EP2247384A2 - Catalyst for the catalytic gas phase oxidation of aromatic hydrocarbons to form aldehydes, carboxylic acids and/or carboxylic acid anydrides, in particular phthalic acid anhydride - Google Patents

Abstract

The invention relates to a catalyst for the catalytic gas phase oxidation of aromatic hydrocarbons to form aldehydes, carboxylic acids and/or carboxylic acid anydrides, in particular phthalic acid anhydride, comprising a plurality of different catalyst layers, wherein the catalyst comprises a hot spot catalyst layer in which the greatest local temperature maximum occurs, relative to the entire catalyst, at least one catalyst layer placed prior to the hot spot catalyst layer, and preferably at least one catalyst layer placed after the hot spot catalyst layer. According to the invention, in the case of antimony and/or bismuth and/or niobium in the catalytically active material of the hot spot catalyst layer the amount of at least one of said elements in the catalytically active material of at least one catalyst layer placed before is reduced compared to the amount of the same element in the catalytically active material of the hot spot catalyst layer by a pre-determined value.

Description

 description

Catalyst for the catalytic gas-phase oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and / or carboxylic anhydrides. in particular too

phthalic anhydride

The invention relates to a catalyst for the catalytic gas-phase oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and / or carboxylic anhydrides, in particular to phthalic anhydride, according to the preamble of claim 1.

It is known that a large number of carboxylic acids and / or carboxylic anhydrides are produced industrially by the catalytic gas-phase oxidation of aromatic hydrocarbons such as benzene, xylene, naphthalene, toluene or durene in fixed bed reactors, preferably tube bundle reactors. For example, benzoic acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid or pyromellitic anhydride are obtained.

For this purpose, in general, a mixture of a molecular oxygen-containing gas, for example air, and the hydrocarbon to be oxidized, for example o-xylene, passed through a so-called tube bundle reactor in which a

Variety of reaction tubes are arranged in parallel and in which there is a bed of a suitable catalyst. Due to the strong formation of heat in such oxidation reactions, it is necessary to surround the reaction tubes with a heat transfer medium, for example a molten salt, in order to produce the reaction mixture

Dissipate heat. The heat transfer medium used is generally a salt melt, preferably a mixture of sodium nitrite and potassium nitrate.

Despite such a temperature control or thermostation, it may come in the catalyst bed to form local temperature maxima, the so-called hot spots ("hot spots").

Such "hot spots" cause a number of undesirable effects, such as

Example, a reduced selectivity by partial total oxidation of the starting material or an increased formation of undesirable by-products such as Benzoic acid, maleic anhydride and phthalide in the oxidation of o-xylene to phthalic anhydride, which can be separated only partially with increased effort with partial loss of the desired end product. Another major disadvantage in the formation of such locally partly greatly increased temperatures ("hot spots") in the catalyst bed is the significant amount of time after initial commissioning, which is needed to achieve the desired maximum throughput of raw material and extreme training of one or several hot spots even the permanent limitation of the load below the desired range, in order to avoid irreversible catalyst damage.

In order to mitigate or avoid such hot spots, so-called multi-layer catalysts have been used in the art, which are arranged in layers in the catalyst bed, in particular with superimposed catalyst layers, and where the least active catalyst is usually closest to the gas inlet and thus comes first in contact with the starting mixture.

Too high an activity of this first catalyst layer would lead to an uncontrolled rise in the hot-spot temperature, which can lead to the selectivity losses already mentioned or to irreversible damage to the catalyst.

The catalyst layer (s) following in the direction of the gas outlet generally has an increased activity in order, on the one hand, to ensure complete conversion of the starting material and, on the other hand, to reduce the content of undesired by-products as far as possible.

US-A 4,356,112 describes a catalyst with two layers of different activity for the production of phthalic anhydride, wherein the antimony trioxide (Sb ₂ O ₃₎ content is 0.5 to 10 wt .-%. The best yields are achieved with an antimony oxide content of 2 wt .-% and a vanadium pentoxide content of 3 wt .-% of the active composition. It is disclosed that antimony oxide improves the heat resistance and selectivity of the catalysts.

EP-A-0 522 871 describes a catalyst for the production of phthalic anhydride with two different catalyst layers by gas-phase oxidation, using a pentavalent antimony compound as antimony source. The disclosed catalysts have a Sb ₂ O ₅ content of 2.5 wt .-% and a Sb ₂ O ₃ content of 2.0 wt .-% and have a V ₂ O ₅ : Sb ₂ θ3 ratio of 2: 2.0 or a V ₂ 0 ₅ : Sb ₂ 0 ₅ - ratio of 2: 2.5. DE 103 23 461 A1 describes a process for the preparation of phthalic anhydride, wherein a catalyst with two different layers with increasing activity from the gas inlet to the gas outlet is used and at the same time the ratio of V ₂ O ₅ to Sb ₂ O ₃ in the first layer at 3, 5: 1 to 5: 1.

DE 198 39 001 describes a shell catalyst for the gas phase oxidation of hydrocarbons with a total content of up to 10 wt .-% Sb ₂ O ₃ , in which the catalytically active layers are applied in two or more layers on the inert support and wherein the inner layer or inner layers have a total content of antimony trioxide of up to 15 wt .-% and the outer layer has a reduced by 50 to 100% Sb ₂ O ₃ content. The proportion of active mass of the inner layer or inner layers is 10 to 90% of the total active mass.

EP 1 084 115 B1 describes a process for the oxidation of o-xylene and / or naphthalene with at least three layers arranged one above the other, in which the catalyst activity increases continuously from the gas inlet side to the gas outlet side. In this case, the content of active mass increases from the reactor inlet to the reactor outlet and the alkali metal content optionally additionally decreases from the gas inlet to the gas outlet.

DE 102 378 18 describes a catalyst for the preparation of phthalic anhydride with at least three successive layers, the activity of the layers continuously increasing from the gas inlet to the gas outlet. The control of the increase in activity is achieved here by using TiO ₂ with different BET surface area. Here, the BET surface area of the titanium dioxide used is lowest in the gas inlet layer and highest in the last gas outlet layer.

EP 1 063 222 describes a process for the preparation of phthalic anhydride, wherein the catalyst consists of three or more different catalyst layers. The reaction may be carried out in one or more consecutive fixed bed reactors. The conversion of the starting material used is 30 to 70% after the first layer and at least 70% after the second layer.

DE 10 2004 026 472 A1 describes a multilayer catalyst for the production of

Phthalic anhydride, in which the catalyst activity increases in the flow direction and at the same time decreases the active mass of the various layers from the gas inlet to the gas outlet.

DE 103 23 817 A1 describes a catalyst for the preparation of phthalic anhydride with at least three different layers, wherein the activity of the layers increases from the gas inlet to the gas outlet and only the last layer closest to the gas outlet contains phosphorus and more than 10% by weight. % V ₂ O ₅ . WO 2006/092304 describes the use of a multilayer catalyst for producing phthalic anhydride with at least three catalyst layers, wherein the first layer located to the gas inlet side has a higher activity than the subsequent second layer.

WO 2006/092305 describes a process for the preparation of phthalic anhydride with an initial catalyst consisting of at least two catalyst layers of different activity, wherein a portion of the first gas inlet layer is replaced by a higher activity catalyst.

Although the use of the above-described structured catalysts having the most varied composition of the active composition permits sufficient control of the local temperature maxima even at relatively high raw material loads, there is still a considerable need for improvement of the catalyst systems with regard to selectivity, suppression of the formation of by-products and also in terms of reliability of the process for the production of phthalic anhydride.

It is therefore an object of the present invention to provide a catalyst for the catalytic gas-phase oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and / or carboxylic anhydrides, in particular to phthalic anhydride, which has a high selectivity by means of which the formation of by-products is suppressed can and allows high reliability of the process for the preparation of phthalic anhydride in particular.

This object is achieved with the features of claim 1. Preferred embodiments thereof are the subject of the dependent claims, the disclosure content of which is expressly included in this description.

According to claim 1, in the case of antimony and / or bismuth and / or niobium in the catalytically active material of the hot-spot catalyst layer, the content of at least one of these elements in the catalytically active composition of at least one upstream catalyst layer over the content of the same element in the catalytically active mass of the hot-spot catalyst layer reduced. Particularly preferred is the antimony, especially antimony oxide content of the catalytically active material in at least one of the hot-spot catalyst layers upstream catalyst alga against the antimony, especially antimony oxide content of the catalytically active material in the hot-spot catalyst layer by a predetermined Value reduced. According to a particularly preferred embodiment, this antimony content is reduced by 20 to 100 wt .-%, most preferably reduced by 40 to 100 wt .-%. The idea of the invention will be described below by way of example in conjunction with a reduced antimony content. Analogously, however, the following also applies to bismuth or niobium, without explicitly mentioning this again. The same applies to the claims. Here, the general inventive concept of claim 1 with the term "antimony" or "antimony content" was further developed in the subclaims merely for reasons of clarity and clarity, although antimony is the preferred element of the three mentioned elements antimony, bismuth and niobium. The term "antimony" or "antimony content" is thus hereby expressly synonymous with "bismuth" or "bismuth content" on the one hand and "niobium" or "niobium content" on the other hand, that is quasi as a generic term for all three elements antimony, bismuth and niobium be valid.

As experiments on the part of the inventor have shown, a significant increase in the selectivity can be achieved with such an inventive reduction of the antimony content in at least one catalyst layer upstream of the hot-spot catalyst layer. At the same time, the formation of by-products is suppressed.

For the purposes of the present inventive idea, the term hotspot refers to the largest locally measured temperature in the entire catalyst bed. In addition to this largest hot spot, which is located in a fresh catalyst usually in a range of 50 to 120 cm deep in the catalyst bed (measured from the gas inlet in the direction of gas outlet), quite a few smaller side hot spots in further catalyst layers occur.

