Methanol synthesis via catalytic hydrogenation of carbon dioxide

Abstract

The invention relates to a method for synthesising methanol via catalytic hydrogenation of CO2, the method comprising at least one step (E1) in which the following are brought into contact within a reactor comprising at least one first reaction chamber (1) provided with at least one first inlet (10) and at least one first outlet (90): - a gas stream containing CO2 and H2 flowing from the first inlet (10) to the first outlet (90); and - a hydrogenation catalyst contained in the reaction chamber, between the first inlet (10) and the first outlet (90), in the form of a fixed bed, whereby part of the CO2 and H2 of the gas stream is converted into methanol and water during step (E1), wherein: • the pressure is at least 20 MPa (200 bar) during step (E1); and • the gas stream obtained at the end of step (E1), which comprises residual CO2 and H2, is reinjected via a second inlet (20) into a reaction chamber containing a hydrogenation catalyst, this chamber being identical to or different from that of step (E1).

Description

Synthesis of methanol by catalytic hydrogenation of carbon dioxide technical field

[0001] The present invention relates to the field of methanol synthesis by hydrogenation of carbon dioxide.

[0002] The invention relates more specifically to a specific process for the catalytic hydrogenation of carbon dioxide, which allows the synthesis of methanol with a particularly high yield. Technological background

[0003] The synthesis of methanol by catalytic hydrogenation of carbon dioxide is a known technique, various versions of which have been described in particular in the article "A Brief Review of Carbon Dioxide Hydrogenation to Methanol over Copper and Iron Based Catalysts" by Obid Tursunov*, Leonid Kustov and Aleksandr Kustov, Oil & Gas Science and Technology - Rev. IFP Energies nouvelles (2017) 72, 30.

[0004] Most publications concerning the synthesis of methanol by catalytic hydrogenation of carbon dioxide implement hydrogenation under moderate pressure, typically between 10 and 90 bars, by circulating a mixture of carbon dioxide CO2 and dihydrogen H2 over a fixed bed of catalyst.

[0005] In particular, to obtain the highest possible CO2-to-methanol conversion rates at the catalytic bed outlet, syntheses at higher pressures have been proposed. For example, the article “Synthesis of methanol from carbon dioxide and hydrogen over copper alumina catalysts. Mechanism of reaction” by V.N. Ipatieff and G.S. Monroe, J. Am. Chem. Soc. (1945), 67, 12, 2168-2171, describes syntheses carried out at pressures ranging from 26 to 414 bar. Application EP0448019 similarly considers pressures ranging from 50 to 200 bar, and application NL2006535 describes pressures ranging from 50 to 200 bar. Summary of the invention

[0006] One object of the present invention is to provide a catalytic hydrogenation process of carbon dioxide which makes it possible to further improve the yields obtained with the high-pressure processes described in the prior art.

[0007] To this end, the present invention proposes to implement the catalytic hydrogenation of CO2 under pressure and to recycle the residual CO2 and H2 which has not been converted at the end of the catalysis, which makes it possible to optimize the yield compared to the processes described so far.

[0008] More specifically, the present invention relates to a process for the synthesis of methanol by catalytic hydrogenation of carbon dioxide, comprising at least one step (E1) where the following are brought into contact within a reactor comprising at least one first reaction chamber equipped with at least one first inlet and at least one first outlet: - a gaseous stream containing carbon dioxide (CO2) and hydrogen (H2) flowing from said first inlet of said first reaction chamber to said first outlet of said first reaction chamber; and - a hydrogenation catalyst contained in said first reaction chamber, between said first inlet and said first outlet, in the form of a fixed catalyst bed, by which at least a portion of the carbon dioxide CO2 and dihydrogen H2 of the gas stream is converted into methanol and water during step (E1), characterized in that: • the pressure is greater than or equal to 20 MPa (200 bar), for example greater than or equal to 26 MPa (260 bar), during step (E1); and • the gas stream obtained at the end of step (E1), which includes residual carbon dioxide CO2 and dihydrogen H2 which did not react during step (E1) is reinjected into a reaction chamber containing a hydrogenation catalyst (this reaction chamber may or may not be identical to that of step (E1); when the two chambers are distinct, they may contain the same catalyst or different catalysts).

[0009] According to an interesting embodiment, the process of the invention can, for example, be carried out according to the following embodiment: - at the end of step (E1), the gas flow exiting the first reaction chamber The first outlet introduces a second reaction chamber containing a hydrogenation catalyst; and - the process includes a step (E2) of bringing the gas stream and the hydrogenation catalyst into contact under a pressure of at least 200 bar (20 MPa) within said second reaction chamber, whereby at least a portion of the carbon dioxide CO2 and dihydrogen H2 of the gas stream is converted into methanol and water during step (E2).

[0010] It should be noted that the reaction chamber implemented in step (E2), which is referred to here as the "second chamber" for brevity, may optionally be the first chamber used for step (E1). Alternatively, the second reaction chamber of step (E2) may be a separate chamber from the one used in step (E1).

[0011] Advantageously, step (E2) is conducted under conditions similar to those of step (E1).

[0012] In step (E1), given the pressure conditions used, the water and methanol produced are in liquid form (under the conditions of step (E1), the resulting water / methanol mixture is liquid), while the residual H2 and CO2 obtained at the end of step (E1) are gaseous. The products formed in step (E1) are therefore easily separable from the residual H2 and CO2. Preferably, in step (E2) the resulting water / methanol mixture is also liquid.

[0013] The process of the invention may advantageously include, following step (E1), a separation step of the methanol and water produced during step (E1) on the one hand, and the unreacted carbon dioxide and hydrogen on the other, carried out under the pressure and temperature conditions obtained at the end of step (E1). This separation is carried out directly at the outlet of the first reaction chamber used in step (E1), under the pressure and temperature conditions at the outlet of this chamber. This embodiment differs from prior art processes that systematically employ a pressure reduction step and / or a cooling step before carrying out the separation. According to the invention, the separation can typically be carried out by simple decantation of the reaction products (liquids), namely methanol and water, which separate by gravity from the compounds residual gases (H2 and CO2) without having to modify the pressure and temperature conditions obtained at the end of step (E1).

