A catalyst for the oxidation of methanol to formaldehyde and a method for its preparation

Abstract

This invention belongs to the field of catalyst technology and relates to a catalyst for the methanol oxidation to formaldehyde and its preparation method. The invention involves a three-step precipitation reaction to prepare a molybdenum trioxide core, an iron molybdate intermediate layer coating the surface of the molybdenum trioxide core, and an outer layer of molybdenum trioxide coating the iron molybdate, forming primary particles with iron molybdate in the middle, and molybdenum trioxide as the core and outermost layer. The outer layer of molybdenum trioxide effectively delays the decomposition of iron molybdate, while the molybdenum trioxide core can diffuse into the iron molybdate phase and undergo a solid-phase reaction with the newly formed iron oxide to reversely generate iron molybdate, further delaying the decomposition of the active phase and thus extending the catalyst's lifespan. The catalyst prepared by this method, when applied to the methanol oxidation to formaldehyde, exhibits extremely high methanol conversion rate, good formaldehyde selectivity, and activity stability.

Description

Technical Field

[0001] This invention belongs to the field of catalyst technology, specifically relating to a catalyst for the oxidation of methanol to formaldehyde and its preparation method. Background Technology

[0002] Formaldehyde is an important basic organic chemical raw material, mainly used in the production of resins, 1,4-butanediol, polyoxymethylene, disinfectants, etc. It is usually produced by the catalytic oxidation of methanol, and the catalyst used is generally an iron-molybdenum catalyst.

[0003] Ferro-molybdenum catalysts are mainly prepared via co-precipitation, as illustrated in patents CN 115007165A, CN 100413584C, and CN111097461A. Studies have shown that ferric molybdate (Fe2(MoO4)3) is the catalytically active phase in the methanol-to-formaldehyde oxidation process. Ferric molybdate exhibits extremely high catalytic activity and product selectivity for methanol-to-formaldehyde oxidation, a fact currently internationally recognized. However, ferric molybdate has low thermal stability and slowly decomposes into molybdenum trioxide and ferric oxide at operating temperatures. Bulk molybdenum trioxide has virtually no catalytic activity and is easily lost, while ferric oxide readily causes deep oxidation of methanol or formaldehyde, generating byproducts such as carbon dioxide. This leads to a continuous decline in catalyst activity and a simultaneous decrease in product selectivity. Patents CN 103933998B and CN107952445B, among others, improve catalyst activity and stability by adding other substances such as Ni, Co, V, Ce, and Cs, reportedly extending catalyst lifespan. Despite numerous improvements, the activity and stability of domestically produced catalysts have not yet been adequately addressed. Summary of the Invention

[0004] To address the shortcomings of the existing technology, this invention provides a catalyst for the oxidation of methanol to formaldehyde and a method for its preparation.

[0005] The specific plan is as follows:

[0006] A catalyst for the oxidation of methanol to formaldehyde includes a molybdenum trioxide core, an iron molybdate intermediate layer coated on the surface of the core, and a molybdenum trioxide outer layer coated on the surface of the intermediate layer; wherein, the molar ratio of Mo / Fe in the catalyst is 1.6 to 5.5; Mo in the molybdenum trioxide core accounts for 5 to 35 wt% of the total Mo in the catalyst, and Mo in the molybdenum trioxide outer layer accounts for 5 to 15 wt% of the total Mo in the catalyst.

[0007] A method for preparing a catalyst for the oxidation of methanol to formaldehyde includes the following steps:

[0008] S1 Obtain the molybdenum trioxide core;

[0009] S2 uses iron salts, molybdates, and a molybdenum trioxide core for deposition and aging to obtain an intermediate with a molybdenum trioxide core and an iron molybdate intermediate layer;

[0010] S3 The methanol oxidation to formaldehyde catalyst is obtained by deposition and aging of molybdate and intermediate obtained in step S2.

[0011] This invention prepares nano-molybdenum trioxide particles under strongly acidic conditions, then uses iron ions and molybdate ions to simultaneously deposit on the outside of molybdenum trioxide under weakly acidic conditions to form an iron molybdate intermediate layer. Finally, molybdate ions are deposited on the surface of iron molybdate under acidic conditions to form an outer layer of molybdenum trioxide, thus obtaining primary particles with an intermediate layer of iron molybdate and an inner core and outer layer of molybdenum trioxide.

