DE102008044890B4 - Catalyst for the catalytic gas phase oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and/or carboxylic anhydrides, in particular to phthalic anhydride, and process for producing such a catalyst - Google Patents

Abstract

Catalyst for the catalytic gas phase oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and / or carboxylic anhydrides, the active mass comprising vanadium oxide, titanium dioxide and at least one mixed element oxide of silver with defined elements, the mixed element oxide of silver having the following formula (I): AgxMyNzOn (Formula I) with Ag = Silver x = 0.01 to 100, M = an element selected from the group Li, Na, K, Rb, Cs, P, Mg, Ca, Sr, Ba, TI, y = 0 to 1N = at least one element selected from the group V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Co, Rh, Ni, Pd, Cu, Au, Zn, Cd, Sn, Pb, B, AI , Bi, Sb, As, Ti, Zr, Hf, Ce, Laz = 1O = oxygenn = a number that is determined by the valence and frequency of the elements other than oxygen in the formula I, whereby in the production of the catalyst this is at least a mixed element oxide of silver and/or at least one precursor compound of the at least one mixed element oxide of silver is used as a raw material source, and wherein the active mass has 0.01 to 15% by weight of the mixed element oxide of silver.

Description

The catalytic gas phase oxidation of aromatic hydrocarbons such as benzene, xylene, naphthalene, toluene or durene in fixed bed reactors, preferably tube bundle reactors, has been known for a long time and has been widely described in the literature. In this way, for example, benzoic acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid and pyromellitic anhydride are produced.

For this purpose, a mixture of a gas containing molecular oxygen, for example air, and the hydrocarbon to be oxidized, for example o-xylene or naphthalene, is generally passed through a large number of tubes arranged in a reactor, in which there is a bed of at least one catalyst .

To regulate the temperature or to dissipate the amount of heat generated during the exothermic reaction, the tubes filled with catalyst are surrounded by a heat transfer medium, for example a molten salt.

Despite such temperature control, local temperature maxima (“hot spots”) can form in the catalyst bed, which can lead to undesirable effects such as total oxidation of the starting material or the formation of by-products that are difficult to separate or even a limited throughput of starting material.

In order to avoid such hot spots, in practice, catalysts with different activities and chemical compositions have been arranged in layers one above the other in the reaction tubes, whereby, according to the prior art, the catalyst generally has the lowest activity in the area of the highest temperature maximum and the catalyst layers following in the direction of the gas outlet have increased activity.

So-called shell catalysts have proven useful here, which generally consist of an inert, non-porous support material to which at least a thin layer of the catalytically active material is applied in the form of a shell.

The composition of the catalytically active material plays a crucial role in the catalytic properties of the catalysts in question.

A large number of catalysts have been proposed in the past for the production of phthalic anhydride from o-xylene and/or naphthalene by oxidation in the gas phase with gases containing molecular oxygen on a fixed bed catalyst.

According to the state of the art, practically all industrially used phthalic anhydride catalysts contain in their catalytically active mass the compounds titanium dioxide in the anatase modification and vanadium pentoxide as well as other components or promoters in mostly small amounts to improve the activity, the conversion, the yield Selectivity and the long-term stability of the catalysts in question.

Such additives include cesium, antimony, phosphorus, boron, molybdenum, tungsten, tin, bismuth, silver, niobium, iron, potassium, rubidium, chromium and calcium.

There is therefore an effort to further increase the performance and stability of the PSA catalysts through targeted doping with various substances. This means that over time, increasingly complex catalyst formulations and combinations of different catalysts are being developed to solve these problems.

So-called “multi-layer” catalysts are also increasingly being used, which differ in terms of activity and selectivity due to the different composition of the active mass. Here, several different catalysts are arranged one after the other in layers in the catalyst bed in order to perform different tasks depending on their position in the catalyst bed. The main issue here is the requirement to safely control the exothermic reaction at high throughputs of starting material, while at the same time maintaining high selectivity and stability of the catalyst.

EP 0 021 325 A1 describes catalysts for the production of phthalic anhydride, the active mass of which consists of 60 to 99% by weight of titanium dioxide, 1 to 40% by weight of vanadium pentoxide and, based on the total amount of TiO ₂ and V ₂ O ₅ , up to 2% by weight of phosphorus and contain up to 1.5% by weight of rubidium and/or cesium, the catalytically active material being applied to the support in two layers. The inner layer contains 0 to 2.0% by weight of phosphorus and no rubidium and/or cesium and the outer layer contains 0 to 0.20% by weight of phosphorus and 0.02 to 1.5% by weight of rubidium and /or cesium. The catalytically active mass of these catalysts can contain other substances in addition to the components mentioned, e.g. B. contain up to 10% by weight of an oxide of the metals molybdenum, tungsten, niobium, tin, silicon, antimony, hafnium. Steatite is used as an inert carrier material.

EP 0 286 448 A2 describes a process for the production of phthalic anhydride in which two different catalysts with similar contents of titanium dioxide and vanadium pentoxide are used. They differ essentially in that one catalyst additionally contains 2 to 5% by weight of a cesium compound, in particular cesium sulfate, but no phosphorus, tin, antimony, bismuth, tungsten or molybdenum compounds, and the second catalyst additionally contains 0 .1 to 3.0% by weight of a phosphorus, tin, antimony, bismuth, tungsten or molybdenum compound

US 4,864,036 A relates to a catalyst for the production of phthalic anhydride, which is produced in several steps. In the first step, a compound of a metal from subgroup 6, preferably a molybdenum or tungsten compound, is applied to titanium dioxide in the anatase modification and then calcined. Then, in a second step, a vanadium compound is added to the calcined precursor and this precursor is calcined again.

GB 1 140 264 A relates to an oxidation catalyst for aromatic and non-aromatic hydrocarbons to carboxylic acids, which consists of an inert, non-porous support and a 0.02 to 2 mm thick layer of active mass, which in turn consists of 1 to 15% V ₂ O ₅ and 85 to 99 % titanium dioxide exists, with the vanadium content of the total catalyst being in the range from 0.05 to 3%. A catalyst is described in which the active material, in addition to titanium dioxide and vanadium pentoxide, also contains 0.1 to 3% by weight of at least one metal oxide from the group of silver, iron, cobalt, nickel, chromium, molybdenum or tungsten.

The EP 0 447 267 A1 describes a catalyst for the production of phthalic anhydride which comprises the following as a catalytically active material: (A) 1 to 20% by weight of V ₂ O ₅ and 99 to 80% by weight of a titanium dioxide in the anatase modification with a BET surface area of 10 to 60m ² /g and (B), based on 100 parts by weight of this mixture (A), 0.05 to 1.2 parts by weight of at least one element from the group K, Cs, Rb and TI as oxide and 0.05 to 2 parts by weight of silver , calculated as silver oxide.

EP 0 522 871 A1 relates to a catalyst which, in addition to titanium dioxide and vanadium pentoxide, also contains 0.01 to 1% by weight of niobium as niobium pentoxide, 0.05 to 2% by weight of at least one element from the group of potassium, cesium, rubidium or thallium as oxide, 0. 2 to 1.2% by weight of phosphorus as P2O5, and 0.55 to 5.5% by weight of antimony oxide and 0.05 to 2% by weight of silver as Ag2O, with a pentavalent antimony compound as the source of the antimony is used.

CN 1 108 996 A relates to a catalyst with two layers based on titanium dioxide/V2O5 which additionally contains at least one rare earth compound and at least one oxide of the elements antimony, phosphorus, zinc and silver, the layer closer to the gas inlet additionally containing at least one alkali metal compound

Silver-vanadium oxide compounds with an atomic ratio Ag: V < 1 are known as silver bronzes. The use of silver vanadium bronzes as oxidation catalysts is well known in the literature.

DE 198 51 786 A1 describes a multimetal oxide of the general formula Ag _ab M _b V ₂ O _x * c H ₂ O, where a has a value of 0.3 to 1.9, M is a metal selected from the group Li, Na, K, Rb, Cs TI, Mg, Ca, Sr, Ba, Cu, Zn, Cd, Pb, Cr, Au, Al, Fe, Co, Ni and/or Mo and b has a value from 0 to 0.5, where ab is or is greater than 0.1. The use of the multimetal oxides in question as precursor compounds for the production of precatalysts and catalysts for the gas phase oxidation of aromatic hydrocarbons is described.

In WO 2005/092496 A1 Finally, a catalyst for the production of aldehydes, carboxylic acids and/or carboxylic anhydrides is described, the active mass of which contains a phase A and a phase B in the form of three-dimensionally extended delimited areas, with phase A being a silver-vanadium oxide bronze and phase B being a mixed element oxide phase based on titanium dioxide and vanadium pentoxide. The molar ratio of Ag:V in phase A is 0.15 to 0.95.

