A highly stable Fischer-Tropsch synthesis iron-based catalyst and its use

Abstract

The present invention discloses a highly stable Fischer-Tropsch synthesis iron-based catalyst and its use. The Fischer-Tropsch synthesis iron-based catalyst disclosed in the present invention is a catalyst with surface carbon modification, comprising a co-precipitated or impregnated catalyst of the Fe / Cu / K / Si or Fe / Mn / K / Si series and a carbonized layer coated on the SiO2 surface of the co-precipitated or impregnated catalyst. In the present invention, organic groups are introduced onto the surface of the catalyst silica by an organosilane reagent, and the organic groups are carbonized under appropriate calcination conditions to form a carbonized layer on the surface of the catalyst silica and at the iron-silica phase interface, inhibiting the transformation of iron carbide to Fe3O4 during the reaction process, inhibiting the CO2 selectivity of the catalyst, and simultaneously improving the reaction activity and long-term stability of the catalyst.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and relates to a highly stable Fischer-Tropsch synthesis iron-based catalyst and its use. Background Art

[0002] Fossil fuels represented by coal and petroleum are the main energy sources currently used by humans. The Fischer-Tropsch synthesis technology is a synthesis method that processes raw materials such as coal, natural gas, or biomass to generate syngas (CO and H2), and then converts it into liquid fuels through the catalytic action of a catalyst. This technology can not only effectively reduce air pollution, but also provide a new way for the development of alternative oil resources, and thus has attracted more and more attention.

[0003] Iron, cobalt, nickel, ruthenium, and rhodium are all active metals for catalyzing the Fischer-Tropsch synthesis reaction. Compared with other metal-based catalysts, the Fischer-Tropsch synthesis iron-based catalyst has the advantages of low price, good toxicity resistance, and wide reaction conditions.

[0004] In the iron-based Fischer-Tropsch synthesis reaction, about 50% of CO is converted into unwanted C1 by-products (CO2 and CH4), which greatly increases the energy consumption during the production process and reduces the product yield. How to suppress the formation of C1 by-products in the Fischer-Tropsch synthesis process, thereby improving the selectivity of the target product, and ensuring that the catalyst has good stability is a challenge in the field of Fischer-Tropsch synthesis.

[0005] CN101811050B discloses an organically hydrophobic modified cobalt-based Fischer-Tropsch synthesis catalyst and its preparation method. In this technology, an organosilane reagent is first used to modify the support, and then metal Co is loaded on the modified support. After loading, the catalyst is organically hydrophobically modified again, and the modification steps are carried out in two steps. CN112007655A discloses a method for coating a hydrophobic shell on the surface of iron-manganese oxides. In this method, a silica shell is first coated on the surface of iron-manganese oxides, and then the silica is silanized for hydrophobic modification. CN107442147B discloses a preparation method for coating a graphite layer on the surface of Fe3C catalysts. The prepared Fe3C particles are evenly embedded in the graphite layer. During the preparation process, sugars are used as carbon sources to form the graphite layer, and the encapsulation structure formed by the graphite layer is used to effectively prevent the agglomeration and deactivation of Fe3C particles. CN112588320A has the same purpose as CN101811050B, which is to introduce organic groups. During the reaction process, through the hydrophobic effect of the organic groups, water is quickly discharged to avoid the oxidation of the active phase due to excessive water pressure, thereby improving the stability of the catalyst. However, although the introduction of organic groups effectively enhances the anti-water oxidation performance of the catalyst, due to the steric hindrance of the organic groups, the reaction activity of the catalyst will also be reduced. CN114054077A introduces carbon species, silica or other metal oxides to shield the strong acidic sites in the zeolite, thereby improving the selectivity of the catalyst. However, in this method, if metal oxides are used, selective adsorption will occur on different strength acidic sites during adsorption by the impregnation method, while the modification of silica by the method or hydrothermal method, the modification of carbon species by chemical vapor deposition method, and the coating method are all non-selective covering processes. To a certain extent, it is difficult to only cover the strong acidic sites, which will not only cover other acidic sites on the zeolite but also cover the active sites of the metal active phase without discrimination.

[0006] There is still a need to find a Fischer-Tropsch synthesis iron-based catalyst with high stability and good reaction activity. Summary of the Invention

[0007] The object of the present invention is to overcome the deficiencies of the prior art and provide a Fischer-Tropsch synthesis iron-based catalyst with high stability and good reaction activity.

