A method and system for the production of a synthetic fuel

Abstract

A method and system for the production of a synthetic fuel Method and system for the production of a synthetic fuel comprising C5+ hydrocarbons which uses gaseous feedstocks of hydrogen and carbon monoxide and / or dioxide to produce a methanol intermediate with an electrochemical reactor and an enzymatic treatment unit before a two-stage reaction process produces the synthetic fuel via dehydration of the methanol to dimethyl ether.

Description

[0001] A method and system for the production of a synthetic fuel

[0002] The present invention relates to a method and system for the production of a synthetic fuel. More particularly, the present invention relates to the production of a synthetic fuel comprising C5+ hydrocarbons using gaseous feedstocks of hydrogen and carbon monoxide and / or dioxide via a methanol intermediate, and as such may be performed sustainably.

[0003] The shift from a fossil fuel-centric economy to one reliant on renewable energy necessitates profound advancements in energy conversion technology. Carbon dioxide (CO2), a byproduct of fossil fuel combustion and a primary contributor to the greenhouse effect has become a global concern.

[0004] Paradoxically, CO2 represents an economical and abundantly available carbon source that holds promise for conversion into valuable fuels and chemicals through electrochemical processes. The electrochemical conversion of CO2 into chemical fuels emerges as a strategic avenue for effectively storing renewable energy. Such synthetic fuels may also be known in the art as electrofuels (e-fuels) and are intended to be used as ‘drop-in’ replacements for the fossil fuel derived alternatives.

[0005] In recent decades, anthropogenic CO2 emissions have surged, triggering concerns related to climate change. To counteract these concerns and the associated rise in greenhouse gas emissions, there is a burgeoning interest in technologies capable of repurposing CO2 into high-value products.

[0006] Policy briefing “Sustainable synthetic carbon based fuels for transport”, published by The Royal Society, September 2019, provides a background on synthetic fuels including e-fuels, their advantages, disadvantages and research challenges.

[0007] Energy Adv.2022, 1 , 580 “A critical review of technologies, costs, and projects for production of carbon-neutral liquid e-fuels from hydrogen and captured CO2” provides a broad overview of the production of e-fuels, including the reactions, catalysts, and costs, to produce two liquid e-fuels (e-methanol and e-kerosene).

[0008] Energies 2024, 17, 3995 “E-Fuels: A Comprehensive Review of the Most Promising Technological Alternatives towards an Energy Transition” investigates the production of carbon-neutral synthetic fuels, focusing on e-hydrogen (e-H2) generated from water electrolysis using renewable electricity and carbon dioxide (CO2) captured from industrial sites or the air. This review discusses how e-methanol can be produced by CO2 catalytic hydrogenation in an adiabatic fixed-bed catalytic reactor, or by the direct conversion of e-methane using water.

[0009] Clean Energy 2024, 8, 5, 1-19 “Beyond fossil: the synthetic fuel surge for a green-energy resurgence” similarly provides an overview of e-fuels, including synthetic fuel production methods from methanol which is prepared from syngas.,ACS Omega 2024, 9, 22858-22870 “Kinetic Modeling and Techno-Economic Analysis of a Methanol-to-Gasoline Production Repurposed Refinery Equipment” provides a background and a simulation of a methanol-to-gasoline conversion process.

[0010] The electrochemical reduction of COx (CO2, CO, or their combinations) demands three essential inputs: COx, a proton source, and electricity. These inputs undergo transformation into fuels, chemicals, and diverse products. However, despite its potential, this technology has yet to attain industrial-scale deployment, primarily due to the absence of a suitable electrochemical reactor.

[0011] The challenge is multifaceted, encompassing inefficient transport of COx to the catalyst surface within the reactor. This inefficiency is rooted in the low solubility of COx in aqueous solutions and the intricate control required to mitigate the competing water reduction reaction, leading to hydrogen production. These competing reactions for protons, crucial to producing hydrocarbons and alcohols, occur at the cathode during both Electrochemical Reduction of carbon dioxide (ERC) and hydrogen formation processes. Unfortunately, a significant amount of energy is diverted towards the Hydrogen Evolution Reaction (HER) instead of the desired reaction: the reduction of CO2. This diversion adversely affects the efficiency of ERC, especially considering that hydrogen evolution necessitates one third the number of protons (2) compared to ERC (6).

[0012] Adding complexity to this competition is the circumstance where both reactions are favoured under a negative equilibrium potential. The hydrogen evolution reaction, with its more negative equilibrium potential, prevails under lower potential conditions. To optimise ERC efficiency, operating at more positive potentials is necessary, although this relationship is not linear. Additionally, reaction rates depend on the concentrations of both reactants and products. Elevated concentrations of CO2 and diminished concentrations of H2 favour ERC, highlighting the intricate balance necessary for maximizing the effectiveness of electrochemical carbon dioxide reduction processes.

[0013] Moreover, when comparing the electrochemical potential applied in methanol production to other products of ERC, as shown in Table 1 , the reduction potentials are all relatively similar.Table 1 - Reduction potential (V) (vs the standard hydrogen electrode (SHE) at 25°C and pH = 7):

[0014] Reduction (electrochemical) Reaction Main ERC product

[0015] potential (V)

[0016] 2H++ 2e- H2-0.42

[0017] CO2 + 2H++ 2e- CO -0.52

[0018] CO2 + 2H++ 2e- HCOOH -0.61

[0019] CO2 + 4H++ 4e- CH2O -0.51

[0020] CO2 + 6H++ 6e~ CH3OH -0.38

[0021] CO2 + 8H++ 4e- CH4-0.24

[0022]

[0023] Current Opinion in Green and Sustainable Chemistry 2020, 23, 10-17 “Recent progress on electrochemical reduction of CO2 to methanol” reviews seven types of electrocatalysts, including metal alloys, metal oxides, metal chalcogenides and carbides, metal-organic complexes, metal-free pyridine and metal-organic framework-based electrocatalysts for the electrochemical reduction of CO2 to methanol, together with the effects of reactor configuration and electrode.

[0024] Clean Energy 2022, 6, 202-210 “Analysis of routes for electrochemical conversion of CO2 to methanol” discusses three technical routes for electrochemical conversion of CO2 to methanol. Route 1 is based on electrolysis of water to H2 and hydrogenation of CO2 to methanol. Route 2 is based on the electrochemical reduction of CO2 in water. Route 3 is based on co-electrolysis of CO2 and water to syngas and subsequent synthesis of methanol therefrom.

[0025] Journal of CO2 Utilization 2023, 71 , 102477 “A comprehensive review of electrochemical reduction of CO2 to methanol: Technical and design aspects” similarly provides an overview of CO2 conversion processes including CO2 hydrogenation, ERC, and CO2 photoelectrochemical reduction.

[0026] Sustainable Energy Fuels 2023, 7, 5445 “Electrochemical CO2 conversion technologies: state-of-the-art and future perspectives” provides a recent review of the state-of-the-art of electrochemical CO2 conversion processes to produce CO, syngas, formic acid and ethylene.

[0027] J. Org. Chem. 2009, 74, 487-498 “Chemical Recycling of Carbon Dioxide to Methanol and Dimethyl Ether: From Greenhouse Gas to Renewable, Environmentally Carbon Neutral Fuels and Synthetic Hydrocarbons” reviews the chemical recycling of carbon dioxide including its capture, conversion to methanol and / or dimethyl ether combining chemical and hydrogenative reduction, or initial electrochemical reduction of CO2 to CO.

[0028] Journal of Catalysis 2016, 343, 232-239 “Cu2O-loaded gas diffusion electrodes for the continuous electrochemical reduction of CO2 to methanol” describes the electrochemical reduction of CO2 toliquid products in a continuous filter-press electrochemical cell equipped with CuzO-based gasdiffusion electrodes (GDEs).

[0029] Catalysts 2020, 10, 713 “Fundamentals of Gas Diffusion Electrodes and Electrolysers for Carbon Dioxide Utilisation Challenges and Opportunities” describes the fundamentals of gas diffusion electrodes and gas-fed electrolysers, their design and their operation within gas-fed electrolysers in both flow-through and flow-by configurations.

[0030] In an alternative aspect in the art, Catal. Sci. Technol. 2021, 11 , 6952 “Improved biocatalytic cascade conversion of CO2 to methanol by enzymes Co-immobilized in tailored siliceous mesostructured cellular foams” describes the enzymatic reduction of CO2 to methanol in a cascade reaction involving three enzymes, a technology investigated as a means to mitigate the climate impact caused by the high emissions of CO2. The cascade reactions were carried out in a closed reactor under a CO2 atmosphere at 5 bar pressure for 1 hour and 37°C under stirring.

[0031] CN 116083211 A describes a system and a method for preparing ethanol by coupling purge gas with methanol prepared by hydrogenation of carbon dioxide. The system comprises a compressor, a reactor, a separator, a rectification system and a biological fermentation system.

[0032] US 2009 / 014336 A1 and US 2010 / 193370 A1 each describe a method for producing methanol by reductive conversion of an available source of carbon dioxide including flue gases of fossil fuel burning power plants, industrial exhaust gases or the atmosphere itself.

[0033] JP 2017-218646 A relates to a CO2 electrolytic treatment system for the simultaneous electrolytic reduction of CO2 stored at a large-scale CO2 emission source (such as a thermal power plant) and the synthesis of methanol.

[0034] WO 2015 / 097020 A1 describes an enzymatic process for reducing CO2 to methanol.

[0035] CN 114369843 A relates to a device containing a photoelectrocatalysis composite membrane, in particular to a photoelectrocatalysis and enzyme catalysis coupled CO2 reduction device capable of realizing cyclic utilization of coenzyme.

