Photochemical process for producing alcohols

Abstract

The invention relates to a photochemical process for selective production of alcohol from CO2 and a hydrogen source, wherein: CO2, a hydrogen source, O2 and a photosensitizer are brought into contact with each other; the photosensitizer is capable of converting O2 to a reactive oxygen species in the presence of light; and light is added to CO2, the hydrogen source, O2 and the photosensitizer until the reaction of CO2 with the hydrogen source to form alcohol is completed.

Description

[0001] Photochemical process for the production of alcohols

[0002] The present invention relates to a photochemical process for producing alcohols from CO2 and water. Furthermore, the invention relates to the use of a photosensitizer for the photochemical production of alcohols. Finally, the invention relates to a system for the photochemical reduction of CO2 to produce alcohols.

[0003] Background of the invention

[0004] Alcohols are among the most widely produced organic chemicals. They are important starting materials and intermediates in the chemical industry and can be used as energy carriers. The simplest alcohols are methanol and ethanol. With the help of fuel cells, methanol can provide electrical energy. Alcohol can also act as a chemical (long-term) storage medium for solar and wind energy, using solar and wind energy to reduce CO2 emissions (power to liquid).

[0005] Converting the greenhouse gas CO2 into energy carriers using water and sunlight would be an ideal way to mitigate the effects of climate change and ensure a supply of sustainable raw materials. Although such sustainable photocatalytic conversion has been intensively researched worldwide for more than 30 years, no system is yet known that can achieve conversions sufficient for industrial application. Controlling the selectivity toward specific products, such as methanol or longer-chain hydrocarbons, has also not been possible to date (D. Krämer, A. Bazzanella, "Material Use of CO2: Motivation, Challenge, Outlook in "Results of the BMBF Funding Measure Technologies for Sustainability and Climate Protection - Chemical Processes and Material Use of CO2" Ed.: DECHEMA Society for Chemical Engineering and Biotechnology eV 2017, Section 1).11 - PhotoKat - Development of active and selective photocatalysts for the reduction of CO2 to Ci basic chemicals).

[0006] There are various systems for reducing CO2 to produce more energy-rich Ci or C2 compounds, which are summarized, for example, in Song et al., "Advances in Clean Fuel Ethanol Production from Electro-, Photo- and Photoelectro-Catalytic CO2 Reduction"; Catalysts 2020, 10, 1287. In addition to electroreduction and photoelectroreduction, photoreduction is a way to produce solar fuels from CO2.

[0007] Known systems for the photoreduction of CO2 to produce Ci or C2 compounds comprise a light source, a photocatalyst, and an electron donor that is consumed during the reaction (Song et al.). Water is the simplest electron donor, although alternative electron donors are also used as sacrificial reagents. In addition to the undesirable consumption of alternative electron donors or sacrificial reagents, known processes for the photoreduction of CO2 require a semiconductor, metal-semiconductor, or semiconductor / metal hybrid catalyst.

[0008] US 10,047,027 B1 describes a process for the formation of methanol by irradiating a reaction mixture comprising H2O, CO2, and a semiconductor photocatalyst with UV light. Exposure to light creates an electron-hole pair in the semiconductor photocatalyst. Holes in the semiconductor's conduction band then oxidize water to O2 and H + . H +Valence band electrons and CO2 react to form methanol and H2O. US Pat. No. 8,986,511 B1 describes a similar process in which CO2 is reduced in the presence of H2O and a photosensitizer, using semiconductors. The catalysts used in the described processes are complex to manufacture and contain metals classified as harmful to health, which also represent valuable resources. For example, the same materials can also be used in solar cells, but are not widely used there due to their toxicity and cost.

[0009] WO 2017 / 091857 A1 describes a process for producing hydrocarbons such as methane or substituted hydrocarbons such as methanol. In the presence of light, a catalyst is brought into contact with H2O and CO2 to catalyze (i) the splitting of H2O into O2 and H2 and (ii) the reaction of H2 with CO2. The catalyst comprises Au and the highly oxidation-sensitive Ru in a supported nanocluster. The reaction conditions are comparatively harsh (high pressure of 20 Torr H2O, Ar atmosphere of 280 Torr). The system appears susceptible to contamination and is resource-intensive in several respects.

