Vanadyl Hydrogen Phosphate Research Articles

Synthesis and properties of VPO catalysts for oxidation of n-butane to maleic anhydride

Bulk and supported vanadium-phosphorus oxide VPO catalysts were synthesized by traditional and barothermal methods. It was shown that the use of aerosil as a support for the VPO phase, depending on the time of its introduction into the reaction mixture. It can lead to the formation of catalyst precursor of vanadyl hydrogen phosphate VOHPO4 0.5H2O, or a phase of vanadyl pyrophosphate (VO)2P2O7 as already the catalytically active phase for selective oxidation of n-butane to maleic anhydride. The use of a modified aerosil gel formed from pyrogenic aerosol, as a support for the VPO phase, leads to the formation of VOHPO4∙0.5H2O phase. It has been found that the nature of support affects the features of formation of VOHPO4∙0.5H2O phase, in particular, the ratio of crystallographic planes in resulting VPO phase. The use of aerosil as a support leads to a decrease in the relative content of the basal plane, while use of aerosil gel leads to an increase in the relative content of the basal plane in applied VPO phase. The catalytic properties of bulk and supported VPO samples were studied in the selective oxidation of n-butane to maleic anhydride in standard (1.7 vol.%) and enriched (3.4 vol.%) n-butane mixtures. It has been found that in an enriched n-butane mixture for bulk samples, the n-butane conversion and selectivity for maleic anhydride are sharply reduced. It has been found that supported VPO samples have a higher specific rate of n-butane oxidation and higher productivity compared to bulk samples. It was shown that use of barothermal synthesis and aerosol gel as a support made it possible to increase the selectivity of maleic anhydride, which is associated with an increase in the relative content of the basal plane of VPO phase. The achieved improved catalytic properties of VPO catalysts supported with aerosol gel make recycling technology promising. This can make the production of maleic anhydride more economical.

Application of vanadyl hydrogen phosphate/KIT-6 composites as a catalyst for dehydration of sucrose

In this research work, the synthesis of well-ordered mesoporous structure KIT-6 silica-supported vanadyl hydrogen phosphate hemihydrate (VHP/KIT-6) catalysts possessing varying surface area was carried out, followed by being utilized in the dehydration of sucrose to 5-hydroxymethylfurfural (5-HMF) as a solid acid catalyst. The fabrication of two different VHP/KIT-6 catalysts was conducted using the impregnation approach through V2O5 reduction in alcoholic media. Then, the catalytic behavior presented by the prepared VHP/KIT-6 samples was compared with unsupported VHP and unsupported vanadyl pyrophosphate (VPP), which was prepared through the calcination of VHP precursor at 400 °C for 24 h. The prepared catalysts were characterized by different analyses, including ICP-OES, BET, XRD, NH3-TPD analysis, FT-IR spectroscopy, as well as SEM and TEM techniques. Furthermore, important parameters, including the catalyst type and weight, sucrose amount, time, temperature, and the type of solvent on sucrose dehydration, were also studied. It was observed that the highest catalytic activity in the dehydration reaction (5-HMF yield: 67%, sucrose conversion: > 94%) could be provided by the catalyst possessing the highest surface area. Moreover, the catalyst reusability was examined for consecutive four times, based on which no considerable change in the catalytic activity was observed.

VOHPO4.5H2O/KIT-6 composites: Preparation and their application in extractive and catalytic oxidation desulfurization of benzothiophene and dibenzothiphene

Abstract The catalytic activity of vanadyl hydrogen phosphate hemihydrate (VPO) for deep desulfurization of a model fuel was explored. For this purpose, a series of composites containing different amounts of vanadyl hydrogen phosphate and well-ordered mesoporous silica (KIT-6) were prepared and their structural and textural characterizations were carried out using SEM, TEM, ICP-AES, FT-IR, N2 adsorption-desorption, XPS and XRD techniques. It was found that the catalyst with maximum vanadium phosphate loading (VPKA) shows the highest catalytic activity in dibenzothiophene (DBT) removal through oxidative desulfurization (ODS) process. Different reaction conditions such as temperature, the mole ratio of H2O2:DBT, and catalyst loading were investigated in the catalytic system.

