JP2010235454A - Method of recovering and reusing n-oxyl compound - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of simply and efficiently recovering an N-oxyl compound at a low cost from waste water after the oxidation treatment of a cellulose-based raw material containing inorganic salts, a water soluble organic substance, the N-oxyl compound and the like. <P>SOLUTION: The waste water produced in oxidizing a cellulose-based raw material in the presence of an N-oxyl compound and a bromide and/or an iodide with the use of an oxidizing agent is brought into contact with a supercritical carbon dioxide fluid to extract the N-oxyl compound from the waste water. The extracted N-oxyl compound can be also reused in the oxidation of the cellulose-based raw material. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for efficiently recovering and reusing an expensive N-oxyl compound from wastewater generated in an oxidized cellulose production process using an N-oxyl compound as a cellulose oxidation catalyst.

  It is known that a primary hydroxyl group of cellulose can be oxidized into a carboxyl group and an aldehyde group by treating a cellulose-based raw material in the presence of a catalytic amount of an N-oxyl compound and a chlorine-based oxidant (Non-patent Document 1).

  The N-oxyl compound, which is a catalyst used in this oxidized cellulose production technology, is very expensive, so it is desirable to recover and reuse it from the waste water after completion of the reaction. However, a method for recovering and reusing N-oxyl compounds from wastewater has not been reported so far.

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

  As a general method for recovering a low molecular weight organic molecular catalyst which is amphiphilic and low in volatility dissolved in both water and an organic solvent, such as an N-oxyl compound, extraction and adsorption can be considered. However, liquid-liquid extraction using an organic solvent such as hexane causes environmental concerns because the solvent remains in the wastewater after the extraction process, and requires a separate process for recovering the organic solvent. It also takes. In addition, the method of selectively adsorbing and recovering a catalyst using a hydrophobic synthetic resin or the like uses an organic solvent when desorbing the catalyst adsorbed on the resin surface. And costly.

  As another method, it may be possible to concentrate and reuse the catalyst by removing components other than the catalyst contained in the wastewater, such as inorganic salts, using a desalting tower filled with an ion exchange resin. It is done. However, in this method, when regenerating the ion exchange resin, a large amount of inorganic ions, regenerative chemicals, and drainage containing organic components are generated, and a separate process for treating the drainage is required. It takes time and money.

  In view of the above, the present invention provides an N-oxyl compound from a wastewater after oxidation treatment of a cellulose-based raw material containing inorganic salts, a water-soluble organic substance, an N-oxyl compound, etc., at low cost, simply and efficiently. The object is to provide a method of recovering well.

  As a result of intensive studies, the inventors of the present invention made efficient by extracting the N-oxyl compound into the supercritical carbon dioxide fluid by contacting the wastewater after the oxidation treatment of the cellulosic raw material with the supercritical carbon dioxide fluid. And it discovered that N-oxyl compound could be collect | recovered easily and came to complete this invention.

  In the present invention, the expensive N-oxyl compound can be selectively recovered by bringing the supercritical carbon dioxide fluid into contact with the waste water containing the N-oxyl compound. The carbon dioxide used is a safe and economical substance that originally exists in nature. Further, since the temperature and pressure at which carbon dioxide changes to a supercritical state, that is, the critical point is relatively low temperature and low pressure, the supercritical state can be easily obtained.

  Extraction of organic substances dissolved in water using supercritical carbon dioxide fluid has been performed so far, but this technology is used to actually extract N-oxyl compounds from wastewater generated by oxidation of cellulosic raw materials. This is the first time that has been extracted. The N-oxyl compound used in the oxidation of cellulosic raw materials is a kind of compound that generates stable radicals, but it is extremely different from general organic substances that have been extracted with supercritical carbon dioxide fluids so far. Because it is an unstable substance, there is a concern that it will be decomposed during processing with supercritical carbon dioxide fluid or modified by reacting with other components in wastewater, and it will lose its ability as an oxidation catalyst. I was expected. It was a surprising result that an unmodified and catalytic N-oxyl compound could be recovered by the extraction process using the supercritical carbon dioxide fluid of the present invention.

