MANUFACTURING METHOD FOR CARBON FIBER MATERIAL FOR THE SEPARATION OF NH3 FROM GAS MIXTURES AND THE USE OF SUCH CARBON FIBER MATERIAL

Description

[0001] The present invention relates to a process for producing a carbon fiber material for separating NH 3 from gas mixtures, and to the use of a carbon fiber material obtainable by the process for separating NH 3 from gas mixtures Background of the invention

[0002] Global warming is currently one of the biggest environmental problems.

[0003] Due to the widespread combustion of fossil fuels for energy production and intensive agriculture, the proportion of CO2 in the atmosphere is continuously increasing, leading to problems such as the greenhouse effect, melting polar ice caps and glaciers, and rising sea levels. Raw natural gas and biogas also contain large amounts of CO2, which must be separated before being fed into the natural gas grid or used as an energy source.

[0004] Therefore, intensive work is being carried out on the development of technologies for removing CO2 from exhaust gases such as power plant or industrial emissions, or from the atmosphere, to avoid these negative environmental effects. The CO2 thus extracted can be stored either underground or underwater (so-called CCS (carbon capture and storage) technology), or it can be further processed into raw materials for industrial use, such as synthesis gas (CO2 / H2), methane, or ethylene.

[0005] Several processes are known for CO2 capture from flue gases, in particular the so-called post-combustion capture, i.e. a scrubbing process (downstream of flue gas desulfurization) using a bicarbonate solution (carbonate scrubbing), an amine solution (amine scrubbing), methanol or other substances that can bind CO2. Another known process is the so-called oxyfuel process, in which the actual combustion is carried out with pure oxygen instead of air, and high doses of CO2 are added to prevent the combustion temperature from rising too high. The flue gas then consists mainly of CO2 and water vapor, which can be easily removed by condensation, so that CO2 can be recovered in a relatively pure form and returned to the process.

[0006] Furthermore, CO2 capture technologies using active materials on which CO2 is selectively adsorbed are already known. These include pressure swing adsorption processes, and membrane technology processes. Cryodistillation technology is also known to be used to separate CO2 from gas mixtures.

[0007] There is also increased interest in the separation of ammonia (NH 3 ) from gas mixtures, partly because ammonia can be used to generate energy in ammonia fuel cells. Ammonia – bound as ammonium – is found in large quantities in production wastewater, municipal wastewater, or fermentation residues, which are previously lost unused and pollute water bodies. Therefore, membrane distillation processes are currently being developed to extract ammonia from gases for ammonia fuel cells (e.g., the "Ammonia-to-Power" project, AEEE). Such a separation process would be advantageous for the post-treatment of ammonia-containing synthesis exhaust gases or for the post-treatment of ammonia fuel cells or electrolyzers in which ammonia is produced (e.g., the "Power-to-Ammonia" project of RWTH Aachen University in collaboration with the applicant).

[0008] In the pressure swing adsorption process, special porous materials (e.g., zeolites, activated carbon) are used as adsorbents. The separation effect can be based on two different principles: separation based on equilibrium adsorption or separation based on molecular sieve action. In the first case, one of the components to be separated is adsorbed more strongly than another, resulting in an enrichment of the less well-adsorbed component in the gas phase. In the second case, certain molecules penetrate the porous structure of the adsorbent more quickly. If the gas mixture flows through the adsorbent in a reactor bed, the component that penetrates the pores less easily requires less time to pass by and thus reaches the reactor bed exit sooner. The gas is introduced under increased pressure (usually 6 bar to 10 bar) into a fixed-bed reactor filled with the adsorbent, causing it to flow through the adsorbent.One or more components of the mixture (referred to as "heavy components") are now adsorbed. At the bed outlet, the so-called "light component" can be removed in concentrated form. After a while, the adsorber bed is largely saturated, and a portion of the heavy component escapes. At this point, the process is switched via valves so that the outlet for the light component is closed and an outlet for the heavy component is opened. This is accompanied by a pressure drop. At the low pressure, the adsorbed gas is desorbed again and can be recovered at the outlet. Two alternately loaded and unloaded adsorbers enable continuous operation. To expel the excess of desorbed heavy component from the adsorber bed, a portion of the desired product is used to avoid contamination.The precise adjustment of the switching times is based on the desired gas purity. Increasing the purity of one component always results in a reduction in the recoverable quantity and the purity of the other component. Pressure swing adsorption is used when the desorption energy is low and the concentration is high. It is used for the separation of air to produce N2 (>99.9%), O2 (<97%), or argon, as well as for the production / purification of hydrogen, e.g., for fuel cells. Another application is the removal of CO2 from biogas or the removal of water from compressed air.

[0009] Membrane technology is considered an energy-efficient alternative for the separation of numerous gases and solvent mixtures. Specific examples include the separation of hydrogen from gasification and hydrocarbons, the upgrading of biogas and landfill gas, exhaust gas treatment, methane purification, and the dehydration of organic solvents. In this context, inorganic microporous membranes have been intensively investigated due to their thermal and chemical stability. Membrane candidates include silica, zeolites, carbon molecular sieves, and graphene / graphene oxide membranes.

[0010] There are three mechanisms by which membrane separation can basically occur: Knudsen diffusion, the molecular sieve effect, or solution diffusion.

[0011] Polymeric membranes are generally non-porous, and therefore gas permeation through them generally occurs through the solution-diffusion mechanism. This mechanism is based on the solubility of specific gases in the membrane and their diffusion through the dense membrane matrix. Polymeric membranes are further divided into rubber-like membranes, which operate above the glass transition temperature (Tg) of the polymer, and glass-like membranes, which operate below the Tg.

[0012] The Knudsen separation is based on the fact that certain gas molecules pass through membrane pores that are small enough to prevent the entire gas from passing through.

[0013] The molecular sieve mechanism relies on size exclusion to separate gas mixtures. Pores within the membrane have a strictly controlled size relative to the kinetic diameter of the gas molecules. This allows the smaller gas molecules to diffuse at a much higher rate than the larger gas molecules.

[0014] Membranes enable the separation of gas mixtures consisting of components with different kinetic molecular diameters because larger molecules are excluded by the membrane while smaller molecules pass through it.

[0015] In addition to MOFs (metal-organic frameworks) and polymers, zeolites and carbon materials are primarily used as adsorbents for gas separation. Carbon adsorbents are usually highly porous activated carbon materials, which, in addition to their high porosity, exhibit a certain selectivity for CO2 adsorption. To increase this selectivity, the surfaces of the carbon materials are usually impregnated with bases such as KOH. Carbon from biomass impregnated in this way is widespread. Examples include carbonized polymers such as carbonized polypyrrole, polyindole, melamine or urea resins, and PVDF.

