A molten iron catalyst, its pretreatment method and application

Abstract

This disclosure relates to a molten iron catalyst, its pretreatment method, and its application. In the molten iron catalyst, the α-Fe grain size is less than 30 nm; the pore volume is 0.10-0.24 mL / g; and the specific surface area is 15-40 m². 2 The molten iron catalyst has an average pore size of 20-50 nm per g. The total content of silicon and aluminum in the molten iron catalyst is 0.5-1.5% by weight relative to the total weight of the catalyst; the content of potassium is 3-6% by weight relative to the weight of iron. The molten iron catalyst disclosed herein exhibits high catalytic performance when used in Fischer-Tropsch synthesis reactions.

Description

Technical Field

[0001] This disclosure relates to the field of Fischer-Tropsch synthesis, specifically to a molten iron catalyst, its pretreatment method, and its application. Background Technology

[0002] Molten iron catalysts are typically prepared using a high-temperature melting method. The main catalyst component is Fe3O4, with a total additive content of approximately 3%–6%. Commercial application in ammonia synthesis is mature, and domestic and international scholars have conducted systematic studies on its reduction conditions, mechanisms, and kinetics in ammonia synthesis. However, its application in Fischer-Tropsch synthesis is less reported. Despite more than half a century of improvements, molten iron catalysts still face numerous challenges, including high deactivation rates caused by sintering, carbon deposition, and iron phase changes, as well as relatively high methane selectivity.

[0003] Alkali metals enhance the activity and product selectivity of Fischer-Tropsch synthesis catalysts, and potassium (K) is one of the most commonly used alkali metals in Fischer-Tropsch catalysts. Freshly prepared catalysts have high K concentrations on their surface, but K migrates or aggregates during the reaction, resulting in an overall K concentration concentration of [missing information]. + Severe loss significantly reduces catalyst activity and stability, severely impacting catalyst lifespan. Summary of the Invention

[0004] The purpose of this disclosure is to provide a molten iron catalyst, its pretreatment method, and its application.

[0005] To achieve the above objectives, the first aspect of this disclosure provides a molten iron catalyst in which the α-Fe grain size is less than 30 nm, the pore volume is 0.10-0.24 mL / g, and the specific surface area is 15-40 m². 2 / g, with an average pore size of 20-50nm; In the molten iron catalyst, the total content of silicon and aluminum is 0.5-1.5% by weight relative to the total weight of the molten iron catalyst; and the content of potassium is 3-6% by weight relative to the weight of iron.

[0006] A second aspect of this disclosure provides a method for pretreating molten iron catalyst, the method comprising: (1) The molten iron catalyst to be treated is subjected to pre-reduction and passivation treatment to obtain the first product; (2) The first product is contacted with an impregnation solution containing a potassium source, an optional silicon source and an optional aluminum source for impregnation treatment to obtain the second product; (3) The second product is roasted to obtain the third product; (4) The third product is subjected to reduction and activation treatment; The pre-reduction process includes a first pre-reduction heating stage, a second pre-reduction heating stage, and a third pre-reduction heating stage performed sequentially. The relationship between the heating rate K1 of the first pre-reduction heating section, the heating rate K2 of the second pre-reduction heating section, and the heating rate K3 of the third pre-reduction heating section is K1 > K2 > K3.

[0007] Optionally, in step (1), 2.0℃ / min≤K1≤8.0℃ / min, 0.5℃ / min≤K2<2.0℃ / min, and 0.1℃ / min≤K3<0.5℃ / min; The atmosphere in the first pre-reduction heating section includes a mixture of hydrogen and nitrogen, wherein the hydrogen content is 10-50% by volume. The atmosphere in the second pre-reduction heating section includes a mixture of hydrogen and nitrogen, wherein the hydrogen content is 30-70% by volume. Preferably, the third pre-reduction heating section includes an alternating first reduction section and a first purging section; Each first reduction phase lasts 5-30 minutes, and the atmosphere consists of a mixture of hydrogen and nitrogen, with hydrogen comprising 60-80% by volume. The duration of each first purging phase is 1-10 minutes, and the atmosphere includes nitrogen gas with a flow rate of 50-300 mL / min. -1 ·g -1 ; Preferably, after the first pre-reduction heating stage, a first pre-reduction isothermal stage is performed, followed by a second pre-reduction heating stage; the temperature of the first pre-reduction isothermal stage is 330-350℃, the time is 5-20 min, the atmosphere includes nitrogen, and the flow rate is 50-200 mL·min. -1 ·g -1 ; Preferably, after the second pre-reduction heating stage, a second pre-reduction isothermal stage is performed, followed by a third pre-reduction heating stage; the temperature of the second pre-reduction isothermal stage is 390-410℃, the time is 20-60 min, the atmosphere includes nitrogen, and the flow rate is 50-300 mL·min. -1 ·g -1 ; Preferably, a third pre-reduction isothermal stage is performed after the third pre-reduction heating stage; the temperature of the third pre-reduction isothermal stage is 400-425℃, the time is 40-200 min, the atmosphere includes a nitrogen atmosphere, and the flow rate is 50-300 mL·min. -1 ·g -1 .

