VPO catalyst in the form of an improved shaped body and method for the production thereof

Abstract

The invention relates to a VPO catalyst in the form of a shaped body for oxidation of hydrocarbons with molecular oxygen, in particular for oxidation of butane to maleic anhydride with molecular oxygen, wherein the VPO catalyst contains 0.05% by weight to 7.0% by weight of Zn, partly in the form of ZnO, and, in transmission infrared spectroscopy, has a first absorption band having a maximum between 790 cm-1 and 810 cm-1 and possibly a second absorption band having a maximum between 820 cm-1 and 840 cm-1, characterised in that only the first absorption band is present or the intensity of the first absorption band is greater than the intensity of the second absorption band, and that the shaped body is a cylindrical shaped body, having a height of 4 mm to 5 mm, an outer diameter of 5 mm to 6 mm and an average axial opening with a diameter of 2 mm to 4 mm. The invention further relates to a method for the production of a VPO catalyst according to the invention, comprising the steps of: a) producing a catalyst precursor containing vanadyl hydrogenphosphate, b) shaping the catalyst precursor to a shaped body, c) activating the shaped bodies in order to form a VPO phase, characterised in that, after step a), ZnO is mixed with the catalyst precursor.

Description

[0001] VPO catalyst in the form of an improved shaped body and process for its preparation

[0002] The invention relates to a VPO catalyst in the form of a shaped body for the oxidation of hydrocarbons with molecular oxygen, in particular for the oxidation of butane to maleic anhydride with molecular oxygen, wherein the VPO catalyst contains between 0.05 wt.% to 7.0 wt.% Zn, which is partly present as ZnO and has a first absorption band with a maximum between 790 cm' in infrared spectroscopy carried out in transmission 1 up to 810 cm -1 and possibly a second absorption band with a maximum between 820 cm' 1 up to 840 cm' 1characterized in that only the first absorption band is present or the strength of the first absorption band is greater than the strength of the second absorption band and that the shaped body is a cylindrical shaped body with a height of 4 mm to 5 mm, an outer diameter of 5 mm to 6 mm and a central axial opening with a diameter of 2 mm to 4 mm.

[0003] The invention also relates to a process for producing a VPO catalyst according to the invention, comprising the steps of: a) producing a catalyst precursor containing vanadyl hydrogen phosphate, b) forming the catalyst precursor into shaped bodies, c) activating the shaped bodies to form a VPO phase, characterized in that after step a) ZnO is mixed with the catalyst precursor.

[0004] The invention further relates to the use of a Zn compound in the form of a solid for stabilizing the VPO catalyst according to the invention in particle form.

[0005] Maleic anhydride is a chemical intermediate of great economic importance. It is used, for example, in the production of alkyd and polyester resins, either alone or in combination with other acids. Furthermore, it is a versatile intermediate for chemical synthesis, for example, in the synthesis of γ-butyrolactone, tetrahydrofuran, and 1,4-butanediol, which in turn are used as solvents or can be further processed into polymers such as polytetrahydrofuran or polyvinylpyrrolidone.

[0006] The production of maleic anhydride is typically carried out by partial oxidation of n-butane in the gas phase with molecular oxygen or with a gas containing molecular oxygen in the presence of a vanadium phosphorus oxide (VPO) catalyst containing vanadyl pyrophosphate (VPP). Vanadyl pyrophosphate in its pure form contains vanadium with a valence of +4 and is particularly suitable for the production of maleic anhydride from unbranched saturated or unsaturated hydrocarbons with at least four carbon atoms. Both fixed-bed reactors and fluidized-bed reactors are used.

[0007] VPO catalysts exhibit only low intrinsic activity in the reaction of n-butane to maleic anhydride. Therefore, a large amount of catalyst is required for sufficient conversion. Furthermore, VPO catalysts are among the most expensive non-precious metal catalysts available, primarily due to the high cost of their starting materials. Consequently, the challenge is to improve the catalytic performance (activity and selectivity) as well as the lifetime and mechanical stability of such catalysts. It is known from the prior art that the performance of VPO catalysts can be improved by adding foreign elements to the vanadium phosphorus oxide (VPO) phase, such as by adding molybdenum (Mo promoter or Mo doping).

[0008] US 5,929,256 discloses the synthesis of an active molybdenum-modified vanadium-phosphorus catalyst for producing maleic anhydride. A compound containing substantially pentavalent vanadium is reacted with a compound containing pentavalent phosphorus in an alcoholic medium suitable for reducing the vanadium to an oxidation state below 5. Molybdenum is incorporated into the reaction product, forming a solid molybdenum-modified precursor composition. The alcohol is removed to obtain a dried, solid molybdenum-modified precursor composition. Shaped bodies containing the dried, solid molybdenum-modified precursor compound are molded. The dried and molded molybdenum-modified precursor compositions are activated to convert them into the active catalyst.

[0009] US 5,070,060 discloses an improvement in the oxidation catalyst used for the partial oxidation of n-butane and containing mixed oxides of vanadium, phosphorus, zinc, and lithium. This improvement comprises the addition of a molybdenum compound modifier in an amount of about 0.005 to 0.025 / 1 Mo / V to the catalyst during the reaction of the reduced vanadium compound with concentrated phosphoric acid. The addition of Mo produces a catalyst that is very stable, a more active system, and has a longer shelf life than the unmodified catalyst. US 3,980,585 discloses a catalyst complex suitable for the conversion of normal C4 hydrocarbons to maleic anhydride in the gas phase, comprising the components vanadium, phosphorus and copper and one of the elements selected from the group Te, Zr, Ni, Ce, W, Pd, Ag, Mn, Cr, Zn, Mo, Re, Sm, La, Hf, Ta, Th, Co, U and Sn, preferably with an alkali metal or an alkaline earth metal.

[0010] US 4,056,487 discloses a suitable catalyst for the partial oxidation of alkanes to the corresponding anhydrides, for example, normal C4 hydrocarbons converted to maleic anhydride in the gas phase, comprising the components vanadium, phosphorus and oxygen, Nb, Cu, Mo, Ni, Co, and Cr. Preferred are the compositions that additionally contain one or more elements selected from Ce, Nd, Ba, Hf, U, Ru, Re, Li, or Mg.

[0011] US 4,515,904 discloses a process for the preparation of a phosphorus-vanadium catalyst and a phosphorus-vanadium co-metal catalyst for use in the production of maleic anhydride from butane, the process comprising reacting a vanadium compound in an organic ether solvent having from about 2 to about 10 carbon atoms with a phosphorus halide at a temperature of from about 0°C to about 200°C in the presence of water or an aliphatic alcohol having from about 1 to about 8 carbon atoms; removing the solvent; and activating the catalyst by the addition of butane or another hydrocarbon starting material and a phosphorus compound at a temperature of from about 300°C to about 500°C.