[0012] Further, in step S1, the molybdenum trioxide core is obtained by deposition and aging using a molybdate solution; the deposition conditions are: pH≤2, 40~90℃; the aging temperature is 40~90℃; the aging time is preferably 1~5h; preferably, the molybdate concentration is 0.1~5mol / L.

[0013] Preferably, in step S1, the molybdate is selected from at least one of orthomolybdate, tetramolybdate, and secondary molybdate, and the molybdate is preferably at least one of ammonium salt, lithium salt, sodium salt, and potassium salt.

[0014] Preferably, in step S1, during the deposition and aging process, the rotation speed is 10,000 to 30,000 rpm.

[0015] Further, in step S2, the deposition conditions are: pH 3-5, temperature 40-90℃; aging temperature 40-90℃, and preferably aging time 1-5h. Specifically, in step S2, molybdate and iron salt are deposited and aged on the surface of the molybdenum trioxide core under these conditions to obtain iron molybdate; preferably, in step S2, the molar ratio of Mo in the molybdate to Fe in the iron salt is 1-5:1.

[0016] Preferably, the molybdate in step S2 is selected from at least one of orthomolybdate, tetramolybdate, and secondary molybdate, and the molybdate is preferably at least one of ammonium salt, lithium salt, sodium salt, and potassium salt.

[0017] Preferably, the iron salt in step S2 is selected from at least one of soluble iron salts, and the soluble iron salts are preferably ferric nitrate, ferric chloride, ferric sulfate, and ferric acetate.

[0018] Further, in step S3, the deposition conditions are: pH ≤ 2, temperature 40~90℃; aging temperature 40~90℃, and aging time preferably 5~10h. Specifically, in step S3, molybdate solution is used to deposit and age molybdenum trioxide on the surface of the intermediate under these conditions, wherein the concentration of the molybdate solution is 0.05~5mol / L.

[0019] Furthermore, step S3 also includes roasting.

[0020] Preferably, in step S3, the calcination temperature is 350~550℃ and the calcination time is 1~20h.

[0021] The beneficial effects of this invention are as follows:

[0022] This invention involves a three-step precipitation reaction to prepare a molybdenum trioxide (Mo) oxide core, an ferric molybdate intermediate layer coating the surface of the Molybdenum trioxide core, and an outer layer of Molybdenum trioxide coating the ferric molybdate, forming primary particles with ferric molybdate in the middle, and Molybdenum trioxide as the core and outermost layer. The outer layer of Molybdenum trioxide effectively delays the decomposition of ferric molybdate, while the Molybdenum trioxide core can diffuse into the ferric molybdate phase and undergo a solid-phase reaction with the newly formed ferric oxide to reversely generate ferric molybdate, further delaying the decomposition of the active phase of the catalyst and thus extending the catalyst's lifespan. The catalyst prepared by this method, when applied to the oxidation of methanol to formaldehyde, exhibits extremely high methanol conversion rate, good formaldehyde selectivity, and activity stability. Detailed Implementation

[0023] The embodiments of the present invention will be described in further detail below with reference to the examples. The following examples are for illustrative purposes only and should not be construed as limiting the scope of the invention. Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0024] Example 1

[0025] A catalyst for the oxidation of methanol to formaldehyde includes a molybdenum trioxide core, an iron molybdate intermediate layer coated on the surface of the core, and a molybdenum trioxide outer layer coated on the surface of the intermediate layer; wherein the molar ratio of Mo / Fe in the catalyst is 2.0; Mo in the molybdenum trioxide core accounts for 10 wt% of the total Mo mass in the catalyst, and Mo in the molybdenum trioxide outer layer accounts for 5 wt% of the total Mo mass in the catalyst.

