JP5879342B2 - Multilayer catalyst for producing phthalic anhydride and method for producing phthalic anhydride - Google Patents

Description

  The present invention relates to a multilayer catalyst for producing phthalic anhydride having a plurality of catalyst layers arranged continuously in a reaction tube, each catalyst layer having an alkali metal content that decreases in the flow direction. . The present invention further provides a method for oxidizing naphthalene or o-xylene / naphthalene mixture through such a multilayer catalyst, and a method for using such multilayer catalyst to oxidize naphthalene or o-xylene / naphthalene mixture to phthalic anhydride. About.

  Many carboxylic acids and / or carboxylic anhydrides are produced industrially in a fixed bed reactor by catalytic gas phase oxidation of hydrocarbons such as benzene, xylene, naphthalene, toluene or durene. Thus, for example, benzoic acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid or pyromellitic anhydride can be obtained. In general, the oxygen-containing gas and the starting material to be oxidized pass through a tube in which a catalyst bed is present. In order to adjust the temperature, the tube is covered with a heat transfer medium such as a molten salt.

  Catalysts found to be suitable for these oxidation reactions are coated catalysts in which the catalytically active composition is applied in the form of a shell to an inert support material such as steatite. In general, titanium dioxide and vanadium pentoxide are used as catalytically active components in the catalytically active compositions of these coated catalysts. In addition, small amounts of many other oxide compounds that act as promoters that affect catalyst activity and selectivity can be included in the catalytically active composition.

  It has been found to be particularly advantageous to use different catalysts in different catalyst layers with respect to their catalytic activity and / or the chemistry of their active composition. When two reaction zones are used, the catalyst preferably used in the first reaction zone, ie the reaction zone closest to the reaction gas inlet, is the second reaction zone, ie the reaction zone closest to the reaction gas outlet. It has a somewhat lower catalytic activity than the catalyst present in In general, the reaction is controlled by a temperature setting such that most of the aromatic hydrocarbons contained in the reaction gas react with maximum yield in the first zone. It is preferred to use a catalyst system of 3 to 5 layers, in particular a catalyst system of 3 and 4 layers.

  The oxidation of o-xylene to phthalic anhydride (PAn) through a vanadium oxide / titanium dioxide catalyst system is usually carried out with an air flow of about 4 standard cubic meters per hour and an o-xylene load of up to 100 g per standard cubic meter. . In order to oxidize an o-xylene / naphthalene mixture, the catalyst is generally developed so that the catalyst is very suitable for a specific o-xylene / naphthalene mixture ratio or a narrow range of o-xylene / naphthalene mixture ratios. ing. If the o-xylene / naphthalene ratio is changed significantly, the yield of PAn is greatly reduced, the product quality is significantly deteriorated, and / or the catalyst's proper working life is adversely affected. This is particularly noticeable at high loads of o-xylene or naphthalene. The higher the total load of o-xylene and naphthalene, the narrower the tolerance range for the o-xylene / naphthalene ratio.

  EP 539878 describes a process for oxidizing an o-xylene / naphthalene mixture through a two-layer catalyst. A mass ratio of 10/90 to 90/10 is used, and the maximum total load in a single pass is 70 grams per standard cubic meter with a gas hourly space velocity (GHSV) of 3000 per hour. Depending on the catalyst and the o-xylene / naphthalene mixing ratio, the PAn yield is in the range of 98.5-111.5% by weight.

  In EP744214, a PAn yield of only 101% by weight was achieved with a naphthalene load of 80 g per standard cubic meter and 4 standard cubic meters of air per hour.

  In the case of a two-layer catalyst as described in EP 1082317, a PAn yield of 110% by weight is 65-80 g per standard cubic meter, a 75% / 25% by weight o-xylene / naphthalene mixture and 4 standards per hour. Achieved in cubic meters of air. The o-xylene / naphthalene ratio was not changed.

