OXOVANADIUM PHOSPHATE CATALYST, AND PREPARATION METHOD AND APPLICATION THEREFOR

Abstract

Provided are an oxovanadium phosphate catalyst, and a preparation method and an application therefor. The method includes: 1) mixing and reacting a vanadium source, a choline chloride-organic carboxylic acid eutectic solvent, and alcohol; 2) mixing the obtained reaction product with a phosphorus source, raising the temperature to a temperature higher than the melting point of the eutectic solvent, and continuing the reaction to obtain an oxovanadium phosphate precursor; and 3) calcining to obtain the oxovanadium phosphate catalyst. The alcohol is: benzyl alcohol or a mixture of C.sub.3-C.sub.8 monohydric alcohol and benzyl alcohol. The present method uses a green and inexpensive eutectic solvent to strengthen the preparation of oxovanadium phosphate catalyst, avoids the disadvantages of the prior art, and overcoming the problems of low yield and poor selectivity when used in a reaction to prepare maleic anhydride by catalytic n-butane selective oxidisation.

Claims

1. A method for preparing a vanadyl phosphate catalyst, comprising: (1) mixing a vanadium source with a deep eutectic solvent and alcohol to obtain a mixture, and reacting the mixture; (2) mixing a reaction product obtained in step (1) with a phosphorus source, raising the temperature to a temperature higher than a melting point of the deep eutectic solvent, and continuing a reaction to obtain a vanadyl phosphate precursor; and (3) carrying out calcination to obtain the vanadyl phosphate catalyst; wherein the deep eutectic solvent is a deep eutectic solvent formed by choline chloride and an organic carboxylic acid; and the alcohol is benzyl alcohol or a mixture of C.sub.3-C.sub.8 monohydric alcohol and benzyl alcohol.

2. The method of claim 1, wherein the organic carboxylic acid comprises any one or a combination of at least two of malonic acid, oxalic acid, and tartaric acid.

3. The method of claim 2, wherein the C.sub.3-C.sub.8 monohydric alcohol is any one or a combination of at least two of propanol, isobutanol, n-butanol, pentanol, hexanol, heptanol, and octanol.

4. The method of claim 3, wherein the alcohol is a mixture of isobutanol and benzyl alcohol.

5. The method of claim 1 wherein when the alcohol is benzyl alcohol, a volume ratio of the deep eutectic solvent to benzyl alcohol is (0.15-0.25):1; or when the alcohol is the mixture of C.sub.3-C.sub.8 monohydric alcohol and benzyl alcohol, a volume ratio of the deep eutectic solvent, C.sub.3-C.sub.8 monohydric alcohol, and benzyl alcohol is (0.15-0.25):(3-5):1.

6. The method of claim 1, further comprising adding either or both of a metal oxide and a metal salt while adding the deep eutectic solvent.

7. The method of claim 6, wherein a metal element in the metal oxide or the metal salt is independently selected from any one or a combination of at least two of Fe, Cu, Co, Mn, Ni, Zr, Zn, Ce, and Mo.

8. The method of claim 7, wherein an atomic molar ratio of the metal element to a vanadium element in the vanadium source is 0.0005-0.035.

9. The method of claim 1, wherein a mass ratio of the vanadium source to the deep eutectic solvent is (50-10):1; in the mixture, the vanadium source has a concentration of 0.02 g/mL to 0.12 g/mL; and a molar ratio of phosphorus in the phosphorus source to vanadium in the vanadium source is (0.8-1.5):1.

10. The method of claim 1, wherein a manner for the mixing in step (1) is placing the vanadium source in a vessel, and then adding a mixed solution of the deep eutectic solvent and the alcohol.

11. The method of claim 1, wherein in step (3), a calcination atmosphere is a nitrogen atmosphere, a mixed atmosphere of n-butane and air, or a mixed atmosphere of n-butane, oxygen, and nitrogen.

