Schiff base modified vanadium phosphorus oxide catalyst as well as preparation method and application thereof

Abstract

The invention provides a Schiff base modified vanadium phosphorus oxide catalyst as well as a preparation method and application thereof, and belongs to the technical field of catalyst preparation. The Schiff base modified vanadium phosphorus oxide catalyst obtained by the invention can be used for heterogeneous selective oxidation of short chain hydrocarbons, and is particularly applied to the process of preparing maleic anhydride by oxidation of n-butane, and experiments prove that the n-butane conversion activity is improved to 91% or above, the selectivity is improved to 74% or above, and the stability is good.

Description

Technical Field

[0001] The invention belongs to the technical field of catalyst preparation, and specifically relates to a Schiff base modified vanadium phosphorus oxygen catalyst, a preparation method thereof and application thereof in heterogeneous selective oxidation of short-chain alkanes, in particular to application thereof in oxidation of n-butane to prepare maleic anhydride. Background Art

[0002] Maleic anhydride is also known as maleic anhydride, which is the third largest organic anhydride after phthalic anhydride and acetic anhydride. Maleic anhydride is an important chemical intermediate. Due to the presence of conjugated maleic groups in its molecular structure, maleic anhydride has high activity and can undergo addition, self-polymerization, copolymerization, amidation, hydrogenation, alkylation, esterification, hydration, oxidation-reduction and other chemical reactions. One of the main uses of maleic anhydride is the production of unsaturated polyester resin (UPR) in the glass fiber reinforced plastic industry; maleic anhydride can also be used to produce 1,4-butanediol (BDO), tetrahydrofuran (THF) and γ-butyrolactone (GBL) in hydrogenation products; it can also be used to produce tartaric acid, succinic acid, malic acid and tetrahydrophthalic anhydride; at the same time, maleic anhydride deep-processing products are also used in the production of high value-added fine chemical products in the fields of coatings, lubricant additives, pesticides, and medicines.

[0003] In the prior art, the main production method of maleic anhydride is the selective oxidation of n-butane. The selective oxidation of n-butane is the only industrialized selective oxidation reaction of low-carbon alkanes. The vanadium phosphorus oxide (VPO) catalyst is currently recognized as the most effective catalyst for the selective oxidation of n-butane to prepare maleic anhydride. Its main component is vanadyl pyrophosphate. Since the selective oxidation of n-butane is a structure-sensitive reaction, the different crystal structures and surface states of the catalyst determine the selectivity and conversion rate of maleic anhydride. Since it was discovered that the complex structure of the vanadium phosphorus oxide catalyst can effectively selectively oxidize n-butane to prepare maleic anhydride, people have conducted a lot of research on this type of catalyst; the catalytic performance of the vanadium phosphorus oxide catalyst is very sensitive to the preparation conditions. Different preparation methods and the introduction of auxiliary agents and additives will lead to a large difference in the catalytic activity of the final catalyst. The vanadium phosphorus oxide catalyst is a composite oxide catalyst with a complex microstructure. Its catalytic activity is closely related to its structure and composition, and its preparation process has a great influence on the catalytic performance. In order to improve the activity and selectivity of the vanadium phosphorus oxide catalyst and improve the economic benefits of the existing device, people have conducted in-depth research on its preparation method.

[0004] The reaction of preparing maleic anhydride by oxidation of n-butane is a lattice oxidation reaction in principle. The adsorption of oxygen and the oxygen storage capacity of the catalyst play a very important role in the oxidation of n-butane. In addition, the reaction of preparing maleic anhydride by oxidation of n-butane is an external diffusion reaction. The specific surface area and pore volume of the vanadium phosphorus oxygen catalyst are relatively small. Therefore, the surface acidity and surface acid distribution of the vanadium phosphorus oxygen catalyst are particularly important.

[0005] The existing technologies have improved the performance of the vanadium phosphorus oxygen catalyst from aspects such as the type of additives, preparation process, and physical properties of the catalyst, but none of them have improved the catalyst from aspects such as the surface acidity and surface acid distribution of the catalyst. Summary of the invention

[0006] One of the purposes of the present invention is to provide a Schiff base modified vanadium phosphorus oxygen catalyst having improved physical properties and catalytic performance. The inventors have found that the Schiff base modified vanadium phosphorus oxygen catalyst is used in the oxidation reaction of hydrocarbons, and the catalytic performance is significantly improved. In particular, in the n-butane oxidation reaction, the n-butane conversion activity (conversion rate) is greatly improved to more than 92%, and the maleic anhydride selectivity is improved to more than 74%.

