VPO catalysts with improved selectivity and stability and methods of making same

Abstract

The invention relates to a VPO catalyst in the form of a molded body for the oxidation of hydrocarbons with molecular oxygen, in particular for the oxidation of butane to maleic anhydride with molecular oxygen, the VPO catalyst containing 0.05 to 7.0 wt.% of Zn, which is partially present as ZnO, and having a first absorption band in the transmission infrared spectrum having a maximum of 790 to 810 cm-1, and possibly a second absorption band having a maximum of 820 cm <-1 > 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. The invention also relates to a method for preparing the VPO catalyst according to the invention, comprising the following steps: a) preparing a catalyst precursor containing vanadium oxyhydrogen phosphate, b) forming the catalyst precursor into a shaped body, c) activating the shaped body to form a VPO phase, characterized in that, after step a), ZnO is mixed with the catalyst precursor.

Description

[0001] 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 0.05 to 7.0% by weight of Zn, partly in the form of ZnO, and has a maximum at 790 cm-1 in the transmission infrared spectrum. -1 Up to 810cm -1 The first absorption band may have a maximum at 820 cm -1 Up to 840cm -1 The second absorption band at is characterized in that only the first absorption band exists or the intensity of the first absorption band is greater than the intensity of the second absorption band.

[0002] The present invention also relates to a method for preparing the VPO catalyst of the present invention, comprising the following steps:

[0003] a) preparing a catalyst precursor containing vanadyl hydrogen phosphate,

[0004] b) shaping the catalyst precursor to obtain a shaped body,

[0005] c) activating the shaped body to form a VPO phase,

[0006] The method is characterized in that, after step a), ZnO is mixed with a catalyst precursor.

[0007] The present invention also relates to the use of a Zn compound in solid form for stabilizing a VPO catalyst in particulate form.

[0008] Maleic anhydride is an economically important chemical intermediate. For example, it is used alone or in combination with other acids in the preparation of alkyd and polyester resins. In addition, it is a versatile intermediate in chemical syntheses, for example for the synthesis of gamma-butyrolactone, tetrahydrofuran and butane-1,4-diol, which in turn are used as solvents or can be further processed into polymers, such as polytetrahydrofuran or polyvinylpyrrolidone.

[0009] Maleic anhydride is usually prepared by partial oxidation of n-butane with molecular oxygen or a molecular oxygen-containing gas in the gas phase in the presence of a vanadium phosphorus oxide catalyst (VPO catalyst) containing vanadyl pyrophosphate (VPP). Vanadyl pyrophosphate in its pure form contains vanadium with a valence of +4 and is particularly suitable for preparing maleic anhydride from unbranched saturated hydrocarbons or unbranched unsaturated hydrocarbons having at least four carbon atoms. Both fixed bed reactors and fluidized bed reactors can be used.

[0010] VPO catalysts have only low intrinsic activity in the reaction of n-butane to maleic anhydride. Therefore, sufficient conversion requires large amounts of catalyst. In addition, VPO catalysts are the most expensive of all non-precious metal catalysts, mainly because of their high raw material costs. This sets the goal of improving the catalyst performance (activity and selectivity) as well as the life 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, for example by adding molybdenum (Mo promoter or Mo dopant).

[0011] US 5929256 discloses the synthesis of an active vanadium phosphorus catalyst modified by molybdenum for the preparation of maleic anhydride. In this synthesis, a compound consisting essentially of pentavalent vanadium reacts with a compound containing pentavalent phosphorus in an alcohol medium suitable for reducing vanadium to an oxidation state lower than 5. This results in the incorporation of molybdenum into the reaction product to form a molybdenum-modified solid precursor composition. The alcohol is removed to obtain a molybdenum-modified dried solid precursor composition. It is shaped into a molded body comprising a molybdenum-modified dried solid precursor composition. The molybdenum-modified dried and shaped precursor composition is activated to convert it into an active catalyst.

[0012] US 5070060 discloses an improvement in an oxidation catalyst for partial oxidation of n-butane comprising a mixed oxide of vanadium and phosphorus, a mixed oxide of zinc and lithium, comprising adding a molybdenum compound modifier in an amount of about 0.005 / 1 to about 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 very stable catalyst, constitutes a more active system, and lasts longer than the unmodified catalyst.

[0013] US 3980585 discloses a catalyst complex suitable for converting normal C4 hydrocarbons to maleic anhydride in the gas phase, which comprises the components vanadium, phosphorus and copper and one of the elements selected from Te, Zr, Ni, Ce, W, Pd, Ag, Mn, Cr, Zn, Mo, Re, Sm, La, Hf, Ta, Th, Co, U and Sn, preferably containing an alkali metal or an alkaline earth metal.

[0014] US 4056487 discloses a catalyst complex suitable for the partial oxidation of alkanes to the corresponding anhydrides, for example the conversion of normal C4 hydrocarbons to maleic anhydride in the gas phase, the catalyst complex comprising the components vanadium, phosphorus and oxygen, Nb, Cu, Mo, Ni, Co and Cr. Preferably, the components further comprise one or more than one element selected from Ce, Nd, Ba, Hf, U, Ru, Re, Li or Mg.

[0015] US 4515904 discloses a process for preparing a phosphorus-vanadium catalyst and a phosphorus-vanadium co-metal catalyst for preparing 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 in the presence of water or an aliphatic alcohol having from about 1 to about 8 carbon atoms at a temperature of from about 0°C to about 200°C; removing the solvent; and activating the catalyst by adding butane or another hydrocarbon feedstock and a phosphorus compound at a temperature of from about 300°C to about 500°C.

[0016] US 5158923 discloses an improvement in an oxidation catalyst for partial oxidation of n-butane comprising a mixed oxide of vanadium and phosphorus, a mixed oxide of zinc and lithium, comprising adding a molybdenum compound modifier in an amount of about 0.005 to about 0.025 Mo / V to the catalyst during the destruction of the reduced vanadium compound by concentrated phosphoric acid. The addition of Mo produces a very stable catalyst, constitutes a more active system, and lasts longer than the unmodified catalyst.

[0017] US 5262548 discloses an improvement in an oxidation catalyst for the partial oxidation of n-butane comprising a mixed oxide of vanadium and phosphorus, a mixed oxide of zinc and lithium, comprising adding a molybdenum compound modifier in an amount of about 0.005 to about 0.025 Mo / V to the catalyst during the destruction of the reduced vanadium compound by concentrated phosphoric acid. The addition of Mo results in a catalyst that constitutes a very stable active system and has a longer life than the unmodified catalyst.

