Process for preparing alcohol from carbon dioxide by reduction in the presence of a photosensitizer

Abstract

The invention relates to a process for reducing CO2 in the presence of O2, of a sacrificial reagent that functions as electron donor and of a hydrogen source, in which reactive oxygen species (ROS) are generated, and in which CO2 is reduced to alcohol, characterized in that, under the action of light, by means of a photosensitizer, at least one reactive oxygen species (ROS) is generated, and the photosensitizer includes lignin, lignocellulose and / or tannin, each of which may be functionalized.

Description

[0001] Process for producing alcohol from carbon dioxide

[0002] BY REDUCTION IN THE PRESENCE OF A PHOTOSENSITISER

[0003] This application claims priority from Austrian patent application No. A 60118 / 2022 and German patent application No. 10 2023 118 732.5.

[0004] The present invention relates to processes for the photochemical reduction of CO2 with at least one reactive oxygen species (ROS) using photosensitizers based on lignin, lignocellulose and / or tannin, photosensitizers and their use for generating reactive oxygen species.

[0005] Photochemical reactions occur via excited electron states caused by light absorption.

[0006] The term photosensitizer has the same meaning in this application as the term photocatalyst. A photocatalyst or photosensitizer refers to a compound that accelerates a reaction under the influence of light or even enables it in the first place. More specifically, in this application, a photosensitizer refers to a compound that is excited under the influence of light, whereby the energy gained by the excitation is transferred to another compound, thereby returning the photosensitizer to its ground energetic state. This process can be repeated because the photosensitizer is not consumed in the process.

[0007] Global warming, caused by the accelerated accumulation of atmospheric carbon dioxide (CO2), poses a major problem for humanity. CO2 is a greenhouse gas that makes up 0.04% of the atmosphere (as of 2019) and is said to contribute between 9 and 26% to the natural greenhouse effect. Global anthropogenic CO2 emissions are in the double-digit gigatons and account for the majority of the additional human-caused greenhouse effect. The greenhouse effect, the depletion of natural fossil fuels, and the growing energy demand have triggered worldwide efforts to research sustainable, environmentally friendly, and economically viable solutions to these problems.According to the Intergovernmental Panel on Climate Change, limiting global temperature rise to 1.5 degrees Celsius will require some form of carbon capture technology. However, simply capturing and storing CO2 will not be sufficient. This will require new technologies.

[0008] Although solar energy is one of the most promising renewable energy sources, its current applications have not yet reached their full potential. The fact that solar energy is not constantly available on demand and the disadvantages associated with its storage in terms of the resources required and currently used are just some of the reasons why it cannot be considered a viable alternative to conventional fossil fuels. A major challenge is to both efficiently convert solar energy and store it appropriately. Storing this energy source in the form of chemically bound energy appears very promising, as it allows large amounts of energy to be stored easily and efficiently transported. So-called solar fuels have gained importance over the last decade.Previous research suggests that CO2 can be used as a valuable feedstock for the continuous and environmentally friendly production of chemicals and solar fuels.

[0009] Several processes for producing alcohol from CO2 are known in the art, including photochemical processes. Known systems for the photoreduction of CO2 to obtain Ci or C2 compounds comprise a light source, a photocatalyst, and an electron donor that is consumed in the reaction. Water is the simplest electron donor, although alternative electron donors are also used as sacrificial reagents. However, the consumption of electron donors or sacrificial reagents other than water is not desirable because they are not as widely available as water and also represent additional resources. Furthermore, most known processes for the photoreduction of CO2 require a semiconductor, metal-semiconductor, or semiconductor / metal hybrid catalyst.

[0010] For example, US Patent No. 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 + . Formal reaction H + , valence band electrons, and CO2 to 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 are complex to produce and contain rare earth elements classified as harmful to health, or metals that represent valuable resources. For example, the same materials can also be used in solar cells, but have not been successful there due to their toxicity and cost.

[0011] 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.

