Ammoxidation Activity Research Articles

In situ synthesis and ammoxidation activity of ammonium salt of molybdophosphoric acid on VOPO 4 catalysts

Two isomorphous VOPO 4 catalysts were prepared and used as support materials for the in situ synthesis of ammonium salt of 12-molybdophosphoric acid (AMPA) from their solid phase phosphate component. Formation of AMPA was confirmed by the XRD and FTIR analyses. The activity of the catalysts in the ammoxidation reaction revealed that the in situ synthesized AMPA catalysts are more active than the parent VOPO 4 catalysts in the conversion of 2-methylpyrazine to 2-cyanopyrazine.

Synthesis, Characterization, and Catalytic Properties of Vanadium Oxide Catalysts Supported on AlPO4

A series of V2O5/AlPO4 catalysts with vanadia loadings ranging from 2.5 to 20 wt % was prepared and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy, electron spin resonance spectroscopy, temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH3, BET surface area, and oxygen chemisorption methods. The catalytic properties were evaluated for the vapor phase ammoxidation of 3-picoline to nicotinonitrile. TPR results indicated that the appearance of a single reduction peak corresponds to V5+ to V3+. The XRD results suggest that, at low vanadia loadings AlPO4 is found to be amorphous. However, at V2O5 loading of 7.38 wt % and above, AlPO4 exist as α-cristobalite form in addition to V2O5. TPD results show that the acidity increases with vanadia loading and decreases at 7.38% and above loadings. The ESR spectra obtained under ambient conditions for the samples reduced at 640 K show the presence of V4+ in axial symmetry. Dispersion of vanadia was determined by oxygen chemisorption at 640 K and by the static method on the samples prereduced at the same temperature. The oxygen uptake measured at 640 K increases with increase in vanadia loading, whereas dispersion of vanadia was decreased. The decrease in dispersion of vanadia was discussed in terms of formation of crystalline forms of V2O5 and AlPO4. The ammoxidation activity and selectivity of nicotinonitrile formation increase with vanadia loading up to 9.5 wt % and did not change appreciably at higher loadings. The catalytic properties during ammoxidation are related to the oxygen chemisorption sites.

Characterization and reactivity of vanadium oxide catalysts supported on niobia

A series of V 2O 5/Nb 2O 5 catalysts with vanadia loading varying from 2 to 12 wt.% were prepared and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), BET surface area and oxygen chemisorption at 640 K. The catalytic properties have been evaluated for vapor phase ammoxidation of 3-picoline to nicotinonitrile. XRD results suggest the formation of β-(Nb,V) 2O 5 phase at higher loadings. TPR profiles showed two peaks: the low temperature peak is due to reduction of surface vanadia species and the high temperature peak is due to reduction of Nb 2O 5. XPS results reveal that both vanadia and niobia are present in fully oxidized state (5+) in all the samples. The intensity ratio V 2p 3/2:Nb 3d 5/2 is found to increase with increase in vanadia loading up to 6 wt.% and to remain constant at higher vanadia loadings. The oxygen uptake increases with increase of vanadia loading on niobia, whereas the dispersion of vanadia decreases. The dispersion of vanadia measured by oxygen chemisorption method is in good agreement with the dispersion determined from XPS. The ammoxidation activity increases with vanadia loading up to 6 wt.%, which corresponds to monolayer coverage and remains constant at higher vanadia loadings. The catalytic properties during ammoxidation of 3-picoline are related to the oxygen chemisorption sites.

Ammoxidation of substituted toluenes on silica-supported VPO catalysts

The ammoxidation of substituted toluenes to their corresponding nitriles over silica-supported vanadium phosphorus oxide (VPO/SiO2) catalysts has been studied. the effects of carrier silica, the addition of phosphorus, the substituents and the loadings have been discussed. Compared with unsupported VPO, the VPO/SiO2 catalysts have higher catalytic activity for ammoxidation of substituted toluenes and much lower reaction temperature. XRD shows that vanadium phosphorus oxides exist as amorphous phase and disperse to a high degree on the silica surface in 10% loading catalyst. When the loadings are over 10%, the crystalline α-VOPO4 would emerge, which would decrease the yield and selectivity. Additional phosphorus can form composite oxides with vanadia and play concerted catalytic function, which increase the selectivity of nitriles remarkably. Different substituents or same substituents on different positions have different influences because of the variant electronic stability of intermediates, the hindered accessibility of methyl group or the chemisorption state of the substrate molecule on the electrophilic catalyst surface.