[0018] WO 2013062919 A1 discloses a method for preparing an upgraded VPO catalyst, wherein the catalyst comprises a mixed oxide of vanadium and phosphorus, and wherein the catalyst is upgraded with at least one selected from niobium, cobalt, iron, zinc, molybdenum or titanium, the method comprising the steps of: (i) preparing a VPO catalyst comprising vanadyl pyrophosphate as a major component and comprising less than 5 wt% of vanadyl phosphate, (ii) contacting the VPO catalyst with a solution comprising a compound containing at least one metal selected from niobium, cobalt, iron, zinc, molybdenum or titanium as a metal source to form a metal-impregnated VPO catalyst, and (iii) drying the metal-impregnated VPO catalyst to form an upgraded VPO catalyst. In one embodiment, a niobium-activated VPO catalyst is prepared.

[0019] US 5280003 discloses an improvement in an oxidation catalyst for partial oxidation of n-butane comprising a mixed oxide of vanadium and phosphorus, a mixed oxide of zinc, lithium and molybdenum, wherein the preparation of the catalyst comprises a crystallization step under static conditions that allow more uniform crystal growth. During the crystallization, the static conditions are maintained by heating the reflux solvent.

[0020] US 4251390 discloses an improvement in an oxidation catalyst for partial oxidation of n-butane comprising a mixed oxide of vanadium and phosphorus, characterized in that a zinc compound is added to the catalyst in an amount of 0.15 to 0.001 Zn / V during the reaction of the reduced vanadium component with concentrated phosphoric acid. The addition of zinc produces a catalyst that is more easily activated and is very stable to heating of the reaction system. Small amounts of lithium compounds and silicon compounds also have additional desirable catalytic effects without reducing the benefits of the zinc compound.

[0021] DE 10 2014 004786 A1 relates to a catalyst comprising vanadium phosphorus oxide and an alkali metal, wherein the weight proportion of the alkali metal in the vanadium phosphorus oxide is 10 ppm to 400 ppm, based on the total weight of the vanadium phosphorus oxide, and also to 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.

[0022] To prepare a VPO catalyst containing a VPP phase, vanadium pentoxide (V2O5) is usually reduced in an organic alcohol solvent in the presence of phosphoric acid using benzyl alcohol as a reducing agent to form vanadyl hydrogen phosphate (VHP) together with benzaldehyde. The redox reaction ("reduction") carried out here is as follows, in which vanadium in oxidation state V (V(V)) reacts to form a VHP phase in which vanadium oxy species (VO) with oxidation state IV (V(IV)) are present. 2+ ):

[0023] (1)V2O5+2H3PO4+Ph-CH2-OH→2VOHPO4*1 / 2H2O+Ph-CHO+2H2O

[0024] In the subsequent activation step, the VHP phase is transformed into the vanadyl pyrophosphate phase by heating with simultaneous elimination of water.

[0025] (2)2VOHPO4*1 / 2H2O→(VO)2P2P7+1 1 / 2H2OThe problem with using VPO catalyst to catalyze the reaction of butane into maleic anhydride is that the process must be carried out under a state limited by pore diffusion. Therefore, the porosity has a direct impact on the catalytic yield here. Therefore, care must be taken to ensure that the pore structure is not adversely affected during the molding (e.g., by tableting). However, this does have the consequence of damaging the mechanical stability of the molded body. However, its mechanical stability must be high enough to keep the tablets intact during the filling process (falling into a reaction tube of about 6m long). Otherwise, smaller fragments will cause the dynamic pressure in the process to be too high, resulting in high compressor costs or reduced production. Therefore, the technical purpose is to increase the mechanical strength of the molded body without adversely affecting the pore structure. At the same time, a VPO catalyst with improved performance, i.e., a VPO catalyst with improved activity, selectivity and stability, is required.

[0026] It was therefore an object of the present invention to provide improved VPO catalysts for the gas phase oxidation of hydrocarbons, in particular for the preparation of maleic anhydride, which have increased catalytic properties, in particular improved selectivity, and at the same time significantly improved mechanical strength compared to the catalysts typically used hitherto.

[0027] 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 0.05 to 7.0% by weight of Zn, partly in the form of ZnO, and has a maximum at 790 cm-1 in the transmission infrared spectrum. -1 Up to 810cm -1 The first absorption band at 820 cm -1 Up to 840cm -1 The second absorption band at is characterized in that only the first absorption band exists or the intensity of the first absorption band is greater than the intensity of the second absorption band.

[0028] The intensity of the absorption band is determined here by drawing in each case a first straight line which touches the spectrum tangentially to the left and right immediately below the band in question, then drawing a second straight line starting from the maximum of the band in question in the direction of constant wavelength and decreasing absorption, and determining the point of intersection of the two straight lines, wherein the intensity of the absorption band is defined as the length between the maximum and the point of intersection. In this case, the ratio of the intensity of the first absorption band to the intensity of the second absorption band is preferably greater than 1.1, preferably greater than 2.

[0029] The lateral compression strength of the shaped bodies according to the invention exceeds 25 N, preferably 25 N to 200 N, which is determined with a Zwick Z0.5 tester using standard ASTM D4179 at a constant force rate of 20.0 N / s, wherein 100 pieces are each placed individually and measured with the cylinder axis parallel to the surface of the measuring jaws to determine the average rupture force, which represents the lateral compression strength.

[0030] When analyzed by powder x-ray diffraction using Cu—Kα radiation, the VPO catalyst according to the present invention shows reflections at 31.7° to 31.9°, 34.3° to 34.5°, and 36.2° to 36.4°.

[0031] The VPO catalyst according to the present invention comprises a VPO phase or contains or consists essentially of a VPO phase. For example, based on the total weight of the VPO catalyst, the VPO catalyst according to the present invention contains a VPO phase in an amount of more than 70 wt%, preferably more than 80 wt%, and particularly preferably more than 90 wt%. In addition, the VPO catalyst may contain VPP, a dopant, and unreacted oxides of the raw materials, such as vanadium pentoxide or phosphorus oxide.

[0032] The VPO catalyst according to the present invention may optionally contain 0.1 wt % to 1 wt %, preferably 0.4 wt % to 0.7 wt %, of Mo, based on the total weight of the VPO catalyst.

[0033] However, the VPO catalyst according to the invention may also contain alkali metals, such as Na and / or K, preferably 80 ppm to 300 ppm of alkali metals. In addition, the VPO catalyst according to the invention may also contain carbon, such as carbon in the form of graphite, for example in an amount of 3% to 5% by weight, based on the total weight of the catalyst. The graphite present is used, for example, as a tableting aid.

[0034] The VPO catalyst according to the invention comprises ZnO, but may also contain other Zn compounds. The Zn content in the VPO catalyst resulting from the presence of ZnO and possibly other Zn compounds must be in the range of 0.05% to 7.0% by weight, preferably 0.1% to 6.0% by weight, more preferably 0.2% to 4.0% by weight, most preferably 0.7% to 3.0% by weight of Zn, in each case based on the total weight of the catalyst.

