Catalyst for preparing formaldehyde through methanol oxidation and preparation method thereof

Abstract

The invention discloses a catalyst for preparing formaldehyde through methanol oxidation and a preparation method thereof, and belongs to the technical field of catalyst processing. The preparation method comprises the following steps: mixing deionized water, an alkali source, a boron source and a molybdenum source, adding a silicon source, uniformly mixing, crystallizing, distilling, filter-pressing, washing and drying the crystallized material to obtain a catalyst precursor, adding the dried precursor into a mixed aqueous solution of nitric acid and ferric nitrate, filtering, washing, drying, forming and roasting the material to obtain the catalyst, the catalyst is used in a reaction for preparing formaldehyde through methanol oxidation, the prepared catalyst is high in strength and long in service life, and the selectivity of a formaldehyde product obtained after the reaction is high.

Description

Technical Field

[0001] This invention belongs to the field of catalyst processing technology, and particularly relates to a synthesis method and application of a catalyst for the oxidation of methanol to formaldehyde. Background Technology

[0002] Formaldehyde is an important downstream product of methanol and a raw material for organic chemicals. It is one of the common platform molecules in C1 and low-carbon chemicals. Its derivatives include formaldehyde polymers, trialdehyde resins, 1,4-butanediol, polyvinyl alcohol acetal fibers, diphenylmethane diisocyanate, hexamethylenetetramine, tetramethylolmethane, methyl acetal, acrylic acid and its esters, polyoxymethylene dimethyl ether, and ethylene glycol. There are two commonly used methods for formaldehyde production both domestically and internationally: the silver catalytic method and the iron-molybdenum catalytic method. The former has been gradually replaced by the iron-molybdenum method due to the high cost of catalysts, high catalytic reaction temperature, and low formaldehyde concentration produced.

[0003] However, due to the formation of hot spots in the oxidation reaction, molybdenum volatilizes from the catalyst, causing catalyst deactivation. Furthermore, the large amount of air in the reaction leads to catalyst pulverization, reducing the catalyst's catalytic activity and selectivity for aldehydes. Chinese invention patent CN 115487818 A uses a co-precipitation method to adjust the catalyst's physical structure by adding organic amines, thereby improving its activity and stability and extending its lifespan. Chinese invention patent CN 112871180 B, by modifying the catalyst's structure, can reduce the loss of effective components, increase catalyst lifespan, and improve reaction selectivity. The above-mentioned literature all modify the physical structure of the catalyst; however, their solutions to the pulverization problem are limited. Component loss during pulverization is the main reason for the continuous decline in conversion rate and selectivity; therefore, improving strength and reducing component loss are necessary. Summary of the Invention

[0004] The purpose of this invention is to provide a catalyst for the oxidation of methanol to formaldehyde and its preparation method. The aim is to integrate the existing iron-molybdenum catalyst and molecular sieve, and add boron as an auxiliary element to synthesize a catalyst with higher strength that can withstand high space velocities. At the same time, it reduces the degree of catalyst pulverization and component loss. During use, the use of magnetic rings can be reduced to achieve heat transfer effect, and the problems of formaldehyde carbonization and polymerization during use can be reduced.

[0005] To solve the above technical problems and achieve the above-mentioned objectives, the present invention adopts the following technical solution: This invention provides a catalyst for the oxidation of methanol to formaldehyde. The main components of the catalyst are ferric molybdate and molybdenum trioxide, and the catalyst is doped with boron oxide and silicon dioxide.

[0006] Furthermore, the atomic ratio of molybdenum to iron in the catalyst is (2.0~2.5):1.

[0007] Furthermore, the atomic ratio of molybdenum to iron in the catalyst is (2.1~2.4):1.

[0008] Furthermore, the atomic ratio of molybdenum to boron atoms in the catalyst is 1:(0.5~2), and the silica accounts for 20%~31% of the total mass of the catalyst.

