Method for the synthesis of solid mixed oxides containing oxides of nickel, magnesium and aluminium and optionally an activator; associated mixed oxides, catalysts and uses

Abstract

The present invention relates to a method for the synthesis of a solid mixed oxide containing oxides of nickel, magnesium and aluminium and optionally further containing an activator. Characteristically, an ionic solution A containing Ni2+, Mg2+ and Al3+ ions and a solvent selected from water, organic solvents and mixtures thereof, in particular alcohols and more particularly methanol is reacted in a basic medium with a solution B of Na2CO3 in a solvent selected from water, organic solvents and mixtures thereof, in particular alcohols and more particularly methanol, and further containing a surfactant, until a suspension is obtained, the solid particles thus obtained are separated and calcined at a temperature equal to or greater than 400°C in particular equal to 550°C.

Description

[0001] PROCESS FOR THE SYNTHESIS OF SOLID MIXED OXIDES CONTAINING NICKEL, MAGNESIUM AND ALUMINUM OXIDES AND OPTIONALLY AN ACTIVATOR - MIXED OXIDES, CATALYSTS AND ASSOCIATED USES

[0002] Technical field

[0003] The present invention relates to a process for the synthesis of a solid mixed oxide of hydrotalcite-type structure, containing nickel, magnesium and aluminium oxides and optionally further containing a promoter (activator). The present invention also relates to the oxides obtainable according to the process of the invention, the catalysts obtained by reductions of the mixed oxides of the invention and their uses.

[0004] State of the art

[0005] Nickel-based mixed oxides are used as a catalyst for the conversion of carbon dioxide into methane, either in the so-called Sabatier reaction (thermal conversion) or from a plasma containing carbon dioxide.

[0006] Some mixed oxides can also be used for the synthesis of methanol from carbon dioxide.

[0007] The publication entitled "On the influence of the preparation routes of NiMgAI-mixed oxides derived from hydrotalcite on their CO2 methanation catalytic activities" by Minh Nguyen-Quang and F. Azzolina-Jury, published in the International Journal of Hydrogen Energy on September 17, 2022, reports on the influence of the synthesis route of mixed oxides on their catalytic activity and physicochemical characteristics. Thus, there are three methods for the synthesis of mixed oxides: coprecipitation, urea hydrolysis, and impregnation. The aforementioned publication indicates that the addition of urea to the reaction solution makes it possible to obtain a structure with a greater number of layers and a larger surface area. Urea hydrolysis improves the total basicity of the mixed oxide and its distribution of medium and weakly basic sites.

[0008] The publication by Tan Mingwu et al entitled "Template-free synthesis of mesoporous gamma alumina supported Ni-Mg oxides and their catalytic properties for prereforming liquefied petroleum gas" and published in the Journal of Catalysis, Academic Press Duluth, Minnesota US, on May 4, 2014 in volume 314, pages 117 to 131 describes a synthesis of mixed oxides in which an aqueous solution is formed by dissolving the following salts AI(NOs)3, Ni(NOs)2 and Mg(NOs)2 in water at 70°C. A solution of (NH4)3CO3 is then added dropwise; the pH of the mixture ranges from 5.2 to 5.8, so it is acidic.

[0009] The publication by Debek R & al, entitled "operando FT-IR study on basicity improvement of Ni(Mg, Al) 0 hydrotalcite-derived catalyst promoted by glow plasma discharge" and published on January 31, 2019 in the journal Plasma science and technology, Institute of physics publishing, vol. 21, pages 45503 describes a synthesis of oxides. The pH during the synthesis is equal to 10 but no surfactant is used.

[0010] Technical issues to be resolved

[0011] An aim of the present invention is to provide a new simple process for the synthesis of mixed oxides containing nickel, magnesium and aluminum oxides.

[0012] Another object of the present invention is to propose a new simple process for the synthesis of mixed oxides such as those mentioned above, doped with another metal by one-step synthesis.

[0013] Another aim of the present invention is to propose new mixed oxides of hydrotalcite type structure, which have a specific surface area and / or porosity and / or basicity giving them good performance in terms of yield and / or specificity in the carbon dioxide methanation reaction (Sabatier reaction under thermal or plasma-assisted conditions) and / or in the transformation of carbon dioxide into methanol.

[0014] Summary of the invention

[0015] The present invention relates to a process for the synthesis of a solid mixed oxide containing nickel, magnesium and aluminium oxides and optionally further containing an activator. Typically, according to the invention, an ionic solution A containing Ni ions is reacted in a basic medium 2+ ,Mg 2+ and Al 3+and a solvent chosen from water, organic solvents and mixtures thereof, in particular alcohols and more particularly methanol with a solution B of Na2COs in a solvent chosen from water, organic solvents and mixtures thereof, in particular alcohols and more particularly methanol and containing, in addition; a surfactant until a suspension is obtained, the solid particles thus obtained are separated and calcined at a temperature equal to or greater than 400°C, in particular equal to 550°C. The duration of the calcination is advantageously equal to or greater than 3 hours and in particular equal to 5 hours, with a temperature ramp ranging from 0.5 to 5°C per minute, preferably equal to 2°C per minute.

