CN111889120B - Fe core-shell nano catalyst, and preparation method and application thereof - Google Patents
Fe core-shell nano catalyst, and preparation method and application thereof Download PDFInfo
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- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J27/00—Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
- B01J27/20—Carbon compounds
- B01J27/22—Carbides
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J35/00—Catalysts, in general, characterised by their form or physical properties
- B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
- B01J35/396—Distribution of the active metal ingredient
- B01J35/397—Egg shell like
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y40/00—Manufacture or treatment of nanostructures
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- C—CHEMISTRY; METALLURGY
- C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
- C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
- C10G2/00—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon
- C10G2/30—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon from carbon monoxide with hydrogen
- C10G2/32—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon from carbon monoxide with hydrogen with the use of catalysts
- C10G2/33—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon from carbon monoxide with hydrogen with the use of catalysts characterised by the catalyst used
- C10G2/331—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon from carbon monoxide with hydrogen with the use of catalysts characterised by the catalyst used containing group VIII-metals
- C10G2/332—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon from carbon monoxide with hydrogen with the use of catalysts characterised by the catalyst used containing group VIII-metals of the iron-group
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Abstract
The invention provides an Fe core-shell nano catalyst, a preparation method and application thereof, wherein the catalyst takes an Fe nanocube with a core-shell structure as an active ingredient, in the core-shell structure, a ferroferric oxide nanocube is taken as a core, and the surface of the ferroferric oxide nanocube is coated with X-Fe 2 C 5 An active phase shell layer. The catalyst has good catalytic activity and high selectivity of short-chain olefin. In the process of preparing olefin by Fischer-Tropsch technology, the conversion frequency of the supported nano catalyst is up to 1.32s ‑1 Far exceeding the highest conversion frequency of the prior reported Fischer-Tropsch olefin catalyst, and simultaneously the short-chain olefin selectivity of the catalyst can reach 63.0C% in the prepared hydrocarbon. Fe loaded on SiC in the invention 3 O 4 @χ‑Fe 5 C 2 The nanocube catalyst is the only simultaneous achievement of a conversion frequency higher than 1.0s at present ‑1 And the selectivity of short-chain olefin exceeds 60.0C percent.
Description
Technical Field
The invention relates to the technical field of catalysts, in particular to an Fe core-shell nano catalyst, a preparation method and application thereof.
Background
Short chain olefins of C2-C4 are basic carbon-based raw materials for some high value added products such as polymers, surfactants, plastics, etc. Currently, most short chain olefins are produced by thermal or catalytic cracking of petroleum derived hydrocarbons; rapid consumption of petroleum has accelerated worldwide research into the production of short chain olefins from other feedstocks. A typical petroleum substitute is synthesis gas, which is produced by oxidation ofCarbon and hydrogen are used as main components and are used as a raw material gas of chemical raw materials; which can easily pass through coal, biomass, CO 2 And natural gas.
The Fischer-Tropsch olefin reaction provides a means for producing short chain olefins directly from synthesis gas, which is a CH x (x=1, 2, 3). To achieve this goal, researchers have developed a number of Fischer-Tropsch olefin catalysts. For example, there are catalysts prepared with Fe nanocrystals supported on carbon nanofibers that have a selectivity to short chain olefins of up to 61C% but a CO conversion of less than 1% in the fischer-tropsch reaction, i.e. the catalyst has high selectivity to short chain olefins but low activity. In addition, there are catalysts for preparing iron carbide embedded porous carbon substrates, which exhibit higher activity and have conversion frequencies of up to 0.11s -1 But the selectivity to C2-C5 short chain olefins is only 28.8C%.
Although both high activity and high selectivity of existing fischer-tropsch olefin catalysts have been achieved separately, none of these reported catalysts have achieved both high activity and high selectivity.
Disclosure of Invention
In view of the above, the present application aims to provide a Fe core-shell nanocatalyst, a preparation method and an application thereof, and the nanocatalyst provided by the present invention has the advantages of good catalytic activity and high selectivity of C2-C4 olefins, and is beneficial for application in fischer-tropsch olefin reaction.
The invention provides an Fe core-shell nano catalyst, which takes an Fe nanocube with a core-shell structure as an active ingredient, wherein in the Fe nanocube core-shell structure, a ferroferric oxide nanocube is taken as a core, and the surface of the ferroferric oxide nanocube is coated with X-Fe 2 C 5 An active phase shell layer.
Preferably, the Fe core-shell nanocatalyst further comprises an inert support, and the Fe nanocube having a core-shell structure is supported on the inert support.
Preferably, the inert support is selected from one or more of silicon carbide, silica and carbon supports.
Preferably, the total content of iron in the Fe core-shell nano-catalyst is 24-32mol%.
Compared with the prior art, the catalyst provided by the invention can be expressed as Fe 3 O 4 @χ-Fe 5 C 2 The active ingredient of the nano cube catalyst is Fe with a core-shell structure 3 O 4 @χ-Fe 2 C 5 Nano-cubes of (i.e. Fe) 3 O 4 X-Fe with one layer of (020) crystal face on nano cube surface 5 C 2 An active phase. Fe according to the invention 3 O 4 @χ-Fe 5 C 2 The nanocube catalyst has high fischer-tropsch olefin selectivity due to the unique reaction pathway, i.e. the hydrogen-assisted CO dissociation pathway reduces the activation energy to activate CO. In addition, the Fe 3 O 4 @χ-Fe 2 C 5 The nanocubes have weak adsorption on C2-C4 olefins, thereby inhibiting the process of preparing paraffin by secondary hydrogenation and preparing C5+ products by carbon-carbon coupling. Therefore, the nano catalyst provided by the invention has better catalytic activity and higher C2-C4 olefin selectivity. In the Fischer-Tropsch olefin process of the embodiment of the invention, fe is loaded on SiC 3 O 4 @χ-Fe 5 C 2 The conversion frequency of the nanocube catalyst is up to 1.32s -1 Far exceeding the highest reported conversion frequency of the Fischer-Tropsch olefin catalyst (0.22 s -1 ) At the same time, the selectivity of the catalyst for C2-C4 olefin can reach 63.0C% in the prepared hydrocarbon. In particular, fe supported on SiC in the present invention 3 O 4 @χ-Fe 5 C 2 The nanocube catalyst is the only simultaneous achievement of a conversion frequency higher than 1.0s at present -1 And the C2-C4 olefin selectivity is over 60.0C percent.
