Vanadium phosphorus oxide catalyst activation method, activated vanadium phosphorus oxide catalyst and application thereof

Abstract

The invention relates to the technical field of catalysts, and discloses an activation method of a vanadium-phosphorus-oxide catalyst, the activated vanadium-phosphorus-oxide catalyst and application of the activated vanadium-phosphorus-oxide catalyst, the method comprises the following steps: (1) performing first activation on a vanadium-phosphorus-oxide catalyst precursor in an atmosphere I to obtain a first activated product; the atmosphere I is first gas; (2) performing second activation on the first activation product in an atmosphere II to obtain a second activation product; the atmosphere II is mixed gas of air and first gas; (3) performing third activation on the third activation product in an atmosphere III to obtain a third activation product; and (4) performing fourth activation on the third activation product in an atmosphere IV. When the activated vanadium phosphorus oxide catalyst provided by the invention is applied to selective oxidation of n-butane, the catalyst has the advantages of high catalytic activity, high n-butane conversion rate, high maleic anhydride selectivity and high maleic anhydride yield.

Description

Technical Field

[0001] The present invention relates to the technical field of catalysts, and particularly relates to an activation method for a vanadium phosphorus oxygen catalyst, the activated vanadium phosphorus oxygen catalyst, and its application. Background Art

[0002] Maleic anhydride (MA) is abbreviated as maleic anhydride and is an important commonly used organic chemical raw material. Its consumption is second only to phthalic anhydride and acetic anhydride. It can be used to prepare a series of chemicals such as unsaturated polyester resin, alkyd resin, 1,4-butanediol (BDO), γ-butyrolactone (GBL), tetrahydrofuran (THF), maleic acid, fumaric acid, and tetrahydroanhydride, and is widely used in the fields of pesticides, pharmaceuticals, coatings, inks, lubricant additives, paper chemicals, textile finishing agents, food additives, and surfactants, etc. The market development prospect is very considerable.

[0003] At present, the industrial production process route of maleic anhydride is mainly divided into benzene oxidation method and n-butane oxidation method according to raw materials.

[0004] Among them, the benzene oxidation method has been gradually phased out due to its disadvantages such as high toxicity and high raw material cost. Currently, only a few countries rich in benzene resources still use this method to prepare maleic anhydride. The n-butane oxidation method gradually replaces the benzene oxidation method and occupies the dominant position in maleic anhydride production due to its advantages such as less environmental pollution and low raw material cost.

[0005] The selective oxidation of n-butane to maleic anhydride is currently the only industrially realized low-carbon alkane selective oxidation reaction, and the vanadium phosphorus oxide (VPO) catalyst is the most effective catalyst for this reaction. Researchers in this field unanimously believe that the active phase of the VPO catalyst is the (VO)2P2O7 phase of V 4+ and the VOPO4 phase of V 5+ When the two reach a suitable ratio, the catalytic performance of the catalyst reaches the best. Generally, it is considered that the valence of vanadium is less than 5, and a catalyst usually between about 3.8 and 4.8 is particularly suitable. The precursor VOHPO4·1 / 2H2O of the VPO catalyst needs to be activated to have activity. The activation atmosphere has a great influence on the crystal phase structure, vanadium valence state and composition of the catalyst.

[0006] Numerous researchers have done a great deal of relevant work on the activation method of VPO catalysts. However, in most of the reported patent literatures, in-situ activation methods are adopted. That is, after the preparation of the precursor is completed, it is placed in a reaction tube and activated by introducing air and n-butane reaction atmosphere. Generally, a low butane concentration (volume fraction not higher than 2%) and a long activation time (more than 100 h) are used to obtain a VPO catalyst with higher steady-state performance. The performance of this catalyst needs to experience a slow process from low to high, resulting in a very low maleic anhydride production capacity of the factory in the initial stage of the reaction. In addition, using the reaction gas as the activation atmosphere during the activation process, reactions will occur simultaneously while activating. During the crystal phase transformation of the catalyst, it is necessary to form crystal nuclei and also participate in the n-butane oxidation reaction, seriously affecting the crystal phase formation process of the catalyst. These changes in crystal phase composition and components have a great impact on the conversion rate of reactants, the selectivity of products, and the stability of the catalyst.

