Method for producing a catalyst, involving a water treatment process

Abstract

The invention relates to a method for producing a catalyst, involving the process of thermally treating one or more powders P or one or more shaped geometric precursor bodies so as to form one or more catalytically active multielement oxides, the one or more catalytically active multielement oxides being treated with a liquid after the thermal treatment, optionally filtered, and dried.

Description

A process for producing a catalyst comprising a water treatment Description The present invention relates to a process for producing a catalyst. Furthermore, the present invention relates to the catalysts obtainable according to the invention and their use for the catalysis of heterogeneously catalyzed partial gas-phase oxidation. Multi-element oxides containing Mo, W, V and Cu are known, for example, from US 2011 / 0275856, US 2014 / 0221683, JP 2018-43197, US 6,994,833, EP 1 138 385 A and WO 2004 / 108267. WO 2013 / 167405 A1 discloses the production of multi-element oxides containing Mo, W, V and Cu using hydrothermal processes. A disadvantage of the production of multi-element oxides according to the aforementioned documents is the low catalytic activity and the associated need for a high temperature during the reaction. The object of the present invention was therefore to provide an improved process for the production of a catalyst. The catalyst should, in particular, exhibit increased activity. Accordingly, a process for producing a catalyst, comprising the thermal treatment of one or more powders P or one or more geometric precursor shapes to form one or more catalytically active multi-element oxides, is provided, characterized in that the one or more catalytically active multi-element oxides are treated with a liquid, optionally filtered, and dried after the thermal treatment. The thermal treatment preferably involves calcination. Conditions for calcination are described further below. The treatment preferably takes place with liquid at a temperature of 5°C to 300°C, more preferably from 10°C to 250°C, further preferably from 15°C to 200°C, particularly preferably from 20°C to 150°C, and especially preferably from 25°C to 100°C. The temperature of the liquid also depends on the liquid used and its boiling point. Preferably, drying after treatment with liquid takes place at a temperature of 50°C to 500°C, preferably from 75°C to 350°C, more preferably from 100°C to 300°C, particularly preferably from 125°C to 250°C, and especially preferably from 150°C to 200°C. Drying after treatment with liquid can be carried out under an inert gas atmosphere, an oxidative (gas) atmosphere such as air (or another mixture of inert gas and oxygen), or a reducing atmosphere (e.g., mixtures of inert gas and reducing gases such as hydrogen, ammonia, carbon monoxide, methane, and / or acrolein, or the aforementioned reducing gases individually). (Naturally, an overall reducing atmosphere can also have a limited molecular oxygen content.) The oxidative (gas) atmosphere preferably contains 0.1 to 20 vol%, particularly preferably 0.5 to 15 vol%, and most preferably 1 to 10 vol% molecular oxygen. The preferred oxidative (gas) atmospheres contain, in addition to molecular oxygen, inert gases such as nitrogen and water vapor. The water vapor content is preferably less than 20 vol%.-%, preferably less than 10 vol%. Oxygen contents above and below the aforementioned limits typically reduce the resulting catalytic activity. Drying can also be carried out under vacuum. The treatment with liquid is preferably carried out for 1 h to 100 h, more preferably for 2 h to 75 h, more preferably for 3 h to 50 h, particularly preferably for 4 h to 25 h, and especially preferably for 5 h to 10 h. Preferably, when treated with liquid, the mass ratio of catalytically active multi-element oxide to liquid is from 0.001 to 1000, preferably from 0.01 to 100, more preferably from 0.1 to 10, and particularly preferably from 0.5 to 5. Preferably, the catalyst is a shell catalyst, wherein an outer surface of a geometric support body is coated with dried, catalytically active multi-element oxide and optionally a binder. Preferably, the one or more catalytically active multi-element oxides are applied to the outer surface of a geometric support body after treatment with liquid, optional filtration, and subsequent drying, as well as optionally a binder. Preferably, the shell catalyst has an active mass fraction of 5 to 50 wt.%, more preferably 6 to 40 wt.%, further preferably 7 to 35 wt.%, particularly preferably 8 to 30 wt.%, and especially preferably 9 to 25 wt.%, based on the total mass. The catalytically active multi-element oxide is hereinafter also referred to as the active mass. Preferably, the treatment with liquid comprises at least suspension or immersion in water. According to the invention, suspension is understood to mean intermittent or continuous stirring. According to the invention, immersion is understood to mean combining the catalytically active multi-element oxide and the liquid without stirring. Preferably, filtration after treatment with water includes separating the liquid from the solid. This is preferably achieved using negative pressure. Filtration can be carried out, for example, in the laboratory using a Büchner funnel with filter paper, or in large-scale production using a filter press or similar equipment. The treatment is preferably carried out with water at a pressure of 75 kPa to 130 kPa, more preferably from 85 kPa to 120 kPa, more preferably from 95 kPa to 110 kPa, and particularly preferably from 98 kPa to 105 kPa. Preferably, the ratio of the BET surface area of ​​the catalyst or the catalytically active multi-element oxide before treatment with liquid, optional filtration, and drying to the BET surface area of ​​the catalyst or the catalytically active multi-element oxide after treatment with liquid, optional filtration, and drying is 0.10 to 0.99, preferably 0.4 to 0.9, more preferably 0.7 to 0.85, and particularly preferably 0.75 to 0.80. Preferably, the catalytically active multi-element oxide contains the elements Mo, V and optionally W. More preferably, the catalytically active multi-element oxide contains the elements Mo, V, W and optionally Cu. Particularly preferably, the catalytically active multi-element oxide contains the elements Mo, V, W, Cu and optionally Sb. Preferably the catalytically active multi-element oxide contains the elements Mo, W, V, Cu and optionally Sb, wherein the ratio of the elements corresponds to the general formula (I) MOl2WaVbCUcSbd (I), where a = 0.4 to 3.0, b = 1.0 to 6.0, c = 0.1 to 3.0 and d = 0.0 to 3.0, and the molar fraction of the element Mo in the total amount of all elements other than oxygen is 5 to 95 mol-%. The stoichiometric coefficient a of element W in the general formula (I) is preferably 0.6 to 2.5, particularly preferably 0.8 to 2.0, and most particularly preferably 1.0 to 1.6. The stoichiometric coefficient b of element V in the general formula (I) is preferably 1.5 to 5.5, particularly preferably 2.0 to 5.0, most preferably 2.5 to 4.5. Cu increases the selectivity to acrylic acid (the CO x -Selectivity decreases (i.e., less total combustion) and the activity reaches a maximum. The stoichiometric coefficient c of the element Cu in the general formula (I) is preferably 0.4 to 2.5, particularly preferably 0.6 to 2.0, most preferably 0.8 to 1.5. Preferably, the stoichiometric coefficient c of the element Cu is lower after treatment of the catalytically active multi-element oxide with liquid, optional filtration, and drying than before. Preferably, the ratio of the stoichiometric coefficient c of the element Cu before treatment with liquid to the stoichiometric coefficient c of the element Cu after treatment with liquid is 1.01 to 1.5, more preferably 1.05 to 1.4, particularly preferably 1.1 to 1.3, and especially preferably 1.15 to 1.35. This is explained by the fact that copper, or a copper component, is preferably leached from the multi-element oxide by the liquid treatment. Sb increases the long-term stability of the catalytically active multi-element oxide. The stoichiometric coefficient d of the element Sb in the general formula (I) is preferably 0.2 to 1.5, particularly preferably 0.25 to 1.2, and most particularly preferably 0.3 to 0.8. The molar fraction of the element Mo in the total amount of all elements other than oxygen is preferably from 20 to 90 mol%, particularly preferably from 35 to 85 mol%, and most preferably from 50 to 80 mol%. In the process according to the invention, an aqueous solution or aqueous suspension is produced using suitable sources of the elemental constituents Mo, W, V, Cu and optionally Sb to produce the catalytically active multi-element oxide. Preferably, the liquid is acetone, acetonitrile, benzene, butanol, chloroform, cyclohexane, diethylene glycol, diethyl ether, dimethylformamide, ethyl acetate, ethanol, propanol, methanol, hexane, heptane, formamide, ethylene glycol dimethyl ether, ethylene glycol, pentane, pyridine, tetrahydrofuran, toluene, triethylamine, triethylene glycol, triethylene glycol dimethyl ether, acetic acid, formic acid, oxalic acid, each of their aqueous solutions, water, or any mixture thereof. Preferably, the liquid is water. The invention further relates to a catalyst comprising one or more catalytically active multi-element oxides, in particular obtainable according to the inventive method, characterized in that the atomic spatial arrangement of the one or more catalytically active multi-element oxides is determined using Cu-Co- Radiation (A = 1.54178 Ω) produces a powder X-ray diffraction