Precipitated theta-iron carbide compound as well as preparation method and application thereof

Abstract

The invention relates to a precipitation type theta-iron carbide compound as well as a preparation method and application thereof, and the method comprises the following steps: (1) under the action of H2, carrying out reduction treatment on a precipitation type precursor at the temperature of 430-520 DEG C; and (2) carrying out carbide forming treatment on the reduced product in a mixed gas atmosphere at the temperature of 290-420 DEG C to obtain a product containing theta-iron carbide. The theta-iron carbide compound disclosed by one embodiment of the invention can be used as a catalyst for a synthesis gas conversion reaction, and halogen ions such as bromine or iodine are introduced into the theta-iron carbide compound, so that the reaction has relatively high CO conversion rate, extremely low total CO2 selectivity and low CH4 selectivity, and comprehensive optimization of a reaction result is realized.

Description

Technical Field

[0001] The present invention belongs to the field of syngas conversion, and relates to a precipitated θ-iron carbide composite that can be used for syngas conversion. Background Art

[0002] Syngas is a mixed gas obtained by gasifying coal, natural gas, and biomass (its composition is CO and H2). Converting syngas into liquid fuels and high-value chemicals through the action of a catalyst can solve the energy problem of shortage of oil and gas.

[0003] The reaction equations for syngas conversion are as follows:

[0004] (2n + 1)H2 + nCO → C n H 2n+2 + nH2O (a)

[0005] 2nH2 + nCO → C n H 2n + nH2O (b)

[0006] (n + 1)H2 + 2nCO → C n H 2n+2 + nCO2 (c)

[0007] nH2 + 2nCO → C n H 2n + nCO2 (d)

[0008] Iron-based catalysts are the cheapest and most readily available catalysts for syngas conversion, and have the advantages of high activity, wide applicable condition window, strong sulfur resistance, simple on-line catalyst replacement, and suitability for industrial continuous production. However, one of the bottleneck problems in the conventional iron-based catalyst syngas conversion technology is the excessive CO2 selectivity.

[0009] The prior art discloses a high-purity iron carbide catalyst, which can reduce the primary CO2 in the Fischer-Tropsch synthesis reaction to nearly zero, and at the same time can achieve a high CO space-time conversion rate, so that the total CO2 selectivity can be reduced to <5% at a low CO conversion rate (usually lower than 35%). However, when the CO conversion rate increases, with the increase of the H2O content in the reaction environment, the WGS reaction (CO + H2O → CO2 + H2) becomes intense, resulting in an increase in secondary CO2, and ultimately leading to an increase in the total CO2 selectivity at a high CO conversion rate.

[0010] On the other hand, in addition to CO2, another main by-product in the field of syngas conversion is methane. In the reaction, a high CH4 selectivity and a high CO2 selectivity will jointly reduce the effective product selectivity of the reaction. Summary of the Invention

[0011] To overcome at least one defect of the above-mentioned prior art, in a first aspect, an embodiment of the present invention provides a method for preparing a precipitated θ-iron carbide composite, comprising the following steps:

[0012] (1) By the action of H2, reducing the precipitated precursor at a temperature of 430 - 520 °C; and

[0013] (2) Subjecting the product of the reduction treatment to carbide formation treatment at a temperature of 290 - 420 °C in a mixed gas atmosphere to obtain a product containing θ-iron carbide;

[0014] wherein, the mixed gas comprises hydrogen and carbon monoxide with a molar ratio of 5 - 90:1;

[0015] Reacting iron ions in a first solution to obtain a basic precipitate of iron; calcining the basic precipitate of iron and then performing an impregnation treatment to make it contain iodide ions and / or bromide ions, and using the impregnated product as the precipitated precursor, and the product containing θ-iron carbide in the step (2) is the precipitated θ-iron carbide composite; or,

[0016] Reacting iron ions in the first solution to obtain a basic precipitate of iron; performing a calcination treatment on the basic precipitate of iron, and using the calcined product as the precipitated precursor; after treating the precipitated precursor through the steps (1) to (2), then performing an impregnation treatment on the obtained product containing θ-iron carbide to make it contain iodide ions and / or bromide ions.

[0017] In a second aspect, an embodiment of the present invention provides a θ-iron carbide composite prepared by the above-mentioned preparation method.

[0018] In a third aspect, an embodiment of the present invention provides a catalyst, comprising the θ-iron carbide composite prepared by the above-mentioned preparation method or the above-mentioned θ-iron carbide composite.

[0019] In a fourth aspect, an embodiment of the present invention provides an application of the θ-iron carbide composite prepared by the above-mentioned preparation method, the above-mentioned θ-iron carbide composite or the above-mentioned catalyst in a reaction for synthesizing C, H fuels and / or chemicals based on the Fischer-Tropsch synthesis principle.

[0020] In a fifth aspect, an embodiment of the present invention provides a syngas conversion process, comprising contacting the above-mentioned catalyst with syngas under reaction conditions for reaction.