[0016] Calcination allows not only the degradation of the surfactant but also the formation of mixed oxides.

[0017] Preferably, the calcination temperature is 550°C, which allows the oxides to form and all the surfactant to be removed. The calcination temperature is preferably less than 800°C.

[0018] The surfactant helps to give spatial organization to the molecules; it will separate the molecules from each other and thus promote their dispersion. The final solid material is porous and has a spatially homogeneous chemical composition. The surfactant also acts on the shape and size of the pores formed in the final material. During calcination, the surfactant is destroyed, leaving empty spaces that form pores. The surfactant, depending on its size, allows for more or less large pores.

[0019] It is also known to use a surfactant for the synthesis of mesoporous silica-based molecular sieves under acidic conditions. The surfactant used (Pluronic P123) gives this molecular sieve a large surface area and a uniform and ordered pore network.

[0020] For the methanation of carbon dioxide, it is necessary to have a basic catalyst. Indeed, the hydroxyl groups on the surface of the catalyst prevent the methanation reaction. It was therefore not obvious to use a surfactant in an acidic medium. Moreover, in the case of the aforementioned molecular sieve, the mixture of reagents is simple (2 components at most and mainly silica) which effectively allows the formation of an ordered network to be envisaged. In the case of a reactive mixture comprising several metal ions, moreover all in equivalent proportions, it was not obvious that the use of the surfactant would make it possible to obtain a network of pores, nor that it could increase the size of the pores.

[0021] Detailed description

[0022] During the reaction between solution A and solution B, solution A is advantageously added in small fractions to solution B.

[0023] The pH of the A+B mixture is maintained at a basic value and preferably at a value greater than or equal to 9 and less than or equal to 10. The solid formed is then rinsed to a pH of 7 in order to eliminate the soda remaining between the grains formed.

[0024] According to a particular embodiment, said solution A further contains cations of at least one metal chosen from the following metals: Co, Cu, Fe, Mn, Y, Gd, Sc, La, Ba, Ca and mixtures of at least two of these cations. These metals are promoters (or activators) of the methanation reaction. They can be introduced into solution A in the form of nitrate salts, for example. A doped mixed oxide is thus obtained which contains an activator ion which makes it possible to promote the catalytic action of the mixed oxide in certain reactions.

[0025] Advantageously, said aqueous ionic solution A has a ratio [AI 3+ ] / ([Neither 2+ ]+[Mg 2+ ]+[AI 3+])] or ([AI 3+ ]+[M 3+ ]) / ([Neither 2+ ]+[Mg 2+ ]+[AI 3+ ]+[M 3+ ]+[M 2+ ])] when solution A contains at least one activating cation M 2+ and / or M 3+ equal to or greater than 0.05 and equal to or less than 0.6 and in particular equal to 0.25 or 0.3. It should be noted that in the above formula M is a metal which can be the same for the cations M 2+ and M 3+ or which may be different, the cations M 2+ being the cations of a first metal M1, the cations M 3+ being the cations of a second metal M2.

[0026] This type of hydrotalcite was synthesized according to the relationship the ratios of x = [M 3+ ] / ([M 2+ ]+[M 3+ ]) = 0.25 with [M 2+ ] = total concentration of M 2+ ; [M 3+ ] = total concentration of M 3+ in the middle.

[0027] In the case of the NMA catalyst (NiMgAI mixed oxides), M 2+= [Ni] and [Mg], while M 3+ = [AI], Thus, the ratio x = [AI 3+ ] / ([Neither 2+ ] + [Mg 2+ ] + [Al 3+ ]) = 0.25.

[0028] For example, in the case of Cobalt, Co (M 2+ ) the total concentration of M 2+ = [Ni]+[Mg]+[Co] while M 3+ = [AI], The previous formula becomes: x = [AI 3+ ] / ([Neither 2+ ] + [Mg 2+ ] + [Co 2+ ] + [Al 3+ ]) = 0.25. For example, in the case of Y (cation 3+), the total concentration of M 2+ = [Ni]+[Mg] while M 3+ = [Y 3+ ] + [Al 3+ ], The previous formula becomes: x = [Al 3+ ] + [Y 3+ ] / ([Neither 2+ ] + [Mg 2+ ] + [Y 3+ ] + [Al 3+]) = 0.25. The value of 0.25 corresponds to a natural mixture. The value of 0.3 gives good results in the use of the mixed oxide obtained for the methanation of carbon dioxide. Advantageously, solution A contains 15% by mass of nickel.

[0029] Advantageously, the mass percentage of said surfactant in said solution B is equal to or greater than 1% and less than or equal to 7% and in particular equal to 1%, 2%, 2.5%, 3%, 3.5%, 4% or 5%.

[0030] These mass percentages of surfactant make it possible to obtain a porous solid body which remains mechanically solid and which does not sinter. Advantageously, the mass percentage of surfactant is less than 5% and equal to or greater than 1% or equal to or less than 3% or 3.5%. The value of 3% makes it possible to obtain a catalyst which provides a high percentage of conversion of carbon dioxide in the plasma methanation reaction of the latter.

