Technical field

The present disclosure relates to a process for producing C2 hydrocarbons from methane that is performed in a solid oxide electrolyser cell.

Technical background

Carbon dioxide (CO2) emissions and their capture and utilization or capture and storage are newly emerging problems lying ahead of society to fulfill its CO2 neutrality commitments.

For utilization as fuels, chemicals, or plastics, CO2 must be nowadays converted first by electrolytic or reverse water gas shift technology into CO, which is then in turn mixed with H2 to form syngas and eventually yield methanol, methane or other, aromatic or olefin hydrocarbons. Because the CO2 reduction is highly endothermic, the known overall processes are highly energy demanding.

To ensure completely green technology, H2 usually comes from water electrolysis that uses renewable energy sources. This is done in a separate process, and the water splitting is again an endothermic and very energy-demanding reaction.

These two processes, namely the production of green hydrogen and carbon dioxide sink technology, are therefore not straightforward in terms of feasibility since an elevated content of energy is needed.

To solve these drawbacks, it was already suggested to couple the carbon dioxide reduction reaction with an exothermic reaction, such as the oxidative coupling of methane (OCM), as illustrated in the following chemical equation:

2CW4 + O ₂ C ₂ W ₄ + 2W ₂ O AH = -282 kJ/mol

The OCM reaction is the conversion of methane to ethylene, and it has been implemented into a solid oxide electrolyser. On the cathode, carbon dioxide is converted to carbon monoxide, releasing O ² ' ions that are directly utilized to in situ electrochemically oxidize methane into C2 hydrocarbons at the anode.

Thus, the study by Zhu et al., entitled “Electrochemical conversion of methane to ethylene in a solid oxide electrolyser” (Nature Comm., 2019, 10, 1173) has shown that the advantage of conducting OCM in a solid oxide electrolyser cells (SOEC) is that there is no oxygen in the gas phase, leading subsequently to higher selectivity into C2 products in comparison with a similar reaction performed in the gas phase. It was thus shown that the C2 products reach 16.7% (11.5% C2H4 + 5.2% C2H6) with 82.2% C2 selectivity while the CH4 conversion ratio is approaching as high as 40.5% at ambient pressure and at a potential of 1.6V.

On the anode side of the SOEC, in addition to the conversion of methane to C2 hydrocarbons, other oxidation reactions are achieved, such as the partial oxidation (POX) of methane, according to the following equation, or the oxidation of hydrogen into water such as

2H ₂ + O ₂ 2H ₂ O.

The release of oxygen from endothermic reactions such as water splitting and/or carbon dioxide reduction reaction is then beneficial to the implementation of the oxidation of methane into C2 hydrocarbons within the same experimental setup

The objective of this disclosure is therefore to provide a technology that uses efficiently the exothermicity of the oxidative coupling of methane as well as the oxygen that is produced from endothermic processes such as water splitting and/or carbon dioxide reduction reaction, in a way to enhance the production of C2 hydrocarbons from methane, and since ethylene is usually the most wanted of the C2 hydrocarbons because it can be further used in downstream processes, in a way to enhance the selectivity into ethylene from methane oxidation.

In addition, it is theoretically possible to co-feed carbon dioxide to the anode and still yield mainly C2 products, according to the following exothermic chemical reaction:

5CH ₄ + 0 ₂ + C0 ₂ ^ 3C ₂ H ₄ + 4H ₂ 0 AH = -153 kJ/mol

It could be therefore also beneficial to use the oxygen that is produced from endothermic processes such as water splitting and/or carbon dioxide reduction reactions in a way to perform such reactions. The benefit of such a process could thus achieve an additional objective, which is the implementation of negative carbon dioxide technology, namely a technology that consumes more carbon dioxide than it produces.

Summary

According to a first aspect, the disclosure relates to a process for producing C2 hydrocarbons from methane, said process is remarkable in that it comprises the steps of a) providing one or more solid oxide electrolyser cells, wherein each of said one or more solid oxide electrolyser cells has an anode and a cathode and comprises a solid oxide electrolyte between the anode and the cathode; further wherein the anode, the cathode and the solid oxide electrolyte are each composed of one or more ceramic materials; b) providing an anode feed stream at the anode, wherein the anode feed stream comprises methane and carbon dioxide; c) providing a cathode feed stream at the cathode, wherein the cathode feed stream comprises one reactant selected from water, carbon dioxide, and any mixture thereof; d) performing a reduction reaction of said cathode feed stream at the cathode, to generate oxygen ions diffusing to the anode through said solid oxide electrolyte; e) performing an oxidation reaction of the said anode feed stream at the anode with the oxygen from said oxygen ions generated at the anode at step (d), to generate a product stream comprising at least C2 hydrocarbons; f) optionally, recovering the at least C2 hydrocarbons from the product stream; wherein the molar content of the reactant in the cathode feed stream is at most twice the molar content of methane in the anode feed stream and wherein the molar content of carbon dioxide in the anode feed stream ranges between 0.1 mol.% and 95 mol.% based on the total molar content of the anode feed stream.