[0007] US4187235A discloses a method for preparing a VPO catalyst with a high specific surface area and uses it for the oxidation of n-butane to maleic anhydride. Vanadium pentoxide is reduced to a valence between 4.0 - 4.6 with anhydrous primary or secondary alcohol, and then contacted with phosphoric acid to obtain a catalyst precursor. The recovered catalyst precursor is first heated in an air stream to 380 °C at a rate of 3 °C / min and maintained for 2 hours; then in a n-butane / air mixture, it is heated to 480 °C at a rate of 4 °C / min. The disclosed activation method adopts an in-situ activation method, and the activation and reaction use the same reactor, which is convenient to operate. However, a long activation time (more than 100 h) is required to obtain a VPO catalyst with higher steady-state performance, resulting in a very low maleic anhydride production capacity in the initial stage of the reaction.

[0008] In the method for preparing maleic anhydride from n-butane in the presence of a catalyst disclosed in US3888886A, a VPO catalyst modified with chromium, iron, hafnium, zirconium, lanthanum, and cerium metal promoters is used. The catalyst preparation method is as follows: phosphoric acid, vanadium oxide, hydrogen halide, and metal promoters are mixed and refluxed to obtain a VPO catalyst precursor doped with alkali metals; the recovered precursor is dried, formed, and calcined and activated by introducing a butane-air mixture at about 490 °C to obtain an active catalyst. The disclosed activation method adopts a single-stage activation process, which is simple to operate. However, the catalyst releases heat significantly, resulting in problems such as difficult control of the bed temperature and low yield of the finished catalyst.

[0009] US3856824A discloses a method for preparing maleic anhydride by oxidizing saturated fatty hydrocarbons to be oxidized in the presence of a catalyst. Hydrocarbons having 4 to 10 carbon atoms are oxidized in the presence of an iron, vanadium, phosphorus, oxygen catalyst and an added chromium catalyst modifier. The active catalyst is prepared as follows: an acid aqueous slurry of a suitable material source is heated under reflux to obtain a catalyst precursor, which is filtered, dried, and calcined at a temperature of about 400°C - 600°C by passing air, oxygen or an inert gas, preferably air. The disclosed activation process is simple, but the activation temperature used is high. Although increasing the temperature improves the activity, it will reduce the service life of the catalyst.

[0010] CN1162273A discloses an improved method for calcining and activating a VPO catalyst for preparing maleic anhydride from n-butane. The activation of the catalyst is carried out by further heating at 340 - 500°C while passing air and nitrogen or oxygen and nitrogen and keeping the partial pressure of butane in the gas stream not exceeding 0.02 atm, at which time the vanadium valence state is between 3.95 and 4.15. The calcination and activation steps are preferably carried out in a fluidized bed type container. Although the disclosed activation conditions control the average valence state of vanadium in the catalyst, in this activation process, the reaction gas is used as the activation atmosphere, and reactions will occur during activation, and the formation of the crystal phase of the catalyst is easily affected, resulting in a decrease in the catalytic activity of the catalyst.

[0011] CN1068053A discloses an activation method for converting a vanadium / phosphorus mixed oxide catalyst precursor containing vanadium hydrogen phosphate and optionally a promoter component into an active catalyst containing vanadium pyrophosphate and optionally a promoter component. The method includes: (a) starting to heat up in an atmosphere selected from air, steam, inert gas and their mixtures; (b) rapidly heating up by programmed temperature increase in a molecular oxygen / steam atmosphere; (c) heating up and maintaining the temperature first in a molecular oxygen / steam atmosphere and then in a non-oxidizing steam-containing atmosphere. Although the disclosed activation atmosphere selects a hydrocarbon-free mixture of air, steam and inert gas and has a n-butane conversion rate of more than 85%, the activation process lacks the regulation of the crystal phase structure of the catalyst, and the selectivity of the catalyst for maleic anhydride is low and the yield is low.

