Dependence of n-Butane Activation on Active Site of Vanadium Phosphate Catalysts

The nature and the role of oxygen species and vanadium oxidation states on the activation of n-butane for selective oxidation to maleic anhydride were investigated. Bi–Fe doped and undoped vanadium phosphate catalysts were used a model catalyst. XRD revealed that Bi–Fe mixture dopants led to formation of α_II-VOPO₄ phase together with (VO)₂P₂O₇ as a dominant phase when the materials were heated in n-butane/air to form the final catalysts. TPR analysis showed that the reduction behaviour of Bi–Fe doped catalysts was dominated by the reduction peak assigned to the reduction of V⁵⁺ species as compared to the undoped catalyst, which gave the reduction of V⁴⁺ as the major feature. An excess of the oxygen species (O^2?) associated with V⁵⁺ in Bi–Fe doped catalysts improved the maleic anhydride selectivity but significantly lowering the rate of n-butane conversion. The reactive pairing of V⁴⁺-O^? was shown to be the centre for n-butane activation. It is proposed that the availability and appearance of active oxygen species (O^?) on the surface of vanadium phosphate catalyst is the rate determining step of the overall reaction.     

Synthesis of Vanadium Phosphate Catalysts by Hydrothermal Method for Selective Oxidation of n-butane to Maleic Anhydride

Two vanadium phosphate catalysts (VPH1 and VPH2) prepared via hydrothermal method are described and discussed. Both catalysts exhibited only highly crystalline pyrophosphate phase. SEM showed that the morphologies of these catalysts are in plate-like shape and not in the normal rosette-type clusters. Temperature-programmed reduction in H₂ resulted two reduction peaks at high temperature in the range of 600–1100 K. The second reduction peak appeared at 1074 K occurred as a sharp peak indicated that the oxygen species originated from V⁴⁺ phase are having difficulty to be removed and their nature are less reactive compared to other methods of preparation. Modified VPH2 gave better catalytic performance for n-butane oxidation to maleic anhydride contributed by a higher BET surface area, high mobility and reactivity of the lattice oxygen associated to the V⁴⁺ which involved in the hydrocarbon’s activation. A slight increased of the V⁵⁺ phase also enhanced the activity of the VPH2 catalyst.     

Influence of Rare-Earth and Bimetallic Promoters on Various VPO Catalysts for Partial Oxidation of n-Butane

Vanadium phosphorous oxide (VPO) catalyst was prepared using dihydrate method and tested for the potential use in selective oxidation of n-butane to maleic anhydride. The catalysts were doped by La, Ce and combined components Ce + Co and Ce + Bi through impregnation. The effect of promoters on catalyst morphology and the development of acid and redox sites were studied through XRD, BET, SEM, H₂-TPR and TPRn reaction of n-butane/He. Addition of rare-earth element to VPO formulation and drying of catalyst precursor by microwave irradiation increased the fall width at half maximum (FWHM) and reduced the crystallite size of the Vanadyl hydrogen phosphate hemihydrate (VOHPO₄ · 1/2 H₂O, VHP) precursor phase and thus led to the production of final catalysts with larger surface area. The Ce doped VPO catalyst which, assisted by the microwave heating method, exhibited the highest surface area. Moreover, the addition of promoters significantly increased catalyst activity and selectivity as compared to undoped VPO catalyst in the oxidation reaction of n-butane. The H₂-TPR and TPRn reaction profiles showed that the highest amount of active oxygen species, i.e., the V⁴⁺–O^? pair, was removed from the bimetallic (Ce + Bi) promoted catalyst. This pair is responsible for n-butane activation. Furthermore, based on catalytic test results, it was demonstrated that the catalyst promoted with Ce and Bi (VPOD1) was the most active and selective catalyst among the produced catalysts with 52% reaction yield. This suggests that the rare earth metal promoted vanadium phosphate catalyst is a promising method to improve the catalytic properties of VPO for the partial oxidation of n-butane to maleic anhydride.     