spectrum (the intensity of the diffracted X-rays plotted as a function of twice the diffraction angle (29)) which contains no diffraction line at 26.1 ± 0.5° 29. The diffraction line at 26.1 ± 0.5° 29 is due to a molybdenum(IV) oxide (tugarinovite; MOO2) secondary phase. Preferably, this secondary phase is removed by treatment with liquid, so that the diffraction line at 26.1 ± 0.5° 29 is present before treatment with liquid and no longer present afterward (see Figs. 1 and 2). This is all the more surprising since this phase is practically insoluble. Preferably, the BET surface area of ​​the catalyst is from 10.0 to 30.0 m². 2 / g, preferably from 12.0 to 28.0 m 2 / g, preferably from 14.0 to 26.0 m 2 / g, particularly preferably from 15.0 to 25.0 m 2 / g, particularly preferably 16.0 to 24 m 2 / g, wherein the BET surface area is determined after treatment with water, optional filtration, and drying. Preferably, the BET surface area after treatment with liquid, optional filtration, and drying is larger than before treatment. The BET surface area was determined by gas adsorption (N2) according to Brunauer-Emmet-Teller (BET). A description of the BET determination method can be found in DIN ISO 9277 and in J. Am. Chem. Soc. Vol. 60, No. 2, pages 309-319 (1938). Preferably, the catalyst is a shell catalyst, wherein one or more catalytically active multi-element oxides are applied to an outer surface of a geometric support body, and optionally binders, and the shell catalyst has an active mass fraction of 5 to 50 wt.% based on the total mass. Another object of the invention is a process for heterogeneously catalyzed partial gas-phase oxidation on a catalyst fixed bed, characterized in that the catalyst fixed bed comprises the catalyst according to the invention. A preferred method is for the production of acrylic acid by gas-phase catalytic oxidation of acrolein. Surprisingly, it was found that the activity of the catalytically active multi-element oxide or catalyst is significantly increased by treatment with a liquid, with optional filtration, and subsequent drying after thermal treatment. The production of the catalytically active multi-element oxides is known to those skilled in the art, for example from US 2011 / 0275856 and US 2014 / 0221683. To prepare the catalytically active multi-element oxide, an aqueous solution or suspension can be generated using suitable sources of the elemental constituents. This is described as an example for a catalytically active multi-element oxide containing the elements Mo, W, V, Cu, and optionally Sb: First, an aqueous solution or suspension is prepared from sources of the elemental constituents V, Mo, W, and optionally Sb. There are no restrictions on the order of addition. The pH value is preferably from 3 to 8, particularly preferably from 4 to 7, and most preferably from 5 to 7. Ammonium paratungstate heptahydrate is the preferred source for the elemental constituent W. Ammonium heptamolybdate tetrahydrate is the preferred source for the elemental constituent Mo. Ammonium metavanadate is the preferred source for the elemental constituent V. Antimony(II) acetate, antimony(II) oxide, or antimony(V) oxide are the preferred sources for the elemental constituent Sb. Furthermore, in addition to oxides, the main sources of elemental constituents to be considered are metallates, polymetallates, halides, nitrates, formates, oxalates, acetates, carbonates and hydroxides. If the solubility of a source of an elemental constituent in an aqueous medium is insufficient on its own for the purposes of the process according to the invention, the pH value of the aqueous medium can, for example, be modified by adding suitable adjusting agents to improve the solubility of the source of an elemental constituent in the aqueous medium. Suitable adjusting agents include, in particular, Brønsted acids and Brønsted bases that decompose into gaseous components under the influence of elevated temperatures, such as those used in the thermal treatment of the geometric precursor shapes to form the desired catalytically active multi-element oxide.Examples of such pH adjusting agents include ammonia, nitric acid, hydrochloric acid, acetic acid, formic acid, oxalic acid, and ammonium salts of strong and weak Brønsted acids such as ammonium nitrate, ammonium chloride, ammonium carbonate, ammonium hydrogen carbonate, ammonium acetate, ammonium formate, and ammonium oxalate. Alternatively and / or additionally, complexing agents soluble in the aqueous medium can be added. These agents decompose into gaseous compounds upon exposure to elevated temperatures, at least in the presence of molecular oxygen, and / or escape as gaseous compounds, complexing elemental constituents present in ionic form in the sources. This generally also leads to improved solubility in the aqueous medium. Examples of such complexing agents include ammonia and ethylenediaminetetraacetic acid, as well as their salts, which are preferably readily water-soluble. Another measure to improve solubility in aqueous media is the application of elevated temperatures. Naturally, more than one of the various methods mentioned can be used simultaneously to improve solubility in aqueous media. The solubility of at least one source of the elemental constituents W depends on the dosage order. Therefore, the source of elemental constituents W should be dosed before the sources of elemental constituents Mo, V, and optionally Sb. However, a different dosage order is also possible in principle. The resulting aqueous solution or suspension is then mixed with sources of the elemental constituents Cu and, optionally, Sb. The source of the elemental constituent Cu is advantageously added in solid form. The pH value is preferably from 3 to 8, particularly preferably from 4 to 7, and most preferably from 5 to 7. For the preparation of multi-element oxides according to the invention, copper(II) sulfate pentahydrate, copper(II) nitrate hydrate (Cu content = 26.1 wt%) and copper(II) acetate monohydrate are particularly suitable sources for the elemental constituent Cu, the latter being preferred. Antimony(II) acetate or antimony(II) oxide are the preferred sources for the elemental constituent Sb. First, in a), an aqueous solution or aqueous suspension is produced from at least one source each of the elemental constituents W, Mo, V, Cu and optionally Sb. Preferably, step a) is divided into two substeps a1) and a2), wherein in a1) an aqueous solution or aqueous suspension is first generated from at least one source each of the elemental constituents W, Mo, and V of the multi-element oxide, and subsequently in a2) the aqueous solution or aqueous suspension obtained in a1) is mixed with sources of the elemental constituents Cu and optionally Sb of the multi-element oxide. This preferred embodiment of the process is described in detail below. The temperature of the aqueous solution or aqueous suspension in a1) is preferably from 60 to 130°C, particularly preferably from 70 to 120°C, and most preferably from 75 to 110°C. The solution or suspension can be preheated or heated only after the addition of the source of the elemental constituent W. The duration of the addition is not subject to any restrictions. The order of adding the sources of the elemental constituents W, Mo, and V is not subject to any restrictions. Advantageously, in a1), the source of the elemental constituent W is added first, followed by the source of the elemental constituent Mo, and finally the source of the Elemental constituents V are dosed. Preferably, an aqueous solution is prepared in a1). The pH value is preferably from 3 to 8, particularly preferably from 4 to 7, and most preferably from 5 to 7. The addition can be carried out at normal pressure, vacuum, or overpressure. The pressure is preferably from 0.5 to 2 bar, particularly preferably from 0.8 to 1.2 bar, and most preferably from 0.9 to 1.1 bar. During dissolving or suspending, the solution or suspension is advantageously stirred or pumped. Preferably, after the addition of the sources of the elemental constituents W, Mo, and V, the solution or suspension is stirred for 1 to 180 minutes, more preferably 2 to 120 minutes, particularly preferably 3 to 60 minutes, and most preferably 5 to 30 minutes, before the addition of the next source. Subsequently, in a2), the aqueous solution or aqueous suspension obtained in a1) is preferably mixed with sources of the elemental constituents Cu and optionally Sb. The order of addition is not restricted. Advantageously, in a2), the source of the elemental constituent Sb is added first. Preferably, an aqueous suspension is prepared in a2). The sources of the elemental constituents Cu and optionally Sb can preferably be set as a solid, aqueous solution or aqueous suspension. The temperature of the aqueous solution or aqueous suspension in a2) is preferably from 60 to 130°C, particularly preferably from 70 to 120°C, and most preferably from 75 to 110°C. When adding the sources of the elemental constituents Cu and Sb in a2), the temperature of the aqueous solution or aqueous suspension should preferably be kept constant. The aqueous solution or aqueous suspension obtained in a) can be cooled or heated before the addition. The addition can be carried out at atmospheric pressure, vacuum, or overpressure. The pressure is preferably from 0.5 to 2 bar, particularly preferably from 0.8 to 1.2 bar, and most preferably from 0.9 to 1.1 bar. During dissolving or suspending, the solution or suspension is advantageously stirred or pumped.Preferably, the solution or suspension is stirred for 1 to 300 minutes, more preferably 5 to 180 minutes, particularly preferably 10 to 120 