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.     

Effect of the oxide support on the propane ammoxidation with Sb–V–O-based catalysts

The role of Nb₂O₅ and γ-Al₂O₃ oxide supports on the ammoxidation of propane on supported mixed Sb–V oxide at different Sb+V surface coverages is studied. Sb and V oxide species on alumina and on niobia support show different structural features that reflect in different performance during the ammoxidation of propane to acrylonitrile. Niobia-supported catalysts are much more selective to acrylonitrile than alumina-supported ones. Alumina interacts weakly with the supported oxides while niobia forms new phases through solid state reactions with the supported oxides during catalytic operation that must account for its higher selectivity values towards acrylonitrile and higher specific rate of acrylonitrile formation per vanadium site.     

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.     

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.     

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.     

High-throughput synthesis and screening of V–Al–Nb and Cr–Al–Nb oxide libraries for ethane oxidative dehydrogenation to ethylene

High-throughput synthesis and screening of mixed metal oxide libraries for ethane oxidative dehydrogenation to ethylene have been developed. A 144-member catalyst library was prepared on a 3 in. quartz wafer. An apparatus for screening catalytic activity and selectivity of a 144-member catalyst library consists of a reaction chamber, where each member can be heated individually by a CO₂ laser and reactant gases can be delivered locally to each member. The reaction products, ethylene and CO₂, are detected by photothermal deflection spectroscopy and by mass spectrometry. A 144-member catalyst library can be screened in slightly more than 2 h. V–Al–Nb oxide and Cr–Al–Nb oxide libraries are illustrated as examples. V–Al–Nb oxide catalysts are high temperature catalysts and Nb did not affect the catalytic activity of the V–Al oxides in contrast to the effect of Nb found in Mo–V–Nb oxides. However, for the Cr–Al–Nb oxide library, the most active catalyst contains about 4% Nb. These results suggest that a fine composition mapping is necessary for discovery of new heterogeneous catalysts in those ternary systems.     

Activation of propane and butanes over niobium- and tantalum-based oxide catalysts

Niobium and tantalum are important elements for the activation of alkanes in the viewpoints of acidic property and the formation of unique mixed metal oxides. And the difference of the ability of alkane activation between niobium- and tantalum-based oxide catalysts is studied. Although hydrated niobium and tantalum oxides show strong acid property, only hydrated tantalum oxide is activated to a solid superacid by the treatment with sulfuric acid, and isomerizes n-butane to isobutane at room temperature. The sulfuric acid treated tantalum oxide activates P–Mo–V heteropolyacid compounds for the selective oxidation of isobutane to methacrolein (MAL) and methacrylic acid (MAA). The difference of ability of alkanes activation between niobium and tantalum is studied by using surface science technique. Mo–V–Nb–Te mixed metal oxide catalysts are active for the ammoxidation of propane to acrylonitrile (AN). However, Mo–V–Ta–Te mixed metal oxide is less active. The effect of catalyst preparation condition is studied. Mo–V–Nb–Te mixed metal oxide catalysts are also active for the oxidation of propane to acrylic acid (AA).     

Mo–V–Nb mixed oxides as catalysts in the selective oxidation of ethane

Mo–V–Nb–O mixed metal oxides, obtained by heat-treatment in N₂ at 425 °C, have been studied as catalysts in the oxidative dehydrogenation of ethane. They present higher catalytic activity, while maintaining the same selectivity to ethylene, than the corresponding metal oxides calcined under air. Both amorphous and crystalline phases are present on active and selective catalysts. The implications of the presence of these phases as well as their physicochemical characteristics on the nature of active and selective sites are discussed.     

The effect of the nature of vanadium species on benzene total oxidation

The nature of the vanadium species present on V₂O₅/Al₂O₃ catalysts was investigated by using solid state ⁵¹V NMR, diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD) and temperature programmed reduction (TPR). ⁵¹V NMR and DRS analyses indicated the presence of V⁵⁺ in tetrahedral symmetry at low vanadium loading. A surface polymeric vanadium species and/or the bulk crystalline V₂O₅ were mainly observed at high vanadium loading as also detected by XRD. The positions of the absorption edges determined through the UV–VIS spectra allowed distinguishing between various tetrahedral symmetries. After TPR, the average oxidation state of vanadium depended on the vanadium content. The nature of vanadium species was related to the catalyst behavior on the benzene oxidation reaction. The catalysts containing high vanadium content were more active suggesting that a high amount of V⁴⁺ is responsible for the higher activity.     

Selective oxidation of methanol to formaldehyde over V–Mg–O catalysts

The results of a complex investigation of V–Mg–O catalysts for oxidative dehydrogenation (ODH) of methanol are presented. The efficiency of vanadium–magnesium oxide catalysts in production of formaldehyde has been evaluated. Strong dependence of the formaldehyde yield and selectivity upon vanadium oxide loading and the conditions of heat treatment of the catalyst were observed. The parameters of the preparation mode for the efficient catalyst were identified. In optimised reaction conditions the V–Mg–O catalysts at the temperature approximate 450 °C ensured the formation of formaldehyde with the yield of 94% at the selectivity of 97%.

