Method for producing a sorption material based on graphene oxide aerogels

Abstract

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

Description

[0001] Description

[0002] Process for producing a sorption material based on graphene oxide aerogels

[0003] The invention relates to a process for producing a sorption material based on functionalized graphene oxide aerogels and to the resulting sorption material, which can be used as a sorption element in adsorption processes, in particular for the separation of carbon dioxide from the ambient air.

[0004] Direct Air Capture (DAC) is a process for extracting carbon dioxide (CO2) directly from the ambient air. The basic principle is that ambient air flows through a filter, which removes some of the CO2. The result of the process is pure CO2, which can then be used for various purposes. Possible uses of 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 result in negative emissions. The latter is known as Direct Air Carbon Capture and Storage (DACCS) and is intended to actively remove carbon dioxide from the atmosphere and permanently store it using carbon capture and storage (CCS) to counteract global warming.

[0005] The DAC process is a very energy-intensive process because the concentration of CO2 in the ambient air is very low at approximately 400 ppm. Therefore, a prerequisite for an efficient process is finding a suitable sorption material that filters CO2 from the air. However, the solutions currently available on the market have various disadvantages that reduce the overall efficiency of the DAC process. Common sorption materials include zeolites, amine-based resins or polymers, or amine-functionalized silica materials. However, these still have considerable potential for improvement, for example, with regard to their aging stability, water affinity, and / or their energy requirements for regeneration. Excessive energy requirements 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 using binding agents or pressed into pellets. This process typically reduces the CO2 adsorption capacity. The core of a DAC system is therefore the adsorbent, whose task is to first capture the carbon from the ambient air and bind it in an adsorption process, and then release the bound amount from the adsorbent in a precise and controlled manner (desorption).

[0006] Amine-functionalized graphene oxides (GFO) for CO2 adsorption are already known from the state of the art (see NPL1, NPL2). Graphene oxide (GO) is first synthesized from graphite residues using the improved Hummers method with potassium permanganate (KMnO4) and sulfuric acid (H2SO4) and 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).

[0007] CN 108745287 A also discloses a three-dimensional graphene oxide-based CO2 adsorbent. Graphene oxide is dispersed by ultracentrifugation and reacted with a long-chain amine, followed by freeze-drying. A precipitate is obtained, which serves as an adsorbent.

[0008] In CN 113511649 A, a high-amine aqueous solution 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. At the same time, freeze-drying is used to dry the hydrogel, allowing the mesoporous material to retain its complete structure.

[0009] CN 113600135 A discloses a three-dimensional porous graphene aerogel material, as well as a manufacturing method and application thereof. The manufacturing method of the three-dimensional porous graphene aerogel material comprises the following steps: mixing the graphene oxide dispersion liquid with an organic amine and conducting a hydrothermal reaction to obtain a hydrogel, performing a washing step, and then freezing and drying with water to obtain the three-dimensional porous graphene aerogel material. The material is used for adsorbing CO2.

[0010] It is also known from the prior art that three-dimensional shapes based on graphene oxide aerogels can be produced using 3D printing, as described, for example, in CN 107555422 A. The invention discloses a 3D printing aerogel based on an unmodified graphene oxide material. The 3D printing raw materials for producing the graphene aerogel consist primarily of an aqueous graphene oxide dispersion. The manufacturing process described therein produces a material that meets the rheological property requirements of 3D printing. A product blank with a freely designable 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.

[0011] However, the powdered graphene oxide aerogels used to date for CO2 adsorption still show room for improvement in their macroscopic structuring. Template-directed deposition and freeze-casting methods can provide some control over the overall size and shape of the synthesized monolith, but the architectural complexity and scalability of these methods are unsatisfactory.

[0012] There is therefore still a need for a customized and scalable method for the production of 3D graphene oxide structures with flexibility in macroscopic structuring and the provision of a diversified material loading, which would improve the utility of these graphene oxide structures for application as sorption elements in a DAC system. The focus is particularly on the development of efficient sorption elements in which the adsorption and desorption of carbon dioxide occurs reproducibly with lower energy consumption and, on the other hand, a comparatively cost-effective system concept can be implemented. The heating and cooling phases, as well as regeneration, in particular, influence the process costs.

