Skip to main content
NCBI home page
ACS Organic & Inorganic Au logoLink to ACS Organic & Inorganic Au
. 2023 Aug 28;3(6):377–383. doi: 10.1021/acsorginorgau.3c00037

Chemical Upcycling of PET into a Morpholine Amide as a Versatile Synthetic Building Block

Yohei Ogiwara 1,*, Kotohiro Nomura 1
PMCID: PMC10704573  PMID: 38075447

Abstract

graphic file with name gg3c00037_0011.jpg

A catalytic chemical upcycling methodology for polyesters has been developed. Commodity polyesters, such as polyethylene terephthalate (PET), are depolymerized with morpholine by using a Cp*TiCl3 catalyst under ambient pressure without any additives, which provides morpholine amides exclusively. The method can also apply to other polyesters, polybutylene terephthalate (PBT), polyethylene adipate (PEA), polybutylene adipate (PBA), and polybutylene succinate (PBS), as well as an actual PET waste of a 50 g postconsumer beverage bottle. The product, morpholine amide, is a versatile building block in organic chemistry, and the synthetic utility has thus been demonstrated by further transformations, such as hydrolysis, selective reductive conversions, and Grignard reaction.

Keywords: chemical upcycling, polyesters, depolymerization, titanium catalysis, aminolysis, morpholine amides

Plastics are indispensable materials for our daily life, and huge amounts of plastics are, thus, produced and consumed worldwide every day. Meanwhile, the energy problem of making plastics from fossil feedstocks and environmental concerns caused by plastic waste are widely recognized as critical and urgent issues that need to be solved on a global scale for the establishment of the circular economy.1−3

Chemical conversion of a polymer into small molecules, depolymerization, is one of the most essential and sustainable solutions to these plastic problems because the plastic wastes can be considered as organic synthetic building blocks. The depolymerization of polymers into starting virgin feedstocks or their derivatives is well known as chemical recycling. The depolymerization products can be used as the monomers for new polymer synthesis again, or repolymerization, so this cycle is called “closed-loop” (Scheme 1 A).2 Another depolymerization concept is chemical upcycling, wherein the polymer is depolymerized along with certain chemical functionalizations to form value-added compounds. Although fine chemicals, rather than simply raw materials, can be obtained directly from plastic waste, this “open-loop” system is challenging and still leaves room for exploration (Scheme 1B).

Scheme 1. Chemical Depolymerization of Polymer.

Scheme 1

Open in a new tab

Polyesters are one of the most popular and consumed plastics, and PET, especially, occupies the greatest market share in polyesters because of its high thermal and mechanical stability. Therefore, PET is an optimal benchmark polyester for demonstrating the availability of novel depolymerization methodology.3

Hydrolysis is the typical depolymerization route of PET. The C(=O)–O bonds in ester repeating units could be cleaved by water, thereby forming the smaller carboxylic acids—oligomerization—and repeating the fragmentation to finally provide the corresponding monomer, terephthalic acid (TPA).4 Transesterification by alcohols, alcoholysis, is another common method for PET depolymerization. Methanol and ethylene glycol (EG), especially, have been used as depolymerization reagents/solvents to give dimethyl terephthalate (DMT) and bis(hydroxyethyl) terephthalate (BHET) through methanolysis and glycolysis, respectively.5−8 Various chemical recycling reactions of PET have been developed via hydrolysis or alcoholysis by homogeneous catalysis; however, these conventional methods are achieved under high-energy conditions (high temperature, high pressure), and/or stoichiometric amounts of additives have been required because of the stability of PET. Furthermore, the products (TPA, DMT, and BHET) are mostly limited to use as monomers for repolymerization (Scheme 2A). In contrast, we recently developed the acid- and base-free depolymerization of polyesters through transesterification with various alcohols in the presence of homogeneous catalysts.8a,8b,8d

Scheme 2. Depolymerization of PET by Homogeneous Catalysis.

Scheme 2

Open in a new tab

We herein communicate a morpholine-based upcycling of PET via titanium-catalyzed aminolysis, which exclusively provides the morpholine amide as a versatile building block for further reactions. Since this conversion proceeded under ambient pressure and weak basic conditions (pH 8–9) without any additives, a large-scale reaction was easily applicable without any special reactors. The product, morpholine amide, was an air-stable solid, easily isolated by simple operations, and also could be used for various chemical transformations into further value-added fine chemicals (Scheme 2B). Aminolysis for a PET depolymerization strategy has been investigated by using ammonia,9b ethanol amine,9c,9d,9f and several primary amines.9a,9e,9g However, catalytic depolymerization by morpholine has not been achieved to date.

On the basis of our previous results of the depolymerization of polyesters via transesterification,8b,8c two half-titanocenes, cyclopentadienyltitanium(IV) trichloride (CpTiCl3) and (pentamethylcyclopentadienyl)titanium(IV) trichloride (Cp*TiCl3), were chosen for morpholine-based depolymerization of PET into morpholine amide 1 (Table 1).10 The reaction was conducted by simply heating a mixture of commercially available PET pellets (IV = 0.80 ± 0.02 dL/g),11 morpholine, and titanium catalyst at 130 °C in a screw cap tube; no particular reaction vessels, such as a pressure tube or autoclave, are required at that temperature because of the boiling point of morpholine (129 °C). The reaction was conducted in the presence of CpTiCl3 (10 mol %), and the expected morpholine amide 1 was generated in 81% yield, along with the formation of ethylene glycol (EG) after 24 h (Table 1, entry 1). However, trace amounts of unknown carbonyl signals were also detected in the 13C{1H} NMR spectrum, which are probably partially depolymerization products—oligomers.12 The efficiency was improved when Cp*TiCl3 was employed, which afforded 90% of 1 after 24 h, even with 5 mol % of catalyst (entry 2). The increased efficiency of Cp*TiCl3 could be explained by the thermal stability of the catalyst due to the stronger electron-donating and higher steric properties of the Cp* ligand than those of Cp.13 A longer reaction time (48 h) showed the quantitative conversion of PET to 1 (entry 3). As in the 13C{1H} NMR spectrum of the crude mixture (after removing volatiles, Figure 1), the depolymerization products (1 and EG), the remaining morpholine, and the internal standard could be assigned without any other remarkable signals.