minutes, and especially preferably 20 to 90 minutes, after the addition of the source of the elemental constituent Sb and before the addition of the source of the elemental constituent Cu. Preferably, the solution or suspension is stirred for a further 1 to 180 minutes, more preferably 3 to 120 minutes, particularly preferably 5 to 90 minutes, and especially preferably 10 to 60 minutes, after the addition of the source of the elemental constituent Cu. The pH is preferably 3 to 8, particularly preferably 4 to 7, and most preferably 5 to 7. Ammonium paratungstate heptahydrate is the preferred source for elemental constituent W. Ammonium heptamolybdate tetrahydrate is the preferred source for elemental constituent Mo. Ammonium metavanadate is the preferred source for elemental constituent V. Furthermore, in addition to oxides, the main sources of elemental constituents to be considered are metallates, polymetallates, halides, nitrates, formates, oxalates, acetates, carbonates and hydroxides. If the solubility of a source of an elemental constituent in an aqueous medium is insufficient on its own for the purposes of the process according to the invention, the pH value of the aqueous medium can, for example, be modified by adding suitable adjusting agents to improve the solubility of the source of an elemental constituent in the aqueous medium. Suitable adjusting agents include, in particular, Brønsted acids and Brønsted bases that decompose into gaseous components under the influence of elevated temperatures, such as those used in the thermal treatment of the geometric precursor shapes to form the desired catalytically active multi-element oxide.Examples of such pH-adjusting agents include ammonia, nitric acid, hydrochloric acid, acetic acid, formic acid, and ammonium salts of strong and weak Brønsted acids such as ammonium nitrate, ammonium chloride, ammonium carbonate, ammonium hydrogen carbonate, ammonium acetate, ammonium formate, and ammonium oxalate. Alternatively and / or additionally, complexing agents soluble in the aqueous medium can be added. These agents decompose into gaseous compounds upon exposure to elevated temperatures, at least in the presence of molecular oxygen, and / or escape as gaseous compounds, complexing elemental constituents present in ionic form in the sources. This generally also leads to improved solubility in the aqueous medium. Examples of such complexing agents include ammonia and ethylenediaminetetraacetic acid, as well as their salts, which are preferably readily water-soluble. Another measure to improve solubility in aqueous media is the application of elevated temperatures. Naturally, within the framework of the inventive method, more than one of the various methods mentioned for improving solubility in aqueous media can be applied simultaneously. Preferably, the resulting aqueous solution or aqueous suspension is mixed with sources of the elemental constituent Cu and optionally Sb. The source of the elemental constituent Cu is advantageously added as a solid. When adding the at least one source of the elemental constituent Cu in c), the temperature of the aqueous solution or aqueous suspension should be kept constant. The aqueous solution or aqueous suspension obtained in b) may be cooled or heated before the addition. The source of the elemental constituent Cu is preferably dosed over less than 5 hours, particularly preferably over 0.1 to 120 minutes, and most preferably over 0.2 to 20 minutes. The addition can be carried out at atmospheric pressure, vacuum, or overpressure. The pressure is preferably from 0.5 to 2 bar, particularly preferably from 0.8 to 1.2 bar, and most preferably from 0.9 to 1.1 bar. During dissolving or suspending, the solution or suspension is advantageously stirred or The solution is pumped over. The time required for dissolving or suspending depends on temperature, energy input, and concentration, and is preferably not longer than 5 hours, particularly preferably from 1 to 120 minutes, and most preferably from 2 to 60 minutes or 2 to 30 minutes. The pH value is preferably from 3 to 8, particularly preferably from 4 to 7, and most preferably from 5 to 7. Suitable sources for the elemental constituent Cu in the preparation of the multi-element oxides according to the invention include, in particular, copper(II) sulfate pentahydrate, copper(II) nitrate hydrate (Cu content = 26.1 wt%), and copper(II) acetate monohydrate, of which the latter is preferred. Antimony acetate or antimony oxide are the preferred sources for the elemental constituent Sb. In addition to the sources of the elemental constituents Mo, W, V, Cu and optionally Sb, further sources of elemental constituents, for example Ta, Cr, Ce, Ni, Co, Fe, Mn, Zn, Nb, Bi, Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Si, Al, Ti and Zr, can be added to the process according to the invention. In b) a powder P is produced by drying the aqueous solution or aqueous suspension obtained in a) and optionally comminuting it. The aqueous solution or aqueous suspension obtained in a) is preferably spray-dried in b). A powder P can be produced directly by spray-drying the aqueous solution or aqueous suspension obtained in a). In spray drying, the aqueous solution or aqueous suspension is advantageously dispersed into fine droplets by means of a nozzle, which can be operated by liquid pressure, compressed air, or inert gas, or by means of rotating atomizing discs, and introduced into a hot gas stream, preferably a hot air stream, which dries it to powder P in fractions of a second. The hot gas stream can flow either counter-currently (against the spray jet) or, preferably, in the same direction as the spray jet (co-current). The spray tower can be operated with a directly or indirectly preheated gas stream. A directly heated gas stream is preferred, in which hot fuel gas, for example, produced by the combustion of a fuel such as methane, is mixed with an additional air stream and fed onto the spray tower.Typical inlet temperatures of the hot gas stream are in the range of 250 to 390°C, preferably in the range of 270 to 380°C, and typical outlet temperatures are in the range of 90 to 150°C. The loss on ignition of the resulting powder P, based on its total mass, is preferably 5 to 35 wt.% and particularly preferably 15 to 25 wt.%. The loss on ignition is determined by the weight loss during a thermal treatment at 400°C in air for 3 hours. As a rule, the powders P available as described have relatively uniform particle diameters. On its way from the point of origin to the spray drying device, the aqueous solution or aqueous suspension to be spray-dried is advantageously passed through at least one filter to remove any coarse particles it may contain, which could, for example, clog the spray nozzles, before they enter the spray drying device. The temperature of the conveying line is expediently maintained at the final temperature of the aqueous solution or aqueous suspension at which it was generated. The temperature is preferably between 60 and 130°C, particularly preferably between 70 and 120°C, and most preferably between 75 and 110°C. The remaining solution or suspension, which has not yet been spray-dried, is advantageously mixed continuously by stirring.The residence time (= average residence time of the aqueous solution / suspension in the container divided by the average addition rate of the solution / suspension into the spray dryer) in the container is preferably from 0.1 to 30 hours, preferably from 0.5 to 15 hours, more preferably from 1 to 5 hours. On an industrial scale, the spray-drying aqueous solution or aqueous suspension is normally produced in stirred tanks made of stainless steel type 1.4541 (DIN EN 10020). It is advantageous for the spray drying device and the stirrer to be made of the same material. The powder P obtained in b) can be directly thermally treated (also called calcination) in d) to form the catalytically active multi-element oxide. However, it is also possible to first produce geometric precursor shapes in c). To produce the geometric precursor shapes to be thermally treated in the process according to the invention from the powder P, different process variants can be used in detail. In a simple embodiment of the method according to the invention, geometric precursor bodies of any desired geometry are formed directly from the powder P by compaction, such as press agglomeration or tableting (e.g. as in documents DE 10 2008 054586 A, DE 10 2008 040093 A and DE 10 2008 040094 A (for comparable powdered mixtures, examples are shown). Examples of typical precursor shapes according to the invention are spheres (whose diameter can be, for example, from 2 to 10 mm), as well as solid cylinders or hollow cylinders (rings) with an outer diameter and a length that are typically from 2 to 10 mm. In the case of hollow cylinders, a wall thickness of 1 to 3 mm is advantageous. Naturally, aids for subsequent shaping (shaping aids) can be mixed into the powder P. These include lubricants such as graphite, carbon black, polyethylene glycol, stearic acid, stearic acid salts, starch, polyacrylic acid, mineral oil, vegetable oil, water, boron nitride, boron trifluoride, glycerin, finely divided Teflon powder, and / or cellulose ethers. The aforementioned lubricants may partially or completely decompose and / or chemically react during the thermal treatment of the geometric precursor molded parts, possibly forming gaseous substances that escape into the atmosphere. As further shaping aids, the mixture to be compacted can contain so-called reinforcing agents, which promote cohesion in the resulting geometric precursor body. Such reinforcing agents can be, for example, microfibers made of glass, asbestos, silicon carbide, and / or potassium titanate. Unlike lubricants, reinforcing aids are normally retained essentially completely during the thermal treatment of the geometric precursor molded bodies according to the invention. Naturally, lubricants and reinforcing agents can also be mixed in together. With reference to the total quantity of a powdered mixture to be compacted into precursor molded bodies according to the invention, the total quantity of forming aids contained will generally not exceed 30 wt.