Alumina-supported catalysts for propane oxidative dehydrogenation from mixed VXM(6−X)O19 (M=W, Mo) hexametalate precursors

γ-Al₂O₃ supported vanadium oxides were modified by tungsten and molybdenum oxides in order to improve dispersion and selectivity towards olefins in propane oxidative dehydrogenation (ODH). Both vanadium–tungsten and vanadium–molybdenum catalysts were obtained by adsorption of mixed isopolyanions (VW₅O₁₉⁵⁻, V₂W₄O₁₉⁴⁻, VMo₅O₁₉⁵⁻ and V₂Mo₄O₁₉⁴⁻) from aqueous solutions. The isopolyanion solutions were characterized by UV-Vis and ⁵¹V NMR spectroscopy. Vanadium, vanadium–tungsten and vanadium–molybdenum precursors and catalysts were also characterized by UV-Vis (diffuse reflectance) and solid state ⁵¹V NMR spectroscopy. An improved selectivity to propene in the presence of tungsten and molybdenum in VO_x/γ-Al₂O₃ was observed and attributed to dilution of vanadium by tungsten or molybdenum oxides on the γ-Al₂O₃ surface.     

Performance and characterization of Ru/Al2O3 and Ru/SiO2 catalysts modified with Mn for Fischer–Tropsch synthesis

Mn effect and characterization on γ-Al₂O₃-, -Al₂O₃- and SiO₂-supported Ru catalysts were investigated for Fischer–Tropsch synthesis under pressurized conditions. In the slurry phase Fischer–Tropsch reaction, γ-Al₂O₃ catalysts showed higher performance on CO conversion and C₅⁺ selectivity than -Al₂O₃ and SiO₂ catalysts. Moreover, Ru/Mn/γ-Al₂O₃ exhibited high resistance to catalyst deactivation and other catalysts were deactivated during the reaction. From characterization results on XRD, TPR, TEM, XPS and pore distribution, Ru particles were clearly observed over the catalysts, and γ-Al₂O₃ catalysts showed a moderate pore and particle size such as 8 nm, where -Al₂O₃ and SiO₂ showed highly dispersed ruthenium particles. The addition of Mn to γ-Al₂O₃ enhanced the removal of chloride from RuCl₃, which can lead to the formation of metallic Ru with moderate particle size, which would be an active site for Fischer–Tropsch reaction. Concomitantly, manganese chloride is formed. These schemes can be assigned to the stable nature of Ru/Mn/γ-Al₂O₃ catalyst.     

Raman spectroscopic evidence for the formation of silicomolybdic entities on a Mo/HY zeolite catalyst

Mo/Ze catalysts prepared by incipient wetness impregnation of a Faujasite zeolite with ammonium heptamolybdate solutions have been characterized after calcination at 500 °C and transfer in wet atmosphere. Raman spectroscopy clearly evidences the formation of [SiMo₁₂O₄₀]⁴⁻ heteropolyanions. This formation, through extraction of Si atoms, is not observed before the calcination, the Anderson [AlMo₆O₂₄H₆]³⁻ entity being the main species formed during the impregnation–maturation.     

Water-tolerant catalysis of a silica composite of a sulfonic acid resin, Aciplex

A silica composite of a perfluorocarbonsulfonic acid resin, Aciplex, has been used as a solid acid catalyst for a variety of reactions concerning water. The Aciplex–SiO₂ composite containing 20 wt% Aciplex has a surface area of 1.3 m² g⁻¹ and possesses an ion-exchanged capacity of 0.46 meq. g⁻¹ after pretreatment at 423 K, which is higher than that of 13 wt% Nafion–SiO₂ (0.12 meq. g⁻¹). The acid strengths estimated from an initial heat of adsorption of NH₃ were similar for these polymer resin composites. It was found that the Aciplex–SiO₂ was more active than typical solid acids such as Cs_2.5H_0.5PW₁₂O₄₀, H-ZSM-5, and SO₄²⁻/ZrO₂ for hydrolysis of ethyl acetate in excess water and esterification of acrylic acid with 1-butanol, while it was less active than Cs_2.5H_0.5PW₁₂O₄₀ for N-alkylation of acrylonitrile with 1-adamantanol and solid–solid hydrolysis of 2-naphthyl acetate. The Aciplex–SiO₂ was superior in activity to Nafion–SiO₂ for all the above reactions and in thermal stability. These results indicate that Aciplex–SiO₂ is a promising solid acid catalyst for reactions involving liquid phase water.     