12. The method of claim 1 comprising: (1) placing vanadium pentoxide in a vessel and then adding a deep eutectic solvent, isobutanol, and benzyl alcohol to be mixed with vanadium pentoxide to obtain a mixture, reacting the mixture at 130 C. to 140 C. for 3 h to 5 h, and then cooling the reaction product to 30-80 C.; (2) adding a phosphorus source to the vessel, raising the temperature to 100 C. to 200 C., continuing a reaction for 10 h to 24 h, filtering, washing, and drying the resultant to obtain a vanadyl phosphate precursor; and (3) carrying out calcination at 350 C. to 550 C. for 10 h to 24 h in a nitrogen atmosphere, a mixed atmosphere of n-butane and air, or a mixed atmosphere of n-butane, oxygen, and nitrogen, to achieve in-situ activation so as to obtain the vanadyl phosphate catalyst; wherein the deep eutectic solvent is a deep eutectic solvent formed by choline chloride and an organic carboxylic acid; a mass ratio of vanadium pentoxide to the deep eutectic solvent is (20-30):1; a volume ratio of the deep eutectic solvent, isobutanol, and benzyl alcohol is (0.15-0.25):(3-5):1; in the mixture, vanadium pentoxide has a concentration of 0.02 g/mL to 0.12 g/mL; and a molar ratio of phosphorus in the phosphorus source to vanadium in the vanadium source is (0.9-1.2):1.

13. A vanadyl phosphate catalyst prepared by the method of claim 1.

14. A use of the vanadyl phosphate catalyst of claim 13 for selective oxidation of n-butane to maleic anhydride, wherein reaction conditions for the selective oxidation of n-butane to maleic anhydride comprise a reaction temperature of 400 C. to 550 C. a pressure of 0.1 MPa to 0.3 MPa, a space velocity of a mixed gas of n-butane of 1000 h.sup.1 to 2500 h.sup.1, and an n-butane concentration of 1.3 wt % to 1.8 wt %.

15. (canceled)

16. The method of claim 1, wherein a temperature for the reaction in step (1) is 100 C. to 180 C.

17. The method of claim 16, wherein a duration for the reaction in step (1) is 2 h to 8 h.

18. The method of claim 1, wherein in step (2), the temperature is raised to 35 C. to 200 C. higher than the melting point of the deep eutectic solvent.

19. The method of claim 18, wherein in step (2), the reaction is continued for a duration of 10 h to 24 h.

20. The method of claim 11, wherein in the mixed atmosphere of n-butane and air, a volume ratio of n-butane to the air is (0.8-1.8):100.

21. The method of claim 1, wherein a temperature for the calcination in step (3) is 350 C. to 550 C.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0061] FIG. 1 is a scanning electron micrograph of a vanadyl phosphate precursor obtained in step (2) in Example 1 of the present application.

[0062] FIG. 2 is a scanning electron micrograph of an activated vanadyl phosphate catalyst obtained in step (4) in Example 1 of the present application.

[0063] FIG. 3 is a scanning electron micrograph of a vanadyl phosphate catalyst precursor obtained in step (2) in Example 2 of the present application.

[0064] FIG. 4 is a scanning electron micrograph of an activated vanadyl phosphate catalyst obtained in step (4) in Example 2 of the present application.

[0065] FIG. 5 is a scanning electron micrograph of a vanadyl phosphate catalyst precursor obtained in step (2) in Example 3 of the present application.

[0066] FIG. 6 is a scanning electron micrograph of an activated vanadyl phosphate catalyst obtained in step (4) in Example 3 of the present application.

DETAILED DESCRIPTION

[0067] Solutions of the present application are described more fully below through specific embodiments in conjunction with the drawings.

[0068] An embodiment of the present application provides a method for preparing a vanadyl phosphate catalyst. The method includes steps described below.