[0026] The preparation method includes the following steps:

[0027] S1 Weigh 20g of ammonium molybdate (NH4)6Mo7O 24·4H2O, dissolve ammonium molybdate in 100mL of deionized water to prepare a solution, adjust the pH to 0.5 under the conditions of stirring speed of 15000rpm and water bath temperature of 80℃ for deposition, and then age under these conditions for 1h to obtain molybdenum trioxide core slurry;

[0028] S2 Weigh out 170g of ammonium molybdate (NH4)6Mo7O 24 ·4H2O, dissolve ammonium molybdate in 1500mL of deionized water to prepare a solution; weigh 228g of ferric nitrate nonahydrate Fe(NO3)3·9H2O, dissolve in 500mL of deionized water to prepare a solution of ferric nitrate; under stirring and at 80℃, add the ammonium molybdate solution and the ferric nitrate solution in parallel to the molybdenum trioxide slurry obtained in step S1 for deposition, control the deposition pH to 3.5, and after the addition is completed, stir and age at 80℃ for 2h to obtain an intermediate slurry with molybdenum trioxide core and ferric molybdate middle layer;

[0029] S3 Weigh 10g of ammonium molybdate (NH4)6Mo7O 24 • 4H2O, dissolve ammonium molybdate in 100mL of deionized water to prepare a solution; add the ammonium molybdate solution to the intermediate slurry obtained in step S2, adjust the deposition pH to 0.5 under stirring and at 80℃, then stir and age at 80℃ for 5h, then filter and wash with water, dry at 120℃ for 10h, finally compress the dried material into tablets, and calcine at 450℃ for 6h to obtain the methanol oxidation to formaldehyde catalyst.

[0030] Example 2

[0031] A catalyst for the oxidation of methanol to formaldehyde includes a molybdenum trioxide core, an iron molybdate intermediate layer coated on the surface of the core, and a molybdenum trioxide outer layer coated on the surface of the intermediate layer; wherein the molar ratio of Mo / Fe in the catalyst is 4.0; Mo in the molybdenum trioxide core accounts for 20 wt% of the total Mo mass in the catalyst, and Mo in the molybdenum trioxide outer layer accounts for 5 wt% of the total Mo mass in the catalyst.

[0032] The preparation method includes the following steps:

[0033] S1 Weigh out 40g of ammonium molybdate (NH4)6Mo7O 24 ·4H2O, dissolve ammonium molybdate in 200mL of deionized water to prepare a solution, and adjust the pH to 1.5 under the conditions of stirring speed of 15000rpm and water bath temperature of 60℃ for deposition. Then, age under these conditions for 2h to obtain molybdenum trioxide core slurry.

[0034] S2 Weigh 150g of ammonium molybdate (NH4)6Mo7O 24·4H2O was dissolved in 1000mL of deionized water to prepare ammonium molybdate solution; 76g of ferric chloride hexahydrate FeCl3·6H2O was weighed and dissolved in 200mL of deionized water to prepare ferric chloride solution; the ammonium molybdate solution and ferric chloride solution were added concurrently to the molybdenum trioxide slurry obtained in step S1 under stirring and deposition temperature of 60℃ for deposition, and the deposition pH was controlled at 4.0. After the addition was completed, the slurry was stirred and aged at 60℃ for 5h to obtain an intermediate slurry with molybdenum trioxide core and ferric molybdate middle layer;

[0035] S3 Weigh 10g of ammonium molybdate (NH4)6Mo7O 24 • 4H2O, dissolve ammonium molybdate in 100mL of deionized water to prepare a solution; add the ammonium molybdate solution to the intermediate slurry obtained in step S2, adjust the deposition pH to 0.5 under stirring at 60℃, then age at 60℃ for 10h, then filter and wash with water, dry at 120℃ for 10h, finally compress the dried material into tablets, and calcine at 400℃ for 6h to obtain the methanol oxidation to formaldehyde catalyst.

[0036] Example 3

[0037] A catalyst for the oxidation of methanol to formaldehyde includes a molybdenum trioxide core, an iron molybdate intermediate layer coated on the surface of the core, and a molybdenum trioxide outer layer coated on the surface of the intermediate layer; wherein the molar ratio of Mo / Fe in the catalyst is 3.0; Mo in the molybdenum trioxide core accounts for 25 wt% of the total Mo in the catalyst, and Mo in the molybdenum trioxide outer layer accounts for 10 wt% of the total Mo in the catalyst.