[0013] The invention is based on the object of separating carbon dioxide from the ambient air in a comparatively simple and cost-effective manner and overcoming the disadvantages known from the prior art. The object is particularly directed at providing a process for producing a sorption material using a graphene oxide dispersion that is stable without the addition of binders or stabilizers such as surfactants.

[0014] This object is achieved in the present invention, first of all, by the features of patent 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 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.

[0015] The above object is further achieved by a sorption material produced by the process according to the invention.

[0016] The sorption material produced according to the invention thus has a carbon framework, which results 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 porosity of the material, its geometry, and surface chemistry, with the interconnected pores ensuring high mass transport and a high adsorption capacity for CO2. Furthermore, the sorption material produced according to the invention is ultralight.

[0017] Further preferred embodiments of the invention emerge from the remaining features mentioned in the subclaims.

[0018] In an advantageous embodiment of the method for producing a sorption material, the extrusion is carried out using a 3D printer.

[0019] 3D printing enables the production of graphene oxide-based aerogels with controllable macrostructures down to submillimeter precision, yet with hierarchical porous structures that cannot be achieved through conventional self-assembly. 3D printing thus enables the continuous fabrication of highly complex physical structures. Direct ink writing (DIW) is preferred, representing a robust, cost-effective, and scalable 3D printing technique. This technique allows for the continuous and robot-assisted extrusion of a 3D product. Furthermore, the gel-based printing process enables binderless macroscopic shaping.In an advantageous embodiment of the method 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 compounds containing amine groups or by physical attachment of organic compounds with amine groups.

[0020] Functionalizing graphene oxide increases the CO2 adsorption capacity. Gelation during hydrogel production can also be facilitated.

[0021] In an advantageous embodiment of the method for producing the sorption 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 ultracentrifuging.

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

[0023] In a further advantageous embodiment of the method 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 additional components, not only can the CO2 adsorption rate be further improved, but the sorbent can also be strengthened, thereby increasing its operational durability. In particular, this can increase the strength and functionality of the sorption material. The sorption material reinforced in this way not only has a high CO2 adsorption capacity but also increased material strength. This makes it more robust and reproducible under cyclical process stress, which in turn has a positive effect on quality and process costs. A further aspect is the use of the sorption material produced by the process according to the invention as a sorption element in technical adsorption processes, in particular for the adsorption or desorption of CO2, preferably in the DAC process.However, it can also be used in all technical areas where, for example, gas separation, gas purification or gas dehumidification is carried out.

[0025] The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless otherwise stated in the individual case.

[0026] The invention is explained below in exemplary embodiments with reference to the accompanying drawings. They show:

[0027] Figure 1 is a schematic representation of the structure of graphene oxide,

[0028] Figure 2 shows a flow diagram for carrying out a method according to the invention for producing a sorption material for separating carbon dioxide from the ambient air and

[0029] Figure 3 shows a simplified representation of possible embodiments of the production of a sorption material according to the invention.

[0030] The term "suspension" refers to a heterogeneous mixture of substances in which a component (here, graphene oxide) is present as a solid phase dispersed in the liquid phase. The generic term "dispersion" is also used here as a synonym for this term.

[0031] 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 oxidant followed by aqueous processing. It is composed of an extended hexagonal carbon framework containing a variable number of point and extended hole defects within the carbon plane. This carbon lattice is equipped with oxygen-containing functional groups on both sides and at the edge.

[0032] Figure 2 shows a flow chart for carrying out the method and Figure 3 shows an example of a method according to the invention for producing a sorption material 10. The method comprises the provision <100> of a graphene oxide 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, for example, finely ground graphite with high crystallinity is first dispersed in a highly concentrated and 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. In the case of permanganate (Hummers method), the reactive species is the manganyl cation (MnOa) formed in situ by its dehydration. +) or dimanganese heptoxide. According to the Hummers method, a graphite sample is chemically oxidized by treating it with potassium permanganate (KMnO4), sodium nitrate (NaNO3), and sulfuric acid (H2SO4) in a predetermined sequence, followed by the addition of deionized water to form graphene oxide 12.

[0033] As can be seen from Figure 3, the process further comprises a functionalization <110> of the 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 group-containing compounds or C) by physical attachment of organic compounds with amine groups.

[0034] Functionalization A) occurs, for example, via a modification of groups introduced during the synthesis by reaction with ammonia or hydroxylamine.