[0012] US 5,158,923 discloses an improvement in the oxidation catalyst used for the partial oxidation of n-butane and containing mixed oxides of vanadium, phosphorus, zinc, and lithium. This improvement comprises the addition of a molybdenum compound modifier in an amount of about 0.005 to 0.025 Mo / V to the catalyst during digestion of the reduced vanadium compound with concentrated phosphoric acid. The addition of Mo produces a catalyst that is very stable, a more active system, and has a longer shelf life than the unmodified catalyst.

[0013] US 5,262,548 discloses an improvement in the oxidation catalyst used for the partial oxidation of n-butane and containing mixed oxides of vanadium, phosphorus, zinc, and lithium. This improvement comprises the addition of a molybdenum compound modifier in an amount of about 0.005 to 0.025 Mo / V to the catalyst during digestion of the reduced vanadium compound with concentrated phosphoric acid. The addition of Mo produces a catalyst that is a very stable active system and has a longer lifetime than the unmodified catalyst.

[0014] WO 2013062919 A1 discloses a process for producing a promoted VPO catalyst, wherein the catalyst comprises the mixed oxides of vanadium and phosphorus and wherein the catalyst is promoted with at least one of niobium, cobalt, iron, zinc, molybdenum or titanium, the process comprising the steps of: (i) preparing a VPO catalyst comprising vanadyl pyrophosphate as the main component and containing less than 5 wt.% vanadyl phosphate, (ii) contacting the VPO catalyst with a solution comprising a compound as metal source with at least one metal selected from the group consisting of niobium, cobalt, iron, zinc, molybdenum or titanium to form a metal-impregnated VPO catalyst, and (iii) drying the metal-impregnated VPO catalyst to form the promoted VPO catalyst. In one embodiment, a niobium-activated VPO catalyst is prepared.

[0015] US 6,407,030 B1 discloses a process for preparing a catalyst that can be used for the synthesis of maleic anhydride by the oxidation of saturated and / or unsaturated C4 hydrocarbons. A vanadium(V) compound is reacted with a mixture of phosphonic and phosphoric acid in a specific ratio in a solvent mixture comprising a structuring agent and an entraining agent. The water of reaction is distilled off together with the entraining agent, and the resulting precursor is subjected to calcination.

[0016] US 4,132,670 A discloses the preparation of a crystalline vanadium(IV) phosphate composition with an intrinsic surface area of ​​more than 10 m 2 / g by the reaction of orthophosphoric acid with a vanadium(IV) oxy compound. The vanadium compound is suspended in a suitable hydroxyl-containing organic medium, for example, isobutanol, and contacted with the acid at a temperature in the range of 20 °C to 210 °C until the conversion is complete.

[0017] US 4,382,876 discloses a process for the preparation of catalysts for the gas-phase oxidation of saturated or unsaturated hydrocarbons to maleic anhydride, comprising the steps of: (a) mixing a vanadium compound containing pentavalent vanadium with orthophosphoric acid in an alcohol from the series of monohydric lower aliphatic alcohols having 2 to 8 C atoms, optionally in the presence of promoters and a reducing agent combination of H3PO3 and alcohol, wherein the sum of H3PO3 and H3PO4 is controlled such that an atomic ratio of P to V of 1.0 to 1.2 is achieved and wherein the H3PO3 is used in substoichiometric amounts such that these are sufficient to reduce the vanadium to an oxidation state of only 4.2 to 4.4, and wherein the reducing agent combination is subsequently used to further reduce the vanadium to an oxidation state between 3.9 and 4.0,wherein any water present is distilled off by heating as an azeotrope with the alcohol; (b) adding titanium dioxide obtained by high-temperature pyrolysis of titanium tetrahalide and optionally additional promoters Ni, Fe, Li, Mg; (c) forming a precipitated catalyst in a known manner; and (d) activating the catalyst in an oxidation reactor at temperatures of about 450°C to 510°C for about 12 to 72 hours in the presence of an air-hydrocarbon stream.

[0018] WO 9529006 A1 discloses a process for the preparation of oxidation catalysts containing vanadium-phosphorus mixed oxides, consisting of contacting a phosphorus compound with a vanadium compound in an organic solvent under conditions which enable the preparation, recovery and drying of the precursor and the calcination of the precursor under an atmosphere containing air, steam or inert gas or a mixture thereof at a temperature between 350 °C and 550 °C for the time required to obtain active catalysts, and use of the catalysts for the preparation of maleic anhydride by oxidation of aliphatic hydrocarbons.

[0019] US 5,185,455 discloses the improvement of a process for producing maleic anhydride by catalytic oxidation of n-butane in the presence of trimethyl phosphate over a fixed-bed catalyst of vanadium phosphorus oxide in a tubular reactor. The trimethyl phosphate content of the gas entering the reactor is in a range between about 0.9 N and about 1.1 N, where N is a normative concentration of trimethyl phosphate in ppm, determined by the following relationship: N = 5xC4 + 6x(H2O-2.4) + 0.75x-(CONV - c) + (SV / (25xP in)), where: C4 times the mole percent of n-butane in the gas entering the reactor; H2O times the mole percent moisture in the gas entering the reactor; CONV = percent butane conversion in the reactor; SV = gas hourly space velocity of the gas at the inlet of the reactor reduced to a pressure of one atmosphere and 60 °F; Pin = the pressure at the inlet of the reactor (psig); and c=84x0.05 [(SVxC4) / Pin]; regardless of the calculation of N, the trimethyl phosphate content is at least about one ppm.

[0020] US 5,280,003 discloses an improvement to an oxidation catalyst used for the partial oxidation of n-butane, containing mixed oxides of vanadium and phosphorus, zinc, lithium, and molybdenum, comprising its preparation by conducting the crystallization step under static conditions that allow for more uniform conditions for crystal growth. The static conditions are maintained by heating the solvent to reflux during the crystallization period.

[0021] US 4,251,390 discloses an improvement in an oxidation catalyst used for the partial oxidation of n-butane, which contains vanadium and mixed phosphorus oxides. This catalyst comprises adding a zinc compound in an amount of 0.15 to 0.001 Zn / V to the catalyst during the reaction of the reduced vanadium portion with concentrated phosphoric acid. The addition of zinc produces a catalyst that is more easily activated and is highly stable to heating of the reaction system. Small amounts of lithium and silicon compounds also have additional desirable catalytic effects without diminishing the advantage of the zinc compound.