Development of VPO catalysts supported on mesoporous modified material based on an aerosil gel

Supports modified with different organic substances and vanadium-phosphorus oxide catalysts on their basis were developed with the use of the barothermal treatment of aerosil. It was found that the anchoring of a modifier to the surface of a support occurred with the participation of OH groups. The modifier content found from differential thermal analysis data depended on the pore structure of the support and the nature of the modifier. It was demonstrated that, with the use of a modified support in the catalysts, a vanadyl hydrogen phosphate hemihydrate phase—the precursor of a catalyst for the selective oxidation of hydrocarbons—was formed; under reaction conditions, this precursor was converted into vanadyl pyrophosphate. On supporting, the phase grew from the micropore bulk and covered only part of the support surface. The synthesized catalysts exhibited high activity and selectivity in the reaction of n-butane oxidation under hydrocarbon-rich conditions and also in the oxidative dehydrogenation of ethane.

Effect of Activation Duration in 1% Oxygen in Nitrogen Balance on Vanadium Phosphate Catalysts Synthesized via Sesquihydrate Route

Four vanadium phosphate catalysts were synthesized via vanadyl hydrogen phosphate sesquihydrate (VOHPO4•1.5H2O) route and activated at different duration (6, 18, 30 and 75 hours) in 1% oxygen/ nitrogen (1% O2/N2) mixture at 733K. The increased activation duration led to a decreased in the specific surface area of the catalysts. The increased activation duration increased the average oxidation state of vanadium. The surface morphologies of the catalysts exhibited plate-like crystals that agglomerated into clusters with folded edges. Temperature-programmed reduction (TPR) showed that increasing activation duration led to an increased in total amount of oxygen removed from the catalysts. Temperature-programmed desorption (TPD) showed that the total amount of oxygen desorbed from the catalysts increased as the activation duration increased. Catalytic performance revealed that all the catalysts produced possessed rather low in terms of activity but high in terms of selectivity.

Effect of varying reflux durations on the physico-chemical and catalytic performance of vanadium phosphate catalysts synthesized via vanadyl hydrogen phosphate sesquihydrate

A series of vanadyl pyrophosphate, (VO) 2P 2O 7, catalysts prepared via vanadyl hydrogen phosphate sesquihydrate precursors (VOHPO 4·1.5H 2O) was calcined in a reaction flow of 0.75% n-butane in air mixture at 733 K for 18 h. The precursors have been synthesized by refluxing vanadyl phosphate dihydrate (VOPO 4·2H 2O) with 1-butanol for different lengths of time, i.e. 8, 15 and 24 h, and the produced catalysts were denoted as VPO s-R8, VPO s-R15 and VPO s-R24, respectively. X-ray diffraction (XRD) patterns of the three catalysts showed similar diffraction pattern, comprised of a well-crystallized (VO) 2P 2O 7 phase. Brunauer–Emmett–Teller (BET) surface area measurements showed that VPO s-R24 has the highest specific surface area, i.e. 31 m 2 g −1 followed by 27 m 2 g −1 and 19 m 2 g −1 for VPO s-R15 and VPO s-R8, respectively. Inductively coupled plasma (ICP) analyses indicated that the P/V atomic ratios of these catalysts were in the optimum range in producing (VO) 2P 2O 7 phase. A small increment in the average oxidation number of the vanadium was observed as the precursor reflux duration increased. Scanning electron microscope showed the secondary structures of the catalysts with plate-like crystals in different sizes, which were agglomerated into rosette-shape clusters. The total amount of oxygen desorbed from the catalysts increased as the precursor reflux duration increased. Temperature-programmed reduction (TPR) in H 2 profiles of all the catalysts gave three reduction peaks. VPO s-R8 gave the highest total amount of oxygen removed from V 5+/V 4+ phase followed by VPO s-R15 and VPO s-R24. Catalytic tests revealed that the catalyst with lower precursor reflux duration exhibited higher selectivity but lower activity and vice versa.