The catalysts described above, which are known in the prior art and have a wide variety of active composition compositions, either achieve unsatisfactory selectivities, yields or activities or have a service life that is too short, or their industrial production involves enormous effort or costs.

There is therefore a continuous need for catalysts with further improved selectivity and yield combined with high raw material throughput and a long service life, which can be produced industrially with an acceptable cost-benefit ratio.

The present invention is therefore based on the object of developing an improved catalyst for the gas phase oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and / or carboxylic anhydrides, in particular to phthalic anhydride, which results in improved selectivity with a long service life at a high raw material throughput. Furthermore, the technical and economic effort for the large-scale production of the catalyst should be kept as low as possible.

This task is solved with the features of independent claims 1 and 25. Preferred embodiments are the subject of the subclaims.

According to claim 1, a catalyst for the catalytic gas phase oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and / or carboxylic anhydrides, in particular to phthalic anhydride, is claimed, in which the active mass is vanadium oxide, preferably vanadium pentoxide, titanium dioxide, preferably in the anatase modification, and at least one Mixed element oxide of silver with defined elements, preferably vanadium and/or molybdenum and/or tungsten and/or niobium and/or antimony, and/or a mixed element oxide of vanadium with defined elements, preferably bismuth and/or molybdenum and/or tungsten and/or Antimony and / or niobium. When producing the catalyst, in particular when producing a catalyst suspension or a powder mixture required for coating a support, at least one mixed element oxide of silver and/or vanadium and/or at least one precursor compound, in particular at least one polynuclear precursor compound, of at least one mixed element oxide of the Silver and/or vanadium is used as a source of raw materials.

It has been found that the use of mixed element oxides, preferably mixed metal oxides, of silver with z. B. vanadium, molybdenum, tungsten, niobium and / or antimony, and / or the use of mixed element oxides, preferably mixed metal oxides, of vanadium with, for example, bismuth, molybdenum, tungsten, niobium and / or antimony, or their precursor compounds as a source of raw materials in the production of the Catalyst suspension or in the production of a powder mixture used for the coating process and / or as part of the active mass of the catalysts, in addition to the components titanium dioxide and vanadium oxide, results in a significant increase in selectivity.

The reason for this is not clearly understood, but it is reasonable to assume that the interaction of the silver with the other catalytically active components such as the titanium dioxide/vanadium oxide monolayer is much more effective and therefore the actual promoter effect of the silver (the suppression of the total oxidation of the organic Raw material to carbon oxides) is more effective.

The use of silver as a component of the active material has been described many times in technology. In addition to silver oxide, corresponding salts such as silver nitrate, silver sulfate, silver halides, silver sulfide, silver phosphate, silver salts of organic acids, silver hydroxide, ammonium salts of silver and amine complexes of silver were used as raw material sources of silver for the production of the catalyst suspension. Most of these raw material sources are converted mononuclear into silver oxide when the catalyst is heated in the reactor and are therefore present in the active mass as silver oxide, while e.g. B. silver phosphates and silver halides are present unchanged in the active mass and are not converted into silver oxide during thermal treatment.

An essential aspect of the catalysts according to the invention is that conventional catalysts which contain at least titanium dioxide and vanadium oxide in the active mass contain mixed element oxides of silver with vanadium and/or other defined elements as additional dopants, with already at least one mixed element oxide of silver and/or vanadium and/or a corresponding precursor compound is added to at least one of these mixing element compounds in the production of the catalyst suspension or in the production of a powder mixture used for coating the support.

All components are advantageously mixed together in an aqueous medium and then applied to a ceramic carrier using a spray process (preferably fluidized bed or drum process). The mixed element oxide phase of silver and/or vanadium or their mixed element oxide precursor compounds are advantageously added directly to the catalyst suspension with the main components titanium dioxide and vanadium oxide as well as other promoters and are generally not present as spatially separate phases from a phase consisting of titanium dioxide and vanadium oxide. All components of the catalyst suspension thus form a uniform phase which, in addition to the components titanium dioxide and vanadium oxide, also contains the relevant mixed element oxides and optionally other components.

However, systems are also conceivable in which different layers with different chemical compositions are applied one after the other to the inert support and at least one layer contains at least one mixed element oxide of silver and/or vanadium with defined elements. The individual coating layers of a catalyst according to the invention can also consist of a combination of compositions according to the invention and compositions not according to the invention.

Likewise, the relevant mixed element oxides can also be used alone or in combination with other compounds in one or more coating layer(s) which do not contain titanium dioxide and/or vanadium oxide in the active composition.

Catalysts or precursors for the production and production of catalysts for the gas phase partial oxidation of aromatic hydrocarbons with a molar oxygen-containing gas can therefore be realized, which are made up of an inert, non-porous carrier material and one or more layer(s) applied thereon in the form of a shell, wherein at least one of these layers contains 0.01 to 15% by weight, based on the total weight of these layers, of one or more of the above-mentioned mixed element oxides or their precursor compounds.

In addition, a specific process for the production of carboxylic acids and/or carboxylic anhydrides is claimed through the partial oxidation of aromatic hydrocarbons in the gas phase with a gas containing molecular oxygen at elevated temperature on a catalyst, the active mass of which is applied in the form of a shell on an inert support material, which thereby is characterized in that one uses a shell catalyst whose catalytically active mass, based on its total weight, is 0.01 to 15% by weight of a mixed element oxide of silver with vanadium and / or a mixed element oxide of silver with other defined elements and / or 0.01 up to 10% by weight of a mixed element oxide of vanadium with bismuth and/or a mixed element oxide of vanadium with other defined elements and at the same time contains titanium dioxide in the anatase modification and a vanadium compound (preferably V2O5), the mixed element oxide compound(s) or their Precursor compound(s) are already used to produce the catalyst suspension and/or the powder mixture from which the active mass is formed after coating the support by thermal treatment.

Here, at least one shell catalyst not according to the invention, which contains vanadium oxide, in particular vanadium pentoxide, and titanium dioxide, in particular in the anatase modification, in its active mass as essential components, but none of the mixed element oxides mentioned below or their precursor compounds, can be used in the oxidation of aromatic hydrocarbons to carboxylic acids and/or carboxylic acid anhydrides may be present or absent and used in a combined bed (i.e. in a single or multi-layer mixture) together with one or more of the catalysts according to the invention described above.

For this purpose, the gaseous stream is preferably passed over a bed of at least two layers of catalysts, the catalyst located closer to the gas inlet containing a catalyst according to the invention and the bed of the catalyst located downstream containing at least one catalyst catalytically active mass contains vanadium oxide and a titanium dioxide in the anatase modification, but no mixed element oxide of silver with elements such as vanadium, molybdenum, tungsten, niobium, antimony and/or no mixed element oxide of vanadium with elements such as bismuth, molybdenum, tungsten, Niobium, antimony.

However, catalyst beds consisting of at least two layers can also be used, with all catalyst layers containing catalysts according to the invention. The catalysts according to the invention used can differ in the type and amount of the relevant mixing element oxides in the active mass.

Preferably, at least some, particularly preferably all, of the catalyst layers used in the catalyst bed, including the catalysts according to the invention, should have a catalytically active mass which contains 1 to 40% by weight of vanadium oxide (calculated as V ₂ O ₅ ), 50 to 99% by weight of titanium dioxide ( calculated as TiO ₂ ), up to 1% by weight of an alkali metal compound (preferably a cesium compound, calculated as alkali metal), up to 1.5% by weight of a phosphorus compound (calculated as P) and up to 10% by weight of one Antimony compound (calculated as Sb ₂ O ₃ ).

According to the invention, the catalysts contain 0.01 to 15% by weight of at least one mixed element oxide of silver with at least one element from the group vanadium, niobium, tantalum, titanium, zirconium, chromium, molybdenum, tungsten, cerium, lanthanum, aluminum, boron, manganese , iron, cobalt, nickel, copper, zinc, gold, cadmium, tin, lead, bismuth, antimony, arsenic, hafnium, rhenium, ruthenium, rhodium and palladium, and optionally 0.01 to 10% by weight of at least one mixed element oxide Vanadium with at least one element from the group of bismuth, antimony, niobium, tantalum, titanium, zirconium, chromium, molybdenum, tungsten, cerium, lanthanum, aluminum, boron, manganese, iron, cobalt, nickel, copper, zinc, gold, cadmium, Tin, lead, arsenic, hafnium, rhenium, ruthenium, rhodium and palladium.

