CN116984008A - Boron-containing iron carbide catalyst and preparation method and application thereof - Google Patents

Abstract

The invention provides a boron-containing iron carbide catalyst and its preparation method and application. The catalyst has improved activation ability of carbon monoxide molecules, high Fischer-Tropsch catalytic activity, and can significantly improve C5+ product selectivity. The preparation method of the boron-containing iron carbide catalyst includes the following steps A1)-A2) or the following step B): A1) performing a reduction reaction on iron salt and sodium borohydride in an inert atmosphere to prepare a catalyst precursor; or, Iron powder and elemental boron are mixed and ball milled in an inert atmosphere to prepare a catalyst precursor; A2) The catalyst precursor is heated and reacted in a gas containing at least CO and/or C ₂ H ₄ at 200-450°C to obtain the A boron-containing iron carbide catalyst, 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

Boron-containing iron carbide catalyst and its preparation method and application

Technical field

The invention relates to the technical field of Fischer-Tropsch synthesis, and in particular to a boron-containing iron carbide catalyst and its preparation method and application.

Background technique

The Fischer-Tropsch synthesis reaction is a catalytic process that hydrogenates carbon monoxide and converts it into long-chain hydrocarbons. It plays an important role in current and future energy conversion and utilization. Among traditional Fischer-Tropsch synthesis catalysts, iron-based catalysts have the characteristics of low cost, high activity, and wide distribution of reaction products, and have been successfully used in industrial applications of Fischer-Tropsch synthesis.

In traditional methods of adding boron-based additives to catalysts, such as boric acid solution impregnation, co-precipitation, etc., a catalyst with boron oxide supported on the iron carbide surface is usually obtained after roasting and reduction carbonization of the catalyst.

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

Disclosed in the document "Effect of boron promotion on the stability of cobalt Fischer-Tropsch catalysts, J. Catal. 280, 50–59 (2011)) (Document 2), The catalyst prepared by dissolving cobalt nitrate and boric acid together on a catalyst carrier is less likely to produce carbon deposits on the surface than a boron-free Fischer-Tropsch synthesis catalyst and has a longer catalyst life.

CN101767010B (Document 3) discloses a highly wear-resistant iron-based catalyst used in a slurry bed reactor and a preparation method thereof. It provides an iron-based Fischer-Tropsch synthesis catalyst prepared by a coprecipitation method. The catalyst Better wear resistance in slurry bed reactors.

It is disclosed in the document "Promotive effect of boron oxide on the iron-based catalysts for Fischer-Tropsch synthesis, Fuel281, 118714 (2020)) (Document 4) that boron oxide is used as An additive that can hinder the reduction of iron-based catalysts from oxides to metals. When used in Fischer-Tropsch synthesis reactions, the surface area of carbon is reduced and the stability is improved.

It is given in the document "Insight of boroninduced single-step synthesis of short-chain olefins from bio-derived syngas, Fuel263, 116663 (2020)) (Document 5) A method for preparing boron-containing iron-based catalysts by hydrothermal method is proposed. The selectivity of low-carbon olefin products on the iron-based Fischer-Tropsch synthesis catalyst prepared by this method is improved.

At present, boron is widely used in cobalt-based catalysts as a Fischer-Tropsch synthesis catalyst promoter. For example, in the above-mentioned Documents 1 and 2, adding boron promoters to cobalt catalysts improves stability, activity, and C5+ product selectivity. Among them, boron has a competitive adsorption relationship with carbon on the surface of the cobalt-based catalyst, which is a key factor in extending the life of the boron-containing cobalt-based catalyst.

The active phases of iron-based catalysts and cobalt-based catalysts are fundamentally different. The active phase of iron-based catalysts is iron carbide, and carbon atoms fill the vacancies formed by iron atoms. There are obvious differences in the interaction between boron and metallic cobalt and iron carbide, so the method of boron modification of cobalt-based catalysts cannot be directly applied to iron-based catalysts.

Since the carbon atom itself has a certain electron-attracting ability, the iron atoms on the surface of the iron carbide will be partially positively charged (the Pauling electronegativity of carbon is 2.5 and that of iron is 1.8), resulting in the π reaction of iron atoms to CO during the CO activation process. The ability of the bond orbital to transfer electrons decreases, which hinders the dissociation of CO on iron carbide. Generally speaking, creating carbon defect sites on the surface of iron carbide and adding electron-donating aids can promote CO dissociation. However, carbon defects cannot exist stably. As CO is activated, they will be filled by new carbon atoms. If electron-donating aids are used, Agents, such as K-containing compounds, will not only improve the Fischer-Tropsch synthesis reaction activity of the catalyst, but also increase CO ₂ selectivity.

