VPO catalysts with improved selectivity and stability and methods for their preparation

Abstract

The present 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 wt.-% to 7.0 wt.-% of Zn present as ZnO and which has a first absorption band in the transmission infrared spectrum with a maximum at 790 cm ‑1 to 810 cm ‑1 and possibly a second absorption band with a maximum at 820 cm ‑1 to 840 cm ‑1 and is characterized by the presence of only the first absorption band or by the intensity of the first absorption band being greater than the intensity of the second absorption band. The present invention also relates to a process for the preparation of the VPO catalyst of the present invention comprising the steps of a) preparing a catalyst precursor containing vanadyl hydrogen phosphate, b) forming the catalyst precursor into a shaped body, c) activating the shaped body to form the VPO phase, characterized in that after step a) ZnO is mixed with the catalyst precursor.

Description

[0001] This invention relates to a VPO catalyst in shaped form for the oxidation of hydrocarbons with molecular oxygen, particularly for the oxidation of butane to maleic anhydride with molecular oxygen, wherein the VPO catalyst contains 0.05 wt% to 7.0 wt% of Zn partially present as ZnO, and has a maximum value at 790 cm⁻¹ in its transmission infrared spectrum. -1 Up to 810cm -1 The first absorption band at the value, and may have a maximum value at 820 cm⁻¹. -1 Up to 840cm -1 The second absorption band at the location is characterized by the presence of only the first absorption band or the strength of the first absorption band being greater than the strength 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) Preparation of catalyst precursors containing vanadium hydrogen oxyphosphate.

[0004] b) The catalyst precursor is shaped to obtain a shaped article.

[0005] c) Activate the molded body to form the VPO phase.

[0006] The feature is that, after step a), ZnO is mixed with the catalyst precursor.

[0007] The present invention also relates to the use of solid-state Zn compounds for stabilizing particulate VPO catalysts.

[0008] Maleic anhydride is a chemical intermediate of significant economic value. For example, it is used alone or in combination with other acids in the preparation of alkyd resins and polyester resins. Furthermore, it is a common intermediate in chemical synthesis, such as for the synthesis of γ-butyrolactone, tetrahydrofuran, and but-1,4-diol, which are then used as solvents or further processed into polymers such as polytetrahydrofuran or polyvinylpyrrolidone.

[0009] Maleic anhydride is typically prepared by the partial oxidation of n-butane in the gas phase with molecular oxygen or an oxygen-containing gas in the presence of a vanadium phosphorus oxide catalyst (VPO catalyst) containing vanadium pyrophosphate (VPP). The pure form of vanadium pyrophosphate contains vanadium in the +4 valence state 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 and fluidized-bed reactors can be used.

[0010] VPO catalysts exhibit only low intrinsic activity in the reaction of n-butane to maleic anhydride. Therefore, sufficient conversion requires large amounts of catalyst. Furthermore, VPO catalysts are among the most expensive of all non-noble metal catalysts, primarily due to their high feedstock costs. This sets the goal of improving catalyst 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 exogenous elements, such as molybdenum (Mo co-catalyst or Mo dopant), to the vanadium phosphorus oxide (VPO) phase.

[0011] US 5929256 discloses the synthesis of a molybdenum-modified active vanadium-phosphorus catalyst 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 alcoholic medium suitable for reducing vanadium to an oxidation state below 5. This results in the incorporation of molybdenum into the reaction product, forming a molybdenum-modified solid precursor composition. The alcohol is removed to obtain a dried molybdenum-modified solid precursor composition. This is then shaped into a molded body containing the dried molybdenum-modified solid precursor composition. The dried and molded molybdenum-modified precursor composition is activated to convert it into an active catalyst.

[0012] US 5070060 discloses an improvement to an oxidation catalyst for the partial oxidation of n-butane, comprising a mixed oxide of vanadium and phosphorus, and 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 of 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, constituting a more active system, and is more durable than the unmodified catalyst.

[0013] US 3980585 discloses a catalyst complex suitable for converting C4 hydrocarbons to maleic anhydride in the gas phase, comprising vanadium, phosphorus, and copper, and one of an element 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 alkaline earth metal.

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

[0015] US 4515904 discloses a method for preparing a phosphorus-vanadium catalyst and a phosphorus-vanadium co-metal catalyst for the preparation of maleic anhydride from butane, the method comprising reacting a vanadium compound with a phosphorus halide in an organic ether solvent having about 2 to about 10 carbon atoms in the presence of water or a fatty alcohol having about 1 to about 8 carbon atoms at a temperature of 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 about 300°C to about 500°C.

[0016] US 5158923 discloses an improvement to an oxidation catalyst for the partial oxidation of n-butane, comprising a mixed oxide of vanadium and phosphorus, and 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, constituting a more active system, and is more durable than the unmodified catalyst.

[0017] US 5262548 discloses an improvement to an oxidation catalyst for the partial oxidation of n-butane, comprising a mixed oxide of vanadium and phosphorus, and 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 catalyst that constitutes a very stable active system and has a longer lifetime than the unmodified catalyst.