%, usually not exceed 20 wt.% and often not exceed 10 wt.% (but frequently at least 0.1 wt.%, or at least 0.2 wt.%, or at least 0.5 wt.%, or at least 1 wt.%). Preferably, the shaping in the production of the geometric precursor shapes can be achieved by extruding a plastically malleable mass (the resulting kneadable material, the resulting kneadable mass) by kneading powder and liquid. An advantageous method of kneading, extruding and drying is applied, for example, in EP 3805 194 A1. If the shaping of the geometric precursor shapes is carried out by extrusion or compression molding, at least one liquid (a liquid binder) is advantageously mixed in. This is preferably water, an aqueous solution, and / or the components of an aqueous solution. Advantageously, at least one of the aforementioned liquid shaping aids is a lower (C2 to C5) organic carboxylic acid (e.g., formic acid, acetic acid (preferred), propionic acid, fumaric acid, and / or maleic acid, or their respective aqueous solutions and / or the components of such aqueous solutions). Calculated as pure lower organic carboxylic acids, these (preferably acetic acid) are advantageously mixed in at a total amount of 5 to 15 wt.%, based on the powder P content in the total mixture. The total water content of the resulting mixture can be from 5 to 45 wt.%, preferably from 10 to 30 wt.%. The water content is preferably adjusted to ensure good formability of the resulting plastically malleable mass, thus guaranteeing advantageous shaping by extrusion. Too little liquid can result in a hard, plastic mass with very high viscosity. Conversely, too much liquid can result in a mass that is insufficiently plastic and has low viscosity. The incorporation of one or more lower organic carboxylic acids (preferably acetic acid) and / or their aqueous solution is expediently carried out by kneading, ensuring homogeneity as much as possible. The kneading temperature is generally not higher than 50°C. Typically, the temperature is in the range of 20 to 50°C, preferably in the range of 30 to 40°C. The kneading process preferably lasts less than 12 hours, particularly preferably from 10 to 360 minutes, and most preferably from 20 to 120 minutes. The resulting plastically malleable mass (the resulting kneadable material, the resulting modeling compound) is then formed into shaped bodies (precursor shapes) of the desired geometry by extrusion. In the simplest case, these can be strands (solid cylinders). Of course, according to the invention, rings are also possible extrudates. In the case of extruded geometric precursor shapes, thermal treatment includes drying. This drying is generally carried out at temperatures below 200°C, preferably at most 150°C, but usually at temperatures of at least 60°C, at least 80°C, or at least 100°C. Drying can be performed under an atmosphere of air, dry air, or nitrogen. The loss on ignition of the resulting precursor mold, based on its total mass, is suitably 5 to 35 wt.% and particularly suitably 15 to 25 wt.%. The loss on ignition is determined by the weight loss during a heat treatment at 400°C in air for 3 hours. Subsequently, the powder P produced in b) or the precursor bodies produced in c) are thermally treated (also called calcination) to form the catalytically active multi-element oxide. Calcination is carried out at final temperatures of 200 to 600°C, preferably 300 to 500°C, particularly preferably 350 to 450°C, and especially preferably 360 to 430°C (in each case, material temperature). According to the invention, the material advantageously has a temperature that is as uniform as possible, particularly during calcination. Calcination can be performed discontinuously or continuously. In discontinuous calcination, temperature programs with one or more temperature plateaus can be used, as described in EP 1 633 467 A. The heating rate is preferably from 0.1 to 20 K / min, particularly preferably from 0.5 to 10 K / min, and most preferably from 1 to 5 K / min. In continuous calcination, the material passes through a furnace. The calcination can be carried out isothermally or using different temperature zones, as described in EP 1 322 585 A. The temperature of the first temperature zone is preferably at least 30°C lower than the highest temperature of the other temperature zones. Calcination can be carried out in a stationary or moving bed of powder P or precursor molds. Calcination of the precursor molds is preferably carried out in a moving bed. Suitable equipment includes rotary kilns, as described in EP 1 633 467 A, or belt calciners, as described in EP 1 322 585 A. Rotary kilns are preferred. The thermal treatment (in particular the calcination) of the powder P or the geometric precursor shapes can be carried out under an inert gas atmosphere, an oxidative (gas) atmosphere such as air (or another mixture of inert gas and oxygen), or a reducing atmosphere (e.g., mixtures of inert gas and reducing gases such as hydrogen, ammonia, carbon monoxide, methane, and / or acrolein, or the aforementioned reducing gases individually). Naturally, an overall reducing atmosphere can also have a limited molecular oxygen content. The oxidative (gas) atmosphere preferably contains 0.1 to 15 vol.%, particularly preferably 0.5 to 10 vol.%, and most preferably 1 to 8 vol.%, molecular oxygen. In addition to molecular oxygen, the preferred oxidative (gas) atmospheres contain inert gases such as nitrogen and water vapor.The water vapor content is preferably less than 20% by volume, particularly preferably less than 10% by volume. Oxygen contents above and below the aforementioned limits typically reduce the resulting catalytic activity. However, the thermal treatment can also be carried out under vacuum. During calcination, uncontrolled heat generation can occur in the powder P or the precursor body, damaging the catalytically active multi-element oxide being produced. For example, when using ammonium salts, ammonia is released during calcination at temperatures of 150 to 350°C and can combust. This uncontrolled heat generation can be limited by sufficient heat and gas exchange. However, it is also possible to adjust the amount of material to be calcined, the quantity and composition of the atmosphere, and the temperature program. If the thermal treatment of the powder P or the geometric precursor shapes takes place under a gaseous atmosphere, this atmosphere can be either stagnant or flowing. The thermal treatment (especially calcination) of the powder P or the geometric precursor bodies can take up to 24 hours or more. Often, the thermal treatment (especially calcination) extends over a period of minutes to several hours, for example, from 0.5 to 10 hours, or from 1 to 5 hours. Elevated temperatures are usually associated with shorter thermal treatment (especially calcination) durations, and lower temperatures generally require longer thermal treatment (especially calcination) durations. High temperatures and long treatment durations (especially calcination) generally reduce the specific surface area of ​​the resulting catalytically active multi-element oxides produced during the thermal treatment of the geometric precursor bodies (the precursor mass). The specific BET surface area of ​​the catalytically active multi-element oxides obtainable according to the invention is typically from 10 to 32 m². 2 / g, preferably from 12 to 28 m 2 / g, especially preferably from 14 to 26 m 2 / g, especially preferred from 16 to 24 m 2 / g (determined by gas adsorption (N2) according to Brunauer-Emmet-Teller (BET)). A description of the BET determination method can be found in DIN ISO 9277 and in J. Am. Chem. Soc. Vol. 60, No. 2, pages 309-319 (1938). Preferably, the thermal treatment (in particular the calcination) of the geometric precursor shapes is carried out in a gas atmosphere containing oxygen and ammonia. The ammonia can develop from within the precursor shapes themselves by incorporating a corresponding amount of ammonium ions into them. The resulting catalytic activity of the catalytically active multi-element oxide produced during thermal treatment usually shows an optimum depending on the oxygen content of the calcination atmosphere. Suitable calcination processes according to the invention are disclosed, for example, in WO 2004 / 108284, EP 0 724 481 A, WO 2008 / 104577, WO 2004 / 108267 and WO 95 / 11081. The (resulting) geometric catalyst bodies produced during the thermal treatment of geometric precursor shapes can be used as such (as so-called full catalysts) in the catalyst fixed bed for the catalysis of the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid. Suitable solid catalyst geometries according to the invention are, for example, solid cylinders or hollow cylinders with an outer diameter and a length of 2 to 10 mm. In the case of hollow cylinders, a wall thickness of 1 to 3 mm is advantageous. Naturally, the solid catalyst can also have a spherical geometry, with the sphere diameter being from 2 to 10 mm. The geometric catalyst bodies obtainable according to the inventive method (the catalytically active multi-element oxides obtainable according to the inventive method; the catalyst obtainable according to the inventive method), especially when they have been produced in a not particularly uniform geometry, can be converted into a finely divided form (for example, crushed into powder or chips) and used for the catalysis of a heterogeneously catalyzed partial oxidation of acrolein to acrylic acid (also in a fluidized bed or fluidized bed). According to the invention, it is particularly advantageous to convert the catalytically active multi-element oxides into a finely divided form (for example, crushed into powder or granules, for example by grinding) and to apply this finely divided form (obtaining a so-called shell catalyst) as a shell of the catalytically active multi-element oxide to the outer surface of a geometric support body. The coating is typically applied using a liquid binder. This acts as an adhesive, which bonds the finely divided, catalytically active multi-element oxide to the outer surface of the geometric carrier body. The adhesive is then at least partially removed from the coated geometric carrier body (for example, by passing hot gas over it, as is done in...). (as described in WO 2006 / 094766). The residual water content of the resulting catalyst is preferably at most 1.0 wt.