Composite silver catalyst and application thereof in preparation of formaldehyde

Abstract

The invention discloses a composite silver catalyst and application of the composite silver catalyst in preparation of formaldehyde, the catalyst is added with silver as well as auxiliary metal, a ligand and a carrier, the ligand plays a role in activating reaction substrate molecules, and potential energy barriers of reaction are reduced; the assistant metal not only plays a role in reducing the main active metal, but also can accurately and quickly desorb the substrate from the catalyst by utilizing the low electron cloud distribution characteristic of the assistant metal, so that the utilization rate and specificity of the catalyst are improved, the defects that the traditional silver catalyst is low in selectivity and easy to inactivate are overcome, and the catalyst is long in service life, high in catalytic activity and reaction selectivity and suitable for industrial production. The performance is excellent; when the catalyst is used for preparing formaldehyde, the reaction temperature is reduced, high-selectivity conversion of methanol into formaldehyde is realized under mild conditions, side reactions at high temperature are inhibited, and the reaction selectivity and yield are improved. The invention belongs to the technical field of formaldehyde preparation.

Description

Technical Field

[0001] The present 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 chemical industry, materials, medicine and other fields. According to research on the global market, the output of formaldehyde in 2020 was 45.6 metric tons. It is expected to further grow at an annual rate of 4.65% during 2021-2028, and will reach a market value of US$37.7 billion by 2028. At the same time, my country 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%. Among the existing formaldehyde production processes in China, the main one is methanol oxidation process, which is divided into "silver method" and "iron-molybdenum method" according to the type of catalyst used; the silver catalyst process has a long history, and its advantages are mature technology, short process, low power consumption, low investment, and large single-series production capacity; its disadvantages are high methanol consumption, short catalyst life and poor activity.

[0003] Compared with the "silver method", the "iron-molybdenum method" has the advantages of lower reaction temperature, lower methanol consumption, longer catalyst service life, and can produce high-concentration formaldehyde solution; 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 the silver method, increasing equipment investment and poor economic benefits. Therefore, the production of formaldehyde with the silver catalyst method is still the first choice of many manufacturers. The silver method operates outside the explosion limit of methanol-air. The reaction process is an excess of methanol and insufficient air. Methanol undergoes oxidation and dehydrogenation reactions at normal pressure and 580-740 ℃. About 50-60% of formaldehyde is generated by oxidation reaction, and the rest of the formaldehyde is generated by dehydrogenation reaction. The main by-products are CO, CO2, HCOOH and HCOOCH3.

[0004] Traditional silver catalysts have the disadvantages of short service life, easy poisoning and deactivation, raw materials need to be pretreated, high requirements on equipment, low catalyst activity, poor reaction selectivity, etc. 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 the present 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 that the catalyst used in the preparation of formaldehyde has a short service life, low catalytic activity, poor reaction selectivity, and high formaldehyde synthesis reaction temperature.

[0006] To achieve the above purpose, the technical solution adopted in this application is: to provide a composite silver catalyst prepared by the following method: (i) adding a silver source, a transition metal salt and a solvent into a container, stirring and mixing to obtain a mixed solution; (ii) adding a molecular sieve carrier to the mixed solution and stirring for adsorption; (iii) adding phenoxyimine ligand and stirring; removing excess solvent and drying to obtain a composite silver catalyst.

[0007] In one embodiment, 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.

[0008] In one embodiment, The manganese source is one or more of the hydrochloride, nitrate, sulfate, acetate or hydrate thereof containing the manganese element; the niobium source is one or more of the oxide, chloride, sodium compound and potassium compound containing the niobium element; the bismuth source is the nitrate containing the bismuth element; 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.

[0009] In one embodiment, Step (iii) The structural formula of the phenoxyimine ligand is:

[0010] Wherein, 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;

[0011]

[0012] In one embodiment, 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 carrier 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 carrier is 0.01-0.03:1.

[0013] In one embodiment, The temperature of step (i) is room temperature; In step (i), the solvent is preferably capable of dispersing the silver source and the transition metal salt, which is not required herein.

[0014] The stirring temperature in step (ii) is 30-60°C, and the stirring time is 2-6 h.

[0015] In one embodiment, The stirring temperature in step (iii) is 30-80°C, and the stirring time is 1-3 h.

[0016] The present application also provides an application of a composite silver catalyst in the preparation of formaldehyde, wherein the composite silver catalyst prepared by any of the above embodiments is added to a fixed bed reactor, air is introduced, and formaldehyde is prepared by oxidation reaction using methanol as a raw material.

