JP7562091B2 - Porous cellulose particles and their manufacturing method - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies http://www3.scej.org/meeting/stu22w/prog/room_M1.html (title published in the program of the 22nd Chemical Engineering Society Student Presentation (Okayama Conference) (lecture number: M22)). Posted on January 15, 2020 [Publications, etc.] http://www3.scej.org/meeting/stu22w/abst/M22.pdf (abstract published in the abstract of the 22nd Chemical Engineering Society Student Presentation (Okayama Conference) (lecture number: M22)). Posted on February 17, 2020 [Publications, etc.] http://www3.scej.org/meeting/stu22w/abst/M22.pdf (abstract published in the abstract of the 22nd Chemical Engineering Society Student Presentation (Okayama Conference) (lecture number: M22)). org/meeting/51f/appl/serial_1.html (title published in the list of presentation applications for the 51st Autumn Meeting of the Society of Chemical Engineers (acceptance number 89). Posting date: June 3, 2020

The present invention relates to porous cellulose particles formed from cellulose nanofibers and a method for producing the same.

As part of research and applications of cellulose nanofibers in the field of biochemistry, there have been reports of studies on adsorption through interactions between biologically active substances such as proteins and TEMPO-oxidized cellulose nanofibers (TOCN) (see Patent Document 1).

Patent No. 6159737

However, when cellulose nanofibers are used as an adsorbent, if they are dispersed in an aqueous medium in the fine fiber form, they are difficult to separate from the cellulose nanofibers and the adsorbate, and the fine fibrous cellulose is difficult to recover and reuse, making them difficult to handle. In order to improve the handleability of cellulose nanofibers, it is possible to form the nano-sized fine fiber form into micro-sized particles. In that case, simply leaving the aqueous dispersion of cellulose nanofibers to dry will cause the cellulose nanofibers to aggregate, reducing the surface area and, for example, reducing performance as an adsorbent.

An embodiment of the present invention aims to increase the surface area of cellulose particles made of cellulose nanofibers.

The present invention includes the embodiments set forth below.
[1] A porous cellulose particle formed from cellulose nanofibers, the particle surface having a plurality of holes that are traces of the removal of a template particle.
[2] The porous cellulose particles according to [1], wherein the average particle size of the porous cellulose particles is 1 to 30 μm, and the average pore size of the holes is 1/200 to 1/5 of the average particle size of the porous cellulose particles.
[3] The porous cellulose particles according to [1] or [2], wherein the cellulose nanofibers have anionic functional groups.
[4] An adsorbent comprising the porous cellulose particles according to any one of [1] to [3].
[5] A method for producing porous cellulose particles, comprising: spray-drying a dispersion containing cellulose nanofibers and template particles to obtain composite particles in which the cellulose nanofibers and the template particles are combined; and dissolving the template particles contained in the obtained composite particles with an organic solvent.
[6] The method for producing porous cellulose particles according to [5], wherein the average particle size of the template particles is 100 to 1000 nm.
[7] The method for producing porous cellulose particles according to [5] or [6], wherein the cellulose nanofibers have anionic functional groups.

In an embodiment of the present invention, the surface area of cellulose particles made of cellulose nanofibers can be increased by providing holes on the particle surface where the template particles were removed. This can improve the adsorption performance when used as an adsorbent, for example.

(A) A schematic diagram of a composite particle before and after dissolving and removing the template particle (B) A schematic diagram of a porous cellulose particle after dissolving and removing the template particle. (A) SEM image of composite particles (PMMA/TOCN=1/1) of PMMA particles and cellulose nanofibers in Example 1. (B) SEM image of porous cellulose particles after dissolving and removing the PMMA particles in Example 1. Graph showing the results of FT-IR measurement of the composite particles of Example 1, the porous cellulose particles, the cellulose particles, and the PMMA particles. (A) SEM image of composite particles of PMMA particles and cellulose nanofibers (PMMA/TOCN=2/1) in Example 2. (B) SEM image of porous cellulose particles after dissolving and removing the PMMA particles in Example 2. (A) SEM image of composite particles (PMMA/TOCN=3/1) of PMMA particles and cellulose nanofibers in Example 3. (B) SEM image of porous cellulose particles after dissolving and removing the PMMA particles in Example 3. (A) SEM image of composite particles of PMMA particles and cellulose nanofibers (PMMA/TOCN=4/1) in Example 4. (B) SEM image of porous cellulose particles after dissolving and removing the PMMA particles in Example 4. (A) SEM image of the porous cellulose particles (TOCN/Fe ₃ O ₄ /PMMA=1/1/4) after dissolving and removing the PMMA particles in Example 5. (B) An enlarged image of a part thereof. (A) SEM image of the porous cellulose particles (TOCN/Fe ₃ O ₄ /PMMA=1/2/4) after dissolving and removing the PMMA particles in Example 6. (B) Enlarged image of a part thereof. Graph showing the adsorption capacity of lysozyme for porous cellulose particles.

