Sb Mixed Oxides Research Articles

Molybdenum–vanadium–antimony mixed oxide catalyst for isobutane partial oxidation synthesized using magneto hydrodynamic forces

A peculiar effect was observed that the oxidation behavior of antimony oxide prepared in presence of a weak permanent magnetic field is changed. Reactivity of α-Sb2O3 (senarmontite) toward oxidation is significantly enhanced after recirculating its suspension in a magneto hydrodynamic (MHD) system. This inspired the MHD synthesis of a molybdenum–vanadium–antimony mixed oxide with superior catalytic activity for selective partial oxidation of isobutane. Traditionally these mixed oxides are synthesized via sol–gel processes involving complexing agents and/or costly alkoxides. Here a new convenient way is presented to synthesize Mo–V–Sb mixed oxide catalysts departing from senarmontite (α-Sb2O3), ammonium heptamolybdate and ammonium vanadate. The MHD treated mixed oxide prepared from suspensions of Sb2O3 with Mo and V salts was evaluated in isobutane partial oxidation and compared with conventionally prepared catalysts. Improved performance was observed for MHD catalysts, with a peak methacrolein selectivity of 40% and decreased COx selectivity as compared to a reference ‘slurry-type’ catalyst. Although the MHD synthesis mechanism cannot yet be explained on a molecular level, it represents significant scientific and economical potential. This report is intended as an invitation to assist in formulating a molecular mechanism capable of explaining these observations.

Li-storage and cycling properties of Sn–Sb-mixed oxides, (M1/2Sb1/2Sn)O4, M = In, Fe

The mixed oxide compounds, (M1/2Sb1/2Sn)O4, M = In and Fe are prepared by the high-temperature solid-state reaction, at 800 °C for M = In and at 1,150 °C for M = Fe. High-energy ballmilling is used to reduce the particle size to nm-range. The compounds are characterized by X-ray diffraction, Rietveld refinement, scanning electron microscopy, and Brunauer–Emmett–Teller surface area methods. The Li-storage and cycling properties of the bare and ballmilled compounds are evaluated by galvanostatic cycling at ~0.15 C and in the voltage ranges 0.005–1.0 and 0.005–1.2 V vs. Li up to 50 (or 100) cycles and by cyclic voltammetry (CV) at room temperature. Effect of electrode heat treatment and carbon nanotube (CNT) addition is also studied. Initial reversible capacities in the range 425–550 mAh g−1 are observed depending on the metal (M) upper cut-off voltage, CNT content and electrode heat treatment. Ballmilled (In1/2Sb1/2Sn)O4 showed a stable capacity of 445 mAh g-1 up to 30 cycles and 5 % capacity fading after 50 cycles. In all other cases, capacity fading is observed ranging from 9 to 60 %. The CV showed that the main cathodic and anodic peaks occur at 0.15–0.25 V and ~0.5 V vs. Li, respectively, for both M. The reaction mechanism involves alloying–de-alloying reactions of Sn and In with Li3Sb or Fe acting as conducting matrix, and corroborated by the ex-situ X-ray diffraction data on (In1/2Sb1/2Sn)O4.

New rutile Ga/V/Nb/Sb mixed oxides as catalysts for propane ammoxidation to acrylonitrile

Here we report on the chemical–physical characteristics and reactivity in propane ammoxidation to acrylonitrile of a new class of rutile-type, GaSbO4-based multicomponent oxides. The GaSbO4 rutile was much less active than the conventional VSbO4-based rutile systems, but showed a good selectivity to acrylonitrile. The incorporation of V in GaSbO4 allowed for an improvement in catalytic activity, but with decreased maximum selectivity to acrylonitrile. However, the further incorporation of Nb (thereby developing a Ga/V/Nb/Sb mixed oxide) led to a mixed oxide that presented an enhanced catalytic behavior, combining both high activity and high selectivity to acrylonitrile. The Nb presence also greatly improved the stability of the catalytic system, hindering the crystallization phenomena observed with rutile Ga/V/Sb/O systems.

ChemInform Abstract: Chemical State Analysis of Sn-Sb-O Solid Surfaces by Auger Electron Spectroscopy: Differentiation of Sb(III) and Sb(V).

Chemical state analysis of Sn–Sb–O solid surfaces was performed by means of Auger electron spectroscopy (AES). Differential and numerically integrated AE spectra were curve-fitted using standard spectra obtained from pure substances. The fitting revealed, for the first time, the possibility of differentiating two antimony oxide components, Sb(III) and Sb(V), by AES. Using the AE spectral parameters obtained for these two chemical states, a correlation between the Sb AE peak positions and the average Sb oxidation number was constructed and it was applied for the determination of the average Sb oxidation state in Sn–Sb mixed oxide systems.