The invention provides a preparation method of an Fe core-shell nano catalyst, which comprises the following steps:
s1, providing a ferroferric oxide nanocube;
s2, reducing the surface of the ferroferric oxide nanocube into an iron simple substance to obtain a precursor nanocube;
s3, includingIn the atmosphere of CO, carrying out surface reconstruction on the precursor nanocubes to form X-Fe on the surfaces of the ferroferric oxide nanocubes 2 C 5 And the active phase shell layer is optionally loaded with an inert carrier substance to obtain the Fe core-shell nano catalyst.
Preferably, in the step S1, ferric acetylacetonate is used as an iron source, and the heating reaction is performed in a solvent in the presence of 4-phenylbenzoic acid and oleic acid to obtain the ferroferric oxide nanocubes.
Preferably, the step S2 specifically includes: and (3) carrying out reduction reaction on the ferroferric oxide nanocubes in a reducing gas atmosphere, wherein the reducing gas is mixed gas of hydrogen and argon, the temperature of the reduction reaction is 300-350 ℃, and the surfaces of the ferroferric oxide nanocubes are reduced into iron simple substances to obtain precursor nanocubes.
Preferably, in the step S2, before the reduction to the elemental iron, the method further includes:
mixing and grinding the ferroferric oxide nanocubes and inert carrier substances to obtain a compound; reducing the surface of a ferroferric oxide nanocube in the compound into an iron simple substance to obtain a precursor substance loaded with a precursor nanocube;
the step S3 is as follows: carrying out surface reconstruction on the precursor substance loaded with the precursor nanocubes in an atmosphere comprising CO to form X-Fe on the surface of the ferroferric oxide nanocubes 2 C 5 And (3) an active phase shell layer to obtain the Fe core-shell nano catalyst.
Preferably, in the step S3, the precursor loaded with the precursor nanocubes is heated to 300-350 ℃ in the synthesis gas atmosphere to perform surface reconstruction, so as to form χ -Fe on the surface of the ferroferric oxide nanocubes 2 C 5 And (3) an active phase shell layer to obtain the Fe core-shell nano catalyst.
The invention provides an Fe core-shell nano-catalyst as described above or an application of the Fe core-shell nano-catalyst obtained by the preparation method in a Fischer-Tropsch olefin reaction.
Compared with the prior art, the method of the invention usesFerroferric oxide nanocubes are used as raw materials, and Fe is mainly reconstructed through the surface 3 O 4 The X-Fe with (020) crystal face is constructed in a nanocube mode 5 C 2 The active phase is optionally loaded with an inert carrier substance to construct the Fe core-shell nano catalyst. The nano catalyst synthesized by the method has the advantages of good catalytic activity, high selectivity of C2-C4 olefin and stable catalytic performance. Meanwhile, the method is simple and easy to implement, and is convenient for large-scale popularization. In the Fischer-Tropsch olefin process of the embodiment of the invention, fe is loaded on SiC 3 O 4 @χ-Fe 5 C 2 The conversion frequency of the nanocube catalyst is up to 1.32s -1 Far exceeding the highest reported conversion frequency of the Fischer-Tropsch olefin catalyst (0.22 s -1 ) Meanwhile, the selectivity of the C2-C4 olefin of the catalyst can reach 63.0C in the prepared hydrocarbon, and the catalyst is beneficial to the application in the Fischer-Tropsch olefin reaction.
Drawings
FIG. 1 shows Fe obtained in example 1 of the present invention 3 O 4 Transmission electron microscopy of nanocubes;
FIG. 2 shows Fe obtained in example 2 of the present invention 3 O 4 @χ-Fe 5 C 2 High resolution transmission electron microscopy of nanocubes;
FIG. 3 shows Fe obtained in example 2 of the present invention 3 O 4 @χ-Fe 5 C 2 Powder X-ray diffraction spectrum of nanocubes;
FIG. 4 shows Fe obtained in example 2 of the present invention 3 O 4 @χ-Fe 5 C 2 X-ray photoelectron spectroscopy of nanocubes;
FIG. 5 shows Fe obtained in example 2 of the present invention 3 O 4 @χ-Fe 5 C 2 A data diagram of catalytic performance of the nano-cube catalyst for catalyzing Fischer-Tropsch olefin reaction;
FIG. 6 shows Fe obtained in example 2 of the present invention 3 O 4 @χ-Fe 5 C 2 The nano cube catalyst is subjected to a 100-hour catalysis test to obtain a catalysis performance data graph which is detected on line;
FIG. 7 shows Fe obtained in example 2 of the present invention 3 O 4 @χ-Fe 5 C 2 Transmission electron microscopy of nanocube catalyst after 100 hours of catalytic testing;
FIG. 8 is a diagram showing Fe obtained in comparative example 1 of the present invention 3 O 4 Transmission electron microscopy of nanooctahedra.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely, and it is apparent that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The invention provides an Fe core-shell nano catalyst, which takes an Fe nanocube with a core-shell structure as an active ingredient, wherein in the Fe nanocube core-shell structure, a ferroferric oxide nanocube is taken as a core, and the surface of the ferroferric oxide nanocube is coated with X-Fe 2 C 5 An active phase shell layer.