The above-mentioned Documents 3 and 5 use boron additives to improve the performance of iron-based Fischer-Tropsch synthesis catalysts. In them, boron exists in the form of boron oxide, which mainly improves the catalyst's anti-wear and anti-carbon deposition capabilities. However, in Document 4, Boron exists in the form of most oxides and a small amount of zero-valent boron, which improves the selectivity of low-carbon olefins.

Contents of the invention

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

In order to achieve its purpose, the present invention provides the following technical solutions:

In one aspect, 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):

A1) Perform a reduction reaction of iron salt and sodium borohydride in an inert atmosphere to prepare a catalyst precursor; alternatively, mix iron powder and elemental boron by ball milling in an inert atmosphere to prepare a catalyst precursor; A2) Prepare the catalyst precursor in an inert atmosphere The boron-containing iron carbide catalyst is obtained by heating the reaction at 200-450°C in a gas containing at least CO and/or C ₂ H ₄ , 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 some embodiments, in step A1), the ratio of the amounts of iron and boron in the catalyst precursor is 100:0.1-100:2.

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 iron hydrochloride, sulfate, nitrate, One or more of acetate, citrate and their hydrates.

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

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

Step A1) also includes the steps of separating and washing the product obtained by the reduction reaction.

In some embodiments, in step A1) or step B), the ball milling is performed until the particle size of the ball-milled material is reduced to less than 100 nm.

In some embodiments, in step A1) or step B), the ball milling conditions 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 250-600rpm, and the ball milling time is 0.5 -40 hours.

In some embodiments, in step B), the amount of each raw material is calculated based on the three elements of iron, carbon, and boron, and the ratio of the amount of materials is 100:33-50:0.05-2.

In some embodiments, in step A2), the CO content in the gas is 0-100% (v/v), the C ₂ H ₄ content is 0-100% (v/v), and CO and C ₂ The total content of H ₄ is not less than 1% (v/v), and the gas optionally contains H ₂ ;

Preferably, in step A2), the volume space velocity is 500-10000h ^-1 ;

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

Preferably, in step A2), the heating reaction is performed by raising the temperature to 200-450°C at a heating rate of 0.5-20°C/min.

In some embodiments, before step A2), a step of pre-treating the catalyst precursor is also optionally included. The pre-treatment step includes: pre-treating the catalyst precursor in a hydrogen and/or nitrogen atmosphere. The body is treated at 200-600℃ for 0.5-24 hours.

The present invention also provides a boron-containing iron carbide catalyst prepared by the above-mentioned preparation method.

In some embodiments, the boron-containing iron carbide catalyst includes a boron-containing iron carbide formed by replacing part of the carbon atoms in the iron carbide with boron atoms. Preferably, in the boron-containing iron carbide, carbon atoms and boron The ratio of the atomic number of atoms is y-n:n, where n:y takes a value of 0.001-0.1.

In some embodiments, the boron-containing iron carbide catalyst optionally contains unavoidable impurities, and the impurities include oxides of Fe and/or B.

The present invention also provides the use of the boron-containing iron carbide catalyst prepared by the above-described preparation method or the above-described boron-containing iron carbide catalyst as a catalyst for the intermediate reaction of CO and hydrogen. In the reaction of substances, for example, it is used as a catalyst in Fischer-Tropsch synthesis reaction.

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

The boron-containing iron carbide catalyst provided by the invention has improved activation ability of carbon monoxide molecules, high Fischer-Tropsch catalytic activity, and can significantly improve the selectivity of C5+ products.

Description of drawings

Figure 1 is the XRD pattern of the catalyst obtained in Example 1.

Figure 2 is the XRD pattern of the catalyst obtained in Example 2.

Figure 3 is the XRD pattern of the catalyst obtained in Example 5.

Detailed ways

In order to facilitate understanding of the present invention, the present invention will be further described below in conjunction with examples. It should be understood that the following examples are only for a better understanding of the present invention, and do not mean that the present invention is limited to the following examples.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

The endpoints of ranges and any values disclosed herein are not limited to the precise range or value, but these ranges or values are to be understood to include values approaching such ranges or values. For numerical ranges, the endpoint values of each range, the endpoint values of each range and individual point values, and the individual point values can be combined with each other to obtain one or more new numerical ranges. These values The scope shall be deemed to be specifically disclosed herein.

Where specific experimental steps or conditions are not specified in the examples, the operations or conditions of the corresponding conventional experimental steps in this technical field may be followed. If the manufacturer of the reagents or instruments used is not indicated, they are all conventional products that can be purchased commercially.

