Method for producing biomethane

Abstract

The present invention relates to a method for producing biomethane from a gas mixture comprising carbon dioxide and methane, which method comprises bringing said mixture into contact with a ruthenium-based catalyst in a hydrogen atmosphere.

Description

[0001] BIOMETHANE PRODUCTION PROCESS

[0002] SUBJECT OF THE INVENTION

[0003] The present invention relates to the field of green energy and relates to a process for producing methane, in particular biomethane. More specifically, the invention relates to a catalytic process for producing methane or biomethane by direct hydrogenation of carbon dioxide contained in a gas mixture containing methane, such as, for example, biogas.

[0004] BACKGROUND OF THE INVENTION

[0005] Biogas is a gas produced by the fermentation of organic matter. This combustible gas is composed primarily of methane and carbon dioxide and can be burned at its production site to generate energy in the form of heat or electricity. To obtain biomethane that can be used as natural gas for vehicles or injected into natural gas transmission and distribution networks, the biogas must be purified to remove the carbon dioxide and retain only the methane. Currently, biomethane can be obtained from biogas by separating the carbon dioxide and methane using membrane techniques. The recovered pure carbon dioxide can then also be converted into biomethane using a catalytic hydrogenation process.

[0006] A methanation process uses pilot plants and demonstration units that implement nickel-based catalysts with a high nickel content (>15% by mass). These nickel catalysts require a high operating temperature (>350°C) to achieve sufficient CO2 conversion. Most of the time, two reactors in series are necessary to obtain an outlet composition that meets the specifications of the gas that can be injected into the network. The investment costs of the installations and units therefore remain very high.

[0007] Among other existing processes, the cold plasma technology developed by the company ENERGO uses heterogeneous catalysts allowing the hydrogenation of pure carbon dioxide at low temperatures (100-200 °C), but also requires prior separation of carbon dioxide. Also, the conversions of carbon dioxide and methane yields remain insufficient and do not reach thermodynamic limits. In addition, this technology consumes a certain amount of electricity to create the plasma, which must be taken into account in the energy balance of this process.

[0008] Along with prior separation of carbon dioxide, catalytic hydrogenation of pure carbon dioxide from methanization after biogas purification or after carbon dioxide capture from combustion fumes can also be mentioned. Three steps are required for carbon dioxide capture from flue gases in the GRTgaz / Teréga project. The first step involves cooling the flue gases. The second is the trapping of up to 90% of the carbon dioxide from the flue gases in a solvent using membrane contractors. Finally, the third step involves separating the carbon dioxide in a stripping column at 98% purity. However, separating carbon dioxide from biogas or combustion fumes represents a significant economic cost.

[0009] Biological methanization processes have also been developed. For example, methanization from the CO2METH project involves the conversion of carbon dioxide from biogas into methane using hydrogen and microorganisms (hydrogenotrophic methanogens) under mild conditions with low temperatures of around 55°C. However, methane production rates remain relatively low and the very long reaction times are difficult to reconcile with an industrial scale.

[0010] Pyrogasification of organic waste is also one of the methane production processes. In this pyrogasification process, organic matter is heated to very high temperatures of around 1000 °C in the presence of small amounts of oxygen, and produces a mixture containing methane, hydrogen, carbon monoxide and carbon dioxide. The CO and CO2 are then separated or used in methanation processes. Pyrogasification eliminates the methanization step in the biodigester but requires very high operating temperatures around 1000 °C. The energy cost of this process is very high and also requires a carbon dioxide separation step or a methanation step of the CO or CO2 present in the synthesis gas produced.Given the cost problems of implementing existing methanation processes and their environmental impact, there remains a real need to develop new methane production processes, particularly biomethane, which are improved in terms of yield and respond to economic and environmental issues.

[0011] SUMMARY OF THE INVENTION

[0012] In this context, the inventors have proposed a methanation process by direct hydrogenation of carbon dioxide using a ruthenium-based catalyst. This process, using a ruthenium catalyst, preferably supported by titanium dioxide, increases the methane content in the gas, and therefore the energy value of the gas. It also has the important advantage of eliminating the costs associated with the separation of carbon dioxide and methane by absorption or membrane. The process of the invention, converting carbon dioxide into biomethane by catalytic hydrogenation, is also more environmentally friendly by reducing carbon dioxide emissions into the atmosphere.