Table 1. Titanium-Catalyzed Depolymerization of PET with Morpholinea.

graphic file with name gg3c00037_0009.jpg

entry catalyst time (h) 1 (%)b
1 CpTiCl3 (10 mol %) 24 81
2 Cp*TiCl3 (5 mol %) 24 90
3 Cp*TiCl3 (5 mol %) 48 98
Open in a new tab
a

Reaction conditions: PET (192.2 mg, 1.0 mmol, repeating unit), catalyst CpTiCl3 (21.9 mg, 0.10 mmol) or Cp*TiCl3 (14.5 mg, 0.05 mmol), and morpholine (1.0 mL, 11.5 mmol).

b

1H NMR yield based on 1,3,5-trimethoxybenzene as an internal standard.

Figure 1.

Figure 1

Open in a new tab

13C{1H} NMR spectrum of the crude mixture (Table 1, entry 3, CDCl3, rt); 1, morpholine amide; mor, morpholine; EG, ethylene glycol; and IS, internal standard (1,3,5-trimethoxybenzene).

It was revealed that the reaction with 0.5 mol % of Cp*TiCl3 enabled sufficient completion of the depolymerization of polyesters under scale-up conditions (10.0 mmol based on repeating units), as summarized in Table 2. The depolymerization methodology by Cp*TiCl3/morpholine system was then applied to polyesters: polybutylene terephthalate (PBT pellet, Mn = 32 000 g/mol), polyethylene adipate (PEA flake, MW = 10 000 g/mol), polybutylene adipate (PBA flake, MW = 12 000 g/mol), and polybutylene succinate (PBS pellet, Mn = 28 000 g/mol) (Figure 2A). Hence, the starting polyesters were completely converted into the corresponding morpholine amides in the presence of a small amount of catalyst, and the resulting compounds other than the product were the highly hydrophilic EG and both hydrophilic and volatile morpholine (see Figure 1). Therefore, only a simple isolation procedure—reduced pressure then liquid–liquid extraction by CH2Cl2/H2O—provided the morpholine amide in high yield in pure form.

Table 2. Depolymerization of Polyesters with Morpholinea.

graphic file with name gg3c00037_0010.jpg

entry polyester conditions conversionb product, yieldc
1 PET 130 °C, 2 d >99% 1, 87%
2 PBT 150 °C, 7 d >99% 1, 99%
3 PEA 130 °C, 2 d >99% 2, 84%
4 PBA 150 °C, 7 d >99% 2, 97%
5 PBS 150 °C, 7 d >99% 3, 83%
Open in a new tab
a

Reaction conditions: polyester (10.0 mmol, repeating unit), Cp*TiCl3 (14.5 mg, 0.05 mmol), and morpholine (10.0 mL, 114.8 mmol).

b

Estimated by 13C{1H} NMR spectra.

c

Isolated yield.

Figure 2.

Figure 2

Open in a new tab

(A) Structures of commodity polyesters. (B) Structures of morpholine amides 1–3 (left) and their ORTEP drawings (50% probability ellipsoids) with hydrogen atoms omitted for clarity (right).

For example, after a gram-scale reaction of PET, morpholine, and Cp*TiCl3 catalyst for 48 h, the morpholine amide 1 was isolated in an 87% yield only by evaporation and then extraction with CH2Cl2 (Table 2, entry 1).14 The reactivity of PBT toward this conversion was slightly lower than that of PET, and then the reaction was not completed under the conditions (130 °C, 2 d). However, when both the reaction temperature and time were increased to 150 °C and 7 d, respectively, the depolymerization of PBT proceeded exclusively, and the corresponding product 1 was isolated in 99% yield (entry 2). Morpholine amides 2 and 3, originating from aliphatic dicarboxylic acids, were also available by adopting this method. In the reactions of PEA or PBA, morpholine amide 2 derived from adipic acid was generated in excellent yield, although the reactivities of these two polyesters were different, and rather harsh conditions were required in the reaction of PBA (entries 3 and 4). The reaction of PBS was also conducted at 150 °C for 7 days, which afforded the corresponding amide of succinic acid 3 in 83% isolated yield (entry 5). A series of these morpholine amides, 1–3, were stable solids in air, and their structures were also confirmed by single crystal X-ray diffraction analysis (Figure 2B).15

To demonstrate the availability of this developed method, the large-scale depolymerization of actual PET bottle waste was then performed (Scheme 3). We emphasize again that our depolymerization can be conducted under ambient pressure because of the boiling point of morpholine and the reaction temperature (129 °C vs 130 °C); thus, it is easily scalable. PET sheets (50.0 g, 260.0 mmol, the molecular weight unknown) were prepared by cutting beverage bottles (Scheme 3 A) and were used as the substrate for the reaction with 250 mL of morpholine and 40.0 mg of Cp*TiCl3 (0.14 mmol, 0.05 mol %) in a standard three-necked round-bottom flask (500 mL) with a reflux condenser. After heating at 130 °C for 72 h, the starting PET was fully consumed, and a homogeneous solution was obtained (Scheme 3 B), which was cooled to room temperature and evaporated under reduced pressure. Then, unreacted morpholine (184 mL, ∼90% v/v) was recovered as the volatile, which could be reused for the reaction again. Conversely, MeOH was added to the residues (Scheme 3 C), which were then filtered, and the resulting precipitate was 63.7 g (209.2 mmol, 80%) of the morpholine amide 1 in pure form, which was available for elemental analysis (Scheme 3 D). Product 1 still remained in the resulting filtrate with EG and titanium species, which were separated again via reduced pressure and liquid–liquid extraction by CH2Cl2/H2O to give 7.2 g (23.7 mmol, 9%) of 1. Although the homogeneous titanium catalyst could not be recovered, with only 0.05 mol % catalyst and a series of simple isolation procedures, a total of 70.9 g (90% yield, TON > 1600) of the morpholine amide 1 could be obtained from the PET bottle waste. Further recrystallization by MeOH finally gave 60.1 g (76% yield) of crystals of 1. The starting PET for beverage bottles may also contain 1,3-cross-linked structures derived from isophthalic acid and not only the 1,4-terephthalate major component. In this case, however, the corresponding 1,3-substituted morpholine amide 1′ was not detected by the 13C{1H} NMR spectrum, and the terephthalate-based 1,4-product 1 was solely obtained.16

Scheme 3. Conversion of Actual PET Bottle Waste and Simple Isolation of Morpholine Amide 1.