[0012] Therefore, in view of the above technical defects existing in the technical field, it is of great significance to develop a method for activating vanadium phosphorus oxygen catalysts with short activation time, mild activation conditions, high catalyst activity and good performance, as well as high n-butane conversion rate and high maleic anhydride selectivity. Summary of the Invention

[0013] The object of the present invention is to solve the problem of the relatively low catalytic activity of vanadium phosphorus oxygen catalysts existing in the prior art, and to provide a method for activating vanadium phosphorus oxygen catalysts to improve the catalytic performance of the catalysts.

[0014] To achieve the above object, a first aspect of the present invention provides a method for activating a vanadium phosphorus oxygen catalyst, the method comprising:

[0015] (1) subjecting a vanadium phosphorus oxygen catalyst precursor to a first activation under atmosphere I to obtain a first activation product; said atmosphere I being a first gas;

[0016] (2) subjecting the first activation product to a second activation under atmosphere II to obtain a second activation product; said atmosphere II being a mixed gas of air and a first gas;

[0017] (3) subjecting the third activation product to a third activation under atmosphere III to obtain a third activation product;

[0018] (4) subjecting the third activation product to a fourth activation in atmosphere IV;

[0019] The first gas is selected from at least one of inert gases and carbon oxides;

[0020] Both atmosphere III and atmosphere IV contain air, water vapor and a first gas;

[0021] The volume content of water vapor in atmosphere IV is at least 10% higher than the volume content of water vapor in atmosphere III.

[0022] A second aspect of the present invention provides an activated vanadium phosphorus oxygen catalyst prepared by the method described in the first aspect above.

[0023] A third aspect of the present invention provides the use of the activated vanadium phosphorus oxygen catalyst described in the second aspect above in the selective oxidation of n-butane to prepare maleic anhydride.

[0024] By the above technical solutions, the present invention has at least the following advantages:

[0025] (1) The method for activating a vanadium phosphorus oxygen catalyst provided by the present invention adopts a stepped segmented activation. Through four activation processes, one or more of air, water vapor, inert gas and carbon oxides are used as activation gases in each stage to activate the catalyst precursor, effectively avoiding the problems of catalyst inactivation or uneven activation caused by untimely heat transfer or heat removal during the catalyst activation process. At the same time, the activation time is shortened, the production efficiency is improved, and it has the advantages of strong controllability, simple operation and high economy, and is suitable for industrial production applications.

[0026] (2) During the four-stage activation process of the present invention, by controlling the activation conditions and the water vapor content and oxygen content in the activation gas, the tetravalent vanadium phase is effectively prevented from being overly oxidized into the pentavalent vanadium phase, the content of the pentavalent vanadium phase in the catalyst is reduced, and the average valence state of vanadium in the vanadium phosphorus oxygen catalyst is effectively regulated to be between 4.1 and 4.3. Moreover, it can promote the transformation of the crystal phase structure, significantly improve the catalyst performance, and enable it to have a high n-butane conversion rate while having a high maleic anhydride selectivity. Detailed implementation manners

[0027] In the ranges disclosed herein, the endpoints and any values of the ranges are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0028] As described above, the first aspect of the present invention provides an activation method for a vanadium phosphorus oxygen catalyst, and the method includes:

[0029] (1) Subjecting the vanadium phosphorus oxygen catalyst precursor to a first activation under atmosphere I to obtain a first activation product; the atmosphere I is a first gas;

[0030] (2) Subjecting the first activation product to a second activation under atmosphere II to obtain a second activation product; the atmosphere II is a mixed gas of air and the first gas;

[0031] (3) Subjecting the third activation product to a third activation under atmosphere III to obtain a third activation product;

[0032] (4) Subjecting the third activation product to a fourth activation in atmosphere IV;

[0033] The first gas is selected from at least one of inert gases and carbon oxides;

[0034] Both atmosphere III and atmosphere IV contain air, water vapor, and the first gas;

[0035] The volume content of water vapor in atmosphere IV is at least 10% higher than the volume content of water vapor in atmosphere III.

