2P2O7 Phase Research Articles

Effect of direct ultrasound synthesis via a sesquihydrate route on bismuth‐promoted vanadyl pyrophosphate catalysts

AbstractA series of 1, 3, and 5% Bi‐doped vanadium phosphate catalyst catalysts were prepared via sesquihydrate route using direct ultrasound method and were denoted as VPSB1, VPSB3, and VPSB5, respectively. These catalysts were synthesized solely using a direct ultrasound technique and calcined in a n‐butane/air mixture. This study showed that catalyst synthesis time can be drastically reduced to only 2 hr compared to conventional 32–48 hr. All Bi‐doped catalysts exhibited a well‐crystallized (VO)2P2O7 phase. In addition, two V5+ phases, that is, β‐VOPO4 and αII‐VOPO4, were observed leading to an increase in the average oxidation state of vanadium. All catalysts showed V2p3/2 at approx. 517 eV, giving the vanadium oxidation state at approx. 4.3–4.6. Field‐emission scanning electron microscopy micrographs showed the secondary structure consisting of thin and small plate‐like crystal clusters due to the cavitation effect of ultrasound waves. VPSB5 showed the highest amount of oxygen species removed associated with the V5+ and V4+ species in temperature‐programmed reduction in H2 analyses. TheX‐ray absorption near edge structure (XANES) measurement showed the occurrence of vanadium oxide reductions in hydrogen gas flow, indicating the presence of V4+ and V5+ species. Higher average valence states of V5+, indicating more V5+ phases, were present. The addition of bismuth has increased the activity and selectivity to maleic anhydride.

Direct ultrasound synthesis of vanadyl pyrophosphate catalyst for partial oxidation of n-butane to maleic anhydride

Three VPO catalyst were prepared via direct ultrasound technique using organic, sesquihydrate and dihydrate routes, denoted as VPOuo, VPOus and VPOud respectively. These catalysts were synthesised solely on direct ultrasound technique and calcined in n-butane/air mixture. All catalysts exhibited well-crystallised (VO)2P2O7 phase. VPOus and VPOud showed αII-VOPO4, which led to an increase in average oxidation state of vanadium. All catalysts were showing O1s approx. 530 eV, similar P2p value and V2p3/2 at approx. 517 eV, giving vanadium oxidation state of approx. 4.0 – 4.2. FE-SEM micrographs showed the secondary structure consisting of thin plate-like crystals in different sizes agglomerate to each other due to cavitation effect. HR-TEM demonstrated the existence of polycrystalline phase. The nature of the oxidants was investigated by TPR in H2. VPOus showed highest amount of total removal of oxygen species suggesting that it had highest activity compared to VPOuo and VPOud. The XANES measurement of these catalysts showed the occurrence of vanadium oxide reductions in flowing hydrogen gas, which indicates the presence of V4+ and V5+ species. Catalytic tests demonstrated that the activity and selectivity to maleic anhydride increased with direct ultrasound technique. This study showed that catalyst synthesis time can be reduced to only 2 hours and giving polycrystalline particles compared to conventional method.

Relationship between bulk phase, near surface and outermost atomic layer of VPO catalysts and their catalytic performance in the oxidative dehydrogenation of ethane