[0036] It is an object of the present invention to provide a complete and improved method and system suitable for the production of synthetic fuel, addressing various limitations in the prior art to provide a more efficient and scalable process, particularly for industrial-scale production. Alternatively, the present invention at least tackles some of the problems in the prior art or provides a commercially useful alternative thereto.Thus, in an overall aspect of the present invention, there is described herein a method for the production of a synthetic fuel comprising C5+ hydrocarbons, the method comprising:

[0037] (i) providing a source of H2;

[0038] (ii) providing a source of COX;

[0039] (iii) providing an electrochemical reactor comprising a cathode and an anode, each immersed in a first and second aqueous electrolyte solution, respectively, the cathode and anode being separated by a membrane, and introducing the COx into the first aqueous electrolyte solution adjacent the cathode and introducing the H2 into the second aqueous electrolyte solution adjacent the anode;

[0040] (iv) recovering a portion of the first aqueous electrolyte solution from the electrochemical reactor comprising electrochemically-produced methanol, H2 and COX;

[0041] (v) passing the recovered first aqueous electrolyte solution through a first gas-liquid separator to recover a first liquid stream comprising methanol and water, and a second gaseous stream comprising H2 and COX;

[0042] (vi) passing the first liquid stream through a first distillation column to recover a first methanol stream, and an aqueous solution which is recycled into the electrochemical reactor as further aqueous electrolyte solution;

[0043] (vii) passing the second gaseous stream through a separation unit to recover a third gaseous stream comprising H2 and a fourth gaseous stream comprising COX;

[0044] (viii) passing the fourth gaseous stream through an enzymatic treatment unit to recover a second methanol stream, wherein the enzymatic treatment unit converts the COXto methanol using one or more oxidoreductase enzymes;

[0045] (ix) combining the first and second methanol streams and passing the combined stream to a two-stage reaction process to produce a synthetic fuel comprising C5+ hydrocarbons, wherein:

[0046] (I) the first stage of the two-stage reaction process comprises dehydrating the methanol to dimethyl ether in the presence of a methanol dehydration catalyst; and

[0047] (II) the second stage of the two-stage reaction process comprises heating the dimethyl ether under pressure in the presence of a zeolitic catalyst to produce a reaction mixture comprising water and the synthetic fuel; and

[0048] (x) passing the reaction mixture from the second stage through a second gas-liquid separator to recover the synthetic fuel from the water and unreacted COX.

[0049] The present disclosure will now be described further. In the following passages, different aspects / embodiments of the disclosure are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspects / embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. In particular, a second aspect of the present invention provides a system suitable for and / or appropriately configured to perform the method of production. It is intended that the features disclosed in relation to the method may be combined with those disclosed in relation to the system.This innovative strategy enables improved efficiency and purity in the methanol production section, combining electrochemical and biocatalytic processes within a single production cycle, as well as improvements in the synthetic fuel production section.

[0050] Electrochemical reduction of COx to methanol

[0051] A first aspect of the present invention relates to the electrochemical reduction of COx to methanol. As described above, the electrochemical reduction of COx is a well-studied field of commercial importance. The present disclosure is based on the electrochemical reduction of COx in water to form methanol as the major product. Suitable apparatuses, chemicals and materials are known in the art, and these may be used and / or adapted accordingly in order to perform the method described herein, which uses a hydrogen source of protons instead of water.

[0052] The method of the present invention utilises a source of Hz and a source of COx. Ideally, the source of each gas consists essentially of Hz and COx (and optionally HzO as described herein, as well as unavoidable contaminants), though it is generally preferred that the source of Hz comprises at least 95 vol% Hz, and / or that the source of COx comprises at least 95 vol% COx - by volume excluding HzO, more preferably at least 99 vol%. In generally preferred embodiments, the COx may consist of COz. Such a relatively high purity is particularly preferred in order to reduce down-time, for example, and reduce operational costs. This may be the result of catalyst poisoning and inactivation, for example, be it the catalyst(s) of the electrolyser and / or the enzymatic treatment unit. This also helps to avoid the formation of unwanted byproducts to provide a purer methanol stream. The method described herein ideally provides a methanol stream which comprises at least 95 mol% methanol.

[0053] The method uses an electrochemical reactor (which may also be referred to as an electrolyser or an electrolytic cell, for example). Thus, the electrochemical reactor comprises a cathode and an anode, the cathode immersed in a first aqueous electrolyte solution and the anode immersed in a second aqueous electrolyte solution. The cathode and the anode, and therefore the first and second aqueous electrolytes, are separated by a membrane, specifically an ion-exchange membrane which is a semi-permeable membrane which allows for transport of certain ions therethrough, whilst inhibiting the flow of other ions and neutral molecules. The electrochemical reactor may further comprise a reference electrode, typically an Ag / AgCI reference electrode, which may be arranged within the cathodic cell.

[0054] The shape of the cathode and anode are not particularly limited, though they are preferably provided in the form of plates or sheets, thereby having two faces with a relatively high surface area. In embodiments described herein, the cathode and / or anode are fully immersed in their respective electrolytes in the sense that both faces contact the electrolyte, with each face of the membrane separating the two electrodes also contacting each of the two electrolytes separating the cathodesection and the anode section of the reactor. In other embodiments, one face of an electrode is immersed so as to contact the electrolyte, and the other face may be in direct contact with the membrane or may be exposed to a gaseous flow of the Hz or COx.

[0055] Regardless of the form of electrolytic reactor used, the method comprises introducing the COx into the first aqueous electrolyte solution adjacent the cathode, whilst also introducing the Hz into the second aqueous electrolyte solution adjacent the anode. As will be readily appreciated by those skilled in the art, during use, an electric current is applied to the cathode and anode to establish an electric potential difference therebetween, with the supply of electrons the driving force for the reduction of COx to methanol. Each of the cathode and anode are generally provided with a catalyst to promote each set of reactions in each cell.

[0056] It is generally preferred that the electrochemical reactor is configured with flow-type filter-press cells. In such embodiments, only one face of each of the cathode and anode are immersed in their electrolyte. Such reactors are particularly effective and suitable for large scale COx reduction to methanol, and preferably each cell has an area of at least 0.5 m2, such as from 1 to 5 m2(e.g. the area of one or more of the cathode, anode and membrane exposed to the electrolyte and / or gas). Filter cells are designed to maximise the efficiency of the electrochemical process, allowing for more effective contact between the reactants and the electrodes. Additionally, filter cells are generally employed on a larger scale, thereby meeting the demands of industrial scale synthetic fuel production.

[0057] The electrochemical reactor may preferably comprise a plurality of cathodes and anodes, each of which is separated by membranes providing an electrolytic stack, for example comprising from 50 to 200 cells. Each of the cathodic and anodic cells may be connected in series or parallel.

[0058] It is also generally preferred that the cathode and / or anode are gas diffusion electrodes (GDEs). A GDE generally consists of a gas diffusion layer (GDL) and a catalyst layer. A GDL generally comprises a microporous substrate, such as porous carbon paper, i.e. interwoven carbon fibers. A GDL may further comprise a microporous layer, such as a thin layer of carbon powder mixed with a hydrophobic polymer (e.g. polytetrafluoroethylene - PTFE). The substrate may have a thickness of about 100 pm to about 500 pm, whereas the microporous layer may be less than 100 pm thick. The catalyst layer is provided on the microporous layer if present. In other embodiments, the GDL may simply be PTFE treated carbon paper. The carbon fiber paper treated with PTFE is classified as a composite material due to the combination of the properties of carbon fiber with the characteristics of PTFE. This material is made of carbon fibers, which are known for their high strength and lightweight, and is treated with a layer of PTFE, which provides non-stick properties and chemical resistance. In particularly preferred embodiments, the cathode further comprises CuzO as the catalyst layer, whichmay optionally be mixed with ZnO. Such catalysts are preferred for favouring methanol production. Preferably the anode is made of platinum-coated titanium.

[0059] In use, the COx diffuses through the GDL to the catalytic layer by the difference in partial pressure of COx (which is lower on the surface of the catalytic layer) and with the help of the electrolyte. As COx diffuses into the catalytic layer, it interacts with the electrolyte present in the diffusion layer. The presence of water in the electrolyte is crucial, as COx needs to be dissolved (to a certain extent) to participate in the electrochemical reaction. The catalytic layer is then the site where COx reduction to methanol occurs with the aid of electrons and ions. The inventors have found that this process is advantageously enhanced by the oxidation of Hz which takes place at the anode. In some preferred embodiments, a molar ratio of Hz to COx that is introduced into the aqueous electrolyte solution in step (iii) is at least 3:1 and / or up to 10:1. Preferably, the COx is bubbled through the electrochemical reactor at a flow rate of at least 10 mL / min, as measured under the operating conditions. The flow rate of COx may be from 0.5 to 2 kg / s, such as from 1.0 to 1.5 kg / s, and the flow rate of Hz may be from 0.1 to 0.5 kg / s. The flow rate(s) may be modified under the operating conditions to optimise Faradic efficiency.

[0060] The performance of the GDE depends on the equilibrium between the water content in the catalytic layer and the electrolyte. One of the main challenges in operating GDEs is the flooding of the catalytic layer (excess water). When there is an excess of water, the pores of the electrode can become blocked, preventing access of COx gas to the catalytic sites. This results in an increase in mass transport resistance, making the desired reaction difficult. Under high current density conditions, flooding can lead to an uneven distribution of COx, resulting in inefficient use of the catalyst.

[0061] Moreover, excess water can promote the formation of hydroxyl groups, which inhibit active sites, further compromising the process efficiency.

[0062] On the other hand, a lack of water also presents significant challenges. Without water, dry pores become inactive due to the absence of aqueous electrolyte and the necessary ionic pathway for the reaction. Although COx is provided to the GDE from the gaseous phase, the reactant at the catalytic site continues to be dissolved COx.