Since the position of the hotspot can shift into lower-lying zones or even into lower-lying (in the direction of gas outlet) catalyst layers in the course of the operating time, the location of the largest hotspot at optimum operating temperature (salt bath temperature) is preferably used in the present invention in which the catalyst achieves its highest selectivity and produces a sufficiently good product quality, preferably after the start-up (break-in period) of the catalyst about three months after the first start up of the catalyst.

For the purposes of the present inventive concept, the term catalyst layer should be understood in a broader sense, that is to say not only as a coating layer applied to carrier rings, but expressly also include those solutions in which the catalyst is applied to a single continuous carrier. In particular, a catalyst layer in a broader sense is understood to mean a reaction region which is distinguished by a substantially uniform catalyst composition or, in the case of a mixture of several catalysts, by a uniform mixture composition in this range. The transitions between the individual catalyst reaction areas can be flowing. Preference is given to the different Catalysts thus formed by catalyst reaction areas, which have a more or less flowing transition to each other. In particular, the different catalyst layers are formed in the conventional sense by individual so-called catalyst rings, in which the catalytically active composition is applied to an inert, non-porous carrier ring, for example.

For example, the catalytic mass of one or more catalyst layers can be applied in two or more layers to a support, in particular a ceramic support. According to a first variant, the antimony content of at least one coating layer of at least one upstream catalyst layer is preferably reduced by a predetermined value, in particular by 20 to 100%, compared to the antimony content of the coating layer with the highest antimony content in the hot-spot catalyst layer. Alternatively or additionally, the hot-spot catalyst layer on at least two coating layers, wherein at least one of the coating layers by a predetermined value, preferably by 20 to 100%, reduced antimony content of the catalytically active material compared to that coating layer having the highest antimony content. In particular, by way of example, the or at least one layer (s) applied to the inner layer can have an antimony content of the catalytically active composition which is reduced by a predetermined value, in particular by 20 to 100%, compared with the inner layer. This also makes it possible analogously to the inventive concept according to the above-described claim 1, the advantages mentioned there, in particular achieve a higher selectivity. This variant of the invention just described in connection with a multilayer structure of the catalyst layers is expressly claimed independently of the subject of claim 1 of the invention. In other words, according to this aspect of the invention, at least one upstream catalyst layer can be applied in several layers to the carrier rings, for example, and any of the layers, in particular the outer layer, have a lower content of antimony than the antimony content of the hot-spot catalyst layer in a single-layer structure the hot-spot catalyst layer; whereas in the case of an inhomogeneous composition in which the hot-spot catalyst layer itself has multiple layers, any layer, most preferably an outer coating layer, of an upstream catalyst layer may have a lower antimony content than the highest antimony content coating layer in the hot spot catalyst layer.

According to another particularly preferred embodiment of the invention, which is also claimed separately here, a mixture of at least two different catalysts, in particular with regard to the composition of the catalytically active composition, can also be used in the hot-spot catalyst layer. The advantages according to the invention are achieved when the antimony content, in particular the antimony oxide content, of the catalytically active composition of at least one catalyst of the hot-spot catalyst layer is reduced by a predetermined value, preferably by 20 to 100%, compared to the antimony content of the catalytically active material of the catalyst having the highest antimony content in the hot-spot catalyst layer. The same applies to the case, which is likewise claimed independently, in which one or more catalysts, which are different in particular with regard to the composition of the catalytically active composition, are used in one or more catalyst layers upstream of the hot-roller catalyst layer, the antimony content, in particular the antimony oxide content, the catalytically active composition of at least one catalyst of this considered, upstream and a catalyst mixture containing catalyst layer is reduced by a predetermined value, preferably by 20 to 100%, compared to the antimony content of the catalytically active material of that catalyst having the highest antimony content in the hot-spot catalyst layer.

Hereinafter, the preferred catalyst structure will always be described in conjunction with the term catalyst layers, where this design is expressly included in the scope of protection where this also in connection with reaction areas, for example, have a fluid transition to each other, technically reasonable.

According to a preferred embodiment, it is provided that in the case of a single upstream catalyst layer, this one catalyst layer has an antimony content of the catalytically active composition, which is reduced by a predetermined value compared to the antimony content of the catalytically active material of the hot-spot catalyst layer. Alternatively, in the case of several upstream catalyst layers, the antimony content of the catalytically active material of at least one of the upstream catalyst layers is reduced by a predetermined value compared with the antimony content of the catalytically active material of the hot-spot catalyst layer. With such a structure, the aforementioned advantages in terms of high selectivity and the very good suppression of the formation of undesirable by-products can be achieved.

In particular, in the latter case with several upstream catalyst layers can be substantially increased with such a catalyst system, the reliability of the process for the production of phthalic anhydride.

According to a particularly preferred embodiment, it is provided that in the case of a single upstream catalyst layer, this single catalyst layer has an active material content which is less than or equal to the active material content of the hot-spot catalyst layer. Alternatively, in the event that multiple upstream catalyst layers are provided, at least one of the reduced antimony content having upstream catalyst layers having an active material content which is less than or equal to the active material content of the hot-spot catalyst layer.

Furthermore, the selectivity can be significantly increased if the catalyst layers having a reduced antimony content have a vanadium oxide content of from 0 to 9% by weight, most preferably from 3 to 6% by weight.

The selectivity of the catalyst can be further increased by the fact that at least one of the reduced antimony content having upstream catalyst layers has a phosphorus content of 0 to 0.3 wt .-%, most preferably from 0 to 0.15 wt .-%, based on having the catalytically active composition of this catalyst layer.

In addition, in the context of the present invention, the terms first catalyst layer, second catalyst layer, third catalyst layer, fourth catalyst layer, fifth catalyst layer and so forth are also used below: The first catalyst layer is always the catalyst layer located directly toward the gas inlet. The following catalyst layer, which adjoins the first catalyst layer in the direction of the gas outlet, is referred to as the second catalyst layer and the subsequent catalyst layer as the third catalyst layer. The adjoining catalyst layers are referred to as the fourth, fifth and optionally as the sixth catalyst layer. In a catalyst system with a total of six catalyst layers thus the sixth and last catalyst layer is directly at the gas outlet.

In the case of a catalyst with a total of only three layers, only one catalyst layer with a reduced antimony content is located in front of the catalyst layer with the highest hot spot and is thus located directly at the gas inlet. In addition to a reduced antimony content, this first catalyst layer has a content of catalytically active compound (= active composition) which is preferably the same size and particularly preferably smaller than the active composition of the second catalyst layer, which exhibits the highest hot spot. The second catalyst layer located downstream and closer to the gas outlet is immediately adjacent to the first catalyst layer located at the gas inlet and has a correspondingly higher antimony content in the active composition. In the case of a catalyst with a total of three catalyst layers, this second catalyst layer is the catalyst layer with the highest hot spot. The subsequent third catalyst layer, which is located further downstream and closest to the gas outlet, usually has a higher activity than the second catalyst layer. Here, the antimony content of this last catalyst layer may be the same size, smaller or larger than the antimony content of the second layer. The possibilities of setting the activity of the individual catalyst layers are familiar to the person skilled in the art, with various measures being available. In general, the increase in activity of this last catalyst layer is preferred by a reduced content G achieved at activity-damping promoters and / or a higher BET surface area of the titanium dioxide used and / or by an increased addition of activity-increasing promoters and / or by a higher content of active material.

In the case of a catalyst with a total of four catalyst layers, a catalyst layer or two catalyst layers with an antimony content reduced by 20 to 100% can preferably be in the direction of the gas inlet before the layer with the highest hot spot. If only one layer with reduced antimony content is present in front of the catalyst layer with the highest hotspot, then this catalyst layer has an equal or, preferably, a smaller active mass, than the catalyst layer with the highest hotspot, regardless of its activity. In this case, according to the invention, the catalyst layer with the highest hot spot is followed by two further catalyst layers in the direction of the gas outlet, the activity of these catalyst layers preferably increasing in the direction of gas outlet.

In a further embodiment with a total of four catalyst layers are two catalyst layers with a reduced Sb content in front of the catalyst layer with the highest hot spot. In this case, the antimony content may be uneven. It is preferred if one of the catalyst layers has a reduced content compared to the catalyst layer with the highest hot spot and is particularly preferred when both catalyst layers have a reduced Sb content. In the present embodiment, the highest hot spot catalyst layer is the third catalyst layer to which a fourth catalyst layer closest to the gas exit is connected.

In a catalyst with a total of five layers, there are several different options in which one, two or three catalyst layers of the catalyst layer with the highest hot spot can be preceded in the direction of gas inlet. According to the teaching of the invention, it is sufficient if only one catalyst layer of the upstream catalyst layers has a reduced antimony content, particularly preferably that of the upstream catalyst layers with the highest filling level, and this has an active mass which is equal to or less than the active mass of the catalyst layer with the highest hot spot. In particular, all three upstream catalyst layers have a reduced Sb content and two of the upstream catalyst layers (with the largest fill levels) have a reduced active mass. According to the invention, particular preference is given to catalysts in which all three upstream catalyst layers have a reduced Sb content of active mass and at the same time also a reduced active mass.

In a catalyst with five catalyst layers are the catalyst layer with the highest

Hot spot in the direction of gas outlet, depending on the number of upstream catalyst layers one, two or three catalyst layers downstream towards the gas outlet. The downstream Catalysts usually have in the direction of gas leakage an increasing activity, which can be achieved as described by various measures. The catalyst layer closest to the gas outlet is referred to herein as the fifth catalyst layer and typically has the highest activity of all layers.

Preferably, in a catalyst with a total of five catalyst layers, only one catalyst layer of the catalyst layer with the highest hot spot upstream and this catalyst layer has, for example, 20 to 100% reduced Sb content and at the same time an equally large or preferably lower active mass than the catalyst layer the highest hot spot. In this embodiment according to the invention then three different catalyst layers of the catalyst layer are followed by the highest hot spot. In this case, in a preferred embodiment, the activity increases from the second catalyst layer to the fifth catalyst layer in the direction of gas outlet. The activity of the upstream first catalyst layer may be greater than or less than or equal to the activity of the second catalyst layer with the highest hot spot.

