COBALT-BASED CATALYTIC SYSTEM, CATALYST AND ITS USE IN CO2 CONVERSION

Abstract

The present invention relates to a catalytic system in the form of a colloidal suspension comprising a plurality of cobalt nanoparticles, characterized in that said nanoparticles have a number average diameter less than or equal to 5 nm, measured by transmission electron microscopy, and in that the standard deviation relative to the size of said nanoparticles is less than or equal to 50% of said average diameter.

Description

Title of the invention: COBALT-BASED CATALYTIC SYSTEM, CATALYST AND ITS USE IN CO2 CONVERSION Field of invention

[0001] The present invention relates to the synthesis of a catalytic system in the form of a colloidal suspension based on cobalt nanoparticles, a supported catalyst obtained by bringing such a suspension into contact with a support comprising at least one refractory oxide, and the use of the catalyst in the conversion of carbon dioxide (CO2) by reverse water gas shift (RWGS) reaction. State of the art

[0002] Current solutions for reducing greenhouse gas emissions and combating global warming are varied and are the subject of much research in the academic and industrial worlds. Improving the energy efficiency of current processes, developing renewable energies, and capturing, storing, and using CO2 are examples.

[0003] CO2 is a thermodynamically stable compound and its use in chemical products remains complex, although some examples exist. Examples include, at the industrial level, the synthesis of urea for the production of fertilizers or plastics, and at an advanced stage of research, the production of polycarbonates.

[0004] The hydrogenation of CO2 is an interesting recovery route which allows the production, depending on the operating conditions of the methanol, of carbon monoxide, which can themselves react and give access to a wide variety of compounds.

[0005] The reverse water gas shift reaction (also called here RWGS or Reverse Water Gas Shift according to the English terminology), makes it possible to transform CO2 into carbon monoxide (CO) in the presence of hydrogen. The main by-product of the hydrogenation of CO2 into CO is methane, the selectivity of which is favored at low temperature and in the presence of an excess of hydrogen. Oxide materials alone or mixed (spinels, CeO2, CuO / ZnO / Al2O3 composites), noble (Pt, Pd) or non-noble (Cu, Ni, Fe, Co) mono or bimetallic metals supported on generally reducible oxides (TiO2, ZrO2, CeO2) are known to selectively catalyze the RWGS reaction (Chen et al. Frontiers in Chemistry, 8,709, 2020).

[0006] The RWGS reaction is reversible and endothermic. It is therefore thermodynamically favored at high temperatures, generally above 600°C. However, operability at high temperatures is expensive and deactivation im- catalysts is observed.

[0007] There is a need to develop active, selective and stable catalysts for CO2 conversion.

[0008] In the literature, the most used catalysts for the RWGS reaction are based on Fe / Al2O3, Fe-Cu / Al2O3, Fe-Cs / Al2O3 and CuO-Fe2O3 doped or not with Cs. Conversions into CO2 are obtained but by implementing the process at high temperature, typically above 800°C. There is therefore an interest in developing a new process for converting CO2 into CO in the presence of a stable catalyst making it possible to obtain significant conversions into CO2, while allowing a gain in terms of selectivity of hydrogenation of CO2 into CO, and operating at a temperature below 800°C.

[0009] Surprisingly, the Applicant has discovered that the use of a supported cobalt-based catalyst comprising a nanoparticle size of very small diameter, i.e. having a number-average diameter less than or equal to 5 nm, with a narrow size distribution, obtained according to a very specific preparation process by colloidal route, makes it possible to significantly improve the selectivity of hydrogenation of CO2 into CO while maintaining good conversion. Subject of the invention

[0010] The subject of the present invention is a catalytic system in the form of a colloidal suspension comprising a plurality of cobalt nanoparticles, characterized in that said nanoparticles have a number average diameter less than or equal to 5 nm, measured by transmission electron microscopy, and in that the standard deviation relative to the size of said nanoparticles is less than or equal to 50% of said average diameter.

[0011] According to one or more embodiments according to the invention, said nanoparticles have a number average diameter less than or equal to 4 nm, and the standard deviation relative to the size of said nanoparticles is between 20% and 50% of said average diameter.

[0012] According to one or more embodiments according to the invention, said nanoparticles are in reduced form.

[0013] Another object according to the invention relates to a process for preparing a catalytic system according to the invention comprising at least the following steps:

[0014] a) at least one cobalt-based organometallic precursor is supplied;

[0015] b) said organometallic precursor supplied at the end of step a) in the presence of a solvent, at least one first stabilizing agent and at least one second stabilizing agent to obtain a colloidal suspension, said step b) comprising the following sub-steps:

[0016] bl) said organometallic precursor supplied at the end of step a) is brought into contact in the presence of the solvent to obtain a first solution;

[0017] b2) the first solution obtained at the end of step b1) is brought into contact with at least minus a first stabilizing agent to obtain a second solution;

[0018] b3) the second solution obtained at the end of step b2) is brought into contact with at at least one second stabilizing agent, different from the first stabilizing agent, to obtain the colloidal suspension;

[0019] c) said colloidal suspension obtained at the end of step b) is heated to a temperature between 25°C and 180°C under a hydrogen pressure between 0.01 MPa and 1 MPa to obtain said catalytic system.

[0020] According to one or more embodiments according to the invention, said organometallic precursor supplied in step a) is chosen from cobalt transition metal complexes, neutral or charged, containing one or more organic or inorganic ligands.

[0021] According to one or more embodiments according to the invention, the oxidation state of the cobalt is 0, 1, 2 or 3.

[0022] According to one or more embodiments according to the invention, said transition metal complexes comprise several ligands, identical or different from each other, linked or not to each other, and chosen from a hydride, fluoride, chloride, bromide, iodide group, an organic compound, preferably an alkyl, cyclic or not, having from 1 to 15 carbon atoms (in C1-C15) comprising or not one or more heteroelements and / or one or more multiple bonds, and / or an organic compound, preferably a substituted or unsubstituted aryl, having between 4 and 15 carbon atoms (in C4-C15) comprising or not one or more heteroelements, or a substituted or unsubstituted aryl having between 4 and 15 carbon atoms (in C4-C15) comprising or not one or more heteroelements.