  In EP286448, a two-layer catalyst was operated at 3000 GHSV per hour using 70 g of naphthalene per standard cubic meter. However, the o-xylene / naphthalene ratio only changed from 100: 0 to 50:50 or from 50:50 to 0: 100 for the individual catalysts. Extensive variations in the mixing ratio using the same catalyst are not described.

  A catalyst with more than two catalyst layers is described to oxidize o-xylene to phthalic anhydride at 4 standard cubic meters of air per hour and even at very high loads of up to 100 g o-xylene per standard cubic meter. Yes. As described in EP 1084115, a three-layer catalyst system for the oxidation of o-xylene to PAn is mentioned as an example. However, these catalysts are suitable for the oxidation of o-xylene / naphthalene mixtures at about 4 standard cubic meters of air per hour, at a total load of at least 80 g per standard cubic meter, and with a wide variation in the o-xylene / naphthalene ratio. It is not suitable.

  There is a continuing need for catalysts for gas phase oxidation that provide very high selectivity and very high conversion.

EP 539878 EP744214 EP1082317 EP286448 EP1084115

  The object of the present invention is a catalyst for the oxidation of naphthalene or o-xylene / naphthalene mixtures with a total load of at least 80 g per standard cubic meter at about 4 standard cubic meters of air per hour, with a high PAn yield and good production It is to develop a catalyst in which the o-xylene / naphthalene ratio can vary over a wide range in terms of product quality.

  The purpose is a multilayer catalyst for oxidizing naphthalene or an o-xylene / naphthalene mixture to phthalic anhydride, each catalyst layer comprising vanadium oxide and titanium dioxide, and the alkali metal content of the catalyst layer being in the direction of flow. This is achieved by a multilayer catalyst that decreases from layer to layer.

Accordingly, the present invention is a multilayer catalyst for oxidizing naphthalene or an o-xylene / naphthalene mixture to phthalic anhydride, comprising at least three catalyst layers,
Each layer comprises vanadium oxide and titanium dioxide, and a) the alkali metal content of one catalyst layer A is the highest,
b) The catalyst layer Z that follows the catalyst layer A in the flow direction has an alkali metal content of 0 to 10% of the alkali metal content of the catalyst layer A, and c) is located between the catalyst layers A and Z Alkali metal selected such that the catalyst layer has an alkali metal content of 30-90% of the alkali metal content of catalyst layer A, and the alkali metal content of each layer is higher than the alkali metal content of the next layer in the flow direction A multilayer catalyst having a content is provided.

  In a preferred embodiment of the invention, the multilayer catalyst layer has 3, 4 or 5 layers. Particular preference is given to three and four layer catalysts.

  The multilayer catalyst according to the invention can be used in combination with suitable upstream and / or downstream beds and intermediate layers as appropriate, for example in order to avoid high hot spot temperatures, the upstream and / or downstream beds thereof. As well, the intermediate layer can include materials that are generally catalytically inert or less active.

A further preferred embodiment of the present invention is a four-layer catalyst for oxidizing naphthalene or o-xylene / naphthalene mixture to phthalic anhydride, each catalyst layer comprising vanadium oxide and titanium dioxide, and the catalyst layer A) the alkali metal content of one catalyst layer A is the highest,
b) Catalyst layer B following catalyst layer A in the flow direction has an alkali metal content of 60 to 90% of the alkali metal content of catalyst layer A;
c) the catalyst layer C following the catalyst layer B in the flow direction has an alkali metal content of 30 to 59% of the alkali metal content of the catalyst layer A;
d) A four-layer catalyst selected such that the catalyst layer Z following the catalyst layer C in the flow direction has an alkali metal content of 0 to 10% of the alkali metal content of the catalyst layer A.

  The catalyst according to the present invention is generally a coated catalyst in which a catalytically active composition is applied to an inert support material in the form of a shell.