[0007] The second object of the present invention is to provide a method for preparing a Schiff base modified vanadium phosphorus oxygen catalyst.

[0008] The third object of the present invention is to provide a Schiff base modified vanadium phosphorus oxygen catalyst for use in the heterogeneous selective oxidation of short chain alkanes, especially in the oxidation of n-butane to prepare maleic anhydride.

[0009] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0010] In the first aspect, the present invention provides a Schiff base modified vanadium phosphorus oxygen catalyst, in which vanadium and phosphorus exist in the form of (VO)2P2O7 and β-VOPO4, respectively, the average valence of vanadium is 4.0 to 4.6, and the molar ratio of phosphorus to vanadium is 0.9 to 1.2:1.

[0011] In a second aspect, the present invention provides a method for preparing a Schiff base modified vanadium phosphorus oxygen catalyst, comprising the following steps:

[0012] The vanadium phosphorus oxygen catalyst or its precursor is subjected to high-temperature treatment with a Schiff base ligand or a metal complex thereof to obtain a Schiff base-modified vanadium phosphorus oxygen catalyst.

[0013] The following is a detailed description:

[0014] The preparation method of the Schiff base modified vanadium phosphorus oxygen catalyst comprises one of the following methods:

[0015] Method 1:

[0016] The vanadium phosphorus oxygen catalyst precursor and an excess of Schiff base ligand or its metal complex are uniformly mixed under the protection of an inert gas, and subjected to high temperature treatment under a vacuum environment. The obtained product is extracted with an organic solvent to remove the unreacted ligand to obtain a modified precursor;

[0017] The modified precursor is activated to obtain a Schiff base modified vanadium phosphorus oxygen catalyst.

[0018] Method 2:

[0019] activating a vanadium phosphorus oxygen catalyst precursor to obtain an activated vanadium phosphorus oxygen catalyst;

[0020] The activated vanadium phosphorus oxygen catalyst is uniformly mixed with an excess of Schiff base ligand or its metal complex under the protection of an inert gas, and subjected to high temperature treatment under a vacuum environment. The obtained product is extracted with an organic solvent to remove unreacted ligands, and a Schiff base-modified vanadium phosphorus oxygen catalyst is obtained after calcination.

[0021] In some embodiments, the method for preparing a vanadium phosphorus oxygen catalyst precursor comprises the following steps:

[0022] Vanadium pentoxide and phosphoric acid are reacted in a mixed solution of benzyl alcohol and isobutyl alcohol, and the product is washed, dried and calcined to obtain the catalyst precursor.

[0023] In some embodiments, the Schiff base ligand or its metal complex comprises one of a Schiff base ligand (H2Schiff), a tetrahydroSchiff base ligand (H2[H4]Schiff), a Schiff base metal complex (MSchiff) or a tetrahydroSchiff base metal complex (M[H4]Schiff).

[0024] Schiff bases include, but are not limited to, amine Schiff bases, ketal Schiff bases, hydrazone Schiff bases, semicarbazide Schiff bases, amino acid derivative Schiff bases, nitrogen heterocyclic Schiff bases, etc. For example, Salen, Salpn, Salicyhexen, etc.

[0025] Exemplarily, the preparation method of Schiff base ligand (H2Schiff) (Schiff = Salen, Salpn, Salicyhexen) can include the following steps: dissolving salicylaldehyde in ethanol, adding dropwise an alcohol solution of diamine (ethylenediamine or 1,3-propylenediamine or 1,2-cyclohexanediamine) under stirring, reflux the mixture for 1 to 7 hours, filter out yellow crystals after cooling, wash with petroleum ether, and then recrystallize with ethanol to obtain ligands H2Salen, H2Salpn and H2Salicyhexen.

[0026] Exemplarily, the preparation method of tetrahydro Schiff base ligand (H2[H4]Schiff) may include the following steps: dissolving the Schiff base ligand in an alcohol solvent, adding sodium borohydride (NaBH4), stirring at room temperature for 2 to 8 hours, removing the solvent by reduced pressure distillation, washing the crude product with water, and then recrystallizing with an alcohol solvent to obtain white crystalline ligands H2[H4]Salen, H2[H4]Salpn, and H2[H4]Salicyhexen.

