Method for decomposing crosslinked rubber

Abstract

The present invention addresses the problem of providing a method for decomposing crosslinked rubber that can improve the yields of monomers and oligomers. The solution to the aforementioned problem is a method for decomposing crosslinked rubber characterized by including a first decomposition step for decomposing diene-rubber-containing crosslinked rubber using a catalyst represented by general formulas (1)-(3): [in the formulas, M is ruthenium, molybdenum, or the like; X1, X2, L1, L2, and L3 each independently represent a ligand; R1, R2, and R3 each independently represent hydrogen, an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, or the like (these groups may be substituted by one or a plurality of alkyl groups, halogens, alkoxy groups, or the like); and L1 and L2, R1 and R2, and L1 and R1 may respectively bond to each other to form rings], and a second decomposition step for thermally decomposing the decomposition product obtained in the first decomposition step at a temperature of 300-950°C inclusive in the presence of a catalyst.

Description

Method for decomposing crosslinked rubber

[0001] The present invention relates to a method for decomposing crosslinked rubber.

[0002] Conventionally, rubber products primarily made of cross-linked rubber such as vulcanized rubber have been difficult to reuse, and after the end of their product life, they have often been reused as fuel, particularly in cement factories. However, with the recent rise in environmental concerns, there has been a demand for the development of methods for recycling materials obtained by decomposing rubber products, rather than burning them as fuel. There are various methods for decomposing cross-linked rubber, and for example, a technique for thermally decomposing cross-linked rubber at high temperatures is known. Furthermore, Patent Document 1 listed below discloses a method for decomposing polyisoprene-based rubber using microorganisms.

[0003] JP 2009-247241 A

[0004] As described above, there are various methods for decomposing crosslinked rubber, but from the viewpoint of further enhancing the recyclability of crosslinked rubber, it is important to increase the yield of monomers and oligomers obtained by decomposition. However, as described above, when crosslinked rubber is thermally decomposed at high temperatures, the decomposition products become aromatic, resulting in a decrease in the yield of monomers and oligomers. Furthermore, when crosslinked rubber is decomposed using microorganisms, as in the technology disclosed in Patent Document 1, there are problems in that the decomposition takes a long time and the yield of monomers and oligomers is low.

[0005] Therefore, an object of the present invention is to provide a method for decomposing crosslinked rubber that can solve the above-mentioned problems of the conventional technology and improve the yield of monomers and oligomers.

[0006] The gist of the method for decomposing crosslinked rubber of the present invention, which solves the above problems, is as follows.

[0007] [1] A crosslinked rubber containing a diene rubber represented by the following general formula (1), (2), or (3): wherein M is ruthenium, titanium, molybdenum, or tungsten; １ and X ２ each independently represents a ligand; １ , L２ and L ３ each independently represents a ligand; R １ , R ２ and R ３ each independently represent a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, a carboxylate group, an alkoxy group, an alkenyloxy group, an alkynyloxy group, an aryloxy group, an alkoxycarbonyl group, an alkylamino group, an alkylthio group, an arylthio group, an alkylsulfonyl group, or an alkylsulfinyl group (wherein these groups are optionally substituted with one or more alkyl groups, halogens, alkoxy groups, aryl groups, or heteroaryl groups); L １ and L ２ may be bonded to each other to form a ring, R １ and R ２ may be bonded to each other to form a ring, １ and R １ and a second decomposition step of thermally decomposing the decomposition product obtained in the first decomposition step at 300°C or higher and 950°C or lower in the presence of a catalyst.

[0008] [2] The method for decomposing a crosslinked rubber according to [1], wherein, after the first decomposition step, 80% by mass or more of the diene rubber in the crosslinked rubber is decomposed into diene oligomers having a weight average molecular weight of 100 to 50,000.

[0009] [3] The method for decomposing a crosslinked rubber according to [2], wherein 30% by mass or more of the diene oligomer is decomposed into hydrocarbons having 12 or less carbon atoms through the second decomposition step.

[0010] [4] The method for decomposing crosslinked rubber according to any one of [1] to [3], wherein 20% by mass or more of the diene rubber in the crosslinked rubber is decomposed into hydrocarbons having 12 or less carbon atoms through the first decomposition step and the second decomposition step.

[0011] [5] The method for decomposing a crosslinked rubber according to any one of [1] to [4], wherein the first decomposition step is carried out at a temperature of 20°C or higher and 200°C or lower.

[0012] [6] The method for decomposing crosslinked rubber according to any one of [1] to [5], wherein the diene rubber comprises at least one selected from the group consisting of isoprene skeleton rubber, styrene-butadiene rubber, and butadiene rubber.

[0013] [7] The second decomposition step is performed by using TiO ２ , ZrO ２ , MgO, La ２ O ３ , CeO ２ , Y ２ O ３ , Li ２ CO ３ , Na ２ CO ３ , Rb ２ CO ３ , and Cs ２ CO ３ The method for decomposing a crosslinked rubber according to any one of [1] to [6], wherein the decomposition is carried out in the presence of at least one basic catalyst selected from the group consisting of:

[0014] [8] The method for decomposing a crosslinked rubber according to any one of [1] to [7], wherein the crosslinked rubber further contains carbon black.

[0015] [9] The method for decomposing a crosslinked rubber according to any one of [1] to [8], wherein the crosslinked rubber further contains sulfur.

[0016] According to the present invention, it is possible to provide a method for decomposing crosslinked rubber that can improve the yield of monomers and oligomers.

[0017] The method for decomposing crosslinked rubber of the present invention will be described in detail below by way of example based on embodiments thereof.

[0018] The method for decomposing a crosslinked rubber of the present invention comprises treating a crosslinked rubber containing a diene rubber with a compound represented by the following general formula (1), (2) or (3): wherein M is ruthenium, titanium, molybdenum, or tungsten; １ and X ２ each independently represents a ligand; L １ , L ２ and L ３each independently represents a ligand; R １ , R ２ and R ３ each independently represent a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, a carboxylate group, an alkoxy group, an alkenyloxy group, an alkynyloxy group, an aryloxy group, an alkoxycarbonyl group, an alkylamino group, an alkylthio group, an arylthio group, an alkylsulfonyl group, or an alkylsulfinyl group (wherein these groups are optionally substituted with one or more alkyl groups, halogens, alkoxy groups, aryl groups, or heteroaryl groups); L １ and L ２ may be bonded to each other to form a ring, R １ and R ２ may be bonded to each other to form a ring, １ and R １ and a second decomposition step of thermally decomposing the decomposition product obtained in the first decomposition step at a temperature of 300°C or higher and 950°C or lower in the presence of a catalyst.