Particularly preferably, in a catalyst with a total of five catalyst layers, two upstream catalyst layers with reduced antimony content and reduced active mass are provided. In this case, according to the invention, the first catalyst layer located at the gas inlet has the lowest activity of all catalyst layers and at the same time has a reduced Sb content of 20 to 100% compared to the catalyst layer with the highest hot spot. The second upstream catalyst layer preferably has a higher activity than the first catalyst layer and has over the catalyst layer with the highest hot spot a 20 to 100% reduced Sb content and an equal or more preferably a lower active mass. The activity of the two catalyst layers following the catalyst layer with the highest hot spot preferably increases in the direction of gas exit. It is particularly preferred if the last catalyst layer lying in the direction of the gas outlet has a very high activity.

For a catalyst with a total of six catalyst layers, there are accordingly further options, but these are only an optimization of the principle according to the invention. Essentially, these are the fine-tuning of activities and selectivities.

With a total of six catalyst layers, up to four catalyst layers can be preceded by the catalyst layer with the highest hot spot. In this case, the composition and activity of the upstream catalyst layers may well be different. For a catalyst according to the invention, it is sufficient if only one of the catalyst layers upstream of the catalyst layer with the highest hot spot has a Sb content reduced by 20 to 100% and an equal or preferred one has lower active mass than the catalyst layer with the highest hot spot. This layer preferably has the highest filling level of all the layers upstream.

In a preferred embodiment, in the case of a catalyst according to the invention having a total of six layers, only one catalyst layer of the layer with the highest hot spot is disposed in front of it. According to the invention, this upstream catalyst layer has an Sb content reduced by 20 to 100% and an active substance of the same size as the catalyst layer with the highest hotspot. Preferably, the active mass is smaller than the active mass of the catalyst layer with the highest hot spot. The highest hotspot layer in this catalyst is the second catalyst layer as defined above. Preferably, the activity of the four subsequent catalyst layers increases gradually towards the gas outlet. Here, the downstream catalyst layers differ in their chemical composition and / or their physical properties and / or in the content of active mass.

In a particularly preferred embodiment, two catalyst layers with a reduced antimony content and reduced active mass are preceded by a catalyst with a total of six catalyst layers of the catalyst layer with the highest hot spot. The first catalyst layer lying directly on the gas inlet has the lowest activity of all catalyst layers and preferably also has the lowest content of active material. According to the invention, the second catalyst layer has a higher activity than the first catalyst layer and preferably has a higher active mass than the first catalyst layer. At the same time, the antimony content in relation to the catalyst layer with the highest hotspot is preferably reduced by 20 to 100% and the active mass is the same size or preferably smaller than the active mass of the catalyst layer with the highest hotspot. The activity of this second catalyst layer may be smaller or preferably greater than that of the subsequent catalyst layer with the highest hot spot. In a preferred embodiment, the activity of the three catalyst outlet situated in the direction of the gas outlet and after the location with the highest hot spot increases gradually in the direction of gas outlet. In this case, in particular the last catalyst layer has a particularly high activity and a lower content of active material optionally as the catalyst layer with the highest hot spot and / or the immediately upstream upstream catalyst layer.

It has surprisingly been found that it is advantageous if no additional hot spots develop in the catalyst layer (s) which are closer to the gas inlet compared to the catalyst layer having the highest temperature maximum. The formation of so-called side hot spots in the upstream catalyst layers with reduced Sb content also has a negative effect on selectivity and service life of the catalysts. Depending on the nature and composition of the catalysts according to the invention with only one upstream catalyst layer having a reduced Sb content, the filling level of the first catalyst inlet located upstream of the catalyst inlet and the highest hot spot upstream of the catalyst layer is 10 to 90 cm and preferably 20 to 70 cm. If only one catalyst layer in the direction of gas inlet in front of the catalyst layer with the highest local temperature maximum, the filling height of this catalyst layer is particularly advantageous 30 to 60 cm.

Preferably, a coordination between activity and level of this upstream catalyst layer is required in order to achieve optimum selectivity and stability of the catalyst.

The lower the activity of the upstream catalyst layer, the longer the

Fill level of this upstream catalyst layer can be selected. Here it is priority

Target, the largest hot spot always in the subsequent catalyst layer with higher

To keep antimony content. On the other hand, the total length of the available fill level must be taken into account in order to complete the conversion of the reaction gas on

Ensure reactor outlet.

According to a particularly preferred embodiment of the invention, two catalyst beds are used in front of the catalyst layer having the highest local temperature maximum, wherein the total fill level of the two upstream catalyst layers is preferably between 15 and 90 cm and more preferably between 30 to 70 cm.

If two catalyst layers in the direction of gas inlet in front of the catalyst layer with the highest temperature maximum, it is advantageous if the filling level of the first closest to the gas inlet catalyst layer is 5 to 25 cm and the lowest

Activity of all catalyst layers has. Particularly advantageous for this first

Catalyst layer selected a filling height of 5 to 15 cm. Preferably, this first inactive serves

Catalyst layer with lowest activity to avoid ignitions when incompletely evaporated raw material on the beginning of the catalyst bed. As a result, with such a catalyst system, the reliability of the process for

Production of phthalic anhydride can be substantially increased.

In a preferred embodiment, the filling level of the second upstream catalyst layer is 20 to 60 cm and in a particularly preferred embodiment 30 to 50 cm.

The filling level of the individual catalyst layers depends decisively on the total filling level of the reaction tubes, which in turn is determined by the tube length. According to the state of the art, today generally total filling heights of 2.80 to 3.40 m are selected.

In a preferred embodiment, the catalysts with a total of four catalyst layers have a filling height of 30 to 60 cm for the first catalyst layer, 80 to 150 cm for the second catalyst layer (catalyst layer with highest hot spot), 40 to 100 cm for third catalyst layer and 40 bis 100 cm for the fourth catalyst layer. Particularly preferred is a filling height of 30 to 50 cm for the first catalyst layer, 90 to 130 cm for the second catalyst layer, 50 to 80 cm for the third catalyst layer and 50 to 80 cm for the fourth catalyst layer.

In a catalyst with a total of five catalyst layers, the catalysts preferably have a filling level of 5 to 15 for the first catalyst layer, 20 to 50 cm for the second catalyst layer, 80 to 130 cm for the third catalyst layer (hot spot), 50 to 100 cm for the fourth catalyst layer and 50 to 100 cm for the fifth catalyst layer, depending on the available Gesamtfüllhöhe.

In a particularly preferred variant of a catalyst with a total of six catalyst layers, the first catalyst layer has a filling height of 5 to 15 cm, the second catalyst layer a filling height of 30 to 50 cm, the third catalyst layer (catalyst bed with the highest hot spot) a filling height of 80 to 130 cm, the fourth catalyst layer has a filling height of 40 to 60 cm, the fifth catalyst layer has a filling height of 60 to 80 cm and the sixth catalyst layer has a filling height of 50 to 70 cm.

Depending on the nature of the PSA catalysts used according to the invention, the activity of the catalyst layers in the direction of gas entry may be smaller and / or greater than the activity of the catalyst layer having the largest local temperature maximum.

The activity of the catalyst layer or the catalyst layers, which lies or lie after the bed with the highest temperature maximum, or usually has a greater activity than the catalyst layer having the highest temperature maximum, the activity usually from catalyst layer to catalyst layer in the direction of gas outlet increases.

Particularly preferably, the first, closest to the gas inlet catalyst layer has a particularly low activity in order to increase the reliability of the process for the production of phthalic anhydride. It is well known to those skilled in the art that in phthalic anhydride technical installations explosions often occur at the reactor inlet, in particular when the reactors are operated at high loadings. One reason for this is often the impact of incompletely evaporated raw material drops on the Beginning of the catalyst bed. This leads to a locally extremely high charge at the beginning of the catalyst bed on individual catalyst rings, which can lead to "run away" conditions if the catalyst present at the reactor inlet has too high an activity inert ceramic bodies can also be used, although in the latter case the bed at the gas inlet does not contribute to the reaction and no conversion of the raw material takes place in this part of the bed, which is disadvantageous in particular for older reactors with a short tube length remaining, filled with catalyst residual length of the tubes is no longer sufficient to ensure complete conversion of the starting material or a sufficient product quality.

Preferably, the activity of the catalyst layers is set as follows:

I AK (LI) «AK (L2) </ = /> AK (L3) <AK (L4) <AK (L5) <AK (L6)

with AK = activity catalyst layer; Li = i-th catalyst layer

For example, the activity of a catalyst layer can be reduced by various measures known to the person skilled in the art by:

- Lowering the content of active material, and / or

Lowering the average BET surface area of the TiO _{2 used} , and / or increasing the content of cesium, and / or

Lowering the bulk density of the catalyst, and / or

- admixture of inert material, and / or

Increasing the content of activity-damping promoters, and / or

Lowering of the content of activity-increasing promoters, and / or lowering of the V ₂ O ₅ content, and / or

- increase of antimony content

or by combining two or more of the above measures.

In a preferred embodiment of the invention, counteracting measures are used for activity adjustment and selectivity adjustment, depending on the position of the respective catalyst layer in the catalyst bed and the task of the respective catalyst layer in the overall play of the various catalyst layers. The active masses of the individual catalyst layers preferably have a different chemical composition and / or different physical properties. In this case, individual catalyst layers can differ in particular only by a different Gehait of active mass.

The preparation methods and individual compositions of the catalyst layers are familiar to the person skilled in the art.

Within the scope of the invention, it has been found to combine the individual catalyst layers with specific properties and compositions in an advantageous manner, in order to achieve an increase in selectivity with simultaneous stable operation and a high raw material throughput compared with the conventional and known catalysts.

In a preferred embodiment of the catalyst according to the invention, the content of active composition is distributed over the individual layers as follows:

AM (LI) «AM (L2) <AM (L3) <AM (L4) </ = AM (L5)> AM (L6)

with AM = active mass content; Li = i-th catalyst layer

It has been found within the scope of the invention that it is advantageous for multilayer catalysts if the gradation of the active composition content is carried out as described in the catalyst according to the invention, depending on the length and position in the catalyst bed.