[0027] Exemplarily, the preparation method of Schiff base metal complex (MSchiff) or tetrahydro Schiff base metal complex (M[H4]Schiff) may include the following steps: dissolving Schiff base ligand or tetrahydro Schiff base ligand in an alcohol solvent, adding an alcohol solution of a metal salt (such as Cu(CH3COO)2·H2O, Co(CH3COO)2·4H2O, Mn(CH3COO)2·4H2O, etc.) and a NaOH solution at a reflux temperature, and then reflux for 1 to 6 hours after the addition is completed. After cooling, the metal complex is filtered out and recrystallized with a chloroform / petroleum ether mixed solvent to obtain the corresponding Schiff base metal complex or tetrahydro Schiff base metal complex.

[0028] In some embodiments, the molar ratio of the Schiff base ligand or its metal complex to vanadium is 1 to 7:1.

[0029] The inert gas may be nitrogen or argon.

[0030] In some embodiments, the high temperature treatment is performed at a temperature of 50 to 350° C. and for a treatment time of 5 to 100 hours.

[0031] The temperature varies depending on the type of Schiff base ligand or tetrahydro Schiff base ligand.

[0032] The organic solvent for extraction can be ethanol, ether, isopropanol, acetone, butanone, etc.

[0033] In some embodiments, activation of the precursor comprises the following steps:

[0034] The precursor powder is mixed evenly with graphite and formed into sheets. The activation atmosphere is a mixed gas of air, water vapor and nitrogen. The activation temperature is 400-450° C. and the activation time is 4-6 hours.

[0035] In a third aspect, the present invention provides an application of a Schiff base modified vanadium phosphorus oxygen catalyst in the heterogeneous selective oxidation of short-chain alkanes, particularly in the oxidation of n-butane to prepare maleic anhydride.

[0036] The oxidation of n-butane to maleic anhydride is a lattice oxygen oxidation reaction. During the oxidation reaction, the oxygen molecules adsorbed on the catalyst surface accept V 4+The electrons in the species form O 2- and O - Lattice oxygen ion, lattice oxygen ion O 2- It is a nucleophilic reagent that inserts into the electron-deficient position of hydrocarbon molecules through nucleophilic addition, resulting in selective oxidation. The acidity and alkalinity of the catalyst will directly affect the performance of the catalyst, thus having a greater impact on the oxidation process.

[0037] Due to the special properties of Schiff base, it can react with the surface of vanadium phosphorus oxide 4+ The species are better combined, affecting the lattice oxygen ion O 2- The formation of vanadium phosphorus oxide reduces the lattice oxygen content on the catalyst surface. Therefore, the modification of vanadium phosphorus oxide with Schiff base can better adjust the acidity and alkalinity of the catalyst, thereby affecting the catalyst activity.

[0038] At the same time, the introduction of Schiff base distorts the local charge density, produces a spatial effect, and affects the intermediate structure formed between the n-butane molecule and the surface active center of vanadium phosphorus oxide. This structure directly determines the type of the final product. Therefore, Schiff base modification has a great influence on the performance of vanadium phosphorus oxide catalysts.

[0039] Through experiments, it was found that the invention of the Schiff base modified vanadium phosphorus oxygen catalyst has significantly improved the activity and selectivity of the original catalyst on the basis of the previous one.

[0040] Beneficial effects:

[0041] The Schiff base-modified vanadium phosphorus oxygen catalyst obtained by the present invention can be used for heterogeneous selective oxidation of short-chain hydrocarbons, especially for the process of preparing maleic anhydride by oxidation of n-butane. Experimental verification shows that the n-butane conversion activity is increased to more than 91%, the selectivity is increased to more than 74%, and the stability is good.

[0042] The present invention has been described in detail above, but the above embodiments are only illustrative in nature and are not intended to limit the present invention. In addition, this article is not limited by any theory described in the above prior art or invention content or the following examples. DETAILED DESCRIPTION

[0043] The present invention is further described below in conjunction with examples. It should be noted that the following examples are provided for illustrative purposes only and do not constitute a limitation on the scope of protection claimed for the present invention.

[0044] Unless otherwise specified, the raw materials, reagents, methods, etc. used in the examples are conventional raw materials, reagents, methods in the art.

