JP6566463B2 - Composite of metal-organic structure and cellulose nanofiber - Google Patents

Description

  The present invention relates to a composite composed of a metal-organic framework (MOF) and cellulose nanofibers.

  In recent years, energy-saving gas separation technology has been desired as a treatment technology for various exhaust gases that cause environmental pollution. Among them, porous materials such as zeolite that separate gases with different molecular sizes are used as molecular sieve membranes. Development of gas separation technology used as

  Metal-Organic Framework (MOF) is a porous material obtained by a self-organization reaction between a metal ion and an organic ligand. MOF has a high specific surface area and has the property of storing and adsorbing gas like zeolite. By changing the combination of MOF metal ions and organic ligands, various designs can be made in terms of shape, pore size, chemical properties, and the like. In recent years, a composite membrane having gas separation ability has been developed by dispersing and supporting MOF having a pore size similar to that of gas molecules on a polymer support.

Non-Patent Document 1 discloses that a composite film of ZIF-90 (zeolitic imidazolate framework-90), which is a kind of metal-organic structure (MOF), and polyimide is used to form a mixture of carbon dioxide gas and methane gas. It is described that methane was excluded, and it was shown that the permeation selectivity (CO ₂ / CH ₄ selectivity) between carbon dioxide gas and methane gas was about 30 to 40%.

Bae et al., "A High-Performance Gas-Separation Membrane Containing Submicrometer-Sized Metal-Organic Framework Crystals," Angew. Chem. Int. Ed. 2010, 49, 9863-9866

  In a composite membrane in which MOF is supported on a polymer support as shown in Non-Patent Document 1, gas may leak from a gap formed between the polymer support and MOF or from the polymer support itself. There is a problem that high gas separation performance (gas selectivity) is difficult to obtain.

  An object of the present invention is to provide a composite having high gas separation performance.

As a result of intensive studies, the present inventors have found that a composite having high gas separation performance can be obtained by using cellulose nanofibers having a specific acid group amount as a support for MOF. The present invention includes, but is not limited to:
(1) A composite comprising cellulose nanofibers having an average fiber length of 0.1 to 10 μm, an average fiber diameter of 1 to 100 nm, and an acid group amount of 0.2 to 2.2 mmol / g, and a metal-organic structure .
(2) The composite according to (1), which is formed into a film.
(3) The composite according to (1) or (2), wherein a part of metal ions in the metal-organic structure is bonded to an acid group of cellulose nanofiber.
(4) The composite according to any one of (1) to (3), wherein the metal-organic structure contains a polar organic ligand.
(5) The composite according to any one of (1) to (4), wherein the metal-organic structure is composed of zinc and 2-imidazolecarbaldehyde.
(6) A filter comprising a composite according to any one of (1) to (5) combined with a gas permeable substrate.
(7) The filter according to claim 6, wherein the base material is cellulose-derived paper having a polyamide polyamine epichlorohydrin provided on the surface.

  By using cellulose nanofibers having acid groups as a support supporting MOF, a composite having high gas separation performance can be provided.

  A membrane (support) made of cellulose nanofibers alone has a very high gas barrier property, but by supporting a MOF having a pore size corresponding to the gas to be separated to form a composite, cellulose nanofibers It was found that pores having a size corresponding to the pore size of the MOF can be formed while maintaining the denseness of the membrane. As a result, only gas molecules having a size smaller than the pore size of the MOF pass through the composite, whereas gas molecules larger than the pore size of the MOF cannot pass through the composite due to the high gas barrier property of the support. Depending on the pore size of the MOF, the gas can be separated with high selectivity.

  In addition, by the interaction between the acid groups of cellulose nanofibers and the metal ions of MOF, high adhesion is obtained between cellulose nanofibers (support) and MOF, so that gas separation membranes using conventional MOFs can be obtained. As can be seen, a gap between the support and the MOF is less likely to occur, and gas leakage from the gap is reduced, so that the gas can be separated with high selectivity.

