Process for producing a sorption material based on graphene oxide aerogels

Abstract

The invention relates to a method for producing a sorption material (10) based on functionalized graphene aerogels, as well as the resulting sorption material (10). The method comprises the production of a water-based gel (18), its extrusion, and subsequent freeze-drying. The sorption material (10) is suitable as a sorption element in adsorption processes, in particular for the separation of carbon dioxide from the ambient air.

Description

[0001] The invention relates to a method for producing a sorption material based on functionalized graphene oxide aerogels and the resulting sorption material, which can be used as a sorption element in adsorption processes, in particular for separating carbon dioxide from ambient air.

[0002] Direct Air Capture (DAC) is a process for extracting carbon dioxide (CO2) directly from ambient air. The basic principle is that ambient air flows through a filter that removes some of the CO2. The result of this process is pure CO2, which can then be used for various purposes. Potential uses for CO2 include its use as a raw material, for example in the chemical industry, the production of CO2-neutral fuels (renewable gas and e-fuels), and the geological storage of carbon dioxide, which can lead to negative emissions. The latter is known as Direct Air Carbon Capture and Storage (DACCS) and aims to actively remove carbon dioxide from the atmosphere and store it permanently via carbon capture and storage (CCS) to counteract global warming.

[0003] The DAC process is very energy-intensive because the concentration of CO2 in ambient air is very low, at approximately 400 ppm. Therefore, a suitable sorption material that filters CO2 from the air is essential for an efficient process. However, the solutions currently available on the market have several drawbacks that reduce the overall efficiency of the DAC process. Known sorption materials include zeolites, amine-based resins or polymers, and amine-functionalized silica materials. These materials, however, still exhibit considerable potential for improvement, particularly regarding their aging stability, water affinity, and / or the energy required for material regeneration. Excessive energy consumption can be caused, for example, by low thermal conductivity of the material and / or high desorption temperatures of water and CO2.Furthermore, these materials are usually in powder form, which must be pre-sintered with binders or pressed into pellets. This process generally reduces the CO2 adsorption capacity.

[0004] The core of a DAC system is therefore the adsorbent, which has the task of first capturing the carbon from the ambient air and binding it in a process of adsorption, and then releasing the bound amount precisely and in a controlled manner from the adsorbent (desorption).

[0005] Amine-functionalized graphene oxides (GFOs) for CO2 adsorption are already known from the prior art (see NPL1, NPL2). In this process, graphene oxide (GO) is first synthesized from graphite residues using potassium permanganate (KMnO4) and sulfuric acid (H2SO4) according to the improved Hummers method. It is then aminated by integrated physical activation and amine grafting under ultrasonic treatment. Functionalized GO can thus be used for CO2 absorption by reacting CO2 with the amine groups of the functionalized graphene oxide (NPL2).

[0006] CN 108745287 A also discloses a three-dimensional graphene oxide-based CO2 adsorbent. In this process, graphene oxide is dispersed by ultracentrifugation, reacted with a long-chain amine, and subsequently freeze-dried. A precipitate is obtained, which serves as the adsorbent.

[0007] In CN 113511649 A, an aqueous solution with a high amine content and a graphene oxide dispersion are mixed, ultrasonically treated, and freeze-dried to obtain a flexible, amine-modified, three-dimensional mesoporous graphene material. The three-dimensional porous structure increases the amine loading capacity of the mesoporous material, thereby improving the adsorption performance of gases such as carbon dioxide. Simultaneously, freeze-drying is used to dry the hydrogel, ensuring that the mesoporous material retains its complete structure.

[0008] CN 113600135 A discloses a three-dimensional porous graphene aerogel material, a manufacturing process, and its application. The manufacturing process for the three-dimensional porous graphene aerogel material comprises the following steps: mixing the graphene oxide dispersion liquid with an organic amine and performing a hydrothermal reaction to obtain a hydrogel; performing a washing step; and subsequently freezing and drying with water to obtain the three-dimensional porous graphene aerogel material. The material is used for adsorbing CO2.

