Method of rapidly depolymerizing a polyester of dicarboxylic acid

Abstract

There is provided a method of depolymerizing a polyester of a dicarboxylic acid, the method comprising: (a-i) depolymerizing a polyester of a dicarboxylic acid source in the presence of: (i) a dual-base system; and (ii) a solvent system comprising two or more different solvents, to obtain a product comprising a mono-salt of a monoester of the dicarboxylic acid and / or derivatives thereof.

Description

[0001] METHOD OF RAPIDLY DEPOLYMERIZING A POLYESTER OF DICARBOXYLIC ACID

[0002] TECHNICAL FIELD

[0003] The present disclosure relates broadly to a method of rapidly depolymerizing a polyester of a dicarboxylic acid to obtain a product comprising a mono-salt of a monoester of the dicarboxylic acid and / or derivatives thereof.

[0004] BACKGROUND

[0005] Polyesters comprising polymer backbone with ester groups are important recyclable thermoplastics. Polyesters derived from dicarboxylic acids such as polyethylene terephthalate (PET) are widely used in various applications, which include the manufacturing of coloured bottles for personal care products (such as hair shampoos, body soaps, and shaving creams), colorless bottles for drinking water / soft drinks and vegetable cooking oil, boxes for packing foods / fruits / eggs, and textiles / fabrics.

[0006] However, the widespread use of such polyesters leads to the generation of large volumes of polyester wastes (e.g., post-consumer PET wastes) globally. Recycling and upcycling of such polyester wastes into monomers and fine chemicals are critical for resource-saving and environmental protection. Therefore, the polyester industry requires efficient, selective and cost- effective / economical recycling and upcycling methods that are capable of operating under mild conditions for the treatment of waste polyesters of dicarboxylic acids (e.g., PET).

[0007] A key technological challenge to recycle or upcycle waste polyesters derived from dicarboxylic acids (e.g., PET) is to develop an efficient method for recycling or upcycling waste PET back into its monomers or fine chemicals. Among these, terephthalic acid (TP A) and dimethyl terephthalate (DMT) are two important monomers used in the production of virgin PET resin in current industries. Currently, there are three known methods to recycle / upcycle waste PET, namely (1 ) hydrolysis into TPA; (2) methanolysis into DMT; and (3) glycolysis into bis-(2-hydroxyethyl) terephthalate (BHET). However, these recycling and / or upcycling methods suffer from several limitations and are far from desirable. In particular, current techniques require high reaction temperature (e.g., 190 °C), making the processes energy-consuming and energy-intensive. Additionally, the known methods typically require high consumption of base (e g., NaOH) and / or a large amount of acid (e.g., hydrochloric acid) in the acidification step for neutralization, therefore resulting in an overall high material and operational costs. Current methods also encounter issues with slow and long reaction time (e.g., of 24 hours). Furthermore, the desired products are often obtained with low selectivity and the final product mixture often includes oligomers or various by-products, reducing process efficiency and complicating downstream purification.

[0008] In view of the above, there is a need to address or at least ameliorate the above-mentioned problems. In particular, there is a need to provide a simple, efficient, fast and highly selective method for recycling and upcycling polyesters derived from dicarboxylic acids.

[0009] SUMMARY

[0010] In one aspect, there is provided a method of depolymerizing a polyester of a dicarboxylic acid, the method comprising:

[0011] (a-i) depolymerizing a polyester of a dicarboxylic acid source in the presence of:

[0012] (i) a dual-base system; and

[0013] (ii) a solvent system comprising two or more different solvents, to obtain a product comprising a mono-salt of a monoester of the dicarboxylic acid and / or derivatives thereof. In one embodiment, the polyester of a dicarboxylic acid source comprises waste polyester of a dicarboxylic acid.

[0014] In one embodiment, the polyester of a dicarboxylic acid comprises a structure that is represented by general formula (1 ): wherein

[0015] A is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heterocycloalkyl, optionally substituted heterocycloalkenyl, optionally substituted heteroaryl, or combinations thereof;

[0016] R1is optionally substituted alkylene, optionally substituted cycloalkylene, optionally substituted alkenylene, optionally substituted alkynylene, or combinations thereof; and n > 1.

[0017] In one embodiment, A comprises 1 ,4-disubstituted benzene and the polyester of a dicarboxylic acid comprises a structure that is represented by general formula (2): wherein

[0018] R2is optionally substituted alkylene, optionally substituted cycloalkylene, optionally substituted alkenylene, optionally substituted alkynylene, or combinations thereof;

[0019] Ra, Rb, Rcand Rdare each independently hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, or combinations thereof; and n > 1.

[0020] In one embodiment, the depolymerizing in (a-i) comprises a plurality of alcoholysis and hydrolysis reactions.

[0021] In one embodiment, the depolymerizing in (a-i) comprises the following reaction:

[0022] A is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heterocycloalkyl, optionally substituted heterocycloalkenyl, optionally substituted heteroaryl, or combinations thereof;

[0023] R1is optionally substituted alkylene, optionally substituted cycloalkylene, optionally substituted alkenylene, optionally substituted alkynylene, or combinations thereof;

[0024] R4and R4’ are each independently optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, or combinations thereof; and

[0025] M is a metal or ammonium-containing group; and n > 1.

[0026] In one embodiment, the mono-salt of a monoester of the dicarboxylic acid comprises a structure that is represented by general formula (3): wherein

[0027] A is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heterocycloalkyl, optionally substituted heterocycloalkenyl, optionally substituted heteroaryl, or combinations thereof;

[0028] R3is optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, or combinations thereof; and M is a metal or ammonium-containing group. In one embodiment, A comprises 1 ,4-disubstituted benzene and the mono-salt of a monoester of the dicarboxylic acid comprises a structure that is represented by general formula (4): wherein

[0029] R4is optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, or combinations thereof;

[0030] R1 a, R1b, R1 Gand R1dare each independently hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, or combinations thereof; and

[0031] M is a metal or ammonium-containing group.

[0032] In one embodiment, M is an alkali metal selected from lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) or francium (Fr).

[0033] In one embodiment, the dual-base system comprises an inorganic base and an organic base.

[0034] In one embodiment, the inorganic base comprises an Arrhenius base selected from the group consisting of metal hydroxides, alkali metal hydroxides, alkaline earth metal hydroxides, ammonium-containing hydroxides and combinations thereof.

[0035] In one embodiment, the organic base comprises a Lewis base selected from the group consisting of metal alkoxides, metal salt of alkoxides, nitrogen- containing organic bases, nitrogen-based organic bases and combinations thereof.

[0036] In one embodiment, the solvent system comprises two or more different solvents selected from the group consisting of:

[0037] (a) an aprotic solvent;

[0038] (b) a protic solvent; and

[0039] (c) a hydrocarbon compound.

[0040] In one embodiment, the aprotic solvent is selected from the group consisting of tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), acetonitrile (ACN), acetone (ACE), methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cyclopentanone, cyclohexanone, 2-pentanone (also known as methyl propyl ketone), 3-pentanone (also known as diethyl ketone), diethyl ether, methyl tert-butyl ether (MTBE), tert-amyl methyl ether (TAME), cyclopentyl methyl ether (CPME), methoxybenzene (also known as anisole), ethoxybenzene, sulfolane (also known as tetramethylene sulfone), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMA), a,a-trifluorotoluene (TFT), dichloromethane (DCM), chlorobenzene, diethyl ether and combinations thereof.

[0041] In one embodiment, the protic solvent is selected from the group consisting of methanol, ethanol, n-propanol, / sopropanol, n-butanol, terf-butanol, allyl alcohol, propargyl alcohol, and combinations thereof.

[0042] In one embodiment, the hydrocarbon compound is selected from the group consisting of optionally substituted pentane, optionally substituted hexane, optionally substituted heptane, optionally substituted octane, optionally substituted benzene, optionally substituted toluene, optionally substituted xylene, and combinations thereof. In one embodiment, the volume ratio of the aprotic solvent to the protic solvent is from 1 : 10 to 50: 1 .

[0043] In one embodiment, the volume ratio of the aprotic solvent plus the protic solvent to the hydrocarbon compound is from 1 :1 to 75:1 .

[0044] In one embodiment, the molar ratio of the organic base to the inorganic base is 0.1 to 0.75.

[0045] In one embodiment, the molar ratio of the number of repeating units in polyester to the inorganic base to the organic base is 1 : 0.5 - 1.5 : 0.1 - 1 .

[0046] In one embodiment, the molar ratio of inorganic base to the number of repeating units in polyester less than 1 .

[0047] In one embodiment, the method further comprises:

[0048] (a-ii) reacting the product comprising the mono-salt of a mono ester of the dicarboxylic acid with a base to obtain a solution comprising di-alkali metal salt of the dicarboxylic acid.

[0049] In one embodiment, the method further comprises:

[0050] (a-iii) reacting the solution comprising the di-alkali metal salt of the dicarboxylic acid obtained from (a-ii) with an oxidant / oxidizing agent to obtain a decoloured solution comprising the di-alkali metal salt of the dicarboxylic acid.

[0051] In one embodiment, the method further comprises:

[0052] (a-iv) reacting the solution obtained from (a-ii) or (a-iii) with an acid to obtain a dicarboxylic acid. In one embodiment, the dicarboxylic acid comprises a structure that is represented by general formula (8): wherein

[0053] R3a, R3b, R3Gand R3dare each independently hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, or combinations thereof.

[0054] In one embodiment, the method further comprises:

[0055] (a-v) isolating ethylene glycol from the solution obtained from step (a-i).

[0056] DEFINITIONS

[0057] The term “polyester” as used herein is to be interpreted broadly to refer to a polymer which contains one or more ester linkage(s) that connect(s) monomeric units in said polymer.

[0058] The term "polymer" as used herein refers to a chemical compound comprising repeating units and is created through a process of polymerization. The units composing the polymer are typically derived from monomers and / or macromonomers. A polymer typically comprises repetition of a number of constitutional units.

[0059] The terms “monomer” or “macromonomer” as used herein refer to a chemical entity that may be covalently linked to one or more of such entities to form a polymer. The term "bond" refers to a linkage between atoms in a compound or molecule. The bond may be a single bond, a double bond, or a triple bond.

[0060] In the definitions of a number of substituents below, it is stated that “the group may be a terminal group or a bridging group”. This is intended to signify that the use of the term is intended to encompass the situation where the group is a terminal group / moiety as well as the situation where the group is a linker between two other portions of the molecule. Using the term “alkyl” having 1 carbon atom as an example, it will be appreciated that when existing as a terminal group, the term “alkyl” having 1 carbon atom may mean -CHs and when existing as a bridging group, the term “alkyl” having 1 carbon atom may mean -CH2- or the like.

[0061] The terms "alkyl" or “alkylene” as a group or part of a group refers to a straight or branched aliphatic hydrocarbon group having 1 to 20 carbon atoms, 1 to 10 carbon atoms, 1 to 6 carbon atoms, or 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19 or 20 carbon atoms. Examples of suitable straight and branched alkyl substituents include methyl, ethyl, n-propyl, 2-propyl, isopropyl, n- butyl, isobutyl, sec-butyl, t-butyl, hexyl, amyl, 1 ,2-dimethylpropyl, 1 ,1 - dimethylpropyl, pentyl, isopentyl, hexyl, 4-methylpentyl, 1 -methylpentyl, 2- methylpentyl, 3-methylpentyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1 ,2- dimethylbutyl, 1 ,3-dimethylbutyl, 1 ,2,2-trimethylpropyl, 1 , 1 ,2-trimethylpropyl, 2- ethylpentyl, 3-ethylpentyl, heptyl, 1 -methylhexyl, 2,2-dimethylpentyl, 3,3- dimethylpentyl, 4,4-dimethylpentyl, 1 ,2-dimethylpentyl, 1 ,3-dimethylpentyl, 1 ,4- dimethylpentyl, 1 ,2,3-trimethylbutyl, 1 ,1 ,2-trimethylbutyl, 1 ,1 ,3-trimethylbutyl, 5- methylheptyl, 1 -methylheptyl, octyl, nonyl, decyl and the like. The group may be a terminal group or a bridging group.

[0062] The terms "alkenyl" or “alkenylene” as a group or part of a group denotes an aliphatic hydrocarbon group containing at least one carbon-carbon double bond and which may be straight or branched having 2 to 20 carbon atoms, 2 to 10 carbon atoms, 2 to 6 carbon atoms, or 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19 or 20 carbon atoms in the chain. The group may contain a plurality of double bonds and the orientation about each double bond is independently E or Z. Exemplary alkenyl groups include, but are not limited to, ethenyl, vinyl, allyl, 1 -methylvinyl, 1 -propenyl, 2-propenyl, 2-methyl-1 -propenyl, 2-methyl-1 -propenyl, 1-butenyl, 2-butenyl, 3-butentyl, 1 ,3-butadienyl, 1 -pentenyl, 2-pententyl, 3-pentenyl, 4-pentenyl, 1 ,3-pentadienyl, 2,4-pentadienyl, 1 ,4- pentadienyl, 3-methyl-2-butenyl, 1 -hexenyl, 2-hexenyl, 3-hexenyl, 1 ,3- hexadienyl, 1 ,4-hexadienyl, 2-methylpentenyl, 1 -heptenyl, 2-heptentyl, 3- heptenyl, 1 -octenyl, 2-octenyl, 3-octenyl, 1 -nonenyl, 2-nonenyl, 3-nonenyl, 1 - decenyl, 2-decenyl, 3-decenyl and the like. The group may be a terminal group or a bridging group.

[0063] The terms "alkynyl" or “alkynylene” as a group or part of a group denotes an aliphatic hydrocarbon group containing at least one carbon-carbon triple bond and which may be straight or branched having 2 to 20 carbon atoms, 2 to 10 carbon atoms, 2 to 6 carbon atoms, or 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15,

[0064] 16, 17, 18, 19 or 20 carbon atoms in the chain. The group may contain a plurality of triple bonds. Exemplary alkynyl groups include, but are not limited to, acetylenyl, propynyl, 1-butynyl, 2-butynyl, 3-butynyl, 1 -pentynyl, 2-pentynyl, 3- methyl-1 -butynyl, 4-pentynyl, 1 -hexynyl, 2-hexynyl, 5-hexynyl, 1 -heptynyl, 2- heptynyl, 6-heptynyl, 1 -octynyl, 2-octynyl, 7-octynyl, 1 -nonynyl, 2-nonynyl, 8- nonynyl, 1 -decynyl, 2-decynyl, 9-decynyl and the like. The group may be a terminal group or a bridging group.

[0065] The term “cyclic” as used herein broadly refers to a structure where one or more series of atoms are connected to form at least one ring. The term includes, but is not limited to, both saturated and unsaturated 5-membered and saturated and unsaturated 6-membered rings. Examples of groups having a cyclic structure include, but are not limited to, cyclopentane, cyclopentene, cyclohexane, cyclohexene, benzene and the like. The term “cyclic” as used herein includes “heterocyclic”. The term “heterocyclic” as used herein broadly refers to a structure where two or more different kinds of atoms are connected to form at least one ring. For example, a heterocyclic ring may be formed by carbon atoms and at least another atom (i.e. heteroatom) selected from oxygen (O), nitrogen (N) or (NR) and sulfur (S), where R is independently a hydrogen or an organic group. The term also includes, but is not limited to, saturated and unsaturated 5-membered, and saturated and unsaturated 6-membered rings. Examples of groups having a heterocyclic structure include, but are not limited to furan, thiophene, 1 H-pyrrole, 2H-pyrrole, 1 -pyrroline, 2-pyrroline, 3-pyrroline, 1-pyrazoline, 2-pyrazoline, 3- pyrazoline, 2-imidazoline, 3-imidazoline, 4-imidazoline, pyrazole, imidazole, oxazole, isoxazole, thiazole, isothiazole, 1 ,2,3-triazole, 1 ,2,4-triazole, 1 ,2,3- oxadiazole, disubstituted 1 ,2,4-oxadiazole, 1 ,2,5-oxadiazole, 1 ,3,4-oxadiazole,

[0066] 1 .2.3-thiadiazole, 1 ,2,4-thiadiazole, 1 ,2,5-thiadiazole, 1 ,3,4-thiadiazole, tetra hydrofuran, tetrahydrothiophene, pyrrolidine, 1 ,3-dioxolane, 1 ,2-oxathiolane,

[0067] 1.3-oxathiolane, pyrazolidine, imidazolidine, pyridine, pyridazine, pyrimidine, pyrazine, 1 ,2-oxazine, 1 ,3-oxazine, 1 ,4-oxazine, thiazine, 1 ,2,3-triazine, 1 ,2,4- triazine, 1 ,3,5-triazine, 2H-pyran, 4H-pyran, 2-pyrone, 4-pyrone, 1 ,4-dioxin, 2H- thiopyran, 4H-thiopyran, tetrahydropyran, thiane, piperidine, 1 ,4-dioxane, 1 ,2- dithiane, 1 ,3-dithiane, 1 ,4-dithiane, 1 ,3,5-trithiane, piperazine, morpholine, thiomorpholine and the like.

[0068] The term "aryl" as a group or part of a group denotes (i) an optionally substituted monocyclic, or fused polycyclic, aromatic carbocycle (ring structure having ring atoms that are all carbon) preferably having from 5 to 20, or 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19 or 20 carbon atoms per ring. Examples of aryl groups include but are not limited to phenyl, tolyl, xylyl, naphthyl, anthracenyl, phenanthrenyl, fluorenyl, indenyl or indanyl and the like.

[0069] The term "heteroaryl" as a group or part of a group refers to groups containing an aromatic ring (preferably a 5- or 6- membered aromatic ring) having one or more carbon atoms (for example 1 to 6 carbon atoms) in the ring replaced by a heteroatom. Suitable heteroatoms may include nitrogen (N) or (NH), oxygen (O) and sulfur (S). Examples of heteroaryl include but are not limited to thiophene, benzothiophene, benzofuran, benzimidazole, benzoxazole, benzothiazole, benzisothiazole, naphtha[2,3-b]thiophene, furan, isoindolizine, xantholene, phenoxatine, pyrrole, imidazole, pyrazole, pyridine, pyrazine, pyrimidine, pyridazine, tetrazole, indole, isoindole, 1 H-indazole, purine, quinoline, isoquinoline, phthalazine, naphthyridine, quinoxaline, cinnoline, carbazole, phenantridine, acridine, phenazine, thiazole, isothiazole, phenothiazine, oxazole, isooxazole, furazane, phenoxazine, 2-, 3- or 4-pyridyl, 2-, 3-, 4-, 5-, or 8-quinolyl, 1 -, 3-, 4-, or 5-isoquinolinyl 1-, 2-, or 3-indolyl, and 2-, or 3-thienyl and the like. The group may be a terminal group or a bridging group.

[0070] The term "heteroalkylene" as used herein refers to alkylene having one or more -CH2- replaced with a heteroatom selected from O, NR, Si, P or S, where R is hydrogen or alkyl as defined herein. The term "heteroalkylene" can be linear, branched or cyclic and containing up to 500 carbon atoms.

[0071] The term "halogen" represents chlorine, fluorine, bromine or iodine. The term "halide" represents chloride, fluoride, bromide or iodide. The term "halo" represents chloro, fluoro, bromo or iodo.

[0072] The term "amine group" or the like is intended to broadly refer to a group containing -NR2, where R is independently a hydrogen or an organic group. The group may be a terminal group or a bridging group.

[0073] The term "amide group" or the like is intended to broadly refer to a group containing -C(=O)NR2, where R is independently a hydrogen or an organic group. The group may be a terminal group or a bridging group.

[0074] The term “optionally substituted,” when used to describe a chemical structure or moiety, refers to the chemical structure or moiety wherein one or more of its hydrogen atoms is optionally substituted with a chemical moiety or functional group such as alcohol, alkoxy, alkanoyloxy, alkoxycarbonyl, alkenyl, alkyl (e.g., methyl, ethyl, propyl, t-butyl), alkynyl, alkylcarbonyloxy (-OC(O)alkyl), amide (-C(O)NH-alkyl- or -alkylNHC(O)alkyl), amine (such as alkylamino, arylamino, arylalkylamino), aryl, aryloxy, azo, carbamoyl (-NHC(O)O-alkyl- or -OC(O)NH-alkyl), carbamyl (e.g., CONH2, as well as CONH-alkyl, CONH-aryl, and CONH-arylalkyl), carboxyl, carboxylic acid, cyano, ester, ether (e.g., methoxy, ethoxy), halo, haloalkyl (e.g., -CCI3, -CF3, -C(CF3)3), heteroalkyl, isocyanate, isothiocyanate, nitrile, nitro, phosphodiester, sulfide, sulfonamido (e.g., SO2NH2), sulfone, sulfonyl (including alkylsulfonyl, arylsulfonyl and arylalkylsulfonyl), sulfoxide, thiol (e.g., sulfhydryl, thioether) or urea (-NHCONH-alkyl-).

[0075] The term "dicarboxylic acid" as used herein refers to an organic compound comprising two carboxyl functional groups (-COOH) covalently bonded to a carbon-based backbone, which may be linear, branched, alicyclic, or aromatic. The carboxyl groups may be positioned at terminal or non-terminal locations within the molecular structure. Dicarboxylic acids may include, but are not limited to, aliphatic dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, and aromatic dicarboxylic acids such as phthalic acid and terephthalic acid. The term encompasses both naturally occurring and synthetically derived compounds, including their salts, esters, and derivatives, unless the context otherwise requires.

[0076] The term "waste" as used herein refers to any material, substance, or byproduct that is unwanted, unusable, or discarded as a result of industrial, commercial, domestic, medical, agricultural, or biological processes, including but not limited to solids, liquid, sludge or particulate matter, regardless of whether such material retains residual utility or value.

[0077] The term “particle” as used herein broadly refers to a discrete entity or a discrete body. The particle described herein can include an organic, an inorganic, a composite particle or a biological particle. The particle used described herein may also be a macro-particle that is formed by an aggregate of a plurality of sub- particles or a fragment of a small object. The particle of the present disclosure may be spherical, substantially spherical, or non-spherical, such as irregularly shaped particles or ellipsoidally shaped particles. The term “size” when used to refer to the particle broadly refers to the largest dimension of the particle. For example, the term “size” when used in the context of nanoparticle can refer to the diameter of the nanoparticle although it is not limited as such. In various embodiments, when the particle is substantially spherical, the term “size” can refer to the diameter of the particle; or when the particle is substantially non- spherical, the term “size” can refer to the largest length of the particle.

[0078] The term "micro" as used herein is to be interpreted broadly to include dimensions from about 1 micron to about 1000 microns, about 1 micron to less than about 1000 microns, about 1 micron to about 900 microns, about 1 micron to about 800 microns, about 1 micron to about 700 microns, about 1 micron to about 600 microns, about 1 micron to about 500 microns, about 1 micron to about 400 microns, about 1 micron to about 300 microns, about 1 micron to about 200 microns, or from about 1 micron to about 100 microns.