[0022] DE 10 2014 004786 A1 relates to a catalyst which contains a vanadium-phosphorus oxide and an alkali metal, wherein the weight fraction of alkali metal in the vanadium-phosphorus oxide is in the range from 10 to 400 ppm, based on the total weight of the vanadium-phosphorus oxide, a process for its preparation and the use of the catalyst for the gas phase oxidation of hydrocarbons, in particular for the preparation of maleic anhydride.

[0023] To produce VPO catalysts containing a VPP phase, a reduction of vanadium pentoxide (V2O5) is usually carried out in the presence of phosphoric acid in an organic alcoholic solvent using benzyl alcohol as the reducing agent, producing vanadyl hydrogen phosphate (VHP) in addition to benzaldehyde. The resulting redox reaction (the “reduction”), in which vanadium with the oxidation state V (V(V)) reacts to form the VHP phase, in which vanadyl species (VO 2+) with vanadium in the oxidation state IV (V(IV)) is:

[0024] (1 ) V2O5 + 2H3PO4 + Ph-CH2-OH 2VOHPO4 * V2H2O + Ph-CHO + 2H2O

[0025] In a subsequent activation step, the VHP phase is converted into the vanadyl pyrophosphate phase by the action of heat, with the elimination of water.

[0026] (2) 2VOHPO4 * V2H2O (VO)2P2O7+ 1 1 / 2H2O

[0027] One problem with the catalytic conversion of butane to maleic anhydride using VPO catalysts is that the process must be operated in a pore diffusion-limited regime. Here, porosity has a direct influence on the catalytic yield. Therefore, care must be taken to ensure that the pore structure is not negatively affected during shaping (e.g., by tabletting). This, however, has the consequence that the mechanical stability of the shaped bodies suffers. This stability must, however, be large enough for the tablets to survive the filling process (falling into an approximately 6 m long reaction tube) intact. Otherwise, the back pressure in the process would be too high due to the smaller fragments, leading to high compressor costs and reduced throughput. One technical challenge is therefore to increase the mechanical strength of the shaped bodies without negatively affecting the pore structure.At the same time, there is a need for VPO catalysts with improved performance, i.e. improved activity, selectivity and stability.

[0028] The object of the present invention was therefore to provide an improved VPO catalyst for the gas phase oxidation of hydrocarbons, in particular for the production of maleic anhydride, which, compared to previously conventional catalysts, has higher catalytic performance, in particular improved selectivity and at the same time significantly improved mechanical strength and is in particular also more abrasion-resistant.

[0029] The object is achieved by a VPO catalyst in the form of a shaped body for the oxidation of hydrocarbons with molecular oxygen, in particular for the oxidation of butane to maleic anhydride with molecular oxygen, wherein the VPO catalyst contains between 0.05 wt.% to 7.0 wt.% Zn, which is partly present as ZnO and has a first absorption band with a maximum between 790 cm -1 up to 810 cm -1 and possibly a second absorption band with a maximum between 820 cm -1 up to 840 cm -1characterized in that only the first absorption band is present or the strength of the first absorption band is greater than the strength of the second absorption band and that the shaped body is a cylindrical shaped body with a height of 4 mm to 5 mm, an outer diameter of 5 mm to 6 mm and a central axial opening with a diameter of 2 mm to 4 mm.

[0030] The strength of the absorption band is determined by setting a first straight line such that it touches the spectrum tangentially immediately to the left and right below the band in question, then drawing a second straight line starting from the maximum of the band in the direction of constant wavelength and decreasing absorption and determining the point of intersection of the two straight lines, whereby the strength of the absorption band is defined as the length between the maximum and the point of intersection. The ratio of the strength of the first absorption band to the strength of the second absorption band is preferably above 1.1, preferably above 2. The shaped bodies according to the invention have a lateral compressive strength of more than 15 N or 25 N, preferably between 15 N and 200 N or 25 N to 200 N, whereby the lateral compressive strength is measured using a Zwick Z0.5 device using ASTM D4179 at a constant force rate of 20.0 N / s, with 100 tablets each placed and measured individually with the cylinder axis parallel to the jaw surface to determine an average breaking force representing the lateral crush strength.

[0031] The VPO catalyst according to the invention, when examined by powder X-ray diffraction using Cu-Ka radiation, exhibits reflections at 31.7° to 31.9°, 34.3° to 34.5° and 36.2° to 36.4°.

[0032] The VPO catalyst according to the invention comprises, or consists of, or consists essentially of, the VPO phase. For example, the VPO catalyst according to the invention comprises the VPO phase in a content of more than 70 wt. %, preferably more than 80 wt. %, more preferably more than 90 wt. %, based on the total weight of the VPO catalyst. Furthermore, the VPO catalyst may comprise VPP, dopants, and unreacted oxides of the starting materials, such as vanadium pentoxide or phosphorus oxide.

[0033] The VPO catalyst according to the invention may optionally contain, for example, between 0.1 wt% and 1 wt%, preferably between 0.4 wt% and 0.7 wt% of Mo, based on the total weight of the VPO catalyst.

[0034] The VPO catalyst according to the invention can also contain alkali metals such as Na and K, preferably 80 to 300 ppm of alkali metal. Furthermore, the VPO catalyst according to the invention can also contain carbon, e.g., in the form of graphite, e.g., in an amount of 3 wt.% to 5 wt.%, based on the total weight of the catalyst. The graphite present serves, for example, as a tabletting aid.

[0035] The VPO catalyst according to the invention comprises ZnO, but may also contain other Zn compounds. The Zn content resulting from the presence of ZnO and any other Zn compounds present must be between 0.05 wt.% and 7.0 wt.%, preferably between 0.1 wt.% and 6.0 wt.%, more preferably between 0.2 wt.% and 4.0 wt.%, and most preferably between 0.7 wt.% and 3.0 wt.% Zn in the VPO catalyst, in each case based on the total weight of the catalyst.

[0036] The VPO catalyst preferably has the following elemental composition:

[0037] 0.05 wt% to 7 wt% Zn, 0 wt% to 0.7 wt% Mo,

[0038] 26 wt% to 31 wt% V,

[0039] 17 wt% to 21 wt% P,

[0040] 3 wt.% to 5 wt.% C, the remainder oxygen, each based on the total weight of the VPO catalyst.

[0041] The VPO catalyst according to the invention contains ZnO, and if the content of these two phases is sufficiently high, the catalyst exhibits reflections typical of the ZnO phase in an XRD powder diffractogram recorded using Cu-Ka radiation. In particular, sharp reflections are detected at 31.7° to 31.9°, 34.3° to 34.5°, and 36.2° to 36.4°.