[0019] The catalyst represented by the general formula (1), (2), or (3) used in the first decomposition step of the method for decomposing a crosslinked rubber of the present invention can promote metathesis decomposition, causing metathesis decomposition at unsaturated bonds in the diene rubber, thereby decomposing the diene rubber. Unlike conventional thermal decomposition, metathesis decomposition does not require high temperatures, allowing the diene rubber to be decomposed under mild conditions. Furthermore, metathesis decomposition can suppress aromatization of the decomposition products, thereby improving the retention rate of the monomer skeleton (isoprene skeleton, butadiene skeleton, etc.) in the diene rubber compared to conventional thermal decomposition. Furthermore, in the second decomposition step of the method for decomposing a crosslinked rubber of the present invention, the decomposition products obtained in the first decomposition step are thermally decomposed in the presence of a catalyst at 300°C or higher and 950°C or lower, thereby decomposing the decomposition products (intermediate decomposition products) retaining the monomer skeleton into diene monomers and diene oligomers. Therefore, the method for decomposing a crosslinked rubber of the present invention can improve the yield of monomers and oligomers ultimately obtained from the crosslinked rubber.

[0020] <First Decomposition Step> The method for decomposing a crosslinked rubber of the present embodiment includes a first decomposition step of decomposing a crosslinked rubber containing a diene rubber using a catalyst represented by the above general formula (1), (2), or (3).

[0021] (Crosslinked Rubber) The crosslinked rubber to be decomposed by the decomposition method of this embodiment contains a diene rubber and may further contain carbon black, sulfur, etc. The form of the crosslinked rubber is not particularly limited and may be, for example, powdered rubber. The powdered rubber can be obtained by cutting and pulverizing used rubber products such as waste tires. The pulverization step may include multiple steps such as a pre-pulverization step and a fine pulverization step, and the particle size of the powdered rubber to be used may be adjusted by a classification step after the pulverization step.

[0022] --Diene-Based Rubber-- The diene-based rubber is a rubber containing units derived from a diene-based monomer (diene-based units) and may further contain units derived from a copolymerizable comonomer. The units derived from the diene-based monomer enable the diene-based rubber to be crosslinked (vulcanized) and to exhibit rubber-like elongation and strength. While the diene-based rubber in the crosslinked rubber is typically present in a crosslinked state, it may be partially uncrosslinked. Specific examples of diene-based monomers (diene-based compounds) include 1,3-butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. On the other hand, examples of the copolymerizable comonomer include aromatic vinyl compounds. Specific examples of the aromatic vinyl compounds include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o,p-dimethylstyrene, o-ethylstyrene, m-ethylstyrene, and p-ethylstyrene. Examples of the diene rubber include isoprene-skeleton rubber, styrene-butadiene rubber (SBR), butadiene rubber (BR), and chloroprene rubber (CR). The isoprene-skeleton rubber is a rubber whose main skeleton is an isoprene unit, and specific examples include natural rubber (NR) and synthetic isoprene rubber (IR). Among these, the diene rubber preferably comprises at least one selected from the group consisting of isoprene-skeleton rubber, styrene-butadiene rubber, and butadiene rubber. When the diene rubber comprises at least one selected from the group consisting of isoprene-skeleton rubber, styrene-butadiene rubber, and butadiene rubber, easily recyclable diene monomers such as isoprene and butadiene can be obtained. The content of the diene rubber in the crosslinked rubber is not particularly limited, and is preferably in the range of 10 to 100% by mass, for example. From the viewpoint of further improving the yield of butadiene and isoprene, a range of 30 to 100% by mass is more preferable.

[0023] --Carbon Black-- The crosslinked rubber may further contain carbon black. In the method for decomposing a crosslinked rubber of the present invention, in the first decomposition step, metathesis decomposition is performed using a catalyst represented by the above general formula (1), (2), or (3), thereby decomposing the diene rubber and reducing its molecular weight, for example, to a liquid polymer or liquid oligomer. Therefore, even if the crosslinked rubber contains carbon black, the carbon black can be easily separated and recovered after the first decomposition step, for example, by solid-liquid separation. By recovering the carbon black before the thermal decomposition in the second decomposition step, it can be reused as high-quality carbon black. Furthermore, even when a catalyst is used in the second decomposition step, the carbon black can be prevented from becoming a catalyst poison or causing coking. Therefore, even if the crosslinked rubber contains carbon black, the crosslinked rubber can be efficiently decomposed. The carbon black content in the crosslinked rubber is not particularly limited, and is, for example, in the range of 10 to 150 parts by mass, preferably 30 to 120 parts by mass, per 100 parts by mass of the diene rubber.

[0024] --Sulfur-- The crosslinked rubber may further contain sulfur. In the crosslinked rubber, sulfur is usually present in a state where the diene rubber is crosslinked (as a crosslinker for the diene rubber), but some of it may be free. In the method for decomposing a crosslinked rubber of the present invention, in the first decomposition step, metathesis decomposition is performed using a catalyst represented by the above general formula (1), (2), or (3), thereby decomposing the diene rubber and reducing its molecular weight, for example, to a liquid polymer or liquid oligomer. Therefore, even if the crosslinked rubber contains sulfur, the sulfur can be easily recovered after the first decomposition step, for example, by solid-liquid separation. By recovering the sulfur before the thermal decomposition in the second decomposition step, the sulfur can be reused. Furthermore, even when a catalyst is used in the second decomposition step, it is possible to prevent sulfur from poisoning the catalyst or from re-crosslinking (re-vulcanizing) the decomposition products. Therefore, even if the crosslinked rubber contains sulfur, the crosslinked rubber can be efficiently decomposed. The content of sulfur in the crosslinked rubber is not particularly limited, and is, for example, in the range of 0.1 to 10 parts by mass, preferably 1 to 5 parts by mass, per 100 parts by mass of the diene rubber.

[0025] --Other Components--The crosslinked rubber may contain, in addition to the diene rubber, carbon black, and sulfur described above, various components commonly used in the rubber industry, such as rubber components other than diene rubbers, fillers other than carbon black (silica, calcium carbonate, etc.), silane coupling agents, antioxidants, softeners, processing aids, resins, surfactants, organic acids (stearic acid, etc.), zinc oxide (zinc white), vulcanization accelerators, crosslinking agents other than sulfur (peroxides, etc.), and the like.