A kinetic study of the formation of 12-molybdosilicate and 12-molybdogermanate in aqueous solutions by ion transfer voltammetry with the nitrobenzene-water interface

A novel electrochemical approach was developed for the kinetic study of the formation of heteropolyanions. The method (dual pulse amperometry, DPA) is based on the detections of currents due to the transfers of polyanions at the nitrobenzene-water interface. In this study, DPA was applied to the kinetic study of the formation of two Keggin anions, viz., [SiMo₁₂O₄₀]⁴⁻ and [GeMo₁₂O₄₀]⁴⁻. Prior to the kinetic study, cyclic voltammetric measurements were performed to confirm that the Keggin anion and its lacunary anion ([H₃SiMo₁₁O₃₉]⁵⁻ or [H₃GeMO₁₁O₃₉]⁵⁻) coexist at equilibrium under certain conditions. In DPA, double voltage pulses of different amplitudes were alternately applied to the interface to follow the concentrations of both the Keggin and the lacunary anions. The concentration-time profiles for the polyanions could be elucidated by the two-step consecutive reactions mechanism. The lacunary anion was then found to be the intermediate of the Keggin anion.     

Molybdocobaltate cobalt salts: New starting materials for hydrotreating catalysts

This paper deals with the use of Anderson heteropolyanions as alternative starting materials to the ammonium heptamolybdate and cobalt nitrate for the preparation of hydrotreatment oxidic precursors. Ammonium and cobalt salts of molybdocobaltate anions were synthesized and impregnated on alumina. The evolution of these compounds along the different steps of preparation of the oxidic precursors has been followed using various physical techniques such as Raman, XAS and UV–vis spectroscopies. It has been shown that the nature of the surface oxomolybdenum phase strongly depends on the nature of the starting salt. After sulfidation under H₂/H₂S, the performances of these new catalysts have been evaluated in hydrodesulfuration of thiophene. It appears that the cobalt salt of the decamolybdocobaltate anion [Co₂Mo₁₀O₃₈H₄]⁶⁻, with a Co/Mo ratio equal to 0.5, allows us to improve the catalytic conversion by comparison to reference catalysts prepared with ammonium heptamolybdate and cobalt nitrate as starting materials. It has been shown that this improvement is due to the preservation of the heteropolyanionic structure up to the drying step.     

Preparation of H5PMo10V2O40 catalyst immobilized on nitrogen-containing mesostructured cellular foam carbon (N-MCF-C) and its application to the vapor-phase oxidation of benzyl alcohol

Nitrogen-containing mesostructured cellular foam carbon (N-MCF-C) was synthesized by a templating method using mesostructured cellular foam silica (MCF-S) and polypyrrole as a templating agent and a carbon precursor, respectively. The N-MCF-C was then modified to have a positive charge, and thus, to provide a site for the immobilization of [PMo₁₀V₂O₄₀]⁵⁻. By taking advantage of the overall negative charge of [PMo₁₀V₂O₄₀]⁵⁻, H₅PMo₁₀V₂O₄₀ (PMo₁₀V₂) catalyst was chemically immobilized on the N-MCF-C support as a charge-matching component. Characterization results showed that the PMo₁₀V₂ catalyst was finely dispersed on the N-MCF-C support via strong chemical interaction, and that the pore structure of N-MCF-C was still maintained even after the immobilization of PMo₁₀V₂. In the vapor-phase oxidation of benzyl alcohol, the PMo₁₀V₂/N-MCF-C catalyst showed a higher conversion and a higher oxidation activity (formation of benzaldehyde) than the unsupported PMo₁₀V₂ and PMo₁₀V₂/MCF-S catalysts.     