%, particularly preferably at most 0.5 wt.%, and most preferably at most 0.2 wt.%, in each case based on the total mass of the catalyst. Low residual water content is advantageous. As a rule, the aforementioned residual water content is typically at least 0.5 wt%, often at least 2 wt%. In this document, information on residual water content generally refers to its determination using the Moisture Analyzer HB43 from Mettler Toledo AG Laboratory & Weighing Technologies in CH-8606 Greifensee, Switzerland. For this purpose, approximately 5 g of catalyst are heated to 120°C using infrared radiation for approximately 50 seconds and held at this temperature. The measurement is terminated when the weight loss within 20 seconds is less than 1 mg. Suitable materials for the geometric support structures include, in particular, aluminum oxide, silicon dioxide, silicates such as clay, kaolin, steatite (preferably steatite from Cerarn Tee (DE) of type C-220, or preferably with a low water-soluble alkali content), pumice, aluminum silicate, magnesium silicate, silicon carbide, and zirconium dioxide. Advantageously, the geometric support structures are largely inert with respect to the relevant partial oxidation (i.e., when used solely as "catalysts" for the corresponding heterogeneously catalyzed partial gas-phase oxidation of, for example, acrolein to acrylic acid, they behave largely inert, i.e., they essentially do not cause any conversion of the acrolein). The outer surface of the geometric carrier body can be either smooth or rough. A rough outer surface is advantageous because increased surface roughness generally results in increased adhesion of the applied catalytically active multi-element oxides. As geometric support bodies with a clearly defined surface roughness, support bodies are particularly suitable which have a layer of gravel on their outer surface (the geometrically preferred support bodies according to the invention are hollow cylinders with a layer of gravel on their outer surface). Preferably the surface roughness R zThe surface roughness of the geometric support body is in the range of 30 to 100 pim, particularly preferably in the range of 50 to 70 pim (determined according to DIN 4768 Part 1 using a "Hommel Tester for DIN-ISO surface parameters" from Hommelwerke). Surface-rough geometric support bodies from Cerarn Tee (DE) made of steatite C220 are particularly preferred. The support materials can be porous or non-porous. Preferably, the support material is non-porous (the total volume of the pores of the geometric support body is advantageously at most 1 vol.% relative to the volume of the respective geometric support body). The specific BET surface area of ​​the support material (relative to the unit of its mass) is preferably small, preferably less than 5, more preferably 3, particularly preferably 1, and especially preferably 0.5 m². 2 / G. The geometric support bodies can be regularly or irregularly shaped, with regularly shaped geometric support bodies being preferred. The longitudinal extent of the geometric support shapes is normally in the range of 1 to 10 mm (the longitudinal extent is the longest direct connecting line between two points located on the outer surface of a support shape). Spheres or (solid) cylinders, especially hollow cylinders (rings) or Berl saddles, are preferably used as geometric support elements. Favorable diameters for support spheres range from 1 to 6 mm. If cylinders are used as geometric support elements, their length is preferably from 2 to 10 mm and their outer diameter preferably from 4 to 10 mm. In the case of rings, the wall thickness is also typically from 1 to 4 mm. Hollow cylindrical geometric support elements with a length of 3 to 8 mm, an outer diameter of 4 to 8 mm, and a wall thickness of 1 to 2 mm are particularly preferred geometric support elements. Examples of suitable ring geometries for carrier shapes include hollow cylinders with geometries of 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter) and geometries of 6 mm x 6 mm x 4 mm, 7 mm x 7 mm x 5 mm, 8 mm x 8 mm x 6 mm, 7 mm x 7 mm x 4 mm, 6 mm x 6 mm x 3 mm and 6.5 mm x 6.5 mm x 5 mm.Favorable geometric support structures also include all those disclosed in Research Disclosure Database Number 532036 in August 2008 (in particular, all those disclosed there as examples). The production of shell catalysts VS and ES disclosed in the present document can also be carried out with any of those disclosed there. The exemplary ring-shaped support body disclosed (in particular those with the geometry 7 mm x 4 mm x 3 mm or 6 mm x 6 mm x 4 mm) can be used. The thickness of the shell of catalytically active multi-element oxide applied to the outer surface of the geometric support bodies (in particular the ring-shaped support bodies listed above, wherein the outer surface also includes the surface enclosing the cavity of the ring) is advantageously generally 10 to 1000 pim. Preferably, this shell thickness is 10 to 500 pim for shell catalysts, particularly preferably 100 to 500 pim, and most preferably 200 to 450 pim. Advantageously, the shell thickness should be as uniform as possible when considering a single shell catalyst. When producing a larger batch of shell catalysts, the shell thickness should also be as uniform as possible when considering several individual shell catalyst rings. The aforementioned uniformity of shell thickness is often appropriately within the range specified in the embodiments described in DE 103 60 058 A. The application of the finely divided, catalytically active multi-element oxide to the outer surface of the geometric carrier body can be achieved, for example, by first moistening the outer surface in a controlled manner with the liquid binder (e.g., by spraying). By bringing the moistened geometric carrier bodies into contact with the finely divided, catalytically active multi-element oxide, a layer of the active mass is subsequently adhered to the moistened surface (e.g., by dusting the moistened geometric carrier bodies with the finely divided, catalytically active multi-element oxide (with the active mass powder) as described in EP 0 714 700 A). In this context, "controlled moistening" means that the substrate surface is moistened appropriately so that it absorbs liquid binder, but no liquid phase is visually apparent on the substrate surface. If the substrate surface is too moist, the finely divided, catalytically active multi-element oxide agglomerates into separate agglomerates instead of adhering to the surface. Detailed information on this can be found in DE 29 09 671 A and in DE 100 51 419 A, EP 0 714 700 A, and WO 2022 / 090019 A1. Naturally, the process can be repeated periodically to achieve an increased layer thickness. In this case, the coated substrate becomes the new "substrate," etc. However, all other application methods recognized as prior art in EP 0 714 700 A can also be used to produce the shell catalysts described above. Suitable liquid binders include, for example, water, an organic solvent, or a solution of an organic substance (e.g., an organic solvent) in water, in an organic solvent, or in an aqueous solution of an organic solvent. Examples include: Organic binders include mono- or polyhydric organic alcohols such as ethylene glycol, 1,4-butanediol, 1,6-hexanediol, or glycerol; mono- or polyhydric organic carboxylic acids such as propionic acid, oxalic acid, malonic acid, glutaric acid, or maleic acid; amino alcohols such as ethanolamine or diethanolamine; and mono- or polyhydric organic amides such as formamide. Suitable organic binder components (binder promoters) soluble in water, an organic liquid, or a mixture of water and an organic liquid include monosaccharides and oligosaccharides such as glucose, fructose, sucrose, and / or lactose. A solution consisting of 20 to 90 wt% water and 10 to 80 wt% an organic compound is particularly advantageously used as a liquid binder. Preferably, the organic content of the aforementioned liquid binders is 10 to 50 wt%, and particularly preferably 20 to 30 wt%. Most particularly preferred liquid binders are solutions consisting of 20 to 90 wt% water and 10 to 80 wt% glycerin. Advantageously, the glycerin content in these aqueous solutions is 10 to 50 wt%, and particularly preferably 20 to 30 wt%. The advantage of preferred binders lies, among other things, in their ability to wet both the finely divided, catalytically active multi-element oxide (or the finely divided precursor mass (see below)) and the outer surface of the geometric support molds in a fully satisfactory manner. The fineness of the finely divided, catalytically active multi-element oxide (or its precursor mass (see below)) to be applied to the outer surface of the geometric support mold is, of course, adapted to the desired shell thickness. For shell thicknesses ranging from 100 to 500 pim, suitable active mass powders are those in which at least 50% of the total number of preferably granular powder particles pass through a sieve with a mesh size (circular mesh) of 1 to 20 pim or, alternatively, 1 to 10 pim, and whose numerical proportion of particles with a longitudinal dimension above 50 pim (particles that no longer pass through a sieve with a mesh size (circular mesh) of 50 pim) is less than 10 wt.