[0012] 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. The [-CO2-H] adduct is taken over by a co-catalyst and further reduced to 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 relies on a co-catalyst, which requires additional effort.

[0013] JPS5988436 A describes the use of metallaporphyrin complexes to produce methanol from CO. The metallaporphyrin complex is oxidized and then reduced again. A platinum catalyst and hydrogen are used as a co-catalyst. This makes the process complex.

[0014] State-of-the-art photochemical processes therefore utilize photocatalysts based on semiconductor materials or redox catalysts in the presence of co-catalysts. The catalyst, activated by light, formally provides electrons for reduction, which ultimately must be provided by an electron donor. Sacrificial reagents act as electron donors in the state-of-the-art technology, as described, for example, in Y. Pellegrin, F. Odobel, Sacrifical electron donor reagents for solar fuel production, Comptes Rendus Chimie (2016), http: / / dx.doi.org / 10.1016 / j.crci.2015.11.026. The photocatalysts used generally require the use of rare earth elements or other valuable metals. They often require comparatively complex production. Previous processes are generally comparatively resource-intensive. From a study published under DOI:10.1016,The article published in 26434 / chemrxiv-2022-pq21j also describes the photochemical conversion of CO2 into methanol or ethanol using a reactive oxygen species in the presence of a MOF-based photosensitizer, dPCN-224(H). In this case, H2O acts as the hydrogen source. dPCN-224(H) is a specific metal-organic framework (MOF) compound that contains a porphyrin photosensitizer as a building block. Depending on the conditions used, methanol or ethanol can be produced. Due to their size, photosensitizers based on metal-organic frameworks enable heterogeneous catalysis while simultaneously providing confined spaces that can significantly influence reactions and their course.However, corresponding materials are still comparatively complex to produce, especially if they must also contain a photosensitizer. Zirconium, the framework metal in the metal-organic framework compound dPCN-224(H), is non-toxic and comparatively inexpensive. However, the extraction of zirconium is resource-intensive. The production of porphyrins also requires resources.

[0015] Lignin is a phenolic macromolecule synthesized in the cells of perennial plants, found in trees, shrubs, bamboo, rattan, cereals, and other grasses, among other plants. Large quantities of lignin are produced worldwide annually: 20–30% of the dry mass of woody plants results in a total annual production of approximately 20 billion tons of lignin. In terms of quantity, lignin is one of the most important biopolymers in the world, alongside cellulose. At the same time, lignin represents a previously underutilized resource.

[0016] The elongated biopolymer cellulose consists of ß-1,4-linked glucose monomers. A number of cellulose polymers are assembled to form fibers with partially crystalline regions, which contribute to the tensile and flexural strength of plants. Hemicellulose consists of various sugars and also has branching linkages that prevent a fiber-like arrangement. Lignin consists of various types of phenylpropanes that are incorporated into the cellulose-hemicellulose framework and linked to the polymer lignin. The two substances are thus tightly bound to form lignocellulose.

[0017] In paper and pulp production, lignin is separated from cellulose because lignin contributes to yellowing under the influence of light, which is undesirable in paper production. When processed, i.e., purified and fractionated, lignin can be used in a variety of ways, for example as a fertilizer, as a transport medium in fertilizer production, as an energy raw material, as a replacement for adhesives previously made from petroleum in the particle board industry, as a biomaterial, and in animal feed as a filler and carrier for food components. A precise explanation of the yellowing and / or the processes occurring during photobleaching of pulp proves to be complicated due to the diverse chemical structures of lignin and the fact that macromolecules are involved. In any case, reactive oxygen species (ROS), including 1O2, may be involved (K. Fischer et al., Holzforschung 49 (1995), 203-210; Ross et al., Can: J. Chem. 76: 1805-1816 (1998)). Several research groups have shown that lignin releases reactive oxygen species (ROS), including 1 O2, can generate (LR Barclay et al., Can. J. Chem. 2003, 81 , 457-467; K. Fischer et al., Ber. 2000, 79, 25-31 ; LR

[0018] C. Barclay et al., Can. J. Chem. 1998, 76, 1805-1816; K. Fischer et al., Holzforschung 2009, 49, 203-210). Blocking the antioxidant functions of lignin through acetylation can even increase the amount of ROS produced under light irradiation, which is why their use as photosensitizers in photodynamic therapy is being considered (Marchand et al., ChemistrySelect 2018, 3, 5512-5516).

