whereas the selectivity to carbon monoxide and that to cyanhydric acid increased.

The control of catalytic performance of rutile-type Sn/V/Nb/Sb mixed oxides, catalysts for propane ammoxidation to acrylonitrile

This paper describes the effect of the composition of rutile-type Sn/V/Nb/Sb mixed oxides catalysts on the catalytic performance in the gas-phase ammoxidation of propane to acrylonitrile. The variation in the atomic ratio between components in catalysts is the key for the control of activity and selectivity. In samples with atomic composition Sn/V/Nb/Sb 1/0.2/1/ x (0 ≤ x ≤ 5) and 1/0.2/ y/3 (0 ≤ y ≤ 3) several compounds formed, i.e., SnO 2, Sb/Nb mixed oxide, Sb 6O 13 and non-stoichiometric rutile-type V/Nb/Sb/O; the latter segregated preferentially at the surface of the catalyst. Tin oxide provided the rutile matrix for the dispersion of the mixed oxides. The main role of Sb was shown to generate mixed oxides containing specific sites for the allylic ammoxidation of propylene intermediately formed. The presence of Nb enhanced the activity and selectivity of these sites.

Support Effect on the Structure and Reactivity of VSbO4 Catalysts for Propane Ammoxidation to Acrylonitrile

A new synthesis method for Sb−V−O based catalysts is proposed. VSbO4 phase is supported both in alumina and in niobia because it is known that such structure is the active phase for acrylonitrile formation. The relevance of the support on the structure and performance of the VSbO4 phase is evaluated by X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy and in the ammoxidation of propane to acrylonitrile. The role of segregated antimony oxide for the VSbO4 phase is confirmed in VSbO4/Al2O3. In this system, segregated antimony oxide is critical for the redox cycle of vanadium sites. However, a new catalytic phase forms in VSbO4/Nb2O5. Niobia support affords an efficient phase other than VSbO4 that is selective to acrylonitrile formation. This phase appears to be a Sb-promoted V−Nb−O one.

Direct ammoxidation of propane over zeolites with traces of iron

The performance of Fe-zeolites in ammoxidation of propane at 540 °C in presence of molecular oxygen, nitrous oxide or their mixture has been investigated. The results presented here clearly show ability of Fe-silicalites to catalyze direct ammoxidation of propane. Ammoxidation of propane takes place on special extraframework Fe ions exclusively generated by hydrolysis of framework Si–O–Fe bonds. Introduction of Fe ions into silicalite matrix by post-synthesis modification does not lead to an active catalyst for propane ammoxidation. The presence of aluminum atoms in the framework is disadvantageous for propane ammoxidation, in contrast to propane oxidative dehydrogenation by nitrous oxide, due to decomposition of acrylonitrile on strong acidic sites.

Comment on “Design of a Propane Ammoxidation Catalyst Using Artificial Neural Networks and Genetic Algorithms”

Comment on “Design of a Propane Ammoxidation Catalyst Using Artificial Neural Networks and Genetic Algorithms”

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