Method for depolymerizing polyester

Abstract

To provide a novel method for depolymerizing polyester utilizing a catalyst that allows for progression of polyester depolymerization reaction and that enables post-reaction recovery, separation, and reuse.SOLUTION: The present invention provides a method for depolymerizing polyester, comprising a step in which polyester is reacted in the presence of a catalyst and solvent to depolymerize, where the catalyst is a boric acid compound.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for depolymerizing polyester. [Background technology]

[0002] Polyesters, such as polyethylene terephthalate (PET), are widely used in fibers, films, sheets, beverage bottles, and other applications.

[0003] In recent years, disposal methods for plastics such as polyester have become an issue, and various methods for recovering and reusing waste plastics have been considered. Mechanical recycling (material recycling) has traditionally been used to reuse waste plastics, and although it is relatively low-cost, it is prone to contamination with impurities and deterioration in quality. Repeated recycling leads to a deterioration in quality and subsequent uses are limited.

[0004] In response to this, chemical recycling has been adopted. Chemical recycling is a recycling method in which plastics such as polyester are depolymerized by chemical methods to convert them into monomers, which are then recovered, and the recovered monomers are used as raw materials for a second polymerization reaction to produce polyesters, etc.

[0005] As a method for depolymerizing polyester monomers, for example, a methanol decomposition method using methanol, a glycol decomposition method using glycol, and an alkali decomposition method using an alkali have been carried out, and a method of irradiating these with microwaves, an enzymatic decomposition method, etc. have also been proposed. However, chemical recycling tends to have more steps than material recycling, and requires high temperatures for depolymerization and equipment for applying the high temperatures, which makes it a high-cost and high-energy-consuming process.

[0006] Among such chemical recycling methods, for example, the method of Patent Document 1 has been proposed. According to the method of Patent Document 1, by using a specific base catalyst and a glycol scavenger, it is possible to depolymerize into monomers at a lower temperature and lower cost than the conventional chemical recycling methods, and it is also possible to obtain a high-purity monomer ester, and further, it is possible to convert the by-produced cyclic compound back into the glycol scavenger and monomer according to a previously reported method, and it is advantageous in that it is possible to achieve polyester production in a closed loop system. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2022-126617 A Summary of the Invention [Problem to be solved by the invention]

[0008] The method described in Patent Document 1 has several advantages as described above, but the preferred base catalysts used are alkali metal alkoxides and alkaline earth metal alkoxides, and these catalysts are difficult to separate and purify after use in depolymerization of polyester. In particular, due to the monohydric alcohol used in combination as an essential component, it is not easy to separate and purify the alkoxide component associated with the catalyst from the mixture after the reaction. In other words, in the method of Patent Document 1, it is difficult to recover or reuse the catalyst used after the reaction.

[0009] Therefore, the inventors of the present application have intensively studied the problems in Patent Document 1, and as a result have found a new method for depolymerizing polyester using a catalyst that is relatively easy to recover and reuse, while partially taking into consideration the depolymerization method as described in Patent Document 1. The inventors have newly discovered that the depolymerization reaction of polyester can actually proceed, that the catalyst can be easily recovered or separated as a solid content even after the reaction, and that the solid content containing the catalyst can be reused as a catalyst, i.e., the depolymerization of polyester can be carried out again, and have thus completed the present invention.

[0010] Therefore, an object of the present invention is to provide a novel method for depolymerizing polyester using a catalyst. The catalyst can promote the depolymerization reaction of polyester, and can be easily recovered or separated as a solid content after the reaction, and further, the solid content containing the catalyst can be reused as a catalyst. No such report has been made so far. [Means for solving the problem]

[0011] That is, the gist of the present invention is as follows. (1) A method for depolymerizing a polyester, comprising the steps of: The method includes the steps of reacting a polyester in the presence of a catalyst and a solvent to depolymerize the polyester, The method for depolymerizing polyester, wherein the catalyst is a borate compound. (2) The method for depolymerizing a polyester according to (1), wherein the borate compound includes an alkali metal borate compound and / or an alkaline earth metal borate compound. (3) The method for depolymerizing a polyester according to (1), wherein the solvent contains a carbonate diester and / or an alcohol solvent as a glycol scavenger. (4) The method for depolymerizing a polyester according to (1), further comprising a cocatalyst in addition to the catalyst. (5) The method for depolymerizing a polyester according to (4), wherein the cocatalyst is at least one selected from the group consisting of cyclic or acyclic ethers, glycols, quaternary ammonium salts, and alkali metal salts. (6) A method for depolymerizing polyester according to any one of (1) to (5), comprising a step of recovering a solid content containing the catalyst used after reacting and depolymerizing the polyester. (7) A method for depolymerizing a polyester according to (6), characterized in that the solid matter recovered by the method according to (6) is used as a catalyst to react with a polyester in the presence of the solvent to depolymerize the polyester. Effect of the Invention

[0012] According to the present invention, the depolymerization reaction of polyester can be promoted, and the depolymerization reaction can be recovered or separated as a solid content even after the reaction, and the recovered solid content can be reused as a catalyst. As a result, the amount of catalyst used can be reduced overall. Also, the effort required for removing the remaining catalyst from the product can be reduced. [Brief description of the drawings]

[0013] [Figure 1] 1 shows the results of GC analysis of the filtrate after the reaction in Example 20. (a) is a representative GC chart of the filtrate after the depolymerization reaction, and (b) is a GC chart of a DMT standard. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The method for depolymerizing polyester of the present invention will be described in detail below. The present invention is not limited to the following embodiments, and various modifications and combinations are possible within the scope of the present invention.

[0015] As described above, the present invention provides a method for depolymerizing a polyester, which comprises a step of reacting a polyester in the presence of a catalyst and a solvent to depolymerize the polyester, and is characterized in that the catalyst used is a borate compound.

