JP2010235669A - Method for removing organic oxidation catalyst remaining in oxidized pulp - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of efficiently removing the remaining N-oxyl compound from an oxidation pulp obtained using the N-oxyl compound as an oxidation catalyst. <P>SOLUTION: The N-oxyl compound remaining in an oxidation pulp is extracted into supercritical carbon dioxide by bringing the oxidation pulp into contact with the supercritical carbon dioxide and is removed. By using the thus obtained oxidation pulp having a low N-oxyl compound content, the cellulose nano-fiber with a low N-oxyl compound content can be prepared. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  INDUSTRIAL APPLICABILITY The present invention can safely and economically remove N-oxyl compounds remaining in pulp, which cannot be removed only by washing with water, from oxidized pulp produced using an N-oxyl compound as an oxidation catalyst by a simple procedure. Regarding the method.

  In the presence of a catalytic amount of 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (hereinafter referred to as TEMPO) and sodium hypochlorite, which is an inexpensive oxidant, When treated, the primary hydroxyl groups of cellulose can be oxidized to carboxyl groups and aldehyde groups, and the resulting oxidized pulp is subjected to a simple mechanical treatment such as a mixer in water to obtain a highly viscous and transparent cellulose nanofiber aqueous dispersion. It is known that it can be prepared (Non-Patent Document 1).

  The cellulose nanofiber obtained by the above method is a novel biodegradable water-dispersed material, and can be freely modified based on a carboxyl group localized on the fiber surface. In addition, since the cellulose nanofibers obtained by the above method are in the form of a dispersion, the quality can be modified by blending with various water-soluble polymers or by combining with organic / inorganic pigments. it can. Furthermore, cellulose nanofibers can be made into sheets or made into fibers. High-performance packaging materials, transparent organic base materials, high-performance fibers, separation membranes, regenerative medical materials, and the like are assumed as applications utilizing such characteristics of cellulose nanofibers. In the future, it is expected to develop new highly functional products that are essential for the formation of a recycling-type safety and security society by making the best use of the characteristics of cellulose nanofibers.

Saito, T., et al., Cellulose Commun., 14 (2), 62 (2007)

  However, in the oxidized pulp produced using the N-oxyl compound such as TEMPO as a catalyst, about 10 ppm of N-oxyl compound remains as a nitrogen content even after sufficient washing with water. When the cellulose nanofiber aqueous dispersion is prepared using oxidized pulp in which the N-oxyl compound remains, naturally, the N-oxyl compound is also mixed in the cellulose nanofiber dispersion, and the cellulose nanofiber is made of a highly functional material. In some cases, the N-oxyl compound remaining in the dispersion may have an undesirable effect depending on the application. TEMPO, which is one of N-oxyl compounds, and derivatives thereof have not yet been clearly toxic to the environment and the human body. Cellulose nanofiber dispersions or films prepared therefrom can be used as cosmetic thickeners and food packaging. Assuming that the N-oxyl compound is not used in the dispersion, it is preferable that the dispersion is used. In addition, for example, when an N-oxyl compound is present in the dispersion, the dispersibility of the cellulose nanofibers is lowered, the transparency of the dispersion is lowered, and the transparency of the film prepared from the cellulose nanofiber dispersion is lowered. Is also possible. Therefore, development of a method capable of removing N-oxyl compounds as much as possible is desired, but at present, there is no report on a method of removing N-oxyl compounds remaining in pulp.

  An object of this invention is to provide the method of removing efficiently the N-oxyl compound which remains in a pulp.

  As a result of diligent studies to solve the problems of the prior art, the present inventors brought oxidized pulp obtained using an N-oxyl compound as an oxidation catalyst into contact with supercritical carbon dioxide. It has been found that the remaining N-oxyl compound can be extracted into supercritical carbon dioxide, and the N-oxyl compound can be efficiently removed from the oxidized pulp. Moreover, it discovered that the cellulose nanofiber dispersion liquid with a very low density | concentration of a N-oxyl compound can be obtained by using the oxidized pulp from which the N-oxyl compound obtained in this way was removed.

  In the present invention, by contacting an oxidized pulp produced using an N-oxyl compound as an oxidation catalyst with supercritical carbon dioxide, the N-oxyl compound is extracted from the supercritical carbon dioxide, thereby remaining in the pulp. -An oxidized pulp having a very low concentration of oxyl compounds can be produced. Moreover, the cellulose nanofiber dispersion liquid with the very low density | concentration of an N-oxyl compound can be obtained by using the oxidized cellulose obtained in this way.

