A metal-doped iron molybdate catalyst, its preparation method and application

Abstract

The present invention belongs to the technical field of catalysts, and more specifically, relates to a metal-doped iron molybdate catalyst, a preparation method thereof, and an application. The chemical formula of the catalyst is Fe x A y B z (MoO4)3, wherein A and B are each independently one of the metal elements Mn, Co, Ni, Cu, 1.8 ≤ x < 2.0, 0 ≤ y ≤ 0.1, 0 ≤ z ≤ 0.1, and y and z are not simultaneously 0, and x + y + z = 2. The metal-doped iron molybdate catalyst provided by the present invention has a low non-noble metal doping amount. When used for the gas-phase catalytic oxidation of ethanol to acetaldehyde, the catalyst has high activity and good stability, and can obtain an acetaldehyde single-pass yield of > 80% and a space-time yield of > 3.0 g g cat ‑1 h ‑1 at 200 °C, and the catalyst does not deactivate after continuous reaction at 225 °C for 100 h.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and more specifically, relates to a metal-doped iron molybdate catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] Acetaldehyde is an important organic chemical intermediate and can be used to produce common chemical raw materials such as acetic acid, ethyl acetate, vinyl acetate, crotonaldehyde, and pyridine compounds. The traditional industrial production of acetaldehyde uses the Wacker oxidation technology of ethylene, which has the disadvantages of high cost, serious corrosion and pollution, and unsustainability. The gas-phase catalytic oxidation of ethanol to acetaldehyde can be carried out through two routes: anaerobic dehydrogenation and oxidative dehydrogenation. The anaerobic dehydrogenation of ethanol to acetaldehyde is an endothermic reaction, and a relatively high reaction temperature often leads to catalyst sintering and carbon deposition deactivation. The oxidative dehydrogenation of ethanol to acetaldehyde is an exothermic reaction, and the catalyst is not easily carbon-deposited, but it is difficult to obtain a high acetaldehyde yield, and the contradiction between catalyst activity and selectivity still exists. At present, the industrial process of catalytic oxidation of ethanol to acetaldehyde using silver foam exists the disadvantages of high energy consumption, low single-pass yield of acetaldehyde, and short catalyst life. There is an urgent need to develop more efficient and stable catalysts for the gas-phase oxidative dehydrogenation of ethanol to acetaldehyde.

[0003] Chinese Patent CN109569647A reported a gold catalyst supported on a copper-containing composite oxide, which can obtain an acetaldehyde yield of >90% at 200 °C and does not deactivate after continuous reaction for 200 hours. However, due to the low specific surface area (<10 m 2 / g) and high Au loading (>1 wt%) of these powder catalysts, the cost is relatively high and it is difficult to form. For metal oxide catalysts with relatively low cost, vanadium-containing composite oxides show relatively high low-temperature (200 - 250 °C) catalytic activity for the selective oxidation of ethanol, but V is toxic and environmentally harmful. Iron molybdenum catalysts are environmentally friendly industrial catalysts that have been widely used in the selective oxidation of methanol to formaldehyde, but this catalyst only shows moderate catalytic activity and selectivity in the reaction of ethanol oxidation to acetaldehyde and has not been taken seriously for a long time. Recently, it was reported that an iron molybdenum catalyst prepared by the co-precipitation method obtained an acetaldehyde yield of >90% at 280 °C at a relatively low space velocity (ChemCatChem, 2021, 9, 1363), but the space-time yield of acetaldehyde was less than 1.0 g g cat -1 h -1 , and the activity of the catalyst decreased significantly within 50 h. Therefore, it is necessary to develop more efficient and stable non-noble metal oxide catalysts to promote the industrial application of the gas-phase oxidative dehydrogenation of ethanol to acetaldehyde. Summary of the Invention

[0004] Aiming at the defects of the prior art, the purpose of the present invention is to provide a metal-doped iron molybdate catalyst, a preparation method thereof and an application, so as to solve the technical problems of low space-time yield and rapid decline in activity when the prior art iron molybdate catalyst is used for catalytic gas-phase oxidation of ethanol to acetaldehyde.

