How to decompose polypropylene

Abstract

To provide a method for decomposing polypropylene, particularly polypropylene, that does not require harsh reaction conditions that raise safety concerns, does not necessitate the use of special equipment, technology, or materials, and can fully decompose polypropylene, while also allowing the use of polypropylene waste. [Solution] A method for decomposing polypropylene, comprising: a first step of modifying the polypropylene using one or more selected from the group consisting of concentrated sulfuric acid, orthophosphoric acid, and pyrophosphoric acid to obtain a modified polypropylene product; and a second step of heat-treating the modified product with an oxidizing agent to obtain a decomposed product of the modified product.

Description

[Technical Field]

[0001] This invention relates to a method for decomposing polypropylene. [Background technology]

[0002] The decomposition of polyolefins is a very important process from the perspective of resource recycling, and the generation of electrical energy through the combustion of polyolefins is well known as one of the methods for solving the marine plastic waste problem.

[0003] Polyolefins are typically very chemically stable because their main chain is composed of a stable saturated hydrocarbon backbone, and their decomposition requires a great deal of energy. Conventional methods for decomposing polyolefins include, for example, a method of decomposing polyolefins under high temperature conditions of 300°C or higher using a high-concentration aqueous hydrogen peroxide solution (see Non-Patent Document 1), and a method of accelerating the degradation of polyolefins by using transition metal stearates in an air atmosphere (see Non-Patent Document 2).

[0004] Conventional methods for decomposing polyolefins also include a method for decomposing polyethylene into carboxylic acids by using a combination of hot nitric acid and microwave irradiation (see Non-Patent Document 3), and a method for depolymerizing polypropylene into propylene using ion-inductively coupled plasma (see Non-Patent Document 4).

[0005] Conventional methods for decomposing polyolefins include a method of decomposing polyisobutylene by oxidation using a ruthenium catalyst and an oxidizing agent (see Non-Patent Document 5), and a method of oxidative decomposition by performing stepwise hydrogen peroxide treatment and hydrogen peroxide decomposition on polyolefins (see Patent Document 1).

[0006] On the other hand, as a method for recycling polypropylene among polyolefins, a method has been disclosed in which polypropylene is sulfonated at a temperature of 130 to 170°C and then carbonized to produce a carbon material (see Non-Patent Document 6). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2023-144614 [Non-patent literature]

[0008] [Non-Patent Document 1] H. Zhong and co-workers, IOP Conf. Series: Earth and Environmental Science 2020, 450, 012049. [Non-Patent Document 2] PK Roy and co-workers, J. Appl. Polym. Sci. 2010, 117, 524-533. [Non-Patent Document 3] M. Hakkarainen and co-workers, Ind. Eng. Chem. Res. 2017, 56, 14814-14821. [Non-Patent Document 4] R. Knight and co-workers, Ind. Eng. Chem. Res. 2000, 39, 1171-1176. [Non-Patent Document 5] JF Hartwig and co-workers, J. Am. Chem. Soc. 2021, 143, 4531-4535. [Non-Patent Document 6] Z. Qiang and co-workers, Adv. Mater. 2023, 35, 2208029. [Overview of the Initiative]

Problems to be Solved by the Invention

[0009] However, the method disclosed in Non-Patent Document 1 has a problem in that it requires very strict reaction conditions. Further, high-concentration oxygen and heated hydrogen peroxide with a concentration of 60% by mass or more are explosive, so there is a problem in that the safety is low.

[0010] In the method disclosed in Non-Patent Document 2, although the reaction conditions are relatively mild, it requires a long time of more than half a month, and further, there is a problem in that it only remains at the surface deterioration of the polyolefin and does not reach the essential decomposition.

[0011] In the method disclosed in Non-Patent Document 3, a microwave irradiation device is required, and in the method disclosed in Non-Patent Document 4, an inductively coupled plasma generator is required. In either case, a special device is required, and further, in these methods, there is a problem in that the use of highly corrosive reagents is required.

[0012] In the method disclosed in Non-Patent Document 5, although the reaction temperature is 100°C or lower and the reaction conditions are mild, there is a problem in that the synthesis of the complex catalyst requires advanced technology and expensive raw materials. Further, most of the polyisobutylene only remains at the functionalization of its surface and does not reach the essential decomposition.

[0013] In the method disclosed in Patent Document 1, since a high temperature of about 110 to 130°C is required during the oxidation reaction, there is a concern about the explosion of hydrogen peroxide. Further, since it requires a long time of about 45 hours, there is a problem in that the energy consumption is large. Further, when the polyolefin to be decomposed is a waste product of polyolefin, unlike the case of pure products, it contains many impurities, and hydrogen peroxide is affected by these impurities, so there is a problem in that the first-stage hydrogen peroxide treatment does not proceed sufficiently.

[0014] Non-patent document 6 discloses a method for producing carbon materials using polypropylene, not a method for decomposing polypropylene.

[0015] As described above, conventional methods for decomposing polyolefins have problems such as: harsh conditions for oxidation reactions; mild reaction conditions that do not lead to the essential decomposition of polyolefins; the need for special equipment, technology, or materials; or the inability to use waste polyolefins.

[0016] The present invention aims to provide a method for decomposing polypropylene, particularly polypropylene, that does not require harsh reaction conditions that raise safety concerns, does not necessitate the use of special equipment, techniques, or materials, and can fully decompose polypropylene, while also allowing the use of polypropylene waste. [Means for solving the problem]

[0017] To solve the above problems, the present invention adopts the following configuration. [1] A method for decomposing polypropylene, The aforementioned decomposition method comprises a first step of obtaining a modified polypropylene by modifying the polypropylene using one or more substances selected from the group consisting of concentrated sulfuric acid, orthophosphoric acid, and pyrophosphoric acid, A method for decomposing polypropylene, comprising a second step of obtaining decomposed products of the modified material by heat-treating the modified material with an oxidizing agent. [2] The method for decomposing polypropylene according to [1], wherein in the second step, the modified product is further heat-treated using a transition metal catalyst. [3] The transition metal catalyst is one or more selected from the group consisting of a transition metal salt of an inorganic acid, cobalt oxide, cobalt acetate, a cluster-type cobalt catalyst introduced into a polyoxoacid, and a composite of the salt and oxoate. The inorganic acid is nitric acid, carbonic acid, phosphoric acid, hydrogen chloride, sulfuric acid, silicic acid, boric acid, tungstic acid, phosphotungstic acid hydrate, silicatungstic acid hydrate, molybdic acid, phosphomolybdic acid hydrate, silicamolybdic acid hydrate, titanic acid, or aluminic acid. The transition metal is copper, silver, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zinc, molybdenum, tungsten, zirconium, niobium, technetium, ruthenium, palladium, silver, or cadmium. The method for decomposing polypropylene according to [2], wherein the oxo salt is a phosphate, borate, aluminate, silicate, tungstate, phosphotungstate, or silicatungstate.

