Carbon hollow fiber membrane

Abstract

The present invention relates to a method for producing asymmetric cellulose hollow fibers and the use of such fibers in the production of asymmetric carbon hollow fiber membranes (CHFMs). In particular, the present invention provides a facile and scalable method for producing asymmetric CHFM by direct pyrolysis of the polymer precursor, without the need for complex pyrolysis pretreatment steps to prevent pore collapse. The present invention also relates to the use of the asymmetric CHFM produced according to the method for separating hydrogen gas from a mixed gas source, particularly for separating hydrogen from CO in a steam methane reforming reaction.

Description

[Technical Field]

[0001] The present invention relates to a method for producing asymmetric cellulose hollow fibers and to the use of such fibers for the production of asymmetric carbon hollow fiber membranes (CHFM). In particular, the present invention provides an easy and scalable method for producing asymmetric CHFM that directly pyrolyzes a polymer precursor without requiring complex pyrolysis pretreatment steps to prevent pore collapse. The present invention also relates to the use of asymmetric CHFM produced according to the above production method for separating hydrogen gas from a mixed gas source, and in particular for separating hydrogen from CO2 in a steam methane reforming reaction. [Background technology]

[0002] As a clean and efficient energy carrier, hydrogen is also a diverse raw material used in the manufacture of a wide range of products, including petrochemicals, semiconductors, ammonia, methanol, and vitamins. Hydrogen production from natural gas is considered one of the most promising and large-scale technologies for realizing a hydrogen economy for a low-carbon energy future and reducing greenhouse gas emissions.

[0003] Compared to conventional hydrogen purification technologies such as pressure swing adsorption (PSA) and fractional / low-temperature distillation, membrane-based separation technologies are now considered a promising alternative due to their low investment costs, inherent high energy efficiency, and environmental compatibility. Various membrane materials have been developed for H2 / CO2 separation, including polymer membranes, inorganic material-based membranes such as graphene oxide (GO) and MoS2, zeolite imidazolate structures (ZIF), and metal-organic structures (MOF). However, realizing commercially viable membranes for H2 purification remains challenging due to low separation performance, complex fabrication processes (high cost), and limited stability under adverse conditions (e.g., high temperatures and pressures in steam methane reforming processes).

[0004] Carbon sieve (CMS) membranes have a rigid pore structure and are manufactured by controlling the carbonization of porous polymer precursors at high temperatures. Therefore, CMS is a promising candidate as a heat-resistant and pressure-resistant material when used in hollow fibers suitable for membrane modules. The two types of pore structures in CMS membranes, consisting of small ultrapores and larger micropores, provide favorable gas selectivity in H2-related separations such as H2 / CH4 and H2 / C2H4. However, relatively low H2 / CO2 selectivity has been reported to date due to strong adsorption between the carbon surface and CO2 molecules.

[0005] Recently, Ma et al. (Non-Patent Literature 1) reported an H2-assisted method for creating "medium-sized" ultrafine pores (5-7 Å) in CMS films by introducing H2 into the carbonization environment. It was found that the introduction of H2 in the carbonization process suppresses aromatization during the thermal decomposition of the polymer network, resulting in a structure with wider ultrafine pores compared to CMS films fabricated using an argon atmosphere. As reported by Qiu et al. (Non-Patent Literature 2), it has been shown that introducing an additional heat treatment step in the temperature range of 90-250°C to newly fabricated CMS films, called "hyperaging," to accelerate aging, creates even smaller ultrafine pores. However, CMS films reported to date still exhibit relatively large ultrafine pores, which does not allow for precise gas separation between H2 and CO2.

[0006] Another challenge frequently encountered in CMS membrane fabrication is pore collapse. Many polymer precursors commonly used in CMS membrane fabrication exhibit pore collapse during carbonization, which leads to increased density in the resulting membrane. This increased density is often detrimental to the membrane's performance in gas separation applications, particularly in terms of membrane permeability. Furthermore, pore collapse can disrupt the asymmetry present in the polymer precursor fibers, potentially leading to symmetrical CMS membranes unsuitable for gas separation applications.

[0007] Several authors have attempted to address the challenge of pore collapse. Bhuwania et al. (Non-Patent Literature 3, Patent Literature 1) have demonstrated a method to reduce pore collapse during thermal decomposition and, consequently, protect the asymmetric structure of polymer precursors. This method (called "V treatment") involves contacting the polymer precursor with a silicon-containing compound such as vinyltrimethylsilane (VTMS) before thermal decomposition, thereby forming a "crosslinking" network within the precursor structure that prevents pore collapse.

[0008] While such methods are effective in preventing pore collapse, the use of additional processing steps is costly and increases the overall complexity of the process. Furthermore, the use of silicon-containing compounds, which can be toxic, is undesirable and can result in a significant amount of residual silicon in the CHFM, which may lead to a decrease in film performance. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] U.S. Patent No. 9211504 [Non-patent literature]

[0010] [Non-Patent Document 1] Angewandte Chemie International, 58th edition, pp. 13259-13265 (2019) [Non-Patent Document 2] Angewandte Chemie International, 58th edition, pp. 11700-11703 (2019) [Non-Patent Document 3] Carbon 76, pp. 417-434 (2014) [Overview of the project] [Problems that the invention aims to solve]

[0011] Therefore, there is a need for a new manufacturing method of asymmetric CHFM to solve such problems. In particular, there is a need for a manufacturing method of asymmetric CHFM that does not require a complicated thermal decomposition pretreatment step to avoid pore collapse and leads to a CHFM suitable for use in separating hydrogen gas from a mixed gas stream. Furthermore, such CHFM needs to have good high-temperature and high-pressure stability required for applications such as separating hydrogen gas from CO2 in the steam methane reforming reaction. It is also desirable that the manufacturing method of such CHFM enables easy adjustment of the pore morphology, that is, the relative ratio of micropores and ultramicropores.

[0012] The inventors have demonstrated that asymmetric CHFM can be manufactured without requiring a complicated thermal decomposition pretreatment step to avoid pore collapse by using an asymmetric cellulose hollow fiber as a polymer precursor. The inventors have demonstrated that suitable asymmetric cellulose hollow fibers can be manufactured by a dry-wet spinning method including careful control of the coagulation bath temperature. Therefore, a manufacturing method of suitable asymmetric cellulose hollow fibers constitutes a further aspect of the present invention.

