A composite heteropolyacid catalyst, preparation method and application

Abstract

The present invention belongs to the technical field of catalysts, and specifically relates to a composite heteropoly acid catalyst, a preparation method and an application. The present invention prepares a composite catalyst of a heteropoly acid salt and a multi-metal oxide by mixing a specific raw material ratio and adopting a special preparation method. The process controls the order and conditions of precipitation to accurately control the composition and ratio of the heteropoly acid salt and the multi-metal oxide, thereby improving the conversion rate of the raw materials and the selectivity of the target product; the structural formula of the catalyst is P a V b Mo 11 Fe c Bi d A e B f C g O h ; Under normal pressure, when the reaction temperature is 270°C, there is no obvious deactivation of the catalyst after the reaction is carried out for 200 hours, the isobutyraldehyde conversion rate is ≤99.5%, and the methacrylic acid selectivity is ≤95.3%.

Description

Technical Field

[0001] The invention belongs to the technical field of catalysts, and in particular relates to a composite heteropolyacid catalyst, a preparation method and an application thereof. Background Art

[0002] Methyl methacrylate (MMA) is an important chemical intermediate, mainly used in the production of organic glass. It can also be polymerized with other monomers to form copolymers, which are used in coating modifiers, polyvinyl chloride modifiers and medical polymer materials. At present, the methods for producing MAA include acetone cyanohydrin method, isobutylene two-stage oxidation method, isobutyraldehyde oxidation method and ethylene method. As a by-product of the butyl octanol unit, isobutyraldehyde has good development prospects for the production of MAA from the perspective of environmental protection and economic technology. The use of isobutyraldehyde to prepare MAA usually uses heteropolyacid salts as catalysts. However, the high activity of the catalyst can easily induce peroxidation reactions, reducing the product yield; and the oxidation reaction is highly exothermic, which can easily lead to the loss of catalyst activity, affecting the overall economic benefits.

[0003] CN104302391A discloses a method for preparing a heteropoly acid salt catalyst. The method first prepares a phosphorus molybdenum vanadium heteropoly acid salt powder, and then makes the powder into a slurry and applies it on a carrier to prepare a catalyst. The heteropoly acid salt is prepared by a conventional coprecipitation method. The catalyst can be used for preparing MAA by oxidizing methacrolein and isobutyraldehyde. However, the method introduces impurities during the coprecipitation process, which greatly reduces the purity of the final catalyst. It is also easy to cause the generated heteropoly acid salt powder to have an uneven particle size distribution, which affects the specific surface area and active site distribution of the catalyst, thereby reducing the catalytic performance. In addition, the coprecipitation method for preparing the heteropoly acid catalyst has poor repeatability, and it is difficult to ensure that the heteropoly acid salt prepared each time has the same quality. In addition, when the heteropoly acid salt is coated on the carrier, the loading stability is poor. In actual use, the heteropoly acid salt is likely to fall off the carrier, resulting in a significant decrease in the activity of the catalyst.

[0004] CN104801312B discloses a multi-metal oxide catalyst for preparing methacrolein by oxidation of isobutylene, which is composed of Mo 12 Bi a Fe b A c B d C e D f O gThe invention adds a sufficient amount of macromolecular organic additives to the reaction salt solution to form micelles in the solution, thereby controlling the precipitation rate and obtaining a composite metal oxide with fine particles. Since the organic additives are wrapped around the outside of the particles and partially embedded inside, the catalyst grains are fine and the internal components are evenly distributed. However, the selectivity of methacrolein in the catalytic reaction is not high, and some methacrylic acid by-products are generated, which increases the difficulty of subsequent separation and treatment. In addition, although the addition of macromolecular organic additives can disperse the catalyst particles, there will also be residues, which are easy to decompose, volatilize or induce side reactions, thereby affecting the performance and stability of the catalyst, and also complicating post-processing and increasing costs.