[0013] The present invention therefore relates to a process for producing methane from a gas comprising methane and carbon dioxide with a molar ratio of methane to carbon dioxide of between 0.50 and 1.80, said process comprising contacting said gas with a ruthenium-based catalyst under a hydrogen atmosphere. More particularly, the methane is produced by direct catalytic hydrogenation of the carbon dioxide contained in the biogas without any prior step of separating the carbon dioxide from the gas. Preferably, the gas is biogas or a natural gas. More preferably, the gas comprises methane and carbon dioxide with a molar ratio of methane to carbon dioxide of between 0.6 and 1.80, preferably between 0.65 and 1.75, between 1.25 and 1.75, and even more preferably about 1.50. Preferably, the process is carried out in a single-chamber reactor.

[0014] According to a particular embodiment of the invention, the process is carried out at a temperature of between 200 and 350°C, preferably between 225 and 300°C, and even more preferably at a temperature of approximately 250°C. According to another particular embodiment, the process is carried out at a pressure of between 1 and 35 bars, preferably between 10 and 30 bars, between 20 and 30 bars, and even more preferably at a pressure of approximately 30 bars. According to another particular embodiment, the process is carried out at a gas hourly space velocity of between 10,000 and 90,000 mL.g'fh' 1 , preferably enters at a gas hourly space velocity of about 15000 mL.g'fh' 1 .

[0015] According to a preferred embodiment, the ruthenium-based catalyst is supported by titanium dioxide. Particularly, the titanium dioxide is in a crystalline form selected from rutile, brookite, anatase or a mixture thereof. Preferably, the titanium dioxide is a mixture of anatase and rutile.

[0016] According to another preferred embodiment, the ruthenium-based catalyst is prepared by wet impregnation, or by deposition-precipitation, or by colloidal deposition. Preferably, the ruthenium-based catalyst is prepared by colloidal deposition.

[0017] According to another preferred embodiment, the ruthenium is in the form of nanoparticles having a size of between 1 and 4 nm, preferably between approximately 2 and 3 nm, and even more preferably of a size of approximately 2 nm or 3 nm. According to an even more preferred embodiment, the ruthenium-based catalyst is in the form of nanoparticles having a size of approximately 2 nm or 3 nm supported by titanium dioxide in the form of a mixture of rutile and anatase.

[0018] Another object of the invention relates to a process for preparing a ruthenium-based catalyst in the form of nanoparticles having a size of about 2 nm or 3 nm and supported by titanium dioxide in the form of a mixture of rutile and anatase, in which said catalyst is prepared by colloidal deposition.

[0019] A preferred subject of the invention is a process for preparing a ruthenium-based catalyst by colloidal deposition comprising the following steps: i) reacting ruthenium (III) chloride trihydrate with a solution comprising polyvinylpyrrolidone, methanol, and water; ii) optionally, adding an aqueous solution of sodium tetrahydruroborate; iii) adding titanium dioxide in the form of a mixture of rutile and anatase; iv) filtering and drying the suspension obtained in step iii); and v) obtaining the ruthenium-based catalyst. Another subject of the invention is the use of a ruthenium-based catalyst as defined in the present application in a process for producing methane from a gas comprising methane and carbon dioxide, in particular in a process for producing biomethane from biogas.

[0020] BRIEF DESCRIPTION OF THE FIGURES

[0021] Figure 1:

[0022] Figure 1 A: Effect of pressure and temperature on the activity of the lRu / TiO2_NP_IMP catalyst. (a) GHSV= 45000 mL.h^g 1 ; CO2 / CH4 / H2 / He = 1 / 1.5 / 4 / 0.16 (mol.). Figure IB: Effect of pressure and temperature on the activity of the lRu / TiO2_NP_IMP catalyst. (b) GHSV = 90000 mEN'g 1 ; CO2 / CH4 / H2 / He = 1 / 1.5 / 4 / 0.16 (mol.).

[0023] Figure 2:

[0024] Figure 2A: Effect of gas space velocity (GHSV) on catalyst activity

[0025] (a) 1.5Ru / TiO2_NP_DP

[0026] Figure 2B: Effect of gas space velocity (GHSV) on catalyst activity

[0027] (b) lRu / TiO2_NP_2 nm COL

[0028] Figure 3: Effect of the CH4 / CO2 ratio on the activity of catalysts:

[0029] (a) lRu / TiO2_NP_2nm_COL, T = 250 °C. (b) 1.5Ru / TiO2_NP_DP, T= 275 °C, P= 30 bars. Figure 4: (a) Methane yield, (b) methane selectivity as a function of the CH4 / CO2 ratio for the lRu / TiO2_NP_2nm_COL catalyst, T = 250 °C, P=30 bars.