[0017] In one embodiment, The temperature of the oxidation reaction is 200-400°C, the feed amount of methanol is 10-40 ml / min, and the air pressure is 0.2-0.5 MPa; preferably, the temperature of the oxidation reaction is 200-300°C, the feed amount of methanol is 10-20 ml / min, and the air pressure is 0.2-0.3 MPa.

[0018] The present application provides a composite silver catalyst. In addition to silver, the catalyst also contains auxiliary metals, ligands and carriers. The ligands activate the reaction substrate molecules and reduce the potential energy barrier of the reaction. The auxiliary metals not only reduce the main active metals, but also utilize their low electron cloud distribution characteristics to accurately and quickly desorb the substrate from the catalyst, thereby improving the utilization rate and specificity of the catalyst and 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 use of the composite silver catalyst in the preparation of formaldehyde. The method for preparing formaldehyde reduces the reaction temperature, realizes the highly selective conversion of methanol into formaldehyde under relatively mild conditions, inhibits side reactions at high temperatures, and improves the reaction selectivity and yield. DETAILED DESCRIPTION

[0019] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clear, the present application is further described in detail. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0020] The methanol conversion and formaldehyde selectivity were calculated by the following formula:

[0021]

[0022] Example 1 (i) At room temperature, add 0.90 g AgNO3 and 7.71 g Bi(NO3)2·5H2O to 60 mL deionized water and stir to obtain a mixed solution after the metal salt is completely dissolved; (ii) Add 30 g of molecular sieve carrier ZSM-5, raise the temperature to 40 °C, and stir and adsorb for 4 h; (iii) After the temperature reaches 60°C, add 17.00 g of L3 ligand, stir at constant temperature for 2 h, and then remove excess water 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, which is recorded as CAT1.