[0005] Wu et al. reported on the research on heteropolyacid catalyst for the oxidation of isobutyraldehyde to methacrylic acid (Journal of Chemistry in Universities, 1985, 6(8):265-266). The research used isobutyraldehyde, a by-product of the propylene carbonyl synthesis unit for the production of butyl octanol, as the raw material to carry out the research work on the one-step oxidation dehydrogenation of isobutyraldehyde to produce methacrylic acid in order to improve the yield of methacrylic acid and solve the separation difficulty. The catalyst was prepared by coprecipitation method, and the primary structure was Keggin structure with a certain characteristic spectral band. After calcination and activation, it was mixed with a carrier to form a reaction. The reaction was carried out in a flow system fixed bed reactor, and the appropriate reaction temperature, space velocity and other conditions were determined. The research also selected a better catalyst carrier and continuously tested the catalyst. The results showed that the catalyst was stable and the activity and selectivity were almost unchanged. However, the research may have the following technical problems: there are many influencing factors in the process of preparing the catalyst by coprecipitation method, and the preparation method is relatively complicated; the specific role and relationship of each component in the catalyst are unclear, which may affect the further optimization of the catalyst performance; although the influence of the properties of the carrier on the selectivity was studied, more in-depth research is needed on how to further improve the selectivity.

[0006] In summary, in the current process of selective oxidation catalysis to prepare methacrylic acid, the existing catalysts have many shortcomings, which not only affect the production efficiency and product quality, but also increase the production cost and environmental burden. Therefore, it is an urgent problem to improve the activity, selectivity and stability of the catalyst at the same time, and make its preparation method simple, easy, economical and environmentally friendly. Summary of the invention

[0007] In view of the shortcomings of the prior art, the present invention provides a composite heteropolyacid catalyst with high activity, good selectivity and stable catalytic performance, a preparation method and a method for selectively oxidizing and catalyzing isobutyraldehyde to prepare methyl methacrylate.

[0008] The object of the present invention can be achieved by the following measures: The present invention adopts a special preparation method to prepare a composite heteropolyacid catalyst, the structural formula of which is P a Vb Mo 11 Fe c Bi d A e B f C g O h , wherein: A is a mixture of one or more alkali metal elements, B is a mixture of one or more Co, Cu, Mn, Pb, Sn, Cr, Au, Ag, Ru elements; C is a mixture of one or more Ti, Sb, As, Ge, Ce, Si, Zr elements; and a, b, c, d, e, f, g, h are the molar ratios of the required elements:

[0009] a=0.25-2.5;

[0010] b = 0.1-4;

[0011] c = 0.5-5;

[0012] d = 0.1-5;

[0013] e = 0.1-5;

[0014] f=0.01-5;

[0015] g = 0.1-5;

[0016] h is the amount of oxygen atoms that satisfy the oxide valence;

[0017] In the XRD spectrum of the catalyst, there are characteristic diffraction peaks of P-Mo heteropolyacid salt at 2θ=10.83°, 21.68°, 26.64°, 30.86°, 36.36°, and there are characteristic diffraction peaks of multi-metal oxide M6Mo at 2θ=12.82°, 23.75°, 27.95°, 28.97°. 12 ikB 1.5 O x The catalyst is used for selectively oxidizing isobutyraldehyde to prepare methacrylic acid, and the isobutyraldehyde conversion rate can reach 99.5%, the methacrylic acid selectivity can reach 95.3%, and the catalyst has no obvious deactivation after the reaction is carried out for 200 hours;

[0018] The present invention also provides a method for preparing the composite heteropolyacid catalyst, wherein the method adopts a special treatment method for the mother liquor, and comprises the following steps:

[0019] Step 1: Preparation of heteropolyacid aqueous solution of phosphomolybdate vanadium I:

[0020] Add high-valent oxides of Mo and V and phosphoric acid to deionized water, stir evenly to form a solution, the stirring rate is 800-1600r / min, heat the solution under reflux for 5-10h at a temperature of 60-120°C to obtain a red transparent phosphomolybdenum vanadium heteropoly acid aqueous solution I, and cool to room temperature for use;