[0023] According to one or more embodiments according to the invention, said organometallic cobalt precursor is chosen from bis-(hexamethyldisilazane)tetrahydrofuran cobalt(II) ICoi / iw / .s JoOhf)]; bis-[N,N'-di(isopropyl)methylcarbodiimide] of cobalt(II), [Co[iPrNC(CH3)NiPr]2]; bis-[N,N'-di(isopropyl)n-butylcarbodiimide] of cobalt(II), [Co[iPrNC(CH2CH2CH2CH3)NiPr]2]; bis-[N,N'-di(cyclohexyl)methylcarbodiimide] of cobalt(II), [Co[CyNC(CH3)NCy]2]; (1,5-cyclooctadiene)(cyclooctadienyl) cobalt (I), Co(q3-C8Hi3)(q4-C8Hi2).

[0024] According to one or more embodiments according to the invention, said first stabilizing agent and / or said second stabilizing agent are chosen from carboxylic acids, carboxylate anions, primary, secondary or tertiary amines, primary, secondary, tertiary or quaternary ammonium cations, amides, esters, alcohols, alcoholates, thiols, thiolates, saturated or unsaturated, having between 1 and 30 carbon atoms.

[0025] According to one or more embodiments according to the invention, the first sta- The bilising agent is palmitic acid and the second stabilizing agent is hexadecylamine.

[0026] Another object according to the invention relates to a process for preparing a catalyst comprising an active cobalt phase and a support comprising at least one refractory oxide, comprising the following steps:

[0027] i) said catalytic system according to the invention is brought into contact with a support comprising at least one refractory oxide to obtain a catalyst precursor;

[0028] ii) the catalyst precursor obtained at the end of step i) is dried at a temperature below 250°C to obtain a dried catalyst precursor;

[0029] iii) the dried catalyst precursor obtained in step ii) is calcined at a temperature between 250°C and 550°C to obtain said catalyst.

[0030] Another subject according to the invention relates to a catalyst comprising an active cobalt phase and a support comprising at least one refractory oxide, characterized in that said active phase is in the form of nanoparticles with a mean diameter in number less than or equal to 5 nm, measured in oxide form, and in that the standard deviation relative to the size of said nanoparticles is less than or equal to 50% of said mean diameter.

[0031] According to one or more embodiments according to the invention, the cobalt content is between 0.1 and 15% by weight of cobalt element relative to the total weight of the catalyst.

[0032] According to one or more embodiments according to the invention, the support is chosen from alumina, silica, and silica-alumina.

[0033] Another subject matter according to the invention relates to a process for hydrogenating CO2 by implementing the reverse water gas conversion (RWGS) process by bringing a feedstock of a gas mixture comprising CO2 and hydrogen into contact with a catalyst according to the invention, which process is carried out at a temperature of between 0°C and 600°C, at a pressure of between 0.1 MPa and 5 MPa, at a hydrogen / CO2 molar ratio of between 0.1 and 10 and at an hourly volumetric flow rate of between 100 h 1 and 40,000 h 1. Detailed description of the invention 1. Definitions

[0034] In the following, the groups of chemical elements are given according to the CAS classification (CRC Handbook of Chemistry and Physics, publisher CRC press, editor-in-chief DR Lide, 81st edition, 2000-2001). For example, group VIII (or VIIIB) according to the CAS classification corresponds to the metals of columns 8, 9 and 10 according to the new IUP AC classification.

[0035] In the present description, according to the IUP AC convention, micropores are understood to mean pores whose diameter is less than 2 nm, i.e. 0.002 pm; mesopores are understood to mean pores whose diameter is greater than or equal to 2 nm, i.e. 0.002 pm and less than or equal to 50 nm, i.e. 0.05 pm and by macropores pores whose diameter is greater than 50 nm, i.e. 0.05 pm.

[0036] The total pore volume is measured by mercury porosimetry according to ASTM D4284-92 with a wetting angle of 140°, for example using an Autopore III™ model device from Microméritics™.

[0037] The BET specific surface area is measured by nitrogen physisorption according to ASTM D3663-03, a method described in the book Rouquerol F.; Rouquerol J.; Singh K. “Adsorption by Powders & Porous Solids: Principle, methodology and applications”, Academie Press, 1999.

[0038] The average diameter of the cobalt nanoparticles in the catalytic suspension is determined by transmission electron microscopy (TEM or Transmission Electron Microscopy according to the Anglo-Saxon terminology).

[0039] The average diameter of the cobalt nanoparticles in the catalyst corresponds to the diameter of the cobalt crystallites in oxide form. The diameter of the cobalt crystallites in oxide form is determined by X-ray diffraction, from the width of the diffraction line located at the angle 2theta=43° (i.e. along the crystallographic direction

[200] ) using the Scherrer relationship. This method, used in X-ray diffraction on powders or polycrystalline samples which relates the width at half-height of the diffraction peaks to the particle size, is described in detail in the reference: Appl. Cryst. (1978), 11, 102-113 “Scherrer after sixty years: A survey and some new results in the determination of crystallite size”, JI Langford and AJC Wilson.

[0040] The cobalt content is measured by X-ray fluorescence or by induction plasma spectroscopy coupled with an atomic emission spectrometer (ICP-AES or Inductively coupled plasma atomic emission spectroscopy according to the English terminology).

[0041] In the present description, the term "comprise" is synonymous with (means the same as) "include" and "contain", and is inclusive or open and does not exclude other elements not recited. It is understood that the term "comprise" includes the exclusive and closed term "consist". Furthermore, in the present description, the term "substantially" corresponds to an approximation of ± 10%, preferably ± 5%, very preferably ± 2%, of a reference value such as a distance, a speed, a flow rate, a compound content, a temperature, a pressure, etc. 2. Catalytic system

[0042] An object according to the invention relates to a catalytic system in the form of a colloidal suspension comprising, preferably consisting of, a plurality of cobalt nanoparticles, characterized in that said nanoparticles have a number-average diameter less than or equal to 5 nm, measured by electron microscopy. electronics in transmission, preferably less than or equal to 4 nm, and more preferably less than or equal to 3 nm, and in that the standard deviation relative to the size of said nanoparticles is less than or equal to 50% of said average diameter, preferably between 20% and 50%, more preferably between 20% and 35%, and even more preferably between 22% and 28%.