[0013] The obtained asymmetric CHFM has been shown to have good permeability and selectivity of hydrogen with respect to large gas molecules such as CO2, and high stability under high-temperature and high-pressure conditions. Therefore, the CHFM manufactured according to the manufacturing method disclosed herein is particularly suitable for use in separating hydrogen gas in the steam methane reforming reaction. The manufacturing method of CHFM provided by the inventors also provides easy adjustment of the pore morphology (and thus the relative permeability / selectivity) of the membrane due to changes in the thermal decomposition temperature.

Means for Solving the Problems

[0014] In one aspect, the present invention a) preparing a dope solution containing cellulose, at least one ionic liquid, and optionally one or more co-solvents; b) co - extruding, in a gas atmosphere, the dope solution with a core liquid comprising water, at least one ionic liquid, and optionally one or more co - solvents; c) quenching the co - extruded dope solution and core liquid in at least one coagulation bath containing water to form a water - wet fiber, wherein the temperature of the coagulation bath is above 40 °C; d) contacting the water - wet fiber with at least one organic solvent having a surface tension lower than that of water, and optionally e) drying the fiber A method for producing an asymmetric cellulose hollow fiber, which comprises the above steps.

[0015] In another aspect, the present invention relates to an asymmetric cellulose hollow fiber produced according to such a production method. In particular, the asymmetric cellulose hollow fiber has a dense outer layer and a porous inner layer.

[0016] In another aspect, the present invention is a method for producing an asymmetric carbon hollow fiber membrane (CHFM), comprising: a) preparing an asymmetric cellulose hollow fiber; b) pyrolyzing the asymmetric cellulose hollow fiber, and optionally <着 c) constructing a module comprising a plurality of the asymmetric cellulose hollow fibers A method for producing an asymmetric carbon hollow fiber membrane (CHFM), which comprises the above steps.

[0017] In a preferred embodiment, step a) of preparing the asymmetric cellulose hollow fiber includes implementing the method for producing an asymmetric cellulose hollow fiber according to the first aspect of the present invention.

[0018] In another aspect, the present invention relates to an asymmetric carbon hollow fiber membrane produced by any of the production methods described herein.

[0019] In another aspect, the asymmetric carbon hollow fiber membrane has a dense outer layer and a porous inner layer.

[0020] In yet another embodiment, the present invention relates to the use of an asymmetric carbon hollow fiber membrane for separating hydrogen gas from a mixed gas stream, for example, for separating H2 from CO2 in a steam methane reforming reaction. [Brief explanation of the drawing]

[0021] [Figure 1] A) Schematic diagram of the method for producing asymmetric cellulose hollow fibers by wet-dry spinning. B) Schematic diagram of the dried cellulose hollow fiber precursor. C) Carbonization procedure for the production of CHFM. D) Schematic diagram of asymmetric CHFM. The key step is the production of asymmetric cellulose hollow fibers by controlling the solidification temperature to >40°C (60°C was used in this example). Step 2 is a non-solvent exchange before drying, using a low surface tension solvent, e.g., isopropanol, n-hexane, to remove any remaining water inside the hollow fibers and prevent the collapse of the pore morphology. Step 3 is a step to adjust the ultrapore and micropore structure of the carbon film by varying the final carbonization temperature from 550 to 850°C. [Figure 2] A and B) Cross-sectional SEM images of dried cellulose hollow fiber precursors after disintegration-resistant treatment. Scale bars: A-200 μm, B-50 μm. [Figure 3] A and B) Cross-sectional SEM images of cellulose hollow fiber carbon film precursors air-dried directly from a wet film. Scale bars: A-200 μm, B-30 μm. [Figure 4] Comparative cross-sectional SEM images of flat sheet films injected at various solidification bath temperatures. Scale bar: 100 μm. [Figure 5] The mechanism for converting cellulose precursors into amorphous carbon films having two types of pore structures. [Figure 6] Carbonization procedure for cellulose hollow fiber precursors. [Figure 7] A-B) Cross-sectional SEM images of CHFM-700. The insert shows the fiber in a bent state. Scale bars: a-100μm, b-20μm. C) XRD patterns of CHFM carbonized at different temperatures, and D) Pore size distribution of CHFM. [Figure 8]A) Performance of CHFM against a single gas. B) Single gas permeability of CHFM-850 as a function of gas dynamic diameter. Insert: Membrane selectivity for H2 against CO2, N2, and CH4. [Figure 9] Diagram illustrating a high-pressure mixed gas permeation apparatus. [Figure 10] A typical module used for measuring the permeability of mixed gases. [Figure 11] Measurement of 50 mol% H2 / 50 mol% CO2 mixed gas using CHFM-700 at different operating pressures (5-18 bar) and 70°C. [Modes for carrying out the invention]

[0022] This invention relates to a method for producing asymmetric cellulose hollow fibers and to the use of such fibers for the production of asymmetric carbon hollow fiber membranes (CHFM).

[0023] In this specification, the term “asymmetric” refers to the cross-sectional structure of cellulose hollow fibers and CHFM characterized by the presence of at least two concentric layers having different levels of porosity within the fiber. For example, in one embodiment, an asymmetric cellulose hollow fiber or CHFM has at least a relatively less porous or “dense” outer layer and a relatively porous “porous” inner layer. This is in contrast to “symmetric” hollow fibers, which do not exhibit a distinctive layered structure characterized by differences in porosity.

[0024] Generally, especially in gas separation applications, the dense outer layer acts as a selective layer, allowing certain chemical species to pass through the membrane while blocking the passage of other larger chemical species. On the other hand, the porous layer primarily acts as a support for the dense outer layer, providing a mechanism through which chemical species can be transported to the dense layer.

[0025] Fabrication of asymmetric cellulose hollow fibers The asymmetric cellulose hollow fibers disclosed herein are produced by a wet-dry spinning process. A schematic diagram of an example of this production method is shown in Figure 1A). The individual steps leading to the production of the asymmetric cellulose hollow fibers are described in detail below.