[0023] The obtained CAT1 was pressed into tablets with a size of 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 300 °C and kept warm for 1 h, methanol was fed in liquid phase at a flow rate of 20 ml / min. After the reaction was continued for 2 h, the formaldehyde and methanol solution in the receiving bottle were analyzed and determined according to the national standard GB / T9009-2011 "Industrial Formaldehyde Solution". The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0024] Example 2 This embodiment is different from embodiment 1 in that ZSM-5 is replaced with zeolite, and other operations are the same to obtain a composite silver catalyst CAT2, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0025] Example 3 This embodiment is different from embodiment 1 in that ZSM-5 is replaced with β molecular sieve, and other operations are the same to obtain a composite silver catalyst CAT3, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0026] Example 4 The present embodiment is different from the embodiment 1 in that 8.5 g of L3 ligand is added, and other operations are the same to obtain a composite silver catalyst CAT4, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0027] Example 5 The present embodiment is different from the embodiment 1 in that 12.75 g of L3 ligand is added, and other operations are the same to obtain a composite silver catalyst CAT5, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0028] Example 6 The present embodiment is different from the embodiment 1 in that 21.25 g of L3 ligand is added, and other operations are the same to obtain a composite silver catalyst CAT6, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0029] Example 7 This embodiment is different from embodiment 1 in that 25.5 g of L3 ligand is added, and other operations are the same to obtain a composite silver catalyst CAT7, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0030] Example 8 This embodiment is different from embodiment 1 in that 13.43 g of L1 ligand is added, and other operations are the same to obtain a composite silver catalyst CAT8, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0031] Example 9 The present embodiment is different from the embodiment 1 in that 15.21 g of L2 ligand is added, and other operations are the same to obtain a composite silver catalyst CAT9, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0032] Example 10 The present embodiment is different from the embodiment 1 in that 18.02 g of L4 ligand is added, and other operations are the same to obtain a composite silver catalyst CAT10, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0033] Embodiment 11 The present embodiment is different from the embodiment 1 in that 12.16 g of L5 ligand is added, and other operations are the same to obtain a composite silver catalyst CAT11, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0034] Example 12 The present embodiment is different from the embodiment 1 in that 13.05 g of L6 ligand is added, and other operations are the same to obtain a composite silver catalyst CAT12, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0035] Embodiment 13 The present embodiment is different from the embodiment 1 in that 12.16 g of L7 ligand is added, and the other operations are the same to obtain a composite silver catalyst CAT13, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0036] Embodiment 14 The present embodiment is different from the embodiment 1 in that 13.05 g of L8 ligand is added, and other operations are the same to obtain a composite silver catalyst CAT14, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0037] Embodiment 15 The difference between this embodiment and embodiment 1 is that the amount of Bi(NO3)2·5H2O added is 2.57 g, the amount of L3 ligand added is 5.67 g, and other operations are the same to prepare a composite silver catalyst CAT15, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0038] Example 16 The difference between this embodiment and embodiment 1 is that the amount of Bi(NO3)2·5H2O added is 12.85 g, the amount of L3 ligand added is 28.33 g, and the other operations are the same to obtain a composite silver catalyst CAT16, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0039] Embodiment 17 The difference between this embodiment and embodiment 1 is that Bi(NO3)2·5H2O is replaced by Nb2O5, the added amount of Nb2O5 is 4.22 g, the added amount of L3 ligand is 17 g, and other operations are the same to obtain a composite silver catalyst CAT17, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0040] Embodiment 18 The difference between this embodiment and embodiment 17 is that Nb2O5 is replaced by NbCl5, and the amount of NbCl5 added is 4.29 g. Other operations are the same to obtain a composite silver catalyst CAT18, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0041] Embodiment 19 The difference between this embodiment and embodiment 17 is that NbCl5 is replaced by NaNbO3, and the amount of NaNbO3 added is 2.6 g. Other operations are the same to obtain a composite silver catalyst CAT19, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0042] Embodiment 20 The difference between this embodiment and embodiment 17 is that NaNbO3 is replaced by KNbO3, and the amount of KNbO3 added is 2.86 g. Other operations are the same to obtain a composite silver catalyst CAT20, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0043] Embodiment 21 The difference between this embodiment and embodiment 17 is that KNbO3 is replaced by MnSO4·H2O, and the amount of MnSO4·H2O added is 2.69 g. Other operations are the same to obtain a composite silver catalyst CAT21, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0044] Embodiment 22 The difference between this embodiment and embodiment 17 is that MnSO4·H2O is replaced by Mn(OAc)2·2H2O, and the added amount of Mn(OAc)2·2H2O is 4.26 g. Other operations are the same to obtain a composite silver catalyst CAT22, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0045] Embodiment 23 The difference between this embodiment and embodiment 17 is that Mn(OAc)2·2H2O is replaced by Mn(OAc)2, and the amount of Mn(OAc)2 added is 2.75 g. Other operations are the same to obtain a composite silver catalyst CAT23, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0046] Embodiment 24 The difference between this embodiment and embodiment 17 is that Mn(OAc)2 is replaced by Mn(NO3)2·4H2O, and the amount of Mn(NO3)2·4H2O added is 3.99 g. Other operations are the same to obtain a composite silver catalyst CAT24, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0047] Embodiment 25 The difference between this embodiment and embodiment 1 is that the amount of Bi(NO3)2·5H2O added is 17 g, the temperature in step (ii) is raised to 30°C, and the other operations are the same to obtain a composite silver catalyst CAT25, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0048] Embodiment 26 The difference between this embodiment and embodiment 25 is that in step (ii), the temperature is raised to 50°C, and the mixture is stirred and adsorbed for 5 hours. The other operations are the same to obtain a composite silver catalyst CAT26, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0049] Embodiment 27 The difference between this embodiment and embodiment 26 is that in step (ii), the temperature is raised to 60°C, and the mixture is stirred and adsorbed for 6 hours. The other operations are the same to obtain a composite silver catalyst CAT27, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0050] Embodiment 28 The difference between this embodiment and embodiment 26 is that the temperature in step (ii) is raised to 40°C, and the temperature in step (iii) is raised to 70°C to add L3 ligand. Other operations are the same to obtain a composite silver catalyst CAT28, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0051] Embodiment 29 The difference between this embodiment and embodiment 28 is that step (ii) is to keep warm and stir the adsorption for 6 hours, and step (iii) is to raise the temperature to 60°C and add the L3 ligand. Other operations are the same to obtain a composite silver catalyst CAT29, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0052] Embodiment 30 The difference between this embodiment and embodiment 29 is that step (ii) is to keep warm and stir the adsorption for 4 hours, and step (iii) is to raise the temperature to 70°C and add the L3 ligand. Other operations are the same to obtain a composite silver catalyst CAT30, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0053] Embodiment 31 The difference between this embodiment and embodiment 30 is that in step (iii), the temperature is raised to 80°C and the L3 ligand is added. The other operations are the same to obtain a composite silver catalyst CAT31, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0054] Embodiment 32 The difference between this embodiment and embodiment 31 is that in step (iii), the temperature is raised to 60°C and the L3 ligand is added. After constant temperature stirring for 1 hour, the excess water is removed by rotary evaporation. The other operations are the same to obtain a composite silver catalyst CAT32. CAT32 is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0055] Embodiment 33 The difference between this embodiment and embodiment 32 is that in step (iii), after constant temperature stirring for 3 h, excess water is removed by rotary evaporation. Other operations are the same to obtain a composite silver catalyst CAT33, which is used to synthesize formaldehyde. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0056] Embodiment 34 The difference between this embodiment and embodiment 1 is that the methanol flow rate is 10 ml / min. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0057] Embodiment 35 The difference between this embodiment and embodiment 1 is that the methanol flow rate is 40 ml / min. The methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0058] Embodiment 36 The difference between this embodiment and embodiment 1 is that the air pressure is 0.2 MPa, the temperature of the fixed bed reactor is raised to 200° C., and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0059] Embodiment 37 The difference between this embodiment and embodiment 1 is that the air pressure is 0.5 MPa, the temperature of the fixed bed reactor is raised to 400° C., and the methanol conversion rate and formaldehyde selectivity are shown in Table 1.