[0030] Figure 5: Diagram illustrating the optimal conditions of the direct methanation process of CO2 contained in biogas

[0031] DETAILED DESCRIPTION

[0032] As illustrated in the examples below, the inventors have implemented a catalytic process for producing methane by direct hydrogenation of CO2 contained in a gas, and more specifically, a catalytic process for producing biomethane by direct hydrogenation of CO2 contained in biogas or in a natural gas. The inventors have also developed a ruthenium-based catalyst supported on a TiO2 nanopowder which, implemented in this process under optimal conditions of temperature, pressure and space velocity (GHSV), allows conversion of the CO2 contained in the biogas and almost total selectivity to methane.The present invention therefore relates to a process or method for producing methane from a gas comprising methane and carbon dioxide with a molar ratio of methane to carbon dioxide of between 0.50 and 1.80, said process comprising contacting said gas with a ruthenium-based catalyst under a hydrogen atmosphere.

[0033] More particularly, methane is produced by direct catalytic hydrogenation of carbon dioxide contained in the gas without any prior step of separating carbon dioxide from the gas. Thus, carbon dioxide in the gas is efficiently converted into methane in the presence of methane contained in the gas. The process of the invention thus eliminates a costly step of separating carbon dioxide and methane in the gas.

[0034] The method according to the invention can be implemented with any type of gas comprising methane and carbon dioxide. For example, the gas can be biogas or a natural gas, such as a natural gas from the pre-salt layer.

[0035] According to the invention, the method makes it possible to produce methane from gas comprising methane and carbon dioxide with a molar ratio of methane to carbon dioxide of between 0.50 and 1.80, preferably between 0.60 and 1.80, between 0.65 and 1.75. Preferably, the molar ratio of methane to carbon dioxide is between 1.25 and 1.75, and even more preferably about 1.50.

[0036] According to a particular embodiment of the invention, the method makes it possible to produce biomethane from biogas comprising methane and carbon dioxide with a molar ratio of methane to carbon dioxide of between 0.50 and 1.80, preferably between 1.25 and 1.75, and even more preferably of approximately 1.50.

[0037] According to another particular embodiment of the invention, the process is therefore carried out from natural gas in which the molar ratio of methane to carbon dioxide is between 0.50 and 1.50, preferably 0.60 and 1.00, and even more preferably 0.67.

[0038] Preferably, the process is carried out in a single-chamber reactor. The process of the invention can be carried out at different temperatures, such as a temperature between 200 and 350°C, preferably between 225 and 300°C, between 225 and 275°C, and even more preferably at a temperature of about 250°C, or 275°C.

[0039] According to a particular embodiment of the invention, the process is carried out at a temperature between 200 and 350°C, preferably between 225 and 300°C, and even more preferably at a temperature of approximately 250°C.

[0040] According to another particular embodiment of the invention, the method is carried out at a pressure of between 1 and 35 bars, preferably between 10 and 30 bars, between 20 and 30 bars, and even more preferably at a pressure of approximately 30 bars.

[0041] According to another particular embodiment of the invention, the method is carried out at a gas hourly space velocity of between 10,000 and 90,000 mL.g'bh' 1 , preferably enters at a gas hourly space velocity of about 15000 mL.g'hh' 1 .

[0042] The different conditions of temperature, pressure, and gas hourly space velocity used in the process of the invention allow a conversion of the carbon dioxide contained in the gas or biogas into methane or biomethane which is very satisfactory from an industrial point of view. Preferably, the process of the invention is carried out at a temperature between 225 and 300 °C, at a pressure between 10 and 30 bars, and at a gas hourly space velocity between 10,000 and 90,000 mL.g'bh' 1 . Even more preferably, the process of the invention is carried out at a temperature of approximately 250°C, at a pressure of approximately 30 bars, and at a gas hourly space velocity of approximately 15000 mL.g'bh'. 1 These highly preferred conditions allow for an almost complete CO2 conversion of 99.26 ± 0.05% with a methane selectivity of 99.25 ± 0.57%, or a methane yield of 98.52 ± 0.6%.

[0043] The method of the invention can be implemented in any type of reactor commonly used in industry. Preferably, the reactor is single-chamber.

[0044] A preferred subject of the invention therefore relates to a process for producing biomethane from biogas comprising the successive steps: a) introduction of the biogas and hydrogen into a reactor comprising a ruthenium-based catalyst; and b) obtaining the biomethane.

[0045] Another preferred subject of the invention relates to a process for producing biomethane from biogas consisting of the successive steps: a) introduction of the biogas and hydrogen into a reactor comprising a ruthenium-based catalyst; and b) obtaining the biomethane.