[0045] Example 1

[0046] Weigh 19g of V2O5 powder, add it to a three-necked flask, add 200mL of isobutanol and 50mL of benzyl alcohol, heat to reflux at a specific stirring rate, reflux for 6h, slowly drop 27.6g of 100wt% industrial-grade phosphoric acid after cooling to 80°C, then heat to reflux, react for 6h and cool naturally. After the slurry temperature drops to room temperature, filter it with suction, wash it with isobutanol, dry it at 120°C for 5h, and then calcine it at 230°C for 5h to obtain a VPO catalyst precursor. Add 3% graphite to the obtained precursor powder and mix it evenly, beat it into pieces, and then place it in an atmosphere oven for activation. The activation atmosphere is controlled to be 50% / 25% / 25% of air / steam / nitrogen, and the heating rate is 5°C / min to 430°C, and keep it at 430°C for 5h to obtain a VPO catalyst.

[0047] 0.6 mol of salicylaldehyde was dissolved in 1.4 L of ethanol. Under stirring, 700 mL of 0.3 mol of 1,3-propylenediamine ethanol solution was added dropwise. The mixture was refluxed for 3 h. After cooling, the yellow crystals were filtered out, washed with petroleum ether, and then recrystallized with ethanol to obtain the H2Salpn ligand.

[0048] The activated VPO catalyst and H2Salpn ligand were mixed evenly under nitrogen protection, then vacuumed, sealed, and then heated to 100°C for 24 hours. Finally, the product was extracted with acetone to remove the unreacted ligand, and then calcined at 200°C for 3 hours under nitrogen protection to obtain a modified catalyst.

[0049] Example 2

[0050] Weigh 19g of V2O5 powder, add it to a three-necked flask, add 200mL of isobutyl alcohol and 50mL of benzyl alcohol, heat to reflux at a specific stirring rate, reflux for 6h, slowly drop 27.6g of 100wt% industrial-grade phosphoric acid after cooling to 80°C, then heat to reflux, react for 6h, and cool naturally. After the slurry temperature drops to room temperature, filter it by suction, wash it with isobutyl alcohol, dry it at 120°C for 5h, and then calcine it at 230°C for 5h to obtain a VPO catalyst precursor.

[0051] 0.4 mol of salicylaldehyde was dissolved in 1.1 L of methanol. Under stirring, 550 mL of 0.2 mol of 1,2-cyclohexanediamine methanol solution was added dropwise. The mixture was refluxed for 4 h. After cooling, the yellow crystals were filtered out, washed with petroleum ether, and then recrystallized with ethanol to obtain the H2Salicyhexen ligand.

[0052] The VPO catalyst precursor and the H2Salicyhexen ligand were mixed evenly under nitrogen protection, then vacuumed and sealed, and then heated to 120°C for 30 hours. Finally, the obtained product was extracted with acetone to remove the unreacted ligand.

[0053] The modified precursor powder was added with 3% graphite and mixed evenly, formed into sheets, and then placed in an atmosphere oven for activation. The activation atmosphere was controlled to be air / water vapor / nitrogen 50% / 25% / 25%, and the temperature was increased to 430°C at a heating rate of 5°C / min and maintained at 430°C for 5h to obtain a modified catalyst.

[0054] Example 3

[0055] Weigh 19g of V2O5 powder, add it to a three-necked flask, add 200mL of isobutanol and 50mL of benzyl alcohol, heat to reflux at a specific stirring rate, reflux for 6h, slowly drop 27.6g of 100wt% industrial-grade phosphoric acid after cooling to 80°C, then heat to reflux, react for 6h and cool naturally. After the slurry temperature drops to room temperature, filter it with suction, wash it with isobutanol, dry it at 120°C for 5h, and then calcine it at 230°C for 5h to obtain a VPO catalyst precursor. Add 3% graphite to the obtained precursor powder and mix it evenly, beat it into pieces, and then place it in an atmosphere oven for activation. The activation atmosphere is controlled to be 50% / 25% / 25% of air / steam / nitrogen, and the heating rate is 5°C / min to 430°C, and keep it at 430°C for 5h to obtain a VPO catalyst.

[0056] 0.8 mol of salicylaldehyde was dissolved in 2.0 L of ethanol. Under stirring, 900 mL of 0.4 mol of ethylenediamine ethanol solution was added dropwise. The mixture was refluxed for 6 h. After cooling, the yellow crystals were filtered out, washed with petroleum ether, and then recrystallized with ethanol to obtain the H2Salen ligand.

[0057] The activated VPO catalyst and H2Salen ligand were mixed evenly under nitrogen protection, then vacuumed and sealed, and then heated to 160°C for 35 hours. Finally, the product was extracted with acetone to remove the unreacted ligand, and then calcined at 250°C for 3 hours under nitrogen protection to obtain a modified catalyst.