[0016] Amine-functionalized or base-impregnated carbons made of polyacrylonitrile fibers (hereinafter referred to as PAN fibers) and their use for the adsorption of CO 2 are also already known. (Hsiao, H.-Y., Huang, C.-M., Hsu, M.-Y. & Chen, H. Preparation of high-surface-area PAN-based activated carbon by solution-blowing process for CO2 adsorption. Separation and Purification Technology 82, 19-27; 10.1016 / j.seppur.2011.08.006 (2011). Such PAN fibers are obtained, for example, by wet spinning of a PAN solution and subsequent carbonization (Hsiao, ibid., Shen, W., Zhang, S., He, Y., Li, J. & Fan, W. Hierarchical porous polyacrylonitrile-based activated carbon fibers for CO2 capture. J. Mater. Chem. 21, 14036; 10.1039 / C1JM12585K (2011); Li, L. et al. Nitrogen-Enriched Porous Polyacrylonitrile-Based Carbon Fibers for CO2 Capture. Ind. Eng. Chem. Res. 57, 11608-11616; 10.1021 / acs.iecr.8b01836 (2018).

[0017] The production of carbonized PAN fibers by electrospinning a PAN solution in dimethylformamide (DMF) and subsequent carbonization of the fibers is also already known (Kim, DW, Jung, DW, Adelodun, AA & Jo, YM Evaluation of CO2 adsorption capacity of electrospun carbon fibers with thermal and chemical activation. J. Appl. Polym. Sci. 134, 45534; 10.1002 / app.45534 (2017). Urea is added to the polymer solution for activation before electrospinning. Carbonization takes place at 200 °C for 4 h in air. This is followed by activation in a CO2 stream at 600-800 °C for 15 min.

[0018] R. Schierholz et al. (RSC Adv., 2019, 9, 6267) describe the carbonization of non-pretreated PAN fibers at in situ-Observation under a microscope in an ultra-high vacuum. The temperature is lowered to room temperature after each carbonization step and then raised to a higher temperature than in the previous step.

[0019] Other known carbon adsorbents are carbonized bulk PAN and carbonized PAN copolymer (Travis, W. Dissertation. University College London, 2014; Nandi, M. et al. Unprecedented CO2 uptake over highly porous N-doped activated carbon monoliths prepared by physical activation. Chemical Communications (Cambridge, England) 48, 10283-10285; 10.1039 / C2CC35334B (2012).

[0020] Carbon fiber adsorbents consist of highly porous carbon fibers processed into a nonwoven (fiber mat). Therefore, in addition to the aforementioned pores of the fiber itself, the fiber mat also has relatively large interfiber spaces.

[0021] The disadvantage of previously known carbon adsorption materials is their relatively low selectivity. The separation efficiency also depends explicitly on the amount of adsorbate on the adsorbent. Adsorption occurs via van der Waals interactions with the pore walls and acid-base interactions.

[0022] Ultramicroporous zeolite adsorbents, on the other hand, are expensive and very sensitive to moisture.

[0023] CN 106 36 26 84 A discloses a process for producing a nitrogen-containing carbon nanofiber material suitable for adsorbing CO2. The process comprises a) adding a carbon nanofiber precursor polymer to a solvent at room temperature and stirring the mixture for 12-24 hours until the precursor polymer is completely dissolved, obtaining a solution with a polymer content of 6-15 wt.%, b) electrospinning this solution to produce a nanofiber material, c) carbonization, d) surface treatment, and e) activation treatment. Polypyrrole, PAN, polyimide, polybenzoxazine, polyvinyl butyral, polyaniline, polybenzimidazole, phenolic resin A, polyvinylpyrrolidone, polyvinyl alcohol, cellulose, cellulose acetate, and ethylcellulose, as well as mixtures thereof, are disclosed as precursor polymers.Carbonization is achieved by heating the precursor nanofiber material in a first step to 150 to 300 °C in air for pre-oxidation treatment for 0.5 to 5 hours, followed by heating to 600 to 1600 °C under protective gas for 0.5 to 20 hours. The surface modification treatment comprises three steps: first, oxidation with concentrated acid, in particular with a mixture of concentrated H 2 SO 4 / concentrated HNO 3 at 50 to 70 °C for 1 to 5 hours, followed by multiple ultrasonic treatments with distilled water and drying to remove residual N- or S-oxides from the surface, and finally an etching treatment in an NH 3 atmosphere at 600 to 1000 °C for 2 to 5 hours.During the activation treatment, the resulting nanofiber is stirred in a solution of potassium hydroxide or potassium carbonate until completely dispersed, filtered, and then heated at 800–1000°C under nitrogen for 0.5 to 2 hours. The resulting nanofiber is then rinsed with HCl, washed with deionized water, and dried for 5 to 10 hours at 50–80°C. Due to the numerous steps, the process is very time-consuming, labor-intensive, and energy-intensive.

[0024] It is known to produce metal oxide-carbon fiber composites for air filter membranes by electrospinning and carbonization, e.g., from CN 106582148 A. The carbon comes from rapeseed, moss, or cabbage, i.e., biomass. The metal oxide is, for example, TiO 2 . After electrospinning, the resulting fibers are immersed in 5 M hydrochloric acid for 10 h at 60 °C, then washed and dried to remove impurities. Additionally, the membrane can also contain silver nanoparticles introduced during electrospinning.

[0025] Carbon fibers containing metal oxides or intermetallic compounds, obtained by electrospinning and carbonization, are also known, for example, from CN 104466168 A, US 2007 / 0048521 A1, and US 2013 / 0126794 A1. Such materials are used as catalysts, e.g., for methane-carbon dioxide reforming, or primarily as electrode materials for lithium-ion batteries.

[0026] US 2016 / 0036037 A1 discloses a method for producing a lithium-ion battery by electrospinning. Among other things, a conductive material is disclosed that is obtained by electrospinning PAN and subsequent carbonization using a laser. It contains elemental sulfur, sulfur-containing compounds such as Li 2 S, silicon, silicon-containing compounds, or various metal mixed oxides.

[0027] WO 2019 / 004974 A2 discloses a process for producing carbon nanofiber bundles by electrospinning, a stabilization step by low-temperature oxidation under tensile stress, and subsequent carbonization. The resulting carbon fibers are intended for use as anode material in lithium-ion batteries.

[0028] CN 107 051 382 A discloses a method for producing a carbon fiber material using a hybrid electrospinning process in which a composite fiber is produced from PAN and a pore former.