[0046] In particular, step a) of the process is carried out at a temperature of between 200 and 350 °C, preferably between 225 and 300 °C, and even more preferably at a temperature of approximately 250 °C. In particular, step a) of the process is carried out at a pressure of between 1 and 35 bar, preferably between 10 and 30 bar, between 20 and 30 bar, and even more preferably at a pressure of approximately 30 bar. In particular, step a) of the process is carried out at a gas hourly space velocity of between 10000 and 90000 mL.g'bh' 1 , preferably enters at a gas hourly space velocity of about 15000 mL.g'bh' 1 .

[0047] A preferred subject of the invention relates to a process for producing biomethane from biogas comprising or consisting of the successive steps: a) introducing the biogas and hydrogen into a reactor comprising a ruthenium-based catalyst at a temperature of approximately 250°C, a pressure of approximately 30 bars, and a gas hourly space velocity of approximately 15000 mL.g'bh' 1 ; and b) obtaining biomethane.

[0048] Furthermore, the inventors have demonstrated that the molar ratio of methane to carbon dioxide does not disturb the methanation process according to the invention and does not have an impact on the performance of the catalysts and on the results in terms of methane yield and selectivity. The process of the invention can therefore be applied to any type of gas comprising methane and carbon dioxide such as biogas or any type of natural gas. Another aspect of the invention is a process for producing methane from carbon dioxide comprising contacting carbon dioxide with a ruthenium-based catalyst under a hydrogen atmosphere, in particular the methane is produced by direct catalytic hydrogenation of carbon dioxide. The process according to this other aspect includes all of the embodiments and all of the preferred modes described for the process for producing biomethane from biogas.Preferably, the process is carried out from a gas mixture comprising carbon dioxide and methane in which the molar ratio of methane to carbon dioxide is between 2.00 and 0.00, preferably between 1.50 and 0.00, and even more preferably 1.50, 1.00, 0.66, or 0.00.

[0049] The catalysts used and developed in the process of the invention are ruthenium-based catalysts.

[0050] According to a particular embodiment, the ruthenium-based catalyst is supported by titanium dioxide. Preferably, the titanium dioxide is in a crystalline form chosen from rutile, brookite, anatase or a mixture thereof. Even more preferably, the titanium dioxide is a mixture of anatase and rutile, advantageously in an anatase / rutile proportion of approximately 80 / 20% or 79 / 21%. A “mixture of anatase and rutile” is also referred to as a “mixture” in the present application.

[0051] The ruthenium-based catalysts used in the process of the invention can be prepared by various preparation methods. In particular, they can be prepared by wet impregnation, or by deposition-precipitation, or by colloidal deposition. These methods are relatively simple and easy to scale up to industrial scale.

[0052] According to a preferred embodiment, the ruthenium-based catalyst is prepared by colloidal deposition.

[0053] In particular, the ruthenium is in the form of nanoparticles having a size of between 1 and 4 nm, preferably between approximately 2 and 3 nm, and even more preferably of a size of approximately 2 nm or 3 nm. According to a particular embodiment, the ruthenium-based catalyst is in the form of nanoparticles having a size of approximately 2 nm or 3 nm supported by titanium dioxide in the form of a mixture of rutile and anatase. An object of the invention is also a ruthenium-based catalyst in the form of nanoparticles having a size of approximately 2 nm or 3 nm and supported by titanium dioxide in the form of a mixture of rutile and anatase. Preferably, such a catalyst is prepared by colloidal deposition.

[0054] A particular subject of the invention thus relates to a process for preparing a ruthenium-based catalyst in the form of nanoparticles having a size of approximately 2 nm or 3 nm and supported by titanium dioxide in the form of a mixture of rutile and anatase comprising the following steps: i) reacting ruthenium (III) chloride trihydrate with a solution comprising polyvinylpyrrolidone, methanol, and water; ii) optionally, adding an aqueous solution of sodium tetrahydruroborate; iii) adding titanium dioxide in the form of a mixture of rutile and anatase; iv) filtering and drying the suspension obtained in step iii); and v) obtaining the ruthenium-based catalyst.

[0055] Preferably, step ii) of adding an aqueous solution of sodium tetrahydruroborate into the solution comprising ruthenium(III) chloride trihydrate, polyvinylpyrrolidone, methanol, and water is carried out in the catalyst preparation process.

[0056] The invention will be better understood in light of the following examples, which are given purely for illustrative purposes and do not in any way limit the scope of the protection sought.