[0010] WO 2013 / 175311 A2 describes a process for producing methanol from CO2 in the presence of light, a ruthenium polypyridine catalyst, and a cocatalyst. The ruthenium catalyst is reduced under the influence of light, accepting electrons from (alternative) electron donors. The reduced catalyst forms an adduct with CO2 and H +and is oxidized in the process. The [-CO2-H] adduct is taken over by a co-catalyst and further reduced with its help to form the final product. The oxidized catalyst must be reduced again. The described co-catalyst contains Co. In other words, the process uses metals as catalytically active centers and is dependent on a co-catalyst, which means additional effort. JPS5988436 A describes the use of metal-porphyrin complexes to produce methanol from CO. The metalloporphyrin complex is oxidized and must then be reduced again. For this purpose, a Pt catalyst and H2 are used as co-catalyst. The process uses metals as catalytically active centers, including a costly co-catalyst.

[0011] The object of the present invention is therefore to provide a simplified system for the photoreduction of CO2, which avoids the use of potentially toxic and rare metals, and which can selectively produce specific short-chain alcohols (Ci to C10, preferably ethanol or methanol) with high yield in a manner as simple, resource-saving and economical as possible.

[0012] The inventors have surprisingly found that a selective reduction of CO2 to alcohol can be achieved using light, a photosensitizer capable of converting oxygen (O2) into reactive oxygen species in the presence of light, and a hydrogen source.

[0013] The above object is achieved according to the invention by a photochemical process for producing alcohol from CO2 and a hydrogen source, in which CO2, a hydrogen source, O2 and a photosensitizer are brought into contact with one another, wherein the photosensitizer is capable of converting O2 into a reactive oxygen species (ROS) in the presence of light, and wherein CO2, the hydrogen source, O2 and the photosensitizer are contacted with light until the reaction of CO2 with the hydrogen source to form the alcohol is complete.

[0014] Any compound capable of converting O2 into reactive oxygen species (ROS) under the influence of light is suitable as a photosensitizer. The photosensitizer is preferably an organic or organometallic compound, particularly preferably an organic compound. Such compounds are known to the person skilled in the art from the literature. These include, for example, porphyrins and other compounds, which will be discussed further below.

[0015] Reactive oxygen species (ROS) is a collective term known to those skilled in the art for highly reactive oxygen compounds - such as singlet oxygen 1O2 (open-shell and / or closed-shell) or the superoxide ion O2 - although this list is not exhaustive. They are short-lived and difficult to detect, which is why, in the absence of more precise information, the term reactive oxygen species or reactive oxygen species (ROS) is commonly used. The person skilled in the art generally understands this to mean one or more reactive forms of oxygen that differ from conventional, comparatively inert triplet oxygen. 3 O2. Reactive oxygen species can be formed by activating ordinary triplet oxygen 3 Ü2 can be produced in a resource-saving manner using a photosensitizer under the influence of light.

[0016] Alcohols that are used are those with C n H2n+iOH with n < 10, preferably with n < 3, particularly preferably with n = 1 or n = 2.

[0017] Preferably, the hydrogen source in the process according to the invention is H2O.

[0018] H2O is not only the most cost-effective but also the easiest to handle hydrogen source. Providing hydrogen eliminates the need for alternative, potentially expensive sacrificial reagents as a hydrogen source. The use of water also has the attractive effect of removing two products of fossil fuel combustion from the natural cycle while reducing CO2.

[0019] Preferably, the light in the method according to the invention has a wavelength of 200 nm to 800 nm.

[0020] This range extends from ultraviolet to infrared radiation and primarily includes the range of visible light.

[0021] Preferably, processes according to the invention are carried out at a temperature of 10 °C to 100 °C, preferably 20 °C to 80, 70 or 60 °C, more preferably 20 °C to 30 °C, and at a pressure of 0.5 to 2 bar.

[0022] Such a process proves to be particularly resource-efficient, as it takes place at almost normal pressure and at comparatively low temperatures.