[0029] It is particularly interesting to produce suitable components for the electrochemical reduction of CO 2 and the synthesis of NH 3 . Since the electrochemical reduction of gases takes place at three-phase boundaries and the efficiency depends on the proportion of the reaction gas in the gas stream, the reaction gas must first be enriched. For this purpose (i.e., for the separation and thus enrichment of, for example, CO 2 or NH 3 ), the inventors sought suitable materials that can be used directly in the reactor. The fiber material is intended to be used for the electrochemical reduction of CO 2 with water or hydrogen to produce synthesis gas, methane, ethylene, formaldehyde, formic acid, and other hydrocarbons and alcohols.

[0030] Furthermore, in the power-to-ammonia method, the NH 3 should ideally be removed from the resulting mixed gas stream on the product side.

[0031] Against this background, the object of the invention is to provide a process for producing carbon fiber material which overcomes the aforementioned disadvantages and is simple and cost-effective to carry out, as well as providing a carbon fiber material which shows a high selectivity for CO 2 or NH 3 and enables the adsorption of large quantities of these gases. Description of the invention

[0032] The inventors have recognized that the carbonization conditions of PAN fibers play an important role in the production of carbon materials from PAN. Their influence on the creation of pores of specific sizes, such as ultramicropores, has previously been ignored. These ultramicropores allow selective adsorption through a molecular sieve effect without the use of acid-base interactions. This increases the reversibility of the adsorption and makes the application more efficient. The present invention has shown that ultramicropores are the key to the successful adsorption of large amounts of CO2 with high reversibility and selectivity, especially at low relative pressures.

[0033] The object of the invention is achieved by the method according to claim 1. It comprises the following steps: a) preparing a solution of polyacrylonitrile in a suitable organic solvent, the solution consisting only of solvent and polyacrylonitrile b) electrospinning the solution obtained in a) and drying the resulting fiber material c) crosslinking the resulting fiber material by heating to 150 to 350 °C in an air or oxygen atmosphere for 1 to 30 hours hd) carbonizing the resulting fiber material in a protective gas atmosphere at a temperature in the range of 600 to 900 °C, more preferably 800 to 875 °C, without the action of tensile stress.

[0034] The PAN solution in step a) contains no metal compounds, intermetallic compounds, silicon compounds, sulfur, sulfur compounds, and / or silicon compounds disclosed, for example, in US 2013 / 0126794 A1, US 2016 / 00306037 A1, CN 106582148 A, CN 104511279 A. These react during the subsequent carbonization, in particular to form metal oxides, mixed metal oxides, or intermetallic compounds contained in the fibers. According to the invention, however, the carbonized fibers consist only of the pyrolysis product of PAN.

[0035] Preferably, the solution in step a) consists of DMF and polyacrylonitrile.

[0036] The fiber material obtained in step b) is crosslinked before the actual carbonization, e.g., at 250 °C for 3 to 30 hours, preferably 8 to 20 hours, even more preferably overnight. Heating causes the cyclization shown in step a) below in Scheme 1, resulting in crosslinking of the fibers, which stabilizes the PAN fibers. Furthermore, surface oxidation and partial elimination of nitrogen compounds occur during the crosslinking process.

[0037] It has surprisingly been found that this process, which is significantly simplified compared to CN 106362684 A, can produce carbon fibers that exhibit particularly good, i.e., selective and high, CO2 absorption. In particular, neither surface modification nor surface activation of the fibers after electrospinning by treatment with chemical reagents and / or application of tensile stress is necessary to achieve these properties. Preferred embodiments of the process according to the invention are defined in claims 2 to 5.

[0038] The invention further encompasses the use of a carbon fiber material with a specific high proportion of ultramicropores with a pore size of <0.4 nm, obtainable by the said process, for separating NH 3 from gas mixtures. The carbon fiber material according to the invention is also referred to below as nanofiber mats. Description of the characters

[0039] The relationship between carbonization temperature and adsorption properties for Ar and CO 2 is shown in Fig. 1 shown. Fig. 2 and Fig. 4 show the adsorbed volume of Ar or CO 2 per g of carbon fiber material at different relative pressures as a function of the carbonization temperature. Fig. 3 represents the dependence of the quotient of adsorbed CO 2 volume and surface of the material as a function of the carbonization temperature. It is clearly visible how the quotient drops steeply at about 850 °C and hardly changes from about 980 °C onwards. Fig. 5 represents the cumulative pore volume as a function of pore size for different carbonization temperatures. Fig. 6 shows the adsorbed or desorbed volume of Ar at different relative pressures for carbonization temperatures of 600 °C and 700 °C, respectively. Fig. 7shows SEM images (30,000 magnification, 20 kV) of carbon fibers according to the invention. Fig. 8 to Fig. 10 show NH 3 isotherms measured at 240 K, 273 K and 298 K, respectively, on samples according to the invention which were carbonized at different temperatures. Fig. 11 shows a comparison of the isotherms of NH 3 and CO 2 , each measured at 273 K on samples according to the invention carbonized at different temperatures. The left part of the figure shows the adsorption of NH 3 , the right part the adsorption of CO 2 . Fig. 12 shows the NH 3 / N 2 selectivity (IAST) of a gas composition with a molar ratio of 0.001:99.999 (NH 3 / N 2 ) with samples according to the invention that were carbonized at different temperatures. Fig. 13 shows the NH 3 / N 2 selectivity (IAST) of a gas composition with a molar ratio of 10:90 with a sample according to the invention that was carbonized at 1000 °C. Fig. 14shows the isosteric adsorption enthalpy of NH 3 , measured for samples according to the invention carbonized at different temperatures. The adsorption enthalpy was determined by the isosteric method. The data used for this purpose were obtained from the isotherms by linear interpolation between two isothermal points. Fig. 15 shows the CO 2 / N 2 selectivity at a gas volume ratio CO 2 :N 2 of 5:95, measured by dynamic gas adsorption on samples according to the invention that were carbonized at different temperatures. Fig. 16 shows the CO 2 / N 2 selectivity at a gas volume ratio CO 2 :N 2 of 10:90, measured by dynamic gas adsorption on samples according to the invention that were carbonized at different temperatures. Fig. 17 shows IAST calculations for CO 2 / N 2 selectivity. Detailed description

[0040] The carbon fiber material obtained by the process according to the invention has predominantly ultramicropores in the fibers, with hardly any mesopores or macropores. The carbon fibers are porous with pores of various sizes.

[0041] According to the invention, ultramicropores represent the largest proportion of the pores present in the fibers, while mesopores or macropores are fewer. This results in the carbon fiber material according to the invention being able to reversibly adsorb a large amount of CO 2 or NH 3.

[0042] After carbonization in step c), the carbon fiber material is obtained as a nonwoven (nanofiber mat), which can be used directly as an adsorption material in gas separation processes without further processing steps.