[0057] EXAMPLES

[0058] 1. Catalyst preparation processes

[0059] Wet impregnation

[0060] Various supports (AI2O3, t-ZrO_tetragonal, m-ZrCh monoclinic, Nb2Os, TiO2_P25 (mixture of anatase and rutile phases), a-TiCh anatase, r-TiCh rutile, TiO2_NP_(mixture of anatase and rutile phases)) commercially available from Sigma-Aldrich were used without any pretreatment except for Nb2O5 which was obtained by calcination of niobic acid obtained from CBMM company. The support was added to an appropriate amount of RuCh solution (0.22 mg.mL' 1 Ru), to obtain the desired Ru content in the final catalyst, then placed under continuous stirring (600 rpm) at room temperature for 12 h. Then, the solution was placed in a water bath (~90 °C) and kept stirring until complete evaporation of the water. The solids thus prepared were then dried at 50 °C for 24 h, before being calcined at 350 °C for 4 h in static air.

[0061] Deposition-precipitation

[0062] The TiO2_NP_ support (mixture of anatase and rutile phases) was added to an appropriate amount of RuCh solution (0.22 mg. mL' 1 Ru) to obtain the desired Ru content in the final catalyst, before the pH was adjusted to 9 with a 25% vol. ammonia solution, and the mixture was stirred (600 rpm) at room temperature for 12 h. The suspension was then filtered and washed several times with ultrapure water and then evaporated to dryness at 50 °C. The collected solid was then calcined at 350 °C for 4 h under static air. The catalysts obtained were denoted xRu / TiCh NP DP where x represents the mass percentage of ruthenium (1 and 1.5% by mass).

[0063] Colloidal deposition

[0064] Preparation of Ru nanoparticles of approximately 2 nm

[0065] In a 500 mL flask, 2.08 g of PVP (polyvinylpyrrolidone) was dissolved in a mixture of methanol (200 mL) and water (160 mL). To this solution the desired amount of RuCh, 3H2O required to obtain a 1% Ru catalyst by mass was added and the solution was allowed to stir (600 rpm) for 30 minutes. Then, a 40 mL aqueous solution containing 0.354 g of NaBEL was poured into the vigorously stirred solution.

[0066] Preparation of Ru nanoparticles of approximately 3 nm

[0067] In a 500 mL flask, 1.04 g of PVP (polyvinylpyrrolidone) was dissolved in a mixture of methanol (200 mL) and water (160 mL). To this solution the desired amount of RuCh, 3H2O required to obtain a 1% Ru catalyst by mass was added and the solution was left stirring (600 rpm) for 30 minutes. Then, a 40 mL aqueous solution containing 0.177 g of NaBEL was poured into the vigorously stirred solution. Preparation of Ru / TiCh catalysts

[0068] 2 g of TiCh NP support was added to the previously prepared ruthenium nanoparticle solutions, the suspension was stirred at 600 rpm for 6 hours followed by filtration before being dried at 80 °C for 24 hours. Two catalysts denoted respectively lRu / TiO2_NP_2nm_COL and lRu / TiO2_NP_3nm_COL were thus prepared.

[0069] Alternative process

[0070] In a 500 mL flask, 2.08 g of PVP (polyvinylpyrrolidone) was dissolved in a mixture of methanol (200 mL) and water (160 mL). To this solution the desired amount of RuCh, 3H2O required to obtain a 1% Ru catalyst by mass was added and the solution was left stirring (600 rpm) for 30 minutes.

[0071] 2 g of TiCh NP support was added to the previously prepared ruthenium nanoparticle solution, the suspension was stirred at 600 rpm for 6 hours followed by filtration before being dried at 80 °C for 24 hours. A catalyst denoted IRu / TiCh NP PVP was thus prepared.

[0072] 2. Characterization of catalysts

[0073] X-ray diffraction (XRD)

[0074] X-ray diffraction (XRD) analysis of the commercial TiO2_NP_support (mixture of anatase and rutile phases) revealed the presence of two crystalline phases of TiCh: anatase and rutile. XRD analyses of the xRu / TiO2_NP_IMP, DP or COL catalysts did not reveal any crystalline phase of RuCh or Ru (0) which suggests that ruthenium is highly dispersed in the form of nanoparticles on the surface of the TiCh NP support.

[0075] Transmission Electron Microscopy (TEM)

[0076] To further examine the size distribution of ruthenium particles on the surface of TiCh NP support (mixture of anatase and rutile phases), the best catalysts in CO2 hydrogenation reaction, i.e., lRu / TiO2_NP_2nm_COL, lRu / TiO2_NP_3nm_COL, lRu / TiO2_NP_PVP and l.5Ru / TiCh_NP_DP, were analyzed by transmission microscopy (TEM). The images clearly revealed the presence of well-dispersed Ru nanoparticles on the surface of TiO2_NP. The average Ru particle size is 1.89 ± 0.51 nm for lRu / TiO2_NP_2nm_COL catalyst, 3.02 ± 0.59 nm for lRu / TiO2_NP_3nm_COL, 2.34 ± 0.72 nm for lRu / TiO2_PVP and 3.35 ± 2.18 nm for l.5Ru / TiO2_DP catalyst, respectively. The particle size distributions were determined by analyzing 129, 106, 205 and 132 ruthenium particles, respectively, for lRu / TiO2_NP_2nm_COL, lRu / TiO2_NP_3nm_COL, lRu / TiO2_NP_PVP and l.5Ru / TiO2_NP_DP from corresponding TEM images.