Scheme 3

Open in a new tab

Reaction conditions: PET (50.0 g, 260.0 mmol, repeating unit), Cp*TiCl3 (40.0 mg, 0.14 mmol), and morpholine (250 mL, 2.9 mol) at 130 °C for 72 h. (A) Starting PET sheets of beverage bottles. (B) The reaction mixture after 72 h. (C) The residues of reduced pressure. (D) The precipitate of filtration, morpholine amide 1.

Morpholine amides have been used as key intermediates/precursors for various organic reactions.17−19 Depolymerization products 1–3 from polyesters, therefore, could become versatile building blocks in organic synthesis, which means that this depolymerization methodology is “open-loop chemical upcycling.” We then investigated the reactivities of compound 1 to demonstrate its potential utility in organic chemistry.

The conversion of polyesters into dicarboxylic acids is considered the most basic depolymerization manner. Although various homogeneous catalyses have been developed, such as hydrolysis by H2O3,4 and hydrogenolysis by H2,20 the efficient and practical conversion of PET into TPA including the product isolation has been still rare because the standard isolation methods, e.g., column chromatography, liquid extraction, and recrystallization, do not apply to TPA because of its poor solubility to common solvents. In contrast to TPA, morpholine amide 1 slightly shows a water solubility. We hypothesized that the unique hydrophilic property of 1 could apply to a practical synthetic method of TPA. It was found that 1 was completely soluble in water at 100 °C, and then HCl was added to the solution. After stirring the mixture at that temperature for 24 h, the precipitation of pure TPA (4) solid was observed, which was then isolated in a 98% yield by just filtration (Scheme 4). This result of the production of TPA monomer indicates that the conversion of 1 also includes chemical recycling and not only chemical upcycling, as described below.

Scheme 4. Hydrolysis of 1 Leading to Terephthalic Acid (4).

Scheme 4

Open in a new tab

Reaction conditions: 1 (3.0 mmol), HCl (1 mL, ∼11.7 mmol), and H2O (15.0 mL) at 100 °C for 24 h.

Selective reductive transformations of 1 were next performed (Scheme 5).19 When the morpholine amide 1 was treated with 4 equiv of DIBAL-H (diisobutylaluminum hydride, 1 M in hexane) in CH2Cl2 at −78 °C for 1 h, both of the morpholyl parts of 1 were predominately substituted by hydride to give the terephthalaldehyde (5) (conditions A).21 By conducting the reaction with DIBAL-H (5 equiv) at 0 °C, however, the deoxygenative reduction occurred to give benzylic amine 6 in high yield/selectivity (conditions B).

Scheme 5. Reactions with DIBAL-H for Selective Conversions of 1.

Scheme 5

Open in a new tab

Reaction conditions A: 1 (3.0 mmol), DIBAL-H (1.0 M in hexane, 12.0 mL, 12.0 mmol), and CH2Cl2 (12.0 mL) at −78 °C for 1 h, then −78 °C to rt. Conditions B: 1 (3.0 mmol), DIBAL-H (1.0 M in hexane, 15.0 mL, 15.0 mmol), and toluene (6.0 mL) at 0 °C for 4 h.

Morpholine amides are also available for ketone synthesis via reactions with organometallic reagents, such as Grignard and organolithium reagents.18 A reaction of 1 with Grignard reagent (PhMgBr, 1 M in THF) selectively provided the corresponding ketone, 1,4-dibenzoylbenzene (7), in 65% isolated yield, which prevented the overaddition reaction (formation of alcohols) (Scheme 6).15,22

Scheme 6. Grignard Reaction of 1 for Ketone Synthesis.

Scheme 6

Open in a new tab

Reaction conditions: 1 (3.0 mmol), PhMgBr (1.0 M in THF, 12 mL, 12 mmol), and THF (9 mL) at rt for 72 h.

In summary, we demonstrated a novel and practical depolymerization of PET and several polyesters into the corresponding morpholine amides through Cp*TiCl3-catalyzed aminolysis by morpholine. The advantageous features of this method are as follows: (i) lower catalyst loading (minimum 0.05 mol %), (ii) inexpensive and recyclable solvent, (iii) ambient pressure (almost 1 atm), (iv) no additives, (v) simple isolation protocol (just filtration), (vi) high yield and high selectivity (up to 99%), and (vii) scalable (50 g scale). PET bottle waste is also applicable to this depolymerization and yields >70 g of the morpholine amide as a pure and stable solid. Several “morpholine amide-specific” transformations are also possible, such as hydrolysis, selective reductions, and the Grignard reaction, which form important chemicals. Overall, our present methodology is a new and practical concept of PET upcycling and, thus, could contribute to solving various global environmental and energy problems in the future.

Acknowledgments

This project was partly supported by JST-CREST (Grant Number JPMJCR21L5). The authors are grateful to Dr. Daisuke Shimoyama (Tokyo Metropolitan University) for the X-ray crystallographic analysis.

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsorginorgau.3c00037.

  • Experimental procedures, characterization data, 1H and 13C{1H} NMR spectra for all products, and crystal structures of 1–3, 6, and 7 (PDF)

The authors declare no competing financial interest.