By way of example, the grading of the active composition in a catalyst with a total of six catalyst layers is disclosed here:

It has been found that it is advantageous if the first catalyst layer has a very low active composition content in order to avoid ignition of the mixture by locally very high raw material concentrations in the event of incomplete evaporation. Preferably, the second catalyst layer has a significantly higher active mass content in comparison to the first catalyst layer and at the same time particularly preferably a lower active material content than the catalyst layer with the highest hot spot. The catalyst layer with the highest hotspot preferably has an average active material content, on the one hand to ensure a moderate hot-spot guidance even at high loadings and on the other hand to convert most of the raw material to not in the subsequent downstream active catalyst layers not one totally oxidize considerable part of the raw material. In the example catalyst, the catalyst layers immediately following the catalyst layer with the largest hot spot have a higher active material content and a higher activity than the catalyst layer with the highest hot spot in order to complete the conversion of the starting material to the largest part. In this case, the active material content of these two catalyst layers is preferably approximately the same, while the activity of the catalyst layer located closer to the hot-spot catalyst layer is smaller than that of the subsequent catalyst layer. This increase in activity can be achieved, for example, by other measures known to the person skilled in the art, preferably by a reduction of the cesium content. Preferably, the last, located to the gas outlet sixth catalyst layer has a very high activity in order to keep the by-product level as low as possible. According to the invention, this last layer has a lower active material content than the catalyst layer with the highest hot spot and / or the immediately preceding catalyst layer. It has been found that with high activity and at the same time high active mass of the catalyst layer located at the gas outlet, a drop in selectivity of the overall catalyst is caused.

According to a preferred embodiment, in a multi-layer catalyst with a total of six catalyst layers and two catalyst layers with the highest hot-spot upstream catalyst layers, the first catalyst layer has an active material content of 3 to 10 wt .-% and particularly preferably from 3 to 6 wt .-% , The second catalyst layer preferably has an active composition content of 4 to 11 wt .-%, and particularly preferably from 5 to 9 wt .-%. It has been found that it is preferred for the catalyst layer with the highest hotspot to have an active material content of from 5 to 12% by weight and more preferably from 6 to 11% by weight and in particular from 7 to 9% by weight , In a preferred embodiment, the fourth and fifth catalyst layers have an active composition content of from 5 to 15% by weight and in particular from 6 to 12% by weight and more preferably from 7 to 11% by weight.

The last and closest to the gas outlet catalyst layer has in a preferred embodiment, an active material content of 4 to 11 wt .-% and particularly preferably from 5 to 10 wt .-%.

However, it should also be explicitly covered by the scope of protection, in which the active material content of the catalyst layer of the highest hot spot to catalyst layer at the gas outlet is consistent and / or decreasing and / or increasing, since the activity design in particular the catalyst layers according to Catalyst situation with the largest hot spot can be achieved in many ways, without having to accept significant loss of selectivity. The active masses of the individual catalyst layers may, for example, have a different chemical composition. Alternatively or additionally, the individual catalyst layers, in particular the first and second upstream Katalysatoriage, differ only by a different content of active mass.

In a preferred embodiment of the catalyst according to the invention, the individual catalyst layers contain as main constituents in each case at least titanium dioxide in the anatase modification and a vanadium compound in the active composition. In addition, an antimony compound is preferably used, preferably antimony trioxide for improving the thermal stability and service life of the catalysts. In addition, a cesium compound is used in the catalysts according to the invention as a selectivity-increasing and activity-damping component. Furthermore, it is advantageous to use in particular a phosphorus compound to increase the activity of the catalyst layers lying to the gas outlet.

In a preferred embodiment of the catalyst according to the invention, the antimony content of the first two catalyst layers is in a range from 0 to 2.5% by weight, in particular 0 to 1.5% by weight and more preferably 0 to 1% by weight. Calculated as Sb ₂ O ₃ , this content is preferably reduced by 20 to 100% compared to the antimony content of the active material of the catalyst layer with the highest hot spot.

The antimony content of the catalyst layer with the highest hot spot is in a preferred embodiment of the invention in a range of 0 to 5 wt .-%, in particular 1 to 4 wt .-% and particularly preferably from 1, 5 to 3.5 wt .-% (calculated as Sb ₂ O ₃ ).

In a preferred embodiment of the catalyst according to the invention, the antimony content in the catalyst layers, which lie after the catalyst layer with the highest hotspot in the direction of gas exit, initially increases further and is preferably in a range from 0 to 5% by weight and particularly preferably between 1 , 5 to 4 wt .-% (calculated as Sb ₂ O ₃ ). However, it is expressly to include catalysts in which the antimony content of the downstream catalyst layer (s) is less than or equal to the antimony content of the hot-spot catalyst layer.

According to the invention, the last catalyst layer closest to the gas outlet has a reduced antimony content compared with the previous catalyst layers, which is preferably 0 to 4% by weight and more preferably 0 to 3.0% by weight. Thus, the catalyst according to the invention preferably has a structured grading with regard to the antimony content of the catalytically active composition in order to adequately fulfill the different tasks of the individual catalyst layers.

In a preferred embodiment, the catalyst layers following the hot-spot catalyst layer also contain antimony oxide. If only one catalyst layer in the direction of the gas outlet after the layer with the highest hot spot, then preferably the antimony content is equal to or more preferably smaller than the antimony content of the catalyst layer with the highest hot spot.

In a preferred embodiment according to the invention of a catalyst with a total of five catalyst layers (with an upstream catalyst layer with reduced antimony content) and three downstream catalyst layers, the antimony content advantageously increases continuously from the first catalyst layer to the fourth catalyst layer and is then reduced again in the last catalyst layer lying to the gas outlet ,

In a particularly preferred embodiment according to the invention of a catalyst with a total of six catalyst layers (two upstream and three downstream catalyst layers), the antimony content in the first two catalyst layers is identical and increases in stages from the second to the fifth catalyst layer and then decreases again in the last catalyst layer. Here, the different antimony content is used essentially for the selectivity control of the various catalyst layers and not for controlling activity, since antimony oxide has only a small influence on the activity of the catalyst. In a preferred embodiment, all catalyst layers contain 0 to 5 wt .-%, particularly preferably 0 to 3.5 wt .-% antimony oxide. However, those catalysts are also covered by the invention in which the antimony content in the hot-spot catalyst layer is greatest and the downstream catalyst layers partly have a smaller and in some cases a higher antimony content in the active composition than the hot-spot catalyst layer.

As antimony sources are advantageously antimony pentoxide and especially antimony trioxide.

It has been found that particularly good selectivities are achieved in the gas phase oxidation of o-xylene and / or naphthalene to phthalic anhydride, when the vanadium content in the first two upstream catalyst layers in a range of 2.5 to 9 wt .-% and more preferably in a range of 3 to 6 wt .-%. It has also been found that according to a preferred embodiment of the invention, the vanadium content advantageously increases from catalyst layer to catalyst layer from the gas inlet in the direction of the gas outlet.

The content of vanadium pentoxide is advantageously in the catalyst layer with the highest hot spot and the two downstream catalyst layers in a range of 3.5 to 12 wt .-% and particularly advantageously at 4 to 9 wt .-%.

It has been found that it is advantageous to use a high vanadium content, especially in the last gas outlet catalyst layer. Here, the vanadium content is advantageously in a range of 7 to 25 wt .-% and particularly advantageously in a range of 7 to 22 wt .-%, in particular 8 to 17 wt .-%.

In principle, in addition to vanadium pentoxide, the vanadium source is a series of further compounds, such as vanadium oxalate, vanadium halides, metavanadic acid, pyrovanadic acid and vanadium carboxylates, such as vanadium formate, vanadium salicylate or vanadium tartrate. In one embodiment of the invention, vanadium oxalate is preferably used to prepare the catalysts.

More preferably, all catalyst layers of the catalyst according to the invention also contain cesium and / or another element of the first main group in the active composition.

In a preferred embodiment, the cesium content of the upstream or downstream catalyst layers increases to the catalyst layer with the highest hot spot and reaches its maximum in this catalyst layer, and then gradually decrease again in the downstream catalyst layers. The cesium content in the present invention is preferably in a range of 0 to 0.7% by weight, and more preferably 0 to 0.5% by weight. In a particularly preferred embodiment, the cesium content in the upstream catalyst layers is in a range of 0.1 to 0.3 wt .-%.

More preferably, no phosphorus compound is used in the preparation of the active material, especially in the upstream layers. In a preferred embodiment, the phosphorus content gradually increases from the catalyst layer with the highest hot spot downstream to the last catalyst layer, wherein it is particularly preferred that the phosphorus content in the last layer is significantly increased compared to the previous upstream catalyst layer.

It has surprisingly been found that it is advantageous if the catalyst layers from the gas inlet in the direction of gas outlet including the catalyst layer with the highest hot Spot bismuth vanadate included. In a particularly preferred embodiment, in particular the catalyst layer with the highest hot spot contains bismuth vanadate (BiVO ₄ ) in the active composition. The preferred content of bismuth vanadate in the active composition in an embodiment according to the invention is in a range from 0 to 4% by weight and in particular in the range from 0 to 2% by weight and particularly preferably from 0 to 1% by weight. , The use of a bismuth compound in the active composition of catalysts for phthalic anhydride preparation has long been known (EP 0 180 335 A1). In this case, however, preferably bismuth compounds are used, which can be converted during calcination in the corresponding oxides such as Bi ₂ O ₃ .

In one aspect of the invention it has now been found that mixed oxides of bismuth with vanadium have a selectivity enhancing effect on the production of phthalic anhydride by gas phase oxidation. The reason for this has not been finally clarified, but it is reasonable to suppose that a better interaction of the bismuth with the active centers of the catalyst, and here in particular with the present vanadium species, leads to the improvement of the selectivity according to the invention.

