A highly active metastable iron carbide catalyst, its preparation method and application

Abstract

The present invention belongs to the technical field of the design and synthesis of high-efficiency Fischer-Tropsch synthesis catalysts, and specifically discloses a super-high-activity iron carbide catalyst, a preparation method thereof, and an application thereof. The active phase in the catalyst is metastable iron carbide nanoparticles confined and stabilized by a two-dimensional structure, and even when operating at a high temperature of 300 °C, the high-activity iron carbide phase will not change. The preparation method of the catalyst is as follows: dissolve an iron salt, sugar powder, and a nitrogen-containing organic substance in water, then dry and carbonize in an oven, then calcine at 600-1200 °C under a nitrogen atmosphere, and finally place it in a reactor for hydrogen reduction and then carbonization to obtain the catalyst. The catalyst of the present invention shows excellent Fischer-Tropsch synthesis catalytic performance due to maintaining the metastable high-activity iron carbide phase.

Description

Technical Field

[0001] The present invention belongs to the technical field of the design and synthesis of Fischer-Tropsch synthesis catalysts, and specifically discloses a highly active metastable iron carbide catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] The Fischer-Tropsch synthesis process is an important means for catalytically converting syngas (H2 + CO) into hydrocarbon products and is a key technology for future alternative fossil energy to produce fuel oil. Iron-based catalysts have become one of the most widely used catalysts in industry due to their low methane selectivity, good anti-toxicity, and low price. Compared with cobalt-based Fischer-Tropsch synthesis catalysts, iron-based catalysts also have some disadvantages: low specific activity, high deactivation rate (sintering, carbon deposition, active phase transformation), poor mechanical strength (molten iron catalyst), and high CO2 selectivity.

[0003] The active phase of the catalyst determines the essence of the catalytic reaction performance. Different from cobalt-based Fischer-Tropsch synthesis catalysts, the phase composition of iron-based catalysts is very complex. Up to 7 phases including elemental iron, iron carbide, and iron oxide are considered to have the activity of carbon monoxide hydrogenation. Under typical Fischer-Tropsch synthesis reaction conditions of iron-based catalysts, up to 5 phases including elemental iron, iron oxide, and different types of iron carbide may coexist, including Fe7C3, χ-Fe5C2, θ-Fe3C, ε-Fe2C, and ε’-Fe 2.2 Five kinds of iron carbides including ε-Fe2C have been found during the Fischer-Tropsch synthesis reaction process. Some studies have shown that the activity of hexagonal ε-Fe2C at low temperatures (<200 °C) is 4.3 times that of χ-Fe5C2. However, ε-Fe2C can only exist stably below 200 °C. When the temperature is higher than 200 °C, it will gradually transform into χ-Fe5C2. When the temperature further increases to 450 °C, it will transform into θ-Fe3C. The stability of iron carbide decreases with the increase of carbon concentration, and the Fischer-Tropsch activity increases with the increase of carbon concentration.

[0004] Under actual reaction conditions (280 °C to 350 °C), stabilizing the metastable iron carbide active phase is not only expected to obtain a highly active iron catalyst but also provides the possibility to explore the performance differences among various iron carbides. Summary of the Invention

[0005] The present invention intends to provide a method for stabilizing the metastable ε-Fe2C phase at high temperatures to obtain a highly active iron-based Fischer-Tropsch synthesis catalyst, a preparation method thereof, and an application thereof.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A highly active metastable iron carbide catalyst, wherein the active substance of the catalyst is metastable iron carbide nanoparticles ε-Fe₂C, the outer layer of the ε-Fe₂C is confined and stabilized by a two-dimensional graphite layer, and the surface of the catalyst contains a large number of N-containing functional groups (including pyridine N, pyrrole N, graphitic N, and N oxides).

[0008] The present invention also provides a preparation method of the above catalyst, and the specific steps are as follows:

[0009] Weigh a certain amount of iron salt, sugar powder, and nitrogen-containing organic matter. After adding water and stirring evenly, transfer it to an oven at 150°C to 250°C, dry and carbonize for 4 - 30 h (most preferably dry in an oven at 200°C for 15 h). After cooling, take it out and grind it into powder. Under the protection of inert gas, calcine it in a tube furnace. After calcination, place it in a fixed-bed reactor for reduction (preferably reduce at 300 - 450°C for 1 - 10 h, most preferably reduce at 350 - 400°C for 3 h). After the reduction is completed, carbonize it at 200 - 350°C for 1 - 10 h to obtain a highly active metastable iron carbide catalyst (more preferably carbonize at 200 - 300°C for 5 - 10 h).

[0010] Further, the iron salt is one or more of iron(III) nitrate nonahydrate, ferric chloride, iron-MOF, iron phthalocyanine, and iron acetate.

