Alumina supported Mo–V–Te–O catalysts for the ammoxidation of propane to acrylonitrile

The effect of Te addition over Mo–V–O catalysts supported on alumina is discussed for the ammoxidation of propane to acrylonitrile. Catalyst composition and atmosphere of activation are evaluated. Catalysts are characterized before and after catalytic reaction by XPS, XRD and in situ Raman spectroscopies. The absence of Te in catalysts formulation and the presence of a high amount of vanadium induce the presence of V⁵⁺ species and the formation of V₂O₅ oxide; associated with a decrease in acrylonitrile selectivity. The presence of Mo-based polyacids structures decreases the selectivity to acrylonitrile. V⁵⁺ sites are responsible of propane activation and of the subsequent -H abstraction to form the intermediate propylene. Then, a Mo–V rutile-like structure in which vanadium species are reduced as V⁴⁺, is responsible for nitrogen insertion and acrylonitrile formation. The formation of such structure is favoured when Te is added to catalysts and is promoted during propane ammoxidation.     

Selective oxidation and ammoxidation of propane on a Mo–V–Te–Nb–O mixed metal oxide catalyst: a comparative study

A comparative study on the selective oxidation and the ammoxidation of propane on a Mo–V–Te–Nb–O mixed oxide catalyst is presented. The catalyst has been prepared hydrothermally at 175 °C and heat-treated in N₂ at 600 °C for 2 h. Catalyst characterization results suggest the presence mainly of the orthorhombic Te₂M₂₀O₅₇ (M = Mo, V and Nb) bronze in samples before and after use in oxidation and ammoxidation, although some little modifications have been observed after its use in ammoxidation reaction. Propane has been selectively oxidized to acrylic acid (AA) in the 340–380 °C temperature range while the ammoxidation of propane to acrylonitrile (ACN) has been carried out in the 360–420 °C temperature interval. The steam/propane and the ammonia/propane molar ratios have an important influence on the activity and the selectivity to acrylic acid and acrylonitrile, respectively. The reaction network in both oxidation and ammoxidation reactions as well as the nature of active and selective sites is also discussed. The catalytic results presented here show that the formation of both ACN and AA goes through the intermediate formation of propene.     

Ammoxidation of methylpyrazine over V–Ti oxide system

The catalytic properties of vanadium–titanium oxide system in ammoxidation of methylpyrazine have been studied. Catalytic activity increases monotonically and yield of selective products passes a wide maximum in the range of V₂O₅ content from 10 to 75 wt.% with increase in the V/Ti relation. The active centers of binary catalysts include V⁵⁺ cations with distorted octahedral coordination strongly bounded with titania apparently owing to formation of V–O–Ti bonds.     

Cr/V/Sb mixed oxide catalysts for the ammoxidation of propane to acrylonitrile: Part I: Nature of the V species

Rutile-type Cr/V/Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile, were prepared and characterized. For atomic ratios between components equal to Cr/V/Sb 1/x/1 and 1/x/2 the systems were monophasic, but different types of compounds formed depending on the ratio between the three metals. The compositional parameter which most affected the nature of the compound formed was the (Cr+V)/Sb atomic ratio. When this ratio was between 2 and ≈1, a rutile Cr³⁺/V⁴⁺/Sb⁵⁺ mixed oxide of composition Cr₁V_xSb₁O_4+2x developed (0<x<1), which in practice corresponds to a solid solution between 1 CrSbO₄ and x VO₂. When the (Cr+V)/Sb ratio was between 0.5 and ≈1, a rutile Cr³⁺/V³⁺/Sb³⁺/Sb⁵⁺ mixed oxide of composition CrV_xSb_1+x+2zO_4+4x+4z developed (0<x<1), which corresponds to a solid solution between 1 Cr³⁺Sb_z³⁺Sb_1+z⁵⁺O_4+4z and x VSbO₄. The distinction between the two classes of compounds was not clear-cut, and when the (Cr+V)/Sb atomic ratio was around 1, mixed oxides containing both V³⁺ and V⁴⁺ formed. Values of the (Cr+V)/Sb atomic ratio lower than ≈0.5 led to the additional formation of antimony oxide.     

