Process for producing a catalytically active multi-element oxide containing the elements mo, w, v, cu and sb

Abstract

The invention relates to a process for producing a catalytically active multi-element oxide containing Mo, W, V, Cu and Sb, wherein an aqueous solution or an aqueous suspension is generated from at least one source of the element constituents W, Mo and V of the multi-element oxide, the obtained aqueous solution or aqueous suspension is mixed with sources of the element constituents Cu and Sb of the multi-element oxide, a powder P is generated by drying the obtained aqueous solution or aqueous suspension, geometrical precursor molded bodies are optionally produced with the obtained powder P, and the powder P or the geometrical precursor molded bodies are thermally treated, forming the catalytically active multi-element oxide, characterized in that copper acetate is used as the source of the element constituent Cu and antimonoxide is used as the source of the element constituent Sb.

Description

[0001] Process for the preparation of a catalytically active multielement oxide containing the elements Mo, W, V, Cu and Sb

[0002] Description

[0003] The present invention relates to a process for producing a catalytically active multielement oxide containing the elements Mo, W, V, Cu and Sb.

[0004] Furthermore, the present invention relates to the catalytically active multielement oxides obtainable according to the invention, their use for catalyzing the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid and their use for producing coated catalysts particularly suitable for this catalysis, as well as the coated catalysts obtainable according to the invention.

[0005] Multielement oxides containing Mo, W, V, Cu and Sb are known, for example, from WO 2021 / 213823, US 5,959,143, US 6,762,148, EP 0 711 745 A1 and EP 3 488 926 A1.

[0006] WO 2021 / 213823 discloses the preparation of multielement oxides, using copper acetate and antimony acetate as sources of the elemental constituents Cu and Sb.

[0007] From US 5,959,143 and EP 3 488 926 A1 the preparation of multielement oxides is known, wherein copper sulfate and antimony oxide were used as sources of the elemental constituents Cu and Sb.

[0008] US 6,762,148 discloses the preparation of multielement oxides, using copper sulfate and antimony acetate as sources of the elemental constituents Cu and Sb.

[0009] EP 0 711 745 A1 discloses the production of multi-element oxides, using copper nitrate or oxide and antimony oxide as sources of the elemental constituents Cu and Sb.

[0010] A disadvantage in the preparation of the multielement oxides of WO 2021 / 213823, US Pat. No. 5,959,143, US Pat. No. 6,762,148, EP 0 711 745 A1, and EP 3 488 926 A1 is the low catalytic activity and the associated high temperature required during the reaction. The object of the present invention was therefore to provide an improved process for the preparation of a catalytically active multielement oxide containing the elements Mo, W, V, Cu, and Sb. The catalytically active multielement oxide should, in particular, exhibit increased activity.

[0011] Accordingly, a process for the preparation of a catalytically active multielement oxide containing the elements Mo, W, V, Cu and Sb, wherein the ratio of the elements corresponds to the general formula (I)

[0012] Moi2W a VbCu cSbd (I), where a = 0.4 to 3.0, b = 1.0 to 6.0, c = 0.1 to 3.0 and d = 0.1 to 3.0, and the molar proportion of the element Mo in the total amount of all elements other than oxygen is from 5 to 95 mol%, wherein a) an aqueous solution or aqueous suspension is produced from at least one source of the elemental constituents W, Mo and V of the multielement oxide, b) the aqueous solution or aqueous suspension obtained in a) is admixed with sources of the elemental constituents Cu and Sb of the multielement oxide, c) a powder P is produced by drying the aqueous solution or aqueous suspension obtained in b) and optionally comminuting, d) optionally with the powder P obtained in c) and optionally with the addition of one or more shaping aids and after uniform mixing, geometric precursor shaped bodies are produced from the resulting mixture,and e) the powder P obtained in c) or the geometric precursor shaped bodies obtained in d) are thermally treated to form the catalytically active multielement oxide, characterized in that copper acetate is used as the source of the elemental constituent Cu and antimony oxide is used as the source of the elemental constituent Sb. The stoichiometric coefficient a of the element W in the general formula (I) is preferably 0.6 to 2.5, particularly preferably 0.8 to 2.0, very particularly preferably 1.0 to 1.6.

[0013] 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.

[0014] Cu increases the selectivity to acrylic acid (which CO x -Selectivity decreases, ie less total combustion) and the activity passes through a maximum.

[0015] 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.

[0016] Sb increases the long-term stability of the catalytically active multielement oxide.

[0017] 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, most preferably 0.3 to 0.8.

[0018] The molar proportion 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%, most preferably from 50 to 80 mol%.

[0019] To produce the catalytically active multielement oxide, an aqueous solution or aqueous suspension is produced in the process according to the invention using suitable sources of the elemental constituents Mo, W, V, Cu and Sb.

[0020] For the preparation of the catalytically active multielement oxide, copper(II) acetate monohydrate (Cu(CH3COO)2 H2O) is preferably used as a source of the elemental constituent Cu and antimony(III) oxide (Sb205) as a source of the elemental constituent Sb.

[0021] Surprisingly, it was found that the activity of the catalytically active multielement oxide is significantly increased when using copper acetate, in particular copper(II) acetate monohydrate, as a source of the elemental constituent Cu and antimony oxide, in particular antimony(III) oxide, as a source of the elemental constituent Sb compared to other sources.

[0022] First, in a), an aqueous solution or aqueous suspension is prepared from at least one source of each of the elemental constituents W, Mo, and V. The temperature of the aqueous solution or aqueous suspension in a) is preferably from 60 to 130°C, more preferably from 70 to 120°C, 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 addition of the sources of the elemental constituents W, Mo, and V is not subject to any restrictions. Advantageously, in a), 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 constituent V. Preferably, an aqueous solution is prepared in a).The pH is preferably from 3 to 8, more preferably from 4 to 7, most preferably from 5 to 7.

[0023] The addition can be carried out under atmospheric pressure, vacuum, or superatmospheric pressure. The pressure is preferably from 0.5 to 2 bar, more preferably from 0.8 to 1.2 bar, most preferably from 0.9 to 1.1 bar. During dissolving or suspending, the solution or suspension is advantageously stirred or circulated. 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, more preferably 3 to 60 minutes, and especially preferably 5 to 30 minutes before the addition of the next source.

[0024] Subsequently, in b), the aqueous solution or aqueous suspension obtained in a) is admixed with sources of the elemental constituents Cu and Sb. The order of addition is not restricted. Advantageously, in b), the source of the elemental constituent Sb is added first. Preferably, in b), an aqueous suspension is prepared.

[0025] The sources of the elemental constituents Sb and / or Cu can preferably be added as a solid, aqueous solution or aqueous suspension.

[0026] The temperature of the aqueous solution or aqueous suspension in b) is preferably from 60 to 130°C, more preferably from 70 to 120°C, most preferably from 75 to 110°C. When adding the sources of the elemental constituents Cu and Sb in b), 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 addition. The addition can be carried out at atmospheric pressure, vacuum, or superatmospheric pressure. The pressure is preferably from 0.5 to 2 bar, more preferably from 0.8 to 1.2 bar, most preferably from 0.9 to 1.1 bar. During dissolution or suspension, the solution or suspension is advantageously stirred or circulated.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, 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 from 3 to 8, particularly preferably from 4 to 7, most preferably from 5 to 7.