%. Otherwise, what is stated on page 18 of WO 2005 / 120702 applies accordingly. Preferably, shell catalysts available as described are produced according to the manufacturing method described and exemplified in EP 0 714 700 A (see also WO 2011 / 134932 and the embodiments in DE 103 60 057 A). An aqueous solution of 75 wt% water and 25 wt% glycerin is the preferred liquid binder. According to the invention, the thermal treatment of the geometric precursor molds is advantageously carried out according to the method described and exemplified in DE 103 60 057 A. The inventive method also includes methods for producing a catalytically active multi-element oxide in which the shaping of geometric precursor bodies with the (finely divided) mixture consisting of a powder P and optionally one or more shaping aids is carried out in such a way that (in a manner corresponding to that described for the application of an active mass shell) one can directly This (finely divided) mixture (from the finely divided precursor mass) is applied as a shell to the outer surface of a geometric support body. During the thermal treatment of the geometric precursor bodies thus produced (which also includes the at least partial removal of the liquid binder used for application), shell catalysts according to the invention are obtained directly, in which a shell of catalytically active multi-element oxide is applied to the outer surface of a (catalytically essentially inert) geometric support body. As already mentioned, the catalytically active multi-element oxides available according to the invention are particularly suitable for catalyzing a heterogeneously catalyzed partial gas-phase oxidation of acrolein to acrylic acid, as described in WO 2007 / 082827, WO 2004 / 085365, WO 2004 / 085367, WO 2004 / 085368, WO 2004 / 085369, WO 2004 / 085370, WO 2005 / 016861, WO 2005 / 047226 and WO 2005 / 042459. They are particularly characterized by the fact that a catalyst bed charged with them exhibits a long lifetime during the partial oxidation, during which the target product formation occurs with high activity. The preferred application form of a catalytically active multi-element oxide obtainable according to the invention is that of a shell catalyst, which preferably has a ring-shaped geometry.The shell catalyst exemplified in the present document is particularly preferred, for example in all embodiments and in all comparative examples of WO documents WO 2007 / 082827, WO 2004 / 085365, WO 2004 / 085367, WO 2004 / 085368, WO 2004 / 085369, WO 2004 / 085370, WO 2005 / 016861, WO 2005 / 047226 and WO 2005 / 042459, in which it is able to replace the catalyst used therein (what has been said there for the shell catalyst from the example in the present document also applies to the shell catalyst from the comparative example in the present document). In principle, catalytically active multi-element oxides available according to the invention are also advantageously suitable for catalyzing the heterogeneously catalyzed partial gas-phase oxidation of methacrolein to methacrylic acid. The above applies especially when the heterogeneously catalyzed partial gas-phase oxidation of acrolein or methacrolein (i.e., abbreviated as "(meth)acrolein") to acrylic acid or methacrylic acid (i.e., abbreviated as "(meth)acrylic acid") is carried out at high (meth)acrolein loads, as described in DE 10307 983 A, DE 199 48523 A, DE 199 10 508 A, WO 2008 / 104577, WO 2011 / 134932, DE 199 27 624 A and DE 10360 057 A. The heterogeneously catalyzed partial gas-phase oxidation can be carried out in a manner known per se. That is, a reaction gas mixture containing (meth)acrolein, molecular oxygen, and at least one inert dilution gas is passed at elevated temperature through a catalyst bed, the catalysts of which comprise at least one catalytically active multi-element oxide obtainable according to the invention as the active mass, and during the residence time of the (meth)acrolein in the catalyst bed, it is converted to (Meth)acrylic acid. A fixed-bed catalyst is preferred. However, a fluidized bed or a fluidized bed are also suitable for the process according to the invention. Generally, water vapor as a component of the reaction gas mixture improves selectivity and activity. Furthermore, inert dilution gases with increased molar specific heat, such as n-propane or carbon dioxide, are advantageous. These are gases that, when the reaction gas mixture passes through the catalyst bed, preferably undergo a chemical change of no more than 5 mol%, particularly preferably no more than 3 mol%, and most preferably no more than 1 mol%, or not at all. Heat exchanger reactors are particularly suitable for carrying out the gas-phase partial oxidation of (meth)acrolein. A heat exchanger reactor has at least one primary chamber and at least one secondary chamber, both separated from each other by a partition. The catalyst charge, comprising at least one catalytically active multi-element oxide obtainable according to the invention, is placed in the at least one primary chamber and is permeated by a reaction gas mixture containing (meth)acrolein. Simultaneously, a fluid heat transfer medium flows through the secondary chamber, and heat exchange takes place between the two chambers through the partition. This heat exchange serves to control and regulate the temperature of the reaction gas mixture as it passes through the catalyst bed. The gas phase partial oxidation of (meth)acrolein is usually carried out in a tube bundle (heat exchanger) reactor having one or more temperature zones, as described in EP 0 700 174 A, EP 0 700 893 A, DE 199 10 508 A, DE 199 48 523 A, DE 199 10 506 A, DE 199 48 241 A, DE 28 30 765 A, DE 25 13 405 A, US 3,147,084, DE 22 01 428 A, EP 0 383 224 A, JP 2007-260588 and JP S58-096041. A catalyst fixed bed, in the form of a packed bed of catalyst media (optionally mixed with thinning inert geometric media), is located within the metal tubes (contact tubes) of the tube bundle reactor. The temperature-controlled medium(s) are circulated around the metal tubes (if there is more than one temperature zone, a corresponding number of spatially separated temperature-controlled media are circulated around the metal tubes). The temperature-controlled medium is typically a molten salt. The reaction gas mixture is circulated through the contact tubes. Alternatively, the catalyst fixed bed can also be located in the spaces between thermoplate plates of a thermoplate reactor through which a heat transfer fluid flows, as recommended by DE 10 2004 017 150 A, DE 199 52 964 A and DE 103 61 456 A. As already mentioned, the catalyst fixed bed can generally consist of catalysts obtainable according to the invention, but also of catalysts with inert geometric shapes in a diluted form. The inert geometric shapes used for the production of shell catalysts according to the invention can be those used as inert geometric shapes. Geometric support bodies (support bodies) are used. A pure inert packing may be located in front of and / or behind the catalyst fixed bed (such pure inert packings are not normally included in the calculation of the load on the catalyst fixed bed with reaction gas or with a reaction gas component). Contact tubes used in a tube bundle reactor are usually made of ferritic steel and typically have a wall thickness of 1 to 3 mm. Their inner diameter is generally 20 to 30 mm, often 21 to 29 mm or 23 to 28 mm. Their length is conveniently 2 to 4 m. Advantageously, the number of contact tubes housed in the tube bundle vessel is at least 5000, preferably at least 10000. Frequently, the number of contact tubes housed in the reactor vessel is between 15000 and 40000. Tube bundle reactors with a number of contact tubes exceeding 50000 are rather the exception. Within the vessel, the contact tubes are normally arranged homogeneously (preferably 6 equidistant neighboring tubes per contact tube), the distribution being advantageously chosen such that the distance between the central inner axes of the nearest contact tubes (the so-called contact pipeline) is 35 to 45 mm (see, for example, EP 0 468290 A). The use of melts of salts such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or of low-melting-point metals such as sodium, mercury and alloys of various metals, is particularly advantageous as a heat exchanger for tube bundle reactors. Charging contact tubes in tube bundle reactors with catalysts available according to the invention, in particular the catalyst exemplarily implemented in the example, but also in the comparative example, of the present document, is particularly advantageous when the tube bundle reactor is operated with a (meth)acrolein load of the catalyst charge that is at least 90 Nl / l'h, or at least 110 Nl / l'h, or at least 130 Nl / l'h, or at least 150 Nl / l'h, or at