Catalyst for preparing dianhydride from durene by gas phase oxidation and preparation method thereof, and method for preparing dianhydride from durene by gas phase oxidation

Abstract

The present application relates to the technical field of catalyst, in particular to a catalyst for preparing pyromellitic dianhydride by gas phase oxidation of durene, a preparation method of the catalyst and a method for preparing pyromellitic dianhydride by gas phase oxidation of durene. The catalyst comprises a carrier, an inner coating and an outer coating loaded on the carrier in sequence; wherein the inner coating and the outer coating each independently contain V2O5, and the content of V2O5 in the inner coating is greater than the content of V2O5 in the outer coating. By limiting the catalytic activity of the inner coating and the outer coating, the overall utilization of the inner coating and the outer coating in the catalyst is improved, and the catalytic activity of the catalyst is effectively improved. When the catalyst is used for preparing pyromellitic dianhydride by gas phase oxidation of durene, the conversion rate of durene and the yield of pyromellitic dianhydride are more improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of catalysts, in particular to a catalyst for preparing pyromellitic dianhydride from durene by gas phase oxidation, a preparation method of the catalyst, and a method for preparing pyromellitic dianhydride from durene by gas phase oxidation. BACKGROUND

[0002] Pyromellitic dianhydride (1,2,4,5-benzenetetracarboxylic dianhydride, hereinafter referred to as pyromellitic dianhydride) is a very important chemical raw material. Pyromellitic dianhydride and its derivatives have very important and wide uses, and pyromellitic dianhydride can be used as one of the main monomers for producing polyimide. Polyimide has comprehensive performance at high temperature, and is a new type of engineering material with the widest temperature range, excellent dimensional stability, radiation resistance, mechanical properties, electrical properties, and corrosion resistance at high temperature among organic polymer materials.

[0003] In the early stage, the liquid phase oxidation process was mainly used to prepare pyromellitic dianhydride from durene, and the raw material was oxidized to form an acid, and then the acid was dehydrated to form an anhydride. The currently widely used method is to use durene as a raw material to prepare pyromellitic dianhydride by air oxidation. The method has the characteristics of simple process, no dehydration process to form an anhydride, air as an oxidizing agent, no catalyst separation process in the liquid phase oxidation process, continuous production, and easy automation operation.

[0004] The catalysts for preparing pyromellitic dianhydride in the prior art are mostly composed of oxides of elements such as V / Ti / Mo / Fe.

[0005] CN107866241A discloses a catalyst for preparing pyromellitic dianhydride from durene by oxidation, which uses an oxide catalyst, the catalyst uses α-Al2O3, silicon carbide, a porcelain ring, or a mixture thereof as a carrier, and the active components include vanadium elements, titanium elements, and at least one of VA group elements and alkali metal elements. CN107866257A discloses a catalyst for preparing pyromellitic dianhydride from durene. The catalyst uses an oxide catalyst, the catalyst uses α-Al2O3, silicon carbide, a porcelain ring, or a mixture thereof as a carrier, and the active components include vanadium elements, iron-based elements, and at least one of IIB group elements and alkali metal elements.

[0006] Since the reaction of preparing pyromellitic dianhydride from durene by oxidation is carried out in the catalyst coating, the concentration of durene contacting the outside of the coating is higher, and the concentration of durene contacting the inside of the coating is lower. The properties of the catalysts required for oxidation are also different, and a single formula design cannot effectively utilize the entire catalyst coating. How to design the catalyst according to the law of the radial distribution of the raw material along the coating to improve the overall selectivity of the product pyromellitic dianhydride is a technical problem to be solved at present. SUMMARY

[0007] The present application aims to overcome the problems of low utilization rate, low conversion rate of durene and low yield of pyromellitic dianhydride in the existing catalyst coating for preparing pyromellitic dianhydride by oxidizing durene, and to provide a catalyst for preparing pyromellitic dianhydride by oxidizing durene in gas phase, a preparation method of the catalyst, and a method for preparing pyromellitic dianhydride by oxidizing durene in gas phase, the catalyst having an inner coating and an outer coating with different catalytic activities, and the conversion rate of durene and the yield of pyromellitic dianhydride are improved by improving the utilization rate of the inner coating and the outer coating.

[0008] To achieve the above-mentioned purpose, the present application provides a catalyst for preparing pyromellitic dianhydride by oxidizing durene in gas phase, which comprises a carrier, and an inner coating and an outer coating sequentially loaded on the carrier.

[0009] The inner coating and the outer coating each independently contain V2O5, and the content of V2O5 in the inner coating is > the content of V2O5 in the outer coating.

[0010] The present application provides a preparation method of a catalyst for preparing pyromellitic dianhydride by oxidizing durene in gas phase, which comprises the following steps:

[0011] (1) loading a first slurry containing a vanadium source, a phosphorus source, a cesium source, a titanium source and an auxiliary compound on a carrier, drying and activating to load an inner coating on the carrier to form a catalyst intermediate;

[0012] (2) loading a second slurry containing a vanadium source, an optional phosphorus source, a cesium source, a titanium source and an auxiliary compound on the catalyst intermediate, drying and activating to load an outer coating on the inner coating of the catalyst intermediate to form a catalyst;

[0013] The vanadium source in the first slurry and the second slurry is such that the content of V2O5 in the inner coating is > the content of V2O5 in the outer coating.

[0014] The present application provides a method for preparing pyromellitic dianhydride by oxidizing durene in gas phase, which comprises: contacting durene and an oxygen-containing gas with a catalyst and reacting to obtain pyromellitic dianhydride.

[0015] The catalyst is the catalyst provided in the first aspect, or the catalyst prepared by the method provided in the second aspect.

[0016] By the technical scheme, the catalyst for preparing dianhydride from durene by gas phase oxidation provided by the application improves the overall utilization of the inner coating and the outer coating in the catalyst, and effectively improves the catalytic activity of the catalyst by limiting the catalyst to include: an inner coating and an outer coating loaded on a carrier in sequence, and limiting the catalytic activity of the inner coating and the outer coating, i.e., the content of V2O5 in the inner coating > the content of V2O5 in the outer coating, and further limiting the difference and the specific content of the content of V2O5 in the inner coating and the outer coating.

[0017] Meanwhile, the catalyst provided by the application is more conducive to improving the conversion rate of durene and the yield of dianhydride when the catalyst is used for preparing dianhydride from durene by gas phase oxidation. DETAILED DESCRIPTION

[0018] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The ranges or values should be construed to be roughly about the ranges or values. The endpoints of the ranges of values will be understood to be roughly about the endpoint value as if both the upper and lower values are preceded with about. For values which are less than one, one unit is considered to be 0.000... with the number of zeros being equal to the least significant digit of the number that follows the decimal point. For values which are greater than one, one unit is considered to be 1.000... with the number of zeros being equal to the least significant digit of the number that precedes the decimal point. These amounts are approximate values that are understood to encompass minor variations.

[0019] In the application, without special circumstances, "first" and "second" do not represent the order of precedence, nor do they limit each material or step, but only serve to distinguish the same material or step. For example, "first" and "second" in "first slurry" and "second slurry" only serve to distinguish that they are not the same slurry.