[0031] The mass percentage of surfactant is preferably equal to 3%, in particular when the surfactant is as indicated below and in particular P123.

[0032] The surfactant is not limited according to the invention. To obtain a mesoporous solid, the surfactant has a neutral or basic pH in the solvent used for the synthesis and / or in solution B. It can be chosen from polymers or copolymers whose longest chain contains from 90 to 130 atoms and in particular linear block copolymers and more particularly the block copolymer corresponding to the following chemical formula (H°): HO(CH2CH2O)2O(CH2CH(CH3)0)7O(CH2CH20)2OH. This block copolymer is marketed under the name Pluronic 123 and is subsequently referenced by the acronym P123. It makes it possible to obtain hexagonal pores. It also makes it possible to obtain a total porosity (total pore volume) of at least 50cm 3 / g and / or a specific surface area of ​​at least 170.00 m 2 / g.

[0033] The present invention also relates to a solid mixed oxide containing nickel, magnesium and aluminum oxides, obtainable according to the process according to the invention. Typically, this mixed oxide has a specific surface area of ​​at least 170.0 m 2 / g and less than or equal to 180.0 m 2 / g and in particular equal to 177.1 m 2 / g, 178.4 m 2 / g or 152.1 m 2 / g and / or a pore volume greater than or equal to 0.50 cm 3 / g and less than or equal to 1.20 cm 3 / g and in particular equal to 0.62 cm 3 / g, 0.73 cm 3 / g or 0.97 cm 3 / g.

[0034] The mixed oxide of the invention comprises both a hydrotalcite layered structure and hexagonal channel structures created by the presence of the surfactant during the synthesis of the mixed oxide.

[0035] The mixed oxide of the invention contains carbonate ions and hydroxyl ions as anions. It thus corresponds to the following general formula: Ni a MgbAl c (CO3)d(OH) e . yhhO in which 2a+2b+3c=2d+e. In the case of the example cited in the present application, a = 0.211533672, b = 0.538466328, c = 0.25, d = (0.125), e = 2, y = 0.5 with the ratio x = M 3+ / [M 2+ + M 3+ ] = (3*0.25) / [2*0.211533672 + 2*0.538466328 + 3*0.25] = 0.25. Advantageously, the mixed oxide of the invention contains by mass from 24.00% to 30% of oxygen and in particular 24.66%, 27.37 and 28.21% of oxygen and / or at least 27% by mass of nickel and in particular 27.37% of oxygen and 28.68% of nickel, 28.21% of oxygen and 27.11% of nickel or 24.66% of oxygen and 27.92% of nickel.

[0036] According to a particular embodiment, the solid mixed oxide of the invention further contains ions of a metal chosen from: Co, Cu, Fe, Mn, Y, Gd, Sc, La, Ba, Ca and mixtures of at least two of these metals. These ions represent a mass fraction of the oxide less than or equal to 3% and greater than or equal to 0.5% and in particular equal to 1%. These ions are catalysis activators. Calcium and lanthanum make it possible to increase the percentage of conversion of carbon dioxide into methane with low discharge powers for the creation of a plasma without the addition of heat, this being generated by the reaction placed under plasma, at a temperature equal to 165°C.

[0037] Iron and manganese allow, in the Sabatier reaction, to increase the percentage of conversion of carbon dioxide into methane for reaction temperatures ranging from 200°C (200°C inclusive) to 395°C (inclusive). Iron and cobalt provide the best percentage of conversion at a temperature substantially equal to 140°C in the plasma reaction. Doping with iron or manganese makes it possible to obtain a catalyst which provides a percentage of conversion of carbon dioxide into methane of more than 90% over a wider range of temperature values ​​than the catalysts of the invention doped with cobalt and iron. The latter provide a percentage of conversion of 90% for a precise temperature value substantially equal to 140°C.

[0038] Whatever the embodiment, the mixed oxide of the invention advantageously has a total basicity greater than or equal to 0.079 mmol / g and less than or equal to 0.2 mmol / g and in particular equal to 0.115 mmol / g, 0.121 mmol / g or 0.079 mmol / g. The total basicity is based on the number of basic sites. The more basic the catalyst, the more basic sites it contains where the methanation reaction can occur.

[0039] Advantageously, whatever the embodiment, the oxide of the invention has less than 13% of strongly basic sites and in particular 11.7%, 19% or 35.3% and / or at least 50% of moderately basic sites, in particular at least 53% or 58% of moderately basic sites and in particular 58.9% of moderately basic sites and / or more than 15% or 20% of weakly basic sites and in particular 22.1% of weakly basic sites. The moderately basic sites represent at most 60% of the basic sites.

[0040] Indeed, surprisingly, the inventors have shown that the use of the surfactant makes it possible to increase the percentage of medium-basic and weakly basic sites, which are involved in the carbon dioxide methanation reaction.