According to the invention, the content of mixed element oxides of silver with defined elements, in particular with, for example, vanadium and/or tungsten and/or molybdenum and/or niobium and/or antimony, in the active mass of catalysts is in a range from 0.01 to 15% by weight .-%, in a preferred embodiment in a range from 0.01 to 10% by weight and in a particularly preferred embodiment in a range from 0.01 to 5% by weight. The content of mixed element oxides of vanadium with defined elements (except silver, see previous information regarding a mixed element oxide of silver with vanadium), in particular with, for example, tungsten and/or molybdenum and/or niobium and/or antimony, in the active mass of catalysts is in in a preferred embodiment in a range from 0.01 to 10% by weight and in a particularly preferred embodiment in a range from 0.01 to 5% by weight.

In a preferred embodiment, a catalyst according to the invention is used in a first catalyst layer located closest to the gas inlet, which is also the layer with the highest hot spot. In the direction of the gas outlet, at least one further catalyst layer is connected, which generally has a higher activity than the first catalyst layer and which is generally a catalyst corresponding to the prior art. The catalyst here contains at least one mixed element oxide of silver, preferably silver vanadate, and preferably one mixed element oxide of vanadium, preferably bismuth vanadate.

It has also proven to be advantageous if, in a multi-layer catalyst system with at least one layer upstream of the hot spot catalyst layer in the direction of the gas inlet, at least one of the catalyst layers from the gas inlet up to and including the layer with the highest hot spot contains a catalyst according to the invention and conventional ones for subsequent layers , catalysts corresponding to the prior art are used and/or catalysts according to the invention with a reduced content of the claimed mixing element oxides in the active mass are used.

In a further particularly preferred embodiment, at least one of the catalyst layers, which is located in front of the layer with the highest hot spot, contains a mixed element oxide of silver, preferably a mixed element oxide of silver with molybdenum and / or tungsten and particularly preferably a mixed element oxide of silver with vanadium, while the layer with the highest hot spot and the layers downstream do not contain any mixed element oxide of silver. The layer with the highest hot spot advantageously contains a mixed element oxide of vanadium with bismuth. The catalyst layers following the hot spot catalyst layer in the direction of the gas outlet preferably correspond to the prior art and generally do not contain any catalysts according to the invention.

In a particularly preferred embodiment of the catalyst, the compounds AgVO ₃ and/or Ag ₂ MoO ₄ and/or Ag ₂ WO ₄ and/or BiVO ₄ are used in the production of the catalyst suspension and/or are contained in the active mass of the catalysts.

It is important here that the content of the mixed element oxides that act as “promoters” is neither too small nor too large. If the content of mixed element oxides of silver is too small, the selectivity-increasing effect is not fully achieved, while if the content of mixed element oxides of silver is too high, increased activity can be observed, which leads to reduced selectivity due to increased total oxidation to CO and CO ₂ .

Furthermore, it is advantageous to use the catalysts according to the invention in particular in the catalyst layers closer to the gas inlet up to and including the layer with the highest hot spot, since otherwise the full selectivity-increasing effect can no longer be achieved since a significant portion of the raw material has already been converted.

Too high a content of bismuth mixed element oxides also has a negative effect on the selectivity of the catalyst and at the same time there can be an increased formation of by-products, which result in poorer color stability of the desired product, phthalic anhydride. On the other hand, a proportion of bismuth mixed element oxides that is too low does not lead to a significant increase in selectivity.

By combining at least two, but particularly advantageously, several catalyst layers in a bed, of which at least one catalyst layer contains a catalyst according to the invention, a significantly higher yield can be achieved overall than is the case when multi-layer catalyst systems with catalysts not according to the invention are used alone is.

According to the prior art, in many cases the catalyst activity increases gradually from the first catalyst layer used at the gas inlet to the last layer closest to the gas outlet. The activity can be set using various methods familiar to those skilled in the art. For example, the catalyst activity is increased in the layers from the gas inlet to the gas outlet by a gradual reduction in the alkali metal content in the active mass, an increase in the average BET surface area of the titanium dioxide used or an increase in the proportion of active mass in the total weight of the catalyst. A combination of different activity cessation measures can also be used.

The catalysts according to the invention advantageously contain in the active mass at least one type of titanium dioxide in the anatase modification, at least one compound of vanadium (preferably V2O5), at least one mixed element oxide of silver and/or vanadium and optionally a compound of antimony, a compound of an alkali element and/or or a phosphorus compound.

The total titanium dioxide content is 50 to 99% by weight in the active mass of the catalysts according to the invention and particularly preferably 80 to 99% by weight.

Only one type of titanium dioxide with defined physical properties or a mixture of several titanium dioxides with different physical properties can be used in the anatase modification to produce the catalyst suspension and/or the active mass of the catalysts.

The average BET surface area of the titanium dioxide of the catalysts according to the invention in the case of one or more types of titanium dioxide is advantageously 10-60 m ² /g and particularly advantageously 12-30 m ² /g, while the BET surface area of the individual types is in the range of 3 to 300 m ² /g.

The average BET surface area of the titanium dioxide used is calculated from the amount and BET surface area of the individual types used.

At least one type of titanium dioxide is preferably used, the average particle size of which is in a range from 0.3 to 0.8 μm.

The content of vanadium oxide in the active mass (calculated as V2O5) is in a preferred range of 1 to 40% by weight and in a particularly preferred embodiment in a range of 1 to 20% by weight.

The raw material sources vanadium pentoxide and/or vanadium oxalate are advantageously used to produce the catalysts according to the invention. However, other vanadium compounds are also suitable in principle, such as ammonium metavanadate, polyvanadic acid, etc. or a mixture of different sources.

As a rule, these precursor compounds or raw material sources of vanadium are converted into vanadium pentoxide during the thermal treatment of the catalyst or when heating the catalyst in the reactor in the presence of molecular oxygen, so that the vanadium is essentially present in the pentavalent oxidation state in the active mass.

Depending on the location of the respective catalyst according to the invention in the catalyst bed, the active masses of these catalysts have, in addition to titanium dioxide, vanadium oxide, one or more mixed element oxides of silver and/or vanadium and additionally one or more elements from the group of alkali metals.

These are usually added to the catalyst suspension as salts (e.g. sulfates, carbonates, nitrates, phosphates) and are converted into the corresponding oxides when the catalyst is heated or are still present unchanged in the active mass (after the catalyst has been thermally treated).

Alkaline metal compounds are used to control activity while improving the selectivity of the catalysts.

In a preferred embodiment, the alkali metal content in the active composition is in a range from 0 to 1.0% by weight and particularly preferably in a range from 0 to 0.6% by weight. A soluble cesium compound such as cesium sulfate is particularly advantageously used to produce the catalyst suspension.

The catalysts according to the invention advantageously also contain, in particular in the active mass of the hot spot catalyst layer, an antimony compound as a component of the active mass to improve the thermal stability. The antimony content in the active mass is advantageously in a range of 0 to 10% by weight (calculated as Sb2O3) and particularly preferably in a range of 0 to 5% by weight, depending on the location of the catalyst in question throughout Catalyst bed and depending on the thermal load on the catalyst in question in the respective position.

Various antimony compounds such as antimony salts or antimony oxides in different oxidation states can be used as starting materials for the production of the catalyst suspension or a powder mixture of the catalysts according to the invention required for coating the support.

In a preferred embodiment of the catalyst according to the invention, antimony III oxide is used in the preparation of the catalyst suspension.

The catalysts according to the invention can also contain a phosphorus compound in the catalytically active mass or a phosphorus compound can be added as a source of raw material in the production of the catalyst suspension or a powder mixture used for coating the support. In a particularly advantageous embodiment of the catalysts according to the invention, the phosphorus content of the active mass is in a range from 0 to 1.5% by weight (calculated as P).

Particularly advantageously, in catalyst systems with several layers with one or more catalysts according to the invention and/or one or more catalysts not according to the invention in a bed, the phosphorus content increases from the gas inlet to the gas outlet. Ammonium dihydrogen phosphate is preferred as a raw material source for the catalysts according to the invention.

Furthermore, the active masses of the catalysts according to the invention can contain small amounts of a large number of other compounds which, as promoters, reduce or increase the activity and/or have an influence on the selectivity of the catalyst.

It has been described in the literature that in order to control activity or to weaken or avoid so-called “hot spots” in technology, technology has moved to using multi-layer catalysts which are layered in layers, in particular with several catalyst layers lying one on top of the other. in the catalyst bed, whereby in the prior art the least active catalyst is usually located closest to the gas inlet and the activity increases from layer to layer in the direction of the gas outlet.

The catalysts in the individual layers have different chemical compositions, which in turn cause different activities and selectivities.

It is sufficient for the invention if, in a multi-layer catalyst system, at least one layer contains a catalyst according to the invention which contains at least one of the relevant mixing element oxides.

