VPO Catalysts Research Articles

Studies on a niobia-supported VPO catalyst for glycerol dehydration

Niobia-supported VPO catalysts were synthesized using the deposition–precipitation method with different VPO loadings to evaluate glycerol dehydration. The activity patterns are consistent with the acidity trends.

Carbon-supported VPO catalysts with fibrous structure: A new family of catalysts for partial oxidation reactions

The present contribution describes the synthesis of a carbon supported VPO catalyst with a fibrous structure. The synthesis method is simple, low-cost, scalable and reproducible that allows the formation of the bulk mixed oxide (VO)2P2O7 (VPP) active phase on the carbon support material. The preparation does not require any special reducing thermal treatments due to the ability of the carbon support to stabilize the VPP active phase since this carbon-based support is stable under oxidizing conditions up to around 400ºC allowing it to be used as for catalytic oxidation reactions. The stability of the carbon-supported VPP catalyst under reaction conditions was examined with in situ Raman under oxidizing/ H2 reducing cycles and n-butane oxidation conditions under both wet and dry conditions. These studies confirm the dynamic character of VPO phases, which interconvert under different conditions, and the stability of carbon-supported VPP phase under different environments. The novel carbon-supported catalyst exhibits an activity behavior like that observed for commercial catalysts that suggests a very promising catalytic material.

Supported VPO Catalysts for Maleic Anhydride by Atomic Layer Deposition

Supported VPO Catalysts for Maleic Anhydride by Atomic Layer Deposition

Increasing maleic anhydride selectivity for n-butane oxidation by Y-modified VPO catalysts

In the present study, a series of VPO catalysts with different amount of Y were prepared by organic phase method and activated in butane-air atmosphere. The catalytic performance was tested by the selective oxidation of n-butane to maleic anhydride (MA). The catalysts were characterized by SEM, TG, XRD, Raman, XPS, H2-TPR and NH3-TPD techniques. The characterization results showed that the addition of appropriate amount of Y can increase the amount and stability of the rose-like flower clusters of VPO precursors. Meanwhile, the addition of Y greatly increased the proportion of lattice oxygen on catalyst surface and changed the surface acidity of VPO catalyst. As a result, the selectivity and yield of MA were greatly improved by introducing the right amount of Y.

Influence of the Structural Characteristics of VPO Catalysts on the Temperature Sensitivity of the Oxidation Reaction of n-Butane to Maleic Anhydride

Influence of the Structural Characteristics of VPO Catalysts on the Temperature Sensitivity of the Oxidation Reaction of <i>n</i>-Butane to Maleic Anhydride

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.

Increasing Maleic Anhydride Selectivity for N-Butane Oxidation by Y-Modified Vpo Catalysts

Increasing Maleic Anhydride Selectivity for N-Butane Oxidation by Y-Modified Vpo Catalysts

Crystal Imperfections of Industrial Vanadium Phosphorous Oxide Catalysts

This study presents information about crystal imperfections in the main phase of industrial vanadium phosphorous oxide catalysts that are used to catalyze the oxidation of n-butane to maleic anhydride, being an important intermediate in the chemical industry. The mechanism of this reaction is still debated, and the catalytically active and selective surface centers have not yet been identified. The results presented are based on X-ray diffraction data obtained by both laboratory-scale and synchrotron powder diffraction experiments, as well as laboratory-scale single-crystal diffraction experiments. It has been proven that pronounced Bragg reflection broadening effects found in laboratory-scale powder diffraction patterns of industrial VPO catalysts are real and not due to an insufficient 2-θ resolution of the apparatus. In the framework of this work, a powder diffraction full profile fitting strategy was developed using the TOPAS software, which was applied to analyze the X-ray diffraction data of four differently activated industrial catalyst samples, originating from one batch after they had been catalytically tested. It was found that the reflection broadening is mainly caused by an anisotropic crystal size, which results in platelet-shaped crystallites of vanadyl pyrophosphate. A further contribution to the reflex broadening, especially for (111), was found to be a result of stacking faults perpendicular to the a direction in the crystal structure of vanadyl pyrophosphate. These results were used to elaborate on possible correlations between structural proxies and catalytic performance. A direct correlation between the extension of coherently scattering domains in the z direction and the catalyst’s selectivity could be proven, whereas the activity turned out to be dependent on the crystallite shape. Regarding the phase contents, it could be shown that sample catalysts containing a higher amount of β-VO(PO3)2 showed increased catalytic activity.

