Atmosphere gradient calcination method for iron-molybdenum catalyst

Abstract

This invention discloses an atmosphere gradient calcination method for iron-molybdenum catalysts, belonging to the field of catalyst technology. The method employs a synergistic strategy of three-stage programmed temperature rise combined with segmented gradient control of the oxygen partial pressure in an air / nitrogen mixture. This precisely matches the differentiated requirements of catalyst phase composition, defect structure, and active site regulation on oxygen content and temperature change rate. Through the synergistic effect of "segmented temperature control + precise oxygen adjustment," the precipitation of free plate-like MoO3 is reduced, and the amorphous MoO3 on the surface of Fe2(MoO4)3 particles is thickened. x The layer not only inhibits the deep oxidation of formaldehyde to improve selectivity, but also strengthens the catalyst's crush resistance through the "rigid skeleton + flexible bonding" effect. At the same time, it achieves precise control of oxygen vacancy concentration and active site distribution, taking into account catalytic activity, selectivity and mechanical stability. Moreover, the process is simple, requires no complex equipment modification, and is suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention relates to the field of catalyst technology, and more particularly to an atmosphere gradient calcination method for iron-molybdenum catalysts. Background Technology

[0002] Formaldehyde, as an irreplaceable basic chemical raw material, occupies a core position in many high-value-added industries due to its high reactivity, including wood processing (urea-formaldehyde resin and phenolic resin synthesis), coatings, textile printing and dyeing (fixing agents and finishing agents), plastics and synthetic resins, and biopharmaceuticals (intermediate synthesis). Its production capacity and quality directly affect the development quality of downstream industrial chains. Currently, global industrial formaldehyde production uses methanol as a raw material and adopts a heterogeneous catalytic oxidation process. Among them, ferric molybdate (Fe2(MoO4)3)-based catalysts have become the mainstream industrial catalysts for this process due to their high catalytic activity and controllable cost, enabling formaldehyde yields to reach 93%~94%.

[0003] Industrial ferric molybdate catalysts are typically prepared using a co-precipitation method. This process involves key steps such as precipitation, aging, washing, drying, and calcination, with calcination being the core step determining the final structure and performance of the catalyst. During calcination, the catalyst precursor undergoes a series of complex physicochemical changes, including phase transformation (conversion from amorphous to crystalline), surface species reconstruction (phase evolution of molybdenum oxide), texture morphology regulation, and solid-phase reactions (formation of the active phase). These changes directly determine the catalyst's active site density, oxygen vacancy concentration, phase composition, and mechanical structural stability, thus decisively influencing the catalyst's catalytic activity, formaldehyde selectivity, and lifespan.

[0004] Currently, the calcination process for ferric molybdate catalysts in industry and existing technologies mostly employs a single method: directly heating to the final calcination temperature in an air atmosphere and then holding at that temperature. While this process is simple to operate, it suffers from a significant technical bottleneck: the abundant oxygen content in an air atmosphere readily promotes the growth and aggregation of free, plate-like crystalline molybdenum trioxide (MoO3), leading to the formation of amorphous molybdenum oxide (MoO3) within the catalyst. x The content of ) is relatively low; while existing research has confirmed that the amorphous MoO coated on the surface of Fe2(MoO4)3 is... x The catalyst layer is a key component for improving formaldehyde selectivity; insufficient content of this layer will exacerbate the deep oxidation reaction of formaldehyde and reduce the selectivity of the target product. At the same time, calcination in an air atmosphere tends to maintain the oxygen vacancy concentration of the catalyst at a low level (5%~10%), resulting in insufficient redox capacity, which restricts the methanol adsorption and dissociation efficiency and further affects the catalytic activity.

[0005] Furthermore, an excessively high proportion of crystalline MoO3 leads to predominantly angular contact between catalyst particles, making plastic deformation difficult during molding. Rigid crystalline particles are also prone to stress concentration under molding pressure, resulting in microcracks and significantly reducing the crushing strength of the molded catalyst. If the catalyst's mechanical strength is insufficient, it is susceptible to breakage and pulverization during the loading process and long-term operation of industrial reactors. This not only significantly increases reactor bed resistance but also directly causes rapid catalytic performance degradation, drastically shortening catalyst lifespan and increasing operating costs and downtime risks in industrial production.