[0008] In one aspect, the present invention provides a Fischer-Tropsch synthesis iron-based catalyst, which is a catalyst with surface carbon modification, comprising an Fe / Cu / K / Si or Fe / Mn / K / Si series co-precipitation or impregnation catalyst and a carbonized layer coated on the SiO2 surface of the co-precipitation or impregnation catalyst. The preparation method of the catalyst includes the following steps:

[0009] Using ethanol or tetrahydrofuran as a solvent, adding a silane coupling agent thereto, and optionally adding an aqueous nitric acid solution or ammonia water to obtain a silane coupling agent solution; in a rotary evaporation device, impregnating the silane coupling agent solution onto the Fe / Cu / K / Si or Fe / Mn / K / Si series coprecipitation or impregnation-type catalyst, performing rotary evaporation, treating at 30 - 50 °C for 1 - 4 h, treating at 50 - 90 °C for 1 - 4 h, and treating at 90 - 120 °C for 0.5 - 4 h; then calcining the treated catalyst in an air atmosphere or an inert atmosphere at 370 - 600 °C for 1 - 8 h to obtain the Fischer-Tropsch synthesis iron-based catalyst.

[0010] In some embodiments, for the above Fischer-Tropsch synthesis iron-based catalyst, the treatment conditions for the rotary evaporation and calcination are: treating at 30 - 40 °C for 1 - 4 h, treating at 60 - 90 °C for 1 - 4 h, and treating at 100 - 120 °C for 0.5 - 4 h; then calcining the treated catalyst in an air atmosphere or an inert atmosphere at 370 - 600 °C for 1 - 8 h to obtain the Fischer-Tropsch synthesis iron-based catalyst;

[0011] Preferably, treating at 35 °C for 1 - 4 h, treating at 60 - 80 °C for 1 - 4 h, and treating at 105 - 120 °C for 0.5 - 4 h; then calcining the treated catalyst in an air atmosphere or an inert atmosphere at 370 - 600 °C for 1 - 8 h to obtain the Fischer-Tropsch synthesis iron-based catalyst;

[0012] For example, treating at 35 °C for 1 - 2 h, treating at 60 - 80 °C for 1 h, and treating at 105 - 120 °C for 1 h; then calcining the treated catalyst in an air atmosphere or an inert atmosphere at 370 - 600 °C for 1 - 4 h to obtain the Fischer-Tropsch synthesis iron-based catalyst.

[0013] In some embodiments, for any of the above Fischer-Tropsch synthesis iron-based catalysts, the amount of the ethanol or tetrahydrofuran used is 1 - 1.05 times the amount of the solvent used for isovolumetric impregnation of the Fe / Cu / K / Si or Fe / Mn / K / Si series coprecipitation or impregnation-type catalyst.

[0014] In some embodiments, for any of the above Fischer-Tropsch synthesis iron-based catalysts, the amount of the silane coupling agent used is such that the molar ratio of the Si contained therein to the amount of SiO2 in the Fe / Cu / K / Si or Fe / Mn / K / Si series coprecipitation or impregnation-type catalyst is 1:(7 - 100).

[0015] In some embodiments, for any of the above Fischer-Tropsch synthesis iron-based catalysts, the amount of the silane coupling agent used is such that the molar ratio of the Si contained therein to the amount of SiO2 in the catalyst is 1:(7 - 90), preferably 1:(7 - 80).

[0016] In some embodiments, in any of the above-described Fischer-Tropsch synthesis iron-based catalysts, the silane coupling agent is one or more of hexamethyldisilazane, trimethylethoxysilane, dimethylethoxysilane, methylethoxysilane, triphenylethoxysilane, diphenylethoxysilane, phenylethoxysilane, vinyl ethoxysilane, and vinyl methoxysilane.

[0017] In some embodiments, in any of the above-described Fischer-Tropsch synthesis iron-based catalysts, the pH of the aqueous nitric acid solution is 5.0 - 5.5, and the dosage is in a volume ratio of (55 - 556):1×10 to ethanol 6 ; for example, when using 14.2 g of ethanol, add 10 μL of an aqueous nitric acid solution with a pH of 5.5;

[0018] The pH of the aqueous ammonia solution is 7 - 8.5, and the dosage is in a volume ratio of (11 - 112):1×10 to ethanol 5 ; for example, when using 14.2 g of ethanol, add 20 μL of an aqueous ammonia solution with a pH of 7.5;

[0019] When the silane coupling agent used is hexamethyldisilazane, the aqueous acid-base solution may not be added. This coupling agent is relatively active and can hydrolyze in the air or in the presence of trace water without the addition of an acid-base catalyst.