[0018] [4] The method for decomposing polypropylene according to any one of [1] to [3], wherein the oxidizing agent is one or more selected from the group consisting of hydrogen peroxide, peracid, peroxomonosulfate, peroxodisulfate, perborate, urea-hydrogen peroxide, and sodium peroxide carbonate adduct. [5] A method for decomposing polypropylene according to any one of [1] to [4], wherein at least malonic acid is obtained as the decomposition product in the second step. [6] The method for decomposing polypropylene according to any one of [1] to [5], wherein in the second step, the modified material is heat-treated at a temperature of 90°C or lower for 24 hours or less. [Effects of the Invention]

[0019] According to the present invention, a method for decomposing polypropylene, particularly polypropylene among polyolefins, is provided that does not require harsh reaction conditions that raise safety concerns, does not necessitate the use of special equipment, techniques, or materials, and can sufficiently decompose polypropylene, and can utilize polypropylene waste. [Brief explanation of the drawing]

[0020] [Figure 1] This is the 1H NMR spectral data of the degradation product obtained in Example 1. [Modes for carrying out the invention]

[0021] In this specification, the unit of concentration "M" means "mol / L", and "mM" means "mmol / L".

[0022] ◇ How to decompose polypropylene A method for decomposing polypropylene according to one embodiment of the present invention comprises a first step of obtaining a modified polypropylene by modifying the polypropylene with one or more acids selected from the group consisting of concentrated sulfuric acid, orthophosphoric acid, and pyrophosphoric acid (in this specification, these acids may be collectively referred to as "oxoacid (A)"), and a second step of obtaining a decomposed product of the modified product by heat-treating the modified product with an oxidizing agent.

[0023] According to the decomposition method of this embodiment, by subjecting the modified polypropylene obtained in the first step to a heat treatment using an oxidizing agent (i.e., an oxidation reaction) in the second step, the polypropylene can be sufficiently decomposed without having to make the oxidation reaction conditions stricter in the second step. Furthermore, according to the decomposition method of this embodiment, by employing the first and second steps, polypropylene can be sufficiently decomposed without using special equipment, technology, or materials, and even when using polypropylene waste as described later.

[0024] <<First step>> In the first step, polypropylene is modified using one or more oxoacids (A), specifically selected from the group consisting of concentrated sulfuric acid (conc. H2SO4), orthophosphoric acid (H3PO4), and pyrophosphoric acid (H4P2O7, also known as diphosphoric acid), to obtain a modified polypropylene product. The modified product is suitable for decomposition in the subsequent second step.

[0025] <Polypropylene> The polypropylene used in the first step may be a known type, for example, homopolypropylene (hPP), which is a homopolymer of propylene, or a propylene copolymer having both structural units derived from propylene and structural units derived from monomers other than propylene. Preferred propylene copolymers include, for example, polypropylene random copolymers (rPP, propylene random copolymer) such as propylene-ethylene binary copolymer, propylene-1-butene-ethylene ternary copolymer, and propylene-1-butene binary copolymer; and polypropylene block copolymer (bPP, propylene block copolymer). Among these, polypropylene is preferably homopolypropylene.

[0026] The weight-average molecular weight (Mw) of polypropylene is preferably 2,000 to 400,000, more preferably 8,000 to 200,000, and even more preferably 10,000 to 100,000. The number-average molecular weight (Mn) of polypropylene is preferably 500 to 100,000, more preferably 1,000 to 50,000, and even more preferably 3,000 to 10,000. Polypropylene can be decomposed more efficiently if its weight-average molecular weight or number-average molecular weight is above the aforementioned lower limit. On the other hand, polypropylene with a weight-average molecular weight or number-average molecular weight below the aforementioned upper limit is more readily available. In particular, the amount of waste generated from such polypropylene is large.

[0027] In this specification, unless otherwise specified, weight-average molecular weight and number-average molecular weight are polystyrene-converted values ​​measured by gel permeation chromatography (GPC).

[0028] The form of polypropylene used in the first step is not particularly limited. For example, polypropylene may be in the form of molded or unmolded articles, but it is preferable that it be in the form of small pieces, pellets, or powder.

[0029] The polypropylene used in the first step may be, for example, a polypropylene product (a product made of polypropylene), a derivative of the said polypropylene product, a polypropylene component (a component made of polypropylene), or a derivative of the said polypropylene component (hereinafter, these may be collectively referred to as "polypropylene products, etc."). The use of these polypropylene products, etc. is extremely important from the standpoint of resource recycling.

[0030] Examples of the source material for the polypropylene product include processed products of the polypropylene product, preferably in the form of small pieces, pellets, or powder, and may also be crushed polypropylene products. Similarly, examples of materials derived from the polypropylene member include processed polypropylene members, preferably in the form of small pieces, pellets, or powders, and may also be crushed polypropylene members.

[0031] The polypropylene used in the first step may be of one type or two or more types, and if two or more types are used, their combination and ratio can be arbitrarily selected according to the purpose. For example, polypropylene waste may be a mixture of two or more types of polypropylene, and the decomposition method of this embodiment is also suitable for decomposing such polypropylene waste.

[0032] Although the structure of the modified product is not certain, it is presumed that the modified product has a structure in which polypropylene is functionalized by oxoacid (A) depending on the type of oxoacid (A), and furthermore, at least a portion of the functionalized sites are dehydrogenated. Furthermore, it is also presumed that the modified product may have a structure in which these dehydrogenated molecules are bonded together. For example, Non-Patent Literature 6 discloses, as described above, the sulfonation of polypropylene at a temperature of 130 to 170°C, followed by carbonization. More specifically, it discloses that sulfonation of polypropylene under heating conditions dehydrogenates the sulfonated sites of polypropylene, and furthermore, that the dehydrogenated molecules bond together. For example, if concentrated sulfuric acid is used in the first step of this embodiment, a similar reaction may proceed in polypropylene, and a similar product may be produced as the modified product.

[0033] <Oxoacid (A)> In the first step, if two or more oxoacids (A) are used in combination, their combination and ratio can be arbitrarily selected according to the purpose. In the first step, it is preferable to use at least concentrated sulfuric acid as the oxoacid (A), and concentrated sulfuric acid alone may also be used. Doing so will result in a higher degree of modification of the polypropylene.

[0034] In the first step, the amount of oxo acid (A) used is preferably 1 to 10 mL per 1 g of polypropylene used, and may be, for example, 1 to 8 mL, 1 to 6 mL, or 1 to 4 mL. If the amount of concentrated sulfuric acid used is above the lower limit, the desired reaction will proceed more easily, and the degree of modification of the polypropylene will be higher. If the amount of concentrated sulfuric acid used is below the upper limit, the use of concentrated sulfuric acid will be suppressed, and as a result, for example, the generation of carbon dioxide associated with excessive modification can be suppressed.

[0035] In this specification, the amount of oxoacid (A) used refers to the total amount of concentrated sulfuric acid, orthophosphoric acid, and pyrophosphoric acid used. If any of concentrated sulfuric acid, orthophosphoric acid, or pyrophosphoric acid is not used, the amount of oxoacid (A) used for that unused component is considered to be 0 parts by mass.

[0036] <Other ingredients (i)> In the first step, other components that do not fall under either polypropylene or oxoacid (A) (referred to as "other component (i)" in this specification) may or may not be used, as long as they do not impair the effects of the present invention.

[0037] The other component (i) used in the first step may be one type or two or more types, and if there are two or more types, their combination and ratio can be arbitrarily selected according to the purpose.

[0038] [Metal catalyst] Other preferred components (i) include, for example, metal catalysts for promoting the modification of polypropylene. The aforementioned metal catalyst is a catalyst that includes a metal element as a constituent element, and may contain only a metal element, or it may contain both a metal element and a nonmetal element. The metal catalyst may be either an anhydrous or a hydrated form.