FIG. 1 is a schematic view of a MOF-TOCN (TEMPO oxidized cellulose nanofiber) composite in which MOF is formed using a metal ion bonded to an acid group on the surface of cellulose nanofiber as a scaffold. The XRD analysis result of a MOF crystal | crystallization, a TOCN film | membrane, and the MOF-TOCN composite film | membrane of this invention is represented. a) is an SEM image of the MOF crystal of Reference Example 1. b) and e) are SEM images of the surface of the MOF-TOCN composite film. c) is an SEM image of a cross section of the MOF-TOCN composite film. d) is an SEM image of the surface of the TOCN film of Comparative Example 2. It is a result of the EDS analysis of the MOF-TOCN composite film | membrane of this invention. It is a schematic diagram of a gas separator. It is the graph which compared the gas permeation | transmission flow rate of the comparative example 1 (paper), Example 2 (MOF-TOCN @ paper), and the comparative example 2 (TOCN @ paper).

  The composite of the present invention comprises cellulose nanofibers having a specific acid group amount and a metal-organic structure (MOF).

(1) Cellulose nanofiber The cellulose nanofiber used in the present invention is 0.2-2 . It has an acid group amount of 2 mmol / g, an average fiber length of 0.1 to 10 μm, and an average fiber diameter of 1 to 100 nm. Here, “acid group” represents a carboxyl group (—COOH) and a carboxylate group (—COOR), and “acid group amount” refers to a carboxyl group and a carboxylate contained in 1 g of cellulose nanofiber. Represents the total amount of groups. Such cellulose nanofibers can be obtained by introducing an acid group into a cellulose raw material (hereinafter sometimes simply referred to as “cellulose”) and then defibrating.

  As a cellulose raw material into which an acid group is introduced, any of cellulose or a derivative thereof, which is a polysaccharide in which glucose is β-1,4-glycoside-bonded, can be used. In particular, it is preferable to use crystalline cellulose because it provides advantages such as high strength when used as a cellulose nanofiber support, difficulty in dissolving in a solvent, and high gas barrier properties. Examples of crystalline cellulose include natural cellulose derived from plants, bacteria, algae, and animals, and natural cellulose derived from plants or animals (particularly derived from sea squirts) is particularly preferable. The crystal structure of crystalline cellulose is not particularly limited. For example, natural cellulose has a cellulose I type (Iα type, Iβ type) crystal structure.

  As a method for introducing an acid group into the cellulose raw material, a known method may be used. For example, an acid group can be introduced into a cellulose raw material by oxidizing the cellulose raw material using an oxidizing agent in the presence of an N-oxyl compound and bromide, iodide, or a mixture thereof. In this oxidation reaction, the primary hydroxyl group at the C6 position of the glucopyrase ring on the cellulose surface is selectively oxidized to a carboxyl group or a carboxylate group. The concentration of cellulose during the oxidation reaction is not particularly limited, but is preferably 5% by mass or less.

  The N-oxyl compound refers to a compound that can generate a nitroxy radical. Any N-oxyl compound may be used as long as it is an N-oxyl compound that can promote the target oxidation reaction. For example, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), its derivatives (for example, 4-hydroxy TEMPO), or an azaadamantane type nitroxy radical can be used. The amount of the N-oxyl compound used is not particularly limited as long as it is a catalytic amount capable of oxidizing cellulose. For example, 0.01 to 10 mmol is preferable, 0.01 to 1 mmol is more preferable, and 0.05 to 0.5 mmol is more preferable with respect to 1 g of absolutely dry cellulose.

  Bromide means a compound containing bromine, and iodide means a compound containing iodine. Examples include alkali metal bromides and iodides that can dissociate and ionize in water. What is necessary is just to select the usage-amount of a bromide and / or iodide in the range which can accelerate | stimulate an oxidation reaction. For example, the total amount of bromide and iodide is preferably 0.1 to 100 mmol, more preferably 0.1 to 10 mmol, and still more preferably 0.5 to 5 mmol with respect to 1 g of absolutely dry cellulose.

