Removal of cellulose-derived impurities from ionic liquid

Abstract

Provided is a method for removing cellulose-derived impurities from an ionic liquid and effectively recovering the ionic liquid.　This method is for recovering a liquid comprising an ionic liquid and a cosolvent from an impurity-containing liquid comprising the ionic liquid, the cosolvent, and cellulose-derived impurities, the method including treating the impurity-containing liquid with an ultrafiltration membrane and thereby separating the liquid comprising the ionic liquid and the cosolvent from the cellulose-derived impurities.

Description

Removal of cellulose-derived impurities in ionic liquids

[0001] The present disclosure relates to a method for removing cellulose-derived impurities from an ionic liquid. In particular, the present disclosure relates to a method for separating a liquid containing an ionic liquid and a co-solvent from the cellulose-derived impurities and recovering the liquid containing the ionic liquid and the co-solvent from an impurity-containing liquid containing the ionic liquid, a co-solvent, and the cellulose-derived impurities. The present disclosure also relates to a method for producing a cellulose ester, including the method.

[0002] Liquids containing ionic liquids have been used as solvents for synthesizing cellulose esters such as cellulose acetate. Ionic liquids have high solubility in cellulose, making it possible to synthesize cellulose esters uniformly. Liquids containing ionic liquids may contain cosolvents. The cosolvents reduce the viscosity of the liquid and can also reduce the amount of ionic liquid used.

[0003] Since ionic liquids are expensive, when synthesizing cellulose esters on an industrial scale, it is desirable to recycle the ionic liquid by reusing the ionic liquid used in the synthesis in the next synthesis. Cellulose esters synthesized in a liquid containing an ionic liquid can be recovered from the solid phase by precipitating and performing solid-liquid separation. On the other hand, liquids containing a liquid phase ionic liquid contain impurities generated during and after the synthesis process. Such impurities include cellulose-derived impurities (e.g., unreacted cellulose, cellulose esters with a low degree of substitution, cellulose esters with a relatively low molecular weight, etc.) and coloring components generated when the liquid after synthesis is heated in a process such as distillation. To recycle an ionic liquid, it is important to properly remove these impurities.

[0004] As methods for removing impurities and recycling ionic liquids, for example, Patent Document 1 discloses a method for removing impurities by extraction, Patent Document 2 discloses a method for removing impurities by recrystallization, and Patent Document 3 discloses a method for removing impurities using an ion exchange membrane.

[0005] JP 2013-177324 A JP 2012-144441 A JP 2015-96255 A

[0006] When impurities are removed from a liquid containing an ionic liquid and a co-solvent by extraction or recrystallization, problems generally arise, such as low recovery rates of the ionic liquid and the co-solvent, and the need to add and recover additional substances for extraction or recrystallization. Furthermore, when impurities are removed from a liquid containing an ionic liquid and a co-solvent using an ion exchange membrane, a problem arises in that the ion exchange membrane has low resistance to high-boiling polar solvents commonly used as co-solvents, such as dimethyl sulfoxide (DMSO), sulfolane, N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), and dimethylacetamide (DMAc). In view of these circumstances, the present disclosure aims to provide a method for effectively recovering an ionic liquid and a co-solvent by effectively removing impurities from a liquid containing an ionic liquid and a co-solvent.

[0007] The present disclosure discloses, for example, a method for recovering a liquid containing an ionic liquid and a co-solvent from an impurity-containing liquid containing an ionic liquid, a co-solvent, and cellulose-derived impurities, the method comprising subjecting the impurity-containing liquid to an ultrafiltration membrane to separate the liquid containing the ionic liquid and the co-solvent from the cellulose-derived impurities.

[0008] According to the present disclosure, it is possible to effectively remove cellulose-derived impurities from a liquid containing an ionic liquid and a co-solvent, thereby enabling the ionic liquid and the co-solvent to be effectively recovered and reused.

[0009] Figure 1 shows (1) the change in breaking strength over time, (2) the change in breaking elongation over time, and (3) the change in elastic modulus over time in the solvent resistance test of the ultrafiltration membrane used in Example 1. Figure 2 shows the ultraviolet / visible light spectrum of the feed solution containing the ionic liquid and the coloring component supplied to the nanofiltration membrane of Example 8, and the permeate that passed through the nanofiltration membrane.

[0010] An embodiment of the present disclosure will be described in detail below. However, the scope of the present disclosure is not limited to the embodiment described herein, and various modifications can be made without departing from the spirit of the present disclosure. Each aspect disclosed in this specification can be combined with any other feature disclosed in this specification. Furthermore, when multiple upper and lower limit values ​​are described for a specific parameter, any of these upper and lower limit values ​​can be combined to form a suitable numerical range. Furthermore, the lower and / or upper limit values ​​of a numerical range described in this disclosure are numerical values ​​within that numerical range and may be replaced with numerical values ​​shown in the examples. The expression "X to Y" indicating a numerical range means "X or more and Y or less." If a specific description described for one embodiment also applies to other embodiments, that description may be omitted in other embodiments.

[0011] <Method for recovering a liquid containing an ionic liquid and a co-solvent> In one embodiment, a method for recovering a liquid containing an ionic liquid and a co-solvent from an impurity-containing liquid containing the ionic liquid, the co-solvent, and cellulose-derived impurities is disclosed, the method comprising subjecting the impurity-containing liquid to an ultrafiltration membrane to separate the liquid containing the ionic liquid and the co-solvent from the cellulose-derived impurities. This method can effectively recover the ionic liquid and the co-solvent. By reusing the recovered ionic liquid, costs associated with the use of expensive ionic liquids can be reduced. Furthermore, by reducing the amount of ionic liquid and co-solvent used, environmental impact can be reduced.

[0012] The impurity-containing liquid includes an ionic liquid, a cosolvent, and cellulose-derived impurities. In one embodiment, the impurity-containing liquid may be a reaction liquid obtained after the synthesis of a cellulose ester is completed and the cellulose ester is recovered as a product when esterifying cellulose. Specific examples of the ionic liquid, the cosolvent, and the cellulose-derived impurities will be described later.

[0013] In an impurity-containing liquid containing an ionic liquid, a cosolvent, and cellulose-derived impurities, and / or a liquid containing an ionic liquid and a cosolvent after removing cellulose-derived impurities using an ultrafiltration membrane, the content of the ionic liquid is preferably 20 to 99.9 wt %, more preferably 30 to 80 wt %, based on the total weight of the liquid.

[0014] In an impurity-containing liquid containing an ionic liquid, a cosolvent, and cellulose-derived impurities, and / or a liquid containing an ionic liquid and a cosolvent after removing cellulose-derived impurities with an ultrafiltration membrane, the content of the cosolvent is preferably 0.1 to 80 wt %, more preferably 20 to 70 wt %, based on the total weight of the liquid. The weight ratio of the ionic liquid to the cosolvent is preferably 20:80 to 99.9:0.1, more preferably 30:70 to 80:20.

[0015] In an impurity-containing liquid containing an ionic liquid, a cosolvent, and a cellulose ester, and / or a liquid containing an ionic liquid and a cosolvent after removing cellulose-derived impurities using an ultrafiltration membrane, the content of the ionic liquid relative to the total amount of the ionic liquid and the cosolvent is preferably 20 to 99.9 wt %, more preferably 30 to 80 wt %.