[0006] To improve the performance of ferric molybdate catalysts, existing technologies have explored various approaches. For example, patents CN120618476A and CN111229242A attempt to enhance catalytic performance by controlling catalyst morphology and optimizing the number and distribution of oxygen vacancies in MoO3. However, these technologies suffer from drawbacks such as complex process control, high parameter sensitivity, and the need for additional specialized equipment or reagents, making them unsuitable for large-scale industrial production. Summary of the Invention

[0007] The purpose of this invention is to overcome the technical bottleneck of the limited performance optimization of iron-molybdenum catalysts by existing programmed temperature calcination technology. By precisely controlling the calcination process parameters, the phase composition, defect structure and active sites of the catalyst can be synergistically optimized, thereby improving its catalytic performance and mechanical strength to meet the needs of industrial production.

[0008] To achieve the above-mentioned objectives, the present invention employs the following technical solution: a gradient calcination method for iron-molybdenum catalysts, which utilizes segmented gradient control of the oxygen partial pressure in an air / nitrogen mixture to achieve a synergistic effect, precisely matching the catalyst's phase composition, defect structure, and active sites to optimize the differentiated requirements for oxygen content and temperature change rate. The calcination method employs a three-stage programmed temperature ramping mode, which specifically includes the following stages:

[0009] (1) The temperature is raised from room temperature to the first constant temperature, which is controlled at 190-280℃;

[0010] (2) The temperature is raised from the first constant temperature to the second constant temperature, and the second constant temperature is controlled at 280-350℃;

[0011] (3) The temperature is raised from the second constant temperature to the final calcination temperature, wherein the final calcination temperature is controlled at 350-500℃;

[0012] Furthermore, within the aforementioned temperature ranges, the oxygen partial pressure in the air / nitrogen mixed atmosphere is precisely controlled to maintain it within the range of 0–159 mmHg.

[0013] As a preferred technical solution, during the entire process of heating from room temperature to the second constant temperature, the oxygen partial pressure of the air / nitrogen mixed atmosphere is maintained at a constant value of 159 mmHg, thereby providing a stable oxygen environment for precursor decomposition and initial active phase formation.

[0014] As a further preferred technical solution, from the end of the second isothermal stage to the first half of the heating period to the final calcination temperature, the oxygen partial pressure of the mixed atmosphere is controlled at 40-80 mmHg, thereby suppressing the precipitation of free flaky MoO3 and thickening the amorphous MoO3 on the surface of Fe2(MoO4)3 particles. x layer.

[0015] As a further preferred technical solution, after heating to the final calcination temperature, from the latter half of that temperature range until the end of the calcination process, the oxygen partial pressure of the mixed atmosphere is controlled to be 0-20 mmHg, thereby controlling the oxygen vacancy concentration and active site distribution of the catalyst.

[0016] As a further preferred technical solution, from the end of the second isothermal stage to the first half of the temperature range when the temperature is raised to the final calcination temperature, the oxygen partial pressure is precisely controlled at 56 mmHg; after the temperature is raised to the final calcination temperature, from the second half of this temperature range until the end of the calcination process, the oxygen partial pressure is precisely controlled at 12 mmHg, thereby achieving the optimal balance of catalyst performance.

[0017] As a preferred technical solution, the volumetric flow rate of the air / nitrogen mixed atmosphere is controlled at 0.5 to 1.5 times the volume of the roasting furnace cavity per hour throughout the roasting process.

[0018] As a preferred technical solution, the heating rate from room temperature to the first isothermal temperature and from the first isothermal temperature to the second isothermal temperature are both controlled at 1 to 5 °C / min, thereby providing a suitable rate for the initial transformation of the precursor; the heating rate from the second isothermal temperature to the final calcination temperature is controlled at 0.5 to 2 °C / min, thereby slowing down the rate to adapt to the kinetic requirements of crystal transformation and defect structure regulation.

[0019] As a preferred technical solution, the holding time for both the first and second constant temperature stages is 0–120 min, and the holding time at the final calcination temperature is 120–360 min.