[0039] Examples of the aforementioned metal catalyst include metal salts and metal oxides. Examples of the aforementioned metal salts include metal salts of inorganic acids and metal salts of organic acids. Inorganic acid metal salts are preferred because they are inexpensive and readily available.

[0040] The metal catalyst is preferably a transition metal catalyst (sometimes referred to as "transition metal catalyst (i)" in this specification), in which the metal species is a transition metal. Examples of transition metal catalysts (i) include transition metal salts and transition metal oxides. Examples of the transition metal salts include transition metal salts of inorganic acids and transition metal salts of organic acids. Inorganic acid transition metal salts are preferred because they are inexpensive and readily available.

[0041] The transition metal species of the transition metal catalyst (i) is preferably cobalt (i) (the transition metal catalyst (i) is a cobalt catalyst).

[0042] Preferred cobalt catalysts include, for example, cobalt salts such as cobalt(II) sulfate (CoSO4), cobalt(II) nitrate (Co(NO3)2), and cobalt(II) phosphate (Co3(PO4)2); and cobalt oxides such as cobalt(II) oxide (CoO), cobalt(III) oxide (Co2O3), and cobalt(II,III) oxide (Co3O4).

[0043] Among metal catalysts, those that exist as metal salts and have hydrates may be anhydrous metal salts or metal salt hydrates. Among the aforementioned metal salt hydrates, an example of a transition metal salt hydrate is cobalt(II) sulfate hydrate (CoSO4·x 1 H2O(x 1 (where is an integer greater than or equal to 1), for example, cobalt sulfate heptahydrate (CoSO4·7H2O), cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O), cobalt(II) phosphate hydrate (Co3(PO4)2·x 2 H2O(x 2 (where is an integer greater than or equal to 1), for example, cobalt salt hydrates such as cobalt(II) phosphate octahydrate (Co3(PO4)2·8H2O)).

[0044] In the first step, the amount of metal catalyst used is preferably 0.01 to 1 mmol per 1 g of polypropylene used, and may be, for example, 0.01 to 0.8 mmol, 0.01 to 0.6 mmol, or 0.01 to 0.45 mmol. When the amount of metal catalyst used is above the lower limit, the effect obtained by using the metal catalyst is enhanced. When the amount of metal catalyst used is below the upper limit, excessive use of the metal catalyst is suppressed.

[0045] [Product or component-derived ingredients] Other components (i) include, for example, components other than polypropylene that do not contribute to promoting the modification of polypropylene and are therefore undesirable. Examples of such components include, for example, components other than polypropylene in polypropylene-containing products (products containing polypropylene and components other than polypropylene) or polypropylene-containing members (members containing polypropylene and components other than polypropylene). Other components besides polypropylene include, for example, various additives blended into the polypropylene product or polypropylene component to improve its properties; and components of other components provided in the polypropylene composite product, polypropylene composite component, polypropylene product, or polypropylene component. The components other than polypropylene are usually blended together with polypropylene in the first step.

[0046] More specifically, components other than polypropylene include, for example, inorganic fibers such as carbon fibers, glass fibers, and alumina fibers; wood-based materials; and polymer compounds other than polypropylene, such as polyvinyl chloride, polyethylene, ethylene copolymers (polymers having structural units derived from ethylene and structural units derived from monomers other than ethylene), polystyrene, polyurethane, polyamide, polyimide polyester, and acrylic resins (polymers having structural units derived from alkyl methacrylate).

[0047] In other words, in the first step, one or more combinations of polypropylene and other components (i) can be selected from the group consisting of the polypropylene composite product, the polypropylene composite product, the polypropylene composite member, and the polypropylene composite member (hereinafter, these may be collectively referred to as "polypropylene composite products, etc."). The use of these polypropylene composite products, etc. is extremely important from the standpoint of resource recycling.

[0048] Examples of the source material for the polypropylene composite product include processed products of the polypropylene composite product, preferably in the form of small pieces, pellets, or powders, and may also be crushed polypropylene composite products. Similarly, examples of the source material for the polypropylene composite member include processed products of the polypropylene composite member, preferably in the form of small pieces, pellets, or powders, and may also be crushed polypropylene composite members.

[0049] On the other hand, when using waste materials as the aforementioned polypropylene products or polypropylene-combined products, impurities such as degraded materials and contaminants may be attached to these waste materials. In this embodiment, these impurities are also listed as other components (i) (components other than polypropylene).

[0050] In this specification, components other than polypropylene as described above may be collectively referred to as "product or component-derived components."

[0051] In the decomposition method of this embodiment, even if the product or component-derived components are introduced directly into the reaction system without being separated from the polypropylene and the first step is performed, the modification of the polypropylene is not adversely affected. For example, in the polyolefin decomposition method disclosed in Patent Document 1, the polyolefin is treated with hydrogen peroxide in the first step. However, if waste polyolefin is used, impurities attached to the waste material can adversely affect the hydrogen peroxide treatment. In contrast, in the decomposition method of this embodiment, oxoacid (A) is used instead of hydrogen peroxide in the first step, so this problem can be avoided even if the aforementioned product or component-derived components are present. Furthermore, the aforementioned product or component-derived components are not adversely affected in the second step by the modification treatment with oxoacid (A) in the first step.

[0052] <Other conditions> In the first step, the temperature (reaction temperature, modification temperature) when reacting polypropylene with oxo acid (A) is preferably 100 to 300°C, more preferably 125 to 250°C, and even more preferably 150 to 225°C. A reaction temperature (modification temperature) above the lower limit results in a higher degree of modification of the polypropylene. A reaction temperature below the upper limit prevents the reaction temperature from becoming excessively high, thereby reducing energy consumption.

[0053] In the first step, the time (reaction time, modification time) for reacting polypropylene with oxo acid (A) is preferably 5 to 120 hours, more preferably 30 to 90 hours, and even more preferably 60 to 80 hours. A reaction time (modification time) above the lower limit results in a higher degree of modification of the polypropylene. A reaction temperature below the upper limit prevents the reaction time from becoming excessively long, thereby reducing energy consumption.

[0054] In the first step, it is preferable that both the reaction temperature and the reaction time are within one of the numerical ranges described above.

[0055] The first step may be carried out in an air atmosphere, or in an atmosphere of a gas other than air, such as an inert gas.

[0056] <<Second process>> In the second step, the modified material obtained in the first step is heat-treated with an oxidizing agent to obtain decomposition products of the modified material. The decomposition products of the modified material are the final products of polypropylene obtained through its decomposition, i.e., the final decomposition products. The modified material decomposes easily in the second step.

[0057] <Oxidizing agent> Examples of oxidizing agents used in the second step include hydrogen peroxide (H2O2), peracids, peroxomonosulfate, peroxodisulfate, perborate (peroxoborate), urea-hydrogen peroxide (also known as a mixture of hydrogen peroxide and urea, UHP), and sodium peroxide carbonate adduct (Na2CO3·1.5H2O2).

[0058] For example, the polyolefin decomposition method disclosed in Non-Patent Document 5 had the problem that the atomic efficiency of the oxidizing agent used was extremely low. However, in the second step of this embodiment, an oxidizing agent with good atomic efficiency can be selected.