  As the oxidizing agent, known ones can be used, and for example, halogen, hypohalous acid, halous acid, perhalogen acid or salts thereof, halogen oxide, peroxide and the like can be used. Of these, sodium hypochlorite is preferable because it is inexpensive and has a low environmental impact. The appropriate amount of the oxidizing agent used is, for example, preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, further preferably 1 to 25 mmol, and most preferably 3 to 10 mmol with respect to 1 g of absolutely dry cellulose. . For example, 1-40 mol is preferable with respect to 1 mol of N-oxyl compounds. This oxidizing agent may be added in the required amount in the system before the start of the oxidation reaction, but it is preferable to add it sequentially during the oxidation reaction.

  Since the oxidation reaction of cellulose proceeds efficiently even under relatively mild conditions, the temperature is preferably 4 to 40 ° C, and may be room temperature of about 15 to 30 ° C. As the reaction proceeds, acid groups are generated in the cellulose, and a decrease in the pH of the reaction solution is observed. In order to advance the reaction efficiently, an alkaline solution such as an aqueous sodium hydroxide solution is added to maintain the pH of the reaction solution at about 8 to 12, preferably about 10 to 11. The reaction medium is preferably water because it is easy to handle and hardly causes side reactions. The oxidation reaction time can be appropriately set according to the progress of oxidation, and is usually 0.5 to 6 hours, for example, about 0.5 to 4 hours.

  The oxidation reaction may be performed in two stages. For example, by oxidizing the oxidized cellulose obtained by filtration after the completion of the first-stage reaction again under the same or different reaction conditions, the efficiency is not affected by the reaction inhibition by the salt generated as a by-product in the first-stage reaction Can be oxidized well.

  The amount of acid group introduced into the cellulose raw material (acid group amount) can be adjusted by controlling the amount of the oxidizing agent added, the reaction time, and the like.

  Cellulose nanofibers can be obtained by defibrating the oxidized cellulose thus obtained by a known method. For example, the oxidized cellulose is suspended in a medium such as water, and a strong shearing force is applied thereto by using a high speed rotating type, colloid mill type, high pressure type, roll mill type, ultrasonic type apparatus, etc. Cellulose fibers can be fibrillated into cellulose microfibrils having nano-order fiber widths. The concentration of oxidized cellulose in the suspension at the time of defibration is not particularly limited, but is preferably 0.3% (w / v) or more, and more preferably about 1 to 8% (w / v).

  The cellulose nanofiber used in the present invention has an acid group content of 0.2 to 2.2 mmol with respect to 1 g of absolutely dry cellulose nanofiber. Preferably it is 0.2-2 mmol, More preferably, it is 0.5-1.8 mmol / g. Since the acid group amount of cellulose nanofiber is usually equal to the acid group amount of oxidized cellulose, as described above, it can be adjusted by controlling the amount of oxidant added and the reaction time during oxidation of the cellulose raw material. .

The amount of acid groups in cellulose nanofiber or oxidized cellulose can be measured by the following method:
A cellulose nanofiber or an oxidized cellulose sample is dried and precisely weighed to prepare 60 mL of a slurry of 0.5 to 1% by mass. The pH of the slurry is adjusted to about 2.5 with 0.1 mol / L aqueous hydrochloric acid. Then, 0.05 mol / L sodium hydroxide aqueous solution is dripped and electrical conductivity measurement is performed. The measurement is continued until the pH is about 11. From the amount of sodium hydroxide consumed (V) until the change in electrical conductivity shows a mild acid neutralization stage, the acid group amount X is determined using the following equation:
X (mmol / g) = V (mL) × 0.05 / mass of sample (g)
The acid group amount X represents the total amount of carboxyl groups and carboxylate groups in 1 g of the dried sample.

Since the acid group on the surface of the cellulose nanofiber serves as a metal-organic structure (MOF) -supported contact, it is preferable that the acid group exists on the surface of the cellulose nanofiber at a high density. The density of acid groups on the surface of the cellulose nanofiber can be represented by the amount of acid groups per unit surface area. The surface area can be measured by a known method such as the BET method. The acid group per unit surface area, can also be represented by charge amount per unit surface area, 0.01~0.6C / ^{m 2} in the range of about preferably. The charge amount can be determined from the acid group amount, surface area, and Faraday constant of cellulose nanofiber.