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):

A1) Prepare a catalyst precursor by carrying out a reduction reaction of iron salt and sodium borohydride in an inert atmosphere; or, prepare a catalyst precursor by mixing iron powder and elemental boron by ball milling in an inert atmosphere;

A2) The catalyst precursor is heated and reacted at 200-450°C in a gas containing at least CO and/or C ₂ H ₄ to obtain a 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.

The preparation method provided by the invention first produces a catalyst precursor by causing a solid-phase reaction between boron and iron through reduction reaction or ball milling, in which the boron and iron are zero-valent or close to zero-valent, and then reacts with CO and/or C ₂ H ₄ is obtained by carbonization at 200–450°C, or by directly introducing carbon elements in the process of preparing the catalyst precursor by ball milling for solid-state reaction. Some of the carbon atoms in the iron carbide phase obtained by the above preparation method are replaced by boron. The inventor unexpectedly found that using such a catalyst has significantly improved Fischer-Tropsch synthesis reaction activity.

In some embodiments, in step A1), the ratio of the amounts of iron and boron in the catalyst precursor is 100:0.1-100:2.

In some embodiments, in step A1), when the iron salt and sodium borohydride are subjected to a reduction reaction in an inert atmosphere to prepare a catalyst precursor, the iron salt is specifically a soluble iron salt, and the specific type of the soluble iron salt is not particularly important. Limitations, for example, can be selected from one or more of the hydrochloride, sulfate, nitrate, acetate, citrate and hydrates thereof, where the iron in the iron salt is Fe ²⁺ and/or Fe ³⁺ . In some embodiments, the iron salt can be selected from one of the compounds consisting of Fe ²⁺ or Fe ³⁺ and Cl ^- , SO ₄ ^2- , NO ₃ ^- , acetate, citrate and the corresponding crystal hydrates or There are many kinds, such as FeCl ₃ , FeCl ₂ , Fe(NO ₃ ) ₃ , etc.

In some embodiments, in step A1), when iron salt and sodium borohydride are reduced in an inert atmosphere to prepare a catalyst precursor, in step 1), the reduction reaction is carried out at 5-60°C, and a black product is obtained. ; The reaction time is not particularly limited, as long as the black product is obtained. In some embodiments, for example, the reaction time is 15-60 minutes.

In some embodiments, in step A1), when the iron salt and sodium borohydride are subjected to a reduction reaction in an inert atmosphere to prepare a catalyst precursor, the reduction reaction is carried out in the presence of a solvent, and the solvent can be selected from water, ethanol, and ethanol. One or a combination of glycol and polyethylene glycol. Specifically, the iron salt and sodium borohydride can be dissolved in a solvent into a solution in advance and then put into the reaction system.

In some specific embodiments, step A1) also includes the step of separating and washing the product obtained by the reduction reaction to remove impurities and solvents. For example, the product is separated by centrifugation or magnet adsorption, and the centrifugal speed can be, for example, 3000-20000 rpm. For example, use absolute ethanol to wash to remove impurities and remove the solvent. In some embodiments, in step A1), the amount of iron salt and sodium borohydride is calculated based on iron element and boron element, and the ratio of the amount of substances is 1:5-1:20. Subsequently, the iron element and the amount of sodium borohydride are obtained by washing the reaction product. The catalyst precursor has a boron element material ratio of 100:0.1-100:2.

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 a catalyst precursor. Specifically, in step A1), ball milling is performed to prepare a catalyst precursor, and iron powder and elemental boron are ball milled to nanoscale, so that a solid-phase reaction occurs between iron and boron to obtain iron boride (catalyst precursor); The catalyst precursor can be obtained by ball milling until the particle size of the ball-milled material is reduced to less than 100 nm. In step B), similarly, solid-phase reaction occurs between each component through ball milling and finally a boron-containing iron carbide catalyst is obtained. Specifically, the particle size of the ball-milled material can be reduced to less than 100 nm through ball milling, and finally A boron-containing iron carbide catalyst is obtained. In some specific embodiments, the ball milling in step A1) or step B) can be carried out under the following conditions: ball milling is carried out in a ball mill, the rotation speed is 250-600 rpm, the grinding medium (such as grinding balls) and the material to be ball milled (i.e. The mass ratio of the powder to be ground) is 5:1 to 20:1, and the ball milling time is 0.5-40 hours. During the ball milling process, ball milling can be carried out intermittently to avoid overheating. For example, after ball milling for a period of time (for example, 30 minutes), the ball milling can be continued for a period of time (for example, 30 minutes). During the ball milling process, the temperature may be, for example, room temperature to 500°C.

