Aqueous NH4VO3 Research Articles

Highly stable VSi-beta zeolite synthesized by a post-synthesis strategy for efficiently catalyzing styrene epoxidation

To enhance the doping of vanadium in zeolites and achieve high stability, the selection of vanadium precursors containing specific vanadium species can effectively overcome this crucial problem. Two series of VxSi-Beta(A) and VxSi-Beta(N) zeolites have been prepared by a two-step post-synthesis method at pH = 1 and pH = 7. The properties and environments of vanadium in samples are investigated by UV–vis, XPS, EPR, H2-TPR and 51V MAS NMR. The bonding between vanadium and framework in VSi-Beta(A) is stronger than that of VSi-Beta(N). After the two are calcined or hydrated, the vanadium species in the former are relatively stable, while the pseudo-tetrahedral V(V) species and pseudo-octahedral V(V) species in the latter are interconverted. The changes of vanadium species in the supernatant during the preparation have been investigated by 51V static NMR. At low concentration and pH = 1, there is a unique mononuclear cation (VO2+) in the aqueous NH4VO3 solution, which is favorable for the interaction with the negatively charged hydroxyl nests generated after the nitric acid dealumination, and enables the V atoms to be tightly inserted into the framework. In styrene epoxidation, the type of V(V) species in the catalyst could significantly affect its catalytic performance, and the catalytic activity and stability of VxSi-Beta(A) are superior than that of VxSi-Beta(N).

Coupled mutual diffusion in aqueous (ammonium monovanadate + butyl-substituted sulfonated resorcinarene) solutions: An experimental and theoretical approach

Binary mutual diffusion coefficients (D) of aqueous solutions of the macrocyclic salt tetrasodium 5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(butyl)resorcin[4]arene (Na4BRA) have been measured at 298.15 K by the Taylor dispersion method. The limiting diffusion coefficient of Na4BRA obtained by extrapolation of the data is used to evaluate the limiting conductivity and hydrodynamic radius of the BRA4− ion. The change in the hydrodynamic radius of sulfonated resorcinarenes with increasing the length of alkyl substituents is analysed. In the second part of this work, the effect of ammonium monovanadate on the diffusion of aqueous Na4BRA was investigated by measuring apparent ternary mutual diffusion coefficients for aqueous NH4VO3(1) + Na4BRA(2) solutions. In binary aqueous Na4BRA solutions (no added NH4VO3), the BRA4− ions diffuse at the same speed as the smaller and relatively mobile Na+ counterions to maintain electroneutrality. As the NH4VO3 concentration is raised, the main diffusion coefficient of NH4BRA drops to the tracer diffusion coefficients of BRA4− ions in supporting NH4VO3 solutions. Coupled diffusion of NH4VO3 and Na4BRA is significant. In particular, Na4BRA concentration gradients produce significant counter-current coupled flows of NH4VO3. Nernst equations are used to provide an electrostatic interpretation of the strongly coupled diffusion of Na4BRA and NH4VO3.

Vanadium incorporation from aqueous NH4VO3 solution into siliceous Beta zeolite determined by NMR with formation of V-single site zeolite catalysts for application in SCR of NO

The speciation of vanadium in initial aqueous NH4VO3 solutions as a function of pH and concentration, in supernatant and in wet solids were investigated by 51V static and MAS NMR. Two series of VSiBeta zeolite catalysts were prepared by a two-step postsynthesis procedure at pH = 2.5 and 6. 51V static and MAS NMR, 51V 3Q MAS NMR, DR UV–vis, XPS and EPR allowed determining the state of vanadium in both series of catalysts. The catalytic activity of VSiBeta catalysts in SCR of NO strongly depended on the state of the vanadium present. The V-single site V1.0SiBeta(I) and V1.4SiBeta(I) catalysts with isolated pseudo-tetrahedral V(V) were active in SCR of NO with NH3 process, with maximum NO conversion about 75 % at 773 K for V1.4SiBeta(I). In contrast, V1.0SiBeta(II) and V7.5SiBeta(I) catalysts containing pseudo-octahedral V(V) were much less active in SCR of NO and high amount of undesired N2O was produced.