[0020] As a further preferred technical solution, the holding time for both the first and second isothermal stages is 10–60 min, and the holding time at the final calcination temperature is 240–360 min, thereby improving process efficiency while ensuring catalytic performance and balancing performance and production costs.

[0021] As a preferred technical solution, the specific operation process is as follows:

[0022] (1) Charging: The iron-molybdenum catalyst precursor prepared by co-precipitation and dried is evenly spread on the support device in the calcination furnace to ensure that the precursor is heated and in contact with the atmosphere.

[0023] (2) Atmosphere replacement: A pre-prepared air / nitrogen mixture is continuously introduced into the calcination furnace until the air in the furnace is completely replaced, thereby eliminating the interference of residual air on the catalyst structure;

[0024] (3) Program start: Adjust the flow rate of the mixed gas in the furnace to the set value, start the segmented roasting heating program, and control the heating rate, constant temperature time and oxygen partial pressure at each stage according to the parameters, so as to achieve the synergy of "segmented temperature control + precise oxygen adjustment";

[0025] (4) Oxygen partial pressure adjustment: When calcination reaches the preset stage, the oxygen partial pressure of the mixed atmosphere is precisely adjusted to the corresponding set value by adjusting the volume ratio of air and nitrogen, thereby controlling the amorphous MoO. x The ratio of crystalline MoO3 to oxygen vacancy concentration;

[0026] (5) Cooling down and shutting down: After the calcination process is completed, turn off the heating system and wait for the temperature inside the furnace to drop to room temperature naturally before stopping the supply of mixed gas to avoid sudden temperature changes that could damage the catalyst structure.

[0027] (6) Unloading: Remove the calcined iron-molybdenum catalyst, seal it and store it for later use;

[0028] It should be noted that the phased oxygen partial pressure regulation of the present invention runs through the entire process from heating to cooling to room temperature, and the air / nitrogen mixture is maintained in a purging state throughout the process.

[0029] During the staged calcination process, the iron-molybdenum catalyst precursor undergoes characteristic phase transformations at different temperature zones: in the 190–280℃ temperature zone, the precursor decomposes to form metastable molybdenum oxide (MoO). x In the temperature range of 280–350℃, the catalysts Fe2(MoO4)3 and MoO3 complete the transformation from an amorphous state to a crystalline state; in the temperature range of 350–500℃, the crystallinity of Fe2(MoO4)3 and MoO3 continues to increase, and the crystal structure is further improved.

[0030] Differentiated heating rate design can effectively avoid problems such as abnormal growth of Fe2(MoO4)3 or MoO3 grains, reduced density of active sites, and lamellar precipitation of MoO3 caused by excessive heating rate, while avoiding the decrease in production capacity caused by excessively low heating rate; optimization of holding time can balance the contradiction between complete conversion of precursor, sufficient development of crystal phase and excessive growth of active component grains. Heating rate of 0.5 to 5 °C / min and total calcination time of 2 to 10 h provide a reasonable parameter range for balancing efficiency and performance in industrial production.

[0031] The synergistic effect of the above-mentioned preferred technical solutions can significantly improve the methanol conversion rate, formaldehyde selectivity and formaldehyde yield in the methanol oxidation to formaldehyde reaction of iron-molybdenum catalyst, while the crushing strength of the catalyst meets the requirements of industrial loading and long-term operation.

[0032] The calcination method provided by this invention is based on the differences in oxygen content thresholds required for the formation of different phases of the catalyst, the oxygen partial pressure sensitivity of oxygen vacancy concentration regulation, and the requirement for calcination timing parameters due to the uniform distribution of active sites. It constructs a three-stage incremental isothermal system, and designs a gradient oxygen partial pressure atmosphere, a differentiated heating rate, and a precise holding time. By gradually heating to the final calcination temperature, the synergistic optimization of the catalyst structure and performance is achieved.

[0033] Compared with the prior art, the present invention has the following technical advantages:

[0034] (1) Precise control of phase composition: Amorphous MoO2 is achieved by controlling the gradient oxygen partial pressure. x The relative ratio of Fe2(MoO4)3 to crystalline MoO3 is controllable, effectively suppressing the formation of free flaky MoO3, while simultaneously thickening the amorphous MoO3 on the surface of Fe2(MoO4)3 particles. x The coating layer significantly inhibits the deep oxidation reaction of formaldehyde and improves the selectivity of the target product, formaldehyde.