Supplementary Material

gg3c00037_si_001.pdf (6.7MB, pdf)

References

  1. a Zhang X.; Fevre M.; Jones G. O.; Waymouth R. M. Catalysis as an Enabling Science for Sustainable Polymers. Chem. Rev. 2018, 118, 839–885. 10.1021/acs.chemrev.7b00329. [DOI] [PubMed] [Google Scholar]; b Basterretxea A.; Jehanno C.; Mecerreyes D.; Sardon H. Dual Organocatalysts Based on Ionic Mixtures of Acids and Bases: A Step Toward High Temperature Polymerizations. ACS Macro Lett. 2019, 8, 1055–1062. 10.1021/acsmacrolett.9b00481. [DOI] [PubMed] [Google Scholar]
  2. a Coates G. W.; Getzler Y. D. Y. L. Chemical recycling to monomer for an ideal, circular polymer economy. Nat. Rev. Mater. 2020, 5, 501–516. 10.1038/s41578-020-0190-4. [DOI] [Google Scholar]; b Worch J. C.; Dove A. P. 100th Anniversary of Macromolecular Science Viewpoint: Toward Catalytic Chemical Recycling of Waste (and Future) Plastics. ACS Macro Lett. 2020, 9, 1494–1506. 10.1021/acsmacrolett.0c00582. [DOI] [PubMed] [Google Scholar]; c Payne J.; Jones M. D. The Chemical Recycling of Polyesters for a Circular Plastics Economy: Challenges and Emerging Opportunities. ChemSusChem 2021, 14, 4041–4070. 10.1002/cssc.202100400. [DOI] [PMC free article] [PubMed] [Google Scholar]; d Thiyagarajan S.; Maaskant-Reilink E.; Ewing T. A.; Julsing M. K.; van Haveren J. Back-to-monomer recycling of polycondensation polymers: opportunities for chemicals and enzymes. RSC Adv. 2021, 12, 947–970. 10.1039/D1RA08217E. [DOI] [PMC free article] [PubMed] [Google Scholar]; e Lee K.; Jing Y.; Wang Y.; Yan N. A unified view on catalytic conversion of biomass and waste plastics. Nat. Rev. Chem. 2022, 6, 635–652. 10.1038/s41570-022-00411-8. [DOI] [PMC free article] [PubMed] [Google Scholar]; f Chu M.; Liu Y.; Lou X.; Zhang Q.; Chen J. Rational Design of Chemical Catalysis for Plastic Recycling. ACS Catal. 2022, 12, 4659–4679. 10.1021/acscatal.2c01286. [DOI] [Google Scholar]
  3. a Paszun D.; Spychaj T. Chemical Recycling of Poly(ethylene terephthalate). Ind. Eng. Chem. Res. 1997, 36, 1373–1383. 10.1021/ie960563c. [DOI] [Google Scholar]; b George N.; Kurian T. Recent Developments in the Chemical Recycling of Postconsumer Poly(ethylene terephthalate) Waste. Ind. Eng. Chem. Res. 2014, 53, 14185–14198. 10.1021/ie501995m. [DOI] [Google Scholar]; c Al-Sabagh A. M.; Yehia F. Z.; Eshaq G.; Rabie A. M.; ElMetwally A. E. Greener routes for recycling of polyethylene terephthalate. Egypt. J. Pet. 2016, 25, 53–64. 10.1016/j.ejpe.2015.03.001. [DOI] [Google Scholar]; d Westhues S.; Idel J.; Klankermayer J. Molecular catalyst systems as key enablers for tailored polyesters and polycarbonate recycling concepts. Sci. Adv. 2018, 4, eaat9669 10.1126/sciadv.aat9669. [DOI] [PMC free article] [PubMed] [Google Scholar]; e Damayanti; Wu H.-S. Strategic Possibility Routes of Recycled PET. Polymers 2021, 13, 1475–1511. 10.3390/polym13091475. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. For selected examples on hydrolysis of PET, see:; a Yoshioka T.; Sato T.; Okuwaki A. Hydrolysis of Waste PET by Sulfuric-Acid at 150 °C for a Chemical Recycling. J. Appl. Polym. Sci. 1994, 52, 1353–1355. 10.1002/app.1994.070520919. [DOI] [Google Scholar]; b Mishra S.; Zope V. S.; Goje A. S. Kinetic and thermodynamic studies of depolymerisation of poly(ethylene terephthalate) by saponification reaction. Polym. Int. 2002, 51, 1310–1315. 10.1002/pi.873. [DOI] [Google Scholar]; c Liu Y.; Wang M.; Pan Z. Catalytic depolymerization of polyethylene terephthalate in hot compressed water. J. Supercrit. Fluid. 2012, 62, 226–231. 10.1016/j.supflu.2011.11.001. [DOI] [Google Scholar]; d Paliwal N. R.; Mungray A. K. Ultrasound assisted alkaline hydrolysis of poly(ethylene terephthalate) in presence of phase transfer catalyst. Polym. Degrad. Satbil. 2013, 98, 2094–2101. 10.1016/j.polymdegradstab.2013.06.030. [DOI] [Google Scholar]; e Zhou H.; Ren Y.; Li Z.; Xu M.; Wang Y.; Ge R.; Kong Z.; Zheng L.; Duan H. Electrocatalytic upcycling of polyethylene terephthalate to commodity chemicals and H2 fuel. Nat. Commun. 2021, 12, 4679–4687. 10.1038/s41467-021-25048-x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. For selected examples on methanolysis of PET, see:; a Kurokawa H.; Ohshima M.; Sugiyama K.; Miura H. Methanolysis of polyethylene terephthalate (PET) in the presence of aluminium Triisopropoxide catalyst to form dimethyl terephthalate and ethylene glycol. Polym. Degrad. Satbil. 2003, 79, 529–533. 10.1016/S0141-3910(02)00370-1. [DOI] [Google Scholar]; b Du J. T.; Sun Q.; Zeng X. F.; Wang D.; Wang J. X.; Chen J. F. ZnO nanodispersion as pseudohomogeneous catalyst for alcoholysis of polyethylene terephthalate. Chem. Eng. Sci. 2020, 220, 115642–115651. 10.1016/j.ces.2020.115642. [DOI] [Google Scholar]; c McKeown P.; Kamran M.; Davidson M. G.; Jones M. D.; Román-Ramírez L. A.; Wood J. Organocatalysis for versatile polymer degradation. Green Chem. 2020, 22, 3721–3726. 10.1039/D0GC01252A. [DOI] [Google Scholar]; d Pham D. D.; Cho J. Low-energy catalytic methanolysis of poly(ethyleneterephthalate). Green Chem. 2021, 23, 511–525. 10.1039/D0GC03536J. [DOI] [Google Scholar]; e Tanaka S.; Sato J.; Nakajima Y. Capturing ethylene glycol with dimethyl carbonate towards depolymerisation of polyethylene terephthalate at ambient temperature. Green Chem. 2021, 23, 9412–9416. 10.1039/D1GC02298A. [DOI] [Google Scholar]