[0077] 3. Performance of catalysts in reaction

[0078] The catalytic performances of methanation of pure CO2 or CO2 contained in biogas (hydrogenation reaction) were measured on the REALCAT high-throughput catalytic screening platform of the UCCS. The 16 reactors of 2 mm internal diameter of a Flowrence unit were used and 16 experiments could be carried out simultaneously at four different temperatures. Before each test, the catalysts were reduced at 300 °C for 1 h, under a flow of pure hydrogen. The reaction temperature was varied between 200 and 350 °C and the pressure between 1 and 30 bar. The total gas flow was adjusted in each reactor to obtain the desired space velocity (GHSV). The molar ratios were set as follows: CH4 / CO2 = 1.50; 1.00; 0.66 and 0.00 and the H2 / CO2 molar ratio was kept constant at 4 (stoichiometric condition). For each condition, four successive analyses of the reactor effluents were carried out.The catalysts therefore remained under flow for 10 hours for each set of operating conditions.

[0079] 4. Calculation of catalytic performance

[0080] CO2 conversion (eq. 1), CLL yield (eq. 2), CLL selectivity (eq. 3) and carbon balance (eq. 4) were calculated from the molar flow rates in and out of each compound by the following equations, using a single run for the rapid screening tests. An average of three runs combined with a measurement uncertainty calculation was performed for the best catalysts.

[0081] CO2 conversion (%):

[0082] (1) 100

[0083] CH4 yield (%)*:

[0084] (2) Rdt 100 Selectivity in CÜ4 (%):

[0085] (3) Salt 100

[0086] Carbon Footprint (%)*: 100

[0087] The CFE yield (Yield cm) and the carbon balance (CB) were calculated taking into account only the methane produced by the methanation reaction, i.e., without taking into account the CH4 contained in the incoming biogas.

[0088] The specific activity of CO2 (mol gRu' 1 h' 1 ) is calculated according to equation (5): pmiet re p r e S between the total volume flow rate (ml. min' 1 ), [CÛ2] and Xco2 denote the volume fraction of CO2 (vol%) and the conversion of CO2), respectively; and WR U is the mass content of metallic ruthenium (wt%).

[0089] Intrinsic activity (Turnover Frequencies or TOF) (s' 1 ) is calculated according to equation (6):

[0090] The values ​​of ruthenium dispersion are calculated according to equation (7): where dvA represents the average ruthenium particle size (nm).

[0091] 5. Results

[0092] 5.1. Effect of support for catalysts prepared by wet impregnation (WPI)

[0093] Catalytic performance measurements for the direct methanation reaction of CO2 contained in biogas were carried out between 200 °C and 350 °C and between 1 and 20 bar. The feed stream consisted of 15.5% CO2, 23.3% CFE and 61.2% H2 (mol %). The gas hourly space velocity (GHSV) used in these experiments was 90000 h' 1 . The results obtained with different supports impregnated with 1% by mass of ruthenium are gathered in Table 1. The best activity was obtained using TiO2_NP_ (mixture of anatase and rutile phases) as support. A conversion of 86.2% of CO2 was obtained at 350 °C and 20 bars.

[0094] Table 1:

[0095] 5.2. Pressure effect for the lRu / TiO2_NP_IMP catalyst

[0096] The effect of pressure on CO2 conversion for the best catalyst from the preliminary study is reported in Figure 1. An increase in pressure from 10 to 30 bar resulted in an increase in CO2 conversion due to an increase in the partial pressures of the reactants. The optimal pressure of 30 bar allows approaching the thermodynamic limits of the reaction.

[0097] 5.3. Effect of catalyst preparation method and Ru content

[0098] Only the best support “TiO2_NP_mixture” was used to prepare catalysts by implementing two other techniques for deposition of ruthenium nanoparticles: the deposition-precipitation (DP) method and the colloidal (COL) method. Indeed, these techniques allow improving the dispersion and interaction of Ru particles with the support. Two different ruthenium contents (1% and 1.5% by mass) were used in the case of the deposition-precipitation method and only one content (1% by mass) for the colloidal method but with two different particle sizes (1.89 and 3.06 nm as measured by TEM image analysis). The performances of these new catalysts were compared with those of the catalysts prepared by impregnation and deposition-precipitation in Table 2 below (direct methanation of CO2 contained in the biogas at P= 30 bars, at a gas hourly space velocity (GHSV) of 15000 h' 1and at a temperature between 225 and 300 °C). An almost complete CO2 conversion of 99.26 ± 0.05% was obtained at a temperature of 250 °C, with a methane selectivity of 99.25 ± 0.57% (i.e. 98.52 ± 0.6% methane yield). These results are very close to the thermodynamic limits, attesting to the excellent performance of the catalyst developed in the present invention.