[0016] [Ultrafiltration Membrane] Ultrafiltration (UF) membranes are membranes that can separate molecules having a molecular weight greater than a specific molecular weight cutoff from molecules having a molecular weight smaller than this. The molecular weight cutoff of the ultrafiltration membrane is preferably 1,000 to 1,000,000, more preferably 5,000 to 100,000, and even more preferably 10,000 to 30,000. Ultrafiltration membranes having a molecular weight cutoff within these ranges tend to be more effective in removing cellulose-derived impurities from impurity-containing liquids containing ionic liquids, cosolvents, and cellulose-derived impurities. The molecular weight cutoff of an ultrafiltration membrane can be determined, for example, by a constant-pressure cross-flow test using an NMP solution of polyethylene glycol (PEG) having a predetermined molecular weight as the test liquid. Specifically, the test can be carried out, for example, as follows. PEGs with molecular weights of 3K, 8K, 12K, 35K, 100K, and 500K are each mixed and dissolved in NMP at 0.1%, and this solution is passed through a hollow fiber membrane module composed of the ultrafiltration membrane to be tested at a flow rate of, for example, 9.9 ml / min. Furthermore, the test is carried out under constant pressure conditions by setting the operating pressure to a predetermined pressure (e.g., 1 bar or 4 bar) using a pressure gauge installed near the inlet of the hollow fiber membrane and a pressure regulator installed near the outlet of the hollow fiber membrane. After the start of the flow (e.g., 6 hours later), the feed solution and filtrate are sampled. The sampled feed solution and filtrate are each analyzed by liquid chromatography using a gel permeation chromatography (GPC) column and a differential refractive index (RI) detector. The feed solution strength (RI) is determined from a chromatogram of RI peak intensity versus molecular weight (calculated from the PEG calibration curve). ｆｅｅｄ ) and filtrate strength (I ｆｉｌｌ ) and calculate the rejection rate of PEG using the following formula 1. The fractionation curve of the ultrafiltration membrane is plotted, and the molecular weight at which 90% rejection rate is obtained is determined as the molecular weight cutoff. Formula 1: Rejection rate (%) = (1 - I ｆｉｌｌ / I ｆｅｅｄ ) x 100

[0017] The ultrafiltration membrane preferably has high resistance to high-boiling-point polar solvents, which allows the ultrafiltration membrane to be used stably even when the co-solvent is a high-boiling-point polar solvent.

[0018] The ultrafiltration membrane preferably has high hydrophilicity. The ionic liquid, cosolvent, and cellulose-derived impurities supplied to the ultrafiltration membrane are all highly hydrophilic. Therefore, when the ultrafiltration membrane has high hydrophilicity, the adsorption of the ionic liquid, cosolvent, and cellulose-derived impurities to the ultrafiltration membrane is suppressed, thereby suppressing a decrease in the permeation rate due to adsorption (membrane fouling), and the ultrafiltration membrane can easily be used stably for a long period of time.

[0019] The ultrafiltration membrane preferably contains a polyamide resin, which may be, for example, polyamide 6, polyamide 66, polyamide 46, polyamide 610, polyamide 612, polyamide 11, polyamide 1, polyamide MXD6, polyamide 4T, polyamide 6T, polyamide 9T, polyamide 10T, or a copolymer of these polyamides, or may contain a plurality of resins selected from these.

[0020] The NMP permeation rate of the ultrafiltration membrane is preferably 1 to 50 L / m ２ / h / bar, more preferably 10 to 20 L / m ２ / h / bar. The NMP permeation rate can be measured, for example, by a constant pressure crossflow test. Specifically, it can be carried out, for example, as follows. NMP is passed through a hollow fiber membrane module composed of the ultrafiltration membrane to be tested, at a flow rate of, for example, 9.9 ml / min. Furthermore, the test is carried out under constant pressure conditions by setting the operating pressure to a predetermined pressure (for example, 1 bar or 4 bar) using a pressure gauge installed near the inlet of the hollow fiber membrane and a pressure-regulating regulator installed near the outlet of the hollow fiber membrane. At this time, the permeation rate of the filtrate discharged from the outlet of the hollow fiber membrane is measured, and the permeation rate per unit time (h) and per unit membrane area (m ２ ), permeation rate (L / m) per unit pressure (bar) ２ / hr / bar) can be calculated.

[0021] The permeation rate of the impurity-containing liquid through the ultrafiltration membrane is preferably 0.1 to 10 L / m ２ / h / bar, more preferably 0.2 to 2 L / m ２The permeation rate of the impurity-containing liquid through the ultrafiltration membrane can be measured in the same manner as the NMP permeation rate described above.

[0022] The rejection rate of impurities in an impurity-containing liquid by an ultrafiltration membrane is preferably 30 to 99%, more preferably 70 to 99%. The rejection rate of impurities by an ultrafiltration membrane can be measured, for example, by a constant-pressure crossflow test. Specifically, the test can be performed, for example, as follows. The impurity-containing liquid is passed through a hollow fiber membrane module composed of the ultrafiltration membrane to be tested at a flow rate of, for example, 9.9 ml / min. The test is then performed under constant pressure conditions by setting the operating pressure to a predetermined pressure (e.g., 1 bar or 4 bar) using a pressure gauge installed near the inlet of the hollow fiber membrane and a pressure regulator installed near the outlet of the hollow fiber membrane. After the start of the liquid passage (e.g., 24 hours later), the feed liquid and the filtrate are each sampled. The cellulose acetate content of the sampled feed liquid and filtrate is quantified by sugar analysis using the phenol-sulfuric acid method. The quantification is preferably carried out using a calibration curve prepared using the same solvent system, taking into account the influence of the solvent on the measured value. ｆｅｅｄ ), the content of cellulose acetate in the filtrate (C ｆｉｌｌ ), the rejection rate of impurities in the solvent can be calculated using the following formula 2: Rejection rate (%) = (1 - C ｆｉｌｌ / C ｆｅｅｄ ) x 100

[0023] The ultrafiltration membrane may be used in the form of a membrane module housed in a container. In this case, the membrane area per membrane module may be any within the range in which the effects of the present invention can be obtained, but is preferably 1 to 200 m. ２ and more preferably 10 to 100 m ２ The surface area of ​​the ultrafiltration membrane may be a value from a manufacturer's catalog or may be a value measured by a standard method. The number of membrane modules required is determined by the throughput and the membrane permeation rate.

[0024] The ultrafiltration membrane is resistant to a liquid containing an ionic liquid and a co-solvent for preferably 3 months, more preferably 6 months, and even more preferably 12 months.

[0025] The thickness of the ultrafiltration membrane is preferably 0.1 to 5 mm, more preferably 0.3 to 3 mm.

[0026] The ultrafiltration membrane may have any shape as long as the effects of the present invention can be obtained, and may be, for example, a hollow fiber membrane or a flat membrane.

[0027] As the ultrafiltration membrane, a commercially available ultrafiltration membrane can be used as long as the effects of the present invention can be obtained. Non-limiting examples of commercially available ultrafiltration membranes include UF50 and UF120 manufactured by Unitika. Furthermore, an ultrafiltration membrane manufactured by a known method can also be used as the ultrafiltration membrane.

[0028] [Cellulose and cellulose ester] Cellulose has the molecular formula (C ６ H １０ O ５ ) ｎ Cellulose is a carbohydrate represented by the formula: Cellulose is the main component of plant cell walls and plant fibers. Cellulose esters are cellulose esterified with organic acid anhydrides. For example, cellulose acetate is cellulose esterified with acetic anhydride. Cellulose esters are biomass materials and are important as environmentally friendly materials, as they are highly biodegradable.

[0029] [Cellulose-Derived Impurities] Cellulose-derived impurities are typically dissolved or contaminated in the reaction solution after the synthesis is completed and the cellulose ester product is recovered during the esterification of cellulose. Cellulose-derived impurities typically include one or more of unreacted cellulose, cellulose esters with a low degree of substitution, and cellulose esters with a relatively low molecular weight. Cellulose-derived impurities can become impurities when a liquid containing an ionic liquid and a cosolvent is reused in a subsequent synthesis. The molecular weight of the cellulose-derived impurities is preferably 10,000 to 1,500,000, more preferably 20,000 to 1,000,000. The molecular weight of the cellulose-derived impurities can be measured, for example, by gel permeation chromatography (GPC). The degree of substitution of the cellulose ester contained as a cellulose-derived impurity is preferably 3.0 or less, more preferably 2.9 or less, or preferably 1.5 to 2.9, more preferably 2.0 to 2.9. The degree of substitution of the cellulose ester can be measured by nuclear magnetic resonance spectroscopy (NMR). The cellulose ester contained as the cellulose-derived impurity is preferably cellulose acetate.

[0030] [Ionic Liquid] The ionic liquid is an ionic liquid capable of dissolving cellulose. The ionic liquid contains a cation component and an anion component.