The nano catalyst provided by the invention has higher catalytic activity and higher C2-C4 olefin selectivity, has stable catalytic performance, and is beneficial to application in the Fischer-Tropsch olefin reaction.
The catalyst provided by the invention comprises Fe nanocubes with a core-shell structure, and the Fe nanocubes are the active ingredients of the catalyst. The Fe nanocubes are nanocubes material containing iron components and have a core-shell structure with a nanocubes morphology, and the phases are mainly Fe 3 O 4 And X-Fe 2 C 5 . In the Fe nanocube core-shell structure, the core containing Fe component is ferroferric oxide (Fe 3 O 4 ) Nanocubes, wherein the shell layer coating the nanocube core is X-Fe 2 C 5 An active phase, which is a cubic shell. In an embodiment of the present invention, the Fe nanocubes having a core-shell structure may be represented as Fe 3 O 4 @χ-Fe 5 C 2 Nanocubes with an average size of 42nm, shellThe layer thickness is generally 2 to 3nm.
Existing catalysts with Fischer-Tropsch olefin activity are typically capable of stabilizing the CO transition state, based on
Evans-Polanyi relation, the activation energy of which is linearly related to the adsorption energy, and the adsorption energy of intermediates are linearly related to each other. In this case, promoting the activation of CO is equivalent to stabilization of CO, i.e. corresponds to strong adsorption of low olefins. The strong bond between the low olefins and the catalyst results in further hydrogenation to paraffins or in carbon-carbon coupling to long chain products. Thus, the high activity of CO conversion is somewhat contradictory with the high selectivity of lower olefins. Therefore, it is difficult to develop an effective Fischer-Tropsch olefin catalyst because of the simultaneous easy dissociation of CO and weak incorporation of low olefins.
In the present invention, the Fe 3 O 4 @χ-Fe 5 C 2 The nanocube catalyst has a unique reaction pathway and thus has high fischer-tropsch olefin selectivity, i.e. the hydrogen-assisted CO dissociation pathway reduces the activation energy to activate CO. In addition, the Fe 3 O 4 @χ-Fe 2 C 5 The nanocubes have weak adsorption on C2-C4 olefins, thereby inhibiting the process of preparing paraffin by secondary hydrogenation and preparing C5+ products by carbon-carbon coupling.
In an embodiment of the present invention, the Fe core-shell nanocatalyst further includes an inert support, and the Fe nanocube having a core-shell structure is supported on the inert support. The inert carrier mainly plays a role in dispersing active ingredients; the inert support according to the present invention may be selected from one or more of silicon carbide (SiC), silica and carbon supports (e.g., activated carbon, carbon nanofibers, carbon nanotubes, etc.), preferably SiC. The catalyst comprising an inert carrier is a supported Fe core-shell nano catalyst, and the Fe nanocubes with a core-shell structure are preferably supported on the surfaces of SiC particles. In the supported Fe core-shell nano catalyst, the total content of iron is preferably 24-32mol%. In the process of implementing the invention, the mixture ratio is adopted, the catalytic activity is better, and the reactionFe in the process 3 O 4 The nanocubes are not prone to agglomeration resulting in catalyst deactivation.
In the Fischer-Tropsch olefin process of the embodiment of the invention, fe is loaded on SiC 3 O 4 @χ-Fe 5 C 2 The conversion frequency of the nanocube catalyst is up to 1.32s -1 Far exceeding the highest reported conversion frequency of the Fischer-Tropsch olefin catalyst (0.22 s -1 ) At the same time, the selectivity of the catalyst for C2-C4 olefin can reach 63.0C% in the prepared hydrocarbon. In particular, fe supported on SiC in the present invention 3 O 4 @χ-Fe 5 C 2 The nanocube catalyst is the only simultaneous achievement of a conversion frequency higher than 1.0s at present -1 And the C2-C4 olefin selectivity is over 60.0C percent.
Correspondingly, the embodiment of the invention provides a preparation method of the Fe core-shell nano catalyst, which comprises the following steps:
s1, providing a ferroferric oxide nanocube;
s2, reducing the surface of the ferroferric oxide nanocube into an iron simple substance to obtain a precursor nanocube;
s3, in the atmosphere comprising CO, carrying out surface reconstruction on the precursor nanocubes to form X-Fe on the surfaces of the ferroferric oxide nanocubes 2 C 5 And the active phase shell layer is optionally loaded with an inert carrier substance to obtain the Fe core-shell nano catalyst.
The Fe core-shell nano catalyst prepared by the method has the advantages of good catalytic activity, high short-chain olefin selectivity and stable catalytic performance, and is simple and easy to implement and beneficial to application.
The embodiment of the invention firstly provides Fe 3 O 4 Nanocubes, which are crystal structures of the order of nanometers in size, have an average size of 40nm. The Fe is 3 O 4 Nanocubes can be prepared using general methods well known to those skilled in the art, and are preferably prepared using the following steps: mainly adopts ferric acetylacetonate (Fe (acac) 3 ) As iron source in 4-phenylbenzoic acid and oilAnd (3) carrying out heating reaction in a solvent in the presence of acid to obtain the ferroferric oxide nanocubes.