  6. For selected examples on glycolysis of PET, see:; a Chen J.-W.; Chen L.-W. The Glycolysis of Poly(ethylene terephthalate). J. Appl. Polym. Sci. 1999, 73, 35–40. . [DOI] [Google Scholar]; b Chen C.-H.; Chen C.-Y.; Lo Y.-W.; Mao C.-F.; Liao W.-T. Studies of Glycolysis of Poly(ethylene terephthalate) Recycled from Postconsumer Soft-Drink Bottles. I. Influences of Glycolysis Conditions. J. Appl. Polym. Sci. 2001, 80, 943–948. 10.1002/app.1174. [DOI] [Google Scholar]; c Shukla S. R.; Kulkarni K. S. Depolymerization of Poly(ethylene terephthalate) Waste. J. Appl. Polym. Sci. 2002, 85, 1765–1770. 10.1002/app.10714. [DOI] [Google Scholar]; d Troev K.; Grancharov G.; Tsevi R.; Gitsov I. A Novel Catalyst for the Glycolysis of Poly(ethylene terephthalate). J. Appl. Polym. Sci. 2003, 90, 1148–1152. 10.1002/app.12711. [DOI] [Google Scholar]; e López-Fonseca R.; Duque-Ingunza I.; de Rivas B.; Arnaiz S.; Gutiérrez-Ortiz J. I. Chemical recycling of post-consumer PET wastes by glycolysis in the presence of metal salts. Polym. Degrad. Stab. 2010, 95, 1022–1028. 10.1016/j.polymdegradstab.2010.03.007. [DOI] [Google Scholar]; f Fukushima K.; Coulembier O.; Lecuyer J. M.; Almegren H. A.; Alabdulrahman A. M.; Alsewailem F. D.; Mcneil M. A.; Dubois P.; Waymouth R. M.; Horn H. W.; Rice J. E.; Hedrick J. L. Organocatalytic Depolymerization of Poly(ethylene terephthalate). J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 1273–1281. 10.1002/pola.24551. [DOI] [Google Scholar]; g Imran M.; Kim D. H.; Al-Masry W. A.; Mahmood A.; Hassan A.; Haider S.; Ramay S. M. Manganese-, Cobalt-, and Zinc-Based Mixed-Oxide Spinels as Novel Catalysts for the Chemical Recycling of Poly(ethylene terephthalate) via Glycolysis. Polym. Degrad. Satbil. 2013, 98, 904–915. 10.1016/j.polymdegradstab.2013.01.007. [DOI] [Google Scholar]; h Chen F.; Wang G.; Li W.; Yang F. Glycolysis of Poly(ethylene terephthalate) over Mg-Al Mixed Oxides Catalysts Derived from Hydrotalcites. Ind. Eng. Chem. Res. 2013, 52, 565–571. 10.1021/ie302091j. [DOI] [Google Scholar]; i Fukushima K.; Coady D. J.; Jones G. O.; Almegren H. A.; Alabdulrahman A. M.; Alsewailem F. D.; Horn H. W.; Rice J. E.; Hedrick J. L. Unexpected Efficiency of Cyclic Amidine Catalysts in Depolymerizing Poly(ethylene terephthalate). J. Polym. Sci. Part A: Polym.Chem. 2013, 51, 1606–1611. 10.1002/pola.26530. [DOI] [Google Scholar]; j Al-Sabagh A. M.; Yehia F. Z.; Eissa A.-M. M. F.; Moustafa M. E.; Eshaq G.; Rabie A.-R. M.; ElMetwally A. E. Glycolysis of Poly(ethylene terephthalate) Catalyzed by the Lewis Base Ionic Liquid [Bmim][OAc]. Ind. Eng. Chem. Res. 2014, 53, 18443–18451. 10.1021/ie503677w. [DOI] [Google Scholar]; k Al-Sabagh A. M.; Yehia F. Z.; Eshaq G.; ElMetwally A. E. Ionic Liquid-Coordinated Ferrous Acetate Complex Immobilized on Bentonite As a Novel Separable Catalyst for PET Glycolysis. Ind. Eng. Chem. Res. 2015, 54, 12474–12481. 10.1021/acs.iecr.5b03857. [DOI] [Google Scholar]; l Wang Q.; Yao X.; Geng Y.; Zhou Q.; Lu X.; Zhang S. Deep eutectic solvents as highly active catalysts for the fast and mild glycolysis of poly(ethylene terephthalate)(PET). Green Chem. 2015, 17, 2473–2479. 10.1039/C4GC02401J. [DOI] [Google Scholar]; m Wang Q.; Geng Y.; Lu X.; Zhang S. First-Row Transition Metal-Containing Ionic Liquids as Highly Active Catalysts for the Glycolysis of Poly(ethylene terephthalate) (PET). ACS Sustainable Chem. Eng. 2015, 3, 340–348. 10.1021/sc5007522. [DOI] [Google Scholar]; n Al-Sabagh A. M.; Yehia F. Z.; Harding D. R. K.; Eshaq G.; ElMetwally A. E. Fe3O4-boosted MWCNT as an efficient sustainable catalyst for PET glycolysis. Green Chem. 2016, 18, 3997–4003. 10.1039/C6GC00534A. [DOI] [Google Scholar]; o Suo Q.; Zi J.; Bai Z.; Qi S. The Glycolysis of Poly(ethylene terephthalate) Promoted by Metal Organic Framework (MOF) Catalysts. Catal. Lett. 2017, 147, 240–252. 10.1007/s10562-016-1897-0. [DOI] [Google Scholar]; p Jehanno C.; Flores I.; Dove A. P.; Müller A. J.; Ruipérez F.; Sardon H. Organocatalysed depolymerisation of PET in a fully sustainable cycle using thermally stable protic ionic salt. Green Chem. 2018, 20, 1205–1212. 