[0043] The person skilled in the art knows the techniques suitable for determining the average particle diameter and also knows the degree of uncertainty existing in these measurements. For example, the average particle diameter of a set, the standard deviation and the size distribution can be determined by statistical studies from microscopy images, and in particular by transmission electron microscopy (TEM). The number average diameter is calculated on at least 250 nanoparticles. The standard deviation is calculated as the square root of the variance.

[0044] Preferably, said cobalt nanoparticles are in reduced form, i.e. in the zero valence state. 3. Process for preparing the catalytic system

[0045] The catalytic system according to the invention is prepared according to a particular preparation process comprising at least the following steps:

[0046] a) at least one cobalt-based organometallic precursor is supplied;

[0047] b) said organometallic precursor supplied at the end of step a) in the presence of a solvent, at least one first stabilizing agent and at least one second stabilizing agent to obtain a colloidal suspension, said step b) comprising the following sub-steps:

[0048] bl) said organometallic precursor supplied at the end of step a) is brought into contact in the presence of the solvent to obtain a first solution;

[0049] b2) the first solution obtained at the end of step b1) is brought into contact with at least minus a first stabilizing agent to obtain a second solution;

[0050] b3) the second solution obtained at the end of step b2) is brought into contact with at least at least one second stabilizing agent, different from the first stabilizing agent, to obtain the colloidal suspension;

[0051] c) said colloidal suspension obtained at the end of step b) is heated to a temperature between 25°C and 180°C under a hydrogen pressure between 0.01 MPa and 1 MPa to obtain said catalytic system.

[0052] Steps a) to c) are described in more detail below. Step a)

[0053] According to step a) of the process for preparing the catalytic system according to the invention, at least one organometallic cobalt precursor is supplied.

[0054] More particularly, said metal precursor supplied in step a) is selected from neutral or charged cobalt transition metal complexes containing one or more organic or inorganic ligands.

[0055] When said metal precursor is selected from cobalt transition metal complexes, the oxidation state of cobalt in the complex is 0, 1, 2 or 3. Preferably the oxidation state of cobalt in the complex is 2.

[0056] Advantageously, the transition metal complexes may contain neutral, anionic or cationic ligands. The ligands may form single, double or triple bonds with the metal center. The ligands may be identical or different from each other, bonded or not to each other, and are preferably chosen from a hydride, fluoride, chloride, bromide, iodide group, an organic compound, preferably an alkyl, cyclic or not, having from 1 to 15 carbon atoms (C1-C15) comprising or not one or more heteroelements and / or one or more multiple bonds, and / or an organic compound, preferably a substituted or unsubstituted aryl, having between 4 and 15 carbon atoms (C4-C15) comprising or not one or more heteroelements, or a substituted or unsubstituted aryl having between 4 and 15 carbon atoms (C4-C15) comprising or not one or more heteroelements.

[0057] The donor atom, that is to say the one which forms the bonds with the metal center, can be chosen from the elements H, C, N, O, Si, P, S, F, Cl, Br, I. Preferably, the donor atom is chosen from N or O.

[0058] In a particular non-limiting case, the organometallic complex may be chosen from multinucleated coordination complexes containing two or more metals, which may be identical or different. The multinucleated complexes may be linked together by one or more bridging ligands, and / or by the formation of metal-metal type chemical bonds.

[0059] In one embodiment according to the invention, said cobalt-based organometallic precursor is chosen from bis-(hexamethyldisilazane)tetrahydrofuran cobalt(II) [Co( / zm6?5)2(thf)]; cobalt(II) bis-[N,N'-di(isopropyl)methylcarbodiimide], [Cop PrNC(CH3)NiPr]2]; cobalt(II) bis-[N,N'-di(isopropyl)n-butylcarbodiimide], |Co| PrNC(CH2CH2CH2CH3)NiPr]2]; cobalt(II) bis-[N,N'-di(cyclohexyl)methylcarbodiimide], [Co[CyNC(CH3)NCy]2]; (1,5-cyclooctadiene)(cyclooctadienyl) cobalt(I), Co(q3-C8Hi3)(q4-C8Hi2). Preferably, said cobalt-based organometallic precursor is the cobalt(II) bis-(hexamethyldisilazane)tetrahydrofuran complex [Co( AmtA^tlif)] whose chemical structure is represented below:

[0060] [Chem.l]

[0061] The solvent may be chosen from organic solvents and in particular from ethers, alcohols, chlorinated solvents and saturated, unsaturated, aromatic or non-aromatic, cyclic or non-cyclic hydrocarbons. Preferably, the solvent is chosen from hexane, cyclohexane, methylcyclohexane, heptane, butane or isobutane, mono-olefins or diolefins preferably comprising 4 to 20 carbon atoms, cycloocta-1,5-diene, benzene, toluene, orthoxylene, mesitylene, ethylbenzene, dichloromethane, chlorobenzene, methanol, ethanol, pure or as a mixture, and ionic liquids. In the case where the solvent is an ionic liquid, it is advantageously chosen from the ionic liquids described in patents US 6,951,831 B2 and FR 2895406 B1. Step b)

[0062] According to step b) of the process for preparing the catalytic system according to the invention, said organometallic cobalt precursor supplied in step a) is brought into contact in the presence of a solvent, at least one first stabilizing agent and at least one second stabilizing agent to obtain a colloidal suspension.

[0063] The role of the first stabilizing agent is to form a stabilizing complex before the nucleation phase but also to stabilize the suspended nanoparticles obtained after the nucleation phase.

[0064] The role of the second stabilizing agent is to initiate the nucleation phase allowing the formation of the suspended bimetallic nanoparticles while playing a stabilizing role once the suspended nanoparticles have been formed.