[0021] Step 2: Prepare salt solution II:

[0022] Add molybdenum salt and alkali metal salt containing A into deionized water and stir evenly to form salt solution II;

[0023] Step 3: Prepare salt solution III:

[0024] Add the compound containing B and the compound containing C into deionized water and stir evenly to form salt solution III;

[0025] Step 4: Preparation of composite heteropolyacid catalyst:

[0026] The salt solution II and the salt solution III are added to the phosphomolybdate alum heteropoly acid aqueous solution I simultaneously under stirring to obtain a slurry containing heteropoly acid salts and multi-metal oxides, and the stirring is continued for 20-100 minutes at a stirring rate of 800-1600 r / min. The mixture is filtered, dried, calcined at 250-450° C. for 3-6 hours, and pressed into tablets to obtain the composite heteropoly acid catalyst.

[0027] Furthermore, in step 1, the high-valent oxide of Mo is one of MoO3, (NH4)2MoO4, Na2MoO4 or a mixture of two or more thereof, and the high-valent oxide of V is one of V2O5, NH4VO3, NaVO3 or a mixture of two or more thereof.

[0028] Further, in step 2, the soluble salt of molybdenum is (NH4)6Mo7O 24 ·4H2O, (NH4)2MoO4, Na2MoO4 or a mixture of two or more thereof; the alkali metal salt containing A is one or a mixture of two or more thereof selected from the group consisting of Na, K and Ce.

[0029] Furthermore, in step 3, the compound containing B is one or a mixture of two or more of the salts or oxides of Co, Cu, Mn, Pb, Sn, Cr, Au, Ag, and Ru, and the compound containing C is one or a mixture of two or more of the salts or oxides of Ti, Sb, As, Ge, Ce, Si, and Zr.

[0030] The present invention also provides a method for preparing methacrylic acid by selectively oxidizing isobutyraldehyde using the composite heteropolyacid catalyst, wherein the catalyst is crushed into particles of 20-40 mesh, loaded into a reactor, and a reaction gas is introduced, and the space velocity is set to 1000-2000h -1 , the reaction pressure is 100-200 kPa, the reaction temperature is 100-300°C, and the catalytic reaction is carried out for 100-300 hours.

[0031] Furthermore, the reactor is any one of a fixed bed reactor or a shell-and-tube reactor.

[0032] Furthermore, the reaction gas is a mixed gas of isobutyraldehyde, air, N2, and water vapor, wherein the volume fraction of isobutyraldehyde is 2-20%, the volume fraction of air is 20-70%, the volume fraction of N2 is 10-50%, and the volume fraction of water vapor is 10-60%.

[0033] The beneficial effects of the present invention are:

[0034] 1. The present invention adopts a special preparation method to prepare a heteropolyacid catalyst, firstly prepares a phosphomolybdenum-vanadium heteropolyacid aqueous solution I, then prepares a molybdenum salt and an alkali metal salt into a salt solution II, and prepares the remaining metal salts into a salt solution III, and simultaneously adds the salt solution II and the salt solution III into the phosphomolybdenum-vanadium heteropolyacid aqueous solution I to form a composite catalyst of a heteropolyacid salt and a multi-metal oxide. The process controls the order and conditions of precipitation, and accurately controls the loading amount and distribution of the heteropolyacid on the carrier, thereby unexpectedly improving the performance of the catalyst. Compared with the heteropolyacid catalyst prepared by the traditional co-precipitation method, the catalyst has a higher isobutyraldehyde conversion rate and a higher selectivity for the target product MAA;

[0035] 2. Heteropolyacid salts have strong acidity and redox properties, while polymetallic oxides also have their own structural characteristics and catalytic properties. The combination of the two unexpectedly forms a composite structure that has both the characteristics of heteropolyacid salts and metal oxides. This structural composite can produce a synergistic effect, thereby improving the performance of the catalyst;

[0036] 3. Compared with the traditional co-precipitation method, the special preparation method adopted in the present invention can better control the precipitation process, reduce the introduction of impurities, and improve the purity of the catalyst. DETAILED DESCRIPTION