With preference, the reactant in the cathode feed stream (i.e., the reactant selected from water, carbon dioxide, and any mixture thereof) is in a molar content ranging between 5% and 200% of the molar content of methane in the anode feed stream, more preferably between 10% and 150%, even more preferably between 15% and 100%, or between 15% and 90%, or between 15% and 80%.

For example, the molar content of methane in the anode feed stream is selected to be in molar excess by comparison to the molar content of the reactant in the cathode feed stream; wherein the reactant is selected from water, carbon dioxide, and any mixture thereof.

In other words, the molar content of the reactant in the cathode feed stream is below the molar content of methane in the anode feed stream.

Surprisingly, it has been found that it is possible to convert methane into C2 hydrocarbons (j.e., C2H4 and also C2H2 and C2H6) into one or more solid oxide electrolyser cells by maximizing either the formation of hydrogen or the formation of carbon monoxide, depending on the composition of the stream provided at the cathode.

During step (e), the part of the methane comprised within the anode feed stream that is oxidized is called the reacting methane; during step (d), the part of the water comprised within the cathode feed stream, or the part of the carbon dioxide comprised within the cathode feed stream, or the part of the mixture of water and carbon dioxide comprised within the cathode feed stream, that is reduced is respectively called the reacting water, or the reacting carbon dioxide, or the reacting mixture.

It is advantageous that during step (e), the part of the methane of the anode feed stream that is oxidized is called the reacting methane, and during step (d), the part of the reactant of the cathode feed stream that is reduced is called the reacting reactant, the ratio between the molar content of the reacting methane and the reacting reactant is ranging between 0.50 and 0.80; preferably between 0.52 and 0.78; more preferably between 0.55 and 0.75.

Thus, it is preferred that:

- when the reactant is selected to be water, the part of the water of the cathode feed stream that is reduced is called the reacting water and the ratio between the content of the reacting methane and the reacting water ranges between 0.50 and 0.80, preferably between 0.52 and 0.78; more preferably between 0.55 and 0.75.

- when the reactant is selected to be carbon dioxide, the part of the carbon dioxide of the cathode feed stream that is reduced is called the reacting carbon dioxide and the ratio between the content of the reacting methane and the reacting carbon dioxide is ranging between 0.50 and 0.80, preferably between 0.52 and 0.78; more preferably between 0.55 and 0.75.

- when the reactant is selected to be a mixture of water and carbon dioxide, the part of the mixture of water and carbon dioxide of the cathode feed stream that is reduced is called the reacting mixture and the ratio between the content of the reacting methane and the reacting mixture is ranging between 0.50 and 0.80, preferably between 0.52 and 0.78; more preferably between 0.55 and 0.75.

For example, the molar content of the methane in the anode feed stream ranges between 4.5 mol.% and 99.9 mol.% based on the total molar content of the anode feed stream, preferably between 5.0 mol.% and 99.5 mol.%, more preferably between 7.5 mol.% and 99.0 mol.%, even more preferably between 10.0 mol.% and 95.0 mol.%.

Advantageously, the anode feed stream is a stream of natural gas, biogas, fuel gas, sour gas, or any mixture thereof. For example, the content of carbon dioxide in the anode feed stream ranges between 0.1 mol.% and 95 mol.% based on the total molar content of the anode feed stream.

For example, the content of carbon dioxide in the anode feed stream ranges between 0.1 mol.% and below 50 mol.% based on the total molar content of the anode feed stream, preferably between 0.5 mol.% and 45 mol.%, more preferably between 1 mol.% and 40 mol.%, even more preferably between 5 mol.% and 35 mol.%, most preferably between 10 mol.% and 30 mol.%. In that case, the CO2 can act as an additional oxidant by providing additional oxygen to boost the single-pass methane conversion.

For example, the content of carbon dioxide in the anode feed stream ranges between 50 mol.% and 95 mol.% based on the total molar content of the anode feed stream, preferably between 55 mol.% and 90 mol.%, or between 60 mol.% and 90 mol.%, or between 65 mol.% and 90 mol.%. In that case, the CO2 can act as a diluent for the methane. The presence of the diluent can boost the selectivity to C2 hydrocarbons up to ten times.

For example, the process further comprises the step of adding carbon dioxide into the anode feed stream.

To favour the formation of syngas, in a preferred embodiment, the cathode feed stream comprises both water and carbon dioxide.

In an embodiment wherein the reactant is selected to be a mixture of water and carbon dioxide; during step (e), the part of the methane of the anode feed stream that is oxidized is called the reacting methane, and during step (d), the part of the mixture of water and carbon dioxide of the cathode feed stream that is reduced is called the reacting mixture. Following this definition, it is advantageous that the molar ratio between water and carbon dioxide in the reacting mixture is higher than 3, preferably higher than 4, more preferably higher than 5, even more preferably higher than 6, most preferably higher than 7, even most preferably higher than 8, or higher than 9.