[0014] According to an advantageous embodiment, the first reaction chamber is provided with at least one second outlet, towards which the water and methanol, produced in liquid form in step (E1), are carried by gravity. Preferably, the process then comprises a step (E2) of the aforementioned type and, between step (E1) and step (E2), the process includes a separation step where the unreacted carbon dioxide CO2 and dihydrogen H2 are separated from the methanol and water formed in step (E1), by means of gravity extraction of the water and methanol through the second outlet of the first reaction chamber, this extraction being carried out under the pressure conditions obtained at the end of step (E1).

[0015] According to the invention, in step (E1), as in the following step (E2), which is conducted under similar conditions, CO2 to methanol conversion rates of less than 80% are obtained, and are generally much lower for each step considered individually. However, and surprisingly, it turns out that when steps (E1) and (E2) are implemented sequentially, with reinjection of the unreacted CO2 and H2, the overall reaction yield is much higher than for a single step with a high conversion rate. Thus, it is possible according to the invention to achieve an overall process yield of more than 20 g of methanol / g of catalyst / hour, compared to less than 2 g of methanol / g of catalyst / hour for currently proposed processes, which nevertheless aim for higher individual conversion rates than those of the invention.Surprisingly, the inventors have now shown that the optimum yield is not obtained by maximizing the conversion rate of each of the stages (E1) and (E2), but on the contrary that, to obtain the highest possible overall yields, it is actually preferable that the individual conversion rate of each of the stages (E1) and (E2) remains below 80%, and preferably below 60% (typically between 20 and 50%).

[0016] According to the invention, high methanol production yields are achieved, which can typically reach at least 20 g of methanol per gram of catalyst per hour, for example yields greater than or equal to 50 g of methanol per gram of catalyst per hour, or even higher.

[0017] As indicated, the reaction chamber implemented in step (E2) can be the first chamber used for step (E1) or a separate chamber.

[0018] Thus, according to a first possible embodiment, the second reaction chamber used in step (E2) is the same as the first reaction chamber of step (E1). In this embodiment, the gas flow exiting the first outlet of the first chamber is reinjected into the first reaction chamber through a second inlet of the reaction chamber upstream of the catalyst contained within the chamber (identical to or different from the first inlet). The reinjection of the gas flow exiting the first outlet of the first chamber to the second inlet of the same first reaction chamber can, in particular, be carried out by means of a pump (or, alternatively, according to the gas flow circulation method induced by an oscillating inlet pressure described at the end of this description). This reinjection can be performed concurrently with a parallel injection of CO2 and H2 through the first inlet of the chamber.In this case, the process can be carried out continuously using a reactor comprising only one reaction chamber, with a continuous supply of "fresh" reagents to this chamber, and a recycling loop for residual reagents (where "fresh" reagents are understood in this description to be H2 and / or CO2 injected into the reactor and not originating from one of the reactor's reaction chambers, as opposed to "residual" H2 or CO2, which does originate from a reaction chamber of the reactor). In this specific case, in continuous mode, steps (E2) and (E1) are combined.

[0019] According to a second possible embodiment, the second reaction chamber used in step (E2) is a separate chamber from the first reaction chamber of step (E1). Preferably, this second chamber is then configured like the first reaction chamber used in step (E1), namely with at least one first inlet of the second reaction chamber allowing the introduction of a gas stream and at least one first outlet of the second reaction chamber allowing the removal of this gas stream and the hydrogenation catalyst between said first inlet and said first outlet. According to this second embodiment, the gas stream exiting the first outlet of the first chamber is injected into said second reaction chamber through a second inlet of the second reaction chamber (identical or different). of the first inlet) located upstream of the catalyst contained in this second chamber. The injection of the gas stream into the second reaction chamber generally does not require a pump. This injection can be ensured by the pressure prevailing within the reactor, combined with sequential injections of gaseous reactants as described below. Advantageously, the fluid connection between the first outlet of the first reaction chamber and the second inlet of the second reaction chamber includes a non-return device (typically a valve or check valve) allowing fluid flow only in the direction from the first reaction chamber to the second reaction chamber (and therefore prohibiting fluid flow in the opposite direction, i.e., from the second chamber to the first).

[0020] Preferably, the gaseous reactants H2 and CO2 that have not reacted at the end of step (E2) are returned from the first outlet of the second reaction chamber to the first reaction chamber, in order to work in a closed circuit, which further optimizes the final yield.

[0021] This principle of recirculating unreacted H2 and CO2 reactants and injecting them into a new reaction chamber, as described above for steps (E1) and (E2), can be extrapolated to the use of more than two separate reaction chambers. The process of the invention can thus advantageously implement the recirculation of the reactants described above in more than two reaction chambers, connected in the same way as the first and second reaction chambers described in steps (E1) and (E2). The number of interconnected reaction chambers can range from 2 to 1000, advantageously being between 5 and 200, and in particular between 10 and 100.

[0022] Thus, according to an interesting embodiment, the process of the invention is a process comprising n successive catalytic hydrogenation steps of carbon dioxide, where n is an integer at least equal to 3 (preferably between 3 and 1000, for example between 5 and 200, in particular between 10 and 100), where, after steps (E1) and (E2), the process further comprises n-2 additional steps denoted (Ei), with i ranging from 3 to n, each of these n-2 additional steps (Ei) being carried out in a separate reaction chamber, comprising a hydrogenation catalyst, called reaction chamber (i) (it being understood that the second chamber of step (E2) is also designated herein by the term "reaction chamber"). reaction (2) >>) where: - each of the reaction chambers (i) is preferably configured like the first reaction chamber used in step (E1), namely with at least: a first inlet of the reaction chamber (i) allowing the introduction of a gas stream and at least a first outlet of the reaction chamber (i) allowing the evacuation of this gas stream; and the catalyst between said first inlet and said first outlet, - all or part of the gas stream exiting the reaction chamber (i-1) is introduced into the reaction chamber (i) and step (Ei) includes bringing the gas stream and the hydrogenation catalyst into contact under a pressure of at least 20 MPa (at least 200 bars) within said reaction chamber (i), whereby at least part of the carbon dioxide CO2 and dihydrogen H2 of the gas stream is converted into methanol and water during step (Ei).

