Production of epsilon-caprolactam from nylon-6 via depolymerization catalysts

Abstract

Methods for depolymerizing a polyamide may comprise combining a polyamide (e.g., Nylon-6) and a boron group catalyst to depolymerize the polyamide to a product (e.g., epsilon-caprolactam), wherein the boron group catalyst comprises a boron group element and ligands bound to the boron group element, wherein the ligands are independently selected from an oxygen-containing ligand other than an oxygen atom, other than a ketone, and other than a sulfonate; hydrogen (H); a carbon-containing ligand; a nitrogen-containing ligand; an alkali metal; and an alkaline earth metal.

Description

Atty. Dkt. No. 00100-0411-PCTPRODUCTION OF EPSILON-CAPROLACTAM FROM NYLON-6 VIA DEPOLYMERIZATION CATALYSTS CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. provisional patent application number 63 / 720,255 that was filed November 14, 2024, the entire contents of which are incorporated herein by reference.REFERENCE TO GOVERNMENT RIGHTS

[0002] This invention was made with government support under grant number DE-SC0022290 awarded by the Department of Energy. The government has certain rights in the invention.BACKGROUND

[0003] Modem society is heavily dependent on plastics-based materials, as evident by the steady growth in their production. Plastics are extremely ubiquitous, versatile, and low-cost polymeric materials that have dramatically enhanced the quality of human life for more than a century. Currently, plastics are being produced worldwide at an annual rate of 450 million tons year, which is projected to double by 2045. One class of commonly used plastics for applications requiring the material / product to withstand harsh mechanical and environmental conditions are engineering plastics such as polyamides.

[0004] Nylon-6 was one of the first synthetic fibers discovered and developed by Schlack in 1938. Nylon-6 is a thermoplastic polyamide produced industrially by water-assisted ring-opening polymerization (ROP) of E-caprolactam on an 8.9 million tons annual scale, with the market size expected to reach $21.5 billion in 2026. Nylons as non-degradable plastics substantially contribute to the pollution in oceans and landfills, reflecting their superior chemical robustness and the paucity of effective recycling technologies. Indeed, it is estimated that by 2050, plastic waste will outweigh fish in the ocean, with Nylon-6 contributing ca. 10% of ocean plastic pollution as discarded or lost fishing nets (so-called "ghost nets"). This amounts to more than 600,000 tons of abandoned fishing nets per year.Atty. Dkt. No. 00100-0411-PCTSUMMARY

[0005] Provided are methods for depolymerizing polyamides, e.g., depolymerizing Nylon-6 to e-caprolactam, using certain boron group catalysts comprising a boron group element, e.g., Al, B. Aluminum, in particular, is the most abundant metal in the Earth's crust, accounting for over 8% of its mass, and the third most prevalent chemical element on Earth. It is also low cost, low toxicity, and enjoys widespread commercial availability. The Example, below, describes experimental results demonstrating that illustrative such catalysts achieve quantitative conversions efficiently (e.g., 10 minutes), under mild temperatures (e.g., 240 °C), and with minimal catalyst loadings (e.g., 0.2 mol%). The methods are compatible with virgin, post-consumer, and mixed plastic waste blends, including Nylon-6 mixed with a wide variety of other polymers, while providing high yields of e-caprolactam monomer. The resulting e-caprolactam can be repolymerized into high-quality Nylon-6, providing a sustainable alternative to conventional Nylon-6 depolymerization methods. These breakthroughs underscore the ability of the disclosed methods using the boron group catalysts to transform Nylon-6 recycling and support broad industrial applications. The Example, below, describes additional experimental results that demonstrate an unexpected synergy between the boron group species within the boron group catalysts and alkali metal species. The alkali metal species may be present within the boron group catalyst itself, present due to a polyamide pretreatment, present as an additive in a reaction mixture comprising the polyamide and the boron group catalyst, or a combination thereof.

[0006] An embodiment 1 is a method for depolymerizing a polyamide, the method comprising combining a polyamide and a boron group catalyst to depolymerize the polyamide to a product, wherein the boron group catalyst comprises a boron group element and ligands bound to the boron group element, wherein the ligands are independently selected from an oxygen-containing ligand other than an oxygen atom, other than a ketone, and other than a sulfonate; hydrogen (H); a carbon-containing ligand; a nitrogen-containing ligand; an alkali metal; and an alkaline earth metal.

[0007] An embodiment 2 is according to embodiment 1, wherein the boron group catalyst is selected from those having formula (MRx)y(BG)Rz, wherein M is the alkali metal or the alkaline earth metal; BG is the boron group element; each R is independently selected from an oxygen atom (O) of the oxygen-containing ligand, H, a carbon atom (C) of the carbon-containing ligand, and a nitrogen atom (N) of the nitrogen-containing ligand; x is 0 or 1; y isAtty. Dkt. No. 00100-0411-PCT0 or 1; and z is 3 or 4; and further wherein, if M is the alkali metal, then x is 0, y is 1, and z is 4; if M is the alkaline earth metal, then x is 1, y is 1, and z is 4; and if y is 0, then z is 3.

[0008] An embodiment 3 is according to any of embodiments 1-2, The method of claim 2, wherein the boron group element is Al or B.

[0009] An embodiment 4 is according to any of embodiments 1-3, wherein the oxygen-containing ligand is an alkoxy; the carbon-containing ligand is an alkyl; and the nitrogen-containing ligand is a nitrogen-substituted cycloalkyl, an amido, or an amidate.

[0010] An embodiment 5 is according to embodiment 2, wherein M is selected from Li, Na, K, and Mg.

[0011] An embodiment 6 is according to embodiment 2, wherein the formula is (BG)R3.

[0012] An embodiment 7 is according to embodiment 6, wherein BG is Al or B.

[0013] An embodiment 8 is according to any of embodiments 6-7, wherein each R is an alkoxy, a nitrogen-substituted cycloalkyl, or an amido.

[0014] An embodiment 9 is according to embodiment 8 wherein the alkoxy is isopropoxy, the nitrogen-substituted cycloalkyl is caprolactamato, and the amido is -N(CH3)2.

[0015] An embodiment 10 is according to embodiment 2, wherein the formula is M(BG)R4, wherein M is the alkali metal.

[0016] An embodiment 11 is according to embodiment 10, wherein M is selected from Na, Li, and K.

[0017] An embodiment 12 is according to any of embodiments 10-11, wherein each R is independently selected from a nitrogen-substituted cycloalkyl and a substituted alkoxy.

[0018] An embodiment 13 is according to embodiment 12, wherein the nitrogen-substituted cycloalkyl is caprolactamato and the substituted alkoxy is 2-methoxy ethoxy.

[0019] An embodiment 14 is according to any of embodiments 14, wherein two R groups are the caprolactamato and two R groups are the 2-methoxy ethoxy.

[0020] An embodiment 15 is according to embodiment 11, wherein each R is H.

[0021] An embodiment 16 is according to any of embodiments 1-15, wherein the polyamide is poly(hexano-6-lactam) (Nylon-6), poly(2-pyrrolidinone) (Nylon-4), or a combination thereof.Atty. Dkt. No. 00100-0411-PCT

[0022] An embodiment 17 is according to any of embodiments 1-16, wherein the product comprises a monomer from which the polyamide was formed.

[0023] An embodiment 18 is according to embodiment 17, wherein the monomer is a cyclic amide.

[0024] An embodiment 19 is according to any of embodiments 1-18, wherein the polyamide and the boron group catalyst are combined in the presence of an alkali metal species.

[0025] An embodiment 20 is according to embodiment 19, wherein the alkali metal species is an alkali metal present in the boron group catalyst; the alkali metal species is present due to pretreating the polyamide with the alkali metal species prior to combining the polyamide and the boron group catalyst; the alkali metal species is an additive combined with the polyamide and the boron group catalyst; or combinations thereof.

[0026] An embodiment 21 is according to any of embodiments 1-20, wherein the polyamide is Nylon-6; the boron group element is Al or B; and the product comprises s-caprolactam.

[0027] An embodiment 22 is according to embodiment 21, wherein the polyamide and the boron group catalyst are combined in the presence of an alkali metal species.

[0028] An embodiment 23 is according to any of embodiments 1 -22, wherein the polyamide is Nylon-6 and the product comprises s-caprolactam, and further wherein the Nylon-6 is provided as a blend with a different type of polymer and the different type of polymer remains intact after the method.

[0029] An embodiment 24 is according to any of embodiments 1-23, wherein the polyamide is Nylon-6 and the product comprises s-caprolactam, and the method further comprises adding additional Nylon-6 an additional time or continuously without adding additional boron group catalyst to produce additional s-caprolactam.

[0030] Other principal features and advantages of the disclosure will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.Atty. Dkt. No. 00100-0411-PCTBRIEF DESCRIPTION OF THE DRAWINGS

[0031] Illustrative embodiments of the disclosure will hereafter be described with reference to the accompanying drawings.

[0032] FIG. 1 illustrates the depolymerization of Nylon-6 to e-caprolactam, using the disclosed boron group catalysts.

[0033] FIGs. 2A-2C show three illustrative boron group catalysts.DETAILED DESCRIPTION

[0034] A method for depolymerizing a polyamide comprises combining a polyamide and a boron group catalyst, which are each further described below, under conditions to depolymerize the polyamide. The polyamide to be depolymerized by the present methods is a polymer composed of monomers covalently bound into an extended chain via amide linking groups. The term “monomer’’ refers to the chemical reactant that is incorporated to form the extended chain and the amide linking groups during a polymerization reaction. The polyamide may be an aliphatic polyamide (i.e., not comprising aromatic rings) or an aromatic polyamide (i.e., comprising aromatic rings). The polyamide may be a homopolymer (i.e., formed from a single type of monomer) or a heteropolymer (i.e., formed from more than one type of monomer, e.g., two; “heteropolymers” may be referred to as copolymers). The term “ty pe” refers to a chemical formula such that a single type means the same chemical formula and a different type means different chemical formulas.

[0035] The polyamide may be one which is formed by ring-opening polymerization of a cyclic amide (i.e., lactam). In such embodiments, the monomer is a cyclic amide and the polymerization reaction is ring-opening polymerization. Illustrative cyclic amides include 2-pyrrolidone, 2-piperidone, s-caprolactam, enantholactam, capryllactam, pelargolactam, azacycloundecan-2-one, and azacyclotri decan-2-one.

[0036] The polyamide may be one which is formed from condensation of an amine (including a diamine) with an acid (including a diacid). The amine and the acid may be provided on a single chemical compound such as 11 -aminoundecanoic acid or o-aminolauric acid. Alternatively, the amine and the acid may be provided on two different chemical compounds such as hexamethylene diamine and adipic acid. In these embodiments, the monomer is the single chemical compound (with the amine and acid groups) or the twoAtty. Dkt. No. 00100-0411-PCTchemical compounds (the diamine and the diacid), and the polymerization reaction is condensation.

[0037] The polyamide may be identified by reference to the monomers (e.g., cyclic amide, diamine, diacid) used to form the polyamide, recognizing that the chemical form of these monomers may be modified by the ensuing polymerization reaction that provides the amide linking groups of the polyamide.

[0038] Illustrative polyamides to be depolymerized in the present methods include poly(2-pyrrolidinone) (Nylon-4), poly(2-piperidone) (Nylon-5), poly(hexano-6-lactam) (Nylon-6), polyenanthamide (Nylon-7), polycapryllactam (Nylon-8), poly(9-aminononanoic acid (Nylon-9), poly(10-aminodecanoic acid) (Nylon-10), poly(l l-aminoundecanoic acid) (Nylon-11), poly(dodecano-12-lactam) (Nylon-12), poly[imino(l,6-dioxo hexamethylene)imino hexamethylene] (Nylon-66), and poly[imino(l,6-dioxohexamethylene) iminotetramethylene] (Nylon-46). In embodiments, the polyamide to be depolymerized is Nylon-6, Nylon-4, or a combination thereof.