  The present invention is a method for recovering and reusing an N-oxyl compound from wastewater generated by oxidation of a cellulosic raw material using an N-oxyl compound, and comprising bromide and / or iodide, sodium chloride, an oxidizing agent, pulp The N-oxyl compound containing a water-soluble polysaccharide derived therefrom as an impurity is brought into contact with the supercritical carbon dioxide, thereby extracting the N-oxyl compound into the supercritical carbon dioxide. .

(Oxidation of cellulosic material using N-oxyl compound)
In the present invention, as wastewater to be brought into contact with the supercritical carbon dioxide fluid, wastewater generated from a reaction of oxidizing a cellulosic raw material using an oxidizing agent in the presence of an N-oxyl compound and bromide and / or iodide is used. be able to.

  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.

  The amount of carboxyl groups in the resulting oxidized pulp was prepared by adding 60 ml of a 0.5% by mass slurry of oxidized pulp, adding 0.1M hydrochloric acid aqueous solution to pH 2.5, and then dropping 0.05N aqueous sodium hydroxide solution dropwise. Then, the electrical conductivity is measured until the pH reaches 11, and the amount of sodium hydroxide consumed in the neutralization step of the weak acid whose change in electrical conductivity is gradual can be calculated using the following equation. it can.

Amount of carboxyl group [mmol / g pulp] = a [ml] × 0.05 / oxidized pulp mass [g]
(Drainage generated during oxidation of cellulosic materials)
In this invention, an N-oxyl compound is collect | recovered from the waste_water | drain generate | occur | produced when oxidizing a cellulose raw material using an oxidizing agent in presence of said N-oxyl compound and bromide and / or iodide. This drainage can be obtained by removing the oxidized cellulose by, for example, filtering the reaction solution after the oxidation reaction of the cellulose-based raw material is completed.

  In addition to the N-oxyl compound, bromine and / or in addition to the N-oxyl compound, the wastewater generated when the cellulosic material is oxidized with an oxidizing agent in the presence of the N-oxyl compound and bromide and / or iodide. Impurities such as iodide, sodium chloride, oxidant, and pulp-derived water-soluble polysaccharide are considered to be contained.

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

  In the present invention, the N-oxyl compound is extracted from the supercritical carbon dioxide fluid by bringing wastewater generated by the oxidation of the cellulose-based raw material into contact with the supercritical carbon dioxide fluid. The reason why the N-oxyl compound can be efficiently recovered by the method of the present invention is unknown in detail, but supercritical carbon dioxide has the same degree of polarity as hexane, which is one of the good solvents for the N-oxyl compound. In addition, it is presumed that the N-oxyl compound dissolved in the wastewater is extracted efficiently and preferentially because it easily diffuses into the wastewater and easily mixes with the wastewater.

  The present invention is characterized in that the N-oxyl compound is selectively extracted and reused by utilizing the difference in solubility in supercritical carbon dioxide between the water-soluble inorganic substance and polymer substance and the N-oxyl compound. Yes. 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. The contact temperature is preferably 35 to 95 ° C, more preferably 40 to 80 ° C.

  The contact treatment between the waste water and the supercritical carbon dioxide can be performed by putting the waste water into an extraction vessel made of stainless steel, etc., supplying supercritical carbon dioxide and bringing it into contact with the waste water. 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.

  After the above contact treatment, the extract containing the N-oxyl compound can be obtained by returning the pressure of carbon dioxide to normal pressure.

(Reuse of N-oxyl compounds)
According to the present invention, an extract containing an N-oxyl compound extracted and recovered using supercritical carbon dioxide has a low concentration of impurities such as inorganic salts and polymer substances, and also retains oxidation catalytic ability. -Since the oxyl compound is contained at a relatively high concentration, it can be directly reused as a whole or a part of the oxidation catalyst in the oxidation of the cellulosic raw material without any further purification / extraction operation of the N-oxyl compound. Can be used.

  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.