[0026] (Catalyst) In the method for decomposing a crosslinked rubber according to the present embodiment, in the first decomposition step, the crosslinked rubber is decomposed by a catalyst represented by the following general formula (1), (2), or (3): The catalyst represented by the general formula (1), (2), or (3) can promote metathesis decomposition, and can decompose diene rubber simply (under mild conditions) and quickly.

[0027] In the above general formulas (1), (2), and (3), M is ruthenium (Ru), titanium (Ti), molybdenum (Mo), or tungsten (W). Among these, ruthenium is preferred as M from the viewpoint of promoting the decomposition reaction of the diene rubber in the crosslinked rubber.

[0028] In the above general formulas (1) and (2), X １ and X ２ Each independently represents a ligand, and is preferably an anionic ligand. １ and X ２ Examples of the X include hydrogen, halogen, pseudohalogen, a linear or branched alkyl group having 1 to 30 carbon atoms, an aryl group having 6 to 24 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, an aryloxy group having 6 to 24 carbon atoms, an alkyldiketonate having 3 to 20 carbon atoms, an aryldiketonate having 6 to 24 carbon atoms, a carboxylate having 1 to 20 carbon atoms, an alkylsulfonate having 1 to 20 carbon atoms, an arylsulfonate having 6 to 24 carbon atoms, an alkylthiol group having 1 to 20 carbon atoms, an arylthiol group having 6 to 24 carbon atoms, an alkylsulfonyl group having 1 to 20 carbon atoms, and an alkylsulfinyl group having 1 to 20 carbon atoms. １ and X ２ may be substituted with one or more further groups, for example, a halogen (preferably fluorine), an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, or an aryl group having 6 to 24 carbon atoms, wherein these groups may also be further substituted with one or more substituents selected from the group consisting of a halogen (preferably fluorine), an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, and a phenyl group. １ and X ２ are the same or different, and each is a halogen (particularly fluorine, chlorine, bromine or iodine), a benzoate, a carboxylate having 1 to 5 carbon atoms, an alkyl group having 1 to 5 carbon atoms, a phenoxy group, an alkoxy group having 1 to 5 carbon atoms, an alkylthiol group having 1 to 5 carbon atoms, an arylthiol group having 6 to 24 carbon atoms, an aryl group having 6 to 24 carbon atoms, or an alkylsulfonate having 1 to 5 carbon atoms. １ and X ２are identical and each represents a halogen (especially chlorine), CF ３ COO, CH ３ COO, CFH ２ COO, (CH ３ ) ３ CO, (CF ３ ) ２ (CH ３ ) CO, (CF ３ ) (CH ３ ) ２ CO, PhO (phenoxy), MeO (methoxy), EtO (ethoxy), tosylate (p-CH ３ -C ６ H ４ -SO ３ ), mesylate (2,4,6-trimethylphenyl) or CF ３ SO ３ (trifluoromethanesulfonate).

[0029] In the above general formulas (1), (2) and (3), L １ , L ２ and L ３ each independently represents a ligand, preferably a neutral (uncharged) electron donor (also called an "electron-donating neutral ligand"). １ , L ２ and L ３ can be, for example, independently of one another, a phosphine, sulfonated phosphine, phosphate, phosphinite, phosphonit, arsine, stibine, ether, amine, amide, aryloxy, sulfonate, sulfoxide, carboxyl, nitrosyl, pyridine, thioether or imidazolidine ligand. １ , L ２ and L ３are each independently an arylphosphine ligand having 6 to 24 carbon atoms, an alkylphosphine ligand having 1 to 10 carbon atoms or a cycloalkylphosphine ligand having 3 to 20 carbon atoms, a sulfonated arylphosphine ligand having 6 to 24 carbon atoms or a sulfonated alkylphosphine ligand having 1 to 10 carbon atoms, an aryl phosphinite ligand having 6 to 24 carbon atoms or an alkyl phosphinite ligand having 1 to 10 carbon atoms, an arylphosphonite ligand having 6 to 24 carbon atoms or an alkylphosphonite ligand having 1 to 10 carbon atoms, an aryl phosphite ligand having 6 to 24 carbon atoms or an alkyl phosphite ligand having 1 to 10 carbon atoms, an aryl arsine ligand having 6 to 24 carbon atoms or an alkyl phosph ... is preferably an alkylarsine ligand having 1 to 10 carbon atoms, an arylamine ligand having 6 to 24 carbon atoms or an alkylamine ligand having 1 to 10 carbon atoms, a pyridine ligand, an arylsulfoxide ligand having 6 to 24 carbon atoms or an alkylsulfoxide ligand having 1 to 10 carbon atoms, an arylether ligand having 6 to 24 carbon atoms or an alkylether ligand having 1 to 10 carbon atoms, or an arylamide ligand having 6 to 24 carbon atoms or an alkylamide ligand having 1 to 10 carbon atoms, each of which may be substituted with a phenyl group, and the phenyl group may also be optionally further substituted with a halogen, an alkyl group having 1 to 5 carbon atoms or an alkoxy group having 1 to 5 carbon atoms. The term "phosphine" includes, for example, PPh ３ , P(p-Tol) ３ , P(o-Tol) ３ , PPh(CH ３ ) ２ , P(CF ３ ) ３ , P(p-FC ６ H ４ ) ３ , P(p-CF ３ C ６ H ４ ) ３ , P(C ６ H ４ -SO ３ Na) ３ , P(CH ２ C ６ H ４ -SO ３ Na) ３ , P(isopropyl) ３ , P(CHCH ３ (CH２ CH ３ )) ３ , P(cyclopentyl) ３ , P (cyclohexyl) ３ , P (neopentyl) ３ and P(neophenyl) ３ The term "phosphinite" includes, for example, triphenyl phosphinite, tricyclohexyl phosphinite, triisopropyl phosphinite, and methyldiphenyl phosphite. The term "phosphite ester" includes, for example, triphenyl phosphite, tricyclohexyl phosphite, tri-tert-butyl phosphite, triisopropyl phosphite, and methyldiphenyl phosphite. The term "stibine" includes, for example, triphenylstibine, tricyclohexylstibine, and trimethylstibine. The term "aryloxy" includes, for example, 2-tert-butyl-4,5-dimethylphenyloxy. The term "sulfonate" includes, for example, trifluoromethanesulfonate, tosylate, and mesylate. The term "sulfoxide" includes, for example, (CH ３ ) ２ S(=O) and (C ６ H ５ ) ２ The term "thioether" includes, for example, CH ３ SCH ３ , C ６ H ５ SCH ３ , C.H. ３ OCH ２ CH ２ SCH ３and tetrahydrothiophene. The term "pyridine" includes, for example, pyridine, picolines (α-, β-, and γ-picoline), lutidines (2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-lutidine), collidine (2,4,6-trimethylpyridine), trifluoromethylpyridine, phenylpyridine, 4-(dimethylamino)pyridine, chloropyridines, bromopyridines, nitropyridines, quinoline, pyrimidine, pyrrole, imidazole, and phenylimidazole. In the imidazolidine ligand, hydrogen atoms bonded to carbon atoms or nitrogen atoms constituting the imidazolidine ring may be substituted with a linear or branched alkyl group having 1 to 30 carbon atoms, a cycloalkyl group having 3 to 20 carbon atoms, an alkenyl group having 2 to 20 carbon atoms, an alkynyl group having 2 to 20 carbon atoms, an aryl group having 6 to 24 carbon atoms, a carboxylate having 1 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, an alkenyloxy group having 2 to 20 carbon atoms, an alkynyloxy group having 2 to 20 carbon atoms, an aryloxy group having 6 to 20 carbon atoms, an alkoxycarbonyl group having 2 to 20 carbon atoms, an alkylthio group having 1 to 20 carbon atoms, an arylthio group having 6 to 20 carbon atoms, an alkylsulfonyl group having 1 to 20 carbon atoms, an alkylsulfonate having 1 to 20 carbon atoms, an arylsulfonate having 6 to 20 carbon atoms, or an alkylsulfinyl group having 1 to 20 carbon atoms.

