Iron carbide catalyst containing boron, its preparation method and application

Abstract

The present invention provides a boron-containing iron carbide catalyst, a preparation method and an application thereof. The catalyst has improved carbon monoxide molecule activation ability, high Fischer-Tropsch catalytic activity, and can significantly improve the selectivity of C5+ products. The preparation method of the boron-containing iron carbide catalyst includes the following steps A1)-A2) or includes the following step B): A1) performing a reduction reaction on an iron salt and sodium borohydride in an inert atmosphere to prepare a catalyst precursor; or mixing and ball-milling iron powder and elemental boron in an inert atmosphere to prepare a catalyst precursor; A2) heating and reacting the catalyst precursor in a gas containing at least CO and / or C2H4 at 200–450 °C to obtain the boron-containing iron carbide catalyst, wherein the gas does not contain oxygen; or B) mixing and ball-milling iron powder, elemental boron and elemental carbon in an inert atmosphere to prepare the boron-containing iron carbide catalyst.

Description

Technical Field

[0001] The present invention relates to the technical field of Fischer-Tropsch synthesis, and particularly relates to an iron carbide catalyst containing boron, a preparation method thereof, and an application thereof. Background Art

[0002] The Fischer-Tropsch synthesis reaction is a catalytic process for converting carbon monoxide hydrogenation into long-chain hydrocarbons, which plays a significant role in current and future energy conversion and utilization. Among traditional Fischer-Tropsch synthesis catalysts, iron-based catalysts have characteristics such as low cost, high activity, and wide reaction product distribution, and have achieved success in industrial applications of Fischer-Tropsch synthesis.

[0003] In traditional methods for adding boron-based promoters to catalysts, such as boric acid solution impregnation, co-precipitation, etc., usually after calcination and reduction and carbonization of the catalyst, a catalyst with boron oxide loaded on the surface of iron carbide is obtained.

[0004] US6727289B2 (Document 1) discloses that adding boron to a cobalt-based Fischer-Tropsch synthesis catalyst prepared by an impregnation method improves the carbon monoxide conversion rate and stability on the catalyst and reduces the methane selectivity.

[0005] The document "Effect of boron promotion on the stability of cobalt Fischer-Tropsch catalysts" (J. Catal. 280, 50–59 (2011)) (Document 2) discloses that by dissolving cobalt nitrate and boric acid together and preparing a catalyst on a catalyst support, compared with a Fischer-Tropsch synthesis catalyst without boron, carbon deposition is less likely to occur on the surface and the catalyst has a longer lifespan.

[0006] CN101767010B (Document 3) discloses a highly abrasion-resistant iron-based catalyst for a slurry bed reactor and a preparation method thereof, and gives an iron-based Fischer-Tropsch synthesis catalyst prepared by a co-precipitation method, which has better abrasion resistance in a slurry bed reactor.

[0007] The document "Promotive effect of boron oxide on the iron-based catalysts for Fischer-Tropsch synthesis" (Fuel 281, 118714 (2020)) (Document 4) discloses that boron oxide as a promoter can hinder the reduction of an iron-based catalyst from an oxide to a metal, and has less surface carbon deposition and improved stability when applied to the Fischer-Tropsch synthesis reaction.

[0008] The literature "Insight of boron-induced single-step synthesis of short-chain olefins from bio-derived syngas" (Fuel 263, 116663 (2020)) (Literature 5) presents a hydrothermal method for preparing a boron-containing iron-based catalyst. The selectivity of low-carbon olefin products on the iron-based Fischer-Tropsch synthesis catalyst prepared by this method is improved.

[0009] Currently, boron is widely used as a promoter in cobalt-based catalysts for Fischer-Tropsch synthesis. As described in the above-mentioned Literature 1 and Literature 2, the addition of boron promoter to cobalt catalysts improves stability, activity, and the selectivity of C5+ products. Among them, boron has a competitive adsorption relationship with carbon on the surface of cobalt-based catalysts, which is the key factor for the extended lifespan of boron-containing cobalt-based catalysts.

[0010] There are fundamental differences in the active phases between iron-based catalysts and cobalt-based catalysts. The active phase of iron-based catalysts is iron carbide, where carbon atoms fill the vacancies formed by iron atoms. The interaction between boron and metallic cobalt with iron carbide will be significantly different. Therefore, the method of modifying cobalt-based catalysts with boron cannot be directly applied to iron-based catalysts.

[0011] Since carbon atoms themselves have a certain electron-withdrawing ability, it will cause the iron atoms on the surface of iron carbide to carry partial positive charges (the Pauling electronegativity of carbon is 2.5, and that of iron is 1.8). This leads to a decrease in the ability of iron atoms to transfer electrons to the π antibonding orbital of CO during the CO activation process, resulting in a certain hindrance to the dissociation of CO on iron carbide. Generally speaking, creating carbon defect sites on the surface of iron carbide and adding electron-donating promoters can promote CO dissociation. However, carbon defects cannot exist stably and will be filled by new carbon atoms as CO is activated. If electron-donating promoters such as K-containing compounds are used, it will not only increase the Fischer-Tropsch synthesis reaction activity of the catalyst but also increase the CO2 selectivity.