Inert support materials include quartz (SiO ₂ ), porcelain, which can be advantageously used in the production of coated catalysts for oxidizing aromatic hydrocarbons to aldehydes, carboxylic acids and / or carboxylic anhydrides. , Magnesium oxide, tin dioxide, silicon carbide, rutile, alumina (Al ₂ O ₃ ), aluminum silicate, steatite (magnesium silicate), zirconium silicate, cerium silicate or a mixture of these carrier materials, etc. All the carrier materials of the prior art can be used. The catalyst support may be used in the form of, for example, a spherical, ring-shaped, pellet-shaped, spiral-shaped, tubular, extrudate-shaped or pulverized material. The dimensions of these catalyst supports correspond to the catalyst supports conventionally used for the production of coated catalysts for the gas phase reaction of aromatic hydrocarbons. It is preferable to use steatite having a spherical shape with a diameter of 3 to 6 mm, or a ring shape having an outer diameter of 5 to 9 mm, a length of 3 to 8 mm, and a wall thickness of 1 to 2 mm.

  The catalyst according to the invention comprises a catalytically active composition comprising at least vanadium oxide and titanium dioxide and can be applied to the support material in one or more layers. The various layers can differ in their composition.

Catalytically active composition, preferably, the total 60 to 99 wt% calculated as 1 to 40% by weight of vanadium oxide and TiO _2, calculated as _V 2 _{O 5} with respect to the amount of titanium dioxide in the catalytic active composition including. In a preferred embodiment, the catalytically active composition comprises up to 1% by weight of cesium compound calculated as Cs, up to 1% by weight of phosphorus compound calculated as P, and up to 10% calculated as Sb ₂ O _3. It further contains mass% antimony oxide. All numerical values of the chemical composition of the catalytically active composition are based on the latter calcination state, for example, the state after calcination of the catalyst at 450 ° C. for 1 hour.

Titanium dioxide is usually used in the anatase form for catalytically active compositions. The titanium dioxide preferably has a BET surface area of 15 to 60 m ² / g, in particular 15 to 45 m ² / g, particularly preferably 13 to 28 m ² / g. The titanium dioxide used may comprise a single titanium dioxide or a mixture of titanium dioxide. In the latter case, the BET surface area value is a weighted average of the individual titanium dioxide contributions. Titanium dioxide used may, for example, advantageously comprises a mixture of TiO ₂ having a BET surface area of TiO ₂ and 15 to 50 m ² / g with a BET surface area of 5 to 15 m ² / g.

  Suitable vanadium sources are in particular vanadium pentoxide and ammonium metavanadate. Suitable antimony sources are various antimony sources, in particular antimony trioxide. Vanadium and antimony can also be used in the form of a vanadium antimonate compound. The vanadium antimonate introduced into the active composition of at least one layer can be produced by the reaction of any vanadium with an antimonate compound. Direct reaction of the oxide to form mixed acid or vanadium antimonate is preferred.

  The vanadium antimonate can have various molar ratios of vanadium to antimony and optionally includes additional vanadium or antimony compounds and can be used in admixture with vanadium or antimony compounds.

  Possible phosphorus sources are in particular phosphoric acid, phosphorous acid, hypophosphorous acid, ammonium phosphate or phosphate esters, in particular ammonium dihydrogen phosphate. Possible cesium sources include their oxides or hydrides, or carboxylates that can be thermally converted to oxides, in particular acetates, malonates or oxalates, carbonates, bicarbonates, sulfuric acids. A salt such as a salt or nitrate.

  Apart from the optional additives cesium and phosphorus, a small amount of many other oxide compounds that act as promoters that affect the activity and selectivity of the catalyst by reducing or increasing its activity, etc. It can be included in the catalytically active composition. Examples of such promoters are alkali metals, in particular lithium, potassium and rubidium which are usually used in the form of cesium and oxides or hydrides mentioned above, thallium (I) oxide, aluminum oxide, zirconium oxide. Iron oxide, nickel oxide, cobalt oxide, manganese oxide, tin oxide, silver oxide, copper oxide, chromium oxide, molybdenum oxide, tungsten oxide, iridium oxide, tantalum oxide, niobium oxide, arsenic oxide, antimony oxide, cerium oxide .

