FR3119555A1 - METHOD FOR PREPARING A FISCHER-TROPSCH CATALYST FROM MOLTEN SALTS AND AN ORGANIC COMPOUND - Google Patents

Abstract

Process for the preparation of a catalyst comprising the following steps: a) bringing the porous support into contact with an organic compound comprising at least oxygen and/or nitrogen; b) drying the catalyst precursor at a temperature less than or equal to 200°C; c) bringing the dried catalyst precursor into contact with a metal salt comprising a group VIII metal whose melting point of said metal salt is between 30 °C and 150°C; d) heating with stirring under atmospheric pressure of the solid mixture to a temperature between the melting temperature of said metal salt and 200°; e) drying the solid at a temperature less than or equal to 200°C for a period greater than 4 hours;f) calcining the dried solid at a temperature greater than 200°C and less than or equal to 1100°C under a flow of inert gas and/or oxidizing gas.

Description

METHOD FOR PREPARING A FISCHER-TROPSCH CATALYST FROM MOLTEN SALTS AND AN ORGANIC COMPOUND

The present invention relates to the field of hydrocarbon synthesis reactions from a mixture of gases comprising carbon monoxide and hydrogen, generally called Fischer-Tropsch synthesis. More particularly, the present invention relates to the field of the preparation of catalysts used in Fischer-Tropsch syntheses.

State of the art

The Fischer-Tropsch synthesis processes make it possible to obtain a wide range of hydrocarbon cuts from the CO + H ₂ mixture, commonly called synthesis gas. The global equation of the Fischer-Tropsch synthesis can be written as follows:

Fischer-Tropsch synthesis is at the heart of processes for converting natural gas, coal or biomass into fuels or intermediates for the chemical industry. These processes are called GTL (“ Gas to Liquids ” according to the Anglo-Saxon terminology) in the case of the use of natural gas as initial charge, CTL (“Coal to Liquids ” according to the Anglo-Saxon terminology) for coal, and BTL (“ Biomass to Liquids ” according to the Anglo-Saxon terminology) for biomass.

In each of these cases, the initial charge is first gasified into a synthesis gas which comprises a mixture of carbon monoxide and dihydrogen. The synthesis gas is then transformed mainly into paraffins thanks to the Fischer-Tropsch synthesis, and these paraffins can then be transformed into fuels by a hydroisomerization-hydrocracking process. For example, transformation processes such as hydrocracking, dewaxing, and hydroisomerization of heavy cuts (C16+) make it possible to produce different types of fuels in the middle distillate range: diesel (180-370°C cut) and kerosene (cut 140-300°C). The lighter C5-C15 fractions can be distilled and used as solvents.

The Fischer-Tropsch synthesis reaction can be carried out in different types of reactors (fixed, moving bed, or three-phase (gas, liquid, solid) for example of the perfectly stirred autoclave type, or bubble column), and the reaction products in particular have the characteristic of being free of sulphur-containing, nitrogen-containing or aromatic-type compounds.

In an implementation in a reactor of the bubble column type (or " slurry bubble column " according to the English terminology, or even " slurry " in a simplified expression), which implements a catalyst divided in the powder state very fine, typically of the order of a few tens of micrometers, this powder forming a suspension with the reaction medium.

The Fischer-Tropsch reaction takes place in a conventional manner between 1 and 4 MPa (10 and 40 bars), at temperatures traditionally comprised between 200°C and 350°C. The reaction is globally exothermic, which requires special attention to the implementation of the catalyst.

The catalysts used for the Fischer-Tropsch synthesis are essentially cobalt- or iron-based catalysts, although other metals can be used. Nevertheless, cobalt and iron offer a good performance/price compromise compared to other metals.

The addition of an organic compound to Fischer-Tropsch catalysts to improve their activity has been recommended by those skilled in the art. Many documents describe the use of different ranges of organic compounds as additives, such as nitrogen-containing organic compounds and/or oxygen-containing organic compounds. For example, document FR3050659 discloses a catalyst containing an active phase of cobalt, deposited on a support comprising alumina, silica or silica-alumina, said support containing a mixed oxide phase containing cobalt and/or or nickel, said catalyst having been prepared by introducing at least one organic compound comprising at least one ester function during its preparation. The precursor of the active phase and the organic compound are introduced onto the support by the dry impregnation technique.

Thus, the conventional methods for preparing the supported metal catalysts used for the Fischer-Tropsch synthesis consist in depositing a metal salt dissolved in an aqueous solution on a porous support, then in carrying out one or more heat treatment(s) carried out( s) in air, followed by a reduction treatment carried out ex-situ or in-situ.

However, other Fischer-Tropsch catalyst synthesis methods are known from the prior art in order to improve the reducibility of the metallic phase or to control the particle sizes. Among these methods, the use of molten salts as precursors is known from the literature.

Thus, it is known from US patent 5,036,032 to propose a method for preparing a supported cobalt-based catalyst by bringing a support into rapid contact (of the order of a few tens of seconds) in a bath of molten salt of cobalt nitrate, followed by a stage of drying and reduction without intermediate calcination. This method allows the preferential localization of the cobalt phase at the periphery of the support. However, the method does not allow precise control of the amount of cobalt deposited due to the very short contact time, and on the other hand the type of catalyst obtained is not suitable for implementation of the slurry type catalyst. due to too much metal loss through attrition. Finally, this method requires handling large quantities of cobalt nitrate (toxic) in liquid form and at temperature, with ratios of approximately 4 grams of cobalt precursor for 1 gram of support.

Application FR3085382 discloses a Fischer-Tropsch process in the presence of a catalyst prepared by the molten salt route, which process comprises bringing the porous support into contact with a cobalt metal salt to form a solid mixture for a period of between 5 minutes and 5 hours, a step of heating the solid mixture with stirring above the melting point of said metal salt and 200°C, optionally a step of drying at a temperature below 200°C, and a step of calcining at a temperature between 200°C and 1100°C. When the drying step is completed, it is carried out for a maximum of 4 hours.

The Applicant has surprisingly discovered that it is possible to improve the catalytic performance in a process of the Fischer-Tropsch type, and more particularly to significantly increase the activity of said catalyst while maintaining good selectivity by using a catalyst prepared according to a specific preparation process involving the use of molten salts and an organic compound according to very specific steps.