[0005] To achieve the above object, the present invention provides a metal-doped iron molybdate catalyst, the chemical formula of which is Fe x A y B z (MoO4)3, where A and B are each independently one of the metal elements Mn, Co, Ni, Cu, 1.8 ≤ x < 2.0, 0 ≤ y ≤ 0.1, 0 ≤ z ≤ 0.1, and y and z are not both 0 at the same time, and x + y + z = 2; and when the metal element corresponding to A or B is Cu, both y and z are not 0; the Fe in the catalyst contains both +3 valence and +2 valence.

[0006] Preferably, 1.9 ≤ x < 2.0, 0 ≤ y ≤ 0.1, 0 ≤ z ≤ 0.1, and y and z are not both 0 at the same time, and x + y + z = 2.

[0007] According to another aspect of the present invention, a preparation method of the above-mentioned metal-doped iron molybdate catalyst is provided, which includes the following steps:

[0008] (1) According to the atomic ratio in the chemical formula composition, an aqueous solution of ammonium molybdate is dropped into a mixed aqueous solution of an iron salt, a metal salt corresponding to metal element A and a metal salt corresponding to metal element B, and stirring is carried out during the dropping to make them react to obtain a reaction precursor solution;

[0009] (2) The reaction precursor solution obtained in step (1) is subjected to a hydrothermal reaction, the reaction product is separated by solid-liquid separation, the separated solid phase is washed to neutrality, and dried to obtain a metal-doped iron molybdate catalyst.

[0010] Preferably, the iron salt, the metal salt corresponding to metal element A and the metal salt corresponding to metal element B in step (1) are each independently a nitrate, an acetate or an acetylacetonate.

[0011] Preferably, in the mixed aqueous solution of the iron salt, the metal salt corresponding to metal element A and the metal salt corresponding to metal element B in step (1), the total concentration of the solute is 0.2 - 1.0 mol / L.

[0012] Preferably, for the stirring and reaction in step (1), the reaction time is 2 - 4 h.

[0013] Preferably, the temperature of the hydrothermal reaction in step (2) is 140 - 180 °C, and the reaction time is 8 - 20 h.

[0014] According to another aspect of the present invention, there is provided an application of the metal-doped iron molybdate catalyst in the reaction of ethanol vapor-phase oxidative dehydrogenation to prepare acetaldehyde.

[0015] Generally speaking, compared with the prior art by the above technical solutions conceived by the present invention, the following

[0016] Advantages are obtained:

[0017] (1) A metal-doped iron molybdate catalyst provided by the present invention is doped with one or two of Mn, Co, Ni, and Cu in the iron molybdate catalyst of the prior art. Experiments have found that, compared with the iron molybdate catalyst of the prior art, the space-time yield of the metal-doped iron molybdate catalyst of the present invention has been significantly improved, and the activity is well maintained.

[0018] (2) The non-noble metal oxide catalyst of the present invention greatly reduces the catalyst cost compared with the supported nano-gold catalyst and noble metal catalysts such as foamed silver used for ethanol oxidative dehydrogenation to prepare acetaldehyde in the past.

[0019] (3) The metal-doped iron molybdate catalyst is prepared by the hydrothermal method in the present invention. The preparation method is simple, has strong controllability and good reproducibility, and effectively reduces the production cost.

[0020] (4) The low-doping amount and high-specific surface area metal-doped iron molybdate catalyst provided by the present invention has a low non-noble metal doping amount. When used for ethanol vapor-phase oxidative catalysis to prepare acetaldehyde, the catalyst has high activity and good stability, and can obtain an acetaldehyde single-pass yield of >80% and a space-time yield of >3.0 g g cat -1 h -1 at 200 °C, and the catalyst does not deactivate after continuous reaction at 225 °C for 100 h. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 XRD patterns of Example 1, Example 5 and Comparative Example 1.

[0022] Figure 2 H2-TPR patterns of Example 1, Example 5 and Comparative Example 1.

[0023] Figure 3 XPS pattern of the catalyst of Example 5.