[0030] In the above general formulas (1), (2) and (3), R １ , R ２ and R ３each independently represent a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, an aralkyl group, a carboxylate group, an alkoxy group, an alkenyloxy group, an alkynyloxy group, an aryloxy group, an alkoxycarbonyl group, an alkylamino group, an alkylthio group, an arylthio group, an alkylsulfonyl group, or an alkylsulfinyl group, wherein these groups are optionally substituted with one or more alkyl groups, halogens, alkoxy groups, aryl groups, or heteroaryl groups. Furthermore, the alkyl group is preferably an alkyl group having 1 to 30 carbon atoms, the cycloalkyl group is preferably a cycloalkyl group having 3 to 20 carbon atoms, the alkenyl group is preferably an alkenyl group having 2 to 20 carbon atoms, the alkynyl group is preferably an alkynyl group having 2 to 20 carbon atoms, the aryl group is preferably an aryl group having 6 to 24 carbon atoms, the aralkyl group is preferably an aralkyl group having 7 to 24 carbon atoms, the carboxylate group is preferably a carboxylate group having 1 to 20 carbon atoms, the alkoxy group is preferably an alkoxy group having 1 to 20 carbon atoms, and the alkenyloxy group is preferably an alkenyloxy group having 2 to 20 carbon atoms. As the alkynyloxy group, an alkynyloxy group having 2 to 20 carbon atoms is preferred, as the aryloxy group, an aryloxy group having 6 to 24 carbon atoms is preferred, as the alkoxycarbonyl group, an alkoxycarbonyl group having 2 to 20 carbon atoms is preferred, as the alkylamino group, an alkylamino group having 1 to 30 carbon atoms is preferred, as the alkylthio group, an alkylthio group having 1 to 30 carbon atoms is preferred, as the arylthio group, an arylthio group having 6 to 24 carbon atoms is preferred, as the alkylsulfonyl group, an alkylsulfonyl group having 1 to 20 carbon atoms is preferred, and as the alkylsulfinyl group, an alkylsulfinyl group having 1 to 20 carbon atoms is preferred. In one embodiment, R １ and R ２one of the groups is hydrogen, and the other is alkyl having 1 to 20 carbon atoms, cycloalkyl having 3 to 10 carbon atoms, alkenyl having 2 to 20 carbon atoms, alkynyl having 2 to 20 carbon atoms, aryl having 6 to 24 carbon atoms, aralkyl having 7 to 24 carbon atoms, carboxylate having 1 to 20 carbon atoms, alkoxy having 1 to 20 carbon atoms, alkenyloxy having 2 to 20 carbon atoms, alkynyloxy having 2 to 20 carbon atoms, aryloxy having 6 to 24 carbon atoms, alkoxycarbonyl having 2 to 20 carbon atoms, alkylamino having 1 to 30 carbon atoms, alkylthio having 1 to 30 carbon atoms, arylthio having 6 to 24 carbon atoms, alkylsulfonyl having 1 to 20 carbon atoms, or alkylsulfinyl having 1 to 20 carbon atoms, each of which is optionally substituted with one or more alkyl groups, halogens, alkoxy groups, aryl groups, or heteroaryl groups.

[0031] In the above general formulas (1), (2) and (3), L １ and L ２ may be bonded to each other to form a ring. １ and L ２ The ring formed by the combination of may be aliphatic or aromatic and may be optionally substituted and contain one or more heteroatoms, such as oxygen, sulfur, nitrogen, phosphorus, etc.

[0032] In the above general formulas (1), (2) and (3), R １ and R ２ may be bonded to each other to form a ring. １ and R ２ However, these R １ and R ２ The ring formed together with the common carbon atom to which it is attached may be aliphatic or aromatic and may be optionally substituted and contain one or more heteroatoms.

[0033] In the above general formulas (1), (2) and (3), L １ and R １ may be bonded to each other to form a ring. １ and R １The ring formed by the combination of may be aliphatic or aromatic and may be optionally substituted and contain one or more heteroatoms, such as oxygen, sulfur, nitrogen, phosphorus, etc.

[0034] The catalyst represented by the general formula (1) above includes those represented by the following structural formulas (1-1) to (1-3): [In the formula, Cy represents a cyclohexyl group, and Mes represents a mesityl group (also called a "2,4,6-trimethylphenyl group")]. The catalyst represented by structural formula (1-1) is called a Grubbs first-generation catalyst, the catalyst represented by structural formula (1-2) is called a Grubbs second-generation catalyst, and the catalyst represented by structural formula (1-3) is called a Grubbs-Hoveyda second-generation catalyst. When a catalyst represented by any of structural formulas (1-1) to (1-3) is used, the decomposition reaction (metathesis decomposition) of the diene rubber in the crosslinked rubber proceeds more rapidly.