The porous cellulose particles according to this embodiment are cellulose particles formed from cellulose nanofibers and have multiple holes on the particle surface.

It is preferable to use cellulose nanofibers that satisfy the following (a) to (c).
(a) The number average fiber diameter is 3 nm or more and 100 nm or less.
(b) Having a cellulose I type crystal structure.
(c) The average aspect ratio is 2 or more and 5,000 or less.

The number average fiber diameter of (a) above is more preferably 50 nm or less, even more preferably 30 nm or less, and may be 10 nm or less. The number average fiber diameter can be measured as follows.

That is, an aqueous dispersion of cellulose nanofibers with a solid content of 0.05 to 0.1% by mass is prepared, and the aqueous dispersion is cast on a carbon film-coated grid that has been hydrophilized to prepare a sample for observation with a transmission electron microscope (TEM). When the sample contains fibers with a large fiber diameter, a scanning electron microscope (SEM) image of the surface cast onto glass may be observed. The observation sample may also be negatively stained with, for example, 2% by mass uranyl acetate. Then, the sample is observed with an electron microscope image at a magnification of 5,000 times, 10,000 times, or 50,000 times depending on the size of the fibers that make up the sample. In this case, an axis of any image width, vertical or horizontal, is assumed in the obtained image, and the sample and observation conditions (magnification, etc.) are adjusted so that 20 or more fibers cross the axis. After obtaining an observation image that satisfies this condition, two random axes are drawn vertically and horizontally on the image, and the fiber diameters of the fibers that cross the axis are visually read. In this way, at least three non-overlapping images of the surface area are taken with an electron microscope, and the fiber diameter values of the fibers intersecting each of the two axes are read (thus, information on the diameters of at least 20 fibers x 2 x 3 = 120 fibers is obtained). The arithmetic mean of the fiber diameters obtained in this way is the number average fiber diameter.

The presence of the cellulose type I crystal structure (b) above can be identified by the presence of typical peaks at two positions, around 2θ = 14° to 17° and 2θ = 22° to 23°, in the diffraction profile obtained by wide-angle X-ray diffraction image measurement.

The average aspect ratio of (c) above is more preferably 50 or more, further preferably 100 or more, and may be 200 or more. The average aspect ratio is more preferably 1000 or less, and may be 500 or less. The average aspect ratio can be measured as follows.

That is, the number average fiber diameter is calculated according to the method described above. The number average fiber length of the cellulose nanofibers is also calculated from the same observation image. Specifically, the length from the start point to the end point of the fiber (fiber length) of at least 10 fibers is visually read. For branched fibers, the length of the longest part of the fiber is taken as the fiber length. The arithmetic mean of the fiber lengths thus obtained is calculated to be the number average fiber length. Using these values, the average aspect ratio is calculated according to the following formula.
Average aspect ratio = number average fiber length (nm) / number average fiber diameter (nm)

Cellulose nanofibers that have anionic functional groups are preferably used. The anionic functional groups have a negative potential in the liquid, and therefore can exhibit advantageous adsorption performance for adsorbates with positive potential.

Examples of anionic functional groups include at least one selected from the group consisting of carboxyl groups, phosphate groups, sulfonic acid groups, nitrate groups, borate groups, and sulfate groups. In this specification, the term "carboxyl group" refers to a concept that includes not only the acid type (-COOH) but also the salt type, i.e., carboxylate group (-COOX, where X is a cation that forms a salt with a carboxylic acid), and the acid type and the salt type may be mixed. Similarly, the term "phosphate group, sulfonic acid group, nitrate group, borate group, and sulfate group" refers to a concept that includes not only the acid type but also the salt type, and the acid type and the salt type may be mixed. The salt is not particularly limited, and examples thereof include alkali metal salts such as sodium salts and potassium salts, alkaline earth metal salts such as magnesium salts and calcium salts, onium salts such as ammonium salts and phosphonium salts, and amine salts such as primary amines, secondary amines, and tertiary amines.