[0031] (Cation Component) The cationic component is preferably one or more cations selected from the group consisting of imidazolium cations, pyridinium cations, pyrrolidinium cations, piperidinium cations, quaternary ammonium cations, and quaternary phosphonium cations.

[0032] Non-limiting examples of imidazolium cations include cations represented by the following formula (1): The cation represented by formula (1) also includes its tautomers and cations represented by structural formulas that have a resonance relationship with formula (1): Formula (1): In formula (1), R １ and R ３are the same or different and are a substituted or unsubstituted alkyl group, alkenyl group, alkoxyalkyl group, or substituted or unsubstituted phenyl group, R ２ , R ４ , and R ５ are the same or different and are a hydrogen atom, a substituted or unsubstituted alkyl group, an alkenyl group, an alkoxyalkyl group, or a substituted or unsubstituted phenyl group. １ ~R ５ Examples of the substituted or unsubstituted alkyl group in the formula (I) include linear or branched alkyl groups having 1 to 20 (preferably 1 to 10, more preferably 2 to 6, and even more preferably 2 to 4) carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, butyl, hexyl, and octyl groups. These alkyl groups may have a sulfo group bonded to the terminal. Examples of the alkenyl group include linear or branched alkenyl groups having 2 to 20 (preferably 2 to 10, more preferably 2 to 6, and even more preferably 2 to 4) carbon atoms, such as vinyl, 1-propenyl, 2-propenyl, 1-butenyl, 2-butenyl, 1-pentenyl, 2-pentenyl, 1-hexenyl, 2-hexenyl, and 1-octenyl groups. Examples of the alkoxyalkyl group include a straight-chain or branched alkoxyalkyl group having 2 to 20 (preferably 2 to 10, more preferably 2 to 6, and even more preferably 2 to 4) carbon atoms, such as a methoxymethyl group, an ethoxymethyl group, a 1-methoxyethyl group, a 2-methoxyethyl group, a 1-ethoxyethyl group, or a 2-ethoxyethyl group. Examples of the substituted or unsubstituted phenyl group include a phenyl group that may be substituted with 1 to 2 groups selected from a hydroxyl group, a halogen atom, a lower alkoxy group, a lower alkenyl group, a methylsulfonyloxy group, a substituted or unsubstituted lower alkyl group, a substituted or unsubstituted amino group, a substituted or unsubstituted phenyl group, a substituted or unsubstituted phenoxy group, and a substituted or unsubstituted pyridyl group. R １ and R ３is preferably an alkyl group, an alkenyl group, or a substituted or unsubstituted phenyl group, and more preferably a linear alkyl group having 1 to 6 carbon atoms. １ and R ３ It is particularly preferred that one of R is a linear alkyl group having 1 to 4 carbon atoms and the other is a linear alkyl group having 2 to 6 carbon atoms, and the numbers of carbon atoms of these alkyl groups are different. ２ , R ４ and R ５ is preferably a hydrogen atom, an alkyl group, an alkenyl group, or a substituted or unsubstituted phenyl group, and more preferably a hydrogen atom or an alkyl group. The imidazolium cation may be preferably one or more imidazolium cations selected from the group consisting of 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-octyl-3-methylimidazolium, 1-decyl-3-methylimidazolium, 1-tetradecyl-3-methylimidazolium, 1-hexadecyl-3-methylimidazolium, 1-octadecyl-3-methylimidazolium, 1-allyl-3-methylimidazolium, 1-ethyl-2,3-dimethylimidazolium, 1-butyl-2,3-dimethylimidazolium, and 1-hexyl-2,3-dimethylimidazolium, and is particularly preferably a 1-ethyl-3-methylimidazolium cation (Emidazolium). ＋ ) may be.

[0033] Non-limiting examples of pyridinium cations include cations represented by the following formula (2): The cations represented by formula (2) also include tautomers thereof and cations represented by structural formulas that have a resonance relationship with formula (2): In formula (2), R ６ is an alkyl group, an alkenyl group, an alkoxyalkyl group, or a substituted or unsubstituted phenyl group, and R ７ ~R １１are the same or different and are a hydrogen atom, an alkyl group, an alkenyl group, an alkoxyalkyl group, or a substituted or unsubstituted phenyl group. ６ ~R １１ The alkyl group, alkenyl group, alkoxyalkyl group, and substituted or unsubstituted phenyl group in the formula (1) are R １ ~R ５ Examples include those similar to those explained as R ６ is preferably an alkyl group, more preferably a linear alkyl group having 1 to 6 carbon atoms. ６ ~R １１ is preferably a hydrogen atom or an alkyl group, and more preferably a hydrogen atom. The pyridinium cation may be a pyridinium cation such as 1-ethylpyridinium, 1-butylpyridinium, 1-hexylpyridinium, 1-butyl-4-methylpyridinium, 1-butyl-3-methylpyridinium, 1-hexyl-4-methylpyridinium, 1-hexyl-3-methylpyridinium, 1-octyl-4-methylpyridinium, 1-octyl-3-methylpyridinium, 1-butyl-3,4-dimethylpyridinium, or 1-butyl-3,5-dimethylpyridinium, and particularly preferably a 1-octyl-4-methylpyridinium cation.

[0034] Non-limiting examples of pyrrolidinium cations include cations represented by the following formula (3): The cation represented by formula (3) also includes its tautomers. Formula (3): In formula (3), R １２ and R １３ are the same or different and are an alkyl group, an alkenyl group, an alkoxyalkyl group, or a substituted or unsubstituted phenyl group; R １４ ~R ２１ are the same or different and are a hydrogen atom, an alkyl group, an alkenyl group, an alkoxyalkyl group, or a substituted or unsubstituted phenyl group. １２ ~R ２１The alkyl group, alkenyl group, alkoxyalkyl group, and substituted or unsubstituted phenyl group in the formula (1) are R １ ~R ５ Examples include those similar to those explained as R １２ and R １３ is preferably an alkyl group, more preferably a linear alkyl group having 1 to 6 carbon atoms. １４ ~R ２１ is preferably a hydrogen atom or an alkyl group, more preferably a hydrogen atom. The pyrrolidinium cation may preferably be a cation of 1-butyl-1-methylpyrrolidinium.

[0035] Non-limiting examples of piperidinium cations include those represented by the following formula (4): The cation represented by formula (4) also includes its tautomers. Formula (4): In formula (4), R ２２ and R ２３ are the same or different and are an alkyl group, an alkenyl group, an alkoxyalkyl group, or a substituted or unsubstituted phenyl group; R ２４ ~R ３３ are the same or different and are a hydrogen atom, an alkyl group, an alkenyl group, an alkoxyalkyl group, or a substituted or unsubstituted phenyl group. ２２ ~R ３３ The alkyl group, alkenyl group, alkoxyalkyl group, and substituted or unsubstituted phenyl group in the formula (1) are R １ ~R ５ Examples include those similar to those explained as R ２２ and R ２３ is preferably an alkyl group, more preferably a linear alkyl group having 1 to 6 carbon atoms. ２４ ~R ３３ is preferably a hydrogen atom or an alkyl group, more preferably a hydrogen atom. The piperidinium cation may preferably be a cation of 1-butyl-1-methylpiperidinium.