[0041] According to a preferred embodiment, the mixed oxide of the invention has at least 53% of moderately basic sites, more than 10% or 20% of weakly basic sites and less than 37% of strongly basic sites or less than 20% of strongly basic sites.

[0042] The preparation of catalysts with the surfactant as mentioned above, in particular P123, makes it possible to increase the basicity of the catalyst and a redistribution of the basic sites of low and medium intensity.

[0043] Such an oxide turns out to be a spatially homogeneous catalyst capable of methanizing carbon dioxide at temperatures of the order of 300°C with an efficiency of at least 70%.

[0044] According to another preferred embodiment, the mixed oxide of the invention has 22.1% weakly basic sites, 58.9% moderately basic sites and 19.0% strongly basic sites.

[0045] The present invention relates in particular to each of the three mixed oxides obtained in the example part of the present application and referenced 1P-NMA, 3P-NMA and 5P-NMA.

[0046] The present invention also relates to a catalyst obtained by reduction of the mixed oxide of the invention. The reduction reaction can be carried out using hydrogen gas or a hydrogen-argon mixture.

[0047] The present invention also relates to the use of this catalyst for the transformation of carbon dioxide into methane or methanol, in particular in the so-called Sabatier reaction or in a plasma reaction. With regard to the transformation of carbon dioxide into methanol, the catalyst of the invention can advantageously be used in a reaction taking place at 30°C, under a pressure equal to 1 bar with a reaction mixture having the molar ratio CO2 / H2= equal to 1 / 2, 1 / 3 or 1 / 4.

[0048] Definitions For the purposes of the present invention, the term hydrotalcite designates a layered double hydroxide of the following formula: Mg6Al2CO3(OH)i6'4(H2O). A fraction of the divalent metal is substituted by an atom of the trivalent metal, thus generating a positive charge on the layers. The positive layers of metal oxides are ordered and are separated from each other by a disordered layer consisting of anions and water molecules, which ensures the electrical neutrality of the structure. The acronym P123 designates the surfactant of the following formula: HO(CH2CH2O)2O(CH2CH(CH3)0)7O(CH2CH20)2OH.

[0049] The term "NMA" refers to NiMgAl mixed oxides, which were synthesized without the use of surfactant, but from the same reaction mixture as that used for the synthesis of the oxides of the invention. These mixed oxides are obtained according to the same synthesis steps as the mixed oxides of the invention.

[0050] The references 1P-NMA, 3P-NMA, 5P-NMA refer to the mixed oxides synthesized with 1% by weight, 3% by weight and 5% by weight of P 123, respectively. The term "catalyst" designates the mixed oxide obtained by reduction of the mixed oxide directly obtained according to the process of the invention. Apart from the basicity and the percentage of basic sites, the catalyst has the same physicochemical characteristics as the starting mixed oxide and is itself a mixed oxide.

[0051] Brief description of the figures

[0052] The characteristics and advantages of the invention will appear on reading the following description based on the appended figures, among which:

[0053] [Fig. 1] represents the evolution of the mass percentage of conversion of carbon dioxide into methane as a function of the discharge power in the reaction of conversion of carbon dioxide contained in a plasma by use of the catalysts according to the invention and obtained with different mass percentages of surfactants during their synthesis (without activator);

[0054] [Fig. 2] represents the evolution of the mass percentage of conversion of carbon dioxide into methane as a function of the discharge power in the reaction of conversion of carbon dioxide contained in a plasma by using the catalysts according to the invention, obtained with 3% by mass of surfactant (P123) and addition of an activator during their synthesis;

[0055] [Fig. 3] represents the evolution of the mass percentage of conversion of carbon dioxide into methane as a function of the reaction temperature, respectively in the reaction of conversion of carbon dioxide contained in a plasma and in the Sabatier reaction (thermal performance) by use of the catalysts according to the invention and obtained with 3% of surfactant (P123), from a solution A containing 15% by mass of nickel and different activators used at 1% by mass during the synthesis of the mixed oxide of the invention;

[0056] [Fig. 4] represents the Lissajous curve of the charge as a function of the applied voltage for each catalyst doped with an activator according to the invention;

[0057] [Fig. 5] represents the isothermal curves of absorption-desorption of dinitrogen obtained according to the BJH method for respectively, the mixed oxide referenced NMA and the three mixed oxides referenced 1 P-NMA, 3P-NMA, 5P-NMA of the invention at 77K;

[0058] [Fig. 6] represents the pore size distribution of the mixed oxide HT (top curve) and the three mixed oxides of the invention determined according to the BJH method;

[0059] [Fig. 7] represents photographs obtained by using a scanning electron microscope of the mixed oxide NMA and the three mixed oxides of the invention;

[0060] [Fig. 8] represents the spectra obtained by wide-angle X-ray diffraction of the three mixed oxides of the invention, before calcination (dotted curves) and after calcination (solid curves);

[0061] [Fig. 9] shows photographs obtained using a scanning electron microscope of the mixed oxide NMA (photographs A and B) and the 3P-NMA oxide (photographs C and D) of the invention;

[0062] [Fig. 10] represents the discrete cosine transform as a function of temperature obtained by gas chromatographic analysis by programmed temperature reduction of H2 for each of the mixed oxides of the invention;

[0063] [Fig. 11] represents the discrete cosine transform as a function of temperature obtained by gas chromatographic analysis by programmed temperature reduction of CO2 for each of the mixed oxides of the invention; and

[0064] [Fig. 12] shows the temperature variations as a function of time during the plasma phase methanation process. EXAMPLES

[0065] Synthesis of mixed oxides according to the invention

[0066] A) Synthesis of mixed oxides without activator (NMA, 1P-NMA, 3P-NMA, 5P-NMA) The synthesis includes 5 steps.