Such a molar ratio between water and carbon dioxide in the reacting mixture at the cathode of the one or more solid oxide electrolyser cells allows (when the molar content of the reactant in the cathode feed stream is at most twice the molar content of methane in the anode feed stream and/or when the molar content of the reactant in the cathode feed stream is below the molar content of methane in the anode feed stream) to generate syngas having a molar ratio between hydrogen and carbon monoxide that is ranging between 1.70 and 2.30, preferably between 1.80 and 2.20, more preferably between 1.90 and 2.10. Such molar ratio between the carbon monoxide and hydrogen that is thus generated from the process allows further transformation of the generated syngas, notably into fuels, olefins and/or aromatics. For example, the process is operated with electricity coming from one or more of solar energy, wind energy, or nuclear energy.

For example, step (b) is performed with a molar feed rate of methane at the anode and the reactant is selected to be carbon dioxide, and step (c) is performed with a carbon dioxide molar feed rate that ranges between 5% and 200% of the molar feed rate of methane at the anode, preferably between 10% and 150%, more preferably between 15% and 100%, or between 15% and 90%, or between 15% and 80%.

For example, step (b) is performed with a molar feed rate of methane at the anode and the reactant is selected to be carbon dioxide, and step (c) is performed with a carbon dioxide molar feed rate that is less than 200% of the molar feed rate of methane at the anode, preferably less than 150%, more preferably less than 100%, even more preferably less than 90% and most preferably less than 80%.

For example, step (b) is performed with a molar feed rate of methane at the anode and the reactant is selected to be water, and step (c) is performed with a water molar feed rate that ranges between 5% and 200% of the molar feed rate of methane at the anode, preferably between 10% and 150%, more preferably between 15% and 100%, or between 15% and 90%, or between 15% and 80%.

For example, step (b) is performed with a molar feed rate of methane at the anode and the reactant is selected to be a mixture of carbon dioxide and water, and step (c) is performed with a mixture molar feed rate that is ranging between 5% and 200% of the molar feed rate of methane at the anode, preferably between 10% and 150%, more preferably between 15% and 100%, or between 15% and 90%, or between 15% and 80%.

For example, the process is carried out at a temperature ranging between 600°C and 1000°C, preferably between 650°C and 950°C, more preferably between 700°C and 900°C, and even more preferably between 750°C and 850°C.

For example, the process is operated at an operating voltage that is below 5 V per one single solid oxide electrolyser cell, preferably below 4 V, more preferably below 3 V, and even more preferably below 2 V.

For example, the one or more solid oxide electrolyser cells are operated at a current density that is ranging between 0.2 A/cm ² and 5 A/cm ² , or between 0.2 A/cm ² and 4 A/cm ² , or between 0.2 A/cm ² and 3 A/cm ² , or between 0.2 A/cm ² and 2 A/cm ² , preferably between 0.4 A/cm ² and 1 .8 A/cm ² , more preferably between 0.6 A/cm ² and 1 .6 A/cm ² , even more preferably between 0.8 A/cm ² and 1 .4 A/cm ² . Advantageously, when step (f) is carried out, said step (f) also comprises recovering a mixture of carbon monoxide, carbon dioxide, hydrogen and water from the said product stream.

For example, the anode feed stream provided at step (b) further comprises hydrogen disulfide. With preference, the content of hydrogen disulfide in the anode feed stream is below 50 ppm. For example, the anode feed stream provided at step (b) further comprises hydrogen disulfide in a content that is up to 20 mol.% based on the total molar content of said anode feed stream, preferably up to 15 mol.%, more preferably up to 10 mol.%, and wherein the one or more ceramic materials of the anode are one or more ceramic materials comprising La and/or Ce- Zr.

For example, the anode feed stream provided in step (b) further comprises ammonia and/or hydrogen; with preference at a content ranging between 0 mol.% and 95 mol.% based on the total molar content of the anode feed stream. With preference, the content of ammonia in the anode feed stream ranges between 0 mol.% and 95 mol.% based on the total molar content of the anode feed stream. With preference, the content of hydrogen in the anode feed stream ranges between 0 mol.% and 95 mol.% based on the total molar content of the anode feed stream.

Advantageously, the one or more ceramic materials are one or more mixed oxides.

Advantageously, the one or more solid oxide electrolyser cells comprise a layer between the anode and the solid oxide electrolyte and/or between the cathode and the solid oxide electrolyte, wherein said layer is made of one or more ceramic materials, more preferably of one or more mixed oxides.