  The present invention is characterized in that an N-oxyl compound remaining as an impurity in oxidized pulp is extracted into supercritical carbon dioxide to remove the N-oxyl compound from the oxidized pulp.

(Production of oxidized pulp using N-oxyl compound)
In the present invention, the oxidized pulp to be contacted with supercritical carbon dioxide is obtained by using an oxidizing agent in the presence of (1) N-oxyl compound and (2) bromide, iodide, or a mixture thereof. Manufactured by oxidation.

  Examples of the N-oxyl compound that is used in the oxidation of the cellulosic raw material and can be extracted according to the present invention include substances represented by the following general formula (formula 1).

(In Formula 1, R ^{1 to} R ⁴ represent the same or different alkyl groups having about 1 to 4 carbon atoms.)
Among the compounds represented by Formula 1, 2,2,6,6-tetramethyl-1-piperidine-oxy radical (hereinafter referred to as TEMPO) and 4-hydroxy-2,2,6,6-tetramethyl-1 A compound that generates a piperidine-oxy radical (hereinafter referred to as 4-hydroxy TEMPO) is preferable. In addition, the radical of the N-oxyl compound represented by any one of the following formulas 2 to 4, that is, the hydroxyl group of 4-hydroxy TEMPO is etherified with alcohol, or esterified with carboxylic acid or sulfonic acid, and has an appropriate hydrophobicity. The imparted 4-hydroxy TEMPO derivative is inexpensive and can obtain uniform oxidized cellulose, so that it can be preferably used for oxidation of cellulosic raw materials, and can be efficiently recovered by the method of the present invention. it can.

(In formulas 2 to 4, R is a linear or branched carbon chain having 4 or less carbon atoms.)
Further, a radical of an N-oxyl compound represented by the following formula 5, that is, an azaadamantane type nitroxy radical is also preferable for the same reason as the 4-hydroxy TEMPO derivative.

(In Formula 5, R ⁵ and R ⁶ represent the same or different hydrogen or a C _{1 to} C ₆ linear or branched alkyl group.)
The amount of the N-oxyl compound used in the oxidation of the cellulosic material is generally 0.01 to 10 mmol, preferably 0.01 to 1 mmol, more preferably with respect to 1 g of the cellulosic material. Is about 0.05 to 0.5 mmol.

  Examples of the bromide or iodide used in oxidizing the cellulosic material include compounds that can be dissociated and ionized in water, such as alkali metal bromide and alkali metal iodide. In general, bromide or iodide is used in an amount of about 0.1 to 100 mmol, preferably 0.1 to 10 mmol, and more preferably about 0.5 to 5 mmol with respect to 1 g of cellulosic raw material.

  Examples of the oxidizing agent used in oxidizing the cellulosic raw material include halogen, hypohalous acid, halous acid, perhalogen acid or salts thereof, halogen oxide, peroxide and the like. Among these, sodium hypochlorite, which is inexpensive and has a low environmental load, is particularly preferably used from the viewpoint of production cost. The oxidizing agent is generally used in an amount of about 0.5 to 500 mmol, preferably 0.5 to 50 mmol, and more preferably about 2.5 to 25 mmol with respect to 1 g of the absolutely dry cellulosic raw material.

  Cellulosic raw materials that are oxidized using an oxidizing agent in the presence of an N-oxyl compound and bromide and / or iodide are not particularly limited. In addition to powdered cellulose pulverized with a slab or mill, or microcrystalline cellulose powder purified by chemical treatment such as acid hydrolysis, plants such as kenaf, hemp, rice, bacus, bamboo, etc. may be used. . Among these, bleached kraft pulp, bleached sulfite pulp, powdered cellulose, or microcrystalline cellulose powder is preferably used from the viewpoint of mass production and cost.

  The oxidation of the cellulosic raw material performed using an oxidizing agent in the presence of an N-oxyl compound and bromide and / or iodide is generally alkaline, such as an aqueous sodium hydroxide solution, at a room temperature of about 15 to 30 ° C. By adding the solution, the reaction is carried out at a reaction time of about 0.5 to 4 hours while maintaining the pH of the reaction solution at 9 to 12, preferably about 10 to 11.