[0021] A θ-iron carbide composite of an embodiment of the present invention can be used as a catalyst for the synthesis gas conversion reaction, especially the Fischer-Tropsch synthesis reaction. By introducing halide ions such as bromine or iodine into the θ-iron carbide composite, the reaction has a high CO conversion rate, an extremely low total CO2 selectivity, and a low CH4 selectivity, achieving a comprehensive optimization of the reaction results. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The drawings are only for the purpose of showing specific embodiments and are not considered to be a limitation of the present invention. Among them:

[0023] Figure 1 It is the XRD pattern of the precipitated θ-iron carbide composite CX1 prepared in Example 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0024] Typical embodiments embodying the features and advantages of the present invention will be described in detail in the following description. It should be understood that the present invention can have various variations in different embodiments, all of which do not depart from the scope of the present invention, and the descriptions therein are for illustrative purposes in nature and not intended to limit the present invention.

[0025] An embodiment of the present invention provides a θ-iron carbide composite, including θ-iron carbide. Based on the molar amount of θ-iron carbide, the composite further includes 0.1 - 45 mol% of halide ions and 0 - 23 mol% of a first cation;

[0026] Among them, the halide ions are bromide ions and / or iodide ions, and the molar amount of θ-iron carbide is calculated based on the molar amount of the iron element contained therein; the composite has an orthorhombic crystal structure, and its average grain diameter is 5 - 40 nm, and further can be 8 - 35 nm.

[0027] In one embodiment, the θ-iron carbide composite includes 0.1 - 45 mol%, further 0.3 - 27 mol%, and still further 7 - 20 mol% of halide ions. For example, the content of halide ions in the composite can be 0.5 mol%, 1 mol%, 5 mol%, 10 mol%, 15 mol%, 20 mol%, 25 mol%, 30 mol%, 40 mol%, 45 mol%.

[0028] In one embodiment, the θ-iron carbide composite includes 0 - 23 mol%, further 0.1 - 20 mol%, and still further 3 - 10 mol% of the first cation. For example, the content of the first cation in the composite can be 0.5 mol%, 1 mol%, 5 mol%, 10 mol%, 15 mol%, 18 mol%.

[0029] In one embodiment, the θ-iron carbide complex includes a second cation, and the second cation can maintain charge balance with the halide ion, that is, the total negative charge (or the total valence shown) of the halide ion is equal to the total positive charge of the second cation.

[0030] In one embodiment, the θ-iron carbide complex includes a second anion, and the first cation can maintain charge balance with the second anion.

[0031] In one embodiment, the second cation includes one or more of a first metal ion and a complex cation. Further, the first metal ion includes one or more of an iron ion (such as divalent and trivalent iron ions), a manganese ion (such as divalent manganese ion), a copper ion (such as monovalent and divalent copper ions), a cobalt ion (such as divalent cobalt ion), a molybdenum ion (such as divalent, trivalent, and tetravalent molybdenum ions), a lanthanum ion (such as trivalent and tetravalent lanthanum ions), a cerium ion (such as trivalent and tetravalent cerium ions), and a neodymium ion (such as trivalent and tetravalent neodymium ions); the complex cation includes one or more of a hexaammine manganese ion, a hexaammine iron ion, and a hexaammine copper ion.

[0032] In one embodiment, the first cation includes one or more of second metal ions. The second metal ion may include one or more of a manganese ion, a copper ion, a cobalt ion, a molybdenum ion, a chromium ion, a rare earth ion, an alkali metal ion, and an alkaline earth metal ion; for example, the second metal ion may be a manganese ion (such as divalent, trivalent, and tetravalent manganese ions), a copper ion (such as monovalent and divalent copper ions), a cobalt ion (such as divalent cobalt ion), a molybdenum ion (such as divalent, trivalent, and tetravalent molybdenum ions), a chromium ion (such as trivalent chromium ion), a lanthanum ion (such as trivalent and tetravalent lanthanum ions), a cerium ion (such as trivalent and tetravalent cerium ions), a neodymium ion (such as trivalent and tetravalent neodymium ions), a sodium ion, a potassium ion, a calcium ion, and a barium ion.

[0033] In one embodiment, the second anion includes one or more of an oxygen ion, a complex ion, and an acid radical ion, such as a nitrate ion, a citrate ion, and a gluconate ion.

[0034] One embodiment of the present invention provides a method for preparing the above-mentioned precipitation-type θ-iron carbide complex, including the following steps:

[0035] (1) Under the action of H2, the precipitation-type precursor is subjected to a reduction treatment at a temperature of 430-520 °C to obtain a reduction product; and

[0036] (2) The reduction product is subjected to a carbide formation treatment at 290-420 °C in a mixed gas atmosphere to obtain a product containing θ-iron carbide;

[0037] Among them, the mixed gas includes hydrogen and carbon monoxide, and the molar ratio of hydrogen to carbon monoxide is 5:1 to 90:1, and further can be 45:1 to 60:1;

[0038] The preparation method of the precipitation-type precursor includes: reacting ferric ions with precipitating agent anions in a first solution to obtain a basic iron precipitate; calcining the basic iron precipitate, and then performing a first impregnation treatment with a second solution to obtain the precipitation-type precursor; wherein, the second solution contains bromide ions and / or iodide ions; or,

[0039] The preparation method of the precipitation-type precursor includes: reacting ferric ions with precipitating agent anions in a first solution to obtain a basic iron precipitate; calcining the basic iron precipitate to obtain the precipitation-type precursor; after the precipitation-type precursor is treated through steps (1) to (2), the obtained product containing θ-iron carbide is then subjected to a second impregnation treatment with a second solution.