Semiconductive properties of Mo–V–M–O (M = Zn, Ni, Cu, Sb) oxides, catalysts for isobutane oxidehydrogenation

Electrical conductivity measurements have been performed on Mo–V–M–O (M = Zn, Ni, Cu, Sb) mixed oxides as a function of temperature under air, nitrogen and isobutane. MoO3, V2O5, Mo–V–O and Mo–V–Zn–O oxides were found to be n-type semiconductors, while Mo–V–Ni–O, Mo–V–Cu–O and Mo–V–Sb–O oxides were found to be p-type semiconductors. The electrical conductivity data were correlated with the catalytic properties in the oxidehydrogenation of isobutane and mechanisms for isobutane transformation over n-type and p-type semiconductors, respectively, were proposed.

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-Sb 2O 4, 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-Sb 2O 4.

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.

The control of catalytic performance of rutile-type Sn/V/Nb/Sb mixed oxides, catalysts for propane ammoxidation to acrylonitrile

This paper describes the effect of the composition of rutile-type Sn/V/Nb/Sb mixed oxides catalysts on the catalytic performance in the gas-phase ammoxidation of propane to acrylonitrile. The variation in the atomic ratio between components in catalysts is the key for the control of activity and selectivity. In samples with atomic composition Sn/V/Nb/Sb 1/0.2/1/ x (0 ≤ x ≤ 5) and 1/0.2/ y/3 (0 ≤ y ≤ 3) several compounds formed, i.e., SnO 2, Sb/Nb mixed oxide, Sb 6O 13 and non-stoichiometric rutile-type V/Nb/Sb/O; the latter segregated preferentially at the surface of the catalyst. Tin oxide provided the rutile matrix for the dispersion of the mixed oxides. The main role of Sb was shown to generate mixed oxides containing specific sites for the allylic ammoxidation of propylene intermediately formed. The presence of Nb enhanced the activity and selectivity of these sites.

The nature of antimony-enriched surface layer of Fe–Sb mixed oxides

Antimony segregation is a common feature in Fe–Sb mixed oxides, which have been widely applied as catalysts in selective oxidation and ammoxidation reactions. This paper attempts to shed a light on the cause of such a common feature and on the nature of the antimony-enriched surface layer over FeSbO 4 by means of XPS surface analysis. Single-phase FeSbO 4 samples prepared by different methods were studied, and the antimony in their surface layer is a mixture of both Sb 5+ and Sb 3+ rather than single Sb 5+. Their surface composition is close to FeSb 2O 6, which could be described as (FeSbO 4)(Sb 2O 4) δ , δ = 0.5, and it is not “Fe(II)Sb(V) 2O 6” as suggested in literature. Fe–Sb mixed oxides with Sb/Fe > 1 (mol/mol) are mixtures of FeSbO 4 and Sb 2O 4, and the surface of FeSbO 4 grains would be a layer of (FeSbO 4)(Sb 2O 4) δ , δ ≥ 0.5. Fe–Sb mixed oxides with Sb/Fe < 1 are mixtures of FeSbO 4 and Fe 2O 3, and the surface of FeSbO 4 grains would be a layer of (FeSbO 4)(Sb 2O 4) δ , δ ≤ 0.5, but the remaining Fe 2O 3 would be encapsulated by a layer of FeSbO 4.

A structural analysis of W–Sb mixed oxide catalyst

An investigation on the structure of W–Sb mixed oxide catalyst, W 12Sb x O y ( x = 1, 3, 5), is proposed. The W–Sb mixed oxide powders were prepared by the calcination of aqueous precursors, antimony tartrate and ammoniummetatungstate, and characterized with scanning electron microscope, X-ray diffractometer, and transmission electron microscope. At low content of Sb ( x = 1), the W–Sb mixed oxide powder consisted of polyhedral particles, and their crystal structure was triclinic WO 3. At higher content ( x = 3, 5), majority of the oxide powders were bar-shaped particles, consisting of triclinic WO 3 and tetragonal WO 3. With electron diffraction pattern and simulation, Sb incorporation into the cuboctahedral sites of perovskite-like WO 3 was proved and its effect on the phase transition from triclinic to tetragonal was discussed.

Selective oxidation of propane to acrylic acid on K-doped MoVSbO catalysts: catalyst characterization and catalytic performance

K-doped Mo V Sb mixed oxides catalysts have been prepared by impregnation of a MoVSbO mixed oxide (previously prepared by hydrothermal synthesis) with an aqueous solution of potassium nitrate, characterized by XRD, SEM-EDX, EPR, XPS, XANES, and FTIR of adsorbed NH 3, and tested in the selective oxidation of propane and propylene in the 593–693 K temperature range. The undoped MoVSbO catalysts presented a selectivity to acrylic acid lower than 15%, while selectivities to acrylic acid of about 40% can be obtained on K-doped catalysts. No appreciable differences between undoped and K-doped catalysts are observed in the nature of crystalline phases present in each case, on the basis of X-ray diffraction analysis. However, important differences in both the oxidation state of surface Sb species (according to XPS evidence) and the number of acid sites (determined from FTIR of adsorbed NH 3) are observed between the undoped and the K-doped catalysts. The high selectivity to acrylic acid on the K-doped catalysts seems to be related to changes in the acid–base properties of the catalysts and most particularly to the elimination of (Brønsted) acid sites. In addition, the role of antimony species in activity and selectivity is also discussed and a reaction network for the partial oxidation reaction is proposed.