[0063] Therefore, the presence of water is essential for its dissolution and for the electrochemical reaction to occur. Thus, two factors that influence the ideal balance between flooding and excessive dryness in the catalytic layer are namely:

[0064] (a) Catalytic Layer Saturation: The fraction of pores flooded, defined as saturation (S), which depends on capillary pressure. At low capillary pressures, only hydrophilic small pores are flooded. As capillary pressure increases, larger hydrophilic pores, then larger hydrophobic pores, and finally smaller hydrophobic pores are flooded in that order; and(b) Catalytic Layer Hydrophilicity: A more hydrophilic catalytic layer (with greater saturation for a given capillary pressure) can enhance performance at low overpotentials but may result in poorer performance at more cathodic potentials due to easier flooding.

[0065] Consequently, in some embodiments, the system may further comprise a humidifier upstream of the electrocatalytic reactor for adjusting the humidity (i.e. water content) of the source of, in particular, COx.

[0066] The electrochemical reactions may be summarised as follows (based on a CO2 source):

[0067] (1 ) In the cathode section: CO2 + 6H++ 6e- CH3OH + H2O

[0068] (2) In the anode section: H2-> 2H++2e- (3) In the membrane: 2H2O -> 2H++ 2OH- (4) Final reaction: CO2 + 3H2CH3OH + H2O

[0069] All reactor configurations rely on an ion-exchange membrane which separates the first electrolyte (catholyte) from the second electrolyte (anolyte). The membrane may be a bipolar membrane, a charge mosaic membrane, a layered mixture of anion-exchange and cation-exchange resins, or an ion-conducting ceramic or polymeric membrane. Preferably, the membrane may be a cationexchange membrane allowing cations (such as protons) to migrate to the cathodic cell (i.e. into the catholyte). Preferably the membrane is a perfluorinated ion-exchange membrane. Such a membrane may include poly (vinylidene fluoride) (PVDF), poly(ether-ether-ketone) (PEEK), poly(sulfone) (PSU), poly(ether imide) (PEI) and / or Nation. A Nafion-type membrane is particularly preferred, which is one based on a copolymer of tetrafluoroethylene (TFE) and perfluorovinyl ether (PFVE) that contains sulfonate groups, i.e. a perfluorinated polymer incorporating perfluorovinyl ether side chains terminated with sulfonate groups. It is widely used in electrochemical applications, particularly in fuel cells and electrolysers, due to its excellent ionic conductivity, chemical stability, and mechanical strength. Nation membranes selectively allow the passage of protons while blocking the flow of gases and other ions, making them essential for efficient ion transport in various energy conversion and storage systems. Their ability to operate at elevated temperatures and in harsh chemical environments further enhances their utility in advanced energy technologies. In general, the thickness of the membrane may be from 0.1 to 1 mm.

[0070] Preferably the electrochemical reactor is operated at a temperature of from 10°C to 30°C, a pH of from 6 to 8, and a pressure of from 1 to 50 bar, preferably 5 to 30 bar. Each of the first and second aqueous electrolytes may be a solution of sodium and / or potassium, carbonate and / or sulfate. Each aqueous electrolyte may have a concentration of from 0.01 to 1 M, such as from 0.05 to 0.5 M. This provides a balanced environment for ionic conductivity and gas solubility while maintainingcompatibility with the catalyst and GDE structure. This concentration ensures sufficient buffering capacity and stability under the expected operating conditions. Such concentrations minimise flooding while supporting efficient COx diffusion and electrochemical conversion. Typically, the electrolyte does not comprise aprotic solvents, as the system is designed to operate in a purely aqueous environment to favour the COx-to-methanol pathway and maintain simplicity and safety in reactor operation.

[0071] Following and / or during electrolysis, the method further comprises recovering at least a portion of the first aqueous electrolyte solution from the electrochemical reactor, which now comprises electrochemically-produced methanol, Hz and COx. The Hz present in this stream is generally the byproduct of the competing HER, whilst the COx is that generally that which remains unreacted. The method may generally also further comprise recovering the second aqueous electrolyte solution which generally comprises unreacted Hz (where the amount of Hz is generally greater in the second aqueous electrolyte solution than in the first). The Hz may be separated from the second aqueous electrolyte solution, both of which may then be recycled into the electrochemical reactor.

[0072] The method further comprises passing the recovered first aqueous electrolyte solution through a gasliquid separator to recover a first liquid stream comprising methanol and water, and a second gaseous stream comprising Hz and COx. The method then further comprises passing the first liquid stream through a first distillation column to recover a first methanol stream and an aqueous solution. The distillations described herein may preferably be performed with a fractionation column, which enhances separation by repeated vaporisation and condensation. The water content of the first liquid stream may be in the region of about 50 mol% (e.g. from 30 to 80 mol%), and the balance may be substantially methanol. Following distillation, the first methanol stream may be greater than 95 mol%, preferably greater than 98 mol%. The aqueous solution recovered from the distillation column is recycled into the electrochemical reactor as further aqueous electrolyte solution (and in principle may be used for either the first or second aqueous electrolytes though will typically be used to make-up the first aqueous electrolyte).

[0073] The method further comprises passing the second gaseous stream through a separation unit to recover a third gaseous stream comprising Hz and a fourth gaseous stream comprising COx. Any conventional apparatus in the art may be used for such a step. For example, the separation unit may be a membrane separation unit or an absorption-based separation unit, e.g. one which uses chemicals such as amines, or uses MOFs, zeolites or activated carbon.

[0074] As described herein, the fourth gaseous stream is then sent to an enzymatic treatment unit for further conversion into methanol. The third gaseous stream of Hz may be recycled with the source of Hz into the electrolytic treatment unit.Hz source production

[0075] In one particularly preferred embodiment, the source of Hz is produced by:

[0076] electrolysing water in an electrolytic cell to produce hydrogen gas and oxygen gas, the electrolytic cell having a first outlet for hydrogen gas;

[0077] passing the hydrogen gas from the first outlet of the electrolytic cell to a reaction chamber, the reaction chamber comprising a first inlet for receiving the hydrogen gas from the electrolytic cell and a second outlet for hydrogen gas passing out of the reaction chamber, the reaction chamber containing one or more pieces of a metal or an alloy thereof at least partially submerged in an alkali solution, wherein the first inlet is arranged so that the hydrogen gas bubbles through the alkali solution;

[0078] passing the hydrogen gas from the second outlet to a gas-cleaning chamber, the gas-cleaning chamber comprising a second inlet for receiving hydrogen gas from the reaction chamber and a third outlet for hydrogen gas passing out of the cleaning chamber, the gas-cleaning chamber containing an aqueous solution, wherein the second inlet is arranged so that the hydrogen gas bubbles through the aqueous solution; and

[0079] recovering hydrogen gas from the third outlet.

[0080] Such a method for producing and purifying a source of hydrogen is described in detail in WO 2020 / 074886 A1 . The process comprises a first step of electrolysing water in an electrolytic cell to produce hydrogen gas and oxygen gas. The electrolysis of water is a well-known technique and involves the application of an electrical potential through an aqueous solution. Any standard electrolysis apparatus can be used.

[0081] A preferred electrolytic cell has an anode at one side and a cathode at the other, with an ionic solution in between. The electrolysis unit relies upon an ionic solution, which is an aqueous solution containing an electrolyte, to improve the electrical conductivity of the solution. The increased conductivity of the solution increases the rate at which the electrolysis reaction can proceed, increasing the decomposition of the water component and increasing the hydrogen yield per unit time. Preferably the ionic solution in the electrolysis unit has conductivity of at least 0.25 S / cm, more preferably at least 0.5 S / cm and most preferably from 0.5 to 1 S / cm. The conductivity of the ionic solution can be measured using conventional equipment and should be measured at 20°C.

[0082] A preferred ionic solution for use in the electrolysis unit is an alkali solution, such as KOH or NaOH. Such a solution would preferably have a concentration of at least 0.1 M, preferably at least 0.2 M and most preferably about 0.3 M. As explained below, this ionic solution can be recycled from later stages of the process.

[0083] According to one embodiment, the ionic solution may be supplemented with aluminium hydroxide or another soluble metal hydroxide (in addition to the alkali such as KOH or NaOH). The amount ofadded metal hydroxide is preferably present in an amount of at least 0.0001 M, such as from 0.001 to 0.01 M. This has a number of key advantages. Firstly, it improves the electrolysis reaction because it has a strong urge to react with oxygen and other impurities. This means that the addition sequestrates some water impurities and slightly increases the purity of Hz production. It can also provide a small process temperature increase. Furthermore, the metal hydroxide can be readily obtained as a byproduct of the process as a whole; for example, aluminium hydroxide can be used where aluminium is used as the metal in the reaction chamber discussed below. Thus the metal hydroxide can be recovered from the reaction chamber. For example, a 0.001 M solution of aluminium hydroxide can be obtained by dissolving 1 g of AIOH in 20 L of a KOH solution.

[0084] Within the ionic solution in the electrolytic cell there are typically provided a plurality of neutral plates. These neutral plates are preferably parallel with each other and the electrodes, and equally spaced. These act to divide the voltage experienced by the ionic solution into smaller steps, like a plurality of individual sub-cells in series, and thereby reduce the corrosive wear on the electrodes. Hydrogen is produced at the cathode and oxygen at the anode. In addition, in each "sub-cell", hydrogen is produced on the cathode-side and oxygen at the anode-side on the neutral plates.