[0020] The first aspect of the application provides a catalyst for preparing dianhydride from durene by gas phase oxidation, which includes: a carrier, and an inner coating and an outer coating loaded on the carrier in sequence.

[0021] The inner coating and the outer coating each independently contain V2O5, and the content of V2O5 in the inner coating > the content of V2O5 in the outer coating.

[0022] In the application, without special circumstances, the inner coating and the outer coating loaded on the carrier in sequence means that the inner coating is loaded on the carrier, and the outer coating is loaded on the inner coating. At the same time, the inner coating and the outer coating refer to the coating loaded on the carrier as the inner coating, and the coating contacting the outside as the outer coating.

[0023] In the application, without special circumstances, the higher the content of V2O5 in the coating, the higher the catalytic activity of the coating.

[0024] According to the present application, preferably, the difference between the content of V2O5 in the inner coating and the content of V2O5 in the outer coating is 0.3-3.5wt%, for example, 0.3wt%, 0.4wt%, 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.4wt%, 3.5wt%, and any value in the range between any two of the numerical values, preferably 0.4-3.4wt%. With the preferred conditions, the catalytic activity of the catalyst is more favorable. Wherein, the difference refers to the difference between the numerical values of the contents, for example, the content of V2O5 in the inner coating is 7.1wt%, the content of V2O5 in the outer coating is 6.7wt%, and the difference is 7.1wt%-6.7wt%=0.4wt%.

[0025] In some embodiments of the present application, preferably, the content of V2O5 is 7-12wt% based on the total weight of the inner coating, for example, 7wt%, 7.1wt%, 7.5wt%, 8wt%, 8.8wt%, 10wt%, 12wt%, and any value in the range between any two of the numerical values, preferably 7.1-8.8wt%.

[0026] In some embodiments of the present application, preferably, the inner coating further contains a phosphorus compound, a cesium compound, an additive, and TiO2.

[0027] In some embodiments of the present application, preferably, the content of the phosphorus compound is 0.1-0.5wt% based on the total weight of the inner coating, preferably 0.2-0.4wt%; the content of the cesium compound is 0.1-0.3wt% based on Cs, preferably 0.1-0.2wt%; the content of the additive is 1-6wt% based on the oxide, preferably 1.5-5wt%; and the content of TiO2 is 85-90wt%, preferably 87.1-89.5wt%.

[0028] In some embodiments of the present application, preferably, the content of V2O5 is 3-6.9wt% based on the total weight of the outer coating, for example, 3wt%, 4wt%, 5.3wt%, 6wt%, 6.5wt%, 6.8wt%, 6.9wt%, and any value in the range between any two of the numerical values, preferably 5.3-6.8wt%.

[0029] In some embodiments of the present application, preferably, the outer coating further contains an optional phosphorus compound, a cesium compound, an additive, and TiO2.

[0030] In some embodiments of the present application, preferably, the content of phosphorus compound is 0-0.3wt% based on the total weight of the outer coating, preferably 0-0.2wt%; the content of cesium compound is 0.1-0.7wt% based on Cs, preferably 0.2-0.5wt%; the content of the auxiliary is 1-6wt% based on oxide, preferably 1.5-5wt%; the content of TiO2 is 90.5-93wt%, preferably 90.7-92.4wt%.

[0031] In the present application, the type of the auxiliary has a wide range of selection. Preferably, the auxiliary contains at least one element selected from rubidium, cerium, niobium, chromium, tungsten, silver, cobalt, gallium, indium, antimony, bismuth, zirconium and erbium.

[0032] In the present application, the type of the carrier has a wide range of selection, i.e. the carrier is of a type conventional in the art. Preferably, the carrier is a non-porous inert carrier, preferably at least one selected from alumina, talc, silicon carbide, aluminum silicate, quartz and ceramic.

[0033] In the present application, the shape of the carrier is not particularly limited, for example, it can be cylindrical, spherical, annular or granular. Preferably, the shape of the carrier is an annular carrier.

[0034] In some embodiments of the present application, preferably, the outer diameter of the carrier is 3-13mm, for example, 3mm, 5mm, 7mm, 9mm, 11mm, 13mm, and any value in the range between any two of the values, preferably 5-9mm; the height is 2-12mm, for example, 2mm, 3mm, 5mm, 7mm, 8mm, 10mm, 12mm, and any value in the range between any two of the values, preferably 3-8mm; the wall thickness is 0.1-10mm, preferably 0.1-5mm.

[0035] In some embodiments of the present application, preferably, in the catalyst, the weight ratio of the inner coating, the outer coating and the carrier is 5.5-6.8:6.9-8.5:100, preferably 5.8-6.7:6.9-7.9:100. When the weight ratio of the inner and outer coatings differs too much, it affects the yield of the product.

[0036] According to a particularly preferred embodiment of the present application, the catalyst comprises a carrier, and an inner coating and an outer coating sequentially loaded on the carrier,

[0037] The inner coating contains V2O5, a phosphorus compound, a cesium compound, an auxiliary agent and TiO2; the content of V2O5 is 7-12 wt%, the content of the phosphorus compound calculated as P is 0.1-0.5 wt%, the content of the cesium compound calculated as Cs is 0.1-0.3 wt%, the content of the auxiliary agent calculated as an oxide is 1-6 wt%, and the content of TiO2 is 85-90 wt%, based on the total weight of the inner coating.

[0038] The outer coating contains V2O5, an optional phosphorus compound, a cesium compound, an auxiliary agent and TiO2; the content of V2O5 is 3-6.9 wt%, the content of the phosphorus compound calculated as P is 0-0.3 wt%, the content of the cesium compound calculated as Cs is 0.1-0.7 wt%, the content of the auxiliary agent calculated as an oxide is 1-6 wt%, and the content of TiO2 is 90.5-93 wt%, based on the total weight of the outer coating.

[0039] The second aspect of the present application provides a method for preparing a catalyst for preparing dianhydride from durene by gas phase oxidation, which comprises the following steps:

[0040] (1) loading a first slurry containing a vanadium source, a phosphorus source, a cesium source, a titanium source and an auxiliary agent compound on a carrier, drying and activating to load an inner coating on the carrier to form a catalyst intermediate;

[0041] (2) loading a second slurry containing a vanadium source, an optional phosphorus source, a cesium source, a titanium source and an auxiliary agent compound on the catalyst intermediate, drying and activating to load an outer coating on the inner coating of the catalyst intermediate to form a catalyst;

[0042] The respective vanadium sources in the first slurry and the second slurry are such that the content of V2O5 in the inner coating > the content of V2O5 in the outer coating.

[0043] In the present application, in order to load the first slurry and the second slurry more uniformly, preferably, the viscosity of the first slurry and the second slurry is independently 10-40 mPa·s, for example, 10 mPa·s, 12 mPa·s, 15 mPa·s, 20 mPa·s, 25 mPa·s, 30 mPa·s, 35 mPa·s, 40 mPa·s, preferably 12-25 mPa·s. In the present application, unless otherwise specified, the viscosity parameter is measured by a cup viscometer (25℃).