[0023] Preferably, between each step (EM) and each step (Ei), the process includes a separation step, where the unreacted carbon dioxide CO2 and dihydrogen H2 are separated from the methanol and water formed during step (Ei-1), by means of a gravity extraction of the water and methanol through the second outlet of the reaction chamber (i-1), this extraction being carried out under the pressure and temperature conditions obtained at the end of step (Ei-1).

[0024] The fluid connection between the first outlet of each reaction chamber (i-1) and the second inlet of each reaction chamber (i) preferably includes a non-return system allowing fluid flow only in the direction from reaction chamber (i-1) to reaction chamber (i). (but therefore prohibiting fluid flow in the opposite direction, i.e. from chamber (i) to chamber (i-1)).

[0025] Furthermore, the H2 and CO2 reactants that have not reacted at the end of step (En) are preferably returned from the first outlet of the last reaction chamber (n) to the first reaction chamber, in order to work in a closed circuit, which further optimizes the final yield.

[0026] According to a more particular embodiment, compatible with all the aforementioned embodiments, the process of the invention can advantageously be carried out by implementing the following specific conditions during step (E1): • the pressure is greater than or equal to 20 MPa (200 bar); and • the inlet velocity of the gas flow into said first reaction chamber is at least 0.4 cm / s, preferably at least 1 cm / s, or even at least 5 cm / s; and • the first reaction chamber is equipped with at least one second outlet, towards which the water and methanol, produced in liquid state, are carried by gravity.

[0027] The inlet velocity of the gas flow (denoted v) referred to in this description can be calculated using the following formula, in the case of a gas flow injected at a given flow rate into a catalytic bed contained in a tube of given cross-section: v=Q / (SxV) where: Q is the gas flow rate at the pressure considered (expressed in cm³). 3 / second) S is the internal cross-sectional area of ​​the tube containing the catalyst (in cm² 2 ) V is the degree of void of the catalytic bed (dimensionless number between 0 and 1 reflecting the volume of void within the catalytic bed).

[0028] The inventors' work has now demonstrated that implementing a high reagent introduction rate combined with gravity drainage optimizes the efficiency of the hydrogenation catalyst. This is achieved schematically by eliminating the liquid film formed from the methanol-water mixture that tends to form on the catalyst's surface and significantly limits the reaction progress when the conditions of step (E1) are not met (under the conditions of step (E1), the resulting water / methanol mixture is a liquid that effectively masks the catalyst). Through the combined action of a high gas flow rate and gravity drainage, the liquid water and methanol formed during step (E1) are at least partially drained, thus making the catalyst more accessible to the gaseous reactants H2 and CO2.

[0029] Thus, by implementing a high gas flow velocity and gravity extraction of the products, the catalyst efficiency and the final reaction yield are further improved, particularly compared to prior art documents that do not consider the specific conditions employed according to the invention. The processes described in the prior art most often involve moderate pressures, typically between 0.5 and 10 MPa (i.e., between 50 and 100 bar), and the few publications concerning work carried out tests under higher pressures were systematically carried out on horizontal reactors and with gas velocities well below 0.4 cm / s.

[0030] Specifically to ensure the desired gravity-driven effect, the first reaction chamber where step (E1) takes place is preferably equipped with at least one second outlet, towards which the water and methanol, produced in liquid form, are carried by gravity. This second outlet, which allows for the removal of the liquid mixture, is typically located below the catalyst in the direction of the gravity field during step (E1). This second outlet also ensures efficient separation of the gas stream, allowing for its recirculation within the reactor.

[0031] According to a preferred embodiment of the invention, the reaction chamber implemented in step (E1) comprises, in its positioning during step (E1): - an upper part (the highest in the direction of the gravity field) comprising the first entrance; - a lower part (the lowest in the direction of the gravitational field) comprising the second outlet towards which the water and methanol are drawn by gravity; and - a middle part, between the upper and lower parts, comprising the hydrogenation catalyst. According to this embodiment, the gas stream comprising CO2 and H2 is introduced through the upper part of the reaction chamber, then passes through its middle part where the catalyst is located which induces the formation of methanol and water by partial consumption of CO2 and H2, and the methanol and water are drained by gravity to the lower part of the chamber where they are carried to the second outlet (the residual CO2 and H2 are discharged through the first outlet).

[0032] According to an advantageous embodiment of the process of the invention, the reaction chamber used in step (E1) may advantageously comprise one or more tubes containing a hydrogenation catalyst (this tube or these tubes are located in the middle part of the chamber in the case of the embodiment presented in the preceding paragraph), and this tube or these tubes each have the shape of a cylinder whose axis is positioned substantially vertically. According to this embodiment, which differs from the high-pressure processes described in the prior art where the flow within the catalytic bed The flow is horizontal, and the gas flow propagates vertically, from top to bottom: each of the cylindrical tubes containing the catalyst receives a gas flow comprising CO2 and H2 through its inlet located at the top of the tube in its position during step (E1); then the methanol and water formed in contact with the catalyst are drained by gravity to the bottom of each tube in its position during step (E1). The cylindrical tubes containing the catalyst preferably have an internal diameter between 100 µm and 3 cm. They can be joined tubes or, more advantageously, parallel channels formed within a metal piece.By "substantially vertical" is meant, for the purposes of this description, that each of the cylinders containing the catalyst has an axis making an angle less than or equal to 60° (and preferably less than or equal to 45°, advantageously less than or equal to 30°, more advantageously less than or equal to 15°, for example ranging from 0 to 10°) with the direction of the gravity field under the conditions of stage (E1).

[0033] When the process includes a step (E2), step (E2) is preferably carried out under conditions similar to those of step (E1). Thus, according to one embodiment, it is preferred that, during step (E2): - the pressure is greater than or equal to 14 MPa (140 bar); and - the inlet velocity of the gas flow into the second reaction chamber is at least 0.4 cm / s, preferably at least 1 cm / s, or even at least 5 cm / s; and - the second reaction chamber is equipped with at least one second outlet towards which the water and methanol produced are carried by gravity. Preferably both step (E1) and step (E2) are conducted under these conditions.