[0009] Furthermore, the strength of the catalyst is 5~7 N / particle. The strength was tested using a particle strength tester to measure the Raschig rings. The dimensions of each Raschig ring were: outer diameter 5 mm ± 1 mm, inner diameter 2.4 mm ± 0.02 mm, and height 2.6 mm ± 0.1 mm.

[0010] The present invention also provides a method for preparing the above-mentioned catalyst, comprising: (1) Prepare an aqueous solution I containing an alkaline source and a boron source, add a molybdenum source solution, stir I, and obtain solution A; (2) Add silicon source dropwise to solution A, stir (II), keep warm (I), and obtain solution B; (3) After crystallizing solution B, cool it down rapidly, distill it under reduced pressure, and add an equal amount of deionized water to the distillate; (4) Filter, wash and dry to obtain dry material; (5) Prepare an aqueous solution containing acid and iron salt II, add dry material III, stir III, and keep warm II; (6) Filter, wash, dry, pre-calcinate, shape, and calcinate to obtain the catalyst.

[0011] Furthermore, in step (1), The alkali source is selected from at least one of piperidine, pyridine, and methylpiperidine; the boron source is boric acid; and the molybdenum source is selected from at least one of ammonium molybdate, ammonium dimolybdate, and ammonium heptamolybdate.

[0012] Further, in step (1), the ratio of the mass of water in aqueous solution I to the total mass of the alkali source and the boron source is 1~5:1; the mass ratio of the alkali source to the boron source is 0.1~5:1.

[0013] Further, in step (1), the ratio of the mass of water in aqueous solution I to the total mass of the alkali source and the boron source is 1.5~2:1; the mass ratio of the alkali source to the boron source is 0.5~1:1.

[0014] Furthermore, in step (1), the mass ratio of molybdenum source to water in the molybdenum source solution is 1:4~8; the atomic ratio of molybdenum atoms to boron atoms is 1:0.5~2.

[0015] Furthermore, in step (1), the mass ratio of molybdenum source to water in the molybdenum source solution is 1:5~7.

[0016] Further, in step (1), the stirring I is carried out at 25~40℃ for 1~8h, and the stirring rate is 5~20Hz.

[0017] Furthermore, in step (2), The silicon source is selected from at least one of silica sol, fumed silica, water glass, type C silica gel, micronized silica gel, and silicate ester; the silicon source is calculated as silicon dioxide, and the mass ratio of silicon dioxide to boron source is 0.5~1.5:1.

[0018] Further, in step (2), the silicon source is calculated as silicon dioxide, and the mass ratio of silicon dioxide to boron source is 0.7~1:1.

[0019] Furthermore, in step (2), the stirring II is carried out at 25~40℃ for 1~8h.

[0020] Furthermore, in step (2), the heat preservation I is performed at 40~85℃ for 2~12h.

[0021] Furthermore, in step (3), The crystallization treatment is performed at 160~185℃ for 45~85 hours.

[0022] Furthermore, in step (3), the conditions for vacuum distillation include 75~80℃ and vacuum to -0.055~-0.07MPa.

[0023] Furthermore, in step (4), the washing water mass is 1:3~5 (solid material:water); the drying temperature is 100~150℃; and the drying time is 4~24h.

[0024] Furthermore, in step (4), the washing water mass is 1:3.5~4.5 for solid material:water; the drying time is 6~12h.

[0025] Furthermore, in step (5), The acid is nitric acid or hydrochloric acid, the iron salt is ferric nitrate or ferric chloride, and the mass ratio of the acid to the iron salt is 0.01 to 1:1; in aqueous solution II, the mass ratio of the iron salt to water is 1:4 to 8; after adding dry material, the atomic ratio of molybdenum to iron is (2.0 to 2.5):1.