[0020] In some embodiments, in any of the above-described Fischer-Tropsch synthesis iron-based catalysts, the calcination conditions are as follows: When the organic group of the silane coupling agent is methyl, the calcination temperature is 370 - 400 °C in an air atmosphere and can be calcined for 1 - 2 h; the calcination temperature is 370 - 600 °C in an inert atmosphere and can be calcined for 1 - 4 h, such as 1, 1.5, 2, 2.5, 3, 3.5, or 4 h; when the organic group of the silane coupling agent is phenyl, the calcination temperature is 400 - 600 °C in an air atmosphere or an inert atmosphere and can be calcined for 1 - 3 h, such as 1, 1.5, 2, 2.5, or 3 h.

[0021] In some embodiments, in any of the above-described Fischer-Tropsch synthesis iron-based catalysts, the inert atmosphere is one or several of nitrogen, helium, and argon.

[0022] In some embodiments, in any of the above-described Fischer-Tropsch synthesis iron-based catalysts, the contents of the components in the Fe / Cu / K / Si or Fe / Mn / K / Si series of coprecipitation or impregnation-type catalysts are such that the weight ratio of the components in the catalyst satisfies Fe:Cu:K:SiO2 = 100:(0 - 10):(0.1 - 9):(5 - 50), preferably 100:(0 - 7):(2 - 7):(5 - 30).

[0023] In some embodiments, in any of the above-mentioned Fischer-Tropsch synthesis iron-based catalysts, the Fe / Cu / K / Si or Fe / Mn / K / Si series of co-precipitated or impregnated catalysts are placed in air for about 1 h to adsorb moisture in the air. Generally, the Fischer-Tropsch iron-based microsphere catalysts can adsorb 2-4 wt% of moisture in the air, and this moisture is used for the hydrolysis of silane coupling agents.

[0024] In a second aspect, the present invention provides the use of any of the above-mentioned catalysts in the catalytic Fischer-Tropsch synthesis reaction.

[0025] In some embodiments, in the above application, the conditions for the catalytic Fischer-Tropsch synthesis reaction are as follows: slurry bed reactor, space velocity 2-14 NL / g-cat / h, 210-290 °C, synthesis gas volume ratio H2:CO = 0.5-3.5, pressure = 1.2-2.8 MPa;

[0026] For example, space velocity 6000 mL / g-cat / h, 260 °C, synthesis gas volume ratio H2:CO = 2, pressure = 2.0 MPa.

[0027] When silica is used as a structural promoter, during the long-term operation of the catalyst, the active phase migrates through the strong iron-silicon interaction, which in turn causes the growth of the active phase particles of the catalyst and leads to a deterioration in the reaction performance, and the catalyst gradually deactivates. While the present invention uses an organosilane reagent to introduce organic groups on the surface of the catalyst silica for silanization modification, by controlling the silane reagent and the calcination temperature, the organic groups are carbonized to form a carbonized layer on the surface of the catalyst silica and at the iron-silicon phase interface. Therefore, the following beneficial effects are achieved:

[0028] (1) Avoid the steric hindrance effect of organic groups, which is beneficial to the re-adsorption of α-olefins in Fischer-Tropsch synthesis and effectively maintains the formation of long-chain alkanes in the oil product. Compared with the prior art, the purpose of the present invention is not to retain organic groups for their hydrophobicity, but to use silane coupling agents to make organic groups act on the silica structural promoter to avoid their coverage of the iron phase.

[0029] (2) The alkanes generated in the Fischer-Tropsch reaction can be adsorbed on the surface of the carbonized layer, resulting in a certain hydrophobic effect. Therefore, it has a good antioxidant effect, slows down the process of the active phase of iron carbide being oxidized by the product water, inhibits the transformation of iron carbide to Fe3O4 during the reaction process, and inhibits the CO2 selectivity of the catalyst.