SiO2/Nb2O5 sol–gel as a support for HRP immobilization in biosensor preparation for phenol detection

A SiO₂/Nb₂O₅ mixed oxide was prepared by a sol–gel processing method based on TEOS and NbCl₅ as precursors and HCl as catalysts. A material having a specific surface area of 703 m² g⁻¹, average pore diameter of 2.4 nm and 5 wt.% of Nb was obtained. An amperometric peroxidase-based biosensor for phenol was constructed by immobilizing the enzyme onto the SiO₂/Nb₂O₅ sol–gel matrix by adsorption and cross-linking with glutaraldehyde and mixing with graphite powder to make a modified carbon paste. The biosensor performance for phenol detection, investigated in a flow injection system, was based on mediated electron transfer of horseradish peroxidase (HRP), avoiding the direct electron transfer of HRP, which was blocked by the sol–gel matrix. With optimized conditions, a linear response range from 5 to 25 μmol dm⁻³ for phenol was obtained with a sensitivity of 3.2 nA dm³ μmol⁻¹. The detection limit of the biosensor for phenol was 0.5 μmol dm⁻³ and the analytical frequency was 27 samples h⁻¹. The biosensor response was tested for various phenol substrates and the highest response was observed for 2-amino-4-chlorophenol. During 200 determinations, the biosensor kept the same response for phenol. The modified carbon paste retained its activity during 6 months of storage under refrigeration.     

Hydrodesulfurization over supported monometallic, bimetallic and promoted carbide and nitride catalysts

The preparation of alumina-supported β-Mo₂C, MoC_1−x (x≈0.5), γ-Mo₂N, Co–Mo₂C, Ni₂Mo₃N, Co₃Mo₃N and Co₃Mo₃C catalysts is described and their hydrodesulfurization (HDS) catalytic properties are compared to conventional sulfide catalysts having similar metal loadings. Alumina-supported β-Mo₂C and γ-Mo₂N catalysts (Mo₂C/Al₂O₃ and Mo₂N/Al₂O₃, respectively) are significantly more active than sulfided MoO₃/Al₂O₃ catalysts, and X-ray diffraction, pulsed chemisorption and flow reactor studies of the Mo₂C/Al₂O₃ catalysts indicate that they exhibit strong resistance to deep sulfidation. A model is presented for the active surface of Mo₂C/Al₂O₃ and Mo₂N/Al₂O₃ catalysts in which a thin layer of sulfided Mo exposing a high density of sites forms at the surface of the alumina-supported β-Mo₂C and γ-Mo₂N particles under HDS conditions. Cobalt promoted catalysts, Co–Mo₂C/Al₂O₃, have been found to be substantially more active than conventional sulfided Co–MoO₃/Al₂O₃ catalysts, while requiring less Co to achieve optimal HDS activity than is observed for the sulfide catalysts. Alumina-supported bimetallic nitride and carbide catalysts (Ni₂Mo₃N/Al₂O₃, Co₃Mo₃N/Al₂O₃, Co₃Mo₃C/Al₂O₃), while significantly more active for thiophene HDS than unpromoted Mo nitride and carbide catalysts, are less active than conventional sulfided Ni–Mo and Co–Mo catalysts prepared from the same oxidic precursors.     

Thermoelectric characterization of NaxMx/2Ti1−x/2O2 (M=Co, Ni and Fe) polycrystalline materials

Layered -titanate materials, Na_xM_x/2Ti_1−x/2O₂ (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na_0.4Ni_0.2Ti_0.8O₂ were ca. 7×10³ S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm⁻¹ K⁻¹ at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na_0.4Ni_0.2Ti_0.8O₂ and Na_0.4Co_0.2Ti_0.8O₂ was about 1.9×10⁻⁴ and 1.2×10⁻⁴ K⁻¹ around 700 °C, respectively.     

Preparation and characterization of heteropolyacid/mesoporous carbon catalyst for the vapor-phase 2-propanol conversion reaction

H₃PMo₁₂O₄₀ catalyst was chemically immobilized on the surface modified CMK-3 (SM-CMK-3) support as a charge compensating component, by taking advantage of the overall negative charge of [PMo₁₂O₄₀]³⁻. The supported H₃PMo₁₂O₄₀/SM-CMK-3 catalyst was characterized to have high surface area (≈1000 m²/g) and relatively large pore volume (0.83 cm³/g). The H₃PMo₁₂O₄₀/SM-CMK-3 catalyst was applied to the vapor-phase 2-propanol conversion reaction. The H₃PMo₁₂O₄₀/SM-CMK-3 catalyst exhibited higher 2-propanol conversion than the unsupported H₃PMo₁₂O₄₀ and the impregnated H₃PMo₁₂O₄₀ on CMK-3. Furthermore, the PMo₁₂/SM-CMK-3 catalyst showed the enhanced oxidation activity (acetone formation) and the suppressed acid catalytic activity (propylene formation) compared to the other two catalysts. It is believed that [PMo₁₂O₄₀]³⁻ species were chemically and finely immobilized on the SM-CMK-3 support as charge matching species, and thus, the PMo₁₂/SM-CMK-3 catalyst showed an excellent oxidation activity.     