Effect of Potassium Doping on the Catalytic Behavior of Mo–V–Sb Mixed Oxide Catalysts in the Oxidation of Propane to Acrylic Acid

Small addition of potassium to a Mo–V–Sb mixed oxide catalyst (previously prepared by hydrothermal synthesis) strongly modifies its catalytic behavior. Thus, while acetic acid is mainly observed in the K-free catalysts, acrylic acid is selectively obtained in K-doped catalysts. In this case, a selectivity to acrylic acid of about 30% is achieved at propane conversions of 30%. This catalytic behavior is apparently not due to modification of the crystalline phases in the K-doped catalysts but to the elimination of the acid sites of the undoped Mo–V–Sb mixed oxide catalyst.

Oxidation and ammoxidation of propane over Mo–V–Sb mixed oxide catalysts

Catalytic oxidation and ammoxidation of propane to acrolein and acrylonitrile, respectively, were carried out over Mo–V–Sb mixed oxide catalysts. V–Sb mixed oxide showed the activity for the oxidative dehydrogenation of propane to propene, and the selectivity to propene remarkably increased with increasing the concentration of cation vacancy in VSbO 4 phase. It is likely that the oxidative dehydrogenation of propane on the VSbO 4 phase is initiated via H-abstraction by acid–base concerted mechanism. The selectivity to acrolein and acrylonitrile increased by the addition of molybdenum species to V–Sb mixed oxide catalyst. Among a series of Mo–V–Sb oxide catalysts, Mo 1V 1Sb 10O x exhibited the highest selectivity to acrolein and acrylonitrile at 430 and 480°C, respectively. The highly dispersed molybdenum suboxide was formed together with the both phases of VSbO 4 and α-Sb 2O 4 in the Mo 1V 1Sb 10O x . The high catalytic activity of Mo 1V 1Sb 10O x can be explained by the bifunctional mechanism of the highly dispersed molybdenum suboxide and the VSbO 4 phases as follows: the oxidative dehydrogenation of propane proceeds on the VSbO 4 phase followed by the oxidation of propene into acrolein or the ammoxidation into acrylonitrile on the molybdenum suboxide phase. When large size of MoO 3 crystallites were formed, cracking reaction, i.e., C–C bond cleavage, occurred leading to non-selective total oxidation, resulting in decreasing the selectivities to acrolein and acrylonitrile.

A crystalline SbRe2O6 catalyst active for selective ammoxidation of isobutylene and propene

This paper reports the first performance of a crystalline SbRe2O6 catalyst, which is a new family of Re mixed oxide, for the selective ammoxidation reactions of isobutylene to methacrylonitrile and propene to acrylonitrile. The SbRe2O6 with alternate (Re2O6)3– and (SbO)+ layers showed much better performance than two other known Re–Sb–O compounds SbOReO4sdot2H2O and Sb4Re2O13 with tetrahedral (ReO4)– anions and cationic (SbO)+ layers. The SbRe2O6 catalyst was also much more active than a coprecipitated SbRe2Ox catalyst, a supported Re2O7/Sb2O3 catalyst, and Re oxides such as Re2O7, ReO3 and ReO2 for the selective ammoxidation. Rhenium is prerequisite to the ammoxidation catalysis of the Re–Sb mixed oxides. Sb oxides such as Sb2O3 and Sb2O4 were inactive, but Sb in the SbRe2O6 catalyst contributes positively to the methacrylonitrile and acrylonitrile syntheses. Neither change nor modification of the surface composition, crystallinity and morphology of SbRe2O6 occurred under the ammoxidation reaction conditions, where the presence of NH3 stabilized the crystalline SbRe2O6 structure.