[0026] Step a) - Preparation of the dope solution In the first step of the method for producing asymmetric cellulose hollow fibers according to the present invention, a doping solution is prepared. The doping solution comprises cellulose, at least one ionic liquid, and optionally one or more cosolvents. The cellulose used may be derived from plant sources such as wood pulp and cotton pulp. The cellulose used is generally in powder form. In preferred embodiments, the cellulose used is "unmodified," i.e., not a cellulose derivative such as cellulose esters and cellulose ethers.

[0027] Therefore, in one embodiment, the doped solution contains less than 5% by mass of a cellulose derivative, such as cellulose acetate, cellulose triacetate, cellulose propionate, methylcellulose, or carboxymethylcellulose. In a preferred embodiment, the doped solution does not contain a cellulose derivative. In a particularly preferred embodiment, the cellulose added to the doped solution is microcrystalline cellulose (MCC).

[0028] Generally, the amount of cellulose dissolved in the dope solution is between 1.0 and 25.0% by mass. In one embodiment, the amount of cellulose in the dope solution is 5.0% by mass or more, for example, 5.0 to 25.0% by mass, more preferably 5.0 to 20.0% by mass, for example, 5.0 to 15.0% by mass. In a particularly preferred embodiment, 10.0 to 15.0% by mass of cellulose is dissolved in the dope solution.

[0029] In addition to cellulose, doped solutions include ionic liquids. Ionic liquids are salts that are liquid at 25°C and atmospheric pressure. Room temperature ionic liquids often contain large, asymmetric organic cations based on heterocyclic compounds such as 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, fluorosulfonyl-trifluoromethanesulfonylimide (FTFSI), N-methyl-N-alkylpyrrolidinium, and ammonium ions. Phosphonium cations are also possible. A wide range of anions are available, from simple halides to inorganic anions such as tetrafluoroborates and hexafluorophosphates, as well as organic anions of all sizes such as bistrifluimides, acetates, cyanamides, triflates, and tosylates.

[0030] Suitable ionic liquids include [Emim][OAc], [Emim][Cl], [Emim][dicyanamide], [Emim][DEP], [Emim][DMP], and 1-butyl-3,5-dimethylpyridinium bromide. In certain embodiments, the ionic liquid comprises 1-ethyl-3-methylimidazolium cation ([Emim]) or 1-butyl-3-methylimidazolium cation. 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) is particularly preferred. Generally, the ionic liquid has a melting point in the range of 25 to 100°C.

[0031] In general, the amount of ionic liquid in the doping solution must be sufficient to dissolve the cellulose components. In one embodiment, the ionic liquid constitutes at least 25% by mass, preferably at least 50% by mass, more preferably at least 60% by mass, for example, at least 75% by mass, of the doping solution. In one embodiment, the ionic liquid constitutes less than 95% by mass, for example, less than 90% by mass, of the doping solution.

[0032] Optionally, the doped solution includes one or more cosolvents in addition to the ionic liquid. The use of a cosolvent can increase the ionic strength of the ionic liquid, thereby increasing the solubility of the cellulose component in the doped solution. In one embodiment, the cosolvent is a polar solvent, preferably an aprotic polar solvent such as dimethylformamide (DMF), 1,3-dimethyl-2-imidazolidinone (DMI), dimethyl sulfoxide (DMSO), and mixtures thereof. DMSO is particularly preferred. A particularly preferred combination of ionic liquid and cosolvent is a mixture of [Emim][OAc] and DMSO.

[0033] When a co-solvent is present, the amount of co-solvent in the doped solution is generally in the range of 10.0% to 90.0% by mass. In a preferred embodiment, the co-solvent is present in an amount of 10.0 to 70.0% by mass, more preferably 10.0 to 50.0% by mass, for example, 15.0 to 30% by mass. In one embodiment, the mass ratio of the amount of ionic liquid to the co-solvent is in the range of 1:10 to 10:1, preferably 1:3 to 5:1, more preferably 2:1 to 5:1. In a particularly preferred embodiment, the doped solution contains [Emim][OAc] and DMSO in a mass ratio of about 3:1.

[0034] Step b) - Co-extrusion with core liquid In the second step of the method for producing asymmetric cellulose hollow fibers according to the present invention, the dope solution prepared in step a) is co-extruded together with the core liquid into a gas atmosphere. The co-extrusion step, also known as the “spinning” step, involves surrounding the core liquid with the dope solution and co-extruding it through a spinneret into a gas atmosphere, usually into air. This is generally referred to as the “dry” step of wet-dry spinning.

[0035] The gap between the spinneret die and the water bath is often called the air gap, and its size can be varied. Generally, the air gap is between 0.5 and 25 cm, for example, between 5 and 15 cm. Generally, the larger the air gap, the greater the elasticity of the resulting fiber, and consequently, the more aligned the cellulose chain orientation.

[0036] The core solution comprises water, at least one ionic liquid, and optionally one or more co-solvents. The types of ionic liquids and co-solvents suitable for use in dope solutions as described herein are also suitable for use in core solutions. The ionic liquids and optional co-solvent components in the core solution may be the same as or different from those present in the dope solution.

[0037] In a preferred embodiment, the ionic liquid and (if present) the co-solvent in the core solution are the same as those used in the doped solution. In a preferred embodiment, the core solution contains an ionic liquid comprising a 1-ethyl-3-methylimidazolium cation, for example, [Emim][OAc]. The (if present) co-solvent is preferably a polar solvent, more preferably an aprotic polar solvent, for example, DMSO.

[0038] The amount of water in the core solution is generally in the range of 5 to 50% by mass, preferably 5 to 30% by mass, more preferably 10 to 30% by mass, for example, 10 to 25% by mass. In order to obtain cellulose hollow fibers having an asymmetric structure, the composition of the core solution must be different from the composition of the dope solution. Therefore, it is preferable that the amount of water in the core solution is different (preferably more) from the amount of water present in the dope solution (if any). In one embodiment, non-solvent water constitutes at least 10% by mass, preferably at least 15% by mass, of the core solution.

[0039] In one embodiment, the ionic liquid constitutes at least 10% by mass of the core liquid, for example, at least 50% by mass, preferably at least 60% by mass. In one embodiment, the ionic liquid constitutes less than 95% by mass of the core liquid, for example, less than 90% by mass. The amount of cosolvent in the core liquid is generally in the range of 1.0 to 90.0% by mass, for example, 10 to 25% by mass. In one embodiment, the mass ratio of the amount of ionic liquid to the cosolvent is in the range of 1:3 to 10:1, preferably 2:1 to 5:1. In a particularly preferred embodiment, the core liquid contains [Emim][OAc] and DMSO in a mass ratio of about 3:1.