[0085] As the oxygen and hydrogen are produced they bubble out of the ionic solution as gases. The electrolytic cell has means for collecting hydrogen gas from each cathode-side of each "sub-cell" and for collecting oxygen gas from each anode-side of each "sub-cell". Preferably a membrane is provided in between the anode and cathode which prevents cross-contamination of the hydrogen with the oxygen gas. Where neutral plates are included within the electrolytic cell, a separate membrane would be provided between each neutral plate and between the anode and the adjacent neutral plate and between the cathode and the adjacent neutral plate.

[0086] The present inventors have found that the use of such a membrane improves the safety and yield of the system. Moreover, a costly type of membrane is not required and the membrane can be a mesh membrane. Such a mesh membrane can be readily selected to be impervious to the gaseous oxygen and hydrogen species, without hindering flow of the liquid electrolyte. This is particularly the case with the fast bubbling of the gases away from the electrode surfaces. The mesh membrane is preferably a polymer mesh membrane and can be selected depending on the ionic solution being employed for chemical compatibility. A nylon mesh is preferred for KOH(aq) as the ionic solution and a polyester mesh is preferred for NaOH(aq) as the ionic solution.

[0087] A preferred Nylon Monofilament Mesh has between 300 to 500 cross-counts per inch. The mesh forms a thin wall that allows the water to pass through it, but not the bubbles. The H and O ions can pass through the water, cross the membrane, and form the gas on the electrode plate they are attracted to (positive or negative). The hydrogen stays on the negative side of the membrane wall,and the oxygen stays on the positive side. The mesh is a dividing wall; it forms / separates two chambers. The gases rise to the top of their respective side of the chamber and collect at the top.

[0088] The electrolytic cell has a first outlet for hydrogen gas. The outlet is for passing the hydrogen gas collected from the electrolytic cell for onwards processing. The outlet may pass to a flash-back inhibitor to prevent damage of the electrolytic cell if a critical level of oxygen enters the system and risks implosion. However, when using the membrane as described above, the oxygen contamination of the hydrogen is reduced or avoided, such that the risk of implosion is reduced.

[0089] The process comprises a further step of passing the hydrogen gas from the first outlet of the electrolytic cell to a reaction chamber. The reaction chamber comprises a first inlet for receiving the hydrogen gas from the electrolytic cell and a second outlet for hydrogen gas passing out of the reaction chamber.

[0090] The reaction chamber contains one or more pieces of metal or an alloy thereof at least partially submerged in an alkali solution. Metals or alloys (which contain metals) react with alkali to produce hydrogen and metal oxides or hydroxides. The metal or alloy selected should be used in combination with an alkali having sufficient concentration for such a reaction to proceed. The metal or alloy preferably comprises aluminium or steel.

[0091] For aluminium, which is preferred, a strong alkali solution is required because this metal has a very thin passive layer of AI2O3 on its surface that prevents the direct attack of water molecules. The alkali is not consumed in the reaction and acts as a catalyst, it can be fully recovered. This is because the aluminate salts produced in the hydrogen generation undergo a decomposition reaction that regenerates the alkali. The reactions of aluminium with potassium hydroxide in aqueous solution to produce hydrogen are as follows:

[0092] (5) 2AI + 6H2O + 2KOH -> 2K[AI(OH)4] + 3H2

[0093] (6) K[AI(OH)4] KOH + AI(OH)3

[0094] The reaction between aluminium and water obeys the following stoichiometry. Thus, only aluminium and water are the consumed raw materials to produce hydrogen:

[0095] (7) 2AI + 6H2O -> 2AI(OH)3+3H2

[0096] If necessary, the K[AI(OH)4] product can also be treated with acid, such as sulphuric acid to recover the aluminium hydroxide.The form of the metal or alloy is not especially important. However, the method provides a useful opportunity to recycle waste or scrap metal sources, such as aluminium cans. In addition, it is desirable that the metal has a high surface area, such that the reaction can proceed at a reasonable rate. Accordingly, the metal is preferably processed to have a high surface to weight ratio. In the case of aluminium scrap, such as cans, these can be crushed or fragmented into shards. Preferably the metal or metal alloy is provided as a plurality of pieces, each piece having a weight of less than 0.1 kg. The alkali solution is preferably KOH or NaOH and preferably has a concentration of at least 1 M, such as from 1 to 5 M.

[0097] The first inlet is arranged so that the hydrogen gas bubbles through the alkali solution. That is, the first inlet is submerged below the level of the alkali solution. The hydrogen gas can preferably enter the alkali solution through a single point, or through a shower-head type nozzle to distribute the bubbles.

[0098] The inventors have found that this is especially important, since the agitation of the metal / alkali system increases the reaction rate and hydrogen production. As will be appreciated, the combination with the fuel cell provides agitation of the reaction chamber without requiring additional complex equipment, such as a stirrer, or further energy input. Without wishing to be bound by theory, it is understood that agitation enhances the reaction as it allows the solution to mix well and creates more surface for reaction. Thus, the agitation creates a higher possibility for the aluminium-water reaction to occur and enables a more efficient process in both hydrogen production and energy efficiency.

[0099] The gas leaving the second outlet is the hydrogen gas obtained from the fuel cell, supplemented by the hydrogen gas produced in the reaction chamber from the alkali reaction with the metal. This means that, in comparison to a conventional fuel cell, the hydrogen production yield can be increased. The reaction chamber is a component of the system which can be replaced when spent and therefore acts as a supplemental battery-like hydrogen source. Furthermore, the alkali-reacted metal in the form of an oxide or hydroxide can be a useful product.

[0100] Aluminium is especially preferred as the metal (and alloys of aluminium can also be employed). There are several advantages of using aluminium. Its by-product, AI(OH)3, may be used to produce other aluminium salts as a useful by-product, including the use of electrolysis to recover Al metal. The aluminium used in the reaction can be obtained from recyclable materials, such as soft drink or beer cans. Moreover, it reacts with readily and cheaply available alkali sources, such as KOH and NaOH. The hydrogen generated by reaction above is pure and suitable for high purity applications.

[0101] Preferably the process further comprises recovering a metal oxide or metal hydroxide from the reaction chamber. Preferably the process further comprises treating the metal oxide or metal hydroxide to recover the metal. Aluminium can be regenerated from aluminium hydroxide, by twoprocesses developed in the late 1800s: the Bayer process which produces pure alumina from bauxite ore, and the Hall-Heroult process which produces aluminium from alumina.

[0102] The process comprises a further step of passing the hydrogen gas from the second outlet of the reaction chamber to a gas-cleaning chamber. The hydrogen gas leaving the reaction chamber may have entrained some of the ionic solution and / or some of the alkali solution as a vapour. This is an undesirable contaminant in the system which reduces the utility of the hydrogen gas produced. The gas-cleaning chamber serves to remove such vapours. The gas-cleaning chamber comprises a second inlet for receiving hydrogen gas from the reaction chamber and a third outlet for hydrogen gas passing out of the cleaning chamber.

[0103] The gas-cleaning chamber contains an aqueous solution. This is typically just water. However, over time vapourised ionic solution and / or alkali solution entrained with the hydrogen will become trapped in the water. Where the system uses the same reagent to provide the ionic solution in the electrolytic cell and the alkali solution in the reaction chamber (such as KOH), the water will become a weak solution of this reagent. Once levels reach a sufficiently high concentration, the water can be swapped. Preferably the process further comprises recycling spent aqueous solution from the gascleaning chamber to the electrolytic cell for use as at least a portion of the ionic solution. That is, the contaminated water can be used to make fresh ionic solution for the electrolysis cell to recycle the reagent, such as KOH.

[0104] The second inlet is arranged so that the hydrogen gas bubbles through the aqueous solution. That is, the first inlet is submerged below the level of the aqueous solution. The hydrogen gas can preferably enter the aqueous solution through a single point, or through a shower-head type nozzle to distribute the bubbles and enhance the cleaning. The process permits the recovery of hydrogen gas from the third outlet. The third outlet represents the product stream from the process. From the gas-cleaning chamber there is obtained a flow of hydrogen between 1 to 5 bar, which is ready to be used as the source for the electrolytic reactor described herein.

[0105] An example of a suitable apparatus comprises:

[0106] an electrolytic cell for the electrolysis of water to produce hydrogen and oxygen gas, the electrolytic cell having a first outlet for hydrogen gas;

[0107] a reaction chamber comprising a first inlet in fluid communication with the first outlet, and a second outlet; and

[0108] a gas-cleaning chamber comprising a second inlet in fluid communication with the second outlet, and a third outlet for the produced hydrogen gas,

[0109] wherein the reaction chamber contains one or more pieces of a metal or an alloy thereof at least partially submerged in an alkali solution, and wherein the first inlet is for bubbling hydrogen gas through the alkali solution;wherein the gas-cleaning chamber contains an aqueous solution, and wherein the second inlet is for bubbling hydrogen gas through the aqueous solution.

[0110] Preferably the electrolytic cell further comprises a magnetron for treating water within the electrolytic cell. Treating the electrolyte with a magnetron allows for the water to be in a natural frequency allowing the hydrolysis cell to add smaller amounts of electricity to break down the water molecule. This reduces the overpotential required for the electrolytic cell.

[0111] Preferably the electrolytic cell comprises a cathode and an anode and a plurality of neutral plates arranged therebetween, each neutral plate being separated from each adjacent neutral plate by a volume for holding an electrolytic solution, wherein said volume contains a mesh membrane to thereby define a cathode-side volume and an anode-side volume, wherein each cathode-side volume is in fluid communication with the first outlet, and wherein the mesh membrane is substantially impervious to gaseous oxygen and hydrogen. Preferably the mesh membrane is a mesh membrane, preferably a nylon mesh membrane. The mesh membrane can significantly lower the costs and largely enhance the stability of chemicals-assisted hydrogen electrocatalytic reaction. The mesh membrane forms a thin wall that allows the water to pass through it, but not the bubbles. Oxygen bubbles form at the anode and hydronium ions pass through the water, cross the membrane, and form the hydrogen gas on the cathode. The hydrogen stays on the negative side of the membrane wall, and the oxygen stays on the positive side. The mesh membrane is a dividing wall forming and separating two chambers. The gases rise to the top of their respective side of the chamber and collect at the top and leave by their respective outlets. Preferably the apparatus further comprises a flashback arrestor arranged between and in fluid communication with the first outlet and the first inlet.