[0009] It is also known from the prior art that three-dimensional shapes based on graphene oxide aerogels can be produced using 3D printers, as described, for example, in CN 107555422 A. The invention discloses a 3D-printed aerogel based on an unmodified graphene oxide material. The 3D printing raw materials for producing the graphene aerogel consist mainly of an aqueous graphene oxide dispersion. In the manufacturing process described therein, a material is obtained that meets the rheological property requirements of 3D printing. A product blank with a freely configurable structure can be produced by 3D printing. The graphene aerogel produced in this way exhibits excellent conductivity, extremely low density, high elasticity, and heat resistance.

[0010] However, the powdered graphene oxide aerogels used so far for CO2 adsorption still show room for improvement with regard to their macroscopic structuring. Template-controlled deposition and freeze-casting methods can allow some control over the overall size and shape of the synthesized monolith, but the architectural complexity and scalability of these methods are not satisfactory.

[0011] There is therefore still a need for a tailored and scalable method for the fabrication of 3D graphene oxide structures with flexibility in macroscopic structuring and the ability to provide a diverse material loading. This would improve the utility of these graphene oxide structures for use as sorption elements in a DAC plant. The primary focus is on developing efficient sorption elements where carbon dioxide adsorption and desorption occur reproducibly with lower energy consumption, and where a comparatively cost-effective plant design can be implemented. The heating and cooling phases, as well as regeneration, particularly influence process costs.

[0012] The invention is based on the objective of separating carbon dioxide from ambient air in a comparatively simple and cost-effective manner and overcoming the disadvantages known from the prior art. In particular, the objective is to provide a process for producing a sorption material in which a graphene oxide dispersion is used that is stable without the addition of binders or stabilizers such as surfactants.

[0013] This problem is initially solved in the present invention by the features of claim 1. The process for producing a sorption material provides that a graphene oxide is first prepared. In a subsequent step, the graphene oxide is functionalized with amines to form an amine-functionalized graphene oxide (FGO). This is followed by the production of a water-based gel from FGO and water, and the production of a three-dimensional shape by extrusion of the water-based gel. Finally, the extruded three-dimensional shape is freeze-dried.

[0014] The above problem is further solved by a sorption material produced according to the inventive method.

[0015] The sorption material produced according to the invention thus has a carbon framework, resulting in comparatively high electrical and thermal conductivity. These properties enable an energy-efficient regeneration process, as the thermal conductivity allows for shorter heating and cooling phases compared to ceramic materials such as zeolites and silica. This increases the overall efficiency of the DAC process. The electrical conductivity also enables direct electrical heating of the material (Joule heating). Furthermore, the process according to the invention makes it possible to adjust the material's porosity, geometry, and surface chemistry, with the interconnected pores ensuring high mass transport and high adsorption capacity for CO2. In addition, the sorption material produced according to the invention is ultralight.

[0016] Further preferred embodiments of the invention result from the other features mentioned in the dependent claims.

[0017] In an advantageous embodiment of the process for producing a sorption material, it is provided that the extrusion is carried out using a 3D printer.

[0018] 3D printing enables the production of graphene oxide-based aerogels with controllable macrostructures down to submillimeter precision, while still allowing for hierarchical porous structures that cannot be achieved through conventional self-assembly. 3D printing thus enables the continuous production of highly complex physical structures. Direct ink writing (DIW) is the preferred method, representing a robust, cost-effective, and scalable 3D printing technique. This technique allows for the continuous, robot-assisted extrusion of a 3D product. Furthermore, the gel-based printing process enables binderless macroscopic shaping.

[0019] In an advantageous embodiment of the process for producing a sorption material, it is provided that the amine functionalization of the graphene oxide is carried out by direct amination with free ammonia and / or at least one ammonia-releasing compound, by covalent attachment of amine-containing compounds, or by physical attachment of organic compounds with amine groups.