[0042] According to the invention, the presence of ZnO stabilizes the catalyst particles, resulting in higher mechanical strength compared to the strength without the presence of ZnO. However, the presence of ZnO also improves the catalytic performance (i.e., activity, selectivity, and stability) of the catalyst particles, particularly increasing selectivity.

[0043] The VPO catalyst according to the invention is in the form of a shaped body. The shape of the shaped body can be varied depending on the desired contact time, flow rate, and back pressure during the catalytic conversion. The VPO catalyst in the form of a shaped body refers to shaped bodies produced by a molding step such as tabletting. A large number of the shaped bodies according to the invention form a layer or catalyst bed in the tube-bundle reactor, through which the reactants butane, in particular n-butane, and air are passed.

[0044] The molded body according to the invention is cylindrical. The cylinder has a height (length along the cylinder axis) of 4 mm to 5 mm and a substantially round base with an outer diameter of 5 mm to 6 mm. The cylinder has a central axial opening, which must have a diameter of 2 mm to 4 mm.

[0045] The central axial opening of the cylinder refers to a channel or cavity that extends from a substantially circular base surface to the opposite substantially circular base surface of the cylinder, i.e., in the axial direction. The central axial opening extends as a channel approximately from the center of the substantially circular base surface to approximately the center of the opposite substantially circular base surface.

[0046] The shaped body according to the invention in the form of a cylinder with a central axial opening preferably has a height of 4.5 mm to 4.8 mm, a substantially round base with an outer diameter of 5.6 mm to 5.8 mm and a central axial opening with a diameter of 2.2 mm to 3.5 mm.

[0047] The catalyst according to the invention is characterized by increased abrasion resistance, i.e., fewer fragments (abrasion debris) break off under friction and mechanical stress than with previous catalysts. In particular, the catalyst according to the invention is characterized in that the shaped bodies exhibit less than 1.5 wt. %, preferably less than 1.0 wt. %, particularly preferably less than 0.5 wt. %, of abrasion, based on the total weight of the catalyst, when the abrasion is measured using an ERWEKA TAR120 device, wherein approximately 25 g of shaped bodies are weighed in and the abrasion measurement is carried out at 500 revolutions at a rotation speed of 25 revolutions per minute.

[0048] The VPO catalyst or the shaped body according to the invention has a lateral crushing strength of more than 15 N, preferably between 15 N and 200 N. Particularly preferred is a lateral crushing strength of the VPO catalyst or the shaped body of more than 30 N to 150 N, even more preferably of more than 35 N to 100 N.

[0049] The VPO catalyst according to the invention or the cylindrical shaped body has a lateral crushing strength of more than 15 N, preferably between 15 N and 50 N. Particularly preferred is a lateral crushing strength of the VPO catalyst or the shaped body of more than 30 N to 45 N, even more preferably of more than 35 N to 40 N.

[0050] The invention further relates to a process for producing a VPO catalyst according to the invention, comprising the steps: a) producing a catalyst precursor containing vanadyl hydrogen phosphate, b) shaping the catalyst precursor into shaped bodies, c) activating the shaped bodies to form a VPO phase, characterized in that after step a) ZnO is mixed with the catalyst precursor. In a process step a), a catalyst precursor containing vanadyl hydrogen phosphate is prepared in a generally known manner. Typically, a V(V) compound in solution is reduced in a reaction mixture with the aid of a reducing agent in the presence of a P(V) compound, optionally a Mo compound, in a reduction step. For example, the reaction mixture can consist of 45% to 90% by weight of solvent, 5% to 15% by weight of reducing agent, 5% to 15% by weight of V(V) compound, up to 1% by weight of Mo compound and between 5 to 25% by weight.-% P(V) compound. More specifically, for example, 60 wt.% to 70 wt.% isobutanol, 5 wt.% to 15 wt.% benzyl alcohol, 5 wt.% to 15 wt.% vanadium pentoxide, 0.05 wt.% to 0.2 wt.% (NH4)2MO2O7, and 10 wt.% to 20 wt.% phosphoric acid, each based on the total weight of the reaction mixture, can be initially introduced as the reaction mixture.

[0051] The V(V) compound used as starting material in the reaction mixture for producing vanadyl hydrogen phosphate is a compound containing vanadium in the oxidation state V and is preferably V2O5. The P(V) compound used as starting material in the reaction mixture for producing vanadyl hydrogen phosphate is a compound containing phosphorus in the oxidation state V and is preferably phosphoric acid or a phosphate salt, such as NasPCU. If phosphoric acid (H3PO4) is used, it is preferably anhydrous (100% phosphoric acid) or phosphoric acid containing only small amounts of water, i.e., phosphoric acid with a concentration of 98 to 100%, preferably 99 to 100% (this information refers to the commonly stated percentage by weight of pure phosphoric acid relative to the weight of the water-phosphoric acid mixture).Alternatively, in the preparation of the reaction mixture for producing the vanadyl hydrogen phosphate, phosphoric acid with more than 100% can be used, which immediately reacts with any water present in the reaction mixture at the beginning to form phosphoric acid with a concentration of 98 to 100%, preferably 99 to 100%, preferably 100%, so that no phosphoric acid with a concentration of more than 100% is present in the reaction mixture and at the same time no more than 0.2% by weight of water, based on the weight of the reaction mixture, remains in the reaction mixture.

[0052] The Mo compound that can optionally be used as starting material in the reaction mixture for producing vanadyl hydrogen phosphate is any compound containing molybdenum, for example, molybdenum trioxide, ammonium heptamolybdate ((NH4)6MO7O24)*4H2O), ammonium paramolybdate ((NH4)6MO7O2*4H2O), metamolybdate, molybdic acid (H2MOO4) and their salts such as (NH4)2MoO4, Na2MoO4, K2MoO4, or (NH4)2MO2O7. The reducing agent present in the reaction mixture for producing vanadyl hydrogen phosphate can be any reducing agent capable of reducing the V(V) compound to at least partially produce vanadyl hydrogen phosphate. Preferably, the reducing agent is an organic reducing agent such as ethanol, isobutanol or an aromatic alcohol, including in particular benzyl alcohol.

[0053] The solvent present in the reaction mixture for preparing the vanadyl hydrogen phosphate is preferably an alcohol, particularly preferably a high-boiling aliphatic alcohol, in particular isobutanol, alternatively ethanol or isopropanol.

[0054] The starting materials are provided in a suitable reaction vessel to carry out the reduction step and to obtain the vanadyl hydrogen phosphate of the catalyst precursor. The reaction mixture is heated above room temperature, for example, up to a temperature of 100 °C. After the reduction is preferably carried out with stirring in a reflux step, the reaction vessel preferably has a reflux condenser and a device for stirring the reaction mixture. Optionally, the reaction vessel is equipped with a device that allows the water formed during the reduction to be removed from the reaction mixture, i.e., a water separator, such as a Dean Stark trap.