[0018] WO 2013062919 A1 discloses a method for preparing an enhanced VPO catalyst, wherein the catalyst comprises a mixed oxide of vanadium and phosphorus, and wherein the catalyst is enhanced with at least one selected from niobium, cobalt, iron, zinc, molybdenum, or titanium. The method comprises the steps of: (i) preparing a VPO catalyst comprising vanadium pyrophosphate as a major component and containing less than 5% by weight of vanadium oxyphosphate; (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 enhanced VPO catalyst. In one embodiment, a niobium-activated VPO catalyst is prepared.

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

[0020] US 4251390 discloses an improvement to an oxidation catalyst for the partial oxidation of n-butane and a mixed oxide comprising 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 more readily activated catalyst that is highly stable to heating. Small amounts of lithium and silicon compounds also provide additional desired catalytic effects without diminishing the benefits of the zinc compound.

[0021] DE 10 2014 004786A1 relates to catalysts comprising vanadium phosphorus oxide and alkali metals, wherein the weight proportion of alkali metals in the vanadium phosphorus oxide is from 10 ppm to 400 ppm based on the total weight of the vanadium phosphorus oxide, and also relates to methods for its preparation and the use of said catalysts for the gas-phase oxidation of hydrocarbons, particularly for the preparation of maleic anhydride.

[0022] To prepare VPO catalysts containing the VPP phase, vanadium pentoxide (V₂O₅) is typically reduced in an organic alcohol solvent in the presence of phosphoric acid using benzyl alcohol as a reducing agent, thereby forming vanadium hydroxide phosphate (VHP) together with benzaldehyde. The redox reaction (“reduction”) performed here is as follows, wherein vanadium in oxidation state V (V(V)) reacts to form the VHP phase, which contains vanadium oxyoxide species (VO₄) having vanadium in oxidation state IV (V(IV)). 2+ ):

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

[0024] In the subsequent activation step, the VHP phase is converted into the vanadium pyrophosphate phase by heating, while water is eliminated.

[0025] (2)2VOHPO4*1 / 2H2O→(VO)2P2P7+1 1The problem with the catalytic reaction of butane to maleic anhydride using a VPO catalyst is that the process must be carried out under conditions of pore diffusion restriction. Therefore, porosity has a direct impact on the catalytic yield. Care must be taken to ensure that the pore structure is not adversely affected during molding (e.g., by tableting). However, this does have the consequence of compromising the mechanical stability of the molded body. Its mechanical stability must be high enough to ensure that the tablets remain intact during the filling process (falling into a reaction tube approximately 6 m long). Otherwise, smaller fragments will lead to excessively high dynamic pressures in the process, resulting in high compressor costs or reduced production. Therefore, the technical objective is to increase the mechanical strength of the molded body without adversely affecting the pore structure. Simultaneously, a VPO catalyst with improved performance is needed, namely, a VPO catalyst with improved activity, selectivity, and stability.

[0026] Therefore, the object of the present invention is to provide an improved VPO catalyst for the gas-phase oxidation of hydrocarbons, particularly for the preparation of maleic anhydride, which has improved catalytic performance, particularly improved selectivity, and significantly improved mechanical strength compared to catalysts commonly used to date.

[0027] The objective is achieved by a VPO catalyst in the form of a molded prototyping medium for the oxidation of hydrocarbons with molecular oxygen, particularly for the oxidation of butane to maleic anhydride with molecular oxygen, wherein the VPO catalyst contains 0.05 wt% to 7.0 wt% of Zn partially present as ZnO, and has a maximum value at 790 cm⁻¹ in its transmission infrared spectrum. -1 Up to 810cm -1 The first absorption band at 820 cm⁻¹, and may have a maximum value at 820 cm⁻¹. -1 Up to 840cm -1 The second absorption band at the location is characterized by the presence of only the first absorption band or the strength of the first absorption band being greater than the strength of the second absorption band.

[0028] The absorption band intensity is determined here by the following steps: In each case, a first straight line is drawn, tangentially touching the spectrum immediately below the band on the left and right sides. Then, starting from the maximum value of the band, a second straight line is drawn along a direction where the wavelength remains constant and absorption decreases. The intersection of the two lines is determined, where the intensity of the absorption band is defined as the length between the maximum value and the intersection point. 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, and more preferably greater than 2.

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

[0030] When analyzed by powder X-ray diffraction using Cu-Kα radiation, the VPO catalyst according to the invention exhibits 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 invention comprises or consists of a VPO phase. For example, based on the total weight of the VPO catalyst, the VPO catalyst according to the invention comprises more than 70% by weight, preferably more than 80% by weight, and particularly preferably more than 90% by weight of the VPO phase. Furthermore, the VPO catalyst may contain VPP, dopants, and unreacted oxides of the feedstock, such as vanadium pentoxide or phosphorus oxide.