[0067] Mixed oxides NiMgAI are synthesized from an aqueous solution containing nickel acetate and magnesium and aluminum nitrates. Co-precipitation is carried out at constant pH. The solution is such that [M 3+ ] / [M 2+ ]+ [M 3+ ] = 0.25, or in this case [AI 3+ ] / ([Neither 2+ ]+[Mg 2+ ]+[AI 3+ ])= 0.25.

[0068] Step 1. We make a solution A containing 1 M Ni cations 2+ , Mg 2+ and Al 3+ ([Neither 2+l +[Mg 2+ ]+[AI 3+ ]= 1 mol / L) by dissolving nickel acetate salts (Ni(Ac)2.6H2O) and magnesium and aluminum nitrate salts (Mg(N)2.6H2O, AI(N)3.9H2O). Care is taken to ensure that the ratio [AI 3+ / (Neither 2+ + Mg 2+ + Al 3+ )] is equal to or greater than 0.2 and less than or equal to 0.3 and in particular equal to 0.25. The mass percentage of nickel is 15%. (preferred mixture)

[0069] Step 2. A solution B is made by dissolving an ad hoc quantity of Na2COs in water so as to extinguish a concentration of COs 2 ' equal to 1.0 mol / L and 1% by mass, 3% by mass or 5% by mass of Pluronic 123® (referenced P123) of formula HO(CH2CH20)2o(CH2CH(CH3)0)7o(CH2CH20)2oH are added respectively. Step 3. Solution A is added dropwise into solution B with vigorous stirring and at a temperature of 65°C. The pH of the mixture is maintained at a value equal to or greater than 9 and equal to or less than 10 for one hour after mixing the two solutions by adding a 1 M aqueous sodium hydroxide solution.

[0070] Step 4. The suspension obtained is filtered under vacuum and washed with deionized water at 65-80°C until a pH of 7 is obtained.

[0071] Step 5. The filtrate obtained is dried overnight at 80°C and then calcined at 550°C for 5 hours with a gradual increase in temperature of 5°C / min. The mixed oxide obtained is referenced xP-NMA or xPNMA, x being the mass percentage of the surfactant used during the synthesis.

[0072] A particular mixed oxide containing 15% by mass of nickel ions and whose synthesis was carried out with 3% of P 123 was synthesized according to the above-mentioned method. It is referred to in the following by the acronym 3PNMA.

[0073] B) Synthesis of a mixed oxide containing an activator (Ni15Mn1-3PNMA) The steps of the synthesis are as follows: Step 1. Preparation of a solution A containing water (solvent) and containing the following cations Ni 2+ , Mg 2+ Al 3+ and M n+ in concentrations such as [Ni 2+ ]+[Mg 2+ ]+[AI 3+ ]+[M n+] = 1 mol / L while keeping the same mass percentage of nickel (15%) and the same ratio x=[M 3+ / (M 2+ +M 3+ )] = 0.25. Manganese is introduced in the form of a manganese nitrate salt.

[0074] Manganese can be replaced by a cation chosen from transition metals (Co, Cu, Fe, Mn), rare earths and lanthanides (Y, Gd, Sc, La) and alkalis (Ba, Ca) or a mixture of at least two of these ions. The activating ion is always added in the form of nitrate salt and represents 1% by mass of the final mixture. For example, 15Ni1 Co-3PNMA corresponds to a composition of reactants: ([Ni 2+ ]+[Mg 2+ ]+[AI 3+ ]+[Co 2+ ] = 1 mol / L), with 15.% by mass of nickel and the ratio x= =[AI 3+ ] / ([Neither 2+ ]+[Mg 2+ ]+[Co 2+ ]+[AI 3+ ])=0.25.

[0075] In the case of the mixed oxide Ni15Y1 -3P-NMA we have ([Ni 2+ ]+[Mg 2+ ]+[AI3+ ]+[Y 3+ ] = 1 mol / L), 15.% by mass of Ni and x=([AI 3+ ]+[Y 3+ ]) / ([Neither 2+ ]+[Mg 2+ ]+[AI 3+ ]+[Y 3+ ])=0.25 Step 2 Preparation of solution B: solution B is prepared by dissolving in water an adequate quantity of Na2CO3 so as to obtain a CO3 concentration 2 ' equal to 1.0 mol / L and addition respectively of Pluronic® P123 in a mass percentage equal respectively to 1%, 3% or 5%.