[0039] The present methods may be used to depolymerize any of the disclosed polyamides. The depolymerization process deconstructs the polyamide into a product. In embodiments, the depolymerization process provides a monomer (e.g., a cyclic amide) from which the polyamide was formed, i.e., the product comprises (or consists of) the monomer of the polyamide. Throughout this disclosure, the terms “a monomer” and “monomer” encompass both a single type of monomer and multiple, different types of monomers, i.e., depending upon the particular polyamide. In such embodiments, this monomer may be recovered and used to reform the polyamide. This is by contrast to depolymerization processes which do not provide a monomer, but rather a product comprising a chemical compound which cannot reform the polyamide absent additional steps to convert the chemical compound to the monomer. However, the present methods encompass both types of depolymerization processes, i.e., those that provide the monomer of the polyamide and those that provide a different type of product.

[0040] In embodiments, the methods depolymerize Nylon-6 to produce s-caprolactam. In embodiments, the methods depolymerize Nylon-4 to produce 2-pyrrolidone. In embodiments, the methods depolymerize Nylon-5 to produce 6-Valerolactam. In embodiments, the methods depolymerize Nylon-7 to produce enantholactam. In embodiments, the methods depolymerize Nylon-8 to produce capryllactam. In embodiments, the methods depolymerizeAtty. Dkt. No. 00100-0411-PCTNylon-9 to produce pelargolactam. In embodiments, the methods depolymerize Nylon- 10 to produce azacycloundecan-2-one. In embodiments, the methods depolymerize Nylon- 11 to produce w-undecalactam. In embodiments, the methods depolymerize Nylon- 12 to produce laurolactam.

[0041] A single type of polyamide or multiple, different types of polyamides may be used (i.e., a blend of different types of polyamides). In embodiments, the polyamide is a blend comprising different types of polyamides and the present method selectively depolymerizes certain of the polyamides. For example, in embodiments, the polyamide is a blend comprising Nylon-6, Nylon-4, or a combination thereof, and a different type of polyamide (e.g., Nylon-66, Nylon-46, Nylon- 12 or combinations thereof) and the present methods depolymerize the Nylon-6 / Nylon-4 to the corresponding monomers (e-caprolactam / 2-pyrrolidone) without depolymerizing Nylon-66 / Nylon-46 / Nylon-12. By "‘selective depolymerization'’ and “without” it is meant that any yield of the Nylon-66 / Nylon-46 / Nylon-12 monomer(s) (or other polyamide monomer) is less than the detection limit of the 'H NMR system used to determine yield as described in the Example, below. This includes measuring a monomer yield via NMR that is less than 5%.

[0042] The boron group catalysts used in the present methods comprise (or consist of) a boron group element (BG) and ligands (L) bound (e.g., covalently) thereto. The boron group catalysts include organometallic compounds in which the ligands are independently selected from hydrogen (H), a carbon atom (C) of a carbon-containing ligand, or both; metal-organic compounds in which the ligands are independently selected from a nitrogen atom (N) of a nitrogen-containing ligand, an oxygen atom (O) of an oxygen-containing ligand, or both; mixed organometallic and metal-organic compounds in which the ligands are independently selected from each of the corresponding ligands for these compounds; and “-ate” complexes in which at least one ligand is selected from an alkali metal or an alkaline earth metal and another ligand(s) is independently selected from the corresponding ligands for the organometallic and metal-organic compounds in this paragraph. In such -ate complexes, the alkali metal ligand can be a counterion associated with the boron group element. Thus, a bound ligand may be bound via an electrostatic association with the boron group element. As noted above and demonstrated in the Example, below, an unexpected synergy between the boron group element within the boron group catalysts and alkali metal species has been discovered. Thus, in embodiments, the boron group catalyst is an “-ate” complex in which at least one ligand is selected from an alkali metal and another ligand(s) is independentlyAtty. Dkt. No. 00100-0411-PCTselected from the corresponding ligands for the organometallic and metal-organic compounds in this paragraph.

[0043] Regarding the organometallic compounds, the metal-organic compounds, the mixed organometallic and metal-organic compounds, and the -ate complexes described above, these are distinguished from oxides of the boron group element, e.g., Al2O3. Such compounds are excluded from the present boron group catalysts. Similarly, boric acid is excluded from the present boron group catalysts. Compounds in which only hydrogen, only oxygen, or only hydrogen and oxygen are ligands may be excluded from the present boron group catalysts. In embodiments, other compounds may be excluded from the present boron group catalysts, e.g., aluminum acetylacetonate, aluminum triflate.

[0044] Thus, the boron group catalysts encompass those having Formula I, (MRx)y(BG)Rz, wherein M is an alkali metal or an alkaline earth metal; BG is the boron group element; each R is independently selected from H, C, N, or O; x is 0 or 1; y is 0 or 1; and z is 3 or 4. In Formula I, if M is an alkali metal, then x is 0. y is 1, and z is 4; if M is an alkaline earth metal, then x is 1, y is 1, and z is 4. However, both x and y may be 0, i.e., no alkali metal or alkaline earth metal is present, in which case, z is 3. Regarding R in Formula I, as noted above, the carbon (C) in Formula I refers to a carbon atom of a carbon-containing ligand bound to the boron group element via the carbon atom, e.g., alkyl. Similarly, the nitrogen (N) refers to a nitrogen atom of a nitrogen-containing ligand bound to the boron group element via the nitrogen atom, e.g., an amido, an amidate, or a nitrogen-substituted cycloalkyl. (The term amido is analogous to “amine” but more accurately reflects the deprotonated or metalated form of the ligand bound to the boron group element via the nitrogen atom. The term amidate is analogous to “amide” but more accurately reflects the deprotonated and metalated form of the ligand bound to the boron group element via the nitrogen atom.) Similarly, the oxygen (O) refers to an oxygen atom of an oxygen-containing ligand bound to the boron group element via the oxygen atom, e.g., an alkoxy. In embodiments, some oxy gen-containing ligands may be excluded, e.g., sulfonates such as triflate and ketones such as acetylacetonate. Of the multiple R groups present, they may be the same or different. This includes all 3 or 4 R groups being the same; two R groups being the same and two other R groups being the same as each other but different from the former two R groups; and three R groups being the same and one R group being different from the former three R groups.Atty. Dkt. No. 00100-0411-PCT

[0045] Certain boron group catalysts having Formula I may be in the form of multimer complexes, e.g., dimers, tetramers, and Formula I and its related formulas encompass such multimers. Certain boron group catalysts having Formula I may be in the form of a salt and Formula I and its related formulas encompass such salts.

[0046] The boron group catalysts include those having Formula IA, (BG)Rv wherein BG and R are as defined above. In some embodiments of Formula IA, each R is independently selected from H and C (e.g., wherein C is part of a carbon-containing ligand such as alkyl).

[0047] In some embodiments of Formula IA, each R is independently selected from N (e.g., N is part of a nitrogen-containing ligand such as an amido, an amidate, or a nitrogensubstituted cycloalkyl) and O (e.g., O is part of an oxygen-containing ligand such as alkoxy).

[0048] In some embodiments of Formula IA, each R is a nitrogen-substituted cycloalkyl (e.g., caprolactamato). In such embodiments, the N atom and the O atom of caprolactamato may be bound to the boron group element (see this ligand in FIG. 2B).

[0049] In some embodiments of Formula IA, each R is an alkoxy (e.g., isopropoxy). An illustrative such boron group catalyst is shown in FIG. 2A. Boron group catalysts such as these may also be represented by Formula IA1 [(BG)Ra]4, wherein each R is an alkoxy.

[0050] In some embodiments of Formula IA, each R is an amido (e.g., -N(CH3)2). An illustrative such boron group catalyst is shown in FIG. 2C.

[0051] The boron group catalysts include those having Formula IB, M(BG)R4, wherein M is an alkali metal and BG and R are as defined above. In some embodiments of Formula IB, each R is independently selected from a nitrogen-substituted cycloalkyl (e.g., caprolactamato) and a substituted alkoxy (e.g., 2-methoxy ethoxy). An illustrative such boron group catalyst is shown in FIG. 2B. In some embodiments, each R is H.

[0052] The boron group catalysts include those having Formula IC MR(BG)R4, wherein M is an alkaline earth metal and BG and R are as defined above.

[0053] In Formula I and its related formulas, various boron group elements may be used, e.g., Al, B. In embodiments, the boron group element in Formula I and its related formulas is Al. Similarly, various alkali metals may be used, e.g., Li, Na, K. Various alkaline earth metals may be used, e.g., Mg.

[0054] In Formula I and its related formulas, “alkyk’ refers to a linear, branched, or cyclic alkyl group in which the number of carbons may range from, e.g., 1 to 8, 1 to 6, 1 to 4, 1 to 3,Atty. Dkt. No. 00100-0411-PCTor 1 to 2. A cyclic alkyl group may be referred to as a cycloalkyl group. The alkyl group may be unsubstituted, by which it is meant the alkyl group contains no heteroatoms. An unsubstituted alkyl group encompasses an alkyl group in which one or more bonds to a carbon(s) or hydrogen(s) are replaced by a bond to an unsubstituted aromatic ring, e.g. benzyl. The alkyl group may be substituted, by which it is meant an unsubstituted alkyl group in which one or more bonds to a carbon(s) or hydrogen(s) are replaced by a bond to nonhydrogen and non-carbon atoms.

[0055] In Formula I and its related formulas, “aryl” refers to a monocyclic aryl group having one aromatic ring (e.g., benzene) or a polycyclic group having more than one aromatic ring (e.g., two, three, etc. rings). Monocyclic aryl groups may be unsubstituted or substituted as described above with respect to alkyl groups. Regarding polycyclic groups, neighboring aromatic rings may be fused or unfused. The aromatic rings of a polycyclic group may be unsubstituted or substituted as described above with respect to monocyclic aryl groups.

[0056] In Formula I and its related formulas, ‘'amido” refers to -NR2, wherein each R is independently selected from hydrogen, alkyl groups, and aryl groups (each of which has been defined herein) and denotes the bond to the boron group element in Formula I and all its related formulas.

[0057] In Formula I and its related formulas, “amidate” refers to -N(R)C(O)R, wherein each R is independently selected from hydrogen, alkyl groups, and aryl groups (each of which has been defined herein) and denotes the bond to the boron group element in Formula I and all its related formulas.

[0058] In Formula I and its related formulas, ‘'nitrogen-substituted cycloalkyl” refers to a cycloalkyl group (which has been defined herein) that contains N as a heteroatom. Other substituents may be included, e.g., a carbon-hydrogen bond may be replaced by a carbonoxygen bond as in caprolactamato (-NC6H10NO), wherein denotes the bond to the boron group element in Formula I and all its related formulas.

[0059] In Formula I and its related formulas, ‘‘alkoxy” refers to -OR, wherein R is an alkyl group (which has been defined herein) and “-” denotes the bond to the boron group element in Formula I and all its related formulas.Atty. Dkt. No. 00100-0411-PCT

[0060] Regarding substituents in the groups described herein (as opposed to unsubstituted groups), non-hydrogen and non-carbon atoms include, e.g., halogen; oxygen; sulfur; nitrogen; phosphorus; and silicon.

[0061] Illustrative boron group catalysts are listed in Table 1 particularly, entries 4-7, 9-10, 12-21. Also see FIGs. 2A-2C as noted above.

[0062] A single type of boron group catalyst or multiple, different types of boron group catalysts may be used. The boron group catalyst being used to catalyze depolymerization may comprise (or consist of) any of the disclosed boron group catalysts or a combination thereof.