[0055] The reduction to vanadyl hydrogen phosphate is preferably carried out under reflux at atmospheric pressure, with the temperature being increased depending on the boiling point of the solvent used; preferably, only a single reflux step is carried out in the process according to the invention. The catalyst precursor preferably contains vanadyl hydrogen phosphate as the main phase or can even consist essentially of a vanadyl hydrogen phosphate phase. The molybdenum possibly also present in the catalyst precursor can be present as a dopant in the vanadyl hydrogen phosphate phase, where molybdenum doping is understood to mean that the molybdenum is either incorporated into the vanadyl hydrogen phosphate phase or is present on its surface. In addition to the vanadyl hydrogen phosphate phase, however, other vanadium-phosphorus mixed oxides can also be formed during the reduction, in which vanadium has an oxidation state of IV or even III.The reduction does not have to be complete, so portions of the V(V) compound and the P(V) compound remain in the catalyst precursor. However, the catalyst precursor typically has an average vanadium oxidation state of 3.8 to 4.2.

[0056] The water formed during the reduction can be removed from the reaction mixture during the reduction. In the prior art, the water is removed either physically, e.g., using a water separator, or chemically by using compounds that bind the water, e.g., drying agents or anhydrides such as phosphoric acid with a concentration of more than 100%. The reaction mixture can, for example, contain anhydrides that react with and bind water; in particular, the reaction mixture can contain phosphoric acid with a concentration of more than 100%.

[0057] The suspension obtained after the reduction step can, for example, be filtered in an inert gas atmosphere such as nitrogen or a noble gas. In this context, an inert gas refers to any gas that does not react with the catalyst precursor under the given conditions during filtration, but simultaneously displaces oxygen from the air, thus minimizing the risk of explosion. Filtration is carried out by means known to those skilled in the art, typically a filter press, a decanter, or a Nutsche filter. Filtration yields an uncalcined catalyst precursor that is still moistened with solvent.

[0058] The catalyst precursor obtained by filtration (the solid residue from the filtration) can then be dried. This is typically done at a temperature above room temperature, e.g., at a temperature up to 150°C, under reduced pressure or vacuum, or under an inert gas, to obtain a dried catalyst precursor. In this context, an inert gas means any gas that does not react with the catalyst precursor under the drying conditions but simultaneously displaces oxygen from the air, thus minimizing the risk of explosion, for example, nitrogen or a noble gas. Drying preferably takes place under reduced pressure or vacuum between 50°C and 150°C, preferably between 90°C and 140°C.

[0059] Alternatively or optionally, calcination can be performed following drying to obtain the catalyst precursor. Calcination takes place at elevated temperatures between 150 °C and 350 °C, preferably between 230 °C and 290 °C, under an inert gas atmosphere. In this context, an inert gas is any gas that does not react with the catalyst precursor under the calcination conditions but displaces oxygen from the air, thus minimizing the risk of explosion, for example, nitrogen or a noble gas.

[0060] Optionally, graphite can be added to the dried catalyst precursor to facilitate shaping in process step b). The dried catalyst precursor can also be compacted and / or granulated to obtain a compacted or granulated dried catalyst precursor. The catalyst precursor can be compacted into plates using a roller compactor with a contact pressure of 190 bar, a gap width of 0.60 mm, and a roller speed of 7 l / min, and then granulated through a 1 mm sieve.

[0061] In process step b), the resulting catalyst precursor is formed into the shaped catalyst bodies. This can be done, for example, by tableting. In this process, the granules are compressed into the desired tablet shape and size using a rotary tablet press.

[0062] In the following process step c), the resulting shaped catalyst bodies are activated at a temperature above 200 °C. Activation typically takes place in a gas mixture consisting of air, inert gas, and steam. Any gas that does not react with the shaped catalyst body under the given conditions during activation can serve as the inert gas; more preferably, the inert gas is nitrogen or a noble gas. Alternatively, activation can also take place in process gas, i.e., in a gas mixture containing air and butane. Activation takes place at a temperature in the range from 300 °C to 500 °C, preferably in the range from 350 °C to 450 °C. Activation produces the finished VPO catalyst as the product of the process. The tableted VPO catalyst typically has a lateral crushing strength of above 15 N, typically between 15 N and 200 N.

[0063] The inventive production process for the VPO catalyst in the form of shaped bodies is characterized in that, after process step a), ZnO is added to the catalyst precursor or mixed with it. The ZnO, i.e. zinc(II) oxide, is used here as a powder promoter solid. The ZnO can also be added to the catalyst precursor using a metering device and a mixer. Generally, sufficient ZnO is added to prepare the inventive catalyst. For example, based on the weight of the dried catalyst precursor (without ZnO), 0.5 wt.% to 7.0 wt.%, preferably 1.0 wt.% to 5.0 wt.%, more preferably 1.5 wt.% to 3.5 wt.% of ZnO can be added in solid form.

[0064] The ZnO is added to the catalyst precursor, i.e., after its preparation in process step a), for example, by a reduction step, but before the formation of the shaped catalyst bodies in process step c). Preferably, the ZnO is added to the dried catalyst precursor, particularly preferably to the dried and calcined catalyst precursor.

[0065] In a further preferred embodiment, in addition to the ZnO, another solid metal compound, preferably a metal compound in powder form, is added. It is preferred that a solid Mg compound, for example MgO, is also present in the binder. The invention also relates to the use of ZnO to stabilize a VPO catalyst in the form of the shaped body according to the invention, as well as to the use of ZnO as a binder for a VPO catalyst according to the invention in the form of a shaped body.

[0066] The invention further relates to a process for the preparation of maleic anhydride by catalytic oxidation of n-butane, wherein a reactant gas comprising oxygen and n-butane is passed through a reactor tube in which a bed of VPO catalysts according to the invention is located.

[0067] The bed of VPO catalysts in the reactor tube consists of the VPO catalysts or shaped bodies according to the invention, which are introduced into the reactor tube and form a bed by resting on it. The reactor tube is preferably part of a plurality of reactor tubes of a tube bundle reactor, as is known to those skilled in the art for the industrial production of maleic anhydride. During the reaction, the bed of VPO catalysts is at a temperature between 300 °C and 420 °C. The reactant gas can, for example, contain between 0.2 and 10 vol. % n-butane and between 5 and 50 vol. % oxygen and can be circulated at a space-time velocity of 1100 h -1 up to 2500 hours -1 , preferably 1300 h' 1 up to 2000 h -1 through the reactor tube.

[0068] Figure 1 : Dimensions of the manufactured molded bodies

[0069] Figure 2: IR spectra of the samples according to Examples 1 and 2.