[0079] The term "nano" as used herein is to be interpreted broadly to include dimensions in a nanoscale, i.e., less than about 1000 nm, about 1 nm to less than about 1000 nm, about 1 nm to about 900 nm, about 1 nm to about 800 nm, about 1 nm to about 700 nm, about 1 nm to about 600 nm, about 1 nm to about 500 nm, about 1 nm to about 400 nm, about 1 nm to about 300 nm, about 1 nm to about 200 nm, or from about 1 nm to about 100 nm. Accordingly, the term “nanostructures”, “nanoparticles”, “nanomaterials” and the like as used herein may include structures that have at least one dimension in the range of no more than said range. The term “nanostructures”, “nanoparticles”, “nanomaterials” and the like as used herein may include structures that have at least one dimension that is no more than about 100 nm, no more than about 90 nm, no more than about 80 nm, no more than about 70 nm, no more than about 60 nm, no more than about 50 nm, no more than about 40 nm, no more than about 30 nm, no more than about 20 nm, or no more than about 10 nm. The term “nanostructure” as used herein broadly refers to an arrangement of interrelated elements in a system having at least one dimension in the nanoscale. The nanostructure described herein can include a nanoparticle, nanorod, nanofiber, nanoneedle, nanoplate, nanotube, and the like. The term “size” when used in the context of nanoparticle can refer to the diameter of the nanoparticle although it is not limited as such.

[0080] The term “particle” as used herein broadly refers to a discrete entity or a discrete body. The particle described herein can include an organic, an inorganic, a composite particle or a biological particle. The particle used described herein may also be a macro-particle that is formed by an aggregate of a plurality of subparticles or a fragment of a small object. The particle of the present disclosure may be spherical, substantially spherical, or non-spherical, such as irregularly shaped particles or ellipsoidally shaped particles. The term “size” when used to refer to the particle broadly refers to the largest dimension of the particle. For example, when the particle is substantially spherical, the term “size” can refer to the diameter of the particle; or when the particle is substantially non-spherical, the term “size” can refer to the largest length of the particle.

[0081] The terms "coupled" or "connected" as used in this description are intended to cover both directly connected or connected through one or more intermediate means, unless otherwise stated.

[0082] The term "associated with", used herein when referring to two elements refers to a broad relationship between the two elements. The relationship includes, but is not limited to a physical, a chemical or a biological relationship. For example, when element A is associated with element B, elements A and B may be directly or indirectly attached to each other or element A may contain element B or vice versa.

[0083] The term "adjacent" used herein when referring to two elements refers to one element being in close proximity to another element and may be but is not limited to the elements contacting each other or may further include the elements being separated by one or more further elements disposed therebetween.

[0084] The term "and / or", e.g., "X and / or Y" is understood to mean either "X and Y" or "X or Y" and should be taken to provide explicit support for both meanings or for either meaning.

[0085] Further, in the description herein, the word “substantially” whenever used is understood to include, but not restricted to, "entirely" or “completely” and the like. In addition, terms such as "comprising", "comprise", and the like whenever used, are intended to be non-restricting descriptive language in that they broadly include elements / components recited after such terms, in addition to other components not explicitly recited. For example, when “comprising” is used, reference to a “one” feature is also intended to be a reference to “at least one” of that feature. Terms such as “consisting”, “consist”, and the like, may in the appropriate context, be considered as a subset of terms such as "comprising", "comprise", and the like. Therefore, in embodiments disclosed herein using the terms such as "comprising", "comprise", and the like, it will be appreciated that these embodiments provide teaching for corresponding embodiments using terms such as “consisting”, “consist”, and the like. Further, terms such as "about", "approximately" and the like whenever used, typically means a reasonable variation, for example a variation of + / - 5% of the disclosed value, or a variance of 4% of the disclosed value, or a variance of 3% of the disclosed value, a variance of 2% of the disclosed value or a variance of 1 % of the disclosed value.

[0086] Furthermore, in the description herein, certain values may be disclosed in a range. The values showing the end points of a range are intended to illustrate a preferred range. Whenever a range has been described, it is intended that the range covers and teaches all possible sub-ranges as well as individual numerical values within that range. That is, the end points of a range should not be interpreted as inflexible limitations. For example, a description of a range of 1% to 5% is intended to have specifically disclosed sub-ranges 1 % to 2%, 1 % to 3%, 1 % to 4%, 2% to 3% etc., as well as individually, values within that range such as 1 %, 2%, 3%, 4% and 5%. It is to be appreciated that the individual numerical values within the range also include integers, fractions and decimals. Furthermore, whenever a range has been described, it is also intended that the range covers and teaches values of up to 2 additional decimal places or significant figures (where appropriate) from the shown numerical end points. For example, a description of a range of 1 % to 5% is intended to have specifically disclosed the ranges 1.00% to 5.00% and also 1.0% to 5.0% and all their intermediate values (such as 1.01 %, 1.02% ... 4.98%, 4.99%, 5.00% and 1.1 %, 1.2% ... 4.8%, 4.9%, 5.0% etc.,) spanning the ranges. The intention of the above specific disclosure is applicable to any depth / breadth of a range.

[0087] Additionally, when describing some embodiments, the disclosure may have disclosed a method and / or process as a particular sequence of steps. However, unless otherwise required, it will be appreciated that the method or process should not be limited to the particular sequence of steps disclosed. Other sequences of steps may be possible. The particular order of the steps disclosed herein should not be construed as undue limitations. Unless otherwise required, a method and / or process disclosed herein should not be limited to the steps being carried out in the order written. The sequence of steps may be varied and still remain within the scope of the disclosure.

[0088] Furthermore, it will be appreciated that while the present disclosure provides embodiments having one or more of the features / characteristics discussed herein, one or more of these features / characteristics may also be disclaimed in other alternative embodiments and the present disclosure provides support for such disclaimers and these associated alternative embodiments.

[0089] It will also be appreciated that where priority is claimed to an earlier application, the full contents of the earlier application are also taken to form part of the present disclosure and may serve as support for embodiments disclosed herein. DESCRIPTION OF EMBODIMENTS

[0090] Exemplary, non-limiting embodiments of a method of depolymerizing a polyester of a dicarboxylic acid are disclosed hereinafter.

[0091] There is provided a method of depolymerizing a polyester of a dicarboxylic acid (or polyester) into depolymerization / recycled / upcycled / degradation product(s). The polyester of a dicarboxylic acid may be derived from a polyester containing source, which includes but is not limited to polyester-based waste, polyester waste, waste polyester, unwanted polyester, waste polyester of a dicarboxylic acid, unwanted polyester of a dicarboxylic acid or the like. In various embodiments, the method comprises recycling / upcycling / breaking down (e.g., chemical recycling / upcycling / breaking down) waste polyester into valuable products / chemicals. Advantageously, in various embodiments, the method disclosed herein allows waste polyester to be recycled / upcycled / converted directly into forms that are useful on their own or useful as feedstocks for further applications.

[0092] In various embodiments, the polyester of a dicarboxylic acid source is provided in the form of a solid, e.g., in the form of an article. The polyester of a dicarboxylic acid source may be a packaging material (such as protective / food packaging, bottle, box, film, mold, fabric, textile, container, lid, tray, disposable cutlery), polyethylene terephthalate (PET) waste, PET post-consumer products, mixed / blended fabrics of PET-cotton or PET-cotton-spandex or PET-cotton- nylon, multi-layer films of PET-PE or PET-PP, mixed plastic waste of PET with other plastics such as polyethylene (PE), propylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), polyethylene furanoate (PEF), poly(butylene adipate) (PBA), poly(butylene succinate) (PBS), and poly(butylene sebacate) (PBSb), and block copolymer of polyesters, the like or combinations thereof. The polyester of a dicarboxylic acid may be obtained or derived from a source such as a waste source containing unwanted polyester of a dicarboxylic acid. The polyester of a dicarboxylic acid source may also comprise additives, impurities, chemicals, materials, components and / or contaminants such as dyes, plasticizers, pigments, colouring agents, printing inks, fillers such as titanium oxide or zinc oxide, flame retardants, lubricants, adhesives, paper, cotton, fabric, nylon, spandex, cellulose material, polyethylene (PE), propylene (PP), or the like or combinations thereof, which may be present in amounts of at least about 1 wt%, at least about 5 wt%, at least about 10 wt%, at least about 15 wt%, at least about 20 wt%, at least about 25 wt%, at least about 30 wt%, at least about 35 wt%, at least about 40 wt%, at least about 45 wt%, at least about 50 wt%, at least about 55 wt%, at least about 60 wt%, at least about 65 wt%, or at least about 70 wt%. Advantageously, in various embodiments, the polyester of a dicarboxylic acid source does not require pre-treatment, e.g., chemical / physical steps to remove said additives, impurities, chemicals, materials, components, and / or any other contaminants (e.g., dirt or food waste) present in the source. Even more advantageously, the presence of additives, impurities, chemicals, materials, components, and / or contaminants in the polyester of a dicarboxylic acid source does not substantially prevent / deter the method from working efficiently and / or effectively. In various embodiments, the method disclosed herein allows the polyester of a dicarboxylic acid source to be recycled / upcycled / converted successfully into depolymerization products, even in the presence of additives, impurities, chemicals, materials, components, and / or contaminants. Advantageously, in various embodiments, the method is minimally affected by presence of additives and dyes. Advantageously, in various embodiments, the efficiency, product / conversion yield and / or selectivity of the method (or depolymerization step) is not impeded by presence of additives and / or dyes. In various embodiments therefore, the method is substantially devoid of a pre-treatment step to remove additives, impurities, chemicals, materials, components, and / or any other contaminants (e.g., dirt or food waste) from the polyester of a dicarboxylic acid source. It will, however, be appreciated that the polyester of a dicarboxylic acid source may be inspected for non-polyester of a dicarboxylic acid components / parts / contam inants and processed to remove such components / parts / contaminants prior to use. For example, a polyester-based packaging may be inspected and processed to remove parts such as food waste before use. In various embodiments, the polyester of a dicarboxylic acid is substantially biodegradable (e.g., PBA, PBS, the like, or combinations thereof). In various embodiments, the polyester of a dicarboxylic acid is substantially non- biodegradable (e.g., PET, PBT, the like, or combinations thereof).

[0093] In various embodiments, the method further comprises, prior to the depolymerizing / degrading / recycling / upcycling step, a pre-depolymerizing step, and / or a simple pre-treatment to reduce the original size (and / or increase the surface area) of the polyester of a dicarboxylic acid source. The predepolymerizing step and / or the pre-treatment step may comprise a step of cutting / breaking / ripping / tearing the polyester of a dicarboxylic acid source into smaller pieces / chunks. It will be appreciated that, in various embodiments, the pre-depolymerizing step and / or the pre-treatment step is / are not a strict requirement of the method although said step increases the rate of conversion / reaction due to the larger exposed surface area (as a result of cutting / breaking / ripping / tearing / cutting the source).

[0094] In various embodiments, there is provided a method of depolymerizing a polyester of a dicarboxylic acid, the method comprising:

[0095] (a-i) depolymerizing a polyester of a dicarboxylic acid source under suitable conditions.

[0096] In various embodiments, the depolymerizing in step (a-i) is performed in the presence of:

[0097] (i) a dual-base system; and

[0098] (ii) a solvent system comprising two or more different solvents.

[0099] Advantageously, in various embodiments, the method is designed or configured to depolymerize / convert / recycle / upcycle polyester of a dicarboxylic acid source into the desired depolymerization products with increased / enhanced / improved / high efficiency, yield and / or selectivity. The method (or depolymerizing step) may be tailored as desired to obtain target product with high product / conversion yield (up to 100%, e g., 92%) and / or high product selectivity (e.g., up to 99.5%). Without being bound by theory, it is believed that the dual-base system works synergistically with the solvent system, and such a synergistic interaction favourably promotes the depolymerization step (a-i). In various embodiments, when the dual-base system is present together with the solvent system, the base contained in the dual-base system reacts or interacts with the solvent system (e.g., especially with the protic solvent of the solvent system) to generate species or ions (e.g., active species or ions) that participate in the depolymerization reaction, thereby facilitating a plurality / series of alcoholysis and hydrolysis reactions to take place at the polyester. Advantageously, the synergistic interaction between the dual-base system and solvent system allows for high depolymerization rate, depolymerization efficiency, depolymerization yield and / or depolymerization selectivity to be achieved. Advantageously, in various embodiments, the speed / rate of the method / depolymerization reaction is very fast, rapid and / or highly efficient. Advantageously, the synergistic interaction between the dual-base system and solvent system also allows the method to be performed under mild conditions. In various embodiments, the method is substantially devoid of harsh conditions such as high reaction temperature and / or pressure.

[0100] In various embodiments, the base system comprises / consists / consist essentially of a dual-base system. Accordingly, in various embodiments, the base system has more than one base but does not have more than two bases. In various embodiments, the base system comprises two different bases. In various embodiments, the dual-base system comprises an inorganic base and an organic base.

[0101] In various embodiments, the dual-base system comprises an inorganic base. In various embodiments, the inorganic base acts / serves / behaves as a reactant and / or participates in the depolymerization reaction. In various embodiments, the inorganic base is a reactant. In various embodiments, the inorganic base and the polyester of a dicarboxylic acid are reactants.

[0102] In various embodiments, the dual-base system comprises an organic base. In various embodiments, the organic base acts / serves / behaves as the catalyst and / or promotes / increases / enhances the rate / speed of the depolymerization reaction. Advantageously, in various embodiments, as the organic base is not consumed during and / or does not participate as a reactant in the depolymerization reaction, the organic base is reusable, recyclable, and is capable of being recycled / reused for repeated round(s) / cycle(s) of depolymerization.

[0103] Advantageously, the inorganic base and organic base in / of the dual-base system work synergistically to generate alkoxide ions and alter the concentration of alkoxide ions during the PET depolymerization process. Advantageously, the combination of the inorganic base and organic base supplies / provides a source or pool of alkoxide ions, which participate in the depolymerization reaction to increase selectivity and yield.

[0104] In various embodiments, the inorganic base comprises an Arrhenius base. In various embodiments, the Arrhenius base may include but is not limited to metal hydroxides, alkali metal hydroxides, alkaline earth metal hydroxides, ammonium-containing hydroxides (e.g., ammonium hydroxide and quaternary ammonium hydroxides), the like and combinations thereof. In various embodiments, the metal contained in the Arrhenius base (or inorganic base) is selected from an alkali metal (e.g., lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr)) or an alkaline earth metal (e.g., beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra)). In various embodiments, the alkali metal hydroxides are selected from the group consisting of potassium hydroxide (KOH), sodium hydroxide (NaOH), lithium hydroxide (LiOH), the like, and combinations thereof.

[0105] In various embodiments, the quaternary ammonium hydroxides comprise tetra-alkylammonium hydroxides. For example, the quaternary ammonium hydroxides may be selected from the group consisting of tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrabutylammonium hydroxide, the like, and combinations thereof.

[0106] In various embodiments, the organic base comprises a Lewis base. In various embodiments, the organic base comprises a Lewis base which includes but is not limited to alkoxides (e.g., metal alkoxides or metal salt of alkoxides), nitrogen-containing organic bases, nitrogen-based organic bases, the like and combinations thereof.

[0107] In various embodiments, the alkoxides are selected from the group consisting of methoxides, ethoxides, n-propoxides, / 'so-propoxides, n-butoxides, fert-butoxides, the like, and combinations thereof. In various embodiments, the alkoxides are selected from the group consisting of metal salt of methoxides, metal salt of ethoxides, metal salt of n-propoxides, metal salt of / so-propoxides, metal salt of n-butoxides, metal salt of fert-butoxides, the like, and combinations thereof, wherein the metal is selected from an alkali metal (e.g., lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr)) or an alkaline earth metal (e.g., beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra)). For example, the alkoxides may be selected from sodium methoxide, sodium ethoxide, sodium n-propoxide, sodium / so-propoxide, sodium n-butoxide, sodium terf-butoxide, potassium methoxide, potassium ethoxide, potassium n-propoxide, potassium / so- propoxide, potassium n-butoxide, potassium fert-butoxide, lithium methoxide, lithium ethoxide, lithium n-propoxide, lithium / so-propoxide, lithium n-butoxide, lithium fe / l-butoxide, caesium methoxide, caesium ethoxide, caesium n- propoxide, caesium / 'so-propoxide, caesium n-butoxide, caesium terf-butoxide, the like, or combinations thereof.

[0108] In various embodiments, the organic base comprises nitrogen-containing and / or nitrogen-based organic bases selected from the group consisting of 1 ,8- Diazabicyclo[5.4.0]undec-7-ene (DBU), 1 ,1 ,3,3-Tetramethylguanidine (TMG), 2- tert-Butyl-1 ,1 ,3,3-tetramethylguanidine (BTMG), 1 ,2,3-triisopropylguanidine (TIPG), 1 ,4-diazabicyclo[2.2.2]octane (DABCO), 1 ,8-Bis(tetramethylguanidino)- naphthalene (BTMGN), 1 ,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD), phosphazene base (P2-Et), 2,6-Lutidine, 4-Dimethylaminopyridine (DMAP), N,N,N’,N’-Tetramethyl-1 ,8-naphthalenediamine (Proton-sponge®), P4-f-Bu, triethylamine (TEA), the like, and combinations thereof.

[0109] In various embodiments, the solvent system comprises two or more, three or more, four or more, five or more, six or more, seven or more, eight or more, nine or more, or ten or more different solvents. For example, the solvent system may comprise a bi-solvent system, tri-solvent system, tetra-solvent system, penta-solvent system, hexa-solvent system, hepta-solvent system, or octasolvent system.

[0110] In various embodiments, the solvent system comprises two or more different solvents selected from the group consisting of:

[0111] (a) an aprotic solvent (or polyester-swelling solvent);

[0112] (b) a protic solvent (or base-reacting solvent); and

[0113] (c) a hydrocarbon compound (or product-precipitating solvent).

[0114] In various embodiments, the solvent system comprises two or more, or three or more different solvents selected from (a), (b) and (c). In various embodiments, it will be appreciated that the two or more solvents do not necessarily need to each come from two or more of (a), (b) and (c) above respectively, although they may do so. For example, two different solvents may all be broadly protic solvents but are different in their specific identities or the first may be an aprotic solvent and the second may be a protic solvent. In various embodiments, the solvent system comprises two different solvents, for e.g., the solvent system may be a bi-solvent system. In various embodiments, the solvent system comprises three different solvents, for e.g., the solvent system may be a tri-solvent system. In various embodiments, the solvent system comprises at least two solvents from both (a) and (b). For example, the solvent system may comprise a bi-solvent mixture having an aprotic organic solvent (or polyester- swelling solvent and an alcohol (or protic solvent, or base-reacting solvent). The solvent system may also comprise a tri-solvent mixture having an aprotic organic solvent (or polyester-swelling solvent); an alcohol (or protic solvent, or basereacting solvent); and an alkane, benzene and / or alkylbenzene (or productprecipitating solvent).

[0115] In various embodiments, the protic solvent (or base-reacting solvent) in the solvent system reacts or interacts with the inorganic base (e.g., dissociated hydroxide ions from the inorganic base) in the dual-base system to produce alkoxide ions, thereby generating / forming a solution of alkoxide ions and hydroxide ions (existing in equilibrium). Advantageously, the combination of protic solvent (or base-reacting solvent) and inorganic base supplies / provides a source or pool of alkoxide ions and hydroxide ions for a plurality / series of alcoholysis and hydrolysis reactions to take place in the depolymerizing step. Therefore, in various embodiments, the solvent system comprises a protic solvent.

[0116] In various embodiments, the aprotic solvent (or polyester-swelling solvent) swells the polyester of a dicarboxylic acid, and improves interaction between the polyester of a dicarboxylic acid and alkoxide ions, thereby facilitating / enhancing / accelerating / speeding up the alcoholysis reaction. Therefore, in various embodiments, the solvent system comprises an aprotic solvent. In various embodiments, the hydrocarbon compound (or productprecipitating solvent) facilitates / enhances / accelerates / speeds up the hydrolysis reaction and / or precipitation speed of a target product that comprises limited solubility in the reaction mixture (e g., a mono-salt of a monoester of the dicarboxylic acid), thereby enhancing reaction rate and increasing product / conversion yield. Therefore, in various embodiments, the solvent system comprises a hydrocarbon compound.

[0117] In various embodiments, the polyester-swelling solvent comprises an aprotic solvent selected from the group consisting of polar aprotic organic solvents, non-polar aprotic organic solvents and combinations thereof.

[0118] In various embodiments, the polyester-swelling solvent comprises a polar aprotic organic solvent which includes but is not limited to tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), acetonitrile (ACN), acetone (ACE), methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cyclopentanone, cyclohexanone, 2-pentanone (also known as methyl propyl ketone), 3-pentanone (also known as diethyl ketone), diethyl ether, methyl tert-butyl ether (MTBE), tertamyl methyl ether (TAME), cyclopentyl methyl ether (CPME), methoxybenzene (also known as anisole), ethoxybenzene, sulfolane (also known as tetramethylene sulfone), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMA), a,a- trifluorotoluene (TFT), the like, and combinations thereof.

[0119] In various embodiments, the polyester-swelling solvent comprises a nonpolar aprotic organic solvent which includes but is not limited to dichloromethane (DCM), chlorobenzene, diethyl ether, the like, and combinations thereof.

[0120] In various embodiments, the base-reacting solvent comprises a protic solvent selected from saturated alcohols, unsaturated alcohols, linear alcohols, branched alcohols or combinations thereof. In various embodiments, the base-reacting solvent comprises an alcohol which includes but is not limited to methanol, ethanol, n-propanol, / sopropanol, n-butanol, te / t-butanol, allyl alcohol, propargyl alcohol, the like, and combinations thereof.

[0121] In various embodiments, the product-precipitating solvent comprises a hydrocarbon compound selected from aliphatic hydrocarbon, aromatic hydrocarbon or combinations thereof. The hydrocarbon compound may be optionally substituted, for e g., one or more of hydrogen atoms in the hydrocarbon compound may be optionally substituted with a chemical moiety or functional group such as alkyl.

[0122] In various embodiments, the product-precipitating solvent comprises an optionally substituted aliphatic hydrocarbon such as alkanes (e.g., a linear alkane, or a branched alkane, or a cycloalkane) which includes but is not limited to optionally substituted pentane, optionally substituted hexane, optionally substituted heptane, optionally substituted octane, optionally substituted cyclohexane, n-pentane, n-hexane, n-heptane, n-octane, 2-methylpentane, 3- methylpentane, cyclohexane, the like, and combinations thereof.

[0123] In various embodiments, the product-precipitating solvent comprises an optionally substituted aromatic hydrocarbon which includes but is not limited to optionally substituted benzene, optionally substituted toluene, xylene (e.g., 1 ,2- dimethylbenzene, 1 ,3-dimethylbenzene, 1 ,4-dimethylbenzene), the like, and combinations thereof.