[0043] The thickness of the nonwoven obtained in step c) when produced on a laboratory scale is typically 100 to 2000 µm, preferably 400 to 1000 µm, more preferably 600 to 900 µm, and particularly preferably 800 µm, depending on the spinning time. However, the size and thickness of the nanofiber mat can be adjusted to much larger values ​​after adjusting the electrospinning system.

[0044] As Polyacrylonitrile (PAN) According to the invention, a polyacrylonitrile homopolymer or a polyacrylonitrile copolymer with up to approximately 5 to 10 wt. % of one or more comonomers can be used. Suitable comonomers include, for example, acrylic acid, methacrylic acid, itaconic acid, and / or acrylamide. The polyacrylonitrile is either commercially available (e.g., from Sigma-Aldrich or BOC Sciences) or can be prepared in a conventional manner. The molecular weight of the PAN polymer in g / mol is preferably 50,000 to 200,000, preferably 80,000 to 180,000, particularly preferably 100,000 or 150,000.

[0045] As solvent Any organic solvent that can dissolve PAN is suitable. Polar solvents such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dimethylacetamide (DMAC), acetone, methyl ethyl ketone, ethanol, or mixtures thereof are particularly suitable. DMF is particularly suitable.

[0046] According to the invention, the PAN solution in step a) has a concentration of 5 to 20 wt.%, preferably 10 to 15 wt.%, particularly preferably approximately 10 wt.%. It is prepared by adding the PAN to the solvent and stirring, optionally with heating, until the PAN is completely dissolved (visual inspection). Stirring is carried out, for example, at room temperature for 2 days.

[0047] Electrospinningis a fiber production process that uses electrical force to pull charged threads from polymer solutions or polymer melts down to a fiber diameter of a few hundred nanometers. The process requires neither the use of coagulation chemistry nor high temperatures to produce solid threads from the solution. This makes the process particularly suitable for the production of fibers from large and complex molecules. The standard laboratory apparatus for electrospinning consists of a spinneret (typically one or more injection needles), for example 4 or 16 needles, connected to a high-voltage direct current supply (e.g. 5 to 100 kV), a syringe pump, and a collector, which can be grounded or used at negative voltages of several kV.A polymer solution, sol-gel, particulate suspension, or melt is loaded into the syringe, and this fluid is extruded from the needle tip by a syringe pump at a constant velocity. Alternatively, the droplet at the tip of the spinneret can be replenished by feeding from a collection tank at a constant discharge pressure. This constant-pressure delivery works better for low-viscosity feedstocks. When a sufficiently high voltage is applied to a liquid droplet, the liquid body becomes charged, and electrostatic repulsion counteracts the surface tension, stretching the droplet; at a critical point, a liquid stream erupts from the surface. This breakout point is known as the Taylor cone. If the molecular cohesion of the liquid is sufficiently high, flow separation does not occur (if it does, the droplets are electrosprayed), and a charged liquid jet is formed.As the jet dries in flight, the current flow mode changes from resistive to convective as the charge migrates to the surface of the fiber. The jet is then extended by an impact process triggered by electrostatic repulsion at small bends in the fiber until it is finally deposited on the grounded collector. The resulting stretching and thinning of the fiber leads to the formation of uniform fibers with nanometer diameters. The collector rotates, and the needle is also moved laterally back and forth, resulting in a uniform thickness of the fiber mat. Drying the fiber during spinning creates chaotic movement of the fiber, resulting in disordered deposition of the spun fiber and formation of a fiber mat. The collector can be a plate or a drum, for example, or even (in large-scale systems) a moving mat.

[0048] In the process according to the invention, electrospinning is carried out according to a standard method. Typically, an electrospinner, e.g. from IME Technologies, is used at 10 to 60%, preferably 15 to 35%, particularly preferably 20 or 30% relative humidity and 20 to 30 °C, preferably 25 °C. However, it has been shown that these environmental parameters have no significant influence on the properties of the carbon fiber material. The flow rate is, for example, 20 to 60, preferably 30 to 50 and particularly preferably 40 µl / min per needle used, the volume 1.5 to 20 ml, preferably 2 to 15 ml. The spinning time depends on the volume and is, for example, 1 h or 2 h for 2.2 ml and 6 h for 14.6 ml. The voltage is, for example, 21 kV at the anode and -4 kV at the cathode. The collector rotates at 1000 to 2000 rpm, preferably 1500 rpm. The distance between the needle and the collector is 100 to 200 mm, preferably 130 to 190 mm, especially 140 or 180 mm.The inner needle diameter is, for example, 0.5 to 1 mm, preferably 0.7 to 0.9 mm, especially 0.8 mm. The lateral movement of the needle occurs over a total distance of 80 to 150 mm, preferably 90 to 130 mm, particularly preferably 100 or 120 mm. The speed of the lateral needle movement is, for example, 20 mm / s, and the reversal delay is, for example, 500 ms. In this way, PAN nanofiber mats are obtained.

[0049] After electrospinning in step b), the nanofiber mats obtained are preferably first dried, e.g. at 100 to 200 °C, preferably about 150 °C, for 1 to 24 h, preferably 1 to 2 h, particularly preferably for about 1 h in an air atmosphere.

[0050] They are then crosslinked in an air or oxygen atmosphere, e.g. at 150 to 300°C, preferably 200 to 300°C, particularly preferably 250°C for 8 to 30 hours, e.g. overnight.

[0051] According to the invention, the crucial step for achieving the desired properties of the carbon fiber material is the carbonization of the resulting nanofiber mats in step c), and in particular their temperature. Temperature is an easily adjustable and controllable parameter, but its influence on the pore structure and gas adsorption properties has not yet been investigated.

[0052] At the Carbonization This is a pyrolysis process. This involves the conversion of PAN fibers through cyclization, dehydrogenation, and N 2 elimination, as shown schematically below:

[0053] The cyclization (step a)) already takes place partly in the crosslinking step b), as explained above.

[0054] Depending on the progress of the reactions, nitrogen atoms can be present on the fiber surface in different hybridization states. These can be determined, for example, by XPS (X-ray photoelectron spectroscopy).

[0055] The Carbonization According to the invention, for example, the reaction takes place in an oven at comparatively low temperatures of 600 to 900 °C, particularly preferably at approximately 800 °C to 850 °C.

[0056] Alternatively, carbonization can also be carried out using laser or IR heating as well as microwave treatment.

[0057] According to the invention, it is not the type of heating that is important, but rather compliance with the specified temperatures during carbonization in order to achieve the claimed properties of the fiber material.

[0058] The inventors have discovered that the pore size in the carbon fiber material obtained after carbonization decreases continuously from low carbonization temperatures (approx. 600 °C) to higher carbonization temperatures (approx. 1000 °C). Above 850 °C, the pore size becomes so small that CO 2 or NH 3 can no longer diffuse into the pores, resulting in a sudden change in the adsorption properties of the material with respect to CO 2 and NH 3. Temperatures between 800 and 850 °C are particularly suitable, as CO 2 is still well adsorbed at this temperature, but the pores for N 2 and Ar are already too small.