In particular, the first layer, which is closest to the gas inlet and which is also the layer with the highest hot spot, preferably contains a catalyst according to the invention. It is particularly advantageous if all layers in the catalyst bed, starting from the gas inlet up to and including the layer with the highest hot spot, contain catalysts according to the invention. The content and type of mixed element oxides of the z. B. Silver can vary in the different layers concerned. It is advantageous to also adapt the other compositions of the active mass according to their position in the catalyst bed.

In a preferred embodiment, in particular the layer with the highest local temperature maximum contains a mixed element oxide of silver and/or a mixed element oxide of bismuth with vanadium.

In particular, the use of a combination of the mixed element oxides of AgVO3 and BiVO4 in the layer with the highest hot spot has proven to increase selectivity in the catalysts according to the invention.

It was surprisingly found that it is particularly advantageous if at least one of the active masses of the catalyst layers, which are located upstream in the direction of the gas inlet in front of the layer with the highest hot spot, has a reduced antimony content compared to the layer with the highest hot spot contained in the active mass.

In a further advantageous embodiment of a catalyst with several layers in one or more beds, at least one catalyst layer, which is upstream of the hot spot catalyst layer, contains at least one mixed element oxide of silver with the specified elements, particularly advantageously with vanadium and / or molybdenum and / or Tungsten.

In a particular embodiment of the catalysts according to the invention, at least one of the layers from the gas inlet to the layer with the highest hot spot contains a mixed element oxide of silver, in particular a mixed element oxide of silver with vanadium and / or molybdenum and / or tungsten, while the layer with the highest hot spot Spot preferably contains a mixed element oxide of vanadium, in particular a mixed element oxide of vanadium with bismuth.

In a further particularly preferred embodiment, all layers from the gas inlet to the layer with the highest hot spot contain a mixed element oxide of silver and only the layer with the highest hot spot contain an additional mixed element oxide of vanadium with bismuth.

The use of polynuclear mixed element oxides or their precursor compounds as a source in the production of the catalyst suspension or a powder mixture used for coating the support material is particularly advantageous.

In addition, mixed element oxides of silver can also be used to produce the catalyst suspension and/or the active mass, which have a smaller or larger atomic ratio to the Sil more than is present in the case of AgVO3. Depending on the atomic ratio of the elements present in the relevant mixed element oxide, the amount of the mixed element oxide and/or their precursor compounds in the catalyst suspension or the powder mixture used for the coating must be adjusted in order to achieve an optimal silver content in the active mass of the finished catalyst to obtain.

It is advantageous if the described mixed element oxide phase or its precursor compound is already formed before it is added to the catalyst suspension or to the powder mixture used for coating the support. However, one or more corresponding precursor compounds can also be added to the catalyst suspension or the powder mixture, which are then converted into the corresponding mixing element oxides during the thermal treatment of the catalyst.

As already described above, not only mixed element oxides of silver present in an integer molar ratio with other elements and/or their precursor compounds can be used as a raw material source in the production of the catalyst suspension or be present in the active mass of the catalysts according to the invention, but also binuclear or polynuclear mixed element oxides of silver Silver with a large atomic deficiency or excess of silver. Depending on the general formula of the mixed element oxides or their silver component, the content of the relevant mixed element oxide in the catalyst suspension and/or the active mass must then be increased or decreased.

The diatomic or polynuclear mixed element oxides of silver are represented by the general formula Ag _x M _y N _z O _n (Formula I) described.

In the mixed metal oxide of the formula I, x particularly preferably has a value of 0.1 to 5, the value of the variable y is preferably 0 to 0.3.

Mixed metal oxides of the following general formula are particularly preferred: Ag _x N _z O _n wherein
Ag = silver
x = a value from 0.1 to 5
N = an element from the group vanadium, molybdenum, tungsten, niobium, antimony
z=1
n = a number that is determined by the valence and frequency of the elements other than oxygen in formula I

It is fundamentally possible for one or more precursor compounds of the relevant mixing element oxides and/or their starting compounds to be used in the production of the catalyst suspension and for the catalysts according to the invention with the relevant mixing element oxides only to be used in the reactor by calcination at an elevated temperature or during the heating period of the catalyst in the reactor be formed.

In a preferred embodiment of the catalyst, the use of the mixed element oxides AgVO3, Ag2MoO4, Ag2WO4 and/or mixed element oxides of these elements with other atomic ratios leads to an improvement in the catalysts

It has now been found that the addition of mixed element oxides of silver with preferably z. B. vanadium, tungsten and molybdenum, for the catalyst suspension instead of "mononuclear" silver compounds results in a significant improvement in selectivity, compared to catalysts that do not have silver in the active mass and/or in which silver is used as an oxide (AgO or Ag2O). or as a salt (nitrate, phosphate, sulfate, chloride, ammonium salt, salt of an organic acid) or as a hydroxide or as an amine complex is added to the catalyst suspension or is contained in the active mass. It is believed that silver in the form of a mixed-element oxide or a mixed-element compound can participate more intensively in the catalytic process than in the form of a salt or a mononuclear oxide.

By way of example, but not by way of limitation, the compound AgVO3 should be mentioned here as a component of the catalysts according to the invention, which is used as an additive in the production of the catalyst suspension or the active mass, the active mass being at least titanium dioxide in the anatase modification and one Contains vanadium compound. The catalyst according to the invention preferably also contains at least one compound of an element from the first main group. Depending on the location of the catalyst according to the invention in the catalyst bed, the catalyst according to the invention also contains an antimony compound, in particular Sb2O3 in the active mass and optionally a phosphorus compound

It has surprisingly been found that in the case of multi-layer catalysts it is particularly advantageous in terms of increasing selectivity to use at least one mixed element oxide of silver with vanadium and/or tungsten and/or molybdenum or their precursor compounds in the production of the catalyst, the catalyst according to the invention is advantageously used in one or more catalyst layers from the gas inlet up to and including the layer with the highest hot spot.

In a particularly preferred embodiment, one or more or all of the catalyst layers mentioned, from the gas inlet up to and including the layer with the highest hot spot, have a mixed element oxide of silver with vanadium and/or tungsten and/or molybdenum of 0.01% by weight to 15% by weight. %, in particular from 0.01 to 10.0% by weight and particularly preferably from 0.01 to 5% by weight in the active mass.

In a special embodiment of the catalyst according to the invention, the silver-vanadium mixed element oxides are compounds with a molar ratio of Ag:V of 1:1, in particular the mixed element oxide AgVO3. This is not a so-called silver-vanadium bronze, in which the atomic ratio of Ag:V is by definition less than 1:1.

It was also found that the use of mixed element oxides of vanadium with bismuth, molybdenum, tungsten, antimony, arsenic, lead, tin, zinc, copper, nickel, cobalt, iron, manganese, chromium, niobium, zirconium, titanium, gold, rhenium, Lantha, cerium, tantalum, rhenium as a raw material source in catalyst production and/or as a component of the active mass improves selectivity.

In particular, the use of bismuth vanadate as a raw material source in the production of the catalyst suspension or as a component of the active composition has led to an improvement in selectivity. It is advantageous here if the catalysts according to the invention contain BiVO4 as a component of the active mass, in particular in one or more layers from the gas inlet up to and including the layer with the highest hot spot.

In addition to titanium dioxide and vanadium oxide, the catalysts can simultaneously contain at least one mixed element oxide of silver with one or more of the elements mentioned and at least one mixed element oxide of vanadium with one or more of the elements mentioned. In a particularly preferred embodiment, silver vanadate and bismuth vanadate are used to produce the catalyst suspension of the catalysts according to the invention and/or are present side by side in the active mass of the catalysts.

Depending on the location of the catalysts according to the invention in the catalyst bed, the catalysts according to the invention additionally contain at least one alkali metal compound, preferably a cesium compound, an antimony compound, preferably antimony III oxide and optionally a phosphorus compound.

Not only can mixed element oxides of vanadium with other elements or their precursor compounds present in an integer atomic ratio be used as a raw material source in the production of the catalyst suspension or be present in the active mass of the catalysts according to the invention, but also two or polynuclear mixed element oxides of vanadium with a large atomic Deficit or excess of vanadium. Depending on the general formula of the mixed element oxides or their vanadium content, the content of the relevant mixed element oxide in the catalyst suspension and/or the active mass must then be increased or decreased.

The dinuclear or polynuclear mixed element oxides of vanadium are described by the following general formula: M _a N _b V _c O _d (Formula II)

For the catalysts according to the invention, so-called shell catalysts are advantageously considered, in which the catalytically active mass is in a shell shape in one or more layers on a support material that is generally inert under the reaction conditions, such as porcelain, magnesium oxide, tin dioxide, silicon carbide, cerium silicate, magnesium silicate (steatite), zirconium silicate , aluminum oxide, quartz, or mixtures of these carrier materials is applied. Carrier materials made of steatite or silicon carbide have proven particularly useful in technology.