[0044] According to a preferred embodiment of the present application, preferably, in step (1), the preparation of the first slurry comprises:

[0045] (1-i) mixing oxalic acid, a vanadium source, a phosphorus source, a cesium source and a solvent to obtain solution A;

[0046] (1-ii) mixing the solution A with a titanium source, an auxiliary compound and a binder to obtain a first slurry.

[0047] In some embodiments of the present application, preferably, in the first slurry, the weight ratio of the oxalic acid, the vanadium source, the phosphorus source, the cesium source, the solvent, the titanium source, the auxiliary compound and the binder is 20-35: 10-15: 0.5-2: 0.1-0.4: 100: 105: 4-6: 11.7, preferably 24-30: 10-14: 0.8-1.8: 0.15-0.35: 100: 105: 4.3-5.1: 11.7.

[0048] According to a preferred embodiment of the present application, preferably, in step (2), the preparation of the second slurry comprises:

[0049] (2-i) mixing oxalic acid, a vanadium source, an optional phosphorus source, a cesium source and a solvent to obtain a solution B;

[0050] (2-ii) mixing the solution B with a titanium source, an auxiliary compound and a binder to obtain a second slurry.

[0051] In some embodiments of the present application, preferably, in the second slurry, the weight ratio of the oxalic acid, the vanadium source, the phosphorus source, the cesium source, the solvent, the titanium source, the auxiliary compound and the binder is 20-30: 7-9.9: 0-0.02: 0.2-0.8: 100: 105: 2-3.5: 11.7, preferably 18-24: 7.5-9.5: 0-0.01: 0.33-0.76: 100: 105: 2.6-3.2: 11.7.

[0052] According to the present application, the mixing method is not particularly limited, and a wet grinding method can be used. In view of the convenience and timeliness of grinding, preferably, the mixing is performed in a ball mill, and further preferably, the grinding time in the ball mill is 1-5 h, preferably 2-4 h.

[0053] In some embodiments of the present application, preferably, the solvent in the first slurry and the second slurry is independently water and a water-soluble organic solvent; wherein the water-soluble organic solvent includes, but is not limited to, at least one of methanol, ethanol, formamide and N,N-dimethylformamide.

[0054] In some embodiments of the present application, preferably, the weight ratio of water and the water-soluble organic solvent in the solvent is 1: 0.1-1, for example, 1: 0.1, 1: 0.2, 1: 0.3, 1: 0.4, 1: 0.5, 1: 0.6, 1: 0.7, 1: 0.8, 1: 0.9, 1: 1, and any value in the range between any two values, preferably 1: 0.1-0.7.

[0055] In some embodiments of the present application, preferably, the adhesive is selected from at least one of vinyl acetate-acrylate copolymer emulsion, vinyl acetate-ethylene copolymer emulsion, vinyl acetate-maleate copolymer emulsion and acrylic acid-maleic acid copolymer emulsion, preferably vinyl acetate-ethylene copolymer emulsion.

[0056] In the present application, the type of the carrier is defined as above, which is not repeated here.

[0057] In the present application, the type of the vanadium source has a wide range of selection. Preferably, the vanadium source is selected from at least one of ammonium metavanadate, vanadium pentoxide and sodium vanadate, preferably ammonium metavanadate.

[0058] In the present application, the type of the phosphorus source has a wide range of selection. Preferably, the phosphorus source is selected from at least one of ammonium dihydrogen phosphate, triammonium phosphate and diphosphorus pentoxide, preferably ammonium dihydrogen phosphate.

[0059] In the present application, the type of the cesium source has a wide range of selection. Preferably, the cesium source is selected from at least one of cesium nitrate, cesium sulfate, cesium chloride and cesium carbonate, preferably cesium sulfate.

[0060] In the present application, the type of the titanium source has a wide range of selection. Preferably, the titanium source is selected from titanium dioxide and / or metatitanic acid, preferably titanium dioxide; further preferably, the titanium dioxide is anatase TiO2, and the specific surface area is 10-30 m 2 / g, for example, 10 m 2 / g, 15 m 2 / g, 17 m 2 / g, 20 m 2 / g, 26 m 2 / g, 30 m 2 / g, and any value within the range of any two numerical values, preferably 17-26 m 2 / g.

[0061] In some embodiments of the present application, preferably, the auxiliary compound is a metal oxide and / or a metal water-soluble salt containing at least one element selected from rubidium, cerium, niobium, chromium, tungsten, silver, cobalt, gallium, indium, antimony, bismuth, zirconium and erbium. Further preferably, the metal oxide is an oxide of silver and / or an oxide of antimony, and the metal water-soluble salt is at least one of water-soluble salts of rubidium, cerium, niobium, chromium, tungsten, iron, cobalt, gold, gallium, indium, bismuth, zirconium, erbium and tungsten.

[0062] In the present application, the metal water-soluble salt can be nitrate, carbonate, sulfate, oxalate, chloride, etc. without special circumstances.

[0063] In the present application, the manner of loading is not particularly limited, as long as the slurry can be loaded onto the carrier. Preferably, the manner of loading is spraying.

[0064] According to the present application, in order to accelerate the volatilization of water and organic solvent in the first slurry and the second slurry, so as to quickly and effectively adhere the active component. Preferably, hot air at 90-160℃ is used for drying during spraying; more preferably, hot air at 100-130℃ is used for drying.

[0065] In some embodiments of the present application, preferably, the spraying rate of the slurry (the first slurry and the second slurry) is 30-60 mL / min, for example, 30 mL / min, 35 mL / min, 40 mL / min, 45 mL / min, 50 mL / min, 55 mL / min, 60 mL / min, and any value in the range consisting of any two of the above values, preferably 35-50 mL / min, per 2000 grams of carrier. Controlling the spraying rate within the above defined range can prevent waste of the slurry and shedding of the active component in the slurry.

[0066] According to the present application, in order to make the first slurry more easily adhere to the carrier. Preferably, the carrier is heated before spraying to maintain the temperature of the carrier at 80-130℃, for example, 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, and any value in the range consisting of any two of the above values, preferably 110-130℃.

[0067] According to the present application, the activation conditions can be selected in a wide range. Preferably, the activation conditions include 390-410℃ for 4-24h.

[0068] Since high temperature can cause the binder to disappear, reduce the adhesion of the active component on the catalyst, and thus cause shedding of the active component during catalyst loading, in order to avoid shedding of the active component during catalyst loading, the catalyst precursor is first loaded into the reactor, and then activated in the reactor to obtain the catalyst.