[0027] Ammonium paratungstate theptahydrate is the preferred source of the elemental constituent W. Ammonium heptamolybdate tetrahydrate is the preferred source of the elemental constituent Mo. Ammonium metavanadate is the preferred source of the elemental constituent V.

[0028] In addition to oxides, sources of the elemental constituents include, in general, metallates, polymetalates, halides, nitrates, formates, oxalates, acetates, carbonates and hydroxides.

[0029] 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 of the aqueous medium can, for example, be suitably modified by adding appropriate adjusting agents to improve the solubility of the source of an elemental constituent in the aqueous medium. Suitable adjusting agents include, in particular, those Bronsted acids and Bronsted bases that decompose into gaseous components under the influence of elevated temperatures, such as those used in the thermal treatment of the geometric precursor bodies to form the desired catalytically active multielement oxide.Examples of such pH adjusters include ammonia, nitric acid, hydrochloric acid, acetic acid, formic acid and ammonium salts of strong and weak Brönsted acids such as ammonium nitrate, ammonium chloride, ammonium carbonate, ammonium bicarbonate, ammonium acetate, ammonium formate and ammonium oxalate.

[0030] Alternatively and / or additionally, soluble complexing agents can also be added to the aqueous medium. These complexing agents decompose into gaseous compounds upon exposure to elevated temperatures, at least in the presence of molecular oxygen, and / or escape as gaseous compounds and are capable of complexing elemental constituents present in ionic form in the sources, which generally also leads to an improvement in solubility in the aqueous medium. Examples of such complexing agents include ammonia and ethylenediaminetetraacetic acid, as well as their salts, preferably those that are highly soluble in water.

[0031] Another measure for improving solubility in an aqueous medium is the use of elevated temperatures. Of course, more than one of the various options for improving solubility in an aqueous medium can be applied simultaneously within the scope of the inventive process.

[0032] In addition to the sources of the elemental constituents Mo, W, V, Cu and 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 in the process according to the invention.

[0033] In c) a powder P is produced by drying the aqueous solution or aqueous suspension obtained in b) and optionally comminuting.

[0034] The aqueous solution or aqueous suspension obtained in b) is preferably spray-dried in c). A powder P can be produced directly by spray-drying the aqueous solution or aqueous suspension obtained in b).

[0035] During spray drying, the aqueous solution or aqueous suspension is advantageously divided 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 atomiser discs and introduced into a hot gas stream, preferably a hot air stream, which dries it to the powder P in fractions of a second. The hot gas stream can in principle flow against the spray jet, i.e. in countercurrent, or preferably with the spray jet, i.e. in cocurrent. The spray tower can be operated with a directly or indirectly preheated gas stream. Preference is given to using a directly heated gas stream in which hot fuel gas produced, for example, by the combustion of a fuel such as methane is mixed, for example, with an additional air stream and fed onto the spray tower.Typical inlet temperatures of the hot gas stream are in the range from 250 to 390°C, preferably in the range from 270 to 380°C, and typical outlet temperatures are in the range from 90 to 150°C. The loss on ignition of the resulting powder P, based on its total mass, is preferably from 5 to 35 wt.% and particularly preferably from 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. Powders P obtainable as described generally have comparatively uniform particle diameters.

[0036] On its way from the point of production to the spray-drying device, the aqueous solution or aqueous suspension to be spray-dried is advantageously passed through at least one filter to separate any coarse particles it may contain, which could, for example, clog the spray nozzles, before it enters the spray-drying device. The temperature of the conveying line is advantageously maintained at the final value of the production temperature of the aqueous solution or aqueous suspension. The temperature is preferably from 60 to 130°C, more preferably from 70 to 120°C, most preferably from 75 to 110°C. The residual solution or suspension not yet spray-dried is advantageously continuously mixed 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.

[0037] On an industrial scale, the aqueous solution or aqueous suspension to be spray-dried is typically produced in stirred vessels made of stainless steel type 1.4541 (DIN EN 10020). The spray-drying device and the stirrer are preferably made of the same material.

[0038] The powder P obtained in c) can be thermally treated (also called calcination) directly in e) to form the catalytically active multielement oxide. However, it is also possible to first produce geometric precursor bodies in d).

[0039] To produce the geometric precursor shaped bodies to be thermally treated from the powder P in the process according to the invention, different process variants can be used in detail.

[0040] In a simple embodiment of the process according to the invention, geometric precursor shaped bodies of any desired geometry are formed directly from the powder P by compaction, such as press agglomeration or tabletting (e.g. as exemplified in the documents DE 10 2008 054586 A, DE 10 2008 040093 A and DE 10 2008 040094 A for comparable powdered mixtures). Examples of precursor shaped body geometries typical 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 is typically from 2 to 10 mm. In the case of hollow cylinders, a wall thickness of 1 to 3 mm is expedient.

[0041] Of course, aids for subsequent shaping (shaping aids) can be mixed into the powder P. Suitable lubricants include, for example, 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 ether.

[0042] The aforementioned lubricants may decompose and / or chemically react during the thermal treatment of the geometric precursor shaped bodies, possibly with the formation of gaseous substances.

[0043] As additional shaping aids, the mixture to be compacted can contain so-called reinforcing agents that 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.

[0044] In contrast to the lubricants, reinforcing aids are normally essentially completely retained during the thermal treatment of the geometric precursor shaped bodies according to the invention.

[0045] Of course, lubricants and reinforcing agents can also be mixed in together.

[0046] Based on the total amount of a powdery mixture to be compacted according to the invention to form precursor shaped bodies, the total amount of shaping aids contained will generally not be more than 30% by weight, usually not more than 20% by weight and often not more than 10% by weight (but often at least 0.1% by weight, or at least 0.2% by weight, or at least 0.5% by weight, or at least 1% by weight).

[0047] Preferably, the shaping during the production of the geometric precursor shaped bodies can be carried out by extruding a plastically formable mass (the resulting kneaded material, the resulting kneading mass) by kneading powder and liquid.

[0048] An advantageous method of kneading, extruding, and drying is used, for example, in EP 3 805 194 A1. If the shaping during the production of the geometric precursor shaped bodies takes place by extrusion or extrusion, at least one liquid (a liquid binder) is advantageously mixed in. This is preferably water, an aqueous solution, and / or the constituents of an aqueous solution. Advantageously, 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 solution and / or the constituents of such an aqueous solution) is mixed in as at least one aforementioned liquid shaping aid.

[0049] Calculated as pure lower organic carboxylic acids, these (preferably acetic acid) are advantageously mixed in a total amount of 5 to 15 wt. %, based on the content of powder P 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 so that the resulting plastically formable mass is readily available, in order to ensure advantageous shaping by extrusion. An excessively low liquid content can lead to a hard, plastic mass with very high viscosity. An excessively high liquid content, on the other hand, can lead to a mass with insufficient plastically formable properties and low viscosity.