[0059] Examples of the aforementioned peracids include inorganic peracids such as superphosphate; and organic peracids such as performic acid (also known as peroxymethane acid, HCOOOH), peracetic acid (CH3COOOH), and m-chloroperbenzoic acid (mCPBA, ClC6H4COOOH) (e.g., percarboxylic acids). Examples of the peroxomonosulfate include alkali metal peroxomonosulfates such as sodium peroxomonosulfate (Na2SO5) and potassium peroxomonosulfate (K2SO5); and ammonium peroxomonosulfate ((NH4)2SO5). Examples of the peroxodisulfate include alkali metal peroxodisulfates such as sodium peroxodisulfate (Na2S2O8) and potassium peroxodisulfate (K2S2O8); and ammonium peroxodisulfate ((NH4)2S2O8). Examples of the aforementioned perborate salts include sodium perborate tetrahydrate (also known as sodium peroxoborate tetrahydrate, NaBO3·4H2O), potassium perborate tetrahydrate (also known as potassium peroxoborate tetrahydrate, KBO3·4H2O), and ammonium perborate tetrahydrate (also known as ammonium peroxoborate tetrahydrate, NH4BO3·4H2O).

[0060] The oxidizing agent used in the second step may be one type or two or more types, and if there are two or more types, their combination and ratio can be arbitrarily selected according to the purpose.

[0061] In terms of being stable at low temperatures of 200°C or below, and especially 150°C or below, it is preferable that the oxidizing agent used in the second step be one or more selected from the group consisting of hydrogen peroxide, peracid, peroxomonosulfate, peroxodisulfate, perborate, urea-hydrogen peroxide, and sodium peroxide carbonate adduct. In particular, hydrogen peroxide is more preferable as the oxidizing agent used in the second step because it is inexpensive and has high atomic efficiency.

[0062] In the second step, the amount of oxidizing agent used is not particularly limited as long as it can sufficiently oxidize and decompose the modified product, but it is preferably 0.01 to 0.2 mmol per 1 g of the modified product used, and may be, for example, 0.01 to 0.17 mmol, 0.01 to 0.12 mmol, or 0.01 to 0.07 mmol. If the amount of oxidizing agent used is above the lower limit, the amount of decomposition of the modified product will increase further. If the amount of oxidizing agent used is below the upper limit, excessive use of the oxidizing agent will be suppressed.

[0063] In the second step, the concentration of the oxidizing agent in the reaction solution (the ratio of the amount of oxidizing agent in the reaction solution (moles) to the volume of the reaction solution (L)) is not particularly limited, but is preferably 1 to 10 M. When the concentration is above the lower limit, the decomposition of the modified product proceeds more efficiently. When the concentration is below the upper limit, the safety of the second step is increased.

[0064] In the second step, when using an aqueous hydrogen peroxide solution as the solution containing an oxidizing agent, the concentration of hydrogen peroxide in the aqueous hydrogen peroxide solution is preferably 3 to 50% by mass, more preferably 3 to 35% by mass. For example, those with a concentration of 30 to 35% by mass are easily available as commercial products. When the concentration of hydrogen peroxide is at least the lower limit value, the decomposition of the modified product proceeds more efficiently. When the concentration of hydrogen peroxide is at most the upper limit value, the safety of the second step is higher.

[0065] <Transition metal catalyst (ii)> In the second step, it is preferable to heat-treat the modified product using a transition metal catalyst (which may be referred to as "transition metal catalyst (ii)" in this specification). Even if the modified product is heat-treated without using the transition metal catalyst (ii), the modified product decomposes, but by using the transition metal catalyst (ii), the decomposition amount of the modified product increases.

[0066] Examples of the transition metal catalyst (ii) used in the second step include salts of transition metals of inorganic acids, cobalt oxides, cobalt acetate ((CH3COO)2Co), cluster-type cobalt catalysts introduced into polyoxo acids, composites of the salts (salts of transition metals of inorganic acids) and oxoacid salts, and the like.

[0067] The inorganic acid constituting the salt as the transition metal catalyst (ii) may be either an anhydride or a hydrate. Examples of the inorganic acid include nitric acid (HNO3), carbonic acid (H2CO3), phosphoric acid, hydrogen chloride (HCl), sulfuric acid (H2SO4), silicic acid, boric acid (also known as orthoboric acid, H3BO3), tungstic acid (H2WO4), phosphotungstic acid hydrate (H3PW 12 O 40 ·x 3 H2O (x 3 is an integer of 1 or more)), silicotungstic acid hydrate (H4SiW 12 O 40 ·x 4 H2O (x 4(where is an integer greater than or equal to 1), molybdic acid (H2MoO4), phosphomolybdic acid hydrate (H3PMo 12 O 40 ·x 5 H2O(x 5 (is an integer greater than or equal to 1), silicic acid hydrate (H4SiMo 12 O 40 ·x 6 H2O(x 6 Examples include titanium dioxide (where is an integer greater than or equal to 1), titanic acid, aluminic acid, etc. The aforementioned phosphoric acid includes orthophosphate (H3PO4) and metaphosphate ((HPO3) p Examples include (where p is an integer greater than or equal to 3). For instance, when p is 3, the cyclic metaphosphate is cyclotrimetaphosphate. The aforementioned silicic acids include orthosilicic acid (Si(OH)4) and metasilicic acid ((H2SiO3)). q Examples include (where q is an integer greater than or equal to 1), and disilicate (H2Si2O5). Examples of the titanic acid mentioned above include orthotitanic acid (Ti(OH)4) and metatitanic acid (H2TiO3). Examples of the aluminic acid mentioned above include orthoaluminic acid (H3AlO3) and metaaluminic acid (HAlO2).

[0068] The transition metal constituting the salt as the transition metal catalyst (ii) is, for example, a Group 4 transition metal such as titanium or zirconium; Group 5 transition metals such as vanadium and niobium; Group 6 transition metals such as chromium, molybdenum, and tungsten; Group 7 transition metals such as manganese and technetium; Group 8 transition metals such as iron and ruthenium; Group 9 transition metals such as cobalt; Group 10 transition metals such as nickel and palladium; Group 11 transition metals such as copper and silver; Examples include zinc, cadmium, and other Group 12 transition metals.

[0069] In the transition metal catalyst (ii), the transition metal is preferably cobalt, manganese, iron, nickel, zinc, or copper, given its low cost. In the transition metal catalyst (ii), the transition metal is preferably cobalt or manganese, and more preferably cobalt, in that it increases the catalytic activity of the transition metal catalyst (ii). In the transition metal catalyst (ii), the transition metal is preferably cobalt or manganese, and more preferably cobalt, in terms of low cost and higher catalytic activity of the transition metal catalyst (ii).

[0070] Examples of the salt (cobalt salt) when the transition metal is cobalt include cobalt(II) nitrate (Co(NO3)2), cobalt(II) carbonate (CoCO3), cobalt(II) phosphate (Co3(PO4)2), cobalt(II) chloride (CoCl2), and cobalt(II) sulfate.

[0071] Examples of the salt (iron salt) when the transition metal is iron include iron(III) nitrate (Fe(NO3)3) and iron(II) nitrate (Fe(NO3)2).

[0072] Examples of the salt when the inorganic acid is nitric acid include cobalt(II) nitrate (Co(NO3)2), iron(III) nitrate (Fe(NO3)3), iron(II) nitrate (Fe(NO3)2), nickel(II) nitrate (Ni(NO3)2), manganese(II) nitrate (Mn(NO3)2), zinc nitrate (Zn(NO3)2), copper(II) nitrate (Cu(NO3)2), and the like.