[0076] Steps 3, 4 and 5: these coprecipitation and heat treatment steps are identical to those implemented for the synthesis of undoped mixed oxides (i.e. not containing an activator ion)

[0077] The following doped oxides, grouped in Table 1, were synthesized. [Table 1]

[0078] Comparison of the catalytic efficiency of the catalysts obtained from the mixed oxides of the invention with those of the known catalysts in the thermal methanation reaction known as the Sabatier reaction

[0079] The gas used for the reduction preceding the reaction is composed of dihydrogen, carbon dioxide and argon (hh / CC^ / Ar = 12 / 3 / 5 in volume percentage. The flow rate is 100cc / min.). The results are grouped in Table 2 below.

[0080] The curve showing the temperature variations as a function of time during the methanation process is shown in Fig. 12.

[0081] [Table 2]

[0082] n

[0083] = z

[0084] " has

[0085] The acronym VSHG refers to the gas hourly space velocity. VSHG = gas flow rate entering the reactor / catalyst volume. The space velocity is determined with volumes measured at T=25°C and P=1.013 bar.

[0086] The acronym WHSV refers to Weight Hourly Space Velocity. WHSV = mass of incoming gas / mass of catalyst in the reactor. From the results in Table 2, it can be seen that the use of surfactant during catalyst synthesis improves the carbon dioxide conversion rate of the catalyst. The selectivity depends on the nature of the activator for the doped catalysts of the invention. Iron, manganese and lanthanum are found to be good activators in the thermal catalytic methanation (Sabatier reaction) of carbon dioxide.

[0087] Comparison of the catalytic efficiency of the catalysts of the invention with those of known catalysts in the methanation reaction of carbon dioxide from a plasma containing carbon dioxide

[0088] The results are grouped in Table 3 below.

[0089] [Table 3]

[0090] Table 3 shows that the use of P123 allows, for an applied voltage of 24 V and for the same discharge power, to considerably increase the carbon dioxide conversion rate.

[0091] Furthermore, it can be seen from Fig. 1 that the addition of 1% or 3% of P123 surfactant during the synthesis of the oxide modifies the latter (in particular the size of the pores) and makes it possible to increase the CO2 conversion rate for the same plasma discharge power.

[0092] Referring to Fig. 2, the zone indicated 1 corresponds to the phase without plasma, zone 2 corresponds to the ionization phase and phase 3 corresponds to the creation of the plasma with a discharge filament. It can be seen from Fig. 2 that the percentage of conversion of carbon dioxide is greatly increased when the oxide contains 1% by mass of Ca. The power to obtain the plasma is lower than for the other doped oxides of the invention. Lanthanum also allows this effect to a lesser extent. For the other activators, greater power is required.

[0093] Referring to Fig. 3, it can be seen that in the reaction taking place in the plasma, for a temperature substantially equal to 150°C, iron and cobalt allow an increase in the conversion rate of carbon dioxide into methane. As for the Sabatier reaction, iron and manganese allow an increased conversion of carbon dioxide into methane for reaction temperatures equal to or greater than 200°C and less than 400°C.

[0094] Correlation between plasma forming capacity and breakdown voltage

[0095] Fig. 4 shows the Lissajous curves obtained for all doped mixed oxides to which a voltage of 24 kV was applied. The mass % of activator ion is equal to 1% for all mixed oxides. It can be seen from Fig. 4 that the Lissajous curves are different (in terms of shape and slope) depending on the activator ion. The ability to form a plasma is directly related to the dielectric constant of the mixed oxide.

[0096] The dielectric constants of doped mixed oxides are arranged in the following order: Ba < Y ~ Mn < Gd < Co ~ Sc < La ~ Fe ~ Ca < Cu) which means that the mixed oxide containing barium as an activating ion will allow plasma formation more easily than the mixed oxide containing iron as an activating ion.

[0097] Characterization of the mixed oxides of the invention

[0098] Determination of specific surface area by BET method - pore size distribution - determination of pore size by BJH method

[0099] The following Table 4 summarizes the specific active surfaces and pore volumes of the mixed oxides of the invention and of hydrotalcite.

[0100] [Table 4]

[0101] Based on the results in the above table, we can see that the specific surface area is the same for a mass percentage of P123 equal to 1 or 3. The higher the percentage of surfactant used, the greater the pore volume of the mixed oxide. The specific surface area is lower when the mass percentage of P123 is equal to 5. The pores are larger but fewer in number, which reduces the specific surface area.

[0102] In Figures 5 and 6, it can be seen that the 3P-NMA mixed oxide has a significant peak for a pore size of approximately 7 mm. This oxide has more pores than the HT mixed oxide and its pores have a relatively homogeneous size, which is not the case for 5P-NMA.

[0103] Furthermore, it has been shown that the use of P123 during synthesis allows the transformation of a H1 type material (porous material) into a layered material (H3 type). The specific surface area is also increased by using the surfactant during the synthesis of the mixed oxide.