[0040] The temperatures of the doping solution and core liquid in the co-extrusion step may be the same or different, preferably in the range of 5 to 80°C, more preferably 15 to 70°C, for example, 20 to 60°C. Doping solution and core liquid temperatures of 20 to 40°C are particularly preferred.

[0041] Step c) - Rapid cooling The doping solution and core liquid are co-extruded, brought into contact with a gas atmosphere, and then rapidly cooled in at least one solidification bath. The solidification bath induces phase separation within the co-extruded doping solution and core liquid, thereby leading to the formation of water-moistened cellulose hollow fibers.

[0042] The coagulation bath may be a mixture with one or more other solvents, or it may consist of water alone. In a preferred embodiment, the coagulation bath consists of water alone.

[0043] The inventors have demonstrated that the temperature of the coagulation bath is important in the production of cellulose hollow fibers having an asymmetric structure. Specifically, the inventors have demonstrated that when the co-extruded dope solution and core liquid are rapidly cooled in a coagulation bath at 40°C or lower, the resulting cellulose hollow fibers exhibit a dense symmetric structure (see Example 2 and Figure 4).

[0044] Therefore, to obtain cellulose hollow fibers having an asymmetric structure, a coagulation bath temperature of over 40°C, preferably over 41°C, for example, over 45°C is required. The coagulation bath temperature may also be as high as 80°C, for example, 41 to 80°C, preferably 41 to 70°C. In a particularly preferred embodiment, the coagulation bath temperature is in the range of 45°C or higher, for example, 45 to 80°C, preferably 45 to 70°C, and especially 45 to 65°C.

[0045] In one embodiment, the co-extruded dope solution and core liquid pass through two or more coagulation baths. In this case, the requirement that the coagulation bath temperature be above 40°C applies only to the first coagulation bath in a series of coagulation baths. The second and subsequent coagulation baths may have temperatures ranging from 10 to 80°C, preferably 20 to 70°C, for example, in the range of 25 to 60°C. If second and subsequent coagulation baths exist, they preferably have the same composition as the first coagulation bath. In a particularly preferred embodiment, two water-containing coagulation baths are used in succession, with the temperature of the first coagulation bath being higher than the temperature of the second coagulation bath.

[0046] Steps d) and e) - Solvent exchange and drying After rapid cooling, the wet fibers are subjected to a solvent exchange process to remove water from the fibers. If such a process is omitted and the wet fibers are dried directly, strong capillary forces may cause the hollow fiber pore structure to collapse (see Figure 3). Therefore, to prevent pore collapse, the wet fibers prepared in step c) are brought into contact with at least one organic solvent having a surface tension lower than that of water in the fibers. The organic solvent must have a surface tension lower than that of water in order to reduce the capillary forces originating from the fluid present in the fiber pores. By replacing the water in the fibers with a solvent having low surface tension, the capillary forces are reduced, and pore collapse during drying is minimized.

[0047] In one embodiment, the fibers are washed with water to remove any remaining ionic liquid and core liquid. Whether or not this washing step is performed, the wet fibers are then brought into contact with at least one organic solvent having a surface tension lower than that of water. In one embodiment, the organic solvent is selected from the group consisting of C1-C6 alcohols, C5-C8 linear or branched aliphatic hydrocarbons, and mixtures thereof. In a preferred embodiment, the organic solvent is selected from isopropanol, n-hexane, and mixtures thereof.

[0048] In a particularly preferred embodiment, the wet fiber is successively contacted with at least two different organic solvents. For example, in one embodiment, the fiber is first contacted with a C1-C6 alcohol solvent such as isopropanol, followed by a second contact step with a solvent selected from the group of C5-C8 linear or branched aliphatic hydrocarbons, such as n-hexane. When two or more contact steps are used, it is particularly preferable that the second and subsequent solvents have a lower surface tension than the solvent used in the preceding step. In this way, the surface tension of the fluid present in the pores of the fiber is gradually reduced until the fiber can be dried without pore collapse.

[0049] Once the solvent exchange process is carried out, the fibers are dried. Generally, the fibers are dried in air at room temperature, but any conventional drying method may be used.

[0050] The resulting dried cellulose hollow fibers have an asymmetric structure, comprising at least a thin, dense outer layer and a more porous inner carrier layer. Figure 2 shows an SEM image of cellulose hollow fibers produced according to the manufacturing method of the present invention. The fiber diameter is generally in the range of 300 to 800 μm, preferably 400 to 600 μm. The fiber wall thickness (i.e., measured from the inner wall to the outer wall of the fiber) is generally in the range of 25 to 200 μm, preferably 50 to 100 μm. The thickness of the dense outer layer is generally less than 50 μm, for example less than 25 μm, preferably less than 10 μm.

[0051] In another aspect, the present invention provides an asymmetric cellulose hollow fiber having a dense outer layer and a concentric porous inner layer, wherein the dense outer layer is substantially free of macrovoids larger than 500 nm, and the inner porous layer contains a plurality of pores having a maximum pore diameter of 500 nm or more.

[0052] In another aspect, the present invention provides an asymmetric cellulose hollow fiber having a dense outer layer less than 10 μm thick and a concentric porous inner layer 25 to 100 μm thick, wherein the dense outer layer is substantially free of macrovoids larger than 500 nm, and the inner porous layer contains a plurality of pores having a maximum diameter of 500 nm or more.

[0053] In another aspect, the present invention provides an asymmetrical cellulose hollow fiber having a dense outer layer less than 10 μm thick and a concentric porous inner layer 25 to 100 μm thick.

[0054] Generally, asymmetric cellulose hollow fibers produced by the methods described herein consist mainly of cellulose II, although the cellulose component in the doping solution is generally cellulose I. In one embodiment, the asymmetric cellulose hollow fiber consists essentially of cellulose II (i.e., at least 95% by mass, e.g., at least 99% by mass). The asymmetric cellulose hollow fiber generally contains substantially no cellulose acetate monomer (e.g., less than 5% by mass, e.g., less than 1% by mass).