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

[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. Furthermore, 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 other possible Zn compounds, must, in each case, be 0.05 wt% to 7.0 wt%, preferably 0.1 wt% to 6.0 wt%, more preferably 0.2 wt% to 4.0 wt%, and most preferably 0.7 wt% to 3.0 wt% Zn, based on the total weight of the catalyst.

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

[0036] -0.05% by weight to 7% by weight of Zn,

[0037] -0% by weight to 0.7% by weight of Mo,

[0038] -26% to 31% V by weight,

[0039] -17% to 21% by weight of P,

[0040] -3% to 5% C by weight,

[0041] The rest is oxygen.

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

[0043] According to the present invention, the presence of ZnO has the effect of stabilizing catalyst particles, giving them higher mechanical strength compared to their strength in the absence of ZnO. However, the presence of ZnO also has the effect of improving the catalytic performance (i.e., activity, selectivity, and stability) of the catalyst particles, particularly increasing selectivity.

[0044] The VPO catalyst according to the invention exists in the form of a molded body; the shape of the molded body can be designed differently depending on the desired contact time, flow rate, and dynamic pressure during the catalytic reaction. The molded body form of the VPO catalyst is understood to refer to a molded body prepared by a molding step such as tableting. When present in large quantities in a tube-bundle reactor, the molded body according to the invention forms a layer or packing material for the reactant butane and air, particularly n-butane.

[0045] According to the present invention, the molded body should not be smaller than a minimum size, which, for example, corresponds to the size of an imaginary cube that cannot be matched to 3 mm × 3 mm × 3 mm at any point without exceeding its limit.

[0046] For example, the molded body according to the invention can 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 mm to 8 mm, and it has a generally circular base with a diameter of 3 mm to 8 mm. Preferably, the cylinder has a central axial opening; it can have a diameter of, for example, 1 mm to 3 mm.

[0047] For example, the molded body according to the present invention can 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. Then the geometric surface area of ​​this molded body is 1.2 cm². 2 The volume is 0.075cm. 3The mass is 0.12g. Filling a large quantity of this molded body into a 21mm reactor produces 0.85g / cm³. 3 Up to 0.89 g / cm 3 The poured density.

[0048] For example, the molded body according to the invention can 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 ​​such molded bodies is then 1.77 cm². 2 The volume is 0.111 cm³. 3 The mass was 0.18 g. Filling a large quantity of this molded body into a 21 mm reactor produced 0.72 g / cm³. 3 Up to 0.76 g / cm 3 The pouring density.

[0049] The preferred molded body for the reactor concept according to the invention is the molded body described in EP 2643086 A1. The preferred double-α shape is characterized in particular by each individual molded body being designed in the form of a cylinder in each case, the cylinder having an outer base [1], a cylindrical surface [2], a cylindrical axis, and at least one continuous opening [3] extending parallel to the cylindrical axis, and the outer base [1] of the cylinder containing at least four lobes [4a, 4b, 4c, 4d], wherein the geometric base surrounding the molded body is a prism having a prism base having a length and a width, wherein the length is greater than the width, and wherein the lobes [4a, 4b, 4c, 4d] are surrounded by prism corners of the prism base. Figure 9 ).

[0050] Preferred shaped catalyst bodies have a height (length along the axis of the cylinder) 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. For example, a preferred shaped catalyst body has 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 cm². 2 The volume is 0.154 cm³. 3 The mass is 0.24 g. When filled into a 21 mm reactor, it produces 0.60 g / cm³. 3 Up to 0.62 g / cm 3 The pouring density.

[0051] The VPO catalyst / molded body of the present invention has a lateral compressive strength exceeding 25 N, preferably 25 N to 200 N. Particularly preferably, the lateral compressive strength of the VPO catalyst / molded body exceeds 30 N to 150 N, and even more preferably exceeds 35 N to 100 N.

[0052] The cylindrical VPO catalyst / molded body of the present invention has a lateral compressive strength exceeding 25 N, preferably 25 N to 50 N. Particularly preferably, the lateral compressive strength of the VPO catalyst / molded body exceeds 30 N to 45 N, and even more preferably exceeds 35 N to 40 N.

[0053] The lateral compressive strength of the double-α shaped VPO catalyst / molded body of the present invention exceeds 50 N, preferably 50 N to 200 N. Particularly preferably, the lateral compressive strength of the VPO catalyst / molded body is more than 100 N to 170 N, and even more preferably 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) Preparation of catalyst precursors containing vanadium hydrogen oxyphosphate.

[0056] b) The catalyst precursor is shaped to obtain a shaped article.

[0057] c) Activate the molded body to form the VPO phase.

[0058] The feature is that, after step a), ZnO is mixed with the catalyst precursor.