According to another aspect of the invention, which is expressly claimed independently of one of the abovementioned aspects, at least one of the catalyst layers, in particular in the catalyst layers up to and including the hot-spot catalyst layer, comprises silver or a silver component, preferably in the form of mixed oxides of silver, most preferably in the form of silver vanadate and / or silver molybdate and / or silver tungstate. In particular, it has been found that in particular the addition of silver vanadate (AgVO ₃ ) in the active mass in one or more catalyst layers from the gas inlet up to and including the catalyst layers with the highest hot spot leads to increase the selectivity. In a preferred embodiment, one or more or all of the catalyst layers mentioned have a silver vandate content of from 0.01 to 4% by weight, in particular from 0.03 to 2.0% by weight and particularly preferably from 0.05 to 1, 0 wt .-% on. The catalyst layers preferably contain at least titanium dioxide and a vanadium compound in the active composition. Particularly preferably, the active composition contains at least one more alkali metal, particularly preferably cesium as a selectivity-enhancing and activity-damping additive.

The silver is attributed a selectivity-enhancing role, namely by the suppression of the total oxidation of o-xylene and / or naphthalene to CO and CO ₂ .

It has now surprisingly been found that a significant improvement in the

Catalysts can be achieved in terms of selectivity, if the invention

Mixed oxide silver metavanadate AgVO ₃ , which is not a so-called silver bronze, is used as an additive in catalyst preparation. Here are In the active composition preferably at least still contain titanium dioxide and a vanadium compound and preferably also cesium and / or another element of the first main group and in the case of the catalyst layer with the highest hot spot additionally an antimony and / or bismuth and / or Phosphorveruindung.

The composition ranges of the active masses in the individual catalyst layers are preferably in the following ranges.

Here, by way of example, a particularly preferred catalyst according to the invention with a total of six catalyst layers is listed, in which the third catalyst layer is the catalyst layer with the highest hot spot:

In a further aspect of the present invention, the remainder of the active composition consists predominantly of titanium dioxide in the anatase modification. In a preferred embodiment, the remainder of the active composition consists of at least 90% by weight of titanium dioxide and in particular of 95% by weight, particularly preferably 100% by weight, of titanium dioxide.

However, in addition to the preferred components in the catalyst according to the invention, depending on the intended use, further components may also be contained in the active material in one or more of the catalyst layers. A number of compounds are known to those skilled in the art (WO 99/61433), which may be suitable to be used in addition to titanium dioxide and vanadium oxide as a constituent of the active composition and optionally have a positive influence on selectivity, activity, heat resistance and service life of the catalysts or as so-called "thinner" in the hot spot area input. In particular, a large number of promoters are described in the prior art, some of which can also be used in the catalysts according to the invention. These include compounds of alkali and alkaline earth metals, thallium, antimony, bismuth, aluminum, zirconium iron, nickel, cobalt, copper, manganese, tin, silver, chromium, molybdenum, tungsten, iridium, tantalum, niobium, arsenic, cerium and phosphorus ,

In particular, the alkali metal compounds act as the activity-reducing and selectivity-increasing promoters, while phosphorus compounds increase the activity but have a negative influence on the selectivity.

In contrast, compounds such as silicon carbide and other carbides and nitrides are suitable as additives for the active mass in the hot-spot region of the catalysts in order to achieve a better control of the reaction, especially at high loadings.

In a preferred embodiment of the catalyst according to the invention, a structuring of the average surface of the titanium dioxide used in the various catalyst layers is also used in order to adjust the activity of the respective catalyst layers advantageous. In this case, an equal or larger mean BET surface area of the titanium dioxide of the at least one upstream catalyst layer is preferred in comparison to the catalyst layer with the largest hot spot. More preferably, the mean surface in the downstream catalyst layers initially decreases again, and then increases again in the last catalyst layer. Accordingly, according to this aspect of the invention, the activity structuring of the catalyst is not achieved exclusively by using titanium dioxides having defined properties, but rather by a combination of partly countercurrent measures known to the person skilled in the art.

It is further known to the person skilled in the art that in order to set the desired mean BET surface area of the titanium dioxide of a particular catalyst layer, only a single type of titanium dioxide having the desired BET surface area and further defined properties can be used and / or a mixture of titanium dioxides having different surfaces is used can be.

In a preferred embodiment of the catalyst according to the invention, different grades of titanium dioxide having different BET surface areas are used to set the desired BET mean surface area for the respective catalyst layers.

An essential component of the catalyst according to the invention is titanium dioxide in the

Anatase modification with a BET surface area of 7 to 50 m ² / g and preferably with a BET surface area of 12 to 30 m ² and particularly preferably with a surface area of 13 to 25 m ^z / g, which preferably has an average particle size of 0.3 ^• 1CT ⁶ to 0.8 ^■ 10 ^"6 m and which is preferably constructed in practice of porous particles, which are composed essentially of aggregated masses of primary particles.

In principle, other titanium dioxides with a different specification with regard to surface area, particle size and particle structure than described here can also be used in the catalyst according to the invention. In this case, one or more types of titanium dioxide with different surface areas and particle sizes and porosities can be used in certain or all catalyst layers.

It is particularly preferred for a catalyst having a total of five or six catalyst layers in which two catalyst layers are located in front of the catalyst layer with the highest hot spot in the direction of gas inlet, the first closest to the gas inlet catalyst layer in comparison to the second catalyst layer a clear lower active material content and / or a lower average BET surface area. Since the BET surface area of the final catalyst depends almost exclusively on the surface of the titanium dioxide species used, in a preferred embodiment the BET surface area of the titanium dioxide used or the average BET surface area of titanium dioxide species of the first catalyst layer is preferably equal to or lower than the BET surface area of the titanium dioxide or the average BET surface area of the titanium dioxide species in the second upstream catalyst layer.

In a preferred embodiment, the mean BET surface area of the titanium dioxide of the third catalyst layer in which the highest hot spot is located is preferably the same size and particularly preferably less than the mean surface area of the second catalyst layer.

According to a further aspect of the invention, the mean BET surface area of the titanium dioxide of the central catalyst layers downstream of the catalyst layer with the highest hot spot is preferably the same or more preferably smaller than the mean BET surface area of the titanium dioxide of the catalyst layer having the highest hot-spot. spot.

In a preferred embodiment, the mean BET surface area of the titanium dioxide of the last catalyst layer closest to the gas outlet is higher than the mean BET surface area of the titanium dioxide of the fourth and fifth catalyst layers, and preferably equal to or more preferably greater than the mean BET surface area of the titanium dioxide all other catalyst layers. Thus, for a particularly preferred catalyst having a total of six catalyst layers and two upstream catalyst layers with reduced antimony content, the following advantageous distribution of the BET surface area results:

OF (LI) </ = OF (L2)> / = OF (UJ)> 0F (L4)> / = OF (L5) <OF (L6)

OF = BET surface area Li = i-th catalyst layer

In a further preferred embodiment of the invention, the following distribution of the BET surface area is selected for a catalyst having a total of five catalyst layers and two upstream catalyst layers:

OF (LI) </ = 0F (L2)> / = 0F (L3)> / = 0F (L4) </ = 0F (L5)

In a further preferred embodiment with a total of five catalyst layers and a catalyst layer located in the direction of the gas inlet and in front of the catalyst layer with the highest hot spot, the following distribution is advantageous:

OF (LI)> / = 0F (L2)> / = 0F (L3)> / = 0F (L4) </ = 0F (L5)

In a further preferred embodiment with a total of four catalyst layers is particularly preferred only one catalyst layer in front of the catalyst layer with the highest hot spot. In this case, the following distribution of the mean BET surface area results advantageously:

OF (LI)> / = 0F (L2)> / = 0F (L3) </ = 0F (L4)

In principle, however, a different distribution of the mean BET surface areas in the catalyst according to the invention is not excluded. Not excluded from the invention are also multi-layer catalysts, the total of only three

Have catalyst layers or more than six catalyst layers. In so-called

Multilayer catalysts can more than two catalyst layers with different

Preceding the composition of the catalyst layer with the highest hot spot and / or be downstream of more than three catalyst layers of the catalyst layer with the highest hot spot.

The catalysts used in the invention are generally shell catalysts in which the catalytically active material in one or more Layers are cupped on an inert carrier. The layer thickness of the catalytically active composition is generally 0.02 to 0.4 mm, more preferably 0.05 to 0.15 mm. In general, however, at least most of the catalyst layers, and preferably the catalyst layers following downstream of the highest Hot Sport catalyst layer, have a homogeneous chemical composition of the active material.

In such coated catalysts, the catalytically active material is usually on a generally inert under reaction conditions, non-porous support material such as quartz, porcelain, magnesium oxide, tin oxide, silicon carbide, rutile, alumina (Al2O3), aluminum silicate, magnesium silicate (steatite), zirconium silicate or mixtures thereof Carrier materials applied.

As inert, non-porous carrier of the active mass balls or in particular rings of steatite have proven particularly useful. In a preferred embodiment of the catalyst according to the invention steatite rings are used with an outer diameter of 5 to 9 mm, a length of 4 to 8 mm and an inner diameter of 3 to 7 mm.

For the preparation of such coated catalysts, various suitable processes are described in the prior art. Reference may be made, for example, to DE-A 1642938 and DE-A 1769998. In this case, an aqueous or organic solvent-containing solution or suspension of the active composition components and / or their precursor compounds is preferably sprayed onto the support material in a heated drum at elevated temperature until the desired active mass fraction of the total catalyst weight is reached. Advantageously, the coating of the inert support can also be carried out in the fluidized bed process (DE 2106796), since the inside of hollow cylinders is also uniformly coated here.

In order to avoid losses when applying the active composition to the inert carrier, the art has been used to advantageously add so-called organic binders, preferably copolymers, to the solution or suspension with the catalytically active constituents in the form of an aqueous dispersion. The binder is added in amounts of about 5 to 20 wt .-% based on the solids content of the catalytically active composition. With the addition of the above-mentioned binders, the coating temperatures are advantageously in a range from 50 to 450 ^° C. and particularly advantageously in a range from 50 to 200 ^° C.