For example, the one or more ceramic materials are one or more mixed oxides; with preference, the one or more mixed oxides are doped with one or more lower-valent cations. With preference, said one or more mixed oxides are selected from:

- one or more oxides having a cubic fluorite structure being at least partially substituted with one or more lower-valent cations, preferentially selected from Sm, Gd, Y, Sc, Yb, Mg, Ca, La, Dy, Er, Eu, Ba; and/or

- one or more ABCh-perovskites with A and B tri-valent cations, being at least partially substituted in A position with one or more lower-valent cations, preferentially selected from Ca, Sr, or Mg, and comprising at least one of Ni, Ga, Co, Cr, Mn, Sc, Fe and/or a mixture thereof in B position; and/or

- one or more ABOa-perovskites with A bivalent cation and B tetra-valent cation, being at least partially substituted with one or more lower-valent cations, preferably selected from Mg, Sc, Y, Nd, or Yb in the B position or with a mixture of different B elements in the B position; and/or - one or more A2B2O7-pyrochlores with A trivalent cation and B tetra-valent cation being at least partially substituted in A position with one or more lower-valent cations, preferentially selected from Ca or Mg, and comprising at least one of Sn, Zr, and Ti in B position.

For example, the one or more ceramic materials of the anode and/or the cathode further comprise one or more metals selected from nickel, molybdenum, cobalt, and iron. With preference, the one or more ceramic materials of the anode and/or the cathode further comprise nickel.

For example, the one or more ceramic materials of the anode are different from the one or more ceramic materials of the cathode. With preference, the one or more ceramic materials of the anode are one or more mixed oxides selected from

- one or more ABCh-perovskites with A and B tri-valent cations, being at least partially substituted in A position with one or more lower-valent cations, preferentially selected from Ca, Sr, or Mg, and comprising at least one of Ni, Ga, Co, Cr, Mn, Sc, Fe and/or a mixture thereof in B position; and/or

- one or more ABCh-perovskites with A bivalent cation and B tetra-valent cation, being at least partially substituted with one or more lower-valent cations, preferably selected from Mg, Sc, Y, Nd or Yb in the B position or with a mixture of different B elements in the B position.

For example, the one or more ceramic materials have a porosity ranging between 15% and 60% according to ASTM C373 standard, or between 30% and 60%.

Description of the figure

Figure 1 : Scheme of the solid electrolyzer cell used in the present disclosure.

Detailed description

For the disclosure, the following definitions are given:

The terms "comprising", "comprises" and "comprised of" as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. The terms "comprising”, "comprises" and "comprised of" also include the term “consisting of”.

The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1 , 2, 3, 4, 5 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of endpoints also includes the recited endpoint values themselves (e.g., from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

The particular features, structures, characteristics, or embodiments may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments.

The present disclosure relates to a process for producing C2 hydrocarbons from methane, said process comprises the steps of a) providing one or more solid oxide electrolyser cells, wherein each of said one or more solid oxide electrolyser cells has an anode and a cathode and comprises a solid oxide electrolyte between the anode and the cathode; further wherein the anode, the cathode and the solid oxide electrolyte are each composed of one or more ceramic materials; b) providing an anode feed stream at the anode, wherein the anode feed stream comprises methane and carbon dioxide; c) providing a cathode feed stream at the cathode, wherein the cathode feed stream comprises one reactant selected from water, carbon dioxide, and any mixture thereof; d) performing a reduction reaction of said cathode feed stream at the cathode, to generate oxygen ions diffusing to the anode through said solid oxide electrolyte; e) performing an oxidation reaction of the said anode feed stream at the anode with the oxygen from said oxygen ions generated at the anode at step (d), to generate a product stream comprising at least C2 hydrocarbons; f) optionally, recovering the at least C2 hydrocarbons from the product stream; wherein the molar content of the reactant in the cathode feed stream 11 is at most twice the molar content of methane in the anode feed stream 9 and wherein the molar content of carbon dioxide in the anode feed stream 9 ranges between 0.1 mol.% and 95 mol.% based on the total molar content of the anode feed stream 9.

As shown in Figure 1 , at least one solid oxide electrolyser cell 1 is provided in the first step (a). The one or more solid oxide electrolyser cells 1 have each an anode 3 and a cathode 5 and comprise a solid oxide electrolyte 7 between the anode 3 and the cathode 5. The anode 3, the cathode 5 and the solid oxide electrolyte 7 are each composed of one or more ceramic materials. In step (b), an anode feed stream 9 is provided at the anode 3. Said anode feed stream 9 comprises methane and carbon dioxide. In step (c), a cathode feed stream 11 is provided at the cathode 5. Said cathode feed stream 11 comprises at least one of water and/or carbon dioxide. In step (d), a reduction reaction of said cathode feed stream 11 is performed at the cathode 5, to generate oxygen ions which are diffusing to the anode 3 through said solid oxide electrolyte 7. In step (e), an oxidation reaction of said anode feed stream 9 is performed at the anode 3 with the oxygen generated to the anode 3 at step (d), to generate a product stream comprising at least C2 hydrocarbons. The molar content of the reactant in the cathode feed stream 11 is at most twice the molar content of methane in the anode feed stream 9. For example, the molar content of the reactant in the cathode feed stream 11 ranges between 5% and 200% of the molar content of methane in the anode feed stream 9, more preferably between 10% and 150%, even more preferably between 15% and 100%, or between 15% and 90%, or between 15% and 80%, or between 20% and 70%.