Molecular structure and reactivity of the Group V metal oxides

The physical, electronic and reactivity properties of bulk and supported Group V metal oxides (V, Nb, Ta and Db) were compared at the molecular level. Dubnium is a very short-lived element, 60 s, whose properties have not been extensively studied, but can be predicted from knowledge of the other members of the Group V metal oxides. Bulk V₂O₅ possesses platelet morphology with the active surface sites only located at the edges: primarily surface redox sites and some surface acidic sites. Bulk Nb₂O₅ and Ta₂O₅, as well as to be expected for bulk Db₂O₅, possess isotropic morphologies and the active surface sites relatively homogeneously dispersed over their surfaces: only surface acidic sites. However, the bifunctional bulk V₂O₅ was found to exhibit a much higher specific acidic catalytic activity than the acidic bulk Nb₂O₅ and Ta₂O₅, the latter being almost identical in their specific acidic catalytic activity. The bulk properties of the Group V metal oxides were essentially transferred to the analogous supported Group V metal oxides, where the active Group V metal oxides were present as a two-dimensional monolayer on various oxide supports (e.g., Al₂O₃, TiO₂, ZrO₂ as well as Nb₂O₅ and Ta₂O₅). For supported vanadia catalysts, the active surface sites were essentially redox sites, with the exception of supported V₂O₅/Al₂O₃ that also contained strong acidic sites. For supported niobia and tantala catalysts, as well as to be expected for supported dubnia catalysts, the active surface sites were exclusively acidic sites. However, the TOF_redox for the supported vanadia catalysts and the TOF_acidic for the supported niobia and tantala catalysts varied over several orders of magnitude as a function of the specific oxide support with the electronegativity of the oxide support cation. However, the TOF_redox varied inversely to that of the TOF_acidic variation because of the opposite requirements of these active surface sites. Surface redox sites are enhanced by reduction and surface acidic sites are enhanced by stabilization (lack of reduction). The current fundamental understanding of the Group V metal oxides allows for the molecular engineering of their metal oxide applied catalytic materials.     

The role of molybdenum in Mo-doped V–Mg–O catalysts during the oxidative dehydrogenation of n-butane

A detailed study on the influence of the addition of molybdenum ions on the catalytic behaviour of a selective vanadium–magnesium mixed oxide catalyst in the oxidation of n-butane has been performed. The catalysts have been prepared by impregnation of a calcined V–Mg–O mixed oxides (23.8 wt% of V₂O₅) with an aqueous solution of ammonium heptamolybdate, and then calcined, and further characterised by several physico-chemical techniques, i.e. S_BET, XRD, FTIR, FT-Raman, XPS, H₂-TPR. MgMoO₄, in addition to Mg₃V₂O₈ and MgO, have been detected in all the Mo-doped samples. The incorporation of molybdenum modifies not only the number of V⁵⁺-species on the catalyst surface and the reducibility of selective sites but also the catalytic performance of V–Mg–O catalysts. The incorporation of MoO₃ favours a selectivity and a yield to oxydehydrogenation products (especially butadiene) higher than undoped sample. In this way, the best catalyst was obtained with a Mo-loading of 17.3 wt% of MoO₃ and a bulk Mo/V atomic ratio of 0.6. From the comparison between the catalytic properties and the catalyst characterisation of undoped and Mo-doped V–Mg–O catalysts, the nature of selective sites in the oxidative dehydrogenation of n-butane is also discussed.     