[0019] Tannins are polyphenolic compounds found in the roots and bark of some trees, as well as in the leaves and fruits of plants. Their chemical structure usually includes one or more gallic acid units. Tannins can exhibit both pro- and antioxidant properties, with reactive oxygen species reportedly involved (R. Bhat and S.M. Hadi, Mutation Research 313 (1994), 39-48 and 49-55; S.M. Hadi et al., Chemico-Biological Interactions 125 (2000), 177-189).

[0020] An object of the present invention is to provide alternative photosensitizers or catalysts that can be prepared as simply and resource-efficiently as possible.

[0021] The invention defined in claim 1 is based on the problem of providing an alternative and at the same time as resource-efficient process for the photochemical reduction of CO2.

[0022] This problem is solved by the features listed in claim 1, which is directed to:

[0023] A process for the reduction of CO2 in the presence of O2, a sacrificial reagent acting as an electron donor and a hydrogen source, in which reactive oxygen species (ROS) are generated and in which CO2 is reduced to alcohol, characterized in that under the action of light by means of a photosensitizer at least one reactive oxygen species (ROS) is generated and the photosensitizer comprises lignin, lignocellulose and / or tannin, which may each be functionalized.

[0024] The term "photosensitizer" has the same meaning in this application as the term photocatalyst. A photocatalyst or photosensitizer refers to a compound that accelerates a reaction under the influence of light or even enables it in the first place. More specifically, in this application, a photosensitizer refers to a compound that is excited under the influence of light, whereby the energy gained by the excitation is transferred to another compound, thereby returning the photosensitizer to its ground energetic state. This process can be repeated because the photosensitizer is not consumed in the process.

[0025] The term "reactive oxygen compounds" or "reactive oxygen species" (ROS) is familiar to those skilled in the art. It refers to unstable and reactive compounds that consist of oxygen or contain oxygen, such as closed- and / or open-shell singlet oxygen. 1 O2, superoxide radical anions, and the like. Reactive oxygen species have in common that they contain oxygen, are unstable, and thus highly reactive. This makes their detection difficult. This gives rise to the need to use the term "reactive oxygen species" (ROS) to describe them.

[0026] The invention relates to a process in which reactive oxygen species (ROS) are generated, and which is simultaneously characterized in that at least one reactive oxygen species (ROS) is generated under the influence of light by means of a photosensitizer. This is to be understood as meaning that, in principle, one or more types of reactive oxygen species are generated. In other words, the invention encompasses both processes in which only one reactive oxygen species is generated and processes in which different oxygen species are generated.

[0027] Oxygen is one of the few molecules with a triplet ground state 3 O2. 3 O2 can quench almost any excited triplet state of a photosensitizer, which can form reactive oxygen species.

[0028] In the process according to the invention, the energy is transferred directly or indirectly to oxygen (triplet oxygen 3 O2). This produces reactive oxygen species (ROS). How exactly this occurs and which reactive oxygen species are formed is not yet clear.

[0029] The process according to the invention enables the resource-efficient reduction of CO2 in the presence of O2 under atmospheric pressure and at room temperature to alcohol, without the need for environmentally harmful, toxic metals. Alcohols are important basic chemicals and solvents, as well as valuable energy carriers. They are easy to store and transport, making them particularly suitable for energy storage and transport.

[0030] Since lignocellulose, lignin and tannin are obtained from naturally renewable resources using established and constantly evolving processes, the provision of corresponding catalysts is comparatively simple and resource-saving.