[0008] Optionally, in step (1), the passivation treatment conditions include: a time of 10-120 min, a temperature of 20-80 °C, and a passivation atmosphere consisting of a mixture of oxygen and nitrogen, wherein the oxygen content is 0.5-2.0 by volume.

[0009] Optionally, in step (2), the impregnation treatment is carried out under vacuum ultrasonic conditions, wherein the vacuum ultrasonic treatment is performed for 0.5-10 minutes every 15 minutes; the total impregnation treatment time is 2-8 hours, the temperature is 200-260℃, and the pressure is 0-4MPa. Optionally, the potassium source includes one or more of K2CO3, KNO3, K2SO4 and KOH; The silicon source includes one or more of silica sol, tetraethyl orthosilicate, water glass, and silica. The aluminum source includes one or more of Al(NO3)3, aluminum isopropoxide, NaAlO2 and Al(OH)3; Optionally, the weight ratio of the first product to the potassium source (calculated as potassium) is 1:(0.03-0.10), the weight ratio of the first product to the silicon source (calculated as silicon) is 1:(0.01-0.10), and the weight ratio of the first product to the aluminum source (calculated as aluminum) is 1:(0.005-0.10).

[0010] Optionally, in step (3), the conditions for the calcination treatment include: a heating rate of 4-10℃ / min, a temperature of 450-900℃, and a total time of 2-8h.

[0011] Optionally, in step (4), the reduction activation process includes a first reduction heating section, a second reduction heating section, and a reduction isothermal section performed sequentially; The heating rate K4 of the first reduction heating section is greater than the heating rate K5 of the second reduction heating section, where 1.0℃ / min ≤ K4 ≤ 3.0℃ / min and 0.2℃ / min ≤ K5 < 1.0℃ / min; The atmospheres in the first and second reduction heating sections each comprise a mixture of hydrogen and nitrogen, wherein the hydrogen content is 60-80% by volume. The space velocity in the first reduction heating stage is 1000-2000 h⁻¹ -1 The target temperature of the first reduction heating section is 340-360℃; The space velocity in the second reduction heating stage is 8000-10000 h⁻¹ -1 ; The temperature of the reduction isothermal section is 410-430℃, the total time is 2-10 hours, and the space velocity is 8000-10000 h⁻¹. -1 ; Preferably, the reduction isothermal section includes an alternating second reduction section and a second purging section; Each second reduction stage lasts 10-60 minutes, and the atmosphere consists of a mixture of hydrogen and nitrogen, with hydrogen comprising 60-80% by volume. Each second purging phase lasts 1-10 minutes, with an atmosphere including nitrogen and a flow rate of 100-800 mL / min. -1 ·g -1 .

[0012] Optionally, the method for preparing the molten iron catalyst to be treated includes a melting method.

[0013] The third aspect of this disclosure provides a molten iron catalyst obtained by processing using the method described in the second aspect of this disclosure.

[0014] The fourth aspect of this disclosure provides a method for Fischer-Tropsch synthesis, comprising contacting syngas with a catalyst to carry out a Fischer-Tropsch synthesis reaction; said catalyst includes the molten iron catalyst described in the first or third aspect of this disclosure.

[0015] Through the above technical solution, the molten iron catalyst disclosed herein possesses a smaller α-Fe grain size, a larger pore volume, specific surface area, and average pore size, while also having a higher potassium content, which enhances the catalyst's catalytic activity. The method disclosed herein involves pre-reducing the molten iron catalyst before impregnation. The pre-reduction treatment generates more channels inside and on the surface of the molten iron catalyst, followed by impregnation. This increases the contact between the impregnation solution and the catalyst, increases the potassium content within the catalyst, and reduces potassium loss during the reaction. During the reaction, potassium slowly migrates from the catalyst's interior to the surface, maintaining the catalyst's surface catalytic activity and stability. This effectively suppresses methane production, shifts the product towards heavier hydrocarbons, and improves olefin selectivity. When the impregnation solution contains a silicon source and / or an aluminum source, impregnating the catalyst with more channels further increases the silicon and aluminum content within the catalyst. SiO2 and Al2O3 can increase the catalyst's specific surface area, limit the α-Fe grain size, reduce potassium agglomeration and deactivation, and improve the catalyst's catalytic performance. After impregnation, calcination and reduction treatments are carried out in sequence to further optimize the grain size (especially α-Fe grain size), specific surface area, pore volume, pore size and distribution of the molten iron catalyst. This solves the problems of easy sintering, carbon deposition and high deactivation rate caused by iron phase change in the molten iron catalyst, increases the catalyst life and stability, and improves the ability to resist carbon deposition and the Fischer-Tropsch synthesis catalytic activity.

[0016] Other features and advantages of this disclosure will be described in detail in the following detailed description section. Detailed Implementation

[0017] The following provides a detailed description of specific embodiments of this disclosure. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit this disclosure.

[0018] The first aspect of this disclosure provides a molten iron catalyst in which the α-Fe grain size is less than 30 nm, the pore volume is 0.10-0.24 mL / g, and the specific surface area is 15-40 m². 2 / g, with an average pore size of 20-50nm; In the molten iron catalyst, the total content of silicon and aluminum is 0.5-1.5% by weight relative to the total weight of the molten iron catalyst; and the content of potassium is 3-6% by weight relative to the weight of iron.