[0035] The catalyst represented by the general formula (2) above is represented by the following structural formula (2-1): The catalyst represented by structural formula (2-1) is called a Grubbs third generation catalyst. When the catalyst represented by structural formula (2-1) is used, the decomposition reaction (metathesis decomposition) of the diene rubber in the crosslinked rubber proceeds more rapidly.

[0036] The catalyst represented by the general formula (3) above is represented by the following structural formula (3-1): Examples of the catalyst include those represented by the following formula:

[0037] The amount of the catalyst used is preferably 0.1 part by mass or more, more preferably 1 part by mass or more, and is preferably 10 parts by mass or less, and more preferably 8 parts by mass or less, per 100 parts by mass of the diene rubber. If the amount of the catalyst used is 0.1 part by mass or more per 100 parts by mass of the diene rubber, the decomposition reaction of the diene rubber will further proceed, and if the amount of the catalyst used is 10 parts by mass or less per 100 parts by mass of the diene rubber, it is preferable from the standpoint of cost.

[0038] (Reaction Conditions, etc.) The first decomposition step is preferably carried out at a temperature of 20° C. or higher and 200° C. or lower. By carrying out the first decomposition step at 20° C. or higher, the rate of the decomposition reaction of the diene rubber in the crosslinked rubber is improved, and by carrying out the first decomposition step at 200° C. or lower, decomposition of the catalyst (metathesis catalyst) represented by the above general formula (1), (2), or (3) can be suppressed, and after decomposition, the retention rate (selectivity) of the monomer skeleton of the diene rubber in the crosslinked rubber is improved. From the viewpoint of improving the decomposition reaction rate of the diene rubber, the first decomposition step is more preferably 25° C. or higher, and from the viewpoint of suppressing decomposition of the metathesis catalyst and improving the selectivity of the product maintaining the monomer skeleton, it is more preferably 100° C. or lower.

[0039] The first decomposition step can be carried out at any pressure, including reduced pressure, normal pressure, and increased pressure. For example, the reaction pressure is preferably 1 kPa to 10 MPa, more preferably 10 kPa to 1 MPa, and even more preferably 50 kPa to 500 kPa.

[0040] In the method for decomposing a crosslinked rubber according to this embodiment, the catalyst represented by the general formula (1), (2), or (3) may be dissolved in a solvent, and the crosslinked rubber may be immersed in the solvent to carry out the first decomposition step. By allowing the catalyst to act on the crosslinked rubber in the solvent, the decomposition reaction of the diene rubber in the crosslinked rubber is facilitated. Any solvent that does not inhibit the decomposition reaction can be used as the solvent, and examples thereof include ethers, aliphatic hydrocarbons, alicyclic hydrocarbons, and aromatic hydrocarbons (aromatic solvents). More specifically, the solvent is preferably tetrahydrofuran (THF), hexane, cyclohexane, pentane, cyclopentane, toluene, or xylene, and more preferably toluene or tetrahydrofuran. When the solvent is selected from tetrahydrofuran, hexane, cyclohexane, pentane, cyclopentane, toluene, or xylene, the decomposition reaction of the diene rubber in the crosslinked rubber is facilitated. The amount of the solvent used is preferably 10 mL or more, more preferably 50 mL or more, and is preferably 500 mL or less, and more preferably 200 mL or less, per gram of the crosslinked rubber. If the amount of the solvent used is 10 mL or more per gram of the crosslinked rubber, the decomposition reaction of the diene rubber in the crosslinked rubber will further proceed, and if the amount of the solvent used is 500 mL or less per gram of the crosslinked rubber, this is preferred from the standpoint of cost.

[0041] The first decomposition step may be carried out in the presence of a chain transfer agent (CTA). Examples of the chain transfer agent include cis-1,4-diacetoxy-2-butene and cis-1,4-dibenzyloxy-2-butene. The amount of the chain transfer agent used is preferably in the range of 1 to 100 mol per 1 mol of the catalyst.

[0042] (Decomposition Products (Intermediate Decomposition Products)) In the method for decomposing a crosslinked rubber of this embodiment, it is preferable to decompose 80% by mass or more of the diene rubber in the crosslinked rubber through the first decomposition step into diene oligomers having a weight average molecular weight of 100 to 50,000, and more preferably into diene oligomers having a weight average molecular weight of 300 to 10,000. By decomposing 80% by mass or more of the diene rubber in the crosslinked rubber into diene oligomers having a weight average molecular weight of 100 to 50,000 through the first decomposition step, the yield of diene monomers and diene oligomers can be further improved through the second decomposition step described below. From the viewpoint of the yield of diene monomers and diene oligomers after the second decomposition step, it is more preferable to decompose 85% by mass or more of the diene rubber in the crosslinked rubber into diene oligomers having a weight average molecular weight of 100 to 50,000 through the first decomposition step. In this specification, the term "oligomer" refers to an oligomer having a weight-average molecular weight of 50,000 or less and containing two or more monomer units. The term "diene-based oligomer" refers to an oligomer containing two or more units derived from a diene monomer (diene-based units). The weight-average molecular weight (Mw) can be measured by gel permeation chromatography (GPC).

[0043] <Second Decomposition Step> The method for decomposing a crosslinked rubber of the present embodiment includes a second decomposition step in which the decomposition product obtained in the first decomposition step is thermally decomposed in the presence of a catalyst at a temperature of 300° C. or higher and 950° C. or lower. Here, the catalyst used in the second decomposition step may be an acidic catalyst, a neutral catalyst, or a basic catalyst, and among these, a basic catalyst is preferred.

[0044] (Reaction Conditions, etc.) By carrying out the second decomposition step at 300° C. or higher, the decomposition reaction rate of the decomposition product obtained in the first decomposition step is improved, and by carrying out the second decomposition step at 950° C. or lower, the selectivity of the product maintaining the monomer skeleton is improved. From the viewpoint of improving the decomposition reaction rate, the second decomposition step is more preferably 500° C. or higher, and from the viewpoint of improving the selectivity of the product maintaining the monomer skeleton, the second decomposition step is more preferably 900° C. or lower.

[0045] The second decomposition step can be carried out at any pressure, including reduced pressure, normal pressure, and increased pressure. For example, the reaction pressure in the second decomposition step is preferably 10 kPa to 5 MPa, and more preferably 100 kPa to 1 MPa.

[0046] The second decomposition step may be carried out in air, but is preferably carried out in an inert gas atmosphere. By carrying out the second decomposition step in an inert gas atmosphere, oxidation of the decomposition products obtained in the first decomposition step can be suppressed. Examples of the inert gas include nitrogen, carbon dioxide, argon, and helium.