[0069] According to the present application, the device used for spraying is not particularly limited as long as it can meet the needs of spraying, preferably, the device used for spraying comprises a hot air blower, a coating host, an exhaust fan, a slurry spraying system, a control system and a strong power supply cabinet. The coating host comprises a coating drum and a power mechanism thereof, the coating drum is enclosed in the coating host and the rotating speed can be adjusted. The coating drum adopts a horizontal hollow columnar structure, which can improve the contact opportunity of the active component and the inert carrier material. A circular mesh with a hole diameter of 1-8 mm, preferably 3-4 mm, is arranged on the horizontal hollow columnar structure, which is used to ensure that the hot air penetrates into the coating drum, evaporates the solvent in the slurry on the carrier, and then penetrates out of the coating drum to take away the solvent. Preferably, the rotating speed of the coating drum is 5-10 rpm. The slurry spraying system is composed of a nozzle and a feeding system, and the feeding system is composed of a barrel, a stirring and feeding pump and a feeding pipeline. The feeding pump can adjust the spraying rate, and the nozzle can ensure that the slurry can be uniformly sprayed on the surface of the inert carrier material after passing through the feeding pump, forming a smooth and flat catalytically active material coating, and a catalyst is prepared.

[0070] According to the present application, the spraying amount of the active component in the catalyst is controlled by the rotating speed of the drum, the spraying rate, the loss rate and the spraying time. If the active component is insufficient, the activity of the catalyst will be affected, the conversion rate of the raw material is relatively low, if the active component is excessive, the catalyst will be seriously oxidized, more by-products will be produced, and the quality yield of the phthalic anhydride will be reduced. By loading the active component within the range of the present application, the phthalic anhydride catalyst of the present application can be obtained after stopping spraying.

[0071] The third aspect of the present application provides a method for preparing phthalic anhydride by gas phase oxidation of durene, which comprises: contacting durene and an oxygen-containing gas with a catalyst and reacting to obtain phthalic anhydride.

[0072] The catalyst is the catalyst provided in the first aspect or the catalyst prepared by the method provided in the second aspect.

[0073] According to the present application, preferably, the conditions of the reaction comprise: the temperature of the molten salt is 300-400℃, preferably 340-360℃; the volume space velocity is 1500-5000h -1 , preferably 3000-4000h -1 ; the concentration of durene is 45-70g / m 3 , preferably 55-65g / m 3 . In the present application, the pressure of the reaction can be negative pressure, normal pressure or pressurized, preferably normal pressure.

[0074] In the present application, unless otherwise specified, the concentration of durene refers to the number of grams of durene contained in a unit volume of air, and the higher the value, the higher the content of durene in the air.

[0075] According to the present application, the gas can be an oxygen-containing gas, preferably, the gas is air.

[0076] According to the present application, the reaction of preparing pyromellitic dianhydride from gas-phase oxidation of durene can be carried out in a fixed bed or in a fluidized bed, preferably, the reaction of preparing pyromellitic dianhydride from gas-phase oxidation of durene is carried out in a fixed bed single tube reactor; further preferably, the length of the tube of the fixed bed single tube reactor is 3000-4800 mm and the inner diameter is 20-30 mm. The reaction tube of the fixed bed single tube reactor is externally forced to exchange reaction heat by circulating molten salt, and the reaction tube has a thermocouple sleeve with an outer diameter of 5-10 mm, and the thermocouple sleeve has a plurality of thermocouples with the same spacing for measuring the temperature of the reaction bed and the temperature of the molten salt. The outlet of the fixed bed single tube reactor is connected to a product trapping device, and a sampling port is arranged at the lower end outlet of the reaction tube.

[0077] The process of the present application uses a fixed bed reactor and adopts molten salt circulation for heat exchange. In the evaluation of the reaction process, there is a temperature distribution in the catalyst bed, and the highest value of the temperature zone is called the reaction hot spot temperature, which is measured by pulling the bed temperature in the present application.

[0078] The catalyst for preparing pyromellitic dianhydride from gas-phase oxidation of durene provided by the present application comprises an inner coating and an outer coating, and the catalytic activity of the inner coating is greater than that of the outer coating, which is beneficial to improve the overall utilization rate of the coating. When the catalyst provided by the present application is used for preparing pyromellitic dianhydride from gas-phase oxidation of durene, the conversion rate of durene is as high as 99.6%, and the yield of pyromellitic dianhydride can reach 95.7%.

[0079] The present application will be described in detail below through examples.

[0080] The molecular formula of ammonium metavanadate is NH4VO3, and its relative molecular weight is 116.98 g / mol;

[0081] The molecular formula of ammonium dihydrogen phosphate is NH4H2PO4, and its relative molecular weight is 115.03 g / mol;

[0082] The molecular formula of cesium sulfate is Cs2SO4, and its relative molecular weight is 361.87 g / mol;

[0083] Titanium dioxide is anatase titanium dioxide, and its specific surface area is 20-30 m 2 / g;

[0084] The catalytic product is analyzed by chromatographic analysis method;

[0085] The calculation formula of the conversion rate (%) of durene is:

[0086]

[0087] The calculation formula of the weight yield (%) of phthalic anhydride is:

[0088]

[0089] The content of each component in the inner coating and the outer coating of the catalyst prepared in the examples and comparative examples is calculated by the amount of the raw material.

[0090] Example 1

[0091] (1-i) 64.59 g of ammonium metavanadate, 146.7 g of oxalic acid, 1.08 g of cesium sulfate, 5.33 g of ammonium dihydrogen phosphate, 220 mL of formamide and 350 g of water were prepared into solution A-1; the weight ratio of formamide to water was 0.7:1;

[0092] (2-i) 47.1 g of ammonium metavanadate, 112.35 g of oxalic acid, 4.56 g of cesium sulfate, 220 mL of formamide and 350 g of water were prepared into solution B-1; the weight ratio of formamide to water was 0.7:1;

[0093] (1-ii) The above solution A-1 was poured into a ball mill together with 630 g of titanium dioxide, 3.26 g of niobium oxalate, 3.68 g of zirconium sulfate, 18.99 g of antimony sesquioxide, 70 g of vinyl acetate / ethylene copolymer emulsion was added, and the first slurry was formed by ball milling for 4 h, and the viscosity of the first slurry was controlled to be 12 mPa·s;

[0094] (2-ii) The above solution B-1 was poured into a ball mill together with 630 g of titanium dioxide, 4.87 g of niobium oxalate, 10.96 g of antimony sesquioxide, 70 g of vinyl acetate / ethylene copolymer emulsion was added, and the second slurry was formed by ball milling for 4 h, and the viscosity of the second slurry was controlled to be 12 mPa·s;

[0095] (1-iii) 2000 g of the carrier magnetic ring (outer diameter 8 mm, height 6 mm, wall thickness 1.5 mm) was placed in a rotating drum, and the rotating drum speed was controlled to be 10 rpm; the above prepared first slurry was added into the stirring tank of the liquid feeding and spraying system for stirring; the hot air blower was started, and the hot air at 100℃ was blown into the rotating drum to preheat the carrier, when the temperature of the carrier reached 130℃, the feeding nozzle was started, and the spraying rate of the first slurry was controlled to be 30 mL / min; the first slurry was sprayed on the surface of the carrier through the nozzle, and was quickly dried by the hot air; the content of the inner coating reached 6.2 wt% of the weight of the carrier, and the spraying was completed; the catalyst intermediate was obtained by activating at 400℃ for 5 h;

[0096] (2-iii) The second slurry prepared above is added to the stirring tank of the feed liquid spraying system for stirring; the hot air blower is turned on, the hot air at 100℃ penetrates into the rotating drum, the feed nozzle is turned on, and the spraying rate of the second slurry is controlled to be 30 mL / min; the second slurry is sprayed on the surface of the catalyst intermediate through the nozzle, and is rapidly dried through the hot air; the content of the outer coating layer reaches 7.3 wt% of the weight of the carrier, the spraying is completed, and the catalyst S1 is obtained by activating at 400℃ for 5 h.