[0020] Functionalizing the graphene oxide increases the adsorption capacity of CO2. It can also facilitate gelation during the production of the hydrogel.

[0021] In an advantageous embodiment of the process for producing the sorbent material, the water-based gel is produced by dispersing freeze-dried FGO in water or by concentrating an aqueous FGO suspension. In a preferred embodiment, the aqueous FGO suspension is concentrated by evaporating water or by ultracentrifugation.

[0022] The production of the water-based gel or hydrogel allows for the introduction of sufficient porosity into the structure of the sorption material. This avoids the need for additional binders to produce an extrudable form, which also leads to an improvement in CO2 adsorption capacity.

[0023] In a further advantageous embodiment of the process for producing a sorption material, it is provided that during the production of the water-based gel, the addition of further components selected from adsorbents, agents for improving the physical properties (such as inorganic metal oxide nanoparticles) or catalysts and mixtures thereof takes place.

[0024] By adding further components, the CO2 adsorption rate can be further improved, and the sorbent's strength can be increased, thus extending its service life. In particular, the strength and functionality of the sorbent material can be enhanced. The enhanced sorbent material therefore possesses not only a high CO2 adsorption capacity but also increased material strength. This makes it more robust and reproducible under cyclic process stress, which in turn has a positive impact on quality and process costs.

[0025] Another aspect is the use of the sorption material produced according to the inventive process as a sorption element in technical adsorption processes, in particular for the adsorption or desorption of CO2, preferably in the DAC process. However, use is also possible in all technical fields where, for example, gas separation, gas purification, or gas dehumidification is carried out.

[0026] The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless otherwise specified in individual cases.

[0027] The invention is explained below using exemplary embodiments with reference to the accompanying drawings. These show: Fig. 1. A schematic representation of the structure of graphene oxide, Fig. 2 a flowchart for carrying out a process according to the invention for producing a sorption material for separating carbon dioxide from ambient air and Fig. 3 a simplified representation of possible embodiments of the production of a sorption material according to the invention.

[0028] The term "suspension" refers to a heterogeneous mixture in which one component (here, graphene oxide) is dispersed as a solid phase within the liquid phase. The generic term "dispersion" is also used synonymously with this term.

[0029] Fig. Figure 1 shows the basic chemical structure of graphene oxide (GO) 12. Graphene oxide (GO) 12 is a carbon-based 2D nanomaterial, typically prepared by reacting graphite with a strong oxidizing agent and subsequent aqueous workup. It consists of an extended hexagonal carbon backbone with a variable number of point and extended hole defects within the carbon plane. This carbon lattice is furnished with oxygen-containing functional groups on both sides and at the edge.

[0030] Fig. 2 shows a flowchart for carrying out the procedure and in Fig. Figure 3 shows an example of a process according to the invention for producing a sorption material 10. The process comprises the provision <100> 12. One of the most widely used methods for the synthesis of graphene oxide in large quantities for industrial purposes is the Hummers method (and the modified Hummers method). To obtain it, finely ground graphite with high crystallinity is first dispersed in a highly concentrated oxidizing acid. Suitable acids include sulfuric acid, nitric acid, orthophosphoric acid, or mixtures thereof. Subsequently, another oxidizing agent is added, such as KMnO4, KClO3, (NH4)2S2O8, or NaNO3. The reactive species, in the case of permanganate (Hummers method), is the manganyl cation (MnO3) formed in situ by its dehydration. +) or dimanganese heptoxide. According to the Hummers process, a graphite sample is chemically oxidized by being treated with potassium permanganate (KMnO4) and sodium nitrate (NaNO3) and sulfuric acid (H2SO4) in a predetermined sequence, and the sample is subsequently exposed to an addition of deionized water to form the graphene oxide 12.