[0012] In the above-mentioned Literature 3 and Literature 5, boron is used as an additive to improve the performance of iron-based Fischer-Tropsch synthesis catalysts. In these, boron exists in the form of boron oxide, mainly playing a role in improving the abrasion resistance and anti-coking ability of the catalyst. In Literature 4, boron exists in the form of mostly oxides and a small amount of zero-valent boron, improving the selectivity of low-carbon olefins. Summary of the Invention

[0013] In view of this, the present invention provides a boron-containing iron carbide catalyst, its preparation method, and application. The boron-containing iron carbide catalyst provided by the present invention has improved carbon monoxide molecule activation ability, high Fischer-Tropsch catalytic activity, and can significantly enhance the selectivity of C5+ products.

[0014] To achieve its objectives, the present invention provides the following technical solutions:

[0015] On the one hand, the present invention provides a method for preparing a boron-containing iron carbide catalyst, which includes the following steps A1)-A2) or includes the following step B):

[0016] A1) Performing a reduction reaction on an iron salt and sodium borohydride in an inert atmosphere to prepare a catalyst precursor; alternatively, performing a mixed ball milling on iron powder and elemental boron in an inert atmosphere to prepare a catalyst precursor; A2) Heating and reacting the catalyst precursor in a gas containing at least CO and / or C2H4 at 200–450 °C to obtain the boron-containing iron carbide catalyst, and the gas does not contain oxygen;

[0017] Or,

[0018] B) Performing a mixed ball milling on iron powder, elemental boron and elemental carbon in an inert atmosphere to prepare the boron-containing iron carbide catalyst.

[0019] In some embodiments, in step A1), in the catalyst precursor, the molar ratio of iron element to boron element is 100:0.1 - 100:2.

[0020] In some embodiments, in step A1), the iron salt is a soluble iron salt, and the iron in the iron salt is Fe 2+ and / or Fe 3 + , for example, selected from one or more of iron hydrochloride, sulfate, nitrate, acetate, citrate and their hydrates.

[0021] In some embodiments, in step A1), the reduction reaction is carried out at 5 - 60 °C, and the reaction time is, for example, 15 - 60 minutes.

[0022] In some embodiments, in step A1), the reduction reaction is carried out in the presence of a solvent, and the solvent is selected from one or more of water, ethanol, ethylene glycol, polyethylene glycol;

[0023] Step A1) further includes the steps of separating and washing the product obtained from the reduction reaction.

[0024] In some embodiments, in step A1) or step B), the ball milling is carried out until the particle size of the material to be ball milled is reduced to below 100 nm.

[0025] In some embodiments, in step A1) or step B), the conditions of the ball milling include: the mass ratio of the grinding medium to the material to be ball milled is 5:1 - 20:1, the ball milling speed is 250 - 600 rpm, and the ball milling time is 0.5 - 40 hours.

[0026] In some embodiments, in step B), the amounts of the raw materials are based on the three elements of iron, carbon, and boron, and the molar ratio is 100:33 - 50:0.05 - 2.

[0027] In some embodiments, in step A2), in the gas, the CO content is 0 - 100% (v / v), the C2H4 content is 0 - 100% (v / v), and the total content of CO and C2H4 is not less than 1% (v / v). The gas may also optionally contain H2;

[0028] Preferably, in step A2), the space velocity is 500 - 10000 h -1 ;

[0029] Preferably, in the reaction system of step A2), the total pressure is 0.1 - 5.0 MPa;

[0030] Preferably, in step A2), the temperature is raised to 200 - 450 °C at a heating rate of 0.5–20 °C / min for the heating reaction.

[0031] In some embodiments, before step A2), an optional step of pre-treating the catalyst precursor is included. The pre-treatment step includes: treating the catalyst precursor at 200 - 600 °C for 0.5 - 24 hours in a hydrogen and / or nitrogen atmosphere.

[0032] The present invention also provides a boron-containing iron carbide catalyst prepared by the preparation method described above.

[0033] In some embodiments, in the boron-containing iron carbide catalyst, it includes boron-containing iron carbide formed by boron atoms substituting for some carbon atoms in iron carbide. Preferably, in the boron-containing iron carbide, the atomic ratio of carbon atoms to boron atoms is y - n:n, where the value of n:y is 0.001 - 0.1.

[0034] In some embodiments, in the boron-containing iron carbide catalyst, it may optionally contain inevitable impurities, and the impurities include oxides of Fe and / or B.

[0035] The present invention also provides the application of the boron-containing iron carbide catalyst prepared by the preparation method described above or the boron-containing iron carbide catalyst described above as a catalyst in a reaction using CO and hydrogen as intermediate reactants, for example, as a catalyst in the Fischer-Tropsch synthesis reaction.

[0036] The technical solution provided by the present invention has the following beneficial effects:

[0037] The boron-containing iron carbide catalyst provided by the present invention has improved carbon monoxide molecule activation ability, high Fischer-Tropsch catalytic activity, and can significantly improve the selectivity of C5+ products. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is the XRD pattern of the catalyst obtained in Example 1.

[0039] Figure 2 It is the XRD pattern of the catalyst obtained in Example 2.

[0040] Figure 3 It is the XRD pattern of the catalyst obtained in Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0041] For the convenience of understanding the present invention, the present invention will be further described below in conjunction with embodiments. It should be understood that the following embodiments are only for better understanding of the present invention, and do not mean that the present invention is limited only to the following embodiments.

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which the present invention belongs. The term "and / or" as used herein may include any and all combinations of one or more of the related listed items.