In step A1) and step B), the inert atmosphere involved may be an inert atmosphere such as nitrogen or argon.

In step A1) or step B), when the catalyst precursor is prepared by ball milling, the iron powder used is not particularly limited, and can be, for example, but not limited to, Raney iron, reduced iron powder, etc.

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

In some embodiments, in step A2), the reaction atmosphere is a gas containing at least CO and/or C ₂ H ₄ , the CO content is 0-100% (v/v), and the C ₂ H ₄ content is 0-100% (v/v). 100% (v/v), and the total content of CO and C ₂ H ₄ is not less than 1% (v/v), the gas optionally also contains H ₂ ; in some embodiments, in step A2), The volume space velocity is 500-10000h ^-1 ; in some embodiments, in the reaction system of step A2), the total pressure is 0.1-5.0MPa; in some embodiments, the gas containing at least CO and/or C ₂ H ₄ , the partial pressure of CO and C ₂ H ₄ is 0.05-10bar, and the partial pressure of H ₂ is 0-20bar. In some embodiments, an inert gas, such as nitrogen, argon, etc., is also added to the gas containing at least CO and/or C ₂ H ₄ . In some embodiments, the reaction gas in step A2) is CO and/or C ₂ H ₄ . In some embodiments, the reaction gas in step A2) consists of hydrogen, CO and/or C ₂ H ₄ . In some embodiments, the reaction gas in step A2) consists of an inert gas, and CO and/or C ₂ H ₄ . In some embodiments, the reaction gas in step A2) consists of inert gas, H ₂ , and CO and/or C ₂ H ₄ .

All pressures mentioned in this article are gauge pressures unless otherwise stated.

In some embodiments, in step A2), the heating reaction is performed at a heating rate of 0.5-20°C/min to 200-450°C. For example, the heating reaction can be carried out from room temperature to 200-450°C according to the above-mentioned heating rate. In step A2), the reaction is heated until the tail gas composition no longer changes, for example, the tail gas composition is monitored through online chromatography, or iron carbide is generated through in-situ XRD detection, and the reaction is carried out until the crystal structure does not change; in some embodiments, in step A2), the heating is performed Reaction time is 4-48 hours.

In some embodiments, between step A1) and step A2), that is, before step A2), a step of pretreating the catalyst precursor is optionally included, but this step is not necessary. The pretreatment steps include: treating the catalyst precursor at 200-600°C for 0.5-24 hours in a hydrogen and/or nitrogen atmosphere; the pretreatment can adjust the crystallinity of the catalyst precursor and adjust the grain size.

In some embodiments, in step B), the amount of each raw material is calculated based on the three elements of iron, carbon, and boron, and the ratio of the amount of materials is 100:33-50:0.05-2. In step B), the specific type of carbon elemental raw material is not particularly limited, and various carbon elemental raw materials can be used, such as but not limited to graphite carbon powder, activated carbon carbon powder, etc.

A 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 the catalyst, part of the carbon atoms in the iron carbide are replaced by boron atoms. In some embodiments, part of the carbon atoms in the iron carbide are replaced by boron atoms. In the boron-containing iron carbide formed by atoms, the ratio of the atomic number of carbon atoms and boron atoms is y-n:n, where the value of n:y is 0.001-0.1.

In some embodiments, the general formula of iron carbide is expressed as F _x C _{y .} In the boron-containing iron carbide catalyst _, its active component can be expressed as the general formula _F The number of atoms; the ratio of the number of atoms of carbon atoms and boron atoms is yn:n; preferably, the value of n:y is 0.001-0.1.

In some embodiments, the active component of the boron-containing iron carbide catalyst may be, but is not limited to, substituted with B by some of the C atoms in Fe ₅ C ₂ , Fe ₂ C, Fe ₇ C ₃ and Fe ₃ C. Iron carbide of different phases can be obtained by adjusting the gas composition (such as the proportion of CO, C ₂ H ₄ , etc.), pressure, and temperature in step A2), or by adjusting the carbon source (carbon elemental substance) in step B) ) and the ratio of iron source (iron powder), this is what those skilled in the art can know or understand based on the existing technology they have mastered, and will not be described in detail.

The boron-containing iron carbide catalyst of the present invention may have a specific crystal structure or may not have a specific crystal structure.