[0034] Similarly, when the process of the invention comprises n steps called (E1), (E2), (E3) ... (En), according to an interesting embodiment, it is preferred that, at each step (Ei), where i goes from 3 to n: - the pressure is greater than or equal to 20 MPa (200 bar); and - the inlet velocity of the gas flow into the reaction chamber (i) is at least 0.4 cm / s; and - the reaction chamber (i) is provided with at least one second outlet towards which the water and methanol produced in the liquid state are carried by gravity. Preferably all steps (E1), (E2) and (E3) to (En) are conducted under these conditions.

[0035] Furthermore, whatever variant is chosen for the process of the invention, it may be advantageous to use a catalyst with hydrophobic properties, in particular a catalyst naturally exhibiting a low affinity for water or a hydrogenation catalyst supported on a hydrophobic support or mixed with a hydrophobic polymer resistant to the conditions of step (E1), for example those described in more detail at the end of this description.

[0036] The choice of such a catalyst further reduces the formation of a liquid film on the catalyst, and further increases the effects related to the implementation of the particular conditions of step (E1). As examples of hydrophobic catalysts usable according to this embodiment of the invention, we can mention in particular the catalysts described for example in the articles "Influence of Cu / Al Ratio on the Performance of Carbon-Supported Cu / ZnO / Al2O3 Catalysts for CO2 Hydrogenation to Methanol" by Xie, Z.; Hei, J.; Cheng, L.; Li, J.; Yin, X.; Meng, S. in Catalysts 2023, 13, 800. or "Physical regulation of copper catalyst with a hydrophobic promoter for enhancing CO2 hydrogenation to methanol" by Li H., Fang W., Wang L.-X., et al, in The Innovation 4(4), 100445.3 (2023).Hydrophobic hydrogenation catalysts of this type have been considered in the past for the synthesis of methanol, but only at low pressure (typically 30 bars, or 0.3 MPa), with yields obtained at low pressure which are poor (less than 1 g of methanol per gram of catalyst per hour), and much lower than those obtained under the conditions of the process of the invention.

[0037] According to another interesting alternative of the process, the catalyst used for the synthesis of methanol in step (E1) does not have hydrophobic properties and it is then the implementation of the conditions of step (E1) alone that brings the desired effects, regardless of the nature of the catalyst.

[0038] Various other aspects and possible embodiments of the invention are described in more detail below.

[0039] Catalytic hydrogenation in steps (E1), (E2) and (Ei)

[0040] Step (E1) is preferably conducted at high pressure, namely at a pressure greater than or equal to 20 MPa (200 bar), preferably greater than or equal to 26 MPa (260 bar). In practice, this pressure most often remains less than or equal to 45 MPa (450 bar), and it may, for example, be between 26 and 45 MPa (between 260 and 450 bar), preferably between 26 and 36 MPa (between 260 and 360 bar), for example between 30 and 35 MPa (between 300 and 350 bar), for example around 32 MPa (320 bars) or 33 MPa (330 bars). Step (E2); and more generally all the aforementioned steps (El) (with i ranging from 3 to n), are advantageously carried out under the same pressure conditions.

[0041] Step (E1) is carried out under temperature conditions where the synthesized water and methanol mixture is liquid. Specifically, at the aforementioned pressures, step (E1) is preferably at or above 150 °C, preferably at or above 170 °C, for example between 180 and 280 °C, particularly between 200 and 250 °C, for example around 220 °C. Step (E2), and more generally all the aforementioned steps (Ei) (with i ranging from 3 to n), are advantageously carried out under the same temperature conditions. The temperature and pressure conditions of the aforementioned steps (E1), (E2), and (Ei) (with i ranging from 3 to n) are such that the water / methanol mixture produced during these steps is in a liquid state at the end of the step.

[0042] The reactants (CO2 and H2) used in step (E1) are gaseous and are introduced as a gaseous stream whose inlet velocity into the first reaction chamber preferably remains greater than or equal to 0.4 cm / s throughout the duration of step (E1). More advantageously, this velocity remains greater than or equal to 1 cm / s, and more preferably greater than or equal to 2 cm / s, advantageously greater than or equal to 5 cm / s. This inlet velocity is most often less than or equal to 25 cm / s, for example less than or equal to 15 cm / s, most often less than or equal to 12 cm / s. In a particular mode, this velocity is, for example, between 2 and 10 cm / s, notably between 5 and 10 cm / s. In an advantageous mode, this velocity is between 5 and 25 cm / s, for example between 5 and 15 cm / s.

[0043] Step (E1) is generally carried out continuously by injecting a gas stream containing dihydrogen (H2) and carbon dioxide (CO2) into the reaction chamber, usually at a constant flow rate. Step (E2), and more generally all the aforementioned steps (Ei) (with i ranging from 3 to n), are advantageously carried out under the same conditions.

[0044] The H2 / CO2 molar ratio in the flow injected into the reaction chamber of step (E1) is typically between 90 / 10 and 60 / 40, notably between 80 / 20 and 70 / 30, for example in the order of 75 / 25.

[0045] The steps (E2), and more generally all the aforementioned steps (Ei) (with i ranging from 3 to n) are advantageously carried out under the conditions preferential conditions described in the preceding paragraphs for step (E1). Preferably, each of the steps (E1), (E2) and (Ei), i ranging from 3 to n, are carried out under the same conditions.