[0043] In the ranges disclosed herein, the endpoints and any value are not limited to the exact range or value, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0044] For those parts in the examples where specific experimental steps or conditions are not indicated, the operations or conditions of the corresponding conventional experimental steps in the technical field of the present invention can be followed. For reagents or instruments whose manufacturers are not indicated, they are all conventional products that can be obtained through commercial purchase.

[0045] On the one hand, the present invention provides a method for preparing a boron-containing iron carbide catalyst, which mainly includes the following steps A1) and A2), or includes the following step B):

[0046] A1) Performing a reduction reaction on an iron salt and sodium borohydride in an inert atmosphere to prepare a catalyst precursor; or, mixing and ball-milling iron powder and elemental boron in an inert atmosphere to prepare a catalyst precursor;

[0047] A2) Heating and reacting the catalyst precursor in a gas containing at least CO and / or C2H4 at 200–450 °C to obtain a boron-containing iron carbide catalyst, and the above gas does not contain oxygen; or,

[0048] B) Mix iron powder, elemental boron, and carbon in an inert atmosphere by ball milling to obtain the iron carbide catalyst containing boron.

[0049] In the preparation method provided by the present invention, a solid-phase reaction occurs between boron and iron through a reduction reaction or ball milling to obtain a catalyst precursor, in which boron and iron are in zero valence or close to zero valence, and then it is carbonized by reacting with CO and / or C2H4 at 200–450 °C, or carbon is directly introduced during the process of preparing the catalyst precursor by solid-phase reaction in ball milling. In the iron carbide phase obtained by the above preparation method, some carbon atoms are replaced by boron. The inventors unexpectedly found that the use of such a catalyst has significantly improved Fischer-Tropsch synthesis reaction activity.

[0050] In some embodiments, in step A1), in the catalyst precursor, the molar ratio of iron element to boron element is 100:0.1 - 100:2.

[0051] In some embodiments, in step A1), when preparing the catalyst precursor by performing a reduction reaction on an iron salt and sodium borohydride in an inert atmosphere, the iron salt is specifically a soluble iron salt, and there is no particular limitation on the specific type of the soluble iron salt. Exemplarily, for example, it can be selected from one or more of iron hydrochlorides, sulfates, nitrates, acetates, citrates, and their hydrates. The iron in the iron salt is Fe 2+ and / or Fe 3+ . In some embodiments, the iron salt can specifically be selected from Fe 2+ or Fe 3+ combined with Cl - , SO4 2- , NO3 - , acetate, citrate, and one or more of the corresponding hydrated compounds, such as FeCl3, FeCl2, Fe(NO3)3, etc.

[0052] In some embodiments, in step A1), when preparing the catalyst precursor by performing a reduction reaction on an iron salt and sodium borohydride in an inert atmosphere, in step 1), the reduction reaction is carried out at 5 - 60 °C to obtain a black product; there is no particular limitation on the reaction time, and the reaction can be carried out until a black product is obtained. In some embodiments, for example, the reaction is carried out for 15 - 60 minutes.

[0053] In some embodiments, in step A1), when preparing the catalyst precursor by performing a reduction reaction on an iron salt and sodium borohydride in an inert atmosphere, the reduction reaction is carried out in the presence of a solvent, and the solvent can be selected from one or more combinations of water, ethanol, ethylene glycol, and polyethylene glycol. Specifically, the iron salt and sodium borohydride can be dissolved in a solvent to form a solution in advance and then added to the reaction system.

[0054] In some specific embodiments, step A1) further includes steps of separating and washing the product obtained from the reduction reaction to remove impurities and solvents. For example, the product is separated by centrifugation or magnetic adsorption, and the rotation speed of centrifugation can be, for example, 3000 - 20000 rpm. For example, anhydrous ethanol is used for washing to remove impurities and the solvent is removed. In some embodiments, in step A1), the amounts of the iron salt and sodium borohydride are based on iron and boron elements, and the molar ratio is 1:5 - 1:20. Subsequently, the catalyst precursor with an iron element to boron element molar ratio of 100:0.1 - 100:2 is obtained by washing the reaction product.

[0055] In some embodiments, in step A1), iron powder and elemental boron are mixed and ball-milled in an inert atmosphere to cause a solid-phase reaction between the two to prepare the catalyst precursor. Specifically, in step A1), when preparing the catalyst precursor by ball-milling, the iron powder and elemental boron are ball-milled to the nanoscale through ball-milling, causing a solid-phase reaction between iron and boron to obtain an iron boride (catalyst precursor); by ball-milling to reduce the particle size of the material to be ball-milled to below 100 nm, the catalyst precursor can be obtained. In step B), similarly, a solid-phase reaction occurs between the components through ball-milling and finally an iron boride carbide catalyst containing boron is obtained. Specifically, by ball-milling to reduce the particle size of the material to be ball-milled to below 100 nm, an iron boride carbide catalyst containing boron is finally obtained. In some specific embodiments, the ball-milling in step A1) or step B) can be specifically carried out under the following conditions: ball-milling is carried out in a ball mill, the rotation speed is 250 - 600 rpm, the mass ratio of the grinding medium (such as grinding balls) to the material to be ball-milled (i.e., the powder to be ground) is 5:1 to 20:1, and the ball-milling time is 0.5 - 40 hours. Intermittent ball-milling can be carried out during the ball-milling process to avoid overheating. For example, after ball-milling for a period of time (such as 30 min), it is paused for a period of time (such as 30 min) and then ball-milling continues. During the ball-milling process, the temperature can be, for example, from room temperature to 500 °C.

