A composite silver catalyst and its application in the preparation of formaldehyde

Abstract

The present application discloses a composite silver catalyst and its application in the preparation of formaldehyde. In addition to silver, the catalyst also contains a promoter metal, a ligand and a carrier. The ligand plays a role in activating the reaction substrate molecules and reducing the potential energy barrier of the reaction. The promoter metal not only plays a role in reducing the main active metal, but also uses its low electron cloud distribution characteristics to accurately and quickly desorb the substrate from the catalyst, improving the utilization rate and specificity of the catalyst, overcoming the disadvantages of low selectivity and easy deactivation of traditional silver catalysts. The catalyst has a long service life, high catalytic activity and reaction selectivity, and excellent performance. Using this catalyst to prepare formaldehyde reduces the reaction temperature, realizes the highly selective conversion of methanol to formaldehyde under mild conditions, inhibits side reactions at high temperatures, and improves the reaction selectivity and yield. The present application belongs to the technical field of formaldehyde preparation.

Description

Technical Field

[0001] This application belongs to the technical field of formaldehyde preparation, and in particular relates to a composite silver catalyst and its application in the preparation of formaldehyde. Background Art

[0002] As an important organic chemical raw material and chemical intermediate, formaldehyde is widely used in many fields such as chemicals, materials, and medicine. According to research and investigation of the global market, the output of formaldehyde in 2020 was 45.6 metric tons, and it is expected to further grow at a rate of 4.65% per year during the period of 2021 - 2028, reaching a market value of 37.7 billion US dollars by 2028. At the same time, China has become the world's largest formaldehyde producer and consumer, with formaldehyde production capacity accounting for 55% of the global total production capacity and consumption accounting for 53%. In the existing domestic formaldehyde production processes, the methanol oxidation process is mainly used, which is divided into the "silver method" and the "iron-molybdenum method" according to different types of catalysts used; the silver catalyst method has a long history, and its advantages are mature technology, shorter process flow, lower power consumption, less investment, and large single-series production capacity; the disadvantages are higher methanol consumption, shorter catalyst life, and poor activity.

[0003] Compared with the "silver method", the "iron-molybdenum method" has the advantages of lower reaction temperature, lower methanol unit consumption, longer catalyst service life, and can produce high-concentration formaldehyde solution, etc.; however, the iron-molybdenum catalyst method cannot be equipped with a methanol recovery tower, and a large amount of air is required to ensure the safe operation of the device (1 mol of methanol vapor requires 13 mol of air). The presence of a large amount of air makes all the equipment and pipelines of the device much larger than those of the silver method, increasing the equipment investment and resulting in poor economic benefits. Therefore, the production of formaldehyde by the silver catalyst method is still the first choice of many manufacturers. The silver method operates outside the explosion upper limit of methanol-air, and the reaction process is that methanol is in excess and air is insufficient. Methanol undergoes oxidation and dehydrogenation reactions at normal pressure and 580 - 740 °C. Approximately 50 - 60% of formaldehyde is generated by the oxidation reaction, and the remaining formaldehyde is generated by the dehydrogenation reaction. The main by-products are CO, CO2, HCOOH, and HCOOCH3.

[0004] Traditional silver method catalysts have the disadvantages of short service life, easy poisoning and inactivation, raw materials need to be pretreated, high requirements for equipment, low catalyst activity, and poor reaction selectivity. Therefore, it is particularly important to seek new process methods and new catalytic technologies for formaldehyde synthesis to improve catalyst efficiency and formaldehyde yield, promote its industrial development, and consolidate and enhance the comprehensive competitiveness of products. Summary of the Invention

[0005] The purpose of the implementation of this application is to provide a composite silver catalyst and its application in the preparation of formaldehyde, so as to solve the technical problems existing in the prior art, such as short service life, low catalytic activity, poor reaction selectivity of the catalyst used in the preparation of formaldehyde, and high reaction temperature in the synthesis of formaldehyde.

[0006] To achieve the above purpose, the technical solution adopted in this application is: to provide a composite silver catalyst, which is prepared by the following method:

[0007] (I). Add a silver source, a transition metal salt, and a solvent into a container, stir and mix to obtain a mixed solution;

[0008] (II). Add a molecular sieve carrier to the mixed solution and stir for adsorption;

[0009] (III). Add a phenoxyimine ligand and stir; remove the excess solvent, dry to obtain the composite silver catalyst.

[0010] In one embodiment,

[0011] In step (I), the silver source is silver nitrate, and the transition metal salt is a metal compound or inorganic salt including one or more of a manganese source, a niobium source, or a bismuth source; in step (II), the molecular sieve carrier is one or more of zeolite, ZSM-5, and β molecular sieve.