Effect of Crystalline Phases for VPO Catalysts on the Oxidation of Butane: Thermodynamics and Kinetics

Serial VPO catalysts with different compositions of crystalline phases were prepared at various activation temperatures. The characterization results revealed that the proportion of V5+ species, the acidity levels, and the surface P/V ratios of VPO catalysts showed an increasing tendency with activation temperature. The performances of n-butane oxidation for serial VPO catalysts were tested at different conditions. It was shown that the S-405 catalyst (activated at 405 °C) presented the highest conversion of n-butane (75.2%) at a GHSV of 1000 h–1 and the highest selectivity for maleic anhydride (MA) (68.9%) at a GHSV of 1800 h–1, which were attributed to S-405’s high intensities at different crystalline faces, the highest amount of acidity (5.52 mL/g), the highest number of total removed oxygen atoms (2.88 × 1021 atom g–1), and the best reducibility. In addition, the kinetic and thermodynamic analyses confirmed that the S-405 catalyst possessed the best activity and selectivity of n-butane oxidation, and the effects of Lat-O and Sur-O on the selectivity for MA were also disclosed.

The obtaining of the anhydride products by oxidation of n-pentane on the VPO catalysts

It was investigated VPMeO catalysts (Me=Fe, Mo, Te, W, Ni, Ti, La, Bi, Zr і Ag) in oxidation of n-pentane. On these catalytic samples the main products of reaction are maleic (MA), phthalic (PhA), citraconic (CA) anhydrides, carbon oxides and in-significant quantity of acetic and of acrylic acids. It was established that a change of physical-chemical properties of the VPMeO catalysts affects a course of reaction of n-pentane oxidation. It was determined that the introduction of additives into the basic VPO composition and its content influences a phase composition, a morphol-ogy, acidic properties of catalyst surface, a crystallization temperature of active component and a oxidation degree of vanadium in it. It was established that addi-tives in the VPO sample may be distributed in two ways: a) evenly, high disperse (Fe, Te, Ni, Ag ions), b) with formation of X-ray amorphous additive phosphate phase (Ti, Bi, La, W, Zr ions). Additives that decrease temperature of the active phase formation of a catalyst and increase temperature of its oxidation (Fe, Ti, Bi, Zr ions) positively influence the life of exploitation of catalytic pattern without losing its se-lectivity in the n-pentane oxidation. Additives that reduce the O 1s-electrons energy and increase an oxygen content (O/(V+P+Me)) on the VPO composition surface en-hance the specific rate of the hydrocarbon oxidation. A growth of phosphorous con-tent on the surface of synthesized compositions also contributes to the increase of the time of their stable work. The influence of ratio of Bronsted and Lewis acid cen-ters on surface of the VPМеO pattern on a selectivity of anhydrides production was established. The growth of acidic centers content on the surface of patterns increas-es the CA selectivity. The rise of quantity of Lewis centers favors the PhA formation while the MA selectivity reduces in the reaction products. According to experimental data the modification of the VPO catalyst is affect its physical-chemical and cata-lytic properties. The change of defined physical-chemical properties allows to regu-late a process of the n-pentane oxidation in the direction of formation of one of the anhydrides.