least 160 Nl / l'h, or at least 170 Nl / l'h, or at least 180 Nl / l'h, or at least 200 Nl / l'h, or at least 220 Nl / l'h, or at least 240 Nl / l'h, or at least 260 Nl / l'h. Naturally, such catalyst loading is also advantageous at smaller (for example, at most 130 Nl / I'h, or at most 100 Nl / I'h, or at most 80 Nl / I'h, or at most 60 Nl / I'h) (meth)acrolein loads. In this document, the term "loading of a catalyst fixed bed with reaction gas inlet mixture" refers to the quantity of reaction gas inlet mixture in standard liters (= NI; the volume in liters that the corresponding amount of gas would occupy under standard conditions, i.e., at 0°C and 101.3 kPa), which is supplied to the catalyst fixed bed per hour, based on the volume of its packing (pack sections made of pure inert material are not included in the volume of the packing; otherwise, the volume of a packing is the volume of the void space occupied by the packing (or by its relevant packing sections)), i.e., based on its packed volume (-> unit = Nl / l h). The load can also refer to only one component of the reaction gas input mixture (for example, only to the organic starting compound to be partially oxidized). In this case, it is the volume of this component (for example, the organic starting compound of the partial oxidation) in standard liters that is supplied to the catalyst fixed bed per hour, relative to the volume of its packing (sections of the packing made of pure inert material are not included in the packing volume; otherwise, the volume of a packing is the volume of the void space occupied by the packing (or by its relevant packing sections)). The volume-specific activity of the catalyst fixed bed is usually designed to increase in the direction of flow of the reaction gas. This can be easily achieved by gradually decreasing the dilution of the catalyst fixed bed with inert shaped elements in the direction of reaction gas flow. The volume-specific activity can also be adjusted by using catalysts with different specific BET surface areas. Furthermore, it is possible to use shell catalysts with different pore volumes or shell thicknesses. The activity increases with increasing specific BET surface area, pore volume, or shell thickness. Furthermore, the heterogeneously catalyzed partial oxidation with shell catalysts available according to the invention can generally be carried out in all aspects as described in DE 10350 822 A. The (meth)acrolein content in the reaction gas input mixture can be in the range of 3 to 15 vol.%, frequently 3.5 to 10 vol.%, or 4 to 8 vol.% (in each case based on the total volume of the reaction gas input mixture). The molar ratio of oxygen to (meth)acrolein in the reaction gas input mixture will normally be at least 1. Typically, this ratio will be at most 3. In many cases, the heterogeneously catalyzed (meth)acrolein partial oxidation to (meth)acrylic acid will be carried out with a (meth)acrolein to oxygen to water vapor to inert gas volume ratio (NI) of 1 : (1 to 3) : (0 to 20) : (3 to 30) in the reaction gas input mixture, preferably 1 : (1 to 3) : (0.5 to 10) : (7 to 10). Suitable inert diluent gases (these are gases or mixtures of such gases that remain chemically unchanged to at least 95 mol%, preferably at least 97 mol% or at least 99 mol%, and best at 100 mol%, after a single pass of the reaction gas mixture through the catalyst bed (e.g., a fixed catalyst bed) include, among others, nitrogen, carbon dioxide, carbon monoxide, noble gases, propane, ethane, methane, butane, and / or pentane (i.e., each as a sole diluent gas or in a mixture with one or more other inert diluent gases). The reaction temperatures of such a heterogeneously catalyzed (meth)acrolein partial oxidation are typically in the range of 200 to 400°C. The operating temperature is typically 220 to 380°C, often 230 to 350°C, and frequently 245 to 320°C. The working pressure (absolute pressure) is normally 101.3 to 350 kPa, or 101.3 to 250 kPa, or 101.3 to 205 kPa (particularly as the inlet pressure to the catalyst fixed bed). Of course, the (meth)acrolein partial oxidation with the catalysts available according to the invention can also be carried out at working pressures below atmospheric pressure. The (meth)acrolein conversion, based on a single pass of the reaction gas mixture through the catalyst fixed bed, is usually at least 90 mol%, often at least 98 mol%, and frequently at least 98.5 mol%, or even at least 99 mol%. Furthermore, the partial oxidation process according to the invention can be carried out in complete accordance with the recommendations of the teachings of DE 10 2007 019 597 A or WO 2008 / 104577, or WO 2011 / 134932. In particular, the product gas mixture containing (meth)acrolein from a heterogeneously catalyzed partial oxidation of a C3 / C4 precursor compound (for example, propene or isobutene) of (meth)acrolein to (meth)acrolein can be used directly as the source of the (meth)acrolein required for the partial oxidation according to the invention, without the need to separate the (meth)acrolein from such a product gas mixture beforehand. The separation of (meth)acrylic acid from the product gas mixture of the partial oxidation can be carried out in a manner known per se, for example, by first transferring the (meth)acrylic acid into the condensed phase by absorptive and / or condensative measures. Subsequent thermal separation processes such as rectification and / or crystallization can then isolate (meth)acrylic acid of any desired purity from the condensed phase (see DE 602004924 T and WO 2006 / 114428 as well as the prior art cited therein). Catalytically active multi-element oxides, such as those used for the oxidation of acrolein to acrylic acid, typically do not exist in a form with all the metallic elements present in their maximum oxidation states. Maximum oxidation states of the metallic elements refer to the oxidation states in which the respective elements typically occur in their oxides. The maximum oxidation states of the relevant elements are V(V), Mo(VI), W(VI), Cu(II), and Sb(V). For example, vanadium may not exist entirely in oxidation state V(V), but may also exist in oxidation states V(IV) or V(III), or with mixed oxidation states. It is possible for some of the vanadium to be in oxidation state V(V) and another part in oxidation state V(IV), or for some of the vanadium to be in oxidation state V(IV) and another part in oxidation state V(III). Other metallic elements in the mixed metal oxides can also be present in different oxidation states. The oxidation states of the other relevant elements are, for example, Cu(l), Mo(V), Mo(IV) and Sb(lll). In principle, delocalized states are conceivable if a relatively high electron mobility leads to the ability to distinguish between non-discrete metal atoms with different oxidation states. Without delving into further theoretical interpretations, the catalytically active multi-element oxides can be analyzed redox-titrimetrically after digestion in aqueous solution. The content of oxidizable electrons is quantitatively determined by titration with KMnÜ4 as the oxidizing agent. For this purpose, the catalytically active multi-element oxides in powder form are used directly before being applied to a substrate. Catalytically active multi-element oxides with a defined ratio R of oxidizable electrons to vanadium exhibit particularly favorable distribution properties in the oxidation of acrolein to acrylic acid. The ratio R is R = e / CV, where e is the specific content of oxidizable electrons per g [mol / g] and CV is the specific content of vanadium per g [mol / g]. The ratio R is preferably 1.1 to 2.2, particularly preferably 1.2 to 2.1, and most preferably 1.3 to 2.0. The titration with KMnÜ4 as the oxidizing agent is carried out as follows: 15 ml of 96 wt% sulfuric acid, 15 ml of water, and 10 ml of 85 wt% phosphoric acid are placed in a long-necked flask on a heated stir plate and purged with argon to remove air. 100 to 200 mg of sample are weighed into a weighing boat and transferred to the long-necked flask with water. The flask is heated to boiling under an argon atmosphere until the volume of the solution is reduced to 40 ml and the sample is completely dissolved (approximately 30 to 45 minutes, depending on the amount of water required). The solution is then transferred to a titration vessel equipped with a combined platinum electrode and a potentiograph, for example, the 808 Titrando (Metrohm AG, Herisau, Switzerland). Titrations are performed at 80°C under an argon atmosphere. The sample is titrated with aqueous KMnO4 solution (0.02 mol / L) until a reddish-violet color is observed (excess of KMnO4). During the titration, the electrochemical potential is measured and recorded using the combined platinum electrode. The titration curve should show a transition point. No transition point means that no oxidizable electrons are present. The volume of aqueous KMnO4 solution at the endpoint is read from the titration curve. The specific content of oxidizable electrons e is e = (V * C * 5 ) / EW, where V is the volume of aqueous KMnO4 solution [I], C is the concentration of aqueous KMnCV solution [mol / l] and z is the weight of the sample [g]. In some cases, the titration curve may show multiple transition points. This indicates that electrons with different oxidation potentials are present. Two transition points may be an indication of the presence of V(III) and V(IV). Examples Production of a ring-shaped shell catalyst with the catalytically active oxide mass MOi2Wl.2V3CUl.2On The manufacturing process was carried out analogously to the example of US 2015 / 0080605 A1 according to steps A) and B) in paragraphs