[0047] When the second decomposition step is carried out under pressure, for example, the decomposition product obtained in the first decomposition step may be thermally decomposed in a solvent. Any solvent that does not inhibit the decomposition reaction can be used as the solvent, and examples thereof include ethers, aliphatic hydrocarbons, alicyclic hydrocarbons, and aromatic hydrocarbons (aromatic solvents). More specifically, the solvent is preferably tetrahydrofuran (THF), hexane, cyclohexane, pentane, cyclopentane, toluene, and xylene, and more preferably toluene and tetrahydrofuran. The amount of the solvent used is preferably 10 mL or more, more preferably 50 mL or more, and more preferably 500 mL or less, and more preferably 200 mL or less, per 1 g of the decomposition product obtained in the first decomposition step (or the intermediate product to be subjected to the second decomposition step). If the amount of the solvent used is 10 mL or more per 1 g of the decomposition product obtained in the first decomposition step (or the intermediate product to be subjected to the second decomposition step), the decomposition reaction will proceed further. Furthermore, if the amount of the solvent used is 500 mL or less per 1 g of the decomposition product obtained in the first decomposition step (or the intermediate product to be subjected to the second decomposition step), this is preferable from the standpoint of cost.

[0048] (Catalyst) The second decomposition step is carried out in the presence of a catalyst. The catalyst may be an acidic catalyst, a neutral catalyst, or a basic catalyst, and among these, a basic catalyst is preferred. In addition, the second decomposition step is carried out in the presence of a catalyst such as TiO ２ , ZrO ２ , MgO, La ２ O ３ , CeO２ , Y ２ O ３ , Li ２ CO ３ , Na ２ CO ３ , Rb ２ CO ３ , and Cs ２ CO ３ It is more preferable to carry out the second decomposition step in the presence of at least one basic catalyst selected from the group consisting of:

[0044] By carrying out the second decomposition step in the presence of such a basic catalyst, the rate of the decomposition reaction (depolymerization reaction) of the decomposition product obtained in the first decomposition step is improved, and the selectivity of the product maintaining the monomer skeleton is also improved. The catalyst may be used alone or in combination of two or more.

[0049] The amount of the catalyst used is preferably 1 part by mass or more, more preferably 10 parts by mass or more, and even more preferably 100 parts by mass or more, relative to 100 parts by mass of the decomposition product obtained in the first decomposition step (or the intermediate product to be subjected to the second decomposition step), and is preferably 8,000 parts by mass or less, more preferably 4,000 parts by mass or less, and even more preferably 500 parts by mass or less. If the amount of the catalyst used is 100 parts by mass or more relative to 100 parts by mass of the decomposition product obtained in the first decomposition step (or the intermediate product to be subjected to the second decomposition step), the decomposition reaction will proceed further, and if the amount of the catalyst used is 500 parts by mass or less relative to 100 parts by mass of the decomposition product obtained in the first decomposition step (or the intermediate product to be subjected to the second decomposition step), it is preferable from the standpoint of cost.

[0050] (Decomposition Products) In the method for decomposing a crosslinked rubber according to the present embodiment, it is preferable that the second decomposition step decomposes 20% by mass or more of the diene-based oligomers having a weight-average molecular weight of 100 to 50,000 obtained in the first decomposition step into hydrocarbons having 12 or less carbon atoms. By improving the yield of hydrocarbons having 12 or less carbon atoms, the yield of reusable monomers and oligomers is improved, further improving the economic and environmental value of the decomposition method. From the viewpoint of the yield of reusable monomers and oligomers, it is even more preferable that the second decomposition step decomposes 30% by mass or more of the diene-based oligomers having a weight-average molecular weight of 100 to 50,000 obtained in the first decomposition step into hydrocarbons having 12 or less carbon atoms.

[0051] Furthermore, in the method for decomposing a crosslinked rubber according to this embodiment, it is preferable to decompose 0.1% by mass or more of the diene rubber in the crosslinked rubber into hydrocarbons having 12 or less carbon atoms through the first decomposition step and the second decomposition step. By improving the yield of hydrocarbons having 12 or less carbon atoms, the yield of reusable monomers and oligomers is improved, further improving the economic and environmental value of the decomposition method. From the viewpoint of the yield of reusable monomers and oligomers, it is even more preferable to decompose 20% by mass or more of the diene rubber in the crosslinked rubber into hydrocarbons having 12 or less carbon atoms through the first decomposition step and the second decomposition step. The hydrocarbons having 12 or less carbon atoms as decomposition products vary depending on the type of diene rubber in the crosslinked rubber to be decomposed, but examples thereof include 1,3-butadiene, isoprene, 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene, and limonene.

[0052] <Others> The method for decomposing a crosslinked rubber according to this embodiment may further include another step in addition to the first decomposition step and the second decomposition step described above. Examples of such steps include a pretreatment step for the crosslinked rubber (for example, a cutting step or a pulverization step). The method for decomposing a crosslinked rubber according to this embodiment can be carried out using either a batch reactor or a flow reactor. The decomposition products after the decomposition reaction can be separated and recovered by filtration, distillation, or the like, or recovered by precipitation using a poor solvent, and can be reused. The diene monomers and diene oligomers finally obtained from the crosslinked rubber can be reused as raw materials for diene rubber (polymer).

[0053] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0054] <Measurement of Molecular Weight> The polystyrene-equivalent weight average molecular weight (Mw), number average molecular weight (Mn), and molecular weight distribution (D=Mw / Mn) of the degradation products (intermediate degradation products) were determined by gel permeation chromatography (hereinafter sometimes referred to as GPC analysis; solution delivery unit: LC-20AB manufactured by Shimadzu Corporation; column: a combination of KF-803 and KF-804 manufactured by Showa Denko K.K. or a combination of G2000HXL and G4000HXL manufactured by Tosoh Corporation; detector: differential refractometer RID-10A manufactured by Shimadzu Corporation; analysis system: LabSolutions manufactured by Shimadzu Corporation; eluent: tetrahydrofuran) using monodisperse standard polystyrene as a standard. The measurement temperature was 40°C.

[0055] <Decomposition rate in the first decomposition step> The decomposition rate in the first decomposition step was calculated based on the following formula, in which the unreacted solid rubber after the reaction was recovered and weighed, and the ratio to the weight of the rubber before the reaction was used. The decomposition rate was set to 100% when no solid rubber remained after the reaction. Decomposition rate in the first decomposition step (%) = {1 - (weight of unreacted solid rubber after the reaction / weight of rubber before the reaction)} x 100

[0056] <Structural Formula of Catalyst Used> The structural formula of the catalyst used in the first decomposition step is as follows.