[0112] Preferably the apparatus comprises a plurality of interchangeable reaction chambers. These can be removed and replaced when spent. Thus the continuous production of hydrogen from the fuel cell can be coupled to the batchwise reaction of the metal. Preferably the apparatus further comprises a first sensor in communication with the first inlet and a second sensor in communication with the second outlet, wherein the first and second sensors are for determining hydrogen gas flow-rates. This allows the operator to determine when the metal has fully reacted.

[0113] According to a further aspect there is provided an apparatus for producing hydrogen, the apparatus comprising a reaction chamber comprising a second outlet; and

[0114] a gas-cleaning chamber comprising a second inlet in fluid communication with the second outlet, and a third outlet for the produced hydrogen gas,

[0115] wherein the reaction chamber contains one or more pieces of a metal or an alloy thereof at least partially submerged in an alkali solution;

[0116] wherein the gas-cleaning chamber contains an aqueous solution, and wherein the second inlet is for bubbling hydrogen gas through the aqueous solution.The apparatus of this aspect relates to the reaction chamber as described above, coupled with a gascleaning system to avoid the carry-over of alkali solution in the hydrogen product.

[0117] Enzymatic reduction of COx to methanol

[0118] In a further aspect, the present invention operates synergistically with an enzymatic treatment unit to reduce the unreacted COx to provide a further methanol stream. Such a method further comprises a step of passing the fourth gaseous stream through an enzymatic treatment unit to recover a second methanol stream, wherein the enzymatic treatment unit converts the unreacted COx to methanol using one or more oxidoreductase enzymes.

[0119] The additional use of an enzymatic reactor allows for an improvement in the overall COx electrochemical reduction process efficiency through the recycling of the unconverted COx stream (such a stream having been isolated as described above as the fourth gaseous stream). Such a method is particularly effective for maximising the conversion of COx to methanol, and may achieve essentially 100% conversion.

[0120] Such a method can be performed without the need to operate at high temperatures. Unlike methods which rely on microorganisms as catalysts, the enzymatic route leverages highly specific oxidoreductase enzymes to facilitate the multi-step conversion of COx into methanol under controlled conditions. This process minimises the complexities associated with microbial cultivation and metabolic pathways, focusing instead on the direct catalytic action of enzymes.

[0121] In generally preferred embodiments, the method uses a cascade of oxidoreductases which are a series of enzymes which work together to catalyse individual steps of the catalytic reaction. That is, the enzymatic treatment unit converts the COx to methanol using oxidoreductase enzymes in a cascade process. For example, one or more of, and generally all three of, Formate dehydrogenase (FateDH), Formaldehyde dehydrogenase (FaldDH), and Alcohol dehydrogenase (ADH) are preferably used in the enzymatic treatment though it will be appreciated that other enzymes and their combinations may be suitable for methanol production. These enzymes have the enzyme commission number EC 1 .2.1 .1 , EC 1 .2.1 .46, and EC 1 .1 .1 .1 , respectively. In some preferred embodiments, such enzymes may be used in a mass ratio of about 1 : 5-20 : 70-90 (FateDH : FaldDH : ADH).

[0122] Particularly where unreacted CO may be present, a further enzyme may preferably include carbon monoxide dehydrogenase which has the enzyme commission number EC 1 .2.7.4.

[0123] In industrial applications, oxidoreductases can catalyse reactions that conventional chemical catalysts are unable to perform. For instance, they can operate under mild conditions and tolerate impurities, such as sulfur compounds present in combustion gases, which deactivate certain chemical catalysts.However, to ensure optimal oxidoreductase activity, key parameters, including temperature, pH, and the ionic strength of the medium, can be controlled. By employing enzymes such as FateDH, FaldDH and ADH as biocatalysts in tandem with cofactors like NADH, the enzymatic method provides a more efficient, scalable, and environmentally friendly pathway for transforming CO2 into valuable chemical products.

[0124] NAD(P)H (p-Nicotinamide adenine dinucleotide (phosphate) in its reduced form) is preferable to serve as an electron donor in the enzymatic reduction of COx to methanol, transferring the necessary reducing equivalents (electrons and protons) to enable the sequential reduction of COx. NADH, for example, initially donates electrons and protons to reduce CO2 to formate. In the second step, NADH donates the reducing equivalents, transitioning from NADH to NAD+to convert formate into formaldehyde, and finally, NADH reduces formaldehyde to methanol. As such, the cofactor enhances the electrochemical reduction by acting as an electron donor, providing electrons that facilitate the reduction steps in each enzymatic reaction. The high-energy electrons from the cofactor can drive thermodynamically challenging reduction processes. Many oxidoreductases, such as those described above, are significantly more effective together with a cofactor such as NADH to undergo conformational changes, enhancing their catalytic efficiency. The enzymes and any cofactor are generally dissolved in a buffer solution, typically phosphate solution which may have a phosphate concentration of from 10 to 500 mM.

[0125] The co-immobilization of enzymes in a siliceous mesostructured cellular foam (MCF) showed an improvement in the yield of methanol. Immobilisation may be achieved using techniques known to those skilled in the art, and may include sequential incubation of the MCF in the presence of each enzyme in a thermomixer. The optimal conditions within which to operate the enzymatic treatment unit are a pH between 6 and 8, for example between 7 and 8, a temperature between 30 and 37°C to maintain enzyme activity, and a pressure of from 1 to 5 bar. The termination process can be induced by altering the temperature to values outside of the optimal enzyme operation range. To make the system more cost-effective, the NAD(P)H may be electrochemically or enzymatically (using glucose dehydrogenase for example) regenerated and recycled to the enzymatic reactor.

[0126] Synthetic fuel production from methanol

[0127] The present invention preferably incorporates a further aspect of synthetic fuel production following the production of methanol via the inventive methods described hereinabove. As discussed in the background, synthetic fuel production from carbon dioxide and hydrogen as feedstocks is as well studied field, with methanol a frequent intermediate feedstock for e-fuel production (since methanol may in other technologies be obtained from sources such as biomass, e.g. biomethanol).The present method for synthetic fuel production comprises passing the methanol stream to a two-stage reaction process. In accordance with the preceding aspects, such a stream is ideally a combination of the first methanol stream obtained from the electrochemical reduction of COx and the second methanol stream obtained from the enzymatic reduction of the unreacted COx recovered from the product stream of the electrochemical reactor. The methanol stream is ideally as pure as possible at this stage, and the methanol streams (or combined stream) may undergo further purification to achieve a methanol content of greater than 98% v / v, preferably greater than 99% v / v, so as to ensure efficient synthetic fuel production avoiding poisoning of the catalysts.

[0128] In a first stage, the methanol stream is dehydrated to dimethyl ether in the presence of a methanol dehydration catalyst. In a second stage, the dimethyl ether produced by the first stage is heated under pressure in the presence of a zeolitic catalyst to produce a reaction mixture comprising water and the synthetic fuel.

[0129] In the context of methanol, dehydration to decrease the effective water content in the stream produces a mixture with a low methanol concentration, a short-chain ether (i.e. dimethyl ether, DME) and water. The reaction results in a gaseous product stream which preferably contains at least 90% v / v DME, more preferably at least 95% v / v. A liquid methanol stream (MeOH feed stream) is generally pressurised, preferably to greater than 5 bar, such as from 10 to 30 bar, and heated. In order to maximise efficiency, it is ideal for heating to be performed using a heat exchanger which recovers heat from the raw synthetic fuel product obtained from the second stage. The dehydration reaction may preferably be performed at a temperature of at least 80°C, and generally it is preferred to minimise the energy input required, and 150°C is generally a suitable upper limit, preferably up to 120°C. A liquid (by-)product stream may also be recovered from the reaction chamber, the stream consisting essentially of unreacted methanol, DME and water. Preferably, the stream is sent for product separation and recovery for recycling.

[0130] Preferably, the first stage is performed in an adiabatic conversion reactor which contains the methanol dehydration catalyst. Alumina (AI2O3) is an oxide widely used as a catalyst or catalyst support, which may be obtained from the partial dehydration of aluminium hydroxide minerals. Alumina is valued for its low production cost and good thermal stability. In the hydro processing units of oil refineries, alumina serves as a support for catalysts that are transition metal sulfides, such as nickel (Ni) and iron (Fe). The selectivity and catalytic activity of y-alumina depends on its porous structure and the presence of acid sites. Catalytic activity increases with the concentration of acid sites on the surface. The pore size is also a crucial factor in the selectivity of the catalyst; the formation of light olefins is maximised in catalysts with small pores, which prevent the entry of aromatic compounds and branched alkanes. DME (CH3OCH3) is produced from methanol at weak acid sites. The activity and selectivity of this catalyst can be altered by the transformation of strong acid sites into weak acid sites due to the water produced during the dehydration of methanol.As an alternative, tungsten oxide (WO3) catalysts are often used in combination with supports such as alumina or silica and the inventors have found that tungsten oxide is particularly preferred for the present method. Tungsten oxide is known for its acidic and reducing properties, and its acidity can be adjusted depending on the preparation of the catalyst, which is important for reactions involving the dehydration of alcohols. Tungsten oxide-based catalysts are more effective at elevated temperatures for the dehydration of methanol to DME. Additionally, tungsten oxide is utilised in hydrogenation and dehydrogenation reactions, where its reducing properties are advantageous. Tungsten oxide-based catalysts offer good catalytic activity and selectivity, and are regenerable.