For example, the molar content of methane in the anode feed stream 9 is selected to be in molar excess by comparison to the molar content of the reactant in the cathode feed stream 11 ; wherein the reactant is selected from water, carbon dioxide, and any mixture thereof.

The ratio between the methane and the water and/or carbon dioxide is ranging between 0.50 and 0.80.

It is, therefore, possible to convert methane into C2 hydrocarbons (/.e., C2H4 and also C2H2 and C2H6) into one or more solid oxide electrolyser cells by maximizing either the formation of hydrogen or the formation of carbon monoxide, depending on the composition of the cathode feed stream (i.e. , provided at the cathode).

For example, the content of the methane in the anode feed stream 9 ranges between 4.5 mol.% and 99.9 mol.% based on the total molar content of the anode feed stream 9, preferably between 5.0 mol.% and 99.5 mol.%, more preferably between 7.5 mol.% and 99.0 mol.%, even more preferably between 10.0 mol.% and 95.0 mol.%.

The oxidation of methane may not be complete so the methane comprised within the anode feed stream 9 is formed by a part of the methane that is oxidized called the reacting methane and a part of the methane that is not oxidized called the unreacted methane.

Similarly, the reduction of the reactant may not be complete so that the reactant comprised within the cathode feed stream 11 is formed by a part of the reactant that is reduced called the reacting reactant and a part of the reactant that is not oxidized called the unreacted reactant.

During step (e), the part of the methane of the anode feed stream 9 that is oxidized is called the reacting methane. During step (d), a part of the reactant of the cathode feed stream 11 that is reduced is called the reacting reactant, the ratio between the molar content of the reacting methane and the reacting reactant ranges between 0.50 and 0.80; preferably between 0.52 and 0.78; more preferably between 0.55 and 0.75.

Thus, it is preferred that:

- when the reactant is selected to be water, the part of the water of the cathode feed stream that is reduced is called the reacting water and the ratio between the content of the reacting methane and the reacting water ranges between 0.50 and 0.80, preferably between 0.52 and 0.78; more preferably between 0.55 and 0.75.

- when the reactant is selected to be carbon dioxide, the part of the carbon dioxide of the cathode feed stream that is reduced is called the reacting carbon dioxide and the ratio between the content of the reacting methane and the reacting carbon dioxide ranges between 0.50 and 0.80, preferably between 0.52 and 0.78; more preferably between 0.55 and 0.75.

- when the reactant is selected to be a mixture of water and carbon dioxide, the part of the mixture of water and carbon dioxide of the cathode feed stream that is reduced is called the reacting mixture and the ratio between the content of the reacting methane and the reacting mixture is ranging between 0.50 and 0.80, preferably between 0.52 and 0.78; more preferably between 0.55 and 0.75.

According to the disclosure, the anode feed stream 9 comprises carbon dioxide in addition to methane. The incorporation of carbon dioxide into the anode feed stream 9, namely along with methane, further favours the carbon dioxide negative technology that is implemented by the present disclosure. For example, the content of carbon dioxide in the anode feed stream 9 ranges between 0.1 mol.% and 95 mol.% based on the total molar content of the anode feed stream 9.

The following chemical equations illustrate the process when the cathode feed stream 11 comprises water in case of an implementation of a process where the production of hydrogen is also desired.

+-O ₂ anode: 6CH ₄ + CO ₂ - -> C ₂ H ₄ + 6H ₂ O + SCO + 4H ₂

- -o ₂ cathode: 9H ₂ O - -> 9H ₂

The following chemical equations illustrate the process when the cathode feed stream 11 comprises carbon dioxide in case of an implementation of a carbon dioxide sink technology.

+-O ₂ anode: 6CH ₄ + CO ₂ C ₂ H ₄ + 6H ₂ O + SCO + 4H ₂ — ~o ₂ cathode: 9CO ₂ 9CO

For example, the content of carbon dioxide in the anode feed stream 9 ranges between 0.1 mol.% and below 50 mol.% based on the total molar content of the anode feed stream 9, preferably between 0.5 mol.% and 45 mol.%, more preferably between 1 mol.% and 40 mol.%, even more preferably between 5 mol. % and 35 mol.%, most preferably between 10 mol.% and 30 mol.%. In that case, the CO2 can act as an additional oxidant by providing additional oxygen to boost the single-pass methane conversion.