[0037] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0038] Example 1: Preparation of composite heteropolyacid catalyst 1

[0039] Step 1: Preparation of heteropolyacid aqueous solution of phosphomolybdate vanadium I:

[0040] 19 g of MoO3, 0.5 g of V2O5 and 1.5 g of 85% phosphoric acid were added to 200 mL of deionized water, and the mixture was heated under reflux at 95 °C for 10 h with a stirring rate of 1000 r / min to obtain a red transparent phosphomolybdenum vanadium heteropoly acid aqueous solution I, which was then cooled to room temperature for later use;

[0041] Step 2: Prepare salt solution II:

[0042] Add 20 g of (NH4)6Mo7O into 100 mL of deionized water. 24 4H2O, 1.5g CsNO3, stir evenly to form salt solution II;

[0043] Step 3: Prepare salt solution III:

[0044] Add 8.5 g of Bi(NO3)3·5H2O, 15 g of Fe(NO3)3·9H2O, 20 g of Co(NO3)2·6H2O, 5 g of Ce(NO3)3·6H2O, and 3 g of As2O5 to 200 mL of deionized water and stir to form salt solution III;

[0045] Step 4: Preparation of composite heteropolyacid catalyst:

[0046] The salt solution II and the salt solution III are added to the phosphomolybdate heteropolyacid aqueous solution I simultaneously under stirring to obtain a slurry containing heteropolyacid salts and multi-metal oxides. The slurry is stirred for 60 minutes at a stirring rate of 1000 r / min, filtered, dried, calcined at 400° C. for 4 hours, and pressed into tablets to obtain the composite heteropolyacid catalyst 1.

[0047] The composition of catalyst 1 is: 0.58 V 0.24 Mo 11 Fe 1.66 Bi 0.78 Cs 0.35 Co 3.10 Ce 0.52 As 1.17 O 45.69 .

[0048] Example 2: Preparation of composite heteropolyacid catalyst 2

[0049] Step 1: Preparation of heteropolyacid aqueous solution of phosphomolybdate vanadium I:

[0050] Add 20g of MoO3, 1g of V2O5 and 2g of 85% phosphoric acid into 120mL of deionized water, heat and reflux at 100℃ for 10h, stir at 1000r / min, and obtain red transparent phosphomolybdenum vanadium heteropoly acid aqueous solution I, and cool to room temperature for use;

[0051] Step 2: Prepare salt solution II:

[0052] Add 20 g of (NH4)6Mo7O into 100 mL of deionized water. 24 4H2O, 3g KNO3, stir evenly to form salt solution II;

[0053] Step 3: Prepare salt solution III:

[0054] Add 20 g of Bi(NO3)3·5H2O, 15 g of Fe(NO3)3·9H2O, 15 g of Cu(NO3)2·3H2O, and 2 g of Sb2O5 to 200 mL of deionized water and stir to form salt solution III.

[0055] Step 4: Preparation of composite heteropolyacid catalyst:

[0056] The salt solution Ⅱ and the salt solution Ⅲ were simultaneously added to the phosphomolybdenum vanadium heteropoly acid aqueous solution Ⅰ under stirring to obtain a slurry containing a heteropoly acid salt and a polymetallic oxide, and stirring was continued for 60min at a stirring rate of 1000r / min, filtered, dried, calcined at 370°C for 5h, and pressed into tablets to obtain the composite heteropoly acid catalyst 2;

[0057] The composition of catalyst 2 is: 0.75 V 0.48 Mo 11 Fe 1.62 Bi 1.80 K 1.29 Cu 2.71 S b0.54 O 45.91 .