[0099] 5.4. Effect of gas space velocity (GHSV)

[0100] The effect of gas space velocity (GHSV) on the performance of 1.5Ru / TiO2_NP_DP and lRu / TiO2_NP_2nm_COL catalysts was studied between 15000 and 90000 mL.g'bh' 1(Figure 2) for three different temperatures: 250, 275 and 300 °C for 1.5Ru / TiO2_NP_DP and 225, 250 and 275 °C for lRu / TiO2_NP_2nm_COL. The results show that for increasing space velocities (shorter contact times), CO2 conversion gradually decreases. The space velocities were maintained between 15000 and 20000 mL.g'bh' 1 at lower temperatures (225-250 °C) in order to maintain high CO2 conversion. At higher temperatures (275-300 C°), a GHSV of 90000 mL.g'hh' 1 was maintained to maintain very high CO2 conversion.

[0101] 5.5. Effect of Ru particle size

[0102] At low temperature (225 °C) and for a space velocity of 15000 mL.Fr'g -1The effect of particle size on catalyst performance is clearly observed. Indeed, a 63.5% conversion of CO2 is obtained with the lRu / TiO2_NP_3nm_COL catalyst with an average particle size of 3.02 nm compared to 73.6% with the catalyst of the same composition but with an average particle size of 1.89 nm (Table 2). This indicates that the size of Ru nanoparticles significantly affects the activity of the catalysts.

[0103] Table 2:

[0104] 5.6. Effect of the CH4 / CO2 ratio

[0105] Catalytic tests were carried out at the same pressure as previously (30 bars) but with different CH4 / CO2 molar ratios (1.50; 1.00 and 0.66) and also with pure CO2, i.e. for a CH4 / CO2 molar ratio = 0.00. These tests were carried out one after the other without changing the catalyst (Figure 3). It was observed that the composition of the feed does not affect the performance of the catalysts and particularly the methane yield and selectivity (Figure 4). As a result, the methanation of CO2 from different sources can be carried out efficiently using the catalytic systems of the present invention.

[0106] The compositions of the gases at the reactor outlet were determined in mol% and collected in Table 3. For the best catalyst and under the best operating conditions, the gas composition is compatible with injection into the natural gas distribution network in Europe. This gas quality was obtained directly from biogas without any prior separation of CEE and CO2. In contrast, the work described in Renew. Energy, vol. 146, pp. 1301-1308, 2020 shows that nickel-based catalysts always require two reactors in series to obtain a composition suitable for injection into the natural gas network. Table 3:

[0107] 5.7. Conclusions

[0108] The optimal results are illustrated in Figure 5. The lRu / TiO2_NP_2nm_COL catalyst enabled almost complete CO2 conversion (99.26±0.05%) with a CFL selectivity of 99.25±0.57%, i.e. a CFL yield of 98.52±0.6%. The composition of the gas mixture at the reactor outlet meets European requirements for injection into city gas networks.

[0109] The results of the process of the invention were compared with the results of existing processes in terms of methane productivity expressed in mmol. gmetai^.min' 1 and which use pure CO2 (Table 4) or biogas without prior separation of methane and CO2 (Table 5).

[0110] Table 4:

[0111] Reference 1: Nizio, M. et al. Hybrid plasma-catalytic methanation of CO2 at low temperature over ceria zirconia supported Ni catalysts. Int. J. Hydrog. Energy 41, 11584-11592 (2016).

[0112] Référence 2 : Benrabbah, R. et al. Plasma DBD activated ceria-zirconia-promoted Ni-catalysts for plasma catalytic CO2 hydrogenation at low temperature. Catal. Commun. 89, 73-76 (2017).

[0113] Référence 3 : Zhou, J. et al. Interfacial compatibility critically controls Ru / TiO2 metal-support interaction modes in CO2hydrogenation. Nat. Commun. 13, 327 (2022).

[0114] Tableau 5 :

[0115] Référence 4 : Zhuang, Y. & Simakov, D. S. A. Single-Pass Conversion of CO2 / CH4 Mixtures over the Low-

[0116] Loading Ru / y-A12O3 for Direct Biogas Upgrading into Renewable Natural Gas. Energy Fuels 35, 10062-10074 (2021).