Objects of the invention

The subject of the present invention is a process for the preparation of a catalyst containing an active phase based on at least one group VIII metal and a porous support based on alumina, silica, or silica-alumina, said catalyst being prepared by at least the following steps:

a) said porous support is brought into contact with at least one organic compound comprising at least oxygen and/or nitrogen to obtain a catalyst precursor;

b) the catalyst precursor obtained at the end of step a) is dried at a temperature less than or equal to 200° C. to obtain a dried catalyst precursor;

c) said dried catalyst precursor obtained at the end of step b) is brought into contact with a metal salt comprising at least one group VIII metal whose melting point of said metal salt is between 30° C. and 150 ° C to form a solid mixture for a period of between 5 minutes to 5 hours, the mass ratio between said metal salt and said porous support being between 0.1 and 1.5;

d) the solid mixture obtained at the end of step c) is heated with stirring under atmospheric pressure to a temperature between the melting temperature of said metal salt and 200° C. for a period of between 5 minutes and 12 hours;

e) the solid obtained at the end of step d) is dried at a temperature less than or equal to 200° C. for a period greater than 4 hours to obtain a dried solid;

f) the dried solid obtained at the end of step e) is calcined at a temperature greater than 200° C. and less than or equal to 1100° C. under a flow of inert gas and/or oxidizing gas, it being understood that the speed of said gas stream, defined as the mass flow rate of said gas stream per volume of catalyst per hour, is greater than 1 liter per gram of catalyst and per hour.

The Applicant has surprisingly discovered that the drying of the catalyst precursor after introduction of the organic compound and the precursor of the active phase beyond 4 hours followed by a specific calcination step makes it possible to significantly increase the activity catalysis of the catalyst while maintaining good selectivity. Without wishing to be bound by any theory, carrying out a step of drying the catalyst precursor after introduction of the organic compound and the precursor of the active phase beyond 4 hours makes it possible to obtain a narrow particle size distribution. and comprised in an interval comprised between 5 and 25 nm, that is to say that more than 99% of the population of the particles of the active phase have a size comprised between 5 and 25 nm. Indeed, it is generally accepted that the catalyst is all the more active as the size of the metal particles is small. In addition, it is important to obtain a particle size distribution centered on the optimal value as well as a distribution around this value. The preparation process according to the invention makes it possible to obtain such a catalyst with increased catalytic activity while maintaining good selectivity. Furthermore, performing a step of calcining the catalyst precursor in the presence of a gas stream at a relatively high speed makes it possible to more quickly eliminate the heat of the exothermic combustion reaction of the organic compound.

According to one or more embodiments, step e) is carried out for a period of between 5 and 20 hours.

According to one or more embodiments, the mass ratio between the metal salt comprising at least the group VIII metal and the porous support is between 0.3 and 0.9.

According to one or more embodiments, step e) is carried out with stirring.

According to one or more embodiments, the group VIII metal content is between 1 and 60% by weight of group VIII element relative to the total weight of the catalyst.

In one or more embodiments, the Group VIII metal is cobalt.

According to one or more embodiments, the speed of the gas flow in step f) is between 1.5 and 5 liters per gram of catalyst and per hour.

According to one or more embodiments, said organic compound is chosen from a compound comprising one or more chemical functions chosen from a carboxylic, alcohol, ester, amine, amide, ether, dilactone, carboxyanhydride, aldehyde, ketone, nitrile, imide, oxime, urea.

According to one or more embodiments, said organic compound comprises at least one carboxylic function chosen from ethanedioic acid (oxalic acid), propanedioic acid (malonic acid), butanedioic acid (succinic acid), 4 -oxopentanoic acid (levulinic acid) and 3-carboxy-3-hydroxypentanedioic acid (citric acid).

According to one or more embodiments, said organic compound comprises at least one alcohol function chosen from methanol, ethanol, phenol, ethylene glycol, propane-1,3-diol, glycerol, sorbitol, diethylene glycol, polyethylene glycols having an average molar mass of less than 600 g/mol, glucose, fructose and sucrose in any of their isomeric forms.

According to one or more embodiments, said organic compound comprises at least one ester function chosen from a γ-lactone or a δ-lactone containing between 4 and 8 carbon atoms, γ-butyrolactone, g-valerolactone, laurate of methyl, dimethyl malonate, dimethyl succinate and propylene carbonate.

According to one or more embodiments, said organic compound comprises at least one amine function chosen from aniline, ethylenediamine, diaminohexane, tetramethylenediamine, hexamethylenediamine, tetramethylethylenediamine, tetraethylethylenediamine, diethylenetriamine and triethylenetetramine.

According to one or more embodiments, said organic compound comprises at least one amide function chosen from formamide, N-methylformamide, N,N-dimethylformamide, 2-pyrrolidone, N-methyl-2-pyrrolidone, gamma -valerolactam and N,N′-dimethylurea.

According to one or more embodiments, said organic compound comprises at least one carboxyanhydride function chosen from the group of O-carboxyanhydrides consisting of 5-methyl-1,3-dioxolane-2,4-dione and acid 2,5 -dioxo-1,3-dioxolane-4-propanoic, or from the group of N-carboxyanhydrides consisting of 2,5-oxazolidinedione and 3,4-dimethyl-2,5-oxazolidinedione.

According to one or more embodiments, said organic compound comprises at least one dilactone function chosen from the group of 4-membered cyclic dilactones consisting of 1,2-dioxetanedione, or from the group of 5-membered cyclic dilactones consisting of 1 ,3-dioxolane-4,5-dione, 1,5-dioxolane-2,4-dione, and 2,2-dibutyl-1,5-dioxolane-2,4-dione, or from the dilactone group 6-membered rings consisting of 1,3-dioxane-4,6-dione, 2,2-dimethyl-1,3-dioxane-4,6-dione, 2,2,5-trimethyl-1,3 -dioxane-4,6-dione, 1,4-dioxane-2,5-dione, 3,6-dimethyl-1,4-dioxane-2,5-dione, 3,6-diisopropyl-1, 4-dioxane-2,5-dione, and 3,3-ditoluyl-6,6-diphenyl-1,4-dioxane-2,5-dione, or from the group of 7-membered cyclic dilactones consisting of 1 ,2-dioxepane-3,7-dione, 1,4-dioxepane-5,7-dione, 1,3-dioxepane-4,7-dione and 5-hydroxy-2,2-dimethyl-1, 3-dioxepane-4,7-dione.

According to one or more embodiments, said organic compound comprises at least one ether function comprising a maximum of two ether functions, and not comprising a hydroxyl group, chosen from the group of linear ethers consisting of diethyl ether, dipropyl ether, dibutyl ether, methyl tert-butyl ether, diisopropyl ether, di-tert-butyl ether, methoxybenzene, phenyl vinyl ether, isopropyl vinyl ether and isobutyl vinyl ether, or from the group of cyclic ethers consisting by tetrahydrofuran, 1,4-dioxane and morpholine.

According to one or more embodiments, the molar ratio of organic compound comprising introduced during step a) relative to the group VIII metal element introduced in step c) is between 0.01 and 2, 0 mol/mol.