Preparation of two series of VxSiBeta zeolite catalysts with V centres in framework and extra-framework positions and their application in selective oxidation of methanol

Two series of V-containing Beta zeolites have been prepared by contacting of the aqueous NH4VO3 solution with two SiBeta zeolites at pH = 2.5 and 6, respectively. Because of the presence in NH4VO3 solution at pH = 2.5 mainly monomeric VO2+ ions, vanadium have been easily incorporated into framework of SiBeta zeolite as pseudo-tetrahedral non hydroxylated (SiO)3V = O and hydroxylated (SiO)2(OH)V = O species. The moderate nucleophilicity of the basic vanadyl oxygen of the latter species play important role in methanol oxidation toward formaldehyde. The selectivity toward formaldehyde on this series of VxSiBeta(I) increases with the amount of pseudo-tetrahedral hydroxylated (SiO)2(HO)V = O species, which act as either redox or acidic and basic centres. In contrast at pH = 6, the aqueous NH4VO3 solution is expected to contain both mononuclear and polynuclear V ions, thus it is more difficult to incorporate V species in SiBeta at this condition as shown by FT-IR and NMR data. The absence of pseudo-tetrahedral V species in framework position of V0.6SiBeta(II) is probably responsible for lack of activity of this catalyst in methanol oxidation. The appearance of this species in the series of VxSiBeta(II) zeolites for high V content leads to their activity in methanol oxidation toward formaldehyde.

Mononuclear pseudo-tetrahedral V species of VSiBEA zeolite as the active sites of the selective oxidative dehydrogenation of propane

This work reports on the investigation of the influence of the nature of the V species in the ODH of propane. For this purpose, VSiBEA catalysts, including framework and/or extra-framework V species, were synthesized via the two-step method by controlling the speciation of the V species of the aqueous NH4VO3 solutions (via the pH of the corresponding solutions) to which the SiBEA zeolite was contacted with. The samples were characterized by N2 physisorption, XRD, FT-IR, 27Al, 29Si and 57V NMR, DR UV–vis and EPR. The TOF of the C3H8 ODH reaction of the samples exhibiting framework V species were found to be in good agreement with those reported in earlier works for which V species were highly dispersed on various silica supports and one order of magnitude higher than those of the samples exhibiting extra-framework V species, suggesting that isolated pseudo-tetrahedral framework V species would be the active sites of the C3H8 ODH catalytic reaction. EPR measurements of the spent catalysts indicated that these pseudo-tetrahedral framework V species were stable under the C3H8 ODH reaction conditions and were much more abundant in the V0.70SiBEA sample (for which incorporation of V in the SiBEA framework was targeted) than in the V1.00SiBEA sample (for which deposition of V occurred both in framework and extra-framework positions of SiBEA).

ChemInform Abstract: Characterization Study on the Promoting Effect of F‐Doping V2O5/TiO2 SCR Catalysts.

AbstractF‐doped V2O5/TiO2 catalysts for selective reduction of NOx by NH3 are prepared by impregnation of a titanium dioxide support with an aqueous NH4VO3 solution (reflux for 4 h, calcination at 623 K for 4 h).

Influence of the preparation procedure on the nature and environment of vanadium in VSiBEA zeolite: XRD, DR UV–vis, NMR, EPR and TPR studies

Two series of VxSiBEA(I) and VxSiBEA(II) zeolites have been prepared by a two-steps postsynthesis method which consists, in the first step, in the dealumination of TEABEA zeolite by two different procedures to obtain SiBEA(I) and SiBEA(II) zeolites and then, in the second step, in contacting the obtained zeolites with an aqueous NH4VO3 solution at pH 2.7. Because the NH4VO3 solution at pH 2.7 predominantly contains mononuclear VO2+ ions, vanadium can be incorporated into SiBEA(I) and SiBEA(II) as framework pseudo-tetrahedral non hydroxylated (SiO)3V(V)O and hydroxylated (SiO)2(OH)V(V)O species by reaction with silanol groups of vacant T-atom sites. Combined use of FTIR, DR UV–vis, 51V NMR, EPR and TPR allows to determine the oxidation state, nature and environment of vanadium in as prepared, calcined, rehydrated, activated and reduced VxSiBEA(I) and VxSiBEA(II) samples. It is shown that the procedure of dealumination has the effect on the properties of the silanol groups of vacant T-atom sites in SiBEA(I) and SiBEA(II) and on the nature and environment of V species in as prepared VxSiBEA(I) and VxSiBEA(II). As evidenced by DR UV–vis and 51V MAS NMR in VxSiBEA(II), vanadium is present only as pseudo-tetrahedral V(V) species, and in contrast in VxSiBEA(I) as pseudo-tetrahedral and octahedral V(V) species. The oxidation state of framework tetrahedral vanadium in VxSiBEA(I) and VxSiBEA(II) easily change upon calcination in oxygen, outgassing at 773K and reducing with hydrogen at high temperature (873K), as shown by EPR. The presence of Brønsted and Lewis acidic centres are evidenced in VxSiBEA(II) by FTIR adsorption investigations of pyridine and CO used as probe molecules.