Reactivity of V2O5/MgF2 Catalysts for the Selective Ammoxidation of 3-Picoline

V2O5/MgF2 catalysts with V2O5 contents ranging from 2.1 to 15.7 wt% were prepared, and the influence of the V2O5 content of the V2O5/MgF2 catalyst on the structure and activity for the ammoxidation of 3-picoline was investigated. XRD data indicate that V2O5 is in a highly dispersed state though segregation of V2O5 into tiny crystallites occurs at and above 8 wt% V2O5. The 3-picoline ammoxidation activity increased with an increase in V2O5 content due not only to the species arising out of interaction of V2O5 and MgF2, but also to the presence of V2O5 microcrystals in the catalysts.

In Situ Synthesis of the Ammonium Salt of 12-Molybdophosphoric Acid on Niobium Phosphate by Salt-Surface Interaction: The Ammoxidation Functionality of the Catalyst in the Transformation of 2-Methylpyrazine to 2-Cyanopyrazine

The ammonium salt of 12-molybdophosphoric acid (AMPA) has been synthesized, in situ, on niobium phosphate by interacting the phosphate of the support with the molybdenum of the salt. The formation of AMPA, with the popular Keggin structure, is confirmed by X-ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. The in situ synthesized catalysts exhibited higher ammoxidation activity than the bulk AMPA, showing optimum conversion at 10 wt% Mo loading. The selectivity to nitrile increased with increasing loading.

Involvement of Redox Sites in Propane Ammoxidation on V-Sb-Oxide Catalysts.

V-Sb-Oxide catalysts were prepared by SS (solid solution) and CP (co-precipitation) methods, and the involvement of redox sites and acid sites in the ammoxidation of propane were investigated by BET, XRD, XPS, NARP (Nitric oxide-Ammonia Rectangular Pulse) and NH3-TPD methods. Both series of catalysts mainly consisted of the SbVO4 phase with minor components of V2O5 and α-Sb2O4 phases. The reaction rate of propane ammoxidation increased linearly with the increase in the number of redox sites measured by NARP technique. There was no significant effect of strength and amount of acid sites on the activity of propane ammoxidation at low conversion levels. The number of redox sites is the determining factor for ammoxidation activity. V-Sb-Oxide catalysts with excess α-Sb2O4 phase exhibited higher turnover frequency.

Characterization and Kinetic Studies on the Highly Active Ammoxidation Catalyst MoVNbTeOx

The catalytic performance and structures of multicomponent “MoVNbTe” oxides have been studied to find the correlation among the performance, active phases, and preparation conditions. The catalyst with the composition MoV0.4Nb0.1Te0.2Ox showed high activity and selectivity for propane ammoxidation reaction (ca. 50% selectivity to acrylonitrile at ca. 90% conversion) when it was pretreated at 830–900 K for 2 h under O2-free or less than 100 ppm O2-contaminated N2 or He flow conditions. Active phases were characterized by XRD, which gave 2θ=14.1°, 24.9°, 26.9°, 29.1°, 32.4°, 33.2°, and 35.4° in addition to the peaks at 22.1°, 28.2°, 36.2°, 44.7°, and 50.0° reported previously. XPS revealed the presence of Mo5+ in addition to Mo6+. In the presence of more than 1000 ppm O2 contamination during the pretreatment, α-MoO3 was formed at the surface region, resulting in reduction of the activity for propane ammoxidation. The activation energy for propane conversion was 32.0 kcal/mol, which was a little smaller than the 36.1 kcal/mol for propene ammoxidation. The reaction rate (v) for propane ammoxidation on the MoVNbTe oxide catalyst at 693 K is given by v=k [propane]0.8 [O2]0.4 [NH3]−0.4. The active phases and reaction mechanism are discussed on the basis of the characterization by XRD and XPS and the kinetic data.