According to one or more embodiments, said method comprises a step a0) in which said porous support is brought into contact with at least one solution containing at least one cobalt and/or nickel precursor, then dried and calcined to a temperature between 700° C. and 1200° C., so as to obtain a mixed oxide phase containing cobalt and/or nickel in the support.

According to one or more embodiments, the temperature of the drying step e) is at least 10° C. higher than the temperature of the heating step of step d).

Another object according to the invention relates to a Fischer-Tropsch process for the synthesis of hydrocarbons comprising bringing a charge comprising synthesis gas into contact with at least one catalyst obtained by the preparation process according to the invention under a pressure total between 0.1 and 15 MPa, under a temperature between 150°C and 350°C, and at an hourly volume rate between 100 and 20,000 volumes of synthesis gas per volume of catalyst and per hour with a molar ratio H ₂ /CO of syngas between 0.5 and 4.

Definitions

In the following description, the groups of chemical elements are given according to the CAS classification (CRC Handbook of Chemistry and Physics, publisher CRC press, editor-in-chief D.R. Lide, 81st edition, 2000-2001). For example, group VIIIB according to the CAS classification corresponds to the metals of columns 8, 9 and 10 according to the new IUPAC classification.

The textural and structural properties of the support and of the catalyst described below are determined by characterization methods known to those skilled in the art. The total pore volume and the pore distribution are determined in the present invention by mercury porosimetry (cf. Rouquerol F.; Rouquerol J.; Singh K. "Adsorption by Powders & Porous Solids: Principle, methodology and applications", Academic Press, 1999). More particularly, the total porous volume is measured by mercury porosimetry according to standard ASTM D4284-92 with a wetting angle of 140°, for example by means of an Autopore III™ model device from the Micromeritics™ brand. The specific surface is determined in the present invention by the B.E.T method, a method described in the same reference work as mercury porosimetry, and more particularly according to the ASTM D3663-03 standard.

Group VIII metal contents are measured by X-ray fluorescence.

The particle size distribution is determined by Abnormal small-angle X-ray scattering (ASAXS or “ Anomalous small-angle scattering of X-rays ” according to English terminology). SUN®. The scattering intensities were measured at three different energies, precisely 7440, 7647 and 7694 eV, values just below the K-edge absorption values of cobalt. The measurements are carried out on the spent catalysts after 150 hours of Fischer-Tropsch synthesis under the operating conditions as described in Example 3.

Catalyst Preparation Process

A first object according to the invention relates to a method for preparing a catalyst containing an active phase based on at least one group VIII metal and a porous support based on alumina, silica, or silica-alumina, said catalyst being prepared by at least the following steps:

a) said porous support is brought into contact with at least one organic compound comprising at least oxygen and/or nitrogen to obtain a catalyst precursor;

b) the catalyst precursor obtained at the end of step a) is dried at a temperature less than or equal to 200° C. to obtain a dried catalyst precursor;

c) said dried catalyst precursor obtained at the end of step b) is brought into contact with a metal salt comprising at least one group VIII metal whose melting point of said metal salt is between 30° C. and 150 ° C to form a solid mixture for a period of between 5 minutes to 5 hours, the mass ratio between said metal salt and said porous support being between 0.1 and 1.5;

d) the solid mixture obtained at the end of step c) is heated with stirring under atmospheric pressure to a temperature between the melting temperature of said metal salt and 200° C. for a period of between 5 minutes and 12 hours;

e) the solid obtained at the end of step d) is dried at a temperature less than or equal to 200° C. for a period greater than 4 hours to obtain a dried solid;

f) the dried solid obtained at the end of step e) is calcined at a temperature greater than 200° C. and less than or equal to 1100° C. under a flow of inert gas and/or oxidizing gas, it being understood that the speed of said gas stream, defined as the mass flow rate of said gas stream per volume of catalyst per hour, is greater than 1 liter per gram of catalyst and per hour.

The stages of the process for preparing the catalyst used in the Fischer-Tropsch synthesis according to the invention are described in detail below.

Step a0) Formation of the mixed oxide phase containing cobalt and/or nickel (optional)

In one embodiment according to the invention, the preparation process further comprises a step of forming the mixed oxide phase containing cobalt and/or nickel in the support comprising alumina, silica or silica-alumina by bringing a solution containing at least one precursor of cobalt and/or nickel into contact, followed by drying and calcination at high temperature.

It is known that the presence of a mixed oxide phase containing cobalt and/or nickel in a support of the alumina, silica or silica-alumina type makes it possible to improve the resistance to the phenomenon of chemical and mechanical attrition in a Fischer-Tropsch process, and therefore to stabilize the support.

The formation of the mixed oxide phase in the support, often called the support stabilization step, can be carried out by any method known to those skilled in the art. It is generally carried out by introducing the cobalt and/or the nickel in the form of a salt precursor, for example of the nitrate type, onto the initial support containing the alumina, silica or silica-alumina. By calcination at very high temperature, the mixed oxide phase containing cobalt and/or nickel is formed and stabilizes the entire support. The cobalt and/or nickel contained in the mixed oxide phase is not reducible during the final activation of the Fischer-Tropsch catalyst (reduction). The cobalt and/or nickel contained in the mixed oxide phase therefore does not constitute the active phase of the catalyst.

According to step a0), a step of bringing a support comprising alumina, silica or silica-alumina into contact with at least one solution containing at least one cobalt and/or nickel precursor is carried out , then dried and calcined at a temperature between 700 and 1200° C., so as to obtain a mixed oxide phase containing cobalt and/or nickel in the support.

More particularly, step a0) of bringing into contact can be carried out by impregnation, preferably dry, of a support comprising alumina, silica or silica-alumina, preformed or in powder form, with at least an aqueous solution containing the cobalt and/or nickel precursor, followed by drying and calcination at a temperature between 700 and 1200°C.

The cobalt is brought into contact with the support via any soluble cobalt precursor in the aqueous phase. Preferably, the cobalt precursor is introduced in aqueous solution, preferably in the form of nitrate, carbonate, acetate, chloride, complexes formed with acetylacetonates, or any other inorganic derivative soluble in aqueous solution, which is brought into contact with said support. The cobalt precursor advantageously used is cobalt nitrate or cobalt acetate.

The nickel is brought into contact with the support via any nickel precursor that is soluble in the aqueous phase. Preferably, said nickel precursor is introduced in aqueous solution, for example in the form of nitrate, carbonate, acetate, chloride, hydroxide, hydroxycarbonate, oxalate, complexes formed with acetylacetonates, or any other inorganic derivative soluble in aqueous solution, which is brought into contact with said support. The nickel precursor advantageously used is nickel nitrate, nickel chloride, nickel acetate or nickel hydroxycarbonate.

The total cobalt and/or nickel content, expressed as a metallic element, is advantageously between 1 and 20% by weight and preferably between 2 and 10% by weight relative to the weight of the final support.