A set of vanadium phosphorous oxide (VPO) catalysts, mainly consisting of (VO)2P2O7, VO(PO3)2 or VOPO4·2H2O bulk crystalline phases, has been investigated for the oxidative dehydrogenation (ODH) of ethane to ethylene, a key potential reaction for a sustainable industrial and socioeconomic development. The catalytic performance on these VPO catalysts has been explained on the basis of the main crystalline phases and the corresponding surface features found by XPS and LEISS at 400°C, i.e. within the temperature range used for ODH reaction. The catalysts based on (VO)2P2O7 phase presented the highest catalytic activity and productivity to ethylene. Nevertheless, the catalysts consisting of VO(PO3)2 structure showed higher selectivity to ethylene, reaching 90% selectivity at ca. 10% ethane conversion. To the best of our knowledge, this is the highest selectivity reported on a vanadium phosphorous oxide at similar conversions for the ethane ODH. In general, catalysts consisting of crystalline phases with vanadium present as V4+, i.e. (VO)2P2O7 and VO(PO3)2, were found to be significantly more selective to ethylene than those containing V5+ phases. The surface analysis by XPS showed an inverse correlation between the mean oxidation state of vanadium near surface and the selectivity to ethylene. The lower averaged oxidation states of vanadium appear to be favoured by the presence of V3+ species near the surface, which was only found in the catalysts containing V4+ phases. Among those catalysts the one based on VO(PO3)2 phase shows the highest selectivity, which could be related to the most isolated scenario of V species (the lowest V content relative to P) found at the outermost surface by low energy ion scattering spectroscopy (LEISS), a “true” surface technique only sensitive to the outermost atomic layer.

Green and selective oxidation of cyclohexane over vanadium pyrophosphate supported on mesoporous KIT-6

The activity and selectivity of vanadium phosphate (VPO) catalysts on the mesoporous KIT-6 support for green oxidation of cyclohexane to cyclohexanone and cyclohexanol were studied using H2O2. The catalysts were characterized using XPS, XRD, ICP-AES, TEM, SEM, EDX, BET, FT-IR and adsorption of pyridine. Small angle X-ray diffraction spectroscopy confirmed formation of well-ordered mesoporous KIT-6 material. The BET analysis showed a high surface area and well-ordered support formed. The XRD analysis demonstrated the loaded VPO existed almost wholly as VOHPO4 0.5H2O phases and after calcination converted mainly to the form of (VO)2P2O7 phase on KIT-6. XPS analysis revealed that a vanadium oxidation state of catalyst is approximately 4+. Within the studied range, the optimum condition to improve cyclohexane conversion and selectivity towards KA-oil (a mixture of cyclohexanone and cyclohexanol) over vanadium containing catalyst was performed in the presence of 27wt% catalyst VP-loading at 65°C with 1:4 (cyclohexane: H2O2) molar ratio, in acetonitrile. Under these reaction conditions 19.3% conversion and 69.9% selectivity for KA-oil were observed after 4h of reaction time. Finally, the recyclability of the catalyst was studied through five consecutive catalytic runs.

Partial oxidation of n-pentane over vanadium phosphorus oxide supported on hydroxyapatites

The selective oxidation of n-pentane to value-added products, maleic anhydride or phthallic anhydride by vanadium phosphorus oxide loaded on hydroxyapatites as catalysts and oxygen as oxidant was investigated. Hydroxyapatite (HAp) and cobalthydroxyapatite (Co-HAp) were prepared by the co-precipitation method and VPO with varying weight percentages (2.5–15.0 %) were loaded on the hydroxyapatite supports by the wet impregnation technique. The catalyst materials were characterized by surface area measurements, elemental analysis, powder X-ray diffraction (XRD), infrared spectroscopy (IR) and temperatureprogrammed reduction (TPR).VPOis present in two phases, viz. (VO)2P2O7 andVOPO4.With increase in theVPOloading on the hydroxyapatites, the (VO)2P2O7 phase also increased. From catalytic results, a conversion of 75 % of n-pentane and selectivity towards maleic anhydride, about 50 % and phthalic anhydride, about 25 %, were consistently achieved with loadings of 5.0 and 7.5 wt.%VPOat 360 °C forGHSVsof 1900 and 2300 h–1. Under optimum conditions, product yields of up to 40%maleic anhydride and 20 % phthalic anhydride were obtained. It is proposed that the products formed through the diene intermediate. KEYWORDS Selective oxidation, n-pentane, vanadium phosphorus oxide, hydroxyapatite, maleic anhydride, phthalic anhydride.