10.1039/C7GC03396F. [DOI] [Google Scholar]; q Veregue F. R.; da Silva C. T. P.; Moisés M. P.; Meneguin J. G.; Guilherme M. R.; Arroyo P. A.; Favaro S. L.; Radovanovic E.; Girotto E. M.; Rinaldi A. W. Ultrasmall Cobalt Nanoparticles as a Catalyst for PET Glycolysis: A Green Protocol for Pure Hydroxyethyl Terephthalate Precipitation without Water. ACS Sustainable Chem. Eng. 2018, 6, 12017–12024. 10.1021/acssuschemeng.8b02294. [DOI] [Google Scholar]; r Liu B.; Fu W.; Lu X.; Zhou Q.; Zhang S. Lewis Acid–Base Synergistic Catalysis for Polyethylene Terephthalate Degradation by 1,3-Dimethylurea/Zn(OAc)2 Deep Eutectic Solvent. ACS Sustainable Chem. Eng. 2019, 7, 3292–3300. 10.1021/acssuschemeng.8b05324. [DOI] [Google Scholar]; s Liu Y.; Yao X.; Yao H.; Zhou Q.; Xin J.; Lu X.; Zhang S. Degradation of poly(ethylene terephthalate) catalyzed by metal-free choline-based ionic liquids. Green Chem. 2020, 22, 3122–3131. 10.1039/D0GC00327A. [DOI] [Google Scholar]; t Wang L.; Nelson G. A.; Toland J.; Holbrey J. D. Glycolysis of PET Using 1,3-Dimethylimidazolium-2-Carboxylate as an Organocatalyst. ACS Sustainable Chem. Eng. 2020, 8, 13362–13368. 10.1021/acssuschemeng.0c04108. [DOI] [Google Scholar]; u Chiaie K. R. D.; McMahon F. R.; Williams E. J.; Price M. J.; Dove A. P. Dual-catalytic depolymerization of polyethylene terephthalate (PET). Polym. Chem. 2020, 11, 1450–1453. 10.1039/C9PY01920K. [DOI] [Google Scholar]; v Fan C.; Zhang L.; Zhu C.; Cao J.; Xu Y.; Sun P.; Zeng G.; Jiang W.; Zhang Q. Efficient glycolysis of PET catalyzed by a metal-free phosphazene base: the important role of EG. Green Chem. 2022, 24, 1294–1301. 10.1039/D1GC03885K. [DOI] [Google Scholar]; w Yang R.; Xu G.; Dong B.; Guo X.; Wang Q. Selective, Selective, Sequential, and “One-Pot” Depolymerization Strategies for Chemical Recycling of Commercial Plastics and Mixed Plastics. ACS Sustainable Chem. Eng. 2022, 10, 9860–9871. 10.1021/acssuschemeng.2c01708. [DOI] [Google Scholar]; x Kaiho S.; Hmayed A. A. R.; Chiaie K. R. D.; Worch J. C.; Dove A. P. Designing Thermally Stable Organocatalysts for Poly(ethylene terephthalate) Synthesis: Toward a One-Pot, Closed-Loop Chemical Recycling System for PET. Macromolecules 2022, 55 (23), 10628–10639. 10.1021/acs.macromol.2c01410. [DOI] [Google Scholar]; y D’Aniello S.; Laviéville S.; Santulli F.; Simon M.; Sellitto M.; Tedesco C.; Thomas C. M.; Mazzeo M. Homoleptic phenoxy-imine pyridine zinc complexes: efficient catalysts for solvent free synthesis and chemical degradation of polyesters. Catal. Sci. Technol. 2022, 12, 6142–6154. 10.1039/D2CY01092E. [DOI] [Google Scholar]
  7. For selected examples on the other alcoholysis of PET, see:; a Mansour S. H.; Ikladious N. E. Depolymerization of poly(ethylene terephthalate) wastes using 1,4-butanediol and triethylene glycol. Polym. Test. 2002, 21, 497–505. 10.1016/S0142-9418(01)00115-5. [DOI] [Google Scholar]; Liu S.; Wang Z.; Li L.; Yu S.; Xie C.; Liu F. Butanol Alcoholysis Reaction of Polyethylene Terephthalate Using Acidic Ionic Liquid as Catalyst. J. Appl. Polym. Sci. 2013, 130, 1840–1844. 10.1002/app.39246. [DOI] [Google Scholar]
  8. For selected examples on transesterification of polyesters by our group, see:; a Abe R.; Komine N.; Nomura K.; Hirano M. La(III)-Catalysed degradation of polyesters to monomers via transesterifications. Chem. Commun. 2022, 58, 8141–8144. 10.1039/D2CC02448A. [DOI] [PubMed] [Google Scholar]; b Nomura K.; Aoki T.; Ohki Y.; Kikkawa S.; Yamazoe S. Transesterification of Methyl-10-undecenoate and Poly(ethylene adipate) Catalyzed by (Cyclopentadienyl)titanium Trichlorides as Model Chemical Conversions of Plant Oils and Acid-, Base-Free Chemical Recycling of Aliphatic Polyesters. ACS Sustainable Chem. Eng. 2022, 10, 12504–12509. 10.1021/acssuschemeng.2c04877. [DOI] [Google Scholar]; c Sudhakaran S.; Siddiki S. M. A. H.; Kitiyanan B.; Nomura K. CaO Catalyzed Transesterification of Ethyl 10-Undecenoate as a Model Reaction for Efficient Conversion of Plant Oils and Their Application to Depolymerization of Aliphatic Polyesters. ACS Sustainable Chem. Eng. 2022, 10, 12864–12872. 10.1021/acssuschemeng.2c04287. [DOI] [Google Scholar]; d Ohki Y.; Ogiwara Y.; Nomura K. Depolymerization of Polyesters by Transesterification with Ethanol using (Cyclopentadienyl)titanium Trichlorides. Catalysts 2023, 13, 421–430. 10.3390/catal13020421. [DOI] [Google Scholar]