[0016] <Polyester> The polyester to be depolymerized may be any polyester. There are no limitations on the composition, but generally it is a copolymer of a saturated dibasic acid and a glycol, and main examples include polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc. Polyesters having other compositions can also be used.

[0017] Other examples of the saturated dibasic acid include aromatic dicarboxylic acids such as terephthalic acid, phthalic acid (ortho form), isophthalic acid, dibromoisophthalic acid, sodium sulfoisophthalate, phenylenedioxydicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 4,4'-diphenylketonedicarboxylic acid, 4,4'-diphenoxyethanedicarboxylic acid, trimellitic acid, pyromellitic acid, etc. Other examples of the saturated dibasic acid include alicyclic dicarboxylic acids such as hexahydroterephthalic acid and hexahydroisophthalic acid, and aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecadicarboxylic acid, and dodecadicarboxylic acid.

[0018] Examples of glycols include ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol, neopentyl glycol, polytetramethylene glycol, 1,4-cyclohexanediol, and 1,4-benzenediol.

[0019] The shape of the polyester is not limited, and may be, for example, a raw material such as chips, or may be a product such as fibers, films, sheets, or beverage bottles, or may be in a state of used goods or waste after use. From the viewpoint of efficiency of the depolymerization reaction and ease of processing after the reaction, it is preferable to adjust the shape and surface area. A relatively small shape is preferable from the viewpoint of reactivity, and it is preferable to use one that has been pulverized to a shape or particle size such as chips, flakes, or powder. In the present invention, since an embodiment in which separation from the catalyst after the reaction is easy is preferable, it is preferable to adjust the shape and particle size of the polyester to be used, especially considering the ease of separation processing. Although the embodiment is not limited, as an example, one that has been adjusted to a size that is called flake-like in appearance (about 1 to 20 mm) or a size that is called powder-like in appearance (for example, less than 1 mm) may be used.

[0020] From the viewpoints of reactivity, separation after the reaction, and purity, it is preferable to use the polyester in a state in which impurities and moisture are as small as possible. It is also preferable that the amount of moisture contained in the solvent described below and the amount of moisture attached to the equipment used are small.

[0021] <Catalyst> In the depolymerization method of the present invention, it is necessary to use a catalyst, and a borate compound is used as the catalyst. There have been no clear reports on the use of a borate chloride as a catalyst in a method for depolymerizing polyester, and this is a novel idea obtained in the present invention.

[0022] It is of course possible to advance the depolymerization of polyester by using a borate compound as a catalyst, but unlike the catalyst used in the above-mentioned Patent Document 1, even after the reaction treatment, it is relatively easy to recover or separate as a solid content from the reaction mixture, and the recovered or separated solid content containing the catalyst can be reused as a catalyst. As described later, in the examples of the present application, it has been confirmed that after separating and recovering the solid content containing the catalyst used in the depolymerization reaction of polyester, the solid content can be used again as a catalyst to advance the depolymerization reaction of polyester with the same yield.

[0023] The mechanism by which the borate compound can promote the depolymerization reaction of polyester is not necessarily clear, but it is speculated to be an ester exchange reaction due to the Lewis acidity of the metal cation and the basicity of the borate ion. That is, the metal cation coordinates to the carbonyl site of the polyester to increase the electrophilicity of the ester group, and the basicity of the borate ion acts on the hydrogen atom derived from the hydroxyl group of the alcohol to increase the nucleophilicity of the alcohol, thereby proceeding with the ester exchange reaction between the ester group of the polyester and the alcohol. Thereafter, the hydrogen atom derived from the hydroxyl group of the alcohol is transferred as a proton to the alkoxide group derived from the glycol site, completing the ester exchange reaction by the catalyst, and it is speculated that the next ester exchange reaction proceeds in the same manner thereafter. In addition, the catalyst related to this borate compound is relatively easy to recover even after the depolymerization reaction, and the mechanism by which it can be used again in the depolymerization reaction of polyester is that the borate compound is an inorganic compound, which is relatively easy to separate and recover as a solid content, or is relatively stable to oxygen and moisture in the air. Regarding the speculation regarding the mechanism of the present invention, even if an explanation different from this speculation is given or different facts are discovered in the future, this will not be an obstacle to the interpretation of the technical scope of the present invention or the implementation of the invention.

[0024] Here, the borate compound as the catalyst is not limited as long as it is composed of borate ions and a pair of cations, and natural minerals can be used. Hydrates are also acceptable. Among these, metal salts of boric acid are preferred. Here, "boric acid" (including borate ions) may be interpreted to include dehydrated metaboric acid or polyboric acid. There is no limitation on the metal salts of boric acid, and examples of the metals include alkali metals, alkaline earth metals, elements of Group 4, Group 12, and Group 13 of the periodic table. Examples of cations other than metals include ammonium salts.

[0025] Examples of the alkali metal borate compound include lithium borates (lithium borate, lithium metaborate, lithium metaborate dihydrate, lithium tetraborate, lithium pentaborate, etc.), sodium borates (sodium metaborate, sodium metaborate dihydrate, sodium metaborate tetrahydrate, sodium diborate, sodium tetraborate, sodium tetraborate decahydrate (borax), sodium pentaborate, sodium hexaborate, sodium octaborate, etc.), and potassium borates (e.g., potassium metaborate, potassium metaborate 3 / 4 tetrahydrate (KBO2·4 / 3H2O), potassium tetraborate, potassium pentaborate, potassium hexaborate, potassium octaborate, etc.). Hydrates of these compounds can also be mentioned.