[0121] until

[0130] , whereby after calcination (thermal treatment) and grinding a finely divided powder of the catalytically active multi-element oxide of composition Mo12W1.2V3Cu1.2On was obtained. Example shell catalyst C1 (according to the invention) 500 g of the resulting fine powder were suspended in 1000 g of deionized water and stirred for 18 h at 25°C (liquid treatment). The mixture was stirred in a 2 L glass vessel using a propeller stirrer. The suspension was placed on a porcelain filter (15 cm diameter) and filtered. A black mother liquor was obtained as a byproduct. The resulting filter cake was then dried for 5 hours at 200°C in an O₂ / N₂ stream (with 5 vol% O₂). The yield of dried filter cake was 85–90% based on the mass of the feedstock. The dried filter cake was then milled in a centrifugal mill (Retsch ZM200) equipped with a 120 mm circular screen at 18,000 rpm. Subsequently, 1600 g of ring-shaped carrier body (7 mm outer diameter, 3 mm length, 4 mm inner diameter, 45 pim surface roughness Rz, 0.035 m) were used. 2 / g BET surface) of type Steatite C 220 (Cerarn Tee GmbH, Plochingen, Germany) with 230 g of finely divided powder (after water treatment). The coating was applied in a Hi-Coater LHC 25 / 36 mixer (Gebrüder Lödige Maschinenbau GmbH, Paderborn, The project was carried out in Germany. The mixer was converted for continuous powder dosing. For this purpose, a funnel-shaped container was connected to the mixer's drum (36 cm diameter) via a hose (11.1 mm outer diameter, 8 mm inner diameter). 230 g of fine powder were placed in the funnel-shaped container for coating. Dosing was achieved using pressure pulses of 50 ms and an overpressure of 0.7 bar. During dosing, the contents of the funnel-shaped container were agitated using a V-shaped modified anchor stirrer (homemade). Stirring was performed for 2 seconds, followed by a 1-second pause. A 25 wt% aqueous solution of glycerin was used as the binder. The solution was metered into the mixer at a rate of 3 g / min using a type 570 S75 two-fluid nozzle (Düsen-Schlick GmbH, Coburg, Germany) in parallel with the powder dosing. The powder dosing unit was located 6 cm below the two-fluid nozzle and inclined downwards at 40°. The powder was metered outside the spray cone of the two-fluid nozzle. The mixer drum rotated clockwise at 15 rpm. The coating process was carried out at 25°C for 40 minutes. The rotation speed was then reduced to 2 rpm, and the drum was dried for 30 minutes at 130°C in an air stream (220 Nl / h). It was then cooled to 25°C. The powder was absorbed by the surface of the substrates. No twinning or agglomeration was observed. The coated substrates were then dried in a UM 400 type forced-air drying oven (Memmert GmbH & Co. KG, Schwabach, Germany) to remove any adhering glycerin. The coated substrates were then homogeneously distributed onto perforated sheets with a layer thickness of 2 cm. The perforated sheets were 0.5 cm thick, had a 60% open area, and measured 35 cm x 26 cm. The forced-air drying oven was heated to 300°C at a rate of 3 K / min and maintained at this temperature for a further 2 hours. The oven was then cooled to 40–50°C over a period of 2 to 3 hours. The ring-shaped shell catalyst C1 had an oxide active mass fraction of 9.90 wt%. The BET surface area of ​​the catalytically active multi-element oxide was 18.6 m². 2 / g. The properties of the ring-shaped shell catalyst C1 are summarized in Table 1. Example shell catalyst C2 (according to the invention) The shell catalyst C2 was produced analogously to the shell catalyst C1, using 1,600 g of ring-shaped support material and 460 g of finely divided powder for coating. The ring-shaped shell catalyst C2 had an oxide active mass fraction of 19.8 wt.%. The BET surface area of ​​the catalytically active multi-element oxide was 20.0 m². 2 / g. The properties of the ring-shaped shell catalyst C2 are summarized in Table 1. Example shell catalyst E1 (comparative example) The shell catalyst E1 was produced analogously to the shell catalyst C1, without any treatment with water (filtration and drying). The ring-shaped shell catalyst E1 had an oxide active mass fraction of 10.0 wt%. The BET surface area of ​​the catalytically active multi-element oxide was 15.7 m². 2 / g. The properties of the ring-shaped shell catalyst E1 are summarized in Table 1. Example shell catalyst E2 (comparative example) The shell catalyst E2 was produced analogously to the shell catalyst C2, without any treatment with water (filtration and drying). The ring-shaped shell catalyst E2 had an oxide active mass fraction of 20.0 wt.%. The BET surface area of ​​the catalytically active multi-element oxide was 15.9 m². 2 / g. The properties of the ring-shaped shell catalyst E2 are summarized in Table 1. Investigation of the shell catalyst: A reaction tube (stainless steel (material 1.4541); 30 mm outer diameter; 2 mm wall thickness; 26 mm inner diameter; 464 cm length) was loaded from top to bottom as follows: Section 1: 80 cm length Conduit; Section 2: 60 cm length Pre-fill of steatite rings with a geometry of 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter; steatite C 220 from Cerarn Tee GmbH); Section 3: 100 cm length Catalyst fixed bed packing consisting of a homogeneous mixture comprising 20 wt.% steatite rings of geometry 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter; steatite C 220 from Cerarn Tee GmbH) and 80 wt.% of the shell catalyst; Section 4: 200 cm length Catalyst fixed bed packing consisting exclusively of the shell catalyst as described in section 3; Section s: 10 cm length Refilling with the same steatite rings as in section 2; Section e: 14 cm length Catalyst chair made of stainless steel (material 1.4541) for holding the catalyst fixed bed. A reaction gas mixture with the following concentrations was passed through the respective reaction tube, which was charged as described above, flowing from top to bottom: 4.3% Acrolein by volume 0.3% vol. propene 0.2% propane by volume 0.3% vol. acrylic acid, 5.1 vol. -% oxygen, 0.4% by volume of carbon oxides, 7% by volume water and 82.3% by volume nitrogen. The feed temperature of the reaction gas mixture (at the inlet to the reaction tube) was 210 °C and the loading of the catalyst fixed bed (as defined in DE 199 27 624 A) with acrolein was 80 Nl / lh. The reaction tube was surrounded along its entire length (except for the last 10 cm of the empty tube in section 1 and the last 3 cm of the tube in section 6) by a stirred and externally electrically heated salt bath (mixture of 53 wt% potassium nitrate, 40 wt% sodium nitrite, and 7 wt% sodium nitrate, 50 kg molten salt). The flow velocity at the tube was 3 m / s. The salt bath temperature TB, at which the salt bath was supplied, was set in all cases to achieve an acrolein conversion of 99.3 mol% based on a single passage of the reaction gas mixture through the catalyst fixed bed. The salt bath temperature did not change along the reaction tube due to the additional heating (more heat was radiated from the salt bath than was transferred from the reaction tube to the salt bath). Under the selectivity of acrylic acid formation (S AS (mol-%)) is understood in this document as: Molzah l Acrolei nu mgesetzt zu Acrylsäure SAS = x 100. Molzah l Acrolei is not implemented as a whole The selectivity of CO x -Education (S COx (mol-%); total combustion) is calculated analogously. An active mass (leading catalyst) that leads to the same conversion under otherwise unchanged reaction conditions at a lower temperature has a higher activity. Under the sales of acrolein (U AC (mol-%)) is understood in this document as: Total number of moles of acrolein implemented x 100 mol%. Total mole number of acrolein used Table 1 below shows the results obtained after 100 operating hours, depending on the shell catalyst used. Table 1. Experimental results *) Comparative example TB [°C] Salt bath temperature U AC [%] Acrolein sales S AS [mol%] Selectivity to acrylic acid Y AS [mol %] Yield to acrylic acid The experimental results shown in Table 1 indicate that the required salt bath temperature TB for an acrolein conversion of 99.3 mol-% of the shell catalysts C1 and C2 according to the invention is significantly lower than for the comparative examples, and that the catalyst according to the invention therefore has a higher activity. The Cu-Ko powder X-ray diffraction spectra were determined by powder X-ray diffraction on the powders after treatment with water, filtration and drying or before coating the substrate as follows: Measuring device: D8 Advance Series 2 (Bruker AXS GmbH) with multiple sample changer Primary side: Cu anode (Cu-Ko radiation with A = 1.54178 Ω; 40 kV and 40 mA), FDS 0.1° with ASS Secondary side: Scatter beam baffle: 8 mm with Ni 0.5 mm, Söller 4° + Lynx-Eye (3° aperture) The geometry was Bragg-Brentano, and air scattering was reduced by an air scattering shield. The sample is milled until a fine powder is obtained. The mill used is an IKA Tube Mill 100. The milling program is 20,000 rpm for 60 seconds, repeated once, and then the sample is pressed into a standard flat sample holder provided by Bruker AXS GmbH for Bragg-Brentano geometry data acquisition. A glass plate was used to create a flat surface, compressing and smoothing the sample powder. The data were acquired from the angular range of 5 to 70° 2Theta with a step size of 0.02° 2Theta, while the variable divergence gap was set to a fixed angle of 0.1°. The crystalline phases of the samples were determined using the software DIFFRAC.EVA (Bruker AXS GmbH, Karlsruhe). Figure 1 shows the experimentally determined Cu-Ko powder X-ray diffraction spectrum, reflecting the atomic spatial arrangement of catalysts C1 and E1 (ordinate: intensity, given as absolute count rate; abscissa: range from 5° to 70°). The spectrum of C1 is shown above, the spectrum of E1 below. Figure 2 shows the experimentally determined Cu-Ko powder X-ray diffraction spectrum, reflecting the atomic spatial arrangement of the catalysts O2 and E2 (ordinate: intensity, given as absolute count rate; abscissa: range from 5° to 79°). The spectrum of O2 is shown above, the spectrum of E2 below. The diffraction line at 26.1 ± 9.5° 29 is visible in the spectra of E1 and E2, but is no longer present in the spectra of C1 and O2. This can be attributed to the removal of the MoO2 side phase by the water treatment.

Claims

Patent claims 1. A process for producing a catalyst, comprising the thermal treatment of one or more powders P or one or more geometric precursor shapes to form one or more catalytically active multi-element oxides, characterized in that the one or more catalytically active multi-element oxides are treated with a liquid, optionally filtered, and dried after the thermal treatment.

2. Method according to claim 1, characterized in that the treatment with liquid takes place at a temperature of 5°C to 300°C.

3. Method according to claim 1 or 2, characterized in that the drying after treatment with liquid takes place at a temperature of 50°C to 500°C.

4. Method according to one of claims 1 to 3, characterized in that the treatment with liquid is carried out for 1 h to 100 h.

5. Method according to one of claims 1 to 4, characterized in that, during treatment with liquid, a mass ratio of catalytically active multi-element oxide to liquid of 0.001 to 1000 is present.

6. Method according to one of claims 1 to 5, characterized in that the catalyst is a shell catalyst, wherein an outer surface of a geometric support body is coated with dried catalytically active multi-element oxide and the shell catalyst has an active mass fraction of 5 to 50 wt.% based on the total mass.

7. Method according to one of claims 1 to 6, characterized in that the ratio of the BET surface area of ​​one or more catalytically active multi-element oxides before treatment with liquid, optional filtration, and drying to the BET surface area of ​​one or more catalytically active multi-element oxides after treatment with liquid, optional filtration, and drying is 0.10 to 0.

99.

8. Method according to any one of claims 1 to 7, characterized in that the treatment with liquid comprises at least a suspension or immersion in the liquid.

9. Method according to any one of claims 1 to 8, characterized in that the treatment with liquid is carried out at a pressure of 75 kPa to 130 kPa.

10. Method according to any one of claims 1 to 9, characterized in that the catalytically active multi-element oxide contains the elements Mo, W, V, Cu and optionally Sb, wherein the ratio of the elements corresponds to the general formula (I) MOl2WaVbCUcSbd (I), where a = 0.4 to 3.0, b = 1.0 to 6.0, c = 0.1 to 3.0 and d = 0.0 to 3.0, and the molar fraction of the element Mo in the total amount of all elements other than oxygen is from 5 to 95 mol-%.

11. Method according to any one of claims 1 to 10, characterized in that the liquid is water.

12. Catalyst comprising one or more catalytically active multi-element oxides, obtainable according to a method of claims 1 to 11, characterized in that the atomic spatial arrangement of the one or more catalytically active multi-element oxides under application of Cu-Ko radiation (η = 1.54178 η) results in a powder X-ray diffraction spectrum (the intensity of the diffracted X-ray radiation plotted as a function of twice the diffraction angle (29)) which contains no diffraction line at 26.1 ± 0.5° 29.

13. Catalyst according to claim 12, characterized in that the BET surface area of ​​the catalyst is 10.0 to 30.0 m² 2 / g, where the BET surface area is determined after treatment with liquid, optional filtration and drying.

14. Catalyst according to claim 12 or 3, characterized in that the catalyst is a shell catalyst, wherein the one or more catalytically active multi-element oxides are applied to an outer surface of a geometric support body, and the shell catalyst has an active mass fraction of 5 to 50 wt.% based on the total mass.

15. A process for heterogeneously catalyzed partial gas-phase oxidation on a catalyst fixed bed, characterized in that the catalyst fixed bed comprises a catalyst according to one of claims 14 or 15.

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