[0131] Another alternative is solid acid catalysts, which include a variety of materials such as acid resins and zeolites that possess acid groups. In this context, acid resins are polymers that contain acidic functional groups, such as sulfonates. The acidity of these catalysts can be adjusted by selecting the material and through chemical modification, allowing for a wide range of catalytic activities. Acid resins and other solid acid catalysts can be used in the dehydration of methanol to DME, providing an alternative to traditional catalysts, especially in processes where water removal is critical. In addition to dehydration, solid acid catalysts are frequently used in esterification and transesterification reactions, where acidity is an important factor. These catalysts can be regenerated and are less susceptible to poisoning by organic compounds compared to metal catalysts, and they can also operate under milder conditions. However, catalytic activity may be lower compared to more traditional catalysts, such as y-alumina and WO3, and selectivity may be a concern in some reactions.

[0132] The resulting mixture from the first stage reaction is similarly pressurised to at least 5 bar, preferably form 10 to 30 bar, and heated to a temperature preferably of 600°C or more. The DME is then converted into hydrocarbons of varying chain lengths, generally ranging from Ci to C12, using zeolitic catalyst.

[0133] The production of hydrocarbons from DME is usually carried out on catalysts from the zeolite family. The most used is ZSM-5, a type of synthetic zeolite belonging to the family of medium-pore zeolites. This material is characterised by excellent catalytic properties, high thermal stability, and a well-defined structure composed of TO4 tetrahedral bonds, where the element T can be aluminium (Al), silicon (Si), or other elements. These properties make ZSM-5 particularly suitable for catalytic processes at high temperatures, such as the methanol-to-gasoline (MTG) conversion process, where its efficiency is directly related to its structural properties and the diffusion control of reactants and products within its pores.

[0134] ZSM-5 has a three-dimensional structure with 10-membered rings, typical of medium-pore zeolites like ZSM-11 and ZSM-48, and the zeolitic catalyst used in the second stage may be selected from these zeolites. However, ZSM-5 differs from these other materials due to the arrangement of itschannels, which include both straight and sinusoidal systems. The empirical formula of ZSM-5 is represented as NanAlnSi96-nOi92'H2O) , wherein n < 27. Another important feature is that the sodium cations (Na+) in ZSM-5 can be exchanged for other monovalent or divalent cations through ion exchange, altering properties like pore size and catalytic activity. The pore size of ZSM-5 typically ranges from 0.5 to 0.6 nm, limiting the diffusion of certain reactants and products within the zeolite. These factors, along with the Si / AI ratio (SAR), temperature, pressure, and catalyst preparation / synthesis, influence the catalyst’s selectivity.

[0135] When comparing ZSM-5 to ZSM-11 , there are significant differences in structural properties and the resulting product composition. ZSM-11 tends to produce fewer C1-C2 (light gases) and favours the formation of Ce+ aliphatic hydrocarbons and Cg-Cw aromatic hydrocarbons. In contrast, ZSM-5 produces an aromatic fraction predominantly composed of xylene. As a result, a resulting xylene to trimethylbenzene ratio may vary from about 2.5 for ZSM-5 (e.g. from 2 to 3) to about 0.5 for ZSM-11 (e.g. from 0.1 to 1). This difference is partly attributed to the differences in the channel intersections between the two zeolites. ZSM-11 has channel intersections that are about 30% larger than those of ZSM-5, which influences the formation of specific compounds.

[0136] Regarding combustion properties, the gasoline composition produced by ZSM-5 is closer to conventional gasoline, making it more suitable for internal combustion engines in terms of performance and emissions. On the other hand, ZSM-11 , with its composition rich in higher aliphatic and C9-C10 aromatic hydrocarbons, may be preferred when specific gasoline characteristics are desired, or to meet stricter environmental regulations.

[0137] Another significant distinction lies in the structural properties of zeolites. ZSM-11 exhibits linear, elliptical channels, whereas ZSM-5 has more tortuous channels, which directly impacts the diffusion of molecules and selectivity in the formation of products. The larger free space at the intersections of the channels in ZSM-11 facilitates the movement of larger molecules, affecting the formation of higher molecular weight hydrocarbons. ZSM-5, with its more restricted channels, favours the production of smaller aromatic products, such as xylene. ZSM-11 is thus a particularly preferred and beneficial catalyst for the reaction.

[0138] For the second stage conversion of DME into hydrocarbons in the C3+ range, three adiabatic reactors may be used, which may be plug-flow or fixed-bed type reactors, for example.

[0139] In a series arrangement, the first reactor in this system may be used to convert DME into ethylene and n-hexene, with an input feed temperature of from 850°C to 925°C (e.g. about 875°C) and a pressure of from 5 to 25 bar, achieving a conversion of essentially 100% for these products. A second reactor is used to convert the ethylene into benzene, toluene, and ethane with an input feed stream temperature of from 350°C to 450°C (e.g. about 400°C), typically achieving a conversion of up to 80%of the ethylene. Finally, a third reactor is used to produce n-hexane from the n-hexene (together with a make-up hydrogen stream). The n-hexene is hydrogenated to n-hexane under equivalent conditions as those used in the second reactor, typically with a similar conversion of up to 80% of the n-hexene. Alternatively, the reactors may be arranged in parallel, resembling three repurposed catalytic reforming packed-bed reactors, with one of them often in the catalyst regeneration phase. The same type of zeolitic catalyst may be used in all reactors.

[0140] Byproducts can include water, carbon monoxide, and carbon dioxide, as well as short-chain alcohols, aliphatic ethers, and short-chain alkanes. Given the desired final product is a synthetic fuel (i.e. e-fuel / gasoline), with typical hydrocarbon chains between C4 and C12, embodiments of the present invention generally comprise a step of refining the resulting gas stream which is performed in a gasliquid separator to remove the liquid byproducts and a light gas stream from the raw gasoline stream.

[0141] The liquid byproduct steam may comprise, or consist essentially of, DME, methanol and a majority by volume of water. By way of example, the liquid by product stream may comprise from about 50 to 70% v / v water, about 20 to 40% v / v DME, and about 5 to 20% v / v methanol. The liquid byproduct stream may be separated in one or more distillation columns to recover purified streams of DME, methanol and water. Typically, in a first distillation column, DME is separated first due to the higher volatility, and the remaining mixture may be separated in a second distillation column to separate methanol from the water.

[0142] In preferred embodiments, the light gas stream is further divided into two gaseous streams: the first a C2 stream, which is generally a majority ethane with the remainder H2, and the second a C2-depleted stream which may then be recycled into the second stage of the two-stage reaction process together with the DME. That is, it is preferred that the C2 stream is not recycled into the process, but is instead recovered as a product stream. Reducing the amount of ethane recirculated in the system results in lower olefin conversion in the reactor(s), which in turn increases the ethane content in the raw gasoline and alters the refining conditions. Consequently, the degree to which ethane is recycled can be controlled to yield hexane production near or at its maximum value and simultaneously minimise the compression requirements for the C2-depleted stream. The inventors have found that by recycling about half of the ethane present in the light gas stream (e.g. from 40% to 60%) is optimal for hexane production providing a more efficient synthetic fuel production process.

[0143] In one particular combination which advantageously increases the overall process efficiency to improve the scalability of the process, the water recovered from the reaction mixture by the gas-liquid separator may be used as the water in the preferred embodiment for the electrolytic production of hydrogen described hereinabove.In one particularly preferred embodiment, the method may also preferably further comprise recycling the COx recovered from the gas-liquid separator to the enzymatic treatment unit described hereinabove, further enhancing overall operational efficiency.

[0144] In one example, the typical weight percentage of the resulting product distribution is illustrated in Table 2, with the hydrocarbon product distribution illustrated in Tables 3 and 4. Each table ideally sums to 100%.

[0145] Table 2:

[0146] Products Amount (wt%)

[0147] Methanol + DME ~0

[0148] Hydrocarbons 30-50

[0149] Water 50-65

[0150] COx 0-2

[0151] C 0-2

[0152]

[0153] Table 3:

[0154] Hydrocarbons Amount (wt%)

[0155] Ci and C2 0-5

[0156] C31-10

[0157] C45-20

[0158] C5+65-94

[0159]

[0160] Table 4:

[0161] Hydrocarbons Amount (% v / v)

[0162] aromatics 15-40

[0163] olefins 8-15

[0164] alkanes 45-70

[0165]

[0166] Compounds like durene (C Hu; 1 ,2,4,5-tetramethylbenzene) can negatively impact fuel quality due to their physical properties (like its melting point), such as causing the formation of undesirable solids in gasoline, compromising its quality and performance. Durene can therefore be separated through a series of separation steps leading to its isomerisation, typically affording isodurene (1 ,3,4,5-tetramethylbenzene) and prehnitene (1 ,2,3,4-tetramethylbenzene) and / or hydrogenation, typically affording n-pentane. Thus, in some embodiments, the method further comprises passing the synthetic fuel through a hydroisomerisation and hydrogenation unit at a temperature of greater than 300°C (e.g.from 320°C to 380°C) in the presence of a precious metal catalyst, and optionally Hz, to reduce the concentration of the C10 hydrocarbon durene. Typically, the reaction pressure is at least 5 bar, such as from 5 to 20 bar.

[0167] The catalysts used in the hydroisomerisation of durene include precious metal catalysts and solid acid catalysts. Precious metal catalysts, such as platinum (Pt), palladium (Pd), and rhodium (Rh), are frequently employed in hydrogenation and isomerisation reactions. These metals are highly active and selective, enabling the conversion of durene into isodurene and prehnite. To increase surface area and catalytic activity, these metals can be supported on materials such as alumina or silica. The advantages of these catalysts include high activity, selectivity, and good stability under operational conditions. This refining process results in a gas stream with a low durene content ensuring the final gasoline quality without compromising the C10 content.