[0059] The conductivity of the resulting carbon fiber material also depends on the carbonization temperature, as the inventors discovered. When carbonized in the temperature range of 800 to 850 °C, the electrical conductivity is higher than, for example, when carbonized at 600 °C or 700 °C, making the material more suitable for use as an electrode.

[0060] According to the invention, the carbonization treatment is carried out under a protective gas (inert gas). Suitable gases include nitrogen, argon, or helium, most preferably argon. The protective gas can be introduced into the furnace, for example, and a gas flow can be maintained throughout the reaction period. Before the carbonization treatment, the furnace is preferably evacuated and flooded with a protective gas, such as argon, to prevent the entry of air. The atmosphere in the furnace is as free as possible from O2 and CO2 by means of a protective gas atmosphere. According to the invention, the carbonization is carried out, for example, at atmospheric pressure in a protective gas atmosphere, typically under argon at a flow rate of, for example, approximately 200 l / h. It is expected that other flows or pressures can be used without significant effects on the product being observed. The duration of the carbonization treatment is typically 1.5 to 5 hours, preferably 2 to 4 hours, particularly preferably 3 hours.The heating rate is typically 200 to 400 °C / h, preferably 250 to 350 °C / h, most preferably 300 °C / h. The cooling rate is typically 150 to 250 °C / h, preferably 200 °C / h. The heating and cooling times are not included in the specified carbonization time, but are additional.

[0061] The carbon fiber material obtained by the process according to the invention shows flat fiber surfaces and no preferential orientation due to the lateral movement of the needle during electrospinning (i.e., the fiber is not rolled up in an orderly fashion or arranged in parallel loops). The smooth surface of the fiber was confirmed by SEM ( Fig. 7 The fiber diameter is uniform and decreases from approximately 250 nm to 220 nm with increasing carbonization temperature. The fiber diameter is estimated using scanning electron microscopy (SEM) and ImageJ ® software and is based on average values ​​of 20-30 measurements.

[0062] The Pore ​​size of the carbon fiber material is defined here as in the IUPAC Technical Report (https: / / doi.org / 10.1515 / pac-2014-1117; part 2, General definitions and terminology). It is Ultramicropores < 0.7 nm Micropores < 2 nm Mesopores 2-50 nm Macropores > 50 nm.

[0063] Under "kinetic diameter" of gases is understood here as the smallest diameter that a molecule can present to its environment. It differs from the atomic diameter,which indicates the atomic size as the size of the electron shell and is generally much smaller than the kinetic diameter. According to the CRS Handbook of Chemistry and Physics, there are several ways to determine the kinetic diameter, namely from viscosity data, the van der Waals equation, and thermal conductivity. Calculations from bond angles, bond lengths, and the van der Waals radii are also possible. In most cases, the kinetic diameters calculated using the various methods differ by only 2-3%.

[0064] In the context of the present application, the following kinetic diameters for gases given in the literature are assumed: CH 4 0.38 nm [1]< , Ar 0.340 nm [2]< , N 2 0.364 nm [2]< , CO 2 0.330 nm [2]< , H 2 0.29 nm [1]< , NH 3 0.260 nm [1]< or, which better matches the experimental results, 0.326 nm [3]< ([1] B. Scholes, CA et al., Recent Patents on Chemical Engineering, 2008, 1, 52-66; [2] DWBeck, Zeolite Molecular Sieves: Structure, Chemistry and Use, Wiley, New York 1974, 593-724, [3] ME van Leeuwen, Fluid Phase Equilib., 1994, 99, 1-18).

[0065] According to the invention, the pore size in the carbon fiber material can be determined by single gas adsorption measurements of several gases with different kinetic diameters, e.g., Ar, N 2 , CO 2 , and O 2 . The basic principles of the measurement methods are given in the aforementioned IUPAC Technical Report. Therefore, measurement with argon is suitable for determining larger pores, but not for measuring ultramicropores. For this, measurement with CO 2 is required.

[0066] For example, the pore size is determined by adsorption of Ar, as in Fig. 2 Samples carbonized at 600 °C and 700 °C show very slow adsorption kinetics and pseudo-irreversibility, which prevents a reasonable measurement (cf. Fig. 6 This indicates pores with very small diameters or strong pore tortuosity. For samples carbonized at 800 °C and higher, the adsorption of argon is completely reversible without hysteresis.

[0067] The BET surface area is determined using the standard method of adsorbing argon gas at 87 K, followed by evaluation of the monolayer adsorption region according to the BET model. The ASiQwin® software (Quantachrome) is used for this purpose.

[0068] The BET surface area measurements with argon are shown in Table 1 below: Table 1: BET surface area of ​​samples from Ar adsorption experiments sample S BET (m 2 < / g) sample S BET (m 2 < / g) C800 13,4 C925 15,0 C825 12,4 C950 14,3 C850 13,1 C975 16,0 C875 13,2 C1000 14,8 C900 14,6 C1100 20,0

[0069] C800 refers to a carbon fiber sample carbonized at 800 °C, the other designations are analogous.

[0070] The BET surface area S BET of the carbon fiber material according to the invention, determined by argon adsorption, is therefore relatively low and lies in the range of 12 to 20 m 2 < / g with a strong variation, which increases slightly from samples carbonized at 800 °C to samples carbonized at 1100 °C due to the reduction of the fiber diameter (therefore larger geometric surface area).

[0071] Fig. 4shows the CO2 adsorption isotherms of the fibers according to the invention. In contrast to the Ar measurements, CO2 adsorption could be measured without any problems in the samples carbonized at 600 °C and 700 °C, which implies a kinetic inhibition of Ar adsorption, since the adsorption of CO2 at 273 K is much faster than that of Ar at 87 K. The isotherms show a similar shape from C600 to C850, namely a steep rise that flattens out at higher pressures. The initial slope is slightly higher for C600 and C700 than for C800 and C850, indicating a higher adsorption energy due to a higher number of narrower pores or a changed surface chemistry. However, the difference is not very large, and the total pore volume at 1 bar is almost identical.C900 exhibits a smaller initial slope but only a slightly reduced amount of adsorbed CO2 at 1 bar, while the initial slope and total adsorbed volume decrease significantly at carbonization temperatures of 950 °C and above. C1100 achieves a slightly larger adsorbed volume than C1000, which is likely due to the slightly increased surface area without a significant change in porosity or surface chemistry.

