Method for preparing formaldehyde through oxidation of dimethyl ether

Abstract

The invention relates to a method for preparing formaldehyde by oxidizing dimethyl ether. According to the method, dimethyl ether is adopted as a reactant, and formaldehyde is prepared through an oxidation dehydration reaction under the action of an iron-molybdenum catalyst. The reaction conditions are as follows: the reaction is carried out in a fixed bed reactor, the reaction temperature is 280-340 DEG C, the reaction pressure is 0.1-0.3 MPa, and the volume space velocity of the dimethyl ether is 4500-25000h <-1 >. The method is characterized in that an iron-molybdenum material with a specific structure is used as a catalyst, the preparation structure of the catalyst is controllable, the cost is relatively low, the dimethyl ether is efficiently catalyzed to oxidize and generate formaldehyde in a high-selectivity manner, the highest selectivity of the formaldehyde can reach 85%, the highest conversion rate of methanol reaches 90%, and the stability is relatively good.

Description

Technical Field

[0001] The invention relates to a method for preparing formaldehyde, and in particular to a process of using dimethyl ether as a reactant to prepare formaldehyde through oxidation of dimethyl ether. Background Art

[0002] Dimethyl ether has a wide range of sources. In addition to natural gas, it can also be obtained from coal and coalbed methane, CO2 hydrogenation, biomass, etc. The reaction performance of dimethyl ether is similar to that of methanol. It can dissociate on the catalyst surface to generate methoxyl, methyl and other species for chemical reactions. However, methanol is toxic in terms of physical properties, while dimethyl ether is environmentally friendly and easy to store and transport. The catalytic conversion of dimethyl ether to replace methanol to produce formaldehyde has great application prospects and feasibility.

[0003] At present, metal oxide-supported Mo and V catalysts are widely used in the oxidation of dimethyl ether to formaldehyde. Supported molybdenum oxide and vanadium oxide catalysts usually have good catalytic performance at low temperatures (240-320°C), and the acid-base properties and specific surface area of ​​the carrier have a great influence on the catalyst performance. On the preferred acidic carrier, the formaldehyde selectivity of dimethyl ether oxidation to formaldehyde can reach 70%, but the conversion rate of dimethyl ether is relatively low, usually less than 20%; there are also organic heterocyclic compounds as raw materials to prepare molybdenum vanadium iron and other catalysts, but the catalyst preparation is cumbersome and the raw materials are expensive, which limits its large-scale synthesis and use. Therefore, it is of great significance to develop an efficient and inexpensive catalytic system to achieve the selective conversion of dimethyl ether to formaldehyde, but it is still very challenging. Summary of the invention

[0004] The significance of the present invention is to overcome the disadvantages of low conversion rate or selectivity, complicated catalyst preparation, high cost, etc. in the current process of preparing formaldehyde by oxidation of dimethyl ether. The preparation method has a simple reaction process, the catalyst is non-toxic, and the selectivity of the catalyst for ethyl acetate can be effectively improved by simple organic base treatment, and the catalyst has good stability and long service life.

[0005] The invention relates to the preparation of formaldehyde by the following scheme: dimethyl ether is used as a raw material, a reaction is carried out in a fixed bed reactor, an iron-molybdenum catalyst is filled in a reaction tube, and the reaction tube is placed in the fixed bed reactor for reaction, the reaction temperature is 280-340°C, and the reaction pressure is 0.1MPa-0.3MPa; the iron-molybdenum catalyst is prepared by a precipitation method, and the precipitant is one, two or more of ammonia water, sodium carbonate, potassium carbonate, sodium bicarbonate and sodium hydroxide; the precipitation pH is controlled to be 1.3-2.2; the precipitate obtained in the preparation process of the iron-molybdenum catalyst is aged or crystallized, and the temperature is room temperature-180°C; the molybdenum-iron ratio of the iron-molybdenum catalyst when feeding is 2-3 (molar ratio); the iron-molybdenum catalyst is prepared by a precipitation method, and the precipitation pH is preferably controlled to be 1.3-1.8; the molybdenum-iron ratio of the iron-molybdenum catalyst when feeding is preferably 2.2-2.6 (molar ratio); in the process of preparing formaldehyde by oxidation of dimethyl ether, the volume space velocity of dimethyl ether is 4500-25000h -1 In the process of oxidation of dimethyl ether to formaldehyde, the volume space velocity of dimethyl ether is preferably 4500-12000h -1 ; The process of preparing formaldehyde by oxidation of dimethyl ether, the reaction temperature is 280-340°C, the reaction pressure is 0.1MPa-0.3MPa; The process of preparing formaldehyde by oxidation of dimethyl ether, the reaction temperature is preferably 300-320°C, the reaction pressure is 0.1MPa-0.15MPa;

[0006] The iron-molybdenum catalyst catalyzes the oxidation of dimethyl ether to produce formaldehyde with the following characteristics: A challenging problem in the oxidation of dimethyl ether to produce formaldehyde is the oxygen supply capacity of the catalyst, and the core lies in the control of the active crystal phase of the catalyst. Amorphous molybdenum species easily lead to the overoxidation of dimethyl ether to produce CO, and the precipitation of iron oxide crystal phase will trigger the oxidation of dimethyl ether to CO2; the appropriate dispersion state of molybdenum species and the formation of iron molybdate crystal phase are the key to the oxidation of dimethyl ether to formaldehyde, and the acidity and alkalinity of the catalyst itself are also key points affecting the selection of products. By controlling the synthesis conditions of the iron-molybdenum catalyst, the precise control of the crystal phase formation and species distribution of the iron-molybdenum catalyst is the core of determining whether dimethyl ether can efficiently and directionally prepare formaldehyde. Therefore, the present invention focuses on protecting the catalytic effect of the iron-molybdenum catalyst and the relevant synthesis parameters of the catalyst.