Innovative process for the synthesis of vanadyl pyrophosphate as a highly selective catalyst for n-butane oxidation

Vanadyl pyrophosphate (VPO) catalysts are used for the selective oxidation of light alkanes, which are based on vanadyl hydrogen phosphate hemihydrate (VOHPO 4·0.5H 2O) as the precursor. Catalyst precursor with well-defined crystal size has been successfully synthesized for the first time, using a simple one-step high-pressure autoclave that was surfactant-free and water-free method with significantly lower temperature and shorter reaction time. VOHPO 4·0.5H 2O was prepared from V 2O 5 using an isobutanol, 1-pentanol, 1-heptanol and 1-dectanol as both solvent and reducing agent at elevated temperatures (100, 120, and 150 °C). This new method significantly reduced the preparation time and lowered production temperature (50%) of catalyst precursor (VOHPO 4·0.5H 2O) when compared to conventional hydrothermal synthesis methods. VOHPO 4·0.5H 2O can be obtained at temperature far below 150 °C. It was found that the presence of 1-heptanol and 1-decanol in the reaction mixtures is crucial for obtaining a well-defined crystal size of precursor phase and solely generated impurity [VO(H 2PO 4) 2]. Our findings show that both the phase composition and morphology of vanadium phosphate can be influenced by the use of different reducing agents and temperatures during the preparation process. This new methodology produces catalysts with a much higher surface area ( ca. 23 m 2 g −1) compared with those materials prepared by slow hydrothermal synthesis ( ca. 9.5 m 2 g −1). Finally, the yield of maleic anhydride was significantly increased from 21.3% for conventional catalyst to 37.9% for the new solvothermal catalyst.

Novel Synthesis Techniques for Preparation of Ultrahigh-Crystalline Vanadyl Pyrophosphate as a Highly Selective Catalyst for n-Butane Oxidation

The vanadyl hydrogen phosphate hemihydrate (VOHPO 4 ·0.5H 2 O), with well-defined crystal size, has been successfully synthesized for the first time, using a simple one-step solvothermal process that was free of surfactants and water and had a short reaction time and low temperature. The synthesis was performed via the reaction of V 2 O 5 and H 3 PO 4 with an aliphatic alcohol (1-propanol or 1-butanol) at high temperatures (373. 393, and 423 K) in a high-pressure autoclave. The mixture of reactions directly gave the VOHPO 4 ·0.5H 2 O, which is a valuable commercial catalyst precursor for the selective oxidation of n-butane to maleic anhydride. The catalyst precursors were dried by microwave irradiation. The reaction conditions (by varying the reducing agent and reaction temperature) were used further for optimization of the crystallite size, surface area, morphology, and activity of the nanostructure of vanadium phosphate oxide [(VO) 2 P 2 O 7 ] catalyst. This new method significantly reduced the preparation time and lowered the production temperature (50%) of catalyst precursor (VOHP0 4 ·0.5H 2 O), when compared to conventional hydrothermal synthesis methods. The as-prepared (VO) 2 P 2 O 7 catalyst under various conditions exhibited remarkably different physical and chemical properties, indicating the potential of the suggested method in tuning the crystalline structure and surface area of (VO) 2 P 2 O 7 to improve its catalytic performance. It was found that the length of the carbon chain in an alcohol and reaction temperature in the solvothermal condition had a great impact on the chemical and physical properties of resulting catalysts. Interestingly, there was no trace of YO(H 2 PO 4 ) 2 , which is an impurity noted to be readily formed under solvothermal preparation conditions. The precursors and catalysts were characterized using a combination of powder X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area measurement, scanning electron microscopy (SEM), and temperature-programmed reduction in hydrogen (H 2 - TPR). A correlation between the surface area of the catalyst and the activity was observed. Finally, the yield of maleic anhydride was significantly increased from 21% for conventional catalyst to 38% for the new solvothermal catalyst.