[0040] In one embodiment, the precipitating agent anions include one or more of carbonate, bicarbonate, and hydroxide. Among them, when there are two kinds of precipitating agent anions, bicarbonate and hydroxide are not included simultaneously, that is, the precipitating agent anions can include carbonate and bicarbonate, or include carbonate and hydroxide.

[0041] In one embodiment, the first solution for preparing the precipitation-type precursor is obtained by dissolving at least two soluble compounds in a first solvent. The at least two soluble compounds include an iron salt and a basic precipitating agent, and the iron salt includes one or more of ferric nitrate, ferric chloride, ammonium ferrous sulfate, and ammonium ferric citrate.

[0042] In one embodiment, the basic precipitating agent includes one or more of potassium hydroxide, potassium bicarbonate, sodium hydroxide, sodium bicarbonate, ammonia water, sodium carbonate, and potassium carbonate. Among them, carbonates and alkalis are not included simultaneously.

[0043] In one embodiment, the first solvent includes water. For example, the first solution is an aqueous first solution.

[0044] In one embodiment, after the basic iron precipitate is dried and then calcined, the drying treatment can include the following process: drying the basic iron precipitate at 35 - 80 °C and a vacuum degree of 250 - 1200 Pa for 6 - 10 h; then drying the dried material at 75 - 180 °C for 3 - 24 h. The temperature of the calcination treatment can be 250 - 580 °C, such as 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C; the time of the calcination treatment can be 1 - 10 h, such as 2 h, 3 h, 5 h, 6 h, 8 h.

[0045] In one embodiment, a second solution is prepared by dissolving at least one solute in a second solvent. The at least one solute includes a halide, and the halide includes a water-soluble bromide and / or iodide.

[0046] In one embodiment, the halide includes one or more of bromides and iodides containing manganese, copper, rare earth metal elements, cobalt, molybdenum, and iron. Further, the halide includes one or more of manganese bromide, ferrous bromide, copper bromide, cobalt bromide, molybdenum bromide, manganese iodide, ferrous iodide, copper iodide, rare earth bromides, rare earth iodides, hexaammine manganese bromide, hexaammine iron bromide, hexaammine copper bromide, hexaammine manganese iodide, hexaammine iron iodide, and hexaammine copper iodide.

[0047] In one embodiment, the at least one solute further includes an auxiliary agent, and the auxiliary agent includes one or more of salts (organic salts and / or inorganic salts) of manganese, copper, cobalt, molybdenum, rare earth metals, alkali metals, and alkaline earth metals. Further, the auxiliary agent includes one or more of potassium nitrate, sodium nitrate, manganese nitrate, copper nitrate, cobalt nitrate, molybdenum nitrate, calcium nitrate, barium nitrate, rare earth nitrates, potassium carbonate, sodium carbonate, potassium citrate, sodium citrate, manganese citrate, copper citrate, cobalt citrate, molybdenum citrate, calcium citrate, barium citrate, potassium gluconate, sodium gluconate, lithium gluconate, rubidium gluconate, cesium gluconate, manganese gluconate, copper gluconate, and calcium gluconate.

[0048] In one embodiment, no chemical reaction occurs between the solute components of the same solution. For example, the solutes of the second solution do not include potassium carbonate and calcium nitrate at the same time.

[0049] In one embodiment, the dosages of the halide and the auxiliary agent can be appropriately selected according to the contents of various ions in the composite to be prepared. Further, the concentrations of both the halide and the auxiliary agent can be 0.7 to 7 mol / L, such as 1 mol / L, 2 mol / L, 3 mol / L, 5 mol / L, and 6 mol / L.

[0050] In one embodiment, the second solvent includes water and / or ethanol. For example, the second solvent can be water or a mixture of ethanol and water.

[0051] In one embodiment, the temperature of the first impregnation treatment or the second impregnation treatment is 0 to 50 °C, further preferably 20 to 30 °C, such as 10 °C, 15 °C, 20 °C, 30 °C, 35 °C, 40 °C, and 45 °C; the time of the first impregnation treatment or the second impregnation treatment can be 0.1 to 12 h, further preferably 0.2 to 10 h, and still further preferably 0.3 to 9 h, such as 0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, and 8 h.

[0052] In one embodiment, the material after the first impregnation treatment or the second impregnation treatment can be dried at 15 to 40 °C, and further dried under light-shielded conditions. The drying temperature can be, for example, 20 °C, 25 °C, 30 °C, 35 °C; the drying time can be 0.5 to 12 h. The drying treatment can be carried out under normal pressure or reduced pressure conditions.

[0053] In one embodiment, the first impregnation treatment or the second impregnation treatment can adopt one of the slurry impregnation method, the saturated impregnation method, the supersaturated impregnation method or other feasible impregnation methods.

[0054] In one embodiment, the temperature of the reduction treatment in step (1) can be 430 to 520 °C, such as 450 °C, 460 °C, 480 °C, 500 °C, 510 °C; the treatment pressure can be 0.1 to 11 atm, further 0.2 to 6 atm, such as 0.15 atm, 0.2 atm, 0.5 atm, 0.8 atm, 1 atm, 1.5 atm, 2 atm, 2.5 atm, 5 atm, 8 atm, 10 atm; the treatment time can be 0.7 to 15 h, further 1 to 12 h, such as 2 h, 3 h, 5 h, 8 h, 10 h, 13 h.