[0024] Figure 4 Stability test result pattern of the catalyst of Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0025] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0026] A metal-doped iron molybdate catalyst provided by the present invention has a chemical formula of Fe x A y B z (MoO4)3, where A and B are each independently one of the metal elements Mn, Co, Ni, and Cu, 1.8 ≤ x < 2.0, 0 ≤ y ≤ 0.1, 0 ≤ z ≤ 0.1, and y and z are not both 0 at the same time, and x + y + z = 2; and when the metal element corresponding to A or B is Cu, both y and z are not 0, and x + y + z = 2; the Fe in the catalyst contains both +3 and +2 valence states.

[0027] In a preferred embodiment, 1.9 ≤ x < 2.0, 0 ≤ y ≤ 0.1, 0 ≤ z ≤ 0.1, and y and z are not both 0 at the same time, and x + y + z = 2.

[0028] The present invention provides a method for preparing the metal-doped iron molybdate catalyst, comprising the following steps:

[0029] (1) According to the atomic ratio in the chemical formula composition, an aqueous solution of ammonium molybdate is dropped into a mixed aqueous solution of an iron salt, a metal salt corresponding to metal element A, and a metal salt corresponding to metal element B, and stirring is carried out during the dropping to make them react to obtain a reaction precursor solution;

[0030] (2) The reaction precursor solution obtained in step (1) is subjected to a hydrothermal reaction, the reaction product is separated into solid and liquid, the separated solid phase is washed to neutrality, and dried to obtain the metal-doped iron molybdate catalyst.

[0031] In some embodiments, the iron salt, the metal salt corresponding to metal element A, and the metal salt corresponding to metal element B in step (1) are each independently a nitrate, an acetate, or an acetylacetonate. In the mixed aqueous solution of the iron salt, the metal salt corresponding to metal element A, and the metal salt corresponding to metal element B in step (1), the total concentration of the solute is 0.2 to 1.0 mol / L.

[0032] In step (1) of the present invention, an aqueous solution of ammonium molybdate is dropped into an aqueous solution mixture of an iron salt, a metal salt corresponding to metal element A, and a metal salt corresponding to metal element B, rather than other feeding sequences, with the aim of maintaining a weakly acidic microenvironment to ensure uniform mixing of each component in the precursor solution. During the experiment, the feeding sequence was also attempted to be changed by dropping the metal salt mixture into the ammonium molybdate solution to promote the precipitation reaction under alkaline conditions. When preparing the catalyst with the chemical composition of the present invention by the co-precipitation method, tests showed that its catalytic activity was significantly lower under the same conditions.

[0033] In some embodiments, in step (1), the stirring for reaction is carried out for 2 to 4 h. In step (2), the temperature of the hydrothermal reaction is 140 to 180 °C, and the reaction time is 8 to 20 h.

[0034] In some embodiments, the slurry after cooling from the hydrothermal reaction is filtered, washed with deionized water until the filtrate is neutral, and then the filter cake is suction-filtered until it is basically dry; then the solid is placed in an oven and dried at 80 to 120 °C, and then ground evenly to obtain a solid powder catalyst.

[0035] The metal-doped iron molybdate catalyst provided by the present invention can be used for the gas-phase oxidative dehydrogenation of ethanol to prepare acetaldehyde. The synthesized catalyst of the present invention and the comparative catalyst are respectively evaluated for activity using a fixed-bed reactor. The evaluation method is as follows: The powder catalyst and α-Al2O3 are mixed evenly and loaded into a quartz reaction tube placed in a fixed-bed reactor. First, a 10% H2 + 90% N2 mixed gas is introduced at a flow rate of 20 mL / min, and the temperature is raised to 200 °C at a heating rate of 10 °C / min and held for 2 hours for catalyst pretreatment. Then, the feed is switched to air, and ethanol is pumped into the vaporization chamber at a certain flow rate by a liquid pump and mixed evenly with air. The mixed gas enters the catalyst bed layer to react with the catalyst, and the reaction products are analyzed online by gas chromatography. The conditions for the gas-phase oxidative dehydrogenation of ethanol are as follows: The ethanol is anhydrous ethanol or 50 to 95% water-containing bioethanol, the liquid hourly space velocity is 5 mL / g cat / h, the molar ratio of oxygen to ethanol is 1 to 5:1, the gas hourly space velocity is 20000 to 50000 mL / g cat / h, and the reaction temperature is 150 to 250 °C.