[0030] (3) At the same time, the carbonized layer on the silicon surface has a weak interaction with iron, iron oxide or iron carbide, changing the strong iron-silicon interaction to a weak iron-carbon interaction, avoiding sintering caused by the migration of the iron active phase due to the strong iron-silicon interaction at the reaction temperature, and improving the reaction activity and long-term stability of the catalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The CO2 desorption peak obtained by temperature-programmed oxidation after calcining the phenyl-modified catalyst of Example 1 at 500 °C.

[0032] Figure 2 The CO2 desorption peak obtained by temperature-programmed oxidation after calcining the methyl-modified catalyst of Example 2 at 370 °C. Detailed implementation manners

[0033] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods.

[0034] Unless otherwise specified, the materials, reagents, etc. used in the following examples can all be obtained from commercial channels.

[0035] The present invention will be further described below in conjunction with specific examples. It should be understood that the following examples are only used to illustrate the present invention and not to limit the scope of the present invention.

[0036] The deactivation rate of the catalyst refers to the rate of decrease in the reaction activity (carbon monoxide conversion rate) of the catalyst from the stable reaction period to the end of the reaction, and the unit is % / d.

[0037] Example 1

[0038] Take 20 g of the coprecipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1), in which the SiO2 content accounts for 12% of the total mass of the catalyst. Use 14.2 g of ethanol and 0.14 g of phenyltriethoxysilane (the molar ratio of Si contained therein to the SiO2 in the Fe / Cu / K / Si catalyst is 1:68.6) to prepare a solution, and add 10 μL of nitric acid aqueous solution with a pH value of 5.5, and mix evenly to obtain a silane coupling agent solution. Put the catalyst into the flask of the rotary evaporation device, impregnate the prepared silane coupling agent solution onto the catalyst in an equal volume, and turn on the rotary evaporation device, treat at 35 °C for 1 h, raise the temperature to 70 °C and treat for 1 h, and then raise the temperature to 120 °C and treat for 1 h. Put the treated catalyst into a muffle furnace and calcine at 500 °C in an air atmosphere for 1 h to obtain the carbonization-modified catalyst 1.

[0039] Example 2

[0040] Take 20 g of the co-precipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1), where the SiO₂ content accounts for 12% of the total mass of the catalyst. Place the catalyst in air for 1 h. Use 14.2 g of ethanol and 0.067 g of ethoxytrimethylsilane (the molar ratio of Si contained therein to the SiO₂ in the Fe / Cu / K / Si catalyst is 1:70) to prepare a solution, and add 10 μL of a nitric acid aqueous solution with a pH value of 5.5. Mix evenly to obtain a silane coupling agent solution. Place the catalyst in the flask of a rotary evaporation device, impregnate the prepared silane coupling agent solution onto the catalyst in an equal volume, and turn on the rotary evaporation device. Treat at 35 °C for 2 h, raise the temperature to 60 °C and treat for 1 h, and then raise the temperature to 110 °C and treat for 1 h. Place the treated catalyst in a tubular furnace and calcine at 370 °C in a nitrogen atmosphere for 2 h to obtain the carbonization-modified catalyst 2.

[0041] Example 3

[0042] Take 20 g of the co-precipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1), where the SiO₂ content accounts for 12% of the total mass of the catalyst. Use 14.2 g of ethanol and 0.20 g of phenyltriethoxysilane (the molar ratio of Si contained therein to the SiO₂ in the Fe / Cu / K / Si catalyst is 1:48) to prepare a solution, and add 20 μL of an ammonia water with a pH value of 7.5. Mix evenly to obtain a silane coupling agent solution. Place the catalyst in the flask of a rotary evaporation device, impregnate the prepared silane coupling agent solution onto the catalyst in an equal volume, and turn on the rotary evaporation device. Treat at 35 °C for 1 h, raise the temperature to 80 °C and treat for 1 h, and then raise the temperature to 120 °C and treat for 1 h. Place the treated catalyst in a muffle furnace and calcine at 400 °C in an air atmosphere for 1 h to obtain the carbonization-modified catalyst 3.