[0063] The conditions being used in the present methods may refer to parameters such as the temperature, period of time, atmosphere, and boron group catalyst loading. The present methods may be conducted using a variety’ types of reactor systems, including batch reactor systems, semi-batch reactor systems, continuous flow reactor systems, and extruder reactor systems (e.g., twin screw extruder reactor system). The conditions may also refer to a specific type of reactor system. These parameters may be adjusted in order to promote depolymerization and thus, to achieve a desired (e.g., maximum) yield of monomer (or other depolymerization product).

[0064] Regarding temperature, the temperature may be within 30 °C. within 20 °C, or within 10 °C of the melting temperature of the polyamide. The temperature may be at or above the melting temperature of the polyamide such that it is in its molten (liquid state) during the depolymerization. In embodiments, the temperature is no more than 300 °C, no more than 280 °C, no more than 260 °C, or no more than 240 °C. This includes a range between any of these values and from 200 °C to 280 °C, from 200 °C to 250 °C, and from 200 °C to 245 °C.

[0065] Regarding time, this may refer to a total period of time over which the polyamide and the boron group catalyst are subjected to the other depolymerization conditions. In embodiments, the period of time is less than 24 hours, less than 10 hours, less than 5 hours, or less than 2 hours. This includes a range between any of these values and from 1 minute to 10 hours and 10 minutes to 5 hours. For continuous flow reactor sy stems, flow rate rather than time is a relevant parameter. In embodiments, the flow rate is from 5 seem to 1500 seem, although higher flow rates may be used. Reactor volume is another relevant parameter for continuous flow reactor systems. In embodiments, the reactor volume is from 50 mL to 1000 L, although greater reactor volumes may be used.Atty. Dkt. No. 00100-0411-PCT

[0066] Regarding the atmosphere, a vacuum may be used, e.g., a pressure of 10-3Torr or less, 10’2Torr or less, 101Torr or less. However, the methods may be conducted at higher pressures, including in a range of from 10-2Torr to 103Torr. The methods may be conducted under an inert atmosphere (e.g., N2, argon) and a pressure of about 1 atm (760 Torr). As demonstrated in Table 1, entry 24, the present methods may exclude the use of air as an atmosphere.

[0067] Regarding boron group catalyst loading, the loading may be no more than 20 mol%, no more than 10 mol%. no more than 8 mol%. or no more than 6 mol%. This includes a range of from 0.1 mol% to 5 mol% and from 0.1 mol% to 2 mol%. Mol% is calculated based on the number of mol of repeating unit of the polyamide. For example, if 1 g of Nylon-6 is used, the number of mol of repeating unit = 0.00885 mol (repeating unit of Nylon-6 is 113 g / mol). Thus, the amount of catalyst used is 0.00885 mol * x mol%.

[0068] The present methods are generally conducted without using any liquid medium, e.g., solvent. This includes the methods being conducted without using water or steam. As such, the methods may be referred to as being “solvent-free’’ and the like.

[0069] The conditions being used in the present methods may further refer to whether or not the polyamide has been subjected to an alkali metal pretreatment step prior to depolymerization. The alkali metal pretreatment comprises exposing the polyamide to an alkali metal species including an alkali metal hydroxide, an alkali metal (bi)carbonate, and an alkali metal alkoxide. Various alkali metal hydroxides may be used, e.g., NaOH, KOH, LiOH. Various alkali metal (bi)carbonates may be used, e.g., Na2CO3, KHCO3. Various alkali metal alkoxides may be used, e.g., sodium methoxide (NaOMe), potassium tert-butoxide (KOtBu). The alkali metal species may be provided as an aqueous solution and the exposure may be conducted by washing the polyamide with the aqueous solution for a period of time followed by drying as described in the Example, below. Due to the synergistic effects of the boron group element and the alkali metal as described herein, the specific conditions of the alkali metal pretreatment step may be selected to achieve a desired (e.g., maximum) yield of monomer (or other depolymerization product). These conditions may include ensuring that a certain amount of alkali metal is present during depolymerization, including up to one alkali metal per polyamide chain. For example, if Nylon-6 has about 100 repeating s-caprolactam units, the conditions may include ensuring that up to about 1 mol% alkali metal is present during depolymerization. However, smaller amounts of alkali metal may be used. InAtty. Dkt. No. 00100-0411-PCTembodiments, the polyamide has been subjected to an alkali metal pretreatment step and the present methods may include such an alkali metal pretreatment step. However, as the alkali metal species may be present in the boron group catalyst itself (as described above) or as a separate additive (as described below), in embodiments, the present methods do not include an alkali metal pretreatment step and the polyamide has not been subjected to such an alkali metal pretreatment step. (See entries 3, 4, 5, and 7 of Table 2.)

[0070] The conditions being used in the present methods may further refer to whether or not an alkali metal additive is present along with the polyamide and the boron group catalyst during depolymerization. The alkali metal additive may be any of the alkali metal species described above with respect to alkali metal pretreatment, including any of the alkali metal hydroxides, alkali metal (bi)carbonates, and alkali metal alkoxides. The alkali metal species may be added to the polyamide and the boron group catalyst in a desired amount, e.g., from 0.1 to 15 mol% alkali metal species. Due to the synergistic effects of the boron group element and the alkali metal as described herein, the specific amount may be selected to achieve a desired (e.g., maximum) yield of monomer (or other depolymerization product). In embodiments, an alkali metal additive is included during depolymerization. However, as the alkali metal species may be present in the boron group catalyst itself (as described above) or due to a pretreatment step (as described above), in embodiments, the present methods do not include an alkali metal additive during depolymerization.

[0071] The polyamide (the specific chemical type of which has been described above) may be “virgin” polyamide, which generally refers to pure, as-synthesized polyamide that has not been further processed for use in a particular application (whether the virgin polyamide has been used or not). Alternatively, the polyamide may be “post-consumer” polyamide, which generally refers to a polyamide derived from a consumer product (whether that consumer product has actually been used or not). Post-consumer polyamide may include other components (e.g., other non-polyamide polymers such as polyolefins) such that it may be considered to be a composite with such other components and may have been processed for use in a particular application (e.g., fishing nets, carpet fibers, clothing, medical gloves). Blends of different types of polyamide polymers have been described above. Illustrative non-polyamide polymers include those listed in Table 3 of the Example, below, including polyolefins, polycarbonates, polyethers, polyesters, polyurethanes, etc. In either embodiment, the morphology of the polyamide is not particularly limited. For example, virgin polyamide may be in the form of a powder, including powder composed of micron-sized particles.Atty. Dkt. No. 00100-0411-PCTHowever, the present methods are capable of achieving high monomer yields from postconsumer polyamide that has been chopped into pieces significantly larger than the particles of virgin polyamide powder. In other words, no milling, e.g., cryogenic milling, is required to achieve high monomer yields from post-consumer polyamide using the present methods. Other morphologies include granules, pellets, films, and fibers.

[0072] When combined for conducting the present methods, the polyamide and the boron group catalyst may be considered to form a reaction mixture comprising (or consisting of) each of these components. As noted above, the reaction mixture need not comprise any liquid medium (e.g., solvent). (This does not preclude the presence of a liquid in the reaction mixture due to the use of molten polyamide.) Similarly, the reaction mixture need not comprise other additives (other than the alkali metal species / additive described above, if used). Thus, in such embodiments, the reaction mixture may be characterized as being free of a liquid medium (other than the polyamide if in its molten state) and free of an additive (other than the alkali metal additive species / described above, if used). Non-alkali metal additives which may be excluded from the reaction mixture, in embodiments, include the following: an ionic liquid; water or steam; ammonia; an amine; N, N-dimethylaminopyridine; acetic anhydride; a phosphoric acid (or salt thereof); a boric acid (or salt thereof); a sulfonic acid (or salt thereof); a carboxylic acid (or salt thereof); a carbonate; an alkaline earth oxide; an alkaline earth hydroxide; an alkaline earth carbonate; an alkaline earth carboxylate; an alcohol. It is noted that these non-alkali metal additives do not work to depolymerize polyamides using the method conditions described herein. In embodiments, the reaction mixture consists of the polyamide, the boron group catalyst, and optionally, the alkali metal additive. These embodiments do not preclude the ultimate presence of monomers provided by the depolymerization, unreacted polyamide, and / or other depolymerized product (e.g., polyamide fragments) in the reaction mixture. These embodiments also do not preclude the presence of components or impurities which may be inherently present in the reaction mixture due to the particular synthetic technique used to form the polyamide. These embodiments also do not preclude the presence of components which may be inherently present in the reaction mixture due to the source of the polyamide (e.g.. post-consumer polyamide).

[0073] The present methods may be conducted using a variety of types of reactor systems with or without stirring, agitation, or extrusion, including batch reactor systems, semi-batch reactor systems, and continuous flow reactor systems. As noted throughout, the presentAtty. Dkt. No. 00100-0411-PCTmethods are capable of achieving continuous operation for a period of time (e.g., by using a continuous flow of polyamide) while maintaining high monomer yields.

[0074] The present methods may further comprise recovering any products and / or unreacted material from the reaction mixture. This includes recovering and / or recycling the boron group catalysts (or a derivative thereof) from the reaction mixture. No liquid medium is required for recovery' of the boron group catalysts. The recovered boron group catalysts may be used to conduct the methods one or more additional times (i. e., they may be recycled). Similarly, the present methods may further comprise recovering the depolymerized products, e.g., monomers, and using them for any desired purpose, including synthesizing a new polymer, including a new polyamide. Recover}' of the depolymerized products may be by vaporization or sublimation.

[0075] The present methods may be characterized by a yield of monomer. The yield is reported as (weight of monomer) / (starting weight of polyamide)*100%. The yield may be determined using1H NMR as described in the Example, below. The yield may be an initial yield obtained by using fresh (i.e., unused) boron group catalyst. The initial yield may be at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, or at least 98%. This includes a range between any of these values, as well as from 70% to 100%, from 80% to 100%, and from 80% to 95%. The yield may be a yield obtained by using a recovered boron group catalyst which has been used one or more times (e.g., 1, 2, 3, 4, etc.). The yield from a recycled / recovered boron group catalyst may be within ±50%, ±20%, ±10%, ±5%, or ±2% of the initial yield. Any of the yield values in this paragraph may refer to a specific polyamide (e.g., Nylon-6), a specific monomer (e.g., s-caprolactam), a specific boron group catalyst (e.g., any of those in entries 4-7, 9-10, 12-21 of Table 1. Also see FIGs. 2A-2C.), and the methods conducted under specific conditions (e.g., any of those described in the Example below).

[0076] As noted above, the polyamide to be depolymerized by the present methods may be provided in a blend with non-polyamide polymers. The methods may depolymerize the polyamide while leaving the other polymer intact. The term 'intact" has a meaning analogous to “without” and “selective depolymerization” as described above. That is, intact polymers are evidenced by a yield of the corresponding monomer(s) that is below the detection limit of the1H NMR system used to measure yield as described in the Example, below. This includes measuring a monomer yield viaNMR that is less than 5%. Thus, the present methods allowAtty. Dkt. No. 00100-0411-PCTfor separation of a polyamide from such a blend. Blends of different types of polyamide polymers have been described above.

[0077] The present disclosure encompasses reaction mixtures comprising (or consisting of) any of the disclosed boron group catalysts with any of the disclosed polyamides, including blends thereof with non-polyamide polymers.EXAMPLE

[0078] Introduction

[0079] This Example includes experimental results that demonstrate that boron group-based catalysts (e.g., those including aluminum and boron) can efficiently depolymerize Nylon-6 to E-caprolactam in short reaction times, at low temperatures, and at low catalyst loadings and function well under vacuum or inert atmosphere such as N2, and can operate in the presence of mixed plastic waste containing Nylon-6 (FIG. 1 and Tables 1-3). These catalysts include: the organometallic AIR3 (R = H or a carbon-based ligand); metal-organic AIR' 3 (R1= nitrogen or oxy gen-containing ligands or a mixture of thereof such as amidates, amidos, and alkoxy ligand); mixed organometallic and metal-organic AIRnR'm (n+m = 3); and aluminum-ate complexes [M] [AIRnR'm] (n+m = 4, M = Li, Na, K, MgR or MgR').,. The catalysts, including illustrative catalysts shown in FIGs. 2A-2C, have the advantage of being suitable for continuous operation, enabling a continuous feed of Nylon-6 during depolymerization reactions. Furthermore, the E-caprolactam produced by the depolymerization reaction can be polymerized back to pristine Nylon-6.