[0059] In step a) of the method, a catalyst precursor containing vanadium hydroxide phosphate is prepared in a manner generally known. Typically, in this case, the V(V) compound in solution is reduced in the reaction mixture in the presence of a P(V) compound and optionally a Mo compound by means of a reducing agent during the reduction step. For example, the reaction mixture may 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, at most 1% by weight of Mo compound, and 5% to 25% by weight of P(V) compound. More specifically, for example, initially, 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, based on the total weight of the reaction mixture in each case, may be added as a reaction mixture.

[0060] The V(V) compound used as a raw material in the reaction mixture for preparing vanadium hydroxide phosphate is a compound containing vanadium in its oxidation state, and preferably V₂O₅. The P(V) compound used as a raw material in the reaction mixture for preparing vanadium hydroxide phosphate is a compound containing phosphorus in its oxidation state, and 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 with a concentration of 98% to 100%, preferably 99% to 100% (the percentage value refers to the weight percentage content of pure phosphoric acid relative to the weight of the water-phosphoric acid mixture, as usually expressed). Alternatively, in order to prepare a reaction mixture for the preparation of vanadium hydroxide phosphate, phosphoric acid with a concentration of more than 100% can be used, which reacts immediately with any water initially present in the reaction mixture to form phosphoric acid with a concentration of 98% to 100%, preferably 99% to 100%, more preferably 100%, resulting in a reaction mixture free of phosphoric acid with a concentration of more than 100%, and at the same time, water with a concentration of no more than 0.2% by weight is retained in the reaction mixture based on the weight of the reaction mixture.

[0061] The molybdenum compound that can optionally be used as a starting material in the reaction mixture for the preparation of vanadium hydroxide phosphate is any desired molybdenum-containing compound, such as molybdenum trioxide, ammonium heptamolybdate ((NH4)6Mo7O 24 (NH4)4H2O), ammonium molybdate ((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 used to prepare vanadium hydroxide phosphate can be any desired reducing agent capable of reducing compound V(V) to at least partially form vanadium hydroxide phosphate. Preferably, the reducing agent is an organic reducing agent, such as ethanol, isobutanol, or aromatic alcohol, particularly including benzyl alcohol.

[0063] The solvent present in the reaction mixture used to prepare vanadium hydroxide phosphate is preferably an alcohol, more preferably a high-boiling-point fatty alcohol, especially isobutanol, or ethanol or isopropanol.

[0064] The feedstock is provided in a suitable reaction vessel for the reduction step to obtain vanadium hydroxide phosphate, a catalyst precursor. In this case, the reaction mixture is heated to a temperature above room temperature, for example, up to 100°C. Because the reduction is preferably carried out with stirring during the 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 removal of water formed during the reduction process from the reaction mixture, i.e., a water separator, such as a Dean-Stark trap.

[0065] The reduction to form vanadium hydroxide phosphate is preferably carried out under reflux at standard pressure, in which case the temperature is raised to match the boiling point of the solvent used; preferably, the method according to the invention comprises only a single reflux step. The catalyst precursor preferably contains vanadium hydroxide phosphate as the main phase, or may even consist substantially of the vanadium hydroxide phosphate phase. Molybdenum, which may also be present in the catalyst precursor, can be present as a dopant of the vanadium hydroxide phosphate phase, where molybdenum doping is understood to mean that molybdenum is either incorporated into the vanadium hydroxide phosphate phase or present on its surface. However, in addition to the vanadium hydroxide phosphate phase, reduction may also result in the formation of additional vanadium-phosphorus mixed oxides, in which vanadium has an oxidation state of IV or even III. The reduction does not need to be complete, meaning that some V(V) compounds and some P(V) compounds also remain in the catalyst precursor. However, in the catalyst precursor, vanadium typically exists with an average oxidation state of 3.8 to 4.2.

[0066] Water formed during the reduction process can be removed from the reaction mixture during the reduction process. In the prior art, water removal during the reduction process is achieved either physically, such as by means of a water separator, or chemically by using a water-binding compound, such as a desiccant or an anhydride like phosphoric acid with a concentration exceeding 100%. The reaction mixture may contain, for example, an anhydride that reacts with water and binds water; in particular, the reaction mixture may contain phosphoric acid with 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 rare gas. In this case, an inert gas is any gas that, under specific conditions, does not react with the catalyst precursor during filtration but simultaneously displaces oxygen in the air to minimize the risk of explosion. The filtration is carried out in a manner known to those skilled in the art, typically by a filter press, decanter, or through a filter funnel. Filtration yields an uncalcined catalyst precursor, which is still wetted by the solvent.

[0068] The catalyst precursor (solid filter residue) obtained by filtration can then be dried; this is typically carried out under reduced pressure / vacuum or in an inert gas atmosphere at temperatures above room temperature, for example, 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 simultaneously displaces oxygen from the air to minimize the risk of explosion, such as nitrogen or a rare gas. Drying is preferably carried out under reduced pressure / vacuum at temperatures between 50°C and 150°C, preferably between 90°C and 140°C.

[0069] Alternatively or optionally, the catalyst precursor may be calcined after drying to obtain the 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 is any gas that does not react with the catalyst precursor under calcination conditions but simultaneously displaces oxygen from the air to minimize the risk of explosion, such as nitrogen or a rare gas.