Preparation of vanadium phosphate catalyst precursors using a high pressure method

The preparation of vanadium phosphate catalysts is known to be an important factor in determining their performance for the oxidation of butane to maleic anhydride. In this paper the preparation of catalyst precursors from V₂O₅ and H₃PO₄ using both aqueous hydrochloric acid and isobutanol as reducing agents are evaluated. In particular, the preparation of materials at higher temperatures using an autoclave method is described and compared with the materials prepared using the typical reflux methodology. The materials were characterised using a combination of powder XRD, BET surface area measurement, laser Raman spectroscopy and scanning electron microscopy. With the reflux method (1 bar pressure) both methods lead to the formation of VOHPO₄·0.5H₂O. The aqueous hydrochloric acid method leads to the formation of VO(H₂PO₄)₂ as a minor impurity that is readily removed by water extraction. The high temperature autoclave route gives significant differences. The aqueous hydrochloric acid route surprisingly does not give V⁴⁺ phases but gives a mixture of hydrated VOPO₄·xH₂O phases. In contrast the isobutanol method at higher temperature and pressure gave VOHPO₄·0.5H₂O as expected. However, for both methods the use of the higher temperature and higher pressure led to lower surface area materials being formed and consequently this will limit the usefulness of this methodology with these reducing agents.     

The key factor determining the anodic deposition of vanadium oxides

This work demonstrates that anodic deposition of vanadium oxide (denoted as VO_x·nH₂O) can be considered as the chemical co-precipitation of V⁵⁺ and V⁴⁺ oxy-/hydroxyl species and the accumulation of V⁵⁺ species at the vicinity of electrode surface is the key factor for the successful anodic deposition of VO_x·nH₂O at a potential much more negative than the equilibrium potential of the oxygen evolution reaction (OER). The results of in situ UV-vis spectra show that the V⁴⁺/V⁵⁺ ratio near the electrode surface can be controlled by varying the applied potential, leading to different, three-dimensional (3D) nanostructures of VO_x·nH₂O. The accumulation of V⁵⁺ species due to V⁴⁺ oxidation at potentials ≥0.4 V (vs. Ag/AgCl) has been found to be very similar to the phenomenon by adding H₂O₂ in the deposition solution. The X-ray photoelectron spectroscopic (XPS) results show that all VO_x·nH₂O deposits can be considered as aggregates consisting of mixed V⁵⁺ and V⁴⁺ oxy-/hydroxyl species with the mean oxidation state significantly increasing with the applied electrode potential.     

Investigation of the catalytic oxidation of benzene to maleic anhydride by wave front analysis with respect to the application of periodic operation

A kinetic analysis of the partial oxidation of benzene to maleic anhydride over a Merck, Sharp and Dohme developed V₂O₅-MoO₃/TiO₂ catalyst shows that partial oxidation proceeds through surface reduction, induction and collapse phases. In each, surface species and reactions are different. The reduction/induction phases correspond to a “ignited” state whereas the collapse phase is similar to an extinguished state familiar in the catalytic oxidation of CO. Selectivity to maleic anhydride is lower in the extinguished state. Periodic operation should offer a means to keep selectivity high provided the cycle period does not exceed just a few minutes. Improvement in selectivity of close to 100% has been attained by periodic switching of the feed composition between mixtures containing oxygen and benzene in ratios of 2.4 and 4.6 with periods between 1/2 and 6 minutes. Benzene conversion, however, is lower.     

Selective oxidation of n-pentane on 12-molybdovanadophosphoric acids

The selective oxidation of n-pentane on 12-molybdovanadophosporic acids with 0, 1, 2, and 3 vanadium atoms was studied and compared with the behaviour of vanadyl pyrophosphate. Catalysts were characterized before and after the catalytic test by electron spin resonance, Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, krypton BET surface area and visible diffuse reflectance spectroscopy. The activity in the n-pentane conversion process increases linearly with increasing number of vanadium atoms, except for the H₆PMo₉V₃O₄₀ sample, for which physico-chemical characterization suggests partial surface degradation during the catalytic tests. A turnover frequency of about 5·10⁻⁴ molecules of n-pentane transformed per atom of vanadium and per second for both 12-molybdovanadophosporic acids and vanadyl pyrophosphate was calculated. Oxidation of n-pentane on heteropoly compounds formed only maleic anhydride and the selectivity increased linearly with increasing number of vanadium atoms present in the Keggin unit. In contrast, under the same reaction conditions both maleic and phthalic anhydride are formed on vanadyl pyrophosphate.     