[0036] Non-limiting examples of quaternary ammonium cations include ammonium cations represented by the following formula (5): The cation represented by formula (5) also includes its tautomers. Formula (5): In formula (5), R ３４ ~R ３７ are the same or different and are an alkyl group, an alkenyl group, an alkoxyalkyl group, or a substituted or unsubstituted phenyl group. ３４ ~R ３７ Examples of the alkyl group in the formula (I) include a linear or branched alkyl group having 1 to 20 (preferably 1 to 10, more preferably 2 to 6, and even more preferably 2 to 4) carbon atoms, such as a methyl group, ethyl group, n-propyl group, isopropyl group, butyl group, hexyl group, or octyl group. Examples of the alkenyl group include a linear or branched alkenyl group having 2 to 20 (preferably 2 to 10, more preferably 2 to 6, and even more preferably 2 to 4) carbon atoms, such as a vinyl group, 1-propenyl group, 2-propenyl group, 1-butenyl group, 2-butenyl group, 1-pentenyl group, 2-pentenyl group, 1-hexenyl group, 2-hexenyl group, or 1-octenyl group. Examples of the alkoxyalkyl group include a straight-chain or branched alkoxyalkyl group having 2 to 20 (preferably 2 to 10, more preferably 2 to 6, and even more preferably 2 to 4) carbon atoms, such as a methoxymethyl group, an ethoxymethyl group, a 1-methoxyethyl group, a 2-methoxyethyl group, a 1-ethoxyethyl group, or a 2-ethoxyethyl group. Examples of the substituted or unsubstituted phenyl group include a phenyl group that may be substituted with 1 to 2 groups selected from a hydroxyl group, a halogen atom, a lower alkoxy group, a lower alkenyl group, a methylsulfonyloxy group, a substituted or unsubstituted lower alkyl group, a substituted or unsubstituted amino group, a substituted or unsubstituted phenyl group, a substituted or unsubstituted phenoxy group, and a substituted or unsubstituted pyridyl group. R ３４ ~R ３７is preferably an alkyl group, more preferably a linear alkyl group having 1 to 6 carbon atoms. The quaternary ammonium cation may be preferably one or more cations selected from the group consisting of trimethylpropylammonium, trimethylbutylammonium, triethylmethylammonium, trioctylmethylammonium, tetramethylammonium, tetraethylammonium, tetrabutylammonium, tetrapentylammonium, and tetrahexylammonium.

[0037] Non-limiting examples of quaternary phosphonium cations include phosphonium cations represented by the following formula (6): The cation represented by formula (6) also includes its tautomers. Formula (6): In formula (6), R ３８ ~R ４１ are the same or different and are an alkyl group, an alkenyl group, an alkoxyalkyl group, or a substituted or unsubstituted phenyl group. ３８ ~R ４１Examples of the alkyl group in the formula (I) include a linear or branched alkyl group having 1 to 20 (preferably 1 to 10, more preferably 2 to 6, and even more preferably 2 to 4) carbon atoms, such as a methyl group, ethyl group, n-propyl group, isopropyl group, butyl group, hexyl group, or octyl group. Examples of the alkenyl group include a linear or branched alkenyl group having 2 to 20 (preferably 2 to 10, more preferably 2 to 6, and even more preferably 2 to 4) carbon atoms, such as a vinyl group, 1-propenyl group, 2-propenyl group, 1-butenyl group, 2-butenyl group, 1-pentenyl group, 2-pentenyl group, 1-hexenyl group, 2-hexenyl group, or 1-octenyl group. Examples of alkoxyalkyl groups include linear or branched alkoxyalkyl groups having 2 to 20 (preferably 2 to 10, more preferably 2 to 6, and even more preferably 2 to 4) carbon atoms, such as methoxymethyl, ethoxymethyl, 1-methoxyethyl, 2-methoxyethyl, 1-ethoxyethyl, and 2-ethoxyethyl groups. Examples of substituted or unsubstituted phenyl groups include phenyl groups that may be substituted with one or two groups selected from a hydroxyl group, a halogen atom, a lower alkoxy group, a lower alkenyl group, a methylsulfonyloxy group, a substituted or unsubstituted lower alkyl group, a substituted or unsubstituted amino group, a substituted or unsubstituted phenyl group, a substituted or unsubstituted phenoxy group, and a substituted or unsubstituted pyridyl group. The quaternary phosphonium cation may preferably be one or more cations selected from the group consisting of tetramethylphosphonium, tetraethylphosphonium, tetrabutylphosphonium, tetrahexylphosphonium, tetraoctylphosphonium, triethylmethylphosphonium, and tributylethylphosphonium.

[0038] (Anion Component) Non-limiting examples of the anion component include halogen anions, pseudohalogen anions, carboxylate anions, phosphate anions, amino acid anions, phenolates, pyrimidine olates, and tetrafluoroborate ions (BF ４－ ), sulfomethyl ion (CH ３ SO ３ －), methylphosphonate, sulfate ion, PF ６ － etc.

[0039] Non-limiting examples of halogen anions include fluoride ion (F － ), chloride ions (Cl － ), iodine ion (I － ), and bromide ion (Br － ) and the like. Non-limiting examples of pseudohalogen anions include cyanide anion, thiocyanate anion, cyanate anion, fulminate anion, azide anion and the like. Non-limiting examples of carboxylate anions include monocarboxylate anions or dicarboxylate anions having 1 to 18 carbon atoms and the like. The carboxylate anion is preferably one or more selected from the group consisting of formate anion, acetate anion, propionate anion, butyrate anion, valerate anion, fumarate anion, oxalate anion, lactate anion, and pyruvate anion.

[0040] Non-limiting examples of phosphate anions include phosphate anion and phosphate ester anions having 1 to 40 carbon atoms. Non-limiting examples of phosphate ester anions include methyl phosphate monoester anion, octyl phosphate monoester anion, octyl phosphate diester anion, lauryl phosphate monoester anion, lauryl phosphate diester anion, stearyl phosphate monoester anion, stearyl phosphate diester anion, eicosyl phosphate monoester anion, eicosyl phosphate diester anion, etc.

[0041] (Combination of Cationic Component and Anionic Component) The ionic liquid is an arbitrary combination of the above cationic component and the above anionic component. For example, an ionic liquid containing an imidazolium cation or a quaternary ammonium cation as the cationic component is preferred. Furthermore, an ionic liquid containing a carboxylate anion as the anionic component is preferred. More specifically, at least one selected from the group consisting of 1-ethyl-3-methylimidazolium acetate (EmimOAc), 1-ethyl-2,3-dimethylimidazolium acetate (EDmimOAc), 1-butyl-3-methylimidazolium acetate, 1-hexyl-3-methylimidazolium acetate, and tetrabutylammonium acetate is more preferred. The ionic liquid may be preferably at least one selected from the group consisting of 1-ethyl-3-methylimidazolium acetate (EmimOAc) and 1-ethyl-2,3-dimethylimidazolium acetate (EDmimOAc), and particularly preferably 1-ethyl-3-methylimidazolium acetate (EmimOAc). Commercially available ionic liquids can be used. Alternatively, the ionic liquid can be produced by known techniques.

[0042] [Co-solvent] A co-solvent is added to reduce the viscosity of a liquid containing an ionic liquid and to reduce the amount of ionic liquid used. The co-solvent is preferably a high-boiling polar solvent. The high-boiling polar solvent is preferably a polar solvent having a boiling point higher than that of the organic acid. For example, when the organic acid is acetic acid, the boiling point is 117.9°C, so a polar solvent having a higher boiling point is preferred. The boiling point is preferably 120°C or higher, and more preferably 150°C or higher. Non-limiting examples of co-solvents include co-solvents selected from the group consisting of dimethyl sulfoxide (DMSO), sulfolane, N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), and combinations thereof, and the co-solvent may contain one or more selected from these.

[0043] <Method for producing cellulose ester> In one embodiment, a method for producing a cellulose ester includes: a first step of dissolving cellulose in a first liquid containing an ionic liquid and a co-solvent to obtain a second liquid; a second step of adding an organic acid anhydride to the second liquid to react the cellulose with the organic acid anhydride and produce a cellulose ester; a third step, following the second step, of adding a poor solvent to the second liquid to precipitate the cellulose ester; a fourth step, following the third step, of separating a solid phase containing the cellulose ester from a liquid phase consisting of the ionic liquid, the co-solvent, and a third liquid containing cellulose-derived impurities, recovering the cellulose ester from the solid phase, and recovering the third liquid from the liquid phase; and a fifth step of supplying the third liquid to an ultrafiltration membrane to separate the fourth liquid containing the ionic liquid and the co-solvent from the cellulose-derived impurities and recovering the fourth liquid. A production method is disclosed in which the first to fifth steps can be repeated by reusing all or part of the fourth liquid as all or part of the first liquid. This cellulose ester production method can effectively produce cellulose ester by using a liquid containing an ionic liquid and a cosolvent as a reaction solvent. Furthermore, this cellulose ester production method can effectively recover the ionic liquid and cosolvent used in the cellulose ester production reaction and reuse them in the next production reaction, thereby reducing the cost associated with the use of expensive ionic liquids. Furthermore, reducing the amount of ionic liquid and cosolvent used can reduce environmental impact.