[0056] In steps A1) and B), the inert atmosphere involved can be an inert atmosphere such as nitrogen or argon.

[0057] In step A1) or step B), when preparing the catalyst precursor by ball-milling, the iron powder used is not particularly limited. For example, it can be, but is not limited to, Raney iron, reduced iron powder, etc.

[0058] In the catalyst precursor obtained through step A1), the valences of iron and boron are both zero or close to zero, and it may inevitably contain a small amount of Fe and / or B in an oxidized state, such as the part oxidized by inevitable contact with oxygen or water vapor in the air during transfer and storage.

[0059] In some embodiments, in step A2), the reaction atmosphere is a gas containing at least CO and / or C2H4, wherein the CO content is 0 - 100% (v / v), the C2H4 content is 0 - 100% (v / v), and the total content of CO and C2H4 is not less than 1% (v / v). The gas may also optionally contain H2. In some embodiments, in step A2), the space velocity is 500 - 10000 h -1 ; In some embodiments, in the reaction system of step A2), the total pressure is 0.1 - 5.0 MPa; In some embodiments, in the gas containing at least CO and / or C2H4, the partial pressures of CO and C2H4 are 0.05 - 10 bar, and the partial pressure of H2 is 0 - 20 bar. In some embodiments, an inert gas such as nitrogen, argon, etc. is incorporated into the gas containing at least CO and / or C2H4. In some embodiments, the reaction gas in step A2) consists of CO and / or C2H4. In some embodiments, the reaction gas in step A2) consists of hydrogen and CO and / or C2H4. In some embodiments, the reaction gas in step A2) consists of an inert gas and CO and / or C2H4. In some embodiments, the reaction gas in step A2) consists of an inert gas, H2, and CO and / or C2H4.

[0060] Unless otherwise specified, all pressures mentioned in the text are gauge pressures.

[0061] In some embodiments, in step A2), the temperature is raised to 200 - 450 °C at a heating rate of 0.5–20 °C / min for the heating reaction. For example, it can be raised from room temperature to 200 - 450 °C at the above heating rate. In step A2), the heating reaction is carried out until the composition of the tail gas no longer changes. For example, the composition of the tail gas is monitored by on-line chromatography, or the formation of iron carbide is detected by in-situ XRD until the crystal structure does not change; In some embodiments, in step A2), the heating reaction is carried out for 4 - 48 hours.

[0062] In some embodiments, between step A1) and step A2), that is, before step A2), an optional step of pre-treating the catalyst precursor is included, and this step is not necessary. This pre-treatment step includes: treating the catalyst precursor at 200 - 600 °C for 0.5 - 24 hours in a hydrogen and / or nitrogen atmosphere; By pre-treatment, the crystallinity of the catalyst precursor can be adjusted, and the grain size can be adjusted.

[0063] In some embodiments, in step B), based on the amounts of the three elements of iron, carbon, and boron, the molar ratio of the raw materials is 100:33 - 50:0.05 - 2. In step B), there is no particular limitation on the specific type of the carbonaceous raw material, and various carbonaceous raw materials can be used, such as but not limited to graphite carbon powder, activated carbon powder, etc.

[0064] The second aspect of the present invention also provides a boron-containing iron carbide catalyst, which is prepared by the method described above. The boron-containing iron carbide catalyst provided by the present invention can be prepared by the method described above, in which part of the carbon atoms in the iron carbide are replaced by boron atoms, and in some embodiments, the ratio of the number of carbon atoms to the number of boron atoms in the boron-containing iron carbide formed by the boron atoms replacing part of the carbon atoms in the iron carbide is yn:n, wherein n:y is 0.001-0.1.

[0065] In some embodiments, the general formula of iron carbide is expressed as Fe x C y In the boron-containing iron carbide catalyst, the active component can be expressed as the general formula Fe x C y-n B n , wherein each subscript value is the atomic number of the corresponding element; the atomic ratio of carbon atoms to boron atoms is yn:n; preferably, n:y is 0.001-0.1.

[0066] In some embodiments, the active component of the boron-containing iron carbide catalyst may be, but is not limited to, the part of C atoms in Fe5C2, Fe2C, Fe7C3, and Fe3C replaced by B. The iron carbide of different phases can be obtained by adjusting the gas composition (such as the ratio of CO, C2H4, etc.), pressure, and temperature in step A2), or by adjusting the ratio of the carbon source (carbon element) to the iron source (iron powder) in step B), which is known or understood by those skilled in the art based on the prior art they have mastered, and will not be elaborated on.

[0067] The boron-containing iron carbide catalyst of the present invention may be in a form having a specific crystal structure or not having a specific crystal structure.

[0068] In the boron-containing iron carbide catalyst of the present invention, zero-valent boron partially replaces carbon atoms in iron carbide, which is significantly different from the boron-containing iron-based catalyst obtained in the prior art in which boron mainly exists in the state of boron oxide. The catalyst of the present invention does not contain or contains only a small amount of oxidized boron. Even if it contains boron, it is an inevitable impurity, such as the side reactions that inevitably occur in the links of material transfer, transportation, storage, etc. Therefore, the boron-containing iron carbide catalyst of the present invention optionally contains inevitable impurities, which include oxides of Fe and / or B. These oxidized Fe and B are impurity amounts (impurity amount means not the main amount, accounting for less than 50wt%), and the catalyst of the present invention mainly contains iron carbide in which some C atoms in the non-oxidized state are replaced by B atoms.