[0031] The sacrificial reagent is preferably H2O. The method according to the invention is particularly resource-efficient when using this globally available sacrificial reagent.

[0032] The preferred hydrogen source is H2O. The process according to the invention is particularly resource-efficient when using this globally available hydrogen source.

[0033] The preferred alcohols are methanol or ethanol. These are particularly in demand as base chemicals and, due to their properties, are particularly well-suited as energy storage and fuel.

[0034] In processes according to the invention, the photosensitizer can preferably be incorporated into a framework composite material such as a polymer, hydrogel, or a metal-organic framework. Suitable materials are known to those skilled in the art. This can increase the surface area of ​​the catalyst and / or facilitate the recovery or separation of the photocatalyst from reaction mixtures.

[0035] In processes according to the invention, preference is given to using lignin, lignocellulose and / or tannin, each of which is functionalized.

[0036] A mixture of corresponding non-functionalized and functionalized components is also possible.

[0037] Functionalization involves blocking the OH groups present in lignin, lignocellulose, and / or tannin. Such functionalizations allow for an increase in the amount or enhancement of the production of reactive oxygen species (ROS). Examples of such functionalizations include acetylation, alkylation, etherification, silylation, and aminomethylation. Applicable processes for this are well known to those skilled in the art. Acetylation is a particularly preferred type of functionalization.

[0038] Lignin, lignocellulose, and / or tannin obtained from larch bark are preferably used. Acetylated larch bark lignin, acetylated larch bark lignocellulose, and / or acetylated larch bark tannin are particularly preferably used as photosensitizers in processes according to the invention.

[0039] The invention further relates to a photosensitizer consisting of or comprising functionalized lignocellulose and / or functionalized larch bark lignin and / or functionalized tannin. Photocatalysts or photosensitizers according to the invention are particularly suitable for providing open-shell and / or closed-shell singlet oxygen. 1 O2 and other reactive oxygen species or oxygen compounds (reactive oxygen species, ROS).

[0040] In processes or photosensitizers according to the invention, the functionalization is preferably selected from a functionalization that blocks one or more OH groups of lignin, lignocellulose, or tannin, such as acetylation, alkylation, etherification, silylation, or aminomethylation. Functionalization by acetylation is particularly preferred.

[0041] A further object of the invention is therefore the use of photocatalysts or photosensitizers according to the invention for providing ROS, preferably in the context of a process according to the invention.

[0042] The photosensitizer may or may not be singlet oxygen 1 O2 is produced, but in any case, it generates reactive oxygen species (ROS). It can also be contained in a composite material. The composite material can, but does not have to, be bonded to the surface of a support material. The surface support material can also consist of a mesoporous material such as TiO2 or mesoporous silica to increase the storage and conversion capacity of the photocatalytic composition, or it can be in the form of a matrix or metal surface.

[0043] The present invention utilizes one or more photosensitizers or photocatalysts based on natural polymers or polyphenol compounds such as lignin, lignocellulose, or tannin. The material may, but need not, be chemically or biologically modified, such as by acetylation, to increase its activity against ROS production.

[0044] The inventors have discovered that the photocatalyst can be used to generate ROS such as singlet oxygen. The photocatalyst is activated by light and can, for example, but not exclusively, convert CO2 in water into alcohols. This can be achieved at mild temperatures of 10–100°C, preferably between 20–50°C, using a light source such as an LED or a UV lamp. The active species generated during this process can also be stored and released in the dark at 30–100°C if an ROS trap is chemically incorporated into the material, thus enabling a second life cycle of so-called photocatalysis in the dark. Suitable ROS traps can be, for example, but are not limited to, aromatic moieties such as anthracene, naphthalene, and benzene. Other compounds can also be converted using the same mechanism.

[0045] The invention is described in more detail below with reference to the figures and examples.