n-butane oxidation to maleic anhydride: effect of Co and Fe addition by the method of incipient wetness on vanadium phosphate catalysts prepared by the aqueous HC1 method

The effect of the addition of Co²⁺ and Fe³⁺ on the catalytic performance and structure of vanadium phosphate catalysts is described and discussed. The catalyst precursor VOHPO₄.0.5H₂O was prepared using the aqueous HC1 route and Co²⁺ and Fe³⁺ were incorporated using the incipient wetness method. Addition of Co²⁺ improves the selectivity to maleic anhydride, but is found to decrease the specific activity for the formation of maleic anhydride, whereas the addition of Fe³⁺ increases the specific activity up to a maximum level at ca. 2 mol% Fe. The effects are not due to changes in the specific surface area of the final catalyst, which remains 6.5 ±0.5 m² g^–1 for all the catalysts studied. Part of the effect is due to the formation of VOPO₄ phases in the catalyst precursor due to the method of addition of the Co²⁺ or Fe³⁺ and this is both related to the effect of the oxidation potential of the additive cation and to the pH of the impregnating solution. It is apparent that the use of the incipient wetness method for the addition of promoter compounds should be used with care for the VPO system at variance with the direct incorporation of the promoter during the preparation of the VOHPO₄.0.5H₂O precursor.     

The contribution of homogeneous and non-oxidative side reactions in the performance of vanadyl pyrophosphate,catalyst for the oxidation of n-butane to maleic anhydride,under hydrocarbon-rich conditions

The reactivity of vanadyl pyrophosphate, catalyst for the selective oxidation of n-butane to maleic anhydride, was examined under n-butane-rich conditions, simulating a feed in which oxygen is the limiting reactant, and a process in which the unconverted n-butane is recycled. A lower selectivity to maleic anhydride was found with respect to the hydrocarbon-lean conditions, due to the higher selectivity to carbon oxides and to the formation of C₈ by-products: tetrahydrophthalic and phthalic anhydrides. The latter compounds formed by a consecutive reaction of maleic anhydride with the unsaturated C₄ intermediates. This occurred under conditions of total oxygen conversion, due to the decreased catalyst oxidizing property. A relevant contribution of radicalic, homogeneous reactions was also observed, which mainly led to the formation of carbon oxides and olefins. This contribution decreased in the presence of the catalyst, which acted as a radical scavenger, but nevertheless remained important at temperatures higher than 400 °C. When conditions were used under which the conversion of oxygen was not total, olefins generated in the gas phase reacted at the catalyst surface yielding maleic anhydride. This homogeneously initiated heterogeneous process led to an unusual effect, of a relevant increase of maleic anhydride yield over 400 °C.     

Surface alteration of (VO)2P2O7 by α-Sb2O4 as a route to control the n-butane selective oxidation