[0044] The above-mentioned fifth step is preferably carried out according to the above-mentioned method for recovering a liquid comprising an ionic liquid and a co-solvent.

[0045] The above-mentioned method for producing cellulose ester may include a distillation step, prior to step 5, in which the third liquid is distilled to remove the poor solvent in the third liquid and / or the organic acid derived from the organic acid anhydride.By including this distillation step, the method for producing cellulose ester tends to be able to more effectively recover the fourth liquid in step 5.

[0046] In this embodiment, the ionic liquid, co-solvent, cellulose, cellulose ester, cellulose-derived impurities, etc. are the same as those described in the above-mentioned method for recovering a liquid containing an ionic liquid and a co-solvent. Furthermore, the composition of each liquid in this embodiment may be the same as that of the liquid described in the above-mentioned method for recovering a liquid containing an ionic liquid and a co-solvent.

[0047] [Organic Acid Anhydride] Organic acid anhydrides can esterify cellulose to produce a cellulose ester. Non-limiting examples of organic acid anhydrides include acid anhydrides of preferably C1-18 carboxylic acids, more preferably C1-10 carboxylic acids, even more preferably C1-6 carboxylic acids, and particularly preferably C2-4 carboxylic acids, as well as acid anhydrides of carboxylic acids such as formic acid, acetic acid, propionic acid, and trifluoroacetic acid. The organic acid anhydride can be appropriately selected depending on the type of cellulose ester to be synthesized. For example, when the cellulose ester to be synthesized is cellulose acetate, acetic anhydride is preferably used as the organic acid anhydride.

[0048] [Organic Acid] Organic acid is produced by hydrolysis of the organic acid anhydride described above.For example, organic acid is produced as a by-product when cellulose is esterified by organic acid anhydride in the above-mentioned reaction for producing cellulose ester.In addition, when the poor solvent described below contains water, organic acid is also produced when organic acid anhydride is hydrolyzed by water.When organic acid anhydride is acetic anhydride, the organic acid produced is acetic acid.

[0049] [Poor Solvent] The poor solvent is a solvent that has low solubility for cellulose ester. Adding the poor solvent to the second liquid containing the dissolved cellulose ester can precipitate the cellulose ester from the second liquid. The poor solvent can also be used to wash the precipitated cellulose ester. In this case, the poor solvent used to wash the precipitated cellulose ester can be combined with the second liquid. Non-limiting examples of poor solvents include water, alcohols such as methanol, ethanol, and 2-octanol, and ketones such as acetone. The second liquid or the third liquid excluding the precipitated cellulose ester from the second liquid may contain the poor solvent in an amount of preferably 30 to 90 wt %, more preferably 40 to 80 wt %, based on the total weight of the second or third liquid.

[0050] [Distillation] Distillation is an operation for purifying a liquid by evaporating and recondensing an impurity-containing liquid to remove impurities based on the difference in boiling points. The distillation temperature is appropriately set depending on the boiling points of the liquid and impurities. Furthermore, when a liquid contains multiple impurities, the liquid can be more effectively purified by sequentially performing multiple distillations at stepwise increasing temperatures to sequentially remove the multiple impurities. For example, in the above-mentioned distillation process, distillation to remove an organic acid and distillation to remove a poor solvent can be performed sequentially.

[0051] [Additional step of removing coloring components] When the liquid containing ionic liquid is heated in the distillation step, coloring components may be generated in the liquid. The above-mentioned method for producing cellulose ester may also include an additional step of removing coloring components generated by the distillation step. The additional step of removing coloring components generated by the distillation step is preferably carried out by subjecting the fourth liquid to a nanofiltration membrane or a reverse osmosis membrane after the above-mentioned fifth step. When the liquid containing ionic liquid containing coloring components generated by the distillation step is reused for the synthesis of cellulose ester, undesirable effects such as coloring of the synthesized cellulose ester may occur. Therefore, the method for producing cellulose ester includes an additional step of removing coloring components generated by the distillation step, which makes it possible to more effectively recover and reuse the ionic liquid.

[0052] (Nanofiltration membrane or reverse osmosis membrane) A reverse osmosis membrane (RO membrane) is a membrane that allows water to pass through but does not allow impurities other than water, such as ions and salts, to pass through. Any reverse osmosis membrane can be used as the nanofiltration membrane according to the present disclosure, as long as the effects of the present invention can be obtained. A nanofiltration (NF) membrane is a membrane that has performance intermediate between a reverse osmosis membrane (RO membrane) and an ultrafiltration membrane (UF membrane), and typically allows particles or molecules of about 1 nm in size to pass through but does not allow particles or molecules larger than this in size to pass through. Any nanofiltration membrane can be used as the nanofiltration membrane according to the present disclosure, as long as the effects of the present invention can be obtained.

[0053] A nanofiltration membrane or a reverse osmosis membrane can effectively remove coloring components contained in a liquid containing an ionic liquid.

[0054] The salt rejection of the nanofiltration membrane or reverse osmosis membrane is preferably 95% or more, 96% or more, 97% or more, 98% or more, 98.5% or more, 99% or more, or 99.5% or more when the liquid containing the ionic liquid does not contain methanol. Furthermore, the salt rejection of the nanofiltration membrane or reverse osmosis membrane is preferably 85% or more, 90% or more, 95% or more, 96% or more, 97% or more, 98% or more, 98.5% or more, 99% or more, or 99.5% or more when the liquid containing the ionic liquid contains methanol. Here, the salt rejection is calculated using the following formula 3: Salt rejection (%)=(1-C ｆｉｌｌ / C ｆｅｅｄ ) × 100 [In Formula 4, C ｆｅｅｄ is the NaCl concentration in the aqueous NaCl solution supplied to the nanofiltration membrane or the reverse osmosis membrane when the aqueous NaCl solution is passed through the nanofiltration membrane or the reverse osmosis membrane, and C ｆｉｌｌ is the NaCl concentration in the aqueous NaCl solution discharged from the nanofiltration membrane or the reverse osmosis membrane when the aqueous NaCl solution is passed through the nanofiltration membrane or the reverse osmosis membrane.

[0055] The nanofiltration membrane or reverse osmosis membrane preferably has a hydraulic conductivity of 1.0 L / m when the liquid containing the ionic liquid does not contain methanol. ２ / h / bar, more preferably 0.5 to 0.95 L / m ２ In addition, the permeability coefficient of the nanofiltration membrane or reverse osmosis membrane is preferably 8.0 L / m when the liquid containing the ionic liquid contains methanol. ２ / h / bar, more preferably 1.0 to 8.0 L / m ２ The hydraulic conductivity can be defined as the permeation rate when a 1600 ppm NaCl aqueous solution passes through the nanofiltration membrane.

[0056] The nanofiltration membrane or reverse osmosis membrane preferably includes an active layer containing a crosslinked polyamide resin. The crosslinked polyamide resin is formed, for example, by interfacial polymerization of an aromatic or alicyclic polyfunctional amine with a polyfunctional acyl halide. The polyfunctional amine is selected from the diamine group consisting of, for example, m-phenylenediamine, p-phenylenediamine, xylylenediamine, 3,5-diaminobenzoic acid, 3-aminobenzylamine, 4-aminobenzylamine, 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, 4-aminopiperidine, and piperazine. The polyfunctional acyl halide is selected from the trifunctional acyl halide group consisting of, for example, trimesic acid chloride, 1,3,5-cyclohexanetricarboxylic acid trichloride, and 1,2,4-cyclobutanetricarboxylic acid trichloride. The nanofiltration membrane or reverse osmosis membrane is more preferably a composite membrane in which an active layer containing a polyamide is formed on the surface of a polyketone or polysulfone membrane.