Study on the supported Cu-based catalysts for the low-temperature water–gas shift reaction

This paper presents a study on the influence of support (Al₂O₃, MgO, SiO₂-Al₂O₃, SiO₂-MgO, β-zeolite, and CeO₂) of Cu-ZnO catalysts for the low-temperature water–gas shift reaction. Supported Cu-ZnO catalysts were prepared by the conventional impregnation method, followed by the H₂ reduction. The activity of Cu-ZnO catalysts for the water–gas shift (WGS) reaction was largely influenced by the kind of support; Cu-ZnO catalysts supported on Al₂O₃, MgO, and CeO₂ showed high activity, while those on SiO₂-Al₂O₃, SiO₂-MgO and β-zeolite showed less activity in the temperature range 423–523 K. XRD analysis demonstrated that the copper species were highly dispersed on the supports used in the present study, except for a MgO support. TPR results of a series of supported CuO-ZnO catalysts suggest that the reducibility of CuO is one of the important factors controlling the activity of the WGS reaction over the supported catalysts.     

In situ FTIR study on reaction pathways in Ni(CO)4/CH3OK catalytic system for low-temperature methanol synthesis in a liquid medium

An in situ infrared spectroscopic study was conducted to elucidate the reaction pathways for low-temperature methanol synthesis in a catalytic system composed of Ni(CO)₄ and CH₃OK (denoted as Ni(CO)₄/CH₃OK). The reaction was conducted in a liquid medium at 313–333 K with an initial pressure of 3.0 MPa. When CH₃OK was added to Ni(CO)₄ solution at 293 K, different carbonylnickelates, [Ni₅(CO)₁₂]²⁻, [Ni₆(CO)₁₂]²⁻ and [Ni(CO)₃(COOCH₃)]⁻, were immediately formed from Ni(CO)₄. The species and the composition of the carbonylnickel complexes varied with temperature. The variations in concentrations of methanol (MeOH) and methyl formate (MF) during the run, which were determined from their IR absorptions, indicated a pattern characteristic of consecutive reactions with MF as an intermediate. Thus, it was shown that methanol was produced through the carbonylation of MeOH to MF and the subsequent hydrogenation of MF to MeOH. Stable hydridocarbonylnickel anions, [HNi(CO)₃]⁻ and/or [HNi₂(CO)₆]⁻, were observed together with a small amount of Ni(CO)₄ throughout the methanol synthesis. Since Ni(CO)₄ alone showed no activity for the hydrogenation of MF, the hydridocarbonylnickel anions generated in the presence of CH₃OK must be responsible for the reaction. The dual role of CH₃OK in the catalytic system was stated.     

Conversion and in situ FTIR studies of direct NO decomposition over Cu-ZSM5

Copper ion-exchanged zeolites ZSM5 with SiO₂:Al₂O₃ molar ratios 33 and 53 have been subjected to activity tests for direct decomposition of NO (2000 ppm, GHSV 560–5400 h⁻¹). In situ infrared measurements were used to follow the reaction and surface and gas phase compositions. IR studies were also done in excess oxygen with rapid NO₂ formation in the gas phase.

A high level of overexchange of copper in the zeolite in combination with a low concentration of acid sites, concurrent with a high SiO₂:Al₂O₃ ratio, enhances the conversion of NO. A vibrational band at 1631 cm⁻¹ is observed below the light-off temperature and interpreted as a bridged nitrato group bound to Cu²⁺–O–Cu²⁺ dimers. This band disappears above the light-off temperature but the intensity below this temperature correlates with the catalytic activity. We interpret that these bridge bound nitrato groups act as siteblockers on the active sites for NO conversion and that a tentative reaction intermediate, N₂O₃, also binds in a bridge configuration to the same Cu²⁺–O–Cu²⁺ dimers.

A second nitrato group with unidentate coordination and vibrational bands at 1598/1575 cm⁻¹ probes isolated copper ions.