[0097] The catalyst S1 comprises a carrier, and an inner coating layer and an outer coating layer sequentially loaded on the carrier; the difference between the content of V2O5 in the inner coating layer and the content of V2O5 in the outer coating layer is 1.7 wt%; in the catalyst S1, the weight ratio of the inner coating layer, the outer coating layer and the carrier is 6.2:7.3:100;

[0098] Based on the total weight of the inner coating layer, the content of V2O5 is 7.1 wt%; the content of the phosphorus compound calculated in terms of P is 0.2 wt%; the content of the cesium compound calculated in terms of Cs is 0.1 wt%; the content of the additive calculated in terms of oxide is 3 wt%; and the content of TiO2 is 89.6 wt%;

[0099] Based on the total weight of the outer coating layer, the content of V2O5 is 5.4 wt%; the content of the cesium compound calculated in terms of Cs is 0.5 wt%; the content of the additive calculated in terms of oxide is 1.8 wt%; and the content of TiO2 is 92.3 wt%.

[0100] Example 2

[0101] (1-i) 64.59 g of ammonium metavanadate, 146.7 g of oxalic acid, 1.08 g of cesium sulfate, 5.33 g of ammonium dihydrogen phosphate, 220 mL of formamide and 350 g of water are prepared into solution A-1; the weight ratio of formamide to water is 0.7:1;

[0102] (2-i) 60.35 g of ammonium metavanadate, 142.32 g of oxalic acid, 1.99 g of cesium sulfate, 220 mL of formamide and 350 g of water are prepared into solution B-2; the weight ratio of formamide to water is 0.7:1;

[0103] (1-ii) The above solution A-1 is poured into a ball mill together with 630 g of titanium dioxide, 3.26 g of niobium oxalate, 3.68 g of zirconium sulfate, 18.99 g of antimony sesquioxide, 70 g of vinyl acetate / ethylene copolymer emulsion is added, and ball milling is performed for 4 h to form a uniform first slurry, and the viscosity of the first slurry is controlled to be 12 mPa·s;

[0104] (2-ii) The above solution B-2 was mixed with 630 g of titanium dioxide, 3.72 g of niobium oxalate, 15.46 g of antimony trioxide, and 70 g of a vinyl acetate / ethylene copolymer emulsion was added into a ball mill, and ball-milling was performed for 4 h to form a uniform second slurry, and the viscosity of the second slurry was controlled to be 12 mPa s;

[0105] (1-iii) 2000 g of the carrier magnetic ring (outer diameter 8 mm, height 6 mm, wall thickness 1.5 mm) was placed in a rotating drum, and the rotating speed of the drum was controlled to be 10 rpm; the above prepared first slurry was added into a stirring tank of a feed liquid spraying system for stirring; a hot air blower was turned on, and hot air at 100°C was blown into the rotating drum to preheat the carrier; when the temperature of the carrier reached 130°C, a feeding nozzle was turned on, and the spraying rate of the first slurry was controlled to be 30 mL / min; the first slurry was sprayed onto the surface of the carrier through the nozzle, and was rapidly dried by the hot air; the content of the inner coating layer reached 5.8 wt% of the weight of the carrier, and the spraying was completed; the catalyst intermediate was obtained by activating at 450°C for 5 h.

[0106] (2-iii) The above prepared second slurry was added into a stirring tank of a feed liquid spraying system for stirring; a hot air blower was turned on, and hot air at 100°C was blown into the rotating drum; a feeding nozzle was turned on, and the spraying rate of the second slurry was controlled to be 30 mL / min; the second slurry was sprayed onto the surface of the catalyst intermediate through the nozzle, and was rapidly dried by the hot air; the content of the outer coating layer reached 6.9 wt% of the weight of the carrier, and the spraying was completed; the catalyst S2 was obtained by activating at 450°C for 5 h.

[0107] The catalyst S2 comprises a carrier, and an inner coating layer and an outer coating layer sequentially loaded on the carrier; the difference between the content of V2O5 in the inner coating layer and the content of V2O5 in the outer coating layer is 0.4 wt%; in the catalyst S2, the weight ratio of the inner coating layer, the outer coating layer and the carrier is 5.8:6.9:100;

[0108] The content of V2O5 is 7.1 wt% based on the total weight of the inner coating layer; the content of the phosphorus compound is 0.2 wt% in terms of P; the content of the cesium compound is 0.1 wt% in terms of Cs; the content of the additive is 3 wt% in terms of oxide; and the content of TiO2 is 89.6 wt%;

[0109] The content of V2O5 is 6.7 wt% based on the total weight of the outer coating layer; the content of the cesium compound is 0.2 wt% in terms of Cs; the content of the additive is 2.3 wt% in terms of oxide; and the content of TiO2 is 90.8 wt%.

[0110] Example 3

[0111] (1-i) 81.66 g of ammonium metavanadate, 178.14 g of oxalic acid, 1.83 g of cesium sulfate, 10.36 g of ammonium dihydrogen phosphate, 220 mL of formamide and 350 g of water were prepared into solution A-3; the weight ratio of formamide to water was 0.7:1;

[0112] (2-i) 47.1 g of ammonium metavanadate, 112.35 g of oxalic acid, 4.56 g of cesium sulfate, 220 mL of formamide and 350 g of water were prepared into solution B-1; the weight ratio of formamide to water was 0.7:1;

[0113] (1-ii) The above solution A-3 was poured into a ball mill together with 630 g of titanium dioxide, 3.36 g of niobium oxalate, 3.57 g of zirconium sulfate, 23.2 g of antimony sesquioxide, 70 g of vinyl acetate / ethylene copolymer emulsion was added, and ball milling was carried out for 4 h to form a uniform first slurry, and the viscosity of the first slurry was controlled to be 12 mPa·s;

[0114] (2-ii) The above solution B-1 was poured into a ball mill together with 630 g of titanium dioxide, 4.87 g of niobium oxalate, 10.96 g of antimony sesquioxide, 70 g of vinyl acetate / ethylene copolymer emulsion was added, and ball milling was carried out for 4 h to form a uniform second slurry, and the viscosity of the second slurry was controlled to be 12 mPa·s;

[0115] (1-iii) 2000 g of carrier magnetic ring (outer diameter 8 mm, height 6 mm, wall thickness 1.5 mm) was placed in a rotating drum, and the rotating drum speed was controlled to be 10 rpm; the above prepared first slurry was added to the stirring tank of the feed liquid spraying system for stirring; the hot air machine was started, and the hot air at 100℃ was passed into the rotating drum to preheat the above carrier, when the carrier temperature reached 130℃, the feeding nozzle was started, and the spraying rate of the first slurry was controlled to be 30 mL / min; the first slurry was sprayed on the surface of the carrier through the nozzle, and was quickly dried through the hot air; the content of the inner coating layer reached 6.5 wt% of the weight of the carrier, the spraying was completed, and the catalyst intermediate was obtained by activating at 450℃ for 5 h;

[0116] (2-iii) The above prepared second slurry was added to the stirring tank of the feed liquid spraying system for stirring; the hot air machine was started, and the hot air at 100℃ was passed into the rotating drum, the feeding nozzle was started, and the spraying rate of the second slurry was controlled to be 30 mL / min; the second slurry was sprayed on the surface of the catalyst intermediate through the nozzle, and was quickly dried through the hot air; the content of the outer coating layer reached 7.7 wt% of the weight of the carrier, the spraying was completed, and the catalyst S3 was obtained by activating at 450℃ for 5 h.