[0058] Example 4

[0059] Weigh 19g of V2O5 powder, add it to a three-necked flask, add 200mL of isobutanol and 50mL of benzyl alcohol, heat to reflux at a specific stirring rate, reflux for 6h, slowly drop 27.6g of 100wt% industrial-grade phosphoric acid after cooling to 80°C, then heat to reflux, react for 6h and cool naturally. After the slurry temperature drops to room temperature, filter it with suction, wash it with isobutanol, dry it at 120°C for 5h, and then calcine it at 230°C for 5h to obtain a VPO catalyst precursor. Add 3% graphite to the obtained precursor powder and mix it evenly, beat it into pieces, and then place it in an atmosphere oven for activation. The activation atmosphere is controlled to be 50% / 25% / 25% of air / steam / nitrogen, and the heating rate is 5°C / min to 430°C, and keep it at 430°C for 5h to obtain a VPO catalyst.

[0060] 0.5 mol of H2Salpn ligand (Example 1) was dissolved in 2.5 L of ethanol, and 0.57 mol of sodium borohydride (NaBH4) was added, stirred at room temperature for 8 h, and the solvent was removed by vacuum distillation. The crude product was washed with water and then recrystallized with methanol to obtain H2[H4]Salpn ligand.

[0061] The activated VPO catalyst and H2[H4]Salpn ligand were mixed evenly under nitrogen protection, then vacuumed and sealed, and then heated to 140°C for 30 hours. Finally, the product was extracted with acetone to remove the unreacted ligand, and then calcined at 250°C for 3 hours under nitrogen protection to obtain a modified catalyst.

[0062] Example 5

[0063] Weigh 19g of V2O5 powder, add it to a three-necked flask, add 200mL of isobutyl alcohol and 50mL of benzyl alcohol, heat to reflux at a specific stirring rate, reflux for 6h, slowly drop 27.6g of 100wt% industrial-grade phosphoric acid after cooling to 80°C, then heat to reflux, react for 6h, and cool naturally. After the slurry temperature drops to room temperature, filter it by suction, wash it with isobutyl alcohol, dry it at 120°C for 5h, and then calcine it at 230°C for 5h to obtain a VPO catalyst precursor.

[0064] Dissolve 0.4 mol H2[H4]Salen ligand in 2.0 L methanol, add 800 mL 0.08 mol Mo(CH3COO)2·4H2O methanol solution and 60 mL NaOH solution (1.5 mol·L -1 ), after the addition was completed, refluxed for 6 hours. After cooling, the metal complex was filtered out and recrystallized from a mixed solvent of chloroform / petroleum ether to obtain Mo[H4]Salen ligand.

[0065] The VPO catalyst precursor and Mo[H4]Salen ligand were mixed evenly under nitrogen protection, then vacuumed and sealed, and then heated to 150°C for 25 hours. The obtained product was extracted with acetone to remove the unreacted ligand, and then 0.1 mol·L -1 The uncomplexed metal ions were removed by ion back exchange with NaCl solution and finally, washed with deionized water until there was no chloride ion.

[0066] The modified precursor powder was added with 3% graphite and mixed evenly, formed into sheets, and then placed in an atmosphere oven for activation. The activation atmosphere was controlled to be air / water vapor / nitrogen 50% / 25% / 25%, and the temperature was increased to 430°C at a heating rate of 5°C / min and maintained at 430°C for 5h to obtain a modified catalyst.

[0067] Comparative Example 1

[0068] Weigh 19g of industrial grade V2O5 powder, add it to a three-necked flask, add 200mL of isobutanol and 50mL of benzyl alcohol, heat to reflux at a specific stirring rate, reflux for 6h, slowly drop 27.6g of 100wt% industrial grade phosphoric acid after cooling to 80°C, then heat to reflux, react for 6h and cool naturally. After the slurry temperature drops to room temperature, filter it by suction, wash it with isobutanol, dry it at 120°C for 5h, and then roast it at 260°C for 5h to obtain a VPO catalyst precursor. Add 3% graphite to the obtained precursor powder and mix it evenly, beat it into pieces, and then place it in an atmosphere oven for activation. The activation atmosphere is controlled to be 50% / 25% / 25% of air / steam / nitrogen, and the heating rate is 3°C / min to 430°C, and keep it at 430°C for 6h to obtain a VPO catalyst product.