Wherein, 4-phenylbenzoic acid and oleic acid are mainly used for controlling the morphology of the ferroferric oxide; the solvent includes, but is not limited to, diphenyl ether. According to the embodiment of the invention, ferric acetylacetonate, 4-phenylbenzoic acid and oleic acid can be dissolved in benzyl ether, oxygen is removed, and the reaction is stirred for 30-60min after the temperature is preferably raised to 250-300 ℃. After the reaction is finished, the material obtained by the reaction is cooled to room temperature, ethanol is added to precipitate a product, and then the product is separated, washed and dried to obtain the Fe 3 O 4 Nanocubes.
In the embodiment of the invention, the weight volume ratio of the ferric acetylacetonate, the 4-phenylbenzoic acid, the oleic acid and the dibenzyl ether is preferably 1-2g:0.9-1.1g:2-3mL:15-25mL; the proportion of the raw materials can ensure that Fe with uniform size is obtained 3 O 4 Nanocubes. Specifically, in the embodiment of the invention, ferric acetylacetonate, 4-phenylbenzoic acid and oleic acid in a certain proportion are dissolved in dibenzyl ether, heated to 110-130 ℃ in a nitrogen atmosphere, stirred for 20-40min to remove oxygen, heated to 280-290 ℃ at a speed of 8-13 ℃ per min preferably, and then reacted at a constant temperature for 30-60min under the condition that the stirring speed is 200-500 rpm. After the reaction was completed, the temperature was cooled to 150℃and 20mg of sodium oleate and 5mg of sulfur powder were dissolved in the solution at a stirring rate of 300rpm and held for 30 minutes; cooling to room temperature, precipitating with ethanol, separating product with magnet, ultrasonic washing with n-hexane for 3-4 times, vacuum drying overnight to obtain dry Fe 3 O 4 Nanocubes.
Wherein, the heating of the raw material solution can ensure the removal of residual oxygen in the solution; specific limitations of other reaction conditions are favorable for obtaining Fe with uniform size 3 O 4 Nanocubes. After the reaction is finished, ethanol is added to dissolve other impurities and allow Fe to be dissolved 3 O 4 The nanocubes precipitate for magnet separation; finally Fe to be obtained 3 O 4 The nanocubes can be dispersed in n-hexane for storage, and can prevent Fe 3 O 4 The nanocubes are oxidized.
After the ferroferric oxide nanocubes are obtained, the ferroferric oxide nanocubes can be filled into a fixed bed reactor, and are heated and reduced in a reducing gas atmosphere, so that the surfaces of the ferroferric oxide nanocubes are reduced into iron simple substances, and the precursor nanocubes are obtained. The preparation method of the precursor nanocubes comprises the following steps: a certain amount of the ferroferric oxide nanocubes are charged into a fixed bed reactor (the inner diameter of which may be 9 mm) commonly used in the art, and are mixed with a gas mixture of hydrogen and argon (H 2 and/Ar) carrying out a reduction reaction in an air flow atmosphere to reduce the surface of the ferroferric oxide nanocube into an iron simple substance. Wherein the reducing gas is preferably H 2 Mixed gas of Ar, H 2 Ar may have a volume ratio of 1:7-12; the temperature of the reduction reaction is preferably 300-350 ℃, and Fe can be added 3 O 4 Fully reducing into iron simple substance.
After the reduction reaction is finished, the obtained precursor nanocube sample is heated in a fixed bed reactor under the atmosphere of a synthetic gas flow comprising CO to carry out surface reconstruction (or surface reconstruction) reaction, so that the surface of the ferroferric oxide nanocube forms X-Fe 2 C 5 Active phase shell layer to obtain Fe with core-shell structure 3 O 4 @χ-Fe 5 C 2 The nanocubes may be abbreviated as Fe nanocubes with core-shell structure.
The Fe nanocubes with the core-shell structure obtained by the embodiment of the invention have higher catalytic activity, and meanwhile, the selectivity of short-chain olefin in the Fischer-Tropsch olefin preparation reaction is high; which can be expressed as Fe 3 O 4 @χ-Fe 5 C 2 Nanocube catalysts. In order to further enhance the stability of the catalytically active component, embodiments of the present invention may also prepare supported Fe 3 O 4 @χ-Fe 5 C 2 Nanocube catalysts.
In some embodiments of the invention, the resulting Fe 3 O 4 @χ-Fe 5 C 2 Mixing the nanocubes with inert carrier substances to obtain supported Fe core-shell nanoA rice catalyst. The inert carrier material can be selected from one or more of silicon carbide, silicon dioxide and carbon carriers, and is preferably SiC powder; the inert carrier material may have a particle size of 20-40 mesh.
In other embodiments of the invention, the method further comprises, prior to the reduction to elemental iron: mixing and grinding the ferroferric oxide nanocubes and inert carrier substances to obtain a compound; then reducing the surface of the ferroferric oxide nanocube in the compound into an iron simple substance to obtain a precursor substance loaded with the precursor nanocube; finally, the precursor substance loaded with the precursor nanocubes is subjected to surface reconstruction in an atmosphere comprising CO, so that the surfaces of the ferroferric oxide nanocubes form X-Fe 2 C 5 And (3) an active phase shell layer to obtain the Fe core-shell nano catalyst.