Influence of V Content on the Nature and Strength of Acidic Sites in VSiβ Zeolite Evidenced by IR Spectroscopy

VSibeta zeolites prepared by a two-step postsynthesis method have been characterized by physical techniques. A significant reduction of intensity of the IR band near 3515 cm(-1) after impregnation of dealuminated beta zeolite with aqueous NH4VO3 indicates that V ions specifically react with hydrogen-bonded SiO-H groups of vacant T sites. IR bands at 3618 and 3645 cm(-1) are assigned to SiO-H groups interacting with V and to VO-H groups, respectively. In VSibeta, diffuse reflectance UV-visible data show that below 1.9 wt % V is present as lattice tetrahedral species and at higher content as extra-lattice octahedral species (mononuclear and polynuclear). VSibeta samples are EPR-silent at 298 or 77 K suggesting that there are no paramagnetic VIV ions. IR studies show that V-OH groups are less acidic than Si-OH-Al groups of parent HAlbeta zeolite. IR results of CO adsorption evidence three kinds of Lewis acidic sites, related to lattice mononuclear and extra-lattice mononuclear and polynuclear V species. Quantitative IR studies of ammonia and pyridine adsorption reveal that only about half of V introduced into zeolite is able to form either Brønsted or Lewis acidic sites.

Optical Properties of Porous Glass Modified with Vanadium(V) Oxide

Porous glasses modified with vanadium(V) oxide were obtained by impregnation with aqueous NH4VO3 solutions and by subsequent thermal decomposition of the salt. Changes in the charge-transfer spectra, resulting from an increase in the content of the deposited oxide and from water sorption, and also the possibility of using vanadium-containing glass for visual monitoring of air humidity were considered.

Preparation and characterisation of titania-supported vanadium–phosphorus oxide catalysts

A series of vanadium–phosphorus oxides (mainly with V $$:$$ P $$ = 1:1 $$ ) supported on pigmentary anatase (10 m2 g-1) has been prepared using aqueous NH4VO3 and (NH4)H2PO4 solutions, with loadings up to 11.3 wt%, equivalent to about 12.7 monolayers. Characterisation by X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy and temperature-programmed reduction suggests that the main phase present at loadings below about 10 wt% is an amorphous V–P oxide which exists chiefly as blocks of disordered material. The presence of small amounts of crystalline $$\beta $$ -VOPO4 and of V2O5 is indicated at the highest loadings, especially when $$1:3$$ and $$3:1$$ V $$:$$ P ratios are used. The two materials having the lowest loadings are active for methanol oxidation at 473–533 K, and show high selectivity to formaldehyde.

Activity measurements and spectroscopic studies of the catalytic oxidation of toluene over vanadium oxides supported on titania

Catalysts containing 0.1–10 wt% V have been prepared by impregnating TiO2(Degussa P25) by aqueous NH4VO3 and calcining at 773 K. The catalytic performance in the oxidation of toluene was investigated and correlated with the features shown by X.r.d., ESCA, u.v.–visible spectroscopy and i.r. spectroscopy of adsorbed CO. Both the activity in the oxidation of toluene and the initial selectivity for benzaldehyde increase rapidly with increasing vanadium loading up to 2 wt% V. The changes between 2 and 10 wt% V are small. Surface areas, pore structure, primary particle size and anatase/rutile ratio change with the vanadium content. Up to 2 wt% V, a small increase in the anatase/rutile ratio is obtained. In these catalysts, surface-bonded vanadium species are present, and it is suggested that they induce a reconstruction in the titania surface. Larger and opposite effects are observed for 10 wt% V, where crystalline V2O5 facilitates the anatase to rutile phase transition. Sintering of the primary particles and a large reduction in the specific surface area are also obtained. U.v.–visible spectroscopy of reduced catalysts indicated the predominance of VIV species in tetrahedral coordination. VIII species could not be observed by i.r. spectroscopy of adsorbed CO. I.r. spectra indicate the presence of V—OH groups. Breakpoints in the spectral data were obtained at ca. 1.1 wt% V corresponding to the consumption of two support OH groups per vanadium species. It is suggested that the isolated vanadium species are oxohydroxyvanadium bonded to titania through two oxygen bridges. These constitute the active site at low loadings. Two adjacent vanadium species are thought to form the reduced tetrahedral structure. New active sites present at higher loadings may be formed by the reaction between V—OH groups and vanadium precursors. The new sites possibly represent a VV/VIV redox couple: It is suggested that its presence is an important factor governing the catalytic activity.