  The cellulose nanofiber used in the present invention has an average fiber length of 0.1 to 10 μm and an average fiber diameter of 1 to 100 nm. The fiber length and fiber diameter of cellulose nanofibers can be adjusted by controlling the acid group amount of oxidized cellulose or the strength of shearing force when deoxidizing oxidized cellulose. The average fiber length and average fiber diameter of the cellulose nanofiber can be determined using a scanning or transmission electron microscope image (SEM or TEM), an atomic force microscope image (AFM), or the like.

(2) Metal-Organic Framework (MOF)
Metal-Organic Framework (MOF) is a porous material obtained by a self-organization reaction between a metal ion and an organic ligand. When the organic ligand connects metal ions, a crystalline polymer structure having a space (pore) inside is obtained. MOF has a very large specific surface area in the crystal. Further, the pore diameter and topology can be adjusted by appropriately selecting and combining metal ions and organic ligands.

The metal ions for forming the MOF used in the present invention may be selected according to the target structural design. In general, Co ²⁺ , Ni ²⁺ , Cu ²⁺ , Zn ²⁺ , Al ³⁺ , Mn ²⁺ , Fe ²⁺ , Fe ³⁺ , Cr ³⁺ , Cr ^{6+ and the} like are often used.

  What is necessary is just to select the organic ligand for forming MOF according to the target structural design. In general, an oxygen donor ligand and a nitrogen donor ligand are often used, and representative ligands include terephthalic acid, 4,4′-bipyridyl, imidazole, 2-imidazole carbaldehyde, 1,4-benzenedicarboxylate, 1,4-diazabicyclo [2.2.2. ] Octane, pyrazine-2,3-dicarboxylate, isophthalate, 1,3,5-benzenetricarboxylate, 2-methylimidazole, purine, 2-aminoterephthalate and the like.

  These metal ions and organic ligands can be arbitrarily selected as long as the MOF crystalline structure can be formed. One type of metal ion and one type of organic ligand may be combined to form MOF, or a MOF including a plurality of types of metal ions and / or a plurality of types of organic ligands.

  When the composite membrane of the present invention is used for gas separation, MOF having an appropriate pore size may be used according to the size of gas molecules to be separated. Thousands of types of MOFs have already been synthesized and reported, and the number is still increasing (Non-Patent Document 1).

  As the MOF used in the composite film of the present invention, hydrophilic MOF is preferable. When the MOF is hydrophilic, the adhesion to the polar cellulose nanofibers that are the support of the composite membrane is increased, and the gas leakage from the gap between the MOF and the support is reduced. Here, the hydrophilic MOF refers to a MOF composed of a polar organic ligand and a metal ion.

  The MOF can be manufactured by a conventionally known method. In particular, in the present invention, as described later, it is preferable to synthesize MOF using a metal ion introduced into an acid group of cellulose nanofiber as a scaffold, since the adhesion between MOF and the cellulose nanofiber support is further increased. .

(3) Composite As described above, the composite of the present invention has an acid group amount of 0.2 to 2.2 mmol / g, an average fiber length of 0.1 to 10 μm, and an average fiber diameter of 1 to 100 nm. It consists of cellulose nanofibers and metal-organic structures (MOF). When MOF is synthesized on cellulose nanofiber using a metal compound bonded to the acid group of cellulose nanofiber as a scaffold, adhesion between MOF and cellulose nanofiber support is preferably increased. Specifically, first, a metal compound that generates metal ions for forming MOF is added to the aqueous dispersion of cellulose nanofibers having acid groups obtained by the above-described method, and metal ions are added to the acid groups. Combine. The metal ions are almost stoichiometrically bound to the acid groups of the cellulose nanofibers (for example, in the case of divalent metal ions, the metal ions are bound in an amount approximately half the amount of the acid groups of the cellulose nanofibers. The present inventors have confirmed that. Therefore, it is considered that the amount of metal ion added to the cellulose nanofiber aqueous dispersion may be appropriately determined according to the acid group amount of the cellulose nanofiber and the valence of the metal ion. Next, the solvent is optionally replaced with an organic solvent (for example, N, N-dimethylformamide), and dispersion, centrifugation, and supernatant removal are repeated to obtain a metal ion-cellulose nanofiber complex. After the obtained composite is dispersed in a solvent and a metal compound that generates a metal ion for forming MOF and an organic compound that becomes an organic ligand of MOF are added and stirred for several hours under heating. Cool to room temperature. A poor solvent (for example, methanol) is added here, and stirring, centrifugation, and removal of the supernatant are repeated to replace the solvent. Thereby, the dispersion liquid of MOF-cellulose nanofiber composite_body | complex is obtained. In the MOF-cellulose nanofiber composite formed as described above, a MOF is formed using a metal ion bonded to an acid group on the surface of the cellulose nanofiber as a scaffold. A schematic view of such a MOF-cellulose nanofiber composite is shown in FIG. FIG. 1 shows an example in which ZIF-90 (a MOF composed of zinc and 2-imidazole carbaldehyde) is used as the MOF. The TOCN in FIG. 1 is a TEMPO-oxidized cellulose nanofiber (TEMPO-oxidized cellulose nanofiber).