Wherein the inert support material may be selected from one or more of silicon carbide, silica and carbon supports. Taking SiC powder as an example, in these examples, fe is used as a material 3 O 4 Mixing nanocubes with SiC powder, preferably adding into solvent, grinding, and drying to obtain surface-loaded Fe 3 O 4 Nanocubes of SiC composite. The solvent is preferably one or more of n-hexane, ethanol, toluene, acetone and tetrahydrofuran; the solvent and Fe 3 O 4 The volume weight ratio of the nanocubes is preferably 7-15mL:1g. The addition of the solvent and the grinding process can lead Fe to 3 O 4 The nanocube loading is more uniform. In addition, the Fe 3 O 4 The weight ratio of the nanocubes to the SiC powder can be 1:8-11.
After loading, the obtained compound is subjected to reduction reaction in a reducing gas atmosphere, and then subjected to surface reconstruction reaction in a synthesis gas atmosphere, so that the Fe core-shell nano catalyst is obtained. Specifically, the compound is preferably heated to 300-350 ℃ at a speed of 1-3 ℃ per minute in a reducing gas atmosphere with a gas flow rate of 80-120mL/min and a pressure of 0.8-1.2bar, and the reduction reaction is carried out at a constant temperature for 10-15h. Wherein the reducing gas is preferably H 2 Mixed gas of Ar, H 2 Ar volume ratio is 1:7-12. The reduction reaction under the above conditions can lead to Fe 3 O 4 Fully reduces the iron into the iron simple substance, and can ensure that Fe is not destroyed when the temperature is increased at a specific temperature increasing rate 3 O 4 The morphology of the nanocubes.
After the reduction reaction is finished, the precursor loaded with the precursor nanocubes is preferably heated to 300-350 ℃ in the synthesis gas atmosphere for surface reconstruction. Specifically, the air speed is 2000-3000 mL.h -1 ·gcat -1 And in the synthesis gas atmosphere with the pressure of 15-25bar, heating to 300-350 ℃ at the speed of 1-3 ℃/min, and carrying out surface reconstruction at constant temperature, and reacting for 10-15h to obtain the supported Fe core-shell nano catalyst.
Wherein the synthesis gas is preferably H 2 Mixed gas of CO and Ar, H 2 The volume ratio of CO to Ar is 10-15:10-15:1. the reaction temperature facilitates carbonization of iron to form Fe 3 O 4 @χ-Fe 5 C 2 The core-shell structure is heated at a specific heating rate, so that Fe is not destroyed 3 O 4 The morphology of the nanocubes. In addition, the reaction gas, pressure and space velocity may be consistent with the conditions of the catalytic reaction.
The embodiment of the invention further provides an application of the Fe core-shell nano-catalyst in a Fischer-Tropsch olefin preparation reaction, namely a method for preparing short-chain olefin by the Fischer-Tropsch reaction, wherein the Fe core-shell nano-catalyst is used as a catalyst.
In a specific embodiment of the invention, the Fischer-Tropsch olefin reaction may be carried out in a fixed bed reactor at 330-350℃in 1bar synthesis gas. For example, 100 to 900 mg of the supported Fe core-shell nanocatalyst was charged into a fixed bed reactor having an inner diameter of 9mm, and then 1bar of synthesis gas (48 vol% H 2 48vol% CO and 4vol% Ar, ar as internal standard) as feed gas, 15-35mL min -1 Is introduced into the reactor.
The examples of the present invention used two gas chromatographs to detect products and reactants on-line and analyzed for H by a carbon molecular sieve column (TDX-1) with a Thermal Conductivity Detector (TCD) 2 ，CO，CO 2 ，CH 4 And Ar; analysis of carbon hydro-hydrates using a PONA capillary column with Flame Ionization Detector (FID), bringing about CH 4 As a reference bridge between TCD and FID. Assuming that the amount of Ar after the reaction was kept constant, the CO conversion was calculated according to the internal standard method.
The CO conversion is calculated based on carbon atoms as follows:
conversion of co= (CO inlet -CO outlet )/CO inlet ×100％；
Wherein CO inlet And CO outlet The mole number of CO at the inlet and outlet, respectively.
Hydrocarbons C n H m The selectivity of (c) is calculated according to the following formula:
wherein C is n H m outlet Represents the moles of single hydrocarbon product at the outlet; the denominator indicates the total number of moles of carbon atoms in all hydrocarbon products at the outlet, m, n, i and j being the number of carbons and hydrogens. The selectivity of the oxygen-containing compound is lower than 1.0%; carbon balance exceeds 94.0%.
The conversion frequency value is calculated based on the following equation:
conversion frequency = CO conversion x moles of CO in the synthesis gas x gas flow rate/moles of surface Fe atoms.
According to detection, the Fe core-shell nano catalyst has excellent catalytic effect and high catalytic activity in the Fischer-Tropsch olefin preparation reaction. In addition, in the continuous measurement process, the selectivity of C2-C4 olefin and the selectivity of CO are basically kept stable, and excellent stability is shown.
For further understanding of the present application, the Fe core-shell nanocatalyst provided by the present invention, its preparation method and application are specifically described below with reference to examples.
Example 1
1.4g of Fe (acac) 3 And 1.0g of 4-biphenylcarboxylic acidDissolved in a mixture of 20.0mL dibenzyl ether and 2.5mL oleic acid, and the resulting mixed solution was degassed at 120℃for 30min. Subsequently, the solution was stirred at 10℃for a min -1 Heating to 290 ℃ and maintaining for 30min, stirring and reacting at a speed of 300rpm to obtain Fe 3 O 4 Solution of nanocubes. After cooling the temperature to 150 ℃, 20mg of sodium oleate and 5mg of sulfur powder were dissolved in the solution at a stirring rate of 300rpm and held for 30 minutes. Cooling the obtained solution to room temperature, adding ethanol to precipitate out product, separating out product with magnet, ultrasonic washing with n-hexane for three times, vacuum drying overnight to obtain dry Fe 3 O 4 Nanocubes, the electron microscope image of which is shown in figure 1. As can be seen from FIG. 1, this example gives Fe of uniform size 3 O 4 Nanocubes.
