Field of the invention

The invention relates to methods for producing sustainable fuel, and to respective sustainable fuels and uses thereof.

Background of the invention

Climate change and the on-going energy transition makes it mandatory to replace fossilbased energy sources. In this context various aspects will be important for society to reach Net Zero by 2050 as currently desired. For example, the valorisation of alternative feedstocks is expected to contribute towards the circular economy and/or reduce the carbon dioxide (CO2) footprint associated with the final product. However, while many alternative fuels and chemicals are sort after, drop in solutions from alternative feedstocks would allow for existing infrastructure to be maintained.

In this context even the valorisation of CO2 as a feedstock is considered, sourced from flue gas, bio sources and even direct air capture. Upgrading of CO2 to chemicals such as methanol has already received some attention to yield sustainable fuel for various applications. However, some applications regularly lead to enhanced requirements for the sustainable fuel. For example, aviation fuel, i.e. , fuel to power aircraft, regularly requires to contain larger hydrocarbons and aromatics. This is because larger hydrocarbons and aromatics regularly improve the cold flow properties of the aviation fuel. Such improved cold flow properties prevent fuel freezing at low temperatures of for example -40°C, which are typical for cruising altitudes of the powered aircrafts. In attempts to upgrade CO2, larger hydrocarbons and aromatics are however regularly more difficult to achieve.

In a different approach, bio conversion and syngas upgrading via Fischer-Tropsch (FT) are also considered as an alternative to a fossil fuel feedstock. Such routes may appear attractive for diesel and gasoline production. However, they are not suitable for 100% aviation fuel, as they crucially lack aromatic content and hence are applied as a blend. Thus far, there are no established routes of producing a 100% sustainable aviation fuel by bio conversion or syngas upgrading via FT.

Further in the search for sustainable fuel and in particular in the search for aviation fuel, hydrodeoxygenation of fatty acids/triglycerides, CO2- or bio-sourced syngas upgrading, or bio-olefin oligomerisation have been researched at various Technology Readiness Levels (TRLs). However, for each of these routes, the major product is regularly long hydrocarbon chains and is typically only suitable for up to 50% blend to meet requirements for aviation fuel. This is because these routes do regularly not yield aromatics which are however also required to improve the cold flow properties of aviation fuel.

Overall, there remains a general desire for an improved method for producing sustainable fuel.

Problem underlying the invention

It is an object of the present invention to provide a method for producing sustainable fuel which at least partially overcomes the drawbacks encountered in the art.

It is in particular an object of the present invention to provide a method for producing sustainable fuel which reduces the CO2 footprint associated with the produced sustainable fuel.

It is furthermore an object of the present invention to provide a method for producing sustainable fuel which allows for existing infrastructure to be maintained.

It is moreover an object of the present invention to provide a method for producing sustainable fuel which has an improved energy efficiency and/or an improved cost efficiency.

It is also an object of the present invention to provide a method for producing sustainable fuel which has an improved carbon efficiency, i.e., a method which minimizes carbon dioxide equivalents emissions to its output. It is additionally an object of the present invention to provide a method for producing sustainable fuel which has an increased aromatics content.

It is in particular an object of the present invention to provide a method for producing sustainable fuel which meets the requirements for aviation fuel.

It is also an object of the present invention to provide a use of sustainable fuel which at least partially overcomes the drawbacks encountered in the art.

It is also an object of the present invention to provide sustainable fuel which at least partially overcomes the drawbacks encountered in the art.

Disclosure of the invention

Surprisingly, it has been found that the problem underlying the invention is overcome by methods, uses and sustainable fuels according to the claims. Further embodiments of the invention are outlined throughout the description.

Subject of the invention is a method for producing sustainable fuel, comprising the steps:

(i) converting CO2 into CO using a reverse water gas shift catalyst,

(ii) converting CO from step (i) into Ci-Ce hydrocarbons using a Fischer-Tropsch catalyst, and

(iii) converting Ci-Ce hydrocarbons from step (ii) into aromatics using a zeolite-based catalyst. further comprising a cooling step in which CO from step (i) is cooled before being converted in step (ii).