[0070] Optionally, graphite can be added to the dried catalyst precursor to facilitate shaping in step b) of the method. The dried catalyst precursor can also be compacted and / or granulated to obtain a compacted or granulated dried catalyst precursor. In this case, the catalyst precursor can be 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 then granulated through a 1 mm sieve.

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

[0072] In step c) of the subsequent method, the obtained shaped catalyst body is activated at a temperature above 200°C. Activation is typically 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 specific conditions during activation, and is particularly preferably nitrogen or a rare gas. Alternatively, activation can be carried out in a process gas, i.e., in a gas mixture containing air and butane. 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 typically has a side compressive strength exceeding 25 N, typically ranging from 25 N to 200 N.

[0073] The method for preparing a VPO catalyst in shaped form according to the present invention is characterized in that, after step a), ZnO is added to or mixed with the catalyst precursor. ZnO, i.e., zinc oxide (II)), is used as a powder medium solid in this case. The addition of ZnO to the catalyst precursor can also be achieved by means of a metering device and a mixer. To prepare the catalyst of the present invention, sufficient ZnO is typically added. 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 solid ZnO can be added.

[0074] ZnO is added to the catalyst precursor after preparation in step a) of the method, for example by a reduction step, but before the shaped catalyst body is formed in step c) of the method. Preferably, ZnO is added to a dried catalyst precursor, more preferably to a dried and calcined catalyst precursor.

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

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

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

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

[0079] Figure 1 IR spectra of samples from Examples 1 to 7 and Example 12.

[0080] Figure 2 IR spectra of samples from Examples 1, 15, and 16.

[0081] Figure 3 IR spectra of samples from Example 5, Comparative Examples 17 and 18.

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

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

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

[0085] Figure 7 XRD diffraction patterns of VPO catalysts from Examples 5 and 8 to 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] Figure 9 A diagram illustrating the preferred catalyst particle “double α shape” from four different perspectives.

[0088] The numbers in parentheses in the figure refer to samples prepared according to the respective embodiments in each case. Example

[0089] Example 1 (Comparative)

[0090] The device used

[0091] The heating mantle of the 2L four-necked flask is placed on a laboratory jack. A crescent-shaped stirrer with a corresponding stirrer seal is located in the central neck of the flask, connected to the stirrer unit via a stirrer coupling. A thermometer is located in the right neck, and a riser tube for the reflux condenser is located in the left neck. The central neck at the front is used for filling with chemicals, to which a nitrogen inlet is then connected. The entire apparatus can also be purged with nitrogen. For this purpose, nitrogen is first passed through a washing bottle, then enters the apparatus, exits at the top of the condenser, and passes through the washing bottle again.

[0092] Preparation of catalyst precursors

[0093] First, add 1069.5 g of isobutanol and 156.0 g of benzyl alcohol. Then, add 150 g of V₂O₅ while stirring. After adding V₂O₅, add 2.52 g of ammonium dimolybdate. Next, add 232.50 g of phosphoric acid (100%, anhydrous) to the suspension and heat the mixture under reflux at 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 then pressed and dried overnight at a pressure of 14 to 18 bar.

[0096] Drying / Calcination

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

[0098] Compaction / Tableting

[0099] Prior to compaction / tableting, 5% by weight of graphite was added to the calcined powdered catalyst precursor product and mixed uniformly using a drum mixer. The powder was then compacted into tablets 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 sieve.

[0100] The granules are compressed into the desired tablet shape using a rotary tablet press, with a suitable height, such as 5.6 mm × 5.6 mm × 2.3 mm, and a suitable lateral pressure strength.

[0101] Activated into pyrophosphate

[0102] The activation of vanadium pyrophosphate was carried out under controlled conditions in a distillation vessel installed in a programmable furnace. The calcined flakes were uniformly loaded into the distillation vessel, which was then tightly sealed. The catalyst was subsequently activated in a humid air / nitrogen mixture (50% atmospheric humidity), initially at over 300°C for 5 hours, followed by activation at over 400°C for 9 hours.

[0103] Examples 2 to 7 (of the present invention)

[0104] The VPO catalyst of the present invention was prepared in a manner similar to that of Example 1, except that 100 g of calcined precursor powder was mixed with 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), and 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, 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] In each case, the fracture strength of the VPO catalysts according to Examples 1 to 7 was tested before and after activation. XRD diffraction patterns 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 indicate that adding ZnO or ZnO / MgO after the calcination step does not lead to a systematic increase in side compressive strength (SCS) observed in measurements immediately after tableting. However, unexpectedly, this is not the case after the VPO catalyst tablets undergo a subsequent activation step. In this case, the particles with added ZnO or ZnO / MgO show a systematic and very significant increase in side compressive strength. Table 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 increased selectivity.