[0055] Manufacturing of asymmetric carbon hollow fiber membranes (CHFM) In one embodiment, the present invention is a) Steps to prepare asymmetric cellulose hollow fibers, b) A step of thermally decomposing the asymmetric cellulose hollow fiber This invention relates to a method for producing asymmetric carbon hollow fiber membranes (CHFM), including [the specified element].

[0056] The pyrolysis step is usually also called "carbonization" and is generally carried out under vacuum or in an inert gas environment such as CO2, N2, Ar, and He.

[0057] In a particularly preferred embodiment, step a) of preparing the asymmetric cellulose hollow fiber includes the step of carrying out the method for producing the asymmetric cellulose hollow fiber described herein. Therefore, in one embodiment, the present invention is a) A step of preparing a doped solution comprising cellulose, at least one ionic liquid, and optionally one or more cosolvents, b) A step of co-extruding the doped solution and a core liquid containing water, at least one ionic liquid, and optionally one or more cosolvents into a gas atmosphere, c) A step of rapidly cooling the co-extruded dope solution and core liquid in at least one coagulation bath containing water in order to form a wet fiber, wherein the temperature of the coagulation bath is greater than 40°C. d) The step of bringing at least one organic solvent having a surface tension lower than that of water into contact with the wet fiber, e) The step of drying the fibers to form asymmetric cellulose hollow fibers, and f) A step of thermally decomposing the asymmetric cellulose hollow fiber The present invention provides a method for producing asymmetric CHFM, including [specific component].

[0058] Figure 5 shows a schematic of the basic steps of the carbonization mechanism of CHFM. Generally, in the carbonization process, the physical removal of free water occurs first below 150°C, followed by dehydration (150-200°C) to remove bound water. Above 200-250°C, 1,6-glycosidic bond cleavage occurs, where cellulose depolymerization forms levoglucosan (b). The carbon plate (c) formed by cellulose chain cleavage due to 1,6-glycosidic bond cleavage and subsequent polymerization by intermolecular rearrangement at 250-300°C leads to the formation of a poorly aligned microstructure, a "carbon lattice" (d), with micropores and ultrapores. Generally, the heating rate in the temperature range of 250-300°C is a maximum of 4°C / min. These micropores can be further narrowed by internal condensation at even higher carbonization temperatures (e.g., >600°C) (e). Finally, a carbon film (f) with two types of pore structures, including both micropores and ultrapores, is obtained.

[0059] In the thermal decomposition process, the cellulose hollow fibers are preferably heated to at least 500°C, for example, 500 to 900°C, preferably at least 600°C, and more preferably at least 800°C.

[0060] Generally, increasing the pyrolysis temperature correlates with improved selectivity for smaller gases such as hydrogen, but a slight decrease in permeability has been observed. While we do not wish to be bound by theory, the observed performance difference between CHFM produced at high and low carbonization temperatures is thought to be a result of the relative ratio of ultrafine pores and (larger) micropores. Ultrafine pores are the gaps and smaller spaces between highly aromatic carbon chains. Ultrafine pores influence gas pair selectivity, while micropores formed by voids between aromatic carbon plates contribute to high gas permeability. In CHFM, increasing the carbonization temperature results in a more aligned graphitic carbon structure (sp). 2 They tend to form hybrid carbons, which is beneficial for packing carbon chains and creates narrower ultrafine pores. On the other hand, the micropores present between aromatic carbon plates form three-dimensional sp² 3 Due to the reduced content of mixed carbon, it becomes more compressible. Therefore, controlling the pyrolysis temperature in the manufacturing process allows for control over the gas separation performance of the resulting CHFM.

[0061] Figures 7A and 7B show the structure of CHFM produced according to the manufacturing method of the present invention. The fiber exhibits a clearly asymmetric structure having a dense outer layer with substantially no pore collapse and a porous carrier inner layer. In one embodiment, the thickness of the dense layer is less than 20 μm, for example, less than 10 μm, most preferably less than 5 μm.

[0062] Pores in the inner layer can generally be considered macropores, as they allow the gas to be separated to pass through without a separation process. Therefore, the porous inner layer does not separate the gases in the gas mixture supplied to the CHFM. The dense layer generally does not contain macropores larger than 50 nm, but may contain micropores and ultrafine pores that enable the separation of the target gas. Suitable pore sizes are 2.0 to 8.0 angstroms.

[0063] In another aspect, the present invention provides an asymmetric carbon hollow fiber membrane having a dense outer layer and a concentric porous inner layer, wherein the dense outer layer substantially contains no pores larger than 8.0 Å.

[0064] In another aspect, the present invention provides an asymmetric carbon hollow fiber membrane having a dense outer layer less than 10 μm thick and a concentric porous inner layer 25 to 100 μm thick, wherein the dense outer layer substantially contains no pores larger than 8.0 Å and the inner porous layer contains a plurality of macropores.

[0065] In another aspect, the present invention provides an asymmetric cellulose hollow fiber membrane having a dense outer layer less than 10 μm thick and a concentric porous inner layer 25 to 100 μm thick.

[0066] Surprisingly, the inventors have demonstrated that asymmetric cellulose hollow fibers can be used as a polymer precursor to produce asymmetric CHFM without requiring an additional thermal decomposition pretreatment step to prevent pore collapse. Since such treatment is often complex and costly, by eliminating the need to perform such a step, the present invention provides a simpler and less expensive method for producing CHFM.

[0067] One example of a pretreatment used to prevent pore collapse during carbonization is the "V treatment" proposed by Buania et al. (Carbon 76, pp. 417-434 (2014), U.S. Patent No. 9211504), which involves contacting polymer precursor fibers with a silicon-containing compound. However, this results in a considerable amount of residual silicon in the CHFM, which can lead to a decrease in the performance of the film.

[0068] Accordingly, in one embodiment, the CHFM according to the present invention contains less than 1.0 atomic percent, for example, less than 0.5 atomic percent, preferably less than 0.1 atomic percent, and most preferably less than 0.05 atomic percent, of silicon when measured by X-ray photoelectron spectroscopy (XPS). In another embodiment, the CHFM contains more than 95 atomic percent of carbon, oxygen, and nitrogen. In one embodiment, the CHFM consists essentially of these elements (i.e., more than 99 atomic percent).

[0069] The CHFM of the present invention may contain at least 85 atomic% C, 5 to 15 atomic% O, and up to 1.0 atomic% N.