The drying is advantageously carried out at a temperature of between 60° C. and 200° C., preferably for a period ranging from 30 minutes to three hours.

The calcination is carried out at a temperature comprised between 700 and 1200° C., preferably comprised between 850 and 1200° C., and more preferably comprised between 850 and 900° C., generally for a period comprised between one hour and 24 hours and for preferably between 2 hours and 5 hours. The calcination is generally carried out under an oxidizing atmosphere, for example under air, or under oxygen-depleted air; it can also be carried out at least partly under nitrogen. It makes it possible to transform cobalt and/or nickel precursors and alumina and/or silica into a mixed oxide phase containing cobalt and/or nickel.

According to a variant, the calcination can also be carried out in two stages, said calcination is advantageously carried out at a temperature of between 300° C. and 600° C. in air for a period of between half an hour and three hours, then at a temperature between 700°C and 1200°C, preferably between 850 and 1200°C and preferably between 850 and 900°C, generally for a period of between one hour and 24 hours, and preferably between 2 hours and 5 hours.

Thus, at the end of said step a0), said support comprising alumina, silica or silica-alumina further comprises a mixed oxide phase containing cobalt and/or nickel.

Step a)

The bringing into contact of the organic compound used for implementing said step a) with said support is carried out by impregnation, in particular by dry impregnation or excess impregnation, preferably by dry impregnation. Said organic compound is preferentially impregnated on said support after solubilization in solution, preferably aqueous.

Here, an organic compound containing at least oxygen and/or nitrogen is understood to mean a compound not comprising any other heteroatom.

Preferably, said organic compound is chosen from a compound comprising one or more chemical functions chosen from a carboxylic function, alcohol, ester, lactone, amine, amide, ether, dilactone, carboxyanhydride, carbonate, aldehyde, ketone, nitrile, imide, oxime , urea.

When said organic compound comprises at least one or more carboxylic functions, said organic compound can be chosen from ethanedioic acid (oxalic acid), propanedioic acid (malonic acid), butanedioic acid (succinic acid), 4-oxopentanoic acid (levulinic acid) and 3-carboxy-3-hydroxypentanedioic acid (citric acid).

When said organic compound comprises at least one or more alcohol functions, said organic compound can be chosen from methanol, ethanol, phenol, ethylene glycol, propane-1,3-diol, glycerol, sorbitol, diethylene glycol, polyethylene glycols having an average molar mass of less than 600 g/mol, glucose, fructose and sucrose in any of their isomeric forms.

When said organic compound comprises at least one or more ester functions, said organic compound can be chosen from an ester, a diester, a γ-lactone, a δ-lactone or a carbonate containing between 4 and 8 carbon atoms, the γ- butyrolactone, g-valerolactone, methyl laurate, dimethyl malonate, dimethyl succinate and propylene carbonate.

When the organic compound comprises at least one or more amine function, said organic compound can be chosen from aniline, ethylenediamine, diaminohexane, tetramethylenediamine, hexamethylenediamine, tetramethylethylenediamine, tetraethylethylenediamine, diethylenetriamine and triethylenetetramine.

When the organic compound comprises at least one or more amide functions, said organic compound can be chosen from formamide, N-methylformamide, N,N-dimethylformamide, 2-pyrrolidone, N-methyl-2-pyrrolidone, gamma-valerolactam and N,N′-dimethylurea.

When the organic compound comprises at least one or more ether functions, said organic compound may be chosen from organic compounds comprising a maximum of two ether functions and not comprising a hydroxyl group, chosen from the group of linear ethers consisting of diethyl ether, dipropyl ether, dibutyl ether, methyl tert-butyl ether, diisopropyl ether, di-tert-butyl ether, methoxybenzene, phenyl vinyl ether, isopropyl vinyl ether and isobutyl vinyl ether, or in the group of cyclic ethers consisting of tetrahydrofuran, 1,4-dioxane and morpholine

When the organic compound comprises a dilactone function, said organic compound may be chosen from the group of 4-membered cyclic dilactones consisting of 1,2-dioxetanedione, or from the group of 5-membered cyclic dilactones consisting of 1,3- dioxolane-4,5-dione, 1,5-dioxolane-2,4-dione, and 2,2-dibutyl-1,5-dioxolane-2,4-dione, or from the cyclic dilactone group of 6 links consisting of 1,3-dioxane-4,6-dione, 2,2-dimethyl-1,3-dioxane-4,6-dione, 2,2,5-trimethyl-1,3-dioxane- 4,6-dione, 1,4-dioxane-2,5-dione, 3,6-dimethyl-1,4-dioxane-2,5-dione, 3,6-diisopropyl-1,4-dioxane -2,5-dione, and 3,3-ditoluyl-6,6-diphenyl-1,4-dioxane-2,5-dione, or from the group of 7-membered cyclic dilactones consisting of 1,2- dioxepane-3,7-dione, 1,4-dioxepane-5,7-dione, 1,3-dioxepane-4,7-dione and 5-hydroxy-2,2-dimethyl-1,3-dioxepane -4,7-dione.

When the organic compound comprises a carboxyanhydride function, said organic compound can be chosen from the group of O-carboxyanhydrides consisting of 5-methyl-1,3-dioxolane-2,4-dione and 2,5-dioxo-acid. 1,3-dioxolane-4-propanoic, or from the group of N-carboxyanhydrides consisting of 2,5-oxazolidinedione and 3,4-dimethyl-2,5-oxazolidinedione. By carboxyanhydride is meant a cyclic organic compound comprising a carboxyanhydride function, that is to say a sequence –CO-O-CO-X- or –X-CO-O-CO- within the cycle with -CO- corresponding to a carbonyl function and X possibly being an oxygen or nitrogen atom. For X=O we speak of O -carboxyanhydride and when X=N we speak of N -carboxyanhydride.

The molar ratio of organic compound introduced during step a) relative to the metallic element from group VIII introduced in step c) is advantageously between 0.01 and 2.0 mol/mol, preferably between 0.05 and 1.5 mol/mol.

Step b)

In accordance with the drying step b) of the implementation for the preparation of the catalyst, the drying is carried out at a temperature less than or equal to 200° C., advantageously between 50° C. and 180° C., preferably between 70 °C and 150°C, very preferably between 75°C and 130°C. The drying step is preferably carried out for a period of between 1 and 4 hours, preferably under an inert atmosphere or under an atmosphere containing oxygen.

The drying step can be carried out by any technique known to those skilled in the art. It is advantageously carried out at atmospheric pressure or at reduced pressure. Preferably, this step is carried out at atmospheric pressure. It is advantageously carried out in a traversed bed using air or any other hot gas. Preferably, when the drying is carried out in a fixed bed, the gas used is either air or an inert gas such as argon or nitrogen. Very preferably, the drying is carried out in a traversed bed in the presence of nitrogen and/or air. Preferably, the drying step has a short duration of between 5 minutes and 4 hours, preferably between 30 minutes and 4 hours and very preferably between 1 hour and 3 hours.