[0050] The mixing in of one or more lower organic carboxylic acids (preferably acetic acid) and / or their aqueous solution is advantageously carried out by kneading and as homogeneously as possible. The temperature during kneading will generally not exceed 50°C. Typically, the aforementioned temperature is in the range of 20 to 50°C, advantageously in the range of 30 to 40°C. Kneading preferably takes less than 12 hours, more preferably from 10 to 360 minutes, most preferably from 20 to 120 minutes.

[0051] The resulting plastically moldable mass (the resulting kneaded material, the resulting putty) is then extruded into shaped bodies (precursor bodies) of the desired geometry. In the simplest case, these can be strands (solid cylinders). Of course, rings are also possible extrudates according to the invention.

[0052] In the case of geometric precursor bodies produced by extrusion, thermal treatment includes drying. This drying is generally carried out at temperatures of less than 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 carried out under air, dry air, or nitrogen atmosphere.

[0053] The loss on ignition of the resulting precursor molded body, based on its total mass, is advantageously from 5 to 35 wt.% and particularly advantageously from 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.

[0054] Subsequently, the powder P produced in c) or the precursor shaped bodies produced in d) are thermally treated to form the catalytically active multielement oxide (also referred to as calcination).

[0055] The 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 (each material temperature). According to the invention, the material advantageously has as uniform a temperature as possible, particularly during calcination.

[0056] Calcination can be carried out discontinuously or continuously.

[0057] 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, most preferably from 1 to 5 K / min.

[0058] In continuous calcination, the material travels 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 remaining temperature zones.

[0059] The calcination can be carried out in a stationary or moving bed of the powder P or the precursor shaped bodies. The calcination of the precursor shaped bodies is preferably carried out in a moving bed. Suitable apparatus are 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 shaped bodies can be carried out both under an inert gas and under an oxidative (gas) atmosphere such as air (or another mixture of inert gas and oxygen) as well as under 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 alone) can be carried out (of course, an overall reducing atmosphere can also have a limited content of molecular oxygen). The oxidative (gas) atmosphere preferably contains from 0.1 to 15 vol.%, particularly preferably from 0.5 to 10 vol.%, very particularly preferably from 1 to 8 vol.%, of 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.%, particularly preferably less than 10 vol.%. Oxygen contents above and below the aforementioned limits normally reduce the resulting catalytic activity. In principle, however, the thermal treatment can also be carried out under vacuum.

[0060] During calcination, uncontrolled heat generation can occur in the powder P or in the precursor molding, damaging the catalytically active multielement oxide being produced. When using ammonium salts, for example, at temperatures of 150 to 350°C, ammonia is released during calcination and can burn. 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 amount and composition of the atmosphere, and the temperature program.

[0061] If the thermal treatment of the powder P or the geometric precursor bodies takes place under a gaseous atmosphere, this can be either static or flowing.

[0062] Overall, the thermal treatment (especially calcination) of the powder P or the geometric precursor bodies can take up to 24 hours or more. The thermal treatment (especially calcination) often lasts from minutes to several hours, for example from 0.5 to 10 hours, or from 1 to 5 hours. Elevated temperatures are normally associated with shorter thermal treatment times (especially calcination), while lower temperatures generally result in longer thermal treatment times (especially calcination). High temperatures and long treatment times (especially calcination) generally reduce the specific surface area of ​​the catalytically active multielement oxides produced during the thermal treatment of the geometric precursor bodies (the precursor mass).

[0063] The specific BET surface area of ​​the catalytically active multielement oxides obtainable according to the invention is typically from 10 to 32 m 2 / g, preferably from 12 to 28 m 2 / g, particularly preferably from 14 to 26 m 2 / g, especially preferred from 16 to 24 m 2 / g (determined by gas adsorption (N2) according to the Brunauer-Emmet-Teller (BET) method). 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).

[0064] The thermal treatment (especially calcination) of the geometric precursor bodies is preferably carried out in a gas atmosphere containing oxygen and ammonia. The ammonia can evolve from the precursor bodies themselves by incorporating an appropriate amount of ammonium ions into them.

[0065] The resulting catalytic activity of the catalytically active multielement oxide produced during the thermal treatment generally shows an optimum depending on the oxygen content of the calcination atmosphere.

[0066] Calcination processes suitable according to the invention are disclosed, for example,

[0067] WO 2004 / 108284, EP 0 724 481 A, WO 2008 / 104577, WO 2004 / 108267 and WO 95 / 11081.

[0068] The (resulting) geometric catalyst bodies obtained during a thermal treatment of geometric precursor bodies can be used as such (so-called full catalysts) in the fixed catalyst bed to catalyze the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid.

[0069] Suitable unsupported catalyst geometries according to the invention include, 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. Of course, the unsupported catalyst can also have a spherical geometry, with the sphere diameter being between 2 and 10 mm.

[0070] The geometric shaped catalyst bodies obtainable by the process according to the invention (the catalytically active multielement oxides obtainable according to the invention; the catalyst obtainable according to the invention), in particular when they were produced in a not particularly uniform geometry, can also be converted into a finely divided form (for example, comminuted to powder or grit) to catalyze a heterogeneously catalyzed partial oxidation of acrolein to acrylic acid (also in a fluidized or moving bed).

[0071] However, it is particularly advantageous according to the invention to convert the catalytically active multielement oxides into a finely divided form (for example, comminuted to powder or grit, for example by grinding) and to apply this finely divided form (to obtain a so-called shell catalyst) as a shell of the catalytically active multielement oxide to the outer surface of a geometric shaped support body.

[0072] Application is typically carried out using a liquid binder. This acts as an adhesive liquid, with the aid of which the finely divided, catalytically active multielement oxide is adhered to the outer surface of the geometric shaped support body. The adhesive liquid is then at least partially removed from the coated geometric shaped support body (for example, by passing hot gas over it, as described in WO 2006 / 094766). The residual water content of the resulting catalyst is preferably at most 1.0 wt. %, more preferably at most 0.5 wt. %, most preferably at most 0.2 wt. %, in each case based on the total mass of the catalyst.

[0073] Low residual water contents are advantageous. The aforementioned residual water content is usually at least 0.5 wt.%, frequently at least 2 wt.%. Data on residual water contents in this document generally refer to their determination using the Moisture Analyzer HB43 from Mettler Toledo AG Laboratory & Weighing Technologies in 8606 Greifensee, Switzerland. For this purpose, approximately 5 g of catalyst is heated to 120°C in approximately 50 seconds using infrared radiation and held at this temperature. The measurement is terminated when the weight loss within 20 seconds is less than 1 mg.

[0074] Suitable materials for the geometric shaped supports include, in particular, aluminum oxide, silicon dioxide, silicates such as clay, kaolin, steatite (preferably steatite from Cerarn Tee (DE) type C-220, or preferably with a low water-soluble alkali content), pumice, aluminum silicate, magnesium silicate, silicon carbide, and zirconium dioxide. The geometric shaped supports are advantageously 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 are largely inert, i.e., they essentially do not require any conversion of the acrolein).