Oxidation of isobutane over Mo–V–Sb mixed oxide catalyst

The catalytic performances of Mo–V–Sb mixed oxide catalysts have been studied in the selective oxidation of isobutane into methacrolein. V–Sb mixed oxide showed the activity for oxidative dehydrogenation of isobutane to isobutene. The selectivity to methacrolein increased by the addition of molybdenum species to the V–Sb mixed oxide catalyst. In a series of Mo–V–Sb oxide catalysts, Mo1V1Sb10Ox exhibited the highest selectivity to methacrolein at 440°C. The structure analyses by XRD, laser Raman spectroscopy and XPS showed the coexistence of highly dispersed molybdenum suboxide, VSbO4 and α-Sb2O4 phases in the Mo1V1Sb10Ox. The high catalytic activity of Mo1V1Sb10Ox can be explained by the bifunctional mechanism of highly dispersed molybdenum suboxide and VSbO4 phases. It is likely that the oxidative dehydrogenation of isobutane proceeds on the VSbO4 phase followed by the oxidation of isobutene into methacrolein on the molybdenum suboxide phase.

Limitations of V–Sb–W and Mo–V–Nb–Te mixed oxides in catalyzing propane ammoxidation

Testing of several catalysts proposed in the literature revealed that the activity and selectivity of Mo–V–Nb–Te mixed oxides for the ammoxidation of propane compares favourably with the activity and selectivity of V–Sb, Bi–Mo, and VPO mixed oxides under oxidizing reaction conditions. In addition to a higher acrylonitrile yield, the required reaction temperature is also lower. Reactor temperature, feed composition, and catalyst composition were optimized. Acrylonitrile yields of >50% were obtained with Mo–V–Nb–Te mixed oxide catalysts using a feed consisting of propane, ammonia, and oxygen in a ratio of 1 : 1.2 : 3. Data on the decomposition of ammonia and the combustion of propane over Mo–V–Nb–Te, V–Sb, and Bi–Mo mixed oxides revealed that the ammoxidation of propane is limited by propane activation for Bi–Mo mixed oxide and by ammonia activation for V–Sb mixed oxide. The high activity and selectivity of Mo–V–Nb–Te mixed oxides was attributed to its ability to activate both, propane and ammonia at low temperatures.

Catalytic properties of iron-based mixed oxides in the oxidation of methanol and olefins

The selective oxidation of alcohols and olefins is carried out commercially on complex systems based on Fe and Mo or Sb mixed oxides. The role of active phases and of the dopant in the catalysts has been elucidated using several characterization techniques and catalytic data.

Ammoxidation of Propane on Nickel Antimonates

The catalytic behaviour of Ni–Sb mixed oxides doped by vanadium has been investigated for the ammoxidation of propane and propene to acrylonitrile. The binary nickel antimonates, with 1:1<Ni:Sb<1:3, were found to be active and selective in the ammoxidation of propene to acrylonitrile (selectivity >80%) but they showed no activity in propane ammoxidation till 470°C. The activity/gram and the yield in acrylonitrile (ACN)/gram presented a maximum at Ni:Sb 1:2 due to a balance between the surface area and the doping effect of antimony. With the addition of vanadium to the Ni–Sb system, the activity and productivity of the catalysts were increased markedly, both in propane and propene ammoxidation. The optimum vanadium loading in terms of ACN yield was found for NiSb2O6to be V:Ni 0.2:1 atomic ratio, a compromise between activity and selectivity. It was found that sites containing vanadium are involved in the selective nitrogen insertion step in propene ammoxidation, as well as in the activation of propane. The ammoxidation of propane is a cleaner reaction than the ammoxidation of propene, as smaller amounts of hydrogen cyanide (HCN) and acetonitrile (AceN) were formed for the same yield of acrylonitrile. X-ray analysis revealed the presence of NiSb2O6and free αSb2O4in all samples. In the Ni–Sb vanadium doped oxides the FTIR characterisation showed that up to a V:Ni ratio of 0.2, vanadium species different from V2O5, and very likely interacting with the NiSb2O6, were formed; these species are the ones involved in propane activation. With higher loadings of vanadium, V2O5species formed which are responsible for the lowering of acrylonitrile selectivity.

Chemical State Analysis of Sn–Sb–O Solid Surfaces by Auger Electron Spectroscopy: Differentiation of Sb(III) and Sb(V)

Abstract Chemical state analysis of Sn–Sb–O solid surfaces was performed by means of Auger electron spectroscopy (AES). Differential and numerically integrated AE spectra were curve-fitted using standard spectra obtained from pure substances. The fitting revealed, for the first time, the possibility of differentiating two antimony oxide components, Sb(III) and Sb(V), by AES. Using the AE spectral parameters obtained for these two chemical states, a correlation between the Sb AE peak positions and the average Sb oxidation number was constructed and it was applied for the determination of the average Sb oxidation state in Sn–Sb mixed oxide systems.

Selective ethane ammoxidation to acetonitrile on alumina-supported niobium–antimony oxides

Acetonitrile can be synthesized from ethane by heterogeneous catalytic ammoxidation using Nb–Sb mixed oxides supported on alumina; a new type of (amm)oxidation active site on the catalyst, which can lead to a selective insertion of O–N atoms on the double bond of the intermediate ethylene is suggested.