[0104] Observation of the structure of mixed oxides using a scanning electron microscope

[0105] In Fig. 7, we notice that the HT (NMA) oxide has a compact structure: the grains touch each other. On the contrary, the mixed oxides of the invention have a porous structure; the grains are far from each other. The higher the percentage of surfactant used during the synthesis of the mixed oxide, the further the grains are apart.

[0106] Determination of the chemical composition of mixed oxides

[0107] The chemical composition was determined by scattered X-ray radiographic analysis coupled with a scanning electron microscope.

[0108] The results of these analyses are summarized in Table 5 below. All mixed oxides prepared with P123 exhibit a brittle and spongy structure compared to the structure of mixed oxides obtained without the use of P123. The use of P123 modifies the morphology of the mixed oxide.

[0109] [Table 5]

[0110] In view of the results of the above table, it can be seen that the use of the surfactant also modifies the chemical composition of the mixed oxide. The mixed oxides of the invention all contain more than 20% by mass of oxygen, more than 25% by mass of magnesium, more than 15% and in particular more than 17% by mass of aluminum. They contain, however, less than 30% by mass of nickel. This may depend on the affinity of P123 with nickel, which is less important than for the other metal atoms. The chlorine and potassium detected come from the water used.

[0111] Wide-angle and small-angle X-ray diffraction analysis Referring to Fig. 8, it can be seen that the oxides of the invention do indeed contain -NiO and / or -NiA^Os groups, -MgO groups and -Ni groups xMgi-xO2. It is noted that calcination also allows the formation of the mixed oxide. The mixed oxides of the invention contain nickel and magnesium oxides, magnesium oxides and nickel or nickel and aluminum oxides (NiAl2O4). The higher the mass percentage of surfactant, the more the peaks corresponding to the species NiO and / or -NiA^Os, -MgO and -Ni x Mgi- x O2 are pointed which seems to indicate that the chemical composition is more homogeneous.

[0112] Study of the microscopic structure using a scanning electron microscope of a mixed oxide according to the invention

[0113] With reference to Fig. 9, photographs A and B represent the mixed oxide synthesized without the use of P 123 while photographs C and D represent the mixed oxide of the invention referenced 3P-NMA. The mixed oxides studied have previously been subjected to a reduction (by passing a flow of hydrogen and argon) and therefore are in their active catalyst form. In view of photographs A and C, it can be clearly deduced that the oxide of the invention comprises the nickel particles Ni 0(metallic nickel) the smallest. For the NMA catalyst, it is observed that the metallic nickel particles form aggregates and are not dispersed in the material. In photograph A, groups are observed. On the other hand, in photographs C and D, the layered structure of the hydrotalcite is observed, confirming its successful synthesis. In photograph D, an ordered arrangement in the shape of a hexagon is observed in the circled area. This arrangement was conferred by the P123 which was completely degraded by calcination, leaving only the observable arrangement in the hydrotalcite.

[0114] Gas chromatographic analysis by temperature-programmed reduction under H2 flow for each of the mixed oxides of the invention (obtaining a catalyst according to the invention) Fig. 10 represents the TCD signal as a function of temperature. In view of this figure, it can be seen that there are 4 primary reduction peaks, at 377-400°C, 494-520°C, 622-726°C (shoulder) and a last one at a higher temperature. It can be seen that the curve relating to the mixed oxide 3P-NMA has a larger peak at the temperature of 377°C compared to the curves relating to 1 P-NMA and 5P-NMA. This peak indicates a better dispersion of the -NiO groups and / or the presence of free NiO blocks on the surface of the pores of the mixed oxide. The last two bands are shifted to higher temperatures for 1P-NMA and 3P-NMA and to a lower temperature for 5P-NMA.The reduction peak at 379°C associated with a broad base peak for high temperatures obtained for the 3P-NMA catalyst shows that the NiO species in this catalyst are uniformly distributed and were easily reduced during the mixed oxide reduction step that makes it active as a catalyst.

[0115] The reduction peak at 379°C of 3P-NMA shifts to higher temperatures for other P123 proportions and decreases in intensity until it spreads out, indicating that the P123 proportion equal to 3% by mass allows the generation of small nickel particles and their uniform distribution.

[0116] The reduction peak around 763°C -767°C and shifted to 795°C for 3P-NMA and to 704°C for 5P-NMA indicates that the mass percentage of P123 equal to 3% increases the interactions between the NiO groups and the other metal oxides: the reduction causes a peak and consumes less hydrogen than for 5P-NMA. Indeed, when the proportion of P123 is equal to 5% by mass, the reduction step consumes 99.6% of the hydrogen. We therefore see that the use of P123 and its mass percentage during the synthesis of the catalyst influences the catalytic properties of the latter.

[0117] The following Table 6 lists the H2 consumption in mmol / g of catalyst. [Table 6] is consumed at the second peak temperature for 1P-NMA and 3P-NMA. The higher the % of surfactant used for the synthesis of the catalyst, the more porous it is and the more dihydrogen it reduces. However, the reduction temperature is also higher. It is the 3P-NMA catalyst which allows for the greatest reduction at 379°C.