Molecular structure and reactivity of the group V metal oxides

The molecular structures and reactivity of the group V metal oxides (V₂O₅, Nb₂O₅ and Ta₂O₅) were compared. Their solid state structural chemistry, physical and electronic properties, number of active surface sites and their chemical reactivity properties were examined. For the bulk oxides, the solid state structural chemistry and the physical and electronic properties are well established. The number of active surface sites and the distribution of surface redox/acid sites were determined with methanol chemisorption and methanol oxidation, respectively. These studies revealed that the active surface sites present in pure V₂O₅ are primarily redox sites and the active surface sites in pure Nb₂O₅ are essentially acidic in nature. Furthermore, the surface redox sites present in pure V₂O₅ are orders of magnitude more active than the surface acid sites in pure Nb₂O₅. Consequently, the catalytic properties of bulk V₂O₅–Nb₂O₅ mixed oxides are dominated by the vanadia component. For the supported metal oxides, where the group V metal oxides are present as two-dimensional metal oxide overlayers, the structural and electronic properties are not well established in the literature. From a combination of molecular spectroscopic characterization methods (e.g., XANES, Raman, IR and UV–Vis DRS), it was possible to obtain this fundamental information. Methanol chemisorption studies demonstrated that a similar number of active surface sites are present in the supported vanadia and niobia catalyst systems. Similar to their bulk oxides, the surface vanadia species possess redox characteristics and the surface niobia species primarily possess acidic characteristics (Lewis acidity). The surface niobia species was a very sluggish redox site during oxidation reactions (e.g., methanol oxidation to formaldehyde and SO₂ oxidation to SO₃), but significantly promoted the surface vanadia redox sites for oxidation reactions that required dual surface redox and acid sites (e.g., butane oxidation to maleic anhydride and selective catalytic reduction of NO_x by NH₃ to produce N₂). These new fundamental insights are allowing for the molecular engineering of group V metal oxide catalysts (especially vanadia and niobia). In contrast, the molecular structure and reactivity properties of Ta₂O₅ catalysts are not yet established and will require significant research efforts.     

High surface area Mo–V–Te–Nb–O catalysts: Preparation, characterization and catalytic behaviour in ammoxidation of propane

Mo–V–Te–Nb mixed oxides with a molar ratio of 1:0.30:0.20:0.15 were prepared by citrate and dry-up method, both associated with hydrothermal treatments in the presence of a cationic surfactant (cetyl trimethylammonium bromide, CTAB), and tested in the ammoxidation of propane. The catalysts were characterized by adsorption–desorption isotherms of nitrogen at 77 K, particle size measurements, XRD, and XPS. By using the surfactant, the surface area increased significantly, and samples with surface area between 110 and 239 m²/g were obtained. These catalysts exhibited a propane conversion near 48% with selectivity to acrylonitrile of about 32% for a space velocity 30 times higher than generally reported.     

Structure-activity relationship in Sb-V-oxide catalysts for the direct synthesis of acrylonitrile from propane

The structure/composition and catalytic behavior in propane ammoxidation to acrylonitrile of Sb-V-oxides prepared by a novel precipitation—deposition method is reported and the nature of the active components in these catalysts is discussed. Results suggest that the active phase for the selective synthesis of acrylonitrile from propane is a trirutile-like V-antimonate with Sb⁵⁺-oxide supported on it. Vanadium plays a catalytic role in the selective activation of propane and reoxidation of antimony, but when present in large amounts on the surface catalyzes the unselective NH₃ → N₂ conversion with a lowering in acrylonitrile selectivity. -Sb₂O₄ is instead a side phase, with a marginal or negligible role in the catalytic behavior.     

Catalytic behaviour of M1, M2, and M1/M2 physical mixtures of the Mo–V–Nb–Te–oxide system in propane and propene ammoxidation

Essentially pure orthorhombic M1 and pseudo-hexagonal M2 phases were prepared using the precursor method. Consistent with literature the M1 phase was shown to be effective for propane ammoxidation to acrylonitrile while the M2 phase was essentially inert for propane activation. Both phases convert propene efficiently to acrylonitrile. Both phases show a significant selectivity dependence on the ammonia and oxygen concentrations in the feed, revealing thereby additional insights into the reaction mechanism.

Physical mixtures of the two separately prepared phases exhibited symbiosis in the ammoxidation of propane when finally divided (5 μm), thoroughly mixed and brought into intimate contact with each other. Acrylonitrile yields significantly higher than those obtained with the M1 phase alone were demonstrated with a 50 wt.% M1/50 wt.% M2 physical mixture having a corresponding surface area ratio of about 4:1. The phase cooperation effect is particularly large at high propane conversions and non-existent when the particle size of the phases is too large (e.g. >250 μm) and the inter-particle contact is poor.     

