Vanadium Phosphate Catalysts Research Articles

Mechanosynthesis and mechanochemical treatment of bismuth-doped vanadium phosphorus oxide catalysts for the partial oxidation of n-butane to maleic anhydride

Three Bi-doped vanadyl pyrophosphate catalysts were prepared via dihydrate route (VPD method), which consisted of different preparation methods including mechanosynthesis, mechanochemical treatment, and the conventional reflux method. The catalysts produced by the above three methods were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and temperature programmed reduction (TPR). Catalytic evaluation for the partial oxidation of n-butane to maleic anhydride (MA) was also carried out. The XRD patterns of all the Bi-doped catalysts showed the main peaks of pyrophosphate phase. Lower intensity peaks were observed for the mechanochemically treated Bi-doped catalyst (VPDBiMill) with two additional small peaks corresponding to the presence of a small amount of V 5+ phase. The TPR profiles showed that the highest amount of active oxygen species, i.e, V 4+−O − pair, responsible for n-butane activation, was removed from VPDBiMill. Furthermore, from the catalytic test results, the graph of selectivity to MA as a function of the conversion of n-butane demonstrated that VPDBiMill was the most selective catalyst. This suggests that the mechanochemical treatment of vanadium phosphate catalyst (VPDBiMill) is a potential method to improve the catalytic properties for the partial oxidation of n-butane to maleic anhydride.

Laboratory set-up to pilot plant investigations on vapour phase ammoxidation of 2,6-dichlorotoluene

The scaling-up of the gas phase catalytic ammoxidation of 2,6-dichlorotoluene (DCT) to 2,6-dichlorobenzonitrile (DCBN) over VPO catalysts in a pilot plant was investigated. The focus was led on the process variables that might be critical to the design of a commercial manufacturing facility. Furthermore, catalyst scale-up, catalyst stability and a development of a product separation section particularly for the solid reaction products was studied. The scaling-up of the gas phase catalytic ammoxidation of 2,6-dichlorotoluene (DCT) to 2,6-dichlorobenzonitrile (DCBN) over vanadium phosphate catalyst in a pilot plant was investigated. The focus was led on the process variables that might be critical for designing a commercial manufacturing unit, in particular on evaluation of reaction temperature that may cause hot spots, temperature gradients, etc. Furthermore, catalyst scale-up, catalyst stability and a development of a product separation section particularly for the solid reaction products was studied. The up-scaling of the process using 100 ml of catalyst was investigated both by reactor simulations and catalytic experiments. For reactor simulations, a pseudo-homogeneous model of a fixed bed tubular reactor with negligible axial and radial diffusion operating in the steady-state was applied. The kinetic model based upon the Langmuir–Hinshelwood mechanism was used. The results showed that the temperature control will be crucial in scaling-up of the process. The influence of the reactor geometry, reactor operating conditions and catalyst stability on the reactor performance is discussed. The possibility of an appearance of hot spots in the reactor is addressed. High selectivity of DCBN (85%) can be achieved for an extended period of time. A good agreement between experimental and simulation results is observed.

Preparation, characterization and catalytic properties of promoted vanadium phosphate catalysts

Abstract Various transition metal promoted (M = Co, Cr, Mo) vanadium phosphate (VPO) catalysts were prepared using two different supports such as TiO 2 (anatase) and γ-Al 2 O 3 . These samples were characterized by N 2 -adsorption, X-ray diffraction (XRD), FTIR-spectroscopy, UV–DRS and acidity measurements by potentiometric titrations. The catalytic properties were checked in the heterogeneous catalytic ammoxidation of 2-chloro benzaldehyde to 2-chloro benzonitrile as a model reaction. The characterization revealed the formation of (VO) 2 P 2 O 7 in all the catalysts. However, the formation of crystalline proportions of mixed oxides was not observed. The catalytic ammoxidation runs revealed a significant effect of the promoter metals on the catalytic performance of the catalysts. A considerable increase in activity and selectivity was observed over promoted VPOs compared to bulk VPO solids. Almost complete conversion of 2-chloro benzaldehyde and about 90% selectivity of nitrile were observed using a 25%VPCrO/TiO 2 catalyst.