[0036] The following are examples:

[0037] Example 1

[0038] Preparation of Fe 1.98 Mn 0.02 (MoO4)3

[0039] Weigh 4.00 g of iron nitrate and 0.025 g of manganese nitrate, dissolve them in 65 mL of deionized water, and place them in a 250 mL three-necked flask. Then weigh 2.65 g of ammonium heptamolybdate, dissolve it in 65 mL of deionized water, pour it into a clean 100 mL constant pressure dropping funnel, and slowly drip it into the metal salt solution at room temperature. While dripping slowly, stir rapidly. After the dripping is completed, continue stirring for 2 h. After the reaction is over, pour it into a hydrothermal reactor and react at a temperature of 140 °C for 12 h. Pour it into a sintered glass funnel, filter it with deionized water until it is neutral, then drain it, place it in an oven and dry it at 80 °C for 12 h, and then grind it evenly to obtain the final catalyst.

[0040] Load a mixture of 60 mg of catalyst and 600 mg of α-Al2O3 into the reaction tube. Use nitrogen to check for leaks. After ensuring that the system has good airtightness, start the pre-reduction treatment of the catalyst. First, introduce 20 mL / min of H2 (10% H2, 90% N2), and then raise the temperature of the fixed bed to 280 °C at a heating rate of 10 °C / min and keep it warm for 2 hours. Then carry out the activity evaluation. The feed ethanol enters the fixed bed through an injection pump, mixes with the feed air after passing through the vaporization chamber, enters the catalyst bed layer to react with the catalyst, and the reaction products enter an on-line gas chromatograph for substance determination. The reaction conditions are as follows: the liquid hourly space velocity of ethanol feed is 5 mL / g cat / h, the molar ratio of oxygen to ethanol is 3:1, the gas hourly space velocity is 50000 mL / g cat / h, the reaction temperature is 150 - 250 °C. All the following examples and comparative examples are under this test condition.

[0041] Example 2

[0042] Preparation of Fe 1.98 Co 0.02 (MoO4)3

[0043] Weigh 4.00 g of iron nitrate and 0.029 g of cobalt nitrate, dissolve them in 65 mL of deionized water, and place them in a 250 mL three-necked flask. Then weigh 2.65 g of ammonium heptamolybdate, dissolve it in 65 mL of deionized water, pour it into a clean 100 mL constant pressure dropping funnel, and slowly drip it into the metal salt solution at room temperature. While dripping slowly, stir rapidly. After the dripping is completed, continue stirring for 2 h. After the reaction is over, pour it into a hydrothermal reactor and react at a temperature of 140 °C for 12 h. Pour it into a sintered glass funnel, filter it with deionized water until it is neutral, then drain it, place it in an oven and dry it at 80 °C for 12 h, and then grind it evenly to obtain the final catalyst.

[0044] Example 3

[0045] Preparation of Fe 1.99 Ni 0.01(Preparation of (MoO4)3)

[0046] Weigh 4.02 g of iron nitrate and 0.015 g of nickel nitrate, dissolve them in 65 mL of deionized water, and place them in a 250 mL three-necked flask. Then weigh 2.65 g of ammonium heptamolybdate, dissolve it in 65 mL of deionized water, transfer it to a clean 100 mL constant-pressure dropping funnel, and slowly drip it into the metal salt solution at room temperature. While dripping slowly, stir rapidly. After the dripping is completed, continue stirring for 2 h. After the reaction is over, pour it into a hydrothermal reactor and react at 140 °C for 12 h. Then pour it into a sintered glass funnel and filter it with deionized water until neutral, and then dry it by suction. Place it in an oven and dry it at 80 °C for 12 h, and then grind it evenly to obtain the final catalyst.

[0047] Example 4

[0048] Fe for the gas-phase oxidation of ethanol to acetaldehyde 1.98 Zn 0.02 (Preparation of (MoO4)3)

[0049] Weigh 4.00 g of iron nitrate and 0.026 g of zinc nitrate, dissolve them in 65 mL of deionized water, and place them in a 250 mL three-necked flask. Then weigh 2.65 g of ammonium heptamolybdate, dissolve it in 65 mL of deionized water, transfer it to a clean 100 mL constant-pressure dropping funnel, and slowly drip it into the metal salt solution at room temperature. While dripping slowly, stir rapidly. After the dripping is completed, continue stirring for 2 h. After the reaction is over, pour it into a hydrothermal reactor and react at 140 °C for 12 h. Then pour it into a sintered glass funnel and filter it with deionized water until neutral, and then dry it by suction. Place it in an oven and dry it at 80 °C for 12 h, and then grind it evenly to obtain the final catalyst.