[0124] In various embodiments, the method comprises a step of chemically converting / depolymerizing / recycling / upcycling / breaking down a polyester of a dicarboxylic acid source. In various embodiments, the method or the step of depolymerizing the polyester into depolymerization / degradation product(s) comprises cleavage of the polymeric ester chains present in the polyester into shorter / smaller / simpler chains, small organic molecules, the like, or combinations thereof. For example, the polyester of a dicarboxylic acid source may be converted / depolymerized / recycled / upcycled / broken down into one or more of the following:

[0125] (1 ) a diester of the dicarboxylic acid;

[0126] (2) a mono-salt of a monoester of the dicarboxylic acid;

[0127] (3) a mono-acid of a monoester of the dicarboxylic acid;

[0128] (4) a di-alkali metal salt of the dicarboxylic acid; and

[0129] (5) a dicarboxylic acid.

[0130] In various embodiments, the method (or depolymerizing step) is designed to be custom izable / tunable to obtain specific depolymerization product(s) from the depolymerization of the polyester of a dicarboxylic acid. For example, the method (or depolymerizing step) may be configured to selectively produce a “mono-salt of a monoester of the dicarboxylic acid” (or “alkali metal salt of a monoester of the dicarboxylic acid”). The method (or depolymerizing step) may also be configured to selectively produce a “mono-acid of a monoester of the dicarboxylic acid”. The method (or depolymerizing step) may also be configured to selectively produce a “di-alkali metal salt of the dicarboxylic acid”. The method (or depolymerizing step) may also be configured to selectively produce a “dicarboxylic acid”. In various embodiments, the depolymerization products comprise both direct products obtained (e.g., a mono-salt of a monoester of dicarboxylic acid) and by-products formed (e.g., a di-alkali metal salt of dicarboxylic acid). In various embodiments, the depolymerization products may include but is not limited to “mono-salt of a monoester of the dicarboxylic acid” (or “alkali metal salt of a monoester of the dicarboxylic acid”), “mono-acid of a monoester of the dicarboxylic acid”, “di-alkali metal salt of the dicarboxylic acid”, “dicarboxylic acid” or the like. In various embodiments, the depolymerization products comprise a product comprising a mono-salt of a monoester of the dicarboxylic acid, a product comprising a mono-acid of a monoester of the dicarboxylic acid, a product comprising a di-alkali metal salt of the dicarboxylic acid, a dicarboxylic acid or derivatives thereof. In various embodiments, the method and / or step (a-i) achieves a conversion / yield (of the mono-salt of a monoester of dicarboxylic acid) of at least about 50.0%, at least about 55.0%, at least about 60.0%, at least about 65.0%, at least about 70.0%, at least about 75.0%, at least about 80.0%, at least about 81 .0%, at least about 82.0%, at least about 83.0%, at least about 84.0%, at least about 85.0%, at least about 86.0%, at least about 87.0%, at least about 88.0%, at least about 89.0%, at least about 90.0%, at least about 91.0%, at least about 92.0%, at least about 93.0%, at least about 94.0%, at least about 95.0%, at least about 96.0%, at least about 97.0%, at least about 98.0%, at least about 99.0%, at least about 99.1 %, at least about 99.2%, at least about 99.3%, at least about 99.4%, at least about 99.5%, at least about 99.6%, at least about 99.7%, at least about 99.8%, at least about 99.9%, or about 100.0%.

[0131] In various embodiments, the method and / or step (a-i) achieves a selectivity (of the mono-salt of a monoester of dicarboxylic acid) of at least about 60.0%, at least about 65.0%, at least about 70.0%, at least about 75.0%, at least about 80.0%, at least about 81.0%, at least about 82.0%, at least about 83.0%, at least about 84.0%, at least about 85.0%, at least about 86.0%, at least about 87.0%, at least about 88.0%, at least about 89.0%, at least about 90.0%, at least about 91.0%, at least about 92.0%, at least about 93.0%, at least about 94.0%, at least about 95.0%, at least about 96.0%, at least about 97.0%, at least about 98.0%, at least about 99.0%, at least about 99.1 %, at least about 99.2%, at least about 99.3%, at least about 99.4%, at least about 99.5%. In various embodiments, the selectivity percentage / value refers to the ratio of the amount of the direct product obtained (i.e. mono-salt of a monoester of dicarboxylic acid) relative to the by-products formed (e.g., di-alkali metal salt of dicarboxylic acid).

[0132] In various embodiments, the step of depolymerizing (a-i) is custom izable / tunable to obtain the desired product(s) by altering the reaction conditions. For example, the reaction conditions employed for the step of PET depolymerization can be designed / adjusted to obtain a single specific product (e.g., mono-salt of a monoester of the dicarboxylic acid) in high selectivity and / or high product / conversion yield.

[0133] In various embodiments, the depolymerizing step (a-i) comprises one or more of the following steps: mixing, stirring, and / or heating the mixture comprising:

[0134] (a) polyester of a dicarboxylic acid source;

[0135] (b) dual-base system; and

[0136] (c) solvent system (e.g, tri-solvent system).

[0137] In various embodiments, the molar ratio of the organic base to the inorganic base is from about 0.1 to about 0.75, from about 0.15 to about 0.7, from about 0.2 to about 0.65, from about 0.25 to about 0.6, from about 0.3 to about 0.55, from about 0.35 to about 0.5, from about 0.4 to about 0.45, or about 0.425. The molar ratio of the organic base to the inorganic base may be from about 0.25 to about 0.5.

[0138] In various embodiments, the molar ratio of the number of repeating units in polyester to the inorganic base to the organic base is about 1 : 0.5 - 1.5 : 0.1 - 1 , about 1 : 0.6 - 1 .4 : 0.2 - 0.9, about 1 : 0.7 - 1.3 : 0.3 - 0.8, about 1 : 0.8 - 1.2 : 0.4 - 0.7, about 1 : 0.9 - 1.1 : 0.5 - 0.6, or about 1 : 0.8 - 1.1 : 0.25 - 0.5. In various embodiments, the molar ratio of the number of repeating units in polyester to the inorganic base to the organic base is about 1 : 0.98 - 1 : 0.25 - 0.5.

[0139] In various embodiments, when water impurity is present, the molar ratio of the number of repeating units in polyester to the inorganic base is 1 : less than 1 , i.e. the molar ratio of the inorganic base to the number of repeating units in polyester is less than 1. In various embodiments, the molar ratio of the inorganic base to the number of repeating units in polyester is less than about 1 , no more than about 0.99, no more than about 0.98, no more than about 0.97, no more than about 0.96, or no more than about 0.95. Advantageously, this allows lesser excess hydroxide ions to be formed from the reaction between the organic base and water, thereby improving the selectivity of the monoester of the dicarboxylic acid formed (e.g. K-MMT).

[0140] In various embodiments, the volume ratio of the aprotic solvent (or polyester-swelling solvent) to the protic solvent (or base-reacting solvent) is from about 1 :10 to about 50:1 , from about 1 :8 to about 40:1 , from about 1 :6 to about 30: 1 , from about 1 :4 to about 20: 1 , from about 1 :3 to about 15: 1 , from about 1 :2 to about 10: 1 , or from about 1 : 1 to about 5: 1 .

[0141] In various embodiments, the volume ratio of the aprotic solvent (or polyester-swelling solvent) plus the protic solvent (or base-reacting solvent) to the hydrocarbon compound (or product-precipitating solvent) is from about 1 :1 to about 75:1 , from about 2:1 to about 70:1 , from about 3:1 to about 65:1 , from about 4:1 to about 60:1 , from about 4:1 to about 55:1 , from about 4:1 to about 50:1 , from about 5:1 to about 45:1 , from about 6:1 to about 40:1 , from about 7:1 to about 35: 1 , from about 8: 1 to about 30:1 , from about 9: 1 to about 25:1 , from about 10:1 to about 20: 1 , or from about 12:1 to about 18: 1 .

[0142] In various embodiments, the volume ratio of the polyester-swelling solvent to the base-reacting solvent to the product-precipitating solvent is about 0.5-0.8 : 0.2-0.5 : 0.1 -0.2. In various embodiments, the volume ratio of the polyester- swelling solvent to the base-reacting solvent to the product-precipitating solvent is about 0.5-0.7 : 0.3-0.4 : 0.1 -0.2, or about 0.5-0.6 : 0.3-0.4 : 0.1 -0.2.

[0143] In various embodiments, the step (a-i) of depolymerizing is performed at a temperature that is at least about 10.0°C, at least about 15.0°C, at least about 20.0°C, at least about 25.0°C, at least about 30.0°C, at least about 35.0°C, at least about 40.0°C, at least about 45.0°C, at least about 50.0°C, at least about 55.0°C, at least about 60.0°C, at least about 65.0°C, at least about 70.0°C, at least about 75.0°C, at least about 80.0°C, at least about 85.0°C, or at least about 90.0°C. The step of depolymerization may be performed at a temperature from about 10.0°C to about 90.0°C, from about 15.0°C to about 85.0°C, from about 20.0°C to about 80.0°C, from about 25.0°C to about 75.0°C, from about 30.0°C to about 70.0°C, from about 35.0°C to about 65.0°C, from about 40.0°C to about 60.0°C, from about 45.0°C to about 55.0°C, or about 50.0°C. The step of depolymerization may be carried out at a temperature of more than about 23°C to no more than about 55°C.

[0144] In various embodiments, the step (a-i) of depolymerizing is performed over a time duration of at least about 10 minutes, at least about 15 minutes, at least about 20 minutes, at least about 25 minutes, at least about 30 minutes, at least about 35 minutes, at least about 40 minutes, at least about 45 minutes, at least about 50 minutes, at least about 55 minutes, at least about 1 hour, at least about 2 hours, at least about 3 hours, at least about 4 hours, at least about 5 hours, at least about 6 hours, at least about 7 hours, at least about 8 hours, at least about 9 hours, or at least about 10 hours. The step of depolymerization may be performed over a time duration of from about 10 minutes to about 10 hours, from about 15 minutes to about 9 hours, from about 20 minutes to about 8 hours, from about 25 minutes to about 7 hours, from about 30 minutes to about 6 hours, from about 35 minutes to about 5 hours, from about 40 minutes to about 4 hours, from about 45 minutes to about 3 hours, from about 50 minutes to about 2 hours, or from about 55 minutes to about 1 hour.

[0145] In various embodiments, the step (a-i) of depolymerizing achieves a high yield of at least about 90% and a high selectivity of at least about 90% at a temperature of at least about 40°C, over a time period of about 1 hour.

[0146] In various embodiments, the polyester of a dicarboxylic acid comprises a structure that is represented by general formula (1 ): wherein

[0147] A is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heterocycloalkyl, optionally substituted heterocycloalkenyl, optionally substituted heteroaryl, or combinations thereof;

[0148] R1is optionally substituted alkylene, optionally substituted cycloalkylene, optionally substituted alkenylene, optionally substituted alkynylene, or combinations thereof; and n > 1.

[0149] In various embodiments, n is an integer that is at least about 30, at least about 300, or at least about 3,000. In various embodiments, n is an integer > 30. For example, n > 35, n > 40, n > 45, n > 50, n > 55, n > 60, n > 65, n > 70, n > 75, n > 80, n > 85, n > 90, n > 95, n > 100, n > 150, n > 200, n > 250, n > 300, n > 350, n > 400, n > 450, n > 500, n > 550, n > 600, n > 650, n > 700, n > 750, n > 800, n > 850, n > 900, n > 950, n > 1 ,000, n > 1 ,500, n > 2,000, n > 2,500, or n > 3,000. In various embodiments, n is a value that is from about 50 to about 3,000, from about 75 to about 2,750, from about 100 to about 2,500, from about 125 to about 2,250, from about 150 to about 2,000, from about 175 to about 1 ,750, from about 200 to about 1 ,500, from about 225 to about 1 ,250, from about 250 to about 1 ,000, from about 275 to about 750, from about 300 to about 500, from about 325 to about 475, from about 350 to about 450, or about 400.

[0150] In various embodiments, the average molecular weight (Mw) of the polyester of a dicarboxylic acid is from about 5,000 to about 500,000, from about 6,000 to about 450,000, from about 7,000 to about 400,000, from about 8,000 to about 350,000, from about 9,000 to about 300,000, from about 10,000 to about 250,000, from about 15,000 to about 200,000, from about 20,000 to about 150,000, from about 25,000 to about 100,000, from about 30,000 to about 95,000, from about 35,000 to about 90,000, from about 40,000 to about 85,000, from about 45,000 to about 80,000, from about 50,000 to about 75,000, from about 55,000 to about 70,000, or from about 60,000 to about 65,000.

[0151] In various embodiments, the average molecular mass (Mn) of the polyester of a dicarboxylic acid is from about 5,000 to about 500,000, from about 6,000 to about 450,000, from about 7,000 to about 400,000, from about 8,000 to about 350,000, from about 9,000 to about 300,000, from about 10,000 to about 250,000, from about 15,000 to about 200,000, from about 20,000 to about 150,000, from about 25,000 to about 100,000, from about 30,000 to about 95,000, from about 35,000 to about 90,000, from about 40,000 to about 85,000, from about 45,000 to about 80,000, from about 50,000 to about 75,000, from about 55,000 to about 70,000, or from about 60,000 to about 65,000.

[0152] In various embodiments, A comprises an optionally substituted 5- membered ring. For example, A may be selected from pyrrole, thiophene, furan, pyrazole, imidazole, imidazoline, oxazole, isoxazole, thiazole, isothiazole, triazole, oxadiazole, thiadiazole, tetrazole, the like or combinations thereof. For example, A may be an optionally substituted furan ring (e g., di-substituted furan such as 2,3-disubstituted furan, 2,4-disubstituted furan, 2,5-disubstituted furan or 3,4-disubstituted furan). In various embodiments, A comprises an optionally substituted 6-membered ring. For example, A may be selected from benzene, toluene, phenol, aniline, nitrobenzene, chlorobenzene, benzoic acid, pyridine, phosphorine, pyrimidine, pyrazine, pyridazine, 1 ,3,5-triazine, 1 ,2,4-triazine, 1 ,2,3-triazine, oxazine, thiazine, quinoline, isoquinoline, the like or combinations thereof.

[0153] In various embodiments, A comprises an optionally substituted 6- membered ring. For example, A may be an optionally substituted benzene ring (e.g., di-substituted benzene such as 1 ,2-disubstituted benzene, 1 ,3- disubstituted benzene or 1 ,4-disubstituted benzene). In various embodiments, A comprises 1 ,4-disubstituted benzene. In such embodiments, the polyester of a dicarboxylic acid comprises a structure that is represented by general formula (2): wherein

[0154] R2is optionally substituted alkylene, optionally substituted cycloalkylene, optionally substituted alkenylene, optionally substituted alkynylene, or combinations thereof;

[0155] Ra, Rb, Rcand Rdare each independently hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, or combinations thereof; and n > 1.

[0156] In various embodiments, A comprises optionally substituted fused polycyclic aryl group(s) selected from the group consisting of optionally substituted naphthalene, optionally substituted anthracene, optionally substituted phenanthrene, optionally substituted fluorene, the like or combinations thereof. For example, A may be an optionally substituted naphthalene (e.g., di-substituted naphthalene such as 1 ,2-disubstituted naphthalene, 1 ,3-disubstituted naphthalene, 1 ,4-disubstituted naphthalene, 1 ,5-disubstituted naphthalene, 1 ,6- disubstituted naphthalene, 1 ,7-disubstituted naphthalene, 1 ,8-disubstituted naphthalene, 2,3-disubstituted naphthalene, 2,6-disubstituted naphthalene, or 2,7-disubstituted naphthalene). In various embodiments, A is optionally substituted alkyl (or alkylene), optionally substituted alkenyl (or alkenylene), optionally substituted cycloalkyl (or cycloalkylene), optionally substituted cycloalkenyl (or cycloalkenylene), optionally substituted alkynyl (or alkynylene), optionally substituted aryl (or arylene), optionally substituted heterocycloalkyl (or heterocycloalkylene), optionally substituted heterocycloalkenyl (or heterocycloalkenylene), optionally substituted heteroaryl (or heteroarylene), or combinations thereof.

[0157] In various embodiments, A is selected from optionally substituted alkylene, optionally substituted alkenylene or optionally substituted alkynylene. In various embodiments, A is optionally substituted alkylene. For example, A may be selected from methanediyl, ethanediyl, propane-1 ,3-diyl, propane-1 ,2-diyl, butane-1 ,4-diyl, 2-methylpropane-1 ,3-diyl, 1 -methylpropane-1 ,3-diyl, 1 ,1 - dimethylethanediyl, hexane-1 ,6-diyl, 1 ,2-dimethylpropane-1 ,3-diyl, 1 ,1 - dimethylpropane-1 ,3-diyl, pentane-1 ,5-diyl, 3-methylbutane-1 ,4-diyl, 4- methylpentane-1 ,5-diyl, 1-methylpentane-1 ,5-diyl, 2-methylpentane-1 ,5-diyl, 3- methylpentane-1 ,5-diyl, 2,2-dimethylbutane-1 ,4-diyl, 3,3-dimethylbutane-1 ,4- diyl, 1 ,2-dimethylbutane-1 ,4-diyl, 1 ,3-dimethylbutane-1 ,4-diyl, 1 ,2,2- trimethylpropane-1 ,3-diyl, 1 ,1 ,2-trimethylpropane-1 ,3-diyl, 2-ethylpentane-1 ,5- diyl, 3-ethylpentane-1 ,5-diyl, heptane-1 ,7-diyl, 1 -methylhexane-1 ,6-diyl, 2,2- dimethylpentane-1 ,5-diyl, 3,3-dimethylpentane-1 ,5-diyl, 4,4-dimethylpentane- 1 ,5-diyl, 1 ,2-dimethylpentane-1 ,5-diyl, 1 ,3-dimethylpentane-1 ,5-diyl, 1 ,4- dimethylpentane-1 ,5-diyl, 1 ,2,3-trimethylbutane-1 ,4-diyl, 1 , 1 ,2-trimethylbutane- 1 ,4-diyl, 1 ,1 ,3-trimethylbutane-1 ,4-diyl, 5-methylheptane-1 ,7-diyl, 1 - methylheptane-1 ,7-diyl, octane-1 ,8-diyl, nonane-1 ,9-diyl, decane-1 ,10-diyl, or the like or combinations thereof. In various embodiments, A comprises -CzFhz-, where z is an integer > 1 . For example, A may be -CzH2z-, where z > 1 , z > 2, z

[0158] > 3, z > 4, z > 5, z > 6, z > 7, z > 8, z > 9, z > 10, z > 11 , z > 12, z > 13, z > 14, z

[0159] > 15, z > 16, z > 17, z > 18, z > 19, or z > 20. In various embodiments, A may be -C2H4-, -C4H8-, or - CsH - . In various embodiments, A is selected from optionally substituted alkynylene. In various embodiments, A comprises - CZH2Z-4- , where z is an integer > 2. For example, A may be -CzH2z-4-, where z > 2, z > 3, z > 4, z > 5, z > 6, z > 7, z > 8, z > 9, z > 10, z > 11 , z > 12, z > 13, z > 14, z > 15, z > 16, z > 17, z > 18, z > 19, or z > 20. In various embodiments, A may be -C=C-, -C3H2-, or -C4H4-.

[0160] In various embodiments, A is selected from optionally substituted cycloalkylene. In various embodiments, A is selected from optionally substituted cyclopropane, optionally substituted cyclobutane, optionally substituted cyclopentane, optionally substituted cyclohexane, optionally substituted cycloheptane, optionally substituted cyclooctane or optionally substituted cyclotridecane. For example, A may be an optionally substituted cyclohexane (e.g., di-substituted cyclohexane such as 1 ,2-disubstituted cyclohexane, 1 ,3- disubstituted cyclohexane or 1 ,4-disubstituted cyclohexane).

[0161] In various embodiments, R1and R2are each independently selected from optionally substituted alkylene, optionally substituted alkenylene or optionally substituted alkynylene. In various embodiments, R1and R2are each optionally substituted alkylene. For example, R1and / or R2may be selected from methanediyl, ethanediyl, propane-1 , 3-diyl, propane-1 ,2-diyl, butane-1 ,4-diyl, 2- methylpropane-1 , 3-diyl, 1 -methylpropane-1 , 3-diyl, 1 ,1 -dimethylethanediyl, hexane-1 ,6-diyl, 1 ,2-dimethylpropane-1 , 3-diyl, 1 , 1 -dimethylpropane-1 , 3-diyl, pentane-1 ,5-diyl, 3-methylbutane-1 ,4-diyl, 4-methylpentane-1 ,5-diyl, 1 - methylpentane-1 ,5-diyl, 2-methylpentane-1 ,5-diyl, 3-methylpentane-1 ,5-diyl, 2,2- dimethylbutane-1 ,4-diyl, 3,3-dimethylbutane-1 ,4-diyl, 1 ,2-dimethylbutane-1 ,4- diyl, 1 ,3-dimethylbutane-1 ,4-diyl, 1 , 2, 2-trimethylpropane-1 , 3-diyl, 1 ,1 ,2- trimethylpropane-1 , 3-diyl, 2-ethylpentane-1 ,5-diyl, 3-ethylpentane-1 ,5-diyl, heptane-1 ,7-diyl, 1 -methylhexane-1 ,6-diyl, 2, 2-dimethylpentane-1 ,5-diyl, 3,3- dimethylpentane-1 ,5-diyl, 4,4-dimethylpentane-1 ,5-diyl, 1 ,2-dimethylpentane- 1 ,5-diyl, 1 ,3-dimethylpentane-1 ,5-diyl, 1 ,4-dimethylpentane-1 ,5-diyl, 1 ,2,3- trimethylbutane-1 ,4-diyl, 1 , 1 ,2-trimethylbutane-1 ,4-diyl, 1 , 1 ,3-trimethylbutane- 1 ,4-diyl, 5-methylheptane-1 ,7-diyl, 1 -methylheptane-1 ,7-diyl, octane-1 , 8-diyl, nonane-1 ,9-diyl, decane-1 , 10-diyl, or the like or combinations thereof. In various embodiments, R1and / or R2comprises -CxH2x-, where x is an integer > 1. For example, R1and / or R2may be -CxH2x-, where x > 1 , x > 2, x > 3, x > 4, x > 5, x > 6, x > 7, x > 8, x > 9, x > 10, x > 11 , x > 12, x > 13, x > 14, x > 15, x > 16, x > 17, x > 18, x > 19, or x > 20. In various embodiments, R1and / or R2is / are ethyl (or ethylene). For example, R1and / or R2may be -C2H4-. In various embodiments, R1and / or R2is / are propyl (or propylene). For example, R1and / or R2may be -C3H6-. In various embodiments, R1and / or R2is / are butyl (or butylene). For example, R1and / or R2may be -C4H8-. In various embodiments, R1= R2.

[0162] In various embodiments, Ra, Rb, Rcand Rdare all H.

[0163] In various embodiments, the polyester of a dicarboxylic acid comprises a polyester derived from a dicarboxylic acid selected from the group consisting of terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 2,5- fu rand icarboxy lie acid, succinic acid, adipic acid, sebacic acid, dodecanedioic acid, 1 ,4-cyclohexanedicarboxylic acid, acetylene dicarboxylic acid, and combinations thereof. In various embodiments, the polyester of a dicarboxylic acid comprises polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), polyethylene furanoate (PEF), poly(butylene adipate) (PBA), poly(butylene succinate) (PBS), and poly(butylene sebacate) (PBSb), the like, or combinations thereof.