[0065] More particularly, step b) comprises the following sub-steps:

[0066] bl) said organometallic precursors supplied at the end of step a) are brought into contact in the presence of a solvent to obtain a first solution;

[0067] b2) the first solution obtained at the end of step b1) is brought into contact with a first stabilizing agent to obtain a second solution;

[0068] b3) the second solution obtained at the end of step b2) is brought into contact with a second stabilizing agent, different from the first stabilizing agent, to obtain the colloidal suspension.

[0069] In step b1) the solvent may be identical or different, preferably identical, to the or solvent(s) optionally used in step a). The solvent may be chosen from organic solvents and in particular from ethers, alcohols, chlorinated solvents and saturated, unsaturated, aromatic or non-aromatic, cyclic or non-cyclic hydrocarbons. Preferably, the solvent is chosen from hexane, cyclohexane, methylcyclohexane, heptane, butane or isobutane, monoolefins or diolefins preferably comprising 4 to 20 carbon atoms, cycloocta-1,5-diene, benzene, toluene, orthoxylene, mesitylene, ethylbenzene, dichloromethane, chlorobenzene, methanol, ethanol, pure or as a mixture, and ionic liquids. Preferably, the solvent is mesitylene. In the case where the solvent is an ionic liquid, it is advantageously chosen from the ionic liquids described in patents US 6,951,831 B2 and FR 2895406 B1.

[0070] In step b2) the first stabilizing agent may be chosen from carboxylic acids, carboxylate anions, primary, secondary or tertiary amines, primary, secondary, tertiary or quaternary ammonium cations, amides, esters, alcohols, alcoholates, thiols, thiolates, saturated or unsaturated, having between 1 and 30 carbon atoms.

[0071] Preferably, said first stabilizing agent is palmitic acid.

[0072] In step b3), the second stabilizing agent may be chosen from carboxylic acids, carboxylate anions, primary, secondary or tertiary amines, primary, secondary, tertiary or quaternary ammonium cations, amides, esters, alcohols, alcoholates, thiols, thiolates, saturated or unsaturated, having between 1 and 30 carbon atoms.

[0073] Preferably, the second stabilizing agent is hexadecylamine.

[0074] Preferably, the molar ratio between the stabilizing agents supplied in step b) and said organometallic precursors supplied in step a) is greater than 0 and less than or equal to 10, preferably between 0.05 and 5, and even more preferably between 0.05 and 1.5.

[0075] Preferably, the molar ratio between the first stabilizing agent and said organometallic precursors supplied in step a) is less than 0.5, more preferably between 0.1 and 0.3.

[0076] Preferably, the molar ratio between the second stabilizing agent and said organometallic precursors supplied in step a) is less than 1.5, more preferably between 0.8 and 1.2. Step c)

[0077] After step b) of contacting, said colloidal suspension obtained at the end of step b) is heated to a temperature of between 25°C and 180°C, preferably between 30°C and 170°C, under a hydrogen pressure of between 0.01 MPa and 1 MPa, preferably between 0.1 and 0.3 MPa, to obtain said catalytic system, preferably for a period of between 1 and 24 hours, preferably between 2 and 20 hours.

[0078] The concentration of said organometallic precursors contained in the solution obtained at the end of step c) is less than or equal to 1 mol / L, preferably between 0.001 mol / L and 0.1 mol / L, and very preferably between 0.025 mol / L and 0.75 mol / L. 4. Process for preparing a catalyst

[0079] Another subject according to the invention relates to a process for preparing a catalyst comprising a cobalt-based active phase and a support comprising at least one refractory oxide, comprising the following steps:

[0080] i) a catalytic system according to the invention is brought into contact with a support comprising at least one refractory oxide to obtain a catalyst precursor;

[0081] ii) the catalyst precursor obtained at the end of step i) is dried at a temperature below 250°C to obtain a dried catalyst precursor;

[0082] iii) the catalyst obtained in step ii) is calcined at a temperature between 250°C and 550°C to obtain said catalyst;

[0083] iv) optionally, the catalyst obtained at the end of step iii) is reduced by contacting with a reducing gas to obtain a reduced catalyst.

[0084] Preferably, step i) of contacting is carried out by impregnating the support with a volume of colloidal suspension of between 0.25 and 1.5 times the pore volume of the support to be impregnated.

[0085] The catalyst precursor obtained at the end of step i) is then dried at a temperature below 250°C, preferably between 15°C and 180°C, more preferably between 30°C and 160°C, even more preferably between 50°C and 150°C, and even more preferably between 70°C and 140°C, for a duration typically between 0.5 hours and 12 hours, and more preferably for a duration between 0.5 hours and 5 hours. Longer durations are not excluded, but do not necessarily provide an improvement.

[0086] The drying step can be carried out by any technique known to those skilled in the art. It is advantageously carried out under an inert atmosphere or under an atmosphere containing oxygen or under a mixture of inert gas and oxygen. It is advantageously carried out at atmospheric pressure or at reduced pressure. Preferably, this step is carried out at atmospheric pressure and in the presence of air or nitrogen.

[0087] After drying, the catalyst obtained at the end of step ii) is calcined (step iii)) at a temperature of between 250°C and 550°C, for a period typically of between 0.5 hours and 24 hours, preferably for a period of between 0.5 hours and 12 hours, and even more preferably for a period of between 0.5 hours and 10 hours, preferably under an inert atmosphere or under an atmosphere containing oxygen. Longer durations are not excluded, but do not necessarily bring an improvement. A calcined catalyst is obtained.

[0088] The calcined catalyst obtained at the end of step iii) can be reduced. This step is preferably carried out in the presence of a reducing gas, either in situ, i.e. in the reactor where the catalytic transformation is carried out, or ex situ. Preferably, this step is carried out at a temperature between 80°C and 450°C, even more preferably between 100°C and 400°C.

[0089] The reduction is carried out in the presence of a reducing gas comprising between 25 vol% and 100 vol% of hydrogen, preferably 100% volume of hydrogen. The hydrogen is optionally supplemented by an inert gas for the reduction, preferably argon, nitrogen or methane.

[0090] The reduction generally includes a temperature increase phase followed by a plateau.

[0091] The duration of the reduction stage is generally between 1 hour and 40 hours, preferably between 2 hours and 20 hours.