[0043] Example 4

[0044] Take 20 g of the co-precipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1), where the SiO₂ content accounts for 12% of the total mass of the catalyst. Place the catalyst in air for 1 h. Use 14.2 g of ethanol and 0.29 g of ethoxytrimethylsilane (the molar ratio of Si contained therein to the SiO₂ in the Fe / Cu / K / Si catalyst is 1:16.7) to prepare a solution, and add 20 μL of an ammonia water with a pH value of 7.5. Mix evenly to obtain a silane coupling agent solution. Place the catalyst in the flask of a rotary evaporation device, impregnate the prepared silane coupling agent solution onto the catalyst in an equal volume, and turn on the rotary evaporation device. Treat at 35 °C for 2 h, raise the temperature to 65 °C and treat for 1 h, and then raise the temperature to 115 °C and treat for 1 h. Place the treated catalyst in a tubular furnace and calcine at 600 °C in a nitrogen atmosphere for 4 h to obtain the carbonization-modified catalyst 4.

[0045] Example 5

[0046] Take 20 g of the co-precipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1), where the SiO₂ content accounts for 12% of the total mass of the catalyst. Use 14.2 g of ethanol and 0.43 g of phenyltriethoxysilane (the molar amount of Si it contains is in a ratio of 1:22.2 to the molar amount of SiO₂ in the Fe / Cu / K / Si catalyst) to prepare a solution, and add 10 μL of nitric acid aqueous solution with a pH value of 5.5, mix evenly to obtain a silane coupling agent solution. Put the catalyst into the flask of the rotary evaporation device, impregnate the prepared silane coupling agent solution onto the catalyst in an equal volume, and turn on the rotary evaporation device. Treat it at 35 °C for 1 h, raise the temperature to 70 °C and treat for 1 h, then raise the temperature to 120 °C and treat for 1 h. Put the treated catalyst into a muffle furnace and calcine it in a nitrogen atmosphere at 600 °C for 2 h to obtain the carbonization-modified catalyst 5.

[0047] Example 6

[0048] Take 20 g of the co-precipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1), where the SiO₂ content accounts for 12% of the total mass of the catalyst. Use 14.2 g of ethanol and 0.64 g of ethoxymethyltrimethoxysilane (the molar amount of Si it contains is in a ratio of 1:7.4 to the molar amount of SiO₂ in the Fe / Cu / K / Si catalyst) to prepare a solution, and add 10 μL of nitric acid aqueous solution with a pH value of 5.5, mix evenly to obtain a silane coupling agent solution. Put the catalyst into the flask of the rotary evaporation device, impregnate the prepared silane coupling agent solution onto the catalyst in an equal volume, and turn on the rotary evaporation device. Treat it at 35 °C for 1 h, raise the temperature to 70 °C and treat for 1 h, then raise the temperature to 120 °C and treat for 1 h. Put the treated catalyst into a muffle furnace and calcine it in an air atmosphere at 400 °C for 1 h to obtain the carbonization-modified catalyst 6.

[0049] Example 7

[0050] Take 20 g of the co-precipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1), where the SiO₂ content accounts for 12% of the total mass of the catalyst. Use 14.2 g of ethanol and 0.97 g of triphenyltriethoxysilane (the molar amount of Si it contains is in a ratio of 1:12.5 to the molar amount of SiO₂ in the Fe / Cu / K / Si catalyst) to prepare a solution, and add 20 μL of ammonia water with a pH value of 7.5, mix evenly to obtain a silane coupling agent solution. Put the catalyst into the flask of the rotary evaporation device, impregnate the prepared silane coupling agent solution onto the catalyst in an equal volume, and turn on the rotary evaporation device. Treat it at 35 °C for 1 h, raise the temperature to 80 °C and treat for 1 h, then raise the temperature to 120 °C and treat for 1 h. Put the treated catalyst into a muffle furnace and calcine it in a nitrogen atmosphere at 450 °C for 3 h to obtain the carbonization-modified catalyst 7.

[0051] Example 8

[0052] Take 20 g of the co-precipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1), in which the SiO2 content accounts for 12% of the total mass of the catalyst. Use 14.2 g of ethanol and 0.1 g of methyltriethoxysilane (the molar ratio of Si contained therein to the SiO2 in the Fe / Cu / K / Si catalyst is 1:71.4) to prepare a solution, and add 20 μL of ammonia water with a pH value of 7.5, mix evenly to obtain a silane coupling agent solution. Put the catalyst into the flask of the rotary evaporation device, impregnate the prepared silane coupling agent solution onto the catalyst in an equal volume, and turn on the rotary evaporation device. Treat at 35°C for 1 h, raise the temperature to 80°C and treat for 1 h, and then raise the temperature to 120°C and treat for 1 h. Put the treated catalyst into a muffle furnace and calcine in a nitrogen atmosphere at 600°C for 1 h to obtain the carbonization-modified catalyst 8.