[0026] Further, in step (5), the mass ratio of the acid to the iron salt is 0.05~0.5:1; in aqueous solution II, the mass ratio of the iron salt to the water is 1:5~7; after adding the dry material, the atomic ratio of molybdenum to iron is (2.1~2.4):1.

[0027] Furthermore, the stirring process III is carried out at 25~40℃ for 1~5 hours.

[0028] Furthermore, the heat preservation II is performed at 100~120℃ for 12~48h.

[0029] Furthermore, in step (6), The washing water ratio is 1:3~5 (solid material:water); the drying temperature is 100~150℃, and the drying time is 4~24h.

[0030] Furthermore, in step (6), the washing water mass is 1:3.5~4.5 for solid material:water; the drying time is 6~12h.

[0031] Furthermore, the pre-calcination temperature is 200~300℃ for 2~10 hours.

[0032] Furthermore, the molding process includes adding 0.5% to 1% graphite and 0.5% to 1.5% cellulose by mass to the pre-calcined powder.

[0033] Furthermore, the calcination is carried out at 300~450℃ for 4~10 hours.

[0034] The present invention also provides an application of the above-mentioned catalyst in the catalytic reaction of methanol to formaldehyde.

[0035] The beneficial effects of this invention include: This invention involves mixing deionized water, an alkaline source, a boron source, and a molybdenum source, then adding a silicon source and mixing thoroughly before crystallization to obtain a catalyst precursor. The dried precursor is then added to a mixed aqueous solution of nitric acid and ferric nitrate. The material is filtered, washed, dried, shaped, and calcined to obtain the catalyst. This catalyst is used in the reaction of methanol oxidation to prepare formaldehyde. The resulting catalyst has high strength, long service life, and high selectivity for the formaldehyde product obtained after the reaction.

[0036] This invention improves the strength and lifespan of catalysts by adding auxiliary elements and changing the synthesis method, using a molecular sieve synthesis method. This makes the main components more robust and improves the conversion rate and selectivity by changing the acidity and alkalinity of the system through auxiliary agents. Detailed Implementation

[0037] The following description, in conjunction with embodiments, aims to make the advantages and features of the present invention more readily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0038] Example 1

[0039] Weigh 116.4 g of ammonium heptamolybdate and dissolve it in 582 g of deionized water (molybdenum source to water mass ratio 1:5). Add 1298.5 g of deionized water to the reactor, along with 216.4 g of piperidine and 43.28 g of boric acid. After stirring, add the molybdenum source solution. Stir at 15 Hz for 4 hours at 30°C, ensuring no solids remain in the reactor. Add a mixed solution of 65 g of fumed silica and 220 g of deionized water to the reactor. After stirring for 6 hours; heat to 60℃ and hold for 8 hours; transfer the material to a crystallization kettle and crystallize at 170℃ for 72 hours; rapidly cool to 30℃; transfer the material to a distillation kettle, and distill under reduced pressure to recover 300g of piperidine solution, distilling off 300g of deionized water equal to the distillate; filter the material under pressure, wash with 5 times the weight of the filter cake of deionized water, and dry at 120℃ for 12 hours to obtain dried material; add 385g of deionized water and 38.5g of... A mixed solution of 60% nitric acid and 76.95g ferric nitrate was added to the dried material, stirred for 5 hours, heated to 100℃ and held for 24 hours, cooled to 30℃, filtered, washed with deionized water at 5 times the mass of the filter cake, dried at 120℃ for 12 hours, pre-calcined at 300℃ for 2 hours, and then 1% graphite and 1% cellulose were added to the pre-calcined powder by mass ratio before mechanical shaping and calcination at 450℃ for 6 hours to obtain the catalyst.