[0012] In one embodiment,

[0013] The manganese source is one or more of manganese-containing hydrochlorides, nitrates, sulfates, acetates, or their hydrates; the niobium source is one or more of niobium-containing oxides, chlorides, sodium compounds, potassium compounds; the bismuth source is bismuth-containing nitrate; more specifically, the manganese source is one of MnSO4·H2O, Mn(OAc)2·2H2O, Mn(OAc)2, Mn(NO3)2·4H2O, the niobium source is one of Nb2O5, NbCl5, NaNbO3, KNbO3, and the bismuth source is Bi(NO3)2·5H2O.

[0014] In one embodiment,

[0015] The structural formula of the phenoxyimine ligand in step (III) is:

[0016]

[0017] Among them, R1 is one of -Ph, -CH3, -C2H5 or n-hexyl, R2 is one of -t-Bu, -CH3 or -i-Pr, and R3 is one of -H, -CH3 or -CH3O; specifically: L1 (MW: 211.26), L2 (MW: 239.31), L3 (MW: 267.37), L4 (MW: 283.36), L5 (MW: 191.27), L6 (MW: 205.30), L7 (MW: 191.27), L8 (MW: 205.30), and the structural formulas of L1-L8 are as follows;

[0018]

[0019]

[0020] In one embodiment,

[0021] The molar ratio of the phenoxyimine ligand to the transition metal salt is 1:2 - 6, the molar mass ratio of the silver source to the transition metal salt is 1:1 - 5, and the mass ratio of the silver source to the molecular sieve support is 0.01 - 0.05:1; preferably, the molar ratio of the phenoxyimine ligand to the transition metal salt is 1:2 - 4, and the mass ratio of the silver source to the molecular sieve support is 0.01 - 0.03:1.

[0022] In one embodiment,

[0023] The temperature of step (i) is room temperature;

[0024] In step (i), the solvent is preferably one that can disperse the silver source and the transition metal salt, and no requirements are made here.

[0025] The temperature of stirring in step (ii) is 30 - 60 °C, and the stirring time is 2 - 6 h.

[0026] In one embodiment,

[0027] The temperature of stirring in step (iii) is 30 - 80 °C, and the stirring time is 1 - 3 h.

[0028] This application also provides an application of the composite silver catalyst in the preparation of formaldehyde. Using the composite silver catalyst prepared in any of the above embodiments, the composite silver catalyst is added to a fixed-bed reactor, air is introduced, and formaldehyde is prepared by an oxidation reaction using methanol as a raw material.

[0029] In one embodiment,

[0030] The temperature of the oxidation reaction is 200 - 400 °C, the feed rate of methanol is 10 - 40 ml / min, and the pressure of air is 0.2 - 0.5 MPa; preferably, the temperature of the oxidation reaction is 200 - 300 °C, the feed rate of methanol is 10 - 20 ml / min, and the pressure of air is 0.2 - 0.3 MPa.

[0031] This application provides a composite silver catalyst. In addition to silver, this catalyst also adds promoter metals, ligands, and carriers. The ligands play a role in activating the reaction substrate molecules and reducing the potential energy barrier of the reaction; the promoter metals not only play a role in reducing the main active metal, but also use their low electron cloud distribution characteristics to accurately and quickly desorb the substrate from the catalyst, improving the utilization rate and specificity of the catalyst, overcoming the disadvantages of traditional silver catalysts with low selectivity and easy deactivation. The catalyst synthesized in this application has a long service life, high catalytic activity and reaction selectivity, and excellent performance; this application also provides the application of the composite silver catalyst in the preparation of formaldehyde. This method for preparing formaldehyde reduces the reaction temperature, realizes the high-selectivity conversion of methanol into formaldehyde under relatively mild conditions, inhibits side reactions at high temperatures, and improves the reaction selectivity and yield. Detailed implementation manners

[0032] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application clearer and more understandable, this application is further described in detail. It should be understood that the specific embodiments described herein are only used to explain this application and are not used to limit this application.

[0033] The conversion rate of methanol and the selectivity of formaldehyde are calculated by the following formulas:

[0034]

[0035]

[0036] Example 1

[0037] (1) At room temperature, 0.90 g of AgNO3 and 7.71 g of Bi(NO3)2·5H2O are added to 60 mL of deionized water and stirred and mixed. After the metal salts are completely dissolved, a mixed solution is obtained;

[0038] (2) 30 g of molecular sieve carrier ZSM-5 is added, the temperature is raised to 40 °C, and the mixture is kept warm and stirred for adsorption for 4 h;

[0039] (3) After the temperature rises to 60 °C, 17.00 g of L3 ligand is added. After stirring at a constant temperature for 2 h, the excess water is removed by rotary evaporation. The obtained solid is placed in a vacuum drying oven at 80 °C and dried to constant weight to obtain a composite silver catalyst, denoted as CAT1.