[0080] Experimental

[0081] General methods and materials

[0082] Methods: All depolymerization reactions were assembled by mixing the polymer and the appropriate catalyst in an Ar-filled MBraun glovebox in cylindrical 100 mL Schlenk tubes. A customized aluminum heating block supplied heating with a fitted hole for the Schlenk tubes.

[0083] General materials: All catalysts were purchased from commercial sources and were used directly without further purification. Nylon-6 was washed with 1 M KOH solution overnight, filtered and washed with H2O, and dried under a vacuum at 100 °C before use. Pristine Nylon-6 powder with a mean particle size of 15-20 pm was obtained from Goodfellow Inc.Atty. Dkt. No. 00100-0411-PCT

[0084] Physical and analytical methods

[0085] NMR spectra were recorded on a Bruker Avance III HD system equipped with a TXO Prodigy probe (500 MHz) spectrometer. Chemical shifts (5) for H-NMR were referenced to the internal solvent.

[0086] General procedure for depolymerization reactions

[0087] In a glove box, a 100 mL oven-dried Schlenk tube was charged with a magnetic stir bar, Nylon-6 polymer, and catalyst. The vessel was sealed, and the polymer and catalyst were thoroughly mixed by stirring at room temperature for approximately 30 seconds. The Schlenk tube was then connected to a vacuum line, subjected to reduced pressure (~10‘3Torr), sealed, and heated to the specified temperature with magnetic stirring for the specified time. Reaction time was recorded starting 2.5 min after the reaction tube was placed in the heating block, since it takes an average of 2.5 min for the reaction to reach the depolymerization temperature and the polymer to begin melting. During the reaction, the products sublimed from the hot reaction zone and deposited as a crystalline layer on the cold wall of the reactor. After cooling to room temperature, the soluble part of the reaction mixture was dissolved in 3-4 mL of deuterated solvent, and mesitylene was added as an internal standard. A sample of this solution was withdrawn for NMR analysis. Yields were determined by 'H-NMR, comparing the signal integrals of s-caprolactam and mesitylene.

[0088] Procedures for reactions in Table 1

[0089] Table 1, entry 1240 °C, 10’3TorrNylon-6 3.0 h e-Caprolactam(KOH treated) 1.6%

[0090] Exactly 0.1 gr of KOH treated Nylon-6 powder was reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. s-caprolactam was obtained in 1.6% yield.

[0091] Table 1, entry 2Atty. Dkt. No. 00100-0411-PCT7.63 mol%NaOH240 °C, 10'3TorrNylon-6E-Caprolactam(KOH treated) 2%

[0092] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0027 gr of NaOH (7.63 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. s-caprolactam was obtained in 2 % yield.

[0093] Table 1, entry 35.04 mol%KOtBu240 °C, 103TorrNylon-6 1.0 h e-Caprolactam(KOH treated) 2%

[0094] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0050 gr of KOtBu (5.04 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. s-caprolactam was obtained in 2 % yield.

[0095] Table 1, entry 44.98 mol%[AI(OCH(CH3)2)3]4240 °C, 10'3TorrNylon-6 1.0 h 8-Caprolactam(KOH treated) 86%

[0096] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0090 gr of [A1(OCH(CH3)2)3]4 (4.98 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. s-caprolactam was obtained in 86 % yield.

[0097] Table 1, entry 52.53 mol%[AI(OCH(CH3)2)3]4240 °C, 10'3TorrNylon-6 c-Caprolactam(KOH treated) 93%Atty. Dkt. No. 00100-0411-PCT

[0098] Exactly 0.197 gr of KOH treated Nylon-6 powder and 0.0090 gr of [A1(OCH(CH3)2)3]4 (2.53 mol%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. 8-caprolactam was obtained in 95 % yield.

[0099] Table 1, entry 60.25 mol%[AI(OCH(CH3)2)3]4240 °C - 260 °CNylon-6 103Torr, 3.0 h e-Caprolactam(KOH treated) 87%

[0100] Exactly 0.2 gr of KOH treated Nylon-6 powder and 0.0009 gr of [A1(OCH(CH3)2)3]4 (0.25 mol%) were reacted according to the general procedure, heating the reaction mixture for 25 min at 240 °C and 130 min at 260 °C. 8-caprolactam was obtained in 87 % yield.

[0101] Table 1, entry 72.53 mol%H [AI(OCH(CH3)2)3]4' ■* _J 280 °C, 10'3Torrn e - Nylon-6 JO min e-Caprolactam(KOH treated) 96%

[0102] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0045 gr of [Al(OCH(CH3)2)3]4(2.53 mol%) were reacted according to the general procedure, heating the reaction mixture to 280 °C for 15 min. 8-caprolactam was obtained in 96% yield.

[0103] Table 1, entry 82.53 mol% OAI2O3240 °C, 10’3TorrNylon-6 1.0 h e-Caprolactam(KOH treated) 1.4%

[0104] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0025 gr of Al2O3(2.53 mol%) were reacted according to the general procedure, heating the reaction mixture to 240°C for 1 h. e-caprolactam was obtained in 1.4% yield.

[0105] Table 1, entry 9Atty. Dkt. No. 00100-0411-PCT4.26 mol%Na[CH3O(CH2)2O]2[(CH2)5CON]2AI280 °C, 10'3Torr, 10 minNylon-6 8-Caprolactam(KOH treated) 88%

[0106] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0200 gr of Na[CH3O(CH2)2O]2[(CH2)5CON]2Al (80 wt%) in toluene, corresponding to 4.26 mol%, were mixed thoroughly. The toluene was then removed under reduced pressure. The resulting mixture was heated for 10 minutes at 280 °C following the general procedure. 8-Caprolactam was obtained in a total yield of 96%, which includes caprolactam derived from both the Nylon-6 and the caprolactam-based ligands in the catalyst. To determine the caprolactam yield derived specifically from Nylon-6, the total yield was corrected by subtracting the theoretical maximum yield of caprolactam that could originate from the catalyst's ligands, which corresponded to 88% yield.

[0107] Table 1, entry 104.26 mol% Na[CH3O(CH2)2O]2[(CH2)5CON]2AIPET240 °C -280 °C, 10’3Torr Nylon-6 E-Caprolactam (KOH treated) 72%

[0108] Exactly 0.1 gr of KOH treated Nylon-6 powder, 0.05 g of PET, and 0.0330 gr of Na[CH3O(CH2)2O]2[(CH2)5CON]2Al (80 wt%) in toluene, corresponding to 4.26 mol%, were mixed thoroughly. The toluene was then removed under reduced pressure. The resulting mixture was heated for 20 minutes at 240 °C and 75 min at 280 °C following the general procedure. 8-Caprolactam was obtained in a total yield of 86%, which includes caprolactam derived from both the Nylon-6 and the caprolactam-based ligands in the catalyst. To determine the caprolactam yield derived specifically from Nylon-6, the total yield was corrected by subtracting the theoretical maximum yield of caprolactam that could originate from the catalyst’s ligands, which corresponded to 72% yield.

[0109] Table 1, entry 11Atty. Dkt. No. 00100-0411-PCT4.99 mol%[CH3CH(OH)COO]3AI280 °C, 10’3Torr, 4.0 hNylon-6 s-Caprolactam(KOH treated) 2%

[0110] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0130 gr of [CH3CH(OH)COO]3A1 (4.99 mol%) were reacted according to the general procedure, heating the reaction mixture for 4 h at 280 °C. e-caprolactam was obtained in 2 % yield.

[0111] Table 1, entry 125.06 mol%LiAlH4280 °C, 10-3TorrNylon-6 20 min e-Caprolactam(KOH treated) 85%

[0112] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0017 gr of LiAlH4 (5.06 mol%) were reacted according to the general procedure, heating the reaction mixture for 20 min at 240 °C. e-caprolactam was obtained in 85 % yield.

[0113] Table 1, entry 135.19 mol%LiBH4240 °C, 10-3TorrNylon-6 -0 e -Caprolactam(KOH treated) 67%

[0114] Exactly 0.1 g of KOH treated Nylon-6 powder and 0.0010 g of LiBH4 (5.19 mol%) were reacted according to the general procedure, with the reaction mixture heated at 240 °C for 1 hour. e-Caprolactam was obtained in a yield of 67%. The solid LiBHt powder was prepared by removing the THF solvent under reduced pressure at 80 °C for 2 hours from a 2M THF solution of LiBHt.

[0115] Table 1, entry 14Atty. Dkt. No. 00100-0411-PCT4.97 mol% O[(Me2N)3AI]2240 °C, 10-3TorrNylon-6 1.0 h s-Caprolactam(KOH treated) 92%

[0116] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0070 gr of [(Me2N)3Al]2 (4.97 mol%) were reacted according to the general procedure, heating the reaction mixture for 1 h at 240 °C. 8-caprolactam was obtained in 92 % yield.

[0117] Table 1, entry 151.42 mol%[(Me2N)3AI]2240 °C, 10-3TorrNylon-6 30 min c-Caprolactam(KOH treated) 92%

[0118] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0020 gr of [(Me2N)3Al]2(1.42 mol%) were reacted according to the general procedure, heating the reaction mixture for 30 min at 240 °C. s-caprolactam was obtained in 92 % yield.

[0119] Table 1, entry 161.42 mol% O[(Me2N)3AI]2240 °C, ArgonNylon-6 1.0 h e-Caprolactam(KOH treated) 84%

[0120] Exactly 0.1 g of KOH treated Nylon-6 powder and 0.0020 g of [(Me2N)3Al]2(1.42 mol%) were mixed in a glove box under an argon atmosphere. The mixture was then sealed and immediately heated without vacuum treatment, following the general procedure, for 1 h at 240 °C. s-Caprolactam was obtained in 84% yield.

[0121] Table 1, entry 17Atty. Dkt. No. 00100-0411-PCT0.57 mol% O[(Me2N)3AI]2240 °C, 10-3TorrNylon-6 1.0 h c-Caprolactam(KOH treated) 93%

[0122] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0008 gr of [(Me2N)3Al]2(0.57 mol%) were reacted according to the general procedure, heating the reaction mixture for 1 h at 240 °C. 8-caprolactam was obtained in 93 % yield.

[0123] Table 1, entry 180.25 mol% O[(Me2N)3AI]2240 °C, 10-3TorrNylon-6 1.0 h c-Caprolactam(KOH treated) 91%

[0124] Exactly 0.2 gr of Nylon-6 powder and 0.0007 gr of [(Me2N)3Al]2 (0.25 mol%) were reacted according to the general procedure, heating the reaction mixture for 1 h at 240 °C. 8-caprolactam was obtained in 91 % yield.

[0125] Table 1, entry 191.99 mol% OAI(NC6H11O)3280 °C, 10-3TorrNylon-6 30 min c-Caprolactam(KOH treated) 67%

[0126] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0300 gr of Al(NC6H11O)3(21.34 wt%) in caprolactam, corresponding to 1.99 mol%, were mixed thoroughly. The mixture was heated for 30 minutes at 280 °C following the general procedure. c-Caprolactam was obtained in a total yield of 92%, including caprolactam derived from both the Nylon-6 and the caprolactam-based ligands in the catalyst and the caprolactam from the catalyst / caprolactam mixture. To determine the caprolactam yield derived specifically from Nylon-6, the total yield was corrected by subtracting the theoretical maximum yield of caprolactam that could originate from the catalyst, which corresponded to 63% yield.