[0072] Fig. 3shows that the adsorbed CO 2 volume divided by the BET surface area is a measure of the surface affinity for CO 2. The value is practically constant at 5 cm 3 / m 2 for C800 to C875, and drops sharply to less than 1 cm 3 / m 2 at higher carbonization temperatures > 950 °C. The value of 5 cm 3 / m 2 is extraordinarily high compared to other carbon materials, which normally require a surface area of ​​several hundred m 2 to achieve a similarly high CO 2 adsorption capacity. This phenomenon, exhibited by the carbon fiber material of the invention, could be attributed to a high chemical affinity for CO 2 or to a high ultramicropore volume, whereby the ultramicropores are not accessible to Ar and therefore do not contribute anything to the BET surface area.

[0073] The texture properties of the carbon fiber materials according to the invention derived from CO 2 adsorption tests in comparison to three known commercially available carbons (carbon blacks) are summarized in the following Table 2: Table 2: Texture properties derived from CO 2 adsorption experiments Carbonization temperature S micro (MC) V<0.4nm (MC) V >0.4nm (MC) V total (MC) °C m 2 < / g cm 3 < / g cm 3 < / g cm 3 < / g 600 626 0,053 0,119 0,172 700 618 0,051 0,114 0,165 800 615 0,039 0,141 0,180 825 599 0,039 0,133 0,172 850 610 0,034 0,145 0,179 875 656 0,035 0,156 0,191 900 551 0,011 0,179 0,190 925 318 0,004 0,132 0,136 950 274 0 0,118 0,118 975 160 0 0,071 0,071 1000 151 0 0,069 0,069 1100 165 0 0,070 0,070 SuperP 299 0 0,134 0,134 GP300 321 0,004 0,126 0,130 BP2000 1173 0,007 0,470 0,477 S micro is the surface area of ​​the micropores V <0.4nm is the volume of pores with a pore size < 0.4 nm V >0.4nm is the volume of pores with a pore size > 0.4 nm V ges is the total pore volume E ads is the adsorption energy MC means determined according to a Monte Carlo simulation (Grand Canonical Monte Carlo Simulation (GC-MC)) SuperP is Carbon black SuperP ®< from Alfaesar GP300 are graphene platelets from Sigma-Aldrich with a stated BET surface area of ​​300 m 2< / g BP2000 is Black Pearls ®< 2000 Carbon Black from Cabot.

[0074] The pore volume was calculated from GC-MC and divided into pores larger and smaller than 0.4 nm. The pore volume <0.4 nm decreases with increasing carbonization temperature, while the total pore volume and the micropore surface area increase slightly from C600 to C900. C1000 shows no pores below 0.4 nm and a sharp decrease in total pore volume and surface area.

[0075] Commercially available carbon materials exhibit no pores <0.4 nm, which explains their low CO 2 uptake at low pressures. The carbon fiber material according to the invention, in contrast, exhibits a high proportion of ultramicropores V <0.4 nm, ranging from 0.01 to 10, preferably 0.02 to 5, even more preferably 0.025 to 0.1, and most preferably 0.03 to 0.06 cm 3 / g, determined by CO 2 adsorption and evaluation using DFT and GC-MC simulation.

[0076] Fig. 8 to 10 show adsorption isotherms of ammonia measured on various samples according to the invention at different temperatures.

[0077] Surprisingly, it was found that NH 3 and CO 2 exhibit the same tendency in the inventive materials: very high adsorption was achieved at C600 to C800. C900 proved to be kinetically hindered. At C1000, only a low adsorption of the respective gas was observed. NH 3 as a Lewis base and CO 2 as a Lewis acid require different surface chemistry for effective adsorption. It is very unlikely that chemically different molecules will exhibit the same adsorption behavior due to changes in the surface chemistry of the adsorbent. Therefore, in the inventive materials, the decisive factor for adsorption is not the surface structure or chemistry, but the accessibility of the ultramicropores.

[0078] The comparison of the adsorption of NH 3 and CO 2 ( Fig. 11) shows that at C600 to C800, NH 3 is adsorbed in larger amounts than CO 2 . At C1000, however, CO 2 is adsorbed more strongly than NH 3 . This could be due to the fact that NH 3 has access to a larger number of pores, the pores can be packed more densely with NH 3 (i.e., more NH 3 molecules fit into the same pore than CO 2 molecules), or that the pores have different surface chemistry.

[0079] It is known from the literature (M. Goncalves et al., Environ. Sc. Technol. 2011, 45, 10605-10610) that the NH 3 adsorption capacity increases linearly with the amount of oxygen surface groups. However, in the samples according to the invention, no correlation was observed between surface chemistry (oxygen or nitrogen) and the NH 3 adsorption capacity, as shown in the following tables.

[0080] The elemental composition of the samples was determined by elemental analysis and XPS and is summarized in the following tables: Table 3: CHNO analysis sample C (wt%) N (wt%) O (wt.%) H (wt%) C600 63,5 23,0 11,8 2,1 C700 67,9 19,9 11,1 1,8 C800 72,6 16,2 10,6 1,4 C900 77,6 11,6 9,5 1,2 C1000 91,5 7,1 3,1 0,4 C1100 95,8 3,7 0,8 0,5 Table 4: XPS analysis sample C (at%) N (at%) O (at%) C600 76,5 19,7 3,8 C700 80,4 16,8 2,8 C800 84,1 13,7 2,2 C900 86,6 11,0 2,4 C1000 92,6 4,3 3,1 C1100 95,1 3,1 1,8

[0081] This provides further evidence that, according to the invention, pore size is the determining factor for adsorption capacity, rather than surface chemistry.

[0082] For the fiber materials according to the invention, the adsorption data correlate strongly with the kinetic diameter of the adsorbate.

[0083] The relationship between kinetic diameter and adsorption capacity of the samples according to the invention is shown in the following table: Table 5: Kinetic diameter and adsorption capacity gas Kinetic diameter (pm) Samples with high adsorption capacity N 2 364 Ar 340 C600* - C700* CO2 330 C600 - C900* NH3 260 / 326 C600 - C900* *: kinetically hindered

[0084] The arrow on the right means that the adsorption capacity increases from top to bottom.

[0085] The smaller the molecules or atoms, the smaller the pores they can penetrate. The pore size decreases with increasing carbonization temperature. The adsorption capacity therefore changes drastically from one sample to another if molecules can no longer penetrate the pores. The kinetic diameter and pore size effects therefore consistently explain the adsorption behavior of the nanofiber mats according to the invention.

[0086] 0.26 nm is a value frequently cited in the literature as the kinetic diameter for NH 3. However, it has been shown that this value cannot adequately explain experimental data, particularly for pore diffusion. The value determined by van Leeuwen (0.326 nm), on the other hand, is in good agreement with the findings of Kanezashi et al. and is therefore used here. (M. Kanezashi, A. Yamamoto, T. Yoshioka, T. Tsuru, AI Ch. E., 2010, 56, 1204-1212, ME van Leeuwen, Fluid Phase Equilib., 1994, 99, 1-18).