Mechanochemical-treated Cr-promoted Vanadyl Pyrophosphate Catalyst for n-Butane Oxidation to Maleic Anhydride

Cr-promoted vanadium phosphate (VPO) catalyst was synthesized by mechanotreating VOHPO4·0.5H2O in cyclohexane for 2 hr using a high energy planetary ball miller followed by calcination in a flow of n-butane/air mixture at 673 K. The physico-chemical properties of the sample were investigated by several characterization techniques such as BET, XRD, redox titration, SEM, and TPR. The data were compared to the unmilled material. BET surface area measurement of the milled catalyst showed that it possesses higher surface area (13.2 m2 g−1) compared to the unmilled catalyst (6.4 m2 g−1). Milling also caused a slight increment in the average oxidation state of vanadium as well as the percentage of V5+ oxidation state. XRD pattern of the milled material revealed that the major diffraction peaks were broadened thus indicating a reduction of particle size. SEM micrographs showed the lost in the blossom morphology and the formation of layer packages, with more circular particles in the milled catalyst. The amount of active lattice oxygen species being removed from V4+-O− pairs increased significantly for mechanochemical treated Cr-doped VPO catalyst leads to the enhancement of the catalytic activity for n-butane oxidation to maleic anhydride.

Influence of Bi–Fe additive on properties of vanadium phosphate catalysts for n-butane oxidation to maleic anhydride

The physico-chemical and catalytic properties of three ways of modified catalysts were studied, i.e. (i) the addition of both Bi and Fe (nitrate form) during the refluxing VOPO 4·2H 2O with isobutanol (Catalyst A), (ii) the simultaneous addition of BiFe oxide powder in the course of the synthesis of precursor VOHPO 4·0.5H 2O (Catalyst B) and (iii) the mechanochemical treatment of precursor VOHPO 4·0.5H 2O and BiFe oxide in ethanol (Catalyst C). It was found that surface area of the modified catalysts has increased except Catalyst B. The reactivity of the oxygen species linked to V 5+ and V 4+ was studied by using H 2-TPR, which also affected the catalytic performance of the catalyst. The conversion of n-butane decreases with an increment of oxygen species associated with V 5+.

Synthesis and Characterization of Ni-Doped Vanadium Phosphorus Oxide Catalysts

The effect of Ni doping (1%, 2%, and 5%) on vanadium phosphate catalysts prepared via VOPO4·2H2O was described and discussed. At low levels, the Ni dopant significantly enhanced the amount of the active lattice oxygen species O− and lowered the reduction peak temperature corresponding to the reduction of V5+ and V4+ phases. A combination of powder X-ray diffraction, temperature-programmed reduction, and chemical analysis data shows that at higher levels, Ni doping induced the formation of the V5+ phase and suppressed the presence of the V4+ phase. Previous studies have indicated that a large amount of oxygen species associated with V5+ inhibit the n-butane conversion but promote the selectivity for maleic anhydride.

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.

Effect of different calcination environments on the vanadium phosphate catalysts for selective oxidation of propane and n-butane

Vanadium phosphate catalysts were synthesized via VOPO 4·2H 2O and were calcined in two different hydrocarbon reaction environments, i.e. n-butane/air and propane/air. Both catalysts are denoted VPDB and VPDP, respectively. Both catalysts exhibited a good crystalline with characteristic peaks of pyrophosphate phase. However, the peaks for VPDP are shown to be more prominent than those of VPDB. BET surface area showed that VPDB gave higher surface area (23 m 2 g −1) compared to VPDP (18 m 2 g −1). The average V valence state for VPDP is 4.08 and the higher V valence state for VPDB is 4.26 due to higher amount of V V for VPDB. Furthermore 14.2% of V III was found for VPDP but none for VPDB. SEM micrographs clearly revealed that the morphologies of both catalysts composed of plate-like crystallite that was arranged into the characteristic of rosette cluster. However, the catalyst calcined in n-butane/air environment (VPDB) resulted in an increment of the amount of plate-like crystal formed in the rosette rosebud agglomerates. TPR in H 2 profiles of both catalysts gave two reduction peaks corresponding to two kinetically different oxygen species which were associated with V V and V IV phases, respectively. VPDB removed larger amount of active oxygen species linked to V IV phase which eventually caused a higher conversion rate in the selective oxidation of n-butane and propane to maleic anhydride and acrylic acid, respectively.