[0055] In one embodiment, the gas flow rate of H2 in step (1) can be 600 to 25000 mL / h / g, further 2800 to 22000 mL / h / g, such as 1000 mL / h / g, 1500 mL / h / g, 2000 mL / h / g, 2500 mL / h / g, 3000 mL / h / g, 5000 mL / h / g, 6000 mL / h / g, 8000 mL / h / g, 10000 mL / h / g, 12000 mL / h / g, 15000 mL / h / g, 18000 mL / h / g, 20000 mL / h / g.

[0056] In one embodiment, the molar ratio of hydrogen to carbon monoxide in the mixed gas in step (2) can be 10:1, 20:1, 30:1, 50:1, 60:1, 80:1. The second gas can be a mixture of hydrogen and carbon monoxide.

[0057] In one embodiment, the treatment temperature in step (2) can be 290 to 420 °C, further can be 350 to 400 °C, such as 300 °C, 320 °C, 330 °C, 350 °C, 380 °C, 400 °C; the treatment pressure can be 0 to 19 atm, further can be 0.01 to 12 atm, such as 0.1 atm, 0.2 atm, 0.5 atm, 0.8 atm, 1 atm, 2 atm, 5 atm, 8 atm, 10 atm, 11 atm, 15 atm, 18 atm; the treatment time can be 3 to 72 h, further can be 5 to 48 h, such as 10 h, 15 h, 20 h, 25 h, 30 h, 35 h, 40 h, 45 h, 50 h, 60 h, 70 h.

[0058] In one embodiment, the gas flow rate of the mixed gas in step (2) can be 200 to 35000 mL / h / g, further can be 1200 to 20000 mL / h / g, such as 500 mL / h / g, 1000 mL / h / g, 1800 mL / h / g, 2000 mL / h / g, 3000 mL / h / g, 4000 mL / h / g, 5000 mL / h / g, 6000 mL / h / g, 7000 mL / h / g, 10000 mL / h / g, 12000 mL / h / g, 15000 mL / h / g, 18000 mL / h / g, 25000 mL / h / g, 30000 mL / h / g.

[0059] In one embodiment, in step (2), the temperature of the system is raised or lowered from 430 to 520 °C in step (1) to 290 to 420 °C at a temperature change rate of 0.2 to 5 °C / min. Further, the temperature of the system is raised or lowered from 430 to 520 °C to 300 to 400 °C at a temperature decrease rate of 0.2 to 2.5 °C / min; the temperature change rate in step (2) can be, for example, 0.5 °C / min, 0.8 °C / min, 1 °C / min, 1.2 °C / min, 1.5 °C / min, 1.8 °C / min, 2 °C / min, 2.2 °C / min, 3 °C / min, 4 °C / min.

[0060] In one embodiment, preferably, steps (1), (2), and the first or second impregnation treatment can all be carried out under light-shielded conditions.

[0061] In one embodiment, a method for preparing a precipitated θ-iron carbide composite includes the following steps:

[0062] (0) React iron ions with precipitant anions in a first solution to obtain a basic iron precipitate; subject the basic iron precipitate to a calcination treatment, and then perform a first impregnation treatment with a second solution to obtain a precipitated precursor; wherein the second solution contains bromide ions and / or iodide ions;

[0063] (1) Under the action of hydrogen, subject the precipitated precursor to a reduction treatment at a temperature of 430-520 °C to obtain a reduction product;

[0064] (2) Subject the reduction product to a carbide formation treatment at 290-420 °C in a mixed gas atmosphere to obtain a product containing θ-iron carbide (i.e., a θ-iron carbide complex containing halogen ions).

[0065] In another embodiment, a method for preparing a precipitated θ-iron carbide complex includes the following steps:

[0066] (0) React iron ions with precipitant anions in a first solution to obtain a basic iron precipitate; subject the basic iron precipitate to a calcination treatment to obtain a precipitated precursor;

[0067] (1) Under the action of hydrogen, subject the precipitated precursor to a reduction treatment at a temperature of 430-520 °C to obtain a reduction product;

[0068] (2) Subject the reduction product to a carbide formation treatment at 290-420 °C in a mixed gas atmosphere to obtain a product containing θ-iron carbide

[0069] (21) Perform a second impregnation treatment on the product containing θ-iron carbide with a second solution to obtain a θ-iron carbide complex containing halogen ions.

[0070] One embodiment of the present invention provides a catalyst comprising the above-mentioned θ-iron carbide complex.

[0071] One embodiment of the present invention provides the application of the above-mentioned θ-iron carbide complex or catalyst in a syngas conversion reaction.

[0072] In one embodiment, the syngas conversion reaction can be a Fischer-Tropsch synthesis reaction or other reactions based on the Fischer-Tropsch synthesis principle, such as a reaction with syngas as the starting material and an alcohol as the final product.

[0073] In one embodiment, the syngas includes CO and H2.

[0074] One embodiment of the present invention provides the application of the above-mentioned iron θ-carbide composite or catalyst in the reaction for synthesizing C and H fuels and / or chemicals based on the Fischer-Tropsch synthesis principle. Among them, the reaction based on the Fischer-Tropsch synthesis principle refers to the reaction of synthesizing hydrocarbons and / or their oxygen-containing derivatives by hydrogenation of CO and carbon chain growth reaction using syngas (a mixture of CO and H2) as raw materials under the action of a catalyst and appropriate conditions.