  A film can be formed by removing the solvent from the obtained dispersion of the composite. The method for forming the membrane is not limited to this. For example, filter paper is set in a funnel with a sieve pan (Buchner funnel, etc.), the dispersion of the composite is poured and suction filtered, and the composite is placed on the filter paper. A composite film (composite film) can be formed by remaining in a film and drying. At the time of drying, for example, it is preferable to dry while applying pressure by pressing a metal plate or the like because a smooth film can be obtained. The composite membrane may be obtained as a single membrane, or may be obtained in a form adhered to a substrate such as filter paper.

  The obtained composite membrane may be used as a filter for gas separation, for example, by arranging a substrate for reinforcing the strength on one side or both sides as desired. The base material for reinforcing the strength is not particularly limited as long as it has gas permeability. For example, paper (cellulose) base material such as filter paper; glass filter; metal or metal oxide mesh , Honeycombs, or foams; or porous polymeric substrates.

  EXAMPLES Next, although an Example demonstrates this invention still in detail, this invention is not limited to this.

Reference Example 1 Synthesis of Metal-Organic Structure (MOF) 2.5 mmol of zinc nitrate (II) hexahydrate (Zn (NO ₃ ) ₂ .6H ₂ O) and 10 mmol of 2-imidazole carbaldehyde were added to N , N-dimethylformamide (DMF) was added to 25 ml and stirred, and further stirred in an oil bath (80 ° C.) for 4 hours. Then, it took out from the oil bath and continued stirring until it became room temperature. After adding 25 ml of methanol and stirring for 30 minutes, it was subjected to centrifugation (25 ° C., 101,000 rpm, 60 minutes), and the supernatant was removed. After adding methanol so that the total volume becomes 50 ml, the resulting MOF is sufficiently dispersed using vortex and sonication, subjected to centrifugation (same conditions), and the supernatant is removed three times. It was. In order to remove DMF in the MOF, methanol was added so that the total volume became 50 ml, and the mixture was stirred for one day, and centrifuged (same conditions), and the supernatant was removed. After drying at room temperature, it was subjected to evaporation at 170 ° C. to obtain MOF powder. The resulting MOF is the MOF known by the name ZIF-90.

(Example 1) Production of MOF-TOCN composite membrane (1) Preparation of metal-TOCN composite TEMPO oxidized cellulose nanofiber (TOCN) manufactured by Nippon Paper Industries Co., Ltd. (acid group amount 1.18 mmol / g, average fiber length) 0.48 .mu.m, the aqueous dispersion 50ml average fiber diameter 8 nm), was added zinc nitrate (II) hexahydrate _{(Zn (NO 3) 2 ·} 6H 2 O) 1mmol, well using a vortex and sonication after dispersing, the supernatant was removed Calls centrifuged (same conditions). Acetone was added to a total volume of 50 ml, dispersed using vortex and sonication, then subjected to centrifugation (same conditions), and the supernatant was removed three times. Add DMF so that the total volume is 50 ml, disperse using vortex and sonication, and then subject to centrifugation (same conditions) and remove the supernatant three times. Metal-TOCN A complex was obtained.