Example 2
1. 0.2g of Fe prepared in example 1 3 O 4 The nanocubes were packed into a fixed bed reactor with an inner diameter of 9mm, the Fe 3 O 4 Nanocubes at H 2 /Ar(10vol％H 2 And 90vol% Ar) at a pressure of 1bar at 100mL min -1 Reducing for 10h at 350 ℃ with a heating rate of 1 ℃ min -1 . The sample obtained is then reacted in a fixed bed reactor with 1bar of synthesis gas (48 vol% H) 2 48vol% CO and 4vol% Ar) and the gas flow rate was 20mL min -1 The heating rate is 1 ℃ min after 10 hours at 340 DEG C -1 Obtaining Fe 3 O 4 @χ-Fe 5 C 2 Nanocubes.
Fe obtained 3 O 4 @χ-Fe 5 C 2 The transmission electron microscope of the nanocube is shown in fig. 2, the powder X-ray diffraction spectrum is shown in fig. 3, and the X-ray photoelectron spectrum is shown in fig. 4. As can be seen from the figure, the average size of the nanocubes obtained is 42nm; it has a core-shell structure, the core is Fe 3 O 4 Nanocubes with shell of X-Fe with (020) crystal face 5 C 2 The thickness of the active phase and the shell layer is 2-3nm.
2. 0.2g of prepared Fe 3 O 4 @χ-Fe 5 C 2 Nanocubes were mixed with 1.8g of 20-40 mesh SiC powder and the resulting composite was loaded into a fixed bed reactor with an inner diameter of 9 mm. Subsequently, 1bar of synthesis gas (48 vol% H) 2 48vol% CO and 4vol% Ar, ar as internal standard) as feed gas, 20mL min -1 Is introduced into the reactor. The Fischer-Tropsch olefin reaction was carried out in a fixed bed reactor at 340℃under 1bar synthesis gas.
The products and reactants of the Fischer-Tropsch olefin reaction were detected and the CO conversion, olefin selectivity, etc. were calculated, respectively, as described above. The results are shown in FIG. 5, and FIG. 5 is catalytic performance data for the Fischer-Tropsch olefin reaction. Fig. 6 is a data of catalytic performance on-line detected through a 100-hour catalytic test, and fig. 7 is a transmission electron microscope image of the catalyst after the 100-hour catalytic test.
Referring to fig. 5-6, in the reaction of preparing olefin by fischer-tropsch, the reaction conditions of the Fe core-shell nano catalyst obtained by the invention include: 1bar of synthesis gas (48 vol% H) 2 48vol% CO,4vol% Ar), flow rate 20mL min -1 The reaction temperature was 340 ℃. As shown in FIG. 5, the C2-C4 olefin selectivity in the product was as high as 63C, the C2-C4 alkylene ratio was as high as 7.2, and there was a CO conversion of 52.2%. Therefore, the Fe core-shell nano catalyst obtained by the invention has excellent catalytic effect and high catalytic activity in the reaction of preparing olefin by Fischer-Tropsch process.
To more accurately evaluate the activity of the catalyst, the conversion frequency was calculated based on the amount of CO conversion per second of Fe atoms per mole of surface Fe 3 O 4 @χ-Fe 5 C 2 nanocubes/SiC 1.32s -1 . As shown in FIG. 6, the C2-C4 olefin selectivity and the CO selectivity remained substantially stable during the 100h continuous measurement, showing excellent stability.
Example 3
1g of Fe (acac) 3 And 1.1g of 4-biphenylcarboxylic acid was dissolved in a mixture of 20.0mL of dibenzyl ether and 2.5mL of oleic acid, and the resulting mixed solution was degassed at 110℃for 40min. Subsequently, the solution was stirred at 8℃for min -1 Heating to 300 ℃ and maintaining for 30min, stirring and reacting at 500rpm to obtain Fe 3 O 4 Solution of nanocubes. After cooling the temperature to 150 ℃, 20mg of sodium oleate and 5mg of sulfur powder were dissolved in the solution at a stirring rate of 300rpm and held for 30 minutes. Cooling the obtained solution to room temperature, adding ethanol to precipitate out product, separating out product with magnet, ultrasonic washing with n-hexane for three times, vacuum drying overnight to obtain dry Fe 3 O 4 Nanocubes.
Example 4
0.2g of Fe prepared in example 3 3 O 4 The nanocubes were packed into a fixed bed reactor with an inner diameter of 9mm, the Fe 3 O 4 Nanocubes at H 2 /Ar(10vol％H 2 And 90vol% Ar) at a pressure of 1bar at 100mL min -1 Reducing for 10h at 350 ℃ with a heating rate of 1 ℃ min -1 . The sample obtained is then reacted in a fixed bed reactor with 1bar of synthesis gas (48 vol% H) 2 48vol% CO and 4vol% Ar) and the gas flow rate was 20mL min -1 The heating rate is 1 ℃ min after 10 hours at 340 DEG C -1 Obtaining Fe 3 O 4 @χ-Fe 5 C 2 Nanocubes.