[0107] The VPO catalyst of this invention is also characterized by its infrared spectral features. Typical non-inventory VPO catalysts without ZnO exhibit infrared spectral characteristics at 790 cm⁻¹. -1 Up to 810cm -1 It has a maximum value at (the first absorption band), at 820 cm⁻¹. -1 Up to 840cm -1 The second absorption band has a second maximum value, where the intensity of the second absorption band is higher than that of the first absorption band in this case. Figure 1 (1) It has been found that the addition of ZnO or ZnO / MgO simultaneously induces a 790 cm⁻¹ -1 Up to 810cm -1 The strength of the band at 820 cm⁻¹ increases (first absorption band). -1 Up to 840cm -1 The intensity of the maximum value of the second absorption band decreases, making the first absorption band in the sample of the present invention stronger than the second absorption band, or the intensity of the second absorption band even decreases to a level that is no longer detectable. Figure 1 (2) to (7)). In the latter case, the VPO catalysts of the present invention are characterized by having only a first absorption band.

[0108] It can also be seen that the VPO catalyst according to the present invention has the following characteristics that can be observed by XRD diffraction ( 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 characteristics become stronger with increasing ZnO content and are very consistent with ZnO (in wurtzite structure / as zincite).

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

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

[0115] Figure 7 This indicates that XRD reflections were observed when ZnO was added after calcination and before tableting, particularly at 31.7° to 31.9°, 34.3° to 34.5°, and 36.2° to 36.4°, which were not visible when ZnO was added at the beginning of the reflux step. Figure 7 , Diffraction pattern (8)).

[0116] Comparative Examples 11 and 12

[0117] The VPO catalysts according to Comparative Examples 11 and 12 were prepared in a manner similar to that of the VPO catalyst according to Example 1, but the activated sheets 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% Zn 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 8XRD diffraction patterns (diffraction pattern (11) and diffraction pattern (12)) of the VPO catalysts according to Comparative Examples 11 and 12 are shown in comparison with the XRD diffraction pattern (diffraction pattern (5)) of the VPO catalyst prepared according to Example 5 according to the present invention. It can be seen that the addition of ZnO compound to the activated sheet does not cause any reflection, especially at 31.7° to 31.9°, 34.3° to 34.5°, and 36.2° to 36.4°.

[0119] Comparative Examples 13 and 14

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

[0121] The stability of a VPO catalyst can be readily determined by measuring its stability with respect to the byproduct water, since most of the water vapor produced by the oxidation reaction is present during the reaction and can damage the catalyst. Therefore, the stability of the VPO catalyst of the present invention with respect to the byproduct water according to Example 4 was tested and compared with that of the VPO catalyst prepared with a conventional binder according to Comparative Example 13. For this purpose, the selectivity of the untreated sample was tested, followed by immersion in water of both samples and a repeat of the selectivity test (Comparative Example 14). It was found that immersion in water did not cause any loss of selectivity in the catalyst of the present invention within the measured tolerance range.

[0122] Examples 15 and 16

[0123] The VPO catalysts according to Examples 15 and 16 were prepared in a manner similar to that of the VPO catalyst according to Example 1, but without the addition of ammonium dimolybdate, so that the obtained VPO catalysts were free of Mo.

[0124] Furthermore, after calcination and before tableting, 1% by weight (Example 15) or 2% by weight (Example 16) of ZnO based on the total weight of the calcined powder was added to the calcined powder. As shown in Table 1, an increase in catalytic selectivity and an increase in side pressure strength were also achieved in the case of a Mo-free VPO catalyst. Figure 2 The transmission IR spectra shown also indicate that, for both samples of the present invention, 790 cm⁻¹ -1 Up to 810cm -1 The strength of the first absorption band at that point is greater than 820 cm⁻¹. -1 Up to 840cm -1 The maximum value at that location.

[0125] Example 17 and Example 18 (Comparison)

[0126] The device used

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

[0128] Preparation of catalyst precursors

[0129] First, add 150 g of V₂O₅ to a four-necked flask. Then add 300 mL of benzyl alcohol and 1200 mL of isobutanol. Inertize the suspension with N₂ and then heat under reflux with stirring for 10 hours. After cooling to a maximum of 40 °C with stirring, add 7.51 g of ammonium dimolybdate, 6.60 g of ferric nitrate (III) nonahydrate, 2.55 g of cerium nitrate (III) hexahydrate, and 4.5 g of ammonium niobate oxalate. While stirring and inertizing with N₂, add 138 mL of phosphoric acid (85%) over 15 minutes, and then heat the suspension under reflux with stirring for 24 hours.

[0130] filter:

[0131] After cooling the suspension containing the catalyst precursor product, 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 washed once with ethanol (100%), and the liquid in the filter funnel was removed again by suction. The same steps were then repeated with double-distilled water.

[0132] Drying / calcination:

[0133] The washed filter cake was transferred to an evaporation flask in a rotary evaporator. The filter cake was dried overnight at 120°C under a water-jet vacuum. The powder dried in this manner was then placed in the furnace of a suitable calcining jar and calcined at 200°C to 300°C for 9 hours in a nitrogen atmosphere.