According to a first variant, the drying is carried out so as to preferably retain at least 30% of the organic compound comprising introduced during step b) of the preparation process, preferably this quantity is greater than 50% and even more preferred, greater than 70%, calculated on the basis of the carbon remaining on the catalyst.

At the end of the drying step b), a dried catalyst precursor is then obtained.

Step c)

According to step c), the dried catalyst precursor obtained at the end of step b) is brought into contact with a metal salt comprising at least a group VIII metal whose melting point of said metal salt is between 30 °C and 150°C to form a solid mixture, the mass ratio between said metal salt and said porous support being between 0.1 and 1.5.

According to step c), a metal salt is provided comprising at least one solid group VIII metal such that its melting point is between 30°C and 150°C. In step c), the metal salt is in solid form, that is to say that the bringing into contact between said porous support and said metal salt is carried out at a temperature below the melting temperature of said metal salt.

When the Group VIII metal is cobalt, said cobalt metal salt is preferably a hydrated metal salt. Preferably, the cobalt metal salt is cobalt nitrate hexahydrate or cobalt acetate tetrahydrate. Very preferably, the cobalt metal salt is cobalt nitrate hexahydrate. The metal salt used in the context of the preparation process according to the invention may be in the form of powder of variable particle size and/or particles of millimetric size.

The mass ratio between the metal salt comprising at least one group VIII metal and the porous support is between 0.1 and 1.5, preferably between 0.3 and 1.0. The preparation process allows an optimized control of the quantity of metal deposited on the catalyst as well as a dangerousness and a cost controlled by the minimization of the quantity of metallic precursor used not exceeding 1.5 grams of metallic precursor for 1 gram of support.

According to step c), the bringing into contact of said porous support and of said metal salt can be done by any method known to those skilled in the art. Preferably, said porous support and the metal salt are brought into contact with contact means chosen from convective mixers, drum mixers or static mixers.

Step c) is carried out for a period of between 5 minutes to 5 hours depending on the type of mixer used, preferably between 10 minutes and 4 hours, and even more preferably between 15 minutes and 3 hours.

According to a variant of step c), any other organic or inorganic solid compound in the form of powder of variable particle size may be added to the mixture. If it is an inorganic compound, it may contain at least one element chosen from groups VIIB, VIII, IB, IIB, IA (alkaline element), IIA (alkaline-earth element), IIIA, alone or in blend. When another organic or inorganic compound is added to the mixture, the mass ratio between the cobalt metal precursor and said compound is between 10 and 50,000, preferably between 50 and 20,000.

Step d)

According to step d), the mixture obtained at the end of step c) is heated with stirring at atmospheric pressure to a temperature between the melting point of the metal salt comprising at least one group VIII metal and 200° vs. In the highly preferred case where the metal salt is cobalt nitrate hexahydrate, the temperature of step d) is between 55°C and 150°C, preferably between 65°C and 130°C. The residence time is between 5 minutes and 12 hours, preferably between 5 minutes and 4 hours.

According to step d), the mechanical homogenization of the mixture can be done by any method known to those skilled in the art. Preferably, convective mixers, drum mixers or static mixers can be used.

Step e)

According to an essential aspect of the invention, the solid obtained at the end of step d) is then dried at a temperature less than or equal to 200° C., advantageously between 50° C. and 180° C., preferably between 70° C. and 150° C., very preferably between 75° C. and 130° C., and for a duration greater than 4 hours, preferably between 5 hours and 20 hours and preferably between 6 hours and 12 hours.

The drying step is preferably carried out under an inert atmosphere or under an atmosphere containing oxygen. The drying step can be carried out by any technique known to those skilled in the art. It is advantageously carried out at atmospheric pressure or at reduced pressure. Preferably, this step is carried out at atmospheric pressure. It is advantageously carried out using air or any other hot gas. Preferably, the gas used is either air or an inert gas such as argon or nitrogen. Very preferably, the drying is carried out in the presence of nitrogen and/or air.

Preferably, step e) of drying is carried out with stirring, by any method known to those skilled in the art. Preferably, convective mixers, drum mixers, static mixers or even an endless screw may be used. More preferably, step e) is carried out with stirring using a drum mixer.

Preferably, the temperature of the drying step e) is higher than the heating temperature of step d). More preferably, the temperature of the drying step e) is at least 10° C. higher than the temperature of the heating step of step d).

Step f)

According to an essential aspect of the preparation process according to the invention, at the end of step e) of drying, a calcination step f) is carried out at a temperature greater than 200° C. and less than or equal to 1100° C. , preferably at a temperature between 250°C and 600°C, under a flow of inert gas and/or of oxidizing gas, preferably under a flow of oxidizing gas (preferably air), it being understood that the speed of said flow gas flow, defined as the mass flow rate of said gas flow per volume of catalyst per hour, is greater than 1 liter per gram of catalyst and per hour, and is preferably between 1.5 and 5 liters per gram of catalyst and per hour.

Without wishing to be bound by any theory, carrying out a step of calcining the dried solid obtained at the end of step e) in the presence of a gas stream at a relatively high speed makes it possible to eliminate the heat of the exothermic combustion reaction of the organic compound.

The duration of this heat treatment is generally between 0.5 hours and 16 hours, preferably between 1 hour and 5 hours. After this treatment, the group VIII metal of the active phase is thus in oxide form and the catalyst no longer contains or contains very little organic compound introduced during its synthesis. However, the introduction of the organic compound during its preparation left an imprint on the catalyst support.

Step f) is advantageously carried out in a traversed bed or in a fluidized bed, preferably in a traversed bed, using air as the gas stream.

Prior to its use in the Fischer-Tropsch synthesis catalytic reactor, the catalyst obtained at the end of step f) generally undergoes a reducing treatment, for example under pure or dilute hydrogen, at high temperature, intended to activate the catalyst. and forming metal particles in the zero valent state (in metallic form). This treatment can be carried out in-situ (in the same reactor as that in which the Fischer-Tropsch synthesis is carried out), or ex-situ before being loaded into the reactor. The temperature of this reducing treatment is preferably between 200° C. and 500° C. and its duration is generally between 2 and 20 hours.

Catalyst

The catalyst obtained by the preparation process described above comprises, preferably consists of, an active phase comprising, preferably consists of, at least one group VIII metal and a porous support based on alumina, silica, or silica-alumina, and optionally a mixed oxide phase containing cobalt and/or nickel.