Example 5
2.0g of ferric acetylacetonate, 0.9g of 4-phenylbenzoic acid and 3.0mL of oleic acid are dissolved in 15.0mL of dibenzyl ether at normal temperature, the obtained solution is heated to 130 ℃ in nitrogen atmosphere, and oxygen is removed by stirring for 20 min; then, the temperature was raised to 250℃at a heating rate of 13℃per minute, and the reaction was stirred for 60 minutes at a stirring rate of 200rpm, to thereby obtain a Fe-containing alloy 3 O 4 Solution of nanocubes. After cooling the temperature to 150 ℃, 20mg of sodium oleate and 5mg of sulfur powder were dissolved in the solution at a stirring rate of 300rpm and held for 30 minutes. Cooling the obtained solution to room temperature, adding ethanol to precipitate out product, separating out product with magnet, ultrasonic washing with n-hexane for three times, vacuum drying overnight to obtain dry Fe 3 O 4 Nanocubes.
Example 6
0.5g of Fe prepared in example 5 3 O 4 Nanocubes are fitted into a fixture with an internal diameter of 9mmIn the bed reactor, the Fe 3 O 4 Nanocubes at H 2 /Ar(10vol％H 2 And 90vol% Ar) at a pressure of 1bar at 100mL min -1 Reducing for 10h at 350 ℃ with a heating rate of 1 ℃ min -1 . The sample obtained is then reacted in a fixed bed reactor with 1bar of synthesis gas (48 vol% H) 2 48vol% CO and 4vol% Ar) and the gas flow rate was 20mL min -1 The heating rate is 1 ℃ min after 10 hours at 340 DEG C -1 Obtaining Fe 3 O 4 @χ-Fe 5 C 2 Nanocubes.
Example 7
1.5g of ferric acetylacetonate, 1.0g of 4-phenylbenzoic acid and 2.5mL of oleic acid are dissolved in 20.0mL of dibenzyl ether at normal temperature, and the obtained solution is heated to 120 ℃ in nitrogen atmosphere and stirred for 30min to remove oxygen; then, the temperature was raised to 280℃at a heating rate of 12℃per minute, and the reaction was stirred for 40 minutes at a stirring rate of 300rpm, to thereby obtain a Fe-containing alloy 3 O 4 Solution of nanocubes. After cooling to 150 ℃, 20mg of sodium oleate and 5mg of sulfur powder are dissolved in the solution at a stirring rate of 300rpm and maintained for 30 minutes. Cooling to room temperature after the reaction is finished, adding 10mL of ethanol to precipitate a product, separating the product by using a magnet, washing the product by using n-hexane for 4 times with ultrasonic waves for 1.5min each time, separating the product by using the magnet, and then drying the product in vacuum at 50 ℃ for overnight to obtain Fe 3 O 4 Nanocubes.
Example 8
0.3g of Fe prepared in example 7 3 O 4 The nanocubes were packed into a fixed bed reactor with an inner diameter of 9mm, the Fe 3 O 4 Nanocubes at H 2 /Ar(10vol％H 2 And 90vol% Ar) at a pressure of 1bar at 100mL min -1 Reducing for 10h at 350 ℃ with a heating rate of 1 ℃ min -1 . The sample obtained is then reacted in a fixed bed reactor with 1bar of synthesis gas (48 vol% H) 2 48vol% CO and 4vol% Ar) and the gas flow rate was 20mL min -1 The heating rate is 1 ℃ min after 10 hours at 340 DEG C -1 Obtaining Fe 3 O 4 @χ-Fe 5 C 2 Nanocubes.
Example 9
0.2g of Fe prepared in example 4, example 6 and example 8 3 O 4 @χ-Fe 5 C 2 The nanocubes were mixed with 1.8g of 20-40 mesh SiC powder, and the resulting composites were loaded into fixed bed reactors with an inner diameter of 9mm, respectively, for Fischer-Tropsch olefin reactions. The reaction conditions include: 1bar of synthesis gas (48 vol% H) 2 48vol% CO,4vol% Ar), flow rate 20mL min -1 The reaction temperature was 340 ℃.
The catalyst of example 4 had a C2-C4 olefin selectivity of 60C%, a C2-C4 olefin to olefin ratio of 6.9, and a CO conversion of 48.3% in the Fischer-Tropsch olefin reaction product. The conversion frequency of the Fe core-shell nano catalyst is 1.12s -1 And the C2-C4 olefin selectivity and CO selectivity remained substantially stable during the 100h continuous measurement, showing excellent stability.
The catalyst of example 6 had a C2-C4 olefin selectivity of 61.5C, a C2-C4 olefin ratio of 7.1 and a CO conversion of 51.6%. The conversion frequency of the Fe core-shell nano catalyst is 1.32s -1 And the C2-C4 olefin selectivity and CO selectivity remained substantially stable during the 100h continuous measurement, showing excellent stability.
The catalyst of example 8 had a C2-C4 olefin selectivity of 62.8C, a C2-C4 olefin ratio of 7.2 and a CO conversion of 50.4% in the Fischer-Tropsch olefin reaction product. The conversion frequency of the Fe core-shell nano catalyst is 1.12s -1 And the C2-C4 olefin selectivity and CO selectivity remained substantially stable during the 100h continuous measurement, showing excellent stability.
Comparative example 1
1.0g of Fe (acac) 3 Dissolved in a mixture of 20.0mL dibenzyl ether, 2.3mL oleylamine and 1.6mL oleic acid, and then degassed at 120℃for 30 minutes. The degassed solution was stirred at 20℃for a min -1 Heated to 220℃and stirred at 300rpm for 1 hour. However, the method is thatThe resulting solution was then heated to 20℃for a minute -1 Heated to 300 c and held for 2 hours. After cooling to 150 ℃, 13mg of sodium oleate and 5mg of sulfur powder are dissolved in the solution at a stirring rate of 300rpm and kept for 30 minutes to obtain Fe 3 O 4 The nano octahedron has an electron microscope image shown in FIG. 8.