In the boron-containing iron carbide catalyst of the present invention, zero-valent boron partially replaces the carbon atoms in the iron carbide, which is significantly different from the boron-containing iron-based catalyst obtained in the prior art where boron mainly exists in the state of boron oxide. , the catalyst of the present invention does not contain or only contains a small amount of oxidized boron. Even if it does, it is an unavoidable impurity, such as caused by side reactions that inevitably occur during material transfer, transportation, storage and other links. Therefore, the boron-containing iron carbide catalyst of the present invention optionally contains unavoidable impurities. These impurities include oxides of Fe and/or B, and the oxidation states of Fe and B are in the amount of impurities (the amount of impurities refers to (not a major amount, accounting for less than 50wt%), 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.

The above-mentioned boron-containing iron carbide catalyst of the present invention can be produced by adopting the preparation method described above.

The above-mentioned boron-containing iron carbide catalyst provided by the present invention has 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 with other components. Electronic additives (such as K ₂ O, etc.) are used, and/or the boron-containing iron carbide catalyst of the present invention is supported on a carrier, such as SiO ₂ and the like.

Another aspect of the present invention also provides the use of the boron-containing iron carbide catalyst prepared by the above-described preparation method or the above-described boron-containing iron carbide catalyst as a catalyst, with CO and hydrogen as the intermediate The reaction of the reactants mainly refers to the hydrogenation of CO ₂ and the reaction of CO and H ₂ O. The former obtains CO and hydrogen through reverse water vapor shift, and the latter obtains CO and hydrogen through water vapor shift. Specifically, for example, as a catalyst in Fischer-Tropsch synthesis Applications in reactions, such as but not limited to Fischer-Tropsch synthesis reactions in fixed bed or slurry bed reactors.

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 inventors performed an exemplary theoretical calculation simulation analysis. According to the calculation simulation, the introduction of boron atoms makes the activity The orientation of the particle crystal planes of physical phase iron carbide changes, and the exposure ratio of (021), (311), (31-1) and (31-2) planes increases. CO dissociation is more likely to occur on these planes, making the catalyst The ability to activate CO has been improved. Secondly, when boron atoms replace carbon atoms, whether on the surface or subsurface of the catalyst, due to the lower electronegativity of boron compared to carbon, the iron atoms on the surface can have less positive charges, and the d electrons on the iron atoms It is easier to enter the π anti-bonding orbital of CO and activate CO, making it easier for the catalyst to activate CO.

Taking Fe ₅ C ₂ as an example, Table 1 shows the predicted changes in the exposed crystal planes of Fe ₅ C ₂ caused by boron atom lattice doping through density functional theory. The surface energy of different crystal faces is calculated through density functional theory (the data in this table is calculated using the VASP program, PAW pseudopotential, and PBE functional). According to the Wulff Construction principle, different crystal face exposure ratios can be obtained. Boron atom substitution can be obtained by changing different crystal faces. The surface energy of a surface affects its relative exposed area in the catalyst particles. As the substitution amount of boron atoms increases, the proportion of crystal faces with lower CO dissociation activation energy increases, improving the overall activity of the catalyst.

Table 1

As can be seen from Table 1, density functional theory calculations were used to prove that as the boron atom replaces the surface carbon atomic weight in Fe ₅ C ₂ increases, crystals such as (021), (311), (31-1) and (31-2) The face exposure ratio increases. These crystal faces have lower CO dissociation activation energy than the (510) crystal face. Considering that the (510) crystal face is the main exposed face of the Fe ₅ C ₂ crystal without adding boron additives (accounting for (ratio 31.6%), the introduction of boron exposes more surfaces with higher CO dissociation activity (lower activation energy), indicating that the CO activation ability of the catalyst in which boron atoms replace lattice carbon atoms is enhanced.

Table 2 shows the CO dissociation energy information after carbon atoms on the (510) surface of Fe ₅ C ₂ are replaced by boron atoms. There are two carbon atoms exposed on the (510) surface. After replacing the two carbon atoms with boron atoms, two calculation models of boron-substituted carbon 1 and boron-substituted carbon 2 were obtained. CO dissociation was performed on them respectively. Calculation and charge analysis, the results are listed in Table 2. Specifically, in Table 2, the data corresponding to boron substituting carbon 1 and boron substituting carbon 2 refers to the surface model obtained by boron substituting carbon atoms at two different positions on the (510) plane of Fe ₅ C ₂ through density functional theory. Data obtained from calculations and charge analysis.