[0031] As also from Fig. As can be seen in section 3, the procedure also includes a functionalization. <110> of graphene oxide 12 with amine groups 14 to form an amine-functionalized graphene oxide (FGO) 16. The functionalization is carried out, for example, by A) direct amination with free ammonia or hydroxylamine and / or at least one ammonia-releasing compound, B) by covalent attachment of amine-containing compounds, or C) by physical attachment of organic compounds with amine groups.

[0032] Functionalization A) is achieved, for example, by modifying groups already introduced during synthesis through reaction with ammonia or hydroxylamine.

[0033] Compounds suitable for covalent functionalization (B) include, for example, compounds suitable for covalent coupling to graphene oxide, such as compounds containing amine groups. Aminosilanes such as 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-aminopropyldiethoxymethylsilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, or 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane are preferred.

[0034] Non-covalent functionalization (C) is possible, for example, with alkylamines via hydrogen bonds between hydroxyl groups and amine groups, or via electrostatic attraction of negatively charged sulfuric acid esters with the positively charged alkylammonium ions, as well as with surface-active surfactants such as sodium dodecyl sulfate or sodium dodecylbenzenesulfonate. Preferably, the non-covalent functionalization (C) involves a physical bonding (impregnation) with compounds containing organic amine groups.

[0035] Examples of organic amines that can be used for impregnation include alkylamines, arylamines, heterocyclic amines, polymeric amines, and spirotetramines. In specific embodiments, the amine is selected from the group consisting of ethylenediamine, diethylenetriamine, triethylamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, diisopropylethylamine, piperazine, and polyethyleneimine, as well as combinations thereof.

[0036] The following steps will be taken <120> A water-based gel 18 is produced from the amine-functionalized graphene oxide 16. For this purpose, the amine-functionalized graphene oxide (FGO) 16 is dispersed in water 20 at a sufficiently high concentration. The exact concentration depends on the size and chemical composition of the FGO platelets. A sufficiently high concentration is reached when the suspension has attained the rheological properties of a gel. The gel 18 is preferably produced by dispersing freeze-dried FGO 16 in water 20 or by concentrating an aqueous FGO suspension. Concentration is achieved, for example, by evaporating water 20 or by ultracentrifugation.

[0037] In step <120> In a preferred embodiment, for example, one or more further components 22 are added to the amine-functionalized graphene oxide (FGO) 16 and the water 20. The FGO 16 acts on the one hand as a rheology modifier and a binder, and on the other hand as a CO2 adsorbent. Further components 22 are, for example, (I) additional CO2 adsorbent, (II) substances that can improve physical properties (strength, conductivity), or (III) catalysts.

[0038] Suitable additional CO2 adsorbents (I) include, for example, other amine-functionalized particles with a high CO2 affinity, such as amine-functionalized activated carbon. Suitable substances (II) include, for example, inorganic metal oxide nanoparticles. Suitable catalysts (III) include, for example, nanoparticles that catalyze the conversion of CO2.

[0039] The other components 22, as well as the amine-functionalized graphene oxide (FGO) 16, are dispersed in water 20, such that the water-based gel (hydrogel) 18 comprises the FGO 16 and one or more other components 22. The concentration and proportion of the other components 22 are variable, provided that the rheology of the gel 18 is ensured.

[0040] The viscosity of the water-based gel 18 is such that it can be used in 3D printing. The minimum viscosity of the gel 18 can be between 10,000 and 100,000 Pa·s, preferably between 20,000 and 50,000 Pa·s, and particularly preferably between 10,000 and 20,000 Pa·s.

[0041] In one embodiment, the method according to the invention also comprises a step of isolating the resulting water-based gel 18, which can be achieved by filtering or centrifuging.

[0042] In one embodiment it is possible to first produce the water-based gel 18 and then to functionalize it by means of an amine functionalization.