Investigation of the Effect of SO2 and H2O on VPO-Cr-PEG/TiO2 for the Low-Temperature SCR de-NOx

A catalyst 0.1VP(0.2)O-Cr(0.01)-PEG(1×10-4)/Ti for low-temperature SCR de-NOx was developed and the effects of SO2 and water vapor on its catalytic activity were investigated. Cr doping increases the molar ratio of V5+ to V4+ on the VPO catalyst, which promote oxidation of NO to NO2 and catalytic activity of the VPO catalyst. An appropriate amount of redox-coupled V5+/V4+ also promotes catalytic activity of the VPO catalyst. The denitration efficiency over 0.1VP(0.2)O-Cr(0.01)-PEG(1×10-4)/Ti was above 98% at 150-350℃. Cr doping also promotes the generation of Lewis acid around V5+ center and BrOnsted acid (P-OH) on the VPO catalysts. The strong surface acidity of 0.1VP(0.2)O-Cr(0.01)-PEG(1×10-4)/Ti could restrain the adsorption of and oxidation of SO2. Characterization (FT-IR, TG) results indicate that nearly no sulfate was deposited on the surface of 0.1VP(0.2)O-Cr(0.01)-PEG(1×10-4)/Ti after activity test. The competitive adsorption of water molecules and nitric oxide on the catalyst surface decreases the catalytic activity of the VPO catalyst. The addition of Cr and PEG could increase the surface area and the exposure of active site of VPO catalyst, which also increase the unoccupied active sites of VPO catalysts in the presence of water vapor. The catalyst 0.1VP(0.2)O-Cr(0.01)-PEG(1×10-4)/Ti for low-temperature SCR de-NOx shows high catalytic activity and exhibits high resistance to SO2 and water vapor.

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.

Synthesis and catalytic activity of vanadium phosphorous oxides systems supported on silicon carbide for the selective oxidation of n-butane to maleic anhydride

VPO catalysts supported on silicon carbide were prepared using the solvothermal method. A decrease the reaction temperature by 70 °C under application of SiC, prepared from sucrose and fumed silica, as a support for an active VPO phase compared to a bulk VPO catalyst prepared under similar synthesis conditions was demonstrated. A twofold increase of the productivity towards maleic anhydride was reached. The higher specific reaction rate of n-butane oxidation over prepared supported VPO-SiC systems compared to the bulk VPO catalyst was shown. Use of an environmentally safe support allowed to reduce an amount of active vanadium-containing phase, and an application of the solvothermal method decreased the synthesis duration and an amount of organic solvent compared to the traditional approaches to the synthesis of VPO systems.

Effect of metal-doped VPO catalysts for the aldol condensation of acetic acid and formaldehyde to acrylic acid.

Mo, W, Cr, La, and Ce additives were introduced into a VPO catalyst, and the resulting catalysts were investigated for the gas-phase aldol condensation reaction of formaldehyde and acetic acid. XRD, FT-IR, SEM, NH3-TPD, Py-IR, and BET were used to characterize the structure and properties of the catalysts. After the addition of the third component, the crystal structure changed to a certain extent; the surface acidity of the catalyst changed, and the conversion of acetic acid and the selectivity of acrylic acid also showed different degrees of influence. The acidity of the catalyst was the main factor affecting the catalytic performance. When La was added to the catalyst, the selectivity for acrylic acid was the highest, and the stability of the catalyst also improved. It is presumed that B acid is the main active site of this reaction, and a moderate amount of acid is favorable to facilitate the reaction.

Deactivation behavior on VPO and VPO-Zr catalysts in the aldol condensation of methyl acetate and formaldehyde

Improvement in activity for the Zr doped VPO catalyst appears to be a general observation in aldol condensation of methyl acetate and formaldehyde to methyl acrylate. The deactivation behavior significantly differs between VPO and VPO-Zr catalysts. In this work, the deactivation/reactivation behavior and carbon formation in aldol condensation on VPO and VPO-Zr catalysts were investigated. After partial deactivation of the conversion of methyl acetate and formaldehyde to methyl acrylate with a fixed-bed reactor, the deactivated catalysts were fractionated from the reactor. The samples were then analyzed by spectroscopic techniques, gas adsorption measurements and thermogravimetry. The VPO-Zr catalyst showed differences in deactivation/reactivation behavior from the VPO catalyst under same reaction conditions. This finding could be partially ascribed to different coke locations in the deactivated catalysts with varied porosity. The IV4+/IV5+ ratio of V atom remarkably differed among fresh, used, and refreshed catalysts, which could be a key factor affecting catalyst deactivation. The relations of carbon formation and reaction time were fitted by empirical Voorhies equation over VPO and VPO-Zr catalysts. 1