Table 2

As can be seen from Table 2, density functional theory calculations have been used to prove 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) surface decreases, and the CO activation energy barrier decline,

During the implementation of the present invention, the inventor carried out XRD characterization of the boron-containing iron carbide catalyst obtained by the present invention and found that its morphology was significantly better than that of the iron carbide catalyst that was not incorporated with boron according to the method of the present invention. Variety. As an example, Figure 1 shows the XRD characterization results in an embodiment. It can be seen that the boron-containing iron carbide catalyst increases the proportion of (021) isocrystalline planes. The intensity of the diffraction peak at 43.5° exceeds the diffraction peak at 44.2°. The former is the (021) plane, and the latter is the (510) plane; the diffraction intensity at 44.2° in the boron-free iron carbide catalyst is stronger. Comparing it with the Fe ₅ C ₂ standard spectrum at the bottom in Figure 1, it can be confirmed that the The solution of the present invention introduces boron promoter to change the morphology of the catalyst.

The present invention will be exemplified by examples below.

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

XRD (X-ray polycrystalline diffraction) characterization: instrument model Bruker D8AX, Cu Kα;

XPS (X-ray photoelectron spectroscopy) characterization: Instrument model ThermoScientific Escalab 250Xi, Al Kα.

ICP-AES (Inductively Coupled Plasma Resonance Atomic Emission Spectroscopy) Characterization: The instrument model is SpectroArcos.

Example 1

The boron-containing iron carbide catalyst of Example 1 was prepared using the following steps:

1) Dissolve 2g of iron salt FeCl ₂ ·4H ₂ O using 50mL of solvent ethylene glycol to obtain solution A. Dissolve 3g of sodium borohydride in 15mL of deionized water to obtain solution B. Solution A is stirred with a magnet and rotated at 500rpm under nitrogen protection. Add solution B to solution A under ambient conditions, control the temperature to 50°C, and obtain a black product after a reduction reaction for 15 minutes; separate the product by centrifugation, discard the centrifuge, and wash the precipitate three times with absolute ethanol to remove impurities and solvents. Catalyst precursor is obtained. Among them, the ratio of the amount of iron element to boron element substance in the obtained catalyst precursor is 100:2.

2) Heat the catalyst precursor obtained in step 1) in CO/H ₂ gas (CO volume content 10%) to 300°C at a heating rate of 5°C/min. The reaction time is 20 hours and the total pressure is 0.1 MPa, the volume space velocity is 5000h ^-1 ; after the reaction, a boron-containing iron carbide catalyst Fe _x C _yn B _n is obtained, in which the values of x, y, and n are 5, 2, and 0.1 respectively.

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

Comparative example 1

The catalyst of Comparative Example 1 was prepared using the following steps:

Dissolve 4g of iron salt Fe(NO ₃ ) ₃ ·9H ₂ O 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 magnet at a rotation speed of 500 rpm. Add solution B to solution A, control the temperature to 50°C, and obtain a red precipitate of the product; separate the product by centrifugation, discard the centrifuge, and wash the precipitate three times with deionized water, add 0.012g of boric acid, and dry at 60°C to obtain Catalyst precursor;

Thereafter, the catalyst precursor was carbonized in the same manner as step 2) in Example 1 to obtain the catalyst Fe ₅ C ₂ ·0.05B ₂ O ₃ . The elemental composition was determined by ICP-AES combined with XRD.

The obtained catalyst was characterized by XPS. The characterization results are shown in Table 3.

Table 3 XPS characterization of boron atom valence state

192.5eV (boron oxide) 189.0eV (low-priced boron) Example 1 28.2 71.8 Comparative example 1 100 0

It can be seen from Figure 1 that the sample obtained in Example 1 has a χ-Fe ₅ C ₂ crystal structure, and its diffraction intensity in the (021) direction exceeds the (510) direction, which is similar to the standard card and common χ-Fe ₅ C ₂ crystal structure. There is a significant difference in the ratio, indicating that the exposure ratio of the crystal face of the catalyst changes, 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 face.

It can be seen from Table 3 that in the catalyst obtained by the method of the embodiment, more than 70% of the boron is in the reduced state, while in the comparative example, no reduced boron can be obtained, indicating that the former boron exists in the reduced state. Combined with the XRD characterization results, it is very likely Boron replaces some of the carbon atoms in the crystal lattice. In the latter, boron exists in an oxidized state and exists separately from the catalyst active phase χ-Fe ₅ C ₂ .

Example 2

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

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

Table 4 XPS characterization of boron atom valence state

192.5eV (boron oxide) 189.0eV (low-priced boron) Example 2 14.6 85.4

Comparative example 2

The difference from Example 2 is that no elemental boron was added.