[0087] CO 2 and NH 3 exhibit similar behavior due to their comparable molecular size. CO 2 and NH 3 have very slow adsorption kinetics on C900 (very small pores). However, surface chemistry has only a minor influence on adsorption.

[0088] NH 3 is more strongly adsorbed than CO 2 on samples with a larger number of accessible pores (C600 to C800). In contrast, CO 2 is slightly favored when the pores are inaccessible (C1000). Surface chemistry slightly favors CO 2 adsorption, at least at C1000.

[0089] Fig. 14 shows the isosteric adsorption enthalpies of NH 3 on the samples. Higher values ​​indicate stronger adsorption of NH 3 on the sample.

[0090] IAST (ideal adsorbed solution theory) calculations predict excellent selectivity of the inventive materials for NH 3 over N 2. The results are presented in Fig. 12 and 13 shown.

[0091] However, these calculations should only be considered approximate, as components with strongly different adsorbing properties can lead to deviations in lAST calculations. The lAST results can be well explained by a molecular sieve effect. C600 to C800 have pores that are accessible to NH 3 but not to N 2 . This results in high selectivity values. At C1000, i.e., a material without accessible pores, the selectivity for NH 3 decreases sharply.

[0092] Figs. 15 and 16 show measured values ​​for CO 2 / N 2 selectivity that are in agreement with the IAST predictions (approximately 25 - 55 for C600 - C800). To the inventors' knowledge, these values ​​are among the highest values ​​measured to date for CO 2 / N 2 on carbon-based samples.

[0093] For comparison, lAST calculations of the CO 2 / N 2 selectivity in Fig. 17These are slightly higher than the measured values ​​(high double-digit to low triple-digit range).

[0094] The carbon fiber material according to the invention is particularly suitable for separating NH 3 from gas mixtures. The gas mixtures can be, for example, air, combustion exhaust gases, natural gas, biogas, or other gas mixtures.

[0095] Due to its small kinetic diameter, H 2 can also be separated from suitable gas mixtures (e.g. CH 4 / H 2 ) using the carbon fiber materials according to the invention.

[0096] The materials according to the invention can be used advantageously as adsorbents in the pressure swing, alternatively in the temperature swing and, with particular advantage due to the particularly high selectivity at low pressures, in the vacuum swing adsorption process.

[0097] The carbon fibers of the invention also adsorb large amounts of water. They could therefore be advantageously used for drying gases.

[0098] For the same reason, it is important to dry CO 2 or NH 3 from gases before separating them, e.g. by cooling, since water vapor competes with CO 2 or NH 3 and would therefore reduce the separation efficiency of the membrane. Examples Synthesis of carbon fibers Example 1 (not according to the invention)

[0099] PAN from Sigma-Aldrich or BOC Sciences with a molecular weight of 150,000 g / mol was used.

[0100] Carbon fibers were produced by electrospinning. First, solutions of 10% polyacrylonitrile (PAN) in dimethylformamide (DMF) were prepared. For this purpose, 4 g of PAN was dissolved in 36 g of DMF while stirring with a magnetic stirrer at room temperature for two days. The different samples were prepared from the same solution. The solution was spun directly into fiber mats in an IME Technologies electrospinner in a controlled atmosphere. The parameters shown in Table 6 were used. Table 6: Standard parameters for electrospinning climate temperature 25°C humidity 20% material flow rate 40 µL / min volume 2.2 mL Spinning time 1 hour / 2 hours PAN wt% 10 Lateral movement Total distance 100 mm Starting position -50 mm distance 100 mm speed 20 mm / s Reverse delay 500 ms Distances needle - collector 140 mm Inner needle diameter 0.8 mm Collector movement rotation 1500 rpm Tension Positive electrode 21 kV Negative electrode -4 kV

[0101] After the electrospinning process, the mats were dried in an oven at 150°C in air overnight and then crosslinked in an air atmosphere at 250°C overnight. Finally, after reaching the target temperature, the crosslinked mats were carbonized for 3 hours at 1000°C in a previously evacuated and argon-flushed oven, and then cooled. The heating rate was 300°C / h, the cooling rate was 200°C / h, and the argon flow rate was 200 L / h. Example 2

[0102] A 10 wt% solution of PAN (Mw 150,000 g / mol) in DMF was prepared by adding 8 g of PAN to 72 g of DMF and stirring at room temperature for two days. This solution was used in the electrospinning process to produce PAN nanofiber mats. The process was carried out in a controlled atmosphere with the parameters given in Table 7: Table 7: Parameters for the electrospinning process with a 10 wt% solution of PAN in DMF climate temperature 25°C humidity 30% material flow rate 40 µL / min volume 14.6 mL Spinning time 6 h PAN wt% 10 Lateral movement Total distance 120 mm Starting position -60 mm distance 120 mm speed 20 mm / s Reverse delay 500 ms Distances needle - collector 180 mm Inner needle diameter 0.8 mm Collector diameter 60 mm Collector movement rotation 1500 rpm Tension Positive electrode 21 kV Negative electrode -4 kV

[0103] The resulting PAN nanofiber mats were then dried at 150 °C for 1 h. Crosslinking was carried out in an air atmosphere at 250 °C overnight (18 h). For carbonization, the samples were heated at a heating rate of 300 °C / h under an argon stream (200 L / h) to the desired carbonization temperature, which was maintained for 3 h. The cooling rate was 200 °C / h. The samples for the temperature series 600 to 1000 °C (C600 to C1000) were prepared from the same solution. Analytics REM

[0104] The SEM images were taken with an ETD detector on an FEI Quanta ®< FEG 650 at a magnification of 30,000x. The accelerating voltage was 20 kV. During sample preparation, the samples were glued to the sample holder and grounded with a copper strip. The fiber diameters were estimated using the ImageJ ®< program. The average diameter of a sample is based on a measurement of 20 to 30 fiber diameters. Gas adsorption

[0105] Gas adsorption experiments were conducted on a Quantachrome Autosorb®< iQ2 instrument equipped with a cryocooler. For preparation, the samples were cut into small strips approximately 1 mm wide, of which 30–100 mg were transferred to the sample tube. The samples were then baked at 300°C for 8 hours. To determine the sample mass, both the empty sample tube and the tube filled with sample were weighed three times after baking. The sample weight was calculated from the difference in the average masses. The gas adsorption measurements with the gases argon, nitrogen, and oxygen were performed in different orders on the same sample. Multiple measurements were taken for each gas and sample at different temperatures. The order of the measurements was not systematically defined and was based on metrological considerations at the time of the measurement. Dynamic gas adsorption

[0106] Dynamic gas adsorption experiments were conducted using a mixSorb ®< SHP from 3P Instruments. Control and analysis were performed using the mixSorb Manager ®< software (version 1.2.3.0) and 3P Sim ®< (version 1.0.7).