[0058] Example 3: Preparation of composite heteropolyacid catalyst 3

[0059] Step 1: Preparation of heteropolyacid aqueous solution of phosphomolybdate vanadium I:

[0060] Add 20g of MoO3, 2g of V2O5 and 3g of 85% phosphoric acid into 150mL of deionized water, heat and reflux at 99°C for 10h, with a stirring rate of 1000r / min, to obtain a red transparent phosphomolybdenum vanadium heteropoly acid aqueous solution I, and cool to room temperature for use;

[0061] Step 2: Prepare salt solution II:

[0062] Add 20 g of (NH4)6Mo7O into 100 mL of deionized water. 24 4H2O, 4g KNO3, stir evenly to form salt solution II;

[0063] Step 3: Prepare salt solution III:

[0064] Add 25 g of Bi(NO3)3·5H2O, 20 g of Fe(NO3)3·9H2O, 20 g of Co(NO3)2·6H2O, and 3 g of Sb2O5 to 200 mL of deionized water and stir to form salt solution III.

[0065] Step 4: Preparation of composite heteropolyacid catalyst:

[0066] The salt solution Ⅱ and the salt solution Ⅲ were added to the phosphomolybdenum vanadium heteropoly acid aqueous solution Ⅰ under stirring to obtain a slurry containing a heteropoly acid salt and a polymetallic oxide, and stirring was continued for 60min at a stirring rate of 1000r / min, filtered, dried, calcined at 450°C for 5h, and pressed into tablets to obtain the composite heteropoly acid catalyst 3;

[0067] The composition of catalyst 3 is: 1.13 V 0.96 Mo 11 Fe 2.16 Bi 2.25 K 1.73 Co 3.00 S b0.81 O 50.73 .

[0068] Comparative Example 1: Preparation of Catalyst 4

[0069] Step 1: Prepare salt solution I:

[0070] Add 19 g of MoO3, 0.5 g of V2O5 and 1.5 g of 85% phosphoric acid into 200 mL of deionized water and stir well to obtain salt solution I;

[0071] Step 2: Prepare salt solution II:

[0072] Add 20 g of (NH4)6Mo7O into 100 mL of deionized water. 24 4H2O, 1.5g CsNO3, stir evenly to form salt solution II;

[0073] Step 3: Prepare salt solution III:

[0074] Add 8.5 g of Bi(NO3)3·5H2O, 15 g of Fe(NO3)3·9H2O, 20 g of Co(NO3)2·6H2O, 5 g of Ce(NO3)3·6H2O, and 3 g of As2O5 to 200 mL of deionized water and stir to form salt solution III;

[0075] Step 4: Preparation of composite heteropolyacid catalyst:

[0076] The salt solution II and the salt solution III are added to the salt solution I at the same time under stirring to obtain an orange slurry, and the stirring is continued for 60 minutes at a stirring rate of 1000 r / min, filtered, dried, calcined at 400° C. for 4 hours, and pressed into tablets to obtain the composite heteropolyacid catalyst 4;

[0077] The composition of catalyst 4 is: 0.58 V 0.24 Mo 11 Fe 1.66 Bi 0.78 Cs 0.35 Co 3.10 Ce 0.52 As 1.17 O 45.69 .

[0078] Comparative Example 2: Preparation of Catalyst 5

[0079] Step 1: Prepare salt solution I:

[0080] Add 20 g of MoO3, 1 g of V2O5 and 2 g of 85% phosphoric acid into 120 mL of deionized water and stir well to obtain salt solution I;

[0081] Step 2: Prepare salt solution II:

[0082] Add 20 g of (NH4)6Mo7O into 100 mL of deionized water. 24 4H2O, 3g KNO3, stir evenly to form salt solution II;

[0083] Step 3: Prepare salt solution III:

[0084] Add 20 g of Bi(NO3)3·5H2O, 15 g of Fe(NO3)3·9H2O, 15 g of Cu(NO3)2·3H2O, and 2 g of Sb2O5 to 200 mL of deionized water and stir to form salt solution III.