[0046] The hydrogenation catalyst

[0047] The catalyst in the reaction chamber of each of the steps (E1), (E2), and (Ei), with i ranging from 3 to n, is most often in the form of a fixed bed contained within a tubular environment, or advantageously in several parallel tubes where the gas flow is distributed. The catalyst may be separate in each chamber, but practically speaking, a single catalyst may be present in each of the reaction chambers of steps (E1), (E2), and (Ei), with i ranging from 3 to n. The catalyst may advantageously be contained within a tube, particularly a cylindrical one, having an internal diameter advantageously between 100 µm and 3 cm, for example between 200 µm and 2 cm (for example, on the order of 1 to 10 mm), and a length that may, for example, be on the order of 1 to 50 cm.When implemented as a fixed bed in a tube of this type, the catalyst is preferably in the form of particles of average size on the order of one-tenth of the tube diameter (typically, for example, a tube with an internal diameter of a few mm contains a catalyst in the form of particles with dimensions less than a few hundred microns). Advantageously, the catalyst is located in parallel tubular channels (typically straight and cylindrical with an internal diameter advantageously between 100 pm and 3 cm, for example between 200 pm and 2 cm (for example on the order of a few mm)) formed within a metal part that can contain between 1 and 10,000, preferably between 1 and 2,000 parallel channels.The chamber may advantageously contain additional channels that are not in fluid connection with the channels comprising the catalyst, which can be used to convey a heat transfer fluid, in particular to ensure thermal regulation during the catalytic hydrogenation reaction.

[0048] In principle, each of the steps (E1), (E2), and (Ei), with i ranging from 3 to n, can employ any type of hydrogenation catalyst capable of catalyzing the hydrogenation of CO2. This could include, for example, well-known prior art catalysts for this reaction, such as catalysts containing copper and / or nickel and / or aluminum. As an example of A suitable catalyst includes, in particular: - Cu / ZnO / ALO type catalysts, typically comprising 55 to 75% by weight of copper, 10 to 35% of ZnO and 2 to 25% of Al2O3; - Cu / ZnO catalysts of the MOF type (for English "metal organic framework" >>); - Cu / SiO2 type catalysts - CU / AI2O3 type catalysts - CuO-ZnO-ZrO2 type catalysts - catalysts based on copper nanoparticles deposited on silica (SiO2) or zirconia (ZrO2), at a concentration of 1-10% Cu - Cu and Pd based catalysts deposited on supports such as silica or alumina with total Cu / Pd contents of 2 to 10% by mass and a Cu / Pd molar ratio of 99 / 1 to 90 / 10.

[0049] According to one possible embodiment, the catalyst present in the reaction chamber of each of the steps (E1), (E2) and (Ei), with i ranging from 3 to n, has a relatively good affinity for water. Alternatively, it may be a hydrophobic catalyst, or at least one exhibiting a low affinity for water.

[0050] When the process of the invention uses hydrophobic catalysts (or at least catalysts with a low affinity for water), this catalyst may in particular be chosen from: (i) catalysts selected from the aforementioned catalysts and mixed with hydrophobic polymers stable at high temperature. The polymer can, for example, be a copolymer of styrene and divinyl benzene, with a molar ratio between these two monomers ranging from 100 / 0 to 0 / 100, preferably 90 / 10 to 40 / 60. It is advantageous during polymer synthesis to use a process that creates porosity, for example, by adding a solvent that is subsequently removed. The polymer can also be a silicone polymer such as poly(dimethylsiloxane) or poly(phenylsilsesquioxane), as this silicone polymer can advantageously be cross-linked. The mixture of the catalyst and the hydrophobic polymer can typically be shaped into granules or objects with dimensions from 100 µm to 5 cm, shaping being possible in particular by one of the following processes: • Powder mixing and compaction, possibly with heating; • Powder mixing and co-granulation by adding a solvent such as benzene or methyl chloride; • polymerization of the copolymer in the presence of a catalyst dispersion, whereby the catalyst particles are encapsulated within the porous polymer network; or (ii) catalysts supported on activated carbon, such as for example a zinc and copper based catalyst supported on activated carbon.

[0051] A hydrogenation catalyst according to the invention is generally activated before its use in the hydrogenation reaction of step (E1). To do this, the catalyst can, for example, be activated by reduction under dihydrogen at 25 bar (2.5 MPa) and at 300 °C for two hours.

[0052] Useful catalysts according to the invention can in particular be obtained according to the preparation protocols A, B and C described below:

[0053] PREPARATION PROTOCOL A A CuO / ZnO / Al2O3 type catalyst can be synthesized by the following coprecipitation method, from copper nitrate (Cu(NOs)2-3H2O), zinc nitrate (Zn(NO3)2-6H2O), aluminum nitrate (Al(NO3)3-9H2O) and sodium carbonate (Na2COs) precursors The necessary proportions of nitrate precursors are mixed and diluted with deionized water to a final volume of 500 ml, to obtain a molar ratio of Cu / Zn / Al of 4 / 2 / 0.5. The aqueous nitrate solution thus obtained is then co-added with an aqueous sodium carbonate solution, drop by drop, in 400 ml of preheated DI water (65-70 °C), to obtain co-precipitation under vigorous stirring (typically 400 rpm), maintaining the temperature at 65-70 °C and the pH at 6.5-7.0 during the co-precipitation process. The pH value is then immediately adjusted to 7.0 by adding an aqueous solution of sodium carbonate at the end of the co-precipitation. The precipitates were then aged at 70 °C for 30 minutes under vigorous stirring (typically 400 rpm). After aging, the precipitates were filtered under reduced pressure and then rinsed several times with hot DI water. Next, the catalyst (solid filtrate obtained by filtration after washing) was dried in an oven at 110 °C (typically overnight) and then air-calcined at 400 °C for 4 h (preferably with a heating rate of 2 °C / min). The catalyst is then ground and sieved to retain only particles with a size between 200 and 600 pm. Just before conducting the hydrogenation reactions, the catalyst can typically be reduced to 250 °C for 10 h, with 20 ml / min of H2.

[0054] PREPARATION PROTOCOL B The catalyst obtained according to protocol A is finely ground to obtain a powder with a particle size of less than 10 µm. This powder is then mixed 50 / 50 by weight with a hydrophobic polymer powder, preferably a poly(styrene-co-divinylbenzene) copolymer, for example, the commercial product Supelco 468312 available from Sigma Aldrich. Four percent by mass of methylene chloride is added to the mixture of the two powders, and the resulting paste is kneaded (typically with a rotor mill for 10 minutes). The paste is then dried overnight at 80 °C. It is subsequently ground and sieved to retain only particles with a size between 200 and 600 µm.