[0069] The boron-containing iron carbide catalyst of the present invention can be prepared by adopting the preparation method described above.

[0070] The above-mentioned iron carbide catalyst containing boron provided by the present invention has a significantly improved ability to dissociate CO molecules. In specific application processes, it can be used alone or in combination with other components that can improve the catalyst activity. For example, it can be used in combination with other electronic promoters (such as K2O, etc.), and / or the boron-containing iron carbide catalyst of the present invention can be supported on a carrier and used, and the carrier is, for example, SiO2, etc.

[0071] On the other hand, the present invention also provides the application of the boron-containing iron carbide catalyst prepared by the above-mentioned preparation method or the boron-containing iron carbide catalyst described above as a catalyst in reactions with CO and hydrogen as intermediate reactants, mainly referring to CO2 hydrogenation and the reaction of CO with H2O. The former obtains CO and hydrogen through the reverse water gas shift, and the latter obtains CO and hydrogen through the water gas shift. Specifically, for example, it is used as a catalyst in the Fischer-Tropsch synthesis reaction, for example, but not limited to, the Fischer-Tropsch synthesis reaction in a fixed bed or slurry bed reactor.

[0072] In order to explore the reason why the boron-containing iron carbide catalyst provided by the present invention can have significantly improved Fischer-Tropsch synthesis catalytic activity, the inventor of the present invention exemplarily carried out theoretical calculation and simulation analysis on it. According to the calculation and simulation, the introduction of boron atoms changes the grain surface orientation of the active phase iron carbide, and the exposure ratios of the (021), (311), (31-1), and (31-2) planes increase. CO dissociation is more likely to occur on these planes, improving the ability of the catalyst to activate CO. Secondly, when boron atoms replace carbon atoms, whether on the catalyst surface or the subsurface, due to the lower electronegativity of boron compared to carbon, the positive charge carried by the surface iron atoms can be reduced, and the d electrons on the iron atoms are more likely to enter the π antibonding orbit of CO to activate CO, making it easier for the catalyst to activate CO.

[0073] Taking Fe5C2 as an example, Table 1 shows the information on the change in the exposed crystal planes of Fe5C2 caused by lattice doping of boron atoms predicted by density functional theory. By calculating the surface energy of different crystal planes through density functional theory (the data in this table is calculated using the VASP program, PAW pseudopotential, and PBE functional), the exposure ratios of different crystal planes can be obtained according to the Wulff Construction principle. The substitution of boron atoms affects the relative area of their exposure in the catalyst particles by changing the surface energy of different crystal planes. As the substitution amount of boron atoms increases, the proportion of crystal planes with lower CO dissociation activation energy increases, improving the overall activity of the catalyst.

[0074] Table 1

[0075]

[0076]

[0077] As can be seen from Table 1, calculations using density functional theory have shown that as the amount of boron atoms replacing surface carbon atoms in Fe5C2 increases, the exposure ratios of crystal planes such as (021), (311), (31-1), and (31-2) increase. These crystal planes have a lower CO dissociation activation energy compared to the (510) crystal plane. Considering that the (510) crystal plane is the main exposed plane of the Fe5C2 crystal without the addition of boron promoter (accounting for 31.6%), the introduction of boron exposes more surfaces with higher CO dissociation activity (lower activation energy), indicating that the ability of the catalyst with boron atoms replacing lattice carbon atoms to activate CO is enhanced.

[0078] Table 2 shows the CO dissociation energy information after the surface carbon atoms on the (510) plane of Fe5C2 are replaced by boron atoms. There are two surface-exposed carbon atoms on the (510) plane. After replacing the two carbon atoms with boron atoms respectively, two calculation models, namely boron-substituted carbon 1 and boron-substituted carbon 2, are obtained. The calculations and charge analyses of CO dissociation are carried out on them respectively, and the results are listed in Table 2. Specifically, in Table 2, the data corresponding to boron-substituted carbon 1 and boron-substituted carbon 2 refer to the data obtained from density functional theory calculations and charge analyses of the surface models obtained by replacing two carbon atoms at different positions on the (510) plane of Fe5C2 with boron atoms.

[0079] Table 2

[0080]

[0081] As can be seen from Table 2, calculations using density functional theory have shown that after the surface carbon atoms are replaced by boron atoms, the positive charge (Bader charge) of the iron atoms related to CO dissociation on the (510) plane decreases, and the CO activation energy barrier decreases.

[0082] During the implementation of the present invention, the present inventors performed XRD characterization on the iron carbide catalyst containing boron obtained by the present invention and found that its morphology has obvious changes compared to the iron carbide catalyst doped with boron in a manner not in accordance with the present invention. As an example, Figure 1 For the XRD characterization results in an embodiment, it can be seen that for the iron carbide catalyst containing boron, the proportion of crystal planes such as (021) increases. The diffraction peak intensity at 43.5° exceeds that at 44.2°. The former is the (021) plane, and the latter is the (510) plane; while in the iron carbide catalyst without boron, the diffraction intensity at 44.2° is stronger. Comparing with Figure 1 the Fe5C2 standard spectrum located at the bottom, it can be confirmed that the introduction of boron promoter by the scheme of the present invention changes the morphology of the catalyst.

[0083] The present invention will be described exemplarily through examples below.

[0084] The catalysts obtained in the following examples and comparative examples were characterized by XRD and XPS, where:

[0085] XRD (X-ray powder diffraction) characterization: Instrument model Bruker D8AX, Cu Kα;

[0086] XPS (X-ray photoelectron spectroscopy) characterization: Instrument model Thermo Scientific Escalab 250Xi, Al Kα.