[0026] Examples of the alkaline earth metal borate compound include magnesium borates (magnesium metaborate, magnesium diborate, trimagnesium tetraborate, pentamagnesium tetraborate, magnesium hexaborate, etc.), calcium borates (calcium metaborate, calcium diborate, tricalcium tetraborate, pentacalcium tetraborate, calcium hexaborate, etc.), barium borates (barium orthoborate, barium metaborate, barium diborate, barium tetraborate, etc.), etc. Hydrates of these compounds can also be mentioned.

[0027] Examples of the other elements of Group 4, Group 12, and Group 13 of the periodic table include zirconium borates, zinc borates, aluminum borates, and hydrates thereof. Examples of elements other than metals include ammonium borates and hydrates thereof. The above borate compounds can be used alone or in mixtures of two or more. In addition, the category of borate compounds in the present invention does not exclude raw materials that inevitably generate the borate compounds by reacting with, for example, a solvent or a component in the solvent as described below in the process of depolymerizing the polyester according to the present invention. In other words, it is intended not to exclude raw materials that are inevitably converted to the borate compounds in the depolymerization process, and the borate compounds are essentially used as catalysts. Such raw materials are not limited, but examples thereof include sodium borohydride and potassium borohydride.

[0028] The amount of the catalyst used in the present invention is not limited as long as it is within the range of the object of the present invention, and any amount may be used. For example, it can be 0.1 to 150 mol% with respect to the polyester to be depolymerized. Here, the moles of polyester correspond to the number of moles of the terephthalate component contained in the polyester used, as shown in the Examples section described later. If the amount of catalyst is too small, the depolymerization reaction tends not to proceed or the reaction rate tends to be slow, so the preferred lower limit is 1 mol%, more preferably 3 mol%, and even more preferably 5 mol%. On the other hand, if the amount of catalyst is too large, the effect tends to plateau with respect to the amount added, so from the viewpoint of reactivity and cost, the preferred upper limit is 100 mol%, more preferably 50 mol%, even more preferably 30 mol%, and even more preferably 20 mol%. The amount used can be appropriately determined based on the polyester used, reaction conditions, etc.

[0029] In the present invention, it is not prohibited to use other known catalysts than the catalyst composed of the borate compound as long as the object of the present invention is not impaired. However, the known catalyst is different from the co-catalyst described below. Examples of such known catalysts include solid base catalysts and acid catalysts. Such known catalysts are preferably those that contribute to the object of the present invention, and those that can increase the reactivity of the depolymerization reaction or do not hinder the recovery and separation after the reaction can be preferably used. Those that deactivate the borate compound used in combination are not preferable. In this case, the catalyst (catalyst mixture) of the other known catalyst can be used in an amount of, for example, 50 mol % or less relative to the borate compound.

[0030] <Solvent> In the depolymerization method of the present invention, a solvent is used. The use of a solvent is preferable because it has the function of a dispersion solvent for the polyester and catalyst used, and can increase dispersibility and also increase reactivity. For such purposes, it is preferable to use a glycol scavenger as the solvent. It is also preferable to use an alcohol-based solvent. More preferably, the glycol scavenger and the alcohol-based solvent are used in combination.

[0031] In the present invention, the detailed mechanism of using the borate compound as a catalyst and using these glycol trapping agents in combination with an alcohol-based solvent cannot necessarily be clearly defined, but by taking into consideration the above-mentioned Patent Document 1 in part, it is possible to infer the use of a glycol trapping agent and an alcohol-based solvent as follows. That is, it is considered that the advantage of using both of these to react with a polyester in the presence of the catalyst is that the depolymerization of the polyester (transesterification reaction by an alcohol-based solvent) is first advanced, and the glycol, which is an intermediate product generated by this, is reacted with the glycol trapping agent, thereby allowing the reaction (equilibrium reaction) to proceed in the direction of the product system. That is, it is considered that the advantage of biasing the equilibrium reaction toward depolymerization is that the depolymerization reaction of the polyester can be efficiently advanced.

[0032] In this reaction, first, a dialkyl compound derived from the saturated dibasic acid component constituting the polyester is obtained. If the saturated dibasic acid component is an aromatic dicarboxylic acid, a dialkyl aromatic dicarboxylate is obtained as a monomer. Specifically, as confirmed in the examples, it can be confirmed that dimethyl terephthalate (DMT) is obtained as a monomer ester derived from the terephthalic acid constituting the polyethylene terephthalate and the methanol used.

[0033] On the other hand, glycol is produced by the depolymerization reaction. The produced glycol can react with a glycol trapping agent to obtain an alkyl carbonate, which is a relatively stable cyclic compound. Various alkyl carbonates can be obtained depending on the glycol trapping agent used and the structure of the resulting glycol. Specifically, as confirmed in the examples, it can be confirmed that ethylene carbonate (EC) can be obtained as an alkyl carbonate derived from ethylene glycol, which constitutes polyethylene terephthalate, and dimethyl carbonate, which serves as a glycol trapping agent.

[0034] The glycol scavenger and the alcohol-based solvent as the solvent can be determined in consideration of the reactivity of the depolymerization reaction of the polyester and the monomers to be obtained.

[0035] Here, the glycol scavenger is not limited as long as it can generate a relatively stable cyclic compound by reacting with the glycol intermediate product. Examples of the glycol scavenger include carbonic acid diesters and tetraalkoxysilanes. Examples of carbonic acid diesters include dimethyl carbonate, diethyl carbonate, diphenyl carbonate, and methyl ethyl carbonate. Preferred are lower carbonic acid diesters having 3 to 10 carbon atoms, and more preferably dimethyl carbonate. On the other hand, examples of tetraalkoxysilanes include, but are not limited to, tetramethoxysilane, tetraethoxysilane, and tetrabutoxysilane. Tetramethoxysilane is more preferred.

[0036] The alcohol solvent is not limited as long as it can promote the depolymerization reaction (transesterification reaction). The alcohol solvent is preferably a monohydric alcohol. Examples of the monohydric alcohol include methanol, ethanol, propanol, butanol, etc., and methanol is preferred.