[0070] Figure 3: Catalysis experiment to determine catalyst activity, Example 1 compared to Examples 3 to 5 according to the invention.

[0071] Figure 4: Catalysis experiment to determine the catalyst selectivity of Example 1 in comparison with Examples 3 to 5 according to the invention.

[0072] Figure 5: Catalysis experiment to determine catalyst activity, Example 1 compared to Examples 6 to 8 according to the invention.

[0073] Figure 6: Catalysis experiment to determine catalyst selectivity, Example 1 compared to Examples 6 to 8 according to the invention.

[0074] Figure 7: XRD diffractograms of the VPO catalysts according to Examples 1 and 2.

[0075] The numbers in parentheses in the figures refer to the sample prepared according to the corresponding example.

[0076] Example 1 (Comparison)

[0077] Equipment used

[0078] A heating mantle is placed on a lab bench, containing a 2L four-necked flask. A half-moon stirrer with a suitable stirrer cap is located in the center opening of the four-necked flask, connected to the stirrer via a stirrer coupling. A thermometer is located in the right opening, and a riser tube to the reflux condenser is located in the left. The opening in the front center is used for filling with the chemicals, after which the nitrogen purge is connected there. The entire apparatus can also be flooded with nitrogen. For this purpose, the nitrogen is first passed through a gas scrubber bottle and then into the apparatus, and then discharged from the condenser at the top through a gas scrubber bottle.

[0079] Preparation of the catalyst precursor

[0080] First, 1069.5 g of isobutanol and 156.0 g of benzyl alcohol are added. 150 g of V2O5 are added while stirring. After the V2O5 addition, 2.52 g of ammonium dimolybdate are added. Subsequently, 232.50 g of phosphoric acid (100%, or anhydrous) are added to the suspension and heated under reflux under N2 for 10 h.

[0081] Filter

[0082] After cooling, the suspension, which contains the catalyst precursor as a solid, is transferred from the four-necked flask to a suction filter, and the liquid is removed by suction. The moist filter cake is pressed dry overnight in a press at 14 to 18 bar.

[0083] Drying / Calcination

[0084] The pressed filter cake is placed in the evaporation flask of a rotary evaporator. The filter cake is dried overnight at 110 °C under a water jet vacuum. The dried powder is placed in a suitable calcining pot in an oven and calcined in a 2-degree atmosphere at temperatures of 200 to 300 °C for 9 hours.

[0085] Compaction / T abletting

[0086] Before compaction / tableting, 5 wt.% graphite is added to the calcined powdered catalyst precursor and homogeneously mixed using a drum wheel mixer. This powder is compacted into plates using a roller compactor with a contact pressure of 190 bar, a gap width of 0.60 mm, and a roller speed of 7 l / min, and granulated through a 1 mm sieve.

[0087] The granules are pressed into the desired tablet shape using a rotary tablet press with dimensions of 5.6 x 5.6 x 2.3 mm [height x outer diameter x inner diameter].

[0088] Activation to pyrophosphate

[0089] The activation, which produces vanadyl pyrophosphate, is carried out under controlled conditions in a retort installed in a programmable furnace. The calcined tablets are evenly filled into the retort, which is then tightly sealed. The catalyst is then activated in a humid air-nitrogen mixture (50% absolute humidity), first at over 300 °C for 5 hours, then at over 400 °C for 9 hours.

[0090] Example 2 (comparison)

[0091] The VPO catalysts were prepared analogously to Example 1, except that the calcined powder of the catalyst precursor was mixed with 2.0 g per 100 g (Example 2) and simultaneously with the graphite. This resulted in calcined precursor powders containing 2.0 wt. % ZnO (Example 2).

[0092] The VPO catalysts according to Examples 1 and 2 were tested for their fracture toughness before and after activation. XRD diffractograms of the VPO catalysts were also obtained and tested for their catalytic selectivity.

[0093] The results show that the addition of ZnO after the calcination step does not result in a systematic increase in the lateral crushing strength (SDF) when measured immediately after tabletting. Surprisingly, however, this is the case after the subsequent activation step of the VPO catalyst tablets. This shows a systematic and very significant increase in the lateral crushing strength of the particles with the addition of ZnO.

[0094] The VPO catalyst is also characterized by infrared spectroscopic features. Conventional non-inventive VPO catalysts without ZnO exhibit a maximum in the IR spectrum at 790 cm -1 up to 810 cm' 1 (first absorption band) and a second maximum between 820 cm' 1 up to 840 cm' 1(second absorption band), whereby the second absorption band is stronger than the first absorption band (Figure 2, Example 1). It was found that with the addition of ZnO there was a simultaneous increase in the strength of the band at 790 cm -1 up to 810 cm' 1 (first absorption band) and a decrease in the strength of the maximum between 820 cm -1 up to 840 cm' 1 (second absorption band) occurs, so that the first absorption band in the samples is stronger than the strength of the second absorption band, or the strength of the second absorption band is even reduced to such an extent that it is no longer detectable (Figure 2, Example 2). In the latter case, the VPO catalysts are characterized by only exhibiting the first absorption band.

[0095] It is also shown that the VPO catalysts exhibit the following features observable by XRD diffraction (Figure 7):

[0096] 31.7 to 31.9° (sharp reflection, ZnO)

[0097] 34.3 to 34.5° (sharp reflection, ZnO)

[0098] 36.2 to 36.4° (sharp reflection, ZnO)

[0099] These features become more intense with increasing ZnO content and are in good agreement with ZnO (in wurzite structure or as zincite).

[0100] The VPO catalysts according to Example 2 were each tested for abrasion resistance after activation, resulting in an abrasion of 2.3 wt.% (based on the total weight of the catalyst used in the test).

[0101] Examples 3 to 5 (test 1 according to the invention)

[0102] The inventive moldings according to Examples 3 to 5 were prepared analogously to the VPO catalyst according to Example 2 (i.e., with the addition of 2 wt.% ZnO), but a different tablet shape was formed in each case using the rotary press. The dimensions of the moldings were as follows:

[0103] Example 3 (Outer) diameter 5.6 mm, height 4.5 mm, inner diameter 2.2 mm

[0104] Example 4 (Outer) diameter 5.6 mm, height 4.8 mm, inner diameter 2.8 mm

[0105] Example 5 (Outer) diameter 5.6 mm, height 4.5 mm, inner diameter 3.4 mm

[0106] Examples 6 to 8 (Test 2 According to the Invention) The inventive molded bodies according to Examples 6 to 8 were prepared analogously to the VPO catalyst according to Example 2 (i.e., with the addition of 2 wt. % ZnO), but a different tablet shape was formed in each case using the rotary press. The dimensions of the molded bodies were: Example 6 (outer) diameter 5.6 mm, height 4.1 mm, inner diameter 2.2 mm.