[0127] Table 1, entry 20Atty. Dkt. No. 00100-0411-PCT1.0 mol% OAl(NC6H11O)3280 °C, 10-3TorrNylon-6 10 min c-Caprolactam(KOH treated) 78%

[0128] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0150 gr of Al(NC6H11O)3(21.34 wt%) in caprolactam, corresponding to 1.00 mol%, were mixed thoroughly. The mixture was heated for 10 minutes at 280 °C following the general procedure. s-Caprolactam was obtained in a total yield of 93%, including caprolactam derived from both the Nylon-6 and the caprolactam-based ligands in the catalyst and the caprolactam from the catalyst / caprolactam mixture. To determine the caprolactam yield derived specifically from Nylon-6, the total yield was corrected by subtracting the theoretical maximum yield of caprolactam that could originate from the catalyst, which corresponded to 78% yield.

[0129] Table 1, entry 214.98 wt.%[AI(OCH(CH3)2)3]4+ PE260 °C, 10-3TorrNylon-6 30 min g-Caprolactam(KOH treated) 87%

[0130] Exactly 0.1 gr of KOH treated Nylon-6 powder, 0.03 gr of polyethylene, and 0.0055 gr of [Al(OCH(CH3)2)3]4(4.98 mol%) were reacted according to the general procedure, heating the reaction mixture for 30 min at 260 °C. 8-caprolactam was obtained in 87 % yield.

[0131] Table 1, entry 222.53 mol% OAI(OTf)3240 °C, 10’3TorrNylon-6 1.0 h s-Caprolactam(KOH treated) 8%

[0132] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0012 gr of Al(OTf)3(2.53 mol%) were reacted according to the general procedure, heating the reaction mixture for 1.0 h at 240 °C. s-caprolactam was obtained in 8 % yield.Atty. Dkt. No. 00100-0411-PCT

[0133] Table 1, entry 232.53 mol% OAl(acac)3240 °C, 10-3TorrNylon-6 1.0 h e-Caprolactam(KOH treated) 1.2%

[0134] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0072 gr of Al(acac)3(2.53 mol%) were reacted according to the general procedure, heating the reaction mixture for 1.0 h at 240 °C. s-caprolactam was obtained in 1.2 % yield.

[0135] Table 1, entry 242.53 mol%[AI(OCH(CH3)2)3]4240 °CNylon-6 Air, 1.0 h s-Caprolactam(KOH treated) negligible

[0136] Exactly 0.1 gr of KOH treated Nylon-6 powder and 0.0045 gr of [A1(OCH(CH3)2)3]4(2.53 mol%) were reacted according to the general procedure, heating the reaction mixture for 1.0 h at 240 °C. 8-caprolactam was obtained in negligible amount.

[0137] Results and Discussion

[0138] The main objective of this study was to develop a selective Nylon-6 depolymerization process using mild conditions and low-cost, non-toxic catalysts. For this reason, the initial focus was on systems containing abundant, cheap, and non-toxic oxygencontaining ligands such as alkoxides. To assess the performance of oxygen-substituted alkali metals, hydroxides and alkoxides NaOH and KOtBu were first studied. Tn an initial set of experiments, a Nylon-6 pretreatment was applied to prevent catalyst deactivation by the carboxylic end group and to obtain more accurate catalytic data on the effects of various catalysts. This involved suspending Nylon-6 powder in an aqueous KOH solution, followed by filtration, thorough washing with water, and drying under reduced pressure. To verify whether residual potassium metal remaining after washing of the Nylon-6 polymer contributes to catalysis under the experimental conditions, the depolymerization of Nylon-6 was conducted without adding any catalyst. Under the current reaction setup, i.e., a sealed batch reactor operated under reduced pressure and heated to 240 °C, the reaction yielded onlyAtty. Dkt. No. 00100-0411-PCTa negligible amount of caprolactam even after 3 h, indicating that depolymerization does not proceed effectively in the absence of a catalyst. These results confirm that the presence of a catalyst is essential under the current reaction step up and conditions (Table 1, entry 1).

[0139] Despite previous reports suggesting that NaOH can depolymerize Nylon-6 to caprolactam, under the current reaction setup and conditions, i.e., a sealed batch reactor under reduced pressure heated to 240 °C, only 2% of caprolactam was obtained after 1 h even using a high catalyst loading of 7.63 mol % (Table 1, entry 2). When tBuOK, an alkali metal alkoxide catalyst, was used, only 2% of caprolactam was produced after 1 h at 240 °C (Table 1, entry 3). Without wishing to be bound to any particular theory, the poor performance of these catalysts under the mild reaction conditions in Table 1 is believed to be due to their limited basicity which may be insufficient for deprotonating and metalating the amide bonds in Nylon-6, a crucial step required for successive ring-closing depolymerization reactions.

[0140] In the next stage of experimentation, aluminum-based catalysts w ere used, and their activity in Nylon-6 depolymerization reactions was studied. To the inventors’ know ledge, there are no reports of aluminum compounds being able to depolymerize Nylon-6 to caprolactam. The tetrameric aluminum isopropoxide (FIG. 2A)was chosen as the first aluminum catalyst. Heating Nylon-6 in the presence of 4.98% aluminum isopropoxide for 1 hour at 240 °C yielded an 86% conversion to caprolactam (Table 1. entry 4). This high conversion w as unexpected, as the basicity of aluminum alkoxides (such as aluminum isopropoxide), which is similar to oxygen-substituted alkali metals, w ould be considered to be insufficient to deprotonate or metalate the amide bonds in the polyamide chain. Thus, the present results suggest that, in the presence of aluminum catalysts, these reactions may be proceeding via a completely different mechanism. Interestingly, decreasing the catalyst loading to 2.53% resulted in a higher yield of 93% under identical reaction conditions (Table 1, entry' 5). Surprisingly, further decreasing the catalyst loading by a factor of 10 to 0.25 % still resulted in high catalytic activity', producing an 87% yield, although a longer reaction time and a higher temperature was required (Table 1, entry 6). The fact that the process can operate with such low catalyst loading is surprising, considering that the standard range of conventionally7used catalysts is typically 1-10 mol% catalyst loading. Thus, the present results suggest that the aluminum isopropoxide catalyst is a stable, robust, and less likely to deactivate catalyst, presenting an attractive option for a continuous Nylon-6 depolymerization process.Atty. Dkt. No. 00100-0411-PCT

[0141] Next, the activity of the commercially available sodium dicaprolactamato bis-(2-methoxyethoxo) aluminate (Na[CH3O(CH2)2O]2[(CH2)5CON]2Al) (FIG. 2B) was studied. Impressively, using 4.26% catalyst loading, a quantitative conversion was observed after 10 min at 280 °C (Table I, entry 9). Excitingly, when Nylon-6 was pre-mixed with polyethylene terephthalate) (PET), and heated in the presence of the catalyst, exclusive formation of caprolactam was observed in high yields (Table 1. entry 10), demonstrating that the process is compatible with mixed plastics waste. Note that Nylon-6 / PET mixtures are a common waste stream in the textile, carpet, and fishing industries, and the abi 1 i ty to selectively extract caprolactam from such mixtures exclusively is attractive. Another attractive feature of the sodium dicaprolactamato bis-(2-methoxyethoxo) aluminate catalyst is that during the catalysis, the aluminum-bounded caprolactam ligands were released and contributed to a higher overall caprolactam yield (Table 1, entries 9-10).

[0142] Next, aluminum L-lactate with Al-0 ligands was tested; however, unlike lanthanum isopropoxide, negligible activity was observed despite using a 4.99 mol % catalyst, yielding only 2% caprolactam (Table, entry 11). Thus, an Al-0 linkage within the catalyst alone was insufficient to enable the transformation of Nylon-6 to caprolactam.

[0143] The effect of various substituents on the aluminum center was subsequently investigated to understand its catalytic properties further. Interestingly, when the aluminum hydride LiAlH4was used, a high yield of 85% of caprolactam was obtained after only 20 min at 240 °C (Table 1, entry 12). Considering the highly -reducing nature of LiAlH4. which is known to deoxygenate amides to amines effectively, the selective formation of caprolactam, a cyclic amide, was very surprising, especially since no detectable amount of amines was observed. This process is not limited to aluminum; it can also proceed with other Group III elements, such as boron. Accordingly, the corresponding boron hydride, LiBH4, also exhibited catalytic activity in Nylon-6 depolymerization (Table 1, entry 13).

[0144] Next, it was investigated whether amido-substituted complexes could catalyze this transformation. The commercially available dimeric tris(dimethylamido)aluminum (FIG. 2C) exhibited exceptional activity, achieving quantitative yields of caprolactam at 240 °C with catalyst loadings as low as 0.25% (Table 1, entries 14-18). The process operated well under a static vacuum or an inert atmosphere (Table 1, entries 15 and 16).

[0145] At 280 °C, the tris(caprolactamato)aluminum complex also demonstrated an impressive catalytic efficiency in Nylon-6 depolymerization, with the reaction reachingAtty. Dkt. No. 00100-0411-PCTcompletion in just 10 minutes at catalyst loadings as low as 1 mol% (Table 1, entries 19-20). It is still unclear why decreasing the catalyst loading and reaction time in entry 16 from 1.99% and 30 min to 1.00% and 10 min resulted in yield improvement (Table 1, entries 19-20). As noted above, similar phenomenon was also observed for the [Al(OCH(CH3)2)3]4catalyst when decreasing the catalyst loading from 4.98% (Table 1, entry 4) to 2.53% (Table 1, entry 5) resulted in a yield increase from 86% to 93%.

[0146] Lastly to demonstrate that the process is compatible with a mixed Nylon-6 waste stream (e.g., as in entry 10 where Nylon-6 was mixed with PET), the [A1(OCH(CH3)2)3]4 catalyst was tested with a Nylon-6 / polyolefin mixture. It was found that the presence of polyethylene did not affect the reaction performance yielding 87% of caprolactam after 30 min at 240 °C (Table 1, entry 21).

[0147] Additional experiments showed that the depolymerization reactions using aluminum triflate (Al(OTf)3) and aluminum acetylacetonate (Al(acac)3) produced only 8% and 1.2% yield of caprolactam, respectively (Table 1. entries 22-23). These results demonstrate that the catalytic activity is highly sensitive to the ligand environment. That is, although efficient depolymerization occurs when aluminum is coordinated to isopropoxide ligands, changing the ligand to triflate or acetylacetonate significantly suppresses the activity. Furthermore, activity is sensitive to atmosphere as performing the reaction under air using aluminium isopropoxide gave no depolymerization products, whereas vacuum or an inert atmosphere did achieve effective catalysis (Table 1, entry 24).