[0043] In another variant of the process according to the invention, the amine-functionalized graphene oxide (FGO) 16 is pressed into a shape using binders, so that the production of a gel with subsequent extrusion is eliminated.

[0044] In one possible embodiment, the water-based gel 18 is used without further treatment (no step). <130> freeze-dried <140> , whereby the water 20 is removed from the gel 18 and a porosity 24 is formed in an aerogel. A sorbent 10 is obtained in a monolithic form. In a preferred embodiment, the water-based gel 18 is extruded <130> brought into a macroscopic form, preferably a three-dimensional structure, and then freeze-dried <140> The extrusion process <130> This process is carried out, for example, using a 3D printer or an extruder at room temperature. Preferably, extrusion is performed with a 3D printer by pushing the gel through a thin nozzle. In this way, virtually any shape of sorbent, such as lattice structures, can be created.Alternatively, shaping can be achieved using the manufacturing processes of primary forming and / or forming, such as casting or pressing.

[0045] In step <140> The extruded gel is freeze-dried, resulting in an aerogel with a porosity of 24. In this way, water 20 is removed from the structure without destroying the porosity 24 of the material. The sorbent 10 produced in this way exhibits the described properties. Non-patented literature NPL1: Liu et al., “Ultrasound-assisted amine functionalized graphene oxide for enhanced CO2 adsorption,” Fuel 247 (2019) 10-18 NPL2: R. Zeleszki, “Carbon Dioxide Capture With Amine Functionalized Graphene Oxide” (2015). Electronic Theses and Dissertations. 917. Reference symbol list 10 Sorption material 12 Graphene oxide 14 Amine functionalization 16 Amine-functionalized graphene oxide 18 Water-based gel 20 Water 22 other components 24 Porosity QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] CN 108745287 A

[0006] CN 113511649 A

[0007] CN 113600135 A

[0008] CN 107555422

[0009] Cited non-patent literature

[0000] Liu et al., “Ultrasound-assisted amine functionalized graphene oxide for enhanced CO2 adsorption”, Fuel 247 (2019) 10-18

[0045] R. Zeleszki, „Carbon Dioxide Capture With Amine Functionalized Graphene Oxide“ (2015). Electronic Theses and Dissertations. 917

[0045]

Claims

[1] Method for producing a sorption material (10), comprising: - Provision of a graphene oxide (12), functionalization of the graphene oxide (12) by amine functionalization (14) to an amine-functionalized graphene oxide, FGO, (16), - Preparation of a water-based gel (18) from amine-functionalized graphene oxide (16) and water (20), - Production of a three-dimensional shape by extrusion of the water-based gel (18) and - Freeze-drying of the extruded three-dimensional shape. [2] Method according to claim 1, wherein the extrusion is carried out using a 3D printer. [3] Method according to claim 1 or 2, wherein the amine functionalization (14) is carried out by direct amination with free ammonia and / or at least one ammonia-releasing compound, by covalent attachment of amine group-containing compounds or by physical attachment of organic compounds with amine groups. [4] Method according to any one of claims 1 to 3, wherein the preparation of the water-based gel (18) is carried out by dispersing freeze-dried amine-functionalized graphene oxide (FGO) (16) in water (20) or by concentrating an aqueous suspension of the amine-functionalized graphene oxide (16). [5] Method according to claim 4, wherein the concentration of the aqueous suspension of the amine-functionalized graphene oxide (16) is carried out by evaporating water (20) or by ultracentrifugation. [6] Method according to any one of claims 1 to 5, wherein in the production of the water-based gel (18) the addition of further components (22) selected from adsorbents, agents for improving physical properties or catalysts and mixtures thereof takes place. [7] Sorption material (10) produced according to a method according to any one of claims 1 to 6. [8] Use of the sorption material (10) according to claim 7 as a sorption element for adsorption or desorption of CO2. [9] Use of the sorption material (10) according to claim 8, wherein the sorption element is installed in a Direct Air Capture (DAC) system.

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