[0073] Among the transition metal catalysts (ii), examples of cobalt oxides include cobalt(II) oxide (CoO), cobalt(III) oxide (Co2O3), and cobalt(II,III) oxide (Co3O4).

[0074] Among the transition metal catalysts (ii), those transition metal salts that have a hydrate may be anhydrous transition metal salts or transition metal salt hydrates. Among the transition metal salt hydrates mentioned above, cobalt salt hydrates include, for example, cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O), cobalt(II) chloride hexahydrate (CoCl2·6H2O), and cobalt(II) sulfate hydrate (CoSO4·x 1 H2O(x 1 (where is an integer greater than or equal to 1), for example, cobalt sulfate heptahydrate (CoSO4·7H2O), cobalt(II) carbonate hydrate (CoCO3·x 7 H2O(x 7 (where is an integer greater than or equal to 1), cobalt(II) phosphate hydrate (Co3(PO4)2·x 2 H2O(x 2 (where is an integer greater than or equal to 1), for example, cobalt(II) phosphate octahydrate (Co3(PO4)2·8H2O) is one such example. Among the transition metal salt hydrates mentioned above, examples of nitrate hydrates include iron(III) nitrate notahydrate (Fe(NO3)3·9H2O), iron(III) nitrate hexahydrate (Fe(NO3)3·6H2O), iron(II) nitrate hexahydrate (Fe(NO3)2·6H2O), nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O), manganese(II) nitrate tetrahydrate (Mn(NO3)2·4H2O), manganese(II) nitrate hexahydrate (Mn(NO3)2·6H2O), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), copper(II) nitrate trihydrate (Cu(NO3)2·3H2O), and copper(II) nitrate hexahydrate (Cu(NO3)2·6H2O).

[0075] The composite of the aforementioned salt (a transition metal salt of an inorganic acid) and oxoate typically has low solubility in solvents, and can therefore be easily reused by recovering it after use in the second step. Furthermore, unlike complex catalysts, the composite is easy to synthesize, and the raw material salts are inexpensive.

[0076] The salt constituting the composite may be any of the above. In particular, the salt is preferably an iron salt or a cobalt salt in terms of higher activity of the transition metal catalyst (ii).

[0077] Examples of oxo salts constituting the composite include salts of oxo acids (such as nitric acid, carbonic acid, phosphoric acid, sulfuric acid, silicic acid, boric acid, tungstic acid, phosphotungstic acid, silicatungstic acid, molybdic acid, phosphomolybdic acid, silicamolybdic acid, titanic acid, aluminic acid, etc.) from among the inorganic acids that constitute the salt (transition metal salt of an inorganic acid). In particular, in terms of the higher activity of the transition metal catalyst (ii), the oxo salt is preferably a phosphate, borate, aluminate, silicate, tungstate, phosphotungstate, or silicatungstate.

[0078] Examples of the aforementioned complexes in which the transition metal is cobalt include, for example, a complex of cobalt salt and phosphate (which may be referred to as a "cobalt-phosphate complex" in this specification; the same applies hereafter when the oxo salt is something other than a phosphate), a complex of cobalt salt and borate (cobalt-borate complex), a complex of cobalt salt and aluminate (cobalt-aluminate complex), a complex of cobalt salt and silicate (cobalt-silicic acid complex), a complex of cobalt salt and tungstate (cobalt-tungstate complex), a complex of cobalt salt and phosphotungstate (cobalt-phosphotungstate complex), and a complex of cobalt salt and silicatungstate (cobalt-silitaungstate complex).

[0079] Examples of the aforementioned complexes in which the transition metal is iron include, for example, a complex of iron salt and phosphate (which may be referred to as "iron-phosphate complex" in this specification; the same applies hereafter when the oxo salt is something other than phosphate), a complex of iron salt and borate (iron-borate complex), a complex of iron salt and aluminate (iron-aluminate complex), a complex of iron salt and silicate (iron-silicic acid complex), a complex of iron salt and tungstate (iron-tungstate complex), a complex of iron salt and phosphotungstate (iron-phosphotungstate complex), and a complex of iron salt and silicatungstate (iron-silitudinal tungstate complex).

[0080] The aforementioned complex is obtained as a precipitate by mixing the salt (a transition metal salt of an inorganic acid) and the oxo salt in water, and stirring the resulting aqueous solution while heating it. In this case, the mass ratio of [amount of transition metal salt of inorganic acid used (parts by mass)] to [amount of oxoate used (parts by mass)] is preferably 60:40 to 40:60, more preferably 55:45 to 45:55, and even more preferably 50:50. The temperature of the aqueous solution during heating and stirring is preferably 70 to 90°C, and the heating and stirring time is preferably 1 to 24 hours. The precipitate is filtered, washed by a known method, and dried to obtain the composite of high purity.

[0081] Among the transition metal catalysts (ii), those in which the transition metal is iron are particularly suitable when the oxidizing agent used in the second step is hydrogen peroxide.

[0082] The transition metal catalyst (ii) used in the second step may be one type or two or more types, and if there are two or more types, their combination and ratio can be arbitrarily selected according to the purpose.

[0083] The transition metal catalyst (ii) is one or more selected from the group consisting of transition metal salts of inorganic acids, cobalt oxides, cobalt acetate, cluster-type cobalt catalysts introduced into polyoxoacids, and composites of the aforementioned salts and oxoates. The inorganic acid is nitric acid, carbonic acid, phosphoric acid, hydrogen chloride, sulfuric acid, silicic acid, boric acid, tungstic acid, phosphotungstic acid hydrate, silicatungstic acid hydrate, molybdic acid, phosphomolybdic acid hydrate, silicamolybdic acid hydrate, titanic acid, or aluminic acid. The transition metal is copper, silver, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zinc, molybdenum, tungsten, zirconium, niobium, technetium, ruthenium, palladium, silver, or cadmium. The oxo salt is preferably a phosphate, borate, aluminate, silicate, tungstate, phosphotungstate, or silicatungstate. Using such a transition metal catalyst (ii) allows the decomposition of the modifier to proceed more efficiently.

[0084] When a transition metal catalyst (ii) is used in the second step, the amount of transition metal catalyst (ii) used in the second step is preferably 0.1 to 5 mg per 1 g of modified polypropylene, for example, 0.1 to 3.5 mg, 0.1 to 2 mg, or 0.1 to 1 mg. When the amount of transition metal catalyst (ii) used is above the lower limit, the effect obtained by using the transition metal catalyst (ii) is enhanced. When the amount of transition metal catalyst (ii) used is below the upper limit, excessive use of the transition metal catalyst (ii) is suppressed.

[0085] <Other ingredients (ii)> In the second step, other components (which may be referred to as "other components (ii)" in this specification) that do not fall under the category of polypropylene modifier, oxidizing agent, or transition metal catalyst (ii) may be used, or not, as long as they do not impair the effects of the present invention.

[0086] The other component (ii) used in the second step may be one type or two or more types, and if there are two or more types, their combination and ratio can be arbitrarily selected according to the purpose.

