Method for decomposing N-oxyl compounds

Abstract

A new method is provided that can decompose N-oxyl compounds in solution. A halogen oxoacid or its salt is added to a solution containing an N-oxyl compound, and then a salt of thiosulfate is added.

Description

[Technical field]

[0001] The present invention relates to a method for decomposing N-oxyl compounds. [Background technology]

[0002] 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) is a water-soluble, stable nitroxide radical that is used as a catalyst for oxidation reactions in organic synthesis. For example, TEMPO is used together with an oxidizing agent to oxidize the primary hydroxyl groups of cellulose to carboxyl and aldehyde groups. After a reaction using an N-oxyl compound such as TEMPO, a waste liquid containing the N-oxyl compound remains. For example, TEMPO corresponds to Class 4 hazardous material under the Fire Service Act, so waste liquid containing N-oxyl compounds such as TEMPO must be properly treated.

[0003] Patent Document 1 discloses a method for decomposing an N-oxyl compound in a solution by adding hydrogen peroxide and an iron salt to the solution containing the N-oxyl compound to cause the Fenton reaction. However, the method described in Patent Document 1 has a problem that iron (III) ions (Fe 3+ ) is thought to result in the generation of insoluble iron hydroxide and iron oxide, which would require time and cost to recover and treat. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2016-97379 A Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a new method for decomposing N-oxyl compounds in a solution such as a waste liquid after an oxidation reaction. [Means for solving the problem]

[0006] As a result of intensive research, the present inventors have found that it is possible to decompose an N-oxyl compound in a solution by adding a halogen oxoacid or its salt to the solution containing the N-oxyl compound and then adding a salt of thiosulfate. The present invention includes, but is not limited to, the following: (1) A method for decomposing an N-oxyl compound, which comprises adding a halogen oxoacid or a salt thereof to a solution containing the N-oxyl compound, and then adding a salt of thiosulfate. (2) The method for decomposing an N-oxyl compound according to claim 1, comprising adding sodium bromide to the solution containing the N-oxyl compound before adding the halogen oxoacid or its salt or before adding the salt of thiosulfate, and / or the solution containing the N-oxyl compound before adding the halogen oxoacid or its salt further contains sodium bromide. (3) The method for decomposing an N-oxyl compound according to (1) or (2), wherein the halogen oxoacid or a salt thereof is at least one selected from sodium hypochlorite and potassium hypochlorite. (4) The method for decomposing an N-oxyl compound according to any one of (1) to (3), wherein the amount of the halogen oxoacid or a salt thereof added is 10 to 400 parts by mass per part by mass of the N-oxyl compound. (5) The method for decomposing an N-oxyl compound according to any one of (1) to (4), wherein the amount of the salt of thiosulfate added is 5 to 100 parts by mass per part by mass of the N-oxyl compound. (6) A method for decomposing an N-oxyl compound according to any one of (1) to (5), comprising adding a salt of thiosulfate 3 minutes or more after the addition of the halogen oxoacid or its salt. Effect of the Invention

[0007] According to the present invention, N-oxyl compounds in a solution can be decomposed. The method of the present invention is useful for treating a solution containing N-oxyl compounds, such as waste liquid after an oxidation reaction.

Mode for Carrying Out the Invention

[0008] The present invention relates to a method for decomposing N-oxyl compounds, which includes adding a halogen oxo acid or its salt to a solution containing N-oxyl compounds and then adding a salt of thiosulfuric acid.

[0009] <Solution containing N-oxyl compound> In the present invention, the N-oxyl compound refers to 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and its derivatives. Examples of TEMPO derivatives include 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-benzoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-acetamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl, N,N-dimethylamino-2,2,6,6-tetramethylpiperidine-1-oxyl, 1-methyl-2-azaadamantane-N-oxyl, and the like. Among them, N-oxyl compounds having a methyl group as an adjacent group to the nitroxyl radical group are preferred.

[0010] The solution containing N-oxyl compounds may contain only one of the above N-oxyl compounds or may contain two or more of them. The solvent in the solution containing the N-oxyl compound to be subjected to the method of the present invention is not particularly limited as long as it can dissolve the N-oxyl compound. Examples of the solvent include water, dimethyl sulfoxide, acetone, ethanol, and the like, or a mixed solvent of two or more of these. As one embodiment of the above solution, but not limited thereto, for example, a liquid used in the oxidation reaction of cellulose using an N-oxyl compound or a waste liquid after the oxidation reaction can be used. This oxidation reaction is usually carried out using water as a medium, so when the liquid used in the oxidation reaction or the waste liquid after the oxidation reaction is used as the "solution containing the N-oxyl compound" in the method of the present invention, the solvent in the solution is usually water.