[0019] The molten iron catalyst disclosed herein has a small α-Fe grain size, a large pore volume, a large specific surface area and an average pore size, and a high potassium content, which can improve the catalytic activity of the catalyst.

[0020] According to one embodiment of this disclosure, the grain size of α-Fe can be, for example, 20-30 nm.

[0021] According to a preferred embodiment of this disclosure, the pore volume is 0.13-0.24 mL / g, and the specific surface area is 20-35 m². 2 / g, with an average pore size of 25-35nm.

[0022] According to a preferred embodiment of the present disclosure, in the molten iron catalyst, the content of potassium is 3.5-5.5% by weight relative to the content of iron; and the total content of silicon and aluminum is 0.8-1.2% by weight.

[0023] A second aspect of this disclosure provides a method for pretreating molten iron catalyst, the method comprising: (1) The molten iron catalyst to be treated is subjected to pre-reduction and passivation treatment to obtain the first product; (2) The first product is contacted with an impregnation solution containing a potassium source, an optional silicon source and an optional aluminum source for impregnation treatment to obtain the second product; (3) The second product is roasted to obtain the third product; (4) The third product is subjected to reduction and activation treatment; The pre-reduction process includes a first pre-reduction heating stage, a second pre-reduction heating stage, and a third pre-reduction heating stage performed sequentially. The relationship between the heating rate K1 of the first pre-reduction heating section, the heating rate K2 of the second pre-reduction heating section, and the heating rate K3 of the third pre-reduction heating section is K1 > K2 > K3.

[0024] To address the problems of easy sintering, carbon deposition, and high deactivation rate caused by iron phase changes in existing molten iron Fischer-Tropsch synthesis catalysts, the inventors of this disclosure have developed a pretreatment method for molten iron catalysts through inventive work. The method involves sequentially pre-reducing, passivating, impregnating, calcining, and activating the prepared catalyst, and the heating method for the pre-reduction treatment is specified, so that the treated catalyst has higher lifespan, stability, resistance to carbon deposition, and Fischer-Tropsch synthesis catalytic activity. The method disclosed herein first pre-reduces the catalyst to achieve a reduction degree of 20-60% (reduction degree = total oxygen loss after pre-reduction / theoretical oxygen content of the catalyst × 100%, where the oxygen content is expressed as Fe3O4 content). Simultaneously, it expands the catalyst pores, increasing the specific surface area and pore volume. After pre-reduction treatment, the potassium source, optionally a silicon source, and optionally an aluminum source in the impregnation solution can enter the catalyst pores to a greater extent, increasing the K content inside the catalyst and reducing K loss during the reaction. During the reaction, K inside the catalyst slowly migrates to the surface, maintaining the catalytic activity and stability of the catalyst surface. This effectively inhibits methane production, shifts the product towards heavier hydrocarbons, and improves olefin selectivity. Furthermore, it promotes a more uniform and reasonable K distribution, avoiding deterioration in catalyst stability due to K migration and loss during the reaction. It can also increase the content of silicon and aluminum elements inside the catalyst. SiO2 and Al2O3 can increase the specific surface area of ​​the catalyst, limit the grain size of α-Fe, reduce the agglomeration and deactivation of K, and further improve the catalytic performance of the catalyst.

[0025] According to one embodiment of this disclosure, in step (1), 2.0℃ / min ≤ K1 ≤ 8.0℃ / min, preferably, 2.0℃ / min ≤ K1 ≤ 4.0℃ / min, more preferably, 2.0℃ / min ≤ K1 ≤ 3.5℃ / min, where K1 includes, but is not limited to, 2.0℃ / min, 2.5℃ / min, 3.5℃ / min, 4.0℃ / min, 5.0℃ / min, 6.0℃ / min, 7.5℃ / min, 8.0℃ / min, or any combination thereof; 0.5℃ / min ≤ K2 < 2.0℃ / min, preferably, 0.5℃ / min ≤ K2 ≤ 1.0℃ / min, more preferably, 0.5℃ / min ≤ K2 ≤ 0.8℃ / min, where K2 includes, but is not limited to, 0.5℃ / min, 0.8℃ / min, 1.0℃ / min. The range of n, 1.5℃ / min, 1.8℃ / min, 2.0℃ / min, or any combination thereof; 0.1℃ / min ≤ K3 < 0.5℃ / min, preferably 0.1℃ / min ≤ K3 ≤ 0.2℃ / min, including but not limited to 0.1℃ / min, 0.2℃ / min, 0.3℃ / min, 0.4℃ / min, 0.5℃ / min, or any combination thereof; the atmosphere of the first pre-reduction heating section includes a mixed atmosphere of hydrogen and nitrogen, wherein the hydrogen content is 10-50% by volume; the atmosphere of the second pre-reduction heating section includes a mixed atmosphere of hydrogen and nitrogen, wherein the hydrogen content is 30-70% by volume; the above heating method combined with the reducing atmosphere can avoid the problem of uneven heating inside and outside of the catalyst particles and uneven pore size and distribution caused by excessively rapid heating.