Logically, steps (i), (ii) and (iii) are carried out in the given order, i.e. , first step (i), thereafter step (ii) and thereafter step (iii). However, additional steps before or after each of steps (i), (ii) and (iii) may also be comprised by the method according to the present invention. In step (i), carbon dioxide (CO2) is converted into carbon monoxide (CO). The conversion occurs in the presence of a reverse water gas shift catalyst. As used herein, a reverse water gas shift catalyst promotes a reverse water gas shift reaction. In other words, the reverse water gas shift catalyst lowers the activation energy of a reverse water gas shift reaction. As used herein, a reverse water gas shift reaction is a reaction which comprises a reaction of CO2 with hydrogen (H2) to yield CO and water (H2O). The reverse water gas shift reaction, and hence step (i), comprises the following reaction: CO2 + H2 — > CO + H2O

In step (ii), CO is converted into short hydrocarbons, namely Ci-Ce hydrocarbons. Ci-Ce hydrocarbons are compounds containing at least one and at most six carbon atoms, i.e., 1 , 2, 3, 4, 5 or 6 carbon atoms, and additionally hydrogen. The Ci-Ce hydrocarbons can be saturated and/or unsaturated hydrocarbons. Saturated Ci-Ce hydrocarbons are regularly linear, branched or cyclic alkyls. Unsaturated Ci-Ce hydrocarbons, more precisely unsaturated C2-C6 hydrocarbons, are regularly linear, branched or cyclic alkenyls or alkynyls. The Ci-Ce hydrocarbons may comprise heteroatoms like oxygen (O), sulfur (S), nitrogen (N) or phosphorous (P). However, according to the present invention, the Ci-Ce hydrocarbons are preferably free from heteroatoms like oxygen (O), sulfur (S), nitrogen (N) and phosphorous (P).

In step (ii), CO is converted using a Fischer-Tropsch catalyst (or FT-catalyst), that is, in the presence of a Fischer-Tropsch catalyst. As used herein, a Fischer-Tropsch catalyst promotes a Fischer-Tropsch reaction. In other words, the Fischer-Tropsch catalyst lowers the activation energy of a Fischer-Tropsch reaction. As used herein, a Fischer-Tropsch reaction is a reaction which comprises a reaction of CO with hydrogen (H2) to yield at least the Ci-Ce hydrocarbons. During the reaction, the hydrocarbons regularly grow in a repeated sequence in which hydrogen atoms are added to carbon and oxygen, the C-0 bond of CO is split and a new C-C bond is formed. For one -CH2- group, the reaction can be given as follows:

CO + 2 H ₂ (CH ₂ ) + H ₂ O

Such a Fischer-Tropsch reaction can for example comprise the following reaction steps: associative adsorption of CO by the FT-catalyst; splitting of the C-0 bond; dissociative adsorption of 2 H2 by the FT-catalyst; transfer of 2 H to the oxygen to yield H2O; desorption of H2O from the FT-catalyst; transfer of 2 H to the carbon to yield CH2.

In step (iii), Ci-Ce hydrocarbons from step (ii) are converted into aromatics. As used herein, aromatics are aromatic in the sense of the IIIPAC Gold Book. More specifically, aromatics are cyclically conjugated molecular entities with a stability (due to delocalization) significantly greater than that of a hypothetical localized structure (e.g. , Kekule structure). Such cyclically conjugated molecular entities have aromaticity. A method for determining aromaticity can in particular be the observation of diatropicity in the ¹ H-NMR spectrum.

In step (iii), Ci-Ce hydrocarbons from step (ii) are converted using a zeolite-based catalyst, that is, in the presence of a zeolite-based catalyst. As used herein, zeolite-based means that the catalyst comprises zeolite. The zeolite-based catalyst is preferably composed of > 50 wt.%, more preferably of > 60 wt.%, still more preferably of > 70 wt.%, even more preferably of > 80 wt.% and in particular preferably of > 90 wt.% of zeolite; the weight percentages are based on the total weight of the zeolite-based catalyst. In a particular case, the zeolite-based catalyst consists of zeolite. As used herein, zeolite is given the same meaning as usual in the art. In particular, a zeolite has an aluminosilicate matrix with a tetrahedral arrangement of silicon (Si ⁴⁺ ) and aluminium (Al ³⁺ ) cations surrounded by four oxygen anions (O ² '). This regularly results in a macromolecular three-dimensional structure of SiC>2 and AIO2 tetrahedral building blocks. As the AIO2 tetrahedral building blocks are negatively charged, zeolites regularly comprise additional chargecompensating cations, e.g., alkali metal cations, alkaline earth metal cations, protons and/or ammonia cations.