[0053] Example 9

[0054] Take 20 g of the co-precipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1), in which the SiO2 content accounts for 12% of the total mass of the catalyst, and place the catalyst in the air for 1 h. Use 14.2 g of ethanol and 0.20 g of hexamethyldisilazane (the molar ratio of Si contained therein to the SiO2 in the Fe / Cu / K / Si catalyst is 1:16) to prepare a solution to obtain a silane coupling agent solution. Put the catalyst into the flask of the rotary evaporation device, impregnate the prepared silane coupling agent solution onto the catalyst in an equal volume, and turn on the rotary evaporation device. Treat at 35°C for 1 h, raise the temperature to 60°C and treat for 1 h, and then raise the temperature to 105°C and treat for 1 h. Put the treated catalyst into a muffle furnace and calcine in an air atmosphere at 400°C for 2 h to obtain the carbonization-modified catalyst 9.

[0055] Comparative Example 1

[0056] (1) Weigh 10 kg of Fe(NO3)3·9H2O and 0.27 kg of Cu(NO3)2·3H2O, add 40 L of deionized water and stir to dissolve to obtain a metal salt solution; weigh 6 kg of Na2CO3, add 30 L of deionized water and stir to dissolve to obtain a precipitant solution; mix the metal salt solution and the precipitant solution in a co-current manner for co-precipitation reaction. Keep the temperature of the reaction kettle jacket at 60°C, control the pH value in the reaction kettle to 5 by adjusting the pump speeds of the two pumps respectively, the stirring rate is 90 rpm, and the reaction time is 40 min; after the co-precipitation reaction is completed, obtain a precipitate slurry and wash and filter it repeatedly with deionized water until the conductivity of the filtrate is below 1 ms / cm to obtain the first filter cake.

[0057] (2) The first filter cake was reslurried with 50 kg of deionized water, and then 1.8 kg of potassium silicate solution (SiO2 content: 20 wt%) was added. After stirring evenly, a 5 kg aqueous solution of 4.5 wt% nitric acid was prepared. The aqueous nitric acid solution and the slurry containing potassium silicate were fed in parallel while maintaining the temperature at 25 °C, pH at 6, and reaction time at 20 min. After the reaction, the slurry temperature was maintained at 25 °C for aging for 60 min, and then water was added to a total weight of 100 kg. After stirring, filtration was carried out to obtain the second filter cake;

[0058] (3) The second filter cake was reslurried with a certain amount of deionized water, and an aqueous solution containing 0.175 kg of potassium nitrate was added. After thorough pulping, a catalyst slurry with a solid content of 14 wt% was obtained. The catalyst slurry was fed into a spray dryer and spray-dried under the conditions of an inlet air temperature of 290 °C and an outlet air temperature of 115 °C to obtain a catalyst precursor. The obtained catalyst precursor was heated in a muffle furnace from room temperature to 120 °C at a rate of 15 °C / min, held at this temperature for 8 h, then heated to 550 °C at a rate of 5 °C / min, and held at 550 °C for 5 h to obtain a Fischer-Tropsch synthesis iron-based catalyst, i.e., Comparative Catalyst 1. Based on 100 wt% of the total mass of the catalyst, it contained 81.1 wt% Fe2O3, 3.0 wt% CuO, 3.9 wt% K2O, and 12 wt% SiO2.

[0059] Comparative Example 2

[0060] 20 g of the co-precipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1) was taken, in which the SiO2 content accounted for 12% of the total mass of the catalyst. 14.2 g of ethanol and 0.14 g of phenyltriethoxysilane were used to prepare a solution, and 10 μL of an aqueous nitric acid solution with a pH of 5.5 was added. After mixing evenly, a silane coupling agent solution was obtained. The catalyst was placed in the flask of a rotary evaporation device, and the prepared silane coupling agent solution was impregnated onto the catalyst in an equal volume. The rotary evaporation device was then turned on and treated at 35 °C for 1 h, heated to 70 °C for 1 h, and then heated to 120 °C for 1 h. The treated catalyst was placed in an oven and dried at 120 °C for 8 h to obtain Comparative Catalyst 2.