Acetalization of glycerol with acetone to bio fuel additives over supported molybdenum phosphate catalysts

The acetalization of glycerol with acetone was carried out over a series of molybdenum phosphate catalysts supported on SBA-15 with varying MoPO loadings ranging from 5-50wt%. These catalysts were characterized by X-ray diffraction, FT-IR, Laser Raman Spectroscopy, Ultraviolet–visible diffuse reflectance spectroscopy (UV DRS), NH3-TPD analysis, ex-situ pyridine adsorbed FT-IR analysis and pore size distribution measurements. The XRD results of unsupported MoPO show the formation of (MoO2)2P2O7 phase and this phase is present in a well dispersed state on the SBA-15. Raman spectra reveal the presence of MoPO species in the form of (MoO2)2P2O7 phase in the samples above 40wt% MoPO/SBA-15. The presence of both isolated tetrahedrally and isolated octahedrally coordinated Mo centers in the unsupported and supported MoPO are confirmed by the UVDRS findings. Ammonia TPD analysis suggests that the total acidity increased with MoPO loading and acidity of the catalysts was proved to be detrimental to assess the catalytic performance. The conversion and selectivity during the acetalization depends strongly on the reaction time, catalyst loading and glycerol to acetone molar ratio. Acetalization of glycerol suggests that 40wt% MoPO/SBA-15 sample exhibited better catalytic properties than other catalysts investigated. The catalytic properties are well correlated with the acidic functionalities of the catalysts.

Structural characteristics of an amorphous VPO monolayer on alumina for propane ammoxidation

The structural properties and oxidation state of vanadium in alumina-supported vanadium-phosphorous oxides at nearly monolayer coverage influence its catalytic performances. The population distribution VVO and VIVO species in the amorphous VPO phase strongly depends on the sample history. By using alumina-supported VPO instead of conventional bulk catalysts, we facilitate enhanced discrimination between active phase and bulk phase signals in the vanadium-phosphorous oxygen system. This brings light on the speciation of VPO catalysts in general. Our observations were achieved by using mainly in situ EPR &operando Raman–GC spectroscopic techniques at relevant temperatures and gas conditions, showing that the (VivO)2P2O7 phase is formed during propane ammoxidation at 753K. Such transformation is less intense in inert atmosphere at 753K. Ammonia adsorption on the VPO catalyst surface leads to the formation of VO2+ sites, probably bearing Brønsted and/or Lewis acidity. Propane ammoxidation appears to promote the formation of mixed valence compounds.

Preparation of vanadium phosphate catalyst precursors for the selective oxidation of butane using α,ω-alkanediols

Vanadium phosphate catalyst precursors were prepared from V2O5 and H3PO4 or VOPO4·2H2O using α,ω-alkanediols (1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol) as both the reducing agent and the solvent. A series of layered VOHPO4·HO(CH2)nOH (n=4–6) materials intercalated with α,ω-alkanediols were obtained. The performance for butane oxidation of the final catalysts formed via the in situ transformation of the VOHPO4·HO(CH2)nOH precursors under reaction conditions is described. The final catalysts derived from the alkanediol intercalated materials were found to exhibit relatively low selectivities to maleic anhydride and comprised of VV (VOPO4) phases with an amorphous VIV phase. However, when a crystalline (VIVO)2P2O7 phase is present, with small amounts of VV phases, as with materials prepared with 1,4-butanediol, the specific and intrinsic catalytic activity are higher.

The Effect of Cr, Ni, Fe, and Mn Dopants on the Performance of Hydrothermal Synthesized Vanadium Phosphate Catalysts for n-Butane Oxidation

Vanadium phosphorus oxide (VPO) catalysts synthesized via hydrothermal method were introduced with some different dopants, Cr, Ni, Fe, and Mn during the preparation of the precursors, VOHPO4 · 0.5H2O. All modified precursors were subsequently transformed under reaction conditions to give the active phase, (VO)2P2O7. Several techniques were used to characterize the physicochemical properties of the catalysts, such as X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), H2-temperature programmed reduction (TPR), laser Raman spectroscopy (LRS), and chemical analysis. The catalytic performance of the catalysts for selective oxidation of n-butane to maleic anhydride has been carried out by using a fixed bed microreactor (673 K, gas hourly space velocity [GHSV] = 2,400 hr−1). The results show that the addition of dopants into the VPO catalysts increased the surface area of the catalysts and induced the formation of V5+ phases as shown in XRD and LRS spectra that promoted the catalytic performance. A further water reflux treatment to all the precursors gave catalysts with only (VO)2P2O7 phase. However, the activity and selectivity of these catalysts were reduced markedly.