[0075] The outer surface of the geometric support body can be either smooth or rough. Rough is advantageous, as increased surface roughness generally results in increased adhesion of the applied catalytically active multielement oxides.

[0076] As geometric carrier moldings with clearly developed surface roughness, carrier moldings which have a chippings coating on their outer surface are particularly suitable (preferred geometric carrier moldings according to the invention are hollow cylinders with a chippings coating on their outer surface).

[0077] The surface roughness Rz of the outer surface of the geometric carrier body is preferably in the range of 30 to 100 μm, particularly preferably in the range of 50 to 70 μm (determined according to DIN 4768 Sheet 1 using a "Hommel Tester for DIN-ISO Surface Measurement Parameters" from Hommelwerke). Surface-roughened geometric carrier bodies made of steatite C220 from Cerarn Tee (DE) are particularly preferred.

[0078] The carrier materials can be porous or non-porous. Preferably, the carrier material is non-porous (the total volume of the pores of the geometric carrier body, based on the volume of the respective geometric carrier body, is advantageously at most

[0079] 1 vol.-%).

[0080] The specific (relative to the unit of its mass) BET surface area of ​​the support material is preferably small, preferably less than 5, more preferably 3, particularly preferably 1, especially preferably 0.5 m 2 / G.

[0081] The geometric carrier bodies can be regularly or irregularly shaped, with regularly shaped geometric carrier bodies being preferred.

[0082] The longitudinal dimension of the geometric carrier bodies is normally in the range of 1 to 10 mm (the longitudinal dimension is the longest direct connecting line between two points on the outer surface of a carrier body).

[0083] Spheres or (solid) cylinders, in particular hollow cylinders (rings) or Berl saddles, are preferably used as geometric carrier molds. Favorable diameters for carrier spheres are from 1 to 6 mm. If cylinders are used as geometric carrier molds, their length is preferably from 2 to 10 mm and their outer diameter is 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 carrier molds 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 carrier molds. Examples of favorable ring geometries for carrier moldings are hollow cylinders with the geometry 7 mm x 3 mm x 4 mm (outer diameter x length x inner diameter) as well as the geometries 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 shaped supports also include all the shaped supports disclosed in Research Disclosure Database Number 532036 in August 2008 (in particular all those disclosed therein by way of example). The preparation of coated catalysts VS and ES disclosed in the present document can also be carried out with any annular shaped support disclosed therein by way of example (in particular with those with the geometry 7 mm x 4 mm x 3 mm or 6 mm x 6 mm x 4 mm).

[0084] The thickness of the shell of catalytically active multielement oxide applied to the outer surface of the geometric shaped support bodies (in particular the annular shaped support bodies listed above, the outer surface of which also includes the surface enclosing the cavity of the ring) is advantageously generally between 10 and 1000 pm. For coated catalysts, this shell thickness is preferably between 10 and 500 pm, particularly preferably between 100 and 500 pm, and most particularly preferably between 200 and 450 pm.

[0085] Advantageously, the shell thickness is as uniform as possible across a single coated catalyst. When manufacturing a larger production batch of coated catalysts, the shell thickness is also as uniform as possible across several individual coated catalyst ring bodies. The aforementioned uniformity of the shell thickness is often within the range of the specifications given in the exemplary embodiments of DE 103 60 058 A.

[0086] The application of the finely divided catalytically active multielement oxide to the outer surface of the geometric shaped support body can be achieved, for example, by first moistening the outer surface with the liquid binder in a controlled manner (e.g., by spraying). By bringing the thus moistened geometric shaped support body into contact with the finely divided catalytically active multielement oxide, a layer of the active material is subsequently adhered to the moistened surface (e.g., by dusting the moistened geometric shaped support body with the finely divided catalytically active multielement oxide (with the active material powder) as described in EP 0 714 700 A).

[0087] In this context, "controlled moistening" means that the support surface is moistened to such an extent that it has absorbed liquid binder, but no liquid phase is visually visible on the support surface. If the support surface is too moist, the finely divided catalytically active multielement 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 DE 100 51 419 A, EP 0 714 700 A, and

[0088] WO 2022 / 090019 A1. Of course, the process can be repeated periodically to achieve an increased layer thickness. In this case, the coated base body becomes the new "carrier body," etc.

[0089] However, all other application processes recognized as prior art in EP 0 714 700 A can also be used to produce the coated catalysts described above.

[0090] Examples of suitable liquid binders include water, an organic solvent, a solution of an organic substance (e.g. an organic solvent) in water, or in an organic solvent, or in an aqueous solution of an organic solvent. Examples of suitable 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) that are soluble in water, in an organic liquid, or in a mixture of water and an organic liquid include monosaccharides and oligosaccharides such as glucose, fructose, sucrose, and / or lactose.

[0091] A particularly advantageous liquid binder is a solution consisting of 20 to 90 wt.% water and 10 to 80 wt.% of an organic compound. The organic content of the aforementioned liquid binders is preferably 10 to 50 wt.% and particularly preferably 20 to 30 wt. Very particularly preferred liquid binders are solutions consisting of 20 to 90 wt.% water and 10 to 80 wt.% glycerol. The glycerol content in these aqueous solutions is advantageously 10 to 50 wt.% and particularly preferably 20 to 30 wt. The advantage of preferred binders is based, among other things, on their ability to satisfactorily wet both the finely divided catalytically active multielement oxide (or the finely divided precursor mass (see below)) and the outer surface of the geometric shaped support bodies.

[0092] The fineness of the finely divided catalytically active multielement oxide (or its precursor mass (see below)) to be applied to the outer surface of the geometric shaped support body is naturally adapted to the desired shell thickness. Suitable active material powders for the shell thickness range of 100 to 500 μm are those of 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 μm or, alternatively, 1 to 10 μm, and whose numerical proportion of particles with a longitudinal dimension above 50 μm (of particles that no longer pass through a sieve with a mesh size (circular mesh) of 50 μm) is less than 10 wt. %. Furthermore, the statements on page 18 of WO 2005 / 120702 apply accordingly.

[0093] Preferably, coated catalysts obtainable as described are produced according to the preparation method described and exemplified in EP 0 714 700 A (see also WO 2011 / 134932 and the working examples of DE 103 60 057 A). An aqueous solution of 75 wt.% water and 25 wt.% glycerol is the preferred liquid binder. According to the invention, the thermal treatment of the geometric precursor shaped bodies is advantageously carried out according to the procedure described and exemplified in DE 103 60 057 A.

[0094] However, the procedure according to the invention also includes those processes for producing a catalytically active multielement oxide in which the shaping of geometric precursor shaped 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) a shell is applied directly from this (finely divided) mixture (from the finely divided precursor mass) as such to the outer surface of a geometric carrier shaped body.During the thermal treatment of the geometric precursor shaped bodies thus produced (which also comprises the at least partial removal of the liquid binder used for application), coated catalysts according to the invention are directly obtained in which a shell of catalytically active multielement oxide is applied to the outer surface of a (catalytically essentially inert) geometric support shaped body.

