VOPO4 Phases Research Articles

Synthesis of an efficient VPO catalyst for the selective oxidation of n-butane to the MA product: Mechanism of crystal transformation

Serial VPO catalysts with different crystal structures were prepared by adjusting the ratios of air and N2 in the calcination process. The precursors and vanadium phosphorus oxide (VPO) catalysts were characterized by various measurements. As the air and N2 ratios were lower than 1:1, the ordered VOHPO4·0.5H2O phase was mainly formed in the precursors during calcination. However, as the ratio of air to N2 reached 3:1, amorphous VOPO4 phases were formed over the surface of the precursor during calcination, which prevented the formation of ordered V5+ species over the surface of the VPO catalyst after activation. Meanwhile, a high proportion of VOHPO4·0.5H2O in the precursor could facilitate more formation of the ordered α1-VOPO4 phase over the surface of the VPO catalyst. Interestingly, the crystal structure over the surface of active VPO catalysts was totally different from the core of VPO catalysts. There were more P species and V species with 5+ existing over the surface of the VPO catalysts. The evaluation results of n-butane oxidation showed that when the ratio of air to N2 was 1:3, the corresponding VPO-1A catalyst presented the highest conversion of n-butane and yield of the MA product (57.6 m% at 412 °C). The reason should be related to the fact that the VPO-1A catalyst possessed the highest acidity and proportions of surface V5+ species and Sur-O species. As the ratio of air to N2 reached 3:1, the VPO-3A catalyst showed the poorest performance in the oxidation of n-butane, which could be due to the formation of inactive and amorphous VOPO4 phases over the surface of the VPO catalyst. Finally, kinetic and thermodynamic models were established and calculated to investigate the characteristics of n-butane oxidation reactions for various VPO catalysts.

Enhancing catalytic performance of phosphorus-modified ceria supported VPO catalysts for n-butane oxidation

Vanadium phosphorus oxide (VPO) catalysts supported on CeO2 and P-modified CeO2 (P-CeO2) have been firstly synthesized for selective oxidation of n-butane to maleic anhydride (MA). The catalysts supported on P-CeO2 showed higher catalytic performance, especially higher MA selectivity than catalysts on pure ceria, and their specific activities (normalized to per unit mass of VPO loading) were also superior to that of bulk VPO. TG, TEM, XRD, Raman, H2-TPR and XPS results revealed that the structure and property of VPO are largely affected by the support. The loaded VPO existed almost wholly as VOPO4 phases on pure ceria, while mainly in the form of (VO)2P2O7 with small amount of VOPO4 phases on phosphorus-modified ceria. Both VPO/P-CeO2 and bulk VPO catalysts have the similar VPO phase structure, but the former showed the enhanced reducibility of V4+ phase. These differences in structure and properties of the catalysts were discussed and related to their different catalytic performances.

Samarium-modified vanadium phosphate catalyst for the selective oxidation of n-butane to maleic anhydride

A series of samarium-modified vanadium phosphate catalysts were prepared and studied in selective oxidation of n-butane to maleic anhydride. The catalytic evaluation showed that Sm modification significantly increased the overall n-butane conversion and intrinsic activity. N2-adsorption, XRD, SEM, Raman, XPS, EPR and H2-TPR techniques were used to investigate the intrinsic difference among these catalysts. The results revealed that the addition of Sm to VPO catalyst can increase the surface area of the catalyst, lead to a significant change in catalyst morphology from plate-like structure into rosette-shape clusters, and largely promote the formation of (VO)2P2O7. All of these were related to the different catalytic performance of Sm-doped and undoped VPO catalysts. The roles of the different VOPO4 phases and the influence of Sm were also described and discussed.

VOPO4 nanosheets as anode materials for lithium-ion batteries.

VOPO4 nanosheets are successfully synthesized using a hydrothermal method followed by calcination. The XRD results reveal that obtained products are crystallized in the orthorhombic VOPO4 phase. SEM and TEM images demonstrate that VOPO4 products possess unique nanosheet morphology. Electrochemical tests show that the VOPO4 nanosheets exhibit an excellent electrochemical performance as anode materials. They can deliver an initial discharge capacity of 1356 mA h g(-1) at 0.1 C, and possess a favorable capacity at rates of 0.2, 1, and 2 C. The synthesized VOPO4 can be used as a good anode material; furthermore, the nanosheet structure can effectively improve the electrochemical performance of this material.