[0087] Other components (ii) can be arbitrarily selected depending on the purpose and are not particularly limited. Other preferred components (ii) include, for example, solvents such as water and organic solvents. The solvent may, for example, be formulated alone in the second step, or it may be mixed with any or all of the modifier, oxidizing agent, and transition metal catalyst (ii) to form a solution or dispersion of these components. For example, when using hydrogen peroxide as an oxidizing agent, it is preferable to use water as the solvent and to incorporate an aqueous solution of hydrogen peroxide. For example, when using a component other than hydrogen peroxide as an oxidizing agent, it is preferable to use an inorganic acid that is liquid at room temperature, such as trifluoroacetic acid (CF3COOH), trichloroacetic acid (CCl3COOH), or phosphoric acid (H3PO4), as a solvent and to incorporate an inorganic acid solution of the oxidizing agent.

[0088] In this specification, unless otherwise specified, the term "solvent" encompasses both a component that is liquid at room temperature for dissolving a solute and a component that is liquid at room temperature for dispersing a dispersed phase.

[0089] In the second step, the ratio of the total amount of oxidizing agent, transition metal catalyst (ii), and solvent used to the total amount of components other than the modified product used (([Amount of oxidizing agent used in the second step (parts by mass)] + [Amount of transition metal catalyst (ii) used in the second step (parts by mass)] + [Amount of solvent used in the second step (parts by mass)]) / [Total amount of components other than the modified product obtained in the first step used in the second step (parts by mass)] × 100) is preferably 80% by mass or more, more preferably 90% by mass or more, and may be, for example, 95% by mass or more, or 98% by mass or more. In this way, by having the total amount of the main components used be above the lower limit and the amount of other components used be below a certain value, the amount of the target decomposition product produced will be sufficiently large. On the other hand, the aforementioned ratio is 100% by mass or less. Here, for example, if transition metal catalyst (ii) is not used, the amount of transition metal catalyst (ii) used in the second step is 0 parts by mass. The same applies to solvents.

[0090] <Other conditions> In the second step, the temperature (reaction temperature, decomposition temperature) when the modified material is heat-treated with an oxidizing agent can be set lower than the temperature used in conventional methods for decomposing polyolefins with an oxidizing agent. This is because the modified material is used. Thus, due to the low reaction temperature (decomposition temperature), the decomposition method of this embodiment has the following advantages. In other words, energy consumption in the second step can be reduced. As will be described later, it is not necessary to have a long reaction time, in which case energy consumption can be reduced even further. Furthermore, the main decomposition products can be kept in their pre-carbon dioxide form rather than becoming stable carbon dioxide, allowing for the reuse of the resulting decomposition products. For example, malonic acid is unstable under high-temperature conditions, and the production of malonic acid through the decomposition of polyolefins has not been reported to date. However, the decomposition method of this embodiment makes it possible to obtain malonic acid. By suppressing the production of carbon dioxide, a greenhouse gas, the adverse impact on the environment can be reduced. Furthermore, even when hydrogen peroxide is used as the oxidizing agent, unlike the decomposition methods disclosed in Patent Document 1 and Non-Patent Document 1, there is no concern about explosion, and the decomposition method of this embodiment is highly safe.

[0091] In the second step, the temperature (reaction temperature, decomposition temperature) when the modified product is heat-treated with an oxidizing agent is preferably 90°C or lower, more preferably 80°C or lower, and even more preferably 75°C or lower. By keeping the reaction temperature below the upper limit, the reaction temperature is prevented from becoming excessively high, and the above-mentioned effects become more pronounced. On the other hand, the reaction temperature is preferably 30°C or higher in that it increases the degree of decomposition of the modified product, and may be, for example, 50°C or higher, or 60°C or higher. In one embodiment, the reaction temperature may be, for example, 30-90°C, 50-80°C, or 60-75°C. However, these are just examples of reaction temperatures.

[0092] In the second step, the time (reaction time, decomposition time) for heat-treating the modified product with an oxidizing agent is preferably 24 hours or less, more preferably 21 hours or less, and even more preferably 18 hours or less. By keeping the reaction time below the upper limit, excessively long reaction times are suppressed, and energy consumption can be reduced. On the other hand, the reaction time is preferably 5 hours or more in that it increases the degree of decomposition of the modified product, and may be, for example, 6 hours or more, 10 hours or more, or 12 hours or more. In one embodiment, the reaction time may be, for example, 5 to 24 hours, 6 to 24 hours, 10 to 21 hours, or 12 to 18 hours. However, these are just examples of reaction times.

[0093] In the second step, it is preferable that both the reaction temperature and the reaction time are within one of the numerical ranges described above. For example, in the second step, it is more preferable to perform the heat treatment of the modified material under conditions of 90°C or lower and for 24 hours or less.

[0094] The second step may be carried out in an air atmosphere, as in the first step, or in an atmosphere of a gas other than air, such as an inert gas.

[0095] In the second step, even with mild reaction conditions, polypropylene can be sufficiently decomposed, unlike the decomposition method disclosed in Non-Patent Document 2. Furthermore, because the reaction conditions are mild, as described above, the main decomposition products obtained can be limited to compounds that do not reach carbon dioxide. Examples of such decomposition products include a carboxylic acid represented by the following general formula (1) (sometimes referred to as "carboxylic acid (1)" in this specification), a carboxylic acid represented by the following general formula (2) (sometimes referred to as "carboxylic acid (2)" in this specification), and acetone.

[0096] [ka] (In the formula, R 1 (where n is a hydrogen atom or an alkyl group; n is an integer of 1 or more.)

[0097] Carboxylic acid (1) is a saturated aliphatic monocarboxylic acid, and preferred carboxylic acid (1) includes, for example, formic acid (HCOOH) and acetic acid (CH3COOH). Carboxylic acid (2) is a saturated aliphatic dicarboxylic acid, and preferred carboxylic acid (2) includes, for example, malonic acid (HOOCCH2COOH) and succinic acid (HOOCCH2CH2COOH). In particular, the decomposition method of this embodiment, which obtains at least malonic acid as a decomposition product in the second step, is completely different from conventional polyolefin decomposition methods, as described above.

[0098] In the decomposition method of this embodiment, it is possible to increase the amount of carboxylic acid (1) and carboxylic acid (2), among the above decomposition products, particularly formic acid, acetic acid, and malonic acid. Therefore, the polypropylene decomposition method of this embodiment can also be said to be a method for producing these components (carboxylic acid (1), carboxylic acid (2)) using polypropylene as a raw material.

[0099] The structure of the decomposition products generated in the second step can be confirmed by known methods such as nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), ultraviolet-visible spectroscopy (UV-Vis), and infrared spectroscopy (IR).

[0100] In this embodiment, after the completion of the second step, the obtained reaction mixture can be post-treated as needed by known methods to extract the target product (decomposition product). That is, post-treatment operations such as filtration, washing, extraction, pH adjustment, dehydration, and concentration can be performed individually or in combination of two or more, and the target product can be extracted by concentration, crystallization, reprecipitation, column chromatography, etc. Furthermore, the extracted target product may be purified by performing operations such as crystallization, reprecipitation, column chromatography, extraction, and stirring and washing of crystals with a solvent, individually or in combination of two or more, one or more times, as needed. Alternatively, after the completion of the second step, the obtained reaction mixture can be post-treated as needed, and the target product can be used for the next intended purpose without being extracted. For example, the target product can be used in the next intended reaction without being extracted.