[0026] According to one embodiment of this disclosure, the third pre-reduction heating stage includes alternating first reduction stages and first purging stages. This disclosure does not specifically limit the number of first reduction stages and first purging stages. For example, the third pre-reduction heating stage may include a first first reduction stage and a first first purging stage performed sequentially; or, the third pre-reduction heating stage may include a first first reduction stage, a first first purging stage, a second first reduction stage, and a second first purging stage performed sequentially. The duration of each first reduction stage is 5-30 minutes, including but not limited to 5 minutes, 6 minutes, 8 minutes, 10 minutes, 15 minutes, 20 minutes, 22 minutes, 25 minutes, 28 minutes, 30 minutes, or any combination thereof. The atmosphere includes a mixed atmosphere of hydrogen and nitrogen, wherein the hydrogen content is 60-80% by volume. The duration of each first purging stage is 1-10 minutes, including but not limited to 1 minute, 2 minutes, 3 minutes, 4 minutes, 5 minutes, 6 minutes, 7 minutes, 8 minutes, 9 minutes, 10 ... min, or a range of any two thereof; atmosphere including nitrogen atmosphere, flow rate of 50-300 mL / min. -1 ·g -1 , including but not limited to 50 mL·min -1 ·g -1 60 mL·min -1 ·g -1 80 mL·min -1 ·g -1 100 mL·min -1 ·g -1 150 mL·min -1 ·g -1 180 mL·min -1 ·g -1 200 mL·min -1 ·g -1 250 mL·min -1 ·g -1 300 mL·min -1 ·g -1 Or a range consisting of any two of them. The time and atmosphere of each first reduction stage can be the same or different, preferably the same; the time and atmosphere of each first purging stage can be the same or different, preferably the same. Nitrogen purging can remove water from the surface and pores of the catalyst, avoiding repeated reduction and oxidation of the catalyst caused by water retention, reducing the α-Fe grain size, and improving catalyst performance.

[0027] According to one embodiment of this disclosure, after the first pre-reduction heating stage, a first pre-reduction isothermal stage is performed, followed by a second pre-reduction heating stage; the temperature of the first pre-reduction isothermal stage is 330-350℃, and the time is 5-20 min, including but not limited to 5 min, 6 min, 8 min, 10 min, 15 min, 20 min, or any combination thereof; the atmosphere includes a nitrogen atmosphere, and the flow rate is 50-200 mL·min. -1 ·g -1 , including but not limited to 50 mL·min -1 ·g -1 60 mL·min -1 ·g -1 80 mL·min -1 ·g -1 100 mL·min -1 ·g -1 150 mL·min -1 ·g -1 180 mL·min -1 ·g -1 200 mL·min -1 ·g -1 , or a range consisting of any two of them.

[0028] According to one embodiment of this disclosure, after the second pre-reduction heating stage, a second pre-reduction isothermal stage is performed, followed by a third pre-reduction heating stage; the temperature of the second pre-reduction isothermal stage is 390-410℃, and the time is 20-60 min, including but not limited to 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or any combination thereof; the atmosphere includes a nitrogen atmosphere, with a flow rate of 50-300 mL·min. -1 ·g -1 , including but not limited to 50 mL·min -1 ·g -1 60 mL·min -1 ·g -1 80 mL·min -1 ·g -1 100 mL·min -1 ·g -1 150 mL·min -1 ·g -1 180 mL·min -1 ·g -1 200 mL·min -1 ·g-1 250 mL·min -1 ·g -1 300mL·min -1 ·g -1 Or a range consisting of any two of these. Nitrogen purging can remove water from the surface and pores of the catalyst, preventing repeated reduction and oxidation of the catalyst caused by water retention, reducing the α-Fe grain size, and improving catalyst performance.

[0029] According to one embodiment of this disclosure, after the third pre-reduction heating stage, a third pre-reduction isothermal stage is performed; the temperature of the third pre-reduction isothermal stage is 400-425℃, and the time is 40-200 min, including but not limited to 40 min, 50 min, 80 min, 100 min, 150 min, 200 min, or any combination thereof; the atmosphere includes a nitrogen atmosphere, and the flow rate is 50-300 mL·min. -1 ·g -1 , including but not limited to 50 mL·min -1 ·g -1 60 mL·min -1 ·g -1 80 mL·min -1 ·g -1 100 mL·min -1 ·g -1 150 mL·min -1 ·g -1 180 mL·min -1 ·g -1 200 mL·min -1 ·g -1 250 mL·min -1 ·g -1 300 mL·min -1 ·g -1 Or a range consisting of any two of these. Nitrogen purging can remove water from the surface and pores of the catalyst, preventing repeated reduction and oxidation of the catalyst caused by water retention, reducing the α-Fe grain size, and improving catalyst performance.

[0030] According to one embodiment of this disclosure, the temperature of the second pre-reduction isothermal section is lower than the temperature of the third pre-reduction isothermal section.

[0031] According to one embodiment of this disclosure, in step (1), the passivation treatment conditions include: a time of 10-120 min, a temperature of 20-80°C, and a passivation atmosphere comprising a mixture of oxygen and nitrogen, wherein the oxygen content is 0.5-2.0% by volume. The above embodiment can avoid rapid oxidation and flammability of the catalyst.