[0148] Table 1. Screening of catalysts in Equation l.aNylon-6 E-CaprolactamEntry Catalyst Catalyst Time Temperature Yield (%)bloading (°C)(mol%)1 No catalyst 2.53 3 h 240 negligible (n d ) 2 NaOH 7.63 1 h 240 23 KOtBu 5.04 1 h 240 24 [Al(OCH(CH3)2)3]44.98 Ih 240 865 [A1(OCH(CH3)2)3]4C2.53 Ih 240 95Atty. Dkt. No. 00100-0411-PCT6 [A1(OCH(CH3)2)3]4C0.25 155 min 240-260d87d7 [A1(OCH(CH3)2)3]4253 15 min 280 968 Al2O3nano powder (5 nm) 2.53 1.5 h 240 1.49 Na[CH3O(CH2)2O]2[(CH2)5CON]2Ale4.26 10 min 280 96 (88)f10® Na[CH3O(CH2)2O]2[(CH2)5CON]2Ale7.03 95 min 240-280h86 (72)f11 [CH3CH(OH)COO]3A1 4.99 4 h 280 212 LiAlH45.06 20 min 240 8513 LiBH45.19 1 h 240 6714 [(Me2N)3Al]24.97 1 h 240 9215 [(Me2N)3Al]21.42 0.5 h 240 9216 [(Me2N)3Al]2‘ 1.42 1 h 240 8417 [(Me2N)3Al]20.57 1 h 240 9318 [(Me2N)3Al]2c025 1 h 240 9119 Al(NC6H11O)31.99 0.5 h 280 63 (92) 20 Al(NC6H11O)31.00 10 min 280 78k(93) 211[A1(OCH(CH3)2)3]43.04 30 min 260 8722 Al(OTf)32.5 1.0 240 823 Al(acac)32.5 1.0 240 1.224 [Al(OCH(CH3)2)3]4nl2.5 1.0 240 negligible_aGeneral depolymerization conditions: static vacuum, 100 mg of KOH treated Nylon-6 powder in 100 mL Schlenk flask, unless otherwise stated.bYields determined by 'H NMR with a mesitylene internal standard.c200 mg of KOH treated Nylon-6 powder.d25min at 240°C and 130 min at 260°C.e80 wt% of Na[CH3O(CH2)2O]2[(CH2)5CON]2Al in toluene.fThe total yield, including caprolactam produced from the catalyst's ligand. The yield, corrected by subtracting caprolactam derived from the catalyst’s ligands, is shown in parentheses.g100 mg of KOH treated Nylon-6 was mixed with 50 mg of PET.hThe reaction was heated for 20 min at 240°C and 75 min at 280°C.1Reaction was carried under glove box atmosphere containing Ar and N2 gases. 21.34 wt% of Al(NC6H11O)3in caprolactam.kThe total yield, including both caprolactam produced from the catalyst’s ligand and the caprolactam present in the catalyst mixture, is reported. The corrected yield, which subtracts these sources of caprolactam, is shown in parentheses.1100 mg of KOH treated Nylon-6 wasAtty. Dkt. No. 00100-0411-PCTmixed with 30 mg of polyethylene.mThe reaction was performed under air atmosphere, n.d. = not detected.

[0149] Additional experiments were performed to examine the role of KOH pretreatment of Nylon-6 powder, which was used in the experiments conducted above and summarized in Table 1. The procedures for these additional experiments are provided below and the results collected in Table 2, below.

[0150] Procedures for reactions in Table 2

[0151] Table 2, entry 15.53 mol% (10wt.%)[AI(OCH(CH3)2)3]4240 °C, 10-3Torr1.0 hNylon-6 (non KOH treated) c-Caprolactamnegligible

[0152] Exactly 0.1 gr of non KOH treated Nylon-6 powder and 0.01 gr of [A1(OCH(CH3)2)3]4 (5.53 mol%, 10 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. s-caprolactam was obtained in negligible amount.

[0153] Table 2, entry 21.0 mol% (1.4 wt.%)[(Me2N)3AI]2240 °C, 10-3Torr1.0 hNylon-6 (non KOH treated) £-Caprolactamnegligible

[0154] Exactly 0.1 gr of non KOH treated Nylon-6 powder and 0.0014 gr of [(Me2N)3Al]2(1 mol%, 1.4 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. 8-caprolactam was obtained in negligible amount.

[0155] Table 2, entry 3Atty. Dkt. No. 00100-0411-PCT2.53 mol% (1.7 wt.%)LiAlH4240 °C, 10-3Torr1.0 hNylon-6 (non KOH treated) ε-Caprolactam88%

[0156] Exactly 0.1 gr of non KOH treated Nylon-6 powder and 0.0009 gr of LiAlH4 (2.53 mol%, 0.9 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. s-caprolactam was obtained in 88% yield.

[0157] Table 2, entry 44.7 mol% (28 wt.%) Na[CH3O(CH2)2O]2[(CH2)5CON]2Al240 °C, 10-3Torr, 1.0 hNylon-6 (non KOH treated) s-Caprolactam98%

[0158] Exactly 0.1 gr of non KOH treated Nylon-6 powder and 0.0230 gr of Na[CH3O(CH2)2O]2[(CH2)5CON]2Al (80 wt.%) in toluene, corresponding to 4.7 mol% (28 wt.% ). were mixed thoroughly. The toluene was then removed under reduced pressure. The resulting mixture was heated for 1 h at 240°C following the general procedure, e-Caprolactam was obtained in a total yield of 98%, which includes caprolactam derived from both the Nylon-6 and the caprolactam-based ligands in the catalyst. To determine the caprolactam yield derived specifically from Nylon-6, the total yield was corrected by subtracting the theoretical maximum yield of caprolactam that could originate from the catalyst’s ligands, which corresponded to 89% yield.

[0159] Table 2, entry 52.53 mol% (4.5 wt.%)[Al(OCH(CH3)2)3]4+ NaOH 240 °C, 10-3Torr Nylon-6 7.5 mol% 2.0 h ε-Caprolactam (non KOH treated) (2.6 wt.%) 92%

[0160] Exactly 0.1 gr of non KOH treated Nylon-6 powder. 0.0037 gr of [Al(OCH(CH3)2)3]4(2.53 mol%, 4.5 wt%) and 0.0022 gr of NaOH (7.5 mol%, 2.6 wt.%).Atty. Dkt. No. 00100-0411-PCTwere reacted according to the general procedure, heating the reaction mixture to 240 °C for 2 h. e-caprolactam was obtained in 92% yield.

[0161] Table 2, entry 62.53 mol% (4.5 wt.%)[Al(OCH(CH3)2)3]4240 °C, 10-2TorrNylon-6 (non KOH treated) c-Caprolactamnegligible

[0162] An oven-dried 5 L three-neck flask (serving as the reactor) equipped with a mechanical stirrer was connected via a bridge to a 3 L cold-trap flask. 100 gr untreated Nylon-6 polymer pellets (i.e., without KOH washing) were placed in the reactor and heated at 120 °C overnight under continuous reduced pressure (~10-2Torr) to remove residual moisture and volatiles. Subsequently, 4.5 g of [Al(OCH(CH3)2)3]4 (2.53 mol%, 4.5 wt.%) was introduced into the reactor under a continuous flow of argon. The system was then reconnected to the vacuum line (~10-2Torr), and the temperature was raised to 260 °C and maintained for 2 h. The reaction time was recorded from the moment depolymerization products were first observed in the cold trap, accounting for the time required for the polymer to melt and reach the depolymerization temperature. During the reaction, the volatile depolymerization products sublimed from the hot reactor and condensed as a crystalline layer on the cold walls of the trap. After cooling to room temperature, the collected product in the cold trap was melted and transferred out of the flask, e-caprolactam was obtained in negligible amount.

[0163] Table 2, entry 72.53 mol% (4.5 wt.%)[Al(OCH(CH3)2)3]4+ NaOH260 °C, 10-2TorrNylon-6 1.2 mol% 1 h s-Caprolactam(non KOH treated) (0.4 wt.%) 90%

[0164] An oven-dried 5 L three-neck flask (serving as the reactor) equipped with a mechanical stirrer was connected via a bridge to a 3 L cold-trap flask. 100 gr untreated Nylon-6 polymer pellets (i.e., without KOH washing) were placed in the reactor and heated at 120 °C overnight under continuous reduced pressure (~ 10-2Torr) to remove residual moistureAtty. Dkt. No. 00100-0411-PCTand volatiles. Subsequently, 4.5 g of [Al(OCH(CH3)2)3]4 (2.53 mol%, 4.5 wt.%) was introduced into the reactor under a continuous flow of argon. The system was then reconnected to the vacuum line (~ 10-2Torr), and the temperature was raised to 260 °C and maintained for 2 h. The formation of depolymerization product was observed in negligible amount in cold trap. After 2 h heating at 260 °C, 0.413 gr of NaOH pellets (1.2 mol%, 0.4 wt.%) were added to the reactor under argon and system was then reconnected to the vacuum line (~10-2Torr), and maintained at 260 °C for 1 h. During the reaction, the volatile depolymerization products sublimed from the hot reactor and condensed as a crystalline layer on the cold walls of the trap. After cooling to room temperature, the collected product in the cold trap was melted and transferred out of the flask, ε-caprolactam was isolated in 90% yield.

[0165] As noted above, additional experiments were conducted to examine the role of KOH pretreatment of Nylon-6 powder. During the KOH pretreatment, the carboxylic acid (R(C=O)OH) end groups in Nylon-6 are deprotonated, generating carboxylate species (R(C=O)OK). The presence of carboxylic acids consumes some of the catalyst, and thus, conducting a KOH pretreatment is an effective way to convert the reactive carboxylic acid species to the more inert carboxylate species. By doing so, a lower amount of catalyst is required. However, when the untreated Nylon-6 (without KOH pretreatment) was subjected to depolymerization using a 5.53 mol% catalyst loading of aluminum isopropoxide, surprisingly, only trace amounts of the monomeric product was obtained (Table 2, entry 1; compare to Table 1, entry 4 showing that KOH pretreatment and using the same catalysts under similar conditions achieves 86% yield). Similar results were obtained with [(Me2N)3Al]2catalytic systems (Table 2, entry 2; compare to Table 1, entry 1 showing that KOH pretreatment and using the same catalyst under similar conditions achieves 84% yield). These findings strongly suggest that the presence of some alkali metal species may be required to depolymerize Nylon-6 — a previously unobserved phenomenon. This hypothesis is further supported by the results showing that by heating non-KOH-treated Nylon-6 in the presence of 2.53% lithium aluminum hydride (LiAlH4) for 1 hour at 240°C yielded an 88% conversion to caprolactam (Table 2, entry 3). Similarly impressively, when 4.7 mol% of sodium dicaprolactamato bis-(2-methoxyethoxo) aluminate (Na[CH3O(CH2)2O]2[(CH2)5CON]2Al) catalyst was heated with non-KOH-treated Nylon-6, a quantitative depolymerization was achieved within 1 h at 240 °C (Table 2, entry 4). Taken together, the remarkable activity strongly supports the surprising hypothesis that the presenceAtty. Dkt. No. 00100-0411-PCTof small amounts of alkali metal (e.g., lithium, sodium, or potassium) species may be required to create an active depolymerization system. That is. it appears that there is an unexpected synergy between aluminum complexes (R3Al, R = O, N, H, C) in combination with RM (M = alkali metal, R = H, C, N, O) that leads to a catalytically active system for Nylon-6 depolymerization.

[0166] Encouraged by these observations, it was further hypothesized that the deliberate in situ addition of a small amount of NaOH to the reaction mixture containing untreated Nylon-6 and the catalyst could promote Nylon-6 depolymerization. To test this hypothesis, depolymerization experiments under identical conditions were conducted with added NaOH to assess whether enhanced depolymerization could be achieved. It is emphasized that simply using NaOH (without Al complexes) does not lead to depolymerization under the reaction conditions (Table 1, entry 2).

[0167] Surprisingly, reacting untreated Nylon-6 (no KOH pretreatment) with 2.5 mol% aluminum isopropoxide catalyst in the presence of 7.5 mol% sodium hydroxide as a "‘polymer activator” resulted in the formation of a substantial yield (92%) of e-caprolactam under solvent-free conditions (Table 2, entry 5). As noted above, control experiments show that 7.5 mol% sodium hydroxide alone is insufficient to induce depolymerization of either untreated or KOH-pretreated Nylon-6 (Table 1, entry 2). It is also emphasized that without the added sodium hydroxide (or KOH pretreatment), aluminum isopropoxide does not catalyze the depolymerization of Nylon-6 (Table 2, Entry 1). To the best of the inventors’ knowledge, these are the first reported results of such cooperative activation under solvent-free conditions, highlighting a simple and effective strategy to promote catalytic depolymerization of Nylon-6. This finding is particularly remarkable because two components that are individually inactive under identical solvent-free conditions collectively enable efficient depolymerization when combined.