In steps (i) to (iii), the afore-listed catalysts are used, namely, a reverse water gas shift catalyst, a Fischer-Tropsch catalyst and a zeolite-based catalyst. According to the present invention, all these catalysts are used in solid form. Here, solid form refers to the aggregation state of the respective catalysts, in particular under normal conditions of 298.15 K and 101.3 kPa.

The method for producing sustainable fuel according to the present invention uses CO2 as a feedstock, which is at least partially converted and is hence not emitted to the environment. The inventive method can thus help to reduce the CO2 footprint associated with the produced sustainable fuel. Herein, the CO2 footprint refers to the amount of carbon dioxide released into the atmosphere.

The method for producing sustainable fuel according to the present invention can be carried out in one or more reaction vessels, especially reactors, which have previously been used for conversion of fossil feedstock. The inventive method may thus allow for existing infrastructure to be maintained.

The method for producing sustainable fuel according to the present invention combines ways of producing larger hydrocarbons and aromatics and yields a sustainable fuel comprising such larger hydrocarbons and aromatics in one single continuous reaction sequence. The inventive method can thereby lead to improved energy efficiency and/or improved cost efficiency of the fuel production.

The method for producing sustainable fuel according to the present invention combines ways of producing larger hydrocarbons and aromatics and regularly yields a sustainable fuel comprising such larger hydrocarbons and aromatics. The inventive method can thereby produce sustainable fuel which meets the requirements for aviation fuel.

The method according to the present invention further comprises a cooling step in which CO from step (i) is cooled before being converted in step (ii). The cooling step lowers requirements to extract heat from the reaction vessel, like a reactor, in which step (ii) is carried out, without however disadvantageously reducing the conversion rate of CO in step (ii). Preferably, during the cooling step between step (i) and step (ii) liquid water is removed resulting in the water-poor CO stream. The Fischer-Tropsch step (ii) is a highly exothermic reaction step and tends to increase the temperature in the reactor, requiring cooling of the reactor content. Hence it is preferred that the CO stream enters step (ii) cooler than it leaves step (i). Moreover, when significant amounts of water are still present in the CO stream, the Fischer-Tropsch catalyst will perform the water-gas shift reaction (CO + CO2 + H2), being also exothermic and producing heat. It is preferred that the CO stream is poor in water content. Further, the conversion rate of CO in step (ii) may even increase. The cooling step thus further improves the energy efficiency and/or the cost efficiency of the fuel production. In this context, cooling of the CO naturally means that the temperature of the CO obtained in step (i) is lowered before the CO is used in step (ii) for a conversion into Ci-Ce hydrocarbons. Hence, also the notation T<co, step ©) > T(co, step(u)) can be used to indicate that there is an active cooling of the CO between step (i) and step (ii). Moreover, with a maintained or even increased conversion rate of CO in step (ii) the conversion thereof into aromatics in subsequent step (iii) may also be improved, i.e. , the aromatics yield can be increased.

It is preferred that in a method according to the present invention, the CO from step (i) is cooled (in the cooling step) from a temperature of > 500°C to a temperature of < 350°C before being converted in step (ii), more preferably from a temperature of > 500°C to a temperature of < 300°C and still more preferably from a temperature of > 500°C to a temperature of < 250°C. An advantageously efficient conversion of CO2 into CO in the reverse water gas shift reaction in step (i) will regularly lead to a product stream of increased temperature, in particular an increased temperature of > 500°C. It has been found that in order to increase the efficiency of the Fischer-Tropsch reaction and to increase the yield of the Ci to Ce hydrocarbons in step (ii), in particular the yield of unsaturated C2 to Ce hydrocarbons, it is advantageous to lower the temperature of CO from 500°C or more to 350°C or less, even better to 300°C or less and especially to 250°C or less. A respective lowering of the temperature of the CO can then ultimately also increase the yield of aromatics obtained in step (iii).

In the context of the cooling step, it is especially preferred that step (i) is carried out in a first reactor and that step (ii) is carried out in a physically separated second reactor, wherein the cooling takes place between the CO leaving the first reactor and the CO entering the second reactor. It is especially preferred that in the cooling step liquid water is removed from the CO stream.