A third infrared band at 2130 cm⁻¹ confirms previous observations of -ions bound to the zeolite. We conclude that these species are coordinated to deprotonated and negatively charged sites on the zeolite and that these sites for adsorption are blocked by Cu²⁺ ion-exchange. The 2130 cm⁻¹ species appear to have no role in direct NO decomposition but the adsorption sites are crucial for the stability of the zeolite and intimately related to ion mobility in the lattice.

Prolonged immersion of the zeolite in dilute solutions of copper ions improves the catalyst performance by copper hydroxylation leading to enhanced formation of the above dimers.

A high SiO₂:Al₂O₃ ratio leads to more stable catalysts, particularly in combination with a modest overexchange of copper ions. Excessive amounts of copper escalates the deactivation of the Cu-ZSM5 catalyst through the migration and sintering of cupric oxide crystallites.     

Photocatalytic oxidation of CO with various oxidants by Mo oxide species highly dispersed on SiO2 at 293 K

The photocatalytic oxidation of CO into CO₂ with oxidants such as NO, N₂O and O₂ proceeded efficiently on a Mo/SiO₂ with high Mo dispersion under UV light irradiation. It was found that the reaction rate greatly depended on the kind and concentration of the oxidant. Photoluminescence investigations reveal the close relationship between the reaction rate and the relative concentration of the photo-excited Mo⁶⁺-oxide species, i.e. charge transfer–excited–triplet state (Mo⁵⁺–O⁻)^*, under steady-state reaction conditions. Moreover, the photocatalytic oxidation of CO with O₂ in excess H₂ was carried out to test suitability for applications to supplying pure H₂. This reaction was seen to proceed efficiently on Mo/SiO₂ with a high CO conversion of 100% and CO selectivity of 99% after 180 min under UV light irradiation, showing higher photocatalytic performance than TiO₂ (P-25) photocatalyst. UV–vis, XAFS, photoluminescence and FT-IR investigations revealed that the high reactivity of the charge transfer–excited–triplet state (Mo⁵⁺–O⁻)^*, with CO as well as the high reactivity of the photoreduced Mo-oxide species (Mo⁴⁺-species) with O₂ to produce the original Mo-oxide species (Mo⁶⁺O²⁻), played a crucial role in the reactions.     

Deactivation studies over NiO/γ-Al2O3 catalysts for partial oxidation of methane to syngas

Two types of NiO/γ-Al₂O₃ catalysts prepared by the impregnation and the sol–gel method were used for the partial oxidation of methane to syngas at 850°C (GHSV1.8×10⁵ lkg⁻¹ h⁻¹). The effects of the carbon deposition, the loss and sintering of nickel and the phase transformation of γ-Al₂O₃ support on the catalytic performance during 80 h POM reaction were investigated with a series of characterization such as XRD, BET, AAS, TG, and XPS. The results indicated that the carbon deposition and the loss and sintering of nickel could not cause the serious decrease of catalytic performance over NiO/γ-Al₂O₃ catalyst during the short-time reaction. However, the slow process of the support γ-Al₂O₃ phase transforming into -Al₂O₃ could slowly decrease the performance of NiO/γ-Al₂O₃ catalysts. Aimed at the reasons of the deactivation, an improved catalyst was obtained by the complexing agent-assisted sol–gel method.     

Palladium-substituted lanthanum cuprates: application to automotive exhaust purification

A series of palladium-substituted La₂CuO₄, corresponding to the formula La₂Cu_{1 −x}Pd_xO₄ (x = 0−0.2) were prepared by metal nitrate decomposition in a polyacrylamide gel. This method allows an easy incorporation of palladium in the mixed-oxides, which are formed at moderate temperature with rather high specific areas (13–17 m²/g). The partial substitution of copper for palladium allows a strong improvement of the three-way catalytic activity, in particular for NO reduction. The light-off temperatures for the conversions of CO, NO and C₃H₆ decreased markedly when increasing the palladium content, the activity of catalysts La₂Cu_0.9Pd_0.1O₄ and La₂Cu_0.8Pd_0.2O₄ being comparable to that of a Pt-Rh/CeO₂–Al₂O₃ catalyst for NO reduction, and higher for CO and C₃H₆ oxidation.