  Of the above promoters, niobium and tungsten oxides as additives in an amount of 0.01 to 0.50% by weight based on the catalytically active composition are preferred.

The layer of coated catalyst is advantageously applied by spraying a suspension of TiO ₂ and V ₂ O ₅ optionally containing the aforementioned promoter component sources onto a fluidized support. Prior to the coating operation, the suspension is preferably sufficiently long, for example 2 to 30 hours, in particular 12 to 12 to disperse the aggregates of suspended material and to obtain a homogeneous suspension. Stir for 25 hours. The suspension typically has a solid content of 20-50% by weight. The suspending medium is generally aqueous, for example water itself or an aqueous mixture with a water-miscible organic solvent such as methanol, ethanol, isopropanol, formamide.

  In general, an organic binder, preferably a copolymer, is advantageously added to the suspension in the form of an aqueous dispersion of acrylic acid-maleic acid, vinyl acetate-vinyl laurate, vinyl acetate-acrylate, styrene-acrylate or vinyl acetate-ethylene. The The binder is commercially available, for example, as an aqueous dispersion having a solid content of 35 to 65% by mass. The amount of binder dispersion used is generally from 2 to 45% by weight, preferably from 5 to 35% by weight, particularly preferably from 7 to 20% by weight, based on the weight of the suspension.

  The support is fluidized in an ascending gas stream, in particular in the atmosphere, for example in a fluidized bed or moving bed apparatus. The apparatus typically comprises a conical or spherical container into which fluidizing gas is introduced from the bottom or top via a dip tube. The suspension is sprayed into the fluidized bed from the top, from the side or from the bottom via a nozzle. The use of risers arranged in the center of the dip tube or concentrically is advantageous.

  The relatively high gas velocity that carries the carrier particles upwards prevails within the riser. In the outer ring, the gas velocity is slightly above the relaxed velocity. The particles thus move vertically in a circulating manner. A suitable fluid bed apparatus is described, for example, in DE-A 4006935.

  When coating the catalyst support with the catalytically active composition, a coating temperature of 20 to 500 ° C. is generally employed and the coating can be carried out under atmospheric pressure or under reduced pressure. In general, the coating is carried out at 0 to 200 ° C., preferably 20 to 150 ° C., in particular 60 to 120 ° C.

  The layer thickness of the catalytically active composition is generally 0.02 to 0.2 mm, preferably 0.05 to 0.15 mm. The ratio of the active composition in the catalyst is usually 5 to 25% by mass, mainly 7 to 15% by mass.

  As a result of the heat treatment of the precatalyst thus obtained at temperatures above 200 ° C. to 500 ° C., the binder is released from the applied layer as a result of pyrolysis and / or combustion. The heat treatment is preferably performed in situ in a gas phase oxidation reactor.

  It is also possible to obtain a pseudo-continuous transition between multiple layers instead of different catalyst delimited layers, so that the continuous catalyst transitions from one layer to the next By inserting a zone containing the mixture, an effective and uniform reduction of the alkali metal content can be obtained.

  The bed length of the catalyst layer A is preferably in the range of 10 to 50%, particularly preferably in the range of 15 to 30% of the total catalyst filling height in the reactor. The bed height of catalyst layers A and B or A, B and C is preferably in the range of 60 to 95% of the total catalyst loading height. A typical reactor has a packing height of 250 cm to 350 cm. The catalyst layer can also be distributed across multiple reactors if desired.

  The catalyst according to the invention is particularly suitable for oxidizing naphthalene or o-xylene / naphthalene mixtures to phthalic anhydride at a total load in the range of 80-100 g per standard cubic meter and an air flow of about 4 standard cubic meters per hour. .