[0011] The content of the N-oxyl compound in the solution containing the N-oxyl compound is not particularly limited. For example, the content of the N-oxyl compound in the solution is preferably 1 to 200 ppm, more preferably 3 to 100 ppm, and further preferably 5 to 80 ppm.

[0012] As described above, the solution containing the N-oxyl compound may be a liquid used in the oxidation reaction of cellulose using the N-oxyl compound or a waste liquid after the oxidation reaction. In this case, the solution may contain substances used in the oxidation reaction or products of the oxidation reaction other than the N-oxyl compound. Examples of such substances include by-products of the oxidation reaction such as NaCl, NaClO3, NaBrO, NaBrO3, halites, and perhalogenates, and alkalis used for pH adjustment such as NaOH. In particular, in the case of a waste liquid after an oxidation reaction of pulp using an N-oxyl compound, it may contain unreacted cellulose, oxidized cellulose, glucose, and the like. It may also contain hydrogen peroxide used for the short fiber treatment of oxidized cellulose. The waste liquid may also be a filtrate generated when removing oxidized cellulose by filtration after the completion of the oxidation reaction of cellulose. The waste liquid may further contain a waste liquid from washing generated when washing the oxidized cellulose after filtration.

[0013] <Addition of halogen oxoacid or its salt> A halogen oxoacid or its salt is added to a solution containing an N-oxyl compound. Examples of halogen oxoacid or its salt include sodium hypochlorite, potassium hypochlorite, sodium chlorite, perchloric acid, sodium perchlorate, sodium hypobromite, sodium bromate, potassium bromate, sodium periodate, sodium iodate, etc., and one or more of these can be added in combination. Among them, it is preferable to use sodium hypochlorite (NaOCl) and / or potassium hypochlorite, and it is most preferable to use sodium hypochlorite.

[0014] The amount of halogen oxoacid or its salt added to the solution is preferably 10 to 400 parts by mass, more preferably 40 to 200 parts by mass, per part by mass of the N-oxyl compound. The temperature during addition is not particularly limited, and can be, for example, at room temperature (e.g., about 15 to 40° C.).

[0015] After adding the halogen oxoacid or its salt, it is preferable to hold the mixture for a certain period of time before adding the thiosulfate salt. The holding time is not limited to this, but is preferably 3 minutes or more, and more preferably 5 minutes or more. The upper limit of the holding time is not particularly limited, but is preferably 120 minutes or less, more preferably 90 minutes or less, and more preferably 60 minutes or less. During the holding time, it is preferable to stir the mixture continuously or intermittently. The temperature during the holding time is not particularly limited, and can be, for example, at room temperature (for example, about 15 to 40°C).

[0016] The radical form of N-oxyl compounds (NO·) can reversibly change form through oxidation and reduction. When reduced, it becomes a hydroxylamine form (N-OH), and when oxidized, it becomes an oxoammonium form (N +=O). The oxoammonium form is unstable and easily changes to the radical form or hydroxyamine form. It is believed that when a halogen oxoacid or its salt is added to an N-oxyl compound, the N-oxyl compound is oxidized to the oxoammonium form. This is believed to promote the decomposition reaction of the N-oxyl compound by the thiosulfate salt added later.

[0017] <Addition of thiosulfate> In the present invention, after the halogen oxoacid or its salt is added, the thiosulfate salt is added. In order to proceed with the reaction uniformly, the thiosulfate salt is preferably added in a state dissolved in water. The concentration of the aqueous solution is not particularly limited. For example, it can be added as an aqueous solution with a concentration of about 0.35% by mass. As described above, after the addition of the halogen oxoacid or its salt, it is preferable to hold the mixture for a certain period of time before adding the thiosulfate salt. In other words, it is preferable to add the thiosulfate salt after a certain period of time, for example, 3 minutes or more, more preferably 5 minutes or more, has elapsed since the addition of the halogen oxoacid or its salt.