(2) Preparation of MOF-TOCN complex Obtained metal-TOCN complex, zinc nitrate (II) hexahydrate (Zn (NO ₃ ) ₂ .6H ₂ O) 1.5 mmol, 2-imidazole carbaldehyde 10 mmol Was added to 25 ml of DMF and stirred, and further stirred in an oil bath (80 ° C.) for 4 hours. Then, it took out from the oil bath and continued stirring until it became room temperature. After adding 25 ml of methanol and stirring for 30 minutes, it was subjected to centrifugation (same conditions), and the supernatant was removed. After adding methanol so that the total amount became 50 ml, the operation of sufficiently dispersing using vortex and sonication and centrifuging (same conditions) and removing the supernatant was repeated three times. In order to remove DMF, methanol was added to a total volume of 50 ml, stirred for 1 day, and centrifuged (same conditions) to remove the supernatant. Methanol was added so that the total amount was 50 ml to obtain 50 ml of MOF-TOCN methanol dispersion. The MOF of the complex is the MOF known by the name of ZIF-90.

(3) Preparation of MOF-TOCN composite membrane The obtained MOF-TOCN methanol dispersion was poured onto a commercially available cellulose filter paper (area 95 cm ² ) set in a Buchner funnel, and suction filtration was performed. Furthermore, it was made to dry, applying a pressure with a metal plate. The composite membrane on the filter paper was not adhered to the filter paper, and a single MOF-TOCN composite membrane was obtained by peeling from the filter paper.

(Example 2) was dipped MOF-TOCn composite membrane preparation commercially available cellulose filter paper (area 95cm ²⁾ Poly A bromide polyamine epichlorohydrin (PAE, paper strength enhancing agent) in an aqueous solution 1.25 wt% 15 minutes, It was dried at 105 ° C. for 1 hour to obtain a PAE-immersed filter paper. The MOF-TOCN methanol dispersion obtained according to (1) and (2) of Example 1 was poured onto PAE-immersed filter paper set in a Buchner funnel, and suction filtration was performed. Thereafter, another PAE-immersed filter paper was further stacked, washed with acetone, and dried at 40 ° C. for 4 hours. The obtained MOF-TOCN composite membrane was adhered to PAE-immersed filter paper on both sides of the membrane. This is probably because the azetidinium ring of PAE was cross-linked with the acid group on the TOCN by heating. In this way, a laminate (MOF-TOCN film @paper) of a paper base material (filter paper) and a MOF-TOCN composite film was obtained.

(Comparative Example 1)
Comparative Example 1 was obtained by stacking two PAE-immersed filter papers obtained by the method described in Example 2.

(Comparative Example 2)
On a PAE-immersed filter paper obtained by the method described in Example 2, TEMPO oxidized cellulose nanofiber (TOCN) manufactured by Nippon Paper Industries Co., Ltd. (acid group amount 1.18 mmol / g, average fiber length 0.48 μm, average fiber) 50 ml of an aqueous dispersion having a diameter of 8 nm) was applied. Thereafter, another PAE-immersed filter paper was further stacked and dried at 40 ° C. for 4 hours. This obtained the laminated body (TOCN @ paper) of a paper base material (filter paper) and a TOCN film | membrane.

<Structural analysis by X-ray diffraction (XRD)>
The MOF (ZIF-90) of Reference Example 1, the MOF-TOCN composite film of Example 2, and the TOCN film of Comparative Example 2 were analyzed by XRD (XD-D1 manufactured by Shimadzu Corporation). The results are shown in FIG.