[0070] CHFM produced according to the manufacturing method of the present invention exhibits good flexibility. In one embodiment, the CHFM may have a bending radius of less than 1.5 cm, for example, less than 1 cm. In some embodiments, the CHFM may have a small bending radius of about 0.5 cm.

[0071] Purpose CHFMs manufactured according to the present invention are particularly suitable for use in gas separation applications. Accordingly, in one embodiment, the present invention provides the use of asymmetric carbon hollow fiber membranes (CHFMs) manufactured by the manufacturing method described herein for separating hydrogen gas from a mixed gas stream, for example, for separating H2 from CO2 in a steam methane reforming reaction. In one embodiment, the CHFM according to the present invention has an H2 permeability of at least 140 GPU and an H2 / CO2 selectivity of at least 10.0, preferably at least 45.0, for example, at least 80.0.

[0072] Multiple carbon hollow fibers may be combined to form a module. In this specification, a single carbon hollow fiber is considered a membrane. However, the present invention also relates to combinations of multiple CHFMs for manufacturing a module.

[0073] The present invention will now be further described with reference to the following embodiments and drawings. [Examples]

[0074] material Microcrystalline cellulose (MCC) powder (Avicel PH-101), isopropanol (≧99.7%, FCC grade), n-hexane (ReagentPlus®, ≧99%), and dimethyl sulfoxide (DMSO, FCC grade) were purchased from Sigma-Aldrich. 1-Ethyl-3-methylimidazolium acetate (EmimOAc, >95%) was purchased from IOLITEC GmbH. All chemicals were used as received. Single gases (e.g., H2, CO2) and 50 mol%·50 mol% H2 / CO2 mixed gases were purchased from AGA Norway. All supplies used for the construction of the membrane module were purchased from Swagelok.

[0075] Characterization SEM images were acquired using a Hitachi SU-6600 field emission scanning electron microscope (FESEM). XRD analysis of CHFM was performed with a 2θ range of 5° to 70° and a time frame of 0.05s. -1 The scan was performed using a Bruker D8 Focus instrument operated at 45kV and 200mA at a scanning speed (Cu-Kα line, λ=0.154nm). CO2 physicoadsorption was measured at 0°C using a Quantachrome® ASiQwin® automated gas adsorption analyzer. XPS spectra were acquired using monochromatic Al-Kα lines with an ESCALAB250 operated at 150W and 200eV. Raman analysis was performed using a Renishaw inVia Raman microscope with a 532nm laser. [Examples]

[0076] Manufacturing of asymmetric cellulose hollow fibers Asymmetric cellulose hollow fibers, precursors for CHFM which are ultimately carbonized, were produced by a wet-dry spinning method as shown in Figure 1A. A 12% by mass MCC / (EmimOAc+DMSO) doped solution was used in the spinning process. 60 g of MCC (cellulose I) was gradually added to 440 g of EmimOAc / DMSO (mass ratio 3:1) cosolvent with mechanical stirring in an N2 atmosphere glove box, and maintained overnight at 50°C to completely dissolve the cellulose. Next, asymmetric cellulose (cellulose II) hollow fibers were produced by a wet-dry spinning method under the conditions shown in Table 1.

[0077] [Table 1]

[0078] The resulting hollow spun fibers were cut into pieces approximately 1.2 m long and placed in a deionized water bath for 48 hours, during which the solvent (EmimOAc + DMSO) was completely replaced with water. The wet cellulose hollow fibers were immersed in pure isopropanol for 2 hours, followed by immersion in n-hexane for 2 hours, after which all the hollow fibers were dried under atmospheric conditions.

[0079] Cross-sectional SEM images of the obtained dried cellulose hollow fibers are shown in Figures 2A and 2B. The hollow fibers exhibit a clearly asymmetric structure, having a relatively dense outer layer, a middle layer rich in macrovoids, and a more porous inner support layer. This is a significant difference from the structure of the comparative cellulose hollow fibers shown in Figures 3A and 3B, which were produced by the same method except that they were air-dried directly from wet fibers without solvent exchange treatment. The cellulose hollow fibers in Figures 3A and 3B have a dense, symmetric structure, which is most likely the result of pore collapse. Therefore, solvent exchange treatment is important to obtain asymmetric cellulose hollow fibers. [Examples]

[0080] Investigation of the effect of coagulation bath temperature on cellulose hollow fiber structure To determine the optimal conditions for cellulose hollow fiber production, the coagulation bath temperature (T cThe effects of the doping temperature (T) were investigated. To achieve this objective, the doping temperature (T) was investigated. d ) is maintained at 25°C, and various flat sheet films are subjected to various temperatures ranging from 25°C to 60°C. c The films were manufactured under the specified conditions. No core liquid was used to produce the flat sheet films. Next, the wet cellulose films were immersed in pure isopropanol for 2 hours, followed by immersion in n-hexane for 2 hours, after which all films were dried under atmospheric conditions.

[0081] Cross-sectional SEM images of the obtained flat sheet membrane are shown in Figure 4 (A) 25°C, B) 35°C, C) 40°C, D) 45°C, E) 50°C, and F) 60°C). The images show the results at lower solidification bath temperatures (T c This shows that temperatures below 40°C produce dense, symmetrical cellulose films (Figures 4A-C). In contrast, T c At temperatures ≥ 45°C, a clearly asymmetrical structure is formed, with a dense upper layer and a more porous carrier layer (Figures 4D-F). In further tests, simply increasing the doping temperature while maintaining the solidification bath temperature below 40°C did not result in the desired asymmetrical structure.

[0082] Therefore, the temperature of the coagulation bath is crucial when obtaining cellulose hollow fibers with an asymmetric structure. Furthermore, it has been shown that adjusting the coagulation bath temperature allows for control over the relative thickness of the dense and porous layers. [Examples]

[0083] Manufacturing of carbon hollow fiber membranes The dried cellulose hollow fibers produced in Example 1 were carbonized in a tubular furnace (Horizontal Split Tube Furnace manufactured by Carbolite Gero Limited) by applying a specific carbonization procedure shown in Fig. 6 under a continuous flow rate of 80 mL / min and a high-purity argon (Ar, 99.999%) purge gas. Considering the significant mass loss due to cellulose depolymerization at 300 °C, a reaction time of 2 hours was adopted. All other carbonization parameters (e.g., heating rate, reaction time, etc.) were kept the same except that the final temperatures were set at different temperatures of 550 °C, 700 °C, and 850 °C, and three types of carbon membranes were obtained (designated as CHFM-550, CHFM-700, and CHFM-850, respectively). The tubular furnace was evacuated overnight at about 3 mbar before purging with Ar. After the carbonization process was completed, the system was cooled naturally, and the obtained carbon hollow fiber membranes (CHFM) were taken out when the temperature dropped below 50 °C.