[0036] In the present invention, the inert gas is at least one of nitrogen, helium, argon, and neon.

[0037] The present invention has no special requirements for the raw materials for preparing the vanadium phosphorus oxygen catalyst precursor. The vanadium phosphorus oxygen catalyst precursor is prepared by an organic synthesis method, and after drying, a dried powder is obtained. It is determined by X-ray powder diffraction analysis that the dried powder mainly contains the VOHPO4·xH2O crystal phase, where 0.5 ≤ x ≤ 4.

[0038] The present invention has no particular limitation on the shape of the vanadium phosphorus oxygen catalyst precursor particles, and they can be various shapes commonly used in the art, and the shape can be a regular sphere, strip, hollow cylinder, cylinder, clover shape, four-leaf clover shape, hollow bow shape or irregular shape.

[0039] Preferably, the outer diameter d1 of the vanadium phosphorus oxygen catalyst precursor particles is 3 - 9 mm, and the inner diameter d2 is 2 - 8 mm.

[0040] More preferably, the outer diameter d1 of the vanadium phosphorus oxygen catalyst precursor particles is 3 - 6 mm, and the inner diameter d2 is 2 - 5 mm. The inventors of the present invention have found that in this preferred case, the activated vanadium phosphorus oxygen catalyst has better catalytic activity.

[0041] Preferably, the bulk density of the vanadium phosphorus oxygen catalyst precursor is 0.5 - 1.5 g / mL.

[0042] The activation process of the present invention is carried out in an activation reactor, and the activation reactor includes a fixed bed reactor and a high-temperature tubular furnace. According to a specific embodiment, when the vanadium phosphorus oxygen catalyst precursor is the aforementioned dried powder, the activation process of the present invention is selected to be carried out in a high-temperature tubular furnace; when the vanadium phosphorus oxygen catalyst precursor is the aforementioned dried powder mixed with graphite and a pore-forming agent and pressed into pellets, the activation process of the present invention is selected to be carried out in a fixed bed reactor. It should be noted that when granulating the catalyst precursor, based on the total weight of the dried powder, the graphite content is 1 - 10% by weight, preferably 2 - 5% by weight, and the pore-forming agent content is 0 - 20% by weight.

[0043] Preferably, in step (1), the conditions for the first activation include: the temperature is 150 - 280 °C; the heating rate is 1 - 15 °C / min; the holding time is 0.1 - 12 h.

[0044] Preferably, the conditions for the first activation include: the holding time is 0.5 - 5 h.

[0045] More preferably, in step (2), the conditions for the second activation include: the temperature is 250 - 380 °C; the heating rate is 1 - 15 °C / min; the holding time is 0.1 - 15 h.

[0046] Preferably, the conditions for the second activation include: a heat preservation time of 5 - 7 h.

[0047] Preferably, in the atmosphere II, the content of air is 5 - 70 vol%, and the content of the first gas is 30 - 95 vol%.

[0048] More preferably, in the atmosphere II, the content of air is 30 - 50 vol%, and the content of the first gas is 50 - 70 vol%.

[0049] In the present invention, the average vanadium valence state of the second activation product is 3.6 - 4.3. According to a specific embodiment, the average vanadium valence state is measured by redox titration.

[0050] More preferably, in step (3), the conditions for the third activation include: a temperature of 400 - 510 °C; a heating rate of 0.5 - 10 °C / min.

[0051] Preferably, the conditions for the third activation include: a temperature of 410 - 445 °C. The inventors of the present invention have found that under this preferred specific embodiment, the activated vanadium phosphorus oxygen catalyst obtained in the present invention has better catalytic activity, and when this catalyst is used in the n - butane oxidation reaction system, it has higher maleic anhydride selectivity and maleic anhydride yield.

[0052] Particularly preferably, in the atmosphere III, the content of air is 5 - 50 vol%, the content of water vapor is 5 - 60 vol%, and the content of the first gas is 5 - 90 vol%.