[0060] Table 1 Methanol conversion and formaldehyde selectivity

[0061] Example 38 Catalyst stability test The obtained CAT1 was pressed into tablets with a mesh size of 20-60 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. Gas chromatography was used for online monitoring. After 200 h, the conversion rate of methanol was 59.3% and the selectivity of formaldehyde was 97.6%.

[0062] Due to the moderate reaction temperature and few side reactions, the catalyst maintains high strength, activity and stability, avoiding bed blockage caused by catalyst pulverization, making the catalyst less likely to deactivate and reducing production costs.

[0063] The present application provides a composite silver catalyst, which comprises the following steps: setting a temperature, adding a silver source and a transition metal salt to a solvent, stirring and mixing to obtain a mixed solution; adding a molecular sieve carrier to the mixed solution, stirring and adsorbing; adding a phenoxyimine ligand and stirring; removing excess solvent, drying, and obtaining a composite silver catalyst; in addition to silver, the catalyst also comprises an auxiliary metal, a ligand and a carrier, wherein the ligand plays a role in activating reaction substrate molecules and reducing the potential energy barrier of the reaction; the auxiliary metal not only plays a role in reducing the main active metal, but also utilizes its low electron cloud distribution characteristics to accurately and quickly desorb the substrate from the catalyst, thereby improving the utilization rate and specificity of the catalyst, overcoming the disadvantages of low selectivity and easy deactivation of traditional silver catalysts, and 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 use of the composite silver catalyst in the preparation of formaldehyde, and the method for preparing formaldehyde reduces the reaction temperature, realizes the highly selective conversion of methanol into formaldehyde under relatively mild conditions, inhibits side reactions at high temperatures, and improves the reaction selectivity and yield.

[0064] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0065] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A composite silver catalyst, characterized in that Prepared by the following method: (i) adding a silver source, a transition metal salt and a solvent into a container, stirring and mixing to obtain a mixed solution; (ii) adding a molecular sieve carrier to the mixed solution and stirring for adsorption; (iii) adding phenoxyimine ligand and stirring; removing excess solvent and drying to obtain a composite silver catalyst.

2. A composite silver catalyst according to claim 1, characterized in that: 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.

3. A composite silver catalyst according to claim 2, characterized in that: The manganese source is one or more of hydrochloride, nitrate, sulfate, acetate or hydrate containing manganese element; the niobium source is one or more of oxide, chloride, sodium compound and potassium compound containing niobium element; the bismuth source is nitrate containing bismuth element.

4. A composite silver catalyst according to claim 1, characterized in that: The structural formula of the phenoxyimine ligand in step (iii) is: 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.

5. A composite silver catalyst according to claim 1, characterized in that: 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 carrier is 0.01-0.05:

1.

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

7. A composite silver catalyst according to claim 1, characterized in that: The stirring temperature in step (iii) is 30-80°C, and the stirring time is 1-3 h.

8. Use of a composite silver catalyst in the preparation of formaldehyde, characterized in that: The composite silver catalyst according to any one of claims 1 to 7 is added into a fixed bed reactor, air is introduced, and formaldehyde is prepared by oxidation reaction using methanol as a raw material.

9. Use of a composite silver catalyst in the preparation of formaldehyde according to claim 8, characterized in that: The temperature of the oxidation reaction is 200-400°C.

10. Use of a composite silver catalyst in the preparation of formaldehyde according to claim 8, 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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