The amount of anionic functional groups in the cellulose nanofibers is not particularly limited, and may be, for example, 0.5 to 3.0 mmol/g or 1.5 to 2.0 mmol/g per dry mass of the cellulose nanofibers. For example, in the case of a carboxy group, the amount of anionic functional groups can be calculated by preparing 60 mL of a cellulose nanofiber-containing slurry adjusted to a concentration of 0.1 to 1% by mass, adjusting the pH to about 2.5 with a 0.1 mol/L aqueous hydrochloric acid solution, dropping a 0.05 mol/L aqueous sodium hydroxide solution, measuring the electrical conductivity, and continuing until the pH reaches about 11. The amount of sodium hydroxide consumed in the neutralization stage of the weak acid where the change in electrical conductivity is gradual (V) according to the following formula. The amount of phosphate groups can also be measured by the same electrical conductivity measurement. Other anionic groups may also be measured by known methods.
Amount of anionic functional group (mmol/g) = V (mL) x [0.05/mass of cellulose nanofiber (g)]

In one embodiment, the cellulose nanofiber preferably has a carboxy group as an anionic functional group. Examples of cellulose nanofibers containing a carboxy group include oxidized cellulose nanofibers obtained by oxidizing the hydroxyl groups of the glucose units in the cellulose molecules, and carboxymethylated cellulose nanofibers obtained by carboxymethylating the hydroxyl groups of the glucose units in the cellulose molecules.

Oxidized cellulose nanofibers include those in which the hydroxyl group at the C6 position of the glucose unit in the cellulose molecule is selectively oxidized to a carboxyl group. The oxidized cellulose nanofibers are obtained by oxidizing natural cellulose such as wood pulp using a co-oxidant in the presence of an N-oxyl compound and then defibrating (refining) the cellulose. As the N-oxyl compound, a compound having a nitroxy radical, which is generally used as an oxidation catalyst, is used, for example, a piperidine nitroxyoxy radical, and in particular, 2,2,6,6-tetramethylpiperidinooxy radical (TEMPO) or 4-acetamide-TEMPO is preferred. Cellulose nanofibers oxidized with TEMPO are generally called TEMPO-oxidized cellulose nanofibers (TOCN). The oxidized cellulose nanofibers may have an aldehyde group or a ketone group in addition to the carboxyl group.

The cellulose nanofibers may be obtained by a defibration treatment. The defibration treatment may be performed after or before the introduction of anionic functional groups. The defibration treatment may be performed by treating a dispersion of cellulose fibers using, for example, a homomixer, a high-pressure homogenizer, an ultrasonic dispersion processor, a beater, a disk refiner, a conical refiner, a double disk refiner, a grinder, or the like under high-speed rotation, and a dispersion of cellulose nanofibers can be obtained.

The porous cellulose particles according to the embodiment have a plurality of holes on the particle surface that are traces of the removal of the template particles. The holes are formed by dissolving and removing the resin template particles with an organic solvent, and are clearly distinguishable from holes caused by unevenness or gaps on the particle surface of cellulose particles obtained by simply spray-drying a dispersion of cellulose nanofibers. By using the template particles, holes with a pore size according to the particle size can be formed. Therefore, holes with a desired pore size can be formed on the surface of cellulose particles made of cellulose nanofibers to make them porous, and porous cellulose particles with an increased surface area can be easily obtained.

The above-mentioned removal holes are preferably provided on the particle surface to enhance the adsorption effect when used as an adsorbent, and may be provided inside the particle as well as on the particle surface.

The shape of the holes corresponds to the shape of the template particles. When spherical particles are used as the template particles, the holes remaining after removal are partially spherical on the particle surface of the porous cellulose particles and spherical inside the particles. Therefore, when spherical template particles are used, the porous cellulose particles have partially spherical holes on their particle surface. Here, partially spherical means a shape in which a part of a sphere is missing. However, since the holes remaining after removal may be deformed due to distortion of the porous cellulose particles caused by processing after dissolving and removing the template particles, the above-mentioned spherical or partially spherical shapes do not necessarily have to have a strictly spherical or partially spherical surface, and also include shapes that are distorted after dissolving and removing.