Operando Raman–GC studies of alumina-supported Sb-V-O catalysts and role of the preparation method

The interactions between Sb and V are studied by operando Raman–GC methodology during propane ammoxidation in order to understand the effect of the preparation method and reaction conditions on the structure and activity/selectivity of alumina-supported Sb-V-O catalysts. Dispersed V(V) species react with antimony species during propane ammoxidation to form VSbO₄; partially reversible transformations towards surface vanadium (V) species may account for the catalytic redox cycle. The catalytic performance is determined by the interaction between Sb and V, which is affected by the preparation method and the reaction conditions.     

Parameters influencing the synergetic effect induced by vanadium incorporation on non-conventional (Al)(Ga)PO supports for the propane ammoxidation

A series of V-oxide supported catalysts (5V/AlPO₄, 5V/Al_0.5Ga_0.5PO₄, 5V/GaPO₄) were prepared in order to study their catalytic behaviour in the ammoxidation of propane. Catalysts were characterized by BET surface area, XRD, Raman spectroscopy, TPR-H₂ and XPS. The presence of vanadium induced the crystallization of the supports (AlPO₄, Al_0.5Ga_0.5PO₄ and GaPO₄). Crystalline V₂O₅ was observed on the V-based catalysts. Catalytic results showed that the impregnation of vanadium enhances the propane conversion and selectivity to acrylonitrile (ACN). At 530 °C 5V/Al_0.5Ga_0.5PO₄ exhibits the highest selectivity to ACN. One assumes that the best performance of 5V/Al_0.5Ga_0.5PO₄ in propane ammoxidation, is due to on the one hand the easiest reduction of V₂O₅ on Al_0.5Ga_0.5PO₄ and to on the other hand a certain tunability of the reduced species stabilized on Al_0.5Ga_0.5PO₄.     

Competition between NO reduction and NO decomposition over reduced V–W–O catalysts

The SCR of NO and NO decomposition were investigated over a V–W–O/Ti(Sn)O₂ catalyst on a Cr–Al steel monolith. The conversions of NO and NH₃ over the reduced and oxidised catalysts were determined. The higher conversion of NO than of NH₃ was observed in SCR over the reduced catalyst and very close conversions of both substrates were found over the oxidised one. The increase of the pre-reduction temperature was found to cause an increase in catalyst activity and its stability in direct NO decomposition. The surface tungsten cations substituted for vanadium ones in vanadia-like active species are considered to be responsible for the direct NO decomposition. The results of DFT calculations for the 10-pyramidal clusters: V₁₀O₃₁H₁₂ (V–V) and V₉WO₃₁H₁₂ (V–W) modelling (0 0 1) surfaces of vanadia and WO₃–V₂O₅ solid solution (s.s.) active species, respectively, show that preferable conditions for NO adsorption exist on W sites of s.s. species and that reduction causes an increase in their ability for electron back donation to the adsorbed molecule. Electron back donation is believed to be responsible for the electron structure reorganisation in the adsorbed NO molecule resulting in its decomposition. The high selectivity of NO decomposition to dinitrogen was considered to be connected with the formation of the tungsten nitrosyl complexes solely via the W–N bond.     

Catalytic dehydrogenation of ethylbenzene with carbon dioxide: promotional effect of antimony in supported vanadium–antimony oxide catalyst

Alumina-supported vanadium oxide, VO_x/Al₂O₃, and binary vanadium–antimony oxides, VSbO_x/Al₂O₃, have been tested in the ethylbenzene dehydrogenation with carbon dioxide and characterized by S_BET, X-ray diffraction, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction and CO₂ pulse methods. VSbO_x/Al₂O₃ exhibited enhanced catalytic activity and especially on-stream stability compared to VO_x/Al₂O₃ catalyst. Incorporation of antimony into VO_x/Al₂O₃ increased dispersion of active VO_x species, enhanced redox properties of the systems and formed a new mixed vanadium–antimony oxide phase in the most catalytically efficient V_0.43Sb_0.57O_x/Al₂O₃ system.     