[0011] Further, the sugar powder is one or more of glucose, fructose, and maltose.

[0012] Further, the nitrogen-containing organic matter is one or more of formamide, dicyandiamide, melamine, and urea.

[0013] Further, the mass ratio of the sugar powder to the nitrogen-containing organic matter is 3:(1 - 10), most preferably 3:(4 - 6), and the mass ratio of the iron salt to the sugar powder is 1:(1 - 15), most preferably 1:(1 - 2).

[0014] Further, the calcination temperature in the tube furnace is 600 - 1200°C, and the calcination time is 1 - 24 h. Preferably, the calcination temperature in the tube furnace is 650 - 900°C, and the calcination time is 3 h.

[0015] Further, the inert gas is one or more of pure nitrogen, pure helium, and pure argon.

[0016] Further, the reduction atmosphere in the fixed-bed reactor is hydrogen or a mixture of hydrogen and a balance gas, where the balance gas is one or more of nitrogen, argon, and helium, and the volume ratio of hydrogen to the balance gas is 1:10 - 5 - 1.

[0017] Further, after the reduction is completed in a fixed-bed reactor, the gas used for carbonization is CO or a mixture of CO and hydrogen, wherein the volume ratio of CO to H2 in the mixture of CO and hydrogen is 1:5 to 5:1, preferably 1:(1 - 3).

[0018] The present invention also provides the application of the above high-activity metastable iron carbide catalyst in the Fischer-Tropsch synthesis reaction.

[0019] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0020] The present invention uses powdered sugar as a carbon source and a nitrogen-containing organic compound soluble in water as a nitrogen source. It dissolves them together with an iron salt in water to fully mix the metal salt ions and the carbon source, and then through carbonization, by using the complexation bonds and hydrogen bonds of different precursors, the regulation of the molecular self-assembly process is achieved. Then through calcination, at high temperature, iron ions will be reduced by carbon to form iron particles. The iron particles catalyze the surrounding carbon atoms to form a graphite coating layer and form θ-Fe3C particles. The particles are evenly embedded in the graphite carbon. The material obtained by this method has a rich pore structure, the particles are very evenly dispersed, the surface is rich in N functional groups, and the special encapsulation structure can effectively prevent particle agglomeration. More importantly, the special structure can make the phase transition process of Fe species occur in the confined environment of the graphite layer. Under a hydrogen atmosphere, θ-Fe3C is reduced to Fe, and then carbonized to iron carbide with a high carbon content in a syngas atmosphere. Protected by the two-dimensional structure, the metastable iron carbide active phase can be stable at high temperature.

[0021] The present invention uses the graphite layer structure to confine iron particles, improving the specific activity of the iron-based Fischer-Tropsch catalyst, and the FTY value can reach 500 μmol CO g Fe -1 s -1 。 Description of the Drawings

[0022] Figure 1 It is a high-resolution transmission electron micrograph of the graphite layer-confined iron carbide catalyst prepared in Example 1. It can be seen that the iron carbide is surrounded by 2 - 3 layers of graphite layers.

[0023] Figure 2 It is an XRD pattern of Fe3C@C prepared in Example 1 after hydrogen reduction under syngas atmospheres at different temperatures. It can be seen that under the protection of the graphite layer, the iron species are converted into ε-Fe2C (a kind of iron carbide species that is difficult to be stable above 250 °C) in the syngas atmosphere.

[0024] Figure 3For the ordinary iron oxide Fe2O3 catalyst and its XRD patterns after reduction and then exposure to syngas at different temperatures, it can be seen that ε-Fe2C still exists in the catalyst at 200 °C, but when the temperature rises to 300 °C, it gradually transforms into χ-Fe5C2.

[0025] Figure 4 For the N 1s XPS pattern of Fe3C@C prepared in Example 1, it can be seen that there are various N functional groups, including pyridine N, pyrrole N, graphitic N, and N oxides. Detailed implementation manners

[0026] The technical solutions of the present invention will be further elaborated through the following specific examples.

[0027]

Example 1

[0028] Dissolve 2 g of ferric nitrate nonahydrate, 4 g of glucose, and 6 g of melamine in 20 ml of deionized water, stir evenly, directly place it in an oven at 200 °C for drying for 15 h, then take it out. The obtained powder is ground and placed in a tubular furnace, and calcined at 650 °C for 3 h in a nitrogen atmosphere, and then taken out to obtain Fe3C@C. The iron loading of the obtained Fe3C@C is measured to be 22.8%. Place the obtained Fe3C@C in a fixed-bed reaction tube, reduce it at 400 °C for 3 h in a pure hydrogen atmosphere, cool down to 200 °C, then switch to syngas (H2:CO = 1, volume ratio, the same below), and carbonize at 200 °C for 10 h to obtain the graphite-layer-confined iron carbide catalyst, that is, the required highly active metastable iron carbide, and its active phase is ε-Fe2C.