Solvothermal synthesis of vanadium phosphate catalysts for n-butane oxidation

In this paper, we have developed a simple, low-cost, template-free and surfactant-free solvothermal process for synthesis of vanadyl hydrogen phosphate hemihydrate (VOHPO 4·0.5H 2O) with well defined crystal size. The synthesis was performed by reaction of VPO 4·2H 2O with an aliphatic alcohol (isobutyl alcohol, 1-pentanol, 1-hexanol, 1-heptanol or 1-decanol). This afforded well crystallized VOHPO 4·0.5H 2O by solvothermal methods at 120 °C temperature. This new method significantly reduced the preparation time and lowered production temperature (50%) of catalyst precursor (VOHPO 4·0.5H 2O) when compared to conventional hydrothermal synthesis methods. By varying the reducing agent, the solvothermal evolution process from layered tetragonal phase VOPO 4·2H 2O to orthorhombic phase VOHPO 4·0.5H 2O was observed. It was found that the length of carbon chain in an alcohol in the solvothermal condition had a great impact on chemical and physical properties of resulting catalysts. Interestingly, there was no trace of VO(H 2PO 4) 2 an impurity noted to be readily formed under solvothermal preparation condition. Therefore, this study introduces a more facile synthetic pathway to V(III) compounds. In addition, the microwave-synthesized catalysts exhibited some properties superior to those of conventionally synthesized catalyst such as better stability, crystallinity, and catalytic activity in the production of maleic anhydride. The characterization of both precursors and calcined catalysts was carried out using X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES), N 2 physisorption, temperature programmed reduction (H 2-TPR) and scanning electron microscopy (SEM). The XRD pattern of the active catalyst prepared by this solvothermal method confirmed the presence of smaller crystal size (between 6 and 13 nm along 0 2 0 planes) of vanadium phosphate catalyst with higher specific surface area. Finally, the yield of maleic anhydride was significantly increased from 29% for conventional catalyst to 44% for the new solvothermal 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.

Transformation of nano-sized vanadyl hydrogen phosphate hemihydrate crystallites to vanadyl pyrophosphate during activation in the presence of n-butane and oxygen

The transformation of nano-sized VOHPO 4⋅0.5H 2O crystallites (340 nm × 40 nm) to (VO) 2P 2O 7 was examined. The crystalline structure of VOHPO 4⋅0.5H 2O quickly collapsed to form an oxidized amorphous phase within 1 h (reaction conditions: 1.5% n-butane, 17% O 2, and 81.5% He at 663 K), followed by gradual crystallization to (VO) 2P 2O 7, accompanied by the formation of sharply angular nano-sized crystallites (about 50 nm). No crystalline phases other than (VO) 2P 2O 7 were formed during this transformation. This is quite different from the transformation of large and thick VOHPO 4⋅0.5H 2O crystallites, in which α II-VOPO 4 and δ-VOPO 4 have been detected in the resulting catalyst. The selectivity to maleic anhydride increases during the transformation as a result of the formation of (VO) 2P 2O 7. The activity increases in the early stages of the transformation, then decreases with time. The decrease in activity is caused by gradual reduction of the catalyst surface during the transformation.

Solid-state reactions in V x O y (NH4VO3)-P2O5 and V x O y (NH4VO3)-(NH4)2HPO4 closed systems

Solid-state reactions in V x O y (NH4VO3)-P2O5 and V x O y (NH4VO3)-(NH4)2HPO4 closed systems can be used to synthesize vanadyl hydrogen phosphate at 300°C and a variety of ammonium vanadium phosphates at lower and higher temperatures.