  According to the X-ray diffraction profile, peaks of (1-10) plane, (110) plane, and (200) plane of cellulose I-type crystals were observed in the TOCN film. Moreover, since the peak of the synthesized MOF (ZIF-90) carrier and the crystal peak of TOCN were also confirmed in the MOF-TOCN composite film, the MOF was synthesized and composited on the TOCN while maintaining the TOCN crystal. It was thought that

<Observation with scanning electron microscope (SEM)>
The MOF (ZIF-90) of Reference Example 1, the MOF-TOCN composite film of Example 2, and the TOCN film of Comparative Example 2 were carbon coated, and each film was subjected to SEM (Zeiss ULTRA55 manufactured by JFE Techno-Research Corporation). The surface was observed. The cross section of the MOF-TOCN composite film was also observed. The SEM image is shown in FIG. 3a shows the MOF crystal of Reference Example 1, and b) and e) show the surface of the MOF-TOCN composite film. c) is a cross section of the MOF-TOCN composite film. d) is the surface of the TOCN film of Comparative Example 2.

  In FIG. 3b), a cube-like substance similar to the MOF crystal (FIG. 3a) was also observed on the TOCN surface, confirming MOF synthesis using TOCN as a matrix. From the cross-sectional view of the MOF-TOCN composite film (FIG. 3c), it became clear that a large number of MOFs were also present inside the composite film.

Further, in FIG. 3e), it was observed that a nanofiber-like substance considered to be TOCN was entangled and adhered to the surface of the immobilized MOF crystal on the MOF-TOCN composite film. It is thought that the physical entanglement of TOCN with a high aspect ratio and the existence of electrostatic interaction between hydrophilic ZIF-90 and polar polymer TOCN contribute to the adhesion between MOF and TOCN. It is done. In the previously reported MOF composite membrane, the previously synthesized MOF crystal is dispersed in a hydrophobic polymer to form a membrane, and the MOF crystal due to the low affinity between the MOF crystal and the hydrophobic polymer support. (See, for example, FIG. 2 (c) on page 9864 of Non-Patent Document 1). Composite film comprising MOF (ZIF-90) and polyimide support (6FDA-DAM) In the SEM image of the above, there is a gap between the MOF and the support) and the gas permeability of the support itself may have decreased the gas permeability, In the composite film of the example, the MOF (ZIF-90) crystal was synthesized using the coordinate bond with the metal ion (Zn ²⁺ ) introduced into the acid group on the TOCN interface, and thus the MOF crystal And improved adhesion to the support Erareru.

<Energy dispersive X-ray analysis (DES)>
The MOF-TOCN composite film was subjected to surface elemental analysis using NSS300 manufactured by Thermo ELECTRON simultaneously with the surface structure analysis of SEM. The results are shown in FIG.

  Zn characteristic X-rays were also detected in the portion where the MOF crystal was not visible in the image (1 in FIG. 4a) (FIG. 4b). It is considered that MOF crystals having a small particle diameter are present on the surface of the MOF-TOCN composite film, or MOF crystals are present inside the composite film.

<Water resistance>
When water was dropped onto the MOF-TOCN composite membrane, it was easily absorbed without forming water droplets on the membrane surface, confirming that the composite membrane was hydrophilic. With respect to the water resistance, only the TOCN film of Comparative Example 2 was immersed in water and allowed to stand for a certain period of time to absorb water and became a gel state. However, the MOF-TOCN composite film collapsed even when immersed in water. There wasn't. Unlike paper and TOCN membranes, which are composed of hydrogen bonds and physical entanglements between cellulose fibers, MOF-TOCN composite membranes are fibers in which metal ions present in acid groups on the TOCN crystal interface are TOCN fibers. This suggests the possibility that dissociation between fibers in water is suppressed.

<Test of gas separation performance>
A mass flow controller MODEL8500 (manufactured by Coflock Co., Ltd.), a mass flow meter MODEL3810DS (manufactured by Coflock Co., Ltd.), and a vacuum controller V-850 (manufactured by Nippon Buch Co., Ltd., GC-8A (manufactured by Shimadzu Corporation) are assembled (shown in FIG. 5). The PAE immersion filter paper of Comparative Example 1 (Paper), the laminate of the PAE immersion filter paper of Example 2 and the MOF-TOCN composite membrane (MOF-TOCN film @paper), and the PAE immersion filter paper and TOCN of Comparative Example 2 Each of the laminates (TOCN @ paper) with a membrane is used as a separation membrane, and a gas mixed at a ratio of CO ₂ : CH ₄ = 1: 1 is allowed to flow at a differential pressure of 0.1 MPa and each gas supply amount of 100 sccm. A separation test was conducted, and the pore size of MOF (ZIF-90) was 0.35 nm. The carbon dioxide molecule has a size of 0.33 nm, the methane molecule has a size of 0.38 nm, and the gas permeability can be expressed by the following equation.