[0023] It is further preferred that, in the process according to the invention, O2 is dissolved in water and treated with light. Preferably, CO2 is also dissolved in water and treated with light.

[0024] According to a preferred first embodiment of the process according to the invention, the alcohol is ethanol. According to a further preferred second embodiment of the process according to the invention, the alcohol is methanol.

[0025] Preferably, the first embodiment of the method according to the invention is characterized in that

[0026] - both O2 and CO2 are dissolved in water and mixed with light,

[0027] - the alcohol is ethanol, where - the molar concentration of CO2 in H2O is between 0.02 and 0.04 mol / L, preferably about 0.023 mol / L. This has proven advantageous for the production of ethanol. The quantitative fraction of O2 in H2O in the process according to the invention according to the first embodiment is preferably between 2.4 and 4.7 ppm.

[0028] Preferably, the second embodiment of the method according to the invention is characterized in that

[0029] - both O2 and CO2 are dissolved in water and mixed with light,

[0030] - the alcohol is methanol, where

[0031] - the molar concentration of CO2 in H2O is between 0.005 and 0.015 mol / L, preferably approximately 0.01 mol / L. This has proven advantageous for the production of methanol. The quantitative fraction of O2 in H2O in the process according to the invention according to the second embodiment is preferably between 3.2 and 4.0 ppm.

[0032] In a preferred embodiment, the alcohol is ethanol. It has been shown that this process can be used to produce ethanol particularly selectively and with good yield. Selective means that the ratio of the desired alcohol to other alcohols is at least 90 mol%. This means that, based on the total amount of alcohols produced, at least 90 mol% is ethanol and a maximum of 10 mol% is other alcohols. The selectivity can be adjusted, in particular, via the ratio of hydrogen source to CO2.

[0033] In one embodiment, the hydrogen source is H2O. This is not only the most cost-effective but also the easiest to handle hydrogen source.

[0034] For a process for producing ethanol, it has proven advantageous if the amount of CO2 in H2O is 0.02 to 0.04 mol / L, preferably about 0.023 mol / L.

[0035] For a process for producing methanol, it has proven advantageous if the amount of CO2 in H2O is 0.005 to 0.015 mol / L, preferably about 0.01 mol / L H2O.

[0036] Preferably, the light has a wavelength of 200 nm to 800 nm.

[0037] Surprisingly, the process conditions do not require high temperatures or pressures. The reaction can be carried out at temperatures ranging from 10 °C to 100 °C, preferably 20 °C to 30 °C, and at pressures ranging from 0.5 to 2 bar.

[0038] As already mentioned above, any compound capable of converting O2 into reactive oxygen species (ROS) under the influence of light is suitable as a photosensitizer. The photosensitizer is preferably an organic or organometallic compound, particularly preferably an organic compound. Such compounds are known to the person skilled in the art from the literature. These include, for example, porphyrins and other compounds called ROS. The photosensitizer is preferably selected from the group consisting of:

[0039] Porphyrins, chlorines, BODIPYs (dipyrromethenboron difluoride 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene and its derivatives), psoralen, fullerenes (Ceo, C70), hypocrellin, cercsporin, methyl violet, rose bengal, methylene blue, eosin blue, Ru(bpy)3 2+ , Ru(phen)3 2+, titanium dioxide, perovskites, halogenated xanthenes, fluoroscein, eosin Y, acetylformic acid, rylenes such as perylene or perylenediimide, PCN-224, PCN-224(H),

[0040] , , , , , , , , , where M = H, Fe, Pd, Pt, Fe, Zn, Ni, Co, Mn, Mg or Cu, where M = H, Fe, Pd, Pt, Fe, Zn, Ni, Co, Mn, Mg or Cu, where the radicals R and R 1 , if present in the compounds according to the above formulas, independently of one another represent a phenyl, phenylcarboxylic acid, hydroxyphenyl or pyridinyl radical, and where R 2 , if present in the compounds according to the above formulas, represent a phenyl, phenylcarboxylic acid, hydroxaphenyl, pyridinyl or Ci-Cio-alkyl radical.