[0040] The obtained CAT1 was tableted into 20 - 60 mesh, and then added into a fixed - bed reactor. Air was introduced into the fixed - bed reactor, and the pressure of the air was 0.3 MPa. After the fixed - bed reactor was heated to 300 °C and kept at this temperature for 1 h, methanol was fed in liquid phase with a flow rate of 20 ml / min. After continuous feeding and reaction for 2 h, for the formaldehyde and methanol solution in the receiving flask, the methanol conversion rate and formaldehyde selectivity were analyzed and determined according to the national standard GB / T9009 - 2011 "Formaldehyde solution for industrial use", and the results are shown in Table 1.

[0041] Example 2

[0042] The difference between this example and Example 1 is that ZSM - 5 was replaced with zeolite, and other operations were the same. The composite silver catalyst CAT2 was prepared. CAT2 was used for the synthesis of formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0043] Example 3

[0044] The difference between this example and Example 1 is that ZSM - 5 was replaced with β - molecular sieve, and other operations were the same. The composite silver catalyst CAT3 was prepared. CAT3 was used for the synthesis of formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0045] Example 4

[0046] The difference between this example and Example 1 is that 8.5 g of L3 ligand was added, and other operations were the same. The composite silver catalyst CAT4 was prepared. CAT4 was used for the synthesis of formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0047] Example 5

[0048] The difference between this example and Example 1 is that 12.75 g of L3 ligand was added, and other operations were the same. The composite silver catalyst CAT5 was prepared. CAT5 was used for the synthesis of formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0049] Example 6

[0050] The difference between this example and Example 1 is that 21.25 g of L3 ligand was added, and other operations were the same. The composite silver catalyst CAT6 was prepared. CAT6 was used for the synthesis of formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0051] Example 7

[0052] The difference between this example and Example 1 is that 25.5 g of L3 ligand was added, and other operations were the same. The composite silver catalyst CAT7 was prepared. CAT7 was used for the synthesis of formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0053] Example 8

[0054] In this example, different from Example 1, 13.43 g of L1 ligand was added, and other operations were the same. The composite silver catalyst CAT8 was prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0055] Example 9

[0056] In this example, different from Example 1, 15.21 g of L2 ligand was added, and other operations were the same. The composite silver catalyst CAT9 was prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0057] Example 10

[0058] In this example, different from Example 1, 18.02 g of L4 ligand was added, and other operations were the same. The composite silver catalyst CAT10 was prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0059] Example 11

[0060] In this example, different from Example 1, 12.16 g of L5 ligand was added, and other operations were the same. The composite silver catalyst CAT11 was prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0061] Example 12

[0062] In this example, different from Example 1, 13.05 g of L6 ligand was added, and other operations were the same. The composite silver catalyst CAT12 was prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0063] Example 13

[0064] In this example, different from Example 1, 12.16 g of L7 ligand was added, and other operations were the same. The composite silver catalyst CAT13 was prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0065] Example 14

[0066] In this example, different from Example 1, 13.05 g of L8 ligand was added, and other operations were the same. The composite silver catalyst CAT14 was prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0067] Example 15

[0068] The difference between this example and Example 1 is that the addition amount of Bi(NO3)2·5H2O is 2.57 g, and the addition amount of L3 ligand is 5.67 g. Other operations are the same. The composite silver catalyst CAT15 is prepared and used for synthesizing formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0069] Example 16

[0070] The difference between this example and Example 1 is that the addition amount of Bi(NO3)2·5H2O is 12.85 g, and the addition amount of L3 ligand is 28.33 g. Other operations are the same. The composite silver catalyst CAT16 is prepared and used for synthesizing formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0071] Example 17

[0072] The difference between this example and Example 1 is that Bi(NO3)2·5H2O is replaced by Nb2O5. The addition amount of Nb2O5 is 4.22 g, and the addition amount of L3 ligand is 17 g. Other operations are the same. The composite silver catalyst CAT17 is prepared and used for synthesizing formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0073] Example 18

[0074] The difference between this example and Example 17 is that Nb2O5 is replaced by NbCl5. The addition amount of NbCl5 is 4.29 g. Other operations are the same. The composite silver catalyst CAT18 is prepared and used for synthesizing formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0075] Example 19