[0069] In S01 a vanadium source was placed in a vessel, a mixed solution of a deep eutectic solvent, isobutanol, and benzyl alcohol with a volume ratio of (0.15-0.25):(3-5):1 was added, the temperature was raised to 100 C. to 180 C. for a reaction of 2 h to 8 h, the resultant was then cooled to 30 C. to 80 C., a phosphorus source was added, the temperature was raised to 100 C. to 200 C. to continue a reaction for 10 h to 24 h, and a product was filtered, washed, and dried to obtain a vanadyl phosphate precursor, where a molar ratio of phosphorus in the phosphorus source to vanadium in the vanadium source was (0.8-1.5):1, the concentration of vanadium pentoxide in the mixed solution of isobutanol and benzyl alcohol was 0.02 g/mL to 0.12 g/mL, and a mass ratio of the vanadium source to the deep eutectic solvent was (50-10):1.

[0070] In S02, the vanadyl phosphate precursor was calcined at 350 C. to 550 C. for 10 h to 24 h to be activated, and cooled to obtain an activated vanadyl phosphate catalyst.

[0071] Specifically, the vanadium source is a vanadium salt or a vanadium oxide, where the vanadium salt was NH.sub.4VO.sub.3, and the vanadium oxide was any one or a combination of at least two of V205, V.sub.2O.sub.4, and V.sub.2O.sub.3; and the phosphorus source was at least one of a phosphoric acid, a phosphate, or a phosphorus oxide, where the phosphate was at least one of (NH.sub.4).sub.3PO.sub.4, (NH.sub.4).sub.2HPO.sub.4, or NH.sub.4H.sub.2PO.sub.4, and the phosphorus oxide is P.sub.2O.sub.5 or P.sub.2O.sub.3. The selected deep eutectic solvent included any one or a combination of at least two of choline chloride-malonic acid, choline chloride-oxalic acid, and choline chloride-tartaric acid.

[0072] The calcination step in the present application may be performed after the vanadyl phosphate precursor was formed, so that the calcined catalyst may be directly used for the evaluation of the effects; or the vanadyl phosphate precursor may also be calcined, followed by forming, and then used for the evaluation of the effects of the catalyst.

[0073] An embodiment of the present application further provides a use of the vanadyl phosphate catalyst obtained by the above method for selective oxidation of n-butane to maleic anhydride.

[0074] An embodiment of the present application further provides a use of the vanadyl phosphate catalyst obtained by the above method for selective oxidation of n-butane to maleic anhydride.

[0075] Reaction conditions for preparing maleic anhydride with vanadium phosphorus oxides include a reaction temperature of 380 C. to 450 C., a normal pressure of 0.1 MPa to 0.5 MPa, a space velocity of a mixed gas of n-butane of 1000 h.sup.1 to 3500 h.sup.1, and an n-butane concentration of 1.0 wt % to 1.8 wt %.

[0076] The method for preparing an vanadyl phosphate catalyst is illustrated below through specific examples. Compounds in the following examples may be directly prepared according to existing methods, and of course, the compounds may also be directly purchased on the market in other examples, but it is not limited thereto.

EXAMPLE 1

[0077] An vanadyl phosphate catalyst was prepared as follows:

[0078] (1) 10 g of V.sub.2O.sub.5 were weighed and placed in a 250 mL three-neck flask, a mixed solution of 1 g of choline chloride-oxalic acid deep eutectic solvent, 80 mL of isobutanol, and 20 mL of benzyl alcohol was added, the mixture was mechanically stirred to be uniformly mixed and refluxed at 135 C. for 3 h, and then the temperature was decreased to 60 C.

[0079] (2) 7.53 mL of 85% H.sub.3PO.sub.4 were added dropwise slowly, and the temperature was raised to 135 C. for continued reflux for 16 h. A product was filtered and washed with absolute ethanol to obtain a blue precipitate which was dried in air for 24 h at 120 C. to obtain vanadyl phosphate catalyst precursor powder.

[0080] (3) The obtained vanadyl phosphate catalyst precursor powder was pressed at a pressure of 15 MPa and crushed, and 20-40 meshes of catalyst particles were sieved.

[0081] (4) The catalyst particles were heated from room temperature at a heating rate of 2 C./min in a mixed reaction atmosphere of n-butane/oxygen/nitrogen with a volume ratio of 1.5:17:81.5 to 430 C. and then activated in situ for 12 h to obtain the vanadyl phosphate catalyst.