From the viewpoint of ease of handling, the porous cellulose particles according to the embodiment are preferably micro-sized. More specifically, the average particle size of the porous cellulose particles is preferably 1 to 30 μm, more preferably 2 to 20 μm, and even more preferably 3 to 15 μm.

The average pore size of the holes formed in the porous cellulose particles is not particularly limited, but is preferably 1/200 to 1/5, and more preferably 1/100 to 1/10, of the average particle size of the porous cellulose particles. For example, the average pore size of the holes may be 100 to 1000 nm, or 200 to 700 nm. Here, the average pore size of the holes is the average value of the pore size (opening size) on the particle surface.

The area ratio of the holes formed on the particle surface of the porous cellulose particles (hole ratio) is not particularly limited, and may be 5 to 75% or 10 to 60% as the ratio of the opening area of the holes to the surface area of the particle.

The nitrogen adsorption specific surface area (BET method) of the porous cellulose particles is not particularly limited, but is preferably 5 to 50 m ² /g, more preferably 8 to 40 m ² /g, and may be 10 to 30 m ² /g.

The zeta (ζ) potential of the porous cellulose particles is not particularly limited. When the cellulose nanofibers have anionic functional groups, the porous cellulose particles have a negative zeta potential, and cationic organic compounds can be adsorbed onto the particle surface by electrostatic attraction. Here, the zeta potential refers to the potential at the slip surface of the electric double layer, and is also called the interfacial dynamic potential. The zeta potential of the porous cellulose particles in pure water may be, for example, -100 to -10 mV, or -70 to -30 mV.

The porous cellulose particles according to this embodiment are formed from cellulose nanofibers, and may be cellulose nanofibers alone, or may contain other components together with the cellulose nanofibers. For example, in order to efficiently collect the porous cellulose particles from the liquid, magnetic particles such as iron oxide may be contained as other components. In addition, if the template particles remain without being completely dissolved and removed, the resin that constitutes the template particles may be contained as other components.

The method for producing porous cellulose particles according to this embodiment includes the following steps.
(Step 1) spray-drying a dispersion containing cellulose nanofibers and template particles; and
(Step 2) A step of dissolving the template particles contained in the composite particles obtained by spray drying in an organic solvent.

In step 1, the cellulose nanofibers and template particles are uniformly mixed and dispersed in a solvent, for example, water, to prepare a dispersion.

The template particles are particles that serve as a mold for forming holes in cellulose particles made of cellulose nanofibers to make them porous particles. There are no particular limitations on the resin particles that can be dissolved and removed by organic solvents, acids, alkalis, etc. after being composited with cellulose nanofibers. Both organic and inorganic materials can be used as the template particles. Examples of organic materials include resin particles such as acrylic resins such as polymethyl methacrylate and polymethyl acrylate, styrene resins such as polystyrene, vinyl resins such as polyvinyl alcohol, and polyester resins. Examples of inorganic materials include calcium carbonate that dissolves in acids, and SiO ₂ that dissolves in alkalis. These may be used alone or in combination of two or more.

The average particle size of the template particles is not particularly limited, but is preferably 100 to 1000 nm, more preferably 200 to 800 nm, and may be 300 to 700 nm.

In the dispersion liquid prepared in step 1, the concentration of the cellulose nanofibers is not particularly limited as long as it is possible to form cellulose particles by spray drying, and may be, for example, 0.01 to 20% by mass, or 0.05 to 10% by mass. The concentration of the template particles is also not particularly limited, and may be, for example, 0.01 to 20% by mass, or 0.1 to 10% by mass.

In the dispersion, the ratio of cellulose nanofibers to template particles is not particularly limited, but may be a mass ratio of template particles/cellulose nanofibers of 0.5 to 10, or 1 to 6. The same applies to the mass ratio of cellulose nanofibers to template particles in the composite particles obtained by step 1.

In step 1, the dispersion is spray-dried to obtain composite particles in which the cellulose nanofibers and template particles are combined. Spray drying is a method for producing a dry powder by spraying the dispersion into a gas and rapidly drying it, and can be carried out using a known spray dryer. The drying temperature in spray drying is not particularly limited, and may be, for example, 150 to 200°C.