[0164] In various embodiments, the mono-salt of a monoester of the dicarboxylic acid is represented by general formula (3): wherein

[0165] A is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heterocycloalkyl, optionally substituted heterocycloalkenyl, optionally substituted heteroaryl, or combinations thereof;

[0166] R3is optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, or combinations thereof; and M is a metal or ammonium-containing group.

[0167] In various embodiments, A comprises 1 ,4-disubstituted benzene. In such embodiments, the mono-salt of a monoester of the dicarboxylic acid is represented by general formula (4): wherein

[0168] R4is optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, or combinations thereof;

[0169] R1 a, R1b, R1 cand R1dare each independently hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, or combinations thereof; and

[0170] M is a metal or ammonium-containing group.

[0171] In various embodiments, R3and R4are each independently selected from optionally substituted alkyl, optionally substituted alkenyl or optionally substituted alkynyl. The alkyl, alkenyl may have at least 1 , at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11 , at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, or at least 20 carbon atoms. In various embodiments, R3and R4are each independently selected from -CyH2y+i or -CyH2y-i, where y is an integer > 1 , y > 2, y > 3, y > 4, y > 5, y > 6, y > 7, y > 8, y > 9, y > 10, y > 11 , y > 12, y > 13, y > 14, y > 15, y > 16, y > 17, y > 18, y > 19, or y > 20. For example, R3and / or R4may be selected from methyl, ethyl, n-propyl, 2-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, hexyl, amyl, 1 ,2-dimethylpropyl, 1 ,1 -dimethylpropyl, pentyl, isopentyl, hexyl, 4-methylpentyl, 1 -methylpentyl, 2-methylpentyl, 3-methylpentyl,

[0172] 2.2-dimethylbutyl, 3,3-dimethylbutyl, 1 ,2-dimethylbutyl, 1 ,3-dimethylbutyl, 1 ,2,2- trimethylpropyl, 1 , 1 ,2-trimethylpropyl, 2-ethylpentyl, 3-ethylpentyl, heptyl, 1 - methylhexyl, 2,2-dimethylpentyl, 3,3-dimethylpentyl, 4,4-dimethylpentyl, 1 ,2- dimethylpentyl, 1 ,3-dimethylpentyl, 1 ,4-dimethylpentyl, 1 ,2,3-trimethylbutyl,

[0173] 1 .1 .2-trimethylbutyl, 1 , 1 ,3-trimethylbutyl, 5-methylheptyl, 1 -methylheptyl, octyl, nonyl, decyl or the like or combinations thereof. In various embodiments, R3and R4may contain one or more C=C double bond(s). For example, R3and / or R4may be selected from ethenyl (vinyl), allyl (2-propenyl), 1 -propenyl, 1-butenyl, 2- butenyl (cis and trans), isobutenyl, 1 -pentenyl, 2-pentenyl (cis and trans), 3- pentenyl, isopentenyl, 1 -hexenyl, 2-hexenyl (cis and trans), 3-hexenyl, 1 - heptenyl, 2-heptenyl, 3-heptenyl, 1 -octenyl, 2-octenyl, 3-octenyl, 1-nonenyl, 2- nonenyl, 3-nonenyl, 1 -decenyl, 2-decenyl, 3-decenyl, 1 -undecenyl, 2-undecenyl, 1 -dodecenyl, 2-dodecenyl, 1 -tridecenyl, 2-tridecenyl, 1 -tetradecenyl, 2- tetradecenyl, 1 -pentadecenyl, 2-pentadecenyl, 1 -hexadecenyl, 2-hexadecenyl,

[0174] 1 -heptadecenyl, 2-heptadecenyl, 1 -octadecenyl (oleyl), 2-octadecenyl, 1 - nonadecenyl, 2-nonadecenyl, 1 -eicosenyl, 2-eicosenyl, or the like or combinations thereof. The alkynyl may have at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11 , at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, or at least 20 carbon atoms. In various embodiments, R3and R4are each independently selected from -CyH2y-3, where y is an integer > 2, y > 2, y > 3, y > 4, y > 5, y > 6, y > 7, y > 8, y > 9, y > 10, y > 11 , y > 12, y > 13, y > 14, y > 15, y > 16, y > 17, y > 18, y > 19, or y > 20. In various embodiments, R3and R4may contain one or more C=C triple bond(s). For example, R3and / or R4may be selected from ethynyl (acetylide), 1 -propynyl, 2-propynyl (propargyl), 1 -butynyl,

[0175] 2-butynyl, 1 -pentynyl, 2-pentynyl, 3-pentynyl, 1 -hexynyl, 2-hexynyl, 3-hexynyl, 1 - heptynyl, 2-heptynyl, 3-heptynyl, 1 -octynyl, 2-octynyl, 3-octynyl, 4-octynyl, 1 - nonynyl, 2-nonynyl, 3-nonynyl, 4-nonynyl, 1 -decynyl, 2-decynyl, 3-decynyl, 4- decynyl, 5-decynyl, 1 -undecynyl, 2-undecynyl, 3-undecynyl, 4-undecynyl, 5- undecynyl, 1 -dodecynyl, 2-dodecynyl, 3-dodecynyl, 4-dodecynyl, 5-dodecynyl, 6- dodecynyl, 1 -tridecynyl, 2-tridecynyl, 3-tridecynyl, 4-tridecynyl, 5-tridecynyl, 6- tridecynyl, 1 -tetradecynyl, 2-tetradecynyl, 3-tetradecynyl, 4-tetradecynyl, 5- tetradecynyl, 6-tetradecynyl, 7-tetradecynyl, 1 -pentadecynyl, 2-pentadecynyl, 3- pentadecynyl, 4-pentadecynyl, 5-pentadecynyl, 6-pentadecynyl, 7-pentadecynyl, 1 -hexadecynyl, 2-hexadecynyl, 3-hexadecynyl, 4-hexadecynyl, 5-hexadecynyl, 6-hexadecynyl, 7-hexadecynyl, 8-hexadecynyl, 1 -heptadecynyl, 2-heptadecynyl, 3-heptadecynyl, 4-heptadecynyl, 5-heptadecynyl, 6-heptadecynyl, 7- heptadecynyl, 8-heptadecynyl, 1 -octadecynyl, 2-octadecynyl, 3-octadecynyl, 4- octadecynyl, 5-octadecynyl, 6-octadecynyl, 7 -octadecynyl, 8-octadecynyl, 9- octadecynyl, 1 -nonadecynyl, 2-nonadecynyl, 3-nonadecynyl, 4-nonadecynyl, 5- nonadecynyl, 6-nonadecynyl, 7-nonadecynyl, 8-nonadecynyl, 9-nonadecynyl, 1 - eicosynyl, 2-eicosynyl, 3-eicosynyl, 4-eicosynyl, 5-eicosynyl, 6-eicosynyl, 7- eicosynyl, 8-eicosynyl, 9-eicosynyl, 10-eicosynyl, the like or combinations thereof.

[0176] In various embodiments, R3and / or R4are each independently selected from -CH3, -C2H5, — CH2CH=CH2, -C3H7, -CH(CH3)2, -C4H9, -C(CH3)3, or -CH2C=CH.

[0177] In various embodiments, the identity of R3and / or R4in general formula (3), (4), (11 ), (12) and (13) corresponds to the type of alcoholysis involved (or the type of protic solvent / base-reacting solvent used) in depolymerization. For example, if methanolysis is performed (or methanol is used as the protic solvent) during depolymerization, then R3and / or R4may be methyl. If ethanolysis is performed (or ethanol is used as the protic solvent) during depolymerization, then R3and / or R4may be ethyl. If n-propanolysis is performed (or n-propanol is used as the protic solvent) during depolymerization, then R3and / or R4may be n-propyl. If / sopropanolysis is performed (or / sopropanol is used as the protic solvent) during depolymerization, then R3and / or R4may be / sopropyl. If n-butanolysis is performed (or n-butanol is used as the protic solvent) during depolymerization, then R3and / or R4may be n-butyl. If fert-butanolysis is performed (or tert-butanol is used as the protic solvent) during depolymerization, then R3and / or R4may be tert-butyl. If allyl alcoholysis is performed (or allyl alcohol is used as the protic solvent) during depolymerization, then R3and / or R4may be allyl. If propargyl alcoholysis is performed (or propargyl alcohol is used as the protic solvent) during depolymerization, then R3and / or R4may be propargyl.

[0178] In various embodiments, M is an alkali metal. For example, M may be selected from lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), francium (Fr), the like, or combinations thereof. In various embodiments, M is an alkaline earth metal. For example, M may be selected from beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), the like, or combinations thereof.

[0179] In various embodiments, M is an ammonium-containing group. For example, M may be selected from ammonium (NFh) or quaternary ammonium, e.g., tetra-alkylammonium such as tetramethylammonium, tetraethylammonium, tetrabutylammonium, the like, and combinations thereof.

[0180] In various embodiments, the identity of M in general formula (3), (4), (5), (6) and (12) corresponds to the type of base (or more particularly, the cation present in said base) used in depolymerization. For example, if potassium hydroxide is used as the inorganic base during depolymerization, then M may be potassium (K). If sodium hydroxide is used as the inorganic base during depolymerization, then M may be sodium (Na). If lithium hydroxide is used as the inorganic base during depolymerization, then M may be lithium (Li). If ammonium hydroxide is used as the inorganic base during depolymerization, then M may be ammonium (NF ). If tetra-alkylammonium hydroxide is used as the inorganic base during depolymerization, then M may be tetra-alkylammonium, e g., tetramethylammonium, tetraethylammonium, tetrabutylammonium hydroxide, the like, or combinations thereof. In various embodiments, A, R1 a, R1 b, R1cand R1dcontain one or more features and / or share one or more properties that are similar to those already described above. In various embodiments, the step of depolymerizing comprises a plurality / series of alcoholysis and hydrolysis reactions. The plurality / series of alcoholysis and hydrolysis reactions may occur at the carbonyl group(s) and / or ester group(s) / linkage(s) of the polyester. In various embodiments, the alcoholysis and hydrolysis reactions may occur in a series / sequence of cascade / tandem / domino / consecutive reactions. That is, the depolymerization may proceed first with an alcoholysis reaction, followed by a hydrolysis reaction. In various embodiments, the alcoholysis and hydrolysis reactions may occur concurrently. That is, the depolymerization may proceed with both an alcoholysis reaction and a hydrolysis reaction occurring at the same time.

[0181] In various embodiments, the depolymerizing step comprises the following wherein A is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heterocycloalkyl, optionally substituted heterocycloalkenyl, optionally substituted heteroaryl, or combinations thereof;

[0182] R1is optionally substituted alkylene, optionally substituted cycloalkylene, optionally substituted alkenylene, optionally substituted alkynylene, or combinations thereof;

[0183] R4and R4’ are each independently optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, or combinations thereof; and

[0184] M is a metal or ammonium-containing group; and n > 1.

[0185] In various embodiments, the polyester of a dicarboxylic acid is represented by general formula (10). In various embodiments, the polyester of a dicarboxylic acid (i.e. , represented by general formula (10)) is converted into a diester of the dicarboxylic acid (i.e., represented by general formula (11 )). In various embodiments, the conversion of general formula (10) into general formula (11 ) comprises alcoholysis. In various embodiments, the diester of the dicarboxylic acid (i.e., represented by general formula (11 ) is converted into a mono-salt of a monoester of the dicarboxylic acid (i.e. represented by general formula (12)). In various embodiments, the conversion of general formula (12) into general formula (13) comprises hydrolysis. In various embodiments, the mono-salt of a monoester of the dicarboxylic acid (i.e. represented by general formula (12) is converted into a mono-acid of a monoester of the dicarboxylic acid (i.e., represented by general formula (13)). In various embodiments, the conversion of general formula (12) into general formula (13) comprises acidification. The acidification may be performed in the presence of an acid e.g., inorganic / mineral acid such as hydrochloric acid (HCI), sulfuric acid (H2SO4) and boric acid. In various embodiments therefore, there is provided a method of producing alkali metal salt of a mono-ester of dicarboxylic acid via alternating alcoholysishydrolysis depolymerization / depolymerizing / recycling / upcycling / breaking down of a polyester of a dicarboxylic acid under suitable conditions to obtain a product comprising a mono-salt of a monoester of the dicarboxylic acid (e.g. M-METT) or derivatives thereof (e.g. TPA). In various embodiments, there is provided a method of alternating alcoholysis-hydrolysis depolymerization of a polyester of a dicarboxylic acid (e.g., a polyester of terephthalic acid such as polyethylene terephthalate). In various embodiments, there is provided a method of selectively converting a polyester of a dicarboxylic acid (e.g., a polyester of terephthalic acid such as polyethylene terephthalate) into a monoester of a dicarboxylic acid (e.g., a monoester of terephthalic acid such as monoalkyl terephthalate) and / or its derivatives thereof. In various embodiments, there is provided a method of preparing / producing a monoester of a dicarboxylic acid (e.g., a monoester of terephthalic acid such as monoalkyl terephthalate) and / or its derivatives thereof via alternating alcoholysis-hydrolysis depolymerization of a polyester of a dicarboxylic acid (e.g., a polyester of terephthalic acid such as polyethylene terephthalate). In various embodiments, there is provided a method of producing an alkali metal salt of a monoester of a dicarboxylic acid via alternating alcoholysis-hydrolysis depolymerization of a polyester of a dicarboxylic acid. In various embodiments, there is provided a method of producing an alkali metal salt of a monoester of terephthalic acid via alternating alcoholysis-hydrolysis depolymerization of PET. In various embodiments, there is provided a method of producing K-MMT via alternating methanolysis-hydrolysis depolymerization of PET.

[0186] In various embodiments, there is provided a method of alternating alcoholysis-hydrolysis depolymerization of a polyester of a dicarboxylic acid, selectively producing an alkali metal salt of a monoester of a dicarboxylic acid. In various embodiments, there is provided a method of alternating alcoholysishydrolysis depolymerization of a PET, selectively producing an alkali metal salt of a monoester of terephthalic acid. In various embodiments, there is provided a method of alternating ethanolysis-hydrolysis depolymerization of PET, selectively producing an alkali metal salt of a monoethyl terephthalate (Metal-MET, where Metal = K, Na, Li). In various embodiments, there is provided a method of alternating methanolysis-hydrolysis depolymerization of PET, selectively producing an alkali metal salt of a monomethyl terephthalate (Metal-MMT, where Metal = K, Na, Li).

[0187] In various embodiments, R4’ is selected from optionally substituted alkyl, optionally substituted alkenyl or optionally substituted alkynyl. The alkyl, alkenyl may have at least 1 , at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11 , at least 12, at least 13, at least 1 , at least 15, at least 16, at least 17, at least 18, at least 19, or at least 20 carbon atoms. In various embodiments, R4’ is selected from -CyH2y+i or -CyH2y-i, where y is an integer > 1 , y > 2, y > 3, y > 4, y > 5, y > 6, y > 7, y > 8, y > 9, y > 10, y > 11 , y s 12, y > 13, y > 1 , y > 15, y > 16, y > 17, y > 18, y > 19, or y > 20. For example, R4’ may be selected from methyl, ethyl, n-propyl, 2-propyl, isopropyl, n- butyl, isobutyl, sec-butyl, f-butyl, hexyl, amyl, 1 ,2-dimethylpropyl, 1 ,1 - dimethylpropyl, pentyl, isopentyl, hexyl, 4-methylpentyl, 1 -methylpentyl, 2- methylpentyl, 3-methylpentyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1 ,2- dimethylbutyl, 1 ,3-dimethylbutyl, 1 ,2,2-trimethylpropyl, 1 , 1 ,2-trimethylpropyl, 2- ethylpentyl, 3-ethylpentyl, heptyl, 1 -methylhexyl, 2,2-dimethylpentyl, 3,3- dimethylpentyl, 4,4-dimethylpentyl, 1 ,2-dimethylpentyl, 1 ,3-dimethylpentyl, 1 ,4- dimethylpentyl, 1 ,2,3-trimethylbutyl, 1 , 1 ,2-trimethylbutyl, 1 ,1 ,3-trimethylbutyl, 5- methylheptyl, 1 -methylheptyl, octyl, nonyl, decyl or the like or combinations thereof. In various embodiments, R4’ may contain one or more C=C double bond(s). For example, R4’ may be selected from ethenyl (vinyl), allyl (2 -propenyl), 1 -propenyl, 1-butenyl, 2-butenyl (cis and trans), isobutenyl, 1 -pentenyl, 2- pentenyl (cis and trans), 3-pentenyl, isopentenyl, 1 -hexenyl, 2-hexenyl (cis and trans), 3-hexenyl, 1 -heptenyl, 2-heptenyl, 3-heptenyl, 1 -octenyl, 2-octenyl, 3- octenyl, 1 -nonenyl, 2-nonenyl, 3-nonenyl, 1 -decenyl, 2-decenyl, 3-decenyl, 1 - undecenyl, 2-undecenyl, 1 -dodecenyl, 2-dodecenyl, 1 -tridecenyl, 2-tridecenyl, 1 - tetradecenyl, 2-tetradecenyl, 1 -pentadecenyl, 2-pentadecenyl, 1 -hexadecenyl, 2- hexadecenyl, 1 -heptadecenyl, 2-heptadecenyl, 1 -octadecenyl (oleyl), 2- octadecenyl, 1 -nonadecenyl, 2-nonadecenyl, 1-eicosenyl, 2-eicosenyl, or the like or combinations thereof. The alkynyl may have at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, at least 11 , at least 12, at least 13, at least 14, at least 15, at least 16, at least 17, at least 18, at least 19, or at least 20 carbon atoms. In various embodiments, R4’ is selected from - CyH2y-3, where y is an integer > 2, y > 2, y > 3, y > 4, y > 5, y > 6, y > 7, y > 8, y > 9, y > 10, y > 11 , y > 12, y > 13, y > 14, y > 15, y > 16, y > 17, y > 18, y > 19, or y > 20. In various embodiments, R4' may contain one or more C=C triple bond(s). For example, R4’ may be selected from ethynyl (acetylide), 1 -propynyl, 2-propynyl (propargyl), 1-butynyl, 2-butynyl, 1 -pentynyl, 2-pentynyl, 3-pentynyl, 1 -hexynyl, 2- hexynyl, 3-hexynyl, 1 -heptynyl, 2-heptynyl, 3-heptynyl, 1 -octynyl, 2-octynyl, 3- octynyl, 4-octynyl, 1-nonynyl, 2-nonynyl, 3-nonynyl, 4-nonynyl, 1 -decynyl, 2- decynyl, 3-decynyl, 4-decynyl, 5-decynyl, 1 -undecynyl, 2-undecynyl, 3- undecynyl, 4-undecynyl, 5-undecynyl, 1 -dodecynyl, 2-dodecynyl, 3-dodecynyl, 4- dodecynyl, 5-dodecynyl, 6-dodecynyl, 1 -tridecynyl, 2-tridecynyl, 3-tridecynyl, 4- tridecynyl, 5-tridecynyl, 6-tridecynyl, 1 -tetradecynyl, 2-tetradecynyl, 3- tetradecynyl, 4-tetradecynyl, 5-tetradecynyl, 6-tetradecynyl, 7-tetradecynyl, 1 - pentadecynyl, 2-pentadecynyl, 3-pentadecynyl, 4-pentadecynyl, 5-pentadecynyl, 6-pentadecynyl, 7-pentadecynyl, 1 -hexadecynyl, 2-hexadecynyl, 3-hexadecynyl,

[0188] 4-hexadecynyl, 5-hexadecynyl, 6-hexadecynyl, 7-hexadecynyl, 8-hexadecynyl,

[0189] 1 -heptadecynyl, 2-heptadecynyl, 3-heptadecynyl, 4-heptadecynyl, 5- heptadecynyl, 6-heptadecynyl, 7-heptadecynyl, 8-heptadecynyl, 1 -octadecynyl,

[0190] 2-octadecynyl, 3-octadecynyl, 4-octadecynyl, 5-octadecynyl, 6-octadecynyl, 7- octadecynyl, 8-octadecynyl, 9-octadecynyl, 1 -nonadecynyl, 2-nonadecynyl, 3- nonadecynyl, 4-nonadecynyl, 5-nonadecynyl, 6-nonadecynyl, 7-nonadecynyl, 8- nonadecynyl, 9-nonadecynyl, 1 -eicosynyl, 2-eicosynyl, 3-eicosynyl, 4-eicosynyl,

[0191] 5-eicosynyl, 6-eicosynyl, 7-eicosynyl, 8-eicosynyl, 9-eicosynyl, 10-eicosynyl, the like or combinations thereof. In various embodiments, R4= R4. In various embodiments, A, R1, R4, M and n contain one or more features and / or share one or more properties that are similar to those already described above.

[0192] In various embodiments, the dual-base system and solvent system (e.g., tri-solvent system) work synergistically to increase / enhance rate, efficiency, product / conversion yield and / or selectivity of the depolymerization reaction. In various embodiments, the inorganic base contained in the dual-base system (e.g., dissociated hydroxide ions from the inorganic base) reacts with the protic solvent to produce alkoxide ions, which in turn attack the carbonyl group(s) or ester group(s) / linkage(s) of the polyester.

[0193] In various embodiments, the method further comprises:

[0194] (a-ii) reacting the product comprising the alkali metal salt of mono-ester of the dicarboxylic acid with a base to obtain a solution comprising di-alkali metal salt of the dicarboxylic acid.

[0195] In various embodiments, the step (a-ii) comprises hydrolysis of the alkali metal salt of mono-ester of the dicarboxylic acid (with a base). In various embodiments, step (a-ii) comprises producing di-alkali metal salt of the dicarboxylic acid. The base may include but is not limited to metal hydroxides, alkali metal hydroxides, alkaline earth metal hydroxides, ammonium-containing hydroxides (e.g., ammonium hydroxide and quaternary ammonium hydroxides), the like and combinations thereof. In various embodiments, the base is separately provided to the reaction mixture, and is not a base from step (a-i) or produced as a byproduct from step (a-i).

[0196] In various embodiments, the di-alkali metal salt of the dicarboxylic acid comprises a structure that is represented by general formula (5): wherein

[0197] A is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heterocycloalkyl, optionally substituted heterocycloalkenyl, optionally substituted heteroaryl, or combinations thereof; and each of M is a metal or ammonium-containing group that may be same or different.

[0198] In various embodiments, A is 1 ,4-disubstituted benzene. In such embodiments, the di-alkali metal salt of the dicarboxylic acid comprises a structure that is represented by general formula (6): wherein

[0199] R2a, R2b, R2cand R2dare each independently hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, or combinations thereof; and each of M is a metal or ammonium-containing group that may be same or different. In various embodiments, R2a, R2b, R2cand R2dare all H.