[0168] In order to ensure that a minimal durene content is achieved, hydrogenation of the remaining durene to n-pentane is ideal and can afford a synthetic fuel with less than 1 wt% durene. This reaction is generally conducted at a temperature of from 200 to 300°C (e.g. about 250°C) under a pressure of at least 5 bar, such as from 5 to 20 bar. This method is particularly beneficial since the combination of isomerisation and hydrogenation allows for the C10 and C5 content to be easily controlled thereby ensuring the desired product distribution in the final product for use as a synthetic fuel (which is subject to strict regulatory requirements) providing a further advantage.

[0169] In view of the foregoing description, another aspect of the present invention provides a corresponding system for use in the method of producing a synthetic fuel comprising C5+ hydrocarbons, the system comprising:

[0170] an electrochemical reactor comprising a cathode and an anode immersible in first and second aqueous electrolyte solutions, respectively, the cathode and anode being separated by a membrane, the reactor configured to introduce COx into the first aqueous electrolyte solution adjacent the cathode and introduce Hz into the second aqueous electrolyte solution adjacent the anode;

[0171] a first gas-liquid separator configured to receive a portion of the first aqueous electrolyte solution and recover a first liquid stream and a second gaseous stream;

[0172] a first distillation column configured to receive the first liquid stream and to recover a first methanol stream and an aqueous solution, the first distillation column configured to recycle the aqueous solution into the electrochemical reactor as further aqueous electrolyte solution;

[0173] a separation unit configured to receive the second gaseous stream and to recover a third gaseous stream comprising Hz and a fourth gaseous stream comprising COX;

[0174] an enzymatic treatment unit configured to receive the fourth gaseous stream and produce a second methanol stream using one or more oxidoreductase enzymes;a first reactor configured to receive a combination of the first and second methanol streams, the first reactor comprising a methanol dehydration catalyst for dehydrating methanol to provide a first reaction mixture comprising dimethyl ether;

[0175] a second reactor configured to receive the first reaction mixture, the second reactor comprising a zeolitic catalyst for dehydrating dimethyl ether to provide a second reaction mixture comprising C5+ hydrocarbons; and

[0176] a second gas-liquid separator configured to receive the second reaction mixture and recover the C5+ hydrocarbons therefrom.

[0177] Figures

[0178] The present invention will now be described further with reference to the following exemplary non-limiting Figures, in which:

[0179] Figure 1 is a cross-sectional schematic of an electrochemical cell which serves to illustrate the electrochemical reduction of COx in accordance with an embodiment of the method described herein.

[0180] Figure 2 is a flow diagram illustrating an embodiment of the electrochemical and enzymatic reduction process for the production of methanol.

[0181] Figure 3 is a flow diagram illustrating an embodiment of synthetic fuel production from methanol.

[0182] Figure 4 is a flow diagram illustrating an embodiment of synthetic fuel refinement.

[0183] Figure 1 is a cross-section illustrating the electrochemical reduction of COx in one example of a flowtype filter-press electrochemical reactor 100. The reactor 100 comprises a cathode 105 and an anode 110 separated by an ion-exchange membrane 115 (e.g. a Nafion-type membrane). Either side of the membrane 115 are two electrolytes, a catholyte 120 and anolyte 130 (which may have the same composition), the catholyte contacting one face of the cathode 105 and the anolyte contacting one face of the anode 110, and being fed by catholyte inlet 125 and anolyte inlet 135, respectively.

[0184] An electric potential difference is established between the cathode 105 and the anode 110 by a power supply (not shown) whilst COx 140 is introduced via a COx inlet 145 which feeds the COx 140 to the opposite face of the cathode 105 and diffuses into the catholyte 120. Simultaneously, H2150 is introduced via an H2 inlet 155 which feeds the H2150 to the opposite face of the anode and diffuses into the anolyte 130. The potential difference drives the electrochemical reduction of the COx to methanol at the catalytic site of the cathode 105, and the methanol is then recovered via the catholyteoutlet 160 and sent for separation as described further herein. The electrochemical reduction is advantageously enhanced by the oxidation of Hz 150 which takes place at the anode 110 providing a source of protons which migrate from the anolyte 130 across the ion-exchange membrane 115 and into the catholyte.

[0185] Figure 2 is a flow diagram illustrating an embodiment of the electrochemical and enzymatic reduction process for the production of methanol using system 200.

[0186] The system 200 comprises an electrochemical reactor 205, such as an electrochemical reactor as described in respect of Figure 1 above. The reactor 205 is fed by a first aqueous electrolyte 210 (catholyte) and a second aqueous electrolyte 215 (anolyte) whilst simultaneously being fed with a source of gaseous COx 220 and a source of gaseous Hz 225. The reactor 205 comprises a first outlet for receiving at least a portion of the catholyte as stream 230. The reactor 205 further comprises a second outlet for receiving at least a portion of the anolyte as stream 235.

[0187] Stream 230 is conveyed to a gas-liquid separation chamber 240 whereupon the catholyte is separated into a first liquid stream 245 and a second gaseous stream 250. The first liquid stream 245 is then conveyed to a first distillation column 255 whereupon the first liquid stream 245 is distilled to yield and recover a first methanol stream 260a and an aqueous solution 265 (which may then be recycled - as stream 265’ - into either the first and / or second aqueous electrolyte input streams 210, 215).

[0188] Returning to the second gaseous stream 250, this is conveyed to a separation unit 270 whereupon the stream 250 is separated to yield a third gaseous stream 275 comprising Hz which generated during the electrolysis at the cathode of the electrolytic reactor 205. The separation unit 270 further yields a fourth gaseous stream 280 comprising COx which remains unreacted from the feed stream 220. Preferably, the fourth gaseous stream 280 consists essentially of COx, and is then conveyed to an enzymatic treatment unit 285 configured to produce methanol from the unreacted COx using one or more oxidoreductase enzymes and a cofactor, such as NADH, introduced into the enzymatic treatment unit 285 as stream 290. The gaseous COx may be recycled within the enzymatic treatment unit (shown as stream 280’). A second methanol stream 260b is recovered from the enzymatic treatment unit 285, which may then be combined with the first methanol stream 260a, providing a combined methanol stream 260.

[0189] Figure 3 is a flow diagram illustrating an embodiment of synthetic fuel production from the combined methanol stream 260 using system 300a. The methanol stream 260 is conveyed to a fixed-bed type, adiabatic reactor 315, via a first pump 305a and then a heat exchanger 310 to heat the methanol stream 260, for example to a temperature of about 100°C and a pressure of about 10 bar. In the first stage reactor 315, the methanol stream 260 is dehydrated to dimethyl ether, which preferablyperformed using a tungsten oxide catalyst. A gas phase stream 320 may comprises greater than 95% v / v DME (or consist essentially of DME), whilst a liquid product stream 325 may also be recovered from the first stage reactor 315, which may consist essentially of unconverted methanol, DME and water, each of which may subsequently be recovered from the liquid product stream 325.

[0190] The DME stream 320 (which may be mixed with recycled gas stream 385 as discussed below) is conveyed to a compressor 330 and compressed, for example up to about 20 bar, and heated in a first heater 335a, such as to about 330°C, before being introduced into a first adiabatic reaction chamber 340a of the second stage reaction process. The first reactor 340a converts the DME predominantly into ethylene and n-hexene using a zeolitic catalyst, preferably ZSM-11. The reaction product stream is conveyed to a first cooler 345a before the second adiabatic reactor chamber 340b whereupon the ethylene is predominantly converted into benzene, toluene and ethane. The reaction product stream from the second reactor 340b is then combined with a first hydrogen gas stream 350a and conveyed to a third adiabatic reaction chamber 340c whereupon the n-hexene is hydrogenated under the same conditions as those used in the second reactor 340b.

[0191] The resulting product stream 355 is conveyed to the heated exchanger 310 to heat the incoming methanol stream 260 as described above and to cool the product stream 355 (which may be further cooled with a second cooler 350b if required). The now cooled product stream 355 is conveyed to a gas-liquid separator 360 to be separated into a raw synthetic fuel stream 365 consisting essentially of hydrocarbons (which may be depressurised via a valve 370 before further refining), and into a light gases stream 375 and an aqueous liquid stream 395.

[0192] The light gases stream 375 comprises (or consists essentially of) Ci to Ce hydrocarbons, preferably Ci to C3 hydrocarbons. The light gases stream is conveyed to a separator 380 in order to separate a C2 hydrocarbon stream 390 from a stream 385 comprising the remainder of the light gases stream 375. The stream 385 without C2 hydrocarbons, primarily ethane, may then be recycled into the second stage reaction process by mixing with the DME stream 320.