It is preferred that in a method according to the present invention, in step (i) the CO2 is at least partially reacted with H2, and/or in step (ii) the CO is at least partially reacted with H2. A reaction with H2 in either step can lead to an increased conversion of CO2 and/or CO, which can further reduce the CO2 footprint. Additionally, an increased conversion of CO2 and/or CO can further improve the energy efficiency and/or the cost efficiency of the fuel production. It is preferred that in a method according to the present invention, the reverse water gas shift catalyst comprises Fe, Co, Cu, Cr, Ni, Ir, Mn or mixtures thereof, preferably in oxidic form (Fe-oxide, Co-oxide, Cu-oxide, Cr-oxide, Ni-oxide, Ir-oxide, Mn-oxide, or mixtures thereof) or in metallic form supported on metal oxide(s) or supported on carbon. It is preferred that the reverse water gas shift catalyst comprises Ni, even more preferably comprises Ni supported on metal oxide, in particular Ni/AhCh. It is also preferred that the reverse water gas shift catalyst comprises Fe, even more preferably comprises an Fe- oxide, in particular Fe2Oa or FesC The use of a reverse water gas shift catalyst which comprises Fe, Co, Cu, Cr, Ni, Ir, Mn or mixtures thereof can lead to an increased conversion of CO2 in step (i), which can further reduce the CO2 footprint. Additionally, such a reverse water gas shift catalyst comprising Fe, Co, Cu, Cr, Ni, Ir, Mn or mixtures thereof may already be used in existing infrastructure. Hence, such a reverse water gas shift catalyst comprising Fe, Co, Cu, Cr, Ni, Ir, Mn or mixtures thereof can help to maintain existing infrastructure. The mentioned effects can be particularly pronounced when the reverse water gas shift catalyst comprises either Ni supported on metal oxide, in particular Ni/AhCh, or an Fe-oxide, in particular Fe2Oa or FesC

It is similarly preferred that in a method according to the present invention, the reverse water gas shift catalyst comprises Fe, Co, Cu, Cr, Ni, Ir, Mn or mixtures thereof in the form of sulphide(s) or carbide(s).

It is further preferred that in a method according to the present invention, the reverse water gas shift catalyst comprises Fe, Co, Cu, Cr, Ni, Ir, Mn or mixtures thereof, preferably in the form of oxides, sulphides or carbides, or preferably in metallic form supported on metal oxide(s) or supported on carbon, wherein the water gas shift catalyst further comprises a promotor metal species selected from alkali metals and alkaline earth metals.

It is also preferred that in a method according to the present invention, the reverse water gas shift catalyst is selected from

- oxide catalysts, preferably selected from CeC>2, CuO, ZnO, AI2O3, Fe2Os, C^Ch, ln2O3, and MnC>2,

- composite oxides, preferably selected from CuO/ZnO/AhCh, NiO/CeCh, ZnO/AhCh, ZnO/Cr ₂ O3, CuO _x /CeC>2, and I^Ch-CeCh,

- spinel oxides, preferably selected from ZnAhOt, ZnCr2O4, Cu AI2O4, and Co AI2O4, - solid solution oxides, preferably selected from Zn _x Zri- _x O2- _y , Ceo.5Zro.5O2, NixCeo.75Zr ₀ .25- _x 02, and

- perovskite-type oxides, preferably selected from BaZrosYo ieZno ^OS, Lao.75Sro.25Co03-d, Lao.7sSro.25Fe03, Lao^sSro^sFei-yCuYOs, LaNiOs, Lao.gSro.iNiOs+d, Lao.gSro.iFeOs-d, Lao.oSro.iNio.sFeo.sOs-d, LaojsSro^sCro.sMno.sOs-d, and SrCeo.gYo.iOs-d.

It is also preferred that in a method according to the present invention, the reverse water gas shift catalyst is selected from metals, which metals are preferably selected from Pt, Pd, Au, Rh, Ru, Cu, Ni, Re, Co, Fe, and Mo, which are immobilized on a metal oxide support material, which support material is preferably selected from CeO2, TiO2, AI2O3, ZnO, ZrO2, and SiO2. It is additionally preferred that such a water gas shift catalyst further comprises a promotor metal species selected from alkali metals and alkaline earth metals.