The content of metal from group VIII is advantageously between 1 and 60% by weight, preferably between 5 and 30% by weight, and very preferably between 10 and 30% by weight of element of the metal from group VIII relative to the total weight of the metal. catalyst.

Preferably, the Group VIII metal is cobalt.

The specific surface of the catalyst is generally between 50 m²/g and 500 m²/g, preferably between 80 m²/g and 250 m²/g, more preferably between 90 m²/g and 150 m²/g. The pore volume of said catalyst is generally between 0.2 ml/g and 1 ml/g, and preferably between 0.25 ml/g and 0.8 ml/g.

Preferably, the active phase of the group VIII metal is distributed homogeneously in the support, i.e. the active phase is not distributed in a crust on the surface of said support.

Support

The catalyst support prepared according to the process according to the invention is based on alumina, silica, or silica-alumina, and optionally a mixed oxide phase containing cobalt and/or nickel.

When the support is a silica-alumina, the silica (SiO ₂ ) content can vary from 0.5% by weight to 30% by weight relative to the weight of the support, preferably between 0.6 and 15% by weight.

According to a variant, said porous support also contains a mixed oxide phase containing cobalt and/or nickel. According to this variant, the content of the mixed oxide phase in the support is between 0.1 and 50% by weight relative to the weight of the support.

Preferably, the mixed oxide phase comprises an aluminate of formula CoAl ₂ O ₄ or NiAl ₂ O ₄ in the case of a support based on alumina or silica-alumina, or a silicate of formula Co ₂ SiO ₄ or Ni ₂ SiO ₄ in the case of a support based on silica or silica-alumina. The term “mixed oxide phase containing cobalt and/or nickel” means a phase in which cobalt and/or nickel cations are combined with the O ^2- oxide ions of the alumina and/or silica support forming thus a mixed phase containing aluminates and/or silicates containing cobalt and/or nickel.

The mixed oxide phase can be in amorphous form or in crystallized form. When the support is based on alumina, the mixed oxide phase may comprise an aluminate of formula CoAl ₂ O ₄ or NiAl ₂ O ₄ , in amorphous or crystallized form, for example in spinel form. When the support is based on silica, the mixed oxide phase may comprise a silicate of formula Co ₂ SiO ₄ or Ni ₂ SiO ₄ (cobalt- or nickelorthosilicate), in amorphous or crystallized form. When the support is based on silica-alumina, the mixed oxide phase may comprise an aluminate of formula CoAl ₂ O ₄ or NiAl ₂ O ₄ in amorphous or crystalline form, for example in spinel form, and/or a silicate of formula Co ₂ SiO ₄ or Ni ₂ SiO ₄ , in amorphous or crystalline form.

The cobalt and/or the nickel contained in the mixed oxide phase is not reducible during the final activation of the Fischer-Tropsch catalyst (reduction). The cobalt and/or the nickel contained in the mixed oxide phase therefore does not constitute the active phase of the catalyst.

The presence of mixed oxide phase in the catalyst according to the invention is measured by temperature programmed reduction RTP (or TPR for "temperature programmed reduction" according to the English terminology) as for example described in Oil & Gas Science and Technology, Rev. IFP, Vol. 64 (2009), No. 1, p. 11-12. According to this technique, the catalyst is heated under a flow of a reducing agent, for example under a flow of dihydrogen. The measurement of the dihydrogen consumed as a function of the temperature gives quantitative information on the reducibility of the species present. The presence of a mixed oxide phase in the catalyst is thus manifested by a consumption of dihydrogen at a temperature above approximately 800°C.

In one embodiment according to the invention, the support consists of a silica-alumina and a mixed oxide phase containing cobalt and/or nickel, preferably containing cobalt.

The specific surface of the support is generally between 50 m²/g and 500 m²/g, preferably between 100 m²/g and 300 m²/g, more preferably between 150 m²/g and 250 m²/g.

The pore volume of said support is generally between 0.3 ml/g and 1.2 ml/g, and preferably between 0.4 ml/g and 1 ml/g.

The porous distribution of the pores of the porous support can be of the monomodal, bimodal or plurimodal type. Preferably, it is of the monomodal type. The pore size is of the order of 2 to 50 nm, with an average pore size between 5 and 25 nm, preferably between 8 and 20 nm.

The support may have a morphology in the form of beads, extrudates (for example of trilobe or quadrilobe form) or pellets, in particular when said catalyst is implemented in a reactor operating in a fixed bed, or have a morphology in the form of powder of variable particle size, in particular when said catalyst is implemented in a bubble column type reactor. Preferably, the support is in the form of a powder with a particle size between 10 and 500 μm.

The support can be provided by any means known to those skilled in the art.

Fischer-Tropsch process

Another object according to the invention relates to a Fischer-Tropsch process in the presence of a catalyst prepared according to the preparation process according to the invention. This Fischer-Tropsch process leads to the production of essentially linear and saturated C ₅ ⁺ hydrocarbons (having at least 5 carbon atoms per molecule). The hydrocarbons produced by the process of the invention are thus essentially paraffinic hydrocarbons, the fraction of which having the highest boiling points can be converted with a high yield into middle distillates (diesel and kerosene cuts) by a process of hydroconversion such as hydrocracking and/or catalytic hydroisomerization(s).

The feed used for implementing the method of the invention comprises synthesis gas. The synthesis gas is a mixture comprising in particular carbon monoxide and hydrogen having H ₂ /CO molar ratios which can vary in a ratio of 0.5 to 4 depending on the process by which it was obtained. The H ₂ /CO molar ratio of the synthesis gas is generally close to 3 when the synthesis gas is obtained from the hydrocarbon or alcohol steam reforming process. The H ₂ /CO molar ratio of the synthesis gas is of the order of 1.5 to 2 when the synthesis gas is obtained from a partial oxidation process. The H ₂ /CO molar ratio of the synthesis gas is generally close to 2.5 when it is obtained from a thermal reforming process. The H ₂ /CO molar ratio of the synthesis gas is generally close to 1 when it is obtained from a CO ₂ gasification and reforming process.

The catalyst used in the process for the synthesis of hydrocarbons according to the invention is preferably implemented in reactors in an ebullated bed or else in a three-phase fluidized bed. The implementation of the catalyst in suspension in a three-phase fluidized reactor, preferably of the bubble column type, is preferred. In this preferred implementation of the catalyst, said catalyst forms a suspension with the reaction medium. This technology is also known by the term "slurry" process by those skilled in the art.

The hydrocarbon synthesis process according to the invention is operated under a total pressure of between 0.1 MPa and 15 MPa, preferably between 0.5 MPa and 10 MPa, at a temperature of between 150° C. and 350° C. , preferably between 180°C and 270°C. The hourly volume rate is advantageously between 100 and 20,000 volumes of synthesis gas per volume of catalyst and per hour (100 to 20,000 h ^-1 ) and preferably between 400 and 10,000 volumes of synthesis gas per volume of catalyst and per hour (400 to 10000 h ^-1 ).