[0134] Compaction / Tableting:

[0135] Before compaction / tableting, 2.1 wt% Zn3(PO4)2 was added, followed by 4 wt% graphite, to the calcined powdered catalyst precursor product, and the mixture was homogenized using a drum mixer. The powder was then compacted into tablets 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 sieve.

[0136] The granules are compressed into the desired tablet shape using a rotary tablet press, with a suitable height, such as 5.6 mm × 5.6 mm × 2.3 mm, and a suitable lateral pressure strength.

[0137] Heat treatment

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

[0139] The VPO catalyst according to Example 18 was prepared in a manner similar to that of the VPO catalyst according to Example 1, except that 2.1% by weight of 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-inventory samples according to Examples 17 and 18 exhibited significantly lower selectivity compared to the samples of the present invention. Meanwhile, in Figure 3 In the transmission IR spectra of these two non-inventory samples, it was observed that at 790 cm⁻¹ -1 Up to 810cm -1 The strength of the first absorption band at the point of absorption is no greater than 820 cm⁻¹. -1 Up to 840cm -1 The maximum value at the location (Example 17), or neither band exists (Example 18).

[0141] result

[0142] Table 1:

[0143]

[0144] method

[0145] Infrared spectroscopy (IR)

[0146] The catalyst was characterized using transmission infrared spectroscopy. This involved determining the intensity of infrared radiation from the source (I0) after passing through the sample (I) in a wavelength-dependent manner. The absorption of IR radiation in the tested material and the resulting excited vibrational modes cause the infrared radiation to decay in a wavelength-dependent manner. For the spectrum, the absorption calculated by log(I0 / I) was plotted against the wavenumber.

[0147] Measurements were performed on a Nicolet Nexus 470FTIR spectrometer, equipped with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector, an "IR Source Everglo" as the IR source, and a HeNe laser for frequency calibration. For sample preparation, flakes of each catalyst were crushed in a mortar and pestle, and thin disks with a diameter of 11 mm were prepared using approximately 10 g of material via a manual hydraulic press. These disks were mounted in a vacuum chamber with a KBr window and positioned in the beam path. Subsequently, the samples were subjected to 200 °C and 10... -4Pretreatment at a pressure of millibars for 2 hours was performed to remove adsorbed water. For measurement, the vacuum chamber containing the sample was cooled to 77 K with liquid nitrogen, and then subjected to [further treatment] at 10 [units of pressure]. -4 millibar pressure, 2cm -1 The spectrum was recorded at a step size of 72, an aperture of 72, and an optical speed of 0.64.

[0148] Determination of the maximum IR absorption value

[0149] At 790cm -1 Up to 810cm -1 820cm -1 Up to 840cm -1 The intensity of two absorption bands, each with a maximum value, corresponds to the absorption intensity of the maximum region of the band relative to the spectral baseline. To determine the absorption intensity of the maximum region, in each case, a first straight line is plotted as the linear baseline, tangentially touching the spectrum immediately below the band on the left and right sides (i.e., within the frequency range up to the adjacent absorption band). This is the baseline of the spectrum. Subsequently, a line is drawn vertically downwards from the maximum value of the band (S0), i.e., in the direction where the wavelength remains constant and absorption decreases, and the intersection point (S1) of this line with the linear baseline is determined. The intensity of the absorption band is defined as the distance from S0 to S1.

[0150] Fracture strength test

[0151] To measure the fracture strength of the molded articles, a Zwick Z0.5 tester was used to determine the force required for fracture. Measurements were performed according to standard ASTM D4179. To dry the molded articles before measurement, they were stored in a drying oven at 100°C for at least 3 hours, and the fracture strength was subsequently measured within 1 hour after drying. The tester was operated at a constant force rate of 20.0 N / s according to standard ASTM D4179. For each embodiment, 100 pieces were placed individually with the cylinder axis parallel to the surface of the measuring jaws (radial crushing in standard ASTM D4179) and measured. The average force required for fracture of the molded article in each case was then determined from the 100 individual values, and this average value corresponds to the average fracture force of the molded article. All data regarding lateral crush strength in this application refer to the lateral crush strength obtained by the methods described herein.

[0152] Powder X-ray diffraction (XRD)

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

[0154] Measurements were performed on a Bruker AXS D4 Endeavor equipped with Cu-Kα radiation and a LYNXEYE detector. Diffraction patterns were recorded in 0.02° steps, with a recording time of 1.5 seconds per step, and a fixed divergence slit of 0.3° within a 2θ angle range of 5° to 50°. For the measurements, the sample was finely ground and compressed in a sample holder. This setup allows for variation of the diffraction angle by tilting the sample. All information regarding XRD reflectance in this application refers to XRD reflectance obtained using this method.