[0037] The amount of glycol scavenger and / or alcohol solvent used as a solvent is not limited, but can be appropriately determined according to the composition and amount of polyester. That is, it can be appropriately determined in consideration of the amount of ester bonds in the polyester and the amount of glycol generated by the depolymerization reaction. The amount of alcohol solvent used is considered to be a theoretical amount or more for proceeding with the depolymerization reaction (ester exchange reaction) based on the structure of the polyester used, but it is preferable to use an excess amount from the viewpoint of reactivity. In addition, it is considered that the amount of glycol scavenger is necessary to be a theoretical amount or more based on the amount of glycol produced, but it is preferable to use an excess amount of this agent from the viewpoint of reactivity. In addition, it is preferable to determine the amount of these agents used taking into consideration the reaction conditions.

[0038] <Reaction conditions> The conditions of the depolymerization reaction of the present invention are not limited, but the reaction can proceed at room temperature to 65°C under normal pressure or pressure. If necessary, the reaction may be further heated, and can usually be carried out at 20°C to 150°C. The reaction temperature and reaction time can be determined in consideration of reactivity and ease of implementation. The higher the reaction temperature, the more advantageous it is for the depolymerization reaction, but it is preferable to determine the reaction temperature in consideration of the boiling point of the solvent used. For example, when methanol is used as the alcohol solvent under normal pressure, it is preferable to set the reaction temperature to 65°C or less. Therefore, a preferred temperature is 20 to 65°C. A longer reaction time is also considered to be advantageous for the depolymerization reaction, but the longer the reaction time, the more costly and laborious it becomes, and the effect tends to plateau, so that the reaction time can be, for example, 1 to 96 hours. The reaction time is preferably 1 to 72 hours, and more preferably 1 to 48 hours.

[0039] <Cocatalyst> In the present invention, it is preferable to use a cocatalyst together with the catalyst. It has been confirmed that the use of a cocatalyst can efficiently advance the depolymerization reaction of polyester and has the effect of improving the yield. As described above, any cocatalyst can be used without limitation as long as it is expected to improve the efficiency and yield of the reaction. Although the detailed mechanism is unclear, it is presumed that it is preferable to focus on a catalyst that has coordination or inclusion properties with the catalyst, or a catalyst that can exchange cationic species. The speculation regarding the mechanism is the same as that described above, and even if an explanation different from the speculation is given or different facts are discovered in the future, this will not be an obstacle to the interpretation of the technical scope of the present invention or the implementation of the invention.

[0040] From this viewpoint, it is presumed that the cocatalyst is preferably a cyclic or non-cyclic ether, a glycol, a quaternary ammonium salt, an alkali metal salt, etc. The cocatalyst may be used alone or in a mixture of two or more kinds. Cyclic ethers include, but are not limited to, those having the general formula (-CH2CH2O-) n Those represented by the formula (I) can be used, and examples thereof include those having a value of n of about 3 to 8. Examples thereof include 9-crown-3, 12-crown-4, 15-crown-5, 18-crown-6, dibenzo-18-crown-6, 21-crown-7, and 24-crown-8. Examples thereof include thiacrown ethers in which oxygen atoms are replaced with sulfur atoms, azacrown ethers in which oxygen atoms are replaced with nitrogen atoms, and further, alicyclic-containing crown ethers in which an alicyclic skeleton is introduced, and aromatic-ring-containing crown ethers in which an aromatic ring is introduced. Any of these may be substituted with other substituents. Among these, those having a value of n of 4 to 6 are particularly preferred in terms of availability and expression of effects.

[0041] Examples of acyclic ethers include, but are not limited to, acyclic monoethers such as dimethyl ether, diethyl ether, diisopropyl ether, dibutyl ether, and anisole; 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-diisopropoxyethane, 1,2-dibutoxyethane, 1,2-diphenoxyethane, 1,2-dimethoxypropane, 1,2-diethoxypropane, 1,2-diisopropoxypropane, 1, Acyclic diethers such as 2-dibutoxypropane, 1,2-diphenoxypropane, 1,3-dimethoxypropane, 1,3-diethoxypropane, 1,3-diisopropoxypropane, 1,3-dibutoxypropane, 1,3-diphenoxypropane, 1,4-dimethoxybutane, 1,4-diethoxybutane, 1,4-diisopropoxybutane, 1,4-dibutoxybutane, and 1,4-diphenoxybutane, and diethylene glycol dimethyl ether are also included. Examples of non-cyclic polyethers include acyclic polyethers such as dimethyl ether (diglyme), dipropylene glycol dimethyl ether, dibutylene glycol dimethyl ether, diethylene glycol diethyl ether, dipropylene glycol diethyl ether, dibutylene glycol diethyl ether, triethylene glycol dimethyl ether (triglyme), tripropylene glycol dimethyl ether, tributylene glycol dimethyl ether, triethylene glycol diethyl ether, tripropylene glycol diethyl ether, tributylene glycol diethyl ether, tetraethylene glycol dimethyl ether (tetraglyme), tetrapropylene glycol dimethyl ether, tetrabutylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetrapropylene glycol diethyl ether, and tetrabutylene glycol diethyl ether. Among these, diglyme is preferred from the viewpoints of availability and effectiveness.

[0042] Examples of glycols include, but are not limited to, alkylene glycols such as ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol, neopentyl glycol, polytetramethylene glycol, 1,4-cyclohexanediol, 1,4-benzenediol, etc. These alkylene glycols may have an alicyclic structure, an aromatic group, or other substituents or atoms.