[0032] To promote the entry of the impregnation liquid into the micropores of the catalyst and to fully expel the gas inside the catalyst, thereby improving the impregnation effect, according to one embodiment of this disclosure, in step (2), the impregnation treatment is carried out under vacuum ultrasonic conditions, wherein the vacuum ultrasonic treatment is performed for 1-10 minutes at 15-minute intervals; the total impregnation treatment time is 2-8 hours, the temperature is 200-260°C, and the pressure is 0-4 MPa, preferably 1.8-2.5 MPa, including but not limited to 0 MPa, 0.5 MPa, 1 MPa, 1.5 MPa, 1.8 MPa, 2 MPa, 2.2 MPa, 2.5 MPa, 3 MPa, 2.8 MPa, 4 MPa, or any combination thereof; wherein, the "total impregnation treatment time" includes the sum of all ultrasonic times and interval times. The above conditions are beneficial to improving the catalyst weight gain and bulk density, and some of the more fragile closed pores are opened under pressure, further improving the impregnation effect, and also preventing the collapse of some pores inside the catalyst.

[0033] According to one embodiment of this disclosure, the impregnation solution contains a potassium source, as well as a silicon source and / or an aluminum source.

[0034] According to one embodiment of this disclosure, the potassium source can be of a type that allows the obtained catalyst to contain K2O. For example, it can include one or more of K2CO3, KNO3, K2SO4 and KOH. When the potassium source includes two or more types, there is no specific limitation on their proportion.

[0035] According to one embodiment of this disclosure, the silicon source can be of a type that allows the resulting catalyst to contain SiO2. For example, it may include one or more of silica sol, tetraethyl orthosilicate, water glass, and fumed silica. When the silicon source includes two or more types, there is no specific limitation on their proportions.

[0036] According to one embodiment of this disclosure, the silicon source can be of a type that allows the resulting catalyst to contain Al2O3. For example, it can include one or more of Al(NO3)3, aluminum isopropoxide, NaAlO2, and Al(OH)3. When the aluminum source includes two or more types, there is no specific limitation on their proportions.

[0037] To ensure the treated catalyst has a suitable elemental content and improve its activity, according to one embodiment of this disclosure, the weight ratio of the first product to the potassium source (calculated as potassium) is 1:(0.03-0.10), preferably 1:(0.05-0.10; the weight ratio of the first product to the silicon source (calculated as silicon) is 1:(0.01-0.10), preferably 1:(0.03-0.08); and the weight ratio of the first product to the aluminum source (calculated as aluminum) is 1:(0.005-0.10), preferably 1:(0.01-0.05).

[0038] According to one embodiment of the present disclosure, in step (3), the conditions for the calcination treatment include: a heating rate of 4-10℃ / min, a temperature of 450-900℃, and a total time of 2-8h, wherein the "total time" includes the sum of the heating time and the constant temperature time.

[0039] In order to stabilize the structure of the catalyst and have a suitable and uniform grain size, pore distribution and pore size, according to one embodiment of the present disclosure, in step (4), the reduction activation treatment includes a first reduction heating section, a second reduction heating section and a reduction isothermal section performed sequentially. The heating rate K4 of the first reduction heating section is greater than the heating rate K5 of the second reduction heating section, where 1.0℃ / min ≤ K4 ≤ 3.0℃ / min, preferably 1.0℃ / min ≤ K4 ≤ 2.0℃ / min, including but not limited to 1.0℃ / min, 1.5℃ / min, 2.0℃ / min, 2.5℃ / min, 3.0℃ / min, or any combination thereof; and 0.2℃ / min ≤ K5 < 1.0℃ / min, preferably 0.2℃ / min ≤ K5 ≤ 0.4℃ / min, including but not limited to 0.2℃ / min, 0.4℃ / min, 0.5℃ / min, 0.8℃ / min, 1.0℃ / min, or any combination thereof. The atmospheres in the first and second reduction heating sections each comprise a mixture of hydrogen and nitrogen, wherein the hydrogen content is 60-80% by volume. The space velocity in the first reduction heating stage is 1000-2000 h⁻¹ -1 The target temperature of the first reduction heating section is 340-360℃; The space velocity in the second reduction heating stage is 8000-10000 h⁻¹ -1 ; The temperature of the reduction isothermal section is 410-430℃, the total time is 2-10 hours, and the space velocity is 8000-10000 h⁻¹. -1"Total time" includes the sum of the times for all second reduction and second purging phases; Preferably, the reduction isothermal section includes alternating second reduction and second purging sections; this disclosure does not specifically limit the number of second reduction and second purging sections. For example, the reduction isothermal section may include sequentially performing a first second reduction section and a first second purging section; or, the reduction isothermal section may include sequentially performing a first second reduction section, a first second purging section, a second second reduction section, and a second second purging section; the time and atmosphere of each second reduction section may be the same or different, preferably the same; the time and atmosphere of each second purging section may be the same or different, preferably the same. Each second reduction stage lasts 10-60 minutes, and the atmosphere consists of a mixture of hydrogen and nitrogen, with hydrogen comprising 60-80% by volume. Each second purging phase lasts 1-10 minutes, with an atmosphere including nitrogen and a flow rate of 100-800 mL / min. -1 ·g -1 .