To produce such coated catalysts, an aqueous and/or organic solvent-containing solution or suspension (referred to here as “catalyst suspension”) of the active mass components and/or their precursor compounds is generally applied to the support material in a fluidized bed process ( DE 2 106 796 A ) applied at elevated temperature until the desired proportion of active material in the total weight of the catalyst is reached. The coating of the inert, non-porous carrier material with the active material components or their precursor compounds as a suspension or powder mixture can also be carried out in a heated coating drum at elevated temperatures.

Since when spraying the catalyst suspension with the active mass components contained therein and/or their precursor compounds in the coating drum or when coating the inert support material in the fluidized bed, high losses sometimes occur due to nebulization of the catalyst suspension and/or due to partial abrasion of the already coated supports The technique has moved to adding organic binders, preferably copolymers, to the catalyst suspension, advantageously in the form of an aqueous dispersion of vinyl acetate/vinyl laurate, vinyl acetate/acrylate, styrene/acrylate, vinyl acetate/maleate and vinyl acetate/ethylene, etc., with binder amounts of 10 20% by weight, based on the solids content of the catalyst suspension, can be used. When one of the binders mentioned is added, the coating temperature is advantageously in a range of 50-450 ° C. After the catalyst has been poured into the reactor, the binders used usually decompose when the reactor is heated up to operating temperature or at the latest when the catalyst is put into operation and are thereby completely removed from the catalyst.

The catalytically active components that remain on the inert carrier material as a thin shell are referred to as active mass. The components of the active mass can sometimes differ from the components or raw material sources or precursor compounds used in the catalyst suspension, since some of these are chemically converted by the heat treatment of the catalyst and are usually converted into the corresponding metal oxides.

It has now surprisingly been found that the performance of the catalysts does not depend exclusively on the composition and amount of the active mass (catalytically active compounds present after heating or tempering) of the catalysts, but rather that in the catalyst suspension or that for coating the support The raw material sources used in the powder mixture have a significant influence on the selectivity, activity and service life of the catalysts.

In addition, a process for producing aldehydes, carboxylic acids and/or carboxylic anhydrides, which comprises contacting a gaseous stream containing an aromatic hydrocarbon and a gas containing molecular oxygen at an elevated temperature with a catalyst as defined above.

The mixed element oxides used as a raw material source for the catalyst suspension can be produced in various ways and added to the catalyst suspension as an isolated compound or directly as a reaction mixture. Corresponding precursor compounds of the mixed metal oxides can also be added to the catalyst suspension. These are advantageously polynuclear mixed element compounds or mixed metal oxide compounds which generally have a different crystal structure than the corresponding thermally treated mixed element oxides present in the active mass and which can also contain water of crystallization.

In many cases, it is also advantageous to react a solution of a metal compound with a suspension of a metal oxide at elevated temperatures in an aqueous or non-aqueous solvent to produce the mixed element oxides and/or their precursor compounds used in the catalysts according to the invention. For example, a silver-vanadium oxide used for the catalysts according to the invention can advantageously be produced by mixing silver nitrate in an aqueous solution at elevated temperatures with vanadium pentoxide in the corresponding desired atomic quantities conditions is implemented. The resulting reaction mixture with the mixed metal compound contained therein can be added directly to the catalyst suspension or the corresponding mixed metal compound can also be isolated beforehand and, if necessary, thermally treated.

In addition to water, polar organic solvents such as polyols, polyethers or amines can also serve as solvents. The reaction of a metal oxide or metal oxides (such as vanadium pentoxide and molybdenum trioxide) with a silver compound and optionally a further compound (such as a phosphorus compound) can generally be carried out at room temperature or elevated temperature. The reaction is preferably carried out at temperatures of 50 to 100 ° C and takes 20 minutes to 5 days, depending on the starting materials used and reaction conditions.

The mixed metal compound formed in this way can be isolated from the reaction mixture and, if necessary, stored until further use or can also be added directly as a suspension solution to the catalyst suspension, which still contains at least titanium dioxide and a vanadium compound. The mixed metal compound obtained and isolated by the reaction can also be subjected to thermal treatment at elevated temperatures before being added to the catalyst suspension in order to rearrange the crystal structure and remove water of crystallization.

If the mixed metal compound is previously isolated, this can be done by filtering off the suspension and drying the resulting solid, whereby the drying can be carried out using various methods. The mixed metal suspension is advantageously dried by spray drying or freeze drying.

As an alternative to the reaction in solution, the mixed metal compounds can also be produced by melting them together, e.g. B. in such a way that various finely mixed metal oxide powders are reacted in a solid state reaction at elevated temperatures of 400 to 800 ° C.

The components of the catalyst suspension are generally used in the form of oxides and/or in the form of compounds, for example salts, which convert into oxides when heated in the presence of oxygen. However, metal salts such as phosphates or halides can also be added to the catalyst suspension, which remain unchanged in the active mass of the catalyst even after thermal treatment.

The catalysts according to the invention are generally produced via a precursor of the finished catalyst, which can be stored as such. This is an inert ceramic support on which the raw materials used in the catalyst suspension are applied by means of a spray process and advantageously fixed using an organic binder.

The active catalyst is usually created through thermal treatment of this precursor or when the catalyst is heated in the reactor. The metal compounds used are usually converted into the corresponding metal oxides. For example, vanadium oxalate is decomposed and converted into V ₂ O ₅ . Other compounds contained in the catalyst suspension can also be converted into their oxidic compounds when the catalyst is heated. It can be assumed that ammonium dihydrogen phosphate is converted to P ₂ O ₅ and is present as such in the active mass. In the event that water-containing mixed metal oxides are produced and used as a raw material source in the production of the catalyst suspension, it can generally be assumed that they will lose the water of crystallization during the thermal treatment of the catalyst and possibly change their crystal structure.

The conversion of the raw material sources used in the catalyst suspension preferably takes place at temperatures of 200 to 500 ° C and particularly preferably in the range of 300 to 500 ° C.

The shape of the inert support material is not crucial for the performance of the catalysts according to the invention, but in the prior art, balls or rings in particular have proven to be advantageous shaped bodies.

The layer thickness of the active mass or the sum of the layer thicknesses of the shells containing the active mass is generally 20-400 μm and varies depending on the position of the catalyst according to the invention in the catalyst bed.

It was further surprisingly found that a spatial separation of the various components of the catalyst suspension from the mixed metal oxides used is generally not necessary in order to obtain improved catalysts. All components used in the catalyst suspension are generally mixed or blended with the corresponding mixing element oxides and the uniformly mixed suspension is applied to the inert support. However, catalysts are also conceivable in which catalyst suspensions of different compositions are applied one after the other to the inert ceramic support, with at least one layer containing a catalyst according to the invention and thus an improved catalyst according to the invention is created.

In addition, improved catalysts are conceivable in which a catalyst suspension not according to the invention, which contains at least titanium dioxide, a vanadium compound and advantageously also an alkali metal compound, is first isolated and, if necessary, thermally treated and then the resulting powder is generally mixed again with a powder (advantageously in Water) which was previously obtained from a catalyst suspension which consists of at least titanium dioxide, a vanadium compound and at least one mixed metal oxide or its precursor compound. This new catalyst suspension can be applied as a layer alone to an inert ceramic support or can also be used in combination with other coating layers according to the invention and/or not according to the invention.

The catalysts according to the invention are generally used for the partial gas phase oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and/or carboxylic anhydrides. These catalysts are particularly suitable for the gas phase oxidation of o-xylene and/or naphthalene to phthalic anhydride with a gas containing molecular oxygen.

As already mentioned, the catalysts according to the invention can be used for this purpose in a catalyst bed alone or in combination with other differently active catalysts, for example catalysts from the prior art (vanadium pentoxide/anatase base without mixed metal oxide component).

Here, the different catalysts according to the invention and those not according to the invention are generally used in separate catalyst beds which are arranged in at least one catalyst bed. Overall, it is advantageous to use the catalysts according to the invention in the catalyst layers closest to the gas inlet up to the layer with the highest hot spot.

The use of the catalysts according to the invention in the catalyst layers following the layer with the highest hot spot and closer to the catalyst layers located closer to the gas outlet also has a positive effect, but already results in a significantly smaller positive effect in terms of selectivity than the use of the catalysts according to the invention the catalyst layers located closer to the gas inlet. The catalysts according to the invention and/or their precursor catalysts according to the invention are filled into the reaction tubes of a reactor in layers. The different layers can consist of catalysts according to the invention and not according to the invention.