[0035] The VPO catalyst preferably has the following elemental composition, based in each case on the total weight of the VPO catalyst:

[0036] - 0.05 to 7 wt. % Zn,

[0037] - 0 to 0.7 wt. % Mo,

[0038] - 26 to 31 wt. % of V,

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

[0040] - 3 to 5 wt% C,

[0041] The rest is oxygen.

[0042] The VPO catalyst according to the invention contains ZnO, and if the content of these two phases is high enough, the catalyst exhibits reflections typical of the ZnO phase in the XRD powder diffraction pattern recorded using Cu-Kα radiation. In particular, sharp reflections are detected at 31.7° to 31.9°, 34.3° to 34.5°, 36.2° to 36.4°.

[0043] According to the present invention, the presence of ZnO has the effect of stabilizing the catalyst particles, so that they have a higher mechanical strength than when ZnO is not present. However, the presence of ZnO also has the effect of improving the catalytic properties (i.e. activity, selectivity and stability) of the catalyst particles, in particular the effect of increasing the selectivity.

[0044] The VPO catalyst according to the invention is present in the form of a shaped body; the shape of the shaped body can be designed differently depending on the contact time, flow rate and dynamic pressure desired during the catalytic reaction. VPO catalysts in the form of shaped bodies are understood to mean shaped bodies prepared by a forming step such as tableting. When present in large quantities in a tube bundle reactor, the shaped bodies according to the invention form a layer or a filling of the catalyst through which the reactants butane and air pass, the butane being especially n-butane.

[0045] According to the invention, the shaped body should not be smaller than a minimum size, which corresponds, for example, to a size in which the shaped body cannot fit into an imaginary cube of 3 mm×3 mm×3 mm without exceeding its limits at any point.

[0046] For example, the shaped body according to the invention may be in the form of a conventional cylinder. In this case, the height of the cylinder (length along the axis of the cylinder) is 3 to 8 mm and has a substantially circular bottom surface with a diameter of 3 to 8 mm. Preferably, the cylinder has a central axial opening; it may have a diameter of, for example, 1 to 3 mm.

[0047] For example, a shaped body according to the invention may have a height of 4.7 mm, an outer diameter of 4.7 mm and a central axial opening with a diameter of 1.3 mm. The geometric surface area of ​​the shaped body is then 1.2 cm 2 , with a volume of 0.075 cm 3, and the mass is 0.12g. A large amount of this molded body is filled into a 21mm reactor to produce 0.85g / cm 3 Up to 0.89g / cm 3 Poured density (poureddensity).

[0048] For example, a shaped body according to the invention may have a height of 5.6 mm, an outer diameter of 5.5 mm and a central axial opening with a diameter of 2.3 mm. The geometric surface area of ​​these shaped bodies is then 1.77 cm 2 , with a volume of 0.111 cm 3 , and the mass is 0.18g. A large amount of this molded body is filled into a 21mm reactor to produce 0.72g / cm 3 Up to 0.76g / cm 3 Pour density.

[0049] Preferred shaped bodies for the reactor concept according to the invention are the shaped bodies described in EP 2643086 A1. The preferred double alpha shape is characterized in particular in that each individual shaped body is designed in each case in the form of a cylinder having an outer base [1], a cylinder surface [2], a cylinder axis and at least one continuous opening [3] extending parallel to the cylinder axis, and the outer base [1] of the cylinder contains at least four corners (lobes) [4a, 4b, 4c, 4d], wherein the geometrical base surrounding the shaped body is a prism, wherein the prism has a prism base with a length and a width, wherein the length is greater than the width, and wherein the corners [4a, 4b, 4c, 4d] are surrounded by the prism corners ( Fig. 9 ).

[0050] Preferred are double alpha shaped bodies with a height (length along the cylindrical axis) of 3 mm to 8 mm, a length of 5 mm to 9 mm, and a width of 4 mm to 8 mm, and an inner hole diameter of 1 mm to 4 mm. For example, preferred are double alpha shaped bodies with a height of 5.6 mm, a length of 6.7 mm, a width of 5.8 mm, and an inner hole diameter of 2.1 mm. The geometric surface area of ​​these shaped catalyst bodies is 2.37 cm 2 , with a volume of 0.154 cm 3 , and the mass is 0.24g. Filling into a 21mm reactor produces 0.60g / cm 3 Up to 0.62g / cm 3 Pour density.

[0051] The lateral pressure strength of the VPO catalyst / shaped body of the present invention exceeds 25 N, preferably 25 N to 200 N. Particularly preferably, the lateral pressure strength of the VPO catalyst / shaped body exceeds 30 N to 150 N, even more preferably exceeds 35 N to 100 N.

[0052] The lateral pressure strength of the cylindrical VPO catalyst / shaped body of the invention exceeds 25 N, preferably 25 N to 50 N. Particularly preferably, the lateral pressure strength of the VPO catalyst / shaped body exceeds 30 N to 45 N, even more preferably exceeds 35 N to 40 N.

[0053] The lateral compression strength of the VPO catalyst / shaped body of the invention in double alpha shape exceeds 50 N, preferably 50 N to 200 N. Particularly preferably, the lateral compression strength of the VPO catalyst / shaped body is from more than 100 N to 170 N, even more preferably from more than 120 N to 150 N.

[0054] The present invention also relates to a method for preparing the VPO catalyst of the present invention, comprising the following steps:

[0055] a) preparing a catalyst precursor containing vanadyl hydrogen phosphate,

[0056] b) shaping the catalyst precursor to obtain a shaped body,

[0057] c) activating the shaped body to form a VPO phase,

[0058] The method is characterized in that, after step a), ZnO is mixed with a catalyst precursor.

[0059] In step a) of the process, a catalyst precursor containing vanadyl hydrogen phosphate is prepared in a generally known manner. Typically, in this case, in the reaction mixture, the V(V) compound in solution is reduced in the reduction step in the presence of the P(V) compound, optionally the Mo compound, by means of a reducing agent. 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 the V(V) compound, up to 1% by weight of the Mo compound and 5% to 25% by weight of the P(V) compound. More specifically, for example, 60% to 70% by weight of isobutanol, 5% to 15% by weight of benzyl alcohol, 5% to 15% by weight of vanadium pentoxide, 0.05% to 0.2% by weight of (NH4)2Mo2O7 and 10% to 20% by weight of phosphoric acid, in each case based on the total weight of the reaction mixture, can be initially added as the reaction mixture.