[0087] ICP-AES (inductively coupled plasma resonance atomic emission spectroscopy) characterization: Instrument model is Spectro Arcos.

[0088] Example 1

[0089] The boron-containing iron carbide catalyst of Example 1 was prepared by the following steps:

[0090] 1) Dissolve 2 g of iron salt FeCl2·4H2O in 50 mL of solvent ethylene glycol to obtain solution A, dissolve 3 g of sodium borohydride in 15 mL of deionized water to obtain solution B. Stir solution A with a magnetic stirrer at a rotation speed of 500 rpm. Under a nitrogen protection environment, add solution B to solution A, control the temperature at 50 °C, and obtain a black product after a reduction reaction for 15 minutes; separate the obtained product by centrifugation, discard the centrifugate, and wash the precipitate three times with absolute ethanol to remove impurities and solvents to obtain a catalyst precursor. Among them, in the obtained catalyst precursor, the molar ratio of iron element to boron element is 100:2.

[0091] 2) Heat and react the catalyst precursor obtained in step 1) in a CO / H2 gas (CO volume content 10%) at a heating rate of 5 °C / min to 300 °C, the reaction time is 20 hours, the total pressure is 0.1 MPa, and the volume space velocity is 5000 h -1 ; After the reaction, a boron-containing iron carbide catalyst Fe x C y-n B n is obtained, where the values of x, y, and n are 5, 2, and 0.1 respectively.

[0092] The obtained catalyst was characterized by XRD and XPS, and the elemental composition was determined by ICP-AES in combination with XRD. The characterization results are as Figure 1 and shown in Table 3 below.

[0093] Comparative Example 1

[0094] The catalyst of Comparative Example 1 was prepared by the following steps:

[0095] Dissolve 4 g of iron salt Fe(NO3)3·9H2O in 50 mL of deionized water to obtain solution A. Dissolve 1.0 g of sodium carbonate in 15 mL of deionized water to obtain solution B. Stir solution A with a magnetic stirrer at a rotation speed of 500 rpm, and add solution B to solution A while controlling the temperature at 50 °C to obtain a red precipitate as the product. Centrifuge the obtained product, discard the centrifugate, wash the precipitate three times with deionized water, then add 0.012 g of boric acid, and dry it at 60 °C to obtain a catalyst precursor.

[0096] Afterwards, carbonize the catalyst precursor in the same manner as in step 2) of Example 1 to obtain the catalyst Fe5C2·0.05B2O3. The elemental composition is determined by ICP-AES in combination with XRD.

[0097] Perform XPS characterization on the obtained catalyst. The characterization results are shown in Table 3.

[0098] Table 3 XPS characterization of the valence state of boron atoms

[0099] 192.5 eV (boron oxide) 189.0 eV (low-valence boron) Example 1 28.2 71.8 Comparative Example 1 100 0

[0100] From Figure 1 it can be seen that: the sample obtained in Example 1 has a χ-Fe5C2 crystal structure, and the diffraction intensity in the (021) direction exceeds that in the (510) direction, showing significant differences compared with the standard card and the common χ-Fe5C2 crystal structure, indicating that the exposed proportion of the crystal plane of the catalyst has changed, which is consistent with the theoretical simulation results. The introduction of boron changes the crystal morphology and increases the content of the highly active (021) crystal plane.

[0101] From Table 3, it can be seen that: in the catalyst obtained by the method of Example, more than 70% of the boron is in the reduced state, while the reduced state of boron cannot be obtained in the comparative example, indicating that boron in the former exists in the reduced state. Combining with the XRD characterization results, it is very likely that boron replaces some carbon atoms in the crystal lattice. While boron in the latter exists in the oxidized state and is discrete from the catalyst active phase χ-Fe5C2.

[0102] Example 2

[0103] 1) Under nitrogen protection, use 20 g of reduced iron powder and 0.038 g of elemental boron to carry out ball milling in a ball mill at a rotation speed of 600 rpm with 200 g of zirconia grinding balls. After ball milling for 15 minutes, pause for 15 minutes to prevent the system from overheating. The total ball milling time is 6 hours, and the particle size of the milled material is less than 100 nm. In the obtained catalyst precursor, the molar ratio of iron element to boron element is 100:1.

[0104] 2) Heat the catalyst precursor obtained in step 1) in a CO / H2 gas (CO volume content 10%) at a heating rate of 5 °C / min to 300 °C for a reaction time of 20 hours, with a total pressure of 0.1 MPa and a space velocity of 5000 h -1 ; After the reaction, an iron carbide catalyst containing boron, Fe x C y-n B n is obtained, where the values of x, y, and n are 5, 2, and 0.05 respectively. For the XRD characterization results of Example 2, see Figure 2 , which has a Fe5C2 crystal phase, and the diffraction peak intensity of its (021) plane is enhanced compared to the standard spectrum. For the XPS characterization results, see Table 4.

[0105] Table 4 XPS Characterization of Boron Atom Valence States

[0106] 192.5 eV (boron oxide) 189.0 eV (low-valence boron) Example 2 14.6 85.4

[0107] Comparative Example 2

[0108] The difference from Example 2 is that no elemental boron is added.