For example, the content of carbon dioxide in the anode feed stream 9 is ranging between 50 mol.% and 95 mol.% based on the total molar content of the anode feed stream 9, preferably between 55 mol.% and 90 mol.%, or between 60 mol.% and 90 mol.%, or between 65 mol.% and 90 mol.%. In that case, the CO2 can act as a diluent for the methane. The presence of the diluent can boost the selectivity of methane conversion into C2 hydrocarbons up to ten times.

For example, the process further comprises the step of adding carbon dioxide into the anode feed stream 9. Not only the carbon dioxide that is intrinsically present along with the methane, depending on the source of the anode feed stream 9 can be used in the negative carbon dioxide technology implemented by the present disclosure, but also, one or more additional sources of carbon dioxide can be further incorporated into the anode feed stream 9, increasing subsequently the effect of carbon dioxide sink.

The process that is implemented by the first and/or the second embodiment further generates, in addition to the C2 hydrocarbons, syngas, which is a mixture of carbon oxides (mainly carbon monoxide) and hydrogen. To favour the formation of syngas, in a preferred embodiment, the cathode feed stream 11 at the cathode 5 comprises both water and carbon dioxide (i.e., the reactant is selected to be a mixture of water and carbon dioxide). It is understood that the stream of water and the stream of carbon dioxide can be two independent streams or a single stream comprising a mixture of water and carbon dioxide.

In an embodiment wherein the reactant is selected to be a mixture of water and carbon dioxide; during step (e), the part of the methane of the anode feed stream 9 that is oxidized is called the reacting methane, and during step (d), the part of the mixture of water and carbon dioxide of the cathode feed stream 11 that is reduced is called the reacting mixture. It is then advantageous that the molar ratio between the water and the carbon dioxide in the reacting mixture is higher than 3, preferably higher than 4, more preferably higher than 5, even more preferably higher than 6, most preferably higher than 7, even most preferably higher than 8, or higher than 9. Such molar ratio between the mixture of water and carbon dioxide reacting at the cathode 5 of the one or more solid oxide electrolyser cells 1 allows (when the molar content of the reactant in the cathode feed stream 11 is at most twice the molar content of methane in the anode feed stream 9 and/or when the molar content of the reactant in the cathode feed stream 11 is below the molar content of methane in the anode feed stream 9) to generate syngas having a molar ratio between hydrogen and carbon monoxide that is ranging between 1.70 and 2.30, preferably between 1.80 and 2.20, more preferably between 1.90 and 2.10.

The following chemical equations illustrate the process in the case where carbon dioxide is added to the anode feed stream 9 and where the process is optimized to generate syngas with an ideal ratio of hydrogen and carbon monoxide amounting to 2.00.

+-O ₂ anode: 6CH ₄ + CO ₂ - -> C ₂ H ₄ + 6H ₂ O + SCO + 4H ₂

-~o ₂ cathode: 8// ₂ O + CO ₂ SH ₂ + CO

The following chemical equations illustrate the process in the case where the process is optimized to generate syngas with an ideal ratio between hydrogen and carbon monoxide that is ranging between 1 .70 and 2.30, preferably between 1 .80 and 2.20, more preferably between 1.90 and 2.10.

+50, anode: 6CH ₄ - > C ₂ H ₄ + 6H ₂ O + 4CO + 4H ₂

-SO ₂ cathode: SH ₂ 0 + 2CO ₂ — > SH ₂ + 2 CO

Such a ratio between the carbon monoxide and hydrogen that is thus generated from the process is an ideal ratio for allowing further transformation of the generated syngas, notably into fuels, olefins and/or aromatics.

The generation of hydrogen or the reduction of carbon dioxide, along with the production of C2 hydrocarbons and syngas, can be further enhanced by working the one or more solid oxide electrolyser cells 1 under environmental-friendly conditions. Thus, green hydrogen can be produced in large amounts if the one or more solid oxide electrolyser cells 1 are worked with renewable energy (e.g., solar energy, wind energy). The same consideration is true when a carbon dioxide reduction reaction is implemented to the cathode of the one or more solid oxide electrolyser cells 1. For example, the process is thus operated with electricity coming from one or more of solar energy or wind energy. In an embodiment, the process can be operated with nuclear energy, so that pink hydrogen is produced. Advantageously, the anode feed stream 9 comprises between 4.5 mol.% and 99.9 mol.% of methane based on the total molar content of said anode feed stream 9 can be a stream of natural gas, biogas, fuel gas, sour gas, or any mixture thereof. Biogas is a stream having a methane content between 50-80 vol.% based on the total volume of the biogas, and a carbon dioxide content between 15-50 vol.% based on the total volume of the biogas. Sour gas is a stream of natural gas having a significant content of hydrogen disulfide, for example up to 50 ppm of hydrogen disulphide.