To illustrate the invention and to enable those skilled in the art to carry it out, we present below various embodiments of the process for preparing supported cobalt-based catalysts and their use in Fischer-Tropsch synthesis; however, this does not limit the scope of the invention.

Examples

Example 1 (comparative): Catalyst A with formula Co / CoAl ₂ O ₄ -Al ₂ O ₃ .SiO ₂ containing γ-valerolactone, calcined with an air velocity of 0.7 liters of air per gram of catalyst and per hour and not dried after deposition of cobalt by molten salt

The spinel present in the support of catalyst A is a simple spinel formed from cobalt aluminate, which is included in a silica-alumina containing 5% by weight of SiO ₂ , with an average particle size equal to 80 μm, and having a specific surface area of 180 m ² /g and a pore volume of 0.55 ml/g. The preparation of the spinel included in the silica-alumina is carried out by dry impregnation with an aqueous solution of cobalt nitrate (Orrion Chemicals Metalchem, ~13% by weight Co) so as to introduce 5% by weight of cobalt into said silica-alumina . After drying at 120° C. for 3 hours, the solid is calcined at 850° C. for 4 hours in air. The support for catalyst A is formed from 5% by weight of cobalt in the form of cobalt aluminate in silica-alumina.

The γ-valerolactone is deposited on the support described above by dry impregnation with a solution of γ-valerolactone, at a concentration such that the γ-valerolactone/Co molar ratio is 1.0 mol/mol. After dry impregnation, the solid undergoes maturation in a water-saturated atmosphere for 9 hours at room temperature, then is dried in an oven at 120°C for 3 hours. 5 grams of this solid and 4 grams of cobalt nitrate hexahydrate (Aldrich, >98%) are then introduced into a drum mixer inclined at 45° and equipped with counter-blades to ensure a cascading movement during the mixing of the powders. The mixer is stirred at 60 revolutions per minute for 1 hour at ambient temperature and pressure. After this homogenization step, the mixer is left stirring at ambient pressure, and the temperature is increased at 5°C/min up to 85°C and maintained at 85°C for 1 hour.

The undried solid is then calcined in air at 400° C. for 4 hours in a traversed bed with an air speed of 0.7 liter of air/gram of catalyst and per hour (l/g.h) in order to obtain the catalyst a.

Example 2 (according to the invention): Catalyst B of formula Co/CoAl ₂ O ₄ -Al ₂ O ₃ .SiO ₂ containing γ-valerolactone, calcined with an air velocity of 4 liters of air per gram of catalyst and per hour and dried for 9 hours after deposition of the cobalt by molten salt

The spinel present in the support of catalyst B is a simple spinel formed from cobalt aluminate, which is included in a silica-alumina containing 5% by weight of SiO ₂ , with an average particle size equal to 80 μm, and having a specific surface area of 180 m ² /g and a pore volume of 0.55 ml/g. The preparation of the spinel included in the silica-alumina is carried out by dry impregnation with an aqueous solution of cobalt nitrate (Orrion Chemicals Metalchem, ~13% by weight Co) so as to introduce 5% by weight of cobalt into said silica-alumina . After drying at 120° C. for 3 hours, the solid is calcined at 850° C. for 4 hours in air. The support for catalyst B is formed from 5% by weight of cobalt in the form of cobalt aluminate in silica-alumina.

The γ-valerolactone is deposited on the support described above by dry impregnation with a solution of γ-valerolactone, at a concentration such that the γ-valerolactone/Co molar ratio is 1.0 mol/mol. After dry impregnation, the solid undergoes maturation in a water-saturated atmosphere for 9 hours at room temperature, then is dried in an oven at 120°C for 3 hours. 5 grams of this solid and 4 grams of cobalt nitrate hexahydrate (Aldrich, >98%) are then introduced into a drum mixer inclined at 45° and equipped with counter-blades to ensure a cascading movement during the mixing of the powders. The mixer is stirred at 60 revolutions per minute for 1 hour at ambient temperature and pressure. After this homogenization step, the mixer is left stirring at ambient pressure, and the temperature is increased at 5°C/min up to 85°C and maintained at 85°C for 1 hour. The temperature is then increased at 5°C/min to 100°C and maintained at 100°C for 9 hours for the drying step. The dried solid is then calcined in air at 400°C for 4 hours in a traversed bed with an air speed of 4 liters of air/gram of catalyst and per hour (l/g.h) in order to obtain catalyst B.

Example 3: Implementation of catalysts A to B in Fischer-Tropsch synthesis

Catalysts A and B, before being successively tested in synthesis gas conversion, are reduced ex situ under a flow of pure hydrogen at 400° C. for 16 hours in a tubular reactor. Once the catalyst has been reduced, it is unloaded under an argon atmosphere and coated in Sasolwax® wax to be stored away from the air before testing. The Fischer-Tropsch synthesis reaction is carried out in a three-phase reactor (also called “slurry” technology according to English terminology) operating continuously and operating with a concentration of 10% (vol) of catalyst in the dispersed phase. The test conditions are as follows: temperature=220° C.; total pressure = 2 MPa; H ₂ /CO=2 molar ratio. The CO conversion is maintained at 60% for the duration of the test. The test conditions are adjusted so as to be at iso conversion of CO whatever the activity of the catalyst. The results, in terms of activity, were calculated after 150 hours of testing for catalysts A and B with respect to catalyst A serving as reference and appear in Table 1 below. Methane formation selectivities are also given.

Catalysts Drying time at 100°C (in hours) Calcining air velocity (in l/g _catalyst /hour) Relative activity after 150 hours of testing Methane formation selectivity (in %) A (non-compliant) 0 0.7 100 (basic) 7.7 B (compliant) 9 4.0 135 8.0

Table 1: Catalytic performances of catalysts A and B

The results in Table 1 above show the catalytic performances of catalysts A and B both in terms of activity and selectivity. It appears that the catalyst prepared by the process according to the invention with a drying time greater than 4 hours after deposition of the cobalt by molten salt and a calcination air speed greater than 1 liter of air/gram catalyst and per hour has, after 150 hours of testing, an improved relative activity compared to catalyst A of example 1 whose drying time after deposition of cobalt by molten salt is less than 4 hours and whose calcining air velocity is less than 1 liter of air/gram of catalyst and per hour, while maintaining good selectivity.