[0075] In one embodiment, the above reaction is a Fischer-Tropsch synthesis reaction, the reaction temperature can be 280-335 °C, such as 285 °C, 290 °C, 300 °C, 310 °C, 320 °C, 330 °C; the reaction pressure can be 2-3.5 MPa, and the molar ratio of H2 / CO can be 1.7-2.15.

[0076] One embodiment of the present invention provides a syngas conversion process, which includes contacting the above catalyst with syngas under syngas conversion reaction conditions for reaction.

[0077] In one embodiment, the syngas conversion can be carried out in a high-temperature and high-pressure continuous reactor.

[0078] The iron θ-carbide composite of one embodiment of the present invention can be used as a catalyst for syngas conversion reaction. By introducing halide ions into iron θ-carbide, the reaction has a high CO conversion rate, an extremely low total CO2 selectivity and a low CH4 selectivity. At the same time, benefiting from the very high CO space-time conversion rate of the iron θ-carbide composite catalyst, it shows considerable activity.

[0079] The iron θ-carbide composite of one embodiment of the present invention can be used as a catalyst for syngas conversion reaction, which can keep the reaction with extremely low CO2 selectivity under high CO conversion rate, while maintaining low CH4 selectivity and high reaction stability, greatly improving the utilization efficiency of carbon atoms and the selectivity of effective products, breaking through the key technical bottleneck, and promoting the high-end, diversification and low-carbon of clean syngas conversion, pointing out a new trend and direction for the development of modern syngas chemical industry.

[0080] The iron θ-carbide composite of one embodiment of the present invention, as a catalyst for Fischer-Tropsch synthesis reaction, can maintain continuous and stable reaction for more than 300 h using a high-pressure continuous reactor under industrial Fischer-Tropsch synthesis reaction conditions, its CO2 selectivity is below 5%, further below 3%; the selectivity of its by-product CH4 can be kept below 8.5%, further below 5.5%; the utilization efficiency of carbon atoms is kept above 95%, further above 97%; the selectivity of effective products can reach above 86.5%, further above 92%.

[0081] In one embodiment, through the Fischer-Tropsch synthesis reaction catalyzed by the θ-iron carbide composite, a CO2 selectivity of <5%, a carbon atom utilization efficiency of >95%, and an effective product selectivity of >90% can be achieved at a CO conversion rate of over 70%.

[0082] As used herein, the "ions" contained in the composite include all particles that are combined with other particles by covalent bonds and / or ionic bonds. For example, the bromide ions in the composite include both Br that interacts with K by ionic bonds + interacting with - and Br atoms that interact with H atoms by covalent bonds.

[0083] All pressure values involved herein are gauge pressures.

[0084] Hereinafter, in conjunction with the accompanying drawings and specific embodiments, a θ-iron carbide composite and its application in one embodiment of the present invention will be further described. Among them, the test methods involved are as follows:

[0085] 1. During the reaction process of the examples or comparative examples, an in-situ XRD detector (Rigaku Corporation, model D / max-2600 / PC) was used to monitor the phase changes of the materials, and the crystal system structure of the θ-iron carbide composite was measured by the X-ray diffractometer.

[0086] 2. The average grain diameter of the θ-iron carbide composite was obtained through XRD testing.

[0087] 3. A Mössbauer spectrometer (Transmission 57 Fe, 57 Co(Rh) source sinusoidal velocity spectrometer) was used to perform Mössbauer spectroscopy detection on the θ-iron carbide composite to obtain the corresponding composition.

[0088] 4. An inductively coupled plasma emission spectrometer (ICP) was used to detect the elements of the θ-iron carbide composite.

[0089] 5. During the synthesis gas conversion reaction process, the products obtained from the reaction were subjected to gas chromatography analysis (Agilent 7890 gas chromatography) for calculating the conversion rate, selectivity, etc. The products refer to the tail gas collected from the reactor end, including the generated hydrocarbon compounds, alcohol compounds, CO2, etc.

[0090] 6. The CO conversion rate %, CO2 selectivity %, CH4 selectivity %, carbon atom utilization efficiency %, and effective product selectivity % were calculated through the following formulas:

[0091] CO conversion rate % = [(moles of CO in feed - moles of CO in product) / moles of CO in feed] × 100%;

[0092] CO2 selectivity % = [moles of CO2 in product / (moles of CO in feed - moles of CO in product)] × 100%;

[0093] CH4 selectivity % = [moles of CH4 in product / (moles of CO in feed - moles of CO in product)] × 100%;

[0094] Carbon atom utilization efficiency % = (1 - CO2 selectivity %) × 100%;

[0095] Effective product selectivity % = (1 - CO2 selectivity % - CH4 selectivity %) × 100%.

[0096] Example 1

[0097] (0) Ferric nitrate with a concentration of 1.0 mol / L was mixed with a 1.5 mo1 / L sodium carbonate solution at 55 °C and pH = 6.5 to obtain a precipitate slurry. After washing with deionized water and filtering, a filter cake was obtained, dried at 107 °C for 16 h, and calcined at 360 °C for 7 h to obtain an iron basic precipitate.

[0098] (1) At a pressure of 450 °C and 2.1 atm, the iron basic precipitate prepared in step (0) was maintained in H2 with a flow rate of 7000 mL / h / g for 12 h for reduction treatment to obtain a reduction product.