[0095] As already mentioned, catalytically active multielement oxides obtainable 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 characterized in particular by the fact that a catalyst bed charged with them has a long service life during the partial oxidation, during which the target product formation occurs with high activity. The preferred application form of a catalytically active multielement oxide obtainable according to the invention is that of a coated catalyst, which preferably has an annular geometry.The coated catalyst exemplified in the example of the present document is particularly preferably used, for example in all working examples 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 capable of replacing the catalyst used therein (what was said there for the coated catalyst from the example of the present document also applies to the coated catalyst from the comparative example of the present document).

[0096] In principle, catalytically active multielement oxides obtainable according to the invention are also suitable in a correspondingly advantageous manner for catalyzing the heterogeneously catalyzed partial gas phase oxidation of methacrolein to methacrylic acid.

[0097] The above applies in particular when the heterogeneously catalyzed partial gas phase oxidation of acrolein or methacrolein (ie, abbreviated to "(meth)acrolein") to acrylic acid or methacrylic acid (ie, abbreviated to "(meth)acrylic acid") is carried out at high (meth)acrolein loads, as described in DE 103 07 983 A, DE 199 48 523 A, DE 199 10 508 A, WO 2008 / 104577, WO 2011 / 134932, DE 199 27 624 A and DE 103 60 057 A.

[0098] The heterogeneously catalyzed partial gas-phase oxidation can be carried out in a manner known per se. This means that a reaction gas mixture comprising (meth)acrolein, molecular oxygen, and at least one inert diluent gas is passed at elevated temperature through a catalyst bed whose catalysts comprise at least one catalytically active multielement oxide obtainable according to the invention as the active material. During the residence time of the (meth)acrolein in the catalyst bed, the catalyst is converted to (meth)acrylic acid. A fixed catalyst bed is preferred as the catalyst bed. In principle, however, a fluidized bed or a moving bed are also suitable for the process according to the invention. As a rule, steam as a constituent of the reaction gas mixture leads to an improvement in selectivity and activity. Inert diluent gases with increased molar specific heat, such as n-propane or carbon dioxide, are advantageous.These are gases which, as the reaction gas mixture passes through the catalyst bed, undergo chemical change, preferably by at most 5 mol%, particularly preferably by at most 3 mol%, and very particularly preferably by at most 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 one another by a dividing wall. The catalyst charge, which comprises at least one catalytically active multielement oxide obtainable according to the invention and through which a reaction gas mixture comprising (meth)acrolein flows, is placed in the at least one primary chamber.At the same time, a fluid heat transfer medium flows through the secondary chamber and heat exchange takes place between the two chambers through the dividing wall, with the purpose of controlling and regulating the temperature of the reaction gas mixture on its way through the catalyst bed.

[0099] As a rule, the gas phase partial oxidation of (meth)acrolein is 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.

[0100] A fixed catalyst bed is located in the metal tubes (contact tubes) of the tube-bundle reactor in the form of a corresponding bed of shaped catalyst bodies (optionally mixed with diluting inert geometric shaped bodies). The temperature medium(s) are passed around the metal tubes (if there is more than one temperature zone, a corresponding number of essentially spatially separated temperature media are passed around the metal tubes). The temperature medium is usually a molten salt. The reaction gas mixture is passed through the contact tubes.

[0101] Alternatively, the fixed catalyst bed can also be located in the spaces between thermoplates of a thermoplate reactor through which a heat transfer medium flows, as recommended by DE 10 2004 017 150 A, DE 199 52 964 A and DE 103 61 456 A.

[0102] As already mentioned, the fixed catalyst bed can generally consist only of catalysts obtainable according to the invention, but also of such catalysts diluted with inert geometric shaped bodies. The geometric shaped supports (support bodies) used to produce coated catalysts according to the invention can be used as inert geometric shaped bodies. A pure inert shaped body bed can be located upstream and / or downstream of the fixed catalyst bed (such pure inert shaped bodies are normally not included in the calculation of the reaction gas or reaction gas component loading of the fixed catalyst bed).

[0103] 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 usually between 20 and 30 mm, frequently between 21 and 29 mm or between 23 and 28 mm. Their length is conveniently between 2 and 4 m.

[0104] The number of catalyst tubes accommodated in the tube bundle vessel is expediently at least 5,000, preferably at least 10,000. Frequently, the number of catalyst tubes accommodated in the reactor vessel is between 15,000 and 40,000. Tube bundle reactors with a number of more than 50,000 catalyst tubes are rather the exception. Within the vessel, the catalyst tubes are normally arranged in a homogeneous distribution (preferably 6 equidistant adjacent tubes per catalyst tube), with the distribution expediently selected such that the distance between the central inner axes of the nearest catalyst tubes (the so-called catalyst pipeline) is between 35 and 45 mm (cf., for example, EP 0 468 290 A).

[0105] The use of melts of salts such as potassium nitrate, potassium nitrite, sodium nitrite and / or sodium nitrate, or of low-melting metals such as sodium, mercury and alloys of various metals is particularly advantageous as heat exchange media for tube bundle reactors.

[0106] Charging catalyst tubes in tube bundle reactors with catalysts obtainable according to the invention, in particular the catalysts exemplified in the example, but also in the comparative example, of the present document is particularly advantageous when the tube bundle reactor is operated at a (meth)acrolein loading of the catalyst charge which 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. Of course, such a catalyst feed is also advantageous for smaller (for example, maximum 130 Nl / l - h, or maximum 100 Nl / l - h, or maximum 80 Nl / l • h, or maximum 60 Nl / l - h) (meth)acrolein loadings.

[0107] In this document, the loading of a fixed catalyst bed with reaction gas input mixture is understood to mean the amount of reaction gas input mixture in standard liters (= NI; the volume in liters that the corresponding amount of gas would occupy under standard conditions, ie at 0°C and 101.3 kPa), which is fed to the fixed catalyst bed per hour, based on the volume of its bed (bed sections made of pure inert material are not included in the volume of the bed; otherwise, the volume of a bed is the volume of the empty space occupied by the bed (or by its relevant bed sections)), ie, based on its bed volume (-> unit = Nl / I-h).

[0108] The loading can also be related 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 fed to the fixed catalyst bed per hour, based on the volume of its bed (bed sections made of pure inert material are not included in the bed volume; otherwise, the volume of a bed is the volume of the empty space occupied by the bed (or its relevant bed sections)) (-> unit = Nl / l-h).

[0109] The volume-specific activity of the fixed catalyst bed will generally be designed so that it increases in the flow direction of the reaction gas.

[0110] This can be easily achieved by decreasing the degree of dilution of the fixed catalyst bed with inert shaped bodies in the direction of reaction gas flow. However, the volume-specific activity can also be adjusted by using catalysts with different specific BET surface areas. Furthermore, it is possible to use coated catalysts with different pore volumes or different shell thicknesses. The activity increases with increasing specific BET surface area, pore volume, or shell thickness.

[0111] Furthermore, the heterogeneously catalyzed partial oxidation with coated catalysts obtainable according to the invention can generally be carried out in all aspects as described in DE 103 50 822 A. The (meth)acrolein content in the reaction gas input mixture can be from 3 to 15 vol.%, frequently from 3.5 to 10 vol.%, or from 4 to 8 vol.% (in each case based on the total volume of the reaction gas input mixture).