[0060] The V(V) compound used as a starting material in the reaction mixture for preparing the vanadyl hydrogen phosphate is a compound containing vanadium in the oxidation state V, and is preferably V2O5. The P(V) compound used as a starting material in the reaction mixture for preparing the vanadyl hydrogen phosphate is a compound containing phosphorus in the oxidation state V, and is preferably phosphoric acid or a phosphate such as Na3PO4. If phosphoric acid (H3PO4) is used, it is preferably anhydrous (100% phosphoric acid) or phosphoric acid containing only a small amount of water, i.e. phosphoric acid having a concentration of 98% to 100%, preferably 99% to 100% (the percentage values ​​refer to the weight percentage content of pure phosphoric acid relative to the weight of the water-phosphoric acid mixture, as usually expressed). Alternatively, for the preparation of the reaction mixture for the preparation of vanadium hydrogen phosphate, phosphoric acid in a concentration of more than 100% may be used which immediately reacts with any water initially present in the reaction mixture to form phosphoric acid in a concentration of 98% to 100%, preferably 99% to 100%, more preferably 100%, resulting in a reaction mixture free of phosphoric acid in a concentration of more than 100% and at the same time not more than 0.2% by weight of water, based on the weight of the reaction mixture, remaining in the reaction mixture.

[0061] Molybdenum compounds which can optionally be used as starting materials in the reaction mixture for preparing the vanadyl hydrogen phosphate are any desired molybdenum-containing compounds, for example molybdenum trioxide, ammonium heptamolybdate ((NH4)6Mo7O 24 )*4H2O), ammonium paramolybdate ((NH4)6Mo7O2*4H2O), metamolybdate, molybdic acid (H2MoO4) and its salts, such as (NH4)2MoO4, Na2MoO4, K2MoO4 or (NH4)2Mo2O7.

[0062] The reducing agent present in the reaction mixture for preparing the vanadyl hydrogen phosphate may be any desired reducing agent capable of reducing the V(V) compound to at least partially form the vanadyl hydrogen phosphate. Preferably, the reducing agent is an organic reducing agent, such as ethanol, isobutanol or an aromatic alcohol, in particular including benzyl alcohol.

[0063] The solvent present in the reaction mixture for preparing the vanadyl hydrogen phosphate is preferably an alcohol, more preferably a high-boiling aliphatic alcohol, especially isobutanol, or ethanol or isopropanol.

[0064] The starting materials are provided in a suitable reaction vessel to carry out the reduction step and obtain the vanadyl hydrogen phosphate of the catalyst precursor. In this case, the reaction mixture is heated to a temperature above room temperature, for example up to 100° C. Since the reduction is preferably carried out with stirring in the reflux step, the reaction vessel preferably has a reflux condenser and means for stirring the reaction mixture. Optionally, the reaction vessel is equipped with means allowing the water formed during the reduction to be removed from the reaction mixture, i.e. a water separator, for example a Dean-Stark trap.

[0065] The reduction to form the vanadyl hydrogen phosphate is preferably carried out under reflux at standard pressure, in which case the temperature is raised to correspond to the boiling point of the solvent used; preferably, the process according to the invention comprises carrying out only a single reflux step. The catalyst precursor preferably contains vanadyl hydrogen phosphate as the main phase, or can even consist essentially of a vanadyl hydrogen phosphate phase. Molybdenum, which may also be present in the catalyst precursor, can be present in the form of a vanadyl hydrogen phosphate phase-doped, molybdenum doping being understood to mean that molybdenum is either incorporated into the vanadyl hydrogen phosphate phase or is present on its surface. However, in addition to the vanadyl hydrogen phosphate phase, the reduction may also lead to the formation of further vanadium-phosphorus mixed oxides, in which the vanadium has an oxidation state of IV or even III. The reduction does not need to be carried out to completion, which means that part of the V(V) compound and part of the P(V) compound also remain in the catalyst precursor. However, in the catalyst precursor, vanadium is generally present in an average oxidation state of 3.8 to 4.2.

[0066] The water formed during the reduction can be removed from the reaction mixture during the reduction. In the prior art, during the reduction, the removal of water is either physically achieved, for example by means of a water separator, or chemically achieved by using compounds that bind water, such as desiccants or anhydrides such as phosphoric acid in a concentration exceeding 100%. The reaction mixture can contain, for example, anhydrides that react with water and bind water, in particular, the reaction mixture can contain phosphoric acid in a concentration exceeding 100%.

[0067] The suspension obtained after the reduction step can be filtered, for example, in an atmosphere of an inert gas such as nitrogen or a noble gas. In this case, an inert gas is any gas which does not react with the catalyst precursor under certain conditions during the filtration process but at the same time displaces the oxygen in the air in order to minimize the risk of explosion. The filtration is carried out in a manner known to those skilled in the art, usually by means of a filter press, a decanter or by means of a filter funnel. The filtration yields an uncalcined catalyst precursor which is still wetted with the solvent.

[0068] The catalyst precursor (solid filter residue) obtained by filtration can subsequently be dried; this is generally carried out under reduced pressure / vacuum or under an inert gas at a temperature above room temperature, for example at a temperature of up to 150° C., to obtain a dried catalyst precursor. In this context, an inert gas is any gas that does not react with the catalyst precursor under drying conditions but at the same time displaces oxygen from the air to minimize the risk of explosion, such as nitrogen or a noble gas. Preferably, the drying is carried out under reduced pressure / vacuum at a temperature of 50° C. to 150° C., preferably 90° C. to 140° C.

[0069] Alternatively or optionally, drying may be followed by calcination to obtain a catalyst precursor. Calcination is carried out under an inert gas at a high temperature of 150° C. to 350° C., preferably 230° C. to 290° C. In this context, an inert gas refers to any gas that does not react with the catalyst precursor under calcination conditions but at the same time displaces oxygen from the air to minimize the risk of explosion, such as nitrogen or a noble gas.

[0070] Optionally, graphite may be added to the dried catalyst precursor to facilitate shaping in step b) of the process. The dried catalyst precursor may also be compacted and / or granulated to obtain a compacted or granulated dried catalyst precursor. In this case, the catalyst precursor may be compacted into a sheet using a roller compactor at a compaction pressure of 190 bar, a gap width of 0.60 mm and a roller speed of 7 rpm and granulated through a 1 mm screen.

[0071] In step b) of the process, the catalyst precursor obtained is shaped into shaped catalyst bodies; this can be achieved, for example, by tabletting. This is achieved, for example, by pressing the granules into the desired tablet shape with an appropriate height using a rotary tablet press.