[0035] (2) Mechanical strength enhancement: amorphous MoO x It has good plastic deformation ability and can fill the "rigid support skeleton" formed by crystalline MoO3 particles. Through the synergistic effect of "rigid skeleton + flexible bonding", it increases the contact area and bonding force between particles, significantly improves the crush resistance of the molded catalyst, and is suitable for industrial reactor filling and long-term operation.

[0036] (3) Synergistic optimization of defect structure and active site: Through the synergistic regulation of oxygen partial pressure, temperature, time and heating rate, the concentration of oxygen vacancy and the distribution of active sites in the catalyst are precisely controlled. This avoids the low activity of methanol adsorption and dissociation caused by insufficient oxygen vacancy, and prevents the formaldehyde desorption lag and deep oxidation caused by excessive oxygen vacancy. At the same time, it ensures the uniform distribution of active sites on the catalyst surface and in the bulk phase, thereby improving the overall catalytic performance of the catalyst.

[0037] (4) Strong industrial adaptability: The core of this invention is to improve performance through segmented oxygen partial pressure regulation. The process parameters are easy to adjust and the control precision is high. No new complex equipment is required. It can be directly adapted to existing catalyst roasting production lines and has good industrial promotion value. Detailed Implementation

[0038] To explain the technical content, objectives, and effects of the present invention in detail, the following specific embodiments are provided to further illustrate the content of the present invention. However, the content of the present invention is far more than the following examples.

[0039] The method for preparing the catalyst precursor before catalyst calcination in this invention is a prior art known in the field.

[0040] In the following examples, the catalyst precursor used was prepared according to the following method: In a molybdenum salt mother liquor reactor, a 3% (w / w) ammonium molybdate solution was prepared, the dissolution temperature was controlled at 50°C, and the pH of the solution was adjusted to 2 with nitric acid; In an iron salt mother liquor reactor, a 10% (w / w) ferric nitrate solution was prepared, the dissolution temperature was controlled at 80°C, the theoretical molar ratio of molybdenum to iron was 2.0, and the ferric nitrate solution was pumped into the molybdenum salt mother liquor reactor for precipitation. The reaction temperature was controlled at 50°C, the reaction pressure at atmospheric pressure, and the stirring speed at 400 rpm. The precipitation was completed after 1 hour of reaction; The precipitate was aged for 3 hours at an aging temperature of 40°C and a stirring speed of 200 rpm; After aging, the precipitate was washed 3 times and separated by filtration; The solid was placed in a drying oven and dried at 120°C for 6 hours to obtain the "catalyst precursor".

[0041] Example 1

[0042] A gradient calcination method for an iron-molybdenum catalyst includes the following steps:

[0043] The catalyst precursor was ground to a mesh size of 60 or less, placed in a quartz boat, and then placed in a tube furnace with a diameter of 80 mm and a length of 1000 mm. Dry air was used to purge the air for 2 minutes to replace the existing air. The air flow rate was 80 mL / min (oxygen partial pressure 159 mmHg). The purge gas flow rate was maintained constant, and the heating program was started at a rate of 2 °C / min, raising the temperature from room temperature to 280 °C and holding it for 30 minutes. The heating rate was then maintained constant, and the temperature was further raised to 350 °C and held for 30 minutes. The heating rate was then adjusted to 1 °C / min, raising the temperature to 400 °C and holding it for 240 minutes. After calcination, the temperature was allowed to cool naturally to room temperature, and the air was turned off to obtain catalyst FM-1.