[0057] The nanofiltration membrane or reverse osmosis membrane may have any shape as long as the effects of the present invention can be obtained, and may be, for example, a hollow fiber membrane or a flat membrane.

[0058] As the nanofiltration membrane or reverse osmosis membrane, commercially available nanofiltration membranes or reverse osmosis membranes can be used as long as the effects of the present invention can be obtained. Non-limiting examples of commercially available nanofiltration membranes include TriSep TS80 manufactured by Aquqsource and NTR7250 manufactured by Nitto Denko Corporation. Non-limiting examples of commercially available reverse osmosis membranes include SWC4 manufactured by Nitto Denko Corporation.

[0059] (Coloring Components and Color Removal Rate) Coloring components are components that are produced in a liquid containing an ionic liquid by a process involving heating, such as distillation. Coloring components that are produced in a liquid containing an ionic liquid by a process involving heating, such as distillation, have an absorption peak in the ultraviolet region and an absorption band that extends into the visible light region ( FIG. 2 ). In the present disclosure, the color removal rate is defined as the change in the average absorbance between 380 and 770 nm in a liquid containing an ionic liquid before and after passing through a nanofiltration membrane or reverse osmosis membrane. Therefore, the effectiveness of a nanofiltration membrane or reverse osmosis membrane in removing coloring components contained in a liquid containing an ionic liquid can be evaluated by the color removal rate of the nanofiltration membrane or reverse osmosis membrane. The color removal rate is expressed by Formula 4: Color removal rate (%) = (1 - Abs ｆｉｌｌ / Abs ｆｅｅｄ ) × 100 [In Formula 4, Abs ｆｅｅｄ is the average absorbance in the range of 380 to 770 nm of a liquid containing an ionic liquid supplied to a nanofiltration membrane or a reverse osmosis membrane when the liquid contains an ionic liquid and is passed through the nanofiltration membrane or the reverse osmosis membrane, and Abs ｆｉｌｌ is the average absorbance at 380 to 770 nm of a liquid containing an ionic liquid that is discharged from a nanofiltration membrane or a reverse osmosis membrane when the liquid containing an ionic liquid is passed through the nanofiltration membrane or the reverse osmosis membrane.] The color removal rate by the nanofiltration membrane or the reverse osmosis membrane is preferably 50% or more, 60% or more, 70% or more, 80% or more, 90% or more, or 90% or more.

[0060] [Additional Step of Adding and / or Removing Methanol] When the additional step of removing the coloring components is carried out in the production method of cellulose ester, the method may include adding methanol to the fifth liquid in an amount of preferably 30 to 70 wt %, more preferably 40 to 60 wt %, based on the total weight, prior to the additional step of removing the coloring components. In this case, the production method may further include a step of removing methanol from the liquid containing the ionic liquid by, for example, distillation or extraction, after the additional step of removing the coloring components.

[0061] A non-limiting list of exemplary embodiments and combinations of exemplary embodiments of the present disclosure is disclosed below. [1] A method for recovering a liquid containing an ionic liquid and a co-solvent from an impurity-containing liquid containing an ionic liquid, a co-solvent, and cellulose-derived impurities, the method comprising: subjecting the impurity-containing liquid to an ultrafiltration membrane to separate the liquid containing the ionic liquid and the co-solvent from the cellulose-derived impurities. [2] The method according to [1], wherein the molecular weight cutoff of the ultrafiltration membrane is 1,000 to 1,000,000. [3] The method according to [1] or [2], wherein the ultrafiltration membrane comprises a polyamide resin. [4] The method according to any one of [1] to [3], wherein the cellulose-derived impurities comprise a cellulose ester. [5] The method according to any one of [1] to [4], wherein the molecular weight of the cellulose-derived impurities is 10,000 to 1,500,000. [6] The method according to any one of [1] to [5], wherein the cellulose-derived impurities comprise a cellulose ester having a degree of substitution of 2.9 or less. [7] The method according to [1], wherein the ionic liquid contains an imidazolium cation or a quaternary ammonium cation as a cation component and a carboxylate anion as an anion component. [8] The method according to any one of [1] to [6], wherein the ionic liquid is selected from the group consisting of 1-ethyl-3-methylimidazolium acetate (EmimOAc), 1-ethyl-2,3-dimethylimidazolium acetate (EDmimOAc), 1-butyl-3-methylimidazolium acetate, 1-hexyl-3-methylimidazolium acetate, tetrabutylammonium acetate, and combinations thereof. [9] The method according to any one of [1] to [8], wherein the co-solvent is a high-boiling point polar solvent.

[10] The method according to [9], wherein the high-boiling polar solvent is selected from the group consisting of dimethyl sulfoxide (DMSO), sulfolane, N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), and combinations thereof.

[11] The method according to any one of [1] to

[10] , wherein in the impurity-containing liquid and / or the liquid containing an ionic liquid and a co-solvent, the content of the ionic liquid relative to the total amount of the ionic liquid and the co-solvent is 20 to 99.9 wt %.

[12] A method for producing a cellulose ester, comprising: a first step of dissolving cellulose in a first liquid containing an ionic liquid and a co-solvent to obtain a second liquid; a second step of adding an organic acid anhydride to the second liquid to react the cellulose with the organic acid anhydride and produce a cellulose ester; a third step, following the second step, of adding a poor solvent to the second liquid to precipitate the cellulose ester; a fourth step, following the third step, of separating a solid phase containing the cellulose ester from a liquid phase consisting of the ionic liquid, the co-solvent, and a third liquid containing cellulose-derived impurities, recovering the cellulose ester from the solid phase, and recovering the third liquid from the liquid phase; and a fifth step of passing the third liquid through an ultrafiltration membrane to separate the fourth liquid containing the ionic liquid and the co-solvent from the cellulose-derived impurities and recovering the fourth liquid.

[14] The method according to

[13] , further comprising, prior to step 5, a distillation step of distilling the third liquid to remove the poor solvent and / or the organic acid derived from the organic acid anhydride in the third liquid.

[15] The method according to

[14] , further comprising, after step 5, an additional step of subjecting the fourth liquid to a nanofiltration membrane or a reverse osmosis membrane to remove colored components produced by the distillation step.

[15] A method for producing a cellulose ester, comprising: a first step of dissolving cellulose in a first liquid containing an ionic liquid and a co-solvent to obtain a second liquid; a second step of adding an organic acid anhydride to the second liquid to react the cellulose with the organic acid anhydride and produce a cellulose ester; a third step, subsequent to the second step, of adding a poor solvent to the second liquid to precipitate the cellulose ester; a fourth step, subsequent to the third step, of separating a solid phase containing the cellulose ester from a liquid phase consisting of the ionic liquid, the co-solvent, and a third liquid containing cellulose-derived impurities, recovering the cellulose ester from the solid phase, and recovering the third liquid from the liquid phase; and a fifth step of subjecting the third liquid to an ultrafiltration membrane according to the method of any one of [1] to

[11] to separate the fourth liquid containing the ionic liquid and the co-solvent from the cellulose-derived impurities and recovering the fourth liquid. A manufacturing method in which steps 1 to 5 can be repeated by reusing all or part of the fourth liquid as all or part of the first liquid.

[16] The method of

[15] , which includes, prior to step 5, a distillation step of distilling the third liquid to remove the poor solvent and / or organic acid derived from the organic acid anhydride in the third liquid.

[17] The method of

[16] , which includes, after step 5, an additional step of subjecting the fourth liquid to a nanofiltration membrane or a reverse osmosis membrane to remove colored components produced by the distillation step. Each configuration and combination thereof in each embodiment is merely an example, and configurations and combinations thereof can be added, omitted, substituted, and otherwise modified as appropriate within the scope of the present disclosure.

[0062] The present disclosure will be explained in more detail below by showing examples, but interpretation of the present disclosure is not limited to these examples.

[0063] <Removal of cellulose-derived impurities by ultrafiltration membrane> To investigate the removal of cellulose-derived impurities by the ultrafiltration membrane according to the present disclosure, the NMP permeation rate, molecular weight cutoff, permeation rate of an impurity-containing liquid, and rejection rate of impurities in the solvent were measured. Nylon resin hollow fiber membranes (UF50 and UF120 manufactured by Unitika) and polyethersulfone (PES) hollow fiber membranes were used as ultrafiltration membranes.