[0085] Step 4: Preparation of composite heteropolyacid catalyst:

[0086] The salt solution II and the salt solution III are added to the salt solution I at the same time under stirring to obtain a blue-green slurry, and the stirring is continued for 60 minutes at a stirring rate of 1000 r / min, filtered, dried, calcined at 370° C. for 5 hours, and pressed into tablets to obtain the composite heteropolyacid catalyst 5;

[0087] The composition of catalyst 2 is: 0.75 V 0.48 Mo 11 Fe 1.62 Bi 1.80 K 1.29Cu 2.71 S b0.54 O 45.91 .

[0088] The catalytic reaction of selective oxidation of isobutyraldehyde to prepare methacrylic acid is carried out using the above catalysts, and the specific steps are: the above catalysts 1-5 are crushed into particles of 20-40 mesh, 10 mL of the catalyst is measured and loaded into a fixed bed reactor, and a mixed gas of isobutyraldehyde, air, N2, and water vapor is introduced, wherein the volume fraction of the isobutyraldehyde is 5%, the volume fraction of the air is 45%, the volume fraction of the N2 is 20%, and the volume fraction of the water vapor is 30%, and the air velocity is set to 1000h -1 , the reaction pressure was normal pressure, the reaction temperature was 270°C, and there was no obvious deactivation of the catalyst after the reaction was carried out for 200 hours. The tail of the product was connected to a gas chromatograph for automatic sampling and online detection of the product. The isobutyraldehyde conversion rate and methacrylic acid selectivity are recorded in Table 1.

[0089] Table 1. Catalyst reaction performance evaluation results

[0090] Catalyst No. Isobutyraldehyde conversion rate (%) Methacrylic acid selectivity (%) 1 99.5 95.2 2 98.8 94.5 3 99.2 95.3 4 88.7 79.3 5 85.3 81.6

[0091] As can be seen from Table 1, the composite heteropolyacid catalyst prepared by the special preparation method of the present invention is more active than the catalyst prepared by the traditional coprecipitation method, and the selectivity of the product is better; In addition, the catalyst of the present invention has no obvious signs of deactivation after the catalytic reaction continues for 200h, and has good stability. This is because the present invention adopts the process of a special preparation method to form a composite catalyst of a heteropolyacid salt and a polynary metal oxide, which accurately controls the loading amount and distribution of heteropolyacid on a carrier by controlling the order and conditions of precipitation, thereby unexpectedly improving the performance of the catalyst, compared to the heteropolyacid catalyst prepared by the traditional coprecipitation method, with higher isobutyraldehyde conversion and higher selectivity of target product MAA; In addition, heteropolyacid salts have strong acidity and redox properties, and are combined with the structural characteristics and catalytic performance of polynary metal oxides, and unexpectedly form a composite structure having both the characteristics of heteropolyacid salts and metal oxide characteristics, and this structural composite can produce synergistic effects, thereby improving the performance of the catalyst.

[0092] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A composite heteropolyacid catalyst, characterized in that: The structural formula of the catalyst is P 0.58 V 0.24 Mo 11 Fe 1.66 Bi 0.78 Cs 0.35 Co 3.10 Ce 0.52 As 1.17 O 45.69 , the preparation method of the catalyst is: Step 1: Preparation of heteropolyacid aqueous solution of phosphomolybdate vanadium I: 19 g of MoO3, 0.5 g of V2O5 and 1.5 g of 85% phosphoric acid were added to 200 mL of deionized water, and the mixture was heated under reflux at 95 °C for 10 h with a stirring rate of 1000 r / min to obtain a red transparent phosphomolybdenum vanadium heteropoly acid aqueous solution I, which was then cooled to room temperature for later use; Step 2: Prepare salt solution II: Add 20 g of (NH4)6Mo7O into 100 mL of deionized water. 24 4H2O, 1.5g CsNO3, stir evenly to form salt solution II; Step 3: Prepare salt solution III: Add 8.5 g of Bi(NO3)3·5H2O, 15 g of Fe(NO3)3·9H2O, 20 g of Co(NO3)2·6H2O, 5 g of Ce(NO3)3·6H2O, and 3 g of As2O5 to 200 mL of deionized water and stir to form salt solution III; Step 4: Preparation of composite heteropolyacid catalyst: The salt solution II and the salt solution III are added to the phosphomolybdate alum heteropoly acid aqueous solution I simultaneously under stirring to obtain a slurry containing heteropoly acid salts and multi-metal oxides, and the stirring is continued for 60 minutes at a stirring rate of 1000 r / min, filtered, dried, calcined at 400° C. for 4 hours, and pressed into tablets to obtain the composite heteropoly acid catalyst; The composite heteropolyacid catalyst is used for selectively oxidizing isobutyraldehyde to prepare methacrylic acid.