  9. For selected examples on aminolysis of PET, see:; a Spychaj T.; Fabrycy E.; Spychaj S.; Kacperski M. Aminolysis and aminoglycolysis of waste poly(ethylene terephthalate). J. Mater. Cycles Waste Manage. 2001, 3, 24–31. 10.1007/s10163-000-0036-5. [DOI] [Google Scholar]; b Soni R. K.; Singh S. Synthesis and Characterization of Terephthalamides from Poly(ethylene terephthalate) Waste. J. Appl. Polym. Sci. 2005, 96, 1515–1528. 10.1002/app.21502. [DOI] [Google Scholar]; c Shukla S. R.; Harad A. M. Aminolysis of polyethylene terephthalate waste. Polym. Degrad. Satbil. 2006, 91, 1850–1854. 10.1016/j.polymdegradstab.2005.11.005. [DOI] [Google Scholar]; d Parab Y. S.; Shah R. V.; Shukla S. R. Microwave irradiated synthesis and characterization of 1,4-phenylene bis-oxazoline form bis-(2-hydroxyethyl) terephthalamide obtained by depolymerization of poly (ethylene terephthalate) (PET) bottle wastes. Curr. Chem. Lett. 2012, 1, 81–90. 10.5267/j.ccl.2012.3.003. [DOI] [Google Scholar]; e Fukushima K.; Lecuyer J. M.; Wei D. S.; Horn H. W.; Jones G. O.; Al-Megren H. A.; Alabdulrahman A. M.; Alsewailem F. D.; McNeil M. A.; Rice J. E.; Hedrick J. L. Advanced chemical recycling of poly(ethylene terephthalate) through organocatalytic aminolysis. Polym. Chem. 2013, 4, 1610–1616. 10.1039/C2PY20793A. [DOI] [Google Scholar]; f Ghosal K.; Sarkar K. Poly(ester amide) derived from municipal polyethylene terephthalate waste guided stem cells for osteogenesis. New J. Chem. 2019, 43, 14166–14178. 10.1039/C9NJ02940K. [DOI] [Google Scholar]; g Teotia M.; Chauhan M.; Khan A.; Soni R. K. Facile synthesis, characterization, and ab-initio DFT simulations of energy efficient NN’ dialkyl 1,4 benzene dicarboxamide monomers recovered from PET bottle waste. J. Appl. Polym. Sci. 2020, 137, 49321–49332. 10.1002/app.49321. [DOI] [Google Scholar]
  10. In our preliminary experiments, Hf(OTf)4-catalyst or no-catalyst conditions were also provided to 1, though these reactions resulted in complex mixtures, including starting PET and unknown byproducts.
  11. The Mw and Mn values of the PET pellet we used could not be measured due to the insolubility, even by a high temperature GPC (in o-dichlorobenzene at 140 °C). This property probably requires high temperatures and a long reaction time for our depolymerization.
  12. At the middle stage of the optimal conditions (12 h, 74% conversion), these carbonyl signals appeared more clearly; 13C{1H} NMR (CDCl3, rt): δ 169.1 (1), 169.0, 168.9, 165.6, 165.2. However, further information, such as molecular weights, has not been determined at this stage.
  13. Although the detailed mechanism is unclear at this stage, the Cp*Ti-amido species is one of the possible intermediates,; a Kissounko D. A.; Guzei I. A.; Gellman S. H.; Stahl S. S. Titanium(IV)-Mediated Conversion of Carboxamides to Amidines and Implications for Catalytic Transamidation. Organometallics 2005, 24, 5208–5210. 10.1021/om050768y. [DOI] [Google Scholar]; b Kissounko D. A.; Hoerter J. M.; Guzei I. A.; Cui Q.; Gellman S. H.; Stahl S. S. TiIV-Mediated Reactions between Primary Amines and Secondary Carboxamides: Amidine Formation Versus Transamidation. J. Am. Chem. Soc. 2007, 129, 1776–1783. 10.1021/ja0650293. [DOI] [PubMed] [Google Scholar]
  14. All reactions were conducted under a nitrogen atmosphere, and morpholine was used without any purification. However, this PET depolymerization can also be carried out in air (see Table S1 in the Supporting Information).
  15. The crystal structures of 1 and 7 have been reported: (1); a Jones P. G.; Ossowski J.; Kus P. Crystal Structures of Seven Terephthaldiamide Derivatives. Z. Naturforsch. B 2002, 57, 914–921. 10.1515/znb-2002-0812. [DOI] [Google Scholar]; (7); b Kolev T.; Preut H.; Bleckmann P.; Juchnovski I. 1,4-Dibenzoylbenzene. Acta Cryst. C 1992, 48, 1715–1717. 10.1107/S0108270192006887. [DOI] [Google Scholar]
  16. The morpholine amide 1′ was synthesized by a typical method from the acid chloride with morpholine. The details of 1′, including the 13C{1H} NMR data, are described in the Supporting Information.
  17. For selected examples on the Vilsmeier–Haack reaction of morpholine amides, see:; a Wallace D. M.; Leung S. H.; Senge M. O.; Smith K. M. Rational Tetraarylporphyrin Syntheses: Tetraarylporphyrins from the MacDonald Route. J. Org. Chem. 1993, 58, 7245–7257. 10.1021/jo00077a056. [DOI] [Google Scholar]; b Bröring M. A Facile and Efficient Method for the Preparation of New meso-Arylbisdipyrrins. Synthesis 2000, 2000, 1291–1294. 10.1055/s-2000-6425. [DOI] [Google Scholar]
  18. For selected examples on the Grignard or organolithium reaction of morpholine amides, see:; a Martín R.; Romea P.; Tey C.; Urpí F.; Vilarrasa J. Simple and Efficient Preparation of Ketones from Morpholine Amides. Synlett 1997, 12, 1414–1416. 10.1055/s-1997-1050. [DOI] [Google Scholar]; b Kurosu M.; Marcin L. R.; Grinsteiner T. J.; Kishi Y. Total Synthesis of (±)-Batrachotoxinin A. J. Am. Chem. Soc. 1998, 120, 6627–6628. 10.1021/ja981258g. [DOI] [Google Scholar]; c Jackson M. M.; Leverett C.; Toczko J. F.; Roberts J. C. An Unexpected Equilibrium Process Associated with a Standard Approach to Ynone Synthesis. J. Org. Chem. 2002, 67, 5032–5035. 