[0064] [NMP permeation rate] The NMP permeation rate was measured at a surface area of ​​0.012 m ２ The measurement was carried out by a constant pressure cross-flow test using a hollow fiber membrane. First, NMP was passed through the hollow fiber membrane at a flow rate of 9.9 ml / min. Furthermore, the operating pressure was set to 1 bar or 4 bar using a pressure gauge installed near the inlet of the hollow fiber membrane and a pressure regulator installed near the outlet of the hollow fiber membrane. At this time, the permeation rate of the filtrate discharged from the outlet of the hollow fiber membrane was measured, and the permeation rate per unit time (h) and unit membrane area (m ２ ), NMP permeation rate (L / m) per unit pressure (bar) ２ / hr / bar (LMH / bar)) was calculated.

[0065] [Molecular weight cutoff] The molecular weight cutoff of the ultrafiltration membrane is ２ The measurement was carried out by a constant pressure cross-flow test using a hollow fiber membrane. That is, PEGs with molecular weights of 3K, 8K, 12K, 35K, 100K, and 500K were each mixed and dissolved in NMP at 0.1%, and this was first passed through the hollow fiber membrane at a flow rate of 9.9 ml / min. Furthermore, the test was carried out under constant pressure conditions by setting the operating pressure to 1 bar using a pressure gauge installed near the inlet of the hollow fiber membrane and a pressure regulator installed near the outlet of the hollow fiber membrane. Six hours after the start of the liquid passage, the feed solution and filtrate were collected. The collected feed solution and filtrate were each analyzed by liquid chromatography using a gel permeation chromatography (GPC) column and a differential refractive index (RI) detector. The feed solution strength (RI) was determined from a chromatogram of RI peak intensity versus molecular weight (calculated from the PEG calibration curve). ｆｅｅｄ ) and filtrate strength (I ｆｉｌｌ) was obtained, and the rejection rate of PEG was calculated using the following formula 1. A fractionation curve of the ultrafiltration membrane was plotted, and the molecular weight at which 90% rejection rate was obtained was determined as the molecular weight cutoff. Formula 1: Rejection rate (%) = (1 - I ｆｉｌｌ / I ｆｅｅｄ ) x 100

[0066] [Permeation Rate of Impurity-Containing Liquid and Rejection Rate of Impurities in Impurity-Containing Liquid] As the impurity-containing liquid, an EmimOAc / NMP mixed solvent (weight ratio 50:50) containing 1000 ppm to 6400 ppm of cellulose acetate with a molecular weight of 50 kD or 700 kD was used. The molecular weight and content of cellulose acetate were determined by gel permeation chromatography (GPC). The permeation rate of the impurity-containing liquid and the rejection rate of impurities in the solvent were measured using a surface area of ​​0.012 m. ２ The measurement was carried out by a constant pressure cross-flow test using a hollow fiber membrane. That is, the solvent was first passed through the hollow fiber membrane at a flow rate of 9.9 ml / min. Furthermore, the test was carried out under constant pressure conditions by setting the operating pressure to 1 bar or 4 bar using a pressure gauge installed near the inlet of the hollow fiber membrane and a pressure regulator installed near the outlet of the hollow fiber membrane. 24 hours after the start of the liquid passage, the flow rate of the filtrate was measured. In addition, the feed liquid and the filtrate were sampled. The permeation rate of the impurity-containing liquid was calculated from the measured flow rate of the filtrate. Furthermore, the cellulose acetate content of the sampled feed liquid and the filtrate was quantified by sugar analysis using the phenol-sulfuric acid method. The quantification was carried out using a calibration curve prepared using the same solvent system, taking into account the effect of the solvent on the measured value. The cellulose acetate content (C) in the quantified feed liquid was measured. ｆｅｅｄ ), the content of cellulose acetate in the filtrate (C ｆｉｌｌ ), the rejection rate of impurities in the impurity-containing liquid was calculated using the following formula 2: Rejection rate (%) = (1 - C ｆｉｌｌ / C ｆｅｅｄ ) x 100

[0067] The results of the above measurements are shown in Table 1.

[0068]

[0069] As shown in Table 1, the ultrafiltration membranes of Examples 1 to 6 had appropriate molecular weight cutoffs and exhibited excellent permeation rates for impurity-containing liquids and excellent rejection rates for impurities in the impurity-containing liquids. On the other hand, the ultrafiltration membrane of Comparative Example 1 was not resistant to solvents and was unable to effectively remove impurities from the impurity-containing liquid. This result demonstrates that the ultrafiltration membrane of the present invention can effectively remove impurities from impurity-containing liquids.

[0070] [Solvent resistance of ultrafiltration membrane] The solvent resistance of the ultrafiltration membrane was examined by immersing the ultrafiltration membrane of Example 1 in an EmimOAc / NMP mixed solvent (weight ratio 50:50) at room temperature. The hollow fiber membrane was removed immediately after immersion, and after 1 month, 2 months, and 3 months, and the strength retention rate was evaluated. That is, after removing the hollow fiber membrane from the mixed solvent, the mixed solvent on the surface was gently removed with filter paper, and the hollow fiber membrane impregnated with the mixed solvent was used as a sample, and the strength and elongation of the tensile test were measured using a benchtop testing machine. The measurement conditions were a sample length of 5 cm and a crosshead speed of 10 cm / min. The elastic modulus was estimated from the breaking strength, breaking elongation, and initial slope. The results are shown in Figure 1.

[0071] As shown in Figure 1, the hollow fiber membrane showed no significant changes in (1) breaking strength, (2) breaking elongation, or (3) modulus over three months of immersion in the mixed solvent. This result indicates that the ultrafiltration membrane of the present invention is resistant to a liquid containing an ionic liquid, a cosolvent, and a cellulose ester, and therefore can effectively recover the liquid containing the ionic liquid and the cosolvent from the liquid.

[0072] <Removal of Colored Components by Nanofiltration Membrane or Reverse Osmosis Membrane> The removal of colored components by the nanofiltration membrane or reverse osmosis membrane according to the present disclosure was investigated. A polyketone-carrier interfacially polymerized composite membrane and a polysulfone-carrier composite membrane were used as the nanofiltration membrane or reverse osmosis membrane. The polyketone-carrier interfacially polymerized composite membrane (IP-PK membrane) was prepared by a non-solvent phase separation method. The surface of a flat substrate membrane made of polyketone (manufactured by Hyosung) was polymerized with a polyamide-containing active layer composed of m-phenylenediamine (MPD) or trimesoyl chloride (TMC) monomers by a known method (IP-PK membrane). The polysulfone-carrier composite membrane used was a commercially available reverse osmosis membrane (SWC4 manufactured by Nitto Denko Corporation), a commercially available nanofiltration membrane (TriSep TS80 manufactured by Aquqsource Inc.), and a commercially available nanofiltration membrane (NTR7250 manufactured by Nitto Denko Corporation). SWC4 and TriSep TS80 are interfacially polymerized composite membranes (IP-PS membranes) whose separation active layer contains polyamide, while NTR7250 is a membrane (VA-PS membrane) whose separation active layer contains polyvinyl alcohol and polyamide. The liquids used for the nanofiltration membrane or reverse osmosis membrane were EmimOAc / NMP mixed liquid and mimOAc / NMP / methanol mixed liquid, which were heated at 120°C for 14 hours to produce colored components. The components of these liquids are expressed as the weight ratio of EmimOAc (ILq) to NMP, or the weight ratio of EmimOAc (ILq), NMP, and methanol (Me).