The binder applied to the support material together with the active ingredients is decomposed after the catalyst has been charged during the heating of the reactor and completely removed from the active mass. The binder addition has the additional advantage that the active material firmly adheres to the carrier and there is no loss of active material by abrasion during transport of the catalyst and when the reactor is charged.

The catalysts thus prepared are preferably used for the gas phase oxidation of o-xylene or naphthalene or o-xylene / naphthalene mixtures to phthalic anhydride.

For this purpose, the catalysts to be used according to the invention are filled into the reaction tubes, which are advantageously thermostated from the outside, for example by means of a salt bath, to the reaction temperature. About the thus prepared from several catalyst layers existing catalyst bed, the reaction gas at temperatures of generally from 300 ⁰ C to 45O ⁰ C, preferably at 320 ° C to 400 ⁰ C and more preferably at 330 ° C to 390 ⁰ C, at an overpressure of 0.1 to 2.5 bar, preferably from 0.3 to 1, 5 bar at a space velocity of generally 750 to 5000 1 / h passed.

The reaction gas supplied to the catalyst is generally mixed by mixing a molecular oxygen-containing gas, preferably air, which may contain, besides oxygen, still suitable reaction moderators and / or diluents such as steam, carbon dioxide, sulfur dioxide and / or nitrogen with the aromatic to be oxidized Hydrocarbon generated, wherein the molecular oxygen-containing gas is generally 1 to 100 vol .-%, preferably 2 to 50 vol .-% and particularly preferably 10 to 30 vol .-% oxygen, 0 to 30 vol .-% and preferably 0 to 10% by volume of water vapor, and 0 to 50% by volume, preferably 0 to 1% by volume of carbon dioxide, may contain nitrogen. To generate the reaction gas, the gas containing the molecular oxygen is generally charged with 25 to 140 grams per Nm ^{3 of} the hydrocarbon to be oxidized.

The catalysts according to the invention are temperature-treated or calcined in the customary manner before use. It has proved to be advantageous if the catalyst for at least 6 h at least 390 ⁰ C, especially between 12 and 24 h at 400 ⁰ C to 430 ⁰ C in a molecular oxygen-containing gas, in particular air is conditioned. It is advantageous if the amount of air used in a range of 0.05 to 1 Nm ³ per tube and hour.

The determination of parameters such as BET surface area, particle size determination and

Catalyst activity of the catalyst according to the invention are familiar to the person skilled in the art. Here, for example, reference can be made to WO 2006/092304. In particular, the measurement of the activity of a catalyst must be carried out under reasonable operating conditions (Salt bath temperature) and can be used for comparison purposes only after the catalyst to be considered has reached its final maximum activity.

When using the catalysts according to the invention in particular phthalic anhydride of o-xylene and / or naphthalene can be prepared with high selectivities and good qualities at the same time high loading. The catalysts of the invention are stable and allow for an economical, industrial production of phthalic anhydride over a long time.

The invention will be explained in more detail with reference to the following, the invention not limiting examples.

Examples

Preparation of the catalysts

The various components of the active composition and / or their course compounds are added successively as solutions and / or as a powder in deionized water and the resulting suspension is stirred advantageously for at least 12 h. Titanium dioxide, vanadium oxalate, cesium sulfate, ammonium dihydrogen phosphate, antimony trioxide, bismuth vanadate and silver vanadate are advantageously used as sources for the components containing the catalyst according to the invention.

Thereafter, an organic binder in the form of an aqueous dispersion of a vinyl acetate copolymer is added to the aqueous suspension and the total of about 20 to 25% suspension is stirred for a further 30 minutes.

Subsequently, an appropriate amount of the aqueous suspension which contains the active substances and / or their precursor compounds and the organic binder is applied by spraying to the inert support (steatite rings with 7x7x4 mm or 8x6x5 mm dimension) to a certain amount of the adhesive-containing suspension is applied to the rings, so that after the calcination, the stated in the examples active mass content.

The active material content or content (proportion of the catalytically active substances without binder) in each case refers to the proportion of the catalytically active composition in the total weight of the catalyst, including the support in the respective catalyst layer, measured after four hours of calcination at 400 ^° C. The details of the BET surface areas of the catalysts or catalyst layers relate to the average BET surface area of the titanium dioxide materials used (dried in vacuo at 150 ^° C.). It is known to the person skilled in the art that the BET surface area of a catalyst is essentially determined by the BET surface area of the TiO.sub.2 used and this is modified to some extent by the addition of further catalytically active components, depending on the amount and surface area of the additional components.

The stated content of phosphorus in the examples refers to the amount of a phosphorus component added in the production of the suspension. It is known to those skilled in the art that the actual content of phosphorus in the active composition may vary depending on how much the TiO 2 used is contaminated with phosphorus.

oxidation reaction

In 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 lapped by a salt bath, the relevant multilayer catalyst system is filled, wherein in the examples described in each case the L1 layer next to the gas inlet and the L4 layer or the L5 layer is closest to the gas outlet. In the reaction tube was centrally arranged a thermowell of 2 mm diameter with built-in tension element for temperature measurement. 4 Nm ^{3 of} air with a loading of about 30 to 70 g-xylene / Nm ^{3 of} air are passed through the reaction tube hourly from top to bottom at a salt bath temperature of 340 to 380 ⁰ C.

In order to determine the catalytic performance data, the reaction gas leaving 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 only partially precipitate.

The raw PSA deposited in the condensates is melted by means of hot oil, collected, weighed and then the content of phthalic anhydride is determined by GC analysis.

The raw PSA yield given in the examples is calculated as follows:

[Amount of raw PSA (g) x purity of raw PSA (%)]

Raw PSA yield (wt%) = -

[Feed o-xylene (g) x purity o-xylene (%)] The yield of PSA thus determined, despite the deduction of the byproducts contained therein, is referred to as the crude PSA yield, since the pure PSA is generally termed a product obtained after the thermal pretreatment and distillative workup. This is also obvious to the person skilled in the art.

Since the conversion of o-xylene in each case is almost 100%, the raw yield thus determined corresponds directly to the selectivity of the catalyst system.

Example 1 (comparative example)

Catalyst A (5 layers)

At a loading of 58 to 61 g of o-xylene per Nm ^{3 of} air and a total amount of air of 4 Nm ³ per hour and 350 to 354 ⁰ C SBT of the catalyst A described in Example 1 was tested.

In this case, an average raw PSA yield (based on 100% o-xylene purity) of 113.1 wt .-% was achieved after the run-in phase and the phthalide content in the raw PSA was 0.01 wt .- %%. Example 2 (according to the invention):

Catalyst B (5 layers)

At a loading of 58 to 61 g of o-xylene per Nm ^{3 of} air and a total air flow of 4 Nm ³ per hour and 347 to 350 ⁰ C SBT the catalyst B described in Example 2 was tested.

In this case, an average raw PSA yield (based on 100% o-xylene purity) was achieved after the run-in phase of 113.8% by weight and the phthalide content in the crude PSA was 0.02-0.03% by weight. %

Example 3 (according to the invention)

Catalyst C (5 layers)

At a loading of 58 to 63 g of o-xylene per Nm ^{3 of} air and a total amount of air of 4 Nm ³ per hour and 345 to 347 ⁰ C SBT of the catalyst C described in Example 3 was tested.

In this case, an average raw PSA yield (based on 100% o-xylene purity) was achieved after the run-in phase of 114.90% by weight, and the phthalide content in the crude PSA was 0.03% by weight.

Example 4 (according to the invention):

Catalyst D (5 layers)

At a loading of 58 to 64 g of o-xylene per Nm ^{3 of} air and a total air flow of 4.0 Nm ³ per hour and 346 to 347 ° SBT of the catalyst D described in Example 4 was tested.

In this case, an average raw PSA yield (based on 100% o-xylene purity) was achieved after the run-in phase of 115.0% by weight, and the phthalide content in the crude PSA was 0.03% by weight.

Example 5 (according to the invention):

Catalyst E (5 layers)

At a loading of 58 to 64 g of o-xylene per Nm ^{3 of} air and a total air flow of 4.0 Nm ³ per hour and 346 to 347 ° C SBT of the catalyst described in Example 5 E was tested.

In this case, an average raw PSA yield (based on 100% o-xylene purity) was achieved after the run-in phase of 115.7 wt .-% and the phthalide content in the raw PSA was 0.02 wt .-%.

Example 6 (Comparative Example)

Catalyst F (4 layers)

In fact, this catalyst system is a multi-layer system with four different layers. Here, the catalyst system can also be considered as a fictitious five-layer system, wherein the first lying at the gas inlet layer is divided into two layers of identical composition

At a loading of 50 to 65 g of o-xylene per Nm ^{3 of} air and a total air flow of 4.0 Nm ³ per hour and 344 to 347 ° C SBT of the catalyst F described in Example 6 was tested.

In this case, an average raw PSA yield (based on 100% o-xylene purity) was achieved after the break-in of 113.4 wt .-% and the phthalide content in the raw PSA was 0.01 wt .-%.

Example 7 (according to the invention)

Catalyst G (4 layers)

At a loading of 58 to 63 g of o-xylene per Nm ^{3 of} air and a total air flow of 4.0 Nm ³ per hour and 346 to 352 ° C SBT of the catalyst G described in Example 7 was tested.

In this case, an average raw PSA yield (based on 100% o-xylene purity) was achieved after the run-in phase of 114.0% by weight, and the phthalide content in the crude PSA was 0.06% by weight.

Example 8 (according to the invention):

Catalyst H (4 layers)

At a loading of 58 to 65 g of o-xylene per Nm ^{3 of} air and a total air flow of 4.0 Nm ³ per hour and 346 to 347 ° C SBT of the catalyst H described in Example 8 was tested.

In this case, an average raw PSA yield (based on 100% o-xylene purity) was achieved after the run-in phase of 115.3 wt .-% and the phthalide content in the raw PSA was 0.04 wt .-%.

Table ^* ! :

Performance data of the catalysts of the invention

From the comparison of Examples 2 to 5 and Comparative Example 1 it can be seen that the reduction of antimony in the layer with the highest hot spot upstream layer or layers a significant improvement in selectivity is achieved.