[0040] According to one embodiment of this disclosure, the method for preparing the molten iron catalyst to be treated is conventional in the art, including but not limited to the high-temperature melting method. Specifically, magnetite and auxiliary precursor are mixed and melted in an electric arc furnace at a temperature of 1650-1780°C. After cooling, the mixture is crushed and ground using a grinder, and sieved to select molten iron catalyst with a particle size of 30-150µm. The types of auxiliary precursors are conventional in the art, such as Al2O3, KNO3, CaCO3, etc., and the ratio of magnetite to auxiliary precursors is conventional in the art.

[0041] According to one embodiment of this disclosure, the molten iron catalyst to be treated has a pore volume of 0.005-0.015 mL / g and a specific surface area of ​​0.5-2.0 m². 2 / g, average pore size 2.0-4.0nm.

[0042] The third aspect of this disclosure provides a molten iron catalyst obtained by processing using the method described in the second aspect of this disclosure.

[0043] The molten iron catalyst described in the third aspect of this disclosure has the same characteristics as the molten iron catalyst described in the first aspect of this disclosure, and will not be described again here.

[0044] The fourth aspect of this disclosure provides a method for Fischer-Tropsch synthesis, comprising contacting syngas with a catalyst to carry out a Fischer-Tropsch synthesis reaction; said catalyst includes the molten iron catalyst described in the first or third aspect of this disclosure.

[0045] According to one embodiment of this disclosure, the type and amount of syngas, and the conditions of the Fischer-Tropsch synthesis reaction are conventional in the art; for example, the syngas is CO and H2.

[0046] The present disclosure is further illustrated by the following examples, but the present disclosure is not limited thereto. All raw materials used in the examples are commercially available.

[0047] Preparation method of the molten iron catalyst to be treated: high-temperature melting method, the molten iron catalyst prepared has a pore volume of 0.005-0.015 mL / g and a specific surface area of ​​0.5-2.0 m². 2 / g, average pore size 2.0-4.0nm.

[0048] Element content testing method: X-ray fluorescence spectrometry (XRF).

[0049] Specific surface area test method: Physical adsorption apparatus (BET).

[0050] Pore ​​volume test method: Physical adsorption apparatus (BET).

[0051] Method for measuring the grain size of α-Fe: in-situ X-ray diffraction.

[0052] In Table 1, the K content is a value relative to the weight of iron in the catalyst, and the total Si and Al content is a value relative to the total weight of the catalyst.

[0053] Example 1 The molten iron catalyst is pretreated using the following steps: (1) Pre-reduction treatment and passivation treatment: ① The temperature is raised from room temperature to 340℃ at a rate of 2.0℃ / min, and the atmosphere is 40% H2 / N2 by volume; ② When the temperature reaches 340℃, switch to pure N2, maintain the temperature for 10 minutes, and keep the N2 flow rate at 100 mL / min. -1 ·g -1 ; ③ The temperature is raised from 340℃ to 400℃ at a rate of 0.5℃ / min, and the atmosphere is 60% H2 / N2 by volume. ④ When the temperature reaches 400℃, switch to pure N2, maintain the temperature for 40 minutes, and keep the N2 flow rate at 200 mL / min. -1 ·g -1 ; ⑤ The temperature is increased from 400℃ to 405℃ at a rate K3 of 0.2℃ / min. The heating process includes a 20-min reduction phase and a 5-min purging phase. The atmosphere in the reduction phase is 70% H2 / N2 (volume), and the atmosphere in the purging phase is nitrogen at a flow rate of 200 mL / min. -1 ·g -1 ; ⑥ After reaching 405℃, maintain the temperature for 60 minutes in a nitrogen atmosphere at a flow rate of 200 mL / min. -1 ·g -1 ; ⑦ After the pre-reduction treatment, passivation treatment was carried out in a 0.5 volume% O2 / N2 atmosphere at 40℃ for 80 minutes to obtain the first product; (2) Impregnation treatment: The first product was contacted with an impregnation solution containing KNO3 and silica sol and impregnated at a temperature of 220°C and a pressure of 2MPa for a total time of 6 hours, with ultrasonication for 5 minutes every 15 minutes to obtain the second product. The weight ratio of the first product to KNO3 (calculated as potassium) is 1:0.06, and the weight ratio of the first product to the silicon source (calculated as silicon) is 1:0.02. (3) Calcination treatment: The second product was heated from room temperature to 800℃ at a heating rate of 4.0℃ / min, and then kept at a constant temperature for a total time of 4 hours to obtain the third product. (4) Reduction and activation treatment: The third product was subjected to reduction and activation treatment, including: in a 65 volume% H2 / Ar atmosphere, at a heating rate of 3 °C / min, with K4 set to 1500 h. -1 The temperature was increased to 350℃ at space velocity, and then increased at a heating rate of 0.2℃ / min for K5 and 8500h. -1 The air velocity raises the temperature to 420°C; After the heating is complete, maintain the temperature at 420℃ for a reduction isothermal phase, which includes an alternating 30-minute second reduction phase and a 5-minute second purging phase. The atmosphere for the second purging phase is nitrogen at a flow rate of 200 mL / min. -1 ·g -1 The atmosphere in the second reduction section was 70% H2 / N2 by volume, and the total time in the reduction isothermal section was 8 hours. The parameters of the pretreated molten iron catalyst are listed in Table 1.