[0117] Reference 5: Stangeland, K., Kalai, DY, Li, H. & Yu, Z. Active and stable Ni based catalysts and processes for biogas upgrading: The effect of temperature and initial methane concentration on CO2 methanation. Appl. Energy 227, 206-212 (2018) The methane productivity obtained with the process of the invention is 4 to 5 times higher than those obtained with the processes from the literature: lwt%Ru / r-TiO2 catalyst (rutile-type TiCh nanorods) and IRu wt% / y-A12O3. Similarly, the methane productivity is 3 times higher with the process of the invention compared to processes using nickel-based catalysts (15%Ni / CeZrO2) used by coupling with a cold plasma (Int. J. Hydrog. Energy 41, 11584-11592 (2016)).

[0118] In Table 6, the compositions at the reactor outlet are compared for our best catalyst in comparison with the industrial nickel-based catalyst tested in a pilot biomethane production plant using biogas produced in a wastewater treatment plant: EDAR Riu Sec in Sabadell (Spain).

[0119] Table 6: Reference 6: Guilera, J. et al. Synthetic natural gas production from biogas in a waste water treatment plant. Renew. Energy 146, 1301-1308 (2020).

[0120] The quality of gas obtained with the process of the invention is higher: 97.07±0.15% of CEE and only 0.27±0.06% of CO2 compared to 95.82% CEE and 1.47% of CO2 for the process using a nickel-based catalyst.

Claims

CLAIMS 1. A process for producing methane from a gas comprising methane and carbon dioxide with a molar ratio of methane to carbon dioxide of between 0.50 and 1.80, said process comprising contacting said gas with a ruthenium-based catalyst under a hydrogen atmosphere.

2. A process according to claim 1, wherein the methane is produced by direct catalytic hydrogenation of carbon dioxide contained in the gas without any prior step of separating the carbon dioxide from the gas.

3. A method according to claim 1 or 2, wherein the gas is a biogas or a natural gas.

4. A method according to any one of claims 1 to 3, wherein the molar ratio of methane to carbon dioxide is between 0.6 and 1.80, preferably between 0.65 and 1.75, between 1.25 and 1.75, and even more preferably about 1.

50.

5. A process according to any one of claims 1 to 4, wherein the process is carried out in a single chamber reactor.

6. A method according to any one of claims 1 to 5, wherein the method is carried out at a temperature between 200 and 350°C, preferably between 225 and 300°C, and even more preferably at a temperature of about 250°C.

7. Method according to any one of claims 1 to 6, wherein the method is carried out at a pressure of between 1 and 35 bars, preferably between 10 and 30 bars, between 20 and 30 bars, and even more preferably at a pressure of approximately 30 bars.

8. A method according to any one of claims 1 to 7, wherein the method is carried out at a gas hourly space velocity of between 10000 and 90000 mL.g'fh'. 1 , preferably enters at a gas hourly space velocity of about 15000 mL.g'fh' 1 .

9. A method according to any one of claims 1 to 8, wherein the ruthenium-based catalyst is supported by titanium dioxide.

10. Method according to any one of claims 1 to 9, in which the titanium dioxide is in a crystalline form chosen from rutile, brookite, anatase or a mixture thereof, preferably the titanium dioxide is a mixture of anatase and rutile.

11. A method according to any one of claims 1 to 10, wherein said catalyst is prepared by wet impregnation, or by deposition-precipitation, or by colloidal deposition.

12. A method according to any one of claims 1 to 11, wherein the ruthenium is in the form of nanoparticles having a size between 1 and 4 nm, preferably between about 2 and 3 nm, and even more preferably a size of about 2 nm or 3 nm.

13. A method according to any one of claims 1 to 12, wherein the ruthenium-based catalyst is in the form of nanoparticles having a size of about 2 nm or 3 nm supported by titanium dioxide in the form of a mixture of rutile and anatase.

14. A process for preparing a ruthenium-based catalyst in the form of nanoparticles having a size of about 2 nm or 3 nm and supported by titanium dioxide in the form of a mixture of rutile and anatase, wherein said catalyst is prepared by colloidal deposition.

15. The method of claim 14, wherein the method comprises the following steps: i) reacting ruthenium(III) chloride trihydrate with a solution comprising polyvinylpyrrolidone, methanol, and water; ii) optionally, adding an aqueous solution of sodium tetrahydruroborate; iii) adding titanium dioxide in the form of a mixture of rutile and anatase; iv) filtering and drying the suspension obtained in step iii); and v) obtaining the ruthenium-based catalyst.

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