[0092] The Hourly Volumetric Velocity (HVV) is generally between 150 and 3000, preferably between 300 and 1500 liters of reducing gas per hour and per liter of catalyst. 5. Catalyst

[0093] Another subject according to the invention relates to a catalyst comprising a cobalt-based active phase and a support comprising at least one refractory oxide, characterized in that said active phase is in the form of cobalt nanoparticles with a number-average diameter less than or equal to 5 nm, measured in oxide form, more preferably less than or equal to 4 nm, and even more preferably less than or equal to 3 nm, and in that the standard deviation relative to the size of said nanoparticles is less than or equal to 50% of said average diameter. Such a catalyst can be obtained by the preparation process as described above.

[0094] The cobalt content is generally between 0.1 and 15% by weight of cobalt element relative to the total weight of the catalyst, preferably between 0.5 and 10% by weight, and even more preferably between 1 and 7.5% by weight.

[0095] The specific surface area of ​​the catalyst is generally between 10 m2 / g and 450 m2 / g, preferably between 25 m2 / g and 400 m2 / g, more preferably between 45 m2 / g and 370 m2 / g, and even more preferably between 65 m2 / g and 350 m2 / g.

[0096] The total pore volume of the catalyst is generally between 0.1 and 1 ml / g, preferably between 0.2 ml / g and 0.9 ml / g, and particularly preferably between 0.3 ml / g and 0.8 ml / g. 6. Support

[0097] The support comprises at least one refractory oxide, preferably selected from the group consisting of magnesium, titanium, zirconium, aluminum and silicon oxides.

[0098] Preferably, the support is based on alumina, silica, or silica-alumina. Preferably, the support is made of alumina, silica, or silica-alumina.

[0099] When the support is based on alumina, it is understood that it comprises at least 95%, preferably at least 98%, and particularly preferably at least 99% by weight of alumina relative to the weight of the support. The alumina generally has a crystallographic structure of the delta, gamma or theta alumina type, alone or as a mixture.

[0100] When the support is silica-based, it is understood that it comprises at least 95%, preferably at least 98%, and particularly preferably at least 99% by weight of silica relative to the weight of the support. The silica generally has an amorphous crystallographic structure, but may contain crystalline silica quartz, tridymite, cristobalite alone or in a mixture. The silica may be fused silica.

[0101] When the support is based on silica-alumina, the silica content is between 1 and 95% by weight, preferably between 5 and 90% and more preferably between 10 and 80% by weight relative to the total weight of the support. The alumina content is between 1 and 95% by weight, preferably between 5 and 90% by weight and more preferably between 10 and 80% by weight relative to the total weight of the support. The silica-alumina can be prepared by any method known to those skilled in the art such as the sol-powder, sol-gel, coprecipitation or cogelation method. The silica-alumina contains, depending on its method of preparation, domains of amorphous crystallographic structure and of delta, gamma or theta alumina type, alone or as a mixture.

[0102] The silica, alumina, or silica-alumina-based support may comprise impurities such as metal oxides from groups IIA, IIIB, IVB, IIB, IIIA, IVA according to the CAS classification, preferably alumina, titanium dioxide, zirconium dioxide, zinc oxide, magnesium oxide and calcium oxide, or alkali metals, preferably lithium, sodium or potassium, and / or alkaline earth metals, preferably magnesium, calcium, strontium or barium or sulfur.

[0103] The specific surface area of ​​the support is generally between 10 m2 / g and 450 m2 / g, preferably between 30 m2 / g and 400 m2 / g, more preferably between 50 m2 / g and 370 m2 / g, and even more preferably between 60 m2 / g and 350 m2 / g.

[0104] The pore volume of the support is generally between 0.1 ml / g and 1.2 ml / g, preferably between 0.3 ml / g and 1 ml / g, and very preferably between 0.5 ml / g and 0.9 ml / g.

[0105] The mesoporous median diameter of the support is advantageously between 3 nm and 25 nm, and preferably between 6 nm and 20 nm, and particularly preferably between 8 nm and 18 nm.

[0106] The catalyst support is generally in the form of grains advantageously having a diameter of between 0.5 mm and 10 mm. The grains may have any shape known to those skilled in the art, for example the shape of balls (preferably having a diameter of between 1 mm and 8 mm), extrudates, tablets, hollow cylinders. Preferably, the support is in the form of extrudates with a diameter of between 0.5 mm and 10 mm, preferably between 0.8 mm and 3.2 mm and very preferably between 1.0 mm and 2.5 mm and a length of between 0.5 mm and 20 mm. The term "diameter" of the extrudates is understood to mean the diameter of the circle circumscribed to the cross section of these extrudates. The support may advantageously be in the form of cylindrical, multi-lobed, preferably tri-lobed or quadri-lobed extrudates. The shape of the lobes can be adjusted according to any method known in the prior art. 7. CO2 hydrogenation process by RWGS

[0107] The present invention also relates to a process for the hydrogenation of CO2 by reverse water gas reaction (RWGS) by bringing a feedstock of a gas mixture comprising CO2 and hydrogen into contact with a catalyst according to the invention, which process is carried out at a temperature of between 0°C and 600°C, at a pressure of between 0.1 MPa and 5 MPa, at a hydrogen / CO2 molar ratio of between 0.1 and 10 and at an hourly volumetric flow rate of between 100 h 1 and 40,000 h 1.

[0108] In the process according to the invention, CO2 is selectively hydrogenated to CO by RWGS reaction in the presence of a specific catalyst as described below. The effluents contain CO, water, and optionally unconverted hydrogen and CO2. By-products of the reaction, such as methane, methanol or other alcohols, alkenes, ethers and / or alkanes may also be present in the effluent from the RWGS reaction. The feedstock

[0109] The CO2 contained in the gas mixture feed can come from various sources. The necessary CO2 can be provided by a unit for separating a stream comprising CO2 (e.g. combustion fumes) or a CO2 capture unit (e.g. CO2 present in the air).