The composite particles obtained by spray drying are integrated in a state in which multiple template particles are dispersed in the cellulose nanofiber matrix, as shown in the conceptual diagram in Figure 1 (A).

In step 2, the composite particles obtained above are treated with an organic solvent, acid, alkali, or the like to dissolve and remove the template particles contained in the composite particles. The organic solvent is not particularly limited as long as it can dissolve the template particles without dissolving the cellulose nanofibers, and examples of the organic solvent include toluene, xylene, methyl ethyl ketone, alcohol, and N,N-dimethylformamide, although this depends on the type of resin that constitutes the template particles. Examples of the acid include hydrochloric acid and sulfuric acid. Examples of the alkali include an aqueous solution of sodium hydroxide and an aqueous solution of potassium hydroxide.

After treating with an organic solvent to dissolve and remove the template particles, the particles may be separated from the organic solvent and dried to obtain porous cellulose particles. Figure 1(B) is a conceptual diagram of a porous cellulose particle, in which the surface of a particle made of cellulose nanofiber has multiple holes that are traces of the removal of the template particles.

The porous cellulose particles according to the embodiment can be used, for example, as an adsorbent or carrier for physiologically active substances such as proteins. More specifically, as an adsorbent for physiologically active substances, it can purify and separate physiologically active substances such as proteins, and can also be used as a packing material for column chromatography to purify physiologically active substances. It may also be used as a carrier for carrying enzymes and carrying out reactions. When cellulose nanofibers having anionic functional groups are used, they are preferably used as an adsorbent for physiologically active substances with an isoelectric point greater than 7.

Biologically active substances are not particularly limited, and examples thereof include proteins, hormones, nucleic acids (DNA, RNA), sugars, sugar chains, vitamins, peptides, etc. Examples of proteins include enzymes, antibodies, receptors, etc.

The following provides a more detailed explanation using examples, but the present invention is not limited to these.

[Measurement method]
(1) Number-average fiber diameter of cellulose nanofibers The number-average fiber diameter of cellulose nanofibers was observed using a transmission electron microscope (TEM) (JEM-1400, manufactured by JEOL Ltd.). That is, a sample was cast on a carbon film-coated grid that had been hydrophilized, and then the number-average fiber diameter was calculated from a TEM image (magnification: 10,000 times) negatively stained with a 2% by mass aqueous solution of uranyl acetate, according to the method described above.

(2) Average aspect ratio of cellulose nanofibers Using the observation samples prepared in the same manner as in the measurement of the number average fiber diameter, the number average fiber length of the cellulose nanofibers was calculated according to the method described above. The average aspect ratio was then calculated according to the above formula using the number average fiber diameter and number average fiber length values.

(3) Crystal structure of cellulose nanofibers The diffraction profile of the sample was measured using an X-ray diffractometer (Rigaku Corporation, RINT-Ultima3). When typical peaks were observed at two positions, around 2θ = 14° to 17° and 2θ = 22° to 23°, the crystal structure (I-type crystal structure) was evaluated as “present,” and when no peaks were observed, the crystal structure was evaluated as “absent.”

(4) Amount of carboxyl groups in cellulose nanofibers 0.25 g of a sample was dispersed in water to prepare 60 mL of an aqueous dispersion. The pH was adjusted to approximately 2.5 with a 0.1 M aqueous hydrochloric acid solution, and then a 0.05 M aqueous sodium hydroxide solution was added dropwise to measure the electrical conductivity, and the amount of carboxyl groups was determined according to the method described above.

(5) Average particle size of porous cellulose particles and template particles The average particle size of the porous cellulose particles was measured by taking pictures at a magnification of 5,000 to 20,000 times using a SEM (scanning electron microscope: S-5000, manufactured by Hitachi High-Technologies Corporation, under conditions of 20 kV), arbitrarily selecting 200 or more particles, measuring their diameters, and calculating the average value. The average particle size of the template particles is the value of the volume particle size (D50) when the cumulative frequency becomes 50% of the total in the cumulative distribution function of the volume particle size obtained from a DLS (dynamic light scattering particle size distribution measuring device), and was measured by dynamic light scattering using a "Zetasizer nano zs" manufactured by Malvern. The sample concentration was 0.2 mg/mL, and pure water was used as the solvent.