For example, step (b) is performed with a molar feed rate of methane at the anode, and the reactant is selected to be carbon dioxide, and step (c) is performed with a carbon dioxide molar feed rate that ranges between 5% and 200% of the molar feed rate of methane at the anode, preferably between 10% and 150%, more preferably between 15% and 100%, or between 15% and 90%, or between 15% and 80%, or between 20% and 70%.

For example, step (b) is performed with a molar feed rate of methane at the anode and the reactant is selected to be carbon dioxide, and step (c) is performed with a carbon dioxide molar feed rate that is less than 200% of the molar feed rate of methane at the anode, preferably less than 150%, more preferably less than 100% or between 15% and 90%, or between 15% and 80%, or between 20% and 70%.

For example, step (b) is performed with a molar feed rate of methane at the anode and the reactant is selected to be water, and step (c) is performed with a water molar feed rate that ranges between 5% and 200% of the molar feed rate of methane at the anode, preferably between 10% and 150%, more preferably between 15% and 100% or between 15% and 90%, or between 15% and 80%, or between 20% and 70%.

For example, step (b) is performed with a molar feed rate of methane at the anode and the reactant is selected to be a mixture of carbon dioxide and water, and step (c) is performed with a mixture molar feed rate that is ranging between 5% and 200% of the molar feed rate of methane at the anode, preferably between 10% and 150%, more preferably between 15% and 100% or between 15% and 90%, or between 15% and 80%, or between 20% and 70%.

For example, the process is carried out at a temperature ranging between 600°C and 1000°C, preferably between 650°C and 950°C, more preferably between 700°C and 900°C, and even more preferably between 750°C and 850°C.

For example, the process is operated at an operating voltage that is below 5 V per one single solid oxide electrolyser cell, preferably below 4 V, more preferably below 3 V, and even more preferably below 2 V. In some embodiment, the solid oxide electrolyser cells 1 can be arranged into a stack to produce sizeable amounts of the product stream comprising at least C2 hydrocarbons. For example, the solid oxide electrolyser cells 1 can be arranged on top of each other or side by side. For example, the number of solid oxides electrolyzer cells 1 in one stack can be at least 2 cells.

For example, the one or more solid oxide electrolyser cells are operated at a current density that is ranging between 0.2 A/cm ² and 5 A/cm ² , or between 0.2 A/cm ² and 4 A/cm ² , or between 0.2 A/cm ² and 3 A/cm ² , or between 0.2 A/cm ² and 2 A/cm ² , preferably between 0.4 A/cm ² and 1 .8 A/cm ² , more preferably between 0.6 A/cm ² and 1 .6 A/cm ² , even more preferably between 0.8 A/cm ² and 1.4 A/cm ² .

Advantageously, when step (f) is carried out, said step (f) also comprises recovering a mixture of carbon monoxide, carbon dioxide, hydrogen and water from said product stream.

For example, the anode feed stream 9 provided at step (b) further comprises hydrogen disulfide. With preference, the content of hydrogen disulfide in the anode feed stream 9 is below 50 ppm, more preferably below 40 ppm, or even more preferably below 30 ppm. However, in some cases, the anode feed stream 9 provided at step (b) can comprise hydrogen disulfide up to 20 mol.% based on the total molar content of the anode feed stream 9. The feasibility of the process is thus dependent on the type of the catalyst. When a higher content of hydrogen disulfide is used, a sulfur-tolerant catalyst should be used.

Interestingly, the presence of up to 20 mol.%, or up to 15 mol.%, or up to 10 mol.%, of hydrogen disulfide based on the total molar content of the anode feed stream 9 does not hinder the process of the present disclosure since the sources of natural gas generally comprises a certain quantity of hydrogen disulfide. The hydrogen disulfide can thus undergo a Claus reaction according to the following equation:

2H ₂ S + 3O ₂ ^ 2SO ₂ + 2 H ₂ O

Indeed, the oxygen resulting from the reduction reaction of the cathode feed stream 11 at the cathode 5 and generated to the anode 3 after diffusion of said oxygen through said solid oxide electrolyte 7 allows for oxidizing the hydrogen sulfide into sulfur dioxide and water.

For example, the anode feed stream 9 provided at step (b) further comprises ammonia and/or hydrogen. With preference, the content of ammonia in the anode feed stream 9 ranges between 0 mol.% and 95 mol.% based on the total molar content of the anode feed stream 9. With preference, the content of hydrogen in the anode feed stream 9 ranges between 0 mol.% and 95 mol.% based on the total molar content of the anode feed stream 9. The presence of ammonia in the anode feed stream 9 provides an oxidation reaction at the anode 3 that is exothermic and subsequently generates energy that can be recovered to ensure the good functioning of the one or more solid oxide electrolyser cells 1 . -632 kJ/mol

The presence of additional components in the anode feed stream 9, such as hydrogen disulfide, ammonia and/or hydrogen preferably requires that the presence of one or more purification units (not shown) be placed downstream of the one or more solid oxide electrolyser cells 1. Examples of purification units comprise a distillation system, pressure-swing adsorption (PSA) device preferably including one or more zeolites and/or one or more molecular sieves, and scrubber apparatus.

The catalyst

The one or more ceramic materials are one or more mixed oxides.

Advantageously, the one or more solid oxide electrolyser cells 1 comprise a layer between the anode 3 and the solid oxide electrolyte 7 and/or between the cathode 5 and the solid oxide electrolyte 7. With preference, said layer is made of one or more ceramic materials, more preferably of one or more mixed oxides. In such configuration, it is preferred that the solid oxide electrolyte is made of one first mixed oxide, the layer between the anode 3 and the solid oxide electrolyte 7 and/or between the cathode 5 and the solid oxide electrolyte 7 being a layer made of a second mixed oxide different from the first mixed oxide, and the anode 3 and/or the cathode 5 being made of a mixture comprising said second mixed oxide and one or more metals selected from nickel, molybdenum, cobalt, and iron.

In an embodiment, the mixed oxides can be one or more oxides having a cubic fluorite structure being at least partially substituted with one or more lower-valent cations, preferentially selected from Sm, Gd, Y, Sc, Yb, Mg, Ca, La, Dy, Er, Eu, Ba.

The mixed oxides can also be one or more ABCh-perovskites with A and B tri-valent cations, being at least partially substituted in A position with one or more lower-valent cations, preferentially selected from Ca, Sr, or Mg, and comprising at least one of Ni, Ga, Co, Cr, Mn, Sc, Fe and/or a mixture thereof in B position.

The mixed oxides can further be one or more ABOa-perovskites with A bivalent cation and B tetra-valent cation, being at least partially substituted with one or more lower-valent cations, preferentially selected from Mg, Sc, Y, Nd or Yb in the B position or with a mixture of different B elements in the B position. The mixed oxides can be in other examples one or more A2B2O7-pyrochlores with A trivalent cation and B tetra-valent cation being at least partially substituted in A position with one or more lower-valent cations, preferentially selected from Ca or Mg, and comprising at least one of Sn, Zr and Ti in B position.

With preference, the degree of substitution in the one or more mixed oxides doped with one or more lower-valent cations is between 1 and 50 atom % based on the total number of atoms present in the one or more oxides having a cubic fluorite structure, in the one or more ABO3- perovskites with A and B tri-valent cations, in the one or more ABCh-perovskites with A bivalent cation and B tetra-valent cation or in the one or more A2B2O?-pyrochlores with A trivalent cation and B tetra-valent cation respectively, preferably between 3 and 20 atom %, more preferably between 5 and 15 atom%.

Said one or more oxides having a cubic fluorite structure, said one or more ABCh-perovskites with A and B tri-valent cations, said one or more ABCh-perovskites with A bivalent cation and B tetra-valent cation or said one or more A2B2O?-pyrochlores with A trivalent cation and B tetra-valent cation being at least partially substituted with lower valent cations, also means that the same element, being a high-valent cation, can be reduced in the lower-valent equivalent, for example, Ti(IV) can be reduced in Ti(lll) and/or Co(lll) can be reduced in Co(ll) and/or Fe(lll) can be reduced in Fe(ll) and/or Cu(ll) can be reduced in Cu(l).

For example, the one or more ceramic materials of the anode 3 and/or the cathode 5 further comprise one or more metals selected from nickel, molybdenum, cobalt, or iron. With preference, the one or more ceramic materials of the anode 3 and/or the cathode 5 further comprise nickel.

For example, the one or more ceramic materials of the anode 3 and/or the cathode 5 are nickel/yttria-stabilized zirconia (Ni-YSZ) or lanthanum strontium manganese oxide-YSZ (LSM- YSZ).

For example, the one or more ceramic materials have a porosity ranging between 15% and 60% according to ASTM C373 standard, or between 30% and 60%. Porosity is defined as the ratio of the volume of the voids or of the pore space divided by the total volume. In other words, it is the percentage of void space in the ceramic material.

When a high content of hydrogen disulfide is present in the anode feed stream 9, for example in the case where the content of hydrogen disulfide in the anode feed stream 9 is above 50 ppm and up to 20 mol.% based on the total molar content of the anode feed stream 9, one or more sulphur-tolerant ceramic materials are to be used at the anode 3. Thus, the one or more ceramic materials of the anode 3 are one or more ceramic materials comprising La and/or Ce- Zr. More particularly, examples of sulfur-tolerant ceramic materials are YSZ/Lao.4Sro.6Ti03-8 in a ratio 1/1 , Lao.sMo.yFeo.yCro.sOs-s (M = Sr, and/or Ca), BaZro.iCeo.yYo.2-xYb _x 03-8, Lao.4Sro.5Bao.iTi03+8, Nio.8Mo.2Ceo.8Zro.2O2/Ni-YSZ (M = Co, Cu and/or Fe).