[0044] Example 2

[0045] A gradient calcination method for an iron-molybdenum catalyst includes the following steps:

[0046] The catalyst precursor was ground to a mesh size of 60 or less, placed in a quartz boat, and then placed in a tube furnace with a diameter of 80 mm and a length of 1000 mm. Dry air was used to purge the air for 2 minutes to replace the existing air. The air flow rate was 80 mL / min (oxygen partial pressure 159 mmHg). The purge gas flow rate was maintained constant, and the heating program was started at a rate of 2 °C / min, raising the temperature from room temperature to 280 °C and holding it for 30 minutes. The heating rate was then maintained constant, and the temperature was further raised to 350 °C and held for 30 minutes. The heating rate was adjusted to 1 °C / min, the nitrogen flow rate was adjusted to 52 mL / min, the air flow rate to 28 mL / min, and the oxygen partial pressure of the mixed gas to 56 mmHg. The temperature was raised to 400 °C and held for 240 minutes. After calcination, the temperature was allowed to cool naturally to room temperature, and the nitrogen and air were turned off to obtain catalyst FM-2.

[0047] Example 3

[0048] A gradient calcination method for an iron-molybdenum catalyst includes the following steps:

[0049] The catalyst precursor was ground to a mesh size of 60 or less, placed in a quartz boat, and then placed in a tube furnace with a diameter of 80 mm and a length of 1000 mm. Dry air was used to purge the air for 2 minutes to replace the existing air. The air flow rate was 80 mL / min (oxygen partial pressure 159 mmHg). The purge gas flow rate was maintained constant, and the heating program was started at a rate of 2 °C / min, raising the temperature from room temperature to 280 °C and holding it for 30 minutes. The heating rate was then maintained constant, and the temperature was further raised to 350 °C and held for 30 minutes. The heating rate was adjusted to 1 °C / min, and the nitrogen and air flow rates were adjusted to 40 mL / min. The oxygen partial pressure of the mixed gas was 80 mmHg. The temperature was raised to 400 °C and held for 240 minutes. After calcination, the temperature was allowed to cool naturally to room temperature, and the nitrogen and air were turned off to obtain catalyst FM-3.

[0050] Example 4

[0051] A gradient calcination method for an iron-molybdenum catalyst includes the following steps:

[0052] The catalyst precursor was ground to a mesh size of 60 or less, placed in a quartz boat, and then placed in a tube furnace with a diameter of 80 mm and a length of 1000 mm. Dry air was used to purge the air for 2 minutes to replace the existing air. The air flow rate was 80 mL / min (oxygen partial pressure 159 mmHg). The purge gas flow rate was maintained constant, and the heating program was started at a rate of 2 °C / min, raising the temperature from room temperature to 280 °C and holding it for 30 minutes. The heating rate was then maintained constant, and the temperature was further raised to 350 °C and held for 30 minutes. The heating rate was adjusted to 1 °C / min, the nitrogen flow rate was adjusted to 60 mL / min, the air flow rate to 20 mL / min, and the oxygen partial pressure of the mixed gas to 40 mmHg. The temperature was raised to 400 °C and held for 240 minutes. After calcination, the temperature was allowed to cool naturally to room temperature, and the nitrogen and air were turned off to obtain catalyst FM-4.

[0053] Example 5

[0054] A gradient calcination method for an iron-molybdenum catalyst includes the following steps:

[0055] The catalyst precursor is ground to a mesh size of 60 or less, loaded into a quartz boat, and placed in a tube furnace with a diameter of 80 mm and a length of 1000 mm. The original air was replaced by dry air purging for 2 minutes at a flow rate of 80 mL / min (oxygen partial pressure of 159 mmHg). The purging gas flow rate was kept constant, and the heating program was started at a rate of 2 °C / min, from room temperature to 280 °C. The temperature was held for 30 minutes, and the heating rate was kept constant. The temperature was then increased to 350 °C and held for 30 minutes. The heating rate was adjusted to 1 °C / min, the nitrogen flow rate was adjusted to 52 mL / min, the air flow rate was adjusted to 28 mL / min, and the oxygen partial pressure of the mixed gas was 56 mmHg. The temperature was increased to 400 °C and held for 240 minutes. When the temperature was held for 120 minutes, the nitrogen flow rate was adjusted to 74 mL / min, the air flow rate was adjusted to 6 mL / min, and the oxygen partial pressure of the mixed gas was 12 mmHg. After calcination, the temperature was allowed to cool naturally to room temperature, and the nitrogen and air were turned off to obtain catalyst FM-5.