[0040] Example 2

[0041] Weigh 116.4 g of ammonium heptamolybdate and dissolve it in 582 g of deionized water (molybdenum source to water mass ratio 1:5). Add 1038 g of deionized water to the reactor, along with 115.8 g of piperidine and 51.94 g of boric acid. After stirring, add the molybdenum source solution. Stir at 15 Hz for 4 hours at 30°C, ensuring no solids remain in the reactor. Add 243 g of 30% silica sol to the reactor. After adding all silica sol, stir for 6 hours. h; heat to 60℃ and hold for 8 hours; transfer the material to a crystallization kettle and crystallize at 180℃ for 72 hours; rapidly cool to 30℃; transfer the material to a distillation kettle, and distill under reduced pressure to recover 170g of piperidine solution, distilling out 170g of deionized water of equal volume; filter the material under pressure, wash with 5 times the weight of the filter cake of deionized water, and dry at 120℃ for 12 hours to obtain dried material; add 368g of deionized water and 36.8g of... A mixed solution of 60% nitric acid and 73.6g ferric nitrate was added to the dried material, stirred for 5 hours, heated to 100℃ and held for 24 hours, cooled to 30℃, filtered, washed with deionized water at 5 times the mass of the filter cake, dried at 120℃ for 12 hours, pre-calcined at 300℃ for 2 hours, and then 1% graphite and 1% cellulose were added to the pre-calcined powder before mechanical shaping and calcination at 450℃ for 6 hours to obtain the catalyst.

[0042] Example 3

[0043] Weigh 116.4g of ammonium heptamolybdate and dissolve it in 582g of deionized water (molybdenum source to water mass ratio 1:5). Add 1299g of deionized water to the reactor, along with 216g of piperidine and 43.28g of boric acid. After stirring, add the molybdenum source solution. Stir at 15 Hz for 4 hours at 30°C, ensuring no solids remain in the reactor. Add 215g of 30% silica sol to the reactor. After addition, stir for 6 hours and heat to 60°C. Maintain this temperature for 8 hours. Transfer the material to a crystallization reactor and crystallize at 180°C for 72 hours. Quickly cool to 30°C. Transfer the material to a distillation reactor and distill under reduced pressure to recover 310g of piperidine solution. Add an equal volume of deionized water to the distillate. Filter the material and wash with 5 times the filter cake mass of deionized water. Dry at 120°C for 12 hours to obtain dried material. Add 368g of deionized water and 35.2g of... A mixed solution of 60% nitric acid and 70.54g ferric nitrate was added to the dried material, stirred for 5 hours, heated to 100℃ and held for 24 hours, cooled to 30℃, filtered, washed with deionized water at 5 times the mass of the filter cake, dried at 120℃ for 12 hours, pre-calcined at 300℃ for 2 hours, and then 1% graphite and 1% cellulose were added to the pre-calcined powder by mass ratio before mechanical shaping and calcination at 450℃ for 6 hours to obtain the catalyst.

[0044] Example 4

[0045] Weigh 116.4 g of ammonium heptamolybdate and dissolve it in 582 g of deionized water (molybdenum source to water mass ratio 1:5). Add 695 g of deionized water to the reactor, along with 103.8 g of piperidine and 34.62 g of boric acid. After stirring, add the molybdenum source solution. Stir at 15 Hz for 4 hours at 30°C, ensuring no solids remain in the reactor. Add 113.5 g of 30% silica sol to the reactor. After adding all silica sol, stir for 6 hours. h; heat to 60℃ and hold for 8 hours; transfer the material to a crystallization kettle and crystallize at 185℃ for 72 hours; rapidly cool to 30℃; transfer the material to a distillation kettle, and distill under reduced pressure to recover 150g of piperidine solution, distilling out 150g of deionized water of equal volume; filter the material under pressure, wash with 5 times the weight of the filter cake of deionized water, and dry at 120℃ for 12 hours to obtain dried material; add 368g of deionized water and 36.8g of... A mixed solution of 60% nitric acid and 73.6g ferric nitrate was added to the dried material, stirred for 5 hours, heated to 100℃ and held for 24 hours, cooled to 30℃, filtered, washed with deionized water at 5 times the mass of the filter cake, dried at 120℃ for 12 hours, pre-calcined at 300℃ for 2 hours, and then 1% graphite and 1% cellulose were added to the pre-calcined powder before mechanical shaping and calcination at 450℃ for 6 hours to obtain the catalyst.