Synthesis of vanadium phosphorus oxide catalysts promoted by iron-based ionic liquids and their catalytic performance in selective oxidation ofn-butane

Catalytic performance of undoped and Fe-doped VPO catalysts.

Effect of Metal Ion in Bulk VPO in Aldol Condensation of Formaldehyde and Methyl Acetate to Methyl Acrylate

The metal ion effect on the bulk VPO was investigated using the aldol condensation of methyl acetate and formaldehyde to methyl acrylate. The VPO catalyst with 2 mol % Zr exhibited the highest methyl acrylate formation rate of 18.6 μmol·g–1·min–1. A combination of X-ray diffraction, BET, TPD, XPS, and EPR, together with activity testing data, was utilized to analyze and explain the origin of the metal ion effects. The promotion of methyl acrylate production by the intercalation of Zr was associated with its interaction with the VPO phase. When Zr concentration attained 2 mol %, the Zr4+ ion was distributed in the surface adjacent layers and partially incorporated into VPO species, giving rise to a higher fraction of the β-VOPO4 entity and a relatively higher amount of the badly distorted (VO)2P2O7 entity. These findings suggested that both β-VOPO4 and distorted (VO)2P2O7 phases were required for the formation of methyl acrylate.

On the Correlation of Structure and Catalytic Performance of VPO Catalysts

Correlations are reported between structure and performance of current, state-of the art, industrial maleic anhydride production catalysts. Characterization was done with fully equilibrated catalysts to achieve reliable correlations of structure and activity. The kinetic analysis of the investigated VPO catalysts revealed differences in activities but comparable activation energies indicating active on-surface sites of similar nature but of different concentrations. One major crystalline component, VPP, and six minority compounds were identified, hence, VPP alone cannot be reasonably correlated with catalytic activity. XPS revealed elevated V5+ concentrations, differences in the P/V ratios, and the P surface content correlates negatively with catalytic activity. Hence, all crystalline phases must have noncrystalline, surface-layers containing V5+ species and varying P amounts. Deconvolution of Raman spectra identified four major vanadyl phosphate species of changing concentrations. One species, αI-VOPO4, was absent from the XRD patterns of the two most active catalysts, indicating that it is noncrystalline and, hence, integral part of the amorphous surface layers. These spectral/structural differences correlated with catalyst activity. The most active catalysts contain relatively more crystalline VPP, in accordance with literature. These active catalysts also have a higher amount of the amorphous V5+ surface species, α-VOPO4. All other V5+ species detected, δ- and g-VOPO4 are crystalline and negatively affect catalyst activity. Again in agreement with literature, the surface P/V ratio negatively correlates with catalyst activity, while the reported correlation of the V5+/V4+ ratio with activity was not observed. The multi-method approach chosen here, always strongly favored by H. Knozinger, generated a detailed picture of the compositional differences between the bulk and surface regions of the investigated catalysts and revealed how these differences correlate with catalytic performance.

Formation of Phthalic Anhydride by Diels–Alder Reaction During n-Pentane Oxidation on VPO Catalysts and Control of the Process Selectivity

The oxidation of n-pentane at VPO catalyst was studied in two-step catalytic reactor, and it was shown that phthalic anhydride is formed by Diels–Alder reaction between maleic anhydride and 1,3-butadiene. The selectivity in phthalic anhydride was increased by the introduction of additives that increase the Lewis acidity of the catalyst surface to the basic VPO composition, by reducing the temperature in the lower part of catalytic reactor, and layer-by-layer loading various VPO catalysts with different acidic properties into the reactor.

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.