[0200] In various embodiments, the method further comprises:

[0201] (a-iii) reacting the solution comprising the di-alkali metal salt of the dicarboxylic acid obtained from step (a-ii) with an oxidant / oxidizing agent to obtain a decoloured (e.g., light colored or substantially colorless) solution comprising the di-alkali metal salt of the dicarboxylic acid.

[0202] In various embodiments, the step (a-iii) comprises the degradation / removal / oxidation of dye(s) from the solution (e.g., decolorization).

[0203] In various embodiments, the oxidant / oxidizing agent includes but is not limited to sodium hypochlorite (NaOCI), calcium hypochlorite (Ca(CIO)2), H2O2, NaaOs, K2O2, CaCh?, MgO2, SrCte, ZnO?, benzoyl peroxide, tert-butyl hydroperoxide (tBuOOH), the like, and combinations thereof.

[0204] In various embodiments, the method further comprises:

[0205] (a-iv) reacting the solution obtained from step (a-ii) or (a-iii) with an acid to obtain a dicarboxylic acid.

[0206] In various embodiments, the acid includes but is not limited to inorganic / mineral acid, organic acid and combinations thereof. For example, the acid may be hydrochloric acid (HCI), sulfuric acid (H2SO4), phosphoric acid (H3PO4), acetic acid, formic acid, citric acid, oxalic acid, the like, and combinations thereof.

[0207] In various embodiments, the dicarboxylic acid comprises a structure that is represented by general formula (7): wherein

[0208] A is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heterocycloalkyl, optionally substituted heterocycloalkenyl, optionally substituted heteroaryl, or combinations thereof.

[0209] In various embodiments, A comprises 1 ,4-disubstituted benzene. In such embodiments, the di-alkali metal salt of the dicarboxylic acid comprises a structure that is represented by general formula (8): wherein

[0210] R3a, R3b, R3cand R3dare each independently hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally alkoxy, or combinations thereof.

[0211] In various embodiments, R3a, R3b, R3cand R3dare all H

[0212] In various embodiments, the dicarboxylic acid is selected from the group comprising terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, succinic acid, adipic acid, sebacic acid, dodecanedioic acid, 1 ,4-cyclohexanedicarboxylic acid, acetylene dicarboxylic acid, or combinations thereof.

[0213] In various embodiments, the method further comprises:

[0214] (a-v) isolating ethylene glycol from the solution obtained from step (a-i). In various embodiments, ethylene glycol is contained in organic solvents used in / present in step (a-i) after the mono-salt of a monoester of the dicarboxylic acid (e.g. K-MMT) is isolated by filtration. It will be appreciated that as steps (a- ii), (a-iii) and / or (a-iv) are carried out / performed in water, there is no ethylene glycol present / involved or formed in these steps.

[0215] In various embodiments, the method further comprises:

[0216] (b-i) a step of isolating the mono-salt of a monoester of the dicarboxylic acid after step (a-i);

[0217] (b-ii) a step of isolating the dicarboxylic acid after step (a-iv); and (b-iii) a step of isolating the ethylene glycol after step (a-i).

[0218] In various embodiments, the isolating step comprises one or more of the following steps: cooling, heating, drying, purifying, centrifuging, quenching, dissolving, washing, filtering, precipitating, extracting, distilling, drying, and / or decanting the desired products to remove impurities such as unreacted materials (e.g., un-depolymerized polymers such as PE, PP or un-depolymerized cotton, fabric etc), residual dyes, excess reactants, excess solvents etc.

[0219] In various embodiments, the step(s) of cooling, heating, drying, purifying, centrifuging, quenching, dissolving, washing, filtering, precipitating, distilling, drying, and / or decanting is / are repeated at least 1 time, at least 2 times, at least 3 times, at least 4 times, at least 5 times, at least 6 times, at least 7 times, at least 8 times, at least 9 times, or at least 10 times with a washing medium (e.g., organic solvents). The washing medium may be an organic solvent such as DCM, methanol, ethanol, acetone, acetonitrile, THF or ethyl acetate.

[0220] Advantageously, embodiments of the method allow polyester of dicarboxylic acid to be recycled / upcycled / converted into depolymerization products, without the need for expensive and / or tedious / complex / sophisticated / harsh chemical upcycling / recycling / recovering techniques that require high energy input. In various embodiments, the method does not require and / or is substantially devoid of the use of expensive and / or complex / sophisticated equipment that requires high energy input. Advantageously, in various embodiments, the method utilizes a simple and straightforward experimental set-up. In various embodiments, the method is cost- effective (i.e. has a low production cost). Advantageously, embodiments of the method disclosed herein have a high production yield, high scalability and / or high versatility (e.g., can be used for a wide range of waste polyester).

[0221] Advantageously, in various embodiments, the method is substantially devoid of / does not require a pre-treatment / pre-processing step to convert the polyester source into powder / dust / particle form via physical / mechanical means (e.g., powderization, pulverization, grinding and / or crushing), prior to the oxidizing / degrading / depolymerization step. While it should be appreciated that such pre-treatment / pre-processing are not strictly required in various embodiments of the method, it is still possible to still carry out such steps to increase the surface area of the polyester for reaction, which can increase the degradation speed.

[0222] Advantageously, embodiments of the method are less energy-intensive as compared to known methods; minimally affected by additives such as dyes commonly present in plastic waste; easy to perform as it is carried out in air under atmospheric pressure; requires reasonably low reaction temperature; and / or provides / achieves comparable yields to currently reported literatures within similar or shorter reaction time.

[0223] In various embodiments, the present technology is different from methods that utilize only an inorganic base (e.g., KOH). In contrast, the present technology uses a dual-base system involving both an inorganic base and an organic base.

[0224] Advantageously, the present application has shown that embodiments of the present technology using a dual-base system can significantly enhance the depolymerization rate and increase the yield to 100% while maintaining high selectivity (97.6%) to K-MMT in one hour reaction time. Such technical advantages / effects / outcomes are unexpected, unpredictable and cannot be derived in an obvious manner from methods known in the art.

[0225] Advantageously, the present application has shown that the objective technical problems arising from presence of water impurity (having a negative effect of reducing selectivity) can be overcome at least by embodiments of the present technology. For example, the present application has shown that a molar ratio of [PET-RU : KOH : Organic Base = 1 : 0.98 : 0.25] can attain a remarkable yield of 96% and a high selectivity of 99.2% to K-MMT. This technical advantage / effect / outcome constitutes another unexpected development that is unpredictable and cannot be derived in an obvious manner from methods known in the art.

[0226] In various embodiments, the present technology is different from methods known in the art in that the present technology involves the utilization of a trisolvent medium to attain a superior yield with high selectivity to K-MMT. Advantageously, the present application has shown that embodiments of the present technology using a tri-solvent medium comprising DCM / methanol / hexanes (volume ratio = 6:3:1 ) results in a 96% yield while maintaining a high selectivity of up to 98.6% in one hour reaction time. Such technical advantages / effects / outcomes are unexpected, unpredictable and cannot be derived in an obvious manner from methods known in the art.

[0227] The examples provided below (see e g., FIG. 2) clearly illustrate a mechanism which explains how KOMe and BTMG generate methoxide ions to increase the yield. Such a mechanism of how the two bases of the dual-base system work together, which accounts for the increases in yield and the retention of high selectivity to K-MMT is unpredictable and cannot be derived in an obvious manner from methods known in the art. In various embodiments, the present technology is completely different from methods that simply combine two known methods (e g., combining a known method of using organic base such as DBU to catalyze the glycolysis of PET and another known method of using NaOCHs to catalyze the methanolysis of PET).

[0228] In various embodiments, the present technology is different from methods that focus on producing DMT as a direct product. In contrast, embodiments of the present technology are capable of producing K-MMT as a direct product.

[0229] In various embodiments, the present technology is different from known methods in the art in the depolymerization mechanism used. In various embodiments, the present technology uses “alternating methanolysis-hydrolysis of PET", which is completely different from a traditional methanolysis of PET and also completely different from a traditional hydrolysis of PET.

[0230] In various embodiments, the present technology is different from methods that uses two bases having the same role / function, e g., using two bases that act as catalysts for the methanolysis of PET, and both bases are not consumed in the methanolysis of PET. Without being bound by theory, it is believed that there is no need as there is simply no advantage in using two bases having the same role / function for the same reaction. In contrast, embodiments of the present technology use two different bases having different roles / functions, namely an Arrhenius base and an organic base. In various embodiments, the Arrhenius base, such as KOH, provides hydroxide ions for the mono-hydrolysis step of DMT which is an intermediate of PET depolymerization. KOH is a reactant and is consumed in the reaction. While the organic base, such as KOCHs, provides methoxide ions. KOCH3 is a catalyst for the methanolysis step in the PET depolymerization. In the term “dual-base” used in the present application, the term “dual” refers to two different bases which act as different roles for different reactions or different elemental steps. Advantageously, the present application has shown that embodiments of the present technology designed with the right molar ratios between PET-RU : KOH : KOMe, can achieve higher reaction rates, higher yields (above 96.0%), while retaining excellent selectivity (above 98.0%). The present application also shows that embodiments of the present technology comprising tri-solvent system can improve the yield while retaining the high selectivity to K-MMT. Such technical advantages / effects / outcomes are unexpected, unpredictable and cannot be derived in an obvious manner from methods known in the art.

[0231] In various embodiments, the present technology is different from those of the art that focuses on removing the colors of PET without depolymerization. In contrast, the present technology successfully develops a new method to depolymerize waste into fine chemicals (K-MMT, MMT, TPA).

[0232] In various embodiments, the present technology is different from methods that focus on decomposition of all the dyes in PET by oxidation using an oxidant without depolymerization. In contrast, in the present technology, the inventor found the dyes in PET are released and dissolved into solvents after depolymerization, and thus most of the dyes are removed by dissolving into solvents. Furthermore, in various embodiments, the residual dyes in the obtained fine chemicals (such as K-MMT) are decomposed by oxidation using an oxidant.

[0233] In various embodiments, the present technology is different from methods that focus on producing Na2-TPA as a direct product. In contrast, embodiments of the present technology are capable of producing K-MMT as a direct product.

[0234] In various embodiments, the present technology is different from methods known in the art in the depolymerization mechanism used. In various embodiments, the present technology uses “alternating methanolysis-hydrolysis of PET”, which is a fine-controlled chemical transformation that is completely different from a photo-catalytic hydrolysis of PET in methods known in the art. In various embodiments, the present technology is different from methods known in the art in the choice of catalyst used. In various embodiments, the present technology uses the organic base (of the dual-base system) which acts as a catalyst, which is completely different from a photocatalyst (which is used / activated under irradiation by black UV light).

[0235] Advantageously, embodiments of the present technology have shown that the dyes (black, green, blue, yellow, etc.) in PET do not slow down the efficiency of PET depolymerization, unlike methods known in the art where dyes have shown to slow down the efficiency of PET depolymerization.

[0236] In various embodiments, the present technology is different from those of the art in the amount of base loading. Advantageously, embodiments of the present technology consume lesser base than methods known in the art. In various embodiments, the molar ratio of PET-ester : KOH (or NaOH) used in the PET depolymerization of present technology is 1 :0.5 for producing K-MMT, which consumes a much lesser base than methods known in the art (e.g., prior art uses a molar ratio of PET-ester : KOH (or NaOH) of about 1 :18).

[0237] In various embodiments, the present technology is different from methods known in the art in the depolymerization efficiency. Advantageously, embodiments of the present technology achieve a high depolymerization efficiency (e.g., 100% yield in 1 hour) that is not shown and cannot be expected from methods known in the art.

[0238] Advantageously, embodiments of the present technology comprising a tri-solvent system improve yield while retaining high selectivity to K-MMT. Such technical advantages / effects / outcomes are unexpected, unpredictable and cannot be derived in an obvious manner from methods known in the art.

[0239] In various embodiments, the present technology is a sustainable PET recycling and upcycling technology (SPRUT). Advantageously, in various embodiments, the present technology can be applied to industrial applications, as increasing the isolated yield while retaining high selectivity to produce K-MMT is useful on an industrial scale.

[0240] In various embodiments, the present application also comprises one or more of the following features:

[0241] (Feature 1 ) A process of selectively producing alkali metal salt of mono-ester terephthalate by alternating alcoholysis-hydrolysis depolymerization of waste PET, wherein the process comprises:

[0242] (1 ) A dual-base system comprising an inorganic base and an organic base.

[0243] (2) A solvent mixture serving as a reaction medium, wherein the solvent mixture is a bi-solvent medium or tri-solvent medium.

[0244] (Feature 2) The process of feature 1 , wherein the selectivity of alkali metal salt of mono-ester terephthalate against di-metal salt of terephthalate is above 80%, preferably above 90%, and further preferably above 95%.

[0245] (Feature 3) The process of features 1 and 2, wherein the alkali metal salt of mono-ester terephthalate is potassium mono-methyl terephthalate, sodium mono-methyl terephthalate, lithium mono-methyl terephthalate, potassium monoethyl terephthalate, sodium mono-ethyl terephthalate, lithium mono-ethyl terephthalate, potassium mono-allyl terephthalate, sodium mono-allyl terephthalate, lithium mono-allyl terephthalate, and the mixture of them.

[0246] (Feature 4) The process of feature 1 , wherein the waste PET feedstocks are post-consumer colourless and colored PET wastes ( PET bottles, PET boxes, PET films, industrial PET molds, PET fabrics / textiles), and mixed / blended fabrics of PET-cotton or PET-cotton-spandex or PET-cotton-nylon, multi-layer films of PET-PE or PET-PP, mixed plastic waste of PET with other plastics such as PE, PP, polyvinyl chloride (PVC), polystyrene (PS), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), poly(butylene adipate) (PBA), poly(butylene succinate) (PBS), and poly(butylene sebacate) (PBSb), and block copolymer of polyesters.

[0247] (Feature 5) The process of feature 1 , wherein the inorganic base is selected from the group of alkali metal hydroxide, consisting of KOH (potassium hydroxide), NaOH (sodium hydroxide), LiOH (lithium hydroxide), ammonium hydroxide, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrabutylammonium hydroxide, and mixtures thereof.

[0248] (Feature 6) The process of feature 1 , wherein the organic base is selected from the group of Lewis bases, consisting of alkoxide ions, nitrogen-based organic bases, and mixtures thereof.

[0249] (Feature 7) The process of features 1 and 6, wherein the alkoxide ions are selected from the group, consisting of methoxide ions, ethoxide ions, n-propoxide ions, iso-propoxide ions, n-butoxide ions, tert-butoxide ions, and mixtures thereof.

[0250] (Feature 8) The process of features 1 , 6, and 7, wherein the alkoxide ions are methoxide ions and ethoxide ions.

[0251] (Feature 9) The process of features 1 and 6-8, wherein the counterions for alkoxide ions are selected from the group, consisting of potassium cations, sodium cations, lithium cations, caesium cations, and mixtures thereof.

[0252] (Feature 10) The process of features 1 and 6, wherein the nitrogen-based organic base is selected from the group, consisting of DBU (1 ,8- Diazabicyclo[5.4.0]undec-7-ene), TMG (1 ,1 ,3,3-Tetramethylguanidine), BTMG (2-tert-B uty 1-1 , 1 ,3,3-tetramethylguanidine), TIPG (1 ,2,3-triisopropylguanidine), DABCO (1 ,4-diazabicyclo[2.2.2]octane), BTMGN (1 ,8- Bis(tetramethylguanidino)naphthalene), TBD (1 ,5,7-Triazabicyclo[4.4.0]dec-5- ene), P2-Et (phosphazene Base), 2,6-Lutidine, DMAP (4-Dimethylaminopyridine), Proton-sponge® (N,N,N’,N’-Tetramethyl-1 ,8-naphthalenediamine), P4-f-Bu, triethylamine (TEA), and mixtures thereof.

[0253] (Feature 11 ) The process of features 1 , 6 and 10, wherein the nitrogen-based organic base is BTMG (2-tert-Butyl-1 ,1 ,3,3-tetramethylguanidine) and DABCO (1 ,4-diazabicyclo[2.2.2]octane).

[0254] (Feature 12) The process of feature 1 , wherein the bi-solvent medium comprises a PET-swelling solvent and a base-reacting solvent.

[0255] (Feature 13) The process of feature 1 , wherein the tri-solvent medium comprises a PET-swelling solvent, a base-reacting solvent, and a product-precipitating solvent.

[0256] (Feature 14) The process of features 1 , 12 and 13, wherein the PET-swelling solvent is chosen from a group of aprotic solvents, which may either be nonpolar or polar. Dichloromethane (DCM) serves as a representative nonpolar aprotic solvent, while examples of polar aprotic solvents include tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), acetonitrile (ACN), acetone (ACE), methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cyclopentanone, cyclohexanone, 2-pentanone (also known as methyl propyl ketone), 3-pentanone (also known as diethyl ketone), diethyl ether, methyl tert-butyl ether (MTBE), tertamyl methyl ether (TAME), cyclopentyl methyl ether (CPME), methoxybenzene (also known as anisole), ethoxybenzene, sulfolane (also known as tetramethylene sulfone), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMA), a,a- trifluorotoluene (TFT), and various combinations thereof.

[0257] (Feature 15) The process of features 1 , 12 and 13, wherein the base-reacting solvent is a protic solvent, selected from a group consisting of methanol, ethanol, n-propanol, isopropanol, n-butanol, tert-butanol, allyl alcohol, and propargyl alcohol. (Feature 16) The process of features 1 and 13, wherein the product-precipitating solvent is selected from a group consisting of pentane, hexane, heptane, octane, benzene, toluene, and mixtures thereof.

[0258] (Feature 17) The process of feature 1 , wherein the mole ratio of the inorganic base to the repeating unit of PET is from 0.80 to 1 .5.

[0259] (Feature 18) The process of feature 1 , wherein the mole ratio of the inorganic base to the repeating unit of PET is from 0.90 to 1.1.

[0260] (Feature 19) The process of feature 1 , wherein the mole ratio of the organic base to the repeating unit of PET is from 0.05 to 0.90.

[0261] (Feature 20) The process of feature 1 , wherein the mole ratio of the organic base to the repeating unit of PET is from 0.10 to 0.50.

[0262] (Feature 21 ) The process of features 1 -20, wherein the mole ratio of PET- repeating unit : KOH : KOMe = 1 : 0.98 : 0.25.

[0263] (Feature 22) The process of features 1 , 12-15, wherein the volume ratio of the aprotic solvent to the protic solvent is from 1 :1 to 10:1 .

[0264] (Feature 23) The process of features 1 , 13 and 16, wherein the volume ratio of the aprotic solvent plus protic solvent to product-precipitating solvent is from 4:1 to 50:1.

[0265] (Feature 24) The process of features 1 -23, wherein said alternating depolymerizing is performed at a temperature ranging from 10 °C to 90 °C, preferably from 25 °C to 65 °C. (Feature 25) The process of features 1 -24, wherein said depolymerizing is undertaken for a time period from 10 minutes to 10 hours or from 30 minutes to 5 hours.

[0266] (Feature 26) The process of features 1 -25, wherein further comprising the hydrolysing of the metal salt of mono-ester terephthalate to di-metal salt terephthalate at room temperature in water with a base selected from the group consisting of potassium hydroxide, sodium hydroxide, lithium hydroxide, ammonium hydroxide, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrabutylammonium hydroxide.

[0267] (Feature 27) The process of features 1 -26, wherein further comprising the degradation of residual dyes in the aqueous solution of di-metal salt terephthalate with an oxidant selected from the group comprising sodium hypochlorite (NaOCI), calcium hypochlorite (Ca(CIO)2), H2O2, Na2O2, K2O2, CaCO2, MgO2, SrO2, ZnO2, benzoyl peroxide, tert-butyl hydroperoxide (tBuOOH), etc., and combination thereof.

[0268] (Feature 28) The process of features 26 and 27, wherein further comprising the acidification of di-metal salt of terephthalate to produce terephthalic acid at room temperature by the addition of the acid solution selected from the group consisting of hydrochloric acid (HCI), sulfuric acid (H2SO4), phosphoric acid (H3PO4), acetic acid, formic acid, citric acid and mixtures thereof.

[0269] (Feature 29) A process to transform colored waste PET to white TPA monomer, comprising five steps:

[0270] I. Conduct the alternating alcoholysis-hydrolysis depolymerization of colored waste PET to M-METT (alkali metal salt of mono-ester terephthalate) using a dual-base system in a bi-solvent medium or tri-solvent medium.

[0271] II. Isolation of M-METT by filtration and washing the crude M-METT with organic solvents to remove the dyes which are released from the colored PET after depolymerization. III. Dissolve M-METT in water, then add stoichiometric (equimolar) amount of MOH (KOH or NaOH or LiOH or ammonium hydroxide) to transform it into M2-TPA.

[0272] IV. An oxidant is added to the K2-TPA aqueous solution to degrade the residual dyes, resulting in a pale-colored or colorless K2-TPA solution.

[0273] V. An acid (such as H2SO4 or HCI) is added to the K2-TPA solution, producing a white TPA monomer, which is isolated by filtration.

[0274] (Feature 30) The process of feature 29, wherein the alkali metal salt of monoester terephthalate is potassium mono-methyl terephthalate, sodium monomethyl terephthalate, lithium mono-methyl terephthalate, potassium mono-ethyl terephthalate, sodium mono-ethyl terephthalate, lithium mono-ethyl terephthalate, potassium mono-allyl terephthalate, sodium mono-allyl terephthalate, lithium mono-allyl terephthalate, and the mixture of them.

[0275] (Feature 31 ) The process of feature 29, wherein the waste PET feedstocks are post-consumer colored PET wastes (PET bottles, PET boxes, PET films, industrial PET molds, PET fabrics / textiles), and mixed / blended fabrics of PET- cotton or PET-cotton-spandex or PET-cotton-nylon, multi-layer films of PET-PE or PET-PP, mixed plastic waste of PET with other plastics such as PE, PP, polyvinyl chloride (PVC), polystyrene (PS), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), poly(butylene adipate) (PBA), poly(butylene succinate) (PBS), and poly(butylene sebacate) (PBSb), and block copolymer of polyesters.

[0276] (Feature 32) The process of feature 29, wherein the dual-base system includes an inorganic base and an organic base.

[0277] (Feature 33) The process of features 29 and 32, wherein the inorganic base is selected from the group of alkali metal hydroxide, consisting of KOH (potassium hydroxide), NaOH (sodium hydroxide), LiOH (lithium hydroxide), ammonium hydroxide, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrabutylammonium hydroxide, and mixtures thereof.

[0278] (Feature 34) The process of features 29 and 32, wherein the organic base is selected from the group of Lewis bases, consisting of alkoxide ions, nitrogenbased organic bases, and mixtures thereof.

[0279] (Feature 35) The process of features 29 and 34, wherein the alkoxide ions are selected from the group, consisting of methoxide ions, ethoxide ions, n-propoxide ions, iso-propoxide ions, n-butoxide ions, tert-butoxide ions, and mixtures thereof.