[0043] The quaternary ammoniums are not limited, and examples thereof include tetraalkylammonium halides such as tetraethylammonium chloride, tetraethylammonium bromide, triethylbenzylammonium chloride, triethylbenzylammonium bromide, trioctylmethylammonium chloride, tributylbenzylammonium chloride, and trimethylbenzylammonium chloride. Here, the alkyl group is not only an alkyl group itself, but also an organic group having about 1 to 10 carbon atoms that may have an alicyclic structure, an aromatic group, or other substituents or atoms, and the four organic groups may be the same or different. Furthermore, not only halides but also those that have been changed to ions derived from inorganic acids such as sulfate ions and nitrate ions may be used. Among these, tetraethylammonium chloride, triethylbenzylammonium chloride, and the like are preferred from the viewpoints of availability and effectiveness.

[0044] Examples of alkali metal salts include, but are not limited to, lithium chloride, lithium bromide, lithium iodide, sodium chloride, sodium bromide, sodium iodide, potassium chloride, potassium bromide, potassium iodide, cesium chloride, cesium bromide, and cesium iodide. Furthermore, in addition to halides, the anion portion may be changed to an ion derived from an inorganic acid, such as a sulfate ion or a nitrate ion. Among these, potassium iodide is preferred from the viewpoints of availability and effectiveness.

[0045] There is no limitation on the amount of these cocatalysts used so long as the object of the present invention is not impaired. For example, they can be used in an amount of 0.1 to 300 mol % based on the polyester.

[0046] <Catalyst recovery and reuse methods> The depolymerization method of the present invention includes, after the depolymerization step of polyester by the above-mentioned method, a step of recovering a solid content containing the catalyst used (which may be referred to as a "solid residue" in the present application) in addition to recovering a reaction product (monomer, etc.) and separating it from unreacted raw materials. As described later, the solid content may contain a part or all of the catalyst used, or may contain a catalyst whose structure has changed, so long as it is reusable. The method for recovering the solid content is not limited, and examples of the method include a method in which one or more operations are appropriately combined from filtration using a commonly used filtration device, centrifugation, sedimentation, coagulation, desolvation, chromatographic separation, recrystallization, sublimation, etc.

[0047] The change in the structure of the catalyst may be, for example, that the anhydrous sodium metaborate used as the catalyst exists in the solid fraction recovered after the reaction as sodium borates such as sodium metaborate dihydrate, tetrahydrate, or borax, or as hydrates thereof, etc. Similarly, the borate hydrate used as the catalyst may exist in the solid fraction recovered after the reaction in a form with a reduced amount of water of hydration.

[0048] In addition, it is also possible that the structure of the catalyst is changed by a part of the catalyst reacting with carbon dioxide in the air to become a carbonate. For example, potassium metaborate is basic, so it is predicted to react with carbon dioxide to become potassium bicarbonate and boron oxide as shown in the following reaction formula. 2 KBO2+ 2 CO2→ 2 KHCO3+ B2O3 On the other hand, the depolymerization reaction of the present invention also proceeds well when a carbonate such as potassium hydrogen carbonate is used as a catalyst. For these reasons, even if the structure of the catalyst is partially changed, the recovered solid content can be used as a catalyst.

[0049] The recovered solid content does not need to be isolated, and may be in a state in which some of the raw materials and products remain. From the viewpoint of reactivity during reuse and prevention of side reactions, it is preferable that the raw materials and products are not contained as much as possible.

[0050] The recovered solid matter may be used as it is in the next depolymerization reaction, or may be subjected to a washing treatment, for example, using water or the solvent used, etc. Furthermore, together with or separately from the washing treatment, other treatments, for example, heating, may be performed.

[0051] The present invention is characterized in that the recovered solid content can be reused as a catalyst. There is no particular limitation on the recovered solid content and the method of reusing it. For example, the depolymerization reaction can be carried out again on another new polyester using the recovered solid content and the above-mentioned solvent. In this case, only the recovered solid content may be used, or the recovered solid content may be used in combination with a new catalyst.

[0052] After carrying out the depolymerization reaction of polyester using the recovered solid content as a catalyst, it is possible to recover (re-recover) the solid content again using the method as described above. In addition, the recovered solid content can be used as is or can be subjected to the above-mentioned washing treatment and / or other treatment to carry out the depolymerization reaction of polyester again. In other words, by using the method of the present invention, it is possible to repeatedly recover and reuse the solid content.

[0053] <Method of recovering and reusing monomer> In the depolymerization method of the present invention, it is possible to recover a monomer produced by depolymerizing a polyester or a monomer produced by reacting the monomer with the solvent in the form of a mixture or by isolation. Such monomers are as described above, and it can be confirmed that, for example, dimethyl terephthalate (DMT) and the like can be obtained as a dialkyl compound derived from a saturated dibasic acid component constituting the polyester. It can also be confirmed that, when the glycol produced by the depolymerization reaction reacts with the glycol trapping agent, for example, ethylene carbonate (EC) can be obtained as an alkyl carbonate corresponding thereto. These monomers have different chemical structures depending on the constituent components of the polyester and solvent (alcohol-based solvent, glycol trapping agent) used. If multiple compounds with different chemical structures are used as the polyester and solvent, the produced monomers can also be obtained accordingly.

[0054] As a method for recovering and separating the monomer, the above-mentioned methods for recovering and separating the solid content can be used.

[0055] The recovered monomer may be reused as a monomer as it is, or may be reused as a raw material for synthesizing polyester. For example, it is possible to synthesize a glycol scavenger such as alkyl carbonate and glycol by reacting the recovered alkyl carbonate (EC, etc.) with an alcohol solvent such as methanol. That is, it is possible to synthesize polyester by reacting the recovered and synthesized glycol with the recovered dialkyl compound of saturated dibasic acid (DMT, etc.). For these synthesis reactions, known methods can be adopted.