[0038] The preparation method includes the following steps:

[0039] S1 Weigh out 50g of ammonium molybdate (NH4)6Mo7O 24 ·4H2O, dissolve ammonium molybdate in 300mL of deionized water to prepare a solution, and adjust the pH to 1.0 under the conditions of stirring speed of 25000rpm and water bath temperature of 40℃ for deposition. Then, age under these conditions for 2h to obtain molybdenum trioxide core slurry.

[0040] S2 Weigh out 130g of ammonium molybdate (NH4)6Mo7O 24 ·4H2O, dissolve ammonium molybdate in 1000mL of deionized water to prepare a solution; weigh 150g of ferric nitrate nonahydrate Fe(NO3)3·9H2O, dissolve in 300mL of deionized water to prepare a solution of ferric nitrate; under stirring and deposition temperature of 40℃, add the ammonium molybdate solution and ferric nitrate solution in parallel to the molybdenum trioxide slurry obtained in step S1 for deposition, control the deposition pH to 4.5, and after the addition is completed, stir and age at 40℃ for 5h to obtain an intermediate slurry with molybdenum trioxide core and ferric molybdate middle layer.

[0041] S3 Weigh 20g of ammonium molybdate (NH4)6Mo7O 24 • 4H2O, dissolve ammonium molybdate in 150 mL of deionized water to prepare a solution; add the ammonium molybdate solution to the intermediate slurry obtained in step S2, adjust the deposition pH to 1.0 under stirring at 40°C, then stir and age at 40°C for 8 h, then filter and wash with water, dry at 120°C for 10 h, finally compress the dried material into tablets, and calcine at 500°C for 4 h to obtain the methanol oxidation to formaldehyde catalyst.

[0042] Comparative Example 1

[0043] Weigh out 200g of ammonium molybdate (NH4)6Mo7O 24 Ammonium molybdate solution was prepared by dissolving 4H2O in 1500 mL of deionized water; 228 g of ferric nitrate nonahydrate Fe(NO3)3·9H2O was weighed and dissolved in 500 mL of deionized water to prepare ferric nitrate solution; the ammonium molybdate solution and ferric nitrate solution were added to the reactor in parallel under stirring and deposition temperature of 80℃ for deposition, and the deposition pH was adjusted to 0.5. Then, the mixture was stirred and aged at 80℃ for 2 h to obtain particulate matter; after filtration and washing with water, the filter cake was dried at 120℃ for 10 h, the dried material was pressed into tablets, and calcined at 450℃ for 6 h to obtain the catalyst.

[0044] Comparative Example 2

[0045] Weigh out 200g of ammonium molybdate (NH4)6Mo7O 24 Ammonium molybdate solution was prepared by dissolving 4H2O in 1500 mL of deionized water; 150 g of ferric nitrate nonahydrate Fe(NO3)3·9H2O was weighed and dissolved in 300 mL of deionized water to prepare ferric nitrate solution; the ammonium molybdate solution and ferric nitrate solution were added to the reactor in parallel under stirring and deposition temperature of 40 °C for deposition, and the deposition pH was adjusted to 1.0. Then, the mixture was stirred and aged at 40 °C for 8 h to obtain particulate matter; after filtration and washing with water, the filter cake was dried at 100 °C for 15 h, the dried material was pressed into tablets, and calcined at 500 °C for 4 h to obtain the catalyst.

[0046] Comparative Example 3

[0047] S1 Weigh 20g of ammonium molybdate (NH4)6Mo7O 24 ·4H2O, dissolve ammonium molybdate in 100mL of deionized water to prepare a solution, adjust the pH to 0.5 under the conditions of stirring speed of 15000rpm and water bath temperature of 80℃ for deposition, and then age under these conditions for 1h to obtain molybdenum trioxide nanoparticle slurry.