Ammoxidation of 3-picoline over V 2O 5/TiO 2 (anatase) system. II. Characterisation of the catalysts by DTA, SEM, FTIR, ESR and oxygen and ammonia chemisorption

In an earlier communication the ammoxidation activity of V 2O 5/TiO 2 catalysts with V 2O 5 loadings in the range 0.4–9.9 mol% was correlated to the average oxidation number of vanadium in the catalysts. In the present work, these catalysts were characterised by SEM, FTIR, ESR, DTA techniques and chemisorption of NH 3 and O 2. The scanning electron micrographs of the catalysts indicate that deposition of vanadium is taking place inside the mesopores of titania (anatase) up to 3.4 mol% V 2O 5 corresponding to a monolayer coverage. Beyond this loading neddle-like and bulk structures of vanadia appear probably on the external surface of the catalysts. The bands at 1010–1020 cm −1 appearing in the FTIR spectra of fresh catalysts are characteristic of highly dispersed monomeric VO x units and two-dimensional structures. The FTIR spectra of the used catalysts are altogether different from those of the fresh catalysts suggesting that the active phase has been drastically modified during the course of the reaction. The ESR spectrum of 0.4 mol% V 2O 5 shows an eightfold well resolved hyperfine structure indicating that V 4+ is in diluted conditions on anatase surface. As V 2O 5 content increases the hyperfine structure of ESR spectrum gets progressively smeared out due to strong coupling between V 4+ dipoles. The results indicate that vanadium is in a highly dispersed distorted octahedral or square pyramidal geometry at 3.4 mol% corresponding to a monolayer coverage. The DTA curves contain endothermic peaks at 100–150°C and 630–675°C corresponding to desorption of adsorbed water and melting of vanadia particles and loss of oxygen from vanadia. Chemisorption of NH 3 and O 2 is observed to exhibit maximum at the monolayer V 2O 5 loading just as the ammoxidation activity of the catalysts.

Ammoxidation of 3-picoline to nicotinonitrile over vanadium phosphorus oxide-based catalysts

Ammoxidation of 3-picoline to nicotinonitrile was investigated on vanadium phosphorus oxide (VPO), VPO/SiO 2 and additive atom (Cu, Zr, Mn and Co) incorporated VPO catalysts under atmospheric pressure and at 673 K. For the purpose of comparison a conventional V 2 O 5 -MoO 3 /Al 2 O 3 catalyst was also studied under identical conditions. These catalysts were characterized by means of X-ray diffraction, electron spin resonance, infrared, ammonia chemisorption and BET surface area methods. The VPO-based catalysts show better performance than the V 2 O 5 -MoO 3 /Al 2 O 3 catalyst. Further, the VPO/SiO 2 and VPO catalysts exhibit better conversion and product selectivities than the additive-containing VPO catalysts. Better activity of VPO and VPO/SiO 2 catalysts was related to their high active surface area, higher surface acidity and lower oxidation state of vanadium. The redox couple between (VO) 2 P 2 O 7 (V 4+ ) and α I -VOPO 4 (V 5+ ) phases appears to be responsible for the ammoxidation activity of VPO catalysts.

Ammoxidation of 3-picoline over [formula omitted] (anatase) system I. Relationship between ammoxidation activity and oxidation state of vanadium

A series of titania (anatase)-supported vanadia catalysts ranging in V2O5 content from 0.4 to 9.9 mol% was prepared by wet impregnation technique, characterized by BET surface area measurement and X-ray diffraction, and evaluated for ammoxidation of 3-picoline. The average oxidation number of vanadium in the fresh and used catalysts was determined by titrimetric methods. The ammoxidation activity and the average oxidation number were observed to increase with vanadia loading up to 3.4 mol% in the catalyst which corresponds to a monolayer coverage. The phase transformation of anatase to rutile after the reaction was observed at a V2O5 loading of 5.9 mol%. The slow decrease of ammoxidation activity beyond 3.4 mol% V2O5 was attributed to the coverage of active monomeric VOx species on the support by bulk vanadia and by other oxides, and also to compound formation with ammonia.

Ammoxidation of propane to acrylonitrile on FeSbO 4

The properties of FeSbO 4 (Fe:Sb = 1:1 and 1:2) for propane (amm)oxidation have been investigated. Both catalysts showed little selective oxidation activity, however, both were active and selective for propane ammoxidation to acrylonitrile. The 1:2 catalyst showed increased selectivity, relative to the 1:1 catalyst, for acrylonitrile but at the expense of propane conversion. XRD and FT-IR showed the major phases present were rutile FeSbO 4 and Sb 2O 4. XPS studies showed the surface of both catalysts to be antimony rich and TPD studies showed the major pathway to acrylonitrile was a two-stage process via propene and an allylic intermediate, although there is a direct contribution from a one-stage mechanism via an alkyl intermediate. It is postulated that the degree of surface oxidation is critical in determining the dominant surface intermediate. The rate limiting step in the ammoxidation reaction is considered to be dissociative adsorption of propane on the surface, the adsorption rate of propane being orders of magnitude lower than for propene, as manifested by TPD experiments. The role of Sb and Fe are discussed, together with the principal surface mechanisms.