[0082] Detection:

[0083] 2.6 g of vanadyl phosphate catalyst were weighed and placed in a fixed-bed reactor with an inner diameter of 14 mm for catalyst performance evaluation. The vanadyl phosphate catalyst reacted with a feed gas whose composition was C.sub.4H.sub.10/O.sub.2/N2=1.4/19.5/79 (v/v/v) at a reaction temperature of 420 C., a reaction pressure of 0.12 MPa, and a gas space velocity of 2000 h.sup.1. A reaction tail gas was analyzed online through gas chromatography to obtain results that a conversion rate of n-butane was 86.94%, the selectivity of maleic anhydride was 60.21%, and a yield of maleic anhydride was 55.24%.

[0084] Crystallographic data of the vanadyl phosphate precursor obtained in step (2) in Example 1 is listed in Table 1, and crystallographic data of the activated vanadyl phosphate catalyst obtained in step (4) in Example 1 is listed in Table 2.

EXAMPLE 2

[0085] A vanadyl phosphate catalyst was prepared as follows:

[0086] (1) 10 g of V.sub.2O.sub.5 were weighed and placed in a 250 mL three-neck flask, a mixed solution of 1 g of choline chloride-tartaric acid deep eutectic solvent, 80 mL of isobutanol, and 20 mL of benzyl alcohol was added, the mixture was mechanically stirred to be uniformly mixed and refluxed at 135 C. for 3 h, and then the temperature was decreased to 60 C.

[0087] (2) 7.53 mL of 85% H.sub.3PO.sub.4 were added dropwise slowly, and the temperature was raised to 135 C. for continued reflux for 16 h. A product was filtered and washed with absolute ethanol to obtain a dark blue precipitate which was dried in air for 24 h at 120 C. to obtain vanadyl phosphate catalyst precursor powder.

[0088] (3) The obtained vanadyl phosphate catalyst precursor powder was pressed at a pressure of 15 MPa and crushed, and 20-40 meshes of catalyst particles were sieved.

[0089] (4) The catalyst particles were heated from room temperature at a heating rate of 2 C./min in a mixed reaction atmosphere of n-butane/oxygen/nitrogen with a volume ratio of 1.5:17:81.5 to 430 C. and then activated in situ for 12 h to obtain the vanadyl phosphate catalyst.

[0090] Detection:

[0091] 2.6 g of vanadyl phosphate catalyst were weighed and placed in a fixed-bed reactor with an inner diameter of 14 mm for catalyst performance evaluation. The vanadyl phosphate catalyst reacted with a feed gas whose composition was C.sub.4H.sub.10/O.sub.2/N.sub.2=1.4/19.5/79 (v/v/v) at a reaction temperature of 420 C., a reaction pressure of 0.12 MPa, and a gas space velocity of 2000 h.sup.1. A reaction tail gas was analyzed online through gas chromatography to obtain results that a conversion rate of n-butane was 88.46%, the selectivity of maleic anhydride was 58.57%, and a yield of maleic anhydride was 52.81%.

[0092] Crystallographic data of the vanadyl phosphate precursor obtained in step (2) in Example 2 is listed in Table 1, and crystallographic data of the activated vanadyl phosphate catalyst obtained in step (4) in Example 2 is listed in Table 2.

EXAMPLE 3

[0093] A vanadyl phosphate catalyst was prepared as follows:

[0094] (1) 10 g of V.sub.2O.sub.5 were weighed and placed in a 250 mL three-neck flask, a mixed solution of 1 g of choline chloride-malonic acid deep eutectic solvent, 80 mL of isobutanol, and 20 mL of benzyl alcohol was added, the mixture was mechanically stirred to be uniformly mixed and refluxed at 135 C. for 3 h, and then the temperature was decreased to 60 C.