[0041] More preferably, the photosensitizer is selected from a PCN-224. Most preferably, the photosensitizer is PCN-224(H).

[0042] BODIPYs are understood by the person skilled in the art as: BODIPY, ie 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene and derivatives thereof in which one or more hydrogen atoms of BODIPY are replaced by other functional groups.

[0043] PCN-224 nanoparticles are generated by coordinating tetrakis(4-carboxyphenyl)porphyrin (TCPP) ligands to zirconium. The H in PCN-224(H) indicates that no metal is present in the porphyrin center. The metal in PCN-224 can be, among others: M = H, Fe, Pd, Pt, Fe, Zn, Ni, Co, Mn, Mg, or Cu.

[0044] Photosensitizers may, but do not necessarily have to, contain a central atom M that is a noble metal or metal.

[0045] Finally, the object is achieved by a system for the photochemical reduction of CO2 to produce alcohol, comprising a CO2 source, a hydrogen source, an O2 source, at least one photosensitizer and a reactor in which CO2, O2 and the photosensitizer can be brought into contact, wherein the photosensitizer is an organic or organometallic compound and is capable of converting O2 into a reactive oxygen species (ROS) in the presence of light, wherein a light source is further provided with which the reactor can be irradiated.

[0046] Photosensitizers as mentioned above are preferably used.

[0047] Detailed description of the invention

[0048] The invention is explained in detail below using examples, figures and the description of the figures.

[0049] Fig. 1 shows the reaction progression of a process according to the invention. Fig. 1a shows the reaction progress using FTIR spectroscopy. Fig. 1b shows the increase in ethanol (EtOH) and the decrease in CO2 during the reaction. Fig. 1c shows the comparison of the CO2 concentration in the presence and absence of O2 during the reaction. Fig. 1d shows the reproducibility of the formation of EtOH using this method.

[0050] Fig. 2 shows NMR spectroscopic investigations of the reaction products according to a process according to the invention, wherein in Fig. 2a the 1 H-NMR spectrum (400MHz, d-DMSO) of formed E- tOH, in Fig. 2b the comparison of the 1 H-NMR spectrum (400MHz, d-DMSO) of EtOH (bottom) and 13 C2- EtOH (top) and in Fig. 2c the 1 H-NMR spectrum (400MHz, d-DMSO) of 13 C2-EtOH is shown.

[0051] Fig. 3 shows the reaction course according to a method according to the invention, wherein Fig. 3a shows the reaction course with Direct Air Capture (DAC) after 1 h and 2 h and Fig. 3b shows the comparison of the reaction with a 0.1 M ethanol reference.

[0052] In the detailed investigations, porphyrin systems were used as photosensitizers. The photochemical process was carried out in a closed container (5 mL) with side illumination. For all experiments, 16 mg of photoactive material PCN-224 was used per mL of reaction medium (H2O). The progress of the photochemical reaction was monitored using FTIR spectroscopy.

[0053] The reaction of CO2 to ethanol was investigated using reactive oxygen activation. Initially, the reaction was monitored using FTIR spectroscopy, pH measurement, and determination of dissolved oxygen (DO) and temperature (see Table 1). Table 1 summarizes pH, temperature, and dissolved oxygen levels during the investigations. The resulting FTIR spectra (shown in Fig. 1 a) show that there is a rapid production of EtOH (recognizable by the increase of the v(CO) band at 1044 cm' 1 ) upon exposure of PCN-224 to simulated solar irradiation (white light, LED, 10 mW) in a CO2-saturated aqueous solution (pH: 4.2; DO: 2.7 ppm; temp.: 22.6 °C) under atmospheric conditions (1 atm). The reaction progress was monitored by FTIR spectroscopy over 5 h. The increase in EtOH concentration is accompanied by CO2 consumption, which is evident in the decrease of the v(C=O) band of CO2 at 2344 cm'1 is visible (Fig. 1 b). After a reaction time of only 1 h, the formation of ethanol was observed and a plateau was reached after 2 h (Fig. 1 a). The comparison of the CO2 decrease in the presence and absence of O2 further confirms the involvement of a reactive O2 species in the CO2 conversion. In the presence of O2, 1.75 mmol / L CO2 was consumed, whereas in the absence of O2 the CO2 concentration only decreased by 0.26 mmol / L (Fig. 1 c), which, however, was not due to a reaction of CO2 but to the outgassing of CO2.