[0084] Cross-sectional SEM images of CHFM-700 are shown in Figs. 7A and 7B. The asymmetric structure in the hollow fiber, with an outer selective layer of about 3 μm and an integrated porous support inner layer, was well maintained. The manufactured CHFM has a bending radius of <1.5 cm and also shows good mechanical flexibility, as shown in the inset of Fig. 7A.

[0085] XRD patterns for these CHFMs are shown in Fig. 7C. The XRD patterns show a characteristic peak at a position where approximately 2θ is 24°, which corresponds to the (002) plane of the graphite phase (sp 2 carbon). The interplanar spacing d was calculated using Bragg's equation. The peak shift to a larger 2θ when the carbonization temperature is increased from 550 °C to 850 °C indicates that the average interplanar spacing (d 002 ) decreases from 3.78 Å to 3.50 Å. This indicates that carbon membranes prepared at higher carbonization temperatures tend to form a more aligned graphite structure and graphite-like carbon (about 3.4 Å) with smaller pores.

[0086] The pore size distribution shown in Figure 7D, calculated by the NLDFT model from CO2 physicoadsorption in the 3–10 Å range at 0°C, shows narrower pore widths in CHFM-850 compared to CHFM-550. This CHFM exhibits a strong peak for ultrafine pores in the 3–4 Å range, which is within the size range required to enable molecular sieving between H2 (2.9 Å) and other larger gas molecules (e.g., CO2, N2, and CH4). As the carbonization temperature increases, the ultrafine pore peak (<5 Å) increases while the fine pore peak (>5 Å) weakens, indicating that the average pore size decreases in CHFM carbonized at higher temperatures.

[0087] The elemental compositions of CHFM were analyzed by XPS and are shown in Table 2. The carbon content increases with increasing carbonization temperature.

[0088] [Table 2] [Examples]

[0089] Gas permeation test To demonstrate suitability for gas separation, single-gas and mixed-gas permeation experiments were performed on the CHFM prepared in Example 3. Single-gas permeation measurements were performed using the constant permeation volume method with a supply pressure of 2 bar. Gas permeability and selectivity were calculated using the following formula (1).

[0090]

number

[0091] In the formula, P / l(GPU, 1GPU=1×10 -6 cm 3 (STP) · cm -2 ·s -1 ·cm Hg -1 = 3.35 × 10 -10 mol·s -1 ·m -2 ·Pa -1) is the permeability of a single gas. V(cm 3 ) is the downstream (permeate) volume (pre-determined using He calibration), and T(K) is the experimental temperature. A(cm 2 ) is the effective outer surface area of ​​the hollow fiber membrane (shell side supply). F p(bar) and p(bar) are the pressures on the supply side and the permeate side, respectively. Δt(s) is the test time in a steady state. The ideal selectivity of H2 / CO2 is calculated by the ratio of H2 permeability to CO2 permeability.

[0092] Figure 8A shows the single-gas performance of the CHFM produced in Example 3 at a supply pressure of 2 bar and temperatures of 25°C, 60°C, 100°C, and 130°C. The hollow symbol represents the expected performance at 200°C. Higher carbonization temperatures result in higher H2 / CO2 selectivity, but at the expense of some H2 permeability. For example, CHFM-850 has an H2 / CO2 selectivity of 46.2 at 25°C, which is about four times higher than the selectivity of CHFM-550, but at the same time, the H2 permeability decreases from 102.1 GPU to 16.2 GPU. The solid and dashed lines drawn in a) are based on Robeson's upper limit line of 2008, obtained by converting transmittance to permeability, assuming a film selectivity layer thickness of 1 μm and 3 μm, respectively.

[0093] Figure 8B shows the single-gas permeability of CHFM-850 as a function of gas kinetic diameter at 130°C and 2 bar. There is a clear cutoff between the gas permeability of small molecules (148.2 GPU for H2 and 139.6 GPU for He) and large molecules, indicating that the gas permeability is primarily governed by the kinetic diameter of the gas molecules, i.e., using a molecular sieve transport mechanism. The inset shows the selectivity of H2 for CO2, N2, and CH4.

[0094] Gas permeability and selectivity change significantly with temperature. A significant increase in gas permeability and selectivity is observed by raising the temperature from 25°C to 130°C, particularly in films fabricated at higher carbonization temperatures (Figure 8A). At 130°C, the H2 / CO2 selectivity and H2 permeability of CHFM-850 increased to 83.9 and 148.2 GPU, respectively, which are approximately twice and nine times higher than the results obtained at 25°C. Higher temperatures promote gas diffusion, which increases gas permeability. Conversely, lower CO2 adsorption at higher temperatures improves H2 / CO2 selectivity. Therefore, considering actual industrial applications, such as H2 purification from natural gas synthesis gas (typically operated at temperatures above 150°C), higher operating temperatures are preferable to improve H2 / CO2 separation performance.

[0095] To test the potential of CHFM for H2 purification in steam methane reforming (typically performed at pressures of up to 15-20 bar), laboratory-scale hollow fiber modules containing CHFM-700 were tested using a high-pressure gas permeator at 70°C and different feed pressures (5-18 bar) with a 50 / 50 mol% H2 / CO2 mixed gas (Figures 9 and 10). All tubing and membrane modules were preheated to the set temperature for the gas permeation test. During the test, the feed flow was controlled to 150 NmL / min. Argon was used as the sweep gas. The permeate gas flow rate and composition were measured using a bubble flow meter and a gas chromatograph (SRI Instruments GC 8610C), respectively. Three CHFM-700 membrane modules (eight carbon hollow fibers per module) were tested to determine experimental errors. Using argon as the sweep gas operated in a reverse flow manner, the gas was supplied to the shell side and the permeate gas exited from the hole side. Selectivity was calculated using the following formula.