[0110] The hydrogen required for the conversion of CO2 can be produced by a water electrolysis unit, said water being able to come from the effluent of the RWGS reaction unit and optionally from downstream units (for example Fischer-Tropsch (FT) or alcohol synthesis units). Preferably, the use of the water electrolysis unit water to treat the water produced by the RWGS reaction unit also makes it possible to minimize the environmental impact of the process. Thus, the process according to the invention does not require an external supply of hydrogen, for example produced by steam reforming of natural gas. The electrolyzer can preferably operate with low-carbon electricity, which contributes to the renewable nature of the synthesis gas and the hydrocarbons that will then be produced from this synthesis gas. In addition, the water used for the production of hydrogen can come at least in part from the recycling of the water produced by the RWGS reaction, which has the advantage of limiting the external supply of water.

[0111] The hydrogen / CO2 molar ratio is preferably between 0.1 and 10, preferably between 0.5 and 5, more preferably between 1 and 3, and even more preferably between 1 and 2.

[0112] The gas mixture constituting the charge may contain, in addition to carbon dioxide and hydrogen, other gaseous elements such as water vapor, alkanes such as methane, propane and / or isobutane. Implementation

[0113] The technological implementation of the RWGS process is for example carried out by injection, in ascending or descending flow, of the feedstock, i.e. the gas mixture comprising CO2 and hydrogen, into at least one fixed-bed or fluid-bed reactor. Preferably, the reactor is a fixed-bed reactor. Said reactor may be of the isothermal type or of the adiabatic type. An adiabatic reactor is preferred.

[0114] The feedstock can advantageously be diluted by one or more reinjections of the effluent, from said reactor where the selective hydrogenation reaction takes place, at various points in the reactor, located between the inlet and the outlet of the reactor in order to limit the temperature gradient in the reactor.

[0115] In an embodiment according to the invention, the RWGS reaction is carried out with a hydrogen / CO2 molar ratio of between 0.1 and 10, preferably between 0.5 and 5, more preferably between 1 and 3, and even more preferably between 1 and 2, a temperature of between 0°C and 600°C, preferably between 25°C and 600°C, and even more preferably between 30°C and 550°C, an hourly volumetric velocity (HVV) of between 100 h 1 and 40000 h1, preferably between 2000 h 1 and 20000 h1, and a pressure of between 0.1 MPa and 5.0 MPa, preferably between 0.1 MPa and 4.0 MPa, and even more preferably between 0.1 MPa and 3.0 MPa.

[0116] Preferably, the catalyst is pre-heat-treated in the reactor under a reducing atmosphere before injection of the charge, at a temperature between 100°C and 600°C, preferably between 150°C and 500°C and for a duration between 30 minutes and 12 hours. Examples

[0117] The following examples illustrate the invention without limiting its scope.

[0118] For all the catalysts mentioned in the examples mentioned below, the support is an alumina A having a specific surface area of ​​80 m2 / g, a pore volume of 0.7 mL / g and a median mesoporous diameter of 12 nm.

[0119] Example 1 (non-compliant: 18 nm): Catalytic system based on Cobalt alone prepared by colloidal route

[0120] In a glove box, a solution of 226 mg (0.5 mmol Co) of the complex [Co( / zmtA)2(thf)] in 4 mL of mesitylene is prepared. This solution is introduced into a 120 mL capacity Fisher-Porter bottle with stirring. The bottle is then closed and removed from the glove box, pressurized to 0.2 MPa of H2, and the reaction medium is heated to 150 °C for 18 hours with stirring.

[0121] At the end of the reaction, the solution obtained is dry impregnated onto the alumina A described above. The solid obtained is then calcined at 650°C for 4 hours.

[0122] For this catalyst A, the average diameter of the Co3O4 particles characterized by X-ray diffraction is 18 nm. The cobalt content measured by ICP-AES is 5% by weight relative to the total weight of the catalyst.

[0123] Example 2 (non-compliant: 13 nm): Catalytic system based on Cobalt alone prepared by colloidal route

[0124] In a glove box, a solution of 226 mg (0.5 mmol Co) of the complex [Co( / zmtA)2(thf)] in 4 mL of mesitylene is prepared. This solution is introduced into a Fisher-Porter bottle of 120 mL capacity while stirring. Then, 25.5 mg (0.1 mmol) of palmitic acid dissolved in 8 mL of mesitylene are added to the medium while stirring. The bottle is then closed and removed from the glove box, pressurized to 0.2 MPa of H2, and the reaction medium is heated to 150°C for 18 hours while stirring.

[0125] At the end of the reaction, the solution obtained is dry impregnated onto the alumina A described above. The solid obtained is then calcined at 600°C for 4 hours.

[0126] For this catalyst B, the average diameter of the Co3O4 particles characterized by X-ray diffraction is 13 nm. The cobalt content measured by ICP-AES is 5% by weight relative to the total weight of the catalyst.

[0127] Example 3 (non-compliant: 11 nm): Catalytic system based on Cobalt alone prepared by colloidal route

[0128] In a glove box, a solution of 226 mg (0.5 mmol Co) of the complex [Co( / zmtA)2(thf)] in 4 mL of mesitylene is prepared. This solution is introduced into a 120 mL capacity Fisher-Porter bottle with stirring. A solution of 241 mg (1 mmol) of 1-hexadecylamine dissolved in 4 mL of mesitylene is introduced in the Fisher-Porter. The bottle is then closed and removed from the glove box, pressurized to 0.2 MPa of H2, and the reaction medium is heated to 150°C for 18 hours with stirring.

[0129] At the end of the reaction, the solution obtained is dry impregnated onto the alumina A described above. The solid obtained is then calcined at 450°C for 4 hours.

[0130] For this catalyst C, the average diameter of the Co3O4 particles characterized by X-ray diffraction is 11 nm. The cobalt content measured by ICP-AES is 5% by weight relative to the total weight of the catalyst.