[0035] Compounds for covalent functionalization B) include, for example, compounds suitable for covalent coupling to graphene oxide, such as compounds containing amine groups. Preferred are aminosilanes such as 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-aminopropyldiethoxymethylsilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, or 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane.

[0036] 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. Non-covalent functionalization C) preferably involves physical bonding (impregnation) with compounds containing organic amine groups.

[0037] Examples of organic amines that can be used for impregnation are alkylamines, arylamines, heterocyclic amines, polymeric amines, or spirotetramines. In specific embodiments, the amine is selected from the group consisting of ethylenediamine, diethylenetriamine, triethylamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, diisopropylethylamine, piperazine, or polyethyleneimine, as well as combinations thereof.

[0038] In the following, in step <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 achieved when the suspension has achieved the rheological properties of a gel. For this purpose, the gel 18 is preferably produced by dispersing freeze-dried FGO 16 in water 20 or by concentrating an aqueous FGO suspension. The concentration is achieved, for example, by evaporating water 20 or ultracentrifugation.

[0039] In step <120> In a preferred embodiment, one or more additional components 22 are added to the amine-functionalized graphene oxide (FGO) 16 and the water 20. The FGO 16 assumes the function of a rheology modifier and a binder, while also acting as a CO2 adsorbent. Other components 22 include, for example, (I) additional CO2 adsorbent, (II) substances that can improve physical properties (strength, conductivity), or (III) catalysts.

[0040] Suitable additional CO2 adsorbents (I) include, for example, other amine-functionalized particles that have 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.

[0041] The other components 22, like the amine-functionalized graphene oxide (FGO) 16, are dispersed in water 20, so 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 the rheology of the gel 18 is ensured.

[0042] The viscosity of the water-based gel 18 is such that it can be used in 3D printing, for example. 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.

[0043] In one embodiment, the method according to the invention further comprises a step of isolating the resulting water-based gel 18, which can be achieved by filtering or centrifuging.

[0044] In one embodiment, it is possible to first prepare the water-based gel 18 and then functionalize it by amine functionalization.

[0045] In a further variant of the process according to the invention, the amine-functionalized graphene oxide (FGO) 16 is pressed into a mold using binders, thus eliminating the need for the production of a gel followed by extrusion.

[0046] In a 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 in a monolithic form is obtained. In a preferred embodiment, the water-based gel 18 is produced by extrusion <130> into a macroscopic form, preferably a three-dimensional structure, and then freeze-dried <140> . The extrusion process <130> For example, extrusion is carried out using a 3D printer or an extruder at room temperature. Extrusion is preferably carried out using a 3D printer by forcing the gel through a thin nozzle. This allows for the creation of virtually any shape of sorbent, such as lattice structures.Alternatively, shaping is possible using the manufacturing processes of primary forming and / or forming, such as casting or pressing.

[0047] In step <140, the extruded gel is freeze-dried, creating an aerogel with a porosity of 24. In this way, the water 20 is removed from the structure without destroying the porosity 24 of the material. The sorbent 10 produced in this way has the described properties. Non-patent literature

[0048] NPL1 : Liu et al., “Ultrasound-assisted amine functionalized graphene oxide for enhanced CO2

[0049] Adsorption”, Fuel 247 (2019) 10-18

[0050] NPL2: R. Zeleszki, "Carbon Dioxide Capture With Amine Functionalized Graphene Oxide" (2015).

[0051] Electronic Theses and Dissertations. 917.

[0052] List of reference symbols Sorption material Graphene oxide Amine functionalization Amine-functionalized graphene oxide Water-based gel Water Other components Porosity

Claims

Patent claims 1. A method for producing a sorption material (10), comprising: Providing a graphene oxide (12), functionalizing 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. The method according to claim 1, wherein the extrusion is carried out using a 3D printer.

3. The 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. The method according to any one of claims 1 to 3, wherein the water-based gel (18) is prepared 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. The method according to claim 4, wherein the concentration of the aqueous suspension of the amine-functionalized graphene oxide (16) is carried out by evaporation of water (20) or ultracentrifugation.

6. The method according to any one of claims 1 to 5, wherein during the preparation of the water-based gel (18) further components (22) selected from adsorbents, agents for improving the physical properties or catalysts and mixtures thereof are added.

7. Sorption material (10) produced by a process according to one of claims 1 to 6.

8. Use of the sorption material (10) according to claim 7 as a sorption element for the 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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