[0056] Example 6

[0057] A gradient calcination method for an iron-molybdenum catalyst includes the following steps:

[0058] The catalyst precursor is ground to a mesh size of 60 or less, loaded into a quartz boat, and placed in a tube furnace with a diameter of 80 mm and a length of 1000 mm. The original air was replaced by dry air purging for 2 minutes at a flow rate of 80 mL / min (oxygen partial pressure of 159 mmHg). The purging gas flow rate was kept constant, and the heating program was started at a rate of 2 °C / min, from room temperature to 280 °C. The temperature was held for 30 minutes, and the heating rate was kept constant. The temperature was then increased to 350 °C and held for 30 minutes. The heating rate was adjusted to 1 °C / min, the nitrogen flow rate was adjusted to 52 mL / min, the air flow rate was adjusted to 28 mL / min, and the oxygen partial pressure of the mixed gas was 56 mmHg. The temperature was increased to 400 °C and held for 240 minutes. When the temperature was held for 120 minutes, the nitrogen flow rate was adjusted to 70 mL / min, the air flow rate was adjusted to 10 mL / min, and the oxygen partial pressure of the mixed gas was 20 mmHg. After calcination, the temperature was allowed to cool naturally to room temperature, and the nitrogen and air were turned off to obtain catalyst FM-6.

[0059] Example 7

[0060] A gradient calcination method for an iron-molybdenum catalyst includes the following steps:

[0061] The catalyst precursor was ground to a mesh size of 60 or less, placed in a quartz boat, and then placed in a tube furnace with a diameter of 80 mm and a length of 1000 mm. Dry air was used to purge the air for 2 minutes to replace the existing air at a flow rate of 80 mL / min (oxygen partial pressure of 159 mmHg). The purge gas flow rate was maintained constant, and the heating program was started at a rate of 2 °C / min, raising the temperature from room temperature to 280 °C and holding it for 30 minutes. The heating rate was then maintained constant, and the temperature was further raised to 350 °C and held for 30 minutes. The heating rate was adjusted to 1 °C / min, and the nitrogen flow rate was adjusted to 52 mL / min. The air flow rate was adjusted to 28 mL / min, and the oxygen partial pressure of the mixed gas was 56 mmHg. The temperature was raised to 400 °C and held for 240 minutes. After holding for 120 minutes, the nitrogen flow rate was adjusted to 80 mL / min (oxygen partial pressure of 0 mmHg), and the air was turned off. After calcination, the temperature was allowed to cool naturally to room temperature, and the nitrogen was turned off to obtain catalyst FM-7.

[0062] Catalyst performance test examples

[0063] Catalysts FM-1, FM-2, FM-3, FM-4, FM-5, FM-6, and FM-7 obtained in Examples 1, 2, 3, 4, 5, 6, and 7 were compressed into tablets to obtain annular particulate catalysts with dimensions of 5.00 × 2.40 × 3.80 mm (outer diameter / inner diameter / height). Under normal pressure and a gas space velocity of 9000 h⁻¹, [the catalysts were tested]. -1The catalytic performance of the above catalysts was evaluated and compared using a fixed-bed reactor with a pipe diameter of 20 mm under the conditions of 7.0 vol% methanol and 9.0 vol% oxygen in the feed. The reaction temperature was 320℃. The product components were analyzed by online chromatography, and the methanol conversion rate, formaldehyde selectivity and formaldehyde yield were calculated.

[0064] The methanol conversion rate (%), formaldehyde selectivity (%), and formaldehyde yield (%) mentioned in this invention are calculated according to the following formula:

[0065]

[0066]

[0067]

[0068] In the above formula, and This indicates the amount of methanol entering and exiting the reactor containing the catalyst. This indicates the amount of formaldehyde produced in the reaction.

[0069] The radial crush resistance of the molded catalyst was tested in accordance with ASTM D6175-24, published by the American Society for Testing and Materials.

[0070] Amorphous MoO₂ for different catalysts was obtained through refined calculations using the XRD Rietveld method. x Content; oxygen vacancy concentration of different catalysts was semi-quantitatively analyzed by XPS.