[0055] PREPARATION PROTOCOL C A zinc-copper catalyst supported on activated carbon can be prepared by the following ammonium evaporation method. A solution is prepared containing 55 g of Cu(NO3)2, 3H2O, 34 g of Zn(NOa)2, 6H2O, and 800 g of demineralized water. 740 g of NH3H2O ​​solution (25% concentration) are added to this solution, and the mixture is stirred at room temperature for 10 minutes until the solution turns dark blue. Next, 100 g of activated charcoal (size: 300 µm) are added, which form a suspension, maintained under vigorous stirring at 80 °C for 5 hours, then at 50 °C for 20 hours. Finally, the temperature is raised to 80 °C for 7 hours to evaporate the ammonia and deposit the metallic species on the activated carbon. After evaporation, the pH value is decreased to a value of 7 to 8, then the resulting suspension is filtered to recover the solid catalyst in suspension, which is washed (typically several times with deionized water), then dried (preferably at 90 °C for 24 hours), then calcined (typically at 360 °C for 4 hours under N2).

[0056] Coupling of multiple reaction chambers

[0057] When the process of the invention uses several reaction chambers, the reinjection of the gas flow exiting one of the chambers to the next can in particular be carried out in two ways.

[0058] In one method, a pump can be used (as in the case where only one reaction chamber is used). The use of a pump allows for the continuous reinjection of the recycled H2 / CO2 mixture, adding "fresh" H2 and / or CO2 as needed to the recycled flow at the inlet of each chamber.

[0059] According to another embodiment, the chambers are connected to each other with a non-return device (of the valve or flap type) of the aforementioned type.

[0060] According to this second mode employing the non-return device, the efficient circulation of the H2 / CO2 mixture between the reaction chambers can be ensured by the sequencing on the one hand of the opening and / or closing of the valves introducing the "fresh" CO2 / H2 through the first inlet of the chambers and on the other hand of the evacuation of the methanol / water reaction products through the second outlet of the chambers.

[0061] In one variant, the methanol-water reaction product is continuously discharged, typically by a level regulator that maintains a constant level of the liquid water / methanol mixture at the bottom of each reaction chamber. In this variant, a "fresh" H2 / CO2 gas stream is introduced sequentially, first into the first chamber, then into the second, and so on, returning to the first. This sequential introduction, along with the use of check valves between the chambers, ensures the circulation of the H2 / CO2 gas between the chambers and thus guarantees a sufficient gas flow velocity.

[0062] In a second variant, the "fresh" H2 / CO2 mixture is introduced continuously, and the methanol / water reaction product is discharged sequentially (first into the first reactor, then the second, and so on). This sequential discharge, along with the use of check valves between the chambers, ensures the circulation of the H2 / CO2 gas between the chambers and thus guarantees a sufficient gas flow velocity.

[0063] According to a third variant, both the introduction of the fresh H2 / CO2 mixture and the removal of the methanol / water reaction product are carried out in a Sequential. Typically, the fresh H2 / CO2 gas mixture is introduced through the first inlet of the first reaction chamber, while the methanol / water reaction product is removed from the last reaction chamber (n). Then, the fresh H2 / CO2 gas mixture is introduced into chamber (n), while the methanol / water reaction product is removed from chamber (n-1). Then, the fresh H2 / CO2 gas mixture is introduced into chamber (n-1) while the methanol / water reaction product is removed from chamber (n-2), and so on. This sequential introduction / removal, along with the use of check valves between the chambers, ensures the circulation of the H2 / CO2 gas between the chambers and thus guarantees a sufficient gas flow velocity.

[0064] Gas flow circulation induced by an oscillating inlet pressure

[0065] According to a particular embodiment, the reinjection of the gas stream exiting the first outlet of the first chamber to the second inlet of the first reaction chamber can be carried out without using a pump between said first outlet of the first chamber and said second inlet of the first reaction chamber. For this purpose: - the fresh H2 / CO2 gas mixture is introduced through the first inlet of the first reaction chamber with an oscillating pressure, namely with an injection pressure value of the fresh H2 / CO2 gas mixture that alternately increases and then decreases (typically according to a sinusoidal variation, or another similar continuous periodic variation); and - the first outlet of the first chamber and the second inlet of the first reaction chamber are connected via a channel comprising two non-return devices, said channel comprising: - a channel inlet connected to the first outlet of the first reaction chamber, equipped with a non-return device (typically a valve or check valve) allowing fluid flow only in the direction of entry into the channel; and; - a channel outlet connected to the second inlet of the first reaction chamber equipped with a non-return device (typically a check valve or check flap) allowing fluid circulation only in the direction of the channel outlet.

[0066] According to this particular embodiment, the reinjection of the gas flow is achieved solely due to the oscillation of the injection pressure of the gas mixture. H2 / CO2 freshness: when the injection pressure increases, it induces an overpressure which is imposed on the two non-return devices of the channel, which induces only an entry of the flow into the channel through the channel inlet (the non-return device at the outlet prevents an entry of the flow through the outlet of the channel); conversely, when the pressure decreases, a depression is imposed on the two non-return devices, which induces an exit of the flow out of the channel through the outlet of the channel, which ultimately leads to the reinjection of the desired gas flow.

[0067] Similarly, an injection of fresh H2 / CO2 gas mixture with an oscillating injection pressure can also be carried out in the case where several separate reaction chambers are connected together, with in this case several connecting channels between the different chambers, each of these channels being equipped with two non-return devices allowing the passage of the flow only in one direction within the channel (namely in the direction from chamber n to the next chamber n+1 and not in the reverse direction). Brief description of the figures

[0068] The figures attached hereto are provided to illustrate possible embodiments of the variants described above. These figures are schematic. In particular, the devices shown are not drawn to scale.

[0069] [Fig. 1] Figure 1 is a schematic cross-sectional representation of a modular device including a reaction chamber adapted to the implementation of step (E1) of the invention;

[0070] [Fig. 2] Figure 2 is a schematic cross-sectional representation of the same modular device where the reaction chamber and its various parts are identified.