[0117] The catalyst S3 comprises a carrier, and an inner coating and an outer coating sequentially loaded on the carrier; the difference between the content of V2O5 in the inner coating and the content of V2O5 in the outer coating is 3.4 wt%; in the catalyst S3, the weight ratio of the inner coating, the outer coating and the carrier is 6.5:7.7:100;

[0118] The content of V2O5 is 8.8 wt% based on the total weight of the inner coating; the content of the phosphorus compound calculated in terms of P is 0.4 wt%; the content of the cesium compound calculated in terms of Cs is 0.2 wt%; the content of the adjuvant calculated in terms of oxide is 3.5 wt%; and the content of TiO2 is 87.1 wt%;

[0119] The content of V2O5 is 5.4 wt% based on the total weight of the outer coating; the content of the cesium compound calculated in terms of Cs is 0.5 wt%; the content of the adjuvant calculated in terms of oxide is 1.8 wt%; and the content of TiO2 is 92.3 wt%.

[0120] Example 4

[0121] (1-i) 81.66 g of ammonium metavanadate, 178.14 g of oxalic acid, 1.83 g of cesium sulfate, 10.36 g of ammonium dihydrogen phosphate, 220 mL of formamide and 350 g of water are prepared into solution A-3; the weight ratio of formamide to water is 0.7:1;

[0122] (2-i) 60.35 g of ammonium metavanadate, 142.32 g of oxalic acid, 1.99 g of cesium sulfate, 220 mL of formamide and 350 g of water are prepared into solution B-2; the weight ratio of formamide to water is 0.7:1;

[0123] (1-ii) The above solution A-3 is poured into a ball mill together with 630 g of titanium dioxide, 3.26 g of niobium oxalate, 3.57 g of zirconium sulfate, 23.2 g of antimony sesquioxide, 70 g of vinyl acetate / ethylene copolymer emulsion is added, and ball milling is carried out for 4 h to form a uniform first slurry, and the viscosity of the first slurry is controlled to be 12 mPa·s;

[0124] (2-ii) The above solution B-2 is poured into a ball mill together with 630 g of titanium dioxide, 3.72 g of niobium oxalate, 15.46 g of antimony sesquioxide, 70 g of vinyl acetate / ethylene copolymer emulsion is added, and ball milling is carried out for 4 h to form a uniform second slurry, and the viscosity of the second slurry is controlled to be 12 mPa·s;

[0125] (1-iii) Put 2000 g of the carrier magnetic ring (outer diameter 8 mm, height 6 mm, wall thickness 1.5 mm) into the rotating drum, control the rotating drum speed to be 10 rpm; add the first slurry prepared above into the stirring tank of the feed liquid spraying system to stir; turn on the hot air machine, the hot air at 100 ℃ penetrates into the rotating drum, and when the temperature of the carrier reaches 130 ℃, turn on the feeding nozzle to control the spraying rate of the first slurry to be 30 mL / min; the first slurry is sprayed on the surface of the carrier through the nozzle and dried rapidly via the hot air; the content of the inner coating layer reaches 6.7 wt% of the weight of the carrier, and after the spraying is completed, the catalyst intermediate is obtained by activating at 450 ℃ for 5 h;

[0126] (2-iii) Add the second slurry prepared above into the stirring tank of the feed liquid spraying system to stir; turn on the hot air machine, the hot air at 100 ℃ penetrates into the rotating drum, and turn on the feeding nozzle to control the spraying rate of the second slurry to be 30 mL / min; the second slurry is sprayed on the surface of the catalyst intermediate through the nozzle and dried rapidly via the hot air; the content of the outer coating layer reaches 6.9 wt% of the weight of the carrier, and after the spraying is completed, the catalyst S4 is obtained by activating at 450 ℃ for 5 h.

[0127] The catalyst S4 comprises a carrier, and an inner coating layer and an outer coating layer sequentially loaded on the carrier; the difference between the content of V2O5 in the inner coating layer and the content of V2O5 in the outer coating layer is 2.1 wt%; in the catalyst S4, the weight ratio of the inner coating layer, the outer coating layer and the carrier is 6.7:6.9:100;

[0128] Based on the total weight of the inner coating layer, the content of V2O5 is 8.8 wt%; the content of the phosphorus compound calculated by P is 0.4 wt%; the content of the cesium compound calculated by Cs is 0.2 wt%; the content of the additive calculated by oxide is 3.5 wt%; the content of TiO2 is 87.1 wt%;

[0129] Based on the total weight of the outer coating layer, the content of V2O5 is 6.7 wt%; the content of the cesium compound calculated by Cs is 0.2 wt%; the content of the additive calculated by oxide is 2.3 wt%; the content of TiO2 is 90.8 wt%.

[0130] Comparative Example 1

[0131] According to Example 1, except that there is no second slurry, that is:

[0132] (1) 64.59 g of ammonium metavanadate, 146.7 g of oxalic acid, 1.08 g of cesium sulfate, 5.33 g of ammonium dihydrogen phosphate, 220 mL of formamide and 350 g of water are prepared into solution A-1; the weight ratio of formamide to water is 0.7:1;

[0133] (2) Pour the above solution A-1 into a ball mill together with 630 g of titanium dioxide, 3.26 g of niobium oxalate, 3.68 g of zirconium sulfate, 18.99 g of antimony trioxide, add 70 g of vinyl acetate / ethylene copolymer emulsion, mill for 4 h to form a uniform slurry, and control the slurry viscosity to be 12 mPa·s;

[0134] (3) Put 2000 g of carrier magnetic rings (outer diameter 8 mm, height 6 mm, wall thickness 1.5 mm) into a rotating drum, control the rotating drum speed to be 10 rpm; pour the above prepared slurry into the stirring tank of the liquid feeding and spraying system for stirring; start the air heater, and the hot air at 100℃ penetrates into the rotating drum to preheat the carrier; when the carrier temperature reaches 130℃, start the feeding nozzle, and control the slurry spraying rate to be 30 mL / min; the slurry is sprayed on the surface of the carrier through the nozzle, and is rapidly dried through the hot air; the coating content reaches 13.3 wt% of the weight of the carrier, the spraying is completed, and the catalyst DS1 is obtained by activating at 400℃ for 5 h.