[0071] The results are shown in Table 1. As can be seen from Table 1, compared with the programmed heating of Example 1, the other examples controlled the oxygen partial pressure at different temperature ranges, which significantly improved the overall performance of the catalyst.

[0072] Table 1 Comparison of performance of different catalysts

[0073]

[0074] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A gradient atmosphere calcination method for an iron-molybdenum catalyst, characterized in that, The roasting is carried out using a three-stage programmed temperature rise mode, specifically... Includes the following stages: (1) The temperature is raised from room temperature to the first constant temperature, which is controlled at 190-280℃; (2) The temperature is raised from the first constant temperature to the second constant temperature, and the second constant temperature is controlled at 280-350℃; (3) The temperature is raised from the second constant temperature to the final calcination temperature, wherein the final calcination temperature is controlled at 350-500℃; Furthermore, within the aforementioned temperature ranges, the oxygen partial pressure in the air / nitrogen mixed atmosphere is precisely controlled to maintain it within the range of 0–159 mmHg.

2. The method according to claim 1, characterized in that, Throughout the entire process of heating from room temperature to the second constant temperature, the oxygen partial pressure of the air / nitrogen mixed atmosphere remains constant at 159 mmHg.

3. The method according to claim 2, characterized in that, From the end of the second isothermal stage until the first half of the heating period to the final calcination temperature, the oxygen partial pressure of the mixed atmosphere is controlled to be 40-80 mmHg.

4. The method according to claim 3, characterized in that, After the temperature is raised to the final calcination temperature, from the latter half of that temperature range until the end of the calcination process, the oxygen partial pressure of the mixed atmosphere is controlled to be 0-20 mmHg.

5. The method according to claim 4, characterized in that, From the end of the second isothermal stage to the first half of the period when the temperature rises to the final calcination temperature, the oxygen partial pressure is precisely controlled at 56 mmHg; after the temperature rises to the final calcination temperature, from the second half of this temperature range until the end of the calcination process, the oxygen partial pressure is precisely controlled at 12 mmHg.

6. The method according to claim 1, characterized in that, Throughout the roasting process, the volumetric flow rate of the air / nitrogen mixed atmosphere is controlled at 0.5 to 1.5 times the roasting furnace cavity volume per hour.

7. The method according to claim 1, characterized in that, The heating rate from room temperature to the first isothermal temperature and from the first isothermal temperature to the second isothermal temperature is controlled at 1–5 °C / min; the heating rate from the second isothermal temperature to the final calcination temperature is controlled at 0.5–2 °C / min.

8. The method according to claim 1, characterized in that, The holding time for both the first and second isothermal stages is 0–120 min, and the holding time at the final calcination temperature is 120–360 min.

9. The method according to claim 8, characterized in that, The holding time for both the first and second isothermal stages is 10–60 min, and the holding time at the final calcination temperature is 240–360 min.

10. The atmospheric gradient calcination method for the iron-molybdenum catalyst according to any one of claims 1 to 9, characterized in that, The specific method is as follows: (1) Charging: The iron-molybdenum catalyst precursor prepared by co-precipitation and dried is evenly spread on the support device in the calcination furnace; (2) Atmosphere replacement: A pre-prepared air / nitrogen mixture is continuously introduced into the roasting furnace until the air in the furnace is completely replaced; (3) Program start-up: Adjust the flow rate of the mixed gas in the furnace to the set value, start the segmented roasting and heating program, and control the heating rate, constant temperature time and oxygen partial pressure at each stage according to the parameters mentioned above; (4) Oxygen partial pressure adjustment: When roasting to the preset stage, the oxygen partial pressure of the mixed atmosphere is precisely adjusted to the corresponding set value by adjusting the volume ratio of air and nitrogen. (5) Cooling down and shutting down: After the calcination process is completed, turn off the heating system and wait for the temperature inside the furnace to drop naturally to room temperature. Stop the supply of mixed gas to avoid sudden temperature changes that could damage the catalyst structure. (6) Unloading: Remove the calcined iron-molybdenum catalyst, seal it and store it for later use; Staged oxygen partial pressure regulation is implemented throughout the entire process, from heating up to cooling down to room temperature, while maintaining an air / nitrogen mixture purging state throughout.

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