The catalytic properties of (VO)₂P₂O₇/α-Sb₂O₄ mixed oxides system for n-butane mild oxidation have been investigated on two mechanical mixtures (M1 and M2) of the same well crystallized (VO)₂P₂O₇ (reference vanadyl pyrophosphate) with two different morphologies of α-Sb₂O₄.The M1 mixture of (VO)₂P₂O₇ with α-Sb₂O₄ (1), prepared by oxidation of Sb₂O₃, leads to the oxidative dehydrogenation (ODH) of n-butane, whereas the M2 mixture of (VO)₂P₂O₇ with a commercial α-Sb₂O₄ (2) (Aldrich) with a different morphology improves the maleic anhydride selectivity as compared to the reference (VO)₂P₂O₇ catalyst (synergetic effect). After reaction, no ternary VPSbO phase is detected by XRD and DTA and it was controlled that the two α-Sb₂O₄ oxides are catalytically inactive.The (VO)₂P₂O₇ reference catalyst which produced only maleic anhydride as mild oxidation product shows by XPS a slightly oxidized surface (14% V⁵⁺–86% V⁴⁺).Contamination of the (VO)₂P₂O₇ phase by migration of Sb species occurs after catalytic reaction in the case of the M1 mixture as shown by XPS, LEIS and TEM–EDX analysis. XPS showed that (VO)₂P₂O₇ is partially superficially reduced (86% V⁴⁺–14% V³⁺). This feature is consistent with the decrease of acidity as observed by pyridine adsorption–desorption.In opposition with the M1 mixture, no contamination of the (VO)₂P₂O₇ phase is observed after catalytic reaction in the case of the M2 mixture. The XPS study shows, in this case, that (VO)₂P₂O₇ is partially oxidized (30% V⁵⁺–70% V⁴⁺) at a higher level than for the reference (VO)₂P₂O₇ catalyst. This situation is associated with the increase of selectivity observed for maleic anhydride (synergetic effect).The difference in the catalytic results for the two M1 and M2 mixtures, as compared to the (VO)₂P₂O₇ reference catalyst, can be explained by the alteration of the surface composition of (VO)₂P₂O₇ and the distribution of vanadium oxidation state due to different interaction between Sb₂O₄and (VO)₂P₂O₇, depending on the orientation of the α-Sb₂O₄ crystals.     

Preparation of Vanadium Phosphate Catalysts from VOPO4 · 2H2O: Effect of Microwave Irradiation on Morphology and Catalytic Property

The present work addresses the influence of microwave irradiation on undoped and doped vanadium phosphate catalysts. These catalysts were prepared via VOPO₄ · 2H₂O. The catalyst’s precursors‚ VOHPO₄ · 0.5H₂O were subjected to microwave irradiation and comparison was made with the conventional heating. The interaction of these complex materials with microwave and the addition of several doponts (Nb, Bi, Co, Mo) provide interesting improvements in catalyst preparation found to be a faster, develop higher surface area, higher activity and selectivity for the oxidation of n-butane to maleic anhydride. All the catalysts were characterized by using a combination of powder XRD, H₂-TPR, BET surface area and SEM.     

First two-dimensional framework material constructed from bicapped Keggin structure clusters: [Cu(en)2(H2O)]{[PMoVI8VIV6O42Cu(en)2][Cu0.5(en)]3} · 5.5H2O

The hydrothermal reaction of CuCl₂ · 2H₂O, Na₂MoO₄ · 2H₂O, NH₄VO₃, ethylenediamine (en) and H₃PO₄ yields a novel two-dimensional open-framework material. The extended structure consists of a network of {PMo₈V₆O₄₂ [Cu(en)₂]}⁵⁻ cages, with each cage connected to three other neighboring units via [Cu(en)₂]²⁺ bridging groups.     

Immobilization of Cs,Cd, Pb and Cr by synthetic zeolites from Spanish low-calcium coal fly ash

This report describes a study of the immobilization of Cs⁺, Cd²⁺, Pb²⁺ and Cr³⁺ by synthetic zeolites formed as a result of the hydrothermal treatment of Spanish class F coal fly ash in a 1 M solution of NaOH. The majority zeolite formed at 150 °C was a gismondine-type P1-Na species (Na₆Al₆Si₁₀O₃₂·12H₂O), which at 200 °C transformed into analcime-C zeolite (Na(Si₂Al)O₆H₂O). The shift in pore size distribution towards pores with diameters of about 2.2 nm observed after the zeolites were washed entailed an increase in the specific surface area, to values nearly double the figures recorded prior to washing. With a high selectivity for Cs, the gismondine type Na zeolite P was found to be the best candidate for immobilizing radioactive waste. Gismondine and analcime-C zeolites also exhibited high Cd selectivity.     