[0046] Example 5

[0047] Weigh 116.4g of ammonium heptamolybdate and dissolve it in 582g of deionized water (molybdenum source to water mass ratio of 1:5). Add 1038g of deionized water to the reactor, along with 155g of piperidine and 51.9g of boric acid. After stirring, add the molybdenum source solution. Stir at 15 Hz for 4 hours at 30°C, ensuring no solids remain in the reactor. Add 173g of 30% silica sol to the reactor. Stir for 6 hours after addition. Heat to 60°C and maintain this temperature for 8 hours. Transfer the material to a crystallization reactor and crystallize at 185°C for 72 hours. Quickly cool to 30°C. Transfer the material to a distillation reactor and distill under reduced pressure to recover 225g of piperidine solution. Add an equal volume of deionized water to the distillate. Filter the material and wash with 5 times the weight of the filter cake of deionized water. Dry at 120°C for 12 hours to obtain dried material. Add 368g of deionized water and 36.8g of... A mixed solution of 60% nitric acid and 73.6g ferric nitrate was added to the dried material, stirred for 5 hours, heated to 100℃ and held for 24 hours, cooled to 30℃, filtered, washed with deionized water at 5 times the mass of the filter cake, dried at 120℃ for 12 hours, pre-calcined at 300℃ for 2 hours, and then 1% graphite and 1% cellulose were added to the pre-calcined powder before mechanical shaping and calcination at 450℃ for 6 hours to obtain the catalyst.

[0048] Example 6

[0049] Weigh 119g of ammonium dimolybdate and dissolve it in 582g of deionized water (molybdenum source to water mass ratio 1:5). Add 1298.5g of deionized water to the reactor, along with 216.4g of pyridine and 43.28g of boric acid. After stirring, add the molybdenum source solution. Stir at 15 Hz for 4 hours at 30°C, ensuring no solids remain in the reactor. Add a mixture of 65g of micronized silica gel and 220g of deionized water to the reactor. Once the addition is complete... Stir for 6 hours; heat to 60℃ and hold for 8 hours; transfer the material to a crystallization vessel and crystallize at 170℃ for 72 hours; rapidly cool to 30℃; transfer the material to a distillation vessel, and distill under reduced pressure to recover 300g of pyridine solution, distilling off 300g of deionized water equal to the distillate; filter the material under pressure, wash with 5 times the weight of the filter cake of deionized water, and dry at 120℃ for 12 hours to obtain dried material; add 385g of deionized water and 38.5g of... A mixed solution of 60% nitric acid and 76.95g ferric nitrate was added to the dried material, stirred for 5 hours, heated to 100℃ and held for 24 hours, cooled to 30℃, filtered, washed with deionized water at 5 times the mass of the filter cake, dried at 120℃ for 12 hours, pre-calcined at 300℃ for 2 hours, and then 1% graphite and 1% cellulose were added to the pre-calcined powder by mass ratio before mechanical shaping and calcination at 450℃ for 6 hours to obtain the catalyst.