Example 3

The boron-containing iron carbide catalyst of Example 3 was prepared using the following steps:

1) Dissolve 4g of iron salt Fe(NH ₄ ) ₂ ·(SO ₄ ) ₂ ·6H ₂ O using 60mL of solvent polyethylene glycol (molecular weight 2000) to obtain solution A, and dissolve 5g of sodium borohydride in 15mL of deionized water to obtain solution A. Solution B and solution A were stirred with a magnet at a rotation speed of 500 rpm. Add solution B to solution A under nitrogen protection environment. Control the temperature to 60°C and obtain a black product after reduction reaction for 25 minutes. Separate the product by centrifugation and discard the centrifuge. liquid, and wash the precipitate three times with absolute ethanol to remove impurities to obtain the catalyst precursor. Among them, the ratio of the amount of iron element and boron element in the obtained catalyst precursor is 100:0.67.

2) Heat the catalyst precursor obtained in step 1) in C ₂ H ₄ /H ₂ gas (C ₂ H ₄ volume content 5%) at a heating rate of 20°C/min to 400°C. The reaction time is 12 hour, the total pressure is 0.5MPa, and the volume space velocity is 3000h ^-1 ; after the reaction, a boron-containing iron carbide catalyst Fe _x C _yn B _n is obtained, in which the values of x, y, and n are 3, 1, and 0.02 respectively.

Example 4

1) Under nitrogen protection, use 20g reduced iron powder and 0.02g elemental boron for ball milling in a ball mill. The rotation speed is 600rpm. The zirconia grinding ball is 400g. After ball milling for 30 minutes, pause for 30 minutes to prevent the system from overheating. The total ball milling time is 40 hours, the particle size of the material after ball milling is less than 100nm. In the obtained catalyst precursor, the ratio of the amount of iron element to boron element material is 100:0.5

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

Example 5

Under nitrogen protection, use 20g reduced iron powder, 2.0g graphite powder, and 0.0038g elemental boron for ball milling in a ball mill. The rotation speed is 600rpm. The zirconia grinding ball is 100g. After 10 minutes of ball milling, pause for 20 minutes to prevent the system from overheating. Complete ball milling. The time is 24 hours, and the particle size of the material after ball milling is less than 100nm. The catalyst Fe _x C _yn B _n was obtained, in which the values of x, y, and n were 7, 3, and 0.01 respectively. The results of XRD and XPS characterization are shown in Figure 3 and Table 5 respectively. XRD diffraction shows a crystalline phase _{of Fe7C3} .

Table 5 XPS characterization of boron atom valence state

192.5eV (boron oxide) 189.0eV (low-priced boron) Example 5 25.6 74.4

Example 6

The boron-containing iron carbide catalyst of Example 1 was prepared using the following steps:

1) Dissolve 4g of iron salt Fe(NO ₃ ) ₃ ·9H ₂ O 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 magnet at a rotation speed of 500 rpm. Add solution B to solution A under nitrogen protection environment, control the temperature to 5°C, and obtain a black product after a reduction reaction for 50 minutes; separate the product by centrifugation, discard the centrifuge, and wash the precipitate three times with absolute ethanol to remove impurities. Catalyst precursor is obtained. Wherein, the ratio of the amount of iron element and boron element in the obtained catalyst precursor is 100:1.

2) Heat the catalyst precursor obtained in step 1) in CO/H ₂ gas (CO volume content 30%) to 200°C at a heating rate of 0.5°C/min. The reaction time is 48 hours and the total pressure is 0.1 MPa, the volume space velocity is 3000h ^-1 ; after the reaction, a boron-containing iron carbide catalyst Fe _x C _yn B _n is obtained, in which the values of x, y, and n are 2, 1, and 0.02 respectively.

Example 7

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

Catalyst performance evaluation:

In a fixed bed reactor, the catalytic reaction performance of the catalysts obtained in each example and comparative example was evaluated.

Evaluation conditions:

^aThe reaction temperature is 270°C, CO: _H2 is 1:2, the total pressure is 3.0MPa, the catalyst loading is 200mg, the gas flow is 3000mL/h, and the reaction time is 24h.

^b The reaction temperature is 260°C, CO: _H2 is 1:2, the total pressure is 2.5MPa, the catalyst loading is 600mg, the gas flow is 3000mL/h, and the reaction time is 24h.

The catalyst performance evaluation results obtained in each of the above examples and comparative examples are shown in Table 6.

Table 6

In Table 3, the analysis and detection methods used in various analyzes of the products obtained by the reaction are: online gas chromatography method to analyze CO, CH ₄ , CO ₂ and C ₂ -C ₄ hydrocarbon products, and C ₅₊ products are separated by gas-liquid Tank collections were detected using off-line gas chromatography.