[0109] Example 3

[0110] The following steps are used to prepare the iron carbide catalyst containing boron in Example 3:

[0111] 1) Dissolve 4 g of iron salt Fe(NH4)2·(SO4)2·6H2O in 60 mL of solvent polyethylene glycol (molecular weight 2000) to obtain solution A, dissolve 5 g of sodium borohydride in 15 mL of deionized water to obtain solution B, stir solution A with a magnetic stirrer at a speed of 500 rpm, and add solution B to solution A under a nitrogen protection environment, control the temperature at 60 °C, and obtain a black product after a reduction reaction of 25 minutes; separate the obtained product by centrifugation, discard the centrifugate, and wash the precipitate three times with absolute ethanol to remove impurities to obtain the catalyst precursor. Among them, in the obtained catalyst precursor, the molar ratio of iron element to boron element is 100:0.67.

[0112] 2) Heat the catalyst precursor obtained in step 1) in a C2H4 / H2 gas (C2H4 volume content 5%) at a heating rate of 20 °C / min to 400 °C for a reaction time of 12 hours, with a total pressure of 0.5 MPa and a space velocity of 3000 h -1 ; After the reaction, an iron carbide catalyst containing boron, Fe x C y-n B n is obtained, where the values of x, y, and n are 3, 1, and 0.02 respectively.

[0113] Example 4

[0114] 1) Under nitrogen protection, 20 g of reduced iron powder and 0.02 g of elemental boron are ball-milled in a ball mill at a rotational speed of 600 rpm with 400 g of zirconia grinding balls. After ball-milling for 30 minutes, it is paused for 30 minutes to prevent the system from overheating. The total ball-milling time is 40 hours, and the particle size of the milled material is less than 100 nm. In the obtained catalyst precursor, the molar ratio of iron element to boron element is 100:0.5

[0115] 2) The catalyst precursor obtained in step 1) is heated and reacted in a CO / H2 gas (CO volume content 50%) at a heating rate of 5 °C / min to 200 °C. The reaction time is 48 hours, the total pressure is 0.1 MPa, and the volume space velocity is 5000 h -1 ; The boron-containing iron carbide catalyst Fe x C y-n B n is obtained, where the values of x, y, and n are 2, 1, and 0.01 respectively

[0116] Example 5

[0117] Under nitrogen protection, 20 g of reduced iron powder, 2.0 g of graphite powder, and 0.0038 g of elemental boron are ball-milled in a ball mill at a rotational speed of 600 rpm with 100 g of zirconia grinding balls. After ball-milling for 10 minutes, it is paused for 20 minutes to prevent the system from overheating. The total ball-milling time is 24 hours, and the particle size of the milled material is less than 100 nm. The catalyst Fe x C y-n B n is obtained, where the values of x, y, and n are 7, 3, and 0.01 respectively. The results of XRD and XPS characterizations are shown in Figure 3 and Table 5. XRD diffraction shows the crystal phase of Fe7C3

[0118] Table 5 XPS Characterization of Boron Atomic Valence

[0119] 192.5 eV (boron oxide) 189.0 eV (low-valence boron) Example 5 25.6 74.4

[0120] Example 6

[0121] The boron-containing iron carbide catalyst of Example 1 is prepared by the following steps:

[0122] 1) Dissolve 4 g of iron salt Fe(NO3)3·9H2O in 50 mL of ethanol to obtain solution A. Dissolve 7.6 g of sodium borohydride in 20 mL of deionized water to obtain solution B. Stir solution A with a magnetic stirrer at a rotation speed of 500 rpm. Under a nitrogen protection environment, add solution B to solution A, control the temperature at 5°C, and obtain a black product after a reduction reaction for 50 minutes. Separate the obtained product by centrifugation, discard the centrifugate, and wash the precipitate three times with absolute ethanol to remove impurities to obtain the catalyst precursor. Among them, in the obtained catalyst precursor, the molar ratio of iron element to boron element is 100:1.

[0123] 2) Heat and react the catalyst precursor obtained in step 1) in a CO / H2 gas (CO volume content 30%) at a heating rate of 0.5°C / min to 200°C, with a reaction time of 48 hours, a total pressure of 0.1 MPa, and a volume space velocity of 3000 h -1 ; After the reaction, obtain a boron-containing iron carbide catalyst Fe x C y-n B n , where the values of x, y, and n are 2, 1, and 0.02 respectively.

[0124] Example 7

[0125] The difference from Example 6 is that before step 2), the catalyst precursor is heated to 600°C at a rate of 20°C / min and maintained for 3 hours in a nitrogen atmosphere (space velocity 3000 h -1 ).

[0126] Catalyst performance evaluation:

[0127] In a fixed-bed reactor, the catalytic reaction performance of the catalysts obtained in each example and comparative example is evaluated respectively.

[0128] Evaluation conditions:

[0129] a Reaction temperature 270°C, CO:H2 = 1:2, total pressure 3.0 MPa, catalyst loading 200 mg, gas flow rate 3000 mL / h, reaction time 24 h.

[0130] b Reaction temperature 260°C, CO:H2 = 1:2, total pressure 2.5 MPa, catalyst loading 600 mg, gas flow rate 3000 mL / h, reaction time 24 h.

[0131] The evaluation results of the catalysts obtained in the above examples and comparative examples are shown in Table 6.

[0132] Table 6

[0133]

[0134] In Table 3, the analytical and detection methods used for the analysis of the products obtained from the reaction are as follows: an on-line gas chromatography method is used to analyze the products of CO, CH4, CO2 and C2-C4 hydrocarbons, and C 5+ The products are collected by a gas-liquid separation tank and detected by off-line gas chromatography.