[0193] Figure 4 is a flow diagram illustrating an embodiment of synthetic fuel refinement using system 300b. The raw synthetic fuel stream 365 obtained from the flow described in Figure 3 is conveyed to a second distillation column 400 and separated into a durene-depleted hydrocarbon stream 405 and a durene-rich hydrocarbon stream 410. The durene-rich hydrocarbon stream 410 may comprise essentially all of the durene present in the raw synthetic fuel stream 365 (particularly since durene has especially low volatility) and will generally comprise a mixture of C2+ hydrocarbons, though a majority (by volume or by weight) is more typically Ce+. The durene-depleted stream 405 may consist essentially of from C2 to Ce hydrocarbons. The key aspect to the second distillation column 400 is the separation of durene.To meet the quality and performance requirements of gasoline, a unit for durene treatment is particularly preferred. Durene has a high melting point (Tmeiting = 79°C), meaning it can solidify at higher temperatures compared to other hydrocarbons. This characteristic can lead to the formation of undesirable solids in gasoline, compromising its quality and performance. To address this issue, the durene-rich hydrocarbon stream 410 is conveyed via a second pump 305b and a second heater 335b to a first reaction chamber 415a, such as at a temperature of about 350°C at about 10 bar. The durene present in the durene-rich hydrocarbon stream 410 undergoes isomerisation in the presence of a precious metal catalyst and is converted into a mixture of isodurene and prehnitene. The reaction mixture may then be mixed with a second hydrogen gas stream 350b and conveyed to a second reaction chamber 415b whereupon any remaining durene undergoes hydrogenation to produce C5 hydrocarbons, ultimately yielding a heavy hydrocarbon stream 420.

[0194] The durene-depleted hydrocarbon stream 405 conveyed to a third distillation column 425 to produce a first (lighter) hydrocarbon stream 430 (e.g. comprising 1 -hexene and n-hexane) and a second (heavier) hydrocarbon stream 435 (e.g. comprising n-hexane and benzene), the first hydrocarbon stream 430 comprising a greater volume of C2 to C4 hydrocarbons than the second hydrocarbon stream 435. A final synthetic fuel stream 445 may then be prepared by combining in a mixer 440, the desired quantities of streams 420, 430 and 435 since it may be desirable to reduce the C2 to C4 concentration for a final fuel product. The fuel stream 445 may then be cooled in a second cooler 345b before being sent for storage.

[0195] As used herein, the singular form of “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features and is also intended to include the option of the features necessarily being limited to those described. In other words, the term also includes the limitations of “consisting essentially of” (intended to mean that specific further components can be present provided they do not materially affect the essential characteristic of the described feature) and “consisting of” (intended to mean that no other feature may be included such that if the components were expressed as percentages by their proportions, these would add up to 100%, whilst accounting for any unavoidable impurities), unless the context clearly dictates otherwise.

[0196] It will be understood that, although the terms "first", "second", etc. may be used herein to describe, for example, various apparatus such as reactors or distillation columns, but should not be limited by these terms. These terms are only used to distinguish one apparatus from another.

[0197] Numerical lower and upper limits of features described herein may preferably be combined to provide a closed range.The foregoing detailed description has been provided byway of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.

[0198] For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference.

Claims

Claims:1 . A method for the production of a synthetic fuel comprising C5+ hydrocarbons, the method comprising:(i) providing a source of H2;(ii) providing a source of COX;(iii) providing an electrochemical reactor comprising a cathode and an anode, each immersed in a first and second aqueous electrolyte solution, respectively, the cathode and anode being separated by a membrane, and introducing the COx into the first aqueous electrolyte solution adjacent the cathode and introducing the H2 into the second aqueous electrolyte solution adjacent the anode;(iv) recovering a portion of the first aqueous electrolyte solution from the electrochemical reactor comprising electrochemically-produced methanol, H2 and COX;(v) passing the recovered first aqueous electrolyte solution through a first gas-liquid separator to recover a first liquid stream comprising methanol and water, and a second gaseous stream comprising H2 and COX;(vi) passing the first liquid stream through a first distillation column to recover a first methanol stream, and an aqueous solution which is recycled into the electrochemical reactor as further aqueous electrolyte solution;(vii) passing the second gaseous stream through a separation unit to recover a third gaseous stream comprising H2 and a fourth gaseous stream comprising COX;(viii) passing the fourth gaseous stream through an enzymatic treatment unit to recover a second methanol stream, wherein the enzymatic treatment unit converts the COXto methanol using one or more oxidoreductase enzymes;(ix) combining the first and second methanol streams and passing the combined stream to a two-stage reaction process to produce a synthetic fuel comprising C5+ hydrocarbons, wherein:(I) the first stage of the two-stage reaction process comprises dehydrating the methanol to dimethyl ether in the presence of a methanol dehydration catalyst; and(II) the second stage of the two-stage reaction process comprises heating the dimethyl ether under pressure in the presence of a zeolitic catalyst to produce a reaction mixture comprising water and the synthetic fuel; and(x) passing the reaction mixture from the second stage through a second gas-liquid separator to recover the synthetic fuel from the water and unreacted COX.

2. The method according to claim 1 , wherein the source of H2 comprises at least 95 vol% H2, and / or wherein the source of COXcomprises at least 95 vol% COX.

3. The method according to claim 1 or claim 2, wherein the source of H2 is produced by:electrolysing water in an electrolytic cell to produce hydrogen gas and oxygen gas, the electrolytic cell having a first outlet for hydrogen gas;passing the hydrogen gas from the first outlet of the electrolytic cell to a reaction chamber, the reaction chamber comprising a first inlet for receiving the hydrogen gas from the electrolytic cell and a second outlet for hydrogen gas passing out of the reaction chamber, the reaction chamber containing one or more pieces of a metal or an alloy thereof at least partially submerged in an alkali solution, wherein the first inlet is arranged so that the hydrogen gas bubbles through the alkali solution;passing the hydrogen gas from the second outlet to a gas-cleaning chamber, the gascleaning chamber comprising a second inlet for receiving hydrogen gas from the reaction chamber and a third outlet for hydrogen gas passing out of the cleaning chamber, the gascleaning chamber containing an aqueous solution, wherein the second inlet is arranged so that the hydrogen gas bubbles through the aqueous solution; andrecovering hydrogen gas from the third outlet.

4. The method according to any preceding claim, wherein a molar ratio of Hz to COx introduced into the aqueous electrolyte solution in step (iii) is at least 3:1.

5. The method according to any preceding claim, wherein the COx is bubbled through the electrochemical reactor at a flow rate of at least 10 mL / min.

6. The method according to any preceding claim, wherein the combined stream comprises at least 95 mol% methanol.

7. The method according to any preceding claim, wherein the cathode is made from porous carbon paper.

8. The method according to claim 7, wherein the cathode further comprises CuzO.

9. The method according to any preceding claim, wherein the anode is made of platinum-coated titanium.

10. The method according to any preceding claim, wherein the membrane is a perfluorinated ion exchange membrane, preferably a Nafion-type membrane.

11. The method according to any preceding claim, wherein the electrochemical reactor is operated at a temperature of from 10°C to 30°C, a pH of from 6 to 8, and a pressure of from 1 to 50 bar, preferably from 5 to 30 bar.

12. The method according to any preceding claim, wherein the electrochemical reactor is configured with flow-type filter-press cells, preferably wherein each cell has an area of at least 0.5 m2.

13. The method according to any preceding claim, wherein the first stage of the two-stage reaction process is conducted at a temperature of from 80°C to 150°C.

14. The method according to any preceding claim, wherein the second stage of the two-stage reaction process is conducted at a temperature of greater than 600°C.

15. The method according to any preceding claim, wherein the methanol dehydration catalyst is a tungsten oxide catalyst.

16. The method according to any preceding claim, wherein the zeolitic catalyst used in the second stage of the two-stage reaction process is ZSM-11.

17. The method according to any preceding claim, wherein one or both stages of the two-stage reaction process is conducted at a pressure of greater than 10 bar.

18. The method according to any preceding claim, wherein the enzymatic treatment unit converts the unreacted COx to methanol using NADH and / or NADPH as a cofactor.

19. The method according to any preceding claim, wherein the one or more enzymes are coimmobilised on a siliceous mesostructured cellular foam (MCF).

20. The method according to any preceding claim, wherein a blend of Formate dehydrogenase, Formaldehyde dehydrogenase, and Alcohol dehydrogenase are used in the enzymatic treatment unit in step (viii).

21. The method according to any preceding claim, wherein the water recovered in step (x) is used as the water for the production of hydrogen in the process of claim 3.

22. The method according to any preceding claim, wherein the method further comprises passing the synthetic fuel through a hydroisomerisation and hydrogenation unit at a temperature of greater than 300°C in the presence of a precious metal catalyst, and optionally Hz, to reduce the concentration of durene (1 ,2,4,5-tetramethylbenzene).

23. The method according to any preceding claim, further comprising recycling the COx recovered from the second distillation column in step (x) to the enzymatic treatment unit in step (viii).

24. A system for use in the method of producing a synthetic fuel comprising C5+ hydrocarbons according to any preceding claim, the system comprising:an electrochemical reactor comprising a cathode and an anode immersible in first and second aqueous electrolyte solutions, respectively, the cathode and anode being separated by a membrane, the reactor configured to introduce COx into the first aqueous electrolyte solution adjacent the cathode and introduce Hz into the second aqueous electrolyte solution adjacent the anode;a first gas-liquid separator configured to receive a portion of the first aqueous electrolyte solution and recover a first liquid stream and a second gaseous stream;a first distillation column configured to receive the first liquid stream and to recover a first methanol stream and an aqueous solution, the first distillation column configured to recycle the aqueous solution into the electrochemical reactor as further aqueous electrolyte solution;a separation unit configured to receive the second gaseous stream and to recover a third gaseous stream comprising Hz and a fourth gaseous stream comprising COX;an enzymatic treatment unit configured to receive the fourth gaseous stream and produce a second methanol stream using one or more oxidoreductase enzymes;a first reactor configured to receive a combination of the first and second methanol streams, the first reactor comprising a methanol dehydration catalyst for dehydrating methanol to provide a first reaction mixture comprising dimethyl ether;a second reactor configured to receive the first reaction mixture, the second reactor comprising a zeolitic catalyst for dehydrating dimethyl ether to provide a second reaction mixture comprising C5+ hydrocarbons; anda second gas-liquid separator configured to receive the second reaction mixture and recover the C5+ hydrocarbons therefrom.

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