Pressure Calcination of VPO Catalyst

Calcination and activation are critical steps in the transformation of vanadyl hydrogen phosphate hemihydrate (VPO precursorVOHPO4·0.5H2O) to vanadyl pyrophosphate (VPO catalyst(VO)2P2O7). Whereas many published kinetics studies for butane oxidation over VPO demonstrate the relationship between catalytic activity and reaction conditions, the relationship between calcination conditions and activity has only received superficial attention in the open literature. This study delineates the operating boundaries in which to optimize catalytic performance. The control variables included temperature, pressure, time, and gas-phase composition (principally, oxygen and water vapor partial pressures), and their effects on surface area, oxidation state, and catalytic activity were measured. The experimental plan included several hundred calcination experiments. The commercial-design operating conditions were chosen as the base-case conditions: temperatures from 330 to 460 °C, pressures up to 6 atm, and oxygen and water concentrations between 0 and 20% and 0−3%, respectively. The optimal temperature was found to be about 390 °C. Catalyst performance declines with increasing pressure, but the pressure−time relationship allows a range of conditions: optimal catalytic performance was achieved within 20 h of calcination at atmospheric pressure, but it took less than 4 h to achieve this high level of performance when precursor was calcined at 3.5 atm (at longer times, the catalytic performance deteriorated). Co-feeding steam (in the presence of oxygen) generally had a deleterious affect on catalyst performance and, in particular, maleic anhydride yield. Moreover, oxidation states higher than 4.5 were achieved with steam/oxygen mixtures in as little as 20 h. Catalyst performance declined linearly with oxidation state for precursor calcined to vanadium oxidation states exceeding 4.5.

Structure and properties of Cr promoted VPO catalysts

In this work, VOHPO 4·1/2H 2O was synthesized from V 2O 5 and anhydrous H 3PO 4 in an alcohol mixture. The Cr(NO 3) 3 in varying proportions was either added during synthesis or impregnated on the dried vanadyl hydrogen phosphate. The solid features and the catalytic behavior of these formulations were affected by both the addition procedure and the Cr load. The effect of Cr upon the catalyst structure and the development of acid and redox sites were studied through XRD, TPR, LRS and FT-IR. Acetonitrile was used as a probe molecule to ascertain the Lewis acid strength of the surface sites present in all the VPO formulations. It is concluded that the impregnation of Cr is more effective to increase the concentration of very strong Lewis acid sites, while this method and co-precipitation are similarly effective in the generation of V 5+/V 4+ redox couples. Both types of sites in adequate balance are needed to optimize the catalytic behavior of VPO formulations.

Novel Method for Synthesis of Well-Crystallized Vanadyl(2+) Hydrogen Phosphate Hemihydrate: Crystallization in Organic Media Containing a Small Amount of Water

Vanadyl(2+) hydrogen phosphate hemihydrate (VOHPO 4 .0.5H 2 O, VHP) that had high crystallinity and showed a strong (001) peak in its X-ray diffractometry (XRD) pattern was synthesized via thermal treatment of a mixture of vanadyl(2+) acetylacetonate and triethyl phosphate at 200°C in organic solvents that contained a small amount of water. Scanning electron microscopy observations revealed that the product consisted of thin plates (2-4 μm wide and 20 μm. The present VHP sample was topotactically transformed to divanadyl(2+) pyrophosphate, which showed a strong (020) XRD peak, via thermal treatment in a nitrogen atmosphere at 500°C.

Water-vapor-induced transformation of two-dimensional vanadyl hydrogen phosphate hemihydrate, VOHPO4·0.5H2O, to one-dimensional tetrahydrate, VOHPO4·4H2O

The catalytically important, two-dimensional vanadyl hydrogen phosphate hemihydrate, VOHPO4·0.5H2O phase was transformed to the one-dimensional vanadyl hydrogen phosphate tetrahydrate, VOHPO4·4H2O for the first time by mild treatment with water vapor. It was also observed that for three samples of VOHPO4·0.5H2O prepared under different conditions, the rate of transformation to the tetrahydrate was markedly affected by morphology and crystallinity. The tetrahydrate phase could also be re-transformed to the hemihydrate by applying high vacuum or by heating at 120 °C, but this was accompanied by significant loss in crystallinity.