From the comparison of Examples 7 and 8 with Comparative Example 6 it is apparent that the use of silver vanadate markedly increases the selectivity of the catalysts according to the invention.

Claims

claims 1. Catalyst for the catalytic gas phase oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and / or carboxylic anhydrides, in particular to phthalic anhydride, with a plurality of different catalyst layers, wherein the catalyst is a hot-spot catalyst layer, in relation to the total catalyst, the largest local temperature maximum occurs , at least one catalyst layer upstream of the hot-spot catalyst layer and preferably at least one catalyst layer downstream of the hot-spot catalyst layer, characterized, in the case of antimony and / or bismuth and / or niobium in the catalytically active mass of the hot-spot catalyst layer, the content of at least one of these elements in the catalytically active composition of at least one upstream catalyst layer compared to the content of the same element in the catalytically active composition the hot spot catalyst layer is reduced by a predetermined value. 2. Catalyst according to claim 1, characterized in that the antimony, in particular Antimonoxidgehalt in the at least one upstream Catalyst layer is reduced by 20 to 100 wt .-%, preferably by 40 to 100 wt .-% compared to the antimony content in the hot-spot catalyst layer. 3. A catalyst according to claim 1 or 2, characterized in that the largest local temperature maximum throughout the catalyst having hot-spot catalyst layer is at least one catalyst layer upstream in the direction of the gas inlet side, wherein a) in the case of a single upstream catalyst layer, this one catalyst layer has an antimony content of the catalytically active material, compared to the Antimony content of the catalytically active mass of the hot-spot catalyst layer is reduced by a predetermined value, or b) in the case of several upstream catalyst layers, the antimony content of the catalytically active material of at least one of the upstream catalyst layers is reduced by a predetermined value compared to the antimony content of the catalytically active material of the hot-spot catalyst layer. 4. Catalyst according to claim 3, characterized in that in case a) the only upstream catalyst layer or in case b) at least one of a reduced antimony content having upstream catalyst layers has an active material content which is less than or equal to the active material content of the hot-spot catalyst layer , 5. Catalyst, in particular according to one of claims 1 to 4, characterized in that the catalytically active composition of one or more catalyst layers in two or more layers on a support, in particular a ceramic support is applied. 6. A catalyst according to claim 5, characterized in that the antimony content of at least one coating layer of at least one upstream catalyst layer by a predetermined value, preferably by 20 to 100%, compared to the antimony content of the coating layer with the highest antimony content in the hot spot -Catalyst layer is reduced. 7. A catalyst according to claim 5 or 6, characterized in that the hot-spot catalyst layer has at least two coating layers, wherein at least one of the coating layers by a predetermined value, preferably by 20 to 100%, reduced antimony content of the active material compared to that coating layer having the highest antimony content. 8. Catalyst, in particular according to one of claims 1 to 7, characterized in that in the hot-spot catalyst layer, a mixture of at least two different in particular with respect to the composition of the catalytically active composition catalysts is used, wherein the antimony content, in particular the antimony oxide Content, the catalytically active composition of at least one catalyst of the hot-spot catalyst layer by a predetermined value, preferably by 20 to 100%, compared to the antimony content of the catalytically active material of that Catalyst is reduced, which has the highest antimony content in the hot-spot catalyst layer. 9. Catalyst, in particular according to one of claims 1 to 8, characterized in that upstream in at least one of the hot-spot catalyst layer Catalyst layer, a mixture of at least two different in particular with respect to the composition of the catalytically active composition different catalysts is used, wherein the antimony content, in particular the antimony oxide content of the catalytically active composition of at least one catalyst of this upstream catalyst layer by a predetermined value, preferably by 20 to 100% is reduced compared to the antimony content of the catalytically active material of the catalyst having the highest antimony content in the hot-spot catalyst layer. 10. Catalyst according to one of claims 1 to 9, characterized in that the individual catalyst layers have a different chemical composition and / or different physical properties and / or the individual catalyst layers differ by a different active material content. 11. A catalyst according to any one of claims 1 to 10, characterized in that at least one of the reduced antimony content having upstream catalyst layers has a phosphorus content of 0 to 0.3 wt .-% based on the catalytically active composition of this catalyst layer. 12. Catalyst according to one of claims 1 to 11, characterized in that the phosphorus content of the individual phosphorus, catalyst layers gradually increases from the gas inlet to the gas outlet from catalyst layer to catalyst layer or unsteadily to form catalyst layer groups with approximately the same phosphorus content. 13. A catalyst according to any one of claims 1 to 12, characterized in that at least one of the upstream catalyst layers with a reduced antimony content vanadium, in particular a vanadium oxide content of 0 to 9 wt .-% (calculated as V ₂ O ₅ ) based on the catalytically active Mass of this catalyst layer having. 14. Catalyst according to one of claims 1 to 13, characterized in that the vanadium, in particular vanadium oxide content of the individual catalyst layers from the gas inlet to the gas outlet gradually increases from catalyst layer to catalyst layer, in particular starting from a directly before the hot spot Catalyst layer lying catalyst layer gradually increases. 15. A catalyst according to any one of claims 1 to 14, characterized in that at least one of the upstream catalyst layers with a reduced antimony content an alkali, in particular a cesium content of 0 to 0.6 wt .-% (calculated as alkali metal), preferably from 0 to 0.3 wt .-% (calculated as the alkali metal), based on the catalytically active composition of this catalyst layer. 16. A catalyst according to any one of claims 1 to 15, characterized in that the alkali, in particular cesium content in the catalytically active material in at least one of the upstream catalyst layers with a reduced antimony content is lower than the alkali, in particular cesium content in the hot spot - Catalyst layer. 17. Catalyst according to one of claims 1 to 16, characterized in that the alkali content, in particular the cesium content, in the hot-spot catalyst layer is greatest and decreases in the downstream in the direction of gas outlet catalyst layers gradually from catalyst layer to catalyst layer. 18. Catalyst according to one of claims 1 to 17, characterized in that at least one of all existing catalyst layers has an alkali content (calculated as alkali metal), in particular cesium content, in the catalytically active composition of greater than 0.15 wt .-%. 19. A catalyst according to any one of claims 1 to 18, characterized in that at least one of all existing catalyst layers comprises phosphorus in the catalytically active composition, in particular in the form of an added during the preparation and / or present as an impurity phosphorus compound. 20. A catalyst according to any one of claims 1 to 19, characterized in that during the catalyst preparation at least one bismuth compound is added and / or at least one of the catalyst layers bismuth, bismuth in particular in the form of bismuth vanadate, preferably bismuth vanadate in the range of 0.01 to 4 wt .-% (calculated as Bi) based on the catalytically active composition is added or is present. 21. Catalyst, in particular according to one of claims 1 to 20, characterized in that at least one of the catalyst layers, in particular at least one of the upstream catalyst layers and / or the hot-spot catalyst layer, silver or a silver content, preferably in the form of mixed oxides of silver , most preferably in the form of silver metavanadate (AgVO ₃ ) and / or silver molybdate (Ag ₂ MoO ₄ ) and / or silver tungstate (Ag ₂ WO ₄ ), and / or at least one or more of these compounds are used as a source of raw material in catalyst preparation , 22. A catalyst according to claim 21, characterized in that the proportion of silver in the respective catalyst layer in the range of 0.01 to 5 wt .-% (calculated as Ag) based on the catalytically active composition of this catalyst layer. 23. A catalyst according to any one of claims 1 to 22, characterized in that antimony oxide having an average particle size of 0.5 to 5 ^• 10 ^"6 m is used as antimony source, in particular antimony trioxide. 24. Catalyst according to one of claims 1 to 23, characterized in that in the case of several of the hot-spot catalyst layer upstream catalyst layers the Level of that upstream catalyst layer, which compared with the hot-spot catalyst layer has the most reduced antimony content of all upstream catalyst layers and also a lower catalytically active mass than the hot-spot catalyst layer having the largest level of all upstream catalysts. 25. Catalyst according to one of claims 1 to 24, characterized in that in two of the hot-spot catalyst layer in the direction of gas inlet upstream catalyst layers, the activity of the closest to the gas inlet located first upstream catalyst layer for a smaller than the activity of it in Direction to the hot-spot catalyst layer subsequent subsequent second catalyst layer and on the other hand is smaller than the activity of the hot-spot catalyst layer. 26. A catalyst according to claim 25, characterized in that the activity of the second upstream catalyst layer is equal to or greater than the activity of the hot-spot catalyst layer. 27. A catalyst according to any one of claims 1 to 26, characterized in that the hot-spot catalyst layer is at least one catalyst layer downstream of the gas outlet, wherein at least one of these at least one downstream catalyst layer has a higher activity than the hot-spot catalyst layer. 28. A catalyst according to claim 27, characterized in that the hot-spot catalyst layer downstream of several catalyst layers are deposited in the direction of gas outlet, whose activity increases from the hot-spot catalyst layer to lying closest to the gas outlet catalyst layer of catalyst layer to catalyst layer. 29. A catalyst according to any one of claims 1 to 28, characterized in that in a single upstream in the direction of gas inlet catalyst layer at least three, preferably four, most preferably five catalyst layers are present. 