[0054] To ensure that only the CO2 present in the solution was converted, the photosensitizer was treated with water overnight to remove any CO2 absorbed by the photosensitizer. The concentration of dissolved CO2 was determined, and the photosensitizer was added to the reaction mixture. The reaction was allowed to proceed for 5 hours.

[0055] The area of ​​the dominant v(C=O) band was used to determine the ethanol concentration in the reaction mixture. The initial CO2 concentration was 23.2 mmol / L, which decreased to 1.1 mmol / L over 5 hours. At the same time, 0.47 mmol / L of EtOH was formed within the first hour, which increased to 8.9 mmol / L after 2 hours and to 10.4 mmol / L after 5 hours of reaction time (Fig. 1 a). This results in a very high CO2-to-ethanol conversion efficiency (CTE) of 90%. IR spectroscopy allows not only quantitative evaluation but also stoichiometric investigation of CO2 activation and conversion. In addition, reproducibility experiments were conducted using 16 mg of photoactive material / ml of reaction medium. The formation of EtOH can be seen from the six reproducibility experiments in Fig. 1 d.The average concentration of EtOH formed during these experiments is 10.73 mM with a standard deviation of 0.94 (Table 2).

[0056] Table 2: Reproducibility experiments.

[0057] Average concentration: 10.73 mM; standard deviation: 0.94. To determine the species potentially involved in the reaction, exclusion experiments were performed: No alcohols were formed (under the same conditions) in the absence of light (pH 4.1; DO: 2.3 ppm; temperature: 22.2 °C) or in the absence of the photosensitizer (pH 4.0; DO: 2.7 ppm, temperature: 22.3 °C). These results rule out the possibility of significant reduction / oxidation of impurities acting as sacrificial reagents. Furthermore, no products were detected in control experiments conducted in aqueous solutions without CO2 (pH: 5.9; DO: 4.4 ppm; temp.: 22.7 °C) or without O2 (purged with N2 for 2 h, pH: 4.2; DO: at the sensor's detection limit of 0.1 ppm; temp.: 22.5 °C).

[0058] These observations indicate that the detected EtOH was generated by photochemical CO2 conversion and not by decomposition of the photocatalyst, and simultaneously highlight the catalytic activity of PCN-224 for solar fuel production. To demonstrate that the photosensitizers, and thus the ROS, are the essential component of the photocatalyst, a so-called blank material was synthesized in which the photosensitizer was replaced with terephthalic acid. Here, too, no product formation was observed under the reaction conditions (pH: 4.0; DO: 2.7 ppm; temperature: 22.3 °C).

[0059] This confirmed the involvement of ROS in the CO2 conversion reaction and the absence of any decomposition products. The formation of ethanol was also confirmed by 1 H-NMR (Fig. 2a). Furthermore, isotope labeling experiments with 13 CO2 the formation of 13C2-EtOH by its characteristic 1 H- 13 C splitting pattern of a doublet at 1.04 ppm compared to the usual triplet visible under atmospheric conditions (Fig 2b and Fig. 2c).

[0060] Photochemical reaction via Direct Air Capture (DAC):