[0155] To improve the comparability of diffraction patterns, normalization was performed in each case. To do this, the data point with the lowest intensity value was first identified in each diffraction pattern. This value was then subtracted from all intensity values ​​in the corresponding diffraction pattern. Subsequently, the maximum intensity of the (024) reflection in the corresponding diffraction pattern (located at the 2θ value between 28.4° and 28.5°) was determined. All intensity values ​​in the corresponding diffraction pattern were then divided by this value.

[0156] Catalytic test reaction

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

Claims

1. A VPO catalyst in the form of a shaped body for the oxidation of hydrocarbons with molecular oxygen, wherein the VPO catalyst contains 0.05 wt.% to 7.0 wt.% of Zn, which is present partly as ZnO, and which has a first absorption band in the transmission infrared spectrum with a maximum at 790 cm -1 to 810 cm -1 and possibly a second absorption band with a maximum at 820 cm -1 to 840 cm -1 , characterized in that The first absorption band is present only or the intensity of the first absorption band is greater than the intensity of the second absorption band, the intensity of the absorption band being determined by drawing in each case a first straight line which is in tangential contact with the spectrum immediately below the left and right side of the band in question, then drawing a second straight line starting from the maximum of the band in a direction in which the wavelength is constant and the absorption decreases, and determining the intersection of the two straight lines, wherein the intensity of the absorption band is defined as the length between the maximum and the intersection; and the analysis of the VPO catalyst by powder X-ray diffraction shows reflections at 31.7° to 31.9°, 34.3° to 34.5° and 36.2° to 36.4° in a measurement using a D4 Endeavor from Bruker AXS with Cu-Ka radiation and LYNXEYE detector, the diffractogram being recorded in the 2 theta 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°.

2. The VPO catalyst of claim 1, wherein The ratio of the intensity of the first absorption band to the intensity of the second absorption band exceeds 1.

1.

3. The VPO catalyst according to any one of the preceding claims, characterized in that The VPO catalyst has 0.1 wt.-% to 1 wt.-% of Mo, based on the total weight of the VPO catalyst, of Mo with a content of 4. The VPO catalyst of claim 1 or 2, characterized in that The VPO catalyst contains 0.1 wt.-% to 6 wt.-% of Zn, based on the total weight of the VPO catalyst.

5. The VPO catalyst of claim 1 or 2, wherein The VPO catalyst has the following elemental composition, based on the total weight of the VPO catalyst: - 0.5 wt.-% to 7 wt.-% of Zn, - 0 wt.-% to 0.7 wt.-% of Mo, - 26 wt.-% to 31 wt.-% of V, - 17 wt.-% to 21 wt.-% of P, - 3 wt.-% to 5 wt.-% of C, the remainder being oxygen.

6. The VPO catalyst of claim 1 or 2, wherein The shaped body has a side crush strength of more than 25 N, determined with a Zwick Z0.5 tester using the standard ASTM D4179 at a constant force rate of 20.0 N / s, wherein 100 tablets are each placed individually and measured with the cylinder axis parallel to the surface of the measuring jaws to determine the average breaking force, which represents the side crush strength.

7. The VPO catalyst of claim 1 or 2, wherein The shaped body has a double alpha 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 bore diameter of 1 mm to 4 mm.

8. The VPO catalyst of claim 1 or 2, wherein The shaped body has a cylindrical shape with a height of 3 mm to 8 mm, a substantially circular base face with a diameter of 3 mm to 8 mm, and a central axial opening with a diameter of 1 mm to 3 mm.

9. A process for the preparation of a VPO catalyst according to any one of claims 1 to 8, 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, characterized in that after step a) ZnO is mixed with the catalyst precursor.

10. The method of preparing a VPO catalyst according to claim 9, characterized in that The preparation of the catalyst precursor containing vanadyl hydrogen phosphate in step a) is carried out by reduction of a V(V) compound in the presence of a P(V) compound with the aid of an organic reducing agent in an organic solvent during the course of a reflux step at standard pressure.

11. The method of preparing a VPO catalyst according to claim 10, 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 of not more than 300°C.

12. The method of preparing a VPO catalyst according to claim 11, 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 from 90°C to 140°C for 1 hour to 24 hours, and, in the second step, calcination is carried out in nitrogen at a temperature of from 230°C to 290°C for 1 hour to 24 hours.

13. The method of preparing a VPO catalyst according to claim 12, 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 from 300°C to 500°C for 1 hour to 24 hours.

14. The method of preparing a VPO catalyst according to any one of claim 13, characterized in that After step a), the catalyst precursor is additionally mixed with a Mg compound.

15. A process for the preparation of 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 8.

16. The method of claim 15, wherein The packing of the VPO catalyst is present in the reaction tube at a temperature of from 300°C to 420°C. The packing of the VPO catalyst is present in the reaction tube at a temperature of from 300°C to 420°C.

17. The method of claim 15 or 16, wherein The reaction gas contains 0.2 to 10 vol% of n-butane and 5 to 50 vol% of oxygen and is passed through the reaction tube at a space velocity of 1100 to 2500 hours -1 -1 -1.​

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