[0280] (Feature 36) The process of features 29 and 34, wherein the nitrogen-based organic base is selected from the group, consisting of DBU (1 ,8- Diazabicyclo[5.4.0]undec-7-ene), TMG (1 ,1 ,3,3-Tetramethylguanidine), BTMG (2-tert-B uty 1-1 , 1 ,3,3-tetramethylguanidine), TIPG (1 ,2,3-triisopropylguanidine), DABCO (1 ,4-diazabicyclo[2.2.2]octane), BTMGN (1 ,8- Bis(tetramethylguanidino)naphthalene), TBD (1 ,5,7-Triazabicyclo[4.4.0]dec-5- ene), P2-Et (phosphazene Base), 2,6-Lutidine, DMAP (4-Dimethylaminopyridine), Proton-sponge® (N,N,N’,N’-Tetramethyl-1 ,8-naphthalenediamine), P4-t-Bu, triethylamine (TEA), and mixtures thereof.

[0281] (Feature 37) The process of feature 29, wherein the bi-solvent medium comprises a PET-swelling solvent and a base-reacting solvent.

[0282] (Feature 38) The process of feature 29, wherein the tri-solvent medium comprises a PET-swelling solvent, a base-reacting solvent, and a productprecipitating solvent.

[0283] (Feature 39) The process of features 1 , 37 and 38, wherein the PET-swelling solvent is chosen from a group of aprotic solvents, which may either be nonpolar or polar. Dichloromethane (DCM) serves as a representative nonpolar aprotic solvent, while examples of polar aprotic solvents include tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), acetonitrile (ACN), acetone (ACE), methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cyclopentanone, cyclohexanone, 2-pentanone (also known as methyl propyl ketone), 3-pentanone (also known as diethyl ketone), diethyl ether, methyl tert-butyl ether (MTBE), tertamyl methyl ether (TAME), cyclopentyl methyl ether (CPME), methoxybenzene (also known as anisole), ethoxybenzene, sulfolane (also known as tetramethylene sulfone), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMA), a,a- trifluorotoluene (TFT), and various combinations thereof.

[0284] (Feature 40) The process of features 29, 37 and 38, wherein the base-reacting solvent is a protic solvent, selected from a group consisting of methanol, ethanol, n-propanol, isopropanol, n-butanol, tert-butanol, allyl alcohol, and propargyl alcohol.

[0285] (Feature 41 ) The process of features 29 and 38, wherein the productprecipitating solvent is selected from a group consisting of pentane, hexane, heptane, octane, benzene, toluene, and mixtures thereof.

[0286] (Feature 42) The process of features 29 and 32, wherein the mole ratio of the inorganic base to the repeating unit of PET is from 0.80 to 1.5, preferably from 0.90 to 1.1.

[0287] (Feature 43) The process of feature 29, wherein the mole ratio of the organic base to the repeating unit of PET is from 0.05 to 0.90, preferably from 0.10 to 0.50.

[0288] (Feature 44) The process of features 29, 32-36, wherein the mole ratio of PET- repeating unit : KOH : KOMe = 1 : 0.98 : 0.25.

[0289] (Feature 45) The process of features 29, 39 and 40, wherein the volume ratio of the aprotic solvent to the protic solvent is from 1 :1 to 10: 1 . (Feature 46) The process of features 29, 39-41 , wherein the volume ratio of the aprotic solvent plus protic solvent to product-precipitating solvent is from 4:1 to 50:1.

[0290] (Feature 47) The process of features 29-46, wherein said alternating depolymerizing is performed at a temperature ranging from 10 °C to 90 °C, preferably from 25 °C to 65 °C.

[0291] (Feature 48) The process of features 29-47, wherein said alternating depolymerizing is undertaken for a time period from 10 minutes to 10 hours or from 30 minutes to 5 hours.

[0292] BRIEF DESCRIPTION OF FIGURES

[0293] FIG. 1 is a schematic diagram showing the molecular structures of exemplary nitrogen-based organic bases in accordance with various embodiments disclosed herein.

[0294] FIG. 2 is a schematic diagram 200 showing a method of depolymerizing a polyester of a dicarboxylic acid (derived from a polyester of a dicarboxylic acid source) into depolymerization product(s) in accordance with various embodiments disclosed herein. As shown, the method comprises dual-base promoted depolymerization of PET.

[0295] FIG. 3 is a schematic diagram 300 showing a method of “alternating alcoholysis-hydrolysis” depolymerization of a polyester of a dicarboxylic acid (derived from a polyester of a dicarboxylic acid source) into depolymerization product(s) in accordance with various embodiments disclosed herein. As shown, the method comprises alcoholysis (e.g., methanolysis) processes 302a, 302b and 302c and hydrolysis processes 304a, 304b and 304c alternating along the polymer chain. EXAMPLES

[0296] Example embodiments of the disclosure will be better understood and readily apparent to one of ordinary skill in the art from the following examples, tables and if applicable, in conjunction with the figures. It should be appreciated that other modifications related to structural, and / or chemical changes may be made without deviating from the scope of the invention. Example embodiments are not necessarily mutually exclusive as some may be combined with one or more embodiments to form new example embodiments. The example embodiments should not be construed as limiting the scope of the disclosure.

[0297] The following examples describe the development of a new process of rapidly producing alkali metal salt of mono-ester terephthalate (M-METT) e g., potassium monomethyl terephthalate (K-MMT) via “alternating methanolysis- hydrolysis” depolymerization of waste polyester of a dicarboxylic acid (e.g., waste polyethylene terephthalate (PET). In the following examples, it is demonstrated that a dual-base system promotes such an alternating depolymerization in a bisolvent medium or a tri-solvent medium. The dual-base system comprises an inorganic base and an organic base. The inorganic base used is an alkali hydroxide including but not limited to KOH, NaOH, and / or LiOH. The alkali hydroxide participates as a reactant in the alternating depolymerization of waste PET. Thus, a molar ratio of 1 :1 of KOH to PET-repeating unit is required to complete the alternating depolymerization. The organic base used is a Lewis base, such as methoxide ion, which acts as a catalyst. The organic base can increase the concentration of methoxide ions and hydroxide ions in the depolymerization system, thus accelerating the alternating depolymerization rate, increasing the isolated yield of K-MMT, and surprisingly retaining the high selectivity to K-MMT. The bi-solvent medium comprises a PET-swelling solvent and a base-reacting solvent. The tri-solvent medium comprises a PET-swelling solvent, a base-reacting solvent, and a product-precipitating solvent. As will be shown in the examples, an isolated yield of 96.0% with a selectivity of 98.6% was advantageously achieved using 1 .0 equimolar KOH (as an inorganic base) and 0.38 equimolar amount of potassium methoxide (as an organic base) relative to PET-repeating unit at 40 °C for 1 h time using a trisolvent medium of DCM, methanol, and hexanes (volume ratio = 6:3:1 ).

[0298] Therefore, a process has been developed that allows for selective production of alkali metal salt of mono-ester terephthalate (M-METT) against the di-metal salt of terephthalate by alternating alcoholysis-hydrolysis depolymerization of waste PET, and further converting M-METT to terephthalic acid (TPA).

[0299] In summary, the following examples show that a highly selective, highly efficient and environmentally benign / sustainable methodology has been developed for converting waste polyester of a dicarboxylic acid into useful smaller molecules. The approach comprises use of a mono-base system and tri-solvent system under substantially mild and / or environmentally-benign conditions. Advantageously, the method allows waste polyester of a dicarboxylic acid (e.g., waste polyester of a dicarboxylic acid source) to be upcycled in a less energy- intensive (and therefore more cost and energy-efficient) manner and under environmentally-sustainable conditions. Advantageously, the method is less energy-intensive than current methods. The method / reaction is also easy to perform as it is carried out under mild reaction conditions. Advantageously, the method is able to degrade different types of polyester products, including coloured polyester products. Advantageously, in various embodiments, the method (or depolymerizing step) is minimally affected by presence of dyes in the source. As shown in the following examples, presence of dyes in colored PET bottles and fabrics does not impede the efficiency of PET depolymerization in embodiments of the presently disclosed method. 1. Main Concepts

[0300] Dual-base Promoted Alternating Depolymerization

[0301] Dual-base promoted alternating depolymerization is demonstrated on PET in the following examples. In particular, a process is developed to selectively produce alkali metal salt of mono-ester terephthalate against di-metal salt of terephthalate by alternating alcoholysis-hydrolysis depolymerization of waste PET, wherein the process comprises:

[0302] (1 ) A dual-base system comprising an inorganic base and an organic base; and

[0303] (2) A solvent mixture serving as a reaction medium, wherein the solvent mixture is a bi-solvent medium or tri-solvent medium.

[0304] The PET depolymerization of the present application allows waste PET feedstocks to be selectively and efficiently converted into alkali metal salt of mono-ester terephthalate at mild conditions. The selectively is high up to more than 90%. One example of the alkali metal salt of mono-ester terephthalate is potassium salt of monomethyl terephthalate (K-MMT), which is produced by alternating methanolysis-hydrolysis depolymerization of waste PET. Another example of the alkali metal salt of mono-ester terephthalate is potassium salt of monoethyl terephthalate (K-MET), which is produced by alternating ethanolysis- hydrolysis depolymerization of waste PET.

[0305] The process designed in accordance with various embodiments disclosed herein further includes converting K-MMT to dipotassium salt of terephthalate (K2- TPA) in an aqueous solution and degrading or decomposing the residual dyes in the K2-TPA aqueous solution with an oxidant. The obtained pale-colored or colorless K2-TPA solution may then be acidified to produce white terephthalic acid (TPA). Therefore, in various embodiments, the present application also relates to a process to transform colored waste PET to white TPA monomer, including five steps as described below:

[0306] I. Conduct the alternating methanolysis-hydrolysis depolymerization of colored waste PET to K-MMT using a dual-base system in a bi-solvent or tri-solvent medium.

[0307] II. Isolation of K-MMT by filtration and washing the crude K-MMT with organic solvents to remove the dyes which are released from the colored PET after depolymerization.

[0308] III. Dissolve K-MMT in water, then add stoichiometric (equimolar) amount of KOH (or NaOH, ammonium hydroxide) to transform it into K2-TPA.

[0309] IV. Add an oxidant to the K2-TPA aqueous solution to degrade the residual dyes, resulting in a pale-colored or colorless K2-TPA solution.

[0310] V. Add an acid (such as H2SO4 or HCI) to the K2-TPA solution, producing a white TPA monomer, which can be isolated by filtration.

[0311] Thus, the waste PET feedstocks may include but are not limited to used colorless and colored PET wastes (PET bottles, PET boxes, PET films, industrial PET molds, PET fabrics / textiles), and mixed / blended fabrics of PET-cotton or PET-cotton-spandex or PET-cotton-nylon, multi-layer films of PET-PE or PET-PP, mixed plastic waste of PET with other plastics such as PE, PP, polyvinyl chloride (PVC), polystyrene (PS), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), poly(butylene adipate) (PBA), poly(butylene succinate) (PBS), and poly(butylene sebacate) (PBSb), and block copolymer of polyesters.

[0312] In the dual-base system, the inorganic base may be an alkali metal hydroxide selected from a group consisting of KOH, NaOH, LiOH, CsOH, ammonium hydroxide, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrabutylammonium hydroxide. The organic base may be a Lewis base, such as alkoxide ions or nitrogen-based organic bases. The examples of alkoxide ions include methoxide ions, ethoxide ions, n-propoxide ions, iso- propoxide ions, n-butoxide ions, and tert-butoxide ions. The counterions are potassium cations, sodium cations, lithium cations, and caesium cations. The examples of nitrogen-based organic bases that may be used (FIG. 1 ) are DBU (1 ,8-Diazabicyclo[5.4.0]undec-7-ene), TMG (1 ,1 ,3,3-Tetramethylguanidine), BTMG (2-tert-Butyl-1 ,1 ,3,3-tetramethylguanidine), TIPG (1 ,2,3- triisopropylguanidine), DABCO (1 ,4-diazabicyclo[2.2.2]octane), BTMGN (1 ,8- Bis(tetramethylguanidino)naphthalene), TBD (1 ,5,7-Triazabicyclo[4.4.0]dec-5- ene), P2-Et (phosphazene Base), 2,6-Lutidine, DMAP (4-Dimethylaminopyridine), Proton-sponge® (N,N,N’,N’-Tetramethyl-1 ,8-naphthalenediamine), and P4-f-Bu.

[0313] The bi-solvent medium may comprise a PET-swelling solvent and a basereacting solvent. The solvent used for swelling PET may be chosen from a group of aprotic solvents, which may be either nonpolar or polar. Dichloromethane (DOM) may serve as a representative nonpolar aprotic solvent, while examples of polar aprotic solvents include tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), acetonitrile (ACN), acetone (ACE), methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cyclopentanone, cyclohexanone, 2-pentanone (also known as methyl propyl ketone), 3-pentanone (also known as diethyl ketone), diethyl ether, methyl tert-butyl ether (MTBE), tert-amyl methyl ether (TAME), cyclopentyl methyl ether (CPME), methoxybenzene (also known as anisole), ethoxybenzene, sulfolane (also known as tetramethylene sulfone), N- methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMA), a,a-trifluorotoluene (TFT), and various combinations thereof. The base-reacting solvents may be protic solvents. Examples of protic solvents include those selected from a group consisting of methanol, ethanol, n-propanol, isopropanol, n-butanol, tert-butanol, allyl alcohol, and propargyl alcohol. Such protic solvents are weak acids. Thus, they react with inorganic and organic bases to generate alkoxide ions.

[0314] The tri-solvent medium may comprise a PET-swelling solvent and a basereacting solvent, as well as a product-precipitating solvent selected from a group consisting of pentane, hexane, heptane, octane, benzene, toluene, and mixtures thereof.

[0315] The oxidant used to degrade the dyes in M2-TPA (or K2-TPA when M is potassium) solution may be selected from the group comprising sodium hypochlorite (NaOCI), calcium hypochlorite (Ca(CIO)2), H2O2, Na2O2, K2O2, CaCh, MgOz, SrOz, ZnOs, benzoyl peroxide, fert-buty I hydroperoxide (tBuOOH), or any combination of them.

[0316] Mechanism of Dual-Base Promoted Alternating Depolymerization

[0317] FIG. 2 shows a depolymerization mechanism proposed for the method of alternating depolymerization of waste PET in the dual-base system in accordance with various embodiments disclosed herein. A two-step cascade reaction mechanism was proposed for the “alternating methanolysis-hydrolysis” depolymerization of waste PET. As shown in the schematic diagram 200, the method comprises a step 202 of alcoholysis, a step 204 of hydrolysis (e.g., monohydrolysis) and a step 206 of acidification (e g., in the presence of a mineral acid). Step 202 may be methanolysis (e.g., by using methanol as choice of a protic solvent or base-reacting solvent) or ethanolysis (e.g., by using ethanol as choice of a protic solvent or base-reacting solvent).

[0318] In step 202, a polyester of a dicarboxylic acid source “A” (e.g., polyethylene terephthalate scrap) is subjected to alcoholysis, which results in the production of an intermediate “B” in-situ. In various embodiments, the intermediate “B” comprises a diester of the dicarboxylic acid. In step 204, the intermediate “B” undergoes dissolution and hydrolysis (e.g., monohydrolysis), and converts into a mono-salt of a monoester of the dicarboxylic acid “C”. Due to its limited solubility in the solvent system, “C” precipitates out as a solid. In step 206, “C” is subjected to acidification, and converts into a mono-acid of a monoester of the dicarboxylic acid “D”. Step 206 may be carried out using sulfuric acid (as mineral acid) at room temperature. In various embodiments, the “R” substituent in the ester (i.e. -C(=O)-O- R) present in “B”, “C” and “D” corresponds to the type of alcoholysis involved (or the type of protic solvent / base-reacting solvent used) in depolymerization. For example, if methanolysis is performed (or methanol is used as the protic solvent) during depolymerization, then R may be methyl. If ethanolysis is performed (or ethanol is used as the protic solvent) during depolymerization, then R may be ethyl.

[0319] In various embodiments, when step 202 comprises methanolysis (e g., methanol), polyethylene terephthalate scrap (i.e. A) is converted into dimethyl terephthalate (DMT) (i.e. B), which undergoes hydrolysis and is precipitated out in step 204 as alkali metal salt of mono-methyl terephthalate (M-MMT) (i.e. C). The alkali metal salt of mono-methyl terephthalate (M-MMT) is isolated from the reaction mixture of PET depolymerization. The M-MMT is then dissolved in water and subsequently acidified in step 206 to form monomethyl terephthalate (MMT) (i.e. D). In various embodiments, C comprises potassium salt of monomethyl terephthalate (K-MMT) when a potassium containing base is used in the dualbase system.

[0320] In various embodiments, when step 202 comprises ethanolysis (e.g., ethanol is used in tri-solvent system), polyethylene terephthalate scrap (i.e. A) is converted into diethyl terephthalate (DET) (i.e. B), which undergoes hydrolysis and is precipitated out in step 204 as alkali metal salt of mono-ethyl terephthalate (M-MET) (i.e. C). The alkali metal salt of mono-ethyl terephthalate (M-MET) is isolated from the reaction mixture of PET depolymerization. The M-MET is then dissolved in water and subsequently acidified in step 206 to form monoethyl terephthalate (MET) (i.e. D). In various embodiments, C comprises potassium salt of monoethyl terephthalate (K-MET) when a potassium containing base is used in the dual-base system.

[0321] This cascade depolymerization mechanism comprises two consecutive basic reactions involving dimethyl terephthalate (DMT) as an intermediate. The first basic reaction is the methanolysis of PET to form a soluble intermediate DMT. The second basic reaction is the mono-hydrolysis of DMT to potassium monomethyl terephthalate (K-MMT) which precipitates quickly from the reaction solution after being generated. The intermediate DMT is in-situ generated and rapidly mono-hydrolyzed into K-MMT.

[0322] FIG. 2 also describes how the two bases work together to enhance the depolymerization efficiency significantly.

[0323] When potassium hydroxide (KOH) is used as the sole base, KOH is dissolved into the solvent mixture of bi-solvent medium or tri-solvent medium. In the solution, KOH dissociates into potassium cations and hydroxide ions. Since hydroxide ion is a strong base, it reacts with methanol which is a weak acid, producing methoxide ions. Thus after dissolving KOH into the solvent mixture, a large fraction of the hydroxide ions reacts with methanol to produce methoxide ions, and generates Equilibrium-1 (FIG. 2) between methoxide ions and hydroxide ions. Although hydroxide ions are a stronger base than methoxide ions, the methoxide ions are a stronger nucleophile due to their positive inductive effect. Therefore, the methoxide ions will attack the carbonyl of the PET-repeating unit (PET-RU) and in-situ generate DMT. Since DMT is soluble in the solvent mixture, it is subsequently mono-hydrolyzed into K-MMT, precipitating out as white powder. Thus, this is a two-step cascade reaction, or tandem reaction, or domino reaction, or concurrent reaction. Overall, this is a highly selective “alternating methanolysis-hydrolysis” depolymerization of waste PET to convert waste PET into K-MMT in a single or one-pot process.

[0324] In the case of using potassium methoxide (KOME) as the organic base, the dissolved KOME dissociates into methoxide ions and potassium cations. The dual-base system (KOME and KOH) contributes to the molar concentration of methoxide ions. Thus, the concentration of methoxide ions is higher than that of using KOH only (Ca2 > Cai, Equilibrium-2, FIG. 2), which enhances the first basic step, i.e. methanolysis of PET into DMT more rapidly. In addition, due to the right shifting of Equilibrium-2, the molar concentration of hydroxide ions is also increased (Cb2 > CM , Equilibrium-2, FIG. 2). Thus, the second basic step is also enhanced, i.e. mono-hydrolysis of DMT into K-MMT is accelerated.

[0325] In the case of using 2-tert-butyl-1 ,1 ,3,3-tetramethylguanidine (BTMG) as the organic base, BTMG can also increase the molar concentration of methoxide ions and hydroxide ions (Ca3 > Cai, Cb3 > CM , Equilibrium-3, FIG. 2) via deprotonating methanol. Similarly, the alternating depolymerization rate is also enhanced.

[0326] The inorganic base potassium hydroxide (KOH) or sodium hydroxide (NaOH) is an Arrhenius base, which participates in PET depolymerization. Thus, it is consumed in the reaction. The molar ratio (1 :1 ) of KOH (or NaOH) to PET- RU is required to complete the depolymerization of PET into K-MMT. This can save a lot of inorganic bases compared to methods in the art, wherein the excess base is used, and the molar ratio of KOH (or NaOH) to PET-RU is 3.8:1 .

[0327] In the acidification step to convert K2-TPA (or disodium salt of terephthalate Na2-TPA) into TPA, two stoichiometric (equimolar) amount of acid (such as HCI) is required. If an excess base is used in the depolymerization step, it will be appreciated that the equimolar amount of acid relative to the excess base is required to neutralize the excess base in the acidification step. Therefore, methods in the art require 3.8 times stoichiometric (equimolar) amount of acid relative to PET-RU in the acidification step. On the contrary, the method designed in accordance with various embodiments disclosed herein has advantageously shown that the method is acid-saving or the amount of acid may be reduced because stoichiometric (equimolar) amount of base and the stoichiometric (equimolar) amount of acid are used.

[0328] The organic base, such as KOME or BTMG, is a Lewis base, which acts as a catalyst. Such catalysts can increase the concentration of methoxide ions, thus enhancing the alternating depolymerization efficiency. The methoxide ions generated by the organic base will not be consumed. After PET depolymerization, the soluble methoxide ions stay in the solution and can be reused for the next round of PET depolymerization.

[0329] For the sole base (KOH or NaOH) promoted “alternating methanolysis- hydrolysis” depolymerization, there is only one portion of methoxide ions generated by the reaction of KOH with methanol. During the PET depolymerization, with the hydroxide ions being consumed, the chemical equilibrium will right-shift to convert the methoxide ions back to hydroxide ions. With the depolymerization proceeding, the molar concentrations of methoxide ions and hydroxide ions are becoming lower and lower. Thus, the reaction is becoming slower and slower.

[0330] For the dual-base promoted PET depolymerization, there are two portions of methoxide ions. The first portion is generated by the inorganic base (Cai, FIG. 2). The second portion is generated by the organic base. In this regard, Ci and C2 are generated by KOME and BTMG, respectively (FIG. 2). During the PET depolymerization, with the hydroxide ions being consumed, the chemical equilibrium will right-shift to convert the first portion of methoxide ions back to hydroxide ions. With the depolymerization proceeding, the molar concentration of hydroxide ions is becoming lower and lower. However, the concentration of methoxide ions generated by the organic base (Ci and C2, FIG. 2) will remain constant, helping to utilize the inorganic base as much as possible and thus enhancing the depolymerization rate and isolated yield.