N-Butane Oxidation over γ-Al 2O 3 Supported Vanadium Phosphate Catalysts

Four vanadium phosphate catalysts supported on γ-Al 2O 3 (20 wt%) were synthesized via wetness impregnation of VOHPO 4·0.5H 2O precursor and calcined for different durations (6, 10, 30 and 75 h) at 673 K in a reaction flow of n-butane/air mixture. The samples calcined for 6 and 10 h produced only a single phase of (VO) 2P 2O 7. However, the VOPO 4 phase (β-VOPO 4) was detected and became more prominent with only a minor pyrophosphate peaks were found after 30 h of calcination. All these pyrophosphate peaks disappeared after 75 h of calcination. The formation of V 5+ phase was also observed in the SEM micrographs. The redox properties and the nature of oxidants of the catalysts employed in this study were investigated by H 2-TPR analysis. Selective oxidation of n-butane to maleic anhydride (MA) over these catalysts shows that the percentage of n-butane conversion decreases with the transformation of the catalysts from V 4+ to V 5+ phases. An appropriate ratio of V 5+/V 4+ can enhance the performance of the VPO catalyst. However, a higher amount of V 5+ and its associated oxygen species are responsible to promote the MA selectivity.

Characterization of Au-based Catalysts Using Novel Cerium Oxide Supports

Gold dispersed on various oxide supports has been shown to exhibit high activity in a variety of important catalytic reactions, the most widely investigated of which remains the low-temperature oxidation of CO to CO2 [1]. The nature of the oxide support can have a profound effect on the activity of the catalysts, and Au-catalysts utilizing nanocrystalline oxide supports such as CeO2 and Y2O3 have been found to possess a significantly greater activity than those dispersed on larger support particles [2]. In the present study, a novel synthesis method involving the use of supercritical CO2 as an anti-solvent was devised to prepare CeO2 (scCeO2) support particles. The rapidity of precipitation using this method makes possible the formation of structures that are not available with traditional precipitation methods. This process has previously been used to produce vanadium phosphate catalysts with unusual morphologies and improved activity [3].

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.

Adventures with vanadium phosphate catalysts: Reflections on a long standing collaboration with J.C. Volta

Abstract This short article provides reflections on a most productive collaboration between the authors and Dr. J.C. Volta as an appreciation to mark Jean Claude's retirement in 2006. The sole topic that we focused on during 15 years of intensive effort was a study of vanadium phosphate catalysts, a topic that all of us have found fascinating and challenging, as have many other scientists in the last 50 years. Aspects of the key research themes, in particular the role of amorphous VPO phases, are revisited.

Vanadyl phosphate catalysts in biodiesel production

Abstract The possibility of using vanadyl phosphate (VOP)-based catalysts in biodiesel production has been investigated. Vanadium phosphate catalysts resulted very active in the transesterification reaction of triglycerides with methanol, despite their low specific surface area. A slow deactivation of the catalysts has been experimentally detected under the reaction conditions, but the catalyst can easily be regenerated by calcinations in air. The influence of the calcination treatment on the surface structure and, consequently, on its catalytic performances was deeply investigated. Both fresh and used catalysts were characterized by using several techniques, such as BET, X-ray diffraction (XRD), UV–vis diffuse reflectance (DRUV) and laser-Raman (LRS). The characterization results showed that the deactivation is due to a progressive reduction of vanadium (V) species from V5+ to V4+ and V3+ by methanol. By comparing the obtained performances of VOP catalysts with the ones of other heterogeneous catalysts reported by the literature, it is possible to conclude that VOP catalysts can already be used industrially for biodiesel production but their performances can probably be greatly improved in perspective.