[0061] In one experimental setup, the photosensitizer was also exposed to direct air capture (DAC) conditions. In this process, CO2 is used directly from the ambient air as a reactant without additional saturation or addition of CO2. After the photosensitizer had adsorbed CO2 from the ambient air for 2 hours, distilled water was added. As shown in Fig. 3, rapid EtOH production occurred when PCN-224 was exposed to simulated solar irradiation (white light, LED, 10 mW) in aqueous solution (pH: 5.3; DO: 4.3 ppm; temp.: 22.8 °C) under atmospheric conditions (1 atm). Significant ethanol formation occurred after just 1 h of reaction time. The produced ethanol concentration was 16.1 mmol / L. This is a significantly higher concentration than in the experiments with dissolved CO2, suggesting that the solubility of CO2 in water is the limiting factor of the previous reaction.Under the same conditions, no products were obtained in the absence of light (pH: 5.4; DO: 4.4 ppm; temp.: 22.6 °C) or the photocatalyst (pH: 5.4; DO: 4.3 ppm; temp.: 22.7 °C). These results again rule out the possibility of significant reduction / oxidation of impurities acting as sacrificial reagents. Furthermore, no products were detected in control experiments conducted in aqueous solutions with the blank material (pH: 5.3; DO: 4.2 ppm; temp.: 22.6 °C). These experiments demonstrate the efficiency and flexibility of this method and the potential of combining DAC with dissolved CO2.

[0062] In summary, a pathway for the activation and conversion of CO2 in both dissolved and gaseous form via ROS was investigated. The method proves to be highly efficient, converting CO2 to ethanol within 2 hours under atmospheric conditions and at room temperature. Using CO2-saturated water, 10.4 mmol / L ethanol solutions were generated, corresponding to a complete conversion efficiency (CTE) of 90%. This performance is achieved by the combined high selectivity and high activity of the noble metal-free catalyst. The system exhibits high stability during the photosynthetic reaction without the need for sacrificial reagents. Furthermore, DAC measurements yielded 16.1 mmol / L EtOH, demonstrating the potential to convert both dissolved and gaseous CO2 from atmospheric conditions.This demonstrates both the inherent flexibility of this process and the efficiency that can be further increased by increasing the CO2 concentration.

[0063] With appropriate ratios between CO2 and water, methanol can also be produced in a similar way, so that longer-chain alcohols up to C10 can also be produced under appropriate conditions.

[0064] Materials and methods:

[0065] Unless otherwise stated, all chemicals were purchased from Sigma-Aldrich and used without further purification. 5,10,15,20-(Tetra-4-carboxyphenyl)porphyrin (CAS: 14609-54-2) was purchased from PorphyChem and used without further purification. Reactions were performed in 5 ml transparent tamper-evident sample tubes purchased from Sigma-Aldrich (Z679038).

[0066] Photosensitizer PCN-224: The nanomaterial PCN-224(H) was used as the photosensitizer. This is a well-known porphyrin-containing metal organic framework (MOF) compound prepared by coordinating tetrakis(4-carboxyphenyl)porphyrin (TCPP) ligands with zirconium; the H indicates that the porphyrin unit does not contain a metal center.

[0067] Photoinactive material / blank material. UiO-66 was used as a photoinactive nanomaterial without a photosensitizer. UiO-66 is also a well-known metal-organic framework compound.

[0068] Photochemical experiments: Photosynthetic reactions were carried out in a closed plastic container with side illumination by Paulmann MaxLED Tunable White 70623 with 2 mW radiant flux and 870 lm luminous flux. For all experiments, 16 mg of photoactive material PCN-224(H) was used per ml of reaction medium. Samples were taken at several time points for FTIR spectroscopic measurements and injected into a flow cell. The reactions were carried out at room temperature and under atmospheric conditions without stirring. The CO2 solutions were prepared by saturating distilled water with CO2 gas. The O2-free solution was prepared by saturating distilled water with N2 and then adding CO2 gas. 13 CO2 solutions were prepared by mixing distilled water with 13CO2 gas was saturated. The various gases were passed into wash bottles containing distilled water. The wash bottles were placed in a temperature-controlled water bath at 20 °C. The saturated CO2 water was then diluted with distilled water (1:1; v / v), resulting in 4–4.2 ppm dissolved O2. The photoactive material was added to 5 ml transparent, tamper-evident sample tubes before the water, sealed, and exposed to irradiation. To perform the exclusion experiments without light, the sample tubes were wrapped with aluminum foil and stored in the dark to avoid any contact with light. To perform the exclusion experiments without CO2, only distilled water was added to the reaction. To perform the exclusion experiments without O2, distilled water was degassed with N2 for 2 h before the CO2 was added.Dissolved O2 was measured with a Voltcraft oxygen sensor to ensure the absence of O2. Furthermore, the container was purged with N2 before adding the photoactive material and water, and a protective layer of nitric oxide was added after adding the photosensitizer and water. To conduct the blank material exclusion experiment, UiO-66 was used instead of the photoactive material PCN-224.