[0061] Comparative Example 3

[0062] Take 20 g of the co-precipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1), where the SiO2 content accounts for 12% of the total mass of the catalyst. Place the catalyst in air for 1 h. Prepare a solution with 14.2 g of ethanol and 0.067 g of ethoxytrimethylsilane, and add 10 μL of nitric acid aqueous solution with a pH value of 5.5. Mix evenly to obtain a silane coupling agent solution. Put the catalyst into the flask of the rotary evaporation device, impregnate the prepared silane coupling agent solution onto the catalyst in an equal volume, and turn on the rotary evaporation device. Treat at 35 °C for 2 h, raise the temperature to 60 °C and treat for 1 h, then raise the temperature to 110 °C and treat for 1 h. Put the treated catalyst into the oven and dry at 130 °C for 6 h to obtain Comparative Catalyst 3.

[0063] Comparative Example 4

[0064] Take 20 g of the co-precipitated and calcined Fe / Cu / K / Si catalyst (Comparative Catalyst 1), where the SiO2 content accounts for 12% of the total mass of the catalyst. Prepare a solution with 14.2 g of ethanol and 0.20 g of phenyltriethoxysilane, and add 20 μL of ammonia water with a pH value of 7.5. Mix evenly to obtain a silane coupling agent solution. Put the catalyst into the flask of the rotary evaporation device, impregnate the prepared silane coupling agent solution onto the catalyst in an equal volume, and turn on the rotary evaporation device. Treat at 35 °C for 1 h, raise the temperature to 80 °C and treat for 1 h, then raise the temperature to 120 °C and treat for 1 h. Put the treated catalyst into the oven and dry at 150 °C for 4 h to obtain Comparative Catalyst 4.

[0065] Test Example 1

[0066] Perform a temperature-programmed oxidation (TPO) test on the carbonization-modified catalyst 1 prepared in Example 1 and the carbonization-modified catalyst 2 prepared in Example 2. The catalyst dosage is 50 mg, the oxidation gas is a 5 vol% O2 - 95 vol% He mixture, the gas flow rate under standard conditions is controlled at 50 mL / min, the heating rate is 10 °C / min, and the mass spectrometry (MS) detects the desorption spectrum of the mass-to-charge ratio m / z = 44 CO2. The results are as Figure 1 and Figure 2 shown.

[0067] Figure 1 and Figure 2 show that after the catalyst modification and calcination of the present invention, carbon substances are generated on the surface.

[0068] The catalysts prepared in Examples 3 - 9 underwent the same temperature-programmed oxidation test and all showed obvious CO2 desorption peaks.

[0069] Test Example 2

[0070] The Fischer-Tropsch synthesis performance of each catalyst prepared in the examples and comparative examples was tested. The reactor was a slurry bed reactor, and the reaction conditions were as follows: space velocity 6000 mL / g-cat / h, 260 °C, synthesis gas volume ratio H2:CO = 2, pressure = 2.0 MPa. The activity data after the reaction stabilized are shown in Table 1.

[0071] Table 1

[0072] Sample CO conversion rate (%) <![CDATA[CO2 selectivity (%)]]> <![CDATA[CH4 Selectivity (%)]]> Deactivation rate (% / d) Carbonization modified catalyst 1 64.2 15.4 3.7 Steady Carbonization modified catalyst 2 63.8 14.8 3.8 Steady Carbonization modified catalyst 3 61.8 14.7 4.0 Steady Carbonization modified catalyst 4 63.0 15.1 3.9 Steady Carbonization modified catalyst 5 56.5 14.2 4.1 0.11 Carbonization modified catalyst 6 57.8 15.0 4.1 0.12 Carbonization modified catalyst 7 53.2 13.5 4.4 0.24 Carbonization modified catalyst 8 55.2 13.2 4.2 0.14 Carbonization modified catalyst 9 54.8 13.7 4.0 0.15 Comparative catalyst 1 63.4 26.8 3.9 0.26 Comparative catalyst 2 52.8 14.1 5.6 Steady Comparative catalyst 3 53.7 13.9 5.3 Steady Comparative catalyst 4 47.4 13.5 5.7 Steady