[0168] To further substantiate the hypothesis, the depolymerization reaction was scaled up to a 100 g batch of non-KOH treated Nylon-6 by heating non-KOH treated Nylon-6 pellets at 260 °C for 1 h with 2.5 mol% of catalyst and 1.2 mol% of sodium hydroxide (Table 2, entry 7). Interestingly, even with a reduced hydroxide loading (1.2 mol% in Table 2 entry 7 vs 7.5 mol% in Table 2 entry' 5), a comparable yield (= 90%) of s-caprolactam was obtained, demonstrating the robustness and scalability' of the process. In contrast, under identical conditions but in the absence of sodium hydroxide, only negligible depolymerization wasAtty. Dkt. No. 00100-0411-PCTobserved at the 100 gr scale (Table 2, entry 6). These findings clearly reinforce the importance of combining alkali metal species (e.g., alkali metal hydroxides) and aluminum complexes even at large reaction scales.

[0169] Table 2. Screening of catalysts for the depolymerization of non-KOH-treated Nylon-6 polymer in Equation 2.aCatalystNylon -6 (non KOH treated) c-CaprolactamEntry Catalyst Catalyst loading Time T (°C) Yield (%)b(mol%)1 [Al(OCH(CH3)2)3]45.53 1h 240 negligible (n.d.) 2 [(Me2N)3Al]21 1h 240 negligible (n.d.) 3 LiAlH42.53 1h 240 884 Na[CH3O(CH2)2O]2[(CH2)5CON]2Al 4.7 1h 240 98 (89)c5 [Al(OCH(CH3)2)3]4+ NaOHd,e2.53 (Al) + 7.5 2h 240 92(NaOH)d6f[Al(OCH(CH3)2)3]42.53 1h 240 negligible (n.d.) 7f[Al(OCH(CH3)2)3]4+ NaOHg2.53 (Al) + 1.2 1h 240 90g(NaOH)gaGeneral depolymerization conditions: static vacuum, 100 mg of non-KOH treated Nylon-6 powder in 100 mL Schlenk flask, unless otherwise stated.bYields determined by1H NMR with a mesitylene internal standard.cThe total yield, including caprolactam produced from the catalyst’s ligand. The yield, corrected by subtracting caprolactam derived from the catalyst’s ligands, is shown in parentheses.dA mixture of [Al(OCH(CH3)2)3]4(2.53 mol%) and NaOH (7.5 mol%) were used.e82 mg of non-KOH treated Nylon-6 powder was used.f100 g of non-KOH treated Nylon-6 pellets.gMixture of [Al(OCH(CH3)2)3]4(2.53 mol%) and NaOH (1.2 mol%) were used.8Isolated yield, n.d. = not detected.

[0170] Additional experiments were performed to evaluate whether the developed catalytic systems would selectively depolymerize Nylon-6 in the presence of other polymers. The procedures for these additional experiments are provided below and the results collected in Table 3, below.

[0171] Procedures for reactions in Table 3Atty. Dkt. No. 00100-0411-PCT

[0172] Table 3, entry 14.5 wt.%[Al(OCH(CH3)2)3]4Nylon-66 + Nylon-66240 °C, 10-3TorrNylon-6 3.0 h s-Caprolactam(KOH treated) 89%

[0173] Exactly of 45 mg of KOH treated Nylon-6 powder, and 55 mg of KOH treated Nylon-66 powder and 4.5 mg of [A1(OCH(CH3)2)3]4 (4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. s-caprolactam was obtained in 89% yield.

[0174] Table 3, entry 24.5 wt.%[Al(OCH(CH3)2)3]4+ Nylon-12 240 °C, 10-3TorrNylon-6 3.0 h s-Caprolactam(KOH treated) 87%

[0175] Exactly of 50 mg of KOH treated Nylon-6 powder, and 50 mg of Nylon-12 powder and 4.5 mg of [A1(OCH(CH3)2)3]4 (4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. e-caprolactam was obtained in 87% yield.

[0176] Table 3, entry 34.5 wt.%[AI(OCH(CH3)2)3]4+ i-PP240 °C, 10’3TorrNylon-6 3.0 h s-Caprolactam(KOH treated) 90%

[0177] Exactly of 80 mg of KOH treated Nylon-6 powder, and 20 mg of i-PP (isotactic polypropylene) powder and 4.5 mg of [Al(OCH(CH3)2)3]4(4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. s-caprolactam was obtained in 90% yield.

[0178] Table 3, entry 4Atty. Dkt. No. 00100-0411-PCT4.5 wt.%[Al(OCH(CH3)2)3]4240 °C, 10-3TorrNylon-6 3.0 h s-Caprolactam(KOH treated) 92%

[0179] Exactly of 80 mg of KOH treated Nylon-6 powder, and 20 mg of PECO (polyethylene co-octene) and 4.5 mg of [Al(OCH(CH3)2)3]4(4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. e-caprolactam was obtained in 92% yield.

[0180] Table 3, entry 54.5 wt.%[Al(OCH(CH3)2)3]4240 °C, 10-3TorrNylon-6 3.0 h e-Caprolactam(KOH treated) 92%

[0181] Exactly of 80 mg of KOH treated Nylon-6 powder, and 20 mg of PE (polyethylene) and 4.5 mg of [Al(OCH(CH3)2)3]4(4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. ε-caprolactam was obtained in 92% yield.

[0182] Table 3, entry 64.5 wt.%[Al(OCH(CH3)2)3]4240 °C, 10-3TorrNylon-6 3.0 h e-Caprolactam(KOH treated) 80%

[0183] Exactly of 50 mg of KOH treated Nylon-6 powder, and 50 mg of PC (polycarbonate) and 4.5 mg of [Al(OCH(CH3)2)3]4(4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. 8-caprolactam was obtained in 80% yield.

[0184] Table 3, entry 7Atty. Dkt. No. 00100-0411-PCT4.5 wt.%[Al(OCH(CH3)2)3]4+ PC240 °C, 10-3TorrNylon-6 1.0 h s-Caprolactam(KOH treated) 79%

[0185] Exactly of 80 mg of KOH treated Nylon-6 powder, and 20 mg of PC (polycarbonate) and 4.5 mg of [A1(OCH(CH3)2)3]4 (4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 1 h. e-caprolactam was obtained in 79% yield.

[0186] Table 3, entry 88.0 wt.%[AI(OCH(CH3)2)3]4PET240 °C, 10’3TorrNylon-6 3.0 h e-Caprolactam(KOH treated) 70%

[0187] Exactly of 100 mg of KOH treated Nylon-6 powder, and 20 mg of PET (polyethylene terephthalate) and 6.6 mg of [Al(OCH(CH3)2)3]4(8.0 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. e-caprolactam was obtained in 70% yield.

[0188] Table 3, entry 94.5 wt.%[Al(OCH(CH3)2)3]4+ PEG-8000 PEG-8000240 °C, 10-3TorrNylon-6 2.5 s-Caprolactam(KOH treated) 91%

[0189] Exactly of 88 mg of KOH treated Nylon-6 powder, and 12 mg of PEG-8000 (polyethylene glycol) and 4.5 mg of [A1(OCH(CH3)2)3]4 (4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 2.30 h. e-caprolactam was obtained in 91% yield.

[0190] Table 3, entry 10Atty. Dkt. No. 00100-0411-PCT4.5 wt.%[Al(OCH(CH3)2)3]4+ Spandex + Spandex 240 °C, 10-3TorrNylon-6 3.0 h 6-Caprolactam(KOH treated) 95%

[0191] Exactly of 88 mg of KOH treated Nylon-6 powder, and 12 mg of spandex and 4.5 mg of [A1(OCH(CH3)2)3]4 (4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. e-caprolactam was obtained in 95% yield.

[0192] Table 3, entry 114.5 wt.%[Al(OCH(CH3)2)3]4+ PTFE + PTFE240 °C, 10-3TorrNylon-6 2.0 h s-Caprolactam(KOH treated) 88%

[0193] Exactly of 80 mg of KOH treated Nylon-6 powder, and 20 mg of PTFE strips, taken from a Teflon tape and 4.5 mg of [Al(OCH(CH3)2)3]4(4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 2 h. e- caprolactam was obtained in 88% yield.

[0194] Table 3, entry 124.5 wt.%[AI(OCH(CH3)2)3]4Kevlar + Kevlar 240 °C, 10’3TorrNylon-6 3.0 h e-Caprolactam(KOH treated) 60%

[0195] Exactly of 90 mg of KOH treated Nylon-6 powder, and 10 mg of Kevlar and 4.5 mg of [A1(OCH(CH3)2)3]4 (4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. e-caprolactam was obtained in 60% yield.

[0196] Table 3, entry 13Atty. Dkt. No. 00100-0411-PCT4.5 wt.%[Al(OCH(CH3)2)3]4Cotton Cotton 240 °C, 10-3TorrNylon-6 3.0 h s-Caprolactam(KOH treated) 62%

[0197] Exactly of 90 mg of KOH treated Nylon-6 powder, and 10 mg of Cotton (from Walmart) and 4.5 mg of [Al(OCH(CH3)2)3]4(4.5 wt.%) were reacted according to the general procedure, heating the reaction mixture to 240 °C for 3 h. e-caprolactam was obtained in 62% yield.

[0198] Table 3. Depolymerization of KOH treated Nylon-6 in the presence of other polymers as shown in Equation 3.aH oCatalyst JIz+ Poymer - 0Nylon-6 (KOH treai ed) e-CaprolactamPolymer: Nylon-66, Nyton-12, i-pp, PECO, PE, PC, PTFE, PET, Spandex, PEG, Kevlar, Cotton) Entry Catalyst Plastics Catalyst Time T (.°C) Yield loading (h) (%)b(wf / o)151 [A1(OCH(CH3)2)3]4Nylon-6: Nylon-66c>d4.5 3 240 89.22 [A1(OCH(CH3)2)3]4Nylon-6: Nylon-12e4.5 3 240 873 [Al(OCH(CH3)2)3]4Nylon-6: i-PPf4.5 3 240 904 [A1(OCH(CH3)2)3]4Nylon-6: PECOf4.5 3 240 925 [Al(OCH(CH3)2)3]4Nylon-6: PEf4.5 3 240 926 [A1(OCH(CH3)2)3]4Nylon-6: PCe4.5 3 240 807 [A1(OCH(CH3)2)3]4Nylon-6: PCf45 1 240 798 [Al(OCH(CH3)2)3]4Nylon-6: PETh,i8.0 3 240 709 [A1(OCH(CH3)2)3]4Nylon-6: PEG-800014.5 2.5 240 9110 [A1(OCH(CH3)2)3]4Nylon-6: Spandexj4.5 3 240 9511 [A1(OCH(CH3)2)3]4Nylon-6: PTFEf4.5 2 240 8812 [Al(OCH(CH3)2)3]4Nylon-6: Kevlarg4.5 3 240 60Atty. Dkt. No. 00100-0411-PCT13 [A1(OCH(CH3)2)3]4Nylon-6: Cotton84.5 3 240 62aGeneral depolymerization conditions: static vacuum (10‘3Torr) and 100 mg of mixed plastic (Nylon-6 (KOH pretreated) and polymers) in 100 mL Schlenk flask, unless otherwise stated.bYields determined by 'H NMR with a mesitylene internal standard.cA mixture of Nylon-6 and Nylon-66 in a 9: 11 (45 mg:55 mg) weight ratio was used.dNylon-66 was washed with KOH.eA plastic mixture of Nylon-6 and other polymers in a 1: 1 (50 mg: 50 mg) weight ratio was used.fA plastic mixture of Nylon-6 and other polymers in a 4:1 (80 mg:20 mg) weight ratio was used.8A plastic mixture of Nylon-6 and other polymers in a 9: 1 (90 mg: 10 mg) weight ratio was used.hNylon-6 and PET in a 21:4 weight ratio was used.i100 mg of Nylon-6 was mixed with 20 mg of PET. J A plastic mixture of Nylon-6 and other polymers in a 22:3 (88 mg: 12 mg) weight ratio was used.kCatalyst loading was reported with respect to the total weight of the polymer mixture.