[0073] [Salt rejection rate and hydraulic conductivity of membrane] The salt rejection rate and hydraulic conductivity of nanofiltration membrane or reverse osmosis membrane were measured using a surface area of ​​0.0008 m ２ The measurement was carried out by a constant pressure cross-flow test using a flat nanofiltration membrane or reverse osmosis membrane. That is, a solution of 1600 ppm NaCl dissolved in pure water was passed through the nanofiltration membrane or reverse osmosis membrane at a flow rate of 9.9 ml / min. Furthermore, the test was carried out under constant pressure conditions by setting the operating pressure to 10 bar using a pressure gauge installed near the inlet of the nanofiltration membrane or reverse osmosis membrane and a pressure regulator installed near the outlet of the nanofiltration membrane or reverse osmosis membrane. One hour after the start of the liquid passage, the feed solution and the filtrate were sampled. The NaCl concentration (C) in the feed solution was measured using an electrical conductivity meter. ｆｅｅｄ ) and the NaCl concentration in the filtrate (Cｆｉｌｌ The salt rejection rate was measured using the following formula: Salt rejection rate (%) = (1 - C ｆｉｌｌ / C ｆｅｅｄ ) × 100 [In Formula 3, C ｆｅｅｄ is the NaCl concentration in the aqueous NaCl solution supplied to the nanofiltration membrane or the reverse osmosis membrane when the aqueous NaCl solution is passed through the nanofiltration membrane or the reverse osmosis membrane, and C ｆｉｌｌ is the NaCl concentration in the NaCl aqueous solution discharged from the nanofiltration membrane or reverse osmosis membrane when the NaCl aqueous solution is passed through the nanofiltration membrane or reverse osmosis membrane. The permeability coefficient was determined as the permeation rate when the NaCl aqueous solution was passed through the nanofiltration membrane or reverse osmosis membrane. [Color removal rate] A constant pressure crossflow test was carried out for each liquid in the same manner as above, and the resulting feed liquid, filtrate, and sample were used. These were each diluted 20-fold with acetonitrile and the average absorbance at 380 to 770 nm was measured, thereby determining the average absorbance (Abs ｆｉｌｌ ) and the average absorbance of the filtrate (Abs ｆｉｌｌ The stain removal rate was calculated using the formula 4: Stain removal rate (%) = (1 - Abs ｆｉｌｌ / Abs ｆｅｅｄ ) × 100 [In Formula 4, Abs ｆｅｅｄ is the average absorbance in the range of 380 to 770 nm of a liquid containing an ionic liquid supplied to a nanofiltration membrane or a reverse osmosis membrane when the liquid contains an ionic liquid and is passed through the nanofiltration membrane or the reverse osmosis membrane, and Abs ｆｉｌｌis the average absorbance at 380 to 770 nm of the ionic liquid-containing liquid discharged from a nanofiltration membrane or reverse osmosis membrane when the liquid is passed through the nanofiltration membrane or reverse osmosis membrane. As shown in FIG. 2, colored components generated in a liquid containing an ionic liquid by a heating process such as distillation have an absorption peak in the ultraviolet region and an absorption band extending into the visible light region. Therefore, as described above, the color removal rate can be appropriately evaluated based on the average absorbance at 380 to 770 nm. [Ionic Liquid Recovery Rate] The recovery rate of the entire ionic liquid was evaluated as follows. The liquid before and after the operation to remove the colored components was evaporated to dryness under reduced pressure (140°C, 5 hours) to remove the volatile solvent (NMP, methanol) from the liquid. The weight of the residual ionic liquid was measured before and after the operation to remove the colored components. The weight of the ionic liquid after the operation of removing the colored components was divided by the weight of the ionic liquid before the operation of removing the colored components, and multiplied by 100 to calculate the recovery rate (%).

[0074] [Removal of Coloring Components with Activated Carbon] As a comparative example for the removal of coloring components, the removal of coloring components with activated carbon was evaluated. Commercially available activated carbons, Shirasagi A, Shirasagi ANOX-1, and Carborafine, manufactured by Osaka Gas Chemicals Co., Ltd., were used as activated carbon. The decolorization test using activated carbon was performed by adding 1.0 g of activated carbon to 50 g of liquid (bath ratio 50), stirring the mixture in a vortex mixer for 1 minute, leaving it to stand for 5 minutes, then centrifuging (3000 rpm x 10 minutes), and filtering the supernatant through filter paper. This liquid was compared with a liquid to which activated carbon had not been added, and each parameter was determined in the same manner as described above.

[0075] A comparison of the color removal rates of nanofiltration membranes or reverse osmosis membranes with those of activated carbon is shown in Table 2.

[0076]

[0077] As shown in Table 2, the nanofiltration membrane or reverse osmosis membrane showed good color removal rates, while the activated carbon did not.

[0078] The parameters of the nanofiltration or reverse osmosis membranes are shown in Table 3.

[0079]

[0080] As shown in Table 3, the nanofiltration membranes or reverse osmosis membranes of the examples showed good color removal rates, while the nanofiltration membranes or reverse osmosis membranes of the comparative examples did not show good color removal rates.

[0081] The method of the present disclosure for recovering a liquid containing an ionic liquid and a co-solvent from an impurity-containing liquid containing an ionic liquid, a co-solvent, and cellulose-derived impurities can be suitably used in the production of cellulose esters, which are widely used as raw polymers, and has industrial applicability.

Claims

1. A method for recovering a liquid containing an ionic liquid and a co-solvent from an impurity-containing liquid containing an ionic liquid, a co-solvent, and cellulose-derived impurities, the method comprising subjecting the impurity-containing liquid to an ultrafiltration membrane to separate the liquid containing the ionic liquid and the co-solvent from the cellulose-derived impurities.

2. The method according to claim 1, wherein the molecular weight cutoff of the ultrafiltration membrane is 1,000 to 1,000,000.

3. The method of claim 1, wherein the ultrafiltration membrane comprises a polyamide resin.

4. The method of claim 1, wherein the cellulose-derived impurities include cellulose esters.

5. The method of claim 1, wherein the cellulose-derived impurities have a molecular weight of 10,000 to 1,500,000.

6. The method of claim 1, wherein the cellulose-derived impurities include cellulose esters having a degree of substitution of 2.9 or less.

7. The method according to claim 1, wherein the ionic liquid contains an imidazolium cation or a quaternary ammonium cation as the cation component and a carboxylate anion as the anion component.

8. The method of claim 1, wherein the ionic liquid is selected from the group consisting of 1-ethyl-3-methylimidazolium acetate (EmimOAc), 1-ethyl-2,3-dimethylimidazolium acetate (EDmimOAc), 1-butyl-3-methylimidazolium acetate, 1-hexyl-3-methylimidazolium acetate, tetrabutylammonium acetate, and combinations thereof.

9. The method of claim 1, wherein the co-solvent is a high boiling polar solvent.

10. The method of claim 9, wherein the high boiling point polar solvent is selected from the group consisting of dimethyl sulfoxide (DMSO), sulfolane, N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), and combinations thereof.

11. The method according to claim 1, wherein in the impurity-containing liquid and / or the liquid containing an ionic liquid and a cosolvent, the content of the ionic liquid relative to the total amount of the ionic liquid and the cosolvent is 20 to 99.9% by weight.

12. A method for producing a cellulose ester, comprising: a first step of dissolving cellulose in a first liquid containing an ionic liquid and a co-solvent to obtain a second liquid; a second step of adding an organic acid anhydride to the second liquid to react the cellulose with the organic acid anhydride and produce a cellulose ester; a third step, following the second step, of adding a poor solvent to the second liquid to precipitate the cellulose ester; a fourth step, following the third step, of separating a solid phase containing the cellulose ester from a liquid phase consisting of the ionic liquid, the co-solvent, and a third liquid containing cellulose-derived impurities, recovering the cellulose ester from the solid phase and recovering the third liquid from the liquid phase; and a fifth step of passing the third liquid through an ultrafiltration membrane to separate the fourth liquid containing the ionic liquid and the co-solvent from the cellulose-derived impurities and recovering the fourth liquid. The manufacturing method, wherein the first to fifth steps can be repeated by reusing all or part of the fourth liquid as all or part of the first liquid.

13. The method according to claim 12, further comprising, prior to the fifth step, a distillation step of distilling the third liquid to remove the poor solvent and / or the organic acid derived from the organic acid anhydride in the third liquid.

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