No visible changes in the performance of the catalyst (methanol conversion, formaldehyde yield and selectivity) were detected during the 60 h of operation in prolonged runs. Characterization of the catalyst by XRD, IR, and UV methods suggests the formation of species of the pyrovanadate type (Mg₂V₂O₇) with irregular structure on the surface of a V–Mg–O catalyst. These species make the catalyst efficient for methanol ODH.     

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.     

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.     

Alumina-supported V–Mo–O mixed oxide catalysts, the formation of phases involving aluminum: AlVMoO7

The structure and reactive properties of alumina-supported molybdena, vanadia and molybdena–vanadia above monolayer coverage are studied by XPS, XRD, Raman spectroscopy and methanol temperature-programmed surface reaction (TPSR). Alumina-supported series are prepared by impregnation. Reference bulk Mo–V–Al oxide systems are prepared. The bulk Mo–V–Al oxide system provides structural references to characterize the alumina-supported series. The Raman bands of AlVMoO₇ are reported here for the first time, to the best of our knowledge. It is shown that the chemistry of the bulk Al–V–Mo system is also present in the alumina-supported Mo–V oxide catalysts. Methanol TPSR data show that the systems possess essentially redox activity.     

A study of the catalytic activity of cobalt–zinc manganites for the reduction of NO by hydrocarbons

In this work the catalytic behaviour of pure zinc manganite, ZnMn₂O₄, and cobalt–zinc manganites for the reduction of NO by propane and propene is reported. The NO and N₂O decomposition as well as the reduction of N₂O by propane and propene were also investigated. The catalysts are prepared starting from carbonate monophasic precursors that are decomposed in air at 973 K for 24 h. In all cases a spinel-like phase is obtained. Pure zinc manganite is an efficient catalyst for the NO reduction with both propane and propene and the selectivity to N₂ and CO₂ was almost one. However the presence of cobalt in the catalyst enhances the catalytic activity, in particular when propene is used as reducing agent of NO. All catalysts are stable up to 873 K upon contacting with the propane containing reactant stream whereas in the case of propene they preserve the original spinel structure up to about 773 K. In fact with propene the catalysts start to lose their stability as the reaction temperature increases above 773 K and disaggregate, by reduction of the spinel framework Mn³⁺ cations to Mn²⁺, forming a complex mixture of ZnO and MnO oxides. Despite the collapsing of the spinel phase, the disaggregated polyphasic catalysts still show a good activity and selectivity. An hypothesis for explaining this unusual behaviour is formulated. Finally, the reaction mechanisms presented in literature are consequently revisited on the basis of the results found in this work.     

Characterization of Mo–Sn–O system by means of Raman spectroscopy and electrical conductivity measurements

Mo–Sn–O systems were characterized by Raman spectroscopy and electrical conductivity measurements. The catalysts were obtained from precipitation of SnCl₄ by ammonia in the presence of (NH₄)₂Mo₇O₂₄ using four different levels of Mo concentration. The electrical conductivity measurements showed that particles are formed by agglomeration of SnO₂ crystals aggregated by polymolybdate. Raman spectroscopy suggested that four-coordinated species are dispersed at the external surface while six-coordinated species are inside the particles. For high Mo concentration (Mo >10%), octahedral coordinated species are also on the surface. Bulk MoO₃ oxide was not observed. These results confirm the model previously proposed.     

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.     

Effect of cobalt promoter on Co–Mo–K/C catalysts used for mixed alcohol synthesis

The structures of sulfided Co–Mo–K/C catalysts were studied by means of X-ray diffraction (XRD), laser Raman spectra (LRS), and X-ray absorption fine structure (XAFS). Activities for alcohol synthesis via CO hydrogenation were used to characterize the catalytic performance of these catalysts. On the activated carbon support, molybdenum is mainly present as MoS₂ species which shrinks with the cobalt loading, while cobalt is mainly present in the form of “Co–Mo–S” phase at the low Co loading and partly in a Co₉S₈-like structure at higher Co loading. The catalysts exhibit outstanding performance for higher alcohol synthesis due to the addition of the promotion of cobalt. The activity for alcohol formation is optimized at a Co/Mo atomic ratio of 0.5. Co species operate as a synergistic system, rather than independently from the MoS₂ phase.     

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.     

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.     

High performance of V–Ga–O catalysts for oxidehydrogenation of propane

The catalytic performance in the oxidehydrogenation (ODH) of propane of vanadium oxide catalysts supported on gallium oxide, VO_x/Ga₂O₃, with vanadium coverages lower or near the theoretical monolayer has been studied as a function of the vanadium content and compared with those of other known effective V–M–O (M=Mg, Ca) catalysts. Catalyst activity was very high and increased with the increase of vanadium loading in the range studied, while the selectivity trend was similar for the studied catalysts, excepting that with the lower V content. FT-Raman and ⁵¹V solid state NMR spectroscopies show that for coverages below the theoretical monolayer vanadium atoms are in tetrahedral co-ordination either in isolated or polymeric species, while the onset of vanadia formation is detected above that coverage. Interestingly, these catalysts show an one order of magnitude higher area-specific rate, similar initial olefin selectivity and slightly higher selectivity decrease with the increase of conversion than the best VMgO catalyst. This is due to the high intrinsic activity of isolated tetrahedral vanadium species. The combination of these factors produces an enhanced olefin productivity of V–Ga–O catalysts.