[0007] The advantages of this method are: (1) Using ferric nitrate and ammonium molybdate as raw materials, the iron-molybdenum catalyst is prepared by precipitation method. By precisely controlling the synthesis conditions, a high-efficiency catalyst is obtained. The catalyst raw materials are widely available and the preparation process is simple. (2) The catalytic system has certain advantages in the process of catalyzing the oxidation of dimethyl ether to formaldehyde. The selectivity of the target product is as high as 84%, and the conversion rate is as high as 90%; (3) The iron-molybdenum catalyst has excellent stability. After continuous operation for 800 hours, the catalyst has no obvious deactivation phenomenon.

[0008] The method is characterized in that: using iron-molybdenum materials with specific structures as catalysts, the species states of molybdenum and iron in the catalyst can be controlled and adjusted by precisely controlling the pH value and precipitation conditions, and the iron-molybdenum catalyst can be obtained with high yield and excellent catalytic activity. It can efficiently catalyze the oxidation of dimethyl ether to generate formaldehyde with high selectivity, and the highest selectivity of formaldehyde can reach 84%, and the conversion rate of dimethyl ether can reach up to 90%, and it has good stability. On the contrary, it is easy to cause the loss of iron ions, resulting in a decrease in the catalyst yield, and due to the loss of iron ions, the excess molybdenum forms a highly active molybdenum oxide phase, which causes the overoxidation of dimethyl ether to generate CO and CO2, reducing the selectivity of formaldehyde. DETAILED DESCRIPTION

[0009] In order to further explain the present invention in detail, several specific implementation cases are given below, but the present invention is not limited to these embodiments.

[0010] Example 1

[0011] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; add the ferric nitrate solution dropwise to the ammonium molybdate solution, and adjust the pH to 1.3 with concentrated ammonia water (containing 25% to 28% ammonia mass concentration, the same below). Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0012] The evaluation process of dimethyl ether oxidation reaction is as follows: the above-mentioned iron-molybdenum catalyst is pressed into 40-60 mesh catalyst particles, and 0.9 mL of catalyst particles are measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0013] Example 2

[0014] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.5 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0015] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0016] Example 3

[0017] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.65 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0018] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0019] Example 4

[0020] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.95 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0021] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0022] Example 5

[0023] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 2.15 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0024] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0025] Example 6

[0026] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with 0.1M sodium carbonate. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0027] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0028] Example 7

[0029] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with 0.1M potassium carbonate. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0030] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0031] Example 8

[0032] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with 0.1M sodium bicarbonate. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0033] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0034] Example 9

[0035] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with 0.1M sodium hydroxide. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0036] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0037] Example 10

[0038] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a single-necked flask, age it at 50°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0039] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 280 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0040] Embodiment 11

[0041] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a single-necked flask, age it at 80°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0042] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 300 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate, etc. are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0043] Example 12

[0044] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 80°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0045] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0046] Example 13

[0047] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 100°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0048] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0049] Embodiment 14

[0050] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 110°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0051] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0052] Embodiment 15

[0053] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 150°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0054] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0055] Example 16

[0056] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 180°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0057] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0058] Embodiment 17

[0059] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 135.8 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 138.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0060] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0061] Embodiment 18

[0062] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 117.7 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0063] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0064] Embodiment 19

[0065] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 90.5 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0066] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0067] Embodiment 20

[0068] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 87.8 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 88.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0069] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0070] Embodiment 21

[0071] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0072] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.4 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0073] Embodiment 22

[0074] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0075] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.3 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0076] Embodiment 23

[0077] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0078] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.2 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0079] Embodiment 24

[0080] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0081] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.4 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is 0.12 MPa, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate, etc. are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0082] Embodiment 25

[0083] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0084] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.4 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is 0.25 MPa, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0085] Embodiment 26

[0086] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 100°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0087] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 300 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 300 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate, etc. are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0088] Embodiment 27

[0089] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; slowly add ferric nitrate to the ammonium molybdate, and adjust the pH to 1.3 with concentrated ammonia water. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 100°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0090] The evaluation process of dimethyl ether oxidation reaction is as follows: the above iron-molybdenum catalyst is pressed into a 40-60 mesh catalyst, and 0.9 mL is measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 300 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 800 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate, etc. are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0091] Comparative Example 1

[0092] The preparation process of the iron-molybdenum catalyst used in dimethyl ether is as follows: weigh 108.6 grams of ferric nitrate nonahydrate and dissolve it in 1.2L of water, weigh 118.6 grams of ammonium molybdate tetrahydrate and dissolve it in 1.3L of water; add the ferric nitrate solution dropwise to the ammonium molybdate solution. Stir the above solution at room temperature for 30 minutes, transfer it to a hydrothermal synthesis reactor, crystallize it at 120°C for 8 hours, filter it, wash it with water until it is neutral, dry it at 100°C, and roast it at 400°C for 10 hours. The iron-molybdenum catalyst is obtained, and the yield of the catalyst is shown in Appendix 1.