10.1021/jo025682i. [DOI] [PubMed] [Google Scholar]; d Dhoro F.; Kristensen T. E.; Stockmann V.; Yap G. P. A.; Tius M. A. Asymmetric Amine-Intercepted Nazarov Cyclization. J. Am. Chem. Soc. 2007, 129, 7256–7257. 10.1021/ja0718873. [DOI] [PMC free article] [PubMed] [Google Scholar]; e Concellón J. M.; Rodríguez-Solla H.; Méjica C.; Blanco E. G. Stereospecific Cyclopropanation of Highly Substituted C–C Double Bonds Promoted by CrCl2. Stereoselective Synthesis of Cyclopropanecarboxamides and Cyclopropyl Ketones. Org. Lett. 2007, 9, 2981–2984. 10.1021/ol070896d. [DOI] [PubMed] [Google Scholar]; f Dittrich N.; Jung E.-K.; Davidson S. J.; Barker D. An acyl-Claisen/Paal-Knorr approach to fully substituted pyrroles. Tetrahedron 2016, 72, 4676–4689. 10.1016/j.tet.2016.06.049. [DOI] [Google Scholar]
  19. For selected examples on DIBAL reaction of morpholine amides, see:; a Wilhelmsen C. A.; Zhang X.; Myhill J. A.; Morken J. P. Enantioselective Synthesis of Tertiary β-Boryl Amides by Conjunctive Cross-Coupling of Alkenyl Boronates and Carbamoyl Chlorides. Angew. Chem., Int. Ed. 2022, 61, e202116784 10.1002/anie.202116784. [DOI] [PMC free article] [PubMed] [Google Scholar]; b Kim S. Y.; Kim Y. R.; Kim H. T.; Jaladi A. K.; An D. K. Diisobutyl(morpholino)aluminum-Enabled Partial Reductions with DIBALH: Highly Practical Synthesis of Aldehydes from Carboxylic Acid Derivatives. ChemistrySelect 2022, 7, e202202351 10.1002/slct.202202351. [DOI] [Google Scholar]
  20. a Kratish Y.; Li J.; Liu S.; Gao Y.; Marks T. J. Polyethylene Terephthalate Deconstruction Catalyzed by a Carbon-Supported Single-Site Molybdenum-Dioxo Complex. Angew. Chem., Int. Ed. 2020, 59, 19857–19861. 10.1002/anie.202007423. [DOI] [PubMed] [Google Scholar]; b Kratish Y.; Marks T. J. Efficient Polyethylene Hydrogenolytic Deconstruction via Tandem Catalysis. Angew. Chem., Int. Ed. 2022, 61, e202112576 10.1002/anie.202112576. [DOI] [PubMed] [Google Scholar]
  21. Terephthalaldehyde (5) has been used as a monomer of poly(p-phenylene vinylene) (PPV):; a Junkers T.; Vandenbergh J.; Adriaensens P.; Lutsen L.; Vanderzande D. Synthesis of poly(p-phenylene vinylene) materials via the precursor routes. Polym. Chem. 2012, 3, 275–285. 10.1039/C1PY00345C. [DOI] [Google Scholar]; b Blayney A. J.; Perepichka I. F.; Wudl F.; Perepichka D. F. Advances and Challenges in the Synthesis of Poly(p-phenylene vinylene)-Based Polymers. Isr. J. Chem. 2014, 54, 674–688. 10.1002/ijch.201400067. [DOI] [Google Scholar]
  22. 1,4-Dibenzoylbenzene (7) and its derivatives have been recognized as versatile precursors for π-conjugated compounds:; a Higuchi M.; Kanazawa H.; Yamamoto K. The First Electroresponsive Phenylazomethine Macrocycles: Highly Preferential Formation and Regular Molecular Packing. Org. Lett. 2003, 5, 345–347. 10.1021/ol0273755. [DOI] [PubMed] [Google Scholar]; b Banerjee M.; Emond S. J.; Lindeman S. V.; Rathore R. Practical Synthesis of Unsymmetrical Tetraarylethylenes and Their Application for the Preparation of [Triphenylethylene–Spacer–Triphenylethylene] Triads. J. Org. Chem. 2007, 72, 8054–5061. 10.1021/jo701474y. [DOI] [PubMed] [Google Scholar]; c Stȩpień M.; Szyszko B.; Latos-Grażyński L. Steric Control in the Synthesis of p-Benziporphyrins. Formation of a Doubly N-Confused Benzihexaphyrin Macrocycle. Org. Lett. 2009, 11, 3930–3933. 10.1021/ol9016222. [DOI] [PubMed] [Google Scholar]; d Chan C. Y. K.; Zhao Z.; Lam J. W. Y.; Liu J.; Chen S.; Lu P.; Mahtab F.; Chen X.; Sung H. H. Y.; Kwok H. S.; Ma Y.; Williams I. D.; Wong K. S.; Tang B. Z. Efficient Light Emitters in the Solid State: Synthesis, Aggregation-Induced Emission, Electroluminescence, and Sensory Properties of Luminogens with Benzene Cores and Multiple Triarylvinyl Peripherals. Adv. Funct. Mater. 2012, 22, 378–389. 10.1002/adfm.201102030. [DOI] [Google Scholar]; e Kaczmarczyk B. FTIR study on conjugation in polyketimines. J. Mol. Struct. 2013, 1040, 149–154. 10.1016/j.molstruc.2013.02.041. [DOI] [Google Scholar]; f Baysec S.; Preis E.; Allard S.; Scherf U. Very High Solid State Photoluminescence Quantum Yields of Poly(tetraphenylethylene) Derivatives. Macromol. Rapid Commun. 2016, 37, 1802–1806. 10.1002/marc.201600485. [DOI] [PubMed] [Google Scholar]; g Chernyy S.; Mahalik J. P.; Kumar R.; Kirkensgaard J. J. K.; Arras M. M. L.; Kim H.; Schulte L.; Ndoni S.; Smith G. S.; Mortensen K.; Sumpter B. G.; Russell T. P.; Almdal K. On the Morphological Behavior of ABC Miktoarm Stars Containing Poly(cis 1,4-isoprene), Poly(styrene), and Poly(2-vinylpyridine). J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1491–1504. 10.1002/polb.24733. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

gg3c00037_si_001.pdf (6.7MB, pdf)

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

Articles from ACS Organic & Inorganic Au are provided here courtesy of American Chemical Society

RESOURCES