(6) Average pore size of holes remaining after removal of template particles The average pore size of the porous cellulose particles was determined by taking photographs at a magnification of 5,000 to 20,000 with an SEM (scanning electron microscope: S-5000, manufactured by Hitachi High-Technologies Corporation, at 20 kV), randomly selecting 350 holes from the particles, measuring their opening sizes, and calculating the average value.

(7) Area ratio of holes on the particle surface of porous cellulose particles (hole ratio)
Photographs of any 10 or more porous cellulose particles were taken at a magnification of 5,000 to 20,000 using an SEM (scanning electron microscope: S-5000, manufactured by Hitachi High-Technologies Corporation, 20 kV conditions), and the area of the particle shown in the photograph (the area inside the outline of the particle) and the sum of the opening area of all holes in the removal traces within the outline (the area of the openings that can be read from the photograph) were calculated using image analysis software, and the average ratio (percentage) of the opening area of the holes to the area of the particle was calculated as the hole ratio.

(8) Zeta potential Using Malvern's "Zetasizer nano zs," the measurement was performed at 0.2 mg/mL in pure water of pH 7.0 at a temperature of 25°C.

(9) BET Value The nitrogen adsorption specific surface area of each particle by the BET method was measured by measuring the nitrogen adsorption isotherm at 77K using a capacity measuring device (BELSORP28SA, manufactured by BELL).

(10) FT-IR Measurement The chemical composition of each particle was analyzed by FT-IR (FT-IR8700, manufactured by Shimadzu Corporation).

[Example 1]
(1) Raw material TOCN: TEMPO oxidized cellulose nanofiber (LeoCrysta I-2SX, Daiichi Kogyo Seiyaku Co., Ltd., cellulose concentration: 2 mass%, cellulose I type crystal structure: "present", number average fiber diameter: 4 nm, average aspect ratio: 280, carboxyl group amount: 1.9 mmol/g)

・PMMA particles: Polymethyl methacrylate particles (manufactured by Sekisui Plastics Co., Ltd., average particle size: 503 nm)

(2) Operation TOCN and PMMA particles were mixed with pure water to prepare an aqueous dispersion (PMMA/TOCN (mass ratio) = 1/1) in which the solid content concentrations of TOCN and PMMA were 0.08% by mass, respectively. At that time, ultrasonic dispersion (T10 ULTRA-TURRAX S0004, manufactured by IKA Corporation) was performed for 1 hour to uniformly disperse the particles. The obtained aqueous dispersion was spray-dried using a spray dryer (Mini Spray Dryer B-290, manufactured by BUCHI Corporation) to obtain composite particles of TOCN and PMMA particles. The spray drying was performed at an inlet temperature of 180°C, a liquid flow rate of 2.5 mL/min, and a gas flow rate of 357 L/hour.

0.12 parts by mass of the obtained composite particles were added to 17.3 parts by mass of toluene and stirred at 100°C for 15 minutes to dissolve and remove the PMMA particles in the composite particles. The particles were then recovered from the toluene and dried to obtain porous cellulose particles. The yield of the porous cellulose particles was 49.1% by mass.

(3) Measurement results Figure 2 shows SEM images of the particles before and after toluene treatment. The image before toluene treatment shown in Figure 2 (A) shows that composite particles were obtained in which multiple PMMA particles were integrated with cellulose nanofibers. The image after toluene treatment shown in Figure 2 (B) shows that the PMMA particles were dissolved and removed, and multiple partially spherical holes were formed on the particle surface, resulting in porous cellulose particles with an increased surface area.

The average particle size and pore size of the porous cellulose particles were measured, and the average particle size was 3.2 μm and the average pore size was 440 nm. The hole ratio of the porous cellulose particles was 9.5%.

The zeta potential was measured for the porous cellulose particles and the composite particles before the toluene treatment. For comparison, cellulose particles were produced by spray drying without adding PMMA particles in the manufacturing method up to the spray drying in the above example, and the zeta potential was measured for the cellulose particles obtained as a comparative example.