[0056] The depolymerization reaction of the present invention includes a method for recovering and reusing such monomers. That is, the present invention includes not only the depolymerization reaction of polyester, but also the above-mentioned method for recovering and reusing solids, and further includes the above-mentioned method for recovering and reusing monomers. EXAMPLES

[0057] Preferred embodiments of the present invention will be specifically described below based on examples and comparative examples, but the present invention should not be construed as being limited to the following embodiments.

[0058] <Preparation of polyester raw material> [Production Example 1] 30 g of polyethylene terephthalate (PET, used PET bottle manufactured by Suntory) was cut into approximately 0.5 to 1 cm square pieces using scissors. The cut PET pieces were used as the raw material for the following depolymerization reaction (hereinafter, these are referred to as "flakes").

[0059] [Production Example 2] A part of the flakes obtained in Production Example 1 was crushed using a crusher (HC-700 manufactured by COLDENWALL), and the crushed product was passed through a sieve with 0.56 mm openings. The resulting undersieve was used as the raw material for the depolymerization reaction (hereinafter, this is referred to as "powder").

[0060] <Depolymerization of polyester using sodium metaborate> [Example 1] 500 mg of the powder obtained in Production Example 2 (molar number of terephthalate components contained in polyester: 2.6 mmol) was weighed into a test tube, and 36 mg of sodium metaborate tetrahydrate (NaBO2·4H2O, manufactured by Kanto Chemical Co., Ltd.) (10 mol% relative to the powder), 7.5 mL of dimethyl carbonate (DMC) (manufactured by Fujifilm Wako Pure Chemical Co., Ltd.) dehydrated with molecular sieves (MS3A manufactured by Kanto Chemical Co., Ltd.), and 2.5 mL of methanol (manufactured by Fujifilm Wako Pure Chemical Co., Ltd.) dehydrated with MS3A were added. The atmosphere in the test tube was replaced with nitrogen gas. An octagonal stirrer was placed in the test tube, and the tube was then covered with a lid, and the depolymerization reaction was carried out by stirring for 2 hours in an aluminum block heater set at 65°C. The number of moles of raw PET is hereinafter expressed as the number of moles converted into the terephthalate component contained in the polyester. The amount of catalyst used is also expressed as the mass and the ratio to the converted number of moles of raw PET used.

[0061] After the reaction, the contents in the test tube were filtered using a filter paper of model No. 5C manufactured by Kiriyama Seisakusho. The solid matter separated by the filtration was washed with DMC, and the washings were added to the filtrate. 20 mg of naphthalene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the filtrate as a standard substance, and the filtrate was analyzed by gas chromatography (GC, manufactured by Shimadzu Corporation GC-2014). Based on the GC analysis, the yield of dimethyl terephthalate (DMT) produced was calculated according to the following formula: Yield (%) = (moles of DMT / moles of terephthalate component contained in polyester used) x 100 The results are shown in Table 1.

[0062] [Example 2] The catalyst was changed to sodium metaborate dihydrate (NaBO2·2H2O) and the amount used was changed to 26 mg (10 mol% based on the raw material PET). Except for this, the depolymerization reaction was carried out in the same manner as in Example 1, and the yield was calculated. The results are shown in Table 1. The NaBO2·2H2O was prepared by drying sodium metaborate tetrahydrate (NaBO2·4H2O, manufactured by Kanto Chemical Co., Ltd.).

[0063] [Example 3] The catalyst was changed to anhydrous sodium metaborate (NaBO2) and the amount used was changed to 17 mg (10 mol % based on the raw material PET), but the depolymerization reaction was carried out in the same manner as in Example 1, and the yield was calculated. The results are shown in Table 1. The NaBO2 was prepared by drying sodium metaborate tetrahydrate (NaBO2·4H2O, manufactured by Kanto Chemical Co., Ltd.).

[0064] [Example 4] The depolymerization reaction was carried out in the same manner as in Example 3 except that the reaction time was changed to 24 hours, and the yield was calculated. The results are shown in Table 1.

[0065] [Example 5] The depolymerization reaction was carried out in the same manner as in Example 3 except that the reaction time was changed to 48 hours, and the yield was calculated. The results are shown in Table 1.

[0066] [Example 6] The depolymerization reaction was carried out in the same manner as in Example 3 except that the reaction time was changed to 96 hours, and the yield was calculated. The results are shown in Table 1.

[0067] [Example 7] The depolymerization reaction was carried out in the same manner as in Example 4, except that the amount of the raw material powder was changed to 2 g (10.4 mmol) and the amount of the catalyst was changed to 205 mg (30 mol % based on the raw material PET), and the yield was calculated. The results are shown in Table 1.

[0068] [Example 8] A depolymerization reaction was carried out in the same manner as in Example 4, except that the amount of the catalyst was changed to 240 mg (140 mol % based on the raw material PET), and the yield was calculated. The results are shown in Table 1.

[0069] [Table 1]

[0070] <Depolymerization of polyester using a cocatalyst> [Example 9] Furthermore, a depolymerization reaction was carried out in the same manner as in Example 4, except that 57 mg (10 mol % based on the raw material PET) of 15-crown-5 (manufactured by Tokyo Chemical Industry Co., Ltd.) was added as a cocatalyst, and the yield was calculated. The results are shown in Table 2. For the sake of clarity, Example 4 from Table 1 is transcribed in Table 2. Similarly, in Tables 2 and onwards, there are some places where examples used in other tables are transcribed.

[0071] [Example 10] Furthermore, a depolymerization reaction was carried out in the same manner as in Example 4, except that 35 mg (10 mol % based on the raw material PET) of diglyme (manufactured by Kanto Chemical Co., Ltd.) was added as a cocatalyst, and the yield was calculated. The results are shown in Table 2.