It is also conceivable to use a bed of a mixture of at least one catalyst according to the invention and at least one catalyst not according to the invention in one layer and to use this bed alone or in combination with further catalyst layers made from catalysts according to the invention and/or not according to the invention.

The reaction tubes are thermostatized from the outside, which is generally done by means of a molten salt that flows around the tubes.

When the reactor is heated with the reaction tubes newly filled with catalyst and/or during the subsequent thermal treatment of the catalyst, the actually active catalyst is created from the precursor of the catalyst by burning out the organic binder and the respective raw material sources used in the catalyst suspension are generally in convert the corresponding oxidic compounds. In addition, depending on the type of mixed metal compounds, their chemical composition and/or their crystalline structure can also change.

Over a catalyst bed thermally treated in this way and consisting of at least one catalyst according to the invention, the reaction gas is heated at temperatures of 250-550 ° C, in particular at 330 to 500 ° C and at an overpressure of generally 0.1 to 2.5 bar, preferably 0.3 to 1.5 bar, the space velocity generally being 1000 to 5000 h (-1).

The reaction gas supplied to the catalyst is generally produced by mixing a gas containing molecular oxygen, preferably air, which in addition to oxygen can also contain suitable reaction moderators and/or diluents such as steam, nitrogen and/or carbon dioxide, with the aromatic hydrocarbon to be oxidized, whereby the gas containing molecular oxygen is generally 1 to 100% by volume and particularly preferably 10 to 30% by volume of oxygen, 0 to 30% by volume of water vapor, preferably 0 to 20% by volume of water vapor and 0 to 50% by volume. -%, preferably 0 to 1% by volume of carbon dioxide, the balance may contain nitrogen. To form the reaction gas, the gas containing molecular oxygen is generally mixed with 20 to 200 g per Nm ³ and particularly preferably with 60 to 120 g per Nm ³ of the aromatic hydrocarbon to be oxidized.

In a preferred embodiment of the process for the partial oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and / or carboxylic acid anhydrides, which proves to be particularly advantageous for the oxidation of o-xylene and / or naphthalene to phthalic anhydride, the aromatic hydrocarbon is first on a bed of the catalyst according to the invention only partially converted to a reaction mixture of starting material, intermediate products and end product and this mixture is reacted with at least one further catalyst, which can also be according to the invention or can be a catalyst corresponding to the prior art.

However, the reaction can also be carried out with more than one reactor, whereby each reactor can be thermostatted to different reaction temperatures and contains at least one catalyst bed with at least one catalyst layer. It is sufficient if at least one of these reactors has a layer with a catalyst according to the invention.

For the production of phthalic anhydride from o-xylene, the partially reacted reaction gas after passing through one or more catalyst layers with a catalyst according to the invention contains, in addition to the desired product phthalic anhydride, also a significant amount of unreacted o-xylene and intermediate products such as o-tolyl aldehyde and o-tolylic acid and Phthalide.

The product mixture is then generally passed without further separation over at least one further catalyst layer, which differs from the catalyst according to the invention in terms of its chemical composition and activity, in order to ensure the complete conversion of the raw material or the oxidation of the under-oxidation products to phthalic anhydride.

Furthermore, a separation of the unreacted o-xylene after passing through the catalyst bed with the catalyst according to the invention is conceivable before the reaction gas is fed to one or more further catalyst beds. However, such a variant can only be technically implemented with relatively great effort.

It is therefore preferred that the reaction mixture passes through several catalyst layers one after the other without separating off starting materials or intermediate products, with at least one of these catalyst layers, preferably the layer(s) located closer to the gas inlet, containing a bed of a catalyst according to the invention.

In general, the catalyst according to the invention is used together with at least one further or several catalyst layers, with at least one of the catalyst layers from the gas inlet up to and including the catalyst layers located with the highest hot spot containing a catalyst according to the invention.

The catalysts used in combination for this purpose, which are generally used in locations that are closer to the gas outlet and follow the location with the highest hot spot, are mostly based on catalysts based on titanium dioxide/V205 that are not according to the invention, but do not contain any mixed element oxides of silver and /or vanadium. However, only catalysts according to the invention can also be used in all situations. The catalysts generally differ in the type and amount of mixed metal oxides present in the active material.

The prior art catalysts not according to the invention used in combination with the catalysts according to the invention generally have a titanium dioxide content of 60-99 % by weight, a vanadium oxide content of 1 to 40% by weight, an alkali metal content of up to 1% by weight, a phosphorus content of up to 1.5% by weight and an antimony content of up to 10% by weight .-% on.

Irrespective of the compositions described above, the active masses of the catalysts according to the invention and/or not according to the invention can also contain small amounts of further oxidic compounds for controlling activity and selectivity.

Examples

Production of the catalysts

The various components of the catalyst suspension are added one after the other as solutions and/or as powder into deionized water and the resulting suspension is advantageously stirred for at least 12 hours. Titanium dioxide in the anatase modification, V2O5 or vanadyl oxalate, cesium sulfate, ammonium dihydrogen phosphate, antimony trioxide, bismuth vanadate and silver vanadate, silver molybdate, silver tungstate and/or other mixed metal oxides of silver and vanadium are advantageously used as raw material sources for the components contained in the catalyst according to the invention.

An organic binder in the form of an aqueous dispersion of a vinyl acetate copolymer is then added to the aqueous catalyst suspension and the suspension, which contains a total of approximately 20 to 25%, is stirred for a further 30 minutes.

A corresponding amount of the aqueous suspension, which contains the active ingredients and/or their precursor compounds and the organic binder, is then sprayed onto the inert carrier (steatite rings with 7x7x4 mm or 8x6x5 mm dimensions) until a certain amount of the Adhesive suspension is applied to the rings, so that after calcination, the active mass given in the examples results.

The active mass proportion (proportion of catalytically active substances without binder) refers to the proportion of the catalytically active mass in the total weight of the catalyst including the support in the respective catalyst layer, determined after calcination for 4 hours at 400 ° C.

The specified phosphorus content refers to the amount of a phosphorus compound added during the production of the catalyst suspension. It is known to those skilled in the art that the actual phosphorus content in the active mass can vary depending on how heavily the TiO2 used is contaminated with phosphorus compounds.

Oxidation reaction

The multi-layer catalyst system in question is filled into a tubular reactor with a reaction tube made of iron with an inner diameter of 25 mm and a length of 3.7 m, which is surrounded by a salt bath, with the R1 layer being closest in the examples described to the gas inlet and the R4 layer or the R5 layer is closest to the gas outlet.

A thermal sleeve of 2 mm with a built-in tension element for temperature measurement was located centrally in the reaction tube. 4 Nm ^{3 of} air with a load of approximately 30 to 70 g of xylene/Nm ³ of air at a salt bath temperature of 340-380° C. were passed through the reaction tube every hour from top to bottom.

To determine the catalytic performance data, the reaction gas emerging from the reaction tube is passed through an oil-cooled condenser, in which, in particular, the phthalic anhydride formed is largely completely separated and by-products such as benzoic acid, maleic anhydride and phthalide are only partially separated. The raw PSA separated in the condensers is melted using hot oil, collected, weighed and the phthalic anhydride content is then determined by GC analysis.

The crude PSA yield given in the examples is calculated as follows: $$\begin{array}{l}\text{Crude PSA yield}\left(\text{wt}\text{.}-\text{\%}\right)=(\text{Amount of raw PSA}\left(\text{G}\right)\times \text{Purity raw PSA}\\ \left(\text{\%}\right))/\left(\text{Feed o-xyol}\left(\text{G}\right)\times \text{Purity o-xyol}\left(\text{\%}\right)\right)\end{array}$$

The yield of solid PSA determined in this way is referred to as the crude PSA yield despite deducting the by-products contained therein, since pure PSA is generally referred to as a product obtained after thermal pretreatment and distillative workup. This is also familiar to those skilled in the art.

Since the conversion of the o-xylene is almost 100% in every case, the crude yield determined in this way corresponds directly to the selectivity of the catalyst system.

Example 1 (comparative example):

Catalyst system A (4 layers)

This catalyst system is a multi-layer system with four different layers. The individual beds of this comparison catalyst do not contain any mixed element oxide of silver. Composition In wt.% 1. Location (Hot Spot) 2. Location 3. Location 4. Location Length: 147cm 45cm 70cm 70cm V2O5 5.0 7.7 8.5 15.0 Sb2O3 2.5 2.2 2.4 0.5 BiVO4 0.31 - - - Cs 0.37 0.20 0.10 0.05 P 0.03 0.05 0.05 0.10 TiO2 Rest 100% Rest 100% Rest 100% Rest 100% Share AM 8.3 8.4 8.4 8.0

The catalyst system A described in Example 1 was tested with a load of 50 to 65g of o-xylene per Nm ³ of air and a total amount of air of 4.0 Nm ³ per hour and 344 to 347 ° C SBT.