[0072] In step c) of the subsequent method, the shaped catalyst body obtained is activated at a temperature above 200° C. Activation is usually carried out in a gas mixture consisting of air, an inert gas and water vapor, wherein the inert gas used can be any gas that does not react with the shaped catalyst body under the specific conditions during the activation process, and it is particularly preferably nitrogen or a noble gas. Alternatively, the activation can also be carried out in a process gas, that is, in a gas mixture containing air and butane. The activation is carried out at a temperature of 300° C. to 500° C., preferably 350° C. to 450° C. Activation provides a finished VPO catalyst as a product of the method. The tableted VPO catalyst usually has a side compression strength of more than 25N, usually 25N to 200N.

[0073] The method according to the invention for preparing a VPO catalyst in the form of a shaped body is characterized in that, after step a) of the method, ZnO is added to the catalyst precursor or mixed with it. ZnO, i.e. (zinc (II) oxide), is used in this case as a powder medium solid. The addition of ZnO to the catalyst precursor can also be achieved with the aid of a metering device and a mixer. In order to prepare the catalyst of the invention, sufficient ZnO is usually added. For example, based on the weight of the dried catalyst precursor (without ZnO), 0.5% to 7.0% by weight, preferably 1.0% to 5.0% by weight, more preferably 1.5% to 3.5% by weight of ZnO in solid form can be added.

[0074] The ZnO is added to the catalyst precursor, i.e. after preparation in step a) of the process, for example by a reduction step, but before shaping the shaped catalyst bodies in step c) of the process. Preferably, the ZnO is added to the dried catalyst precursor, more preferably to the dried and calcined catalyst precursor.

[0075] In other preferred embodiments, in addition to the addition of ZnO, further solid metal compounds are added, preferably in powder form. Preferably, a solid Mg compound is also present in the binder, such as MgO.

[0076] The invention further relates to the use of ZnO for stabilizing VPO catalysts in the form of shaped bodies and to the use of ZnO as a binder for VPO catalysts in the form of shaped bodies.

[0077] The invention also relates to a process for preparing maleic anhydride by catalytic oxidation of n-butane, wherein a reaction gas comprising oxygen and n-butane is passed through a reaction tube containing a packing of the VPO catalyst according to the invention.

[0078] The filling of the VPO catalyst in the reaction tube consists of the VPO catalyst / molded body according to the invention, which is poured into the reaction tube and forms a filling of the catalyst after standing. The reaction tube is preferably part of a large number of reaction tubes of a tube bundle reactor, such as are known to those skilled in the art for the industrial preparation of maleic anhydride. During the reaction, the filling of the VPO catalyst is present at a temperature of 300° C. to 420° C. The reaction gas may contain, for example, 0.2% to 10% by volume of n-butane and 5% to 50% by volume of oxygen and is heated at 1100 h. -1 Up to 2500 hours -1 , preferably 1300 hours -1 Up to 2000 hours -1 The space velocity passes through the reaction tube.

[0079] Figure 1 : IR spectra of the samples according to Examples 1 to 7 and 12.

[0080] Figure 2 : IR spectra of samples according to Example 1 and Example 15 and Example 16.

[0081] Figure 3 : IR spectra of the samples according to Example 5 and Comparative Examples 17 and 18.

[0082] Figure 4 : Effect of using ZnO solid and ZnO / MgO solid in the synthesis on the stability of the VPO catalysts according to Examples 1 to 7.

[0083] Figure 5 : Changes in side pressure strength and selectivity of VPO catalysts with different Zn addition amounts.

[0084] Figure 6 : XRD diffraction patterns of the VPO catalysts according to Examples 1 to 5.

[0085] Figure 7 : XRD diffraction patterns of the VPO catalysts according to Example 5 and Example 8 to Example 10.

[0086] Figure 8 : Comparison of the XRD diffraction patterns of the VPO catalysts according to Examples 11 and 12 with the XRD diffraction pattern of the VPO catalyst according to Example 5 prepared according to the present invention.

[0087] Fig. 9 : Illustrations of the preferred catalyst particle "double alpha shape" presented from four different viewing angles.

[0088] The numbers in brackets in the figures refer in each case to a sample prepared from the corresponding example. Example

[0089] Example 1 (comparative)

[0090] Devices used

[0091] The heating jacket with the 2-liter four-necked flask inside is placed on a laboratory jack. In the center neck of the four-necked flask there is a half-moon-shaped stirring paddle with a corresponding stirrer seal, which is connected to the stirrer unit via a stirrer coupling. In the right neck there is a thermometer and in the left neck there is a riser for the reflux condenser. The center neck at the front is used for filling with chemicals, after which the nitrogen inlet is connected to it. The entire apparatus can also be flushed with nitrogen. For this purpose, the nitrogen first passes through a wash bottle and then enters the apparatus and flows out at the top of the condenser, passing through the wash bottle again.

[0092] Preparation of catalyst precursor

[0093] First, 1069.5 g of isobutyl alcohol and 156.0 g of benzyl alcohol were added. 150 g of V2O5 were added with stirring. After the addition of V2O5, 2.52 g of ammonium dimolybdate were added. Subsequently, 232.50 g of phosphoric acid (100%, anhydrous) were added to the suspension, and the mixture was heated under reflux under N2 for 10 hours.

[0094] filter

[0095] After cooling the suspension containing the catalyst precursor product in solid form, the suspension was transferred from the four-necked flask to a filter funnel and the liquid was removed by suction. The moist filter cake was press dried at a pressure of 14 to 18 bar overnight.

[0096] Drying / calcination

[0097] The pressed filter cake was transferred to the evaporating flask of a rotary evaporator. The filter cake was dried overnight at 110° C. under a water jet vacuum. The powder dried in this way was placed in the furnace in a suitable calcining pot and calcined at a temperature of 200° C. to 300° C. in a N2 atmosphere for 9 hours.

[0098] Compacting / tabletting

[0099] Before compaction / tabletting, 5 wt% of graphite was added to the calcined powdered catalyst precursor product and mixed uniformly using a drum hoop mixer. The powder was compacted into a plate using a roller compactor at a compaction pressure of 190 bar, a gap width of 0.60 mm and a roller speed of 7 rpm and granulated through a 1 mm screen.

[0100] The granules are compressed into a desired tablet shape using a rotary tablet press, which has a suitable height, for example, 5.6 mm×5.6 mm×2.3 mm, and a suitable side compression strength.

[0101] Activated to pyrophosphate

[0102] The activation of the vanadium-forming pyrophosphate is carried out under controlled conditions in a retort installed in a programmable furnace. The calcined pieces are uniformly loaded into the retort and the retort is tightly sealed. The catalyst is then activated in a humid air / nitrogen mixture (50% atmospheric humidity), initially at over 300° C. for 5 hours and then at over 400° C. for 9 hours.