[0107] Example 7 (Outer) diameter 5.6 mm, height 4.2 mm, inner diameter 2.8 mm

[0108] Example 8 (Outer) diameter 5.6 mm, height 4.1 mm, inner diameter 3.4 mm

[0109] Table 1 shows that, despite the inventive geometry of the molded bodies with thinner walls, advantageous lateral crush strength can be maintained. Example 9 (Comparison)

[0110] The shaped bodies according to the invention according to Example 9 were produced analogously to the VPO catalyst according to Example 1 (ie without ZnO addition), but a different tablet shape was formed in each case using the rotary press.

[0111] Example 9 (Outer) diameter 5.6 mm, height 4.2 mm, inner diameter 3.36 mm. Figures 3 to 6 show that improved catalytic properties are achieved with the inventive geometry of the molded bodies, ie, with thinner cylinder walls. Table 1 shows that, despite the inventive geometry of the molded bodies with thinner walls, an advantageous lateral crush strength can be maintained.

[0112] Table 1 :

[0113] Methods

[0114] Infrared spectroscopy (IR)

[0115] The catalysts were characterized using infrared spectroscopy (IR) in transmission. This involves determining the wavelength-dependent intensity of infrared radiation from a source (Io) after passing through a sample (I). Absorption of the IR radiation and the resulting excitation of vibrational modes in the material under investigation leads to a wavelength-dependent attenuation of the infrared radiation. The absorption, calculated using log(lo / l), is plotted against the wavenumber for the spectrum.

[0116] The measurements were performed on a Nicolet Nexus 470 FTIR spectrometer equipped with a liquid nitrogen-cooled mercury cadmium tellurium (MCT) detector, an Everglo IR Source as the IR source, and a HeNe laser for frequency calibration. For sample preparation, tablets of the respective catalysts were finely ground, and a hand-stamped pellet with a diameter of 11 mm was produced from approximately 10 mg of material. This pellet was mounted in a vacuum chamber with KBr windows and placed in the beam path. The sample was then heated for 2 h at 200°C and a pressure of 10' 4 mbar to remove adsorbed water. For the measurements, the vacuum chamber containing the sample was cooled to 77 K using liquid nitrogen and maintained at a pressure of 10' 4 mbar with a step size of 2 cm' 1 , an aperture of 72 and an optical velocity of 0.64 the spectrum was recorded.

[0117] Determination of the IR absorption maximum

[0118] The strength of the two absorption bands with a maximum in the range between 790 cm' 1 up to 810 cm -1 and 820 cm' 1 up to 840 cm' 1corresponds to the strength of the absorption in the region of the maximum of the band in comparison to the spectral background. To determine the strength of the absorption in the region of the maximum, a first straight line was set as a linear background in such a way that it tangentially touches the spectrum, i.e. the background of the spectrum, immediately (i.e. within the frequency range up to a neighboring absorption band) to the left and right below the band in question. Then a line is drawn vertically downwards from the maximum of the band (S0), i.e. in the direction of constant wavelength and decreasing absorption, and the intersection point of this line with the linear background (S1) is determined. The strength of the absorption band is defined as the length of the distance S0 to S1.

[0119] Breaking strength test

[0120] A Zwick Z0.5 device was used to measure the breaking strength of the molded specimens and determine the force required to fracture. The measurements were carried out according to the ASTM D4179 standard. To dry the molded specimens prior to measurement, they were stored in a drying cabinet at 100°C for at least 3 hours, and the subsequent breaking strength measurement was carried out within a maximum of 1 hour after the end of drying. The device was operated according to the ASTM D4179 standard at a constant force rate of 20.0 N / s. For each example, 100 tablets were successively placed one after the other with the cylinder axis parallel to the measuring jaw surface (referred to as the "RADIAL CRUSH" in the ASTM D4179 standard) and measured. The mean value, which corresponds to the average breaking force of the molded specimen, was then determined from the 100 individual values ​​of the force required to fracture the molded specimen.All data on lateral compressive strength in this application refer to lateral compressive strengths obtained by the method described here.

[0121] Abrasion resistance test

[0122] An ERWEKA TAR120 measuring device was used to measure the abrasion of the tablets. Approximately 25 g of tablets were weighed for each sample and placed in the drum of the test device, which was then installed in the device. To measure abrasion resistance, 500 revolutions were used at a rotation speed of 25 revolutions per minute. This results in a time of 20 minutes for the abrasion measurement. Upon completion, the drum automatically empties into the collecting tray located below. After separating the abrasion from the intact tablets using a sieve, the intact tablets are weighed. The abrasion is then determined using the quotient of the difference between the initial weight and the intact tablets after abrasion measurement and the initial weight: Abrasion in % = (initial weight - intact tablets after abrasion) / initial weight * 100%

[0123] All information on abrasion in this application refers to the method described.

[0124] Powder X-ray diffraction (XRD)

[0125] The catalysts were characterized using X-ray powder diffraction (XRD). X-rays are diffracted at different angles by the crystalline regions of the sample. The diffraction angle is measured or plotted in 2°. Depending on the phase present, characteristic reflections are observed. These diffraction patterns can be used to assign the diffraction pattern to the phases present using a database.

[0126] The measurements were performed on a Bruker AXS D4 Endeavor with Cu-Ka radiation and a LYNXEYE detector. The diffraction patterns were recorded in an angle range of 20° from 5 to 50° with a step size of 0.02° and an acquisition time of 1.5 s per step with a fixed divergence aperture of 0.3°. For the measurements, the samples were finely ground and pressed into a sample holder. The diffraction angle was varied by tilting the sample. All information on XRD reflections in this application refers to XRD reflections obtained using this method.

[0127] To improve comparability, the diffractograms were standardized. For each diffractogram, the data point with the lowest intensity value is first found. This value is subtracted from all intensity values ​​of the respective diffractogram. The maximum intensity of the (024) reflection (located at a 20 value of 28.4-28.5°) in the respective diffractogram is then determined. All intensity values ​​of the respective diffractogram are divided by this value.

[0128] Catalytic test reaction

[0129] To determine catalyst performance, all catalysts were tested for their catalytic properties after complete preparation (reflux, filtration, vacuum drying, calcination, compaction, tabletting, activation) in a bench-scale reactor at 1.5 mol% butane in air in a dilute catalyst bed (1:9 catalyst:ceramic inert ring mixture). To determine catalyst activity, butane conversion was plotted against the modified residence time (quotient of catalyst mass and inlet flow rate—the otherwise usual contact time cannot be used because these measurements were taken on a dilute catalyst bed). The selectivity to maleic anhydride (MA) was measured at a reaction temperature of 410°C (salt bath) by varying the mass-based GHSV in the range 1300 to 5500 l / kg / h.