[0076] The difference between this example and Example 17 is that NbCl5 is replaced by NaNbO3. The addition amount of NaNbO3 is 2.6 g. Other operations are the same. The composite silver catalyst CAT19 is prepared and used for synthesizing formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0077] Example 20

[0078] The difference between this example and Example 17 is that NaNbO3 is replaced by KNbO3. The addition amount of KNbO3 is 2.86 g. Other operations are the same. The composite silver catalyst CAT20 is prepared and used for synthesizing formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0079] Example 21

[0080] The difference between this example and Example 17 is that KNbO3 is replaced by MnSO4·H2O, and the addition amount of MnSO4·H2O is 2.69 g. Other operations are the same. The composite silver catalyst CAT21 is prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0081] Example 22

[0082] The difference between this example and Example 17 is that MnSO4·H2O is replaced by Mn(OAc)2·2H2O, and the addition amount of Mn(OAc)2·2H2O is 4.26 g. Other operations are the same. The composite silver catalyst CAT22 is prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0083] Example 23

[0084] The difference between this example and Example 17 is that Mn(OAc)2·2H2O is replaced by Mn(OAc)2, and the addition amount of Mn(OAc)2 is 2.75 g. Other operations are the same. The composite silver catalyst CAT23 is prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0085] Example 24

[0086] The difference between this example and Example 17 is that Mn(OAc)2 is replaced by Mn(NO3)2·4H2O, and the addition amount of Mn(NO3)2·4H2O is 3.99 g. Other operations are the same. The composite silver catalyst CAT24 is prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0087] Example 25

[0088] The difference between this example and Example 1 is that the addition amount of Bi(NO3)2·5H2O is 17 g, and the temperature is raised to 30 °C in step (ii). Other operations are the same. The composite silver catalyst CAT25 is prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0089] Example 26

[0090] The difference between this example and Example 25 is that the temperature is raised to 50 °C in step (ii), and the mixture is kept warm and stirred for adsorption for 5 h. Other operations are the same. The composite silver catalyst CAT26 is prepared and used for the synthesis of formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0091] Example 27

[0092] In this example, different from Example 26, in step (ii), the temperature is raised to 60 °C, and it is kept warm and stirred for adsorption for 6 h. Other operations are the same, and the composite silver catalyst CAT27 is prepared. CAT27 is used for synthesizing formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0093] Example 28

[0094] In this example, different from Example 26, in step (ii), the temperature is raised to 40 °C, and in step (iii), when the temperature rises to 70 °C, the L3 ligand is added. Other operations are the same, and the composite silver catalyst CAT28 is prepared. CAT28 is used for synthesizing formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0095] Example 29

[0096] In this example, different from Example 28, in step (ii), it is kept warm and stirred for adsorption for 6 h, and in step (iii), when the temperature rises to 60 °C, the L3 ligand is added. Other operations are the same, and the composite silver catalyst CAT29 is prepared. CAT29 is used for synthesizing formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0097] Example 30

[0098] In this example, different from Example 29, in step (ii), it is kept warm and stirred for adsorption for 4 h, and in step (iii), when the temperature rises to 70 °C, the L3 ligand is added. Other operations are the same, and the composite silver catalyst CAT30 is prepared. CAT30 is used for synthesizing formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0099] Example 31

[0100] In this example, different from Example 30, in step (iii), when the temperature rises to 80 °C, the L3 ligand is added. Other operations are the same, and the composite silver catalyst CAT31 is prepared. CAT31 is used for synthesizing formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0101] Example 32

[0102] In this example, different from Example 31, in step (iii), when the temperature rises to 60 °C, the L3 ligand is added, and after constant temperature stirring for 1 h, the excess water is removed by rotary evaporation. Other operations are the same, and the composite silver catalyst CAT32 is prepared. CAT32 is used for synthesizing formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0103] Example 33

[0104] In this example, different from Example 32, in step (iii), after constant temperature stirring for 3 h, the excess water is removed by rotary evaporation. Other operations are the same, and the composite silver catalyst CAT33 is prepared. CAT33 is used for synthesizing formaldehyde, and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0105] Example 34

[0106] The difference between this example and Example 1 is that the methanol flow rate is 10 ml / min. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0107] Example 35

[0108] The difference between this example and Example 1 is that the methanol flow rate is 40 ml / min. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0109] Example 36

[0110] The difference between this example and Example 1 is that the air pressure is 0.2 MPa, and the fixed-bed reactor is heated to 200 °C. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0111] Example 37