[0050] Example 5

[0051] Fe for the gas-phase oxidation of ethanol to acetaldehyde 1.98 Mn 0.01 Cu 0.01 (Preparation of (MoO4)3)

[0052] Weigh 4.00 g of iron nitrate, 0.015 g of manganese nitrate and 0.012 g of copper nitrate, dissolve them in 65 mL of deionized water, and place them in a 250 mL three-necked flask. Then weigh 2.65 g of ammonium heptamolybdate, dissolve it in 65 mL of deionized water, transfer it to a clean 100 mL constant-pressure dropping funnel, and slowly drip it into the metal salt solution at room temperature. While dripping slowly, stir rapidly. After the dripping is completed, continue stirring for 2 h. After the reaction is over, pour it into a hydrothermal reactor and react at 140 °C for 12 h. Then pour it into a sintered glass funnel and filter it with deionized water until neutral, and then dry it by suction. Place it in an oven and dry it at 80 °C for 12 h, and then grind it evenly to obtain the final catalyst.

[0053] Example 6

[0054] Preparation of Fe 1.98 Co 0.01 Cu 0.01 (MoO4)3

[0055] Weigh 4.00 g of iron nitrate, 0.015 g of cobalt nitrate, and 0.012 g of copper nitrate and dissolve them in 65 mL of deionized water in a 250 mL three-necked flask. Then weigh 2.65 g of ammonium heptamolybdate and dissolve it in 65 mL of deionized water. Pour it into a clean 100 mL constant-pressure dropping funnel and slowly drip it into the metal salt solution at room temperature. Stir rapidly while dripping slowly. After dripping, continue to stir for 2 h. After the reaction is completed, pour it into a hydrothermal reaction kettle and react at 140 °C for 12 h. Pour it into a sintered glass funnel and filter it with deionized water until neutral, then dry it by suction. Place it in an oven and dry it at 80 °C for 12 h, and then grind it evenly to obtain the final catalyst.

[0056] Example 7

[0057] Preparation of Fe 1.98 Ni 0.01 Cu 0.01 (MoO4)3

[0058] Weigh 4.00 g of iron nitrate, 0.015 g of nickel nitrate, and 0.012 g of copper nitrate and dissolve them in 65 mL of deionized water in a 250 mL three-necked flask. Then weigh 2.65 g of ammonium heptamolybdate and dissolve it in 65 mL of deionized water. Pour it into a clean 100 mL constant-pressure dropping funnel and slowly drip it into the metal salt solution at room temperature. Stir rapidly while dripping slowly. After dripping, continue to stir for 2 h. After the reaction is completed, pour it into a hydrothermal reaction kettle and react at 140 °C for 12 h. Pour it into a sintered glass funnel and filter it with deionized water until neutral, then dry it by suction. Place it in an oven and dry it at 80 °C for 12 h, and then grind it evenly to obtain the final catalyst.

[0059] Example 8

[0060] Preparation of Fe 1.98 Zn 0.01 Cu 0.01 (MoO4)3

[0061] Weigh 4.00 g of iron nitrate, 0.013 g of nickel nitrate, and 0.012 g of copper nitrate, dissolve them in 65 mL of deionized water, and place them in a 250 mL three-necked flask. Then weigh 2.65 g of ammonium heptamolybdate, dissolve it in 65 mL of deionized water, transfer it to a clean 100 mL constant-pressure dropping funnel, and slowly drip it into the metal salt solution at room temperature while stirring rapidly. After the dropping is completed, continue stirring for 2 h. After the reaction is over, pour it into a hydrothermal reactor and react at 140 °C for 12 h. Then pour it into a sintered glass funnel and filter it with deionized water until it is neutral, and then drain it. Place it in an oven and dry it at 80 °C for 12 h, and then grind it evenly to obtain the final catalyst.