The present invention is a method of oxidizing naphthalene or an o-xylene / naphthalene mixture to phthalic anhydride using a multilayer catalyst further comprising at least three layer catalysts,
Each layer comprises vanadium oxide and titanium dioxide, and a) the alkali metal content of one catalyst layer A is the highest,
b) the catalyst layer Z that follows the catalyst layer A in the flow direction has an alkali metal content of 0 to 10% of the alkali metal content of the catalyst layer A;
c) The catalyst layer located between the catalyst layers A and Z has an alkali metal content of 30 to 90% of the alkali metal content of the catalyst layer A, and the alkali metal content of each catalyst layer is the next layer in the flow direction. A process having an alkali metal content selected to be higher than the alkali metal content of the present invention.

A preferred embodiment of the present invention is a method of oxidizing naphthalene or an o-xylene / naphthalene mixture to phthalic anhydride using a four-layer catalyst comprising:
Each catalyst layer contains vanadium oxide and titanium dioxide, and the alkali metal content of the catalyst layer is
a) One catalyst layer A has the highest alkali metal content,
b) Catalyst layer B following catalyst layer A in the flow direction has an alkali metal content of 60 to 90% of the alkali metal content of catalyst layer A;
c) the catalyst layer C following the catalyst layer B in the flow direction has an alkali metal content of 30 to 59% of the alkali metal content of the catalyst layer A;
d) The process is selected such that the catalyst layer Z following the catalyst layer C in the flow direction has an alkali metal content of 0 to 10% of the alkali metal content of the catalyst layer A.

The present invention further provides a method for using a multi-layer catalyst comprising at least three catalyst layers for oxidizing naphthalene or an o-xylene / naphthalene mixture to phthalic anhydride, comprising:
Each layer comprises vanadium oxide and titanium dioxide, and a) the alkali metal content of one catalyst layer A is the highest,
b) the catalyst layer Z that follows the catalyst layer A in the flow direction has an alkali metal content of 0 to 10% of the alkali metal content of the catalyst layer A;
c) The catalyst layer located between the catalyst layers A and Z has an alkali metal content of 30 to 90% of the alkali metal content of the catalyst layer A, and the alkali metal content of each catalyst layer is the next catalyst layer in the flow direction. A method of using a multi-layer catalyst having an alkali metal content selected to be higher than the alkali metal content of the present invention is provided.

A preferred embodiment of the present invention is a method of using a four-layer catalyst to oxidize naphthalene or an o-xylene / naphthalene mixture to phthalic anhydride,
Each catalyst layer contains vanadium oxide and titanium dioxide, and the alkali metal content of the catalyst layer is a) the alkali metal content of one catalyst layer A is the highest,
b) Catalyst layer B following catalyst layer A in the flow direction has an alkali metal content of 60 to 90% of the alkali metal content of catalyst layer A;
c) the catalyst layer C following the catalyst layer B in the flow direction has an alkali metal content of 30 to 59% of the alkali metal content of the catalyst layer A;
d) Use of a four-layer catalyst selected such that the catalyst layer Z following the catalyst layer C in the flow direction has an alkali metal content of 0 to 10% of the alkali metal content of the catalyst layer A.

Production of the first catalyst layer CL1:
In a fluidized bed coater, a steatite (magnesium silicate) ring 2000 g having an outer diameter of 8 mm, a length of 6 mm and a wall thickness of 1.5 mm, 662.8 g of anatase (Fuji TA 100C , BET surface area of 20 m ² / g), 29.52 g vanadium pentoxide, 78.48 g oxalic acid, 0.62 g potassium sulfate, 8.31 g cesium sulfate, 1.39 g niobium pentoxide, 0.79 g ammonium dihydrogen phosphate, 212.9 g Formamide, 1000 g water and 67.5 g binder (75/25 weight ratio acrylic acid-maleic acid copolymer as an aqueous polymer solution having a solids content of 49.4% by weight; the preparation of this binder is described in EP1091806 900 g) of the suspension was sprayed at 90 ° C.