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.

Water modification of PEG-derived VPO for the partial oxidation of propane

Abstract In this study, the PEG-derived VPO precursors were subject to water refluxing (90 °C, 8 h) and in situ activation in a steam-containing environment. For comparison, the VPO precursors without water refluxing were also activated in a similar manner. The IR measurement indicated that the majority of PEG in the precursor has been removed during water refluxing, and the VPO is essentially unenwrapped by PEG. The consequence is an increase of particle size and crystallinity of VPO as well as decreases in surface acidity and site density. The activation of catalysts in a stream-containing environment has an influence on the content of V5+ species and on the reduction behavior of the VPO catalysts. The VPO catalyst activated with 20% water vapor (by volume) in the feed shows the highest crystallinity. Compared with the non-PEG-derived VPO precursor, the PEG-derived one undergoes structure changes of higher severity during the water/steam treatments. The VPO catalyst generated from the PEG-derived precursor with water refluxing and activated with steam (20%) exhibited a propane conversion of 25% and a (AA + HAc) selectivity of ∼70%. The superior catalyst behavior can be interpreted in terms of the higher crystallinity of the (VO)2P2O7 phase, the lower content of V5+ species, and the milder surface acidity as caused by the water/steam treatments.

Physico-chemicals and catalytic properties of manganese-promoted vanadium phosphate (VPO) catalyst

The addition of 1% Mn promoter to vanadium phosphate catalyst led to doubling of the specific surface area from 20.3 (unpromoted) to 39.4 m2 g−1. The XRD pattern of the Mn-promoted catalyst gave only the characteristics of the (VO)2P2O7 phase, indicating that the Mn was incorporated into the crystal lattice of the catalyst. The Mn-promoted catalyst was also twice as active in removing the total amount of oxygen. However, since the only oxygen species related to V4+ being removed and no oxygen species associated with V5+ was observed, the n-butane conversion was not much improved as compared to the unpromoted counterpart. A necessary amount and distribution of the V5+ phase in a well crystalline V4+ phase is essential in order to enhance the catalytic performance in the mild oxidation of n-butane to maleic anhydride.

Gallium-doped VPO catalysts for the oxidation of n-butane to maleic anhydride

Vanadium phosphate (VPO) catalyst materials have been doped with gallium and subsequently tested for the mild oxidation of n-butane to maleic anhydride. At low Ga concentrations, this impurity is shown to be beneficial for n-butane conversion. For low dopant levels (Ga/V ⩽ 1 at%), the crystallinity of the hemihydrate (VOHPO4·0.5H2O) precursor phase is improved and its specific area is increased relative to the undoped material as a result of decreased platelet thickness. Electron diffraction and energy dispersive X-ray analysis (XEDS) revealed that the Ga is uniformly distributed in a substitutional manner throughout the hemihydrate structure. The presence of Ga also significantly shortens the activation time required to convert the hemihydrate precursor into a well crystallized vanadyl pyrophosphate (VO)2P2O7 phase under an n-butane/air gas flow at 400 °C. The intimate presence of Ga distributed within the VOHPO4·0.5H2O unit cell has also been confirmed by XANES and EXAFS. Such studies also show that the Ga is partially redistributed within the (VO)2P2O7 structure after catalyst activation. A complementary electrical conductivity study on these materials revealed that Ga3+ substitutes for (VO)2+ species in (VO)2P2O7 giving rise to an n-type semiconductivity which partially compensates the natural p-type conductivity character of the (VO)2P2O7 phase. For higher Ga doping levels (Ga/V ≈ 5 at%), the excess of Ga concentrates as a GaPO4 impurity phase, which is shown to have a detrimental effect on the catalytic performance of the Ga-doped VPO catalyst.