It is preferred that in a method according to the present invention, the Fischer-Tropsch catalyst comprises Fe and/or Co, preferably Co. It is more preferred that the Fischer- Tropsch catalyst comprises Fe or Co which is supported on an oxide, in particular Co supported on an oxide. The use of a Fischer-Tropsch catalyst which comprises Fe or Co can lead to an increased conversion of CO in step (ii), which can further improve the energy efficiency and/or the cost efficiency of the fuel production. Additionally, such a Fischer-Tropsch catalyst comprising Fe or Co may already be used in existing infrastructure. Hence, such a Fischer-Tropsch catalyst comprising Fe or Co can help to maintain existing infrastructure. The mentioned effects can be particularly pronounced when the Fischer-Tropsch catalyst comprises Co. Optionally, the Fe comprised by the Fischer-Tropsch catalyst is metallic Fe, the Co comprised by the Fischer-Tropsch catalyst is metallic Co, or the Fe comprised by the Fischer-Tropsch catalyst is metallic Fe and the Co comprised by the Fischer-Tropsch catalyst is metallic Co.

It is preferred that in a method according to the present invention, the zeolite-based catalyst in step (iii) comprises MFI-type zeolite (especially a ZSM-5 zeolite or an HZSM-5 zeolite), a CHA-type zeolite, a BEA-type zeolite, an MOR-type zeolite, an FAU-type zeolite, an MEL-type zeolite, an FER-type zeolite, an MTT-type zeolite, a TON-type zeolite, an ERI-type zeolite, an MTW-type zeolite, an MWW-type zeolite or a mixture thereof. More preferably, the zeolite-based catalyst in step (iii) comprises a ZSM-5 zeolite or an HZSM-5 zeolite, particularly preferable an HZSM-5 zeolite (an HZSM-5 zeolite is a proton-exchanged form of a ZSM-5 zeolite). The listed zeolite types are indicated here by the codes attributed by the International Zeolite Association. The use of a zeolite-based catalyst which comprises a zeolite of the listed types can lead to an increased conversion of Ci-Ce hydrocarbons into aromatics in step (iii), which can further improve the energy efficiency and/or the cost efficiency of the fuel production. Additionally, such a zeolitebased catalyst of the listed types may already be used in existing infrastructure. Hence, such a zeolite-based catalyst of the listed types can help to maintain existing infrastructure. The mentioned effects can be particularly pronounced when the zeolitebased catalyst in step (iii) comprises a ZSM-5 zeolite or an HZSM-5 zeolite, in particular an HZSM zeolite. Herein, a ZSM-5 zeolite and an HZSM-5 zeolite may be combinedly referred to as (H)ZSM-5 zeolite.

It is preferred that in a method according to the present invention, the zeolite-based catalyst in step (iii) comprises a metal-modified zeolite. As used herein, “metal-modified” means that the zeolite contains metal cations different from alkali metal cations and alkaline earth metal cations. Preferred metal cations are Zn-cations, Ga-cations, Ag-cations, Mo-cations and/or Re-cations. Accordingly, it is particularly preferred that the zeolite-based catalyst in step (iii) comprises a Zn-modified zeolite, a Ga-modified zeolite, an Ag-modified zeolite, an Mo-modified zeolite and/or a Re-modified zeolite. The metal- modified zeolite can be an MFI-type zeolite, a CHA-type zeolite, a BEA-type zeolite, an MOR-type zeolite, an FAU-type zeolite, an MEL-type zeolite, an FER-type zeolite, an MTT-type zeolite, a TON-type zeolite, an ERI-type zeolite, an MTW-type zeolite, an MWW-type zeolite or a mixture thereof. The use of a metal-modified zeolite can lead to an increased conversion of Ci-Ce hydrocarbons into aromatics in step (iii), which can further improve the energy efficiency and/or the cost efficiency of the fuel production. Additionally, such a metal-modified zeolite may already be used in existing infrastructure. Hence, such a metal-modified zeolite can help to maintain existing infrastructure.

It is preferred that in a method according to the present invention, another zeolite-based catalyst is present in step (i), more preferably a metal-modified zeolite, still more preferably a zeolite selected from a Zn-modified zeolite, a Ga-modified zeolite, an Ag-modified zeolite, an Mo-modified zeolite and/or a Re-modified zeolite. The “another” zeolite-based catalyst is different from the zeolite-based catalyst used in step (iii), i.e. , it is a further zeolite-based catalyst. The presence of such a further zeolite-based catalyst in step (i) may further promote the reverse water gas shift reaction, which is especially the case for a metal-modified zeolite.