2. A composite heteropolyacid catalyst, characterized in that: The structural formula of the catalyst is P 0.75 V 0.48 Mo 11 Fe 1.62 Bi 1.80 K 1.29 Cu 2.71 S b0.54 O 45.91 , the preparation method of the catalyst is: Step 1: Preparation of heteropolyacid aqueous solution of phosphomolybdate vanadium I: Add 20g of MoO3, 1g of V2O5 and 2g of 85% phosphoric acid into 120mL of deionized water, heat and reflux at 100℃ for 10h, stir at 1000r / min, and obtain red transparent phosphomolybdenum vanadium heteropoly acid aqueous solution I, and cool to room temperature for use; Step 2: Prepare salt solution II: Add 20 g of (NH4)6Mo7O into 100 mL of deionized water. 24 4H2O, 3g KNO3, stir evenly to form salt solution II; Step 3: Prepare salt solution III: Add 20 g of Bi(NO3)3·5H2O, 15 g of Fe(NO3)3·9H2O, 15 g of Cu(NO3)2·3H2O, and 2 g of Sb2O5 to 200 mL of deionized water and stir to form salt solution III. Step 4: Preparation of composite heteropolyacid catalyst: The salt solution II and the salt solution III are added to the phosphomolybdate vanadium heteropoly acid aqueous solution I simultaneously under stirring to obtain a slurry containing a heteropoly acid salt and a polymetallic oxide, and the stirring is continued for 60 min at a stirring rate of 1000 r / min, filtered, dried, calcined at 370 ° C for 5 h, and pressed into tablets to obtain the composite heteropoly acid catalyst; The composite heteropolyacid catalyst is used for selectively oxidizing isobutyraldehyde to prepare methacrylic acid.

3. A composite heteropolyacid catalyst, characterized in that The structural formula of the catalyst is P 1.13 V 0.96 Mo 11 Fe 2.16 Bi 2.25 K 1.73 Co 3.00 S b0.81 O 50.73 , the preparation method of the catalyst is: Step 1: Preparation of heteropolyacid aqueous solution of phosphomolybdate vanadium I: Add 20g of MoO3, 2g of V2O5 and 3g of 85% phosphoric acid into 150mL of deionized water, heat and reflux at 99°C for 10h, with a stirring rate of 1000r / min, to obtain a red transparent phosphomolybdenum vanadium heteropoly acid aqueous solution I, and cool to room temperature for use; Step 2: Prepare salt solution II: Add 20 g of (NH4)6Mo7O into 100 mL of deionized water. 24 4H2O, 4g KNO3, stir evenly to form salt solution II; Step 3: Prepare salt solution III: Add 25 g of Bi(NO3)3·5H2O, 20 g of Fe(NO3)3·9H2O, 20 g of Co(NO3)2·6H2O, and 3 g of Sb2O5 to 200 mL of deionized water and stir to form salt solution III. Step 4: Preparation of composite heteropolyacid catalyst: The salt solution II and the salt solution III are added to the phosphomolybdate vanadium heteropoly acid aqueous solution I simultaneously under stirring to obtain a slurry containing a heteropoly acid salt and a polymetallic oxide, and the stirring is continued for 60 min at a stirring rate of 1000 r / min, filtered, dried, calcined at 450 ° C for 5 h, and pressed into tablets to obtain the composite heteropoly acid catalyst; The composite heteropolyacid catalyst is used for selectively oxidizing isobutyraldehyde to prepare methacrylic acid.