The catalytically active composition thus applied consists of 0.03% by weight phosphorus (calculated as P), 4.22% by weight vanadium (calculated as V ₂ O ₅ ), 0.87% by weight cesium (Cs ), 0.2 wt% Nb (calculated as Nb ₂ O ₅ ), 0.04 wt% K (calculated as K) and 94.68 wt% titanium dioxide. The active composition content after firing at 450 ° C. for 1 hour was 8.9%.

Production of second catalyst layer CL2:
The catalyst was prepared by changing the composition of the suspension compared to CL1. The catalytically active composition thus applied was 0.03% by weight phosphorus (calculated as P), 4.22% by weight vanadium (calculated as V ₂ O ₅ ), 0.67% by weight cesium (calculated as Cs). ), 0.2 wt% Nb (calculated as Nb ₂ O ₅ ), 0.03 wt% K (calculated as K) and 94.85 wt% titanium dioxide. The active composition content after calcination at 450 ° C. for 1 hour was 8.8%.

Production of third catalyst layer CL3:
The catalyst was prepared by changing the composition of the suspension compared to CL1. The catalytically active composition thus applied was 0.03% by weight phosphorus (calculated as P), 4.22% by weight vanadium (calculated as V ₂ O ₅ ), 0.45% by weight cesium (calculated as Cs). ), calculated as _Nb (Nb 2 _{O 5} of 0.2 wt%) contained 0.02 wt% of K (calculated as K), and 95.1 wt% titanium dioxide. The content of the active composition after baking at 450 ° C. for 1 hour was 9.0% by mass.

Production of fourth catalyst layer CL4:
The catalyst was prepared by changing the composition of the suspension compared to CL1. The catalytically active material thus applied was 0.02% by weight phosphorus (calculated as P), 4.22% by weight vanadium (calculated as V ₂ O ₅ ), 0.00% by weight cesium (calculated as Cs) , (calculated as _Nb 2 _{O 5)} 0.2 wt% of Nb, contained 0.00 (calculated as K) wt% K and 95.56% by weight of titanium dioxide. The content of the active composition after baking at 450 ° C. for 1 hour was 9.6% by mass.

Description of the oxidation of o-xylene to phthalic anhydride The catalytic oxidation of o-xylene to phthalic anhydride is carried out in a tubular reactor having a 25 mm tube inner diameter and cooled using a salt bath. did. The reaction tube was equipped with a thermocouple to record the temperature profile. 4.0 standard cubic meters of air per hour with 0-85 g o-xylene (purity approx. 99 wt.%) Load or naphthalene (purity approx. 97.5 wt.%) Load per standard cubic meter was passed through the tube. The PAn yield was measured at the outlet gas of the reactor and recorded as mass% relative to 100% pure o-xylene or 100% pure naphthalene (PAn (kg per reacted o-xylene or naphthalene (kg)) )).

Results and examples Example 1 (according to the invention):
Floor length distribution: Steatite preliminary floor / CL1 / CL2 / CL3 / CL4 5 cm / 80 cm / 80 cm / 90 cm / 90 cm
With a naphthalene load of 80 g per standard cubic meter, 4 standard cubic meters of air per hour and a salt bath temperature of 360 ° C., a PAn yield of 105.6% by weight and a phthalide and naphthoquinone content of 0.00% and 0.53% by weight, respectively. Was achieved. With a total load of 80 g per standard cubic meter, a 50:50 mixture of o-xylene and naphthalene, 4 standard cubic meters per hour and a salt bath temperature of 362 ° C., 110.1% by weight PAn yield and 0.06% by weight respectively. A phthalide and naphthoquinone content of 0.41% by weight was achieved. 111.0 wt% PAn yield at 30 g naphthalene load per standard cubic meter and 55 g o-xylene load per standard cubic meter (total load: 85 g per standard cubic meter), 4 standard cubic meters of air per hour and a salt bath temperature of 362 ° C. And phthalide and naphthoquinone contents of 0.11% and 0.34% by weight, respectively, were achieved. Therefore, o-xylene / naphthalene ratio while maintaining high PAn yield and good product range (low yield of phthalide and naphthoquinone) with 4 standard cubic meters of air per hour and a total load of at least 80 g per standard cubic meter In the range of 0: 100% to 65: 35%. The hot spot temperature was less than 450 ° C. for all feed compositions.