[0112] The difference between this example and Example 1 is that the air pressure is 0.5 MPa, and the fixed-bed reactor is heated to 400 °C. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0113] Table 1 Methanol conversion rate and formaldehyde selectivity

[0114]

[0115] Example 38 Catalyst stability experiment

[0116] The obtained CAT1 was tableted into 20 - 60 mesh, and then added into a fixed-bed reactor. Air was introduced into the fixed-bed reactor with an air pressure of 0.3 MPa. After the fixed-bed reactor was heated to 400 °C and kept warm for 1 h, methanol was fed in liquid phase with a flow rate of 20 ml / min. Online monitoring was carried out using gas chromatography. After 200 h, the methanol conversion rate was 59.3% and the formaldehyde selectivity was 97.6%.

[0117] Due to the moderate reaction temperature and few side reactions, the catalyst maintained high strength, activity and stability, avoiding the phenomenon of bed plugging caused by catalyst pulverization, making the catalyst not easily deactivated, and reducing the production cost.

[0118] The present application provides a composite silver catalyst. Set the temperature, add a silver source and a transition metal salt to a solvent, stir and mix to obtain a mixed solution; add a molecular sieve carrier to the mixed solution, stir and adsorb; add a phenoxyimine ligand and stir; remove the excess solvent and dry to obtain the composite silver catalyst; in addition to silver, this catalyst also adds a promoter metal, a ligand and a carrier. The ligand plays a role in activating the reaction substrate molecules and reducing the potential energy barrier of the reaction; the promoter metal not only plays a role in reducing the main active metal, but also uses its low electron cloud distribution characteristic to accurately and quickly desorb the substrate from the catalyst, improving the utilization rate and specificity of the catalyst, overcoming the disadvantages of low selectivity and easy deactivation of traditional silver catalysts. The catalyst synthesized in the present application has a long service life, high catalytic activity and reaction selectivity, and excellent performance; the present application also provides the application of the composite silver catalyst in the preparation of formaldehyde. This method for preparing formaldehyde reduces the reaction temperature and realizes the high-selectivity conversion of methanol to formaldehyde under relatively mild conditions, inhibits side reactions at high temperatures, and improves the reaction selectivity and yield.

[0119] In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present application, "a plurality of" means two or more unless otherwise specifically defined.

[0120] The foregoing are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present application shall be included within the protection scope of the present application.

Claims

1. A composite silver catalyst for the oxidative reaction of methanol as a raw material to prepare formaldehyde, characterized in that, The composite silver catalyst is prepared by the following method: (1) Add a silver source, a transition metal salt, and a solvent into a container, stir and mix to obtain a mixed solution; (2) Add a molecular sieve support to the mixed solution and stir for adsorption; (3) Add a phenoxyimine ligand and stir, remove the excess solvent, and dry to obtain the composite silver catalyst; In step (1), the silver source is silver nitrate, and the transition metal salt is an inorganic salt including one or more of a manganese source, a niobium source, or a bismuth source; The specific structure of the phenoxyimine ligand is one of the following structures: ； The molar ratio of the phenoxyimine ligand to the transition metal salt is 1:2 - 6, and the mass ratio of the silver source to the molecular sieve support is 0.01 - 0.05:

1.

2. The composite silver catalyst according to claim 1, characterized in that, In step (2), the molecular sieve support is one or two of ZSM-5 and β zeolite.

3. The composite silver catalyst according to claim 1, characterized in that, The manganese source is one or more of a manganese-containing hydrochloride, nitrate, sulfate, or acetate; the niobium source is a niobium-containing chloride; the bismuth source is a bismuth-containing nitrate.

4. The composite silver catalyst according to claim 1, characterized in that, In step (2), the stirring temperature is 30 - 60 °C, and the stirring time is 2 - 6 h.

5. The composite silver catalyst according to claim 1, wherein In step (3), the stirring temperature is 30 - 80 °C, and the stirring time is 1 - 3 h.

6. Use of a composite silver catalyst in the preparation of formaldehyde, characterized in that, Using the composite silver catalyst according to any one of claims 1 - 5, add the composite silver catalyst into a fixed-bed reactor, introduce air, and oxidize methanol as a raw material to prepare formaldehyde.

7. Use of a composite silver catalyst according to claim 6 in the preparation of formaldehyde, characterized in that, The temperature of the oxidation reaction is 200 - 400 °C.

8. Use of a composite silver catalyst according to claim 6 in the preparation of formaldehyde, characterized in that, The feed rate of the methanol is 10 - 40 ml / min, and the pressure of the air is 0.2 - 0.5 MPa.

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