Gas phase hydrogenation of maleic anhydride to tetrahydrofuran by Cu/ZnO/TiO2 catalysts in the presence of n-butanol

Cu/ZnO/TiO₂ catalysts were prepared via the coprecipitation method. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectrometry, temperature programmed reduction, and N₂ adsorption. The catalytic activity of Cu/ZnO/TiO₂ catalyst in gas phase hydrogenation of maleic anhydride in the presence of n-butanol was studied at 235–280 °C and 1 MPa. The conversion of maleic anhydride was more than 95.7% and the selectivity of tetrahydrofuran was up to 92.7%. At the same time, n-butanol was converted to butyraldehyde and butyl butyrate via reactions, namely, dehydrogenation, disproportionation, and esterification. There were two kinds of CuO species present in the calcined Cu/ZnO/TiO₂ catalysts. At a lower copper content, the CuO species strongly interacted with ZnO and TiO₂; at a higher copper content, both the surface-anchored and bulk CuO species were present. The metallic copper (CuO) produced by the reduction of the surface-anchored CuO species favored the deep hydrogenation of maleic anhydride to tetrahydrofuran. The deep hydrogenation activity of Cu/ZnO/TiO₂ catalyst increased with the decrease of crystallite sizes of CuO and the increase of microstrain values. Compensations of reaction heat and H₂ in the coupling reaction of maleic anhydride hydrogenation and n-butanol dehydrogenation were distinct.     

Hydrothermal synthesis and structural characterization: a novel α-Keggin unit supported zinc-bipyridyl complex [Zn(2,2′-bipy)3]2[ZnW12O40Zn(2,2′-bipy)2] · H2O

The hydrothermal reaction of Na₂WO₄ · 2H₂O, ZnCl₂, Zn(Ac)₂ · 2H₂O, 2,2^′-bipyridine, and H₂O gives rise to a novel Keggin unit supported zinc-bipyridyl complex [Zn(2,2^′-bipy)₃]₂[ZnW₁₂O₄₀Zn(2,2^′-bipy)₂] · H₂O consisting of a novel heteropolyanion [ZnW₁₂O₄₀Zn(2,2^′-bipy)₂]⁴⁺ in which the Keggin anion [ZnW₁₂O₄₀]⁶⁻ acts as a bidentate ligand towards the transition metal cation Zn²⁺ through a terminal oxygen and a bridging oxygen from the same WO₆ octahedron.     

Functionalization of ethylene-propylene-diene terpolymer via the alder ene reaction

An ethylene-propylene-diene rubber (EPDM), containing 5 wt% 5-ethylidene-2-norbornene, was functionalized with maleic anhydride through the Alder Ene reaction at temperatures above 200°C in a co-rotating twin screw extruder. Characterization of the maleated product included FT-IR, ¹H and ¹³C NMR, and GPC. The degree of functionalization was determined by infrared analysis and by the mechanical properties of an ionic network formed by neutralizing the maleated rubber with zinc oxide. Increased temperature and maleic anhydride reactant concentration were found to improve the extent of reaction. Several Lewis acid species (SnCl₂?2H₂O, RuCl_n?xH₂O and AlCl₃) were tested as catalysts, and they were found to have a small effect on the degree of functionalization. This effect improved with reduced acid concentration. Among the Lewis acids examined, AlCl₃ gave rise to the greatest improvement of succinyl anhydride incorporation into the rubber.     

Preparation of V doped lanthanum silicate electrolyte ceramics by combustion method and study on conductance mechanism