[0095] (2) 7.53 mL of 85% H.sub.3PO.sub.4 were added dropwise slowly, and the temperature was raised to 135 C. for continued reflux for 16 h. A product was filtered and washed with absolute ethanol to obtain a blue precipitate which was dried in air for 24 h at 120 C. to obtain vanadyl phosphate catalyst precursor powder.

[0096] (3) The obtained vanadyl phosphate catalyst precursor powder was pressed at a pressure of 15 MPa and crushed, and 20-40 meshes of catalyst particles were sieved.

[0097] (4) The catalyst particles were heated from room temperature at a heating rate of 2 C./min in a mixed reaction atmosphere of n-butane/oxygen/nitrogen with a volume ratio of 1.5:17:81.5 to 430 C. and then activated in situ for 12 h to obtain the vanadyl phosphate catalyst.

[0098] Detection:

[0099] 2.6 g of vanadyl phosphate catalyst were weighed and placed in a fixed-bed reactor with an inner diameter of 14 mm for catalyst performance evaluation. The vanadyl phosphate catalyst reacted with a feed gas whose composition was C.sub.4H.sub.10/O.sub.2/N.sub.2=1.4/19.5/79 (v/v/v) at a reaction temperature of 420 C., a reaction pressure of 0.12 MPa, and a gas space velocity of 2000 h.sup.1. A reaction tail gas was analyzed online through gas chromatography to obtain results that a conversion rate of n-butane was 94.31%, the selectivity of maleic anhydride was 56.27%, and a yield of maleic anhydride was 53.07%.

[0100] Crystallographic data of the vanadyl phosphate precursor obtained in step (3) in Example 3 is listed in Table 1, and crystallographic data of the activated vanadyl phosphate catalyst obtained in step (4) in Example 3 is listed in Table 2.

EXAMPLE 4

[0101] The preparation method and conditions were the same as those in Example 1, except for the following content:

[0102] Addition amounts of vanadium pentoxide, the choline chloride-tartaric acid deep eutectic solvent, isobutanol, benzyl alcohol, and the phosphoric acid were adjusted to 5 g, 1 g, 10 mL, 40 mL, and 3.77 mL, respectively.

[0103] A reflux condition in step (1) was adjusted to reflux at 100 C. for 8 h.

[0104] Step (2) was adjusted to raising the temperature to 150 C. for continued reflux for 12 h.

[0105] Step (4) was adjusted to being heated to 350 C. and being activated in situ for 24 h.

[0106] Detection was carried out by the same method under the same conditions as in Example 1. Detection results were that a conversion rate of n-butane was 92.31%, the selectivity of maleic anhydride was 51.27%, and a yield of maleic anhydride was 47.32%.

EXAMPLE 5

[0107] The preparation method and conditions were the same as those in Example 1, except for the following content:

[0108] Addition amounts of vanadium pentoxide, the choline chloride-tartaric acid deep eutectic solvent, isobutanol, benzyl alcohol, and the phosphoric acid were adjusted to 3 g, 1 g, 15 mL, 40 mL, and 2.26 mL, respectively.

[0109] A reflux condition in step (1) was adjusted to reflux for 5 h at 140 C.

[0110] Step (2) was adjusted to raising the temperature to 180 C. for continued reflux for 10 h.

[0111] Step (4) was adjusted to being heated to 550 C. and being activated in situ for 10 h.

[0112] Detection was carried out by the same method under the same conditions as in Example 1. Detection results were that a conversion rate of n-butane was 92.05%, the selectivity of maleic anhydride was 55.36%, and a yield of maleic anhydride was 50.96%.

EXAMPLE 6

[0113] The preparation method and conditions were the same as those in Example 1, except for the following content:

[0114] Addition amounts of vanadium pentoxide, the choline chloride-tartaric acid deep eutectic solvent, isobutanol, benzyl alcohol, and the phosphoric acid were adjusted to 1.5 g, 1 g, 15 mL, 35 mL, and 1.3 mL, respectively.

[0115] A reflux condition in step (1) was adjusted to reflux for 2 h at 170 C.

[0116] Step (2) was adjusted to raising the temperature to 160 C. for continued reflux for 15 h.