Structural transformation sequences occurring during the activation of vanadium phosphorus oxide catalysts

The structural transformations which occur when a VOHPO4·0.5H2O precursor is activated in a mixture of n-butane–air at 400 °C have been studied using a combination of XRD, TEM and 31P NMR spin–echo mapping. The VOHPO4·0.5H2O precursor was prepared by reducing V2O5 with isobutyl alcohol and had a plate-like morphology with a [001] normal. A systematic series of ‘activated’ catalysts were then prepared in which the activation time was varied between 0.1 and 132 h. Structural characterisation studies on these samples show the structural evolution of the catalyst during the activation procedure. At the periphery of the platelet a direct topotactic transformation from [001] VOHPO4·0.5H2O to [100](VO)2P2O7 occurs. In the interior of the platelet a more complex indirect transformation sequence occurs. Regions are shown to exist where the VOHPO4·0.5H2O precursor initially transforms epitaxially into δ-VOPO4. As the activation time increases the domains of δ-VOPO4, which are embedded in a disordered matrix, shrink and further transform to the final (VO)2P2O7 phase. An attempt has also been made to correlate measured catalytic performance data with the catalyst microstructure at various stages of the transformation process. It is found that there is a distinct parallel between improving catalytic performance and a decrease in the amount of V5+ phases present. A further sample of the same hemihydrate precursor was activated in a pure N2 atmosphere for comparison. In this case, the whole VOHPO4·0.5H2O platelet was observed to undergo a direct topotactic transformation to the (VO)2P2O7 phase. The transformation mechanism, however, involved the homogeneous nucleation of discrete epitaxial patches of crystalline (VO)2P2O7 over the surface of the platelet. This is followed by the growth and eventual coalescence of the patches upon extended activation.

On the Role of the VO(H2PO4)2 Precursor for n-Butane Oxidation into Maleic-Anhydride

The catalytic role of VO(H2PO4)2, the precursor of the VO(PO3)2 phase, has been studied for n-butane oxidation to maleic anhydride. By comparison with the activated VPO catalyst, derived from the VOHPO4 · 0.5H2O precursor phase, VO(H2PO4)2gives a highly selective final catalyst. The total oxidation products CO and CO2 are not observed under any of the conditions examined, a result confirmed by extensive catalyst testing and carbon mass balances. The final catalyst derived from VO(H2PO4)2 has a low surface area, ca. 1 m2/g, and consequently demonstrates low specific activity on the basis of n-butane conversion per unit mass. However, the intrinsic activity (activity per unit surface area) is found to be higher than that for catalysts derived from VOHPO4 · 0.5H&2;O. Since some VO(H2PO4)2 is present in VOHPO4 · 0.5H2O, which is the precursor of the industrial catalyst, the results of this study complicate the simple model in which the (VO)2P2O7 phase derived from VOHPO4 . 0.5H2O is responsible for the selective oxidation of n-butane. The observation that the precursor VO(H2PO4)2 can generate catalysts of high specific activity and of total selectivity to partial oxidation products might provide a useful insight into the design of a new series of high activity and high selectivity partial oxidation catalysts.

X-Ray study of a vanadium–phosphorus mixed oxide catalyst for selective butane oxidation to maleic anhydride

The radial electron distribution obtained from X-ray patterns has been used to study the structure of a poorly-crystalline vanadium–phosphorus mixed oxide (VPO) catalyst after selective oxidation of n-butane; the effective catalyst consists of a mixture of a crystallized (VO)2P2O7 phase (V4+) and an amorphous VPO phase (V5+) showing many corner-sharing VO6 octahedra.