[0112] 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) present in the reaction gas input mixture of 1 : (1 to 3) : (0 to 20) : (3 to 30), preferably 1 : (1 to 3) : (0.5 to 10) : (7 to 10).

[0113] Suitable inert diluent gases (these are gases or mixtures of such gases which remain chemically unchanged to an extent of at least 95 mol%, preferably to an extent of at least 97 mol% or to an extent of at least 99 mol%, and most preferably to an extent of 100 mol%, when the reaction gas mixture passes through the catalyst bed (e.g. a fixed catalyst bed) once) include nitrogen, carbon dioxide, carbon monoxide, noble gases, propane, ethane, methane, butane and / or pentane (i.e. each as the sole diluent gas or in a mixture with one or more other of these inert diluent gases). The reaction temperatures of such a heterogeneously catalyzed (meth)acrolein partial oxidation are usually in the range of 200 to 400°C, generally from 220 to 380°C, often from 230 to 350°C, frequently from 245 to 320°C. The working pressure (absolute pressure) is normally 101.3 to 350 kPa, or101.3 to 250 kPa, or 101.3 to 205 kPa (especially as the inlet pressure into the fixed catalyst bed). Of course, the (meth)acrolein partial oxidation can also be carried out with the catalysts obtainable according to the invention at working pressures below atmospheric pressure.

[0114] The (meth)acrolein conversion, based on a single pass of the reaction gas mixture through the fixed catalyst bed, is usually at least 90 mol%, frequently at least 98 mol%, and often at least 98.5 mol%, or even at least 99 mol%.

[0115] Furthermore, the partial oxidation process according to the invention can be carried out in full accordance with the recommendations of the teachings of DE 10 2007 019 597 A or WO 2008 / 104577, or WO 2011 / 134932.

[0116] In particular, the (meth)acrolein-containing product gas mixture of 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 a source of the (meth)acrolein required for the partial oxidation according to the invention, without the (meth)acrolein having to be separated beforehand from such a product gas mixture.

[0117] The separation of (meth)acrylic acid from the product gas mixture of the partial oxidation can be carried out in a conventional manner, for example, by first converting 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 (cf. DE 602004924 T and WO 2006 / 114428 as well as the prior art cited in these documents).

[0118] The present invention further relates to catalytically active multielement oxides containing the elements Mo, W, V, Cu and Sb, wherein the ratio of the elements corresponds to the general formula (I),

[0119] Moi2W a bCu cSbd (I), where a = 0.4 to 3.0, b = 1.0 to 6.0, c = 0.1 to 3.0 and d = 0.1 to 3.0, and the molar proportion of the element Mo in the total amount of all elements other than oxygen is from 5 to 95 mol%, obtainable by one of the aforementioned processes.

[0120] The BET surface area of ​​the catalytically active multielement oxide is preferably from 10 to 32 m 2 / g, preferably from 12 to 28 m 2 / g, particularly preferably from 14 to 26 m 2 / g, especially preferred from 16 to 24 m 2 / G.

[0121] The stoichiometric coefficient a of the element W in the general formula (I) is preferably 0.6 to 2.5, particularly preferably 0.8 to 2.0, most preferably 1.0 to 1.6.

[0122] 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.

[0123] 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.

[0124] 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, very particularly preferably 0.3 to 0.8. The molar proportion 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%, very particularly preferably from 50 to 80 mol%.

[0125] Catalytically active multielement oxides, such as those used for the oxidation of acrolein to acrylic acid, are typically not present in a single form with all of 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 are typically present in their oxides. The maximum oxidation states of the relevant elements are V(V), Mo(VI), W(VI), Cu(II), and Sb(V).

[0126] For example, vanadium may not be present, or may not be present entirely, in the V(V) oxidation state, but may also be present in the V(IV) or V(III) oxidation state, or in mixed oxidation states. It is possible for some of the vanadium to be present in the V(V) oxidation state and another part in the V(IV) oxidation state, or for some of the vanadium to be present in the V(IV) oxidation state and another part in the V(III) oxidation state.

[0127] Other metallic elements in mixed metal oxides can also be present in various oxidation states. The oxidation states of the other relevant elements include Cu(I), Mo(V), Mo(IV), and Sb(III).

[0128] In principle, delocalized states can be conceivable if a relatively high electron mobility leads to the fact that non-discrete metal atoms with different oxidation states cannot be distinguished.

[0129] Without going into further theoretical interpretations here, the catalytically active multielement oxides can be analyzed by redox titration 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 multielement oxides are used directly in powder form before being applied to a molded support.

[0130] Catalytically active multielement oxides with a defined ratio R of oxidizable electrons to vanadium have particularly advantageous properties in the oxidation of acrolein to acrylic acid. The ratio R is

[0131] 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].

[0132] The ratio R is preferably 1.1 to 2.2, particularly preferably 1.2 to 2.1, most preferably 1.3 to 2.0.

[0133] The titration with KMnO4 as oxidizing agent is carried out as follows:

[0134] 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 mounted on a heated stirrer plate and flushed with argon to remove any air. 100 to 200 mg of sample are weighed into a weighing boat and flushed into 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).

[0135] The solution is then transferred to a titration vessel equipped with a combined Pt 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 (excess KMnO4) is obtained. During the titration, the electrochemical potential is measured and recorded using the combined Pt electrode.

[0136] 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.

[0137] 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 the aqueous KMnO4 solution [mol / l] and z is the sample weight [g].

[0138] In some cases, the titration curve may exhibit multiple transition points. This means that electrons with different oxidation potentials are present. Two transition points may indicate the presence of V(III) and V(IV). The present invention further provides processes for producing a coated catalyst, wherein a catalytically active multielement oxide according to the invention and optionally a binder are applied to the outer surface of a geometric shaped support body.

[0139] The present invention further provides coated catalysts consisting of a geometric shaped support body and a catalytically active multielement oxide according to the invention applied to the outer surface of the geometric shaped support body, and optionally binders.

[0140] The present invention further provides processes for preparing acrylic acid by gas-phase catalytic oxidation of acrolein over a fixed catalyst bed, wherein the fixed catalyst bed comprises a catalytically active multielement oxide according to the invention or a coated catalyst according to the invention.

[0141] Examples

[0142] Example 1

[0143] Ring-shaped coated catalyst C1 with the catalytically active oxide mass

[0144] MO12W1 ,2VsCUl ,1 Sbo,40n

[0145] Preparation of the coated catalyst:

[0146] 3,000 g of water were added to a 10 L flask at 95°C with stirring (using a paddle stirrer, 250 rpm) with 171 g of ammonium paratungstate heptahydrate (W content = 70.65 wt.%) and stirred for a further 5 minutes. A clear solution with a temperature of approximately 95°C and a pH of 6.3 was obtained. Subsequently, 1,161 g of ammonium heptamolybdate tetrahydrate (Mo content = 54.3

[0147] wt.%) and stirred for a further 5 minutes. A clear solution with a temperature of approximately 95°C and a pH of 6.1 was obtained.