[0117] Step (4) was adjusted to being heated to 450 C. and being activated in situ for 18 h.

[0118] Detection was carried out by the same method under the same conditions as in Example 1. Detection results were that a conversion rate of n-butane was 93.02%, the selectivity of maleic anhydride was 54.17%, and a yield of maleic anhydride was 50.38%.

EXAMPLE 7

[0119] The preparation method and conditions were the same as those in Example 1, except for the following content:

[0120] Types and amounts of solvents added were adjusted. Addition amounts of vanadium pentoxide, the choline chloride-tartaric acid deep eutectic solvent, isobutanol, benzyl alcohol, and the phosphoric acid were 5 g, 1 g, 10 mL, 40 mL, and 3.77 mL, respectively.

[0121] A reflux condition in step (1) was adjusted to reflux for at 100 C. for 5.5 h.

[0122] Step (2) was adjusted to raising the temperature to 150 C. for continued reflux for 12 h.

[0123] Step (4) was adjusted to being heated to 350 C. and being activated in situ for 48 h.

[0124] Detection was carried out by the same method under the same conditions as in Example 1. Detection results were that a conversion rate of n-butane was 90.52%, the selectivity of maleic anhydride was 55.26%, and a yield of maleic anhydride was 50.02%.

EXAMPLE 8

[0125] A vanadyl phosphate catalyst was prepared as follows:

[0126] (1) 10 g of V.sub.2O.sub.5 were weighed and placed in a 250 mL three-neck flask, a mixed solution of 1 g of choline chloride-oxalic acid deep eutectic solvent, 80 mL of isobutanol, and 20 mL of benzyl alcohol was added, the mixture was mechanically stirred to be uniformly mixed and refluxed at 145 C. for 3.5 h, and then the temperature was decreased to 40 C.

[0127] (2) 7.53 mL of 85% H.sub.3PO.sub.4 were added dropwise slowly, and the temperature was raised to 165 C. for continued reflux for 14 h. A product was filtered and washed with absolute ethanol to obtain a blue precipitate which was dried in air for 18 h at 100 C. to obtain vanadyl phosphate catalyst precursor powder.

[0128] (3) The vanadyl phosphate catalyst precursor powder was heated from room temperature at a heating rate of 2 C./min in a mixed reaction atmosphere of n-butane/oxygen/nitrogen with a volume ratio of 1.5:17:81.5 to 475 C. and then activated in situ for 15 h to obtain the vanadyl phosphate catalyst.

[0129] (4) The obtained catalyst was pressed at a pressure of 15 MPa and crushed, and 20-40 meshes of catalyst particles were sieved.

[0130] Detection was carried out by the same method under the same conditions as in Example 1. Detection results were that a conversion rate of n-butane was 93.42%, the selectivity of maleic anhydride was 55.26%, and a yield of maleic anhydride was 51.62%.

COMPARATIVE EXAMPLE 1

[0131] 10 g of V.sub.2O.sub.5 were weighed and placed in a 250 mL three-neck flask, a mixed solution of 80 mL of isobutanol and 20 mL of benzyl alcohol was added, the mixture was mechanically stirred to be uniformly mixed and refluxed at 135 C. for 3 h, and then the temperature was decreased to 60 C. 7.53 mL of 85% H.sub.3PO.sub.4 were added dropwise slowly, and the temperature was raised to 135 C. for continued reflux for 16 h. A product was filtered and washed with absolute ethanol to obtain a blue precipitate which was dried in air for 24 h at 120 C. to obtain vanadyl phosphate catalyst precursor powder. The obtained vanadyl phosphate catalyst precursor powder was pressed at a pressure of 15 MPa and crushed, and 20-40 meshes of catalyst particles were sieved. The catalyst particles were heated from room temperature at a heating rate of 2 C./min in a mixed reaction atmosphere of n-butane/oxygen/nitrogen with a volume ratio of 1.5:17:81.5 to 430 C. and then activated in situ for 12 h to obtain the activated vanadyl phosphate catalyst.