[0053] More preferably, in the atmosphere III, the content of air is 20 - 40 vol%, the content of water vapor is 20 - 35 vol%, and the content of the first gas is 30 - 60 vol%.

[0054] Even more preferably, in step (4), the conditions for the fourth activation include: a heat preservation time of 5 - 12 h.

[0055] Preferably, in the atmosphere IV, the content of air is 5 - 40 vol%, the content of water vapor is 5 - 70 vol%, and the content of the first gas is 5 - 70 vol%.

[0056] More preferably, in the atmosphere IV, the content of air is 20 - 40 vol%, the content of water vapor is 30 - 50 vol%, and the content of the first gas is 20 - 50 vol%.

[0057] Particularly preferably, the method further includes: cooling the intermediate obtained after the fourth activation in the presence of a second gas to obtain an activated vanadium phosphorus oxygen catalyst with a temperature not higher than 40 °C;

[0058] The content of water vapor in the second gas is not higher than 0.1% by volume.

[0059] Preferably, the second gas is selected from at least one of air, inert gas, and carbon oxides.

[0060] Preferably, the carbon oxides are carbon monoxide and / or carbon dioxide.

[0061] The present invention has no particular requirement for the gas hourly space velocity of each atmosphere in the activation stage. Exemplarily, in the present invention, the gas hourly space velocity of each atmosphere is 1 - 4000 h -1 .

[0062] As described above, the second aspect of the present invention provides an activated vanadium phosphorus oxygen catalyst prepared by the method described in the first aspect.

[0063] Preferably, the average vanadium valence state of the activated vanadium phosphorus oxygen catalyst is 4.1 - 4.3.

[0064] As described above, the third aspect of the present invention provides the use of the activated vanadium phosphorus oxygen catalyst described in the second aspect in the selective oxidation of n-butane to prepare maleic anhydride.

[0065] Preferably, the use of the activated vanadium phosphorus oxygen catalyst in the selective oxidation of n-butane to prepare maleic anhydride at least satisfies the following conditions: the reaction raw materials are a mixed gas of air, n-butane with a molar content of 1 - 2%, and water vapor with a molar content of 2 - 7%, the space velocity is 900 - 2500 h -1 , the reaction temperature is 360 - 430 °C, and the reaction pressure is 0.05 - 0.25 MPa.

[0066] The present invention will be described in detail below through examples. In the following examples, unless otherwise specified, the instruments, reagents, materials, etc. involved are all conventional instruments, reagents, materials, etc., and can be obtained through regular commercial channels. Among them, unless otherwise stated, the reagents used are all ordinary commercially available analytical pure products.

[0067] Example 1

[0068] 1) First activation: Place the vanadium phosphorus oxygen catalyst precursor in an activation reactor. Under the condition that the nitrogen gas hourly space velocity is 900 h -1 , heat it at a heating rate of 5 °C / min until the temperature reaches 180 °C, and the heat preservation time is 4 h to obtain the first activation product.

[0069] 2) Second activation: Place the first activation product in a mixed atmosphere of air - nitrogen (the air content is 40% by volume), and adjust the gas hourly space velocity to 2500 h -1, heating at a heating rate of 5 °C / min until the temperature reaches 330 °C, and holding for 5 h to obtain a second activated product.

[0070] 3) Third activation: Place the second activated product in a mixed atmosphere of air-steam-nitrogen (air content is 40 vol%, steam content is 20 vol%), and adjust the gas hourly space velocity to 3000 h -1 , heating at a heating rate of 3 °C / min until the temperature reaches 430 °C to obtain a third activated product.

[0071] 4) Fourth activation: Place the third activated product in a mixed atmosphere of air-steam-nitrogen (air content is 40 vol%, steam content is 40 vol%), and adjust the gas hourly space velocity to 3500 h -1 , maintaining the temperature at 430 °C and holding for 12 h to obtain an activated intermediate.