[0101] The decomposition method of this embodiment, by employing the first and second steps described above, can decompose polypropylene without requiring special equipment and highly corrosive chemicals, unlike the decomposition methods disclosed in, for example, Non-Patent Documents 3 and 4. Furthermore, unlike the decomposition method disclosed in Non-Patent Document 5, the oxoacid (A) used in the first step, the oxidizing agent used in the second step, the metal catalyst used in the first step if necessary, and the transition metal catalyst (ii) used in the second step if necessary can be selected without requiring advanced technology or expensive raw materials for their preparation. Therefore, the decomposition method of this embodiment has high practicality and versatility. According to the decomposition method of this embodiment, unlike the decomposition methods disclosed in, for example, Non-Patent Documents 3-4, polypropylene can be efficiently decomposed.

[0102] <<An example of a method for decomposing polypropylene>> A preferred method for decomposing polypropylene in this embodiment is a first step of modifying the polypropylene using one or more selected from the group consisting of concentrated sulfuric acid, orthophosphoric acid, and pyrophosphoric acid to obtain a modified polypropylene product. The process includes a second step of obtaining decomposition products of the modified product by heat-treating the modified product with an oxidizing agent, One example of a method for decomposing polypropylene is in which the oxidizing agent is one or more selected from the group consisting of hydrogen peroxide, peracid, peroxomonosulfate, peroxodisulfate, perborate, urea-hydrogen peroxide, and sodium peroxide carbonate adduct. In the second step of this decomposition method, the modified product is further heat-treated using a transition metal catalyst (ii), wherein the transition metal catalyst (ii) is one or more selected from the group consisting of a transition metal salt of an inorganic acid, cobalt oxide, cobalt acetate, a cluster-type cobalt catalyst introduced into a polyoxoacid, and a composite of the salt and oxoate. The inorganic acid is nitric acid, carbonic acid, phosphoric acid, hydrogen chloride, sulfuric acid, silicic acid, boric acid, tungstic acid, phosphotungstic acid hydrate, silicatungstic acid hydrate, molybdic acid, phosphomolybdic acid hydrate, silicamolybdic acid hydrate, titanic acid, or aluminic acid. The transition metal is copper, silver, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zinc, molybdenum, tungsten, zirconium, niobium, technetium, ruthenium, palladium, silver, or cadmium. The oxo salt is preferably a phosphate, borate, aluminate, silicate, tungstate, phosphotungstate, or silicatungstate. In the first step of this decomposition method, it is preferable to further modify the polypropylene using a transition metal catalyst (i). In this decomposition method, it is preferable that the transition metal catalyst (i) is one or more selected from the group consisting of cobalt(II) sulfate heptahydrate, cobalt(II) nitrate hexahydrate, anhydrous cobalt(II) phosphate, cobalt(II) phosphate hydrate, and cobalt(II) oxide. In the second step of this decomposition method, it is preferable to perform the heat treatment of the modified material under conditions of 90°C or lower for 24 hours or less.

[0103] Another example of a preferred polypropylene decomposition method in this embodiment is a first step of modifying the polypropylene using one or more selected from the group consisting of concentrated sulfuric acid, orthophosphoric acid, and pyrophosphoric acid to obtain a modified polypropylene product, The process includes a second step of obtaining decomposition products of the modified product by heat-treating the modified product with an oxidizing agent and a transition metal catalyst (ii), The transition metal catalyst (ii) is one or more selected from the group consisting of a transition metal salt of an inorganic acid, cobalt oxide, cobalt acetate, a cluster-type cobalt catalyst introduced into a polyoxoacid, and a composite of the salt and oxoate. The inorganic acid is nitric acid, carbonic acid, phosphoric acid, hydrogen chloride, sulfuric acid, silicic acid, boric acid, tungstic acid, phosphotungstic acid hydrate, silicatungstic acid hydrate, molybdic acid, phosphomolybdic acid hydrate, silicamolybdic acid hydrate, titanic acid, or aluminic acid. The transition metal is copper, silver, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zinc, molybdenum, tungsten, zirconium, niobium, technetium, ruthenium, palladium, silver, or cadmium. A method for decomposing polypropylene is provided in which the oxo salt is a phosphate, borate, aluminate, silicate, tungstate, phosphotungstate, or silicatungstate. In the first step of this decomposition method, it is preferable to further modify the polypropylene using a transition metal catalyst (i). In this decomposition method, it is preferable that the transition metal catalyst (i) is one or more selected from the group consisting of cobalt(II) sulfate heptahydrate, cobalt(II) nitrate hexahydrate, anhydrous cobalt(II) phosphate, cobalt(II) phosphate hydrate, and cobalt(II) oxide. In the second step of this decomposition method, it is preferable to perform the heat treatment of the modified material under conditions of 90°C or lower for 24 hours or less. [Examples]

[0104] The present invention will be described in more detail below with reference to specific examples. However, the present invention is not limited in any way to the examples shown below.

[0105] << Manufacturing of transition metal catalysts >> [Manufacturing Example 1] In a 100 mL round-bottom flask, iron(III) nitrate nonahydrate (1.98 g, 49 mmol) and sodium cyclotrimetaphosphate (1.50 g, 49 mmol) were placed, and distilled water (12 mL) was added. The mixture was then stirred at 80°C for 16 hours using a stirrer tip. The resulting white precipitate was then filtered, washed with distilled water, and vacuum-dried to obtain the transition metal catalyst, a complex of sodium cyclotrimetaphosphate and iron(III) nitrate (the iron-phosphate complex), as a white powder (1.31 g).

[0106] <<Decomposition of Polypropylene>> [Example 1] Polypropylene (hPP, manufactured by Sigma-Aldrich, weight-average molecular weight 12,000, number-average molecular weight 5,000) (8g), cobalt(II) sulfate heptahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (900mg), and concentrated sulfuric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (16mL) were placed in a small pressure-resistant autoclave (manufactured by Parr, 43mL) under an air atmosphere and atmospheric pressure, sealed, and heated at 200°C for 60 hours. This reaction was carried out in 5 batches simultaneously, and the resulting black solids from the 5 batches were filtered together, washed with distilled water (300mL), and then vacuum-dried. The dried black solids were crushed using a crusher, and distilled water (200mL) was added to the resulting pulverized material and stirred, filtered, and the solid was washed with distilled water (300mL) and then dried. By doing so, residual substances such as cobalt(II) sulfate remaining in the black solid were removed, and a modified polypropylene product was obtained as a black powder (Step 1).

[0107] The modified polypropylene (2000 mg) obtained above, the transition metal catalyst (1 mg) obtained in Production Example 1, and a 30% by mass aqueous hydrogen peroxide solution (8 mL) were placed in a pressure-resistant miniature autoclave (Huanyu, 50 mL) under an air atmosphere and atmospheric pressure, sealed, and heated at 70°C for 15 hours. After the reaction was complete, the reaction mixture was allowed to cool to approximately 50°C, then cooled with water to approximately room temperature, and the reaction mixture was filtered to separate the liquid containing the decomposition products of the modified polypropylene from the solid containing the residue of the modified polypropylene (second step).

[0108] The obtained solid was washed with water, dried, and the mass (mg) of the dried material was measured. Then, the amount of decomposition of the modified polypropylene was calculated using the following formula. Here, the amount of modified polypropylene used (mg) in the formula below is 2000 (mg). The results are shown in Table 1. [Amount of decomposition of modified polypropylene (mg)] = [Amount of modified polypropylene used (mg)] - [Mass of dry material (mg)]

[0109] A quantitative solution was prepared by weighing 9.18 mg of sodium 4,4-dimethyl-4-silapentane-1-sulfonate (DSS) into a 50 mL volumetric flask, adding heavy water (D2O) to dissolve the DSS, and adjusting the total volume to 50 mL. The liquid (filtrate) (300 μL) obtained by the above filtration and the quantitative solution (300 μL) are mixed, and phosphoric acid (10 μL) is added to the resulting mixed solution (600 μL). The resulting solution is then measured using a nuclear magnetic resonance spectrometer (Bruker "AV400"). 1 The sample was subjected to 1H NMR spectral measurement. The spectral data obtained at this time is shown in Figure 1. Based on the measurement results, the concentration of the decomposition products of the modified polypropylene in the solution was quantified using an internal standard. The results are shown in Table 1.