It is preferred that in a method according to the present invention, step (ii) additionally produces saturated C7+ hydrocarbons, preferably saturated C8+ hydrocarbons. Saturated C7+ hydrocarbons are saturated hydrocarbons which contain seven or more (>7) carbon atoms. Saturated C8+ hydrocarbons are saturated hydrocarbons which contain eight or more (>8) carbon atoms. Saturated C7+ hydrocarbons are particularly suitable for combustion in aircraft engines. Accordingly, when the saturated hydrocarbons comprise saturated C7+ hydrocarbons, the inventive method can be particularly suitable for producing sustainable fuel which meets the requirements for aviation fuel. The mentioned effect can be particularly pronounced when the saturated hydrocarbons comprise saturated C8+ hydrocarbons.

It is preferred that in a method according to the present invention, the Ci-Ce hydrocarbons comprise unsaturated hydrocarbons, more preferably alkenyls, still more preferably C2-C4 alkenyls. When the Ci-Ce hydrocarbons comprise such unsaturated hydrocarbons, the subsequent conversion thereof into aromatics in step (iii) can be particularly effective.

It is more preferred that in a method according to the present invention, the Ci-Ce hydrocarbons comprise ethylene, propylene and/or butylene. When the Ci-Ce hydrocarbons produced in step (ii) comprise ethylene, propylene and/or butylene, the subsequent conversion thereof into aromatics in step (iii) can have a higher conversion rate and/or may require less energy. Accordingly, when the Ci-Ce hydrocarbons comprise ethylene, propylene and/or butylene, the energy efficiency and/or the cost efficiency of the fuel production can be improved.

It is preferred that in a method according to the present invention, methane (CH4) is produced in step (i) which is subsequently at least partially converted into aromatics in step (iii). It is known that a conversion of CO2 into CO over a reverse water gas shift catalyst may yield CH4 as a by-product. Like the Ci-Ce hydrocarbons produced in step (ii), which may also comprise CH4, additional CH4 produced in step (i) can subsequently be at least partially converted into aromatics in step (iii). This additional synthesis of aromatics in the inventive method can improve the aromatics content of the produced sustainable fuel, which can further help to meet the requirements for aviation fuel. Moreover, the byproduct CH4 from step (i) is not lost, but is rather valorised.

It is preferred that in a method according to the present invention, step (i) is performed at a temperature of 250 to 1000°C, more preferably of 300 to 750°C and still more preferably of 300 to 600°C. An increased temperature of 250 to 1000°C in step (i) can lead to an increased conversion of the CO2. This can help to further reduce the CO2 footprint of the sustainable fuel produced by the inventive method. At the same time, too high temperatures may lead to a reduced energy efficiency and/or cost efficiency of the fuel production. Temperatures of 300 to 750°C and in particular of 300 to 600°C are therefore particularly preferred.

It is preferred that in a method according to the present invention, step (i) is performed at an absolute pressure of 0.1 to 10 MPa, more preferably of 1 to 4 MPa. An absolute pressure of 0.1 to 10 MPa in step (i) can lead to an increased conversion of the CO2. This can help to further reduce the CO2 footprint of the sustainable fuel produced by the inventive method. At the same time, too high pressures may lead to a reduced energy efficiency and/or cost efficiency of the fuel production. Absolute pressures of 1 to 4 MPa are therefore particularly preferred.

It is preferred that in a method according to the present invention, a feed is fed in step (i) to the reverse water gas shift catalyst which has a molar ratio of hydrogen to carbon oxides, i.e., a molar ratio H2:CO _X , of 0.5 to 12, more preferably of 1 to 3, with x being 1 and/or 2, with x preferably being 2. With such a molar ratio of hydrogen to carbon oxides of 0.5 to 12 an increased conversion of CO2 in step (i) may be achieved which can help to further reduce the CO2 footprint of the sustainable fuel produced by the inventive method. These effects can be particularly pronounced when the ratio of hydrogen to carbon oxides is 1 to 3.

It is preferred that in a method according to the present invention, H2O is produced in step (i) to yield a mixture comprising CO and H2O, wherein the H2O is at least partially separated from the CO in the cooling step. The separation of H2O can help to maintain or even increase the conversion rate of CO in step (ii). The separation of H2O can thus further improve the energy efficiency and/or the cost efficiency of the fuel production. It is preferred that in a method according to the present invention, in step (i) a feed comprising the CO2 is fed to the reverse water gas shift catalyst, wherein the feed is substantially free of CO. When the feed is substantially free of CO, preferably free of CO, the absent CO cannot interfere with the reverse water gas shift reaction. Accordingly, a substantial absence of CO and in particular a complete absence of CO in the feed fed to the reverse water gas shift catalyst can lead to an increased conversion of CO2 in step (i), which can further reduce the CO2 footprint.