[0018] Examples of thiosulfate include sodium thiosulfate, potassium thiosulfate, and ammonium thiosulfate. One or more of these can be added in combination. Among these, it is preferable to use sodium thiosulfate (Na2S3O3).

[0019] The amount of thiosulfate added to the solution is preferably 5 to 100 parts by mass, more preferably 6 to 70 parts by mass, per part by mass of the N-oxyl compound. The temperature during addition is not particularly limited, and can be, for example, at room temperature (for example, about 15 to 40° C.).

[0020] When a halogen oxoacid or its salt is added, followed by a thiosulfate salt, these oxidation-reduction reactions proceed, sulfuric acid is produced in the system, and the solution gradually becomes acidic. For example, when sodium hypochlorite is used as the halogen oxoacid or its salt, and sodium thiosulfate is used as the thiosulfate salt, sulfuric acid is produced by the reaction of the following formula 1, and the solution gradually becomes acidic. 4NaOCl+Na2S3O3+H2O → Na2SO4+H2SO4+4NaCl (formula 1) Since sodium hypochlorite decomposes under acidic conditions and generates chlorine gas, for safety reasons, when adding a salt of thiosulfate, it is preferable to maintain the pH of the solution at 7 or higher by adding an aqueous alkali solution (e.g., an aqueous sodium hydroxide solution) in parallel, more preferably maintaining the pH at a value greater than 7, and even more preferably maintaining the pH at 7.5 or higher.

[0021] In order to adjust the pH of the solution, the method of adding the thiosulfate is preferably a sequential addition method in which the salt is added in small amounts in multiple batches, or in small amounts over a certain period of time, rather than adding the entire amount at once. The rate of addition is not particularly limited as long as it is a rate that allows the pH to be adjusted, but for example, 0.005 to 0.020 g of thiosulfate in solid content can be added per minute to 1 L of the reaction system, more preferably 0.007 to 0.015 g.

[0022] In the present invention, it is necessary to add the salt of thiosulfate after the addition of the halogen oxoacid or its salt. If the order of addition is reversed, that is, if the salt of thiosulfate is added first and then the halogen oxoacid or its salt, the decomposition of the N-oxyl compound hardly proceeds.

[0023] Although the reason why the method of the present invention can decompose N-oxyl compounds is not clear, the present inventors speculate as follows: By adding a halogen oxo acid or its salt and then a thiosulfate salt, the intermediate product of the reaction between the halogen oxo acid or its salt and the thiosulfate salt, polythionate ion (SO4) 2- and / or (S3O6) 2- It is believed that the methyl group structure that stabilizes the radical form of the N-oxyl compound is destroyed, resulting in the decomposition of the N-oxyl compound.

[0024] <Sodium bromide> The solution containing the N-oxyl compound to be used in the method of the present invention may further contain sodium bromide. As described above, in the method of the present invention, in order to prevent the generation of chlorine gas derived from sodium hypochlorite as much as possible, it is preferable to maintain the pH of the solution at 7 or higher by adding an alkaline aqueous solution (e.g., an aqueous sodium hydroxide solution) in parallel with the addition of a salt of thiosulfate. In this case, if sodium bromide is added in advance to the solution containing the N-oxyl compound, the pH fluctuation during the reaction is gradual, and therefore the pH can be easily controlled and adjusted.

[0025] Sodium bromide may be added to a solution containing an N-oxyl compound before the halogen oxoacid or its salt is added, or may be added to a solution before the addition of a thiosulfate salt after the addition of a halogen oxoacid or its salt. A solution further containing sodium bromide may be used as the solution containing an N-oxyl compound before the addition of a halogen oxoacid or its salt. Alternatively, both of these (i.e., the addition of further sodium bromide to a solution containing sodium bromide) may be performed. Of these, it is more preferable that the solution containing an N-oxyl compound before the addition of a halogen oxoacid or its salt contains sodium bromide in advance by performing either or both of adding sodium bromide to a solution containing an N-oxyl compound before the addition of a halogen oxoacid or its salt, or using a solution containing sodium bromide as the solution containing an N-oxyl compound before the addition of a halogen oxoacid or its salt. The pH of the reaction system may be increased by the addition of a halogen oxoacid or its salt. If the solution before adding the halogen oxoacid or its salt contains sodium bromide, in addition to the above-mentioned advantages, the increase in pH due to the addition of the halogen oxoacid or its salt can be reduced, and the risk of damage to the device due to high pH can be reduced. When the solution contains sodium bromide, the amount of sodium bromide is not particularly limited, but is preferably 1 to 200 parts by mass, more preferably 5 to 150 parts by mass, per part by mass of the N-oxyl compound. EXAMPLES

[0026] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited thereto. Unless otherwise specified, % and ppm refer to % and ppm (ppmw) on a mass basis.