[0118] Chromatographic analysis as a function of temperature in the gas phase by programmed temperature reduction of CO2 for each of the mixed oxides of the invention

[0119] From Fig. 11, we see that there are three carbon dioxide desorption peaks for the three catalysts: one at 150°C, a second at 249°C and the third at 470°C.

[0120] Table 7 below lists the calculated basicity values.

[0121] [Table 7]

[0122] Based on the results in the above table, it can be seen that the basicity is reduced for 5P-NMA. The higher the mass percentage of surfactant used, the lower the basicity, the higher the % of weakly basic sites and the lower the percentage of strongly basic sites. The 3P-NMA catalyst has the highest percentage of moderately basic sites; it is the most homogeneous in terms of % basicity of the sites of the three catalysts tested. Since the moderately and weakly basic sites are considered the most active in the methanation reaction, the 3P-NMA catalyst is therefore the most effective.

Claims

Claims 1. Process for the synthesis of a solid mixed oxide containing nickel, magnesium and aluminium oxides and optionally also containing an activator, characterized in that an ionic solution A containing Ni ions is reacted in a basic medium 2+ ,Mg 2+ and Al 3+and optionally an activator and a solvent chosen from water, organic solvents and their mixtures, in particular alcohols and more particularly methanol with a solution B of Na2CO3 in a solvent chosen from water, organic solvents and their mixtures, in particular alcohols and more particularly methanol and containing, in addition; a surfactant until a suspension is obtained, in that the pH of the mixture A + B is maintained at a basic value, with a reference value greater than or equal to 9 and less than or equal to 10, in that the solid particles thus obtained are separated and calcined at a temperature equal to or greater than 400°C, in particular equal to 550°C.

2. Method according to claim 1, characterized in that said solution A also contains cations of a metal chosen from the following metals: Co, Cu, Fe, Mn, Y, Gd, Sc, La, Ba, Ca and mixtures of at least two of these cations.

3. Method according to claim 1 or 2, characterized in that said solution A has a ratio [AI 3+ ] / ([Neither 2+ ]+[Mg 2+ ]+[AI 3+ ]) or ([AI 3+ ]4-[M 3+ ]) / ([Neither 2+ ]4-[Mg 2+ ]4-[AI 3+ ]4-[M 3+ ]4-[M 2+ ]) when said solution A contains at least one activating cation M 2+ and / or M 3+ equal to or greater than 0.05 and equal to or less than 0.6 and in particular equal to 0.25 or 0.

3.

4. Method according to any one of the preceding claims, characterized in that the mass percentage of said surfactant in said solution B is equal to or greater than 1% and less than or equal to 7% and in particular equal to 1%, 2%, 2.5% 3%, 3.5%, 4% or 5%.

5. Method according to any one of the preceding claims, characterized in that said surfactant is a polymer or copolymer whose longest chain contains from 90 to 130 atoms and in particular a linear block copolymer and more particularly the block copolymer corresponding to the following chemical formula (II): (II) HO(CH2CH2o)2o(CH2CH(CH3)0)7o(CH2CH20)2oH.

6. Solid mixed oxide containing nickel, magnesium and aluminium oxides, obtainable according to the process according to any one of claims 1 to 5, characterized in that it comprises carbonate ions, in that it has a hydrotalcite layered structure and hexagonal channel structures and in that that it has a specific surface area measured according to the BET method of at least 170.0 m 2 / g and less than or equal to 180.0 m 2 / g and in particular equal to 177.1 m 2 / g, 178.4 m 2 / g or 152.1 m 2 / g and / or a pore volume greater than or equal to 0.50 cm 3 / g and less than or equal to 1.20 cm 3 / g and in particular equal to 0.62 cm 3 / g, 0.73 cm 3 / g or 0.97 cm 3 / g.

7. Solid mixed oxide according to claim 6, characterized in that it contains by mass from 24.00% to 30% of oxygen and in particular 24.66%, 27.37 and 28.21% and / or at least 27% by mass of nickel and in particular 27.37% of oxygen and 28.68% of nickel, 28.21% of oxygen and 27.11% of nickel or 24.66% of oxygen and 27.92% of nickel. Ni 2+ .

8. Solid mixed oxide according to claim 6 or 7, characterized in that it further contains cations of a metal chosen from: Co, Cu, Fe, Mn, Y, Gd, Sc, La, Ba, Ca and mixtures of at least two of these metals.

9. Mixed oxide according to any one of claims 6 to 8, characterized in that it has a total basicity greater than or equal to 0.079 mmol / g and less than or equal to 0.2 mmol / g and in particular equal to 0.115 mmol / g, 0.121 mmol / g or 0.079 mmol / g and / or in that it has less than 13% of strongly basic sites and in particular 11.7%, 19% or 35.3% and / or at least 50% of moderately basic sites, and in particular 58.9% of moderately basic sites and / or more than 15% or 20% of weakly basic sites and in particular 22.1% of weakly basic sites.

10. Use of a mixed oxide according to any one of claims 6 to 9, as a catalyst for the transformation of carbon dioxide into methane or methanol, in particular in the so-called Sabatier reaction or in a plasma reaction.

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