0.2g of the obtained Fe 3 O 4 The nano octahedron is loaded on 0.8g of 40-60 mesh silicon carbide carrier and is filled into a fixed bed reactor with the inner diameter of 9 mm. Fe obtained 3 O 4 Nano octahedral load on H 2 /Ar(10vol％H 2 And 90vol% Ar) gas atmosphere at a pressure of 1bar and a pressure of 100 mL.min -1 Is reduced at 350 ℃ for 10h at a heating rate of 1 ℃ min -1 . The sample obtained is then reacted in a fixed bed reactor with 1bar of synthesis gas (48 vol% H) 2 48vol% CO and 4vol% Ar) and the gas flow rate was 20mL min -1 The heating rate is 1 ℃ min after 10 hours at 340 DEG C -1 Obtaining Fe 3 O 4 @χ-Fe 5 C 2 Octahedral nanocatalyst.
By using the Fe obtained 3 O 4 @χ-Fe 5 C 2 Catalytic performance test of Fischer-Tropsch olefin reaction with octahedral nanocatalyst, 1bar synthesis gas (48 vol% H) 2 48vol% CO,4vol% Ar), flow rate 20mL min -1 The reaction temperature was 340 ℃. The selectivity of C2-C4 olefin in the product is only 49.1C, the CO conversion rate is 11.2 percent, and the conversion frequency is only 0.21s -1 。
Comparative example 2
0.2g of Fe-MIL-88BNH2 is put into a tube furnace and heated for 4 hours at 500 ℃; the obtained sample was charged into a fixed bed reactor having an inner diameter of 9 mm. At 60 mL/min -1 Is reduced at 340℃for 4 hours. Then 1bar of synthesis gas (48 vol% H) was introduced 2 48vol% CO and 4vol% Ar) and the gas flow rate was 20mL min -1 The heating rate is 1 ℃ min after 10 hours at 340 DEG C -1 Obtaining Fe 3 O 4 @χ-Fe 5 C 2 Spherical nano-catalysisAnd (3) an agent.
By using the Fe obtained 3 O 4 @χ-Fe 5 C 2 The spherical nano catalyst is used for testing the catalytic performance of the Fischer-Tropsch olefin reaction, and the reaction condition is 20bar of synthesis gas (CO/H) 2 Volume ratio=1), flow rate 30ml·min -1 The reaction temperature was 300 ℃. The selectivity of C2-C4 olefin in the product is only 21.4C, the CO conversion rate is 81.8 percent, and the conversion frequency is also only 0.091s -1 。
From the above examples, it can be seen that during the Fischer-Tropsch olefin process according to the examples of the present invention, fe is supported on SiC 3 O 4 @χ-Fe 5 C 2 The conversion frequency of the nanocube catalyst is up to 1.32s -1 Far exceeding the highest reported conversion frequency of the Fischer-Tropsch olefin catalyst (0.22 s -1 ) At the same time, the selectivity of the catalyst for C2-C4 olefin can reach 63.0C% in the prepared hydrocarbon.
In particular, fe supported on SiC in the present invention 3 O 4 @χ-Fe 5 C 2 The nanocube catalyst is the only simultaneous achievement of a conversion frequency higher than 1.0s at present -1 And the C2-C4 olefin selectivity is over 60.0C percent. In addition, in the continuous measurement process, the selectivity of C2-C4 olefin and the selectivity of CO are basically kept stable, and excellent stability is shown.
The foregoing is merely a preferred embodiment of the present invention and it should be noted that modifications to these embodiments can be made by those skilled in the art without departing from the technical principles of the present invention, and these modifications should also be considered as the scope of the present invention.
Claims (4)
1. The application of the Fe core-shell nano catalyst in the Fischer-Tropsch olefin reaction is characterized in that the Fe core-shell nano catalyst takes an Fe nanocube with a core-shell structure as an active ingredient, in the Fe nanocube core-shell structure, a ferroferric oxide nanocube is taken as a core, and the surface of the ferroferric oxide nanocube is coated with X-Fe 2 C 5 Activity(s)A phase shell layer;
the Fe core-shell nano catalyst also comprises an inert carrier, wherein the Fe nanocubes with the core-shell structure are loaded on the inert carrier, the inert carrier is selected from silicon carbide, and the total content of iron in the Fe core-shell nano catalyst is 24-32mol%.
2. The use according to claim 1, wherein the preparation method of the Fe core-shell nanocatalyst comprises the following steps:
s1, providing a ferroferric oxide nanocube;
s2, reducing the surface of the ferroferric oxide nanocube into an iron simple substance to obtain a precursor nanocube;
s3, in the atmosphere comprising CO, carrying out surface reconstruction on the precursor nanocubes to form X-Fe on the surfaces of the ferroferric oxide nanocubes 2 C 5 And (3) loading the active phase shell layer and an inert carrier substance to obtain the Fe core-shell nano catalyst.
3. The use according to claim 2, wherein in step S1, ferric acetylacetonate is used as an iron source, and the heating reaction is performed in a solvent in the presence of 4-phenylbenzoic acid and oleic acid to obtain the ferroferric oxide nanocubes.
4. The use according to claim 2, wherein said step S2 is specifically: and (3) carrying out reduction reaction on the ferroferric oxide nanocubes in a reducing gas atmosphere, wherein the reducing gas is mixed gas of hydrogen and argon, the temperature of the reduction reaction is 300-350 ℃, and the surfaces of the ferroferric oxide nanocubes are reduced into iron simple substances to obtain precursor nanocubes.
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