[0131] Example 4 (non-compliant: 6 nm): Catalytic system based on Cobalt alone prepared by colloidal route

[0132] In a glove box, a solution of 226 mg (0.5 mmol Co) of the complex [Co( / zmtA)2(thf)] in 4 mL of mesitylene is prepared. This solution is introduced into a Fisher-Porter bottle of 120 mL capacity while stirring. Then, 25.5 mg (0.1 mmol) of palmitic acid dissolved in 8 mL of mesitylene are added to the medium while stirring. Then, a solution of 120 mg (0.5 mmol) of 1-hexadecylamine dissolved in 4 mL of mesitylene is introduced into the Fisher-Porter. The bottle is then closed and removed from the glove box, pressurized to 0.2 MPa of H2, and the reaction medium is heated to 150°C for 18 hours while stirring.

[0133] At the end of the reaction, the solution obtained is dry impregnated onto the alumina A described above. The solid obtained is then calcined at 450°C for 4 hours.

[0134] For this catalyst D, the average diameter of the Co3O4 particles characterized by X-ray diffraction is 6 nm. The cobalt content measured by ICP-AES is 5% by weight relative to the total weight of the catalyst.

[0135] Example 5 (compliant: 5 nm): Catalytic system based on Cobalt alone prepared by colloidal route

[0136] In a glove box, a solution of 226 mg (0.5 mmol Co) of the complex [Co( / zmtA)2(thf)] in 4 mL of mesitylene is prepared. This solution is introduced into a Fisher-Porter bottle of 120 mL capacity while stirring. Then, 51 mg (0.2 mmol) of palmitic acid dissolved in 8 mL of mesitylene are added to the medium while stirring. Then, a solution of 241 mg (1 mmol) of 1-hexadecylamine dissolved in 4 mL of mesitylene is introduced into the Fisher-Porter. The bottle is then closed and removed from the glove box, pressurized to 0.2 MPa of H2, and the reaction medium is heated to 150°C for 18 hours while stirring.

[0137] At the end of the reaction, the solution obtained is dry impregnated onto the alumina A described above. The solid obtained is then calcined at 450°C for 4 hours.

[0138] For this catalyst E, the average diameter of the Co3O4 particles characterized by X-ray diffraction is 5 nm. The cobalt content measured by ICP-AES is 5% by weight relative to the total weight of the catalyst.

[0139] Example 6 (compliant: 4 nm): Catalytic system based on Cobalt alone prepared by colloidal route

[0140] In a glove box, a solution of 226 mg (0.5 mmol Co) of the complex [Co( / zm6?5)2(thf)] in 4 mL of mesitylene is prepared. This solution is introduced into a Fisher-Porter bottle of 120 mL capacity under stirring. Then, 51 mg (0.2 mmol) of palmitic acid dissolved in 8 mL of mesitylene are added to the medium under stirring. Then, a solution of 241 mg (1 mmol) of 1-hexadecylamine dissolved in 4 mL of mesitylene is introduced into the Fisher-Porter. The bottle is then closed and removed from the glove box, pressurized to 0.2 MPa of H2, and the reaction medium is heated to 150°C for 18 hours under stirring.

[0141] At the end of the reaction, the solution obtained is dry impregnated onto the alumina A described above. The solid obtained is then calcined at 350°C for 4 hours.

[0142] For this catalyst F, the average diameter of the Co3O4 particles characterized by X-ray diffraction is 4 nm. The cobalt content measured by ICP-AES is 5% by weight relative to the total weight of the catalyst.

[0143] Example 7: Catalytic tests, performance in hydrogenation of carbon dioxide

[0144] The catalysts described in the examples above are tested for the CO2 hydrogenation reaction by RWGS.

[0145] The catalysts are tested with an Avantium® unit comprising 16 parallel stainless steel reactors, with an internal diameter of 2 mm and a length of 560 mm. The reaction is carried out in the gas phase, at a pressure of 1.2 bar absolute (0.12 MPaa). The feed gases are CO2 and hydrogen with an H2 / CO2 molar ratio of 1.1. The CO2 and H2 are co-injected in a downflow and mixed upstream of the reactor head. The feed comprising the gas mixture containing CO2 and hydrogen is then heated to 300°C in the first part of the reactor. The catalyst is loaded in the form of extrudates with a diameter of 1.6 mm. The mass of catalyst loaded per reactor is 100 mg, corresponding to a loading height of 6 cm to 15 cm depending on the catalyst.

[0146] At the unit outlet, the products are analyzed by gas chromatography.

[0147] The products observed are mainly carbon monoxide (CO), hydrogen (H2), unconverted carbon dioxide (CO2), methanol (CH3OH), methane (CH4), and traces of hydrocarbon products (alkenes, alkanes) and alcohols other than methanol.

[0148] The performance of the catalyst is defined by the conversion to CO2 and the selectivity to CO.

[0149] The conversion to CO2 is defined according to the following formula:

[0150] CO2 Conversion (%) = 100 X [Q (CO2) in - Q (CO2) out] / [Q (CO2) in]

[0151] with: - Q (CO2)inlet ■ molar flow rate of CO2 inlet in NmL / min - Q (CO2)output: molar flow rate of CO2 output in NmL / min

[0152] The molar selectivity in CO (also called here SCO) is defined according to the following formula:

[0153] SCO (%) = Q (CO) output / (Q (CO2) input - Q (CO2) output)

[0154] For each test, catalysts A to F are treated at atmospheric pressure at 300°C, for 4 hours with a hydrogen flow rate of 12.5 Nml / min per reactor corresponding to a VVH of approximately 10,000 h1.

[0155] The H2 / CO2 =1.1 molar mixture charge is then injected into the reactor at 300°C for 4 hours with a gas flow rate of 12.5 Nml / min per reactor, i.e. a vvh of approximately 10 OOOh *.

[0156] The temperature is then increased to 500°C in temperature increments of 25°C with a ramp of 300°C between each stage. For each new temperature, the analyses are triggered after 1 hour of stabilization under load. The total duration of each stage is 4 hours.

[0157] The test protocol makes it possible to know the evolution of conversion with temperature and of CO selectivity with conversion.

[0158] A first series of tests is carried out on catalysts A to F. The results are shown in Table 1 below.