[0071] [Fig. 3] Figure 3 is a schematic cross-sectional representation of the same modular device shown in operating mode with reactant and product flows

[0072] [Fig. 4] Figure 4 is a schematic cross-sectional representation of the same modular device, illustrating a first embodiment of the process of the invention, with recirculation of the residual CO2 / H2 mixture within a single reaction chamber.

[0073] [Fig. 5] Figure 5 is a schematic cross-sectional representation illustrating the coupling principle of two modular devices

[0074] [Fig. 6] Figure 6 is a schematic cross-sectional representation of three modular devices illustrating the embodiment of the process of the invention with recirculation of the residual CO2 / H2 mixture within several coupled reaction chambers. Embodiments shown in the figures

[0075] Figure 1 shows a modular device 1 comprising a reaction chamber useful in the process of the invention and intended for vertical use in the configuration shown in the diagram. The device has a first inlet 10 (located at the highest point of the device) and a second inlet 20, as well as tubular conduits 30, 40, 50, and 60 filled with a hydrogenation catalyst, each of these conduits having an inlet (31 for conduit 30) and an outlet (32 for conduit 30). These tubular conduits can typically be channels formed in a metal part. The conduits open into a cavity having two outlets: outlet 70 at the lowest point of the device and outlet 80, this outlet 80 being connected by a conduit 85 to the outlet 90 of the device.

[0076] Figure 2 identifies the reaction chamber in the device, designated by the dashed-line frame 95. This reaction chamber has, in its upper part, the first inlet 10 and the second inlet 20, and in its lower part, the outlet 80, corresponding to the "first outlet" in this description, and the outlet 70, corresponding to the "second outlet" in this description. The middle section consists of conduits equipped with inlets and outlets of the type shown in inlet and outlet 31 and 32.

[0077] Figure 3 illustrates the operating mode of the modular device during step (E1), with the introduction of the gaseous flow of carbon dioxide and dihydrogen CO2 / H2 at the top of the device through the first inlet 10, the outlet of the water and methanol CH3OH / H2O produced by drainage at the bottom of the device through the second outlet 70, and the outlet of the device of the residual carbon dioxide and dihydrogen CO2 / H2 through the outlet 90 via the first outlet 80.

[0078] Figure 4 illustrates the embodiment of the invention where the residual carbon dioxide and dihydrogen CO2 / H2 exits the device through outlet 90 as shown. in the previous figure 3 are reinjected through the second inlet 20 by means of a pump 95, with a continuous injection of CO2 / H2 through the first inlet 10 and recovery of the methanol formed through the second outlet at the bottom of the reactor.

[0079] Figure 5 shows a coupling of the device 1 described in Figure 1, comprising an additional non-return device 87 in the conduit 85, with a second similar modular device 100 comprising a reaction chamber useful in the process of the invention and also intended for vertical use according to the configuration shown in the diagram. The device 100 comprises a first inlet 110 (located at the highest point of the device) and a second inlet 120, as well as tubular conduits packed with a hydrogenation catalyst, and two outlets: outlet 170 at the lowest point of the device and, at the end of the conduit 185, outlet 190. The two devices are coupled by connecting the outlet 90 of the first device with the inlet 120 of the second.With the illustrated coupling, the CO2 / H2 gas flow entering through the first inlet 10 of the first chamber reacts with the catalyst to form products discharged through outlet 70. The flow containing the residual CO2 / H2 is conveyed to the second device via conduit 85 (the non-return device allows passage in the direction of flow indicated in the diagram). The flow arrives in the second device through inlet 120, reacts again with the catalyst (with or without the addition of CO2 / H2 through inlet 110) to form products discharged through outlet 170. The flow containing the residual CO2 / H2 is sent through conduit 185 to outlet 190. The non-return device 87 prevents the gas flow from returning to the first device.

[0080] Figure 6 shows a coupling of three devices implementing a recirculation of the unreacted CO2 / H2 flow. The coupling uses the two previous devices 1 and 100, plus another similar device 200, which has a first inlet 210 (located at the highest point of the device) and a second inlet 220, as well as tubular conduits filled with a hydrogenation catalyst, and two outlets: outlet 270 at the lowest point of the device and, at the end of conduit 285, outlet 290. The devices are equipped with a non-return system 87, 187, and 287 similar to that of the previous figure. EXAMPLE

[0081] The advantages of the process of the invention are illustrated by the following example which uses a solid catalyst (commercial catalyst supplied by Alfa Aesar), having the following composition (as a percentage by mass relative to the total mass of the catalyst): CuO: 63.5% Zn: 25% Al: 10% Mg: 1.5%7

[0082] The catalyst was ground and sieved to obtain a particle size of 200 to 600 pm

[0083] The catalyst was then activated under a flow of hydrogen at a flow rate of 0.4 Nl / min for 10 hours at a pressure of 0.4 MPa.

[0084] Comparative test

[0085] For comparison with the process of the invention, the catalyst was used for the synthesis of methanol by catalytic hydrogenation of carbon dioxide, at 45 bar and 240 °C on a hydrogenation bench equipped with micro-GC analysis.

[0086] The conversions, calculated based on the concentration of methanol produced and the decrease in hydrogen as a function of the flow rate, are shown in Table 1 below: TABLE 1: Comparative test

[0087] These results illustrate a known effect of the state of the art, namely that at a relatively moderate pressure (45 bars): - Increased speed and flow rates lead to a significant reduction in conversion; and - the amount of methanol produced per gram of catalyst is (very) low.

[0088] Test according to the invention

[0089] The same catalyst was used in a synthesis of methanol by catalytic hydrogenation of carbon dioxide, at high pressure (330-360 bars) under the conditions of the process of the invention.