[0069] <Catalyst Performance Evaluation Method>

[0070] Take 3mL of VPO catalyst, dilute it with quartz sand of the same mesh size, the dilution ratio is 1:3 (catalyst / quartz sand), and place it in a fixed bed reactor with an inner diameter of 15mm to evaluate the catalyst performance. The raw gas composition of the reaction is 1.5% n-butane + 98.5% air, and the volume space velocity is 2000h -1 , reaction temperature 430℃.

[0071] The results are shown in Table 1.

[0072] Table 1

[0073]

[0074] As can be seen from Table 1, the Schiff base modified vanadium phosphorus oxide catalyst of the present invention is applied to the oxidation of n-butane to prepare maleic anhydride, and the n-butane conversion activity is greatly improved to more than 91%, and at the same time, the maleic anhydride selectivity is improved to more than 74%.

[0075] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents, without departing from the spirit and essence of the claims of the present invention; and these modifications or replacements are still within the scope defined by the claims of the present invention.

Claims

1. A Schiff base modified vanadium phosphorus oxygen catalyst, characterized in that: In the Schiff base modified vanadium phosphorus oxygen catalyst, vanadium and phosphorus exist in the form of (VO)2P2O7 and β-VOPO4 respectively, the average valence of vanadium is 4.0-4.6, and the molar ratio of phosphorus to vanadium is 0.9-1.2:

1.

2. A method for preparing the Schiff base modified vanadium phosphorus oxygen catalyst according to claim 1, characterized in that: The following steps are involved: The vanadium phosphorus oxygen catalyst or its precursor is subjected to high-temperature treatment with a Schiff base ligand or a metal complex thereof to obtain a Schiff base-modified vanadium phosphorus oxygen catalyst.

3. The preparation method according to claim 2, characterized in that: The preparation method of the Schiff base modified vanadium phosphorus oxygen catalyst comprises one of the following methods: Method 1: The vanadium phosphorus oxygen catalyst precursor and an excess of Schiff base ligand or its metal complex are uniformly mixed under the protection of an inert gas, and subjected to high temperature treatment under a vacuum environment. The obtained product is extracted with an organic solvent to remove the unreacted ligand to obtain a modified precursor; activating the modified precursor to obtain a Schiff base-modified vanadium phosphorus oxygen catalyst; Method 2: activating a vanadium phosphorus oxygen catalyst precursor to obtain an activated vanadium phosphorus oxygen catalyst; The activated vanadium phosphorus oxygen catalyst is uniformly mixed with an excess of Schiff base ligand or its metal complex under the protection of an inert gas, and subjected to high temperature treatment under a vacuum environment. The obtained product is extracted with an organic solvent to remove unreacted ligands, and a Schiff base-modified vanadium phosphorus oxygen catalyst is obtained after calcination.

4. The preparation method according to claim 3, characterized in that: The preparation method of the vanadium phosphorus oxygen catalyst precursor comprises the following steps: Vanadium pentoxide and phosphoric acid are reacted in a mixed solution of benzyl alcohol and isobutyl alcohol, and the product is washed, dried and calcined to obtain the catalyst precursor.

5. The preparation method according to claim 3, characterized in that: The Schiff base ligand or its metal complex includes one of a Schiff base ligand, a tetrahydroSchiff base ligand, a Schiff base metal complex or a tetrahydroSchiff base metal complex; the Schiff base includes one of an amine Schiff base, a ketal Schiff base, a hydrazone Schiff base, a semicarbazide Schiff base, an amino acid derivative Schiff base, and a nitrogen heterocyclic Schiff base.

6. The preparation method according to claim 3, characterized in that: The molar ratio of the Schiff base ligand or its metal complex to vanadium is 1 to 7:

1.

7. The preparation method according to claim 3, characterized in that: The temperature of the high temperature treatment is 50 to 350° C., and the treatment time is 5 to 100 hours.

8. The preparation method according to claim 3, characterized in that: The activation of the precursor includes the following steps: The precursor powder is mixed evenly with graphite and formed into sheets. The activation atmosphere is a mixed gas of air, water vapor and nitrogen. The activation temperature is 400-450° C. and the activation time is 4-6 hours.

9. Use of the Schiff base-modified vanadium phosphorus oxygen catalyst according to claim 1 or the Schiff base-modified vanadium phosphorus oxygen catalyst prepared by the preparation method according to any one of claims 2 to 8 in the heterogeneous selective oxidation of short-chain alkanes, especially in the oxidation of n-butane to prepare maleic anhydride.

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