[0072] 5) Cooling treatment: Place the obtained activated intermediate in a mixed atmosphere of air-nitrogen (air content is 25 vol%), and adjust the gas hourly space velocity to 3000 h -1 , cooling to below 40 °C to obtain the activated vanadium phosphorus oxygen catalyst A1.

[0073] The specific conditions for each step in the activation processes of Example 1, Example 2, and Example 3 are shown in Table 1.

[0074] Example 4

[0075] This example is carried out by a method similar to that of Example 1. The difference is that the atmosphere III in the third activation is adjusted to: air-steam-nitrogen (air content is 40 vol%, steam content is 10 vol%), and the atmosphere IV in the fourth activation is adjusted to: air-steam-nitrogen (air content is 40 vol%, steam content is 25 vol%), as specifically shown in Table 1.

[0076] Example 5

[0077] This example is carried out by a method similar to that of Example 1. The difference is that the atmosphere IV in the fourth activation is adjusted to: air-steam-nitrogen (air content is 50 vol%, steam content is 40 vol%), as specifically shown in Table 1.

[0078] Comparative Example 1

[0079] This comparative example is carried out by a method similar to that of Example 1. The difference is that in the first activation, the atmosphere I is adjusted to air, as specifically shown in the continued Table 1.

[0080] Comparative Example 2

[0081] This comparative example was carried out in a similar manner to Example 1, except that the atmosphere IV in the fourth activation was adjusted to: air - steam - nitrogen (air content is 40% by volume, steam content is 20% by volume), as specifically shown in the continued Table 1.

[0082] Comparative Example 3

[0083] This comparative example was carried out in a similar manner to Example 1, except that the atmosphere III in the third activation was adjusted to: air - nitrogen (air content is 40% by volume), and the atmosphere IV in the fourth activation was adjusted to: air - nitrogen (air content is 40% by volume), as specifically shown in the continued Table 1.

[0084] Table 1

[0085]

[0086]

[0087] Continued Table 1

[0088]

[0089]

[0090] Comparative Example 4

[0091] The vanadium phosphorus oxygen catalyst precursor was placed in an activation reactor, and under a mixed atmosphere of air - steam - nitrogen (air content is 40% by volume, steam content is 20% by volume), it was heated at a heating rate of 5°C / min until the temperature reached 430°C, and the holding time was 12 h.

[0092] Subsequent cooling treatment was carried out with reference to the method of Example 1 to obtain the activated vanadium phosphorus oxygen catalyst D - A4.

[0093] Test Example

[0094] The above - obtained activated vanadium phosphorus oxygen catalyst was applied to the n - butane oxidation reaction system. The reaction temperature was set at 405°C, the gas hourly space velocity was 2000 h -1 , the reaction pressure was 0.1 MPa, and the reaction system was a mixture of n - butane - steam - air. Among them, the molar content of n - butane was 1.7%, and the molar content of steam was 3%. The gas - phase composition of the tail gas was analyzed by gas chromatography, and the obtained results are shown in Table 2.

[0095] The calculation formulas for the n - butane conversion rate, maleic anhydride selectivity, and maleic anhydride yield in Table 2 are:

[0096] n-Butane conversion rate % = [(Amount of n-butane at the inlet of the reaction tube - Amount of n-butane at the outlet of the reaction tube) / Amount of n-butane at the inlet of the reaction tube] × 100%

[0097] Maleic anhydride selectivity % = [Amount of maleic anhydride at the outlet of the reaction tube / (Amount of n-butane at the inlet of the reaction tube - Amount of n-butane at the outlet of the reaction tube)] × 100%

[0098] Maleic anhydride yield mol% = n-Butane conversion rate × Maleic anhydride selectivity × 100%

[0099] Table 2

[0100]

[0101] It can be seen from the results in Table 2 that the activated vanadium phosphorus oxygen catalyst obtained by using the present invention in the catalytic n-butane selective oxidation reaction system has a high n-butane conversion rate, better maleic anhydride selectivity and maleic anhydride yield.