[0148] Subsequently, 192 g of ammonium metavanadate (V content = 43.56 wt%) was added, and stirring was continued for another 5 minutes. An orange solution with a temperature of approximately 95°C and a pH of 6.5 was obtained.

[0149] Subsequently, 32 g of antimony trioxide (Sb20s; Sb content = 83.54 wt%) was added, and stirring was continued for another 30 minutes. A black suspension with a temperature of approximately 95°C and a pH of 6.6 was obtained.

[0150] Subsequently, 120.4 g of copper(II) acetate monohydrate (Cu(CH3COO)2 H2O; Cu content = 31.8 wt%) was added, and stirring was continued for a further 10 minutes. A black suspension with a temperature of approximately 95°C and a pH of 6.2 was obtained.

[0151] The resulting suspension was then continuously introduced into a Mobile Minor 2000 spray tower with spray head No. F0 A1 (GEA Niro, Soeborg, Denmark) using a rotary atomizer at 30,000 rpm for approximately 2 hours. Drying was carried out in a hot air stream (9 Nm 3 / h) at an inlet temperature of 310°C and an outlet temperature of 120°C. A powder was obtained.

[0152] 1,000 g of the powder was introduced into a ZS1-80 kneader (Coperion Werner & Pfleiderer GmbH & Co. KG; Stuttgart, Germany). The powder was kneaded with 370 g of an aqueous solution of acetic acid (acetic acid content 35 wt%) at 15 rpm for 30 minutes at ambient temperature. The material was then extruded (1 to 10 cm length, 6 mm diameter). The strands were dried in a forced-air drying cabinet for 16 hours at 120°C under lean air flow (5 vol% O2 in N2, 300 NL / h).

[0153] 400 g of the precursor mass removed from the forced-air drying cabinet was calcined discontinuously in a rotary kiln (analogous to US 9,149,799 B2). Calcination was carried out under a gas stream of air and nitrogen (total amount of 186 NL / h) with an oxygen content of 2.3 vol.%. The rotary kiln was heated to 400°C within two hours and held at this temperature for another hour. The heating was then turned off and the material was cooled to ambient temperature while continuing to rotate. The material removed from the rotary kiln was then ground to a fine powder in a ZM 200 mill (Retsch GmbH, Haan, Germany).

[0154] The finely divided powder was used to coat 1,500 g of a ring-shaped support body (7 mm outer diameter, 3 mm length, 4 mm inner diameter, 45 pm surface roughness Rz, 0.035 m 2 / g BET surface area) of the type Steatite C 220 (Cerarn Tee GmbH, Plochingen, Germany). The coating was carried out in a Hi-Coater LHC 25 / 36 mixer (Gebrüder Lödige Maschinenbau GmbH, Paderborn, Germany). The mixer was converted for continuous powder dosing. For this purpose, a funnel-shaped container was connected to the drum (36 cm diameter) of the mixer via a hose (11.1 mm outer diameter, 8 mm inner diameter). For coating, 300 g of finely divided powder were filled into the funnel-shaped container. Dosing was carried out 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 carried out for 2 s and paused for 1 s in each case.

[0155] 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) parallel to the powder metering unit. The powder metering unit was located 6 cm below the two-fluid nozzle and tilted 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 was applied at 25°C for 40 minutes. The rotation speed was then reduced to 2 rpm and dried for 30 minutes at 130°C in an air stream (220 Nl / h). The mixture was then cooled to 25°C. The powder was absorbed by the surface of the substrates. No formation of twinning or agglomeration was observed.

[0156] The coated supports were then freed of adhering glycerol in a UM 400 circulating air drying oven (Memmert GmbH & Co. KG, Schwabach, Germany). The coated supports were homogeneously distributed onto perforated plates in a layer thickness of 2 cm. The perforated plates had a thickness of 0.5 cm, an aperture ratio of 60%, and an area of ​​35 cm x 26 cm. The circulating air drying oven was heated to 300°C at 3 K / min and held at this temperature for a further 2 hours. Subsequently, the catalyst was cooled to 40 to 50°C within 2 to 3 hours. The annular coated catalyst C1 had an oxide active mass fraction of 14.9 wt.%. The BET surface area of ​​the catalytically active multielement oxide was 18.2 m². 2 / g. The ratio R was 1.8.

[0157] Investigation of the shell catalyst:

[0158] A reaction tube (stainless steel (material 1.4541); 30 mm outer diameter; 2 mm wall thickness; 26 mm inner diameter; 464 cm length) was charged from top to bottom as follows:

[0159] Section 1 : 80 cm length

[0160] empty pipe;

[0161] Section 2: 60 cm length

[0162] Pre-filling 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);

[0163] Section 3: 100 cm length

[0164] Fixed catalyst bed consisting of a homogeneous mixture consisting of 20 wt.% 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) and 80 wt.% of the shell catalyst;

[0165] Section 4: 200 cm length

[0166] Fixed catalyst bed consisting exclusively of the shell catalyst as in Section 3;

[0167] Section 5: 10 cm length

[0168] Refill from the same steatite rings as in Section 2;

[0169] Section 6: 14 cm length

[0170] Catalyst chair made of stainless steel (material 1.4541) to accommodate the fixed catalyst bed.

[0171] A reaction gas mixture containing the following contents was passed through the reaction tube charged as described above, flowing from top to bottom: 4.3 vol.% acrolein,

[0172] 0.3 vol.% propene,

[0173] 0.2 vol% propane,

[0174] 0.3 vol% acrylic acid,

[0175] 5.1 vol% oxygen,

[0176] 0.4 vol.% carbon oxides,

[0177] 7 vol% water and

[0178] 82.3 vol% nitrogen.

[0179] The feed temperature of the reaction gas mixture (at the inlet into the reaction tube) was 210 °C and the loading of the fixed catalyst bed (as defined in DE 199 27 624 A) with acrolein was 80 Nl / lh.

[0180] The reaction tube was surrounded by a stirred and externally electrically heated salt bath (a mixture of 53 wt% potassium nitrate, 40 wt% sodium nitrite, and 7 wt% sodium nitrate, 50 kg of molten salt) 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). The flow velocity through the tube was 3 m / s. The salt bath temperature TB, at which the salt bath was fed, was adjusted in all cases to result in an acrolein conversion of 99.3 mol% based on the single passage of the reaction gas mixture through the fixed catalyst bed. The salt bath temperature did not change along the reaction tube due to additional heating (more heat was radiated from the salt bath than was transferred from the reaction tube to the salt bath).

[0181] The selectivity of acrylic acid formation (S AS (mol-%)) is understood in this document:

[0182] Number of moles of acrolein converted to acrylic acid S AS = - x 100.

[0183] Total number of moles of acrolein converted

[0184] The selectivity of CO x -formation (total combustion) is calculated analogously.

[0185] An active mass (leading catalyst) that leads to the same conversion at a lower temperature under otherwise unchanged reaction conditions has a higher activity.