[0132] 2.6 g of activated catalyst were weighed and placed in a fixed-bed reactor with an inner diameter of 14 mm for catalyst performance evaluation. The activated catalyst reacted with a feed gas whose composition was C.sub.4H.sub.10/O.sub.2/N.sub.2=1.5/19.5/79 (v/v/v) at a reaction temperature of 420 C., a reaction pressure of 0.12 MPa, and a gas space velocity of 2000 h.sup.1. A reaction tail gas was analyzed online through gas chromatography to obtain results that a conversion rate of n-butane was 80.86%, the selectivity of maleic anhydride was 55.74%, and a yield of maleic anhydride was 45.07%.

[0133] FIG. 1 is a scanning electron micrograph of the vanadyl phosphate precursor obtained in step (2) in Example 1; FIG. 2 is a scanning electron micrograph of the activated vanadyl phosphate catalyst obtained in step (4) in Example 1; FIG. 3 is a scanning electron micrograph of the vanadyl phosphate catalyst precursor obtained in step (2) in Example 2; FIG. 4 is a scanning electron micrograph of the activated vanadyl phosphate catalyst obtained in step (4) in Example 2; FIG. 5 is a scanning electron micrograph of the vanadyl phosphate catalyst precursor obtained in step (2) in Example 3; and FIG. 6 is a scanning electron micrograph of the activated vanadyl phosphate catalyst obtained in step (4) in Example 3.

[0134] It can be seen from FIGS. 1 to 6 that the vanadyl phosphate catalyst precursor enhanced by the deep eutectic solvent becomes more dispersed, and has a larger sheet thickness and a larger specific surface area; and the improved catalyst, after being activated, has relatively stable structure and is not easy to collapse.

TABLE-US-00001 TABLE 1 Catalyst Half-peak Width Crystal Size precursor I.sub.(001)/I.sub.(130) (001) () (001) (nm) Comparative 51.2 0.396 20.9 example 1 Example 1 68.5 0.214 42.5 Example 2 71.5 0.251 34.8 Example 3 82.6 0.247 35.4

TABLE-US-00002 TABLE 2 Half-peak Width Grain Size Activated Catalyst I.sub.(020)/I.sub.(204) (020) () (020) (nm) Comparative 59.3 0.381 22.0 example 1 Example 1 56.9 0.538 15.3 Example 2 74.4 0.557 14.8 Example 3 65.2 0.502 16.5

[0135] From the crystallographic data of the precursors in Table 1, it can be seen that the vanadyl phosphate catalysts in Examples 1 to 3, relative to the vanadyl phosphate catalyst prepared without adding the deep eutectic solvent in Comparative Example 1, have I(001)/I(130) improved to different degrees, which indicates that the addition of the deep eutectic solvent can induce the growth of a precursor (001) plane, and this crystal plane is a main crystal plane conversed into an active plane. From the crystallographic data of the activated catalysts in Table 2, it can be seen that the catalysts with the addition of the deep eutectic solvent have significantly increased I(020)/I(204) intensity, which proves that the deep eutectic solvent has an effect of inducing the growth of the active plane, and their grain sizes are significantly reduced, which is beneficial to expose more active sites. Especially in Example 2, the relative content of (020) plane is the highest. The exposure of (020) plane increases the activity of the vanadyl phosphate catalyst. While, in Comparative Example 1, the crystal size is large and the active plane has low crystallinity, which are the reasons why the catalyst in Comparative Example 1 has low activity and easily loses activity in a reaction of selective oxidation of n-butane to maleic anhydride.

[0136] The applicant has stated that although the detailed method of the present application is described through the examples described above, the present application is not limited to the detailed method described above, which means that implementation of the present application does not necessarily depend on the detailed method described above. It should be apparent to those skilled in the art that any improvements made to the present application, equivalent replacements of raw materials of the product of the present application, additions of adjuvant ingredients in the product of the present application, and selections of specific manners, etc., all fall within the protection scope and the disclosed scope of the present application.