  After the oxidation reaction, the mixture is filtered using a filter such as a glass filter, thoroughly washed with water, dehydrated and concentrated to obtain wet sheet-like oxidized pulp having a solid content concentration of about 10 to 40% by mass.

(Treatment of oxidized pulp with supercritical carbon dioxide)
In the present invention, the oxidized pulp obtained by the above oxidation method is brought into contact with supercritical carbon dioxide, whereby the N-oxyl compound remaining in the oxidized pulp is extracted into supercritical carbon dioxide and removed.

  In the present invention, supercritical carbon dioxide (also referred to as supercritical carbon dioxide fluid) refers to carbon dioxide in a supercritical state, that is, carbon dioxide under conditions of a temperature of 31.1 ° C. or higher and a pressure of 7.38 MPa or higher. .

  The present invention is characterized in that the N-oxyl compound is selectively extracted and removed by utilizing the difference in solubility in supercritical carbon dioxide between cellulose and the N-oxyl compound. It is preferable to use supercritical carbon dioxide near the supercritical point because the N-oxyl compound can be extracted efficiently. For example, the pressure of supercritical carbon dioxide is preferably 8 to 50 MPa, and more preferably 10 to 20 MPa. Further, the contact temperature is preferably 35 to 95 ° C, more preferably 40 to 80 ° C.

  In the contact treatment with supercritical carbon dioxide, wet sheet-like oxidized pulp having a solid content concentration of about 10 to 40% by mass obtained by the above method is placed in a stainless steel extraction container or the like, and supercritical carbon dioxide is supplied. For example, by contacting with oxidized pulp. The conditions for the treatment with supercritical carbon dioxide can be arbitrarily set. Further, the treatment with supercritical carbon dioxide may be performed batchwise or continuously, and may be appropriately selected.

  The reason why the N-oxyl compound in the oxidized pulp can be efficiently removed by the method of the present invention is not known in detail, but supercritical carbon dioxide is as polar as hexane, which is one of the good solvents for the N-oxyl compound. It is presumed that N-oxyl compounds adsorbed not only on the surface of the fiber but also on the inside can be extracted and removed efficiently because it has a low viscosity and easily diffuses and penetrates into the voids existing in the pulp fiber. Is done.

(Preparation of cellulose nanofiber)
Cellulose nano, which is a single microfibril of cellulose having a width of about 2 to 5 nm and a length of about 1 to 5 μm, by defibrating and dispersing oxidized pulp having a very low content of N-oxyl compound obtained by the method of the present invention Fiber dispersions can be prepared.

  Examples of the type of equipment used for defibration / dispersion include high-speed rotary type, colloid mill type, high-pressure type, roll mill type, ultrasonic type, etc., but cellulose nanofiber dispersion with excellent transparency and fluidity can be used. In order to obtain efficiently, it is preferable to treat with a wet high pressure or ultra high pressure homogenizer that can be dispersed under conditions of 50 MPa or more, preferably 100 MPa or more, more preferably 140 MPa or more.

  The cellulose nanofiber dispersion prepared by using the oxidized pulp obtained according to the present invention has a very low dispersibility of the cellulose nanofiber because the residual amount of the N-oxyl compound is extremely low, and has high transparency. The transparency of the cellulose nanofibers of the present invention is preferably 85% or more, more preferably 90% or more, based on the transmittance of light having a wavelength of 660 nm with respect to a 0.1% (w / v) cellulose nanofiber dispersion. Most preferably, it is 95% or more.

  Next, the present invention will be specifically described based on examples. The following examples are specific examples of the present invention, and the present invention is not limited to these examples. Is not to be done.

<Preparation of oxidized pulp>
5 g (absolutely dried) of bleached unbeaten sulfite pulp (Nippon Paper Chemical Co., Ltd.) derived from coniferous tree was added to 500 ml of an aqueous solution in which 78 mg (0.5 mmol) of TEMPO (Sigma Aldrich) and 754 mg (7 mmol) of sodium bromide were dissolved, Stir until the pulp is uniformly dispersed. After adding 18 ml of an aqueous sodium hypochlorite solution (effective chlorine 5%) to the reaction system, the pH was adjusted to 10.3 with an aqueous 0.5N hydrochloric acid solution to initiate the oxidation reaction. During the reaction, the pH in the system was lowered, but a 0.5N aqueous sodium hydroxide solution was successively added to adjust the pH to 10. After reacting for 2 hours, the mixture was filtered through a glass filter, sufficiently washed with water, then dehydrated and concentrated to obtain wet sheet-like oxidized pulp having a solid content concentration of 12% by mass.