[0331] The “alternating alcoholysis-hydrolysis” processes / reactions may occur at the ester linkages as shown in FIG. 3. In FIG. 3, a schematic diagram 300 is shown which illustrates a method of depolymerizing a polyester of a dicarboxylic acid (derived from a polyester of a dicarboxylic acid source) into depolymerization product(s) through a combination of alcoholysis and hydrolysis reactions. As shown, the method comprises alcoholysis (e.g., methanolysis) processes 302a, 302b and 302c, and hydrolysis processes 304a, 304b and 304c alternating along the polymer chain. The alcoholysis (e.g., methanolysis) processes and hydrolysis processes may occur at the ester linkages, or more specifically at the carbonyl groups (-C(=O)-) along the polyester chain. Through the “alternating methanolysis-hydrolysis” reactions, the polyester of a dicarboxylic acid is converted / depolymerized into an alkali metal salt of mono-ester (AM-MMT, where AM is an alkali metal).

[0332] Effect of Water Impurity

[0333] In the examples, PET depolymerization is carried out using commercial solvents in air. It would be appreciated that these aprotic solvents and protic solvents always contain small amounts of water as an impurity. The air, the PET waste, and the bases used also bring a small amount of water to the reaction. When using the stoichiometric (equimolar) amount of KOH (or NaOH) as the sole base to depolymerize PET, the water impurity has no evident effect on the selectivity to K-MMT, which is high up to 99.4%.

[0334] However, when the dual-base system, such as KOH with KOME, is used to depolymerize PET, it is believed that the KOME will react with the water impurity to produce hydroxide ions. Therefore, if the stoichiometric (equimolar) amount of KOH relative to PET-RU is added to the reaction, the total number of hydroxide ions will exceed one stoichiometric amount of PET-RU. The excess hydroxide ionswill increase the chances of further hydrolyzing K-MMT to K2-TPA, resulting in the decrease of the selectivity to K-MMT.

[0335] The experimental results proved that when using 1 .0 equimolar KOH (as the inorganic base) and 0.25 equimolar amount of KOME (as the organic base) relative to PET-RU in one hour at 40 °C using a bi-solvent medium of DCM and methanol (volume ratio = 2:1 ), an isolated yield of 92.6% was achieved, but the selectivity was decreased to 99.0%. To improve the selectivity, the KOH loading was reduced to 0.98 equimolar amount relative to PET-RU, while keeping other conditions unchanged. The inventors surprisingly found out that both high selectivity (99.3%) and high isolated yield (91.9%) were achieved for a one-hour reaction.

[0336] Furthermore, when THF was used to replace DCM as the aprotic solvent and the PET depolymerization was run at 55 °C, an isolated yield of 96.0% was achieved while retaining the high selectivity of 99.2% for a one-hour reaction. Therefore, embodiments of the presently disclosed method represent a practical technology with great potential for industrial applications.

[0337] Role of Solvents

[0338] In the examples, the PET-swelling solvent (such as DCM) helps enhance the interaction between PET and bases (such as methoxide ions), thus accelerating the reaction rate. The base-reacting solvent (such as methanol) reacts with the inorganic base (such as KOH) to generate methoxide ions, creating a solution containing both methoxide ions and hydroxide ions. The product-precipitating solvent (such as hexane and heptane) accelerates the precipitating rate of K-MMT from the reaction solution, thus leading to a high reaction rate and isolated yield.

[0339] Isolation of Ethylene Glycol (EG) from PET Depolymerization

[0340] In the examples, the solid product K-MMT from PET depolymerization is precipitated from the reaction solution and isolated by filtration. Another product from PET depolymerization is ethylene glycol, which is a liquid and miscible with the solvent mixture. Ethylene glycol can be isolated by removing the low-boiling- point solvents and purified by distillation under reduced pressure. Ethylene glycol is stable at the depolymerization concentrations. Decolorization in case of Using Coloured Waste PET as Feedstock

[0341] It has been shown in the examples that the dyes in the colored PET bottles and fabrics did not slow down the PET depolymerization efficiency in this application. Thus, various colored PET bottles and various colored PET fabrics can be used as feedstock.

[0342] For example, black PET fabric can be depolymerized rapidly, affording a black slurry. When the K-MMT product is filtered and washed with DCM, the obtained K-MMT is a pale black powder, and the filtrate is a black solution. The result showed that, after depolymerization, the organic dyes were released and could be removed by washing K-MMT with organic solvents, such as DCM, methanol, THF, ethyl acetate, etc.

[0343] When the obtained K-MMT is dissolved into water, and equimolar KOH (or NaOH) is added to transform K-MMT to K2-TPA, a pale black aqueous solution is formed. An oxidant may be added to the above solution and stirred at room temperature to degrade or decompose the residual dyes, affording a pale-yellow solution or nearly colorless solution. The oxidant may be selected from the group comprising sodium hypochlorite (NaOCI), calcium hypochlorite (Ca(CIO)2), H2O2, Na2O2, K2O2, CaCO2, MgO2, SrO2, ZnO2, benzoyl peroxide, tert-butyl hydroperoxide (tBuOOH), etc., and any combination of them.

[0344] To the obtained pale-yellow solution or nearly colorless solution, an acid solution (such as H2SO4 or HCI) may be added to acidify K2-TPA to the white TP A monomer. Therefore, this application provides a chemical process to produce white TPA from colored waste PET.

[0345] Using Unsorted Plastic Waste as Feedstock

[0346] In real life, plastic waste is usually a mixture of PET with other materials such as PP, PE, cotton fabric, paper, nylon, and spandex. The process designed in accordance with various embodiments disclosed herein can selectively depolymerize PET composition while keeping other compositions (PP, PE, cotton fabric, paper, nylon, spandex) intact. Thus, various unsorted plastic waste mixtures can be used as feedstocks and the PET composition can be selectively depolymerized and transformed to K-MMT and further to TPA for PET recycling.

[0347] Unique features of the presently disclosed method

[0348] The method disclosed in accordance with various embodiments disclosed herein is different from conventional methods in one or more of the following features:

[0349] (1 ) Conventional methods do not use a dual-base system involving both an inorganic base and an organic base for depolymerization of polyester of a dicarboxylic acid.

[0350] (2) Conventional methods fail to appreciate that the dual-base system of the present technology can significantly enhance the depolymerization rate and increase the yield to 100% while maintaining high selectivity (97.6%) to K-MMT (E-6, Table 1 ) in one hour reaction time. This unexpected outcome is non-obvious and unpredictable from conventional methods.

[0351] (3) Conventional methods do not appreciate that water impurity has a negative effect of reducing selectivity. On the contrary, the present application has established an optimal molar ratio of [PET-RU : KOH : Organic Base = 1 : 0.98 : 0.25] to attain a remarkable yield of 96% and a high selectivity of 99.2% (E-4, Table 1 ) to K-MMT. This outcome constitutes another unexpected development that is non-obvious and unpredictable from conventional methods.

[0352] (4) Conventional methods do not utilize a tri-solvent medium to attain a superior yield and increased selectivity to K-MMT compared to the use of a bisolvent medium. This represents a novel concept. The present application shows that the tri-solvent medium comprising DCM / methanol / hexanes (volume ratio = 6:3:1 ) results in a 96% yield while maintaining a high selectivity of up to 98.6% (E-7, Table 1 ) in one hour reaction time. (5) There is no elucidation in conventional methods on the mechanism of how the two bases of the dual-base system work together, which accounts for the increases in yield and the retention of high selectivity to K-MMT. In this application, FIG. 2 clearly illustrates the mechanism which explains how KOMe and BTMG generate methoxide ions to increase the yield, but the retention of high selectivity to K-MMT is a surprise.

[0353] 2. Experiments and Results

[0354] 2.1. General Considerations

[0355] Preparation of PET or PET-containing feedstocks:

[0356] In general, all kinds of waste PET or PET-containing waste can be used as feedstock in this application, including but not limited to colorless and colored PET bottles, colorless and colored PET boxes, colorless and colored PET films, various colored PET fabrics or textiles, blended or mixed fabrics of PET with cotton, rayon, nylon, and spandex.

[0357] Feedstock-1’, scraps of colorless PET bottles. Without undergoing any cleaning process, diverse brands of post-consumer PET water bottles (colorless and transparent) were collected and manually cut into small pieces measuring approximately 1 .5x1 .5 cm and used as feedstock.

[0358] Feedstock-2’, mixed scraps of colorless PET, PE, and PP scraps. Three kinds of colorless plastic bottles (PET, PE, PP) were cut into small pieces measuring approximately 1.5x1.5 cm and mixed as feedstock. The weight ratio of PET:PE:PP is about 2:1 :1.

[0359] Feedstock-3, scraps of red PET bottles. The red PET bottles were cut into small pieces measuring approximately 1 .5x1 .5 cm and used as feedstock.

[0360] Feedstock-4: scraps of white PET-cotton blended fabric (PET polyester 28%, cotton 72%, on the label). The white PET-cotton blended fabric was cut into small pieces measuring approximately 1 .5x1 .5 cm and used as feedstock. Feedstock-5, scraps of deep black PET fabric (100% PET polyester on the label). The black PET fabric waste was cut into small pieces measuring approximately 1 .5x1 .5 cm using a pair of scissors and used as feedstock.

[0361] Feedstock-6: scraps of colored PET-containing fabric waste. The waste fabrics include black fabric (PET polyester 100%), black blended fabric (PET polyester 95%, spandex 5%), black blended fabric (PET polyester 80%, cotton 20%), and brown blended fabric (PET polyester 80%, rayon 20%). These four waste fabrics were cut into small pieces measuring approximately 1.5x1.5 cm and mixed as feedstock. The weight ratio of these four colored fabrics is about 1 :1 :1 :1 .

[0362] Feedstock-7: scraps of colored PET-containing fabric waste. The waste fabrics include blue blended fabric (PET polyester 91 %, spandex 9%), red blended fabric (PET polyester 91%, spandex 9%), yellow blended fabric (PET polyester 83%, rayon 11 %, spandex 6%), and cyan blended fabric (PET polyester 83%, rayon 11 %, spandex 6%). These four waste fabrics were cut into small pieces measuring approximately 1.5x1.5 cm and mixed as feedstock. The weight ratio of these four colored fabrics is about 1 : 1 : 1 : 1 .

[0363] Determination of selectivity:

[0364] The molar ratio between K-MMT and K2-TPA is determined by calculating the integral area of the Ph-H peaks of K-MMT (at 7.92 ppm and 8.07 ppm) relative to the Ph-H peak of the impurity K2-TPA (at 7.88 ppm) in the1H-NMR spectrum of the resulting K-MMT product.

[0365] Solvents and chemicals:

[0366] All chemicals and solvents bought from the specified providers were utilized as obtained without further purification in all the examples and comparative examples.

[0367] Dichloromethane (DCM): >99.5%, Avantor Performance Materials.

[0368] Tetrahydrofuran: >99.5%, VWR Chemicals.

[0369] Methanol: >99.9%, VWR Chemicals. Hexanes: Assay (sum of 5 isomers, total hexanes plus methylcyclopentane) (by GC) >98.5%, Avantor Performance Materials.

[0370] Potassium hydroxide (KOH): >85.0%, Merck KGaA.

[0371] Potassium methoxide (KOME): 95%, Aldrich.

[0372] 2-tert-Butyl-1 ,1 ,3,3-tetramethylguanidine (BTMG): >95.0%, Tokyo Chemical Industry.

[0373] 1 ,4-diazabicyclo[2.2.2]octane (DABCO): >99%, Sigma-Aldrich.

[0374] Deuterium oxide (D2O, 99.96% D): Cambridge Isotope Laboratories.

[0375] Nuclear magnetic resonance (NMR) measurement:

[0376] 1H-NMR (500MHz) and13C-NMR (125MHz) spectra were recorded with JEOL 500MHz NMR spectrometer.

[0377] 2.2. Experimental procedure for two comparative examples (CE-1 and CE-2)

[0378] Feedstock-1 (scraps of colorless PET bottles, 5 g, repeating unit 26.0 mmol), DCM (26.7 mL), methanol (13.3 mL), and KOH (1 .72 g, 26.0 mmol) were added into a 100 mL RBF in order in air. The resulting mixture was vigorously stirred with a magnetic stirring bar at room temperature (23 °C) for CE-1 , or at 40 °C by heating in a silicon oil bath for CE-2. A white slurry was obtained after 1 hour. The white solid was collected by filtration and washed with DCM (20 mL). No unreacted PET was observed. The white K-MMT product was first dried in air in a fumehood overnight, then dried in an oven at 60 °C for 6 hours. The obtained K-MMT was characterized by1H and13C-NMR. The results are listed in Table 1.

[0379] CE-1 : Obtained K-MMT 3.93 g, 69.2%. The selectivity of K-MMT against K2-TPA is 99.5%.

[0380] CE-2: Obtained K-MMT 4.76 g, 83.8%. The selectivity of K-MMT against K2-TPA is 99.4%.

[0381] 1H-NMR (500 MHz, D2O, ppm) of K-MMT: 3.95 (s, 3H, -CHs), 7.92 (d, 2H, ph-H, JC-H = 8Hz), 8.07 (d, 2H, ph-H, JC-H = 8Hz).13C-NMR (125 MHz, D2O, ppm) of K-MMT: 52.80, 128.77, 129.39, 131.48, 141.16, 169.10, 174.80.

[0382] 2.3. Eleven Examples (E-1 ~ E11, Table 1)

[0383] Experimental procedure for E-1 :

[0384] Feedstock-1 (scraps of colorless PET bottles, 5 g, repeating unit 26.0 mmol), DCM (26.7 mL), methanol (13.3 mL), KOH (1.72 g, 26.0 mmol), and potassium methoxide (0.456 g, 6.5 mmol) were added into a 100 mL RBF in order in air. The resulting mixture was vigorously stirred with a magnetic stirring bar at room temperature (23 °C). A white slurry was obtained after 1 hour. The white solid was collected by filtration and washed with DCM (20 mL). No unreacted PET was observed. The white K-MMT product was first dried in air in a fumehood overnight, then dried in an oven at 60 °C for 6 hours. The obtained K-MMT was characterized by1H and13C-NMR. The results are listed in Table 1.

[0385] Obtained K-MMT 4.48 g, yield 78.9%. The selectivity of K-MMT against K2-TPA is 99.4%.

[0386] 1H-NMR (500 MHz, D2O, ppm) of K-MMT: 3.95 (s, 3H, -CW3), 7.92 (d, 2H, ph-H, JC-H = 8Hz), 8.07 (d, 2H, ph-H, JC-H = 8Hz).

[0387] 13C-NMR (125 MHz, D2O, ppm) of K-MMT: 52.80, 128.77, 129.39, 131.48, 141.16, 169.10, 174.80.

[0388] Experimental procedure for E-2:

[0389] Feedstock-1 (scraps of colorless PET bottles, 5 g, repeating unit 26.0 mmol), DCM (26.7 mL), methanol (13.3 mL), KOH (1.72 g, 26.0 mmol), and potassium methoxide (0.456 g, 6.5 mmol) were added into a 100 mL RBF in order in air. The resulting mixture was vigorously stirred with a magnetic stirring bar at 40 °C by heating in a silicon oil bath. A white slurry was obtained after 1 hour. The white solid was collected by filtration and washed with DCM (20 mL). No unreacted PET was observed. The white K-MMT product was first dried in air in a fumehood overnight, then dried in an oven at 60 °C for 6 hours. The obtained K-MMT was characterized by1H and13C-NMR. The results are listed in table 1 .

[0390] Obtained K-MMT 5.26 g, yield 92.6%. The selectivity of K-MMT against K2-TPA is 99.0%.

[0391] 1H-NMR (500 MHz, D2O, ppm) of K-MMT: 3.95 (s, 3H, -CH3), 7.92 (d, 2H, ph-H, JC-H = 8Hz), 8.07 (d, 2H, ph-H, JC-H = 8Hz).

[0392] 13C-NMR (125 MHz, D2O, ppm) of K-MMT: 52.80, 128.77, 129.39, 131.48, 141.16, 169.10, 174.80.

[0393] Experimental procedure of E-3:

[0394] Feedstock-1 (scraps of colorless PET bottles, 5 g, repeating unit 26.0 mmol), DCM (26.7 mL), methanol (13.3 mL), KOH (1.68 g, 25.5 mmol), and potassium methoxide (0.456 g, 6.5 mmol) were added into a 100 mL RBF in order in air. The resulting mixture was vigorously stirred with a magnetic stirring bar at 40 °C by heating in a silicon oil bath. A white slurry was obtained after 1 hour. The white solid was collected by filtration and washed with DCM (20 mL). No unreacted PET was observed. The white K-MMT product was first dried in air in a fumehood overnight, then dried in an oven at 60 °C for 6 hours. The obtained K-MMT was characterized by1H and13C-NMR. The results are listed in Table 1.

[0395] Obtained K-MMT 5.22 g, yield 91.9%. The selectivity of K-MMT against K2-TPA is 99.3%.

[0396] 1H-NMR (500 MHz, D2O, ppm) of K-MMT: 3.95 (s, 3H, -CW3), 7.92 (d, 2H, ph-H, JC-H = 8Hz), 8.07 (d, 2H, ph-H, JC-H = 8Hz).

[0397] 13C-NMR (125 MHz, D2O, ppm) of K-MMT: 52.80, 128.77, 129.39, 131.48, 141.16, 169.10, 174.80.

[0398] Feedstock-1 (scraps of colorless PET bottles, 5 g, repeating unit 26.0 mmol), THF (40 mL), methanol (20 mL), KOH (1 .68 g, 25.5 mmol), and potassium methoxide (0.456 g, 6.5 mmol) were added into a 150 mL RBF in order in air. The resulting mixture was vigorously stirred with a magnetic stirring bar at 55 °C by heating in a silicon oil bath. A white slurry was obtained after 1 hour. The white solid was collected by filtration and washed with DCM (20 mL). No unreacted PET was observed. The white K-MMT product was first dried in air in a fumehood overnight, then dried in an oven at 60 °C for 6 hours. The obtained K-MMT was characterized by1H and13C-NMR. The results are listed in Table 1.

[0399] Obtained K-MMT 5.54 g, yield 96.0%. The selectivity of K-MMT against K2-TPA is 99.2%.

[0400] 1H-NMR (500 MHz, D2O, ppm) of K-MMT: 3.95 (s, 3H, -CHs), 7.92 (d, 2H, ph-H, JC-H = 8Hz), 8.07 (d, 2H, ph-H, JC-H = 8Hz).

[0401] 13C-NMR (125 MHz, D2O, ppm) of K-MMT: 52.80, 128.77, 129.39, 131.48, 141.16, 169.10, 174.80.

[0402] Experimental procedure of E-5:

[0403] Feedstock-1 (scraps of colorless PET bottles, 5 g, repeating unit 26.0 mmol), DCM (26.7 mL), methanol (13.3 mL), KOH (1.72 g, 26.0 mmol), and potassium methoxide (0.912 g, 13.0 mmol) were added into a 100 mL RBF in order in air. The resulting mixture was vigorously stirred with a magnetic stirring bar at 40 °C by heating in a silicon oil bath. A white slurry was obtained after 1 hour. The white solid was collected by filtration and washed with DCM (20 mL). No unreacted PET was observed. The white K-MMT product was first dried in air in a fumehood overnight, then dried in an oven at 60 °C for 6 hours. The obtained K-MMT was characterized by1H and13C-NMR. The results are listed in Table 1 .

[0404] Obtained K-MMT 5.62 g, yield 99.0%. The selectivity of K-MMT against K2-TPA is 95.7%.

[0405] 1H-NMR (500 MHz, D2O, ppm) of K-MMT: 3.95 (s, 3H, -CW3), 7.92 (d, 2H, ph-H, JC-H = 8Hz), 8.07 (d, 2H, ph-H, JC-H = 8Hz).

[0406] 13C-NMR (125 MHz, D2O, ppm) of K-MMT: 52.80, 128.77, 129.39, 131.48, 141.16, 169.10, 174.80. Experimental procedure of E-6 and E-7:

[0407] Feedstock-1 (scraps of colorless PET bottles, 5 g, repeating unit 26.0 mmol), DCM (24 mL), methanol (12 mL), hexanes (4 mL), KOH (1.72 g, 26.0 mmol), and potassium methoxide (E-6: 0.91 g, 13.0 mmol; E-7: 0.68 g, 9.8 mmol) were added into a 100 mL RBF in order in air. The resulting mixture was vigorously stirred with a magnetic stirring bar at 40 °C by heating in a silicon oil bath. A white slurry was obtained after 1 hour. The white solid was collected by filtration and washed with DCM (20 mL). No unreacted PET was observed. The white K-MMT product was first dried in air in a fumehood overnight, then dried in an oven at 60 °C for 6 hours. The obtained K-MMT was characterized by1H and13C-NMR. The results are listed in Table 1 .

[0408] CE-6: Obtained K-MMT 5.69 g, yield 100%. The selectivity of K-MMT against K2- TPA is 97.6%.

[0409] CE-7: Obtained K-MMT 5.45 g, yield 96.0%. The selectivity of K-MMT against K2- TPA is 98.6%.

[0410] 1H-NMR (500 MHz, D2O, ppm) of K-MMT: 3.95 (s, 3H, -CHs), 7.92 (d, 2H, ph-H, JC-H = 8Hz), 8.07 (d, 2H, ph-H, JC-H = 8Hz).

[0411] 13C-NMR (125 MHz, D2O, ppm) of K-MMT: 52.80, 128.77, 129.39, 131.48, 141.16, 169.10, 174.80.

[0412] Experimental procedure of E-8 and E-9:

[0413] Feedstock-1 (scraps of colorless PET bottles, 5 g, repeating unit 26.0 mmol), DCM (26.7 mL), methanol (13.3 mL), KOH (1.72 g, 26.0 mmol), and an organic base (E-8: BTMG 2.228 g, 13.0 mmol; E-9 DABCO 1 .459 g, 13.0 mmol) were added into a 100 mL RBF in order in air. The resulting mixture was vigorously stirred with a magnetic stirring bar at 40 °C by heating in a silicon oil bath. A white slurry was obtained after 1 hour. The white solid was collected by filtration and washed with DCM (20 mL). No unreacted PET was observed. The white K-MMT product was first dried in air in a fumehood overnight, then dried in an oven at 60 °C for 6 hours. The obtained K-MMT was characterized by1H and13C-NMR. The results are listed in Table 1 . E-8: Obtained K-MMT 5.24 g, yield 92.2%. The selectivity of K-MMT against K2- TPA is 99.0%.

[0414] E-9: Obtained K-MMT 4.91 g, yield 86.5%. The selectivity of K-MMT against K2- TPA is 99.4%.

[0415] 1H-NMR (500 MHz, D2O, ppm) of K-MMT: 3.95 (s, 3H, -CW3), 7.92 (d, 2H, ph-H, JC-H = 8Hz), 8.07 (d, 2H, ph-H, JC-H = 8Hz).

[0416] 13C-NMR (125 MHz, D2O, ppm) of K-MMT: 52.80, 128.77, 129.39, 131.48, 141.16, 169.10, 174.80.