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

CO conversion rate:

CH ₄ selectivity:

CO ₂ selectivity:

C ₂ -C ₄ selectivity:

C ₅₊ selectivity:

It can be seen from the results obtained in Table 6 that the catalyst obtained in the example has higher CO conversion rate and C ₅₊ product selectivity, while the selectivity of by-product methane and CO ₂ is suppressed, which is higher than the catalyst of the comparative example. Efficient Fischer-Tropsch synthesis catalysts. By comparing Example 2 and Comparative Example 2, it can be seen that adding elemental boron improves the C ₅₊ product selectivity and catalytic activity of the iron catalyst prepared by ball milling.

It is easy to understand that the above-mentioned embodiments are only examples for clear explanation, 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 based on the above description. An exhaustive list of all implementations is neither necessary nor possible. The obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims (14)

1. A process for the preparation of a boron-containing iron carbide catalyst, characterized in that it comprises the following steps A1) -A2) or comprises the following step B):

a1 Preparing a catalyst precursor by carrying out reduction reaction on ferric salt and sodium borohydride in inert atmosphere; or, mixing and ball milling iron powder and elemental boron in an inert atmosphere to prepare a catalyst precursor; a2 Will) beThe catalyst precursor contains at least CO and/or C ₂ H ₄ Heating and reacting the gas at 200-450 ℃ 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.

2. The method of claim 1, wherein in step A1), the ratio of the amount of elemental iron to the amount of elemental boron material in the catalyst precursor is from 100:0.1 to 100:2.

3. The 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⁺ For example one or more selected from the group consisting of iron hydrochloride, sulfate, nitrate, acetate, citrate and hydrates thereof.

4. A method according to any one of claims 1-3, wherein in step A1) the reduction reaction is carried out at 5-60 ℃, for example 15-60 minutes.

5. A process according to any one of claims 1 to 3, wherein in step A1) the reduction reaction is carried out in the presence of a solvent selected from one or more of water, ethanol, ethylene glycol, polyethylene glycol;

step A1) further comprises the step of separating and washing the product obtained by the reduction reaction.

6. A method according to any one of claims 1 to 3, wherein in step A1) or step B) the ball milling is performed to reduce the particle size of the material being ball milled to below 100nm.

7. A method according to any one of claims 1 to 3, wherein in step A1) or step B), the ball milling conditions include: the mass ratio of the grinding medium to the ball-milled material is 5:1-20:1, the ball milling rotating speed is 250-600rpm, and the ball milling time is 0.5-40 hours.

8. The process according to claim 1, wherein in step B), the amount of each raw material is calculated as three elements of iron, carbon and boron, and the ratio of the amounts of the substances is 100:33 to 50:0.05 to 2.

9. A process according to any one of claims 1 to 3, wherein in step A2) the CO content of the gas is 0-100% (v/v), C ₂ H ₄ Is 0-100% (v/v) and CO and C ₂ H ₄ Not less than 1% (v/v), said gas optionally further comprising H ₂ ；

Preferably, in step A2), the volume space velocity is from 500 to 10000h ^-1 ；

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

preferably, in step A2), the heating reaction is performed by raising the temperature to 200-450 ℃ at a heating rate of 0.5-20 ℃/min.

10. A method according to any one of claims 1 to 3, characterized in that prior to step A2) it further optionally comprises a step of pre-treating the catalyst precursor, said pre-treatment step comprising: the catalyst precursor is treated at 200-600 ℃ for 0.5-24 hours under hydrogen and/or nitrogen atmosphere.

11. A boron-containing iron carbide catalyst produced by the production process of any one of claims 1 to 10.

12. The boron-containing iron carbide catalyst as set forth in claim 11, wherein the boron-containing iron carbide catalyst comprises boron-containing iron carbide formed by substituting a part of carbon atoms in the iron carbide with boron atoms, preferably wherein the ratio of the number of carbon atoms to the number of boron atoms in the boron-containing iron carbide is y-n: n, wherein n: y has a value of 0.001-0.1.

13. The boron-containing iron carbide catalyst of claim 11 or 12, wherein the boron-containing iron carbide catalyst optionally contains unavoidable impurities including oxides of Fe and/or B.

14. Use of the boron-containing iron carbide catalyst produced by the production process according to any one of claims 1 to 10 or according to any one of claims 11 to 13 as a catalyst, characterized in that it is used as a catalyst in a reaction with CO and hydrogen as intermediate reactants, for example as a catalyst in a fischer-tropsch synthesis reaction.