[0046] Figures 1 to 9 show in

[0047] Fig. 1 : An FTIR spectrum of acetylated lignin;

[0048] Fig. 2: A UV / IS spectrum of acetylated lignin;

[0049] Fig. 3: A 1 H-NMR spectrum of acetylated lignin;

[0050] Fig. 4: A UV-VIS spectrum to detect ROS generation by methylene blue degradation;

[0051] Fig. 5: A UV-VIS analysis of different reaction mixtures with ethanol comparison;

[0052] Fig. 6: HPLC analysis of the reaction mixture with ethanol comparison and CO2 saturated H2O.

[0053] Fig. 7: 1 H-NMR spectrum reaction mixture in D2O

[0054] Fig. 8: An FT-IR analysis of a reaction mixture;

[0055] Fig. 9: The FT-IR analysis from Fig. 8 with ethanol comparison;

[0056] Examples

[0057] The examples described below are not to be interpreted as limiting the invention, but merely as exemplary embodiments.

[0058] Example 1 : Photosensitizer based on larch bark

[0059] The inventors extracted lignin from larch bark as described in A. Meindl et al., Polymers 14 (2022), 4319-4329 and further modified it by acetylation as follows:

[0060] Lignin extracted from untreated larch bark is acetylated by reaction with acetic acid and 1% H2SO4 for 4 min at 400 W in a microwave reactor. The acetylated lignin is precipitated in H2O, washed with H2O, and dried. The dried material is analyzed by FTIR, UV / VIS, and 1 H-NMR spectroscopy. The respective spectra are shown in Figures 1 to 3. They show:

[0061] Fig. 1 : An FTIR spectrum of acetylated lignin

[0062] Fig. 2: A UVA / UVS spectrum of acetylated lignin;

[0063] Fig. 3: A 1 H-NMR spectrum of acetylated lignin in methanol-ds.

[0064] Example 2: Detection of photosensitization

[0065] Acetylated lignin according to Example 1 was tested for its ability to generate ROS upon irradiation in the presence of triplet oxygen. For this purpose, material produced according to Example 1 was placed in a methylene blue solution. Methylene blue is a common compound for determining the presence of ROS. It degrades upon exposure to ROS, resulting in a decrease in its characteristic absorption band. Within 50–60 minutes, all of the methylene blue was degraded, indicating that oxygen was effectively converted to ROS. The degradation of methylene blue is documented by UVA / UVS spectroscopy and shown in Figure 4.

[0066] Example 3: Use as a photocatalyst to reduce CO2

[0067] Lignin acetylated according to Example 1 was added to CO2-saturated H2O and irradiated in the presence of triplet oxygen for up to 4 hours. The reaction mixture was analyzed by UV / vis spectroscopy (Fig. 5), FTIR spectroscopy, and HPLC (Fig. 6), demonstrating the formation of ethanol. The experiment was repeated five times. The photocatalyst concentration ranged from 5 mg / ml to 100 mg / ml of reaction medium, with the reaction medium being CO2-saturated H2O. By varying the reaction conditions, such as the concentrations of CO2 and photocatalyst, the reaction product can be changed, for example, to methanol.

[0068] Figure 5 shows the reaction mixture of three independent reactions compared to a commercial ethanol standard. Figure 6 shows the starting solution (CO2-saturated H2O), a 20% aqueous ethanol solution in CO2-saturated H2O as a reference, and the reaction product. The ethanol standard exhibits not only the alcohol signal but also the signal corresponding to CO2-saturated H2O. After the reaction, the signal correlated with dissolved CO2 is no longer visible, but the peak corresponding to ethanol is.

[0069] Example 4:

[0070] 100 mg of acetylated lignin according to Example 1 was added to a 1:1 v / v mixture of CO2-saturated water and distilled water in the presence of triplet oxygen. The reaction was then irradiated for 4 hours. The reaction mixture was 1H-NMR (Fig. 7) and FTIR (Figs. 8 and 9) were analyzed and compared with an ethanol standard. Ethanol formation was confirmed in both cases. Since the modified lignin used is present as a suspension / solid in the reaction, the photosensitizer can be easily separated and reused after the reaction. To demonstrate that the catalyst can be reused, five independent reactions were set up with the same catalyst, and ethanol was detected spectroscopically in all reactions.