[0110] [Examples 2-4] In the first step, cobalt(II) sulfate heptahydrate (900 mg) was replaced with cobalt(II) nitrate hexahydrate (931 mg) (Example 2), cobalt(II) phosphate hydrate (468 mg) (Example 3), or cobalt(II,III) oxide (256 mg) (Example 4), except that the polypropylene was decomposed in the same manner as in Example 1. The decomposition product of the modified polypropylene obtained at this time 1 The 1H NMR spectral data was similar to that obtained in Example 1, except for the peak intensities of the individual degradation products. 1 Table 1 shows the quantitative results of the concentration of the decomposition products in the solution on which 1H NMR measurements were taken. Although the hydration number of the cobalt(II) phosphate hydrate used here is unknown, for convenience, we assumed it to be a tetrahydrate and determined its formula weight accordingly.

[0111] [Example 5] In the first step, polypropylene was decomposed in the same manner as in Example 1, except that cobalt(II) sulfate heptahydrate (900 mg) was not used. The decomposition product of the modified polypropylene obtained at this time 1 The 1H NMR spectral data was similar to that obtained in Example 1, except for the peak intensities of the individual degradation products. 1 Table 2 shows the quantitative results of the concentration of the decomposition products in the solution on which 1H NMR measurements were taken.

[0112] [Example 6] In the second step, polypropylene was decomposed in the same manner as in Example 1, except that the transition metal catalyst obtained in Production Example 1 was not used. The decomposition product of the modified polypropylene obtained at this time 1 The 1H NMR spectral data was similar to that obtained in Example 1, except for the peak intensities of the individual degradation products. 1 Table 2 shows the quantitative results of the concentration of the decomposition products in the solution on which 1H NMR measurements were taken.

[0113] [Example 7] In the second step, polypropylene was decomposed in the same manner as in Example 5, except that the transition metal catalyst obtained in Production Example 1 was not used. The decomposition product of the modified polypropylene obtained at this time 1 The 1H NMR spectral data was similar to that obtained in Example 1, except for the peak intensities of the individual degradation products. 1 Table 2 shows the quantitative results of the concentration of the decomposition products in the solution on which 1H NMR measurements were taken.

[0114] In the first step of Examples 1-4 and 6, the amount of metal catalyst used was 0.13-0.4 mmol per 1 g of polypropylene used.

[0115] [Comparative Example 1] Except for using polypropylene (sigma-aldrich, weight-average molecular weight 12,000, number-average molecular weight 5,000) (2,000 mg) instead of modified polypropylene (2,000 mg), the decomposition of polypropylene was attempted in the same manner as in Example 1. In other words, in this comparative example, the first step described above was omitted, and polypropylene was used instead of modified polypropylene in the second step described above. In this comparative example, unlike the examples described above, the first step was not performed. Therefore, instead of using [amount of modified polypropylene used (mg)], we used [amount of polypropylene used (mg)] (=2000 mg), and instead of using [amount of modified polypropylene decomposed (mg)], we calculated [amount of polypropylene decomposed (mg)]. The results are shown in Table 2.

[0116] [Table 1]

[0117] [Table 2]

[0118] As is clear from the results above, in Examples 1 to 7, mild reaction conditions were adopted in the second step using an oxidizing agent, rather than harsh reaction conditions that raised safety concerns. In both the first and second steps, polypropylene was sufficiently decomposed without the use of special equipment, techniques, or materials. In Examples 1 to 7, the amount of decomposed polypropylene modified product was high, at 412 mg or more (412 to 631 mg). Furthermore, in Examples 1 to 7, various carboxylic acids and acetone were obtained as decomposition products, and in particular, a large amount of malonic acid was produced (high concentration), which could not be obtained by conventional decomposition methods.

[0119] A comparison of Examples 1-4 confirmed that when a catalyst is used in the first step, polypropylene was decomposed in a generally similar manner, even if the type of catalyst differed.

[0120] A comparison of Examples 1-4 and Example 5 revealed that using a metal catalyst in the first step increased the production of formic acid, acetic acid, and malonic acid, and tended to increase the production of carboxylic acid (1) and carboxylic acid (2).

[0121] A comparison between Example 1 and Example 6, and between Example 5 and Example 7, showed that using a transition metal catalyst in the second step tended to increase the production amounts of various carboxylic acids and acetone.

[0122] In contrast, in Comparative Example 1, because the first step was not performed, the polypropylene could hardly be decomposed, and almost none of the decomposition products obtained in Examples 1-7 were obtained. More specifically, formic acid, malonic acid, succinic acid, and acetone were below the detection limit when their concentrations were quantified. [Industrial applicability]

[0123] This invention can be used for the decomposition of polypropylene, and can also be used for the decomposition of polypropylene waste and the reuse of the decomposed products.

Claims

1. A method for decomposing polypropylene, The aforementioned decomposition method comprises a first step of obtaining a modified polypropylene by modifying the polypropylene using one or more substances selected from the group consisting of concentrated sulfuric acid, orthophosphoric acid, and pyrophosphoric acid, A method for decomposing polypropylene, comprising a second step of obtaining decomposed products of the modified material by heat-treating the modified material with an oxidizing agent.

2. The method for decomposing polypropylene according to claim 1, wherein in the second step, the modified product is further heat-treated using a transition metal catalyst.

3. The transition metal catalyst is one or more selected from the group consisting of a transition metal salt of an inorganic acid, cobalt oxide, cobalt acetate, a cluster-type cobalt catalyst introduced into a polyoxoacid, and a composite of the aforementioned salt and oxoate. The inorganic acid is nitric acid, carbonic acid, phosphoric acid, hydrogen chloride, sulfuric acid, silicic acid, boric acid, tungstic acid, phosphotungstic acid hydrate, silicatungstic acid hydrate, molybdic acid, phosphomolybdic acid hydrate, silicamolybdic acid hydrate, titanic acid, or aluminic acid. The transition metal is copper, silver, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zinc, molybdenum, tungsten, zirconium, niobium, technetium, ruthenium, palladium, silver, or cadmium. The method for decomposing polypropylene according to claim 2, wherein the oxo salt is a phosphate, borate, aluminate, silicate, tungstate, phosphotungstate, or silicatungstate.

4. The method for decomposing polypropylene according to any one of claims 1 to 3, wherein the oxidizing agent is one or more selected from the group consisting of hydrogen peroxide, peracid, peroxomonosulfate, peroxodisulfate, perborate, urea-hydrogen peroxide, and sodium peroxide carbonate adduct.

5. The method for decomposing polypropylene according to claim 1 or 2, wherein at least malonic acid is obtained as the decomposition product in the second step.

6. The method for decomposing polypropylene according to claim 1 or 2, wherein in the second step, the heat treatment of the modified material is carried out under conditions of 90°C or lower and for 24 hours or less.

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