Partial oxidation and ammoxidation of isobutane over Bi-Mo-based composite oxide with Nb 2O 5

Conversion of isobutane to methacrylonitrile via ammoxidation was studied using mixed catalysts composed of BiMo-based composite oxides and various type of Nb 2O 2. The activity for ammoxidation of isobutane on Nb 2O 5 is fairly poor, and that the products were methacrolein without formation of methacylonitriIe. The activity of BiMo-based composite oxides for the ammoxidation of isobutane was also fairly low and that the selectivity to methacylonitrile and methacrolein was 60–80%. The mixed catalyst composed of BiMo-based composite oxides and amorphous Nb 2O 5 and that supported by γ-Al 2O 3 and SiO 2 which have strong acid properties were improved their catalytic activity with keeping of their selectivity for ammoxidation of isobutane to methacrylonitrile.

Highly selective V-P-O/γ-Al2O3catalysts in the ammoxidation of toluene to benzonitrile

γ-Al2O3-supported vanadium phosphorus oxide catalysts prepared by solid-solid wetting for the first time are highly active and selective for the ammoxidation of toluene to benzonitrile at 400 °C and the ammonia uptake of the catalysts correlates directly with their ammoxidation activity.

Effect of basic additives on properties of V−Sb−Bi oxide catalysts for ammoxidation of xylenes

The introduction of K2O and Bao additives into supported V−Sb−Bi oxide catalysts enhances their activity in ammoxidation of xylenes. Bond energy and inhomogeneity character of surface oxygen, oxidation states and relative contents of metal cations remain practically unchanged, whereas the concentration of basic sites significantly increases.

Dispersion and 3-picoline ammoxidation investigation of V2O5/α-Al2O3catalysts

The effect of changing the precursor on the dispersion and 3-picoline ammoxidation activity of various α-Al2O3 supported V2O5 catalysts has been investigated by the techniques of X-ray fluorescence, AES, SEM, EPR, XRD, oxygen chemisorption at 195 K and ammonia chemisorption at 423 K. The nature of the precursor plays an important role in obtaining a highly selective catalyst for 3-picoline ammoxidation to nicotinonitrile. Using maximum oxygen uptake at 195 K and also surface-specific AES and SEM vanadium(III) acetyl-acetonate derived catalysts were found to give a highly dispersed and highly selective catalyst series for nicotinonitrile production in the title reaction. From AES study the oxygen peak intensity, which also reflects the extent of oxidation of the surface, was found to decrease in the order vanadium(III) acetylacetonate > vanadyl oxalate > ammonia metavanadate. The same trend was also observed in their catalytic activity in 3-picoline ammoxidation. The acidity of various catalysts, measured by ammonia uptake, correlated with their selectivity to pyridine.

Acio-Base properties and actvity of complex oxide catalysts for m-xylene ammoxidation

Introduction of small quantities of alkaline and alkalineearth metal oxides into supported Sb−Bi−V oxide catalysts increasses the concentration of surface basic centers and increases significantly its activity in m-xylene ammoxidation. Promoting effect is suggested to be due to the heterolytic type of the primary activation of hydrocarbon.

Fe [sbnd]Se-Tellurates as ammoxidation catalysts

Acrylonitrile is a versatile intermediate and is produced by the Sohio ammoxidation process world-wide at a level of 8 billion pounds per year. In the catalytic process, propylene, ammonia and air are converted to acrylonitrile directly. The most widely used catalysts are mixed metal molybdates or antimonates. The current investigation deals with metal tellurates as possible low temperature ammoxidation catalysts. Of the catalysts investigated, iron-selenium-tellurates are of particular interest since they exhibit good ammoxidation activity already at 320°C which is about 100°C lower than the temperature required for molybdate or antimonate systems. Mechanistically, the tellurates behave more similarly to the antimonates than the molybdates.