[0062] Comparative Example 1

[0063] Preparation of Fe2(MoO4)3 for gas-phase oxidation of ethanol to acetaldehyde (hydrothermal method)

[0064] Weigh 4.04 g of iron nitrate, dissolve it in 65 mL of deionized water, and place it in a 250 mL three-necked flask. Then weigh 2.65 g of ammonium heptamolybdate, dissolve it in 65 mL of deionized water, transfer it to a clean 100 mL constant-pressure dropping funnel, and slowly drip it into the iron nitrate solution at room temperature while stirring rapidly. After the dropping is completed, continue stirring for 2 h. After the reaction is over, pour it into a hydrothermal reactor and react at 140 °C for 12 h. Then pour it into a sintered glass funnel and filter it with deionized water until it is neutral, and then drain it. Place it in an oven and dry it at 80 °C for 12 h, and then grind it evenly to obtain the final catalyst.

[0065] Comparative Example 2

[0066] Fe for gas-phase oxidation of ethanol to acetaldehyde 1.98 Mn 0.01 Cu 0.01 Preparation of (MoO4)3 (calcined at 400 °C)

[0067] Grind the catalyst prepared in Example 5 evenly, put it into a muffle furnace, and calcine it at 400 °C for 5 h to obtain the final catalyst of Comparative Example 2.

[0068] Comparative Example 3

[0069] Fe for gas-phase oxidation of ethanol to acetaldehyde 1.98 Mn 0.01 Cu 0.01 Preparation of (MoO4)3 (calcined at 500 °C)

[0070] Grind the catalyst prepared in Example 5 evenly, put it into a muffle furnace, and calcine it at 500 °C for 5 h to obtain the final catalyst of Comparative Example 3.

[0071] For Comparative Example 2 and Comparative Example 3, to investigate the effect of the catalyst calcination temperature on its catalytic activity, experiments found that after the calcination treatment of Comparative Example 2 and Comparative Example 3, their catalytic performance decreased significantly compared to without calcination. It may be because the calcination of the catalyst makes the catalyst have better crystallinity and fewer surface defect sites of the catalyst, which is not conducive to the activation of the substrate and molecular oxygen during the reaction process, resulting in the reduction of its catalytic performance. Except for changing the catalyst calcination temperature, other operation steps and activity evaluation conditions of Comparative Example 2 and Comparative Example 3 are the same as those of Example 5, and their activity results are listed in Table 1.

[0072] Comparative Example 4

[0073] Preparation of Fe2(MoO4)3 for the gas-phase oxidation of ethanol to acetaldehyde (coprecipitation method)

[0074] Weigh 2.65 g of ammonium heptamolybdate and dissolve it in 65 mL of deionized water in a 250 mL three-necked flask. Then weigh 4.04 g of ferric nitrate and dissolve it in 65 mL of deionized water, pour it into a clean 100 mL constant-pressure dropping funnel, and slowly drip it into the metal salt solution at room temperature while stirring rapidly. After the dropping is completed, continue to stir in an oil bath at 100 °C for 2 h. After the reaction is completed, pour it into a Buchner funnel and filter it with deionized water until neutral, then drain it, place it in an oven and dry it at 80 °C for 12 h. Then grind it evenly and put it into a muffle furnace and calcine it at 500 °C for 5 h to obtain the final catalyst.

[0075] Comparative Example 5

[0076] Fe for the gas-phase oxidation of ethanol to acetaldehyde 1.98 Mn 0.01 Cu 0.01 (MoO4)3 preparation (coprecipitation method)

[0077] Weigh 2.65 g of ammonium heptamolybdate and dissolve it in 65 mL of deionized water in a 250 mL three-necked flask. Then weigh 4.00 g of ferric nitrate, 0.015 g of manganese nitrate, and 0.012 g of copper nitrate and dissolve them in 65 mL of deionized water, pour it into a clean 100 mL constant-pressure dropping funnel, and slowly drip it into the ammonium molybdate solution at room temperature while stirring rapidly. After the dropping is completed, continue to stir in an oil bath at 100 °C for 2 h. After the reaction is completed, pour it into a Buchner funnel and filter it with deionized water until neutral, then drain it, place it in an oven and dry it at 80 °C for 12 h. Then grind it evenly and put it into a muffle furnace and calcine it at 500 °C for 5 h to obtain the final catalyst.