[0107] The composition of the introduced gases was adjusted using several mass flow controllers (MFCs). The gas composition downstream of the sample column was measured using a Pfeiffer mass spectrometer (MS) (Pfeiffer Omnistar®< GSD 320). The sample column was heated either using a water / ethylene glycol bath (-10 to 70 °C) or a heating jacket (90 to 400 °C). The stainless steel sample column is filled with sample to a height of 8.5 cm, has an inner diameter of 0.45 cm, and a sample volume of approximately 1.0 cm³< . A temperature sensor is inserted 2.5 cm into the packing from the top.

[0108] Before measurement, 110–160 mg of the sample was first cut into approximately 1 cm long and 1 mm wide strips and introduced into the sample column using a funnel. The voids were filled by carefully pushing a rod through the strips. The empty weight and the weight of the sample column with sample were each determined three times.

[0109] The first annealing of each sample was carried out at 300 °C for 2 h under vacuum and a carrier gas flow (in this case, helium) of 7 mL / min to remove adsorbed water and other gases. A further annealing after a measurement sequence was carried out for 1 h at 70 °C under vacuum and a carrier gas flow of 7 mL / min.

[0110] To start a measurement, the specified pressure was built up using the carrier gas helium. Once temperature and pressure were stable, the experiment was started by adding the adsorptive CO2 and N2 to the sample column. The corresponding parameters can be found in Table 8. After completing a breakthrough experiment, the next breakthrough experiment in the sequence was started by increasing the adsorptive fraction to the specified value for the next measurement point. After the end of a sequence, the system was baked out as described, and the next sequence was carried out analogously. Table 8: Parameters of the measurement sequences. Helium was used as carrier gas, CO 2 and N 2 as adsorptive gases. Total pressure [bar] Temperature [°C] 1. Measuring point 2. Measuring point 1. Sequence 5,0 0 CO2: 0.5% CO2: 1.0% N 2 : 9,5 % N 2 : 19,0 % He: 90.0% He: 80.0% 2nd sequence 5,0 0 CO2: 1.0% CO2: 2.0% N 2 : 9,0 % N 2 : 18,0 % He: 90.0% He: 80.0%

[0111] For each sequence, a reference measurement was also performed using glass beads (0.4–0.6 mm diameter) as filler material. The adsorbed amount of CO 2 and N 2 was determined from the difference between the sample and reference measurements.

[0112] The selectivity was determined using the following formula: no - adsorbed amount of substance i in the adsorbed phase yi - Volume fraction of i in the gas phase BET

[0113] The quantitative evaluation of the measured gas adsorption isotherms according to the BET method was performed using the function implemented in the ASiQwin ® (Quantachrome) instrument software. The value range used for the evaluation was determined using the software's "Micropore BET assistant" to achieve reproducible point selection according to the recommended criteria. Since the parallel measurement of the saturation vapor pressure using the cryocooler (actually recommended for maximum accuracy, especially for the determination of mesopores) requires sacrificing half the measurement capacity and is not possible for vapor pressures > 1 bar / 760 Torr anyway, the saturation vapor pressures for all measurements were taken from the NIST database. They are summarized in Table 9 below. Torr is the usual unit for adsorption methods, so this unit is used here rather than the corresponding SI unit. Table 9: Saturation vapor pressures of the gases argon, nitrogen, oxygen and carbon dioxide as a function of temperature. Temperature [K] P 0 (Ar) [Torr] P 0 (N 2 ) [Torr] P 0 (O 2 ) [Torr] 77,35 212,3* 759,55 155,61 87 771,94** 2072,1 537,44 273 CO2: 26 141 *own measurement, as below the triple point (only literature values ​​for solid or gaseous phase available) **value for 87.45 K

[0114] To calculate the BET surface areas, the following values ​​for the area of ​​a gas molecule were also used: DFT and GC-MC

[0115] The measured isotherms were analyzed using the DFT (Density Functional Theory) and GC-MC (Grand Canonical Monte Carlo) methods using functions implemented in the Quantachrome ASiQwin ® software (version 5.0). The predefined model for carbon dioxide at 273 K was used for slit pores. For Ar at 87 K, the QS-DFT model for slit pores was used. The carbon reference stored in the software was selected as the material. CHNO

[0116] A Varion EL cube elemental analyzer (Elementar, Germany) was used for elemental analysis. A 2 mg sample of each fiber material was combusted in CHN mode, and a 10 mg sample was combusted in O mode. In CHN mode, the samples were combusted, and the combustion products were separated and detected. In O mode, the pores were treated in a reducing atmosphere, where O-containing fragments were converted to CO and detected as such. This procedure was repeated three times in each mode for each sample. Polyethylene was added to the samples carbonized at 900 °C to improve combustion. XPS analysis

[0117] XPS analyses were performed using a Phi5000 VersaProbe II (ULVAC-Phi Inc., US). Monochromatic 1.486 keV Al κα radiation was used for individual measurements. Peak analysis was performed using CasaXPS with a Shirley background and instrument-specific corrections. The spectra were calibrated to 284.4 eV using the C1s signal.

Claims

1. A method for producing a carbon fiber material comprising the steps of a) preparing a solution of polyacrylonitrile in a suitable organic solvent, the solution consisting only of solvent and polyacrylonitrile, b) electrospinning of the solution obtained in a) and drying of the obtained fiber material, c) crosslinking of the obtained fiber material by heating to 150 to 350 °C in an air or oxygen atmosphere for 1 to 30 h d) carbonization of the obtained fiber material in an inert gas atmosphere at a temperature in the range of 600 to 900 °C without applying tensile stress.

2. The method of claim 1, wherein the organic solvent is selected from DMF, DMSO, DMAC, acetone, methyl ethyl ketone, and ethanol and mixtures thereof.

3. The method according to claim 1 or 2, wherein the solvent is DMF.

4. The method of one or more of claims 1 to 3, wherein the polyacrylonitrile is a polyacylnitrile homopolymer or a polyacrylonitrile copolymer having a comonomer content of from 5 to 10 wt%.

5. The method of one or more of claims 1 to 4, wherein the polyacrylonitrile has a molecular weight in g / mol of from 50,000 to 200,000.

6. Use of the carbon fiber material obtained by the process according to one or more of claims 1 to 5 for separating NH3 from a gas mixture.

7. Use according to claim 6, wherein the gas mixture is exhaust gas from combustion engines, natural gas, biogas or air.