[0050] Example 7

[0051] Weigh 116g of ammonium heptamolybdate and dissolve it in 582g of deionized water (molybdenum source to water mass ratio 1:5). Add 1298.5g of deionized water to the reactor, along with 216.4g of methylpiperidine and 43.28g of boric acid. After stirring, add the molybdenum source solution. Stir at 15 Hz for 4 hours at 30°C, ensuring no solids remain in the reactor. Add a mixture of 65g of silica gel and 220g of deionized water to the reactor. Once the addition is complete... Stir for 6 hours; heat to 60℃ and hold for 8 hours; transfer the material to a crystallization kettle and crystallize at 170℃ for 72 hours; rapidly cool to 30℃; transfer the material to a distillation kettle, and distill under reduced pressure to recover 300g of methylpiperidine solution, distilling off 300g of deionized water equal to the distillate; filter the material under pressure, wash with 5 times the weight of the filter cake of deionized water, and dry at 120℃ for 12 hours to obtain dried material; add 385g of deionized water and 38.5g of... A mixed solution of 60% nitric acid and 76.95g ferric nitrate was added to the dried material, stirred for 5 hours, heated to 100℃ and held for 24 hours, cooled to 30℃, filtered, washed with deionized water at 5 times the mass of the filter cake, dried at 120℃ for 12 hours, pre-calcined at 300℃ for 2 hours, and then 1% graphite and 1% cellulose were added to the pre-calcined powder by mass ratio before mechanical shaping and calcination at 450℃ for 6 hours to obtain the catalyst.

[0052] Comparative Example Weigh 116.4g of ammonium heptamolybdate and dissolve it in 582g of deionized water (molybdenum source to water mass ratio of 1:5). Add 1298.5g of deionized water and 194g of piperidine to the reactor. Start stirring and add the molybdenum source solution. Stir at 15 Hz for 4 hours at 30°C, ensuring no solids remain in the reactor. Add a mixture of 65g of fumed silica and 220g of deionized water to the reactor. Stir for 6 hours after addition. Heat to 60°C and maintain this temperature for 8 hours. Transfer the material to a crystallization reactor and crystallize at 170°C for 72 hours. Quickly cool to 30°C. Transfer the material to a distillation reactor and distill under reduced pressure to recover 300g of piperidine solution. Add an equal volume of deionized water to the distillate. Filter the material and wash with 5 times the weight of the filter cake of deionized water. Dry at 120°C for 12 hours to obtain dried material. Add 385g of deionized water and 38.5g of... A mixed solution of 60% nitric acid and 76.95g ferric nitrate was added to the dried material, stirred for 5 hours, heated to 100℃ and held for 24 hours, cooled to 30℃, filtered, washed with deionized water at 5 times the mass of the filter cake, dried at 120℃ for 12 hours, pre-calcined at 300℃ for 2 hours, and then 1% graphite and 1% cellulose were added to the pre-calcined powder by mass ratio before mechanical shaping and calcination at 450℃ for 6 hours to obtain the catalyst.

[0053] Test case The catalysts obtained in Examples 1-7 and the comparative example were tested for methanol oxidation to formaldehyde. The results are shown in Table 1. The steps are as follows: The reactor is 2.4m long and 20mm in inner diameter. 20g of ceramic rings are packed at the bottom of the pipeline. 280g of catalyst is packed inside the reactor and the top is filled with ceramic rings. The reactor is sealed and heated to 280℃ under nitrogen protection. Feeding begins with an air flow rate of 17L / min, a nitrogen flow rate of 15L / min, and a methanol flow rate of 6.0mL / min to maintain the reaction.

[0054] Among them, the strength test was carried out using a particle strength tester to test the Raschig rings. The dimensions of each Raschig ring were 5mm ± 1mm ​​outer diameter, 2.4mm ± 0.02mm inner diameter, and 2.6mm ± 0.1mm height.

[0055] Table 1

[0056] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A catalyst for the oxidation of methanol to formaldehyde, characterized in that, The main components of the catalyst are ferric molybdate and molybdenum trioxide, and the catalyst is doped with boron oxide and silicon dioxide.

2. The catalyst according to claim 1, characterized in that, The atomic ratio of molybdenum to iron in the catalyst is (2.0~2.5):1, preferably (2.1~2.4):1, and the atomic ratio of molybdenum to boron is 1:(0.5~2). The silica accounts for 20%~31% of the total mass of the catalyst.