[0417] Experimental procedure of E-10:

[0418] Feedstock-2 (mixed scraps of colorless PET, PE, and PP scraps, total 10 g which contains PET 5g, PET-RU 26.0 mmol), DCM (30 mL), methanol (15 mL), hexanes (5 mL), KOH (1.72 g, 26.0 mmol), and potassium methoxide (0.91 g, 13.0 mmol) were added into a 100 mL RBF in order in air. The resulting mixture was vigorously stirred with a magnetic stirring bar at 40 °C by heating in a silicon oil bath. A white slurry (with un-depolymerized PE and PP scraps) was obtained after 1 hour. The white powder (K-MMT) and solid PE and PP scraps were collected by filtration and washed with DCM (25 mL). A powder sample of K-MMT was taken and dried at 40 °C for 5 hours and then analyzed by running1H-NMR, which showed that the selectivity to K-MMT against K2-TPA is 97.5%.

[0419] The obtained solid mixture contains K-MMT powder, PE scraps, and PP scraps. The mixture was extracted with water (2x40 mL) and filtered. Undepolymerized PE scraps and PP scraps were collected, and the total weight was 5 g, indicating that the PE and PP plastics were kept intact under depolymerization conditions. The filtrate is K-MMT aqueous solution, to which KOH (1.72 g, 26.0 mmol) was added and stirred for 10 minutes at room temperature, transferring K-MMT to K2-TPA. Then, the K2-TPA solution was acidified by adding H2SO4 (25 wt.%) till pH reached 2, affording a white slurry of TPA. The TPA white powder was isolated by filtration and dried in an oven at 60 °C overnight. Obtained TPA 4.18 g, yield 96.8%.

[0420] 1H-NMR (100 MHz, DMSO-d6, ppm) of TPA: 7.58 (s, 4H, Ph-H).

[0421] Experimental procedure of E-11 :

[0422] Feedstock-3 (scraps of red PET bottles, 5 g, PET-RU 26.0 mmol), DCM (24 mL), methanol (12 ml_), hexanes (4 mL), KOH (1.72 g, 26.0 mmol), and potassium methoxide (0.91 g, 13.0 mmol) were added into a 100 mL RBF in order in air. The resulting mixture was vigorously stirred with a magnetic stirring bar at 40 °C by heating in a silicon oil bath. A pink slurry was obtained after 1 hour. The powder (K-MMT) was collected by filtration and washed with DCM (3><10 mL), affording a pale pink powder and a red solution. The result showed that, after PET depolymerization, most of the dyes were dissolved into the organic solution. A powder sample of K-MMT was taken and dried at 40 °C for 5 hours and then analyzed by running1H-NMR, which showed that the selectivity to K-MMT against K2-TPA is 97.7%.

[0423] The obtained K-MMT powder was dissolved in water (2x40 mL), forming a pale red solution. Then, KOH (1.72 g, 26.0 mmol) was added and stirred for 10 minutes at room temperature, transferring K-MMT to K2-TPA. Sodium hypochlorite pentahydrate (0.2 g) was added and stirred overnight at room temperature; then, the color turned pale yellow. After that, the K2-TPA solution was acidified by adding H2SO4 (25 wt.%) till pH reached 2, affording a white slurry of TPA. The TPA white powder was isolated by filtration and dried in an oven at 60 °C overnight.

[0424] Obtained TPA 4.20 g, yield 97.2%.

[0425] 1H-NMR (100 MHz, DMSO-d6, ppm) of TPA: 7.58 (s, 4H, Ph-H).

[0426] Experimental procedure of E-12:

[0427] Feedstock-4 (scraps of white PET (28%)-cotton (72%) blended fabric, 18.9 g which contains PET 5g, PET-RU 26.0 mmol), DCM (60 mL), methanol (30 mL), hexanes (10 mL), KOH (1.72 g, 26.0 mmol), and potassium methoxide (0.91 g, 13.0 mmol) were added into a 200 mL RBF in order in air. The resulting mixture was vigorously stirred with an overhead mechanical stirrer at 40 °C by heating in a silicon oil bath. A white slurry (with un-depolymerized cotton fabric) was obtained after 1 hour. The white powder (K-MMT) and cotton scraps were collected by filtration. A powder sample of K-MMT was taken and dried at 40 °C for 5 hours and then analyzed by running1H-NMR, which showed that the selectivity to K-MMT against K2-TPA is 97.1 %.

[0428] The obtained mixture contains K-MMT powder and un-depolymerized cotton fabric, which was extracted with water (2><50 mL) and filtered. Un- depolymerized cotton fabric was collected and dried, and the total weight was 12.40 g, indicating that the blended PET-cotton fabric lost 5.5 g of PET component after depolymerization.

[0429] The filtrate is K-MMT aqueous solution, to which KOH (1 .81 g, 27.4 mmol) was added and stirred for 10 minutes at room temperature, transferring K-MMT to K2-TPA. Then, the K2-TPA solution was acidified by adding H2SO4 (25 wt.%) till pH reached 2, affording a white slurry of TPA. The TPA white powder was isolated by filtration and dried in an oven at 60 °C overnight.

[0430] Obtained TPA 4.30 g, yield 99.5%.

[0431] 1H-NMR (100 MHz, DMSO-de, ppm) of TPA: 7.58 (s, 4H, Ph-H).

[0432] Experimental procedure of E-13:

[0433] Feedstock-5 (scraps of deep black PET fabric, 5g, PET-RU 26.0 mmol), DCM (24 mL), methanol (12 mL), hexanes (4 mL), KOH (1.72 g, 26.0 mmol), and potassium methoxide (0.91 g, 13.0 mmol) were added into a 150 mL RBF in order in air. The resulting mixture was vigorously stirred with an overhead mechanical stirrer at 40 °C by heating in a silicon oil bath. A black slurry was obtained after 1 hour. A pale yellow powder (K-MMT) was collected by filtration and washed with DCM (2x20mL). The filtrate is a black solution containing the black dyes. A powder sample of K-MMT was taken and dried at 40 °C for 5 hours and then analyzed by running1H-NMR, which showed that the selectivity to K-MMT against K2-TPA is 97.5%.

[0434] The pale yellow K-MMT powder was dissolved in water (80 mL), affording a pale yellow aqueous solution, to which KOH (1.81 g, 27.4 mmol) was added and stirred for 10 minutes at room temperature. Thus, K-MMT was transformed into K2-TPA in aqueous solution. Then, the K2-TPA solution was acidified by adding H2SO4 (25 wt.%) till pH reached 2, affording a white slurry of TPA. The TPA white powder was isolated by filtration and dried in an oven at 60 °C overnight.

[0435] Obtained TPA 4.33 g, yield 100%.

[0436] 1H-NMR (100 MHz, DMSO-d6, ppm) of TPA: 7.58 (s, 4H, Ph-H).

[0437] Experimental procedure of E-14:

[0438] Feedstock-6 (scraps of colored PET-containing fabric waste, 8 g which contains about PET 7.1 g, PET-RU 36.9 mmol), DCM (36 mL), methanol (18 mL), hexanes (6 mL), KOH (2.44 g, 36.9 mmol), and potassium methoxide (1.3 g, 18.5 mmol) were added into a 150 mL RBF in order in air. The resulting mixture was vigorously stirred with an overhead mechanical stirrer at 40 °C by heating in a silicon oil bath. A brown slurry (with un-depolymerized fabrics) was obtained after 1 hour. The brown powder (K-MMT) and un-depolymerized fabrics were collected by filtration and washed with DCM (5x20 mL). The filtrate is a brown solution which contains the dyes. A powder sample of K-MMT was taken and dried at 40 °C for 5 hours and then analyzed by running1H-NMR, which showed that the selectivity to K-MMT against K2-TPA is 96.9%.

[0439] The obtained mixture contains K-MMT powder and un-depolymerized fabrics, which was extracted with water (3x50 mL) and filtered. Un-depolymerized cotton fabrics were collected and dried, and their total weight was 1 .0 g. The filtrate is a pale brown solution of K-MMT, to which KOH (2.44 g, 36.9 mmol) was added and stirred for 10 minutes at room temperature, transferring K- MMT to K2-TPA. Then, sodium hypochlorite pentahydrate (0.5 g) was added to the solution and stirred at room temperature overnight. The color of the solution turned from pale brown to pale yellow. The obtained K2-TPA solution was acidified by adding H2SO4 (25 wt.%) till pH reached 2, affording a white slurry of TPA. The TPA white powder was isolated by filtration and dried in an oven at 60 °C overnight.

[0440] Obtained TPA 5.52 g, yield 90.0%.

[0441] 1H-NMR (100 MHz, DMSO-d6, ppm) of TPA: 7.58 (s, 4H, Ph-H).

[0442] Experimental procedure of E-15:

[0443] Feedstock-7 (scraps of colored PET-containing fabric waste, 8 g which contains about PET 7.0 g, PET-RL) 36.4 mmol), DCM (36 mL), methanol (18 mL), hexanes (6 mL), KOH (2.40 g, 36.4 mmol), and potassium methoxide (1.3 g, 18.5 mmol) were added into a 150 mL RBF in order in air. The resulting mixture was vigorously stirred with an overhead mechanical stirrer at 40 °C by heating in a silicon oil bath. A brown slurry (with un-depolymerized fabrics) was obtained after 1 hour. The brown powder (K-MMT) and un-depolymerized fabrics were collected by filtration and washed with DCM (5*20 mL). The filtrate is a brown solution which contains the dyes. A powder sample of K-MMT was taken and dried at 40 °C for 5 hours and then analyzed by running1H-NMR, which showed that the selectivity to K-MMT against K2-TPA is 96.7%.

[0444] The obtained mixture contains K-MMT powder and un-depolymerized fabrics, which were extracted with water (3x50 mL) and filtered. Un- depolymerized cotton fabrics were collected and dried, and their total weight was 1.1 g.

[0445] The filtrate is a pale brown solution of K-MMT, to which KOH (2.40 g, 36.4 mmol) was added and stirred for 10 minutes at room temperature, transferring K- MMT to K2-TPA. Then, sodium hypochlorite pentahydrate (0.5 g) was added to the solution and stirred at room temperature overnight. The color of the solution turned from pale brown to pale yellow. The obtained K2-TPA solution was acidified by adding H2SO4 (25 wt.%) till pH reached 2, affording a white slurry of TPA. The TPA white powder was isolated by filtration and dried in an oven at 60 °C overnight.

[0446] Obtained TPA 5.32 g, yield 87.9%.

[0447] 1H-NMR (100 MHz, DMSO-d6, ppm) of TPA: 7.58 (s, 4H, Ph-H).

[0448] Table 1. Dual-base promoted alternating depolymerization of waste PET (feedstock-1 )

[0449] Note:aPET-RU is PET repeating unit.&The ratio is the molar ratio of [PET-RU : KOH : Organic Base].eThe selectivity is the molar ratio of K-MMT to K2*TPA. TThe yield is the isolated yield of white K-MMT powder.

[0450] 3. Discussions and Comparative Analysis

[0451] Dual-base system significantly increases yield while retaining hiqh selectivity

[0452] In organic transformations, achieving high selectivity is always important but challenging. In particular, when enhancing the reaction rate by tuning the conditions, achieving high selectivity becomes more challenging. Therefore, achieving both high selectivity and high isolated yield in organic reactions is always desirable.

[0453] When using KOH as the sole base in a bi-solvent medium of DCM and methanol at r.t. for 1 h run, the yield is 69.2% with 99.5% selectivity to K-MMT (CE-1 , Table 1 ). When adding KOMe (6.5 mmol) as the organic base while retaining other conditions, the yield was increased from 69.2% to 78.9% with 99.4% selectivity to K-MMT (E-1 , Table 1 ).

[0454] When using KOH as the sole base in a bi-solvent medium of DCM and methanol at 40 °C for 1 h run, the yield is 83.8% with 99.4% selectivity to K-MMT (CE-2, Table 1 ). When adding KOMe (6.5 mmol) as the organic base while retaining other conditions, the yield was increased from 83.8% to 92.6% with 99.0% selectivity to K-MMT (E-2, Table 1 ).

[0455] The results of E-1 vs CE-1 and E-2 vs CE-2 showed that the dual-base system can significantly increase yield while retaining a high selectivity to K-MMT.

[0456] Dual-base system reduces the negative effect of water impurity

[0457] Four factors (see below) bring a small amount of water into the alternating depolymerization of PET.

[0458] • The waste PET feedstocks are not dried, which contain water moisture.

[0459] • The solvents (such as DCM, methanol, and hexanes) contain water moisture.

[0460] • The bases (KOH, KOMe, BTMG, DABCO) contain water moisture. The PET depolymerization is conducted in air, which also contains moisture.

[0461] The small amount of water can increase the chance to hydrolyze K-MMT into K2-TPA, thus slightly reducing the selectivity to K-MMT.

[0462] To maintain a high selectivity to K-MMT when using KOMe to increase the reaction rate and yield, the loading of KOH was reduced by 2% to compensate for the hydroxyl ions generated by the reaction of KOMe with water impurity. Thus, 0.98 equimolar KOH relative to PET-RU is used, and both high selectivity (99.3%) and high isolated yield (91 .9%) were achieved (E-3, Table 1 ).

[0463] In addition, the inventors surprisingly found out that when THF was used to replace DCM as the aprotic solvent, the isolated yield was increased to 96.0% with a selectivity of 99.2% under 55 °C using 0.98 equimolar KOH and 0.25 equimolar KOMe relative to PET-RU (E-4, Table 1 ).

[0464] The results of E-3 and E-4 clearly showed that a proper molar ratio of PET- RU : inorganic base : organic base can reduce the negative effect of water impurity, thus achieving higher yields and higher selectivity to K-MMT.

[0465] The outstanding results of using tri-solvent medium

[0466] In the PET depolymerization using KOH as the inorganic base in a bisolvent medium of DCM and methanol at 40 °C for 1 h run, the addition of KOMe (13.0 mmol) can lead to 99.0% yield, but the selectivity to K-MMT dropped to 95.7% (E-5, Table 1 ).

[0467] When the tri-solvent medium of DCM / methanol / hexanes (volume ratio 6:3:1 ) was used to replace the bi-solvent medium using the same amount of KOME (13.0 mmol), the inventors surprisingly found out that the yield was further increased to 100% while maintaining a higher selectivity (97.6%) to K-MMT (E-6, Table 1 ). When the loading of KOME was reduced from 13.0 mmol to 9.8 mmol while maintaining other factors of E-6, the yield was 96.0% with a relatively higher selectivity of 98.6% to K-MMT (E-7, Table 1 ).

[0468] The results of E-6 and E-7 clearly showed that the tri-solvent medium can afford higher yields and higher selectivity to K-MMT than using the bi-solvent medium in the dual-base promoted alternating depolymerization of PET.

[0469] The use of the nitrogen-containing base as the organic base

[0470] The dual-base systems of KOH-BTMG and KOH-DABCO are also efficient in promoting the alternating depolymerization of PET.

[0471] In the alternating depolymerization of PET using KOH as the inorganic base in a bi-solvent medium of DCM and methanol at 40 °C for 1 h run, the addition of BTMG (13.0 mmol) resulted in 92.3 % yield and 99.0 selectivity to K- MMT (E-8, Table 1 ); the addition of DABCO (13.0 mmol) resulted in 86.5 % yield and 99.4 selectivity to K-MMT (E-9, Table 1 ).

[0472] The use of various PET-containing feedstocks

[0473] Apart from colorless and transparent PET, unsorted plastics (containing PET, PE, and PP) and PET-cotton blended fabrics can be used as feedstocks. PE, PP, and cotton were kept intact after PET depolymerization (E-10, E-12). The dyes in the colored PET bottles and PET fabrics did not slow down the PET depolymerization (E-11 , E-13). Even the mixtures of colored PET-containing blended fabrics can be used as feedstocks to produce TPA.

[0474] After PET depolymerization, the dyes are released into the solvents. The residual dyes in the crude K-MMT can be degraded with oxidants in aqueous solution. The oxidants are selected from the group comprising sodium hypochlorite (NaOCI), calcium hypochlorite (Ca(CIO)2), H2O2, Na2O2, K2O2, CaCO2, MgO2, SrO2, ZnO2, , benzoyl peroxide, tert-butyl hydroperoxide (tBuOOH), etc.. It will be appreciated by a person skilled in the art that other variations and / or modifications may be made to the embodiments disclosed herein without departing from the spirit or scope of the disclosure as broadly described. For example, in the description herein, features of different exemplary embodiments may be mixed, combined, interchanged, incorporated, adopted, modified, included etc. or the like across different exemplary embodiments. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

CLAIMS1 . A method of depolymerizing a polyester of a dicarboxylic acid, the method comprising:(a-i) depolymerizing a polyester of a dicarboxylic acid source in the presence of:(i) a dual-base system; and(ii) a solvent system comprising two or more different solvents, to obtain a product comprising a mono-salt of a monoester of the dicarboxylic acid and / or derivatives thereof.

2. The method of claim 1 , wherein the polyester of a dicarboxylic acid source comprises waste polyester of a dicarboxylic acid.

3. The method of any one of the preceding claims, wherein the polyester of a dicarboxylic acid comprises a structure that is represented by general formula (1 ):whereinA is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heterocycloalkyl, optionally substituted heterocycloalkenyl, optionally substituted heteroaryl, or combinations thereof;R1is optionally substituted alkylene, optionally substituted cycloalkylene, optionally substituted alkenylene, optionally substituted alkynylene, or combinations thereof; and4. The method of claim 3, wherein A comprises 1 ,4-disubstituted benzene and the polyester of a dicarboxylic acid comprises a structure that is represented by general formula (2):whereinR2is optionally substituted alkylene, optionally substituted cycloalkylene, optionally substituted alkenylene, optionally substituted alkynylene, or combinations thereof;Ra, Rb, Rcand Rdare each independently hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, or combinations thereof; and n > 1.

5. The method of any one of the preceding claims, wherein the depolymerizing in (a-i) comprises a plurality of alcoholysis and hydrolysis reactions.

6. The method of any one of the preceding claims, wherein the depolymerizing in (a-i) comprises the following reaction:(13) (12) whereinA is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heterocycloalkyl, optionally substituted heterocycloalkenyl, optionally substituted heteroaryl, or combinations thereof;R1is optionally substituted alkylene, optionally substituted cycloalkylene, optionally substituted alkenylene, optionally substituted alkynylene, or combinations thereof;R4and R4’ are each independently optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, or combinations thereof; andM is a metal or ammonium-containing group; and n > 1.

7. The method of any one of the preceding claims, wherein the mono-salt of a monoester of the dicarboxylic acid comprises a structure that is represented by general formula (3):whereinA is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted cycloalkyl, optionally substituted cycloalkenyl, optionally substituted alkynyl, optionally substituted aryl, optionally substituted heterocycloalkyl, optionally substituted heterocycloalkenyl, optionally substituted heteroaryl, or combinations thereof;R3is optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, or combinations thereof; andM is a metal or ammonium-containing group.

8. The method of claim 7, wherein A comprises 1 ,4-disubstituted benzene and the mono-salt of a monoester of the dicarboxylic acid comprises a structure that is represented by general formula (4):whereinR4is optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, or combinations thereof;R1 a, R1 b, R1 cand R1dare each independently hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, or combinations thereof; andM is a metal or ammonium-containing group.

9. The method of any one of claims 6 to 8, wherein M is an alkali metal selected from lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) or francium (Fr).

10. The method of any one of the preceding claims, wherein the dual-base system comprises an inorganic base and an organic base.

11. The method of claim 10, wherein the inorganic base comprises an Arrhenius base selected from the group consisting of metal hydroxides, alkali metal hydroxides, alkaline earth metal hydroxides, ammonium- containing hydroxides and combinations thereof.

12. The method of any one of claims 10 to 11 , wherein the organic base comprises a Lewis base selected from the group consisting of metal alkoxides, metal salt of alkoxides, nitrogen-containing organic bases, nitrogen-based organic bases and combinations thereof.

13. The method of any one of the preceding claims, wherein the solvent system comprises two or more different solvents selected from the group consisting of:(a) an aprotic solvent;(b) a protic solvent; and(c) a hydrocarbon compound.

14. The method of claim 13, wherein the aprotic solvent is selected from the group consisting of tetrahydrofuran (THF), 2-methyltetrahydrofuran (2- MeTHF), acetonitrile (ACN), acetone (ACE), methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cyclopentanone, cyclohexanone, 2- pentanone (also known as methyl propyl ketone), 3-pentanone (also known as diethyl ketone), diethyl ether, methyl tert-butyl ether (MTBE), tert-amyl methyl ether (TAME), cyclopentyl methyl ether (CPME), methoxybenzene (also known as anisole), ethoxybenzene, sulfolane (also known as tetramethylene sulfone), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMA), a,a-trifluorotoluene (TFT), dichloromethane (DCM), chlorobenzene, diethyl ether and combinations thereof.

15. The method of any one of claims 13 to 14, wherein the protic solvent is selected from the group consisting of methanol, ethanol, n-propanol, / sopropanol, n-butanol, tert-butanol, allyl alcohol, propargyl alcohol, and combinations thereof.

16. The method of any one of claims 13 to 15, wherein the hydrocarbon compound is selected from the group consisting of optionally substituted pentane, optionally substituted hexane, optionally substituted heptane, optionally substituted octane, optionally substituted benzene, optionally substituted toluene, optionally substituted xylene, and combinations thereof.

17. The method of any one of claims 13 to 16, wherein the volume ratio of the aprotic solvent to the protic solvent is from 1 :10 to 50:1 .

18. The method of any one of claims 13 to 17, wherein the volume ratio of the aprotic solvent plus the protic solvent to the hydrocarbon compound is from 1 :1 to 75:1.

19. The method of any one of the preceding claims, wherein the molar ratio of the organic base to the inorganic base is 0.1 to 0.75.

20. The method of any one of the preceding claims, wherein the molar ratio of the number of repeating units in polyester to the inorganic base to the organic base is 1 : 0.5 - 1 .5 : 0.1 - 1 .21 . The method of any one of the preceding claims, wherein the molar ratio of inorganic base to the number of repeating units in polyester less than 1 .

22. The method of any one of the preceding claims, wherein the method further comprises:(a-ii) reacting the product comprising the mono-salt of a mono ester of the dicarboxylic acid with a base to obtain a solution comprising dialkali metal salt of the dicarboxylic acid.

23. The method of claim 22, wherein the method further comprises:(a-iii) reacting the solution comprising the di-alkali metal salt of the dicarboxylic acid obtained from (a-ii) with an oxidant / oxidizing agent to obtain a decoloured solution comprising the di-alkali metal salt of the dicarboxylic acid.

24. The method of any one of claims 22 to 23, wherein the method further comprises:(a-iv) reacting the solution obtained from (a-ii) or (a-iii) with an acid to obtain a dicarboxylic acid.

25. The method of claim 24, wherein the dicarboxylic acid comprises a structure that is represented by general formula (8):wherein R3a, R3b, R3cand R3dare each independently hydrogen, halogen, optionally substituted alkyl, optionally substituted cycloalkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted alkoxy, or combinations thereof.

26. The method of any one of the preceding claims, wherein the method further comprises:(a-v) isolating ethylene glycol from the solution obtained from step (a-i).

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