[0029] The catalyst is prepared according to the above steps. The difference is that the temperature in the syngas stage after reduction is changed to 200 °C, 250 °C, and 300 °C respectively, and the treatment time in the syngas stage is 5 h. The XRD patterns of each catalyst are as Figure 2 shown. It can be seen that under the protection of the graphite layer, the active iron species are transformed into an iron carbide species - ε-Fe2C that is difficult to be stable above 250 °C in the syngas atmosphere.

[0030] The commercial ordinary iron oxide Fe2O3 catalyst is reduced according to the above conditions, and then carbonized at 200 °C and 300 °C syngas for 5 h respectively. Its XRD pattern is shown in Figure 3 , and it can be seen that ε-Fe2C still exists in the catalyst at 200 °C, but when the temperature rises to 300 °C, it gradually transforms into χ-Fe5C2.

[0031]

Example 2

[0032] Dissolve 3 g of ferric nitrate nonahydrate, 3 g of glucose, 3 g of urea, and 2 g of melamine in 20 ml of deionized water, stir evenly, and directly place it in an oven at 200 °C for drying for 15 h. After that, grind the obtained powder and place it in a tubular furnace. After calcining at 650 °C for 3 h under a nitrogen atmosphere, take it out to obtain Fe3C@C. Test the iron loading amount in the obtained Fe3C@C to be 31.4%. Place the obtained Fe3C@C in a fixed-bed reaction tube, reduce it at 350 °C for 3 h under a pure hydrogen atmosphere, switch to syngas (H2:CO = 1) after cooling to 200 °C, and carbonize it at 200 °C for 10 h to obtain a graphite-layer-confined iron carbide catalyst.

[0033]

Example 3

[0034] Dissolve 3 g of ferric nitrate nonahydrate, 3 g of glucose, 3 g of urea, and 2 g of dicyandiamide in 20 ml of deionized water, stir evenly, and directly place it in an oven at 200 °C for drying for 15 h. After taking out the powder, grind it and place it in a tubular furnace. After calcining at 800 °C for 3 h under a nitrogen atmosphere, take it out to obtain Fe3C@C. Test the iron loading amount in the obtained Fe3C@C to be 33.7%. Place it in a fixed-bed reaction tube, reduce it at 350 °C for 3 h under a pure hydrogen atmosphere, switch to syngas (H2:CO = 1) after cooling to 200 °C, and carbonize it at 200 °C for 10 h to obtain a graphite-layer-confined iron carbide catalyst.

[0035]

Example 4

[0036] Dissolve 3 g of ferric nitrate nonahydrate, 3 g of maltose, 3 g of melamine, and 2 g of dicyandiamide in 20 ml of deionized water, stir evenly, and directly place it in an oven at 200 °C for drying for 15 h. After taking out the powder, grind it and place it in a tubular furnace. After calcining at 800 °C for 3 h under a nitrogen atmosphere, take it out to obtain Fe3C@C. Test the iron loading amount in the obtained Fe3C@C to be 32.5%. Place it in a fixed-bed reaction tube, reduce it at 350 °C for 3 h under a pure hydrogen atmosphere, switch to syngas (H2:CO = 2) after cooling to 200 °C, and carbonize it at 200 °C for 10 h to obtain a graphite-layer-confined iron carbide catalyst.

[0037]

Example 5

[0038] Dissolve 3 g of ferric nitrate nonahydrate, 3 g of maltose, 3 g of melamine, and 1 g of dicyandiamide in 20 ml of deionized water, stir evenly, and directly place it in an oven at 200 °C for drying for 15 h. After taking out the powder, grind it and place it in a tubular furnace. After calcining at 750 °C for 3 h under a nitrogen atmosphere, take it out to obtain Fe3C@C. Test the iron loading amount in the obtained Fe3C@C to be 31.3%. Place it in a fixed-bed reaction tube, reduce it at 350 °C for 3 h under a pure hydrogen atmosphere, switch to syngas (H2:CO = 3) after cooling to 200 °C, and carbonize it at 200 °C for 10 h to obtain a graphite-layer-confined iron carbide catalyst.

[0039]

Example 6

[0040] Dissolve 4 g of ferric nitrate nonahydrate, 4 g of fructose and 6 g of melamine in 20 ml of deionized water, stir evenly, directly place it in an oven at 200 °C for drying for 15 h, then take out the powder, grind it and place it in a tubular furnace. After calcining at 750 °C for 3 h under a nitrogen atmosphere, take out to obtain Fe3C@C. The iron loading in the obtained Fe3C@C is tested to be 38.7%. Place it in a fixed-bed reaction tube, reduce it at 400 °C for 3 h under a pure hydrogen atmosphere, switch to syngas (H2:CO = 1) after cooling to 200 °C, and carbonize it at 200 °C for 10 h to obtain a graphite-layer-confined iron carbide catalyst.