[0027] Example 1 To 49.66 g of an aqueous solution containing TEMPO at 5 ppm and NaBr at 659 ppm, NaClO (12% concentration, manufactured by Takasugi Pharmaceutical) was added so that the NaClO concentration was 1000 ppm, and the pH of the solution was adjusted to 10 with 3 M NaOH.

[0028] Sixty minutes after the addition of NaClO, a 0.35% by mass aqueous solution of sodium thiosulfate (Na2S3O3) was added at a rate of 0.1 mL / min (amount of sodium thiosulfate added per 1 L of reaction system: 0.007 g / min). During the addition of the aqueous sodium thiosulfate solution, the pH of the solution was kept at 7 or higher by adding an aqueous NaOH solution as needed. The reaction was carried out at room temperature. When the pH no longer fluctuated, it was considered that all NaClO had disappeared, and the addition of the sodium thiosulfate solution was terminated. The time from the start to the end of the addition of the aqueous sodium thiosulfate solution (called the reaction time) was approximately 36 minutes.

[0029] After the addition of the aqueous sodium thiosulfate solution, 5 mL of the solution was left to stand overnight and filtered through a membrane filter (GL Chromatodisk 25A, pore size 0.45 μm, manufactured by GL Sciences) to obtain a sample. The amount of TEMPO (radical and hydroxylamine forms) contained in the sample was measured using high performance liquid chromatography (HPLC) equipped with an electrochemical detector under the following conditions. Electrochemical detector (COULOCHEM III, Thermo Fisher Scientific) Analytical Carousel: Model 5010 Pump: LC-20AD (Shimadzu Corporation) Column oven: CTO-20AC (Shimadzu Corporation) Degasser: DGU-20A (Shimadzu Corporation) System controller: CBM-20A (Shimadzu Corporation) Sample injection volume: 10μL ·Flow rate: 1.0mL / min Column oven temperature: 40℃ Eluent: 10 mM lithium perchlorate aqueous solution:methanol = 70:30 Using the peak area values ​​of the TEMPO radical and TEMPO hydroxylamine (OH) obtained under the above conditions, the concentrations of the TEMPO radical and TEMPO hydroxylamine in the sample were calculated based on the peak area values ​​of these standards. Note that TEMPO oxoammonium is highly reactive and is quickly converted to either the radical or hydroxylamine, so TEMPO is not detected as an oxoammonium.

[0030] Example 2 The procedure was the same as in Example 1, except that the TEMPO concentration in the initial aqueous solution was changed from 25 ppm to 5 ppm.

[0031] Example 3 The procedure was the same as in Example 2, except that the aqueous sodium thiosulfate solution was added 5 minutes after the addition of NaClO.

[0032] Example 4 The procedure was the same as in Example 2, except that the concentration of TEMPO in the initial aqueous solution was 50 ppm.

[0033] Example 5 The same procedure as in Example 2 was carried out except that the NaBr concentration in the initial aqueous solution was 0 ppm. Example 6 500g (bone dry) of bleached unbeaten kraft pulp (85% brightness) derived from coniferous trees as a cellulose raw material, TEMPO (Sigma Aldrich) (0.025mmol / g relative to the cellulose raw material), sodium bromide (1.0mmol / g relative to the cellulose raw material), and water were mixed to make 20L, and the pulp was stirred until it was uniformly dispersed. A commercially available low-salt sodium hypochlorite aqueous solution (effective chlorine concentration 12% by mass, sodium chloride content 4% by mass or less) was added to the reaction system so that the effective chlorine concentration was 3.2mmol / g relative to the cellulose raw material, and the oxidation reaction was started. As the reaction progressed, the pH in the system decreased, so 3M sodium hydroxide aqueous solution was added successively to adjust the pH to 10. The reaction was terminated 80 minutes after the start of the reaction. Hydrochloric acid was added to the reaction mixture after the reaction to a pH of 2.4 or less, and the pulp was separated by filtration through a glass filter. The pulp was dispersed in ion-exchanged water again, and then hydrochloric acid was added to adjust the pH to below 2.5. The pulp was then separated by filtration through a glass filter. This procedure was repeated to remove excess salt and impurities, yielding oxidized pulp (TEMPO-oxidized cellulose). The carboxyl group content of this TEMPO-oxidized cellulose was 0.99 mmol / g.