An average raw PSA yield (based on 100% o-xylene purity) of 113.4% by weight was achieved after the run-in phase and the phthalide content in the raw PSA was 0.01% by weight.

Example 2 (according to the invention):

Catalyst system B (4 layers)

Composition In wt.% 1. Location 2. Location 3. Location 4. Location Hot spot Length: 140cm 45cm 70cm 70cm V2O5 5.0 7.7 8.5 15.0 Sb2O3 2.5 2.2 2.4 0.5 BiVO4 0.31 - - - AgVO3 0.17 - - - Cs 0.40 0.20 0.10 0.05 P 0.03 0.05 0.05 0.10 TiO2 Rest 100% Rest 100% Rest 100% Rest 100% Share AM 8.6 8.4 8.4 8.0

The catalyst system B described in Example 2 was tested with a load of 58 to 63 g of o-xylene per Nm ³ of air and a total amount of air of 4.0 Nm ³ per hour and 346 to 352 ° C SBT. An average raw PSA yield (based on 100% o-xylene purity) of 114.0% by weight was achieved after the run-in phase and the phthalide content in the raw PSA was 0.06% by weight.

Example 3 (according to the invention):

Catalyst system C (4 layers)

Composition In wt.% 1. Location (Hot Spot) 2. Location 3. Location 4. Location Length: 140cm 45cm 70cm 70cm V2O5 5.0 7.7 8.5 15.0 Sb2O3 2.5 2.2 2.4 0.5 BiVO4 0.31 - - - AgVO3 0.34 - - - Cs 0.42 0.20 0.10 0.05 P 0.03 0.05 0.05 0.10 TiO2 Rest 100% Rest 100% Rest 100% Rest 100% Share AM 8.7 8.4 8.4 8.0

The catalyst system C described in Example 3 was tested with a load of 58 to 65g of o-xylene per Nm ³ of air and a total amount of air of 4.0 Nm ³ per hour and 346 to 347 ° C SBT. An average crude PSA yield (based on 100% o-xylene purity) of 115.3% by weight was achieved after the run-in phase and the phthalide content in the raw PSA was 0.04% by weight.

Example 4 (according to the invention):

Catalyst system D (4 layers)

Composition In wt.% 1. Location (Hot Spot) 2. Location 3. Location 4. Location Length: 145cm 45cm 70cm 70cm V2O5 5.0 7.7 8.5 15.0 Sb2O3 2.5 2.2 2.4 0.5 BiVO4 0.31 - - - Ag2WO4 0.39 - - - Cs 0.40 0.20 0.10 0.05 P 0.03 0.05 0.05 0.10 TiO2 Rest 100% Rest 100% Rest 100% Rest 100% Share AM 8.7 8.4 8.4 8.0

The catalyst system D described in Example 4 was tested with a load of 57 to 69g of o-xylene per Nm ³ of air and a total amount of air of 4.0 Nm ³ per hour and 346 to 348 ° C SBT. An average raw PSA yield (based on 100% o-xylene purity) of 113.9% by weight was achieved after the run-in phase and the phthalide content in the raw PSA was on average 0.02% by weight.

Example 5 (according to the invention):

Catalyst system E (4 layers)

Composition In wt.% 1st layer (hot spot) 8x6x5mm 2nd layer 7x7x4mm 3rd layer 7x7x4mm 4th layer 7x7x4mm Length: 140cm 45cm 70cm 70cm V2O5 5.0 7.7 8.5 15.0 Sb2O3 2.5 2.2 2.4 0.5 BiVO4 0.31 - - - Ag2MoO4 0.16 - - - Cs 0.40 0.20 0.10 0.05 P 0.03 0.05 0.05 0.10 TiO2 Rest 100% Rest 100% Rest 100% Rest 100% Share AM 8.6 8.4 8.4 8.0

The catalyst system E described in Example 5 was tested with a load of 52 to 62 g of o-xylene per Nm ³ of air and a total amount of air of 4.0 Nm ³ per hour and 347 to 348 ° C SBT. An average raw PSA yield (based on 100% o-xylene purity) of 114.2% by weight was achieved after the run-in phase and the phthalide content in the raw PSA was on average 0.01% by weight.

Example 6 (comparative example):

Catalyst system F (5 layers)

Composition In wt.% 1. Location 2nd location (hot spot) 3. Location 4. Location 5. Location Length: 45cm 95cm 50cm 65cm 70cm V2O5 5.0 5.0 7.7 8.5 15.0 Sb2O3 2.5 2.5 2.2 2.4 0.5 BiVO4 0.31 0.31 - - - Cs 0.36 0.42 0.21 0.10 0.05 P 0.03 0.03 0.05 0.05 0.10 TiO2 Rest too Rest too Rest too Rest too Rest too 100% 100% 100% 100% 100% Share AM 8.8 7.8 8.4 8.4 8.0

The catalyst system F described in Example 6 was tested at a load of 58 to 61 o-xylene per Nm ³ of air and a total amount of air of 4 Nm ³ per hour and 350 to 354 ° C SBT. An average raw PSA yield (based on 100% o-xylene purity) of 113.1% by weight was achieved after the run-in phase and the phthalide content in the raw PSA was 0.01% by weight.

Example 6 (according to the invention):

Catalyst system G (5 layers)

Composition In wt.% 1. Location 2nd location (hot spot) 3. Location 4. Location 5. Location Length: 50cm 90cm 45cm 70cm 70cm V2O5 4.0 5.0 7.7 8.5 15.0 Sb2O3 0.2 2.5 2.2 2.4 0.5 BiVO4 - 0.31 - - - AgVO3 - 0.17 - - - Cs 0.28 0.40 0.21 0.10 0.05 P - 0.03 0.05 0.05 0.10 TiO2 Rest 100% Rest 100% Rest 100% Rest 100% Rest 100% Share AM 6.9 7.8 8.4 8.4 8.0

The catalyst system G described in Example 6 was tested with a load of 58 to 75 g of o-xylene per Nm ³ of air and a total amount of air of 4 Nm ³ per hour and 348 to 350 ° C SBT. An average raw PSA yield (based on 100% o-xylene purity) of 114.8% by weight was achieved after the run-in phase and the phthalide content in the raw PSA was 0.03% by weight. Table 1: Performance data of the catalysts according to the invention Catalyst system m Salt bath temperature °C Ql crude PSA yield (after run-in phase) (% by weight) 0 Phthalide content in raw PSA (% by weight) A (comparison) 344-347 113.4 0.01 b 346-352 114.0 0.06 C 346-347 115.3 0.04 D 346-348 113.9 0.02 E 347-348 114.2 0.01 F (comparison) 350-354 113.1 0.01 G 348-350 114.8 0.03

From the comparison of Examples 2 to 5 and Comparative Example 1 it can be seen that the use of mixed element oxides of silver as a raw material source for producing a catalyst suspension for a catalyst in the first catalyst layer closest to the gas inlet leads to a significant increase in selectivity.

From the comparison of Example 7 with Comparative Example 6 it can be seen that the use of silver vanadate in the active mass of the hot spot catalyst layer results in an improvement in selectivity even when used in at least one catalyst layer following the first catalyst layer.