Liquid Phase Oxidation of p-cymene by (VO)2P2O7 and VO(PO3)2 Catalysts

Vanadium phosphate oxide (VPO) catalysts derived from VOHPO4·0.5H2O and VO(H2PO4)2 precursors showed improved rates in the catalysed liquid phase oxidation of p-cymene. Both catalysts showed high performance with conversions of up to 30% achieved within 3 h with a selectivity towards the tertiary cymene hydroperoxide (TCHP) of 75–80% when compared to lower conversions and poorer TCHP selectivity in the non-catalysed industrial oxidation processes that takes about 8–12 h reaction time. The temperature dependent structural changes during activation within the catalysts were studied by in situ XRD and TGA-MS. The in situ XRD showed that the VO(H2PO4)2 form mainly an amorphous phase at temperature up to 600 °C while above 650–750 °C a stable VO(PO3)2 phase was obtained. The in situ XRD studies of the (VO)2P2O7 phase derived from VOHPO4·0.5H2O showed to became more crystalline with increasing temperature from 400 to 750 °C. No formation of VOPO4 phases were observed in the activated (VO)2P2O7 catalyst even at high temperature under the in situ XRD conditions. VPO catalysts derived from VOHPO4·0.5H2O and VO(H2PO4)2 precursors improve oxidation rates and improved tert-cymene hydroperoxide selectivity during the liquid-phase oxidation of p-cymene compared to non-catalyzed oxidations.

Comparison of vanadium phosphate catalysts derived from VOPO4·2H2O prepared from H3PO4 and H4P2O7 Electronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b105304n/

Vanadium phosphate catalysts prepared from VOPO4 ·2H2O are described and discussed. The use of H4P2O7 as a phosphorus source, as compared with H3PO4, is shown to give an improved synthesis of VOPO4·2H2O. Reduction of VOPO4·2H2O with alcohols from both the materials leads to the formation of VOHPO4·0.5H2O. However, the material derived from H4P2O7 has a higher intrinsic activity (mol maleic anhydride formed per unit surface area) than that derived from H3PO4. The final catalysts are shown to comprise (VO)2P2O7 together with a range of V(V) VOPO4 phases and the origin of the enhanced activity is discussed in terms of the catalyst structure.

The effect of the duration of n-butane/air pretreatment on the morphology and reactivity of (VO)2P2O7 catalysts

Three vanadium pyrophosphate catalysts have been prepared by calcining vanadium hydrogen phosphate hemihydrate (VOHPO4⋅0.5H2O, prepared in an organic medium) for different lengths of time (40, 100 and 132 h) in a n-butane (0.75%)/air mixture at 473 K. The catalysts were designated VPO40, VPO100 and VPO132. Increasing the duration of reaction with n-butane/air mixture led to an increase in the total surface area from 21.3 m2 g−1 (VPO40) to 24.9 m2 g−1(VPO100) and to 27.0 m2 g−1(VPO132). It also led to the complete removal of the VOPO4 phase from catalysts VPO100 and VPO132, this VOPO4 phase having seen as a minor component of catalyst VPO40. Scanning electron microscopy showed that longer periods of pretreatment in the n-butane/air mixture produced catalysts with increasing amounts of a characteristic rosette-type of agglomerate. Temperature-programmed reduction with H2 resulted in the removal of ∼11 monolayers equivalent of oxygen from all three of these catalysts at a peak maximum temperature of ∼1000 K with the development of a second reduction peak at ∼1100 K which increases with increasing time of n-butane/air pretreatment. The morphology produced by extended pretreatment in the n-butane/air mixture at 673 K is therefore predisposed to reaction with H2 (and probably with n-butane). Apparently paradoxically, increasing the duration of n-butane/air pretreatment results in catalysts which on temperature-programmed desorption desorb less oxygen.

New Insights into the Nature of the Active Phase of VPO Catalysts—A Quantitative Static LEIS Study

The atomic composition of the outermost atomic layer of equilibrated VPO catalysts was studied with low-energy ion scattering. Using external reference samples, absolute numbers for the atomic densities of both P and V could be determined. VPO catalysts prepared in either aqueous or organic media showed large amounts of carbonaceous surface species. From catalysts prepared in an aqueous medium the carbonaceous species could be removed using a mild oxidation treatment without significantly changing the average valence of the vanadium atoms in the catalyst as evidenced from XPS. After this cleaning treatment the surface concentration of vanadium (atoms/cm2) agrees well with that expected for a vanadyl pyrophosphate structure. However, the surface phosphorus concentration is twice as high as that in vanadyl pyrophosphate, leading to a surface P/V ratio of 2.0±0.2. This shows that VPO catalysts may be terminated by a distorted vanadyl pyrophosphate structure, where the excess amount of phosphorus is positioned between the vanadyl units and the phosphate groups. The catalyst surface is also compared to VOPO4 phases. A significant contribution of a phase such as αII-VOPO4 could explain the observed surface P/V ratio of 2.0.