[0135] In Table 3, the reaction effects involved are calculated by the following formula:

[0136] CO conversion rate:

[0137] CH4 selectivity:

[0138] CO2 selectivity:

[0139] C2-C4 selectivity:

[0140] C5+ selectivity:

[0141] As can be seen from the results in Table 6: the catalyst obtained in the example has a higher CO conversion rate and C 5+ product selectivity, while the selectivities of the by-products methane and CO2 are inhibited, and it is a Fischer-Tropsch synthesis catalyst with higher efficiency compared to the catalyst of the comparative example. By comparing Example 2 and Comparative Example 2, it can be seen that adding elemental boron improves the C 5+ product selectivity and catalyst activity of the iron catalyst prepared by the ball milling method.

[0142] It is easy to understand that the above examples are merely examples given clearly and do not mean that the present invention is limited thereto. For those of ordinary skill in the art, other different forms of changes or modifications can be made on the basis of the above description. It is not necessary and impossible to enumerate all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A preparation method of a boron-containing iron carbide catalyst, characterized in that, It includes the following steps A1)-A2) or includes the following step B): A1) Conduct a reduction reaction between an iron salt and sodium borohydride in an inert atmosphere to prepare a catalyst precursor; or, mix and ball-mill iron powder and elemental boron in an inert atmosphere to prepare a catalyst precursor; A2) Heat and react the catalyst precursor in a gas containing at least CO and / or C2H4 at 200–450 °C to obtain the boron-containing iron carbide catalyst, and the gas does not contain oxygen; Or, B) Mix and ball-mill iron powder, elemental boron and elemental carbon in an inert atmosphere to prepare the boron-containing iron carbide catalyst; In the boron-containing iron carbide catalyst, it includes boron-containing iron carbide formed by boron atoms substituting some carbon atoms in iron carbide. In the boron-containing iron carbide, the atomic ratio of carbon atoms to boron atoms is y - n:n, where the value of n:y is 0.001 - 0.

1.

2. The preparation method according to claim 1, wherein, In step A1), in the catalyst precursor, the molar ratio of iron element to boron element is 100:0.1 - 100:

2.

3. The preparation method according to claim 1, wherein In step A1), the iron salt is a soluble iron salt, and the iron in the iron salt is Fe 2+ and / or Fe 3+ .

4. The preparation method according to claim 1, characterized in that, In step A1), the iron salt is selected from one or more of iron hydrochlorides, sulfates, nitrates, acetates, citrates and their hydrates.

5. The preparation method according to any one of claims 1-4, characterized in that, In step A1), the reduction reaction is carried out at 5 - 60 °C.

6. The preparation method according to claim 5, characterized in that, In step A1), the reaction time of the reduction reaction is 15 - 60 minutes.

7. The preparation method according to any one of claims 1-4, characterized in that, In step A1), the reduction reaction is carried out in the presence of a solvent, and the solvent is selected from one or more of water, ethanol, ethylene glycol, polyethylene glycol; Step A1) also includes the steps of separating and washing the product obtained from the reduction reaction.

8. The preparation method according to any one of claims 1-4, characterized in that, In step A1) or step B), the ball-milling is carried out until the particle size of the material to be ball-milled is reduced to below 100 nm.

9. The preparation method according to any one of claims 1-4, characterized in that, In step A1) or step B), the conditions of the ball-milling include: the mass ratio of the grinding medium to the material to be ball-milled is 5:1 - 20:1, the ball-milling speed is 250 - 600 rpm, and the ball-milling time is 0.5 - 40 hours.

10. The preparation method according to any one of claims 1-4, characterized in that, In step B), based on the amounts of the three elements of iron, carbon and boron, the molar ratio is 100:33 - 50:0.05 - 2.

11. The preparation method according to any one of claims 1-4, characterized in that, In step A2), in the gas, the content of CO is 0 - 100% (v / v), the content of C2H4 is 0 - 100% (v / v), and the total content of CO and C2H4 is not less than 1% (v / v), and the gas may also optionally contain H2.

12. The preparation method according to claim 11, characterized in that, In step A2), the space velocity is 500 - 10,000 h -1 ; And / or, in the reaction system of step A2), the total pressure is 0.1 - 5.0 MPa; And / or, in step A2), the heating reaction is carried out by heating to 200–450 °C at a heating rate of 0.5–20 °C / min.

13. The preparation method according to any one of claims 1-4, characterized in that, Before step A2), it may also optionally include a step of pretreating the catalyst precursor, and the pretreatment step includes: treating the catalyst precursor at 200 - 600 °C for 0.5 - 24 hours in a hydrogen and / or nitrogen atmosphere.

14. A boron-containing iron carbide catalyst prepared by the preparation method according to any one of claims 1 - 13.

15. The iron carbide catalyst containing boron according to claim 14, characterized in that, In the boron-containing iron carbide catalyst, it may optionally contain inevitable impurities, and the impurities include oxides of Fe and / or B.

16. Use of the boron-containing iron carbide catalyst prepared by the preparation method according to any one of claims 1-13 or the boron-containing iron carbide catalyst according to any one of claims 14-15 as a catalyst, characterized in that, It is used as a catalyst in reactions with CO and hydrogen as intermediate reactants.

17. The application according to claim 16, characterized in that, The boron-containing iron carbide catalyst is used as a catalyst in the Fischer-Tropsch synthesis reaction.

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