[0057] <Preparation of Crosslinked Rubber Samples> (Crosslinked Rubber Sample A) A rubber composition was prepared by blending 50.0 parts by mass of carbon black, 1.0 part by mass of antioxidant 6PPD [N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine], 2.0 parts by mass of stearic acid, 2.5 parts by mass of zinc oxide, 1.5 parts by mass of vulcanization accelerator (N-cyclohexyl-2-benzothiazolylsulfenamide), and 4.5 parts by mass of sulfur with 100.1 parts by mass of natural rubber. The rubber composition was then heated and crosslinked to prepare a crosslinked rubber. The resulting crosslinked rubber was cut to approximately 2 to 6 mm on a side to prepare a crosslinked rubber sample. Furthermore, crosslinked rubber samples (approximately 1 mm on a side or less) were also prepared by further freeze-pulverization after cutting.

[0058] (Crosslinked Rubber Sample B) A rubber composition was prepared by blending 50.0 parts by mass of carbon black, 2.0 parts by mass of antioxidant 6PPD [N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine], 1.0 part by mass of antioxidant trimethyldihydroquinoline polymer, 2.0 parts by mass of stearic acid, 2.25 parts by mass of zinc oxide, 0.6 parts by mass of vulcanization accelerator (N-cyclohexyl-2-benzothiazolylsulfenamide), and 1.8 parts by mass of sulfur with 100.0 parts by mass of natural rubber, and the rubber composition was heated and crosslinked to prepare a crosslinked rubber. The obtained crosslinked rubber was cut and further freeze-pulverized to prepare crosslinked rubber samples (approximately 1 mm on a side).

[0059] <Examples of the First Decomposition Step> (Example 1) 10 mL of tetrahydrofuran (THF) was added to 0.125 g of the crosslinked rubber sample A (a sample that had only been cut) and stirred at 25°C for 24 hours to swell the rubber. 24 mg of a second-generation Grubbs catalyst (G2) was added, and the mixture was stirred at 25°C for an additional 24 hours to carry out a metathesis decomposition reaction. After the reaction, 10 mL of methanol and 2.5 mL of ethyl vinyl ether were added to terminate the reaction. Since no rubber sample remained after the reaction, the decomposition rate of the rubber sample was calculated to be 100%. Subsequently, the solvent was distilled off under reduced pressure to obtain a liquid polymer and carbon black. A portion of the mixture was taken, tetrahydrofuran was added, and the carbon black was removed by filtration using a syringe filter. The molecular weight of the liquid polymer was measured by GPC analysis. The results are shown in Table 1.

[0060] (Examples 2 to 10) The reaction conditions (type and amount of catalyst, type and amount of solvent, reaction scale) were changed, and the reaction in the first decomposition step was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0061] Example 11: 10 mL of tetrahydrofuran and 24 mg of the second-generation Grubbs catalyst (G2) were added to 0.125 g of the crosslinked rubber sample A (a sample that was chopped and then freeze-pulverized), and the mixture was stirred at 25°C for 24 hours to carry out a metathesis decomposition reaction. After the reaction, the reaction was stopped by adding 10 mL of methanol and 2.5 mL of ethyl vinyl ether. Since no rubber sample remained after the reaction, the decomposition rate of the rubber sample was calculated to be 100%. Subsequently, the solvent was distilled off under reduced pressure to obtain a liquid polymer and carbon black. A portion of the mixture was taken, and tetrahydrofuran was added. The carbon black was removed by filtration using a syringe filter, and the molecular weight of the liquid polymer was measured by GPC analysis. The results are shown in Table 1.

[0062] Example 12: 10 mL of tetrahydrofuran was added to 0.125 g of the crosslinked rubber sample A (a sample that had only been cut), and the mixture was stirred at room temperature for 24 hours to swell the rubber. Subsequently, 40 mg of cis-1,4-diacetoxy-2-butene was added as a chain transfer agent (CTA), and 24 mg of a second-generation Grubbs catalyst (G2) was added, and metathesis degradation was carried out for 24 hours. After the reaction, the reaction was terminated by adding 10 mL of methanol and 2.5 mL of ethyl vinyl ether. Since no rubber sample remained after the reaction, the decomposition rate of the rubber sample was calculated to be 100%. Subsequently, the solvent was distilled off under reduced pressure to obtain a liquid polymer and carbon black. A portion of the reaction mixture was sampled, tetrahydrofuran was added, and the carbon black was removed by filtration using a syringe filter. The molecular weight of the liquid polymer was measured by GPC analysis. The results are shown in Table 1.

[0063] Example 13: 10 mL of tetrahydrofuran and 8 mg of second-generation Grubbs catalyst (G2) were added to 0.125 g of the crosslinked rubber sample B (a sample that was freeze-pulverized after cutting), and the mixture was stirred at 25°C for 24 hours to carry out a metathesis decomposition reaction. Since no rubber sample remained after the reaction, the decomposition rate of the rubber sample was calculated to be 100%. The reaction solution was diluted with tetrahydrofuran, and the carbon black was separated by centrifugation. The carbon black was washed with tetrahydrofuran. The initial supernatant and the washings were combined and concentrated under reduced pressure to obtain a liquid polymer. A portion was sampled and the molecular weight of the liquid polymer was measured by GPC analysis. The results are shown in Table 1.

[0064] (Examples 14 to 22) The reaction conditions (amount of catalyst, type and amount of solvent, and reaction scale) were changed, and the reaction of the first decomposition step was carried out in the same manner as in Example 13. The results are shown in Table 1.

[0065]

[0066] (Notes for Table 1) 1) In Examples 1 to 10 and 12, samples were used in which the crosslinked rubber was only cut, while in Examples 11 and 13 to 22, samples were used in which the crosslinked rubber was cut and then freeze-pulverized. When freeze-pulverized samples were used, the step of swelling the rubber was omitted. 2) GPC was not measured.