[0102] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for activating a vanadium phosphorus oxygen catalyst, characterized in that, The method includes: (1) First activating a vanadium phosphorus oxygen catalyst precursor under Atmosphere I to obtain a first activation product; Atmosphere I is a first gas; (2) Second activating the first activation product under Atmosphere II to obtain a second activation product; Atmosphere II is a mixed gas of air and the first gas; (3) Third activating the third activation product under Atmosphere III to obtain a third activation product; (4) Fourth activating the third activation product in Atmosphere IV; The first gas is selected from at least one of inert gases and carbon oxides; Both Atmosphere III and Atmosphere IV contain air, water vapor, and the first gas; The volume content of water vapor in Atmosphere IV is at least 10% higher than the volume content of water vapor in Atmosphere III.

2. The method according to claim 1, wherein In step (1), the conditions for the first activation include: a temperature of 150 - 280 °C; a heating rate of 1 - 15 °C / min; a heat preservation time of 0.1 - 12 h; Preferably, the conditions for the first activation include: a heat preservation time of 0.5 - 5 h.

3. The method according to claim 1 or 2, wherein In step (2), the conditions for the second activation include: a temperature of 250 - 380 °C; a heating rate of 1 - 15 °C / min; a heat preservation time of 0.1 - 15 h; Preferably, the conditions for the second activation include: a heat preservation time of 5 - 7 h; Preferably, in Atmosphere II, the content of air is 5 - 70 vol%, and the content of the first gas is 30 - 95 vol%; Preferably, in Atmosphere II, the content of air is 30 - 50 vol%, and the content of the first gas is 50 - 70 vol%.

4. The method according to claim 1 or 2, wherein In step (3), the conditions for the third activation include: a temperature of 400 - 510 °C; a heating rate of 0.5 - 10 °C / min; Preferably, the conditions for the third activation include: a temperature of 410 - 445 °C; Preferably, in Atmosphere III, the content of air is 5 - 50 vol%, the content of water vapor is 5 - 60 vol%, and the content of the first gas is 5 - 90 vol%; Preferably, in Atmosphere III, the content of air is 20 - 40 vol%, the content of water vapor is 20 - 35 vol%, and the content of the first gas is 30 - 60 vol%.

5. The method according to claim 1 or 2, wherein In step (4), the conditions for the fourth activation include: a heat preservation time of 5 - 12 h; Preferably, in Atmosphere IV, the content of air is 5 - 40 vol%, the content of water vapor is 5 - 70 vol%, and the content of the first gas is 5 - 70 vol%; Preferably, in Atmosphere IV, the content of air is 20 - 40 vol%, the content of water vapor is 30 - 50 vol%, and the content of the first gas is 20 - 50 vol%.

6. The method according to claim 1 or 2, wherein The method further includes: cooling the intermediate obtained after the fourth activation in the presence of a second gas to obtain an activated vanadium phosphorus oxygen catalyst with a temperature not higher than 40 °C; The content of water vapor in the second gas is not higher than 0.1 vol%.

7. The method according to claim 6, wherein The second gas is selected from at least one of air, inert gases, and carbon oxides.

8. The method according to claim 1 or 2, wherein The carbon oxides are carbon monoxide and / or carbon dioxide.

9. The activated vanadium phosphorus oxygen catalyst prepared by the method according to any one of claims 1-8; Preferably, the average vanadium valence state of the activated vanadium phosphorus oxygen catalyst is 4.1-4.

3.

10. Use of the activated vanadium phosphorus oxygen catalyst according to claim 9 in the selective oxidation of n-butane to prepare maleic anhydride; Preferably, the application of the activated vanadium phosphorus oxygen catalyst in the selective oxidation of n-butane to maleic anhydride satisfies at least the following conditions: the reaction raw materials are air, a mixed gas of n-butane with a molar content of 1-2% and water vapor with a molar content of 2-7%, and the volume space velocity is 900-2500 h -1 , the reaction temperature is 360-430 °C, and the reaction pressure is 0.05-0.25 MPa.

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