[0072] [Example 11] Furthermore, a depolymerization reaction was carried out in the same manner as in Example 4, except that 1 mL (6.98 mmol, 269 mol % based on the raw material PET) of diglyme (manufactured by Kanto Chemical Co., Ltd.) was added as a cocatalyst, and the yield was calculated. The results are shown in Table 2.

[0073] [Example 12] Furthermore, a depolymerization reaction was carried out in the same manner as in Example 4, except that 50 mg (10 mol % based on the raw material PET) of tetraethylene glycol (manufactured by Tokyo Chemical Industry Co., Ltd.) was added as a cocatalyst, and the yield was calculated. The results are shown in Table 2.

[0074] [Example 13] Furthermore, a depolymerization reaction was carried out in the same manner as in Example 4, except that 72 mg (10 mol % based on the raw material PET) of tetrabutylammonium chloride (TBAC, manufactured by Tokyo Chemical Industry Co., Ltd.) was added as a cocatalyst, and the yield was calculated. The results are shown in Table 2.

[0075] [Example 14] Furthermore, a depolymerization reaction was carried out in the same manner as in Example 4, except that 59 mg (10 mol % based on the raw material PET) of triethylbenzylammonium chloride (TEBA, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) was added as a cocatalyst, and the yield was calculated. The results are shown in Table 2.

[0076] [Example 15] Furthermore, a depolymerization reaction was carried out in the same manner as in Example 4, except that 43 mg (10 mol % based on the raw material PET) of potassium iodide (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) was added as a cocatalyst, and the yield was calculated. The results are shown in Table 2.

[0077] [Table 2]

[0078] <Depolymerization of polyester using different catalysts> [Example 16] The catalyst was changed to borax anhydride (Na2B4O7, manufactured by Tiangen Aerospace Materials (Yingkou) Technology Co., Ltd., China) and the amount used was changed to 52 mg (10 mol% based on the raw material PET). Except for this, the depolymerization reaction was carried out in the same manner as in Example 4, and the yield was calculated. The results are shown in Table 3.

[0079] [Example 17] The depolymerization reaction was carried out in the same manner as in Example 4, except that the catalyst was changed to sodium borohydride (NaBH4, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and the amount used was changed to 10 mg (10 mol% based on the raw material PET), and the yield was calculated. The results are shown in Table 3.

[0080] [Example 18] The depolymerization reaction was carried out in the same manner as in Example 4, except that the catalyst was changed to lithium metaborate dihydrate (LiBO2·2H2O, manufactured by Sigma-Aldirch Japan LLC) and the amount used was changed to 22 mg (10 mol % based on the raw material PET), and the yield was calculated. The results are shown in Table 3.

[0081] [Example 19] The depolymerization reaction was carried out in the same manner as in Example 4, except that the catalyst was changed to potassium metaborate tetrahydrate (KBO 4 / 3H O, manufactured by Kanto Chemical Co., Inc.) in an amount of 28 mg (10 mol % based on the raw material PET), and the yield was calculated. The results are shown in Table 3.

[0082] [Example 20] The catalyst was changed to potassium metaborate tetrahydrate (KBO 4 / 3H O, manufactured by Kanto Chemical Co., Inc.) in an amount of 28 mg (10 mol % based on the raw material PET) and the reaction temperature was changed to 75°C. Except for this, a depolymerization reaction was carried out in the same manner as in Example 4, and the yield was calculated. The results are shown in Table 3.

[0083] [Comparative Example 1] The depolymerization reaction was carried out in the same manner as in Example 4, except that the catalyst was changed to a solid base catalyst, cerium oxide (CeO, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), and the amount used was changed to 45 mg (10 mol % based on the raw material PET), and the yield was calculated. The results are shown in Table 3.

[0084] [Comparative Example 2] The depolymerization reaction was carried out in the same manner as in Example 4, except that the catalyst was changed to a solid base catalyst, magnesium oxide (MgO, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), and the amount used was changed to 10 mg (10 mol % based on the raw material PET), and the yield was calculated. The results are shown in Table 3.

[0085] [Comparative Example 3] The depolymerization reaction was carried out in the same manner as in Example 4, except that the catalyst was changed to an acid catalyst, boric acid (HBO, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), and the amount used was changed to 16 mg (10 mol % based on the raw material PET), and the yield was calculated. The results are shown in Table 3.

[0086] [Comparative Example 4] The depolymerization reaction was carried out in the same manner as in Example 4, except that the catalyst was changed to sodium hypochlorite pentahydrate (NaClO 5H2O, manufactured by Nippon Light Metal Co., Ltd.) and the amount of the catalyst was changed to 43 mg (10 mol % based on the raw material PET), and the yield was calculated. The results are shown in Table 3.

[0087] [Table 3]

[0088] <Depolymerization reaction using flakes> [Example 21] A depolymerization reaction was carried out in the same manner as in Example 4, except that 500 mg (2.6 mmol) of the flakes obtained in Production Example 1 were used as the PET raw material, and the yield was calculated. The results are shown in Table 4.

[0089] [Example 22] A depolymerization reaction was carried out in the same manner as in Example 4, except that 500 mg (2.6 mmol) of the flakes obtained in Production Example 1 were used as the PET raw material and the reaction temperature was set to 75° C., and the yield was calculated. The results are shown in Table 4.

[0090] [Example 23] A depolymerization reaction was carried out in the same manner as in Example 4, except that 500 mg (2.6 mmol) of the flakes obtained in Production Example 1 were used as the PET raw material and the reaction temperature was set to 25° C., and the yield was calculated. The results are shown in Table 4.

[0091] [Example 24] A depolymerization reaction was carried out in the same manner as in Example 20, except that 500 mg (2.6 mmol) of the flakes obtained in Production Example 1 were used as the PET raw material, and the yield was calculated. The results are shown in Table 4.