(Oxidation of cellulosic materials)
Bleached unbeaten sulfite pulp (Nippon Paper Chemical Co., Ltd., carboxyl group content: 0.003 mmol / g) 5 g (absolutely dried) derived from coniferous tree is 2,2,6,6-tetramethyl-1-piperidine-N-oxy The solution was added to 500 ml of an aqueous solution in which 78 mg (0.5 mmol) of radical (TEMPO) and 754 mg (7 mmol) of sodium bromide were dissolved, and stirred until the pulp was uniformly dispersed. Subsequently, 12.5 ml of an aqueous sodium hypochlorite solution (2 mol / l) was added, and then the pH was adjusted to 10.3 with a 0.5N aqueous hydrochloric acid solution to start the oxidation reaction. During the reaction, the pH in the system was lowered, but a 0.5N sodium hydroxide aqueous solution was successively added and reacted for 2 hours while adjusting the pH to 10. The oxidized pulp was filtered off using a glass filter. The carboxyl group content of the oxidized pulp was 1.35 mmol / g.

  The filtrate after separating the oxidized pulp was used as wastewater containing TEMPO in the following examples.

  500 ml of the waste water was put into a 1000 ml stainless steel extraction vessel and heated to 40 ° C., then carbon dioxide was supplied and pressurized to 20 MPa to obtain a supercritical state. After contacting the wastewater and supercritical carbon dioxide for 2 hours under the same conditions, the supercritical carbon dioxide fluid was recovered. The pressure of carbon dioxide was returned to normal pressure, and the amount of TEMPO in the obtained extract was quantified by high performance liquid chromatography (column: Nucleosil C-18 [15 cm], eluent: acetonitrile / water = 30/70, (Flow rate 1.5 ml / min, detection wavelength 240 nm). Further, the amount of TEMPO in the waste water remaining in the stainless steel container was measured in the same manner. As a result, the recovery rate of TEMPO (that is, the ratio of TEMPO extracted in the extract from the TEMPO contained in the waste water) was 82%.

  TEMPO was extracted in the same manner as in Example 1 except that the contact between the waste water and supercritical carbon dioxide was performed at 80 ° C. The recovery rate of TEMPO was 87%.

  50 ml of an extract containing TEMPO recovered in Example 1 (containing 0.42 mmol of TEMPO) was added to 500 ml of an aqueous solution in which 754 mg (7 mmol) of sodium bromide was dissolved. 5 g (absolutely dry) of sulfite pulp (manufactured by Nippon Paper Chemical Co., Ltd., carboxyl group amount: 0.003 mmol / g) was added and stirred until the pulp was uniformly dispersed. Next, 12.5 ml (25 mmol) of 2M aqueous sodium hypochlorite solution was added, and then the pH was adjusted to 10.3 with 0.5N aqueous hydrochloric acid solution to start the oxidation reaction. During the reaction, the pH in the system was lowered, but a 0.5N sodium hydroxide aqueous solution was sequentially added and reacted for 2 hours while adjusting the pH to 10. After completion of the reaction, the pulp was filtered off with a glass filter and sufficiently washed with water to obtain oxidized pulp. The amount of carboxyl groups in the oxidized pulp was 1.11 mmol / g.

  The same operation as in Example 3 was performed except that the extract containing TEMPO recovered in Example 2 (corresponding to 0.44 mmol of TEMPO) was used instead of the extract containing TEMPO recovered in Example 1. An oxidized pulp was obtained. The amount of carboxyl groups in the oxidized pulp was 1.23 mmol / g.

Claims (4)

(1) N-oxyl compound, and (2) N from wastewater generated when oxidizing a cellulosic raw material using an oxidizing agent in the presence of a compound selected from the group consisting of bromide, iodide or a mixture thereof. -A method for recovering an oxyl compound, wherein the N-oxyl compound is extracted from wastewater by bringing the wastewater into contact with a supercritical carbon dioxide fluid. The method according to claim 1, wherein the contact between the waste water and the supercritical carbon dioxide fluid is performed under a pressure of 8 to 50 MPa and a temperature of 35 to 95 ° C. Extracted N-oxyl compound 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. In the reaction of oxidizing a cellulosic raw material using an oxidizing agent in the presence of (1) an N-oxyl compound and (2) a compound selected from the group consisting of bromide, iodide or a mixture thereof, the N- A method for reusing an N-oxyl compound, wherein the N-oxyl compound recovered by the method according to any one of claims 1 to 3 is used as all or part of the oxyl compound.