[0034] When the entire filtrate was collected after the filtration and washing performed after the oxidation reaction of cellulose with TEMPO, the combined concentration of the TEMPO radical and hydroxylamine forms in the filtrate was 12 ppm. The combined concentration here is the sum of the concentrations of the TEMPO radical and hydroxylamine forms in the filtrate, which were calculated by HPLC measurement. The NaBr concentration in the filtrate was 659 ppm.

[0035] The same procedure as in Example 1 was repeated, except that 50 g of the filtrate was used instead of "49.66 g of an aqueous solution having a TEMPO concentration of 5 ppm and a NaBr concentration of 659 ppm" used in Example 1.

[0036] (Reference example 1) The same procedure as in Example 2 was carried out except that the aqueous sodium thiosulfate solution was not added. Comparative Example 1 The procedure was the same as in Example 2, except that no NaClO was added and the same amount of aqueous sodium thiosulfate solution as in Example 5 was added.

[0037] Comparative Example 2 The same procedure was followed as in Example 2, except that the order of addition of NaClO and the aqueous sodium thiosulfate solution was reversed (i.e., NaClO was added 60 minutes after the addition of the aqueous sodium thiosulfate solution).

[0038] Comparative Example 3 The same procedure was followed as in Example 2, except that a common reducing agent, NaBH4, was used instead of the aqueous sodium thiosulfate solution.

[0039] The results are shown in Table 1.

[0040] [Table 1]

[0041] As shown in Table 1, by adding a certain amount of NaClO and sodium thiosulfate in this order, it was possible to promote the decomposition of N-oxyl compounds (Examples 1 to 5). Similarly, when using a waste liquid from an oxidation reaction of cellulose using an N-oxyl compound, it was possible to promote the decomposition of N-oxyl compounds (Example 6). On the other hand, when only sodium thiosulfate was added without adding NaClO, the decomposition did not proceed (Comparative Example 1).

[0042] By adding only NaClO without adding sodium thiosulfate, about 40% of the N-oxyl compounds could be decomposed (Reference Example 1). It is possible that NaClO acts as an oxidizing agent, which decomposes the N-oxyl compounds. In contrast, it was found that the decomposition of the N-oxyl compounds can be further promoted by adding sodium thiosulfate in addition to NaClO (Examples 1 to 5).

[0043] When the order of addition of NaClO and sodium thiosulfate was reversed, decomposition hardly progressed (Comparative Example 2). This is thought to be because the neutralization reaction between sodium thiosulfate and NaClO took precedence.

[0044] When sodium borohydride, a common reducing agent, was used instead of sodium thiosulfate, decomposition did not proceed (Comparative Example 3). It is believed that thiosulfate is closely related to the decomposition of N-oxyl compounds.

Claims

1. A method for decomposing an N-oxyl compound, comprising adding a halogen oxoacid or a salt thereof to a solution containing the N-oxyl compound, and then adding a salt of thiosulfate.

2. The method for decomposing an N-oxyl compound according to claim 1, comprising adding sodium bromide to a solution containing an N-oxyl compound before adding a halogen oxoacid or a salt thereof or before adding a salt of thiosulfate, and / or the solution containing an N-oxyl compound before adding a halogen oxoacid or a salt thereof further contains sodium bromide.

3. 3. The method for decomposing an N-oxyl compound according to claim 1, wherein the halogen oxoacid or a salt thereof is at least one selected from the group consisting of sodium hypochlorite and potassium hypochlorite.

4. 3. The method for decomposing an N-oxyl compound according to claim 1, wherein the amount of the halogen oxoacid or its salt added is 10 to 400 parts by mass per part by mass of the N-oxyl compound.

5. 3. The method for decomposing an N-oxyl compound according to claim 1, wherein the amount of the salt of thiosulfate added is 5 to 100 parts by mass per part by mass of the N-oxyl compound.

6. 3. The method for decomposing an N-oxyl compound according to claim 1, which comprises adding a salt of thiosulfate 3 minutes or more after the addition of the halogen oxoacid or its salt.

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