Claims

Claims 1 . VPO catalyst in the form of a shaped body for the oxidation of hydrocarbons with molecular oxygen, in particular for the oxidation of butane to maleic anhydride with molecular oxygen, wherein the VPO catalyst contains between 0.05 wt.% to 7.0 wt.% Zn, which is partly present as ZnO and has a first absorption band with a maximum between 790 cm' in infrared spectroscopy carried out in transmission 1 up to 810 cm -1 and possibly a second absorption band with a maximum between 820 cm' 1 up to 840 cm' 1 characterized in that only the first absorption band is present or the strength of the first absorption band is greater than the strength of the second absorption band and that the shaped body is a cylindrical shaped body with a height of 4 mm to 5 mm, an outer diameter of 5 mm to 6 mm and a central axial opening with a diameter of 2 mm to 4 mm.

2. VPO catalyst according to claim 1, characterized in that the strength of the absorption band is determined by setting a first straight line in such a way that it touches the spectrum tangentially immediately to the left and right below the band in question, then a second straight line is drawn starting from the maximum of the band in the direction of constant wavelength and decreasing absorption and the intersection point of the two straight lines is determined, the strength of the absorption band being defined as the length between the maximum and the intersection point.

3. VPO catalyst according to claim 1 or 2, characterized in that the ratio between the strength of the first absorption band and the strength of the second absorption band is above 1.1, preferably above 2.

4. VPO catalyst according to one of the preceding claims, characterized in that the VPO catalyst contains a Mo content between 0.1 wt.% and 1 wt.%, preferably between 0.4 wt.% and 0.7 wt.% Mo, in each case based on the total weight of the VPO catalyst.

5. VPO catalyst according to one of the preceding claims, characterized in that the VPO catalyst contains between 0.1 wt% and 6 wt%, preferably between 0.2 wt% and 4 wt% Zn, more preferably between 0.5 wt% and 3.5 wt%, based on the total weight of the VPO catalyst.

6. VPO catalyst according to one of the preceding claims, characterized in that the VPO catalyst has the following elemental composition: - 0.5 wt% to 7 wt% Zn, - 0 wt% to 0.7 wt% Mo, - 26 wt% to 31 wt% V, - 17 wt% to 21 wt% P, - 3 wt% to 5 wt% C, the remainder oxygen, each based on the total weight of the VPO catalyst.

7. VPO catalyst according to one of the preceding claims, characterized in that the shaped body has a lateral crushing strength of more than 15 N, preferably between 15 N and 200 N, wherein the lateral crushing strength is measured with a Zwick Z0.5 device using the ASTM D4179 standard with a constant force rate of 20.0 N / s, wherein 100 tablets are each individually placed with the cylinder axis parallel to the measuring jaw surface and measured in order to determine an average breaking force which represents the lateral crushing strength.

8. VPO catalyst according to one of the preceding claims, characterized in that the VPO catalyst, when examined by powder X-ray diffraction, has reflections at 31.7 ° to 31.9 °, 34.3 ° to 34.5 ° and 36.2 ° to 36.4 °, when measured with a D4 Endeavor from Bruker AXS with Cu-Ka radiation and a LYNXEYE detector, the diffraction patterns being recorded in an angular range 20 from 5 ° to 50 ° with a step size of 0.02 ° at an acquisition time of 1.5 s per step with a fixed divergence aperture of 0.3 °.

9. VPO catalyst according to one of claims 1 to 8, characterized in that the shaped body has a cylindrical shape with a height of 4.5 mm to 4.8 mm, a substantially round base with an outer diameter of 5.6 mm to 5.8 mm and a central axial opening with a diameter of 2.2 mm to 3.5 mm.

10. VPO catalyst according to one of claims 1 to 9, characterized in that the VPO catalyst has less than 1.5 wt.%, preferably less than 1.0 wt.% abrasion, based on the total weight of the catalyst, when the abrasion is measured with an ERWEKA TAR120 device, wherein approximately 25g of molded body is weighed and the abrasion measurement is carried out at 500 revolutions at a rotation speed of 25 revolutions per minute.

11. A process for producing a VPO catalyst according to any one of claims 1 to 10, comprising the steps of: a) preparing a catalyst precursor containing vanadyl hydrogen phosphate, b) forming the catalyst precursor into shaped bodies, c) activating the shaped bodies to form a VPO phase, characterized in that after step a) ZnO is mixed with the catalyst precursor.

12. A process for preparing a VPO catalyst according to claim 11, characterized in that the preparation of the catalyst precursor containing vanadyl hydrogen phosphate in step a) is carried out by the reduction of a V(V) compound in the presence of a P(V) compound by an organic reducing agent during a reflux step at atmospheric pressure in an organic solvent.

13. A process for producing a VPO catalyst according to claim 1 1 or 12, characterized in that the catalyst precursor obtained in step a) is filtered in a step ai) and dried and / or calcined in a step a2) at a temperature of not more than 300 °C.

14. A process for producing a VPO catalyst according to claim 13, characterized in that step a2) is carried out in two steps, wherein in a first step drying is carried out in vacuum at a temperature in the range of 90 °C to 140 °C for a period of 1 h to 24 h and in a second step calcination is carried out in nitrogen at a temperature in the range of 230 °C and 290 °C for a period of 1 h to 24 h.

15. A process for producing a VPO catalyst according to claim 11 to 14, characterized in that the activation step is carried out in a gas mixture consisting of air, inert gas and water vapor, at a temperature in the range of 300 °C to 500 °C, preferably in the range of 350 °C to 450 °C, for a period of 1 h to 24 h.

16. A process for producing a VPO catalyst according to any one of claims 11 to 15, characterized in that after step a) the catalyst precursor is additionally mixed with a Mg compound, preferably MgO.

17. A process for the preparation of maleic anhydride by catalytic oxidation of n-butane, wherein a reactant gas comprising oxygen and n-butane is passed through a reactor tube in which a bed of VPO catalysts according to one of claims 1 to 10 is located.

18. Process according to claim 17, characterized in that the bed of VPO catalysts in the reactor tube is at a temperature between 300 °C and 420 °C.

19. A process according to claim 17 or 18, characterized in that the reactant gas contains between 0.2 and 10 vol.% n-butane and between 5 and 50 vol.% oxygen and is carried out at a space-time velocity of 1 100 h -1 up to 2500 h' 1, preferably 1300 h' 1 until 1600 h -1 through the reactor tube.

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