<Measurement of amount of residual TEMPO in pulp>
The amount of TEMPO remaining in the pulp was measured as the amount of nitrogen in the pulp using a trace nitrogen meter (Mitsubishi Chemical, TN-10).

  The nitrogen content of the bleached unbeaten sulfite pulp, which is the raw material pulp used for the preparation of the oxidized pulp, was 21 ppm. Moreover, the nitrogen content of the oxidized pulp obtained by preparation of said oxidized pulp was 32 ppm. The difference between the amount of nitrogen in the oxidized pulp and the amount of nitrogen in the raw pulp is considered to be based on the amount of nitrogen in the TEMPO used for the oxidation reaction. Therefore, the amount of nitrogen derived from TEMPO remaining in the pulp by the preparation of the oxidized pulp is , 32 ppm-21 ppm = 11 ppm.

  25 g of the oxidized pulp (solid content concentration: 12% by mass) was placed in a 50 ml stainless steel extraction container, heated to 40 ° C., then supplied with carbon dioxide and pressurized to 20 MPa to obtain a supercritical state. After contacting the oxidized pulp with supercritical carbon dioxide for 2 hours, the supercritical carbon dioxide was recovered. After returning the pressure to normal pressure, the oxidized pulp in the container was taken out. After the obtained oxidized pulp was dried at 70 ° C., the amount of nitrogen in the pulp was measured by the above method. As a result, the nitrogen content of the oxidized pulp was 22 ppm. As described above, since the amount of nitrogen in the raw material pulp was 21 ppm, it was found that the amount of nitrogen derived from residual TEMPO was 22 ppm-21 ppm = 1 ppm. As described above, since the amount of nitrogen derived from TEMPO that existed before the supercritical carbon dioxide treatment was 11 ppm, it can be seen that the treatment of Example 1 was able to remove TEMPO with a removal rate of 91%.

  TEMPO in the pulp was removed in the same manner as in Example 1 except that the contact between the oxidized pulp and supercritical carbon dioxide was performed at 80 ° C. As a result of measuring the nitrogen content in the obtained pulp, it was 21 ppm. As described above, since the amount of nitrogen in the raw pulp was 21 pm, it can be seen that TEMPO in the oxidized pulp could be removed with a removal rate of 100% by the treatment in Example 2.

  The 0.1% (w / v) slurry of oxidized pulp obtained in Example 1 was treated at 12,000 rpm for 15 minutes using a mixer, and further treated 5 times at a pressure of 140 MPa using an ultrahigh pressure homogenizer. Thus, a dispersion of cellulose nanofibers, which is a transparent gel dispersion, was obtained. The cellulose nanofiber dispersion thus obtained had good dispersibility and high transparency. When the transmittance of 660 nm light in a 0.1% (w / v) aqueous dispersion was measured using a UV-VIS spectrophotometer UV-265FS (Shimadzu Corporation), it was 95.8%.

Claims (4)

(1) N-oxyl compound and (2) N-oxyl compound from oxidized pulp obtained by oxidizing cellulosic raw material using oxidizing agent in the presence of bromide, iodide or a mixture thereof. A method for removing an N-oxyl compound, comprising extracting the N-oxyl compound from the oxidized pulp by bringing the oxidized pulp into contact with supercritical carbon dioxide. The method according to claim 1, wherein the contact between the oxidized pulp and the supercritical carbon dioxide is performed under a pressure of 8 to 50 MPa and a temperature of 35 to 95 ° C. N-oxyl compound to be removed generates 2,2,6,6-tetramethyl-1-piperidine-N-oxy radical (TEMPO), 4-hydroxy-2,2,6,6-tetramethyl A compound that generates a 1-piperidine-N-oxy radical (4-hydroxy TEMPO), a compound that generates a 4-hydroxy TEMPO derivative obtained by etherification or esterification of a hydroxyl group of 4-hydroxy TEMPO, or an azaadamantane type nitro The method according to claim 1, wherein the compound is a compound that generates a xyl radical, or a mixture thereof. Cellulose nanofibers obtained by defibrating and dispersing the oxidized pulp from which the N-oxyl compound obtained by the method according to any one of claims 1 to 3 has been removed.