[0099] (2) The system of the reduction product was cooled from 420 °C to 400 °C at a rate of 1.5 °C / min and contacted with a mixed gas of H2 and CO at this temperature for the preparation of precipitated carbide; wherein, the pressure of the system was 3.5 atm, the molar ratio of H2 and CO was 45:1, the flow rate of the mixed gas was 10000 mL / h / g, and the treatment time was 7 h.

[0100] (21) Manganese bromide and potassium nitrate were dissolved in water to prepare an impregnation solution with a manganese bromide concentration of 5.7 mol / L; the impregnation solution was mixed with the product of step (2), and impregnation treatment was carried out by the slurry impregnation method with an impregnation ratio of Fe:Br:K = 100:7:2, an impregnation temperature of 32 °C, and an impregnation time of 2 h; the impregnated solid material was dried at 35 °C for 7 h. After the treatment was completed, a precipitated θ-iron carbide composite was prepared and labeled as CX1.

[0101] Examples 1-1 to 3-8 were all prepared with substantially the same raw materials and processes as in Example 1, except that: the content or type of halide ions or the first cation in the impregnating solution was different, and the prepared composites were sequentially labeled as CX1-1 to CX3-8 in the same manner as in Example 1. Since the loss of materials during the preparation process was extremely small, the content of each substance in the obtained composite was substantially the same as the dosage of the corresponding raw material. For specific content values, see Table 1.

[0102] Example 4

[0103] (0) Ferric nitrate with a concentration of 1.0 mol / L was mixed with a 1.5 mo1 / L sodium carbonate solution at 55 °C and pH = 6.5 to obtain a precipitate slurry. After washing with deionized water and filtering, a filter cake was obtained, dried at 107 °C for 16 h, and calcined at 360 °C for 7 h to obtain a basic iron precipitate.

[0104] (1) Under a pressure of 450 °C and 2.1 atm, the basic iron precipitate prepared in step (0) was maintained in H2 with a flow rate of 7000 mL / h / g for 12 h for reduction treatment to obtain a reduction product.

[0105] (2) The system of the reduction product was cooled from 420 °C to 400 °C at a rate of 1.5 °C / min, and contacted with a mixed gas of H2 and CO at this temperature for the preparation of precipitated carbide; wherein, the pressure of the system was 3.5 atm, the molar ratio of H2 and CO was 45:1, the flow rate of the mixed gas was 10000 mL / h / g, and the treatment time was 7 h.

[0106] (21) Manganese bromide, potassium citrate, and sodium nitrate were dissolved in water to prepare an impregnating solution; the impregnating solution was mixed with the product of step (2), and impregnation treatment was carried out by the slurry impregnation method. The impregnation ratio was Fe:Br:K:Na = 100:15:3:2, the impregnation temperature was 32 °C, and the impregnation time was 2 h; the impregnated solid material was dried at 35 °C for 7 h. After the treatment was completed, a precipitated θ-iron carbide composite was prepared and labeled as CX4.

[0107] Example 4-1

[0108] This example was prepared with substantially the same raw materials and processes as in Example 1, except that: in step (2), the molar ratio of H2 to CO was 6:1. The finally prepared θ-iron carbide composite was labeled as CX4-1.

[0109] Example 4-2

[0110] This example uses substantially the same raw materials and process as Example 1 to prepare the θ-iron carbide composite, with the only difference being that in step (2), the molar ratio of H2 to CO is 85:1. The finally prepared θ-iron carbide composite is labeled as CX4-2.

[0111] Example 4-3

[0112] This example uses substantially the same raw materials and process as Example 1 to prepare the θ-iron carbide composite, with the only difference being that in step (2), the molar ratio of H2 to CO is 60:1. The finally prepared θ-iron carbide composite is labeled as CX4-3.

[0113] Example 4-4

[0114] This example uses substantially the same raw materials and process as Example 1 to prepare the θ-iron carbide composite, with the only difference being that in step (2), the mixed gas of H2 and CO is cooled from 420 °C to 290 °C at a cooling rate of 1.5 °C / min and the carbonization step is completed at 290 °C. The finally prepared θ-iron carbide composite is labeled as CX4-4.

[0115] Example 4-5

[0116] This example uses substantially the same raw materials and process as Example 1 to prepare the θ-iron carbide composite, with the only difference being that in step (2), the mixed gas of H2 and CO is cooled from 420 °C to 410 °C at a cooling rate of 1.5 °C / min and the carbonization step is completed at 410 °C. The finally prepared θ-iron carbide composite is labeled as CX4-5.

[0117] Example 4-6

[0118] This example uses substantially the same raw materials and process as Example 1 to prepare the θ-iron carbide composite, with the only difference being that in step (2), the mixed gas of H2 and CO is cooled from 420 °C to 350 °C at a cooling rate of 1.5 °C / min and the carbonization step is completed at 350 °C. The finally prepared θ-iron carbide composite is labeled as CX4-6.

[0119] Example 4-7

[0120] This example uses substantially the same raw materials and process as Example 1 to prepare the θ-iron carbide composite, with the only difference being that in step (1), the iron-containing precipitate is reduced with H2 at a temperature of 510 °C. The finally prepared θ-iron carbide composite is labeled as CX4-7.

[0121] Example 4-8

[0122] In this example, the θ-iron carbide composite is prepared using substantially the same raw materials and process as in Example 1, with the only difference being that in step (2), the pressure of the mixed gas of H2 and CO is 0.1 atm. The finally obtained θ-iron carbide composite is labeled as CX4-8.