[0069] IR measurements: The setup for the IR measurements is based on a previously published setup (S. Freitag et al., Lab Chip, 2021 , 21 , 1811-1819).

[0070] NMR measurements: The 1 H-NMR spectrum was recorded on a Bruker AVANCE 400 spectrometer.

[0071] The invention is the result of work carried out at the COMET Center CHASE, which is funded by the Federal Ministry of Education, Research and Culture (BMK), Federal Ministry of Health (BMDW), and the federal states of Upper Austria and Vienna as part of COMET - Competence Centers for Excellent Technologies. The COMET program is administered by the Austrian Research Promotion Agency (FFG).

[0072] This application claims priority from Austrian patent application No. A 50189 / 2022.

Claims

Claims Photochemical process for the selective production of alcohol from CO2 and a hydrogen source, wherein CO2, a hydrogen source, O2, and a photosensitizer are brought into contact with each other, wherein the photosensitizer is capable of converting O2 into a reactive oxygen species in the presence of light, and wherein CO2, the hydrogen source, O2, and the photosensitizer are exposed to light until the reaction of CO2 with the hydrogen source to form the alcohol is complete. Photochemical process according to claim 1, characterized in that the hydrogen source is H2O. Photochemical process according to claim 1 or claim 2, characterized in that the light has a wavelength of 200 nm to 800 nm. Photochemical process according to any one of claims 1 to 3, characterized in that the reaction is carried out at a temperature of 10 °C to 100 °C, preferably 20 °C to 30 °C, and a pressure of 0.5 to 2 bar.A photochemical process according to any one of claims 2 to 4, characterized in that O₂ is present in solution in water and is exposed to light. A photochemical process according to any one of claims 2 to 5, characterized in that CO₂ is present in solution in water and is exposed to light. A photochemical process according to claim 6, characterized in that the alcohol is ethanol, wherein the molar concentration of CO₂ in H₂O is between 0.02 and 0.04 mol / L, preferably about 0.023 mol / L. A photochemical process according to claim 7, characterized in that the molar fraction of O₂ in H₂O is between 2.4 and 4.7 ppm. A photochemical process according to claim 5, characterized in that the alcohol is methanol, wherein the molar concentration of CO₂ in H₂O is between 0.005 and 0.015 mol / L, preferably about 0.01 mol / L.

10. Photochemical process according to claim 9, characterized in that the proportion of O₂ in H₂O is between 3.2 and 4.0 ppm.

11. Photochemical process according to any one of claims 1 to 10, characterized in that the photosensitizer is selected from the group consisting of porphyrins, chlorins, BODIPYs, psoralens, fullerenes (Ceo, C70), hypocrellin, cercsporin, methyl violet, rose bengal, methylene blue, eosin blue, Ru(bpy)₃ 2+ , Ru(phen)3 2+ , Titanium dioxide, perovskites, halogenated xanthenes, fluoroscein, eosin Y, acetylamic acid, rylenes such as perylene or perylenediimide, PCN-224, PCN-224(H), , where M = H, Fe, Pd, Pt, Fe, Zn, Ni, Co, Mn, Mg or Cu, where R and R 1 If present, they can independently mean: phenyl, phenylcarboxylic acid, hydroxyphenyl or pyridinyl residue, and where R 2"If present" means: phenyl, phenylcarboxylic acid, hydroxaphenyl, pyridinyl, or Ci-Cio alkyl group. System for the photochemical reduction of CO2 to produce a solar fuel, comprising a CO2 source, a hydrogen source, an O2 source, at least one photosensitizer, and a reactor in which CO2, O2, a hydrogen source, and the photosensitizer can be brought into contact, further comprising a light source with which the reactor can be irradiated. System according to claim 12, characterized in that the photosensitizer is defined as in claim 11.

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