[0092] [Example 25] A depolymerization reaction was carried out in the same manner as in Example 20 except that 500 mg (2.6 mmol) of the flakes obtained in Production Example 1 were used as the PET raw material and the reaction temperature was set to 25° C., and the yield was calculated. The results are shown in Table 4.

[0093] [Reference example 1] The depolymerization reaction was carried out in the same manner as in Example 24, except that the catalyst was changed to potassium bicarbonate (KHCO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and the amount used was changed to 26 mg (10 mol% based on the raw material PET), and the yield was calculated. The results are shown in Table 4.

[0094] [Reference example 2] The depolymerization reaction was carried out in the same manner as in Example 24, except that the catalyst was changed to potassium carbonate (K2CO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and the amount used was 36 mg (10 mol % based on the raw material PET), and the yield was calculated. The results are shown in Table 4.

[0095] [Table 4]

[0096] <Catalyst recovery and reuse> [Example 26] The solid matter recovered from the filtration in Example 21 was used to carry out the same depolymerization reaction as in Example 21. The raw materials used were the same. No new catalyst was added at this time. Thereafter, filtration was carried out in the same manner, and the yield of dimethyl terephthalate was calculated by carrying out GC analysis. The results are shown in Table 5. For the sake of clarity, Example 21 described in Table 4 is transcribed in Table 5.

[0097] [Example 27] A depolymerization reaction using borax was carried out as reaction 27-(1). That is, the catalyst was changed to borax anhydride (Na2B4O7, manufactured by Tiangen Aerospace Materials (Yingkou) Technology Co., Ltd., China), and the amount of the catalyst was changed to 26 mg (5 mol% based on the raw material PET), but the depolymerization reaction was carried out in the same manner as in Example 21, and the yield was calculated. After reaction 27-(1), the suspension in the test tube was filtered (recovered) after removing unreacted polyethylene terephthalate. Next, the solid matter recovered from the filtration was used to carry out a depolymerization reaction similar to that in Reaction 27-(1). The raw materials used were the same, and this reaction is designated as Reaction 27-(2). No new catalyst was added at this time. After that, filtration was carried out in the same manner, and the yield of dimethyl terephthalate was calculated by performing GC analysis. The results of reactions 27-(1) and 27-(2) are shown in Table 5.

[0098] [Example 28] A depolymerization reaction using potassium metaborate was carried out as reaction 28-(1). That is, the catalyst was changed to potassium metaborate tetrahydrate (KBO2·4 / 3H2O, manufactured by Kanto Chemical Co., Ltd.), the amount of the catalyst was changed to 28 mg (10 mol% based on the raw material PET), and the reaction temperature was changed to 75°C. The depolymerization reaction was carried out in the same manner as in Example 21, and the yield was calculated. In addition, in this reaction 28-(1), after the depolymerization reaction, the reaction solution was cooled to room temperature, water was added, and the mixture was stirred for 20 minutes. After confirming the precipitation of a solid, the mixture was filtered. The subsequent treatment and GC analysis of the filtrate were the same. Next, the solid matter recovered from the filtration was used to carry out a depolymerization reaction similar to that in Reaction 28-(1). The raw materials used were the same, and this reaction is designated as Reaction 28-(2). No new catalyst was added at this time. After that, filtration was carried out in the same manner, and the yield of dimethyl terephthalate was calculated by performing GC analysis. The results of reactions 28-(1) and 28-(2) are shown in Table 5.

[0099] [Reference example 3] A depolymerization reaction using sodium methoxide was carried out as reaction 3-(1). That is, the catalyst was changed to sodium methoxide (NaOMe, manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.), and the amount used was changed to 14 mg (10 mol% relative to the raw material PET). Except for this, a depolymerization reaction was carried out in the same manner as in Example 21, and the yield was calculated. After reaction 3-(1), the suspension in the test tube, from which unreacted polyethylene terephthalate had been removed, was filtered (recovered). Next, the solid matter recovered from the filtration was used to carry out a depolymerization reaction similar to that in Reaction 3-(1). The raw materials used were the same, and this reaction is called Reaction 3-(2). No new catalyst was added at this time. After that, filtration was carried out in the same manner, and the yield of dimethyl terephthalate was calculated by performing GC analysis. The results of reactions 3-(1) and 3-(2) are shown in Table 5.

[0100] [Table 5]

[0101] As can be seen from the results in Tables 1 to 5, it is possible to advance the depolymerization reaction of polyester by using the borate compound according to the present invention as a catalyst. In addition, it is possible to confirm that the catalyst made of the borate compound used can be recovered (including re-recovery) as a solid content containing the catalyst, and that the recovered solid content can be used to carry out the depolymerization reaction of polyester again.

Claims

1. 1. A method for depolymerizing a polyester, comprising: The method includes the steps of reacting a polyester in the presence of a catalyst and a solvent to depolymerize the polyester, The method for depolymerizing polyester, wherein the catalyst is a borate compound.

2. 2. The method for depolymerizing polyester according to claim 1, wherein the borate compound comprises an alkali metal borate compound and / or an alkaline earth metal borate compound.

3. 2. The method for depolymerizing polyester according to claim 1, wherein the solvent contains a carbonate diester and / or an alcohol solvent as a glycol scavenger.

4. The method for depolymerizing a polyester according to claim 1, further comprising a cocatalyst in addition to the catalyst.

5. 5. The method for depolymerizing a polyester according to claim 4, wherein the cocatalyst is at least one selected from the group consisting of cyclic or acyclic ethers, glycols, quaternary ammonium salts, and alkali metal salts.

6. The method for depolymerizing polyester according to any one of claims 1 to 5, further comprising a step of recovering a solid content containing the catalyst used after the polyester is reacted and depolymerized.

7. 7. The method for depolymerizing polyester according to claim 6, wherein the solid matter recovered by the method for depolymerizing polyester is used as a catalyst to react with the polyester in the presence of the solvent.

Images