[0090] The results, shown in the table below, demonstrate that at high pressure, increasing the gas velocity and flow rate allows for high conversion rates. Furthermore, the amount of methanol produced per gram of catalyst is 20 times greater than in the comparative example, when the temperature is around 280 °C. TABLE 2: Test according to the invention

Claims

Demands

1. Process for the synthesis of methanol by catalytic hydrogenation of carbon dioxide, comprising at least one step (E1) where the following are brought into contact, within a reactor comprising at least one first reaction chamber (1) provided with at least one first inlet (10) and at least one first outlet (80, 90): - a gaseous flow containing carbon dioxide (CO2) and hydrogen (H2) flowing from said first inlet (10) of the first reaction chamber to said first outlet (80, 90) of the first reaction chamber; and - a hydrogenation catalyst (30, 40, 50, 60) contained in said first reaction chamber, between said first inlet (10) and said first outlet (80, 90), in the form of a fixed catalyst bed, whereby at least a portion of the carbon dioxide CO2 and dihydrogen H2 of the gas stream is converted into methanol and water during step (E1), characterized in that: • the pressure is greater than or equal to 20 MPa (200 bar) during step (E1); and • the gas stream obtained at the end of step (E1), which includes residual carbon dioxide CO2 and dihydrogen H2 which did not react during step (E1) is reinjected into a reaction chamber containing a hydrogenation catalyst.

2. A process according to claim 1, comprising, following step (E1), a step of separating the methanol and water produced during step (E1) on the one hand, and the unreacted carbon dioxide and hydrogen on the other hand, carried out under the pressure and temperature conditions obtained at the end of step (E1).

3. A method according to claim 1 or 2, wherein, at the end of step (E1), the gas stream exiting the first reaction chamber (1) through the first outlet (90) is introduced into a second reaction chamber (100) comprising a hydrogenation catalyst, and wherein the method comprises a step (E2) of bringing the gas stream and the hydrogenation catalyst into contact under a pressure of at least 20 MPa (200 bar) within said second reaction chamber (100), thereby at least some of the carbon dioxide CO2 and dihydrogen H2 in the gas stream is converted into methanol and water during step (E2).

4. A process according to claim 3, wherein the first reaction chamber (1) is provided with at least one second outlet (70), towards which the water and methanol, produced in liquid form in step (E1), are carried by gravity and wherein the process comprises, between said step (E1) and said step (E2), a separation step, wherein the unreacted carbon dioxide CO2 and dihydrogen H2 are separated from the methanol and water formed in step (E1) by means of a gravity extraction of the water and methanol through the second outlet (70) of the first reaction chamber (1), this extraction being carried out under the pressure and temperature conditions obtained at the end of step (E1).

5. A method according to claim 3 or 4, wherein the second reaction chamber used in step (E2) is the same as the first reaction chamber of step (E1), and wherein the gas flow exiting the first outlet (90) of the first chamber (1) is reinjected into the first chamber through a second inlet (20) of the reaction chamber upstream of the catalyst contained in the chamber.

6. A method according to claim 3 or 4, wherein the second reaction chamber used in step (E2) is a chamber (100) separate from the first reaction chamber of step (E1), preferably configured like the first reaction chamber used in step (E1), namely with at least one first inlet (110) of the second reaction chamber allowing the introduction of a gas stream and at least one first outlet of the second reaction chamber (190) allowing the discharge of this gas stream and the hydrogenation catalyst between said first inlet (110) and said first outlet (190), and wherein the gas stream exiting the first outlet (90) of the first chamber (1) is injected into said second reaction chamber (100) through a second inlet (120) of the second reaction chamber located upstream of the catalyst contained in this second chamber

7. A method according to claim 6, wherein the fluid connection between the first outlet (80) of the first chamber (1) and the second inlet (120) of the second reaction chamber (100) comprises a non-return device (87) allowing fluid circulation only in the direction from the first reaction chamber to the second reaction chamber.

8. A process according to claim 6 or 7, comprising n successive catalytic hydrogenation steps of carbon dioxide, where n is an integer at least equal to 3, wherein, after steps (E1) and (E2), the process further comprising n-2 additional steps (Ei), with i ranging from 3 to n, each of these n-2 steps (Ei) being carried out in a separate reaction chamber (200), comprising a hydrogenation catalyst, referred to as reaction chamber (i), where: - each of the reaction chambers (i) is preferably configured like the first reaction chamber used in step (E1), namely with at least: a first inlet (210) of the reaction chamber (i) allowing the introduction of a gas stream and at least a first outlet (290) of the reaction chamber (i) allowing the evacuation of this gas stream; and the catalyst between said first inlet and said first outlet, - all or part of the gas stream exiting the reaction chamber (i-1) is introduced into the reaction chamber (i) through a second inlet (220) of the reaction chamber (i) and the step (Ei) comprises bringing the gas stream and the hydrogenation catalyst into contact under a pressure of at least 20 MPa (200 bars) within said reaction chamber (i), whereby at least part of the carbon dioxide CO2 and dihydrogen H2 of the gas stream is converted into methanol and water during the step (Ei).

9. A process according to claim 8, wherein, between each step (Ei-1) and each step (Ei), the process comprises a separation step, wherein the unreacted carbon dioxide CO2 and dihydrogen H2 are separated from the methanol and water formed during step (Ei-1), by means of a gravity extraction of the water and methanol through the second outlet (170) of the reaction chamber (i-1), this extraction being conducted under the pressure conditions obtained at the end of step (Ei-1).

10. A method according to any one of claims 8 or 9, wherein the fluid connection between the first outlet (190) of each reaction chamber (i-1) and the second inlet (220) of each reaction chamber (i) comprises a non-return system (187) permitting fluid flow only in the direction of reaction chamber (i-1) to reaction chamber (i). [Claim 1 1] A method according to any one of claims 1 to 10, wherein, in step (E1) and, where applicable, in each of steps (E2) and (Ei) where i goes from 3 to n: • the pressure is greater than or equal to 20 MPa (200 bar); and • the inlet velocity of the gas flow into the first reaction chamber (1) is at least 0.4 cm / s; and • The first reaction chamber (1) is provided with at least one second outlet (70) towards which the water and methanol, produced in liquid form, are carried by gravity

12. The method according to claim 1 1 , wherein the inlet velocity of the gas flow into the reaction chamber (1 ) is greater than or equal to 1 cm / s and less than or equal to 25 cm / s, for example between 5 and 25 cm / s.

13. A process according to any one of claims 1 to 12 wherein the catalyst used has hydrophobic properties.

14. A process according to any one of claims 1 to 12 wherein the catalyst used does not have hydrophobic properties.

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