[0123] Examples 4-9

[0124] In this example, the θ-iron carbide composite is prepared using substantially the same raw materials and process as in Example 1, with the only difference being that in step (1), the reduction time of the iron-containing precipitate with H2 is 2 h. The finally obtained θ-iron carbide composite is labeled as CX4-9.

[0125] Examples 4-10

[0126] In this example, the θ-iron carbide composite is prepared using substantially the same raw materials and process as in Example 1, with the only difference being that in step (2), the carbonization time is 25 h. The finally obtained θ-iron carbide composite is labeled as CX4-10.

[0127] Examples 4-11

[0128] In this example, the θ-iron carbide composite is prepared using substantially the same raw materials and process as in Example 1, with the only difference being that in step (2), the iron-containing precipitate is reduced with H2 under the condition of a pressure of 0.3 atm. The finally obtained θ-iron carbide composite is labeled as CX4-11.

[0129] Example 5

[0130] (0) Ferric nitrate with a concentration of 1.0 mol / L is mixed with a 1.5 mo1 / L sodium carbonate solution at 55 °C and pH = 6.5 to obtain a precipitate slurry, which is washed with deionized water, filtered to obtain a filter cake, dried at 107 °C for 16 h, and calcined at 360 °C for 7 h to obtain an iron basic precipitate.

[0131] Manganese bromide and potassium gluconate are dissolved in water to prepare an impregnation solution with a manganese bromide concentration of 5.7 mol / L; the impregnation solution is mixed with the iron basic precipitate, and impregnation treatment is carried out by the slurry impregnation method, with the impregnation ratio (molar ratio) of Fe:Br:K = 100:7:2, the impregnation temperature is 32 °C, and the impregnation time is 2 h; the impregnated solid material is dried at 35 °C for 7 h, and after the treatment is completed, a precursor is obtained.

[0132] (1) At a temperature of 450 °C and a pressure of 2.1 atm, the precursor prepared in step (0) is maintained in H2 with a flow rate of 7000 mL / h / g for 12 h for reduction treatment to obtain a reduction product.

[0133] (2) The system of the reduction product was cooled from 420 °C to 400 °C at a rate of 1.5 °C / min and contacted with a mixed gas of H2 and CO at this temperature for the preparation of precipitated carbide; wherein, the pressure of the system was 3.5 atm, the molar ratio of H2 to CO was 45:1, the flow rate of the mixed gas was 10,000 mL / h / g, and the treatment time was 7 h. After the treatment, a precipitated θ-iron carbide composite was prepared and labeled as CX5.

[0134] Comparative Example 1

[0135] In this example, a θ-iron carbide composite was prepared using substantially the same raw materials and process as in Example 1, with the only difference being that in step (21), the impregnation ratio was Fe:Br:K = 100:76.9:33.8. The finally prepared θ-iron carbide composite was labeled as DX1.

[0136] Comparative Example 2

[0137] In this example, a θ-iron carbide composite was prepared using substantially the same raw materials and process as in Example 1, with the only difference being that in the impregnating solution of step (21), manganese bromide was not added. The finally prepared θ-iron carbide composite was labeled as D2.

[0138] Comparative Example 3

[0139] In this example, a θ-iron carbide composite was prepared using substantially the same raw materials and process as in Example 1, with the only difference being that in step (21), manganese chloride was used in an equimolar amount instead of manganese bromide. The finally prepared θ-iron carbide composite was labeled as DX3.

[0140] Comparative Example 4

[0141] Using exactly the same raw materials and steps as in steps (0) to (2) of Example 1, but without performing the impregnation step of (21), θ-iron carbide was prepared and labeled as D4.

[0142] Comparative Example 5

[0143] In this example, a θ-iron carbide composite was prepared using substantially the same raw materials and process as in Example 1, with the only difference being that in step (2), the mixing ratio of H2 and CO was 140:1. The finally prepared θ-iron carbide composite was labeled as DX5.

[0144] The θ-iron carbide composites, θ-iron carbide, etc. prepared in each example and comparative example were subjected to XRD, Mössbauer spectroscopy, and ICP measurements. Among them, the content of θ-iron carbide was calculated based on 100 mol, and the relevant contents all refer to the number of moles. The specific results are shown in Table 1.

[0145] In a slurry bed continuous reactor, the catalytic reaction performance of the θ-iron carbide composites and θ-iron carbide prepared in each example and comparative example was evaluated respectively. The catalyst loading was 9.0 g. Evaluation conditions: T = 283 °C, P = 2.75 MPa, H2:CO = 2.2:1, total amount of (H2+CO) = 15000 mL / h / g- Fe (standard state flow rate, relative to Fe element), and the circulation ratio was 1.2. The reaction was carried out, and the reaction products were analyzed by gas chromatography. The evaluation data of the reaction performance at 24 h and 300 h of the reaction are shown in Tables 2 and 3.