[0054] Example 2 The molten iron catalyst was pretreated using the steps of Example 1, except that steps ② and ④ were not performed. The parameters of the pretreated molten iron catalyst are listed in Table 1.

[0055] Example 3 The molten iron catalyst was pretreated using the steps of Example 1, except that steps ② and ④ were omitted, and in step ⑤, the heating process included a first reduction stage of 25 minutes but did not include a first purging stage. The parameters of the pretreated molten iron catalyst are listed in Table 1.

[0056] Example 4 The molten iron catalyst was pretreated using the steps of Example 1, except that steps ② and ④ were omitted, and in step ⑤, the heating process included a first reduction stage of 25 minutes but did not include a first purging stage, and the impregnation solution did not contain a silicon source. The parameters of the pretreated molten iron catalyst are listed in Table 1.

[0057] Example 5 The molten iron catalyst was pretreated using the steps of Example 1, except that steps ② and ④ were omitted, and in step ⑤, the heating process included a first reduction stage of 25 minutes but did not include a first purging stage. The impregnation solution did not contain silica sol, and the impregnation pressure was atmospheric pressure. The parameters of the pretreated molten iron catalyst are listed in Table 1.

[0058] Example 6 The molten iron catalyst was pretreated using the steps of Example 1, except that the heating rate K1 in step ① was 4℃ / min and the heating rate K2 in step ③ was 1℃ / min; the parameters of the pretreated molten iron catalyst are listed in Table 1.

[0059] Example 7 The molten iron catalyst was pretreated using the steps of Example 1, except that the impregnation pressure was 0.5 MPa; the parameters of the pretreated molten iron catalyst are listed in Table 1.

[0060] Comparative Example 1 The molten iron catalyst was pretreated using the method of the embodiment, except that steps (1) and (2) were not performed. The parameters of the pretreated molten iron catalyst are listed in Table 1.

[0061] Comparative Example 2 The molten iron catalyst was pretreated using the method of the embodiment, except that step (2) was not performed. The parameters of the pretreated molten iron catalyst are listed in Table 1.

[0062] Table 1

[0063] Test case The performance of the treated molten iron catalysts obtained in the examples and preparation examples was tested. Specifically, CO and H2 were introduced into a 100 mL fixed bed device and reacted at 340 °C for 100 h. The molar ratio of CO to H2 was 1:3.5. The results are listed in Table 2.

[0064] The gas composition of the product was tested by online chromatography, and the CO conversion rate and CH4 selectivity were calculated according to the following formula.

[0065]

[0066]

[0067] Table 2

[0068] Based on the above data, it can be seen that the molten iron catalyst treated by the method of this disclosure has a small α-Fe grain size, a large specific surface area and pore volume, and a high K element content. When used in the Fischer-Tropsch synthesis reaction, it has high catalytic activity and can achieve a high CO conversion rate and a low methane selectivity.

[0069] Furthermore, a comparison of Examples 1 with Examples 2 and 3 shows that, during the pre-reduction process, a constant-temperature stage following the heating stage can further increase the specific surface area, pore volume, average pore size, and silicon, aluminum, and potassium content of the catalyst. This results in higher CO conversion and lower CH4 selectivity for the Fischer-Tropsch synthesis reaction. A comparison of Examples 1 and 7 shows that controlling the impregnation pressure within the preferred range of 1.8-2.5 MPa can further improve the impregnation effect, promote the formation of pore structures, and thus increase CO conversion and reduce CH4 selectivity. A comparison of Examples 1 and 6 shows that when K1 and K2 are within the preferred ranges, 2.0℃ / min ≤ K1 ≤ 3.5℃ / min, and 0.5℃ / min ≤ K2 ≤ 0.8℃ / min, CH4 selectivity can be further reduced and CO conversion increased.

[0070] The preferred embodiments of this disclosure have been described in detail above. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0071] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0072] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A molten iron catalyst, characterized in that, In the molten iron catalyst, the grain size of α-Fe is 30 nm or less; the pore volume is 0.10-0.24 mL / g, the specific surface area is 15-40 m 2 / g, and the average pore diameter is 20-50 nm. In the molten iron catalyst, the total content of silicon and aluminum is 0.5-1.5% by weight relative to the total weight of the molten iron catalyst; and the content of potassium is 3-6% by weight relative to the weight of iron.

2. A pretreatment method for molten iron catalyst, characterized in that, The method includes: (1) The molten iron catalyst to be treated is subjected to pre-reduction and passivation treatment to obtain the first product; (2) The first product is contacted with an impregnation solution containing a potassium source, an optional silicon source and an optional aluminum source for impregnation treatment to obtain the second product; (3) The second product is roasted to obtain the third product; (4) The third product is subjected to reduction and activation treatment; The pre-reduction process includes a first pre-reduction heating stage, a second pre-reduction heating stage, and a third pre-reduction heating stage performed sequentially. The relationship between the heating rate K1 of the first pre-reduction heating section, the heating rate K2 of the second pre-reduction heating section, and the heating rate K3 of the third pre-reduction heating section is K1 > K2 > K3.