Chemically Induced Fast Solid-State Transitions of ω-VOPO 4 in Vanadium Phosphate Catalysts

Vanadium phosphates are important catalysts for the oxidation of alkanes, and commercial catalysts comprise a complex range of V4+ and V5+ phosphates. We used three complementary in situ characterization methodologies-powder x-ray diffraction and laser Raman and electron paramagnetic resonance spectroscopies-to show that the metastable phase omega-VOPO4 is very sensitive to many of the reactants and products of butane oxidation. A rapid transformation from omega-VOPO4 to delta-VOPO4 occurs on exposure to butane at the reaction temperature, and hence the metastable omega-VOPO4 may play a role in the formation of commercial catalysts.

Effects of mechanochemical treatment to the vanadium phosphate catalysts derived from VOPO 4·2H 2O

Modification by using mechanochemical treatment of vanadium phosphate catalysts on the microstructure, morphology, oxygen nature and catalytic performance for n-butane oxidation is described and discussed. In this work, the precursor, VOHPO 4·0.5H 2O prepared by reduction of VOPO 4·2H 2O by isobutyl alcohol was subjected to a high energy planetary ball mill for 30, 60 and 120 min in ethanol. The ball milling process reduced the crystallite size of the catalysts and consequently increased their surface area. The morphologies of the milled catalysts are dependent on milling time. The highest reactivity and mobility of the lattice oxygen species has been achieved by the catalyst milled for 60 min with lower reduction peak temperature and higher amount of oxygen atoms removed. The oxygen species removed from the active V 4+ phase was shown to be correlated with the rate of reaction. A good relationship was also found between the oxygen species associated with V 5+ and maleic anhydride selectivity. However, a larger amount of this oxygen species will give a deleterious effect on the conversion rate. The present study demonstrate that the mechanochemical method (with an appropriate duration) effectively enhanced the catalytic activity by increasing the surface area and controlling the reactivity, and that the amount of oxygen species contributed to the partial oxidation of n-butane to maleic anhydride.

Effect of Cr and Co Promoters Addition on Vanadium Phosphate Catalysts for Mild Oxidation of n-Butane

In this study, Cr and Co promoted, as well as unpromoted vanadium phosphate (VPO) catalysts were synthesized by the reaction of V 2O 5 and o-H 3PO 4 in organic medium followed by calcination in n-butane/air environment at 673 K. The physico-chemical properties and the catalytic behavior were affected by the addition of Cr and Co dopants. H 2-TPR was used to investigate the nature of oxidants in the unpromoted and promoted catalysts. The results showed that both the Cr and Co promoters remarkably lowered the temperature of the reduction peak associated with V 5+. The amount of oxygen species originated from the active phase, V 4+, removed was significantly increased for Co and Cr-promoted catalysts. Both Cr and Co dopants improve strongly the n-butane conversion without sacrificing the MA selectivity. A good correlation was observed between the amount of oxygen species removed from V 4+ phase and the activity for n-butane oxidation to maleic anhydride. This suggested that V 4+-O was the center for the activation of n-butane.

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.

Oxidation of Butane to Maleic Anhydride using Vanadium Phosphate Catalysts: Comparison of Operation in Aerobic and Anaerobic Conditions using a Gas-gas Periodic Flow Reactor

The use of a periodic flow reactor is described for the oxidation of butane to maleic anhydride to compare the catalytic performance of vanadium phosphate catalysts operating under aerobic and anaerobic conditions. It is found that for the catalyst prepared via a standard VPO method, operation in the absence of oxygen leads to a very small enhancement in selectivity when butane concentrations in the range 0.9–2.9% are used. Operation in the absence of oxygen leads to very small differences in conversion such that the overall yield is enhanced and this effect is maximised for reactor feeds containing 1.5% butane. However, the enhancement is negligible when the catalyst is operated at high conversion required for commercial operation, indicating that reactors operating with continuous flow with aerobic conditions are preferred. Similar experiments are conducted for a catalyst prepared by the VPD method and, in contrast, this catalyst gives lower butane conversion and maleic anhydride selectivity when operated in the absence of oxygen.

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.

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 V 2O 5 and H 3PO 4 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 VOHPO 4·0.5H 2O. The aqueous hydrochloric acid method leads to the formation of VO(H 2PO 4) 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 V 4+ phases but gives a mixture of hydrated VOPO 4· xH 2O phases. In contrast the isobutanol method at higher temperature and pressure gave VOHPO 4·0.5H 2O 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.