[0103] Embodiment 2 to Embodiment 7 (the present invention)

[0104] The VPO catalyst of the invention was prepared in a manner similar to Example 1, except that 4.5 g (Example 2), 1.8 g (Example 3), 5.0 g (Example 4), 2.0 g (Example 5), 1.0 g (Example 6), 0.5 g (Example 7) of ZnO powder (commercial ZnO, particle size <100 μm, impurity content less than 0.05 wt%) and 0.5 g (Example 2) and 0.2 g (Example 3) of MgO powder were mixed per 100 g of the calcined powder of the catalyst precursor, and simultaneously mixed with graphite. This provided calcined precursor powders containing 5.0 wt% ZnO / MgO (Example 2), 2.0 wt% ZnO / MgO (Example 3), 5.0 wt% ZnO (Example 4), 2.0 wt% ZnO (Example 5), 1.0 wt% ZnO (Example 6) and 0.5 wt% ZnO (Example 7).

[0105] The fracture strength of the VPO catalysts according to Examples 1 to 7 was tested in each case before and after activation. XRD diffractograms of the VPO catalysts were produced and the catalytic selectivity of these VPO catalysts was tested.

[0106] Table 1 and Figure 5 The results presented in show that the addition of ZnO or ZnO / MgO after the calcination step does not lead to a systematic increase in the side compression strength (SCS) observed in the measurements carried out immediately after tabletting. However, surprisingly, this is not the case after the VPO catalyst tablets have undergone a subsequent activation step. In this case, the particles with the addition of ZnO or ZnO / MgO show a systematic and very clear increase in the side compression strength. Tables 1 and Figure 5 It was also shown that under the selected catalytic conditions (85% conversion), the addition of ZnO or ZnO / MgO resulted in an increase in the selectivity.

[0107] The VPO catalysts of the present invention are also characterized by infrared spectral characteristics. A typical non-inventive VPO catalyst that does not contain ZnO has an IR spectrum at 790 cm -1 Up to 810cm -1 The first absorption band has a maximum value at 820 cm -1 Up to 840cm -1 (second absorption band) has a second maximum, wherein the second absorption band in this case is more intense than the first absorption band ( Figure 1 , (1)). It has been found that the addition of ZnO or ZnO / MgO simultaneously causes the -1 Up to 810cm -1 The intensity of the band at 820 cm (the first absorption band) increases and -1 Up to 840cm -1 The intensity of the maximum value at (the second absorption band) is reduced, so that the first absorption band in the samples of the invention is stronger than the intensity of the second absorption band, or the intensity of the second absorption band is even reduced to such an extent that it is no longer detectable ( Figure 1 , (2) to (7)). In the latter case, the VPO catalysts of the invention are characterized in that they have only the first absorption band.

[0108] It can also be seen that the VPO catalyst according to the present invention has the following characteristics observable by XRD diffractometry: Figure 6 ):

[0109] 31.7° to 31.9° (sharp reflection, ZnO)

[0110] 34.3° to 34.5° (sharp reflection, ZnO)

[0111] 36.2° to 36.4° (sharp reflection, ZnO)

[0112] These features become more intense with increasing ZnO content and are very consistent with ZnO in the wurtzite structure / as zincite.

[0113] Comparative Example 8, Example 9, Example 10

[0114] In order to examine the correlation between the presence of a stable ZnO phase and the time of addition of the binder, XRD diffractograms were recorded after different addition times. In Examples 2 to 7, ZnO was always added after calcination and before tableting, whereas ZnO was added during the preparation of the reaction mixture during step a) of the process, i.e. together with the addition of V2O5 (Comparative Example 8), immediately after filtration according to step c) of the process (Example 9), or during step d) of the process, i.e. after vacuum drying and before calcination (Example 10). In these examples, 5.2 g of ZnO were added in each case, which means that the finished catalyst contained nominally 2% by weight of ZnO in each case.

[0115] Figure 7 It is shown that XRD reflections are observed when ZnO is added after calcination and before tableting, especially reflections at 31.7° to 31.9°, 34.3° to 34.5°, 36.2° to 36.4°, which are not seen when ZnO is added at the beginning of the reflux step. Figure 7 , diffraction pattern (8)).

[0116] Comparative Example 11 and Comparative Example 12

[0117] The VPO catalysts according to Comparative Examples 11 and 12 were prepared in a similar manner to the VPO catalyst according to Example 1, but the activated tablets were impregnated with a Zn(OAc)2 solution. In this case, two different concentrations were used to obtain a Zn / V ratio of 0.008 (=0.29 wt.-% Zn based on the total catalyst weight) and a Zn / V ratio of 0.016 (corresponding to 0.58 wt.-% based on the total catalyst weight), respectively. After impregnation, the catalysts were dried in air, first at 100°C for 18 hours and then at 350°C for 20 minutes.

[0118] Figure 8The XRD diffraction patterns of the VPO catalysts according to Comparative Examples 11 and 12 (diffraction patterns (11) and (12)) are compared with the XRD diffraction pattern of the VPO catalyst according to Example 5 prepared according to the present invention (diffraction pattern (5)). It can be seen that the addition of the ZnO compound to the activated flakes does not cause any reflections, particularly at 31.7° to 31.9°, 34.3° to 34.5°, and 36.2° to 36.4°.

[0119] Comparative Example 13 and Comparative Example 14

[0120] The VPO catalyst according to comparative example 13 was prepared in a similar manner to the VPO catalyst according to example 2, but 10% by weight of a binder of the Secar 71 type and 4% by weight of graphite were added to the calcined powder.

[0121] The stability of the VPO catalyst can be determined very easily by measuring the stability with respect to byproduct water, since most of the water vapor produced by the oxidation reaction is present during the reaction, which can damage the catalyst. Therefore, the stability of the VPO catalyst of the invention according to Example 4 with respect to byproduct water was tested in comparison with the VPO catalyst prepared with a conventional binder according to Comparative Example 13. For this purpose, the selectivity of the untreated sample was tested, and then both samples were impregnated with water and the selectivity test was repeated (Comparative Example 14). It was found that the impregnation with water did not lead to any loss of selectivity of the catalyst of the invention within the tolerance range of the measurement.

[0122] Example 15 and Example 16

[0123] The VPO catalysts according to Example 15 and Example 16 were prepared in a similar manner to the VPO catalyst according to Example 1, but without the addition of ammonium dimolybdate, so that the obtained VPO catalysts contained no Mo.

[0124] In addition, 1 wt% (Example 15) or 2 wt% (Example 16) of ZnO based on the total weight of the calcined powder was added to the calcined powder after calcination and before tableting. As shown in Table 1, in the case of the VPO catalyst without Mo, an increase in catalytic selectivity and an increase in lateral pressure strength were also achieved. Figure 2 The transmission IR spectra shown also show that for the two samples of the present invention, 790 cm -1 Up to 810cm -1 The intensity of the band at 820 cm (the first absorption band) is greater than -1 Up to 840cm -1 The maximum value at .