[0048] S2 Weigh 180g of ammonium molybdate (NH4)6Mo7O24 Ammonium molybdate solution was prepared by dissolving 4H2O in 1500 mL of deionized water; 228 g of ferric nitrate nonahydrate Fe(NO3)3·9H2O was weighed and dissolved in 500 mL of deionized water to prepare ferric nitrate solution; under stirring and at a temperature of 80 °C, the ammonium molybdate solution and ferric nitrate solution were added concurrently to the molybdenum trioxide slurry obtained in step S1 for deposition, and the deposition pH was controlled at 3.5; after the addition was completed, the mixture was stirred and aged at 80 °C for 2 h to obtain composite particles with molybdenum trioxide core and ferric molybdate shell. After filtration and washing, the filter cake was dried at 120 °C for 10 h, the dried material was pressed into tablets, and calcined at 450 °C for 6 h to obtain the catalyst.

[0049] test

[0050] The catalysts prepared in Examples 1-3 and Comparative Examples 1-3 were applied to the methanol oxidation reaction to prepare formaldehyde in a fixed-bed reactor with a catalyst loading of 10 mL and a gas space velocity of 15000 h⁻¹. -1 The methanol content was 7% (V / V), with the remainder being air. After continuous reaction at 280℃ for 10 hours, the initial conversion rate of methanol and the formaldehyde selectivity were calculated, and the results are shown in Table 1. Then, the reaction temperature was raised to 600℃ for high-temperature catalytic deactivation of the catalyst. After deactivation for 10 hours, the temperature was lowered again to 280℃ and reacted continuously for 10 hours. The conversion rate of methanol, the formaldehyde selectivity, and the methanol conversion retention rate after catalytic deactivation were obtained, and the results are shown in Table 2.

[0051] Table 1 Results of the first catalysis

[0052]

[0053] Table 2 Results of the second catalysis

[0054]

[0055] The above data demonstrate that the catalyst of this invention has high activity, good selectivity for formaldehyde, and significantly better stability than the comparative catalyst.

[0056] 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 principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A catalyst for the oxidation of methanol to formaldehyde, characterized in that The catalyst comprises a molybdenum trioxide inner core, an iron molybdate intermediate layer coated on the surface of the inner core, and a molybdenum trioxide outer layer coated on the surface of the intermediate layer; wherein the molar ratio of Mo / Fe in the catalyst is 1.6~5.5; Mo in the molybdenum trioxide inner core accounts for 5~35 wt% of the total Mo in the catalyst, and Mo in the molybdenum trioxide outer layer accounts for 5~15 wt% of the total Mo in the catalyst. The preparation method of the methanol oxidation to formaldehyde catalyst includes the following steps: S1 Obtain the molybdenum trioxide core; S2 uses iron salts, molybdates, and a molybdenum trioxide core for deposition and aging to obtain an intermediate with a molybdenum trioxide core and an iron molybdate intermediate layer; S3 The methanol oxidation to formaldehyde catalyst is obtained by deposition and aging of molybdate and intermediate obtained in step S2.

2. The process for preparing a catalyst for the oxidation of methanol to formaldehyde according to claim 1, characterized in that, Includes the following steps: S1 Obtain the molybdenum trioxide core; S2 uses iron salts, molybdates, and a molybdenum trioxide core for deposition and aging to obtain an intermediate with a molybdenum trioxide core and an iron molybdate intermediate layer; S3 The methanol oxidation to formaldehyde catalyst is obtained by deposition and aging of molybdate and intermediate obtained in step S2.

3. The production method according to claim 2, characterized by, In step S1, the molybdenum trioxide core is obtained by deposition and aging using a molybdate solution.

4. The production method according to claim 3, characterized by, In step S1, the deposition conditions are: pH ≤ 2, 40~90℃; aging temperature is 40~90℃; and the rotation speed is 10000~30000 rpm during deposition and aging.

5. The preparation method according to claim 2, characterized in that, In step S2, the deposition conditions are: pH 3~5, temperature 40~90℃; aging temperature 40~90℃.

6. The preparation method according to claim 2, characterized in that, In step S3, the deposition conditions are: pH ≤ 2, temperature 40~90℃; aging temperature 40~90℃.

7. The preparation method according to claim 2, characterized in that, Step S3 also includes roasting.

8. The preparation method according to claim 7, characterized in that, In step S3, the calcination temperature is 350~550℃ and the calcination time is 1~20h.

Images