[0135] The catalyst DS1 comprises a carrier and a coating loaded on the carrier; in the catalyst DS1, the weight ratio of the coating to the carrier is 13.3:100;

[0136] The content of V2O5 is 7.1 wt% based on the total weight of the coating; the content of the phosphorus compound calculated as P is 0.2 wt%; the content of the cesium compound calculated as Cs is 0.1 wt%; the content of the additive calculated as an oxide is 3 wt%; and the content of TiO2 is 89.6 wt%.

[0137] Comparative Example 2

[0138] According to Example 3, except that there is no second slurry, namely:

[0139] (1) Prepare solution A-3 by mixing 81.66 g of ammonium metavanadate, 178.14 g of oxalic acid, 1.83 g of cesium sulfate, 10.36 g of ammonium dihydrogen phosphate, 220 mL of formamide and 350 g of water; the weight ratio of formamide to water is 0.7:1;

[0140] (2) Pour the above solution A-3 into a ball mill together with 630 g of titanium dioxide, 3.36 g of niobium oxalate, 3.57 g of zirconium sulfate, 23.2 g of antimony trioxide, add 70 g of vinyl acetate / ethylene copolymer emulsion, mill for 4 h to form a uniform slurry, and control the slurry viscosity to be 12 mPa·s;

[0141] (3) 2000 g of the carrier magnetic rings (outer diameter 8 mm, height 6 mm, wall thickness 1.5 mm) were placed in a rotating drum, and the rotating drum speed was controlled to be 10 rpm; the slurry prepared above was added into the stirring tank of the feed liquid spraying system for stirring; a hot air blower was turned on, and hot air at 100 ℃ was blown into the rotating drum to preheat the carrier; when the temperature of the carrier reached 130 ℃, the feed nozzle was opened, and the spraying rate of the slurry was controlled to be 30 mL / min; the slurry was sprayed onto the surface of the carrier through the nozzle, and was rapidly dried through the hot air; the content of the coating reached 14.1 wt% of the weight of the carrier, and the spraying was completed; the catalyst DS2 was obtained by activating at 400 ℃ for 5 h.

[0142] In the catalyst DS2, the weight ratio of the coating to the carrier was 14.1:100;

[0143] Based on the total weight of the coating, the content of V2O5 was 8.8 wt%; the content of the phosphorus compound calculated as P was 0.4 wt%; the content of the cesium compound calculated as Cs was 0.2 wt%; the content of the additive calculated as the oxide was 3.5 wt%; and the content of TiO2 was 87.1 wt%.

[0144] Comparative Example 3

[0145] According to the method of Example 1, except that there was no first slurry, namely:

[0146] (1) 47.1 g of ammonium metavanadate, 112.35 g of oxalic acid, 4.56 g of cesium sulfate, 220 mL of formamide and 350 g of water were prepared into a solution B-1; the weight ratio of formamide to water was 0.7:1;

[0147] (2) The above solution B-1 was poured into a ball mill together with 630 g of titanium dioxide, 4.87 g of niobium oxalate, 10.96 g of antimony sesquioxide, and 70 g of ethyl vinyl acetate / ethylene copolymer emulsion was added, and ball milling was performed for 4 h to form a uniform second slurry, and the viscosity of the second slurry was controlled to be 12 mPa·s;

[0148] (3) 2000 g of the carrier magnetic rings (outer diameter 8 mm, height 6 mm, wall thickness 1.5 mm) were placed in a rotating drum, and the rotating drum speed was controlled to be 10 rpm; the slurry prepared above was added into the stirring tank of the feed liquid spraying system for stirring; a hot air blower was turned on, and hot air at 100 ℃ was blown into the rotating drum to preheat the carrier; when the temperature of the carrier reached 130 ℃, the feed nozzle was opened, and the spraying rate of the slurry was controlled to be 30 mL / min; the slurry was sprayed onto the surface of the carrier through the nozzle, and was rapidly dried through the hot air; the content of the coating reached 14.1 wt% of the weight of the carrier, and the spraying was completed; the catalyst DS2 was obtained by activating at 400 ℃ for 5 h.

[0149] The catalyst DS3 comprises a carrier and a coating layer supported on the carrier; the weight ratio of the coating layer to the carrier in the catalyst DS3 is 13.8:100.

[0150] The content of V2O5 is 5.4% by weight based on the total weight of the coating layer; the content of the cesium compound calculated as Cs is 0.5% by weight; the content of the auxiliary calculated as an oxide is 1.8% by weight; and the content of TiO2 is 92.3% by weight.

[0151] Test Example

[0152] The single pipe reactor simulating the industrial production condition is used to test the catalyst performance. The fixed bed single pipe reactor has an inner diameter of 29 mm and a length of 4400 mm. The molten salt circulation is used to remove the heat from the outside of the reaction pipe. The multi-point temperature measuring system is arranged in the reaction pipe. The piperidine catalyst is loaded in a single stage with a loading height of 3100 mm. The reactor outlet is connected to the analysis system and the reactant collection system.

[0153] The catalysts prepared in Examples 1-4 and Comparative Examples 1-3 are evaluated under the same conditions. The catalysts are activated in the oxidation atmosphere for 15 h. The reaction conditions include a pressure of 0.1 MPa and a space velocity of 3500 h -1 The concentration of durene is gradually increased. The samples under different conditions are analyzed at the outlet of the reactor. The performance indicators such as the highest load and the optimal yield of the catalyst are tested. The test results are shown in Table 1.

[0154] Table 1

[0155]

[0156] According to the data in Table 1, when the catalyst provided by the application is used in the reaction of the gas phase oxidation of durene to prepare piperidine, the concentration and conversion rate of durene and the yield of piperidine can be effectively improved. Especially, by further adjusting the difference range between the content of V2O5 in the coating layer and the content of V2O5 in the outer coating layer, the catalytic effect of the catalyst can be further improved.

[0157] Compared with Comparative Example 1, the content of V2O5 in the catalyst prepared in Comparative Example 2 is too high, which causes excessive oxidation. That is, the higher the content of V2O5 in the catalyst, the lower the selectivity, so that the conversion rate of durene and the yield of piperidine are low.

[0158] Compared with Example 1, the catalyst prepared in Comparative Example 3 does not have an inner coating layer, and the outer coating layer with high selectivity is reserved. Therefore, the data of the conversion rate of durene and the yield of piperidine in Comparative Example 3 are close to those in Example 1.

[0159] The preferred embodiments of the present application are described in detail above, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including that each technical feature is combined in any other suitable manner. These simple modifications and combinations should also be considered as disclosed by the present application and fall within the protection scope of the present application.