Example 2 (not according to the invention):
Floor length distribution: Steatite preliminary floor / CL1 / CL2 / CL3 / CL4 20 cm / 100 cm / 0 cm / 90 cm / 100 cm
With a naphthalene load of 80 g per standard cubic meter, 4 standard cubic meters of air per hour and a salt bath temperature of 358 ° C., a PAn yield of 104.7% by mass and a phthalide and naphthoquinone content of 0.00% and 0.55% by mass, respectively. Achieved. With a total load of 80 g per standard cubic meter, a 50:50 mixture of o-xylene and naphthalene, 4 standard cubic meters of air per hour and a salt bath temperature of 364 ° C., 109.6% by weight PAn yield and 0.03% by weight respectively. A phthalide and naphthalene content of 0.31% by weight. The hot spot temperature was less than 450 ° C. for all feed compositions. When the feed composition was further changed to 30 g naphthalene load per standard cubic meter and 55 g o-xylene load per standard cubic meter (total load: 85 g per standard cubic meter), the hot spot temperature rose above 465 ° C. The catalyst could not function stably with this feed composition. Thus, if it is necessary to obtain a high PAn yield and a good product range (low yields of phthalide and naphthoquinone), at a total load of at least 80 grams per standard cubic meter and 4 standard cubic meters of air per hour, o-xylene / The naphthalene ratio could only be changed within the range of 0: 100% to 50: 50%.

Claims (6)