HDS and HDN activity of molybdenum and nickel–molybdenum catalysts supported on alumina–titania carriers

Alumina–titania supports containing 5–50 wt.% of TiO₂ were prepared by coprecipitation method using inorganic precursors (sodium aluminate and titanium chloride). DTA-TGA, XRD, SEM, TPD_NH3, and IR spectroscopy were used to characterise these materials. The study shows that the promoting effect of nickel on the HDS activity of molybdenum catalysts supported on Al₂O₃TiO₂ is significantly lower than that for molybdenum catalyst supported on Al₂O₃, and depends on the TiO₂ content. The SEM results show that in the case of rich Al support (20 wt.% of TiO₂) molybdenum was aggregated on the external surface of the catalyst, whereas it was uniformly dispersed on the external surface of alumina. Results also show that molybdenum is preferably supported on aluminum oxide. Application of Al₂O₃TiO₂ oxides enhances the HDN activity of nickel–molybdenum catalysts. The highest HDN efficiency was obtained for the NiMo/Al₂O₃TiO₂ catalyst containing 50 wt.% of TiO₂. HDN activity was found to depend on protonic acidity and anatase content.     

Partial oxidation and ammoxidation of isobutane over Bi-Mo-based composite oxide with Nb2O5

Conversion of isobutane to methacrylonitrile via ammoxidation was studied using mixed catalysts composed of Bi---Mo-based composite oxides and various type of Nb₂O₂. The activity for ammoxidation of isobutane on Nb₂O₅ 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 Nb₂O₅ and that supported by γ-Al₂O₃ and SiO₂ which have strong acid properties were improved their catalytic activity with keeping of their selectivity for ammoxidation of isobutane to methacrylonitrile.     

In situ TPR/TPO-Raman studies of dispersed and nano-scaled mixed V-Nb oxides on alumina

Various catalysts containing niobium and vanadium oxides supported on alumina were prepared by wet impregnation via aqueous solution using several precursors. The total loading of V and Nb oxides were below their dispersion limit on alumina. Vanadyl sulfate, ammonium metavanadate and ammonium niobate(V) oxalate were the precursors for supported vanadia and niobia. The reduction/oxidation properties were studied by conventional TPR/TPO and TPR/TPO-Raman. Surface vanadium oxide species tend to increase their polymerization degree upon TPR/TPO cycles. A broad weak feature near 900 cm⁻¹ appears associated to V³⁺–O–Al³⁺ bond vibration in the reduced vanadia-alumina catalysts. Niobia appears to retard vanadia reduction. Regarding supported niobia, a fraction of surface niobia is significantly more reducible than surface vanadia and another fraction is significantly less reducible. The more reducible niobia appears associated to an incipient Nb–Al–O phase that may account for a fluorescence background observed in the Raman spectra. The less reducible niobia phases appears associated to dispersed niobium oxide species on alumina. Niobium has an effect on vanadia reduction profiles in VNb/Al₂O₃ system.     

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-Sb₂O₄, 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-Sb₂O₄.     

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.     

Selective ammoxidation of propane over multicomponent oxide catalysts

The ammoxidation of propane to acrylonitrile was investigated over various multicomponent mixed oxidecatalysts. Bi₃GaMo₂ oxide, Bi_0.85V_0.55Mo_0.45 oxide and Bi₂TiMo oxide showed a good selectivity to acrylonitrile. The catalytic performance was strongly dependent on the components constituting the oxides due to the concentration of defects, the nature of the component and the mixed oxide phases. The effect of support type on the catalytic performance of Bi_0.85V_0.55Mo_0.45 oxide showed that selectivity to acrylonitrile decreased in the order silica > alumina > magnesia. The catalytic performance of Bio_0.85V_0.55Mo_0.45 oxide was improved by the addition of a small amount of a component such as magnesium, calcium and tellurium. The catalytic performance of Bi₃GaMo₂, oxide catalyst, however, decreased significantly with the addition of a smallamount of alkali metal (Li, Na, K, Cs).