Claims (20)

Process for preparing a catalyst containing an active phase based on at least one group VIII metal and a porous support based on alumina, silica, or silica-alumina, said catalyst being prepared by at least the steps following:
a) said porous support is brought into contact with at least one organic compound comprising at least oxygen and/or nitrogen to obtain a catalyst precursor;
b) the catalyst precursor obtained at the end of step a) is dried at a temperature less than or equal to 200° C. to obtain a dried catalyst precursor;
c) said dried catalyst precursor obtained at the end of step b) is brought into contact with a metal salt comprising at least one group VIII metal whose melting point of said metal salt is between 30° C. and 150 ° C to form a solid mixture for a period of between 5 minutes to 5 hours, the mass ratio between said metal salt and said porous support being between 0.1 and 1.5;
d) the solid mixture obtained at the end of step c) is heated with stirring under atmospheric pressure to a temperature between the melting point of said metal salt and 200° C. for a period of between 5 minutes and 12 hours;
e) the solid obtained at the end of step d) is dried at a temperature less than or equal to 200° C. for a period greater than 4 hours to obtain a dried solid;
f) the dried solid obtained at the end of step e) is calcined at a temperature greater than 200° C. and less than or equal to 1100° C. under a flow of inert gas and/or oxidizing gas, it being understood that the speed of said gas stream, defined as the mass flow rate of said gas stream per volume of catalyst per hour, is greater than 1 liter per gram of catalyst and per hour. Process according to Claim 1, in which step e) is carried out for a period of between 5 and 20 hours. Process according to one of Claims 1 or 2, in which the mass ratio between the metal salt comprising at least the group VIII metal and the porous support is between 0.3 and 0.9. Process according to any one of Claims 1 to 3, in which step e) is carried out with stirring. Process according to any one of Claims 1 to 4, in which the content of metal from group VIII is between 1 and 60% by weight of element from group VIII with respect to the total weight of the catalyst. A method according to any of claims 1 to 5, wherein the Group VIII metal is cobalt. Process according to any one of Claims 1 to 6, in which in step f) the speed of the gas flow is between 1.5 and 5 liters per gram of catalyst and per hour. Process according to any one of Claims 1 to 7, in which the said organic compound is chosen from a compound comprising one or more chemical functions chosen from a carboxylic, alcohol, ester, amine, amide, ether, dilactone, carboxyanhydride, aldehyde, ketone, nitrile, imide, oxime, urea. Process according to Claim 8, in which the said organic compound comprises at least one carboxylic function chosen from ethanedioic acid (oxalic acid), propanedioic acid (malonic acid), butanedioic acid (succinic acid), 4 -oxopentanoic acid (levulinic acid) and 3-carboxy-3-hydroxypentanedioic acid (citric acid). Process according to Claim 8, in which the said organic compound comprises at least one alcohol function chosen from methanol, ethanol, phenol, ethylene glycol, propane-1,3-diol, glycerol, sorbitol, diethylene glycol, polyethylene glycols having an average molar mass of less than 600 g/mol, glucose, fructose and sucrose in any of their isomeric forms. Process according to Claim 8, in which the said organic compound comprises at least one ester function chosen from a γ-lactone or a δ-lactone containing between 4 and 8 carbon atoms, γ-butyrolactone, g-valerolactone, laurate of methyl, dimethyl malonate, dimethyl succinate and propylene carbonate. Process according to Claim 8, in which the said organic compound comprises at least one amine function chosen from aniline, ethylenediamine, diaminohexane, tetramethylenediamine, hexamethylenediamine, tetramethylethylenediamine, tetraethylethylenediamine, diethylenetriamine and triethylenetetramine. Process according to Claim 8, in which the said organic compound comprises at least one amide function chosen from formamide, N-methylformamide, N,N-dimethylformamide, 2-pyrrolidone, N-methyl-2-pyrrolidone, gamma -valerolactam and N,N′-dimethylurea. Process according to Claim 8, in which the said organic compound comprises at least one carboxyanhydride function chosen from the group of O-carboxyanhydrides consisting of 5-methyl-1,3-dioxolane-2,4-dione and acid 2,5 -dioxo-1,3-dioxolane-4-propanoic, or from the group of N-carboxyanhydrides consisting of 2,5-oxazolidinedione and 3,4-dimethyl-2,5-oxazolidinedione. Process according to Claim 8, in which the said organic compound comprises at least one dilactone function chosen from the group of cyclic 4-membered dilactones constituted by 1,2-dioxetanedione, or from the group of cyclic 5-membered dilactones constituted by 1 ,3-dioxolane-4,5-dione, 1,5-dioxolane-2,4-dione, and 2,2-dibutyl-1,5-dioxolane-2,4-dione, or from the dilactone group 6-membered rings consisting of 1,3-dioxane-4,6-dione, 2,2-dimethyl-1,3-dioxane-4,6-dione, 2,2,5-trimethyl-1,3 -dioxane-4,6-dione, 1,4-dioxane-2,5-dione, 3,6-dimethyl-1,4-dioxane-2,5-dione, 3,6-diisopropyl-1, 4-dioxane-2,5-dione, and 3,3-ditoluyl-6,6-diphenyl-1,4-dioxane-2,5-dione, or from the group of 7-membered cyclic dilactones consisting of 1 ,2-dioxepane-3,7-dione, 1,4-dioxepane-5,7-dione, 1,3-dioxepane-4,7-dione and 5-hydroxy-2,2-dimethyl-1, 3-dioxepane-4,7-dione. Process according to Claim 8, in which the said organic compound comprises at least one ether function comprising a maximum of two ether functions, and not comprising a hydroxyl group, chosen from the group of linear ethers consisting of diethyl ether, dipropyl ether, dibutyl ether, methyl tert-butyl ether, diisopropyl ether, di-tert-butyl ether, methoxybenzene, phenyl vinyl ether, isopropyl vinyl ether and isobutyl vinyl ether, or from the group of cyclic ethers consisting by tetrahydrofuran, 1,4-dioxane and morpholine. Process according to any one of Claims 1 to 16, in which the molar ratio of the organic compound introduced during step a) relative to the group VIII metal element introduced in step c) is between 0 .01 and 2.0 mol/mol. Process according to any one of Claims 1 to 17, comprising a step a0) in which said porous support is brought into contact with at least one solution containing at least one cobalt and/or nickel precursor, then dried and calcined at a temperature between 700°C and 1200°C, so as to obtain a mixed oxide phase containing cobalt and/or nickel in the support. Process according to any one of claims 1 to 18, in which the temperature of the drying step e) is at least 10°C higher than the temperature of the heating step of step d). Fischer-Tropsch process for the synthesis of hydrocarbons comprising bringing a charge comprising synthesis gas into contact with at least one catalyst obtained by the process according to any one of Claims 1 to 19 under a total pressure of between 0, 1 and 15 MPa, at a temperature of between 150°C and 350°C, and at an hourly volume rate of between 100 and 20,000 volumes of synthesis gas per volume of catalyst and per hour with an H ₂ /CO molar ratio of syngas between 0.5 and 4.