[0093] The evaluation process of dimethyl ether oxidation reaction is as follows: the above-mentioned iron-molybdenum catalyst is pressed into 40-60 mesh catalyst particles, and 0.9 mL of catalyst particles are measured for the reaction of dimethyl ether oxidation to formaldehyde. The flow rate of the mixed gas of nitrogen and dimethyl ether is 50 ml / min, the content of dimethyl ether gas is 3.5 vol%, and the oxygen flow rate is 7.0 mL / min. The reaction temperature is 310 ° C, the reaction pressure is normal pressure, and the product analysis is carried out after 10 hours of reaction. The unreacted dimethyl ether and gas products are analyzed by online gas chromatography, and the products such as formaldehyde, water, methylal, formic acid, methyl formate are analyzed by offline chromatography. After analysis, the dimethyl ether conversion rate and formaldehyde selectivity are shown in Appendix 1.

[0094] Appendix 1. Evaluation of the reaction performance of iron-molybdenum catalysts for the oxidation of dimethyl ether to formaldehyde

[0095]

[0096]

[0097] It can be seen from the results in the table that the iron-molybdenum catalyst can be obtained with high yield through precise control of pH, and the catalytic activity is excellent, the highest conversion rate of dimethyl ether reaches 90%, and the selectivity of formaldehyde is up to 85%; at the same time, this type of catalyst has high catalytic stability, and the catalyst has no obvious deactivation after continuous operation for 300 h (Example 26) or even 800 h (Example 27). Without pH adjustment, it is easy to cause the loss of iron ions, resulting in a catalyst yield of less than 90%, and due to the loss of iron ions, excess molybdenum forms a highly active molybdenum oxide phase, which causes dimethyl ether to be peroxidized to generate CO and CO2, reducing the selectivity of formaldehyde.

Claims

1. A method for preparing formaldehyde by oxidation of dimethyl ether, characterized in that: The process of preparing formaldehyde by oxidation of dimethyl ether is as follows: dimethyl ether is used as raw material, an iron-molybdenum catalyst is filled in a reaction tube, and then the reaction is carried out in a fixed bed reactor, the reaction temperature is 280-340° C., and the reaction pressure is 0.1MPa-0.3MPa.

2. The method according to claim 1, characterized in that: The iron-molybdenum catalyst is prepared by a precipitation method, and the precipitant type is one, two or more of ammonia water, sodium carbonate, potassium carbonate, sodium bicarbonate and sodium hydroxide; a soluble iron precursor and a molybdenum precursor are precipitated in a solution, and the pH of the precipitation system is controlled to be 1.3-2.

2.

3. The method according to claim 2, characterized in that: The iron-molybdenum catalyst, during the preparation process, the precipitated solution obtained is aged or crystallized at a temperature of room temperature-180°C, preferably 100-130°C; for a time of 5-24h, preferably 6-12h; After the aged or crystallized solution is separated from the solid and the liquid, the solid is washed with water, dried and then calcined at a temperature of 300-400° C., preferably 340-390° C., for 5-48 hours, preferably 6-24 hours.

4. The method according to claim 2, characterized in that: The iron-molybdenum catalyst has a molybdenum-to-iron ratio of 2-3 (molar ratio) when feeding.

5. The method according to claim 2, characterized in that: The iron-molybdenum catalyst is prepared by a precipitation method, and the precipitation pH is preferably controlled at 1.3-1.

8.

6. The method according to claim 2 or 4, characterized in that: The molybdenum to iron ratio of the iron-molybdenum catalyst when feeding is preferably 2.2-2.6 (molar ratio).

7. The method according to claim 1, characterized in that: In the process of oxidizing dimethyl ether to produce formaldehyde, the volume space velocity of dimethyl ether is 4500-25000h -1 .

8. According to claim 1 or 7, it is characterized in that: In the process of oxidizing dimethyl ether to produce formaldehyde, the volume space velocity of dimethyl ether is preferably 4500-12000h -1 .

9. According to claim 1, it is characterized in that: In the process of oxidizing dimethyl ether to produce formaldehyde, the reaction temperature is 280-340° C. and the reaction pressure is 0.1 MPa-0.3 MPa; The volume ratio of dimethyl ether to oxygen is 1:1-10:1, preferably 3:1-5:

1.

10. According to claim 1 or 9, it is characterized in that: In the process of oxidizing dimethyl ether to produce formaldehyde, the reaction temperature is preferably 300-320° C. and the reaction pressure is 0.1 MPa-0.15 MPa.

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