[0071] Apart from lignin or modified lignin, the following can also be used in the respective reaction: lignocellulose, modified (for example, modified by acetylation) lignocellulose, tannin and modified (for example, modified by acetylation) tannin.

[0072] The presence of triplet oxygen is required. Otherwise, no reactive oxygen species are formed, and the reaction does not occur. Triplet oxygen can therefore formally be considered an energy-absorbing substrate, although it is unclear exactly how the reaction proceeds and whether the triplet oxygen reacts directly or indirectly to form reactive oxygen species.

[0073] The fact that the photosensitizers mentioned can be used in the claimed reaction is in good agreement with the known photochemical properties of lignin(s), lignocellulose(s), and tannin(s): These compounds are known to generate reactive oxygen species. Modification, such as acetylation, can increase the amount of reactive oxygen species, as already mentioned above with reference to relevant literature.

[0074] The aforementioned materials, lignin, lignocellulose, and tannin, are readily available worldwide and can be modified without great effort (e.g., by acetylation). Given their widespread use, the durability of these materials, or the resulting photosensitizers, is of secondary importance. Therefore, they can be replaced without too much effort—should, contrary to expectations, they lose any catalytic activity as photosensitizers after just a few cycles.

[0075] The fact that no metals, especially transition metals, heavy metals, and / or precious metals, are involved results in economic and ecological advantages. This is because the production, handling, and disposal of these photosensitizers is less complex and poses less environmental risks than with other photosensitizers.

[0076] The photosensitizers used not only promote the conversion of CO2 into valuable basic chemicals and fuels, but also represent an economically and ecologically advantageous alternative to known photosensitizers used in comparable reactions.

[0077] At least methanol and ethanol can be obtained from CO2 using processes according to the invention in a highly environmentally friendly, resource-saving manner. There is a worldwide demand for these basic chemicals. This circumstance increases the usefulness of the present invention.

[0078] Although the photosensitizers of the invention have been described in the context of their use as photocatalytic compositions, those skilled in the art will readily recognize that the novel photosensitizers can also be used in other areas, such as antimicrobial and antibacterial materials, or for phototherapeutic applications.

Claims

Claims 1 . A process for the reduction of CO2 in the presence of O2, a sacrificial reagent acting as an electron donor and a hydrogen source, in which reactive oxygen species (ROS) are generated and in which CO2 is reduced to alcohol, characterized in that under the action of light by means of a photosensitizer at least one reactive oxygen species (ROS) is generated and the photosensitizer comprises lignin, lignocellulose and / or tannin, which may each be functionalized.

2. Method according to claim 1, characterized in that the sacrificial reagent is H2O.

3. Process according to claim 1 or 2, characterized in that the hydrogen source is H2O. 4 Process according to one of the preceding claims, characterized in that the alcohol is methanol or ethanol.

5. A method according to any one of the preceding claims, wherein the photosensitizer is incorporated into a scaffold composite material such as a polymer, hydrogel or metal-organic framework.

6. Process according to one of the preceding claims, characterized in that the photosensitizer comprises lignin, lignocellulose and / or tannin, each of which is functionalized.

7. Process according to one of the preceding claims, characterized in that the photosensitizer is functionalized larch bark lignin, functionalized tannin or functionalized lignocellulose. A photosensitizer consisting of or comprising functionalized lignocellulose and / or functionalized larch bark lignin and / or functionalized tannin. The process according to any one of claims 1 to 7 or the photosensitizer according to claim 8, characterized in that the functionalization is selected from a functionalization that blocks one or more OH groups of lignin, lignocellulose, or tannin, such as acetylation, alkylation, etherification, silylation, or aminomethylation. Use of a photosensitizer according to claim 8 or 9 for generating reactive oxygen species (ROS).

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