[0078] Comparative Example 4 and Comparative Example 5 are iron-molybdenum catalysts obtained by different preparation methods (co-precipitation method). To compare the catalytic performance of the same catalyst prepared by different methods, except for changing the catalyst preparation method, the activity evaluation conditions of Comparative Example 4 and Comparative Example 5 are the same as those of Example 1, and the activity results are listed in Table 1. It can be seen that the performance of the catalyst prepared by the co-precipitation method is significantly lower than that of the catalyst prepared by the hydrothermal method in the examples of the present invention.

[0079] Table 1

[0080]

[0081] Figure 1 and Figure 2 are the XRD patterns and H2-TPR patterns of Example 1, Example 5 and Comparative Example 1 respectively. It can be seen from the figures that the doping of transition metals makes the crystallinity of the catalyst worse, but due to the small doping amount, the catalyst is still mainly in the structure of iron molybdate, and the reducibility of the catalyst is enhanced, which may be the reason for the increase in the activity of the catalyst after doping. Figure 3 is the XPS pattern of the catalyst of Example 5. Analysis shows that the valence state of iron element in the catalyst contains both +3 and +2 valence.

[0082] Figure 4 is the stability test result graph of the catalyst of Example 5. The catalyst Fe 1.98 Mn 0.01 Cu 0.01 (MoO4)3 of Example 5 was used for the stability test. The reaction conditions were: the liquid hourly space velocity of ethanol feed was 5 mL / g cat / h, the molar ratio of oxygen to ethanol was 3:1, the gas hourly space velocity was 50000 mL / g cat / h, and the reaction temperature was 225 °C. It can be seen that the catalyst of Example 5 could maintain its activity without deactivation for 115 h.

[0083] It can also be seen from Table 1 that when one or two of Mn, Co, Ni, and Cu are doped into iron molybdate by the hydrothermal method, the obtained doped catalyst has a significant improvement in catalytic performance compared with iron molybdate.

[0084] It is easy for those skilled in the art to understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. Application of a metal-doped iron molybdate catalyst in the gas-phase oxidative dehydrogenation of ethanol to prepare acetaldehyde, characterized in that, The chemical formula of the metal-doped iron molybdate catalyst is Fe x A y B z (MoO4)3, where A and B are each independently one of the metal elements Mn, Co, Ni, and Cu, 1.9 ≤ x < 2.0, 0 ≤ y ≤ 0.02, 0 ≤ z ≤ 0.02, and y and z are not both 0 at the same time, and x + y + z = 2; and when the metal element corresponding to A or B is Cu, neither y nor z is 0; the Fe in the catalyst contains both +3 and +2 valence states; The preparation method of the metal-doped iron molybdate catalyst comprises the following steps: (1) According to the atomic ratio in the chemical formula composition, an aqueous solution of ammonium molybdate is dropped into a mixed aqueous solution of an iron salt, a metal salt corresponding to metal element A, and a metal salt corresponding to metal element B, and stirring is carried out during the dropping to make them react to obtain a reaction precursor solution; in the mixed aqueous solution of the iron salt, the metal salt corresponding to metal element A, and the metal salt corresponding to metal element B, the total concentration of the solute is 0.2 to 1.0 mol / L; (2) The reaction precursor solution obtained in step (1) is subjected to a hydrothermal reaction, the reaction product is separated into solid and liquid, the separated solid phase is washed to neutrality, and after drying, a metal-doped iron molybdate catalyst is obtained; When the metal-doped iron molybdate catalyst is used for catalytic gas-phase oxidation dehydrogenation of ethanol to prepare acetaldehyde, the reaction temperature is 200-225 °C.

2. The application according to claim 1, characterized in that, The iron salt, the metal salt corresponding to metal element A, and the metal salt corresponding to metal element B in step (1) are each independently a nitrate, an acetate, or an acetylacetonate.

3. The application according to claim 1, characterized in that, In step (1), the stirring to make them react, the reaction time is 2 to 4 h.

4. The application according to claim 1, characterized in that, The temperature of the hydrothermal reaction described in step (2) is 140~180 o °C, and the reaction time is 8~20 h.

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