The measurement results of the zeta potential are as follows: the absolute value of the zeta potential of the porous cellulose particles was slightly smaller than that of the composite particles before the toluene treatment. The zeta potential of the porous cellulose particles was equivalent to that of the cellulose particles of the comparative example that were not composited with PMMA particles, indicating that the particles are made of cellulose nanofibers.
Porous cellulose particles: -54.3 mV
Composite particles before toluene treatment: -59.5 mV
Comparative Example Cellulose Particles: -55.3 mV

FT-IR measurements were carried out on each of the above particles. The results are shown in Figure 3. In Figure 3, "PMMA/TOCN=1" shows the composite particles before toluene treatment, "TOCN porous" shows the porous cellulose particles after toluene treatment, "TOCN" shows the cellulose particles of the comparative example, and "PMMA" shows the PMMA particles. There was absorption derived from PMMA near the wave number exceeding 1700 cm ^-1 , which was smaller in the porous cellulose particles, confirming the dissolution and removal of the PMMA particles.

[Examples 2 to 4]
The amount of PMMA particles was adjusted so that the mass ratio PMMA/TOCN of the solid content of PMMA particles to TOCN was 2/1 in Example 2, 3/1 in Example 3, and 4/1 in Example 4, and otherwise porous cellulose particles were prepared in the same manner as in Example 1. The yield of the porous cellulose particles was 57.5 mass% in Example 2, 51.2 mass% in Example 3, and 48.0 mass% in Example 4.

Figures 4 to 6 show SEM images of the particles before and after toluene treatment. Compared to the images of the composite particles before toluene treatment shown in Figures 4(A), 5(A), and 6(A), the images after toluene treatment shown in Figures 4(B), 5(B), and 6(B) show that the PMMA particles have been dissolved and removed, and multiple partially spherical holes have been formed on the particle surface, resulting in porous cellulose particles with an increased surface area.

The results of measuring the average particle size, average pore size, nitrogen adsorption specific surface area, and hole ratio for the porous cellulose particles of Examples 2 to 4 are shown in Table 1 below. Table 1 also shows the results of measuring the average particle size and nitrogen adsorption specific surface area for the cellulose particles of the above comparative example.

[Examples 5 and 6]
(1) Raw materials: TOCN, PMMA particles: same as in Example 1.
_{Fe3O4 particles} : manufactured by Toda Kogyo Co., Ltd. (average particle size: 100 nm)

(2) Operation TOCN and PMMA particles were mixed with pure water and subjected to ultrasonic dispersion (IKA Corporation, T10 ULTRA-TURRAX S0004) for 20 minutes to prepare an aqueous dispersion with a _TOCN solid concentration of 0.08 mass% and a PMMA concentration of 0.32 mass%. Next, _Fe3O4 particles were added to the aqueous dispersion to a concentration of 0.08 mass%, and similar ultrasonic dispersion was performed for ₄₀ minutes to prepare an aqueous dispersion containing TOCN, _Fe3O4 particles, and PMMA particles (TOCN/ _Fe3O4 / _PMMA (mass ratio) = 1/1/4).

The obtained aqueous dispersion was spray-dried using a spray dryer (Mini Spray Dryer B-290, manufactured by BUCHI Corporation) to obtain composite particles of TOCN, _{Fe3O4 particles} , and PMMA particles. The spray drying was performed at an inlet temperature of 180°C, a liquid flow rate of 2.5 mL/min, and a gas flow rate of 357 L/hr. 0.12 parts by mass of the obtained composite particles were added to 17.3 parts by mass of toluene in a vial and stirred at 100°C for 30 minutes to dissolve and remove the PMMA particles in the composite particles.

The particles were then collected from the toluene using a magnet and dried to obtain the porous cellulose particles of Example 5 (yield: 62.5% by mass). For magnetic collection, a neodymium magnet (diameter 2.5 cm, height 15 cm) with a magnetic flux density of 0.5 Tesla was brought into contact with the side of the vial at room temperature for 30 to 1,080 minutes to collect the porous cellulose particles. After confirming that the supernatant was transparent, the toluene in the vial was removed. The porous cellulose particles in the vial were washed with toluene, redispersed in ethanol, and dried at normal pressure at 60°C for 1 hour.

The amount of _Fe3O4 particles was adjusted so that the mass ratio of TOCN, _{Fe3O4 particles} , and PMMA particles ₍ TOCN/ _Fe3O4 / _PMMA ) was 1/2/4, and the rest of the procedure was the same as in Example 5 to prepare porous cellulose particles of Example 6 (yield: 48.5% by mass).