3. The preprocessing method according to claim 2, wherein, In step (1), 2.0℃ / min≤K1≤8.0℃ / min, 0.5℃ / min≤K2<2.0℃ / min, and 0.1℃ / min≤K3<0.5℃ / min; The atmosphere in the first pre-reduction heating section includes a mixture of hydrogen and nitrogen, wherein the hydrogen content is 10-50% by volume. The atmosphere in the second pre-reduction heating section includes a mixture of hydrogen and nitrogen, wherein the hydrogen content is 30-70% by volume. Preferably, the third pre-reduction heating section includes an alternating first reduction section and a first purging section; Each first reduction phase lasts 5-30 minutes, and the atmosphere consists of a mixture of hydrogen and nitrogen, with hydrogen comprising 60-80% by volume. The time of each first purging section is 1-10 min, the atmosphere includes nitrogen atmosphere, and the flow rate is 50-300 mL min -1 ·g -1 ; Preferably, after the first pre-reduction temperature rising stage, a first pre-reduction constant temperature stage is carried out, and then a second pre-reduction temperature rising stage is carried out; the temperature of the first pre-reduction constant temperature stage is 330-350℃, the time is 5-20min, the atmosphere includes a nitrogen atmosphere, and the flow rate is 50-200mL·min -1 ·g -1 ; Preferably, after the second pre-reduction temperature rising stage, a second pre-reduction constant temperature stage is carried out, and then a third pre-reduction temperature rising stage is carried out; the temperature of the second pre-reduction constant temperature stage is 390-410℃, the time is 20-60min, the atmosphere includes a nitrogen atmosphere, and the flow rate is 50-300mL·min -1 ·g -1 ; Preferably, after the third pre-reduction temperature rising stage, a third pre-reduction constant temperature stage is carried out; the temperature of the third pre-reduction constant temperature stage is 400-425℃, the time is 40-200min, the atmosphere includes a nitrogen atmosphere, and the flow rate is 50-300mL·min -1 ·g -1 .

4. The preprocessing method according to claim 2, wherein, In step (1), the passivation conditions include: a time of 10-120 min, a temperature of 20-80 °C, and a passivation atmosphere consisting of a mixture of oxygen and nitrogen, wherein the oxygen content is 0.5-2.0 by volume.

5. The preprocessing method according to claim 2, wherein, In step (2), the impregnation treatment is carried out under vacuum ultrasonic conditions, and the vacuum ultrasonic treatment is performed for 0.5-10 minutes every 15 minutes; the total impregnation treatment time is 2-8 hours, the temperature is 200-260℃, and the pressure is 0-4MPa. Optionally, the potassium source includes one or more of K2CO3, KNO3, K2SO4 and KOH; The silicon source includes one or more of silica sol, tetraethyl orthosilicate, water glass, and silica. The aluminum source includes one or more of Al(NO3)3, aluminum isopropoxide, NaAlO2 and Al(OH)3; Optionally, the weight ratio of the first product to the potassium source (calculated as potassium) is 1:(0.03-0.10), the weight ratio of the first product to the silicon source (calculated as silicon) is 1:(0.01-0.10), and the weight ratio of the first product to the aluminum source (calculated as aluminum) is 1:(0.005-0.10).

6. The preprocessing method according to claim 2, wherein, In step (3), the conditions for the calcination treatment include: a heating rate of 4-10℃ / min, a temperature of 450-900℃, and a total time of 2-8h.

7. The preprocessing method according to claim 2, wherein, In step (4), the reduction activation process includes a first reduction heating section, a second reduction heating section, and a reduction isothermal section performed sequentially; The heating rate K4 of the first reduction heating section is greater than the heating rate K5 of the second reduction heating section, where 1.0℃ / min ≤ K4 ≤ 3.0℃ / min and 0.2℃ / min ≤ K5 < 1.0℃ / min; The atmospheres in the first and second reduction heating sections each comprise a mixture of hydrogen and nitrogen, wherein the hydrogen content is 60-80% by volume. The airspeed of the first reduction temperature rising section is 1000-2000h -1 The target temperature of the first reduction temperature rising section is 340-360℃. The airspeed of the second reduction temperature rising section is 8000-10000h -1 ; The temperature of the reduction constant temperature section is 410-430 DEG C, the total time is 2-10h, and the airspeed is 8000-10000h -1 ; Preferably, the reduction isothermal section includes an alternating second reduction section and a second purging section; Each second reduction stage lasts 10-60 minutes, and the atmosphere consists of a mixture of hydrogen and nitrogen, with hydrogen comprising 60-80% by volume. The time of the second purging section is 1-10 min each time, the atmosphere includes nitrogen atmosphere, and the flow rate is 100-800 mL min -1 ·g -1 .

8. The preprocessing method according to claim 2, wherein, The method for preparing the molten iron catalyst to be treated includes the melting method.

9. The molten iron catalyst obtained by the pretreatment method according to any one of claims 2 to 8.

10. A Fischer-Tropsch synthesis method, comprising contacting syngas with a catalyst to carry out a Fischer-Tropsch synthesis reaction; characterized in that, The catalyst includes the molten iron catalyst as described in claim 1 or 9.

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