Claims (25)

Catalyst for the catalytic gas phase oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and/or carboxylic anhydrides, the active mass comprising vanadium oxide, titanium dioxide and at least one mixed element oxide of silver with defined elements, the mixed element oxide of silver having the following formula (I): Ag _x M _y N _z O _n (Formula I) with Ag = silver x = 0.01 to 100, M = one element selected from the group Li, Na, K, Rb, Cs, P, Mg, Ca, Sr, Ba, TI, y = 0 to 1 N = at least one element selected from the group V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Co, Rh, Ni, Pd, Cu, Au, Zn, Cd, Sn, Pb, B, Al, Bi, Sb, As, Ti, Zr, Hf, Ce, La z = 1 O = oxygen n = a number that is determined by the valence and frequency of the elements other than oxygen in the formula I, whereby in the production of the catalyst the at least one mixed element oxide of silver and / or at least one precursor compound of the at least one mixed element oxide of silver is used as a raw material source, and wherein the active mass has 0.01 to 15% by weight of the mixed element oxide of silver. Catalyst after Claim 1 , characterized in that the active mass has 0.01 to 10% by weight, preferably 0.01 to 5% by weight, of the mixed element oxide of silver. Catalyst after Claim 1 or 2 , characterized in that the at least one mixed element oxide of silver is a mixed element oxide with at least one further element from the group of vanadium, molybdenum, tungsten, niobium, tantalum, chromium, titanium, manganese, iron, cobalt, nickel, copper, zinc, cadmium, Gold, tin, zirconium, antimony, arsenic, cerium, lanthanum, bismuth, hafnium, lead, boron, aluminum, ruthenium, rhenium, palladium, rhodium. Catalyst according to one of the Claims 1 until 3 , characterized in that the active mass has 0.01 to 10% by weight, preferably 0.01 to 5% by weight, of a mixed element oxide of vanadium. Catalyst after Claim 4 , characterized in that the at least one mixed element oxide of vanadium is a mixed element oxide with at least one further element from the group of bismuth, antimony, molybdenum, tungsten, chromium, lanthanum, cerium, iron, manganese, niobium, tantalum, rhenium, cobalt, nickel, Copper, zinc, gold, cadmium, lead, tin, boron, titanium, zirconium, hafnium, aluminum, arsenic, ruthenium, rhodium, palladium. Catalyst according to one of the Claims 1 until 5 , characterized in that the vanadium oxide content of the active mass is 1 to 40% by weight, calculated as V ₂ O ₅ , and the titanium dioxide content of the active mass is 50 to 99% by weight, calculated as TiO ₂ . Catalyst according to one of the Claims 1 until 6 , characterized in that the active mass is 0 to 10% by weight of an antimony compound, calculated as Sb ₂ O ₃ , and/or 0 to 1% by weight of a compound of at least one element from the group of lithium, sodium, potassium, Rubidium, cesium, each calculated as an alkali metal, and / or 0 to 1.5% by weight of a phosphorus compound, calculated as P. Catalyst according to one of the Claims 1 until 7 , characterized in that the active mass has AgVO ₃ and/or Ag ₂ MoO ₄ and/or Ag ₂ WO ₄ and/or BiVO ₄ . Catalyst after Claim 8 , characterized in that a predetermined amount of AgVO ₃ and/or Ag ₂ MoO ₄ and/or Ag ₂ WO ₄ and/or BiVO ₄ and/or at least one precursor compound of these compounds is used as a raw material source in the production of the catalyst. Catalyst according to one of the Claims 4 or 5 or after one of the Claims 6 until 8th , provided this is on the Claims 4 or 5 are referred back, characterized in that the mixed element oxide of vanadium has the following formula (II): M _a N _b V _c O _d (Formula II) with M = at least one element selected from the group Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Co, Rh, Ni, Pd, Cu, Au, Zn, Cd, Sn, Pb, B, Al, Bi, Sb, As, Ti, Zr, Hf, Ce, La a = 0.01 to 100 N = at least one element selected from the group Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba , TI, P b = 0 to 1, V = vanadium c = 1 O = oxygen d = a number that is determined by the valence and frequency of the elements other than oxygen in formula II. Catalyst according to one of the Claims 1 until 10 , characterized in that V ₂ O ₅ and/or vanadium oxalate and/or V ₆ O ₁₃ and/or NH ₄ VO ₃ and/or polyvanadic acid is used as the vanadium source for producing the catalyst, at least part of the vanadium being in the active mass after the thermal treatment of the catalyst is present as vanadium pentoxide. Catalyst according to one of the Claims 1 until 11 , characterized in that at least one type of titanium dioxide in the anatase modification is used to produce the catalyst and is present as such in the active mass, the average BET surface area, calculated from the quantity and surface ratios of the individual types, of the total titanium dioxide being between 10 to 60 m ² /g. Catalyst after Claim 12 , characterized in that a mixture of several different types of titanium dioxide in the anatase modification is used in the active mass, with at least one type of titanium dioxide in the anatase modification having an average particle size of 0.3 µm to 0.8 µm and has a BET surface area of 13 to 60 m ² /g and at least one further type of titanium dioxide in the anatase modification has a BET surface area of 2 to 15 m ² /g. Catalyst according to one of the Claims 1 until 13 , characterized in that the antimony compound used to produce a catalyst suspension or a powder mixture required for coating a support and/or the antimony compound present in the active mass of the catalyst is an antimony III oxide and/or antimony V oxide. Catalyst according to one of the Claims 1 until 14 , characterized in that the catalyst consists of at least three layers, the antimony content of at least one layer upstream of the hot spot catalyst layer in the direction of the gas inlet being reduced by 20 to 100% compared to the antimony content of the hot spot catalyst layer. Catalyst according to one of the Claims 1 until 15 , characterized in that the active mass of the catalyst has the following composition after heating the catalyst at 400 ° C over a defined, preferably several, in particular approximately 4 hour period of time: Vanadium oxide: 1-40% by weight (calculated as V ₂ O ₅ ) Titanium dioxide: 50-99% by weight (calculated as TiO ₂ ) AgxMyNzOn: 0.01-15% by weight (calculated as AgxMyNzOn) ManbVcOd: 0-10% by weight (calculated as MaNbVcOd) Alkaline metal: 0-1.0% by weight (calculated as alkali metal) Antimony oxide: 0-10% by weight (calculated as Sb ₂ O ₃ ) Phosphorus: 0-1.5% by weight (calculated as P) Catalyst after Claim 16 , characterized in that the active mass of the catalyst has the following composition after tempering the catalyst at 400 ° C over a defined, preferably several, in particular approximately 4 hour period of time: Vanadium oxide 1-20% by weight (calculated as V ₂ O ₅ ) Titanium dioxide: 80-99% by weight (calculated as TO ₂ ) Silver vanadate: 0-5% by weight (calculated as AgVO ₃ ) Silver tungstate: 0-5% by weight (calculated as Ag ₂ WO ₄ ) Silver molybdate: 0-5% by weight (calculated as Ag ₂ MoO ₄ ) Bismuth vanadate: 0-3% by weight (calculated as BiVO ₄ ) Cesium: 0-1% by weight (calculated as Cs) Antimony oxide: 0-5% by weight (calculated as Sb ₂ O ₃ ) Phosphorus: 0-1.5% by weight (calculated as P) Catalyst after Claim 16 or 17 , characterized in that the active mass of the catalyst after tempering the catalyst at at least 400 ° C and a period of at least 4 hours is a proportion of 2 to 25 wt .-% of the total weight of the catalyst. Catalyst according to one of the Claims 1 until 18 , characterized in that the layer thickness of the active mass on an inert support material is 20 to 400 µm and the catalytically active mass is applied to an inert, non-porous support, which is preferably a support made of steatite, in particular steatite rings. Catalyst according to one of the Claims 1 until 19 , characterized in that at least two different layers are applied in the form of a shell to a preferably inert support, at least one of the layers containing titanium dioxide, vanadium oxide and a mixed element oxide of silver. Catalyst according to one of the Claims 1 until 20 , characterized in that at least two different layers are applied in the form of a shell to a preferably ceramic support, with at least one layer containing at least titanium dioxide and vanadium oxide and at least one further layer containing at least one silver mixed element oxide with or without additional titanium dioxide and vanadium oxide. Catalyst according to one of the Claims 1 until 21 , characterized in that in at least one catalyst layer from the gas inlet to the layer with the highest hot spot, a shell catalyst is used, the active mass of which has the mixing element oxide of silver and in which catalyst in at least one catalyst layer from the layer with the highest hot spot to the A shell catalyst is used at the gas outlet for the oxidation of aromatic hydrocarbons, which contains vanadium pentoxide and titanium dioxide as essential components in its active mass. Catalyst according to one of the Claims 1 until 22 , characterized in that the binder for producing at least one catalyst layer is an organic polymer or copolymer, in particular a vinyl acetate copolymer, which is used in particular in the form of an aqueous dispersion. Catalyst according to one of the Claims 1 until 23 , characterized in that the catalytically active mass contains, based on its total weight, 0.01 to 15% by weight of the mixed element oxide of silver with vanadium and in which the ratio Ag: V is greater than 0.95 (> 0.95) :1 to 10:1. Process for the production of aldehydes, carboxylic acids and/or carboxylic anhydrides by partial oxidation of aromatic hydrocarbons at elevated temperature on a catalyst whose catalytically active mass is applied to an inert, non-porous support, characterized in that a shell catalyst whose catalytically active mass is obtained on their total weight, 0.01 to 15% by weight of a mixed element oxide of silver with vanadium and / or molybdenum and / or tungsten and / or niobium and / or antimony and at the same time contains titanium dioxide and a vanadium compound, the mixed element oxide compound or its Precursor compound is used in the production of the catalyst suspension and / or the powder mixture for the later active mass, in the presence or absence of at least one further shell catalyst, which contains vanadium pentoxide and titanium dioxide in the anatase modification as essential components in its active mass.