[0186] Among the sales of acrolein (U AC (mol-%)) is understood in this document as: total number of moles of acrolein converted

[0187] U AC = - x 100 mol%

[0188] Total moles of acrolein

[0189] The following Table 1 shows the results after 100 hours of operation depending on the shell catalyst used:

[0190] Example 2 (comparison example)

[0191] Ring-shaped coated catalyst C2 with the catalytically active oxide mass

[0192] MO12W1 ,2 sCUl ,1 Sbo,40n

[0193] The procedure was as in Example 1. 48.4 g of copper(II) oxide (CuO; Cu content = 79.1 wt.%) was used as the copper source instead of copper(II) acetate monohydrate. The annular coated catalyst C2 had an oxide active mass fraction of 14.9 wt.%. The BET surface area of ​​the catalytically active multielement oxide was 5.5 m². 2 / g. The ratio R was 1.7.

[0194] Example 3 (comparison example)

[0195] Ring-shaped coated catalyst C3 with the catalytically active oxide mass

[0196] MO12W1 ,2VsCUl ,1 Sbo,40n

[0197] The procedure was as in Example 1. 143 g of copper(II) nitrate hydrate (Cu(NO 2 ) 2.5H 2 O; Cu content = 26.8 wt.%) was used as the copper source instead of copper(II) acetate monohydrate. The annular coated catalyst C3 had an oxide active mass fraction of 14.7 wt.%. The BET surface area of ​​the catalytically active multielement oxide was 16.2 m². 2 / g. The ratio R was 1.5. Example 4 (Comparative Example)

[0198] Ring-shaped coated catalyst C4 with the catalytically active oxide mass

[0199] MO12W1 ,2 sCUl ,1 Sbo,40n

[0200] The procedure was as in Example 1. 150.7 g of copper(II) sulfate hydrate (Cu(SO4) 5H2O; Cu content = 25.4 wt.%) was used as the copper source instead of copper(II) acetate monohydrate. The annular coated catalyst C4 had an oxide active mass fraction of 14.9 wt.%. The BET surface area of ​​the catalytically active multielement oxide was 16.5 m². 2 / g. The ratio R was 1.7.

[0201] Example 5 (comparison example)

[0202] Ring-shaped coated catalyst C5 with the catalytically active oxide mass

[0203] MO12W1 ,2VsCUl ,1 Sbo,40n

[0204] The procedure was as in Example 1. 63.5 g of antimony(III) acetate (Sb content = 42 wt.%) was used as the antimony source instead of antimony(III) oxide. The ring-shaped coated catalyst C5 had an oxide active mass fraction of 14.7 wt.%. The BET surface area of ​​the catalytically active multielement oxide was 12.8 m². 2 / g. The ratio R was 1.7.

[0205] Table 1. Test results not according to the invention

[0206] TB [°C] Salt bath temperature U AC [%] Acrolein conversion

[0207] S AS [mol%] Selectivity to acrylic acid

[0208] Y AS [mol%] Yield to acrylic acid

[0209] The experimental results shown in Table 1 show that the required salt bath temperature TB at an acrolein conversion of 99.3 mol% in Example 1 according to the invention is significantly lower than in the comparative examples, and the catalyst according to the invention thus has a higher activity. This is all the more surprising since the composition of the catalysts is the same.

Claims

Patent claims 1. A process for the preparation of a catalytically active multielement oxide containing the elements Mo, W, V, Cu and Sb, wherein the ratio of the elements corresponds to the general formula (I) Moi2W a VbCu cSbd (I), where a = 0.4 to 3.0, b = 1.0 to 6.0, c = 0.1 to 3.0 and d = 0.1 to 3.0, and the molar proportion of the element Mo in the total amount of all elements other than oxygen is from 5 to 95 mol%, wherein a) an aqueous solution or aqueous suspension is produced from at least one source of the elemental constituents W, Mo and V of the multielement oxide, b) the aqueous solution or aqueous suspension obtained in a) is admixed with sources of the elemental constituents Cu and Sb of the multielement oxide, c) a powder P is produced by drying the aqueous solution or aqueous suspension obtained in b) and optionally comminuting, d) optionally with the powder P obtained in c) and optionally with the addition of one or more shaping aids and after uniform mixing, geometric precursor shaped bodies are produced from the resulting mixture,and e) the powder P obtained in c) or the geometric precursor shaped bodies obtained in d) are thermally treated to form the catalytically active multielement oxide, characterized in that copper acetate is used as the source of the elemental constituent Cu and antimony oxide is used as the source of the elemental constituent Sb.

2. Process according to claim 1, characterized in that the aqueous solution or aqueous suspension obtained in b) is spray-dried in c). Process according to claim 1 or 2, characterized in that the stoichiometric coefficient a of the element W in the general formula (I) is from 1.0 to 1.6 and / or the stoichiometric coefficient b of the element V in the general formula (I) is from 2.5 to 4.

5. Process according to one of claims 1 to 3, characterized in that copper(II) acetate monohydrate is used as a source of the elemental constituent Cu and antimony(III) oxide is used as a source of the elemental constituent Sb. Process according to one of claims 1 to 4, characterized in that water-soluble salts are used as a source of the elemental constituents Mo, V and / or W. Process according to one of claims 1 to 5, characterized in that the stoichiometric coefficient c of the element Cu in the general formula (I) is from 0.8 to 1.5.Process according to one of claims 1 to 6, characterized in that the stoichiometric coefficient d of the element Sb in the general formula (I) is from 0.3 to 0.

8. A catalytically active multielement oxide containing the elements Mo, W, V, Cu, and Sb, wherein the ratio of the elements corresponds to the general formula (I). Moi2W a VbCu c Sbd (I), where a = 0.4 to 3.0, b = 1.0 to 6.0, c = 0.1 to 3.0 and d = 0.1 to 3.0, and the molar proportion of the element Mo in the total amount of all elements other than oxygen is from 5 to 95 mol%, obtainable by a process of claims 1 to 7. Catalytically active multielement oxide according to claim 8, wherein the stoichiometric coefficient a of the element W in the general formula (I) is from 1.0 to 1.6 and / or the stoichiometric coefficient b of the element V in the general formula (I) is from 2.5 to 4.

5. Catalytically active multielement oxide according to claim 8 or 9, wherein the BET surface area of ​​the catalytically active multielement oxide is from 16 to 24 m 2 / g. Catalytically active multielement oxide according to one of claims 8 to 10, wherein the stoichiometric coefficient c of the element Cu in the general formula (I) is from 0.8 to 1.

5. Catalytically active multielement oxide according to one of claims 8 to 11, wherein the stoichiometric coefficient d of the element Sb in the general formula (I) is from 0.3 to 0.

8. Process for producing a coated catalyst, characterized in that a catalytically active multielement oxide according to one of claims 8 to 12 and optionally a binder is applied to the outer surface of a geometric shaped support body. Coated catalyst consisting of a geometric shaped support body and a catalytically active multielement oxide according to one of claims 8 to 12 and optionally a binder applied to the outer surface of the geometric shaped support body.A process for preparing acrylic acid by gas-phase catalytic oxidation of acrolein over a fixed catalyst bed, characterized in that the fixed catalyst bed comprises a catalytically active multielement oxide according to any one of claims 8 to 12 or a coated catalyst according to claim 14.

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