(3) Measurement results SEM images of the porous cellulose particles of Examples 5 and 6 are shown in Figures 7 and 8, respectively. As is clear from these SEM images, the porous cellulose particles of Examples 5 and 6 had multiple partially spherical holes on the surface, which were traces of the PMMA particles removed. In addition, as shown in Figures 7(B) and 8(B), it was confirmed that iron oxide ( _Fe3O4 particles) was attached to the particle surface and inside the holes of the _porous cellulose particles, and that the iron oxide was composited.

The average particle size, average pore size, nitrogen adsorption specific surface area, hole ratio, and zeta potential of the porous cellulose particles of Examples 5 and 6 were measured, and the results are shown in Table 2 below.

[Evaluation of protein adsorption performance]
(1) Raw Materials: Lysozyme, derived from chicken egg white: manufactured by MP Biomedical (zeta potential: 7.8 mV, size: 4.5 × 3.0 × 3.0 nm, isoelectric point: 11).
Porous cellulose particles: Porous cellulose particles prepared in Example 5.
Cellulose particles 1: Comparative cellulose particles prepared in Example 1.
Cellulose particles 2: Cellufine C-500 (cellulose particles with carboxymethyl groups introduced), manufactured by JNC Corporation

(2) Operation 10 mg of porous cellulose particles, cellulose particles 1, or cellulose particles 2 was added to 50 mL of 0.4 g/L lysozyme solution dissolved in phosphate buffer, and the mixture was stirred using a shaker, and samples were taken at stirring times of 5, 10, 20, 30, 60, 90, and 120 minutes. The supernatant was separated from the sampled samples. Separation of the particles and the supernatant was performed by centrifugation (8000 rpm, 5 minutes) for cellulose particles 1 and cellulose particles 2. Meanwhile, for the porous cellulose particles, the sampled sample was separated into the porous cellulose particles and the supernatant using a neodymium magnet.

The absorbance of the obtained supernatant was measured by ultraviolet-visible-near infrared spectroscopy (281 nm), and the amount of free protein in the supernatant (C ₁ ) was quantified by comparing with a separately prepared calibration curve, and the adsorption capacity of lysozyme was calculated from the result. The adsorption capacity of lysozyme was calculated by subtracting the amount of protein in the supernatant (C ₁ ) from the mass of lysozyme added (C ₀ ) to obtain the adsorption amount of lysozyme (mg), and the adsorption amount of lysozyme per unit mass of the adsorbent added was calculated from the following formula.
Adsorbed amount of lysozyme (mg) = C ₀ - _{C 1}
Adsorption capacity of lysozyme (mg/g)=(C ₀ −C ₁ ) (mg)/mass of adsorbent added (g)

(3) Measurement results The results are shown in Figure 9. The porous cellulose particles of Example 5 had an adsorption capacity of lysozyme of 852 mg/g. This was approximately 1.4 times the adsorption capacity of cellulose particles 1 of the comparative example, which were not porous using template particles, and commercially available cellulose particles 2, and the particles had excellent protein adsorption performance.

Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. These embodiments and their omissions, substitutions, modifications, etc. are included within the scope and gist of the invention as well as the invention and its equivalents as set forth in the claims.

Claims (7)

Porous cellulose particles formed from cellulose nanofibers, with multiple holes on the particle surface that are traces of the removal of template particles. The porous cellulose particles according to claim 1, wherein the average particle size of the porous cellulose particles is 1 to 30 μm, and the average pore size of the holes is 1/200 to 1/5 of the average particle size of the porous cellulose particles. The porous cellulose particles according to claim 1 or 2, wherein the cellulose nanofibers have anionic functional groups. An adsorbent comprising the porous cellulose particles of any one of claims 1 to 3. spray-drying a dispersion liquid containing cellulose nanofibers and template particles to obtain composite particles in which the cellulose nanofibers and the template particles are composited; and
dissolving the template particles contained in the obtained composite particles in an organic solvent;
A method for producing porous cellulose particles, comprising: The method for producing porous cellulose particles according to claim 5, wherein the average particle size of the template particles is 100 to 1000 nm. The method for producing porous cellulose particles according to claim 5 or 6, wherein the cellulose nanofibers have anionic functional groups.