[0146] Table 1

[0147]

[0148]

[0149] Table 2

[0150]

[0151]

[0152] Table 3

[0153]

[0154]

[0155] Based on the above results, using the θ-iron carbide composite containing halogen ions such as bromine or iodine prepared in the examples of the present invention as a catalyst for the syngas conversion reaction under industrial conditions can exhibit an ultra-low CO2 selectivity, a relatively low CH4 selectivity, an extremely high carbon atom utilization efficiency, and an effective product selectivity while maintaining a high CO conversion rate (>60%). Further, a long-term experiment was carried out. From the data of the reaction at 300 h in Table 3, it can be seen that after the θ-iron carbide composite of the examples of the present invention was continuously operated in a stirred tank for a long period as a catalyst, its CO conversion rate, product selectivity, carbon atom utilization efficiency, and effective product selectivity all remained stable without obvious changes, showing good operation stability. Therefore, by using the θ-iron carbide composite of the examples of the present invention as a catalyst for the syngas conversion reaction, the comprehensive optimization of the reaction results can be achieved.

[0156] Unless otherwise specified, the terms used in the present invention have the meanings commonly understood by those skilled in the art.

[0157] The embodiments described in the present invention are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Those skilled in the art can make various other substitutions, changes, and improvements within the scope of the present invention. Therefore, the present invention is not limited to the above embodiments and is only defined by the claims.

Claims

1. A method for preparing a precipitated θ-iron carbide composite, comprising the following steps: (1) Under the action of H2, reducing the precipitated precursor at a temperature of 430-520 °C; and (2) Subjecting the product of the reduction treatment to carbide formation treatment at a temperature of 290-420 °C in a mixed gas atmosphere to obtain a product containing θ-iron carbide; Wherein, The mixed gas comprises hydrogen and carbon monoxide with a molar ratio of 5 to 90:1; React iron ions in the first solution to obtain a basic precipitate of iron; after calcining the basic precipitate of iron, perform an impregnation treatment to make it contain iodide ions and / or bromide ions, and use the product after impregnation as the precipitation-type precursor. The product containing θ-iron carbide in step (2) is the precipitation-type θ-iron carbide composite; or, React iron ions in the first solution to obtain a basic precipitate of iron; perform a calcination treatment on the basic precipitate of iron and use the calcined product as the precipitation-type precursor; after the precipitation-type precursor is treated through steps (1) to (2), perform an impregnation treatment on the obtained product containing θ-iron carbide to make it contain iodide ions and / or bromide ions.

2. The preparation method according to claim 1, wherein, The basic precipitate of iron is prepared by reacting iron ions with a precipitating agent anion in the first solution; the precipitating agent anion includes one or two of carbonate, bicarbonate, and hydroxide, and does not simultaneously contain hydroxide and bicarbonate; and / or, The calcined basic precipitate of iron or the product containing θ-iron carbide is impregnated through a second solution. The raw materials for preparing the second solution include a halide and an optional auxiliary agent. The halide includes a bromine-containing compound and / or an iodine-containing compound.

3. The preparation method according to claim 2, wherein, The raw materials for preparing the first solution include an iron salt and a basic precipitating agent; the iron salt includes at least one of iron nitrate, ammonium ferric citrate, ammonium ferrous sulfate, and ferric chloride, and the basic precipitating agent includes at least one of potassium hydroxide, potassium bicarbonate, sodium hydroxide, sodium bicarbonate, ammonia water, sodium carbonate, and potassium carbonate; and / or, The halide includes one or more of bromides and iodides containing manganese, copper, rare earth metal elements, cobalt, molybdenum, and iron; and / or, The mixed gas comprises hydrogen and carbon monoxide with a molar ratio of 45 to 60:1; and / or, The auxiliary agent includes one or more of salts of rare earth metals, alkali metals, manganese, cobalt, molybdenum, alkaline earth metals, and copper.

4. The preparation method according to claim 1, wherein, The treatment conditions in step (1) include: a pressure of 0.1 to 11 atm and a time of 0.7 to 15 h; and / or, The treatment temperature in step (2) is 350 to 400 °C; and / or, The treatment conditions in step (2) include: a pressure of 0 to 19 atm, preferably 0.01 to 12 atm, and a time of 3 to 72 h.

5. A θ-iron carbide composite prepared by the preparation method according to any one of claims 1 to 4.

6. The θ-iron carbide composite according to claim 5, comprising θ-iron carbide. Compared with 100 mol% of the θ-iron carbide, the composite further comprises 0.1 mol% to 45 mol% of bromide ions and / or iodide ions and 0 to 23 mol% of a first cation; Wherein, The molar number of θ-iron carbide is calculated based on the molar number of iron element it contains; the composite has an orthorhombic crystal structure and its average grain diameter is 5 nm to 40 nm.

7. The θ-iron carbide composite according to claim 6, which comprises 0.3 mol% to 27 mol% of bromide ions and / or iodide ions and 0.1 mol% to 20 mol% of a first cation; and / or, The first cation comprises one or more of second metal ions. Further, the first cation comprises one or more of alkali metal ions, cobalt ions, alkaline earth metal ions, molybdenum ions, copper ions, manganese ions, chromium ions, rare earth ions.

8. A catalyst, comprising the θ-iron carbide composite prepared by the preparation method according to any one of claims 1 to 4 or the θ-iron carbide composite according to any one of claims 5 to 7.

9. Use of the θ-iron carbide composite prepared by the preparation method according to any one of claims 1 to 4, the θ-iron carbide composite according to any one of claims 5 to 7, or the catalyst according to claim 8 in a reaction for synthesizing C and H fuels and / or chemicals based on the Fischer-Tropsch synthesis principle.

10. A syngas conversion process, comprising reacting the catalyst according to claim 8 in contact with syngas under reaction conditions.

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