Catena-Poly[[oxo(1,10-phenanthroline)vanadium(IV)]-di-μ-hydrogenphosphato

The crystal structure of [VO(HPO4)(C12H8N2)]n consists of a bis­(vanadyl hydrogen phosphate) skeleton that assumes a linear ribbon conformation; the nitro­gen heterocycles that chelate to the VIV atoms are connected to the sides of the flat ribbon. The compound is isostructural with the hydrogen arsenate, whose structure has been reported by Hou et al. [Inorg. Chem. Commun. (2004). 7, 128–130]. The asymmetric unit, with the exception of one O atom, lies on a mirror plane

Synthesis and characterisation of vanadyl pyrophosphate catalysts via vanadyl hydrogen phosphate sesquihydrate precursor

Vanadyl pyrophosphate catalysts prepared via VOHPO 4·1.5H 2O precursor were calcined in a reaction flow of n-butane(0.75%)/air mixture at 673 K for a duration of 10, 30 and 75 h. These catalysts were denoted as VPS10, VPS30 and VPS75. Increasing the duration of calcination process led to the appearance of V 5+ phase (VOPO 4), which could be seen in XRD patterns at 2 θ=29.1° and an increment of the average oxidation number of the vanadium. It also led to a decrease in the total surface area from 27.2 (VPS10) to 22.1 (VPS30) and 17.7 m 2 g −1 (VPS75). Scanning electron microscope showed the secondary structure of the catalysts produced, consisting of plate-like crystals in different sizes, which were agglomerated into the characteristics of rosette-shape clusters. However, at closer and higher magnification show the surface of each platelet getting cracked and rougher as the pretreatment time increased. The nature of the oxidants in/on the catalysts was investigated by using O 2 TPD and TPR in H 2. VPS10 gave a broad peak having an onset at 830 K and a peak maximum at 970 K with a shoulder at 1005 K. An increment of the calcination duration to 30 h (VPS30) gave a peak at 974 K and the shoulder at 1005 K (VPS10) becomes a peak at 1006 K. This new phase peak was found to be more dominant at 1018 K as the calcination duration increased to 75 h (VPS75), whereas the first peak reduced to a shoulder at 980 K. The forming of a new peak may be due to the additional phase (i.e. V 5+), which is shown in the XRD and chemical analysis. TPR profiles of all the catalysts gave three peaks. VPS10 gave the highest total amount of oxygen removed, i.e. 2.15×10 21 atom g −1 compared to 1.94×10 21 and 1.72×10 21 atom g −1 for VPS30 and VPS75, respectively.

In situ X-ray diffraction studies of the crystallization of VOHPO4·0.5H2O

Time resolved in situ X-ray diffraction was used to study the formation of vanadyl hydrogen phosphate hemihydrate (VOHPO4·0.5H2O) in organic media. This technique has identified a new phase at early synthesis times (less than 300s) at a d-spacing of 7.5 and 3.1Å. The feature at 7.5Å shifted to 6.7Å during the first 20min of synthesis and this phase collapsed as VOHPO4·0.5H2O formed. Thin symmetrical platelets of 10μm×10μm dimensions were observed when samples were recovered after short synthesis times. Energy dispersive X-ray analysis indicated that these platelets contained vanadium and phosphorus. The platelets appeared to delaminate possibly associated with the strain generated when the d-spacing shifted from 7.5 to 6.7Å, from which growth of the VOHPO4·0.5H2O into the familiar rosette morphology occurs.

Surfactant assisted organization of an exfoliated vanadyl ortho phosphate to a mesostructured lamellar vanadium phosphate phase

The catalytically important vanadyl hydrogen phosphate hemihydrate, VOHPO 4 · 0.5H 2O phase with strong interlayer bonding has been exfoliated for the first time in DMF-water mixture and the exfoliated hemihydrate phase has been reorganized into a novel mesostructured lamellar VPO phase having the same V–P–O connectivity as in VOHPO 4 · 0.5H 2O, using octadecyl amine as the structure directing template.