N-Butane oxidation using VO(H2PO4)2 as catalyst derived from an aldehyde/ketone based preparation method

A detailed study of n-butane oxidation over vanadium phosphate catalysts derived from in situ activation of VO(H2PO4)2 is described and discussed. New methods of preparation of VO(H2PO4)2 are described using the reaction of V2O5 and H3PO4 or VOPO4 ·2H2O with an aldehyde or ketone as a reducing agent. It is proposed that the enol form of the aldehyde and ketone act as the reducing agent. VO(H2PO4)2 can also be obtained by reduction of VOPO4 phases with alcohols. The catalysts derived from VO(H2PO4)2 by in situ activation in 1.5% butane in air at 400°C for 72 h are largely disordered and, by detailed powder X-ray diffraction and laser Raman spectroscopy, are shown to be comprised mainly of disordered material containing some untransformed VO(H2PO4)2, VOPO4 phases and possibly some VO(PO3)2. The catalysts derived from all the forms of VO(H2PO4)2 investigated are found to exhibit maleic anhydride selectivities in the range 20–30% and it is concluded that the low selectivity is due to the presence of VOPO4 phases in the activated catalysts.

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.

New Insights into VOPO4 Phases Through Their Hydration

Some VOPO4 phases can be involved when the V-P-O catalysts used to oxidize n-butane to maleic anhydride are activated. Evidence is given that correct characterization can only be achieved if the catalysts are protected from hydration. Hydration Of VOPO4 phases (αI-, αII-, β-, γ-, δ-) was monitored by Raman spectroscopy. It is shown that, with the exception of β-VOPO4, all VOPO4 phases can be hydrated, with hydration leading to the same VOPO4 · 2H2O. NH3 was also intercalated within αI-VOPO4. These experiments give clues to a unified description of all (with the exception of β-VOPO4) VOPO4 phases as layered arrangements of VO6 distorted octahedra linked by PO4 tetrahedra. Proposals are given for the structure of γ- and δ-VOPO4. The proposed structure of δ-VOPO4 allows epitaxy of this peculiar phase onto (VO)2P2O7 as well as growth of αII-VOPO4 onto δ-VOPO4.

Investigation of the structures formed when vanadium and phosphorus are grafted on to a silica surface

V-P/Si02 samples obtained by impregnation and grafting with different contents of vanadium and phosphorus have been studied by ESR, scanning electron microscopy (SEM) and fine-structure x-ray absorption (FSXA). It is shown that, independent of the method of synthesis, the vanadium concentration and the P/V ratio, the surface region of the carrier consists of a crystalline β- VOPO4 phase with a low concentration of V4+ ions.

Role of surface atomic arrangements of well defined phosphates in partial oxidation and oxidative dehydrogenation reactions

Various vanadium and iron phosphates have been synthesized and characterized physically before and after catalytic reaction has taken place. Vanadium phosphate was used for butane oxidation into maleic anhydride and iron phosphate for isobutyric acid dehydrogenation to methacrylic acid. Vanadium phosphate catalyst in its working state corresponds to a mixture of the pyrophosphate phase (VO)2P2O7 and a given VOPO4 phase (αII, β or δ). 31P MASNMR spectra showed that the most efficient catalysts contained VOPO4 entities in interaction with (VO)2P2O7, while an X-ray electronic radial distribution study showed that VOPO4 entities are present but not necessarily organized in sufficiently long-range order to be detected by classical X-ray diffraction. The iron phosphates Fe2P2O7, FePO4 and Fe7(PO4)6 were transformed under catalytic reaction into Fe7(PO4)6 and an unknown phase. The latter phase was tentatively assigned from Mössbauer spectroscopy to Fe6P6O23 with an Fe3+/Fe2+ ratio equal to two. The best catalysts including the industrial catalyst were those containing the Fe6P6O23 phase. A similarity has been found in the behaviour of vanadium and iron phosphates. The best catalysts correspond to those which exhibit short- or long-range order between the reduced pyrophosphate phase and a more oxidized form as VOPO4 or Fe6P6O23. It is suggested that the ease of transformation of two PO4 groups, at least locally, into one P2O7 group is the clue to obtaining an efficient catalyst. This is a case of topotactic transformation.