[0096]

number

[0097] In the formula, y i and x iThese represent the concentrations of the permeate gas component and the supply gas component, respectively.

[0098] The results of the mixed gas test are shown in Figure 11. As the total supply pressure increases from 5 to 18 bar, the H2 permeability gradually decreases (approximately 15.8%), but the H2 / CO2 selectivity increases from 31.8 to 37.7 (an increase of 18.6%). Therefore, the CHFM according to the present invention is particularly suitable for high-pressure gas separation conditions in steam methane reforming reactions and the like.

Claims

1. a) providing a dope solution comprising cellulose, at least one ionic liquid comprising a 1-ethyl-3-methylimidazolium cation, and optionally one or more co-solvents; b) co-extruding the dope solution with a core liquid comprising water, at least one ionic liquid comprising 1-ethyl-3-methylimidazolium cations, and optionally one or more co-solvents into a gas atmosphere; c) quenching the co-extruded dope solution and core liquid in at least one coagulation bath containing water to form a water-wet fiber, wherein the temperature of the coagulation bath is greater than 40°C; d) contacting the water-wet fibers with at least one organic solvent having a surface tension lower than that of water; and optionally e) drying the fibers; A method for producing an asymmetric cellulose hollow fiber, comprising:

2. 2. The method for producing an asymmetric cellulose hollow fiber according to claim 1, wherein the cellulose in the dope solution is microcrystalline cellulose (MCC).

3. 3. The method for producing an asymmetric cellulose hollow fiber according to claim 1, wherein the amount of the cellulose in the dope solution is 1.0 to 25.0% by mass.

4. A method for producing an asymmetric cellulose hollow fiber described in any one of claims 1 to 3, wherein the amount of cellulose in the dope solution is 5.0 to 25.0 mass%.

5. A method for producing an asymmetric cellulose hollow fiber described in any one of claims 1 to 4, wherein the amount of cellulose in the dope solution is 5.0 to 20.0 mass%.

6. A method for producing an asymmetric cellulose hollow fiber described in any one of claims 1 to 5, wherein the amount of cellulose in the dope solution is 10.0 to 15.0 mass%.

7. A method for producing an asymmetric cellulose hollow fiber described in any one of claims 1 to 6, wherein the ionic liquid is selected from the group consisting of [Emim][OAc], [Emim][Cl], [Emim][dicyanamide], [Emim][DEP], and [Emim][DMP].

8. A method for producing an asymmetric cellulose hollow fiber described in any one of claims 1 to 7, wherein the ionic liquid is [Emim][OAc].

9. The method for producing an asymmetric cellulose hollow fiber according to claim 1 , wherein the co-solvent is a polar solvent.

10. A method for producing an asymmetric cellulose hollow fiber described in any one of claims 1 to 9, wherein the co-solvent is an aprotic polar solvent.

11. A method for producing an asymmetric cellulose hollow fiber described in any one of claims 1 to 10, wherein the co-solvent is dimethyl sulfoxide (DMSO) or DMF.

12. 12. The method for producing an asymmetric cellulose hollow fiber according to claim 1, wherein the temperature of the coagulation bath is in the range of 41 to 80°C.

13. A method for producing an asymmetric cellulose hollow fiber described in any one of claims 1 to 12, wherein the temperature of the coagulation bath is in the range of 41 to 70°C.

14. A method for producing an asymmetric cellulose hollow fiber described in any one of claims 1 to 13, wherein the temperature of the coagulation bath is in the range of 45 to 70°C.

15. A method for producing an asymmetric cellulose hollow fiber described in any one of claims 1 to 14, wherein the temperature of the coagulation bath is in the range of 45 to 60°C.

16. 16. The method for producing an asymmetric cellulose hollow fiber according to any one of claims 1 to 15, wherein the organic solvent having a surface tension lower than that of water is selected from the group consisting of C1 to C6 alcohols, C5 to C8 linear or branched aliphatic hydrocarbons, and mixtures thereof.

17. 17. The method for producing an asymmetric cellulose hollow fiber according to claim 1, wherein step d) comprises contacting the water-wet fiber successively with at least two organic solvents having a surface tension lower than the surface tension of water.

18. 18. An asymmetric cellulose hollow fiber produced by the process according to any one of claims 1 to 17, wherein the asymmetric cellulose hollow fiber consists essentially of cellulose II.

19. a) providing an asymmetric cellulose hollow fiber; b) pyrolyzing the asymmetric cellulose hollow fibers; A method for producing an asymmetric carbon hollow fiber membrane (CHFM), comprising:

18. A method for producing an asymmetric carbon hollow fiber membrane (CHFM), wherein step a) of providing the asymmetric cellulose hollow fiber comprises carrying out the method of any one of claims 1 to 17.

20. 20. The method for producing an asymmetric carbon hollow fiber membrane (CHFM) according to claim 19, wherein the asymmetric cellulose hollow fibers are directly pyrolyzed without being subjected to any additional processing steps before pyrolysis.

21. 21. The method for producing an asymmetric carbon hollow fiber membrane (CHFM) according to claim 19 or 20, wherein the pyrolysis step b) comprises heating the asymmetric cellulose hollow fibers to a temperature of at least 500°C.

22. An asymmetric carbon hollow fiber membrane (CHFM) having a dense outer layer and a concentric porous inner layer, comprising: An asymmetric carbon hollow fiber membrane (CHFM) comprising at least 85 atomic % C, 5 to 15 atomic % O, and up to 1.0 atomic % N.

23. 23. The asymmetric CHFM of claim 22, having a silicon content of less than 1.0 atomic percent as determined by X-ray photoelectron spectroscopy (XPS).

24. At 130°C and 2 bar pressure, at least 140 GPU H ２ and has a H of at least 10.0 ２ / CO ２ 24. The asymmetric CHFM of claim 22 or 23, which has selectivity.

25. A module comprising a CHFM according to claims 22 to 24.

26. 26. Use of an asymmetric carbon hollow fiber membrane (CHFM) according to any one of claims 22 to 24, or a module according to claim 25, for separating hydrogen gas from a mixed gas stream.

27. Use of the asymmetric carbon hollow fiber membrane or module of claim 26 for separating H 2 from CO 2 in a steam methane reforming reaction.

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