30. Catalyst according to one of claims 1 to 29, characterized in that in a single upstream catalyst layer, the filling level of this catalyst layer is 10 to 90 cm. 31. A catalyst according to any one of claims 1 to 28, characterized in that there are at least four, preferably five, most preferably six catalyst layers at two upstream catalyst layers in the direction of gas inlet. 32. Catalyst according to one of claims 1 to 31, characterized in that in a total of two upstream catalyst layers, the entire level of the two upstream layers is in a range of 10 to 90 cm, wherein the filling level of the closest to the gas inlet upstream catalyst layer 5 bis 45 cm and the gas outlet in the following upstream catalyst layer has a filling height of 5 to 85 cm. 33. Catalyst according to one of claims 1 to 32, characterized in that the filling height of the hot-spot catalyst layer is 50 to 170 cm. 34. A catalyst according to any one of claims 1 to 33, characterized in that at a total of two of the hot-spot catalyst layer in the direction of gas outlet downstream catalyst layers, the filling level of the two downstream catalyst layers is in each case in a range of 30 to 120 cm. 35. A catalyst according to any one of claims 1 to 33, characterized in that at a total of three of the hot-spot catalyst layer downstream catalyst layers, the filling height of the first downstream gas outlet catalyst layer is 25 to 90 cm, the filling level of the second in the direction of gas outlet downstream catalyst layer 35 to 110 cm and the filling level of the third downstream and closest to the gas outlet catalyst layer is 35 to 110 cm. 36. Catalyst according to one of claims 1 to 35, characterized in that in a catalyst with a total of five catalyst layers in one of the hot-spot catalyst layer upstream catalyst layer and three of the hot-spot catalyst layer downstream catalyst layers, the active material content of the first to fourth catalyst layer rises and in the last, lying closest to the gas outlet Catalyst layer, the active material content is less than the active material content in the immediately preceding catalyst layer and / or in the hot-spot catalyst layer. 37. A catalyst according to any one of claims 1 to 36, characterized in that in a catalyst with a total of six catalyst layers upstream of two hot-spot catalyst layer and three of the hot-spot catalyst layer downstream catalyst layers, the active material from the first to the fifth catalyst layer increases and the last lying in the direction of gas outlet catalyst layer has a lower active mass content than the immediately preceding catalyst layer and / or the hot-spot catalyst layer. 38. Catalyst according to one of claims 1 to 37, characterized in that the catalytically active composition of the catalyst layers vanadium, in particular vanadium oxide, and / or titanium dioxide and / or antimony, in particular antimony oxide comprises. 39. A catalyst according to any one of claims 1 to 38, characterized in that in at least one of the catalyst layers more at least with respect to the BET surfaces differing titanium dioxide varieties are present in the catalytically active material, so that they have an average BET surface area of the titanium dioxide as the BET surface of titanium dioxide. 40. Catalyst according to claim 38 or 39, characterized in that the BET surface area of the titanium dioxide in the hot spot catalyst layer immediately upstream catalyst layer is equal to or greater than the BET surface area of titanium dioxide in the hot spot catalyst layer. 41. A catalyst according to any one of claims 38 to 40, characterized in that the BET surface of the titanium dioxide is at least in one of the hot-spot catalyst layer subsequent catalyst layers smaller than the BET surface of titanium dioxide in the hot-spot catalyst layer. 42. A catalyst according to any one of claims 38 to 41, characterized in that the BET surface area of the titanium dioxide of the catalyst layer lying closest to the gas outlet is greater than the BET surface area of the titanium dioxide of the upstream catalyst layer. 43. Catalyst according to one of claims 1 to 42, characterized in that in at least one of the catalyst layers, a titanium dioxide variety is used in the anatase modification, which preferably has an average particle size of 0.3 ^• 10 ^'6 to 0.8 ^• 10 ^{' 6} m and a BET surface area of 13 to 50 m ² / g. 44. A catalyst according to any one of claims 38 to 43, characterized in that in at least one of the catalyst layers, a titanium dioxide variety is used in the Anastasmodifikation having a BET surface area of 2 to 15 m ² / g. 45. Catalyst according to one of claims 1 to 44, characterized in that the catalytically active composition of each individual catalyst layer at least 1 to 30% by weight of vanadium pentoxide (V ₂ O ₅ ) 0 to 5% by weight of antimony trioxide (Sb ₂ O ₃ ) 0 to 5% by weight of silver metavanadate (AgVO ₃ ) 0 to 4% by weight bismuth vanadate (BiVO ₄ ) 0 to 4% by weight of niobium pentoxide (Nb ₂ O ₅ ) 0 to 1% by weight cesium (Cs) 0 to 2% by weight phosphorus (P) contains, wherein the remainder of the catalytically active composition to at least 90 Gew. -%, prefers to 95 Gew. -%, particularly prefers to 99 Gew. -% and highly prefers to 100 Gew. -% by Titand dioxide is formed. 46. Catalyst according to claim 45, characterized in that the BET surface area of the titanium dioxide used in at least one catalyst layer, optionally also on average, is between 7 and 50 m ² / g. 47. A catalyst according to any one of claims 1 to 46, characterized in that the proportion of the catalytically active material in the total weight of the catalyst in at least one catalyst layer is between 2 and 20 wt .-%. 48. Catalyst according to one of claims 1 to 47, characterized in that the at least one of the hot-spot catalyst layer upstream in the direction of gas inlet Catalyst layer has an active composition content of about 2 to 15 wt .-%, preferably 3 to 11 wt .-%, wherein preferably the catalytically active composition has a content of From 2 to 12% by weight, preferably from 2.5 to 9% by weight of vanadium oxide (calculated as V ₂ O ₅ ), in particular vanadium pentoxide, 0 to 4 wt .-%, preferably 0 to 2 wt .-%, most preferably 0 to 1 wt .-% antimony oxide (calculated as Sb ₂ O ₃ ), in particular antimony trioxide, 0 to 0.6 wt .-%, preferably 0.1 to 0.5 wt .-% alkali (calculated as alkali metal), in particular cesium, 0 to 0.6 wt.%, Preferably 0 to 0.3 wt.%, Most preferably 0 to 0.1 wt.% Phosphorus (calculated as P), 0 to 4 wt .-%, preferably 0 to 2 wt .-% silver (calculated as Ag) ₁ in particular silver metavanadate, 0 to 4 wt .-%, preferably 0 to 2 wt .-% bismuth (calculated as Bi), in particular bismuth vanadate, 0 to 4 wt .-%, preferably 0 to 1 wt .-% niobium oxide (calculated as Nb ₂ O ₅ ), in particular niobium pentoxide, with the remainder being titanium dioxide. 49. Catalyst according to one of claims 1 to 48, characterized in that the hot-spot catalyst layer has an active material content of 5 to 15% by weight, preferably from 6 to 11 wt .-%, wherein preferably the catalytically active material one Salary of 3 to 12 wt .-%, preferably 4 to 10 wt .-% vanadium oxide (calculated as V ₂ O ₅ ), in particular vanadium pentoxide, 0 to 5 wt .-%, preferably 1 to 3.5 wt .-% antimony oxide (calculated as Sb ₂ O ₃ ), in particular antimony trioxide, 0 to 0.8 wt .-%, preferably 0.2 to 0.5 wt .-% alkali (calculated as alkali metal), in particular cesium, 0 to 0.6% by weight, preferably 0 to 0.3% by weight of phosphorus (calculated as P), From 0 to 4.0% by weight, preferably from 0 to 2.0% by weight of bismuth (calculated as Bi), in particular bismuth vanadate, 0 to 4 wt .-% niobium oxide (calculated as Nb ₂ O ₅ ), in particular niobium pentoxide, 0 to 5.0 wt.%, Preferably 0 to 3.0 wt.% Silver (calculated as Ag), preferably silver metavanadate, with the remainder being titanium dioxide. 50. Catalyst according to one of claims 1 to 49, characterized in that the first of the hot-spot Katalysatorlage immitteibar πac'ngeiagerte catalyst layer has an active material content of about 5 to 15 wt .-%, in particular from 6 to 12 wt .-% , wherein preferably the catalytically active material has a content of 5 to 15% by weight of vanadium oxide (calculated as V ₂ O ₅ ), in particular vanadium pentoxide, 0 to 4% by weight of antimony oxide (calculated as Sb ₂ O ₃ ), in particular antimony trioxide, 0 to 0.5% by weight of alkali (calculated as alkali metal), in particular cesium, 0 to 2% by weight of silver (calculated as Ag), in particular silver metavanadate, 0 to 0.8% phosphorus (calculated as P), 0 to 2 wt .-% bismuth (calculated as Bi), in particular bismuth vanadate, with the remainder being titanium dioxide. 51. Catalyst according to one of claims 1 to 50, characterized in that a second, the hot-spot catalyst layer downstream layer, which is not closest to the gas outlet, an active material content of 5 to 15 wt .-%, in particular of 6 to 12 wt .-%, wherein preferably the catalytically active Mass a salary of From 6 to 15% by weight of vanadium oxide (calculated as V ₂ O ₅ ), in particular vanadium pentoxide, 0 to 4 wt .-% antimony oxide (calculated as Sb ₂ O ₃ ), in particular antimony trioxide, 0 to 0.4% by weight of alkali (calculated as alkali metal), in particular cesium, 0 to 0.8% by weight of phosphorus (calculated as P) with the remainder being titanium dioxide. 52. Catalyst according to one of claims 1 to 51, characterized in that the catalyst layer lying closest to the gas outlet has an active composition content of 4 to 13 wt .-%, preferably from 5 to 11 wt .-%, wherein preferably the catalytically active composition has a content of 7 to 25% by weight of vanadium oxide (calculated as V ₂ O ₅ ), in particular vanadium pentoxide, 0 to 3.5 wt .-% antimony oxide (calculated as Sb ₂ O ₃ ), in particular antimony trioxide, O to 0.3% by weight of alkali (calculated as alkali metal), in particular cesium, 0 to 2% by weight of phosphorus (calculated as P) with the remainder being titanium dioxide. 53. Catalyst according to one of claims 1 to 52, characterized in that the catalytically active composition is applied to an inert, non-porous carrier material, in particular steatite. 54. Catalyst according to one of claims 1 to 53, 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. 55. A process for preparing a catalyst according to any one of claims 1 to 54, characterized in that the catalytically active composition is applied in a fluidized bed or preferably in a fluidized bed to the inert carrier. 56. The method according to claim 55, characterized in that the catalyst is calcined in the reaction space for at least 6 h at temperatures of 380 to 430 ⁰ C in a molecular oxygen-containing gas, in particular air prior to commissioning. 57. A process for the preparation of phthalic anhydride, wherein a gaseous substance containing o-xylene and / or naphthalene and at least molecular oxygen, at elevated temperature via a multilayer catalyst, in particular a catalyst according to one of claims 1 to 54, passes.