[0125] Example 17 and Example 18 (Comparison)

[0126] Devices used

[0127] The same apparatus as in Example 1 was used.

[0128] Preparation of catalyst precursor

[0129] First, 150 g of V2O5 were added to a four-necked flask. 300 mL of benzyl alcohol and 1200 mL of isobutanol were added thereto. The suspension was inerted with N2 and then heated under reflux for 10 hours with stirring. After cooling to a maximum of 40°C with stirring, 7.51 g of ammonium dimolybdate, 6.60 g of iron (III) nitrate nonahydrate, 2.55 g of cerium (III) nitrate hexahydrate and 4.5 g of ammonium niobium oxalate were added. While stirring and inerting with N2, 138 mL of phosphoric acid (85%) was added over 15 minutes, and the suspension was heated under reflux for 24 hours with stirring.

[0130] filter:

[0131] After cooling the suspension containing the catalyst precursor product, transfer the suspension from the four-necked flask to a filter funnel and remove the liquid by suction. Wash the wet filter cake once with ethanol (100%) and remove the liquid in the filter funnel by suction again. Repeat the same steps with double distilled water.

[0132] Drying / Calcination:

[0133] The washed filter cake was transferred to the evaporating flask of a rotary evaporator. The filter cake was dried overnight at 120° C. under a water jet vacuum. The powder dried in this way was placed in the furnace in a suitable calcining pot and calcined at a temperature of 200° C. to 300° C. in a N2 atmosphere for 9 hours.

[0134] Compacting / Tableting:

[0135] Before compaction / tabletting, 2.1 wt% Zn3(PO4)2 and then 4 wt% graphite were added to the calcined powdered catalyst precursor product and mixed uniformly using a drum hoop mixer. The powder was compacted into plates using a roller compactor at a compaction pressure of 190 bar, a gap width of 0.60 mm and a roller speed of 7 rpm and granulated through a 1 mm screen.

[0136] The granules are compressed into a desired tablet shape using a rotary tablet press, which has a suitable height, for example, 5.6 mm×5.6 mm×2.3 mm, and a suitable side compression strength.

[0137] Heat Treatment

[0138] The following heat treatment is carried out under controlled conditions in a distillation pot installed in a programmable furnace. The calcined pieces are uniformly loaded into the distillation pot, and the distillation pot is tightly sealed. The catalyst is then treated at 120°C for 18 hours in an air stream.

[0139] The VPO catalyst according to Example 18 was prepared in a manner similar to the VPO catalyst according to Example 1, but 2.1 wt. % Zn3(PO4)2, based on the total weight of the calcined powder, was added to the calcined powder after calcination and before tableting.

[0140] As shown in Table 1, the non-inventive samples according to Examples 17 and 18 showed much lower selectivity than the inventive samples. Figure 3 In the transmission IR spectra of the two non-inventive samples, it was observed that the -1 Up to 810cm -1 The intensity of the band at 820 cm (the first absorption band) is not greater than -1 Up to 840cm -1 , or both bands do not exist (Example 18).

[0141] result

[0142] Table 1:

[0143]

[0144] method

[0145] Infrared spectroscopy (IR)

[0146] The catalyst was characterized by transmission infrared spectroscopy. This involves determining the intensity of the infrared radiation from a source (I0) after passing through a sample (I) in a wavelength-dependent manner. The absorption of the IR radiation in the material under examination and the vibrational modes thus excited result in the attenuation of the infrared radiation in a wavelength-dependent manner. For the spectrum, the absorption calculated by log(I0 / I) is plotted against the wave number.

[0147] The measurements were performed on a Nicolet Nexus 470 FTIR spectrometer with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector, an "IR Source Everglo" as IR source and a HeNe laser for frequency calibration. For sample preparation, a piece of each catalyst was crushed in a mortar and pestle and about 10 g of material was used to prepare a thin disk of 11 mm diameter by a manual hydraulic press. This was mounted in a vacuum chamber with a KBr window and positioned in the beam path. Subsequently, the samples were heated at 200 °C and 10 -4The samples were pretreated at a pressure of 100 mbar for 2 h to remove adsorbed water. For the measurement, the vacuum chamber containing the samples was cooled to 77 K with liquid nitrogen and heated at 10 -4 mbar pressure, 2cm -1 The spectra were recorded at a step size of 1.5, an aperture of 72, and an optical speed of 0.64.

[0148] Determination of IR absorption maximum

[0149] At 790cm -1 Up to 810cm -1 820cm -1 Up to 840cm -1 The intensity of two absorption bands, each with a maximum at , corresponds to the absorption intensity in the region of the band maxima relative to the spectral baseline. To determine the absorption intensity in the region of the maxima, in each case a first straight line is drawn as a linear baseline so that it touches the spectrum tangentially to the left and right below the band in question (i.e. in the frequency range up to the adjacent absorption band), i.e. the spectral baseline. Subsequently, a line is drawn vertically downwards from the band maximum (S0), i.e. in the direction of constant wavelength and decreasing absorption, and the point of intersection of this line with the linear baseline is determined (S1). The intensity of the absorption band is defined as the distance from S0 to S1.

[0150] Breaking strength test

[0151] In order to measure the breaking strength of the molded body, a Zwick Z0.5 tester was used to determine the force required for breaking. The measurement was carried out according to standard ASTM D4179. In order to dry the molded bodies before measurement, they were stored in a drying oven at 100°C for at least 3 hours, and then the breaking strength was measured within a maximum of 1 hour after the drying was completed. The tester was operated at a constant force rate of 20.0N / s according to standard ASTM D4179. For each embodiment, 100 pieces were placed individually one by one, with the cylinder axis parallel to the measuring claw surface (radial crushing in standard ASTM D4179) and measured. The average value of the force required for the molded body to break in each case was then determined from the 100 individual values, which corresponds to the average breaking force of the molded body. All data on lateral compression strength in this application refer to the lateral compression strength obtained by the method described herein.

[0152] Powder X-ray Diffraction (XRD)

[0153] The catalysts were characterized using X-ray powder diffraction (XRD). In this technique, X-rays are diffracted at different diffraction angles in the crystalline areas of the sample. The diffraction angles are measured / plotted in 2θ units. Characteristic reflections appear as a function of the diffraction angle, depending on the phases present. Based on these diffraction patterns, the diffraction patterns can be matched with the phases present with the help of a database.