[0067] Example 23: A 3.0 mg portion of the reaction mixture obtained in the first decomposition step (a reaction mixture obtained by concentrating the filtrate obtained by filtering the reaction solution of Example 11 under reduced pressure) was dissolved in 0.60 mL of tetrahydrofuran. A 5.0 μL portion of the solution was added to a stainless steel container containing the catalyst La. ２ O ３ After the solvent was distilled off, the La 1000 sachets with 0.025 mg of the reaction mixture from the first decomposition step attached thereto was obtained. ２ O ３ was heated to 650°C using a pyrolyzer (PY-3030D, manufactured by Frontier Labs) to pyrolyze the reaction mixture of the first decomposition step, and the resulting pyrolysis products were analyzed using a gas chromatograph mass spectrometer and a gas chromatograph analyzer (GCMS-QP2010Plus and GC-2014, manufactured by Shimadzu Corporation, respectively, both equipped with an UltraALLOY UA-1 capillary column manufactured by Frontier Labs). As a result, it was confirmed that isoprene, limonene, hydrocarbons having 1 to 4 carbon atoms, and hydrocarbons having 5 to 10 carbon atoms (other than isoprene and limonene) were produced in yields of 19%, 3%, 8%, and 25%, respectively (see Table 2).

[0068] (Examples 24 to 47) The reaction conditions (type of catalyst, temperature) in the second decomposition step were changed, and the reaction and analysis of the reaction mixture obtained in the first decomposition step were carried out in the same manner as in Example 23. The results are shown in Table 2.

[0069]

[0070] (Notes for Table 2) 1) In Examples 23 to 32 and 47, the solution of the reaction mixture obtained in the first decomposition step was impregnated into a catalyst placed in a stainless steel container, and the solvent was distilled off under reduced pressure to carry out thermal decomposition. On the other hand, in Examples 33 to 46, the solution of the reaction mixture obtained in the first decomposition step was placed in a stainless steel container, the solvent was distilled off under reduced pressure, and then the catalyst was added to the stainless steel container to carry out thermal decomposition. 2) Details of the catalysts in Examples 33 to 46 are as follows. CP811-E75: CP811-E75 (H-beta zeolite) manufactured by Zeolyst CBV780: CBV780 (H-SDUSY zeolite) manufactured by Zeolyst CBV8020: CBV8020 (H-ZSM-5 zeolite) manufactured by Zeolyst CBV8011G: CBV8011G (Na-ZSM-5 zeolite) manufactured by Zeolyst CBV28011G: CBV28011G (Na-ZSM-5 zeolite) manufactured by Zeolyst CBV100: CBV100 (Na-Y zeolite) manufactured by Zeolyst HSZ-320NAA: HSZ-320NAA (Na-Y zeolite) manufactured by Tosoh Corporation Activated carbon (032-18091): Activated carbon 032-18091 manufactured by Fujifilm Wako Co., Ltd. Activated carbon (035-18101): Activated carbon 035-18101 manufactured by Fujifilm Wako Co., Ltd. Activated carbon (242276): Activated carbon 242276 manufactured by Merck (Sigma-Aldrich) Pd / C (Pd 5 wt%): Activated carbon-supported palladium (5 wt%) 205680 manufactured by Merck (Sigma-Aldrich) Silica gel (37559-79): Silica gel 37559-79 manufactured by Kanto Chemical Co., Ltd. Activated alumina (019-08295): Activated alumina 019-08295 manufactured by Fujifilm Wako Co., Ltd. Montmorillonite K10 (281522): Montmorillonite K10 manufactured by Merck (Sigma-Aldrich) 281522 3) The yields of isoprene and limonene were calculated using the absolute calibration curve method. The yields of hydrocarbons with 1 to 4 carbon atoms and hydrocarbons with 5 to 10 carbon atoms (other than isoprene and limonene) were calculated based on the area ratios of the respective hydrocarbon peaks in gas chromatographic analysis. 4) Hydrocarbons with 1 to 4 carbon atoms. 5) Hydrocarbons with 5 to 10 carbon atoms (other than isoprene and limonene).

Claims

1. The crosslinked rubber containing a diene rubber is a compound represented by the following general formula (1), (2), or (3): 【Chemistry 1】 wherein M is ruthenium, titanium, molybdenum, or tungsten; X １ and X ２ each independently represents a ligand, L １ , L ２ and L ３ each independently represents a ligand, R １ , R ２ and R ３ each independently represent a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group, an aryl group, a carboxylate group, an alkoxy group, an alkenyloxy group, an alkynyloxy group, an aryloxy group, an alkoxycarbonyl group, an alkylamino group, an alkylthio group, an arylthio group, an alkylsulfonyl group, or an alkylsulfinyl group (wherein these groups are optionally substituted with one or more alkyl groups, halogens, alkoxy groups, aryl groups, or heteroaryl groups); L １ and L ２ may be bonded to each other to form a ring, R １ and R ２ may be bonded to each other to form a ring, L １ and R １ and a second decomposition step in which the decomposition product obtained in the first decomposition step is thermally decomposed at a temperature of 300°C or higher and 950°C or lower in the presence of a catalyst; A method for decomposing crosslinked rubber, comprising:

2. 2. The method for decomposing crosslinked rubber according to claim 1, wherein after the first decomposition step, 80 mass% or more of the diene rubber in the crosslinked rubber is decomposed into diene oligomers having a weight average molecular weight of 100 to 50,000.

3. The method for decomposing crosslinked rubber according to claim 2 , wherein 30% by mass or more of the diene oligomer is decomposed into hydrocarbons having 12 or less carbon atoms through the second decomposition step.

4. 2. The method for decomposing crosslinked rubber according to claim 1, wherein 20% by mass or more of the diene rubber in the crosslinked rubber is decomposed into hydrocarbons having 12 or less carbon atoms through the first decomposition step and the second decomposition step.

5. The method for decomposing a crosslinked rubber according to claim 1, wherein the first decomposition step is carried out at a temperature of 20°C or higher and 200°C or lower.

6. 2. The method for decomposing crosslinked rubber according to claim 1, wherein the diene rubber comprises at least one rubber selected from the group consisting of isoprene skeleton rubber, styrene-butadiene rubber, and butadiene rubber.

7. The second decomposition step is carried out by ２ , ZrO ２ , MgO, La ２ O ３ , CeO ２ , Y ２ O ３ , Li ２ CO ３ , Na ２ CO ３ , Rb ２ CO ３ , and Cs ２ CO ３ The method for decomposing a crosslinked rubber according to claim 1, wherein the decomposition is carried out in the presence of at least one basic catalyst selected from the group consisting of:

8. The method for decomposing a crosslinked rubber according to claim 1 , wherein the crosslinked rubber further contains carbon black.

9. The method for decomposing a crosslinked rubber according to claim 1 , wherein the crosslinked rubber further contains sulfur.

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