Further, the CO ₂ / CH ₄ permeation selectivity can be expressed by a transmissivity ratio.

The measurement result of the permeation | transmission flow rate of each gas is shown in FIG. Compared to the filter paper of Comparative Example 1 alone, in the TOCN single membrane of Comparative Example 2 (TOCN @ paper), both the CH ₄ permeation flow rate and the CO ₂ permeation flow rate are reduced by 99%, which is high in the TOCN support. It turns out that there is a gas barrier property. In the composite membrane of Example 2 (MOF-TOCN @ paper), the CH ₄ permeation flow rate was reduced by 99% compared to the filter paper alone (Paper) of Comparative Example 1, but the CO ₂ permeation flow rate was maintained at 67%. . The decrease in the CH ₄ permeation flow rate is believed to be due to the gas barrier properties of the TOCN support. On the other hand, it is considered that CO ₂ has a molecular size smaller than the pore size of MOF introduced into the TOCN support and has passed through the MOF portion of the composite membrane.

The CO ₂ permeability calculated from the permeate flow rate in the MOF-TOCN @ paper of the present invention is 11800 barrer, which is the ZIF-90 and polyimide support described in Table 1 on page 9865 of Non-Patent Document 1. The composite membrane of the present invention corresponds to about 16 times the CO ₂ permeation flow rate of 720 barr, and it can be seen that the composite membrane of the present invention exhibits extremely high CO ₂ permeability compared to the previously reported MOF composite membrane. As described above, since the TOCN support itself has a high gas barrier property, it is suggested that by introducing MOF into TOCN, pores corresponding to the pore size of MOF could be formed in the dense film of TOCN. Is done.

When the CO ₂ / CH ₄ permeation selectivity is calculated, it can be seen that only the filter paper of Comparative Example 1 is almost 1, and the filter paper itself has no CO ₂ / CH ₄ separation ability. On the other hand, the MOF (ZIF-90) -TOCN @ paper of Example 2 was 68. This is because CO ₂ / CH ₄ (CO ₂ : CH ₄ = 1: 1) permeation selectivity of the composite membrane of ZIF-90 and polyimide support described in Table 1 on page 9865 of Non-Patent Document 1 The MOF composite membrane using the TOCN support of the present invention is significantly higher than the MOF composite membrane already reported even when the same type of MOF (ZIF-90) is used. Thus, it can be seen that the CO ₂ / CH ₄ permeation selectivity is remarkably high. In the previously reported MOF composite membrane, CH ₄ may permeate through the polyimide support portion or the gap portion between the support and the MOF, which may reduce the permeation selectivity. On the other hand, the MOF composite film using the TOCN of the present invention as a support has a high gas barrier property in the TOCN itself, so that gas leakage at the TOCN support can be prevented, and the affinity between MOF and TOCN By improving the adhesion due to the property, gas leakage from the gap around the MOF is also reduced, and it is considered that the permeation selectivity is greatly improved.

Claims (6)

A composite comprising cellulose nanofibers having an average fiber length of 0.1 to 10 μm, an average fiber diameter of 1 to 100 nm, and an acid group amount of 0.2 to 2.2 mmol / g, and a metal-organic structure. A composite in which some of the metal ions in the metal-organic structure are bound to the acid groups of the cellulose nanofibers , and
Gas permeable cellulose substrate
The filter which combines . Complex is formed into a film filter according to claim 1. The filter according to claim 1 or 2, wherein the metal-organic structure contains a polar organic ligand. The filter according to any one of claims 1 to 3, wherein the metal-organic structure is composed of zinc and 2-imidazole carbaldehyde. Kamiryoku enhancer that is applied to the surface of the substrate, a filter according to any one of claims 1 to 4.   The filter according to claim 5, wherein the substrate is a cellulose-derived paper having a polyamide polyamine epichlorohydrin added to the surface.