[0199] As noted above, subsequent experiments were performed to evaluate whether the developed catalytic systems could selectively depolymerize Nylon-6 in the presence of other polymers, including polyamide 66 (Nylon 66), polyamide 12 (Nylon 12), isotactic polypropylene (i-PP), polyethylene-co-1 -octene (PECO), polyethylene (PE), polycarbonate (PC), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), polyurethanepolyurea copolymer (Spandex), polyethylene glycol (PEG), poly -paraphenylene terephthalamide (Kevlar), and cotton, as illustrated in Equation 3.

[0200] Remarkably, even in mixtures containing other polyamides such as Nylon 66 or Nylon 12, the catalyst maintained high activity and selectivity for converting Nylon-6 to caprolactam, despite the presence of additional amide functionalities. For instance, a 9: 11 (w / w) blend of Nylon-6 and Nylon-66 treated with 2.53 mol% of the aluminum isopropoxide produced caprolactam in 89% yield (Table 3, entry 1). Similarly, a 1: 1 mixture of Nylon-6 and Nylon-12 afforded a 87% yield of caprolactam (Table 3, entry 2). In reactions in which Nylon-6 was fully converted to caprolactam, the accompanying polyamide remained intact, demonstrating a solvent-free and efficient method for the chemical separation of mixed polyamides.

[0201] When Nylon-6 was blended with polyolefins such as isotactic polypropylene (i-PP), polyethylene (PE), or polyethylene-co-1 -octene (PECO), caprolactam was recovered inAtty. Dkt. No. 00100-0411-PCT90-92% yield (Table 3, entries 3-5), while the polyolefin components remained unreacted at the bottom of the reactor.

[0202] Encouragingly, the system also showed excellent performance with more challenging plastics such as polycarbonate (PC). In mixtures of Nylon-6 and PC (1: 1 or 4: 1 w / w), high yields of caprolactam up to 80% were achieved (Table 3, entries 6-7). However, in the 1:1 mixture heated for 3 h, a minor unidentified impurity appeared in the aromatic region of the NMR spectrum (Table 3, entry 6). In contrast, when the reaction was performed using a 4: 1 ratio of Nylon-6 to PC and a shorter reaction time of 1 h, pure caprolactam was obtained (= 79%) without any detectable by-products (Table 3, entry 7).

[0203] The catalytic system also demonstrated remarkable compatibility with oxygenrich polymers such as polyesters and polyethers. When polyethylene terephthalate (PET) and polyethylene glycol-8000 (PEG-8000) were employed as co-components, caprolactam was obtained in 70, 91% yield, respectively (Table 2, entries 8-9). Notably, the depolymerization of Nylon-6 proceeded efficiently even in the presence of polyurethane-based materials such as Spandex-commonly found in Nylon-6 blends-affording 95% yield of caprolactam (Table 2, entry 10).

[0204] Finally, the method proved effective with post-consumer, high-melting point polymers including polytetrafluoroethylene (PTFE, Teflon tape), Kevlar fibers (sourced from protective gloves), and cotton. In the case of PTFE, an 88% yield of caprolactam was achieved (Table 3, entry 11). However, when Kevlar and cotton were used, the yields were comparatively lower (up to 62%), likely due to mass-transfer limitations and restricted mixing during the reaction (Table 3, entries 12-13).

[0205] To evaluate the reusability of the [Al(OCH(CH3)2)3]4catalyst, continuous depolymerization was carried out in a 100 mL reactor using a single charge of 9.0 mg of [Al(OCH(CH3)2)3]4catalyst (2.5 mol%) with 200 mg of KOH treated Nylon-6. After each run, the caprolactam produced was removed and weighed, and a fresh 200 mg of Nylon-6 was added to the reactor containing the original catalyst load, no additional catalyst was added between runs. This sequence was repeated over two consecutive batches. Caprolactam production indicating the catalyst retained activity and stability for at least 3 runs; the cumulative monomer yield was 69%. Results are summarized in Table 4.

[0206] Table 4. Catalyst Recyclability Evaluation in Multi-Batch ContinuousDepolymerization.Atty. Dkt. No. 00100-0411-PCT1stRun 2ndRun 3rdRun Totalyield (%)aNylon-6 +200 mg +200 mg +200 mg [Al(OCH(CH3)2)3]4+9.0 mg 0 0Reaction time 25 min 90 min 17. O hMonomer recovery 137 mg 146 mg 104 mg 69%mass (mg)aThe total yield was determined by combining the isolated yield with the NMR-derived yield of the residual product remaining in the reaction flask after the monomer was isolated in the 3rdrun. NMR yield was determined by1H NMR analysis using mesitylene as an internal standard.

[0207] Conclusions

[0208] In conclusion, this Example demonstrated a pioneering approach to Nylon-6 depolymerization using non-toxic, earth-abundant, and cost-effective commercially available boron group catalysts. The method enabled rapid, quantitative conversion of Nylon-6 into e-caprolactam with remarkable efficiency — achieving full depolymerization in minutes under mild conditions and with low catalyst loadings. Notably, the process's compatibility with mixed plastic waste, such as Nylon-6 / Nylon-66, Nylon-6 / Nylon-12, Nylon-6 / i-pp, Nylon-6 / PECO, Nylon-6 / PE, Nylon-6 / PC, Nylon-6 / PTEF, Nylon-6 / PET, Nylon-6 / Spandex, Nylon-6 / PEG, Nylon-6 / Kevlar, and Nylon-6 / Cotton blends, and its ability to yield high-purity caprolactam make it an attractive solution for sustainable recycling.

[0209] For the first time, this Example further revealed a cooperative, synergistic effect between boron group elements, such as aluminum, and alkali metal species that enables efficient solvent-free depolymerization of Nylon-6 to e-caprolactam. Although each component may be inactive on their own, the combination can produce an active catalytic system for Nylon-6 depolymerization. This strategy provides a sustainable route for Nylon-6 depolymerization under mild, solvent-free conditions. Interestingly, catalysts such as LiAlH4or Na[CH3O(CH2)2O]2[(CH2)5CON]2Al that have both aluminum and alkali metals in the catalyst itself are catalytically active, further highlighting the synergy between the two types of metal species.

[0210] The present approach enables a circular economy for Nylon-6, where caprolactam monomers can be seamlessly reintroduced into industrial polymerization processes. The insights and outcomes of this Example demonstrate the transformative potential of earth-Atty. Dkt. No. 00100-0411-PCTabundant catalysts in advancing green chemical recycling practices and establishing scalable, eco-friendly pathways for polymer waste management.

[0211] The word "illustrative" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, "a" or "an" means "one or more.”

[0212] If not already included, all numeric values of parameters in the present disclosure are proceeded by the term ’‘about” which means approximately. This encompasses those variations inherent to the measurement of the relevant parameter as understood by those of ordinary skill in the art. This also encompasses the exact value of the disclosed numeric value and values that round to the disclosed numeric value.

[0213] The foregoing description of illustrative embodiments of the disclosure has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosure. The embodiments were chosen and described in order to explain the principles of the disclosure and as practical applications of the disclosure to enable one skilled in the art to utilize the disclosure in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the claims appended hereto and their equivalents.

[0214] In recognition of the inherent nature of chemical synthesis and chemical processes, throughout the present disclosure, terms and phrases such as “absence,” “free,” “does not comprise,” etc. encompass, but do not require a perfect absence of the referenced entity.

[0215] The term “type” as used herein refers to chemical formula such that a single type means the same chemical formula and different type means different chemical formula. The terms “same” and “different” in reference to elements or groups as used herein also refers to chemical formula such that same means the same chemical formula and different means different chemical formula.

[0216] Terms such as “comprising” and the like may be replaced with terms such as “consisting” and the like.

Claims

Atty. Dkt. No. 00100-0411-PCTWHAT IS CLAIMED IS:

1. A method for depolymerizing a polyamide, the method comprising combining a polyamide and a boron group catalyst to depolymerize the polyamide to a product, wherein the boron group catalyst comprises a boron group element and ligands bound to the boron group element, wherein the ligands are independently selected from an oxygen-containing ligand other than an oxygen atom, other than a ketone, and other than a sulfonate; hydrogen (H); a carbon-containing ligand; a nitrogen-containing ligand; an alkali metal; and an alkaline earth metal.

2. The method of claim 1, wherein the boron group catalyst is selected from those having formula (MRx)y(BG)Rz, wherein M is the alkali metal or the alkaline earth metal; BG is the boron group element; each R is independently selected from an oxygen atom (O) of the oxy gen-containing ligand, H, a carbon atom (C) of the carbon-containing ligand, and a nitrogen atom (N) of the nitrogen-containing ligand; x is 0 or 1; y is 0 or 1; and z is 3 or 4; and further wherein, if M is the alkali metal, then x is 0, y is 1, and z is 4; if M is the alkaline earth metal, then x is 1, y is 1, and z is 4; and if y is 0, then z is 3.

3. The method of claim 2. wherein the boron group element is Al or B.

4. The method of claim 2, wherein the oxygen-containing ligand is an alkoxy; the carbon-containing ligand is an alky l; and the nitrogen-containing ligand is a nitrogen-substituted cycloalkyl, an amido, or an amidate.

5. The method of claim 2, wherein M is selected from Li, Na, K, and Mg.

6. The method of claim 2. wherein the formula is (BG)Rs.

7. The method of claim 6, wherein BG is Al or B.

8. The method of claim 7. wherein each R is an alkoxy, a nitrogen-substituted cycloalkyl, or an amido.

9. The method of claim 8, wherein the alkoxy is isopropoxy, the nitrogen-substituted cycloalkyl is caprolactamato, and the amido is -N(CH3)2.Atty. Dkt. No. 00100-0411-PCT10. The method of claim 2, wherein the formula is M(BG)R4, wherein M is the alkali metal.

11. The method of claim 10, wherein M is selected from Na, Li, and K.

12. The method of claim 11, wherein each R is independently selected from a nitrogen-substituted cycloalkyl and a substituted alkoxy.

13. The method of claim 12, wherein the nitrogen-substituted cycloalkyl is caprolactamato and the substituted alkoxy is 2-methoxyethoxy.

14. The method of claim 13, wherein two R groups are the caprolactamato and two R groups are the 2-methoxyethoxy.

15. The method of claim 11, wherein each R is H.

16. The method of claim 1, wherein the polyamide is poly(hexano-6-lactam) (Nylon-6), poly(2-pyrrolidinone) (Nylon-4), or a combination thereof.

17. The method of claim 1, wherein the product comprises a monomer from which the polyamide was formed.

18. The method of claim 17, wherein the monomer is a cyclic amide.

19. The method of claim 1, wherein the polyamide and the boron group catalyst are combined in the presence of an alkali metal species.

20. The method of claim 19, wherein the alkali metal species is an alkali metal present in the boron group catalyst; the alkali metal species is present due to pretreating the polyamide with the alkali metal species prior to combining the polyamide and the boron group catalyst; the alkali metal species is an additive combined with the polyamide and the boron group catalyst; or combinations thereof.

21. The method of claim 1. wherein the polyamide is Nylon-6; the boron group element is Al or B; and the product comprises s-caprolactam.Atty. Dkt. No. 00100-0411-PCT22. The method of claim 21, wherein the polyamide and the boron group catalyst are combined in the presence of an alkali metal species.

23. The method of claim 1, wherein the polyamide is Nylon-6 and the product comprises e-caprolactam, and further wherein the Nylon-6 is provided as a blend with a different type of polymer and the different type of polymer remains intact after the method.

24. The method of claim 1, wherein the polyamide is Nylon-6 and the product comprises e-caprolactam, and the method further comprises adding additional Nylon-6 an additional time or continuously without adding additional boron group catalyst to produce additional e-caprolactam.

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