Vanadium doped La_9.33Si_6−xV_xO_26+0.5x (x = 0.5, 1.0, 1.5) (LSVO) electrolyte powder was prepared by combustion method at 600°C for 5-7 min. The powder was sintered at 1500°C for 3 hours to prepare LSVO ceramics. XPS, IR, XRD, and EIS analysis show that V⁵⁺ doping replaces Si⁴⁺ in [SiO₄] to form [Si(V)O₄] tetrahedron. With the increase in x, the lattice volume increase. When x = 2.0, the LaVO₄ phase was formed, indicating that the limit doping amount of V⁵⁺ replacing Si⁴⁺ is x ≤ 1.5. The conductivity of LSVO increases significantly with the increase in x (x ≤ 1.0), which attributed to the defect reaction caused by V⁵⁺ doping. The addition of the interstitial oxygen Oi* in 6₃ channels and the increase of lattice volume leads to increased conductivity. When x = 1.0, the highest conductivity is 1.46 × 10⁻² S·cm⁻¹ (800°C). The doping enhancement conductivity mechanism is the Interstitial oxygen defect-Lattice volume composite enhancement mechanism.     

Decavanadate with a novel coordination complex: Synthesis and characterization of (NH4)2[Ni(H2O)5(NH3)]2(V10O28)·4H2O

A new compound (NH₄)₂[Ni(H₂O)₅(NH₃)]₂[V₁₀O₂₈]·4H₂O (1) containing a {V₁₀O₂₈} ⁶⁻ anionic cluster and a novel complex cation, [Ni(H₂O)₅(NH₃)]^2 +, has been synthesized and fully characterized by single crystal X-ray crystallography, spectroscopy and thermogravimetric analysis. The presence of the ammonia ligand in the complex cation in 1 was established unambiguously by X-ray crystallography and variable temperature (200–400 °C) thermogravimetric analyses in combination with FTIR spectroscopic studies. The formation of the novel complex species {Ni(H₂O)₅(NH₃)}^2 + during the synthesis of 1 can be rationalized in terms of ligand substitution involving {Ni(H₂O)₆}^2 +.     

Selective oxidation of H2S to sulfur over vanadia supported on mesoporous zirconium phosphate heterostructure

Vanadium oxide supported on mesoporous zirconium phosphate catalysts has been synthesized, characterized and tested in the selective oxidation of H₂S to sulfur. The nature of the vanadium species depends on the V-loading of catalyst. Catalysts with a V-content lower than 4wt% present both isolated vanadium species and V₂O₅ crystallites. However, V₂O₅ crystallites have been mainly observed in catalysts with higher V-content, although the presence of isolated V-species on the surface of the metal oxide support cannot be completely ruled out. The catalytic behaviour also depends on V-loading of catalysts. Thus, while the catalytic activity of catalysts can be related to the number of V-sites, the catalyst decay is clearly observed in samples with low V-loading. The characterization of catalysts after the catalytic tests indicates the presence of sulfur on the catalyst, which is favoured on catalysts with low V-loading. However, a clear transformation of V₂O₅ to V₄O₉ can be proposed according to XRD and Raman results of used catalysts with high V-loading. The importance of V⁵⁺–O–V⁴⁺ pairs in activity and selectivity is also discussed.     

Tuning the valence of 3,3-di(1H-tetrazol-5-yl)pentanedioic acid: Solvothermal synthesis of Eu(III) and bimetallic Eu(III)/Cu(II) compounds

3,3-Di(1H-tetrazol-5-yl)pentanedioic acid (H₄dtzpda) with four acidic hydrogen atoms can display different valence when reacted with different metal ions. Solvothermal reactions of H₄dtapda with Eu(NO₃)₃·6H₂O or Eu(NO₃)₃·6H₂O/Cu(NO₃)₂·6H₂O afford [Eu(Hdtzpda)(H₂O)₄]·4H₂O (1) and [Eu₂Cu(dtzpda)₂(H₂O)₁₀]·6H₂O (2), respectively, where only three acidic hydrogen atoms of H₄dtzpda are deprotonated in compound 1 while all the four acidic ones are deprotonated in compound 2. In compound 1, Hdtzpda^3 − acts as a pentadentate ligand to bridge Eu(III) centers via the oxygen atoms of the carboxylate group while in compound 2, dtzpda^4 − is a hepadentate one via not only the oxygen atoms of the carboxylate group but also the nitrogen atoms of the tetrazole rings. The luminescence properties of the two compounds in the solid state show both intraligand and characteristic peaks of Eu^3 + at room temperature.