A multilayer catalyst for oxidizing naphthalene or an o-xylene / naphthalene mixture to phthalic anhydride,
Including at least three catalyst layers;
Each layer comprises a niobium oxide of the total amount 0.01 to 0.50 wt% of the amount based on the catalytically active composition in vanadium oxide and titanium dioxide and layers (calculated as _Nb 2 _{O 5),} and a) reacting The alkali metal content based on mass of one catalyst layer A having a bed length in the range of 15 to 30% of the total catalyst filling height in the vessel is the highest,
b) the catalyst layer Z following the catalyst layer A in the flow direction has a mass-based alkali metal content of 0-10% of the mass-based alkali metal content of the catalyst layer A, and c) the catalyst layer A catalyst layer disposed between the Z has an alkali metal content of 30% to 90% of the weight of the alkali metal content of the mass of the catalyst layer a, the direction of the next alkali metal content of the mass of each catalyst layer flows A multilayer catalyst characterized by having an alkali metal content selected to be higher than the mass-based alkali metal content of said catalyst layer. A multilayer catalyst according to claim 1 having four layers,
The alkali metal content based on the mass of the catalyst layer is
a) The mass-based alkali metal content of one catalyst layer A having the bed length in the range of 15 to 30% of the total catalyst filling height in the reactor is the highest,
b) a catalyst layer followed by a catalyst layer A in the flow direction B has an alkali metal content of 60% to 90% of the weight of the alkali metal content of the mass of the catalyst layer A,
c) the catalyst layer C following the catalyst layer B in the flow direction has an alkali metal content on a mass basis of 30 to 59% of the alkali metal content on a mass basis of the catalyst layer A;
catalyst layer Z followed catalyst layer C in d) flow direction, selected multilayer catalyst so as to have an alkali metal content of 0% of the weight of the alkali metal content of the mass of the catalyst layer A. A method of oxidizing naphthalene or an o-xylene / naphthalene mixture to phthalic anhydride using a multilayer catalyst comprising at least three catalyst layers,
Each layer comprises a niobium oxide of the total amount 0.01 to 0.50 wt% of the amount based on the catalytically active composition in vanadium oxide and titanium dioxide and layers (calculated as _Nb 2 _{O 5),} and a) reacting The alkali metal content based on mass of one catalyst layer A having a bed length in the range of 15 to 30% of the total catalyst filling height in the vessel is the highest,
b) The catalyst layer Z that follows the catalyst layer A in the flow direction has an alkali metal content on a mass basis of 0 to 10% of the alkali metal content on a mass basis of the catalyst layer A,
c) has an alkali metal content of 30% to 90% of the weight of the alkali metal content of the weight of the catalyst layer is the catalyst layer A located between the catalyst layer A and Z, alkali mass of each catalyst layer A method having an alkali metal content selected such that the metal content is higher than the mass-based alkali metal content of the next catalyst layer in the flow direction. The method of claim 3, wherein a four-layer catalyst is used,
The alkali metal content based on the mass of the catalyst layer is
a) The mass-based alkali metal content of one catalyst layer A having the bed length in the range of 15 to 30% of the total catalyst filling height in the reactor is the highest,
b) a catalyst layer followed by a catalyst layer A in the flow direction B has an alkali metal content of 60% to 90% of the weight of the alkali metal content of the mass of the catalyst layer A,
c) the catalyst layer C following the catalyst layer B in the flow direction has an alkali metal content on a mass basis of 30 to 59% of the alkali metal content on a mass basis of the catalyst layer A;
How catalyst layer Z is selected to have an alkali metal content of 0% of the weight of the alkali metal content of the mass of the catalyst layer A followed by the catalyst layer C in d) the flow direction. A method for using a multilayer catalyst comprising at least three catalyst layers for oxidizing naphthalene or an o-xylene / naphthalene mixture to phthalic anhydride, comprising:
Each layer comprises a niobium oxide of the total amount 0.01 to 0.50 wt% of the amount based on the catalytically active composition in vanadium oxide and titanium dioxide and layers (calculated as _Nb 2 _{O 5),} and a) reacting The alkali metal content based on mass of one catalyst layer A having a bed length in the range of 15 to 30% of the total catalyst filling height in the vessel is the highest,
b) the catalyst layer Z following the catalyst layer A in the flow direction has a mass-based alkali metal content of 0-10% of the mass-based alkali metal content of the catalyst layer A, and c) the catalyst layer A catalyst layer located between Z and has an alkali metal content of 30% to 90% of the weight of the alkali metal content of the mass of the catalyst layer a, the direction of the next alkali metal content of the mass of each catalyst layer flows A method of use characterized by having an alkali metal content selected to be higher than the alkali metal content of the catalyst layer based on mass . A method of using a four-layer catalyst for oxidizing naphthalene or an o-xylene / naphthalene mixture to phthalic anhydride, comprising:
Each catalyst layer comprises vanadium oxide and titanium dioxide and niobium oxide in an amount of 0.01 to 0.50 mass% ( calculated as Nb ₂ O ₅ ) based on the total amount of catalytically active composition in each layer , and The alkali metal content based on the mass of the catalyst layer is
a) The mass-based alkali metal content of one catalyst layer A having the bed length in the range of 15 to 30% of the total catalyst filling height in the reactor is the highest,
b) a catalyst layer followed by a catalyst layer A in the flow direction B has an alkali metal content of 60% to 90% of the weight of the alkali metal content of the mass of the catalyst layer A,
c) the catalyst layer C following the catalyst layer B in the flow direction has an alkali metal content on a mass basis of 30 to 59% of the alkali metal content on a mass basis of the catalyst layer A;
Use of the catalyst layer Z followed catalyst layer C in d) the flow direction, characterized in that it is selected to have an alkali metal content of 0% of the weight of the alkali metal content of the mass of the catalyst layer A Method.