All the La₂Cu_{1 − x} Pd_xO₄ catalysts are activated under reacting conditions. This activation corresponds to the destruction of the mixed-oxide structure, with formation of reduced Pd⁰ ions atomically dispersed, surrounded by Cu⁺ and Cu²⁺ species on a lanthanum oxycarbonate matrix. This high dispersion state of the two transition metals in various oxidation states is supposed to originate from the initial La₂Cu_{1 −x}Pd_xO₄ structure.     

Oxidative conversion of methane to syngas over NiO/MgO solid solution supported on low surface area catalyst carrier

Influence of time-on-stream (0.5–15 h), CH₄/O₂ ratio in feed (1.8–8.0), space velocity (6000–510,000 cm³ g⁻¹ h⁻¹), catalyst particle size (22–70 mesh), and catalyst dilution by inert solid particles (diluent/catalyst weight ratio=4) on the performance at different temperatures (600–900°C) of the NiO/MgO solid solution deposited on SA-5205 [which is a low surface area macroporous silica-alumina catalyst carrier] in the oxidative conversion of methane to syngas (a mixture of CO and H₂) has been investigated. The dependence of conversion and selectivity on the space velocity is strongly influenced by the temperature. Both the conversion and selectivity for H₂ and CO are decreased markedly by increasing the CH₄/O₂ ratio in the feed. The catalyst dilution resulted in a small but significant decrease in both the conversion and selectivity for H₂ and CO. The increase in the catalyst particle size had also a small but significant effect on both the conversion and selectivity in the oxidative conversion process. Both the heat and mass transfer processes seem to play significant roles in the oxidative conversion of methane to syngas at a very low contact time or very high space velocity (5.1×10⁵ cm³ g⁻¹ h⁻¹).     

Poly(ethylene oxide)–LiX rubbery electrolytes with X–X disulfonamide anions having two or three ether groups in their structures

This work deals with poly(ethylene oxide), PEO–MX (M=Li, K and Cs) amorphous electrolytes with ⁻X–X⁻, [CF₃SO₂NCH₂(CH₂OCH₂)₂CH₂NSO₂CF₃]²⁻ (EDSA) and [CF₃SO₂NCH₂CH₂(CH₂OCH₂)₃CH₂CH₂NSO₂CF₃]²⁻ (TTSA) disulfonamide anions. These dianions have X⁻ end-groups identical to anions [CF₃SO₂N(CH₂)₂OCH₃]⁻ (MESA) and [CF₃SO₂N(CH₂)₃OCH₃]⁻ (MPSA), one of which (MPSA) was reported to yield chelate-like associated species (presumably LiX₂⁻ triplets) at concentrations above EO/Li=20 in PEO. This feature of LiMPSA, evidenced through glass transition temperature (T_g) measurements, does not apply to Li₂EDSA and Li₂TTSA. Though none of these lithium salts form crystalline intermediate compounds with PEO, the limit of solubility of LiMESA (EO/Li=16) does not allow a clarification of this point for this salt. At lower concentrations, however, a conductivity comparison with the potassium and caesium salts shows that the apparent degree of dissociation (=C_Li+/C_Li) of LiMESA is comparable to that of LiMPSA. As opposed to both these salts and to some extent to Li₂EDSA, a much greater dissociation takes place for Li₂TTSA, the anion of which contains an inner, third ether group in its structure.     

Water-soluble niobium peroxo complexes as precursors for the preparation of Nb-based oxide catalysts

In the frame of research aimed at developing new synthetic procedures of multimetallic Nb-based catalysts, peroxo complexes of niobium(V) of general formula A^I₃[Nb(O₂)₄] and A^I₃[Nb(O₂)_x(H_yL)]·nH₂O (A^I: NH₄⁺, CN₃H₆⁺ (gu); L: oxalate, tartrate, citrate) have been prepared and characterized on the basis of elemental and thermal analysis, FTIR and ¹³C-NMR spectra. The crystal structure of (gu)₃[Nb(O₂)₄] and (gu)₃[Nb(O₂)₂(C₂O₄)₂]·2H₂O have been determined. The application of the obtained Nb complexes as precursors for the preparation of silica-supported Nb–Mo–O catalysts has been demonstrated. Combining Nb peroxo-carboxylato compounds with analogous Mo(VI) compounds in a silica-impregnation method carried out in aqueous medium leads to the formation of the supported Nb₂Mo₃O₁₄ phase.