[0159] [Tables 1] Catalyst Average diameter (nm) Conversion 500°C (%) CO selectivity 500°C (%) A (non-compliant) 18 32 60 B (non-compliant) 13 32 61 C (non-compliant) 11 32 60 D (non-compliant) 6 32 59 E (compliant) 5 32 93 F (compliant) 4 32 97

[0160] Catalysts A, B, C, D have good CO2 conversions but very limited CO selectivity unlike catalysts E and F. Particle sizes that are too large, particularly due to the absence of stabilizing ligands during the preparation of the catalytic suspension or a too limited quantity of these (cf. catalysts A, B and C) leads to limited CO selectivity. Furthermore, too high a calcination temperature (catalysts A and B) leads to sintering particles on the support while the colloidal suspension had particle sizes less than or equal to 5 nm. Catalysts E and F prepared by impregnation of a colloidal cobalt solution on an alumina support show the best results in terms of CO conversion and selectivity.

Claims

Claims

1. Catalytic system in the form of a colloidal suspension comprising a plurality of cobalt nanoparticles, characterized in that said nanoparticles have a number average diameter less than or equal to 5 nm, measured by transmission electron microscopy, and in that the standard deviation relative to the size of said nanoparticles is less than or equal to 50% of said average diameter.

2. System according to claim 1, characterized in that said nanoparticles have a number average diameter less than or equal to 4 nm, and in that the standard deviation relative to the size of said nanoparticles is between 20% and 50% of said average diameter.

3. System according to any one of claims 1 or 2, characterized in that said nanoparticles are in reduced form.

4. A process for preparing a catalytic system according to any one of claims 1 to 3, comprising at least the following steps: a) at least one cobalt-based organometallic precursor is supplied; b) said organometallic precursor supplied at the end of step a) is brought into contact in the presence of a solvent, at least one first stabilizing agent and at least one second stabilizing agent to obtain a colloidal suspension, said step b) comprising the following sub-steps: b1) said organometallic precursor supplied at the end of step a) is brought into contact in the presence of the solvent to obtain a first solution; b2) the first solution obtained at the end of step b1) is brought into contact with at least one first stabilizing agent to obtain a second solution;b3) the second solution obtained at the end of step b2) is brought into contact with at least one second stabilizing agent, different from the first stabilizing agent, to obtain the colloidal suspension; c) said colloidal suspension obtained at the end of step b) is heated to a temperature of between 25°C and 180°C under a hydrogen pressure of between 0.01 and 1 MPa to obtain said catalytic system.;

5. A method according to claim 4, wherein said organometallic precursor supplied in step a) is selected from complexes of neutral or charged cobalt transition metals, containing one or more organic or inorganic ligands.

6. The method of claim 5, wherein the oxidation state of cobalt is 0, 1, 2 or 3.

7. Method according to one of claims 5 or 6, in which said transition metal complexes comprise several ligands, identical or different from each other, linked or not to each other, and chosen from a hydride, fluoride, chloride, bromide, iodide group, an organic compound, preferably an alkyl, cyclic or not, having from 1 to 15 carbon atoms (in C1-C15) comprising or not one or more heteroelements and / or one or more multiple bonds, and / or an organic compound, preferably a substituted or unsubstituted aryl, having between 4 and 15 carbon atoms (in C4-C15) comprising or not one or more heteroelements, or a substituted or unsubstituted aryl having between 4 and 15 carbon atoms (in C4-C15) comprising or not one or more heteroelements.

8. A method according to any one of claims 4 to 7, wherein said organometallic cobalt precursor is selected from bis-(hexamethyldisilazane)tetrahydrofuran cobalt(II) |Co( / iw / .s)2(tiif)]; cobalt(II) bis-[N,N'-di(isopropyl)methylcarbodiimide], [Co[' PrNC(CH3)NiPr]2]; cobalt(II) bis-[N,N'-di(isopropyl)n-butylcarbodiimide], [Co[iPrNC(CH2CH2CH2CH3)NiPr]2]; cobalt(II) bis-[N,N'-di(cyclohexyl)methylcarbodiimide], [Co[CyNC(CH3 )NCy]2]; (l,5-cyclooctadiene)(cyclooctadienyl) cobalt (I), Co(q3-C8H 13)(if-C8H12).

9. A method according to any one of claims 4 to 8, wherein said first stabilizing agent and / or said second stabilizing agent are selected from carboxylic acids, carboxylate anions, primary, secondary or tertiary amines, primary, secondary, tertiary or quaternary ammonium cations, amides, esters, alcohols, alcoholates, thiols, thiolates, saturated or unsaturated, having between 1 and 30 carbon atoms.

10. A method according to claim 9, characterized in that the first stabilizing agent is palmitic acid and the second stabilizing agent is hexadecylamine.

11. A process for preparing a catalyst comprising an active cobalt phase and a support comprising at least one refractory oxide, comprising the following steps: i) said catalytic system according to any one of claims 1 to 3 is brought into contact with a support comprising at least one refractory oxide to obtain a catalyst precursor; ii) the catalyst precursor obtained at the end of step i) is dried at a temperature below 250°C to obtain a dried catalyst precursor; iii) the dried catalyst precursor obtained in step ii) is calcined at a temperature between 250°C and 550°C to obtain said catalyst.

12. Catalyst comprising an active cobalt phase and a support comprising at least one refractory oxide, characterized in that said active phase is in the form of nanoparticles with an average diameter in number less than or equal to 5 nm, measured in oxide form, and in that the standard deviation relative to the size of said nanoparticles is less than or equal to 50% of said average diameter.

13. Catalyst according to claim 12, characterized in that the cobalt content is between 0.1 and 15% by weight of cobalt element relative to the total weight of the catalyst.

14. Catalyst according to one of claims 12 or 13, characterized in that the support is chosen from alumina, silica, and silica-alumina.

15. A process for the hydrogenation of CO2 by reverse water gas reaction (RWGS) by contacting a feedstock of a gas mixture comprising CO2 and hydrogen with a catalyst according to any one of claims 12 to 14, which process is carried out at a temperature of between 0°C and 600°C, at a pressure of between 0.1 MPa and 5 MPa, at a hydrogen / CO2 molar ratio of between 0.1 and 10 and at an hourly volumetric flow rate of between 100 h 1 and 40000 h %