Claims

1. A catalyst for the gas phase oxidation of durene to pyromellitic dianhydride, characterized in that The catalyst comprises a carrier, and an inner coating and an outer coating successively loaded on the carrier; The inner coating and the outer coating each independently contain V2O5, and the content of V2O5 in the inner coating is > the content of V2O5 in the outer coating; The difference between the content of V2O5 in the inner coating and the content of V2O5 in the outer coating is 0.3-3.5 wt%; the content of V2O5 is 7-12 wt% based on the total weight of the inner coating; the content of V2O5 is 3-6 wt% based on the total weight of the outer coating; and the weight ratio of the inner coating, the outer coating and the carrier in the catalyst is 5.5-6.8:6.9-8.5:

100. The inner coating further contains a phosphorus compound, a cesium compound, an auxiliary agent and TiO2; the content of the phosphorus compound is 0.2-0.4 wt% based on the total weight of the inner coating; the content of the cesium compound is 0.1-0.2 wt% based on Cs; the content of the auxiliary agent is 1.5-5 wt% based on the oxide; and the content of TiO2 is 87.1-89.5 wt%. The outer coating further contains an optional phosphorus compound, a cesium compound, an auxiliary agent and TiO2; the content of the phosphorus compound is 0-0.2 wt% based on P; the content of the cesium compound is 0.2-0.5 wt% based on Cs; the content of the auxiliary agent is 1.5-5 wt% based on the oxide; and the content of TiO2 is 90.7-92.4 wt%.

2. The catalyst of claim 1, wherein, The difference between the content of V2O5 in the inner coating and the content of V2O5 in the outer coating is 0.4-3.4 wt%.

3. The catalyst of claim 1, wherein, The content of V2O5 is 7.1-8.8 wt% based on the total weight of the inner coating.

4. The catalyst of claim 1, wherein, The weight ratio of the inner coating, the outer coating and the carrier in the catalyst is 5.8-6.7:6.9-7.9:

100.

5. The catalyst of claim 1, wherein, The auxiliary agent contains at least one element selected from rubidium, cerium, niobium, chromium, tungsten, silver, cobalt, gallium, indium, antimony, bismuth, zirconium and erbium.

6. The catalyst of any one of claims 1-5, wherein, The carrier is a non-porous inert carrier.

7. The catalyst of claim 6, wherein, The carrier is selected from at least one of alumina, talc, silicon carbide, aluminum silicate, quartz and ceramic.

8. A process for the preparation of a catalyst for the gas phase oxidation of durene to dianhydride according to any one of claims 1 to 7, characterized in that The method comprises the following steps: (1) loading a first slurry containing a vanadium source, a phosphorus source, a cesium source, a titanium source and an auxiliary agent compound on a carrier, drying and activating to load an inner coating on the carrier to form a catalyst intermediate; (2) loading a second slurry containing a vanadium source, an optional phosphorus source, a cesium source, a titanium source and an auxiliary agent compound on the catalyst intermediate, drying and activating to load an outer coating on the inner coating of the catalyst intermediate to form a catalyst; The respective vanadium sources in the first slurry and the second slurry are such that the content of V2O5 in the inner coating is > the content of V2O5 in the outer coating.

9. The method of claim 8, wherein, The viscosity of the first slurry and the second slurry is independently 10-40 mPa·s.

10. The method of claim 9, wherein, The viscosity of the first slurry and the second slurry is independently 12-25 mPa·s.

11. The method of claim 8, wherein, In step (1), the preparation of the first slurry comprises: (1-i) mixing oxalic acid, a vanadium source, a phosphorus source, a cesium source and a solvent to obtain a reaction solution A; (1-ii) mixing the solution A with a titanium source, an auxiliary compound and a binder to obtain a first slurry.

12. The method of claim 8, wherein, In step (2), the preparation of the second slurry comprises: (2-i) mixing oxalic acid, a vanadium source, an optional phosphorus source, a cesium source and a solvent to obtain a reaction solution B; (2-ii) mixing the solution B with a titanium source, an auxiliary compound and a binder to obtain a second slurry.

13. The method of claim 11, wherein, In the first slurry, the weight ratio of the oxalic acid, the vanadium source, the phosphorus source, the cesium source, the solvent, the titanium source, the auxiliary compound and the binder is 20-35:10-15:0.5-2:0.1-0.4:100:105:4-6:11.

7.

14. The method of claim 13, wherein, In the first slurry, the weight ratio of the oxalic acid, the vanadium source, the phosphorus source, the cesium source, the solvent, the titanium source, the auxiliary compound and the binder is 24-30:10-14:0.8-1.8:0.15-0.35:100:105:4.3-5.1:11.

7.

15. The method of claim 12, wherein, In the second slurry, the weight ratio of the oxalic acid, the vanadium source, the phosphorus source, the cesium source, the solvent, the titanium source, the auxiliary compound and the binder is 20-30:7-9.9:0-0.02:0.2-0.8:100:105:2-3.5:11.

7.

16. The method of claim 15, wherein, In the second slurry, the weight ratio of the oxalic acid, the vanadium source, the phosphorus source, the cesium source, the solvent, the titanium source, the auxiliary compound and the binder is 18-24: 7.5-9.5：0-0.01：0.33-0.76：100：105：2.6-3.2：11.7。 17. The method of claim 8, wherein, The solvent in the first slurry and the second slurry is independently selected from water and a water-soluble organic solvent.

18. The method of claim 17, wherein, In the solvent in the first slurry and the second slurry, the weight ratio of water and the water-soluble organic solvent is 1:0.1-1.

19. The method of claim 18, wherein, In the solvent, the weight ratio of water and the water-soluble organic solvent is 1:0.1-0.

7.

20. The method of claim 17, wherein, The water-soluble organic solvent is selected from at least one of methanol, ethanol, formamide and N,N-dimethylamide.

21. The method of any one of claims 11-16, wherein, The binder is selected from at least one of a vinyl acetate-acrylate copolymer emulsion, a vinyl acetate-ethylene copolymer emulsion, a vinyl acetate-maleate copolymer emulsion and an acrylic acid-maleic acid copolymer emulsion.

22. The method of claim 8, wherein, The vanadium source is selected from at least one of ammonium metavanadate, divanadium pentoxide and sodium vanadate.

23. The method of claim 8, wherein, The phosphorus source is selected from at least one of ammonium dihydrogen phosphate, triammonium phosphate and divanadium pentoxide.

24. The method of claim 8, wherein, The cesium source is selected from at least one of cesium nitrate, cesium sulfate, cesium chloride and cesium carbonate.

25. The method of claim 8, wherein, The titanium source is selected from titanium dioxide and / or metatitanic acid.

26. The method of claim 25, wherein, The titanium source is titanium dioxide.

27. The method of claim 26, wherein, The titanium dioxide is anatase TiO2, and has a specific surface area of 10-30 m 2 / g.

28. The method of claim 27, wherein, The titanium dioxide has a specific surface area of 17-26 m 2 / g.

29. The method of claim 8, wherein, The auxiliary compound is a metal oxide and / or a metal water-soluble salt containing at least one of rubidium, cerium, niobium, chromium, tungsten, silver, cobalt, gallium, indium, antimony, bismuth, zirconium and erbium elements.

30. A process for the gas phase oxidation of durene to pyromellitic dianhydride, characterized in that, The method comprises: contacting durene and an oxygen-containing gas with a catalyst and reacting to obtain pimelic anhydride; The catalyst is the catalyst according to any one of claims 1-7.

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