[0041]

Example 7

[0042] Dissolve 3 g of iron phthalocyanine, 3 g of glucose and 6 g of urea in 20 ml of deionized water, stir evenly, directly place it in an oven at 200 °C for drying for 15 h, then take out the powder, grind it and place it in a tubular furnace. After calcining at 900 °C for 3 h under a nitrogen atmosphere, take out to obtain Fe3C@C. The iron loading in the obtained Fe3C@C is tested to be 36.4%. Place it in a fixed-bed reaction tube, reduce it at 350 °C for 3 h under a pure hydrogen atmosphere, switch to syngas (H2:CO = 1) after cooling to 200 °C, and carbonize it at 200 °C for 10 h to obtain a graphite-layer-confined iron carbide catalyst.

[0043]

Example 8

[0044] Dissolve 3 g of iron phthalocyanine, 3 g of glucose, 1 g of urea and 3 g of melamine in 20 ml of deionized water, stir evenly, directly place it in an oven at 200 °C for drying for 15 h, then take out the powder, grind it and place it in a tubular furnace. After calcining at 900 °C for 3 h under a nitrogen atmosphere, take out to obtain Fe3C@C. The iron loading in the obtained Fe3C@C is tested to be 34.9%. Place it in a fixed-bed reaction tube, reduce it at 350 °C for 3 h under a pure hydrogen atmosphere, switch to syngas (H2:CO = 1) after cooling to 200 °C, and carbonize it at 200 °C for 10 h to obtain a graphite-layer-confined iron carbide catalyst.

[0045]

Example 9

[0046] Dissolve 3 g of ferrous acetate, 3 g of glucose, 1 g of urea and 3 g of melamine in 20 ml of deionized water, stir evenly, directly place it in an oven at 200 °C for drying for 15 h, take out the powder, grind it and place it in a tubular furnace, and calcine it at 900 °C for 3 h in a nitrogen atmosphere, then take it out to obtain Fe3C@C (the iron loading amount measured is 35.5%). Place it in a fixed-bed reaction tube, reduce it at 350 °C for 3 h in a pure hydrogen atmosphere, cool down to 200 °C, then switch to syngas (H2:CO = 1), and carbonize it at 200 °C for 10 h to obtain a graphite-layer-confined iron carbide catalyst.

[0047]

Comparative Example 1

[0048] Dissolve 2 g of ferric nitrate nonahydrate, 4 g of glucose and 6 g of melamine in 20 ml of deionized water, stir evenly, directly place it in an oven at 200 °C for drying for 15 h, take out the powder, grind it and place it in a tubular furnace, calcine it at 800 °C for 3 h in a nitrogen atmosphere, then take it out, and then calcine it in a muffle furnace at 400 °C in an air atmosphere for 4 h to obtain Fe3C@C. The iron loading amount of the obtained Fe3C@C is measured to be 57.6%. Place it in a fixed-bed reaction tube, reduce it at 400 °C for 3 h in a pure hydrogen atmosphere, cool down to 300 °C, then switch to syngas (H2:CO = 1), and carbonize it at 300 °C for 10 h to obtain a catalyst.

[0049] Perform Fischer-Tropsch synthesis performance tests on the catalysts in the examples and Comparative Example 1. The reactor is a fixed-bed reactor, and the reaction conditions are as follows: the pressure is 1.0 MPa; the syngas ratio is H2 / CO = 1.0; the reaction space velocity is 30 L / (h·g cat .); the reaction temperature is 300 °C.

[0050] Table 1

[0051]

Claims

1. A preparation method of a highly active metastable iron carbide catalyst, the specific steps are as follows: Dissolve 4 g of iron(III) nitrate nonahydrate, 4 g of fructose and 6 g of melamine in 20 ml of deionized water, stir evenly, directly place it in an oven at 200 °C for drying for 15 h, take out the powder, grind it and place it in a tube furnace, and calcine it at 750 °C for 3 h in a nitrogen atmosphere, then take it out to obtain Fe3C@C; place it in a fixed-bed reaction tube, reduce it at 400 °C for 3 h in a pure hydrogen atmosphere, switch to syngas after cooling to 200 °C, and carbonize it at 200 °C for 10 h to obtain a highly active metastable iron carbide catalyst; The syngas is a mixture of CO and hydrogen with a volume ratio of 1:

1.

2. Application of the highly active metastable iron carbide catalyst prepared by the preparation method described in Claim 1 in the Fischer-Tropsch synthesis reaction.

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