It is preferred that in a method according to the present invention, step (ii) and/or (iii) is performed at a temperature of 200 to 500°C, more preferably of 250 to 350°C. An increased temperature of 200 to 500°C in step (ii) can lead to an increased conversion of the CO. An increased temperature of 200 to 500°C in step (iii) can lead to an increased conversion of the Ci-Ce hydrocarbons. At the same time, too high temperatures may lead to a reduced energy efficiency and/or cost efficiency of the fuel production. A temperature of 250 to 350°C is therefore particularly preferred.

It is preferred that in a method according to the present invention, step (ii) and/or (iii) is performed at an absolute pressure of 0.1 to 5 MPa, more preferably of 1 to 3 MPa. An absolute pressure of 0.1 to 5 MPa in step (ii) can lead to an increased conversion of the CO. An absolute pressure of 0.1 to 5 MPa in step (iii) can lead to an increased conversion of the unsaturated hydrocarbons. At the same time, too high pressures may lead to a reduced energy efficiency and/or cost efficiency of the fuel production. Absolute pressures of 1 to 3 MPa are therefore particularly preferred.

It is preferred that in a method according to the present invention, in step (i) the CO2 is reacted with H2 at a temperature of 250 to 1000°C using a reverse water gas shift catalyst which comprises Ni/AhCh to yield a mixture comprising CO and H2O, the H2O is at least partially separated from the CO in a subsequent cooling step before the CO is converted in step (ii), in step (ii) CO from step (i) is reacted with H2 using a Fischer-Tropsch catalyst which comprises Co to yield saturated C8+ hydrocarbons and unsaturated hydrocarbons which comprise at least one of ethylene, propylene and/or butylene, and in step (iii) the zeolite-based catalyst comprises an MFI-type or a BEA-type zeolite.

Such a preferred method for producing sustainable fuel according to the present invention can in particular help to reduce the CO2 footprint associated with the produced sustainable fuel, may allow for existing infrastructure to be maintained, can lead to an improved energy efficiency and/or an improved cost efficiency of the fuel production and can produce sustainable fuel which meets the requirements for aviation fuel.

Subject of the invention is also a use of sustainable fuel obtained by a method as described herein as aviation fuel. The preferred embodiments of the method described herein including the claims are likewise preferred for this inventive use in an analogous manner. Any use of sustainable fuel obtained by a method as described herein as aviation fuel may also be considered as a corresponding method of using such sustainable fuel as aviation fuel following a method as described herein. In other words, a use according to the present invention can also be seen as a method according to the present invention which comprises a step iv) after step iii), wherein step iv) is a step of using the aromatics obtained in step iii) as aviation fuel. Here, the aromatics are either used alone or in admixture with other components as aviation fuel.

Also disclosed herein is a sustainable fuel obtainable by a method as described herein. The preferred embodiments of the method described herein including the claims are likewise preferred for this sustainable fuel in an analogous manner.

Brief description of the drawings

Fig. 1 shows an exemplary reactor system in which a method according to the present invention can be carried out. embodiment

The present invention is further described with reference to the accompanying Fig. 1 which shows an exemplary reactor system 10. The reactor system 10 has two reactors, namely a first reactor 1 and a second reactor 2, which are in fluid communication with each other. A first feed 3 is fed to the first reactor 1. Feed 3 comprises CO2 and H2, and depending on the source of the CO2 potentially minor amounts of CO (typically however 0% CO) and light hydrocarbons. The first reactor 1 contains a first catalyst bed 4 which contains Ni/AhOa as a reverse water gas shift catalyst. The first reactor bed 1 is operated at elevated temperatures between 250 and 1000°C and absolute pressures between 0.1 and 10 MPa. The thereby produced CO and H2O are contained in a first product 5, together with unreacted H2 and potentially unreacted CO2. Methane (CH4) in low amounts may also be produced in reactor 1 , in which case the first product s additionally contains CH4. The first product 5 is withdrawn from reactor 1 and is sent to reactor 2. Inbetween reactor 1 and reactor 2, a cooling device 6 may be arranged. The cooling device 6 cools the first product 5 to the reaction temperature of the second reactor 2. The cooling device 6 further separates at least some of the H2O produced in the first reactor 1 to give the second feed 7 which is fed to the second reactor 2. The second feed 7 contains CO, H2 and potentially CO2 and/or CH4. The second reactor 2 contains a second catalyst bed 8 which contains a Co-based Fischer-Tropsch catalyst (metallic Co on support) and an HZSM-5 zeolite catalyst. The second reactor 2 is operated at elevated temperatures between 200 and 500°C and absolute pressures between 0.1 and 5 MPa. In the second reactor 2, unreacted H2 and CO generated from the first reactor are converted by the Fischer-Tropsch catalyst to Ci-Ce hydrocarbons, which contain C2-C4 alkenyls, and additionally to saturated C7+ hydrocarbons. The Ci-Ce hydrocarbons are converted to aromatics by the HZSM-5 zeolite catalyst. As a result, a second product 9 is obtained which contains long hydrocarbons and aromatics, and potentially residual CO, H2, CO2 and/or CH4. The second product 9 can be withdrawn from the second reactor as the overall product, i.e. , as a sustainable (raw) fuel for further use or refinement thereof.