[0154] The measurements were performed on a D4 Endeavor from Bruker AXS with Cu-Kα radiation and a LYNXEYE detector. The diffraction patterns were recorded in the 2θ angle range from 5° to 50° with a step size of 0.02°, a recording time of 1.5 seconds per step and a fixed divergence slit of 0.3°. For the measurements, the samples were finely ground and compressed in a sample holder. The device allows the diffraction angle to be changed by tilting the sample. All information on XRD reflections in this application refers to XRD reflections obtained with this method.

[0155] In order to make the diffraction patterns more comparable, normalization was performed in each case. For this reason, the data point with the lowest intensity value was first identified in each diffraction pattern. This value was subtracted from all intensity values ​​of the corresponding diffraction pattern. The maximum intensity of the (024) reflection in the corresponding diffraction pattern was then determined (present at 28.4 ° to 28.5 ° 2θ values). All intensity values ​​of the corresponding diffraction pattern were divided by this value.

[0156] Catalytic test reaction

[0157] To determine the performance of the catalysts, after the catalyst preparation (reflux, filtration, vacuum drying, calcination, compaction, tableting, activation), the catalytic performance of all catalysts was tested in a laboratory scale reactor with air containing 1.5 mol% of butane in a diluted catalyst packing (a mixture of catalyst and inert ceramic rings of 1:9). The selectivity for maleic anhydride (MA) was determined by interpolation at a reaction temperature of 410°C (salt bath) by varying the mass GHSV in the range of 1300 l / kg / h to 5500 l / kg / h at a conversion of 85%.

Claims

1. 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 with molecular oxygen to maleic anhydride, wherein the VPO catalyst contains 0.05 to 7.0% by weight of Zn, partly in the form of ZnO, and has a maximum at 790 cm-1 in the transmission infrared spectrum. -1 Up to 810cm -1 The first absorption band at 820 cm -1 Up to 840cm -1 The second absorption band at Only the first absorption band exists or the intensity of the first absorption band is greater than the intensity of the second absorption band.

2. The VPO catalyst according to claim 1, characterized in that The intensity of an absorption band is determined by drawing in each case a first straight line which touches the spectrum tangentially to the left and right below the band in question, subsequently drawing a second straight line from the maximum of the band in question in the direction of constant wavelength and decreasing absorption, and determining the point of intersection of the two straight lines, wherein the intensity of the absorption band is defined as the length between the maximum and the point of intersection.

3. The VPO catalyst according to claim 1 or 2, characterized in that The ratio of the intensity of the first absorption band to the intensity of the second absorption band exceeds 1.1, preferably exceeds 2.

4. The VPO catalyst according to any one of the preceding claims, characterized in that The VPO catalyst has a Mo content of 0.1% to 1% by weight, preferably 0.4% to 0.7% by weight, based in each case on the total weight of the VPO catalyst.

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

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

7. The VPO catalyst according to any one of the preceding claims, characterized in that The molded body has a side compression strength of more than 25N, preferably 25N to 200N, which is measured with a Zwick Z0.5 tester using standard ASTM D4179 at a constant force rate of 20.0N / s, wherein 100 pieces are each placed individually and measured with the cylinder axis parallel to the surface of the measuring jaws to determine the average rupture force, which represents the side compression strength.

8. The VPO catalyst according to any one of the preceding claims, characterized in that Analysis of the VPO catalyst by powder X-ray diffraction showed reflections at 31.7° to 31.9°, 34.3° to 34.5° and 36.2° to 36.4° in measurements using a D4 Endeavor from Bruker AXS with Cu-Kα radiation and a LYNXEYE detector, the diffraction pattern being recorded in the 2θ angle range of 5° to 50° with a step size of 0.02°, a recording time of 1.5 seconds per step and a fixed divergence slit of 0.3°.

9. The VPO catalyst according to any one of the preceding claims, characterized in that The molded body has a double α shape with a height of 3 mm to 8 mm, a length of 5 mm to 9 mm, a width of 4 mm to 8 mm, and an inner hole diameter of 1 mm to 4 mm.

10. The VPO catalyst according to any one of claims 1 to 8, characterized in that The molded body has a cylindrical shape with a height of 3 mm to 8 mm, a substantially circular bottom surface with a diameter of 3 mm to 8 mm, and a central axial opening with a diameter of 1 mm to 3 mm.

11. A method for preparing a VPO catalyst according to any one of claims 1 to 10, comprising the following steps: a) preparing a catalyst precursor containing vanadyl hydrogen phosphate, b) shaping the catalyst precursor to obtain a shaped body, c) activating the shaped body to form a VPO phase, The method is characterized in that after step a), ZnO is mixed with the catalyst precursor.

12. The method 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 reducing the V(V) compound by means of an organic reducing agent in the presence of the P(V) compound during a reflux step in an organic solvent at standard pressure.

13. The method for preparing a VPO catalyst according to claim 11 or 12, characterized in that The catalyst precursor obtained in step a) is filtered in step a1) and dried and / or calcined in step a2) at a temperature not exceeding 300°C.

14. The method for preparing a VPO catalyst according to claim 12 or 13, characterized in that Step a2) is carried out in two steps, wherein in the first step, drying is carried out under vacuum at a temperature of 90 to 140° C. for 1 to 24 hours, and in the second step, calcination is carried out in nitrogen at a temperature of 230 to 290° C. for 1 to 24 hours.

15. The method for preparing a VPO catalyst according to claims 11 to 14, characterized in that The activation step is carried out in a gas mixture consisting of air, an inert gas and water vapor at a temperature of 300 to 500° C., preferably 350 to 450° C., for 1 to 24 hours.

16. The method for preparing a VPO catalyst according to any one of claims 11 to 15, characterized in that After step a), the catalyst precursor is additionally admixed with a Mg compound, preferably MgO.

17. Use of ZnO for stabilizing VPO catalysts in particle form.

18. The use according to claim 17, wherein the VPO catalyst is prepared in the form of a shaped body by a process according to claim 11.

19. Use of ZnO as a binder for VPO catalysts in the form of shaped bodies.

20. A process for preparing maleic anhydride by catalytic oxidation of n-butane, wherein a reaction gas comprising oxygen and n-butane is passed through a reaction tube containing a packing of a VPO catalyst according to any one of claims 1 to 9.

21. The method according to claim 20, characterized in that The packing of the VPO catalyst is present in the reaction tube at a temperature of 300°C to 420°C.

22. The method according to claim 20 or 21, characterized in that The reaction gas contains 0.2 volume % to 10 volume % of n-butane and 5 volume % to 50 volume % of oxygen, and is heated at 1100 hours. -1 Up to 2500 hours -1 , preferably 1300 hours -1 Up to 1600 hours -1 The space velocity passes through the reaction tube.

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