3. The catalyst according to claim 1 or 2, characterized in that, The strength of the catalyst is 5~7 N / particle.

4. A method for preparing the catalyst according to any one of claims 1 to 3, characterized in that, include: (1) Prepare an aqueous solution I containing an alkaline source and a boron source, add a molybdenum source solution, stir I, and obtain solution A; (2) Add silicon source dropwise to solution A, stir (II), keep warm (I), and obtain solution B; (3) After crystallizing solution B, cool it down rapidly, distill it under reduced pressure, and add an equal amount of deionized water to the distillate; (4) Filter, wash and dry to obtain dry material; (5) Prepare an aqueous solution II containing acid and iron salt, add dry material, stir III, and keep warm II; (6) Filter, wash, dry, pre-calcine, shape, and calcine to obtain the catalyst.

5. The preparation method according to claim 4, characterized in that, In step (1), The alkali source is selected from at least one of piperidine, pyridine, and methylpiperidine; the boron source is boric acid; and the molybdenum source is selected from at least one of ammonium molybdate, ammonium dimolybdate, and ammonium heptamolybdate. And / or, the mass ratio of water to the sum of the masses of the alkali source and the boron source in aqueous solution I is 1~5:1, preferably 1.5~2:1; the mass ratio of the alkali source to the boron source is 0.1~5:1, preferably 0.5~1:1; And / or, the mass ratio of molybdenum source to water in the molybdenum source solution is 1:4~8, preferably 1:5~7; the atomic ratio of molybdenum atoms to boron atoms is 1:0.5~2; And / or, the stirring I is carried out at 25~40℃ for 1~8h, and the stirring rate is 5~20Hz.

6. The preparation method according to claim 4 or 5, characterized in that, In step (2), The silicon source is selected from at least one of silica sol, fumed silica, water glass, type C silica gel, micronized silica gel, and silicate ester; the silicon source is calculated as silicon dioxide, and the mass ratio of silicon dioxide to boron source is 0.5~1.5:1, preferably 0.7~1:1; And / or, the stirring II is carried out at 25~40℃ for 1~8 hours; And / or, the heat preservation I is performed at 40~85℃ for 2~12 hours.

7. The preparation method according to any one of claims 4 to 6, characterized in that, In step (3), The crystallization treatment is performed at 160~185℃ for 45~85 hours; And / or, the conditions for vacuum distillation include a temperature of 75~80℃ and a vacuum degree of -0.055~-0.07MPa; And / or, in step (4), the washing water mass is solid material: water = 1:3~5, preferably 1:3.5~4.5; the drying temperature is 100~150℃, and the drying time is 4~24h, preferably 6~12h.

8. The preparation method according to any one of claims 4 to 7, characterized in that, In step (5), The acid is nitric acid or hydrochloric acid, the iron salt is ferric nitrate or ferric chloride, and the mass ratio of the acid to the iron salt is 0.01~1:1, preferably 0.05~0.5:1; in aqueous solution II, the mass ratio of the iron salt to water is 1:4~8, preferably 1:5~7; after adding dry material, the atomic ratio of molybdenum to iron is (2.0~2.5):1, preferably (2.1~2.4):1; And / or, the stirring III is carried out at 25~40℃ for 1~5 hours; And / or, the heat preservation II is performed at 100~120℃ for 12~48h.

9. The preparation method according to any one of claims 4 to 8, characterized in that, In step (6), The washing water ratio is 1:3~5 for solid material:water, preferably 1:3.5~4.5; the drying temperature is 100~150℃, and the drying time is 4~24h, preferably 6~12h. And / or, the pre-calcination temperature is 200~300℃ for 2~10h; And / or, the molding process includes adding 0.5% to 1% graphite and 0.5% to 1.5% cellulose by mass to the pre-calcined powder; And / or, the calcination is carried out at 300~450℃ for 4~10h.

10. The use of the catalyst according to any one of claims 1 to 3 in the catalytic reaction of methanol to formaldehyde.

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