[0073] Table 1 shows that compared with the comparative catalyst 1 without silanization modification, the catalyst of the present invention has comparable CO conversion, while the CO2 selectivity is significantly reduced and the stability is significantly improved. This is because the catalyst of the present invention is calcined after silanization modification, so that a carbonized layer is formed on the silica surface and the iron-silicon phase interface. The alkanes generated in the Fischer-Tropsch reaction can be adsorbed on the surface of the carbonized layer, producing a certain hydrophobic effect, which has a good antioxidant effect and inhibits the CO2 selectivity of the catalyst. At the same time, the carbonized layer on the silicon surface has a weak interaction with iron, iron oxide or iron carbide, avoiding the sintering caused by the migration of the iron active phase due to the strong iron-silicon interaction at the reaction temperature and improving the long-term stability of the catalyst. Compared with the comparative catalysts 2-4 after silanization modification, the catalyst of the present invention has a higher CO conversion and significantly inhibits the CH4 selectivity. This is because the carbonized layer on the silicon surface avoids the sintering caused by the migration of the iron active phase due to the strong iron-silicon interaction at the reaction temperature, so it has higher reaction activity. And calcination carbonizes the organic groups on the silica surface, avoiding its steric hindrance effect and reducing the CH4 selectivity.

Claims

1. A Fischer-Tropsch synthesis iron-based catalyst, which is a catalyst with surface carbon modification, and includes one of an Fe / Cu / K / Si series coprecipitation catalyst, an Fe / Cu / K / Si series impregnated catalyst, an Fe / Mn / K / Si series coprecipitation catalyst or an Fe / Mn / K / Si series impregnated catalyst, and a carbonized layer coated on the SiO2 surface of the coprecipitation or impregnated catalyst. The preparation method of the catalyst comprises the following steps: Using ethanol or tetrahydrofuran as a solvent, adding a silane coupling agent thereto, and optionally adding an aqueous nitric acid solution or ammonia water to obtain a silane coupling agent solution; in a rotary evaporation device, impregnating the silane coupling agent solution onto the coprecipitation or impregnated catalyst, performing rotary evaporation, treating at 30 - 50 °C for 1 - 4 h, treating at 50 - 90 °C for 1 - 4 h, and treating at 90 - 120 °C for 0.5 - 4 h; then calcining the treated catalyst in an air atmosphere or an inert atmosphere at 370 - 600 °C for 1 - 8 h to obtain the Fischer-Tropsch synthesis iron-based catalyst.

2. The catalyst according to claim 1, wherein: The dosage of the ethanol or tetrahydrofuran is 1 - 1.05 times the solvent amount used for equal-volume impregnation of the coprecipitation or impregnated catalyst.

3. The catalyst according to claim 1, wherein: The dosage of the silane coupling agent is such that the molar ratio of Si contained therein to the amount of SiO2 in the coprecipitation or impregnated catalyst is 1:(7 - 100).

4. The catalyst according to any one of claims 1-3, characterized in that: The silane coupling agent is one or more of hexamethyldisilazane, trimethylethoxysilane, dimethylethoxysilane, methylethoxysilane, triphenylethoxysilane, diphenylethoxysilane, phenylethoxysilane, vinyl ethoxysilane, vinyl methoxysilane.

5. The catalyst according to any one of claims 1 to 3, characterized in that: The pH of the aqueous nitric acid solution is 5.0 - 5.5; or The pH of the ammonia water is 7 - 8.

5.

6. The catalyst according to any one of claims 1-3, characterized in that: The calcination conditions are as follows: when the organic group of the silane coupling agent is methyl, the calcination temperature in an air atmosphere is 370 - 400 °C, and the calcination temperature in an inert atmosphere is 370 - 600 °C; when the organic group of the silane coupling agent is phenyl, the calcination temperature in an air atmosphere or an inert atmosphere is 400 - 600 °C.

7. The catalyst according to any one of claims 1-3, characterized in that: The inert atmosphere is one or several of nitrogen, helium, argon.

8. The catalyst according to any one of claims 1 to 3, characterized in that: The coprecipitation or impregnated catalyst satisfies the following weight ratio: Fe:Cu:K:SiO2 = 100:(0 - 10):(0.1 - 9):(5 - 50).

9. Use of the catalyst according to any one of claims 1 - 8 in a catalytic Fischer-Tropsch synthesis reaction.

10. The use according to claim 9, wherein: The conditions for the catalytic Fischer-Tropsch synthesis reaction are: a slurry bed reactor, an airspeed of 2 - 14 L / g-cat / h, 210 - 290 °C, a synthesis gas volume ratio of H2:CO = 0.5 - 3.5, and a pressure = 1.2 - 2.8 MPa.

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