List of reference signs 1 : First reactor 2: Second reactor 3: First feed 4: First catalyst bed 5: First product 6: Cooling device 7: Second feed 8: Second catalyst bed 9: Second product 10: Reactor system Further disclosure

The present invention further provides the following items:

1. A method for producing sustainable fuel, comprising the steps:

(i) converting CO2 into CO using a reverse water gas shift catalyst,

(ii) converting CO from step (i) into Ci-Ce hydrocarbons using a Fischer-Tropsch catalyst, and

(iii) converting Ci-Ce hydrocarbons from step (ii) into aromatics using a zeolitebased catalyst.

2. The method according to item 1 , wherein in step (i) the CO2 is at least partially reacted with H2, and/or wherein in step (ii) the CO is at least partially reacted with H ₂ .

3. The method according to item 1 or 2, wherein the reverse water gas shift catalyst comprises Fe, Co, Cu, Cr or Ni.

4. The method according to any of the preceding items, wherein the Fischer-Tropsch catalyst comprises metallic Fe and/or metallic Co, preferably metallic Co.

5. The method according to any of the preceding items, wherein the zeolite-based catalyst in step (iii) comprises an MFI-type zeolite, a CHA-type zeolite, a BEA-type zeolite, a MOR-type zeolite, an FAU-type zeolite, an MEL-type zeolite, an FER-type zeolite, an MTT-type zeolite, a TON-type zeolite, an ERI-type zeolite, an MTW-type zeolite, an MWW-type zeolite or a mixture thereof.

6. The method according to any of the preceding items, wherein another metal- modified zeolite-based catalyst is present in step (i).

7. The method according to any of the preceding items, wherein step (ii) additionally produces saturated C7+ hydrocarbons, preferably saturated C8+ hydrocarbons. 8. The method according to any of the preceding items, wherein the Ci-Ce hydrocarbons comprise ethylene, propylene and/or butylene.

9. The method according to any of the preceding items, wherein step (i) is performed at a temperature of 250 to 1000°C.

10. The method according to any of the preceding items, further comprising a cooling step in which CO from step (i) is cooled before being converted in step (ii).

11. The method according to item 10, wherein H2O is produced in step (i) to yield a mixture comprising CO and H2O, wherein the H2O is at least partially separated from the CO in the cooling step.

12. The method according to any of the preceding items, wherein in step (i) a feed comprising the CO2 is fed to the reverse water gas shift catalyst, wherein the feed is substantially free of CO.

13. The method according to any of the preceding items, wherein in step (i) the CO2 is reacted with H2 at a temperature of 250 to 1000°C using a reverse water gas shift catalyst which comprises Ni/AhOa to yield a mixture comprising CO and H2O, wherein the H2O is at least partially separated from the CO in a subsequent cooling step before the CO is converted in step (ii), wherein in step (ii) CO from step (i) is reacted with H2 using a Fischer-Tropsch catalyst which comprises Co to yield saturated C8+ hydrocarbons and unsaturated hydrocarbons which comprise at least one of ethylene, propylene and/or butylene, and wherein in step (iii) the zeolite-based catalyst comprises an MFI-type zeolite or a BEA-type zeolite.

